Telomerase and Wnt signaling

Now that research into telomeres and telomerase has (finally) garnered a Nobel Prize, it's a good time to write about recent research on the subject.

Seminal work on telomeres by Elizabeth Blackburn, one of the Nobel winners, was published way back in 1978, and active studies have been going on ever since. So perhaps it's not surprising that the rate of new findings is not so rapid as occurs in newer areas – such as stem cells.

But fascinating new results on telomeres and telomerase do still appear, and one of them connects with more recent research areas – such as Wnt signaling and... stem cells.

Here's the press release:

Discovery pinpoints new connection between cancer cells, stem cells (7/1/09)

A molecule called telomerase, best known for enabling unlimited cell division of stem cells and cancer cells, has a surprising additional role in the expression of genes in an important stem cell regulatory pathway, say researchers at the Stanford University School of Medicine. The unexpected finding may lead to new anticancer therapies and a greater understanding of how adult and embryonic stem cells divide and specialize.

Don't bother getting excited about the "new anticancer therapies" bit. That's just boilerplate that about 77.3% of all press releases dealing with cell biology contain, presumably to impress the rubes. If you need something to get excited about, you might recall that telomerase is also being investigated intensively in connection with issues of aging and longevity, independently from cancer. However, while it's possible that something of medical significance may come from this research, that's probably way down the road.

Before discussing the new research, let's review some of the background on telomeres, telomerase, and Wnt signaling.

To begin with, a telomere is a series of short repeated segments of DNA found at the ends of chromosomes in all eukaryotic cells. In cells of vertebrate animals the repeated segment is TTAGGG (where the letters represent nucleobases: T=thymine, A=adenine, G=guanine).

The total number of nucleotides in these repeated segments varies a great deal from species to species, but in humans it is (initially) about 10,000 nucleotides (or ~1700 complete segments). I say "initially", because part of the telomere is lost every time a cell divides – perhaps 50 to 200 nucleotides per division. Obviously, this means that an adult cell newly derived from a stem cell can divide at most 50-200 times before the telomere is all gone. In practice, the number is a lot less than the maximum, perhaps 40 to 60 times, which is called the Hayflick limit. Cells are programmed to stop dividing on reaching this limit, since otherwise useful DNA would be lost or damaged upon further division.

Why does this loss occur? It seems to be somewhat of an accident of the nitty-gritty details of how DNA replication occurs during cell division. I won't go into that, since it's best explained with some diagrams; you can read about it at Wikipedia. In fact, in the early days of molecular biology (around 1972), what happens at the end of chromosomes during DNA replication was rather puzzling, and the puzzle was called the "end replication problem". Now it's pretty well understood, though somewhat messy.

In any case, the loss of DNA from the telomeres during cell division is a fact, and it conveniently explains the existence of the Hayflick limit, which had been recognized since 1965. What happens when the teleomere is all used up and the limit is reached? Cells that have reached the limit don't necessarily die (though they might), but they do stop dividing, and they enter a static phase of cell life known as senescence.

If you think about it, senescence can be a problem, especially in certain tissues that need to continually replenish their cells, such as skin and the lining of the intestines, as well as hair, fingernails, etc. How is senescence circumvented in such tissues? The answer is (adult) stem cells. It turns out that stem cells are not subject to the Hayflick limit. It's not clear whether they are subject to limits at all.

Like any other cell type, stem cells also divide (by the process technically known as mitosis). But there is one difference from the way "ordinary" cells divide. Stem cells can divide asymmetrically, where one daughter cell is another stem cell, but the other daughter is a "progenitor cell", which is close to a normal, fully-differentiated adult cell of a fixed tissue type. Progenitor cells differentiate further into the final form when they divide, and they are able to divide only a limited number of times.

So how is it that stem cells are able to escape the Hayflick limit, dividing an indefinite number of times, even though they are also subject to the same loss of telomere nucleotides with each division? The answer is the enzyme telomerase, which is able to rebuild shortened teleomeres. It's fortunate for the longevity of complex organisms that telomerase exists, otherwise stem cells would not be able to divide often enough to allow tissues exposed to harsh conditions (such as skin and intestinal lining) to be replenished.

In order to explain the results of the research to be described here we need to say more about telomerase, but first let's summarize the function of telomeres. At first it may seem that all they do is compensate for the sloppy way that DNA replication works, by providing long stretches of expendable DNA at the ends of chromosomes. But there is one other notable function: teleomeres are a useful "cap" at the end of a chromosome that is recognizable to cellular mechanisms responsible for detecting DNA damage. If it weren't for the telomeres, the ends of chromosomes would be indistinguishable from DNA damage, which can result from errors in the replication process, as well as damage due to external agents such as ionizing radiation or harmful chemicals. Cells have various mechanisms for repairing many kinds of damage, but even if repair isn't possible, other mechanisms exist that recognize the damage and prevent further cell division or cause programmed cell death (apoptosis).

Not all DNA damage can be repaired or compensated for by apoptosis or cessation of cell division. Unrepaired DNA damage (however it occurs) is the main cause of cancer (though not the only one). So the existence of the Hayflick limit as a result of teleomere shortening acts as one defense against cancer. Cancer can be defined as the uncontrolled proliferation of cells as a result of DNA damage (affecting existing mechanisms that normally control proliferation) or other causes. So fixed limits on the number of times a cell can divide is one of a number of mechanisms organisms have to guard against cancer.

Before moving on, here's a quick summary of the functions served by telomeres: (1) compensate for the chromosome "end replication problem"; (2) make it possible for cells to distinguish chromosome ends from damaged chromosomes; (3) provide natural limits to the number of times ordinary cells can divide, as protection against cancer.

As noted above, the third of these functions is a problem for stem cells that do need the ability to divide an indefinite number of times. Repair of tissues exposed to harsh conditions is not the only circumstance this ability is needed. Another very important case is that of embryonic development. Multicellular, sexually-reproducing organisms start from a single cell (zygote). Yet there are close to 10¹⁴ cells in an adult human.

It is true that a single cell that underwent 47 cycles of cell division could theoretically produce that many cells. But that's really pushing the limits, since some cell types are needed in much larger numbers than others. The bottom line is that embryonic development is the other main circumstance when limits on cell division need to be overcome.

Telomerase is what makes this overcoming of telomere limits possible. And it does it in a pretty straightforward way. Telomerase is a complex molecule with three distinct parts. Two of these are proteins: Telomerase Reverse Transcriptase (TERT) and dyskernin, which are coded for by distinct genes. TERT does most of the work. The other part is a short piece of RNA, called the telomerase RNA component (TERC), which contains and is somewhat longer than the repeat unit (TTAGGG in vertebrates). Like any other reverse transcriptase enzyme (other examples of which occur in RNA viruses such as HIV), TERT simply translates a piece of RNA into DNA and inserts it into a chromosome. In the case of telomerase, the RNA is carried inside the enzyme complex itself. Telomerase does its job simply by replacing the telomere DNA that is lost from chromosomes during mitosis.

So telomerase performs the function of rebuilding telomeres, and this is essential in tissues where cells must proliferate rapidly, such as in embryonic development and tissue regeneration. But as we noted above, if the restriction on cell division is circumvented, the risk of cancer goes up. Although telomerase serves an essential purpose in specialized contexts, it also makes it possible for cells to become cancerous. Consequently, telomerase is not expressed in most adult body cells. However, telomerase is expressed in about 90% of tumor cells.

It might seem as though one approach to controlling or even destroying cancer could involve either inhibiting telomerase or developing a vaccine using telomerase to induce an immune response against telomerase-rich cancer cells. There are in fact various clinical trials exploring both techniques. But this is tricky and rather risky, because as we've observed, telomerase is needed in stem cells required for tissue regneration, at least once the cells have begun proliferating. Those cells need to be protected while they are simply doing their job – replenishing skin and intestinal linings, for example. We need to understand how such cells are controlled so that they work without leading to cancer.

Anyhow, TERT is a protein that is an essential component of telomerase, which plays an important role in cell proliferation. The new research we're finally almost ready to discuss shows that, surprisingly, TERT also plays a role in a completely different aspect of cell proliferation having nothing to do with telomeres. That's where Wnt signaling comes in.

Wnt signaling is a subject we've looked at several times before. Some of the relevant articles are here, here, here, and here. Wnt was first noticed in connection with embryonic development and tissue regeneration. This article has many examples. The name Wnt alludes to a gene called "wingless", because the gene causes fruit flies to lack wings when the gene is mutated.

Recently Wnt's importance in stem cells has also received a lot of attention. Some of our discussion of that may be found here, here, here, and here. Wnt's relevance to cancer and cancer stem cells is often touched on in most of all the articles listed.

Basically, Wnt is the name of a family of proteins that play an important role in signals promoting cell proliferation, especially in the context of embryonic development and tissue renewal (skin, intestines, hair, and immune system cells). Since Wnt proteins promote proliferation, they also play a role in cancer.

The fact, then, that new research shows teleomerase can enhance Wnt signaling is signficant as a second, entirely separate route through which it plays a role in both normal stem cell function and cancer.

Here's the research abstract:

Telomerase modulates Wnt signalling by association with target gene chromatin (7/2/09)

Stem cells are controlled, in part, by genetic pathways frequently dysregulated during human tumorigenesis. Either stimulation of Wnt/β-catenin signalling or overexpression of telomerase is sufficient to activate quiescent epidermal stem cells in vivo, although the mechanisms by which telomerase exerts these effects are not understood. Here we show that telomerase directly modulates Wnt/β-catenin signalling by serving as a cofactor in a β-catenin transcriptional complex. The telomerase protein component TERT (telomerase reverse transcriptase) interacts with BRG1 (also called SMARCA4), a SWI/SNF-related chromatin remodelling protein, and activates Wnt-dependent reporters in cultured cells and in vivo. TERT serves an essential role in formation of the anterior–posterior axis in Xenopus laevis embryos, and this defect in Wnt signalling manifests as homeotic transformations in the vertebrae of Tert^-/- mice. Chromatin immunoprecipitation of the endogenous TERT protein from mouse gastrointestinal tract shows that TERT physically occupies gene promoters of Wnt-dependent genes. These data reveal an unanticipated role for telomerase as a transcriptional modulator of the Wnt/β-catenin signalling pathway.

That's a pretty good summary of the paper, but I imagine most people would like a bit more explanation of what's going on.

Previous research had disclosed some interesting "coincidences" involving stems cells and embryonic development, in which both telomerase and Wnt signaling seemed to have similar effects, even though no obvious connection was known. For example, epidermal (skin) stem cells spend most of their time in a quiescent (non-dividing) state. The main function of Wnt proteins is to carry signals between cells that inform target cells of a need to start dividing, for example during various stages of embryonic development or to heal wounds. The curious thing is that telomerase was known to have a similar effect as Wnt signaling on some stem cells. This coincidence is enough to motivate looking for a connection.

If you are really into this sort of thing, you might want to refer to this diagram of various cell signaling pathways, including that of Wnt. One of the things it illustrates is the position of β-catenin in the Wnt pathway. β-catenin operates at the end of the pathway, where it becomes a part of a protein complex that includes transcription factors (known as TCF/LEF), and the complex causes expression of a variety of Wnt-regulated genes, which go on to enable cell proliferation.

One of the main findings of the research is that TERT interacts with another protein called BRG1 (or SMARCA4), and together these become important parts ("cofactors") of the gene-regulating protein complex. It was also determined that TERT is the only component of telomerase that is involved with Wnt signaling.

Apparently TERT is more important for some instances of Wnt signaling than for others. For example, low levels of TERT in embryos of the frog Xenopus laevis resulted in very abnormal development of the frog embryos (in terms for the anterior-posterior axis of the body). But low levels of TERT at a later stage of development caused only somewhat more subtle defects in formation of ribs in the embryo. Similar somewhat minor effects also occured with low levels of TERT in mouse embryos, and these defects were much like the effects of low levels of β-catenin.

So what, then, is so interesting about this research? It's satisfying to know the reasons for what were previously just observed coincidences, such as the fact that telomerase is not only very important for the ability of stem cells to divide freely (during embryonic development and tissue regeneration) but that it also can help stimulate stem cell division.

But what makes this research especially significant is the importance of the biological processes in which both telomerase and Wnt signaling play major roles – namely embryonic development, tissue regeneration and repair, and cancer. The latter two processes are especially important for medical reasons, although we're still a long way from being able to use this knowledge about telomerase and Wnt signaling for therapeutic purposes.

As far as cancer is concerned, we need to understand much more about how telomerase and Wnt signaling work in various types of stem cells, given that stem cells may play a big role in some types of cancer (though not in others).

But tissue regeneration and repair are also of significant medical interest, since it is the eventual inability of various types of tissues to replenish themselves in old age that is responsible for the many debilities that appear in old age. Many people have speculated that telomerase could help alleviate this problem – provided it does not also lead to facilitating the development of cancer.

Finally, we are left with the puzzle of how it is that TERT happens to play important – yet quite distinct – roles in two very separate processes that are, neverthelesss, both important for cell proliferation. We don't know the answer to this, but we can speculate. Perhaps it's because TERT has to be around anyway for stem cells during embryogenesis and tissue regeneration. That being the case, perhaps at some point in evolution, TERT also happened to help boost Wnt signaling a little. The effect of amplifying Wnt signaling in the same contexts that telomerase was needed would be advantageous and worth conserving. This kind of double duty should lead to more efficient use of cell resources.

Park, J., Venteicher, A., Hong, J., Choi, J., Jun, S., Shkreli, M., Chang, W., Meng, Z., Cheung, P., Ji, H., McLaughlin, M., Veenstra, T., Nusse, R., McCrea, P., & Artandi, S. (2009). Telomerase modulates Wnt signalling by association with target gene chromatin Nature, 460 (7251), 66-72 DOI: 10.1038/nature08137

Further reading:

Cell biology: The not-so-odd couple (7/2/09) – expository article in Nature about the telomerase-Wnt research

Nobel in medicine honors discoveries of telomeres and telomerase (10/5/09) – news article in Science News

Nobel for insights into ageing and cancer (10/5/09) – news article in New Scientist

Chromosome protection scoops Nobel (10/5/09) – news article in Nature

Three Americans Win Physiology or Medicine Nobel (10/5/09) – news article at ScienceNOW.com

Work on Telomeres Wins Nobel Prize in Physiology or Medicine for 3 U.S. Genetic Researchers (10/5/09) – news article in Scientific American

Nobel Winners Isolate Protein Behind Immortality, Cancer (10/5/09) – news article at Wired.com

Tags:

telomeres, telomerase, Wnt signaling

Cancer and Wnt signaling

We've discussed the Wnt signaling pathway several times before, most extensively here. The pathway involves a variety of proteins and their corresponding genes.

Wnt signaling is especially important in embryonic development. In fact, the name is derived from the name of a gene in fruit files that, when mutated, results in wingless flies.

Wnt signaling is also important in stem cells (see here, here, here). As a special case, Wnt is involved in cancer stem cells (see here).

It is far from clear that cancer stem cells are an important factor in many types of cancer. But it appears that Wnt signaling does sometimes play a role itself. Wnt has been found to assist metastasis of colon cancer, and new research suggests it also helps lung cancer to spread quickly to bones and the brain.

Research reveals what drives lung cancer's spread (7/2/09)

Researchers discovered that the same cellular pathway that has been shown to be involved with the spread of colorectal cancer is also responsible for providing lung cancer with an enhanced ability to infiltrate and colonize other organs without delay and with little need to adapt to its new environment. This is a dramatic departure from other cancers, like breast cancer, in which recurrences tend to emerge following years of remission, suggesting that such cancer cells initially lack - and need time to acquire - the characteristics and ability to spread to other organs.

The investigators hypothesized that because not all lung tumors have spread before diagnosis and removal, metastasis may depend on some added feature beyond the mutations that initiate these tumors.

Researchers used bioinformatics to interrogate large collections of lung tumor samples. They found that the WNT cell-signaling pathway was the only one out of the six pathways tested that was hyperactive in lung tumors that went on to metastasize and was normal in those that did not spread. They also observed that WNT hyperactivity was associated with aggressive biological tumor characteristics and poor clinical outcome, suggesting that cancer metastasis is linked to poor survival.

Other research groups, only a few months ago, had found evidence of Wnt involvement in a type of brain cancer, though not one due to metastasis from another location.

Study finds biological clue in brain tumour development (3/18/09)

Clinician -scientists at the University's Children's Brain Tumour Research Centre, working on behalf of the Children's Cancer and Leukaemia Group (CCLG), have studied the role of the WNT biological pathway in central nervous system primitive neuroectodermal tumours (CNS PNET), a type of brain tumour that predominantly occurs in children and presently has a very poor prognosis.

In a paper published in the British Journal of Cancer, they have shown that in over one-third of cases, the pathway is 'activated', suggesting that it plays a role in tumour development. The research also highlighted a link between WNT pathway activation and patient survival — patients who had a CNS PNET tumour that was activated survived for longer than those without pathway activation.

The reason for the link between WNT pathway activation and better patient prognosis is as yet unclear. It could be that these tumours represent a less aggressive subset or that pathway activation itself actually harms the tumour.

Tags: cancer, Wnt signaling

Induced pluripotent stem cells III

There is still some news from here that we need to look at. It has to do with reducing the risk of tumorigenicity by using a signaling protein Wnt3a – a member of the Wnt family of proteins – in place of the c-Myc transcription factor for inducing pluripotency in differentiated adult cells.

This press release gives the executive summary:

Embryonic-like Stem Cells Can Be Created Without Cancer-causing Gene (8/6/08)

Currently, IPS cells can be created by reprogramming adult cells through the use of viruses to transfer four genes (Oct4, Sox2, c-Myc and Klf4) into the cells' DNA. The activated genes then override the adult state and convert the cells to embryonic-like IPS cells.

However, this method poses significant risks for potential use in humans.

First, the viruses employed in the process, called retroviruses, are associated with cancer because they insert DNA anywhere in a cell's genome, thereby potentially triggering the expression of cancer-causing genes, or oncogenes. Second, c-Myc is a known oncogene whose overexpression can also cause cancer. For IPS cells to be employed to treat human diseases such as Parkinson's, researchers must find safe alternatives to reprogramming with retroviruses and oncogenes.

Earlier research has shown that c-Myc is not strictly required for the generation of IPS cells. However, its absence makes the reprogramming process time-consuming and highly inefficient.

To bypass these obstacles, the Whitehead researchers replaced c-Myc and its retrovirus with a naturally occurring signaling molecule called Wnt3a. When added to the fluid surrounding the cells being reprogrammed, Wnt3a promotes the conversion of adult cells into IPS cells.

What amounts to a crude form of gene therapy has been used to make IPS cells. The idea is to insert extra copies of genes for 4 different transcription factors into a cell's DNA in order to raise the expression level of those factors. The problem is that every insertion of a gene into a cell's DNA risks damage to some other random gene in the DNA. Wnt3a, on the other hand, is a signaling protein that normally affects cells only by attaching to receptors on the cell surface.

So what has been accomplished here is that the number of transcription factor genes that need to be inserted into the DNA is reduced from 4 to 3. In addition, the factor that is eliminated, c-Myc, has tumorigenicity risks of its own. Therefore, this research represents a small but useful improvement. However, it is probably only a first step.

More about the present research: here

We discussed earlier research aimed at eliminating use of c-Myc in making IPS cells here.

In fact, just a little bit earlier than the research discussed above, a team from Germany reported, on July 31 in Nature, making IPS cells by adding just two transcription factors. However, they didn't start with adult somatic cells, but with neural stem cells that already had higher expression levels of Sox2 and c-Myc. Given that, they needed to add only Oct4 and either Klf4 or more c-Myc. Here's the abstract:

Pluripotent stem cells induced from adult neural stem cells by reprogramming with two factors

Here we show that adult mouse neural stem cells express higher endogenous levels of Sox2 and c-Myc than embryonic stem cells, and that exogenous Oct4 together with either Klf4 or c-Myc is sufficient to generate iPS cells from neural stem cells. These two-factor iPS cells are similar to embryonic stem cells at the molecular level, contribute to development of the germ line, and form chimaeras. We propose that, in inducing pluripotency, the number of reprogramming factors can be reduced when using somatic cells that endogenously express appropriate levels of complementing factors.

Keep in mind that there's a further variable that's important here: the efficiency of the process, i. e. the yield of IPS cells obtained as a percentage of original cells at the beginning. It should be obvious that a fair amount of work still needs to be done to find a method of making IPS cells that's both efficient and produces cells that are potentially safe to use in therapeutic applications (as opposed to pure research).

OK, enough of that. Let's move on to something new.

One of the interesting questions about IPS cells is about exactly how close they are to actual embryonic stem cells, which are pluripotent by definition. The best way to measure the degree of closeness is by comparing gene expression levels between embryonic stem cells and IPS cells.

The next research has done exactly that. In fact, it studies gene expression levels for stem-like cells obtained from a wide variety of sources:

A new test distinguishes embryonic stem cells and those with equal therapeutic potential (8/24/08)

To distinguish adult stem cells from pluripotent cells, Loring’s team compared the gene activity of about 150 stem cell samples of various types, including reprogrammed cells, embryonic stem cells and neural stem cells. Out of this comparison popped 299 interacting genes that form what the researchers call a pluripotency network, or PluriNet. Measuring the activity of these genes could reliably distinguish the different kinds of stem cells, the team reports.

Here's the abstract for this research:

Regulatory networks define phenotypic classes of human stem cell lines

We report here the creation and analysis of a database of global gene expression profiles (which we call the 'stem cell matrix') that enables the classification of cultured human stem cells in the context of a wide variety of pluripotent, multipotent and differentiated cell types. Using an unsupervised clustering method to categorize a collection of ∼150 cell samples, we discovered that pluripotent stem cell lines group together, whereas other cell types, including brain-derived neural stem cell lines, are very diverse. Using further bioinformatic analysis we uncovered a protein–protein network (PluriNet) that is shared by the pluripotent cells (embryonic stem cells, embryonal carcinomas and induced pluripotent cells). Analysis of published data showed that the PluriNet seems to be a common characteristic of pluripotent cells, including mouse embryonic stem and induced pluripotent cells and human oocytes. Our results offer a new strategy for classifying stem cells and support the idea that pluripotency and self-renewal are under tight control by specific molecular networks.

Tags: stem cells, embryonic stem cells, induced pluripotent stem cells

Cancer stem cells II

Since we've just had a discussion of generalities about cancer stem cells (here), it seems like it would be fun to have a summary of research on CSCs that was presented at the recent meeting of the American Association for Cancer Research in San Diego, or reported since then. So here it is.

As general background, keep in mind that cell surface proteins CD44 and CD24 are considered to be markers of cancer stem cells in various (but not necessarily all) types of cancer. Also, certain cell signaling pathways are thought to be especially important in the activity of cancer stem cells. The list includes Wnt, Sonic hedgehog, Notch, and Bmi1.

Stem Cell-Like Cancer Cells Resistant To Standard Therapy, Responsive To Targeted Therapy (4/29/08)

Previous research had identified a subset of cells in breast tumors that have the ability to form colonies in culture and give rise to tumors in mouse models. Such cells are thought to be cancer stem cells. They express the cell surface glycoprotein CD44, but not CD24, and they appear to be resistant to standard chemotherapeutic agents. However, the drug lapatinib, which inhibits the HER2 pathway, seems to selectively kill these cells.

Getting To The Roots Of Breast Cancer (4/29/08)

This is another report on the research described in the previous item. It provides additional details on the research protocol.

Stem Cell Type Supposed To Be Crucial For Angiogenesis And Cancer Growth Does Not Exist? (4/22/08)

This study casts doubt on the existence of a certain type of bone-marrow derived stem cell that has been suspected of circulating in the blood and acting as a precursor to endothelial cells that make up blood vessel walls. Such cells, if they existed, would be an important target for inhibiting angiogenesis in tumors. The researchers showed, using advanced techniques with mouse models, that endothelial differentiation is not a typical function of bone-marrow derived stem cells.

Ovarian Cancer Stem Cells Identified, Characterized (4/17/08)

Researchers have identified, characterized and cloned ovarian cancer stem cells and have shown that these stem cells may be the source of ovarian cancer's recurrence and its resistance to chemotherapy. They isolated cells from samples of either peritoneal fluid or solid tumors. The cancer stem cells that were identified had traditional cancer stem cell markers including CD44 and MyD88 (which interacts with toll-like receptors to activate NF-κB).
The cells also showed a high capacity for repair and self-renewal. Such cells, when isolated, were capable of forming tumors 100 percent of the time. Within those tumors, 10 percent of the cells were CD44 positive, while 90 percent were CD44 negative, indicating that some cancer stem cells had undergone differentiation.

Stem Cells: The Role Of Cancer-initiating Cells In Diagnosis And Treatment (4/15/08)

This press release describes research presented at the AACR meeting related to stem cells and pancreatic, bladder, ovarian, and breast cancer, and glioma.

Research in pancreatic cancer found that in addition to CD44 and CD24, the enzyme aldehyde dehydrogenase was expressed in a small population of tumor cells. Cells expressing aldehyde dehydrogenase had greater growth capacity than those that didn't, and they were also associated with poorer overall survival.

In a study of breast cancer and glioma, surface markers were not found be sufficient as markers of stem cell activity. However, cells with low proteasome activity did have notably greater capacity for self-renewal and tumor production capacity. (Proteasomes are large protein complexes that degrade unneeded or damaged proteins.)

Researchers studying bladder transitional cell carcinomas found, in 40% of cases, CD44+ cells with other stem cell self-renewal patterns. In these cells, 85% had active Gli1, a part of the Hedgehog pathway, originally discovered in human glioblastoma. A relatively small percentage had active Bmi1, Stat3, or β-catenin (part of the Wnt pathway). None had active Oct4 or Nanog (pluripotent stem cells markers).

The research on ovarian cancer (noted above) involving CD44 and MyD88 markers is referenced again here.

Stem Cell Marker Controls Two Key Cancer Pathways (4/14/08)

Research into breast cancer stem cells has identified, for the first time, another gene that may be involved, Msi1. The investigators showed that Msi1 activated Wnt and Notch signaling. Other studies have shown that Msi1 is a marker of human adult stem cells in general because it has been found in human breast, colon, brain, skin, and other cells. Msi1 was found to affects mammary cells to influence whether they develop into muscle, milk duct linings, etc. Further, Msi1 was found to be expressed in particularly aggressive tumors.

Stem Cells And Cancer: Scientists Investigate A Fine Balancing Act (4/11/08)

This is a report of a general talk about how the mechanisms normally involved in balancing different functions of stem cells may also contribute to cancer. For example, research shows that Bmi1 is important for maintaining stocks of stem cells, and without it stocks of stem cells are depleted. But also Bmi1 is overactive in various cancers including brain tumors.

Secrets of cellular signaling shed light on new cancer stem cell therapies (4/10/08)

Researchers are beginning to study inhibition of signaling pathways that seem to be active in tumors fed by cancer stem cells. In this case, inhibition of the Notch pathway is being investigated as part of a treatment, together with chemotherapy, for metastatic breast cancer. An important question is whether cancer stem cells are sufficiently different from normal adult stem cells so that inhibition of Notch signaling is not harmful. Results of testing in mice indicate that Notch signals are not required for the maintenance of blood-forming stem cells in adult mice.

Stem cells and cancer: cancer pathways that also control the adult stem cell population (4/10/08)

Apc (adenomatosis polyposis coli) is a tumor-suppressing protein that controls β-catenin and hence affects Wnt signaling. When intestinal crypts are damaged and need to be regenerated, Wnt signaling directs stem cells to generate replacement cells. Apc normally turns off Wnt signaling of stem cells when it is no longer needed. The research here showed that if Apc is lost or damaged, Wnt signaling may continue and result in tumor formation

Cancer Stem Cells Created With New Technique (4/9/08)

One of the most important unresolved questions about cancer stem cells is how they originate to begin with. For instance, are they mutations of existing stem cells, or instead precancerous cells that have acquired stem-cell-like capabilities? The research here supports the latter scenario. Starting with normal skin cells, the researchers activated three genes associated with embryonic stem cells. The result closely resembled known cancer stem cells. And they also had more resemblance to normal embryonic stem cells than to normal adult stem cells. One of the genes was Myc, which has also been used to create pluripotent stem cells from skin cells. In addition to the scientific significance of this work, it should also facilitate study of cancer stem cells, which are otherwise hard to locate.

Module Map Links Embryonic Stem Cells And Cancer Stem Cells (4/9/08)

The researchers involved in the work described in the preceding report have additional related findings. They systematically compared gene expression patterns between embryonic stem cells and multiple types of human cancer cells. Gene expression patterns in diverse human epithelial cancers were much like patterns in embronic stem cells. Further, presence of these patterns in cancer cells strongly predicted metastasis and death. On the other hand, normal adult tissue stem cells had an opposite pattern, which was repressed in various human cancers compared to normal tissues. The researchers additionally demonstrated that c-Myc, but not other oncogenes, was sufficient to reactivate the ESC-like program in normal and cancer cells.

And here's some earlier research that features the Nanog and Bmi1 proteins:

To Evade Chemotherapy, Some Cancer Cells Mimic Stem Cells (9/19/07)

Anti-cancer treatments often effectively shrink the size of tumors, but some might have an opposite effect, actually expanding the small population of cancer stem cells believed to drive the disease, according to new findings.

"Our experiments suggest that some treatments could be producing more cancer stem cells that then are capable of metastasizing, because these cells are trying to find a way to survive the therapy," said one of the study's investigators, Vasyl Vasko.

When the researchers applied various anti-cancer drugs to experimental cancer cells, they found that surviving cells expressed more Nanog and Bmi1:

They selected a rare form of cancer, mesenchymal chondrosarcoma (MCS), which has not been well described and for which there is no effective treatment. The researchers first determined that Nanog and BMI1 stem cell markers were more highly expressed in metastatic tumors compared to primary tumors. ...

They then applied various therapies - from VEGF inhibitors such as Avastin to the proteasome inhibitor Velcade - in mice implanted with human MSC, and analyzed the effects on tumors. Some of the treatments seemed to work, because they led to a dramatic decrease in the size of the tumors, Dr. Vasko said. But analysis of stem cell expression before and after treatment revealed that even as some anti-cancer treatments shrank tumors, they increased expression of Nanog and BMI1. "These treatments were not enough to completely inhibit tumor growth, and the cancer stem cell markers were still present," Dr. Vasko said.

Tags: cancer, stem cells, cancer stem cells

Induced Pluripotent Stem Cells II

In this article from the April 4 Science, which I mentioned here, several research reports dealing with induced pluripotent stem cells were discussed. One of these I covered in the post I just noted.

Another just as important report apparently has not yet been formally published, but is (at least temporarily) available online since February 14 at Science Express:

Generation of Pluripotent Stem Cells from Adult Mouse Liver and Stomach Cells

Induced pluripotent stem (iPS) cells have been generated from mouse and human fibroblasts by the retroviral transduction of four transcription factors. However, the cell origins and molecular mechanisms of iPS cell induction remain elusive. This report describes the generation of iPS cells from adult mouse hepatocytes and gastric epithelial cells. These iPS cell clones appear to be equivalent to ES cells in gene expression and are competent to generate germ-line chimeras.

It's not surprising that this is significant research, as it's from the same team of Shinya Yamanaka that was the first to report successful creation of induced pluripotent stem cells. (See here.)

So what is this research about? Well, the investigators used the same four transcription factors (Oct3/4, Sox2, Klf4, and c-Myc) as employed in the majority of previous iPS studies. However, instead of applying the transcription factors to fibroblast cells, they were applied to two types of epithelial cells instead.

Fibroblasts are part of a body's connective tissue. They are involved in structure and support for other tissues and contain large amounts of the protein collagen. They do not divide for the most part, and so it is especially significant that it was possible to reprogram them into a stemcell-like state at all.

Epithelial cells, on the other hand, line the inner and outer surfaces of various body structures, including skin and the gastrointestinal tract. Such cells divide more frequently. They have to, in order to replace other cells of the same kind that are exposed to hostile environments. Epithelial cells also tend to be more adherent to other cells, because they more highly express an adherence protein called E-cadherin.

In some sense, then, epithelial cells are a little more like stem cells to begin with, so one might expect better results when attempting to reprogram them.

This expectation seems to have been met. One of the key differences the researchers found is that reprogrammed epithelial cells had less tendency to form cancerous tumors in mice into which they were included. Certainly not an inconsiderable advantage. This characteristic may be related to the finding that c-Myc seems to play a less essential role in reprogramming epithelial cells.

Specifically, reprogramming of epithelial cells was almost as efficient when c-Myc was not used as when it was included with the other three transcription factors. Yet it was not possible to accomplish reprogramming if any of the other three factors was omitted. In contrast, the efficiency of reprogramming fibroblasts dropped by 90% when c-Myc was omitted.

Another intriguing difference was that reprogrammed epithelial cells contained higher levels of expression of β-catenin than reprogrammed fibroblasts did. (You may recall – see here – that β-catenin is an important part of the Wnt signaling pathway.) In this regard, the reprogrammed epithelial cells are more like true embryonic stem cells than reprogrammed fibroblasts are. It's probably not a coincidence that expression of Nanog is stimulated by β-catenin, (see here), since Nanog is considered important for maintaining stem cell pluripotency.

A further advantage of the use of epithelial cells is that many fewer retroviral "integration sites" were needed to include the transcription factor genes into the cell genome, in comparison with fibroblasts. This is another way the risk of cancer is reduced.

Further reading:

• Stomach and liver cells reprogrammed – excellent 2/28/08 open access overview of the research from Nature Reports Stem Cells
  
  
• Epithelial cells made pluripotent – 2/14/08 blog post at The Scientist giving a good summary of the research findings
  
  
• Epithelial cells made pluripotent – blog post at Wisconsin Stem Cell Now on the preceding article, but noted mainly because it's by the owner of another interesting stem cell blog: Stem Cell Nation
  

Tags: stem cells, induced pluripotent stem cells

Embryonic stem cells and Klf4

There's now some additional information on one of the transcription factors written about here, which are able to reprogram adult skin cells into embryonic stem cells. To review, one of the teams responsible for this research used Oct3/4, Sox2, c-Myc, and Klf4 for the reprogramming, while another team used Oct3/4, Sox2, Nanog and Lin28.

Of the transcription factors in the first list, all but Klf4 have been well-studied. So it is of some interest to know more about Klf4, and why it seems to be somewhat less essential than the others.

Some of the interesting details are reported on here: Molecular Alliance That Sustains Embryonic Stem Cell State Identified.

Klf4 is normally active in real embryonic stem cells. To investigate the role Klf4 might be playing in the reprogramming of skin cells, the researchers investigated embryonic stem cells that had been artificially depleted of Klf4. To their surprise, the team found that the cells maintained their pluripotency.

The question then was how to explain this. What was found is that two closely-related transcription factors – Klf2 and Klf5 – took over the role of Klf4:

"Most important, the data showed that the other Klfs were bound to the target sites when one of them was depleted." said Dr. Ng. "These Krüppel-like factors form a very powerful alliance that work together on regulating common targets. The impact of losing one of them is masked by the other two sibling molecules."

This family of transcription factors, called Kruppel-like factors, gets its name from a homology to the Drosophila Krüppel protein. Members of this family have been studied for their roles in cell proliferation, differentiation and survival, especially in the context of cancer.

Interestingly enough, according to the research press release,

Klfs were found to regulate the Nanog gene and other key genes that must be active for ES cells to be pluripotent, or capable of differentiating into virtually any type of cells. Nanog gene is one of the key pluripotency genes in ES cells.

"We suggest that Nanog and other genes are key effectors for the biological functions of the Klfs in ES cells," Dr. Ng said.

"Together, our study provides new insight into how the core Klf circuitry integrates into the Nanog transcriptional network to specify gene expression unique to ES cells.

Nanog, of course, is one of the transcription factors in the set of transcription factors which was found to be an alternative, for reprogramming adult cells, to the set that contained Klf4.

The Nanog protein, too, is known to be critically important in pluripotent stem cells. It is a homeobox transcription factor that appears to play an essential role in self-renewal of undifferentiated embryonic stem cells. It also appears to be connected with cancer, because (according to Wikipedia) "It has been shown that the tumour suppressor p53 binds to the promoter of NANOG and suppresses its expression after DNA damage in mouse embryonic stem cells. p53 can thus induce differentiation of embronic stem cells into other cell types which undergo efficient p53-dependent cell-cycle arrest and apoptosis."

The connection of Klf proteins with cancer is not only through Nanog. According to Wikipedia, "Klf4 also interacts with the p300/CBP transcription co-activators." The closely-related p300 and CBP "interact with numerous transcription factors and act to increase the expression of their target genes." And they too are involved with cancer:

Mutations in the p300 gene have been identified in several other types of cancer. These mutations are somatic, which means they are acquired during a person's lifetime and are present only in certain cells. Somatic mutations in the p300 gene have been found in a small number of solid tumors, including cancers of the colon and rectum, stomach, breast and pancreas. Studies suggest that p300 mutations may also play a role in the development of some prostate cancers, and could help predict whether these tumors will increase in size or spread to other parts of the body. In cancer cells, p300 mutations prevent the gene from producing any functional protein. Without p300, cells cannot effectively restrain growth and division, which can allow cancerous tumors to form.

Another intriguing connection of p300 is that it can be inhibited by the action of the sirtuin deacetylase Sirt1. (See here.)

P300/CBP themselves are targets of intense research activity. Their physical structure has only very recently been determined. (See here, here, here.)

Finally (for now), it's interesting that p300 plays a role in stem cell signaling through one of our favorite signaling pathways – Wnt (see here). According to this report: Stem Cell Signaling Mystery Solved, a small molecule called IQ-1 interferes with Wnt signaling via p300:

What IQ-1 does, Kahn explains, is to block one arm of a cell-signaling pathway called the Wnt pathway, while enhancing the signal coming from the other arm of the Wnt pathway. The Wnt pathway is known to have dichotomous effects on stem cells i.e. both proliferative and differentiative. More specifically, IQ-1 blocks the coactivator p300 from interacting with the protein ß-catenin; this prevents the stem cells from being 'told' to differentiate into a more specific cell type.

Additional reading:

A core Klf circuitry regulates self-renewal of embryonic stem cells – research abstract published online 2/10/08

Molecular Alliance Identified that Sustains Embryonic Stem-Cell State – another summary of the Klf4 study

Tags: stem cells, cancer

Wnt signaling

We've discussed Wnt signaling a couple of times before, here, and here.

Wnt refers to a family of proteins now numbering perhaps 20 or more, which have been found in a wide range of multicellular animals, from fruit flies, to fish, to mice and humans. Wnt proteins carry messages between cells, and are especially important in embryogenesis. They are known to play a large role in the control of stem cells and regeneration of body parts (in species where this occurs). In mammals, including humans, Wnt signaling, when it malfunctions, also seems to be involved in many types of cancer, degenerative diseases of aging, and other aging-related problems such as insulin resistance. It may be possible to ameliorate a number of these disease conditions once we have a better understanding of the details of Wnt signaling.

The "Wnt signaling pathway" refers to a sequence of proteins that, in the presence of earlier members of the pathway, change in behavior to affect proteins later in the pathway. The pathway begin when a Wnt protein (secreted by a nearby cell) binds to a cell surface protein, such as the whimsically-named Frizzled. Various other proteins in the pathway then interact, and eventually result in the build-up of a protein called β-catenin, which enters the cell nucleus, where it combines with various transcription factors to affect gene expression.

The name "Wnt" originates from the realization that two genes discovered earlier were homologous – the "wingless" gene in fruit flies (which, when mutated, yields flies without wings), and the Int genes found in mouse tumors.

Although Wnt genes and proteins have now been studied for nearly 20 years, the pace of discovery continues to increase. This is because of the large number of very interesting processes heavily influenced by Wnt signaling – from proliferation and differentiation of stem cells to embryonic development, regeneration of body parts, cancer, and degenerative diseases of aging.

The following summaries of research reports from just the past half year or so will give a buffet-style sample of Wnt-related investigations.

Carbohydrate Regulates Stem Cell Potency (2/1/08)

Embryonic stem cells are characterized by an ability to continually self-renew, but also to give rise to any adult cell type. Stem cell renewal is driven by several external signaling proteins and growth factors, including Wnt, FGF (fibroblast growth factor), and BMP (bone morphogenetic protein). In particular, Wnt signaling stimulates β-catenin to produce the transcription factor Nanog, which maintains pluripotency. However, the ability of these proteins to attach to stem cell surface proteins in order to induce a response seems to depend on the presence of a carbohydrate molecule called heparan sulfate (HS). Stem cells were found to reproduce less frequently but differentiate more frequently in proportion to experimental inhibition of HS production.

Beta-catenin Gradient Linked To Process Of Somite Formation (12/27/07)

In a developing vertebrate embryo somites are masses of a type of tissue (mesoderm) that will eventually develop into such adult tissue types as skeletal muscle and vertebrae. This research on mouse embryos demonstrates the importance of β-catenin as the principal mediator of the Wnt-signaling pathway, in the process of somite formation. In particular, there is a gradient in levels of β-catenin found in cells of the presomitic mesoderm (PSM), and this gradient is critical in regulating mesoderm maturation. This leads to the development of the characteristic vertebral column in embryos of vertebrate animals.

Certain Diseases, Birth Defects May Be Linked To Failure Of Protein Recycling System (12/20/07)

The Wnt signaling protein, like other proteins, is produced in the nuclei of certain cells, and it must be transported to the cell surface, so it can be secreted into the extracellular environment to regulate the growth of tissues during (and after) embryonic development. Another protein, called Wntless (Wls), acts as a cargo container for Wnt, and plays a key role in the transport process. Another protein, called Vps35, which makes up an important part of the "retromer complex", is responsible for moving empty Wls molecules (like freight cars) to where they are needed in the cell. But mutated Vps35 proteins can fail to perform their function, and consequently lead to the failure to transport Wnt out of the cell where it has been produced.

Grape Powder Blocks Genes Linked To Colon Cancer (11/14/07)

Previous research has found that the Wnt signaling pathway is linked to more than 85 percent of sporadic (i. e. not caused by a hereditary defect) colon cancers. Additionally, in vitro studies have shown that resveratrol is capable of blocking the Wnt pathway. The present research showed that in some colon cancer patients who consumed grape powder (which contains resveratrol and possibly other active ingredients), Wnt signaling in biopsied colon tissue was significantly reduced.

Odd protein interaction guides development of olfactory system (10/29/07)

The olfactory system of fruit flies has been shown to develop abnormally when the signaling protein Wnt5 is absent. However, if large amounts of Wnt5 but no Wnt5 receptors called "derailed" are present, development is even more abnormal. Specifically, structures called glomeruli in fruit fly antennal lobes (which are analogous to human olfactory bulbs) grow abnormally when Wnt5 is absent. But if Wnt5 is present in large amounts and there are no derailed receptors, malformed glomeruli develop in locations where they should not be.

Cilia: Small Organelles, Big Decisions (10/3/07)

Research into the development of zebra fish (a favorite of developmental biologists) has shown that organelles called cilia in the cells of developing embryos play a large role in the transduction of Wnt signaling proteins that guide the development process. By blocking the production of three proteins used by cilia, researchers were able to disrupt proper balances in the interpretation of Wnt signals, resulting in developmental defects.

New Insights into the Control of Stem Cells: Keeping the Right Balance (9/15/07)

The Wnt signaling pathway plays a crucial role in embryonic development, cell growth (proliferation), and maturation of cells into specialized cells (differentiation). It is also an important regulator of stem cells. An interaction between Wnt signaling and tyrosine kinases enables the proliferating cells to mature into specialized (differentiated) cells. Normally this interaction strikes a proper balance between proliferation and differentiation. Cancers, such as breast and colon cancer, result when the interaction gets unbalanced. In 90% of human cancers the tumor suppressor APC (adenomatous polypolis coli), one of the core components of the Wnt pathway, is deregulated. This results in excessive amounts of β-catenin, which triggers the onset of breast and colon cancer when it gets into the cell nucleus and affects gene expression.

Reactivating A Critical Gene Lost In Kidney Cancer Reduces Tumor Growth (8/15/07)

Studies of an important tumor-suppressor protein, sFRP-1 (secreted frizzled-related protein 1), in clear cell renal cell carcinoma, the most common type of kidney cancer, may reveal a means to defeat the cancer. sFRP-1 was found to control 13 tumor-promoting genes along the Wnt signaling pathway, which has been linked to a number of cancers, especially colon cancer. Several close relatives of sFRP-1 are also known to affect at least 20 Wnt-related proteins, and up-regulation of members of the sFRP-1 family may be an effective way to control cancers linked to Wnt signaling. In one experiment, increasing sFRP-1 expression in human renal cancer cells was effective, and Wnt regulated oncogenes, such as c-myc, were suppressed compared to untreated cells.

Why Aging Muscles Heal Poorly (8/9/07)

Stem cells normally found in muscle tissue are responsible for repair to muscles damaged by injury or age-related degeneration. But in aged muscle tissue, stem cells tend to produce scar-tissue cells called fibroblasts, instead of normal muscle cells (myoblasts). The overproduction of fibroblasts is a condition known as fibrosis. New research shows that it isn't the age of the muscle stem cells that is the problem, but rather the age of the cellular environment itself, including blood supply to the tissue. The malfunction appears to be a problem with Wnt signaling in the aged environment rather than with the actual stem cells. Muscle stem cells from young mice exhibited the same problems when exposed to an enviroment from older animals.

Related research found that Wnt signaling increased, with detrimental effect, due to age-related deficiency of a hormone called klotho. Klotho seems to inhibit Wnt signaling, and also has some control over insulin sensitivity. However, production of klotho seems to decline with age, possibly leading to age-related problems such as cancer, arterial disease, and insulin resistance.

Not A Relay Race, But A Team Game: New Model For Signal Transduction In Cells (6/27/07)

Details of the inner workings of the Wnt signal transduction process have remained incomplete, but are gradually coming into focus. Member of the Wnt family of proteins may dock with a variety of cell-surface proteins, including LRP6 (low density lipoprotein receptor-related protein 6) and members of the family of G protein-coupled receptors known as Frizzled. After the docking, a signaling cascade is triggered that transmits molecular messages via the cytoplasm to the nucleus. This research shows that the first step after docking involves large protein complexes formed from proteins already known to be part of the signaling pathway, such as phosphorylated LRP6, axin, and Dishevelled (Dvl).

Further reading:

The Wnt Homepage

Regeneration for Repair's Sake

The answer is blowing in the Wnt

Miller on Wnt and Klotho

A hazy shade of Wnt

Tags: Wnt, embryogenesis, stem cells, signal transduction, cancer, developmental biology, aging

Cancer and Myc

Myc is a gene that has been studied for over 20 years and is found to be overexpressed in many human cancers. The protein it codes for (which for simplicity we'll also refer to as Myc) is a transcription factor, like members of the FoxO family, which means it can enable or disable the expression of many other genes. How many? The estimate is about 15% of all human genes. Since there are at least 20,000 genes, the number regulated by Myc is 3000 or more. That's a lot of leverage.

Obviously, Myc is essential for cell function or it wouldn't have so much influence. The biological functions Myc affects include cell proliferation, cell growth, apopotosis, cell differentiation, and stem-cell self-renewal. It is possible for a single transcription factor to have such varied effects because other transcription factors (coactivators or corepressors) must also be present in order to activate or repress the expression of some specific other gene. That is, the expression of any particular gene depends on the presence (or absence) of a particular set of transcription factors.

Since Myc affects the functions mentioned above, it's hardly surprising to find that Myc is overexpressed in many cancers. In such cases, the difference between a normal cell and a cancer cell is the overexpression of Myc in the latter and possibly the abnormal presence of other coactivators or corepressors. The result in a cancer cell is excessive proliferation and/or avoidance of cell death by apoptosis.

Surely you've wondered why development of cancer therapies has taken so long. A large part of the reason is that so many genes involved in cancer interact with so many other genes that have essential functions. So it's not possible to interfere with the cancer-related genes without disrupting vital biological functions elsewhere in the body.

The expression of Myc itself is triggered by signals that are usually external to the cell and normally serve to stimulate cell growth and proliferation when needed, as in lymphocyte production, normal growth, or wound healing. Such signals are called mitogens, because they can initiate mitosis. Wnt, Shh, and EGF are mitogens that can lead to Myc expression.

It is probably almost impossible to hope to combat cancer by directly affecting Myc or its related mitogens, because all of these have essential normal functions themselves. Instead, anti-cancer research needs to examine how Myc may be overexpressed or complemented in harmful ways by other transcription factors.

One promising line of research involves cancer stem cells – cells that, like normal stem cells, can proliferate and differentiate into other cell types, and that also carry cancer-causing mutations. There is now thought to be a "signature" consisting of 11 genes that may be characteristic of cancer stem cells. One of these genes has an effect on Myc:

Scientists Uncover Role Of Cancer Stem Cell Marker: Controlling Gene Expression (1/18/08)

Scientists at Jefferson's Kimmel Cancer Center in Philadelphia have made an extraordinary advance in the understanding of the function of a gene previously shown to be part of an 11-gene "signature" that can predict which tumors will be aggressive and likely to spread. The gene, USP22, encodes an enzyme that appears to be crucial for controlling large scale changes in gene expression, one of the hallmarks of cancer cells. ...

In one example, they looked at the relationship between MYC and USP22. MYC, which is among the most commonly overexpressed genes in cancer, encodes a protein that controls the expression of thousands of other genes. The scientists showed that USP22 is a critical partner of MYC and that by depleting cells of USP22, they could prevent MYC from working properly, stopping it from inducing the invasive growth of cancer cells.

It turns out that Myc affects not only 3000 or so ordinary genes, but also a number of DNA sequences that code for microRNA – and some of these microRNAs play an important role in suppressing cancer. In fact, Myc can stop the production of at least 13 microRNAs:

Silencing Small But Mighty Cancer Inhibitors (12/12/07)

Researchers from Johns Hopkins and the University of Pennsylvania have uncovered another reason why one of the most commonly activated proteins in cancer is in fact so dangerous. As reported in Nature Genetics recently, the Myc protein can stop the production of at least 13 microRNAs, small pieces of nucleic acid that help control which genes are turned on and off.

Furthermore, additional observations showed that some of these microRNAs have an inhibiting effect on cancer, a striking result in itself:

[I]n several instances, re-introducing repressed miRNAs into Myc-containing cancer cells suppressed tumor growth in mice, raising the possibility that a sort-of gene therapy approach could be effective therapy for treating certain cancers.

Since the microRNAs repressed by Myc affect many other genes, there could be thousands of other genes indirectly affected by Myc:

"This study expands our understanding of how Myc acts as such a potent cancer-promoting protein," says Mendell. "We already knew that it can directly regulate thousands of genes. Through its repertoire of miRNAs, Myc likely influences the expression of thousands of additional genes. Activation of Myc therefore profoundly changes the program of genes that are expressed in cancer cells."

More: Researchers zero in on the tiniest members in the war on cancer (12/13/07)

Myc is actually a member of a family of genes, the Myc family. Most members of the family have similar functions. Separate members undoubtedly appeared in the course of evolution from the duplication of earlier members and later mutation. One member, called N-myc, plays a role in both normal development of the retina, as well as cancers of the retina (retinoblastoma).

Recent research has shown that N-myc seems to be responsible for the surprising fact that the retinas in all vertebrates have about the same thickness, regardless of the size of the entire animal or its eyes:

Eye Cancer Gene's Role In Retinal Development Defined (1/18/08)

"A series of complex developmental processes must be carefully orchestrated for the eye to form correctly," said Michael Dyer, Ph.D., associate member in the St. Jude Department of Developmental Neurobiology. "One important aspect of this coordination is that retinal thickness be the same, irrespective of eye size. For example, the mouse eye is about 5,000 times smaller than that of the elephant eye, but the retinal thickness in these two species is comparable."

Working with mice, the researchers found that a gene called N-myc coordinates the growth of the retina and other eye structures to ensure the retina has the proper thickness necessary to convert light from the lens into nerve impulses that the brain transforms into images.

Tags: cancer, Myc, microRNA

New Hope For Baldness Treatment: Hair Follicles Created For First Time In Mouse Study

At last! Medical science addresses a problem of great concern to (some) men under 50.

New Hope For Baldness Treatment: Hair Follicles Created For First Time In Mouse Study

Researchers at the University of Pennsylvania School of Medicine have found that hair follicles in adult mice regenerate by re-awakening genes once active only in developing embryos. These findings provide unequivocal evidence for the first time that, like other animals such as newts and salamanders, mammals have the power to regenerate. A better understanding of this process could lead to novel treatments for hair loss, other skin and hair disorders, and wounds.

Though tested so far only in mice, it's intersting that the technique involves recruiting the body's own stem cells – but not cells that are direct precursors of hair follicles. This means that other components of skin might be regenerated as well:

In this study, researchers found that wound healing in a mouse model created an "embryonic window" of opportunity. Dormant embryonic molecular pathways were awakened, sending stem cells to the area of injury. Unexpectedly, the regenerated hair follicles originated from non-hair-follicle stem cells.

"We've found that we can influence wound healing with wnts or other proteins that allow the skin to heal in a way that has less scarring and includes all the normal structures of the skin, such as hair follicles and oil glands, rather than just a scar," explains Cotsarelis.

Even more interesting, the technique involves an important family of protein known as "Wnt":

By introducing more wnt proteins to the wound, the researchers found that they could take advantage of the embryonic genes to promote hair-follicle growth, thus making skin regenerate instead of just repair. Conversely by blocking wnt proteins, they also found that they could stop the production of hair follicles in healed skin.

For a lot more on the Wnt signaling pathway, see here.

Tags: Wnt signaling, regenerative medicine

News dump: the Wnt signaling pathway

Here's one of the interesting things about biology: sometimes small number of genes, present in a large number of animal species, can affect a very diverse range of biological processes – including a number we'd like to have better control over.

In this case, we're concerned with a family of about 19 related genes, called Wnt. Genes of this family code for proteins that are important in embryonic development. They also play an important role in the regeneration of body parts, in those species where such regeneration has been known to be possible – and perhaps someday even in humans.

Here are some recent news releases on the topic, most recent first.

Rare Mutation Causes Early Heart Disease And Metabolic Syndrome (3/14/07)

A very rare mutation of the LRP6 gene, whose protein affects the Wnt signaling pathway, has been found to cause high rates of early-onset coronary artery disease in a family that carries the mutation. Family members having the mutation were also at greater risk for other components of metabolic syndrome, as well as osteoporosis.

How Does A Zebrafish Grow A New Tail? The Answer May Help Treat Human Injuries (12/28/06)

Signaling pathways involving various Wnt proteins and Beta-catenin have been shown to control the regeneration of the fins of zebrafish. Some Wnt/Beta-catenin signals promote fin regeneration. A different pathway involving Wnt5b inhibits regeneration so it doesn't get out of hand. But a mutant form of Wnt5b speeds up regeneration, while an excess of Wnt8 also increases cell proliferation.

Researchers Discover Initial Steps In Development Of Taste (12/6/06)

Researchers have shown that Wnt signaling pathways regulates the development of taste buds in mice. They have also determined that Wnt proteins are required for hooking up the wiring of taste signals to the brain.

Control Mechanism For Biological Pattern Formation Decoded (11/30/06)

Using a mathematical model based on protein reactions and diffusions, researchers have been able to explain the dynamics and parameters of hair formation in mice, based on Wnt signaling.

Scientists Regenerate Wing In Chick Embryo (11/19/06)

This research provides direct evidence that limb regeneration in (some) vertebrates is affected by the Wnt signaling system. By activating Wnt signaling scientists were able to stimulate wing regeneration in chick embryos (where it does not normally occur), and by deactivating Wnt signaling in frogs, zebrafish, and salamanders it was possible to prevent regeneration of missing legs and tails.

Adult Stem Cells May Be Just Remnants Of Evolution (11/2/05)

At least some adult stem cells could be the mere remnants of former embryonal differentiation processes. In this research mesenchymal stem cells of mice we stimulated by Wnt signaling, but their transformation into muscle cells was not complete.

Wnt Signaling Controls The Fate Of Stem Cells In Adult Brains (10/31/05)

The Wnt3 protein affects whether neural stem cells in mice, upon division, continue as stem cell, become neurons, or become support cells, such as astrocytes or oligodendrocytes.

Prostate Cancer Uses Wnt Signaling Proteins To Promote Growth Of Bone Tumors (9/7/05)

Some Wnt proteins play a central role in regulating normal skeletal development in an embryo, but they may also be hijacked by prostate cancer cells to spread the cancer into bone tissue.

Mice With Hyperactive Gene Eat All They Want, But Have Half The Body Fat Of Normal Mice (6/30/04)

When the protein Wnt10b is present in artificially high amounts in fat tissue of experimental mice, the mice appear to be able to eat as much as they like without an increase in body fat.

Tags: Wnt signaling, regenerative medicine