SIRT1 and cancer

In the past weve 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. Well 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 thats 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, heres 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

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Inflammation microRNA and cancer

If theres just one single point worth making about the biology of cancer, it would have to be "its complicated".

Cells in general, and animal cells in particular, are extremely intricate Rube-Goldberg-like mechanisms. Their correct functioning depends on the integrity of 20,000 or so genes (in the case of humans), and at least 5 times as many proteins whose form is specified by the genes. Damage to even one of a few thousand important genes can put a cell on the road to becoming cancerous. So the first fact about cancer isnt really all that hard to understand: cancer (in all of its many forms) is a disease that begins with damage to the DNA of one or more genes.

This damage, which is necessary but not sufficient, can occur in many ways. Sometimes it happens because of the action of external agents, like carcinogenic chemicals or high-energy radiation (including ultraviolet light). Other times it happens simply because of occasional errors made in copying DNA during the process of cell division. These are just a few of many ways in which DNA can suffer damage. Its estimated that from 10,000 to a million DNA mutations can occur in a single human cell per day.

Fortunately, only a few percent of the 3 billion fundamental units (base pairs) of DNA actually occur within genes – everything else is "noncoding DNA". Although much of this noncoding DNA serves some useful purpose, we have little idea at present what that might be. However, its certainly less critical to cell function than the DNA of actual genes. Even so, 10,000 or so genes in every cell could suffer mutations every day.

Of course, complex multicellular life couldnt exist unless nature had evolved some means for coping with all this random genetic damage. And so, there are a large number of ways that cells have of detecting and repairing the damage that does occur. Then in the relatively small number of cases where damage cannot be repaired, cells have additional fail-safe mechanisms to avoid malfunctions which lead to unlimited proliferation – i. e. cancer. One such mechanism is for a cell to enter a state of "senescence", where it ceases to be able to divide at all. A more drastic, but common, mechanism is for the cell to undergo "apoptosis" – orderly cell death.

A necessary condition, therefore, for a cell to become cancerous, even after DNA damage remains unrepaired (perhaps because of damage to part of the repair mechanism), is that the damage occurs in a gene that codes for proteins needed for one of the various fail-safe mechanisms. Consequently, in almost every case of cancer where a tumor has begun to form, one finds problems in some part of the cells anti-proliferation machinery.

Well look at a recent piece of research that identifies one particular way this can happen, and its interesting for the variety of different cell processes that become involved.

Many of the known "causes" of cancer are fairly easy to understand. Certainly, the cancer risk from DNA-damaging carcinogenic chemicals is obvious enough. And once one understands how important a key protein known as p53 is in crucial cellular processes such as detection of unrepaired DNA damage and invocation of apoptosis if necessary, its not hard to understand why more than 50% of human tumors have mutated genes for p53.

But there are other factors which have been found, in epidemiological studies, to be statistically associated with cancer development. One of these is inflammation, which is a very normal part of the bodys immunological defenses against infection. Inflammation itself is a highly complex process – too complex to outline here. Chronic infections by various agents can cause a state of persistent inflammation. An example is the result of H. pylori bacterial infections. In addition to being responsible for stomach ulcers, such infections are also found in cases of stomach cancer. Obesity is also known as an epidemiological factor in various cancers, and the reason is now thought to be the state of chronic inflammation that obesity often causes.

What is not clear is exactly what mechanism connects inflammation with cancer. Theres undoubtedly a variety of mechanisms, given how complicated cellular processes turn out to be when you get down to the finer details. The recent research mentioned above illustrated one such mechanism, in one single type of cancer.

Anti-inflammatory drugs may defeat a treatment-resistant type of cancer (6/24/09)

The research focused on a type of non-Hodgkin lymphoma called diffuse large B-cell lymphoma. In some patients with the disease, chemotherapy works well. In a recent study of 40 patients more than 75 percent of patients with one form of this type of lymphoma survived five years or longer.

But that study also identified a group of patients whose cancer proved difficult to treat. Their tumors failed to respond to chemotherapy, and only 16 percent of patients with this form of lymphoma survived more than five years after they were diagnosed.

Several molecular flags mark this treatment-resistant lymphoma, but the links between them were unknown until now. The new paper reports that tumor cells isolated from these patients have depressed levels of a protein called SHIP1, which was known to suppress tumors. In fact, patients with the lowest levels of SHIP1 are the least likely to survive.

SHIP1 is a phosphatase enzyme. That means it removes phosphate groups from proteins. So a phosphatase has the opposite effect of enzymes known as kinases, which attach phosphate groups to proteins. Having a phosphