New anti-cancer role for p53

I suppose that just about everyone knows of the important role the p53 protein plays in protecting cells from becoming cancerous. The protein was identified 30 years ago and its gene (TP53) cloned soon thereafter. What's not so widely known is just how complex the operation of p53 in protecting against cancer really is. And very recent research shows the complexity is even more than previously thought.

However, the complexity is to be expected, because evolution doesn't "design" cellular mechanisms to work in a straightforward way. The mechanisms are simply the result of about a billion years of trial and error. Being pretty and elegant was not a criterion for success.

Nature is "hairy", knowing nothing of Occam's Razor, and caring even less. Simplicity is for wimps.

But one thing is clear: p53 plays a large role in preventing, or at least suppressing, the development of cancer. In many types of cancer, p53 is found to have mutations more than 50% of the time. Even if p53 isn't mutated, cancer cells generally have other p53 abnormalities, such as low levels of the protein or the presence of various factors that interfere with its activity.

Until the latest research, there have been two principal ways known in which p53 works against cancer, and several additional minor ways. The two main ways p53 has been known to act are binding to DNA as a transcription factor, and binding directly to certain proteins. And each of these mechanisms can lead to either of two main types of tumor suppression: apoptosis (cell death) and temporary or permanent suspension of the cell cycle, which is the process a cell goes through in order to divide and proliferate.

P53 is primarily a transcription factor. In this role it is found in a cell nucleus and binds to various specific DNA gene promoter regions, in order to direct transcription of the associated gene – the first step in production of proteins from a gene.

The proteins that are expressed as a result of this p53 activity can play a part in either apopotosis or cell cycle control (as well as other functions not directly related to cancer – see here, here, here). Which function is invoked depends on the type of signal that activates the p53. Among the possible conditions that may be signaled are detection of correctable or uncorrectable damage to DNA and detection of chromosome telomeres that are too short.

In addition to binding to DNA as a transcription factor, p53 is also capable of binding directly to other proteins in order to control their behavior. Mainly these proteins are involved with apoptosis, such as members of the Bcl2 family.

P53 itself is actually a family of proteins – there are at least 9 different RNA transcripts that can be derived from the TP53 gene. But one thing that each of these family members have in common is a segment, called the DNA binding domain. It is this part of the p53 that is capable of binding to either DNA or other proteins. (In general, a protein domain is a more-or-less self-sufficient component of a protein. Often the same domain appears in different members of a family of proteins.)

One indication of the importance of this p53 domain is the fact that point mutations (errors involving only a single nucleotide pair) in the part of TP53 that code for the binding domain are the only type of point mutations of p53 that are commonly found in tumors. Errors that affect portions of p53 outside of the binding domain are not associated with cancer.

There's one more thing to note about p53's role as a transcription factor. Namely, the RNA that is transcribed under the direction of p53 is not always messenger RNA (mRNA) that will eventually code for the production of a protein. P53 can also initiate the transcription of genes that code for microRNA (miRNA), which is a single-stranded RNA molecule that's normally only 21 to 23 nucleotides in length. Over 500 different types of miRNA have been found in human cells.

MicroRNA is never translated into a protein. Instead, miRNA molecules regulate the translation of messenger RNA for many different proteins (by binding with the mRNA to prevent translation). It has been known for some time that p53 acts as a transcription factor for the miRNA family known as miR-34. It has also been learned that among the proteins regulated by miR-34 are some found in pathways that lead to apoptosis or cell cycle arrest. The net effect is that miR-34 has tumor-suppressing properties, so this is another way that p53, as a transcription factor, helps suppress tumors.

Many other miRNA molecules, on the other hand, are found at high levels in cancer cells. Such miRNAs most likely inhibit expression of tumor suppressing genes, whose proteins might otherwise control cell proliferation or migration. We've discussed a number of miRNAs associated with cancer, mostly of the sort that promote cancer, here and here.

Nevertheless, there are miRNAs besides miR-34 that have anti-cancer effects. Three in particular are miR-16-1, miR-143, and miR-145. It has been observed that these miRNAs, and several others, are found at higher levels in cells where p53 has been activated as a result of DNA damage. (Normally, p53 formed in non-cancer cells is either quickly degraded or else inhibited by certain proteins, especially MDM2, so as not to unnecessarily promote apoptosis or cell cycle arrest. The presence of DNA damage results in the removal of these inhibitions on p53.)

It therefore appears that p53 is doing something to help produce a number of miRNAs, some of which are tumor suppressors. The curious thing, though, is that it can be shown that p53 is not a transcription factor for the genes that encode these miRNAs.

So what is it that p53 is doing instead to help produce these miRNAs? New research published in the July 23, 2009 issue of Nature answers this question – and it uncovers an entirely new mechanism through which p53 (and its binding domain, in particular) acts as a tumor suppressor. Here's the research abstract:

Modulation of microRNA processing by p53

MicroRNAs (miRNAs) have emerged as key post-transcriptional regulators of gene expression, involved in diverse physiological and pathological processes. Although miRNAs can function as both tumour suppressors and oncogenes in tumour development, a widespread downregulation of miRNAs is commonly observed in human cancers and promotes cellular transformation and tumorigenesis. This indicates an inherent significance of small RNAs in tumour suppression. However, the connection between tumour suppressor networks and miRNA biogenesis machineries has not been investigated in depth. Here we show that a central tumour suppressor, p53, enhances the post-transcriptional maturation of several miRNAs with growth-suppressive function, including miR-16-1, miR-143 and miR-145, in response to DNA damage. ... These findings suggest that transcription-independent modulation of miRNA biogenesis is intrinsically embedded in a tumour suppressive program governed by p53. Our study reveals a previously unrecognized function of p53 in miRNA processing, which may underlie key aspects of cancer biology.

To understand what's going on, it's necessary to explain a few things about how miRNAs are produced. It's not a simple 1-step process of transcribing an miRNA gene into the final short piece of RNA.

There are, instead, three steps. The first step is transcription, done just as is done for any other gene. The RNA produced in this step is many nucleotides long, and is called the "primary transcript" or pri-miRNA. This pri-miRNA is then cut into smaller pieces having a hairpin shape, called pre-miRNA. The pre-miRNA, in turn, is further processed to produce the final "mature" miRNA.

The intermediate step that converts pri-miRNA to pre-miRNA is performed by a protein complex known as the "microprocessor complex" (having nothing to do with computers, of course). One of the key proteins in this complex is an enzyme called Drosha. The final step, which is performed by another enzyme called Dicer, splits the pre-miRNA apart to yield the mature miRNA.

The main contribution of p53 in this process is to facilitate the action of Drosha. It seems that, although Drosha can do the job by itself (since miRNAs are needed even if p53 isn't active), p53 helps by binding (via its binding domain) with parts of the microprocessor complex. This is indicated by the observation that mutations in the binding domain disable p53 binding to the complex, resulting in lower levels of miRNA production.

So there you have it: an essentially novel way that p53 acts as a tumor suppressor, by facilitating production, non-transcriptionally, of tumor-suppressing miRNAs.

Suzuki, H., Yamagata, K., Sugimoto, K., Iwamoto, T., Kato, S., & Miyazono, K. (2009). Modulation of microRNA processing by p53 Nature, 460 (7254), 529-533 DOI: 10.1038/nature08199

Further reading:

Protein plays three cancer-fighting roles (7/22/09) – Science News article on the research

Link between p53 and miRNA – editor's summary in Nature of the research

Cancer: Three birds with one stone (7/23/09) – Nature news article on the research

Tags: p53, microRNA, cancer

SIRT1 and cancer

In the past we've had some discussion of the histone deacetylase (HDAC) enzyme SIRT1 and other related sirtuin proteins, especially with respect to their possible relationship with longevity. (See here, for example.)

Much of the focus has been on the HDAC properties of SIRT1 that can switch off various genes. But there have also been findings of more direct relations between SIRT1 and cancer. Some indicate that sirtuins, including SIRT1, may help suppress cancer in certain circumstances, while others suggest it may actually help promote cancer. We'll have to save a general discussion of this relationship for later.

But now we have some research that shows how SIRT1 is directly involved, and has a beneficial effect, in an important pathway that's quite relevant to breast cancer.

The background is that the BRCA1 gene (short for breast-cancer-associated gene 1) is a tumor suppressor gene that, when mutated, may lose its ability to suppress tumors. Defective BRCA1 is sometimes inherited, which helps explain familial tendencies to breast cancer.

So what does BRCA1 normally do to suppress tumors? Well, apparently it maintains expression of SIRT1, which in turn inhibits the expression of another protein, called Survivin. The latter is an inhibitor of programmed cell death (apoptosis), and therefore, when it is active, helps protect cancer cells, which might otherwise be killed by the immune system, chemotherapy, or radiation.

In a nutshell: defective BRCA1 leads to insufficient SIRT1, which leads to an inadequate ability to kill cancer cells.

New Findings May Improve Treatment Of Inherited Breast Cancer (10/9//08)

About 8% of breast cancer cases are caused by mutations in tumor suppressor genes, such as breast cancer associated gene-1 (BRCA1). BRCA1 is the most frequently mutated tumor suppressor gene found in inherited breast cancers and BRCA1 mutation carriers have a 50-80% risk of developing breast cancer by age 70. "Although work with animal models of BRCA1 mutation has provided some insight into the many biological processes linked with BRCA1, very little is known about the downstream mediators of BRCA1 function in tumor suppression," says lead study author Dr. Chu-Xia Deng from the Genetics of Development and Diseases Branch at the National Institutes of Health.

Dr. Deng and colleagues were interested in investigating the relationship among BRCA1, SIRT1 and Survivin. SIRT1 is a protein and histone deacetylase involved in numerous critical cell processes including metabolism, DNA repair and programmed cell death, known as apoptosis. Although SIRT1 has been implicated in tumorigenesis, no concrete role in cancer initiation or progression has been identified. Survivin is an apoptosis inhibitor that is dramatically elevated in many types of tumors. Research has suggested that Survivin may serve to maintain the tumor and promote growth.

The researchers found that BRCA1 functioned as a tumor suppressor by maintaining SIRT1 expression, which in turn inhibited Survivin expression. When BRCA1 was not functioning properly, SIRT levels decreased and Survivin levels increased, allowing BRCA1-deficient cells to overcome apoptosis and undergo malignant transformation.

This leads one to ask whether there are other ways that SIRT1 activation could be maintained when BRCA1 is defective. Fans of resveratrol will observe that this is something that resveratrol can do. And so the researchers gave it a try:

They went on to show that the compound resveratrol strongly inhibited BRCA1-mutant tumor growth in cultured cells and animal models. ... In the current paper, resveratrol enhanced SIRT1 activity, this leading to reduced Survivin expression and subsequent apoptosis of BRCA1 deficient cancer cells.

Ironically, previous research had indicated circumstances in which SIRT1 might promote growth of other types of cancers. It might, for instance, inhibit expression of other tumor-suppressor genes.

Another news account goes into this a little more:

Gene thought to promote tumor growth has opposite role in a kind of breast cancer (10/9/08)

These results were surprising in light of previous reports showing that high levels of SIRT1 enhance growth of other types of tumors. It now appears that SIRT1 can enhance or inhibit tumor growth — it all depends on the context, says Deng. ...

The researchers also found that a red wine chemical called resveratrol, recently touted as a powerful antiaging compound, was effective in combating BRCA1-associated tumor formation specifically.

How resveratrol is able to do this is unclear. “The work in this case is that SIRT1 has an antitumor effect, and this paper provides mechanistic insights into that,” comments Pere Puigserver, a Harvard biologist who studies SIRT1. But the resveratrol data should be taken with caution, he notes. While this new research clearly shows the direct relationship between BRCA1 and SIRT1, the direct link between resveratrol and SIRT1 is more difficult to demonstrate.

Nonetheless, molecular details of BRCA1-related breast cancer are emerging, and this new data places SIRT1 squarely inside the complex web of molecules that impact tumor growth.

One of the main reasons that sirtuins are suspected of having cancer-promoting properties in some circumstances is that they may inhibit the highly important p53 tumor suppressor gene. (P53, when functioning properly, promotes cell apoptosis when DNA defects are detected during cell division.) In just one example of many, here's research from earlier this year that suggests a tumor-promoting property of sirtuins:

Switching on cancer killer gene (5/8/08)

Scottish scientists have discovered how to control a major anti-tumour gene that could lead to more effective chemotherapy. According to a report in the Cancer Cell Journal, research conducted by the Universities of St Andrews and Dundee may eventually lead to the development of new cancer drugs.

The gene, called p53 and known as "the guardian of the genome", is damaged or switched off in most cancers. But the resrchers found that they could reboot it using two new biological compounds called "tenovins".

In a laboratory study, the academics found that these compounds could kick-start p53 by turning off enzymes called sirtuins. Sirtuins act like genetic switches and keep p53 under control, ensuring that the cells stay alive.

Other news accounts of this research: here, here.

Tags: cancer, BRCA1, SIRT1, sirtuin, p53, resveratrol

P53 and skin pigmentation

Since we just mentioned skin coloration (here, in passing), and we have previously discussed how the p53 protein is involved with it (here), it's interesting that there is now more news on the subject.

Protein Linked To Bone Marrow Failure In Humans Found Through Study Of Dark-skinned Mice (7/20/08)

McGowan, Barsh, and their colleagues found that skin from the feet of the mutant mice exhibited elevated levels of p53. This elevation, or "activation," of p53 stimulated the production of a protein called Kit ligand that stimulates the growth of pigment cells, which turned the mice's skin darker than normal. In contrast, mutant mice unable to express p53 had normal levels of Kit ligand. They also had light-colored feet and unaffected numbers of red blood cells. ...

The researchers hypothesize that increased activation of p53 affects different types of cells in the body in different ways. In skin cells, it increases the amount of Kit ligand and causes darker skin, whereas in bone marrow cells it causes anemia by causing the death of red blood cell precursors.

Is there some lesson in this? Well, p53 is generally regarded as a "good" protein, because it helps ward off cancer that would otherwise result from DNA damage. But it does this by promoting apoptosis of cells affected by the DNA mutation. Anemia is the result when too many red blood cells die.

And the skin darkening? Earlier studies indicate that is also a side effect of increased levels of p53. Such unexpected – and not always desirable – side effects are the reason that developing drugs to treat disease is so difficult. There is so much unexpected interconnection of our cellular machinery, adjusting something in one place can lead to problems in quite different places.

Tags: p53

P53, a versatile gene

P53 is well-known for its role in regulating the cell cycle so as to suspend the cycle or even lead to cell death via apoptosis in case damage to a cell's DNA is detected. This function is especially important in forestalling cancer.

And as we noted here, p53 is also involved with skin tanning.

But that's not all p53 is good for. It also plays a role in fertility, which has recently been reported by one of the co-discoverers (Arnold Levine) of p53:

Cancer Fighter May Be Fertility Helper

A protein known primarily for its role in fighting cancer also helps embryos implant in the womb, according to a study in mice. The find may explain why some women have difficulty becoming pregnant.

More information: here, here

But the list of p53's goodness doesn't stop there. It also slows aging, apart from deterring cancer, but via the same mechanism:

Anti-cancer gene p53 doubles up as anti-ageing agent

The latest research suggests that one of the genes that protects us from cancer may also help delay the ageing process.

A new study has found that a particular gene, p53 which has been previously linked to premature ageing, along with one of its cellular regulators, called Arf, may boost the body's antioxidant activity to keep cells younger longer and thereby slow down the aging process.

The regulatory chemical Arf, lets p53 know that a particular cell is in trouble and marked for elimination.

More information: here, here, here, here

But, surprisingly, at least in fruit flies, reducing p53 activity may also increase lifespan, and apparently in the same way that calorie restriction does:

Key To Longer Life (in Flies) Lies In Just 14 Brain Cells

Two years ago, Brown University researchers discovered something startling: Decrease the activity of the cancer-suppressing protein p53 and you can make fruit flies live significantly longer.

Now the same team reports an intriguing follow-up finding. The p53 protein, they found, may work its lifespan-extending magic in only 14 insulin-producing cells in the fly brain.

How was this connected with calorie restriction? Simply by noting that calorie restriction in fruit flies didn't increase longevity when p53 activity was suppressed in only 14 insulin-producing cells of the flies' brains:

Studies have shown that low-calorie diets can significantly increase the lifespan of flies, worms, mice and rats. The phenomenon is of intense interest to researchers who study aging. They want to know if caloric restriction works in people and if drugs could be made to mimic its effects.

So researchers restricted the diets of the flies and ran the same experiments. The calorie-restricted flies didn't live any longer when p53 was reduced in the insulin-producing cells. This evidence supports the notion that p53 reduction is one of the direct effects of caloric restriction.

Even more intriguing, Helfand said, is the fact that the 14 insulin-producing cells that seem to be critical for lifespan extension are the equivalent of beta cells in the human pancreas. Beta cells make and release insulin, the hormone that controls the level of glucose in the blood. The research team found that when p53 activity drops, so does insulin-responsive activity in the fat body, the major metabolic organ in the fruit fly.

The involvement of insulin in this effect is especially interesting, as insulin signaling has also been found to be involved in the mechanism by which sirtuin proteins extend longevity in nematodes and fruit flies (and perhaps other organisms).

One wonders just how p53 came to play such a prominent role in cellular processes. Some researchers think they have found the answer – endogenous retroviruses that have actually proven beneficial to the host genome:

Ancient Retroviruses Spurred Evolution Of Gene Regulatory Networks In Humans And Other Primates

Scientists have long wondered how a master regulator such as p53 gained the ability to turn on and off a broad range of other genes related to cell division, DNA repair, and programmed cell death. How did p53 build its complex and powerful empire, so to speak?

Using the tools of computational genomics, the UCSC team gathered compelling evidence that retroviruses helped out. ERVs jumped into new positions throughout the human genome and spread numerous copies of repetitive DNA sequences that allowed p53 to regulate many other genes, the team contends.

"This would have provided a mechanism to quickly establish a gene regulatory network in a very short evolutionary time frame," said Ting Wang, a post-doctoral researcher at UCSC and lead author of the paper.

It's hard to avoid a suspicion that there's a lot to the story of p53 left to be discovered.

Tags: p53, aging, longevity, calorie restriction, insulin signaling

P53 protein and tanning

At first this may seem an odd coincidence, but maybe it isn't. P53 is the protein which plays a critical role in preventing runaway division in cells with damaged DNA, and hence inhibiting cancer. Unless p53 itself becomes faulty – which happens in the majority of cancerous cells. It does its job by stopping the cell division cycle if damaged DNA is detected.

But apparently p53 is also implicated in tanning of human skin by the sun.

'Guardian Of The Genome' Protein Found To Underlie Skin Tanning

A protein known as the "master watchman of the genome" for its ability to guard against cancer-causing DNA damage has been found to provide an entirely different level of cancer protection: By prompting the skin to tan in response to ultraviolet light from the sun, it deters the development of melanoma skin cancer, the fastest-increasing form of cancer in the world.

In a study in the March 9 issue of the journal Cell, researchers at Dana-Farber Cancer Institute report that the protein, p53, is not only linked to skin tanning, but also may play a role in people's seemingly universal desire to be in the sun -- an activity that, by promoting tanning, can reduce one's risk of melanoma.

"The number one risk factor for melanoma is an inability to tan; people who tan easily or have dark pigmentation are far less likely to develop the disease," says the study's senior author, David E. Fisher, MD, PhD, director of the Melanoma Program at Dana-Farber and a professor in pediatrics at Children's Hospital Boston. "This study suggests that p53, one of the best-known tumor-suppressor proteins in our body, has a powerful role in protecting us against sun damage in the skin."

Of course, people who tan easily or have dark pigmentation may also be less inclined to spend time in the sun for the purpose of acquiring a tan, so any other factors in an individual that might be responsible for tannning or dark pigmentation would also indirectly reduce the statistical liklihood of melanoma.

However, the research shows that p53 does influence tanning directly.

Other reports:

• Gene behind tanning comes out of hiding
  
• A Protein Twofer That Triggers Tanning and Protects against Skin Cancer
  
• Anti-Cancer Gene Triggers Tanning
  

Update 8/3/08: There is related news about this here.

Tags: p53, sun tanning, melanoma, skin cancer, cell cycle