mTOR, MAPK, and cancer

Recent studies are making it increasingly apparent that cancer is really many different diseases – hundreds actually – in the sense that there are hundreds of distinct problems at a molecular level that can result in the symptoms of cancer in a large variety of tissue types. It is necessary to regard all these problems as distinct diseases, because different techniques will be necessary to deal effectively with each one.

One of the things we are now understanding is that it is not simply mutations in a few specific genes that account for most different cancers. Instead, each different type of cancer can be attributed to mutations in one or several genes randomly chosen from a larger set that collectively defines some specific signaling "pathway" in a cell. Or perhaps even several interrelated pathways.

See here for one account of some of the latest research on this. I'd like to discuss the papers that cover this research, but first I'd like to discuss some slightly earlier research that provides a simpler look at the issue.

I've already written about one particular pathway of special importance, the one associated with mTOR. That discussion, from last April, is here.

As you recall, mTOR is a serine/threonine kinase. The pathway in which it plays a prominent part regulates the growth, proliferation, motility, and survival of cells. And also angiogenesis. From that list it should be obvious why malfunctions in the pathway can give rise to cancer. The pathway, in turn, integrates input from a number of upstream pathways, such as those involving intercellular signaling molecules like insulin, IGF-1, and mitogens.

The name mTOR is short for mammalian target of rapamycin. Rapamycin, also known as sirolimus, is a bacterial product that was originally of interest for its antifungal properties. It was subsequently found to have immunosuppressive and antiproliferative properties. These properties, in turn, are a consequence of the fact that rapamycin binds to a protein complex called mTOR complex 1 (mTORC1). The antiproliferative properties, of course, are due to the importance of mTOR in regulating cell proliferation and motility.

All this stuff is well known to cancer biologists and not new. In particular, much research has been devoted to finding useful inhibitors of mTOR. Unfortunately, however, the research hasn't been as successful at actually treating cancer as might have been hoped:

A Role For MAPK Inhibitors Combined With MTORC1 Inhibitors (8/21/08)

Nearly a decade ago, while it was being tested as an immunosuppressive agent to prevent organ rejection in transplant patients, the drug rapamycin was also discovered to have anti-tumor properties. Since then, several rapamycin analogs known as mTOR (mammalian target of rapamycin) inhibitors have been tested in clinical trials for the treatment of various types of cancer.

But despite promising early results, mTOR inhibitors have proven less successful than originally expected.

The problem is that the mTOR inhibitors that have been tried as anti-cancer drugs also seem to stimulate another pathway that promotes cell growth and proliferation:

Now research led by scientists at Beth Israel Deaconess Medical Center (BIDMC) identifies a previously unrecognized problem faced by these agents when it comes to attacking cancers. ... [T]he new findings show that at the same time that rapamycin analogs are halting tumor growth by inhibiting the mTOR protein complex 1 (mTORC1), they are activating the MAPK (mitogen-activated protein kinase) pathway -- thereby encouraging cancer cell survival.

The MAPK pathway has also been under intensive investigation in connection with cancer. As the name implies, kinases in this pathway are activated by mitogens – external signals that promote mitosis. These kinases also affect cell survival and apoptosis. So it's reasonable to guess that adding a MAPK inhibitor to an mTOR inihibitor might counteract the MAPK-stimulating effect of the mTOR inhibitors.

Are you with me? Anyhow, what the new research does is show how there's a feedback loop that connects mTOR inhibition with MAPK activation.

Inhibition of mTORC1 leads to MAPK pathway activation through a PI3K-dependent feedback loop in human cancer

Numerous studies have established a causal link between aberrant mammalian target of rapamycin (mTOR) activation and tumorigenesis, indicating that mTOR inhibition may have therapeutic potential. In this study, we show that rapamycin and its analogs activate the MAPK pathway in human cancer, in what represents a novel mTORC1-MAPK feedback loop. ... We further show that rapamycin-induced MAPK activation occurs in both normal cells and cancer cells lines and that this feedback loop depends on an S6K-PI3K-Ras pathway.

PI3K is another important signaling kinase about which there is a lot of other interesting current research – which we'll get around to discussing at some point. Ras is a G protein known to be very important in cancer because it activates MAPK pathways.

Are you beginning to get the picture of how complicated cancer can be, due to the interaction of pathways?

Fortunately, the research also shows that MAPK inhibition can offset problems due to mTOR inhibition:

[P]harmacological inhibition of the MAPK pathway enhanced the antitumoral effect of mTORC1 inhibition by rapamycin in cancer cells in vitro and in a xenograft mouse model. Taken together, our findings identify MAPK activation as a consequence of mTORC1 inhibition and underscore the potential of a combined therapeutic approach with mTORC1 and MAPK inhibitors.

Another research group has already confirmed the same thing, using the same MAPK inhibitor (PD0325901):

Anti-tumor Effects Are Enhanced By Inhibiting Two Pathways Rather Than One (8/21/08)

In the second study, Cory Abate-Shen and colleagues, at Columbia University College of Physicians and Surgeons, New York, and the University of Medicine & Dentistry of New Jersey, Piscataway, show that simultaneous inhibition of the mTOR and MAPK signaling pathways inhibited the in vitro growth of prostate cancer cell lines and the in vivo growth of prostate tumors in a mouse model of prostate cancer.

Here's their research paper:

Targeting AKT/mTOR and ERK MAPK signaling inhibits hormone-refractory prostate cancer in a preclinical mouse model

The AKT/mammalian target of rapamycin (AKT/mTOR) and ERK MAPK signaling pathways have been shown to cooperate in prostate cancer progression and the transition to androgen-independent disease. We have now tested the effects of combinatorial inhibition of these pathways on prostate tumorigenicity by performing preclinical studies using a genetically engineered mouse model of prostate cancer. We report here that combination therapy using rapamycin, an inhibitor of mTOR, and PD0325901, an inhibitor of MAPK kinase 1 (MEK; the kinase directly upstream of ERK), inhibited cell growth in cultured prostate cancer cell lines and tumor growth particularly for androgen-independent prostate tumors in the mouse model.

AKT is yet another family of serine/threonine kinases, often associated with mTOR, that is deeply involved in tumorigenicity. There's a lot of recent research on it that should also be discussed... some other time.

Further reading:

From Metabolism to Oncogenes and Back - Part II – 3/21/08 blog post that discusses many cancer-related signaling pathways, including mTOR, AKT, PI3K, Ras, and their connection with metabolism

Arkaitz Carracedo, Li Ma, Julie Teruya-Feldstein, Federico Rojo, Leonardo Salmena, Andrea Alimonti, Ainara Egia, Atsuo T. Sasaki, George Thomas, Sara C. Kozma, Antonella Papa, Caterina Nardella, Lewis C. Cantley, Jose Baselga, Pier Paolo Pandolfi (2008). Inhibition of mTORC1 leads to MAPK pathway activation through a PI3K-dependent feedback loop in human cancer Journal of Clinical Investigation DOI: 10.1172/JCI34739

Tags: cancer, mTOR

Choanoflagellates II

No sooner than we do an article on a topic that may seem esoteric to some – namely tyrosine kinase signaling in choanoflagellates – than new information comes along to add to the picture.

The previous discussion was about how remarkable it is that a complete set of sophisticated intercellular signaling proteins exists in a single-celled organism. The new research partially echoes the previous findings:

Primitive Single-Celled Microbe Expert In Cellular Communication Networks (7/7/08)

When it comes to cellular communication networks, a primitive single-celled microbe that answers to the name of Monosiga brevicollis has a leg up on animals composed of billions of cells. It commands a signaling network more elaborate and diverse than found in any multicellular organism higher up on the evolutionary tree, researchers at the Salk Institute for Biological Studies have discovered.

Their study, which will be published during the week of July 7-11 in the online edition of the Proceedings of the National Academy of Science, unearthed the remarkable count of 128 tyrosine kinase genes, 38 more than found in humans.

But it also points out that M. brevicollis actually has a more extensive tyrosine kinase system than metazoa do:

"We were absolutely stunned," says Manning. "Based on past work, we had expected maybe a handful of these kinases but instead discovered that this primitive organism has a record number of them. Two other essential parts of the tyrosine kinase network - PTP and SH2 genes - are also more numerous than in any other genome, showing that it is the whole network that is elaborated here." ...

The Monosiga kinases are more divergent than anything previously seen in animals, which may help scientists understand the fundamentals of how all tyrosine kinase signaling works. Despite their extreme diversity, Monosiga kinases time and again arrive at the same solution to a problem, as do animal kinases, but using a distinct method for instance to create a sensor structure that emerges from the cell, or to target a kinase to a specific part of the cell. "This convergent evolution suggests that there are only a limited number of ways build a functional network from these components," says Manning.

And as was pointed out before, this discovery merely suggests new questions that need to be answered:

With all this new information, one obvious question remains unanswered: what is a single-celled organism doing with all this communications gear? "We don't have a clue!" says Manning, "but this discovery is the first step in finding out."

A possible answer, though one for which no evidence exists, as far as I know, is that M. brevicollis actually developed from the type of cell it strongly resembles ("collar cells") found in sponges. Perhaps the tyrosine kinase signaling system actually evolved in sponges, but then some of the constituent cells decided to go it alone. It would still need to be explained why the kinases didn't disappear in subsequent evolution. Since they are still present, they must serve some useful purpose.

Further reading:

The protist, Monosiga brevicollis, has a tyrosine kinase signaling network more elaborate and diverse than found in any known metazoan – research article from PNAS that reports the research discussed (open access)

Tags: choanoflagellates, cell signaling, tyrosine kinases

Choanoflagellates

Choanoflagellates are single-celled eukaryotic organisms – like amoebae, yeasts, or slime molds, as opposed to prokaryotic organisms like bacteria. Eukaryotic cells are different from prokaryotic ones in having a variety of internal and external structures, including a cell nucleus that contains the organism's genome.

One distinctive feature of a choanoflagellate is its flagellum, a whiplike structure made up of microtubules, which a choanoflagellate uses to propel itself in water. A choanoflagellate rather resembles an animal sperm cell.

But the type of animal cell that a choanoflagellate most strongly resembles is called a choanocyte (meaning "collared cell") and is found in sponges. Modern sponges were, until quite recently, considered to be lineal descendants of the earliest type of multicellular animal (metazoan). (As discussed here and here, comb jellies may be descended from an even earlier metazoan.)

A metazoan is more than simply a collection of cells living together in a cohesive colony, such as slime molds and some choanoflagellates. The cells of a metazoan are of different types, and they communicate among themselves in order to effect whatever behavior the organism has evolved to sustain itself.

With that in mind, it is quite interesting that the genome of choanoflagellates contains genes for three proteins that are used ubiquitously in metazoa for intercellular communication:

New Evidence That Ancient Choanoflagellates' Form Evolutionary Link Between Single-celled And Multi-celled Organisms (7/1/08)

What do humans and single-celled choanoflagellates have in common? More than you'd think. New research into the choanoflagellate genome shows these ancient organisms have similar levels of proteins that cells in more complex organisms, including humans, use to communicate with each other.

According to a paper published in the Proceedings of the National Academies of Science, these findings help confirm choanoflagellates' role as an evolutionary link between single-celled and multi-celled organisms. They also contend that these insights into the organism's genome may mean that the proteins used to help cells communicate may have other roles as well. ...

Choanoflagellates, or at least their ancestors, have long been suspected as being the bridge between microorganisms with only one cell and metazoan, or multi-cellular organisms. There are many clues that lead to this conclusion, including the fact that choanoflagellates are similar to the individual cells in ocean sponges and unlike most other flagellates, they use their flagellate, or tail, to push themselves through water, rather than being pulled by it.

By analyzing the recently-sequenced choanoflagellate genome, the researchers discovered another similarity between choanoflagellates and most metazoans--their genetic code carries the markers of three types of molecules that cells use to achieve phospho-tyrosine signaling proteins.

The type of signaling in question here utilizes phosphorylation – the addition of a phosphate (PO₄) group to a protein at one or more of its constituent amino acids. It is tyrosine phosphorylation when the the phosphate is attached to a tyrosine unit. (Tyrosine is one of the 20 kinds of amino acids that make up proteins.)

This process is much like reading, writing, and erasing a single bit of information in a computer memory. It is the proteins that perform these operations that are found to be shared by choanoflagellates and most metazoa.

Animals depend on tyrosine phosphorylation to conduct a number of important communications between their cells, including immune system responses, hormone system stimulation and other crucial functions. These phospho-tyrosine signaling pathways utilize a three-part system of molecular components to make these communications possible.

Tyrosine kinases (TyrK) 'write' messages between cells by adding phospho-tyrosine modifications, protein tyrosine phosphatases (PTP) are molecules that modify or 'erase' these modifications, and Src Homology 2 (SH2) molecules 'read' these modifications so the recipient cell gets the message.

What is intriguing is that all three of these signaling proteins are found in choanoflagellates in significant amounts. Although the proteins exist in other single-celled organisms, they aren't found together or in the same amounts as they are in choanoflagellates. This is the sort of thing that makes a researcher think, "Hmmm, that's strange. Wonder what's up with that?"

The researchers conclude that the presence of the full three-component signaling system may have played a role in the development of metazoan organisms whose cells could communicate with each other in complex ways.

It would be interesting to find out whether choanoflagellates actually use the proteins to communicate among themselves, and if so, for what purposes. But those are questions that remain to be answered.

Further reading:

The genome of the choanoflagellate Monosiga brevicollis and the origin of metazoans – an earlier (2/08) journal article on the choanoflagellate genome, and possible links to metazoa (sub. rqd. for full access)

The Premetazoan Ancestry of Cadherins – companion research article to the preceding, from Science, 2/15/08 (sub. rqd. for full access)

Genome Of Marine Organism Tells Of Humans' Unicellular Ancestors (2/14/08) – press release that describes the preceding research

Tags: choanoflagellates, cell signaling, tyrosine kinases, phosphorylation

TOR signaling and cancer

Another recent development pertinent to the discussion of TOR signaling and cancer (see here), is the announcement of preclinical findings about a potential anti-cancer drug that may act against ovarian cancer. The drug works by inhibiting the mTOR signaling pathway. (mTOR is the mammalian form of TOR.)

This is not at all the first anti-cancer drug that's come along with a similar mechanism of action. But it's still interesting, because any drug that affects TOR signaling has the potential of also causing unwanted side effects, since TOR signaling is involved in so many cell processes. Presumably some effort has been made to find reasons why the effect of the drug should be limited to cancer cells.

The drug is called NV-128, and has been developed by an Australian biotech company called Novogen. Since the drug hasn't yet entered clinical trials in humans, it could take a decade or so (as usual) to perform enough testing to determine that NV-128 is actually effective, and relatively safe.

Anyhow, here's the news release:

Drug Compound Leads To Death Of Ovarian Cancer Cells Resistant To Chemotherapy (4/17/08)

In a discovery that may be useful for maintaining remission in chemo-resistant ovarian cancer, Yale scientists report that pre-clinical studies have shown the drug compound NV-128 can induce the death of ovarian cancer cells by halting the activation of a protein pathway called mTOR.

Many traditional cancer drugs work by triggering cell death via apoptosis. Unfortunately, apoptosis needs enzymes called caspases to work, as explained here. And cancer cells may develop a circumvention of this mechanism by turning down the production of caspases, which are needed to allow mitochondria to respond to apoptosis signals. NV-128, however, is able to overcome this problem by triggering caspase-independent cell death.

In cancer cells, mTOR signals enhance tumor growth and may be associated with resistance to conventional therapies. Inhibition of mTOR could shut down many of these survival pathways, including proteins that protect the mitochondria of cancer cells.

Here's the Novogen press release:

Novogen’s NV-128 shown to target the akt-mTOR receptor in chemoresistant cancer cells (4/15/08)

NV-128 is unique in that it does not induce caspase-mediated apoptosis which can be non-functional in chemoresistant cancer cells due to accumulated mutations in tumour suppressor/promoter genes and over-expression of anti-apoptotic proteins. Rather, NV-128 uncouples the akt-mTOR­P70S6K signal transduction cascade which has a key role in driving protein translation and uncontrolled cancer cell proliferation. Further, NV-128 induces mitochondrial depolarization via a novel pathway involving the autophagy protein Beclin-1 and Bcl-2, thereby resulting in endonuclease G translocation to the nucleus and cell death.

The same research group that presented the findings just mentioned has also done work on ovarian cancer itself, and been able to locate cancer stem cells for this type of cancer:

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

Researchers at Yale School of Medicine 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.

As already mentioned, NV-128 is not the only drug under investigation for attacking cancer by targeting the TOR pathway. In fact, almost a year ago, the first anti-cancer mTOR-inhibitor received FDA approval. It's Toricel (temsirolimus), an intravenous drug from Wyeth Pharmaceuticals, for kidney cancer. Novartis has an oral drug (everolimus) for kidney cancer in Phase III trials. (It's already been approved by the FDA as an immunosuppressant to prevent rejection of organ transplants.) Interestingly, and unsurprisingly, everolimus is a derivative of Rapamycin (sirolimus) – an anti-fungal and immunosuppressive compound – which led to the original discovery of mTOR. Everolimus works similarly to Rapamycin as an mTOR inhibitor.

The American biotech company Ariad Pharmaceuticals has a small molecule anti-cancer mTOR inhibitor called deforolimus in intermediate clinical trials for a variety of solid cancers, such as sarcomas, endometrial, prostate, breast and non-small cell lung cancers. The company describes the drug as "a novel small-molecule inhibitor of the protein mTOR, a “master switch” in cancer cells. Blocking mTOR creates a starvation-like effect in cancer cells by interfering with cell growth, division, metabolism, and angiogenesis." Last summer Ariad entered into a major partnership with Merck to develop and test the drug, so this is an indication that the drug has definite promise.

Ariad has a nice video you can download, which explains a bit about how their drug works, and about TOR signaling in general. I highly recommend having a look at it, since it covers upstream signals that activate mTOR (growth factors, amino acids, oxygen, energy), downstream effects (synthesis of proteins for cell growth, cell division, metabolism, and angiogenesis). It notes that certain other signaling proteins (PTEN, Akt, PI3K) cause overactivation of mTOR, and it points out that mTOR stimulates the production of the cyclin D cell division protein.

Tags: cancer, TOR signaling

Calorie restriction, TOR signaling, and aging

Now that I've given some pointers to information about how TOR signaling is involved with metabolism (see here), it seems like an opportune time to mention a recent research announcement in this general area.

How Dietary Restriction Slows Down Aging (4/17/08)

University of Washington scientists have uncovered details about the mechanisms through which dietary restriction slows the aging process. Working in yeast cells, the researchers have linked ribosomes, the protein-making factories in living cells, and Gcn4, a specialized protein that aids in the expression of genetic information, to the pathways related to dietary response and aging.

Here's the key background:

Previous research has shown that the lifespan-extending properties of dietary restriction are mediated in part by reduced signaling through TOR, an enzyme involved in many vital operations in a cell. When an organism has less TOR signaling in response to dietary restriction, one side effect is that the organism also decreases the rate at which it makes new proteins, a process called translation.

The researchers investigated various strains of yeast cells that had low rates of protein production, but increased lifespan. They found that a common characteristic of such cells was mutations to one part of the cell's ribosomes, the complex of RNA and certain proteins which manufactures all new proteins in the cell. The result of these ribosome changes was a decrease in the production of most proteins, except for one, called Gcn4, a transcription factor, whose production increased. The effect seems to depend on the same pathway affected by reduced TOR signaling. Gcn4 is associated with control of amino acid synthesis, and is activated when a cell is starved for amino acids.

To make the link between Gcn4 and longevity, the scientists then asked whether preventing the increase of Gcn4 would block life span extension. In every case, cells lacking Gcn4 did not respond as strongly as Gcn4-positive cells.

"The increased production of Gcn4 in long-lived yeast strains, combined with the requirement of Gcn4 for full life-span extension, makes a compelling case for Gcn4 as an important downstream factor in this longevity pathway," Kaeberlein said.

One might speculate that increased Gcn4 production somehow helps the cell cope with lack of nutrients, and one effect is that the cell takes steps to conserve its resources and slow the rate of aging.

Since reduction of TOR signaling is one way to bring about this effect, TOR inhibitors might help slow aging and increase lifespan, at least in yeast. However, since TOR affects so many other cell functions, the chance for harmful side effects of reduced TOR signaling is high.

"The role of TOR and translation in aging is known to be conserved across many different species, so it's plausible that this function of Gcn4 is conserved as well," Kennedy said. Future research will be aimed at testing this hypothesis.

"Clearly TOR signaling is one component, and perhaps the major component, of the beneficial health effects associated with dietary restriction," said Kaeberlein. "The difficulty with TOR as a therapeutic target, however, is the potential for negative side effects. As we learn more of the mechanistic details behind how TOR regulates aging, we will hopefully be able to identify even better targets for treating age-associated diseases in people."

Tags: calorie restriction, TOR signaling

Cancer, metabolism, and oncogenes

I want to call attention (somewhat belatedly) to a series of three very good tutorial blog posts at The Daily Transcript. Although they are nominally about changing views regarding cancer and its causes, they actually provide a nice overview of a number of important topics in molecular biology. Reading these posts will be a big help in understanding a lot of things written about here, in particular topics such as:

• cancer, and how it is "caused" by various factors like metabolism and genetic mutations, and indirectly affected by other biological systems like the immune system
  
• metabolism in general, and how problems with metabolism lead to disease conditions like diabetes and metabolic syndrome, perhaps even Alzheimer's disease
  
• calorie restriction, and how it seems to play a role in longevity
  
• stem cells – what makes them special, how they function biologically and may play a role in the process of cancer
  
• important processes in cell biology, such as apoptosis, autophagy, and (of course) the cell cycle itself
  
• general topics in molecular biology, such as growth factors, transcription factors, signaling cascades, and cell surface receptors
  

So here are the links, with a brief summary of each:

From Metabolism to Oncogenes and Back - Part I (3/17/08)

Historical introduction to the subject. Explains how Otto Warbug had the idea, 100 years ago, that the way to understand cancer was through metabolism. Somewhat later, the discovery of the Rous Sarcoma Virus (1916), and much later, after the revolutionary understanding of DNA and modern molecular biology came about, the focus shifted to the role of oncogenes, tumor suppressors, and genetic mutations in cancer.

From Metabolism to Oncogenes and Back - Part II (3/21/08)

More detailed look at the molecular biology of cancer, protein signaling pathways in general, and TOR signaling in particular. This part includes a great diagram of some of the more important signaling pathways as far as metabolism and cancer are concerned. Besides TOR, it clearly emphasizes the importance of the MAP kinase Ras, and the phosphoinositide signaling proteins PI3K, PTEN, and AKT.

From Metabolism to Oncogenes and Back - Part III (4/2/08)

An even more technical summary of recent discoveries about metabolism, and the peculiar kind of metabolic activity found in cancer cells. It appears that a type of enzyme called pyruvate kinase, which occurs in various forms, plays a big role in cell metabolism and whether a cell uses available energy for making sugars, fats, or DNA.

Tags: cancer, TOR signaling

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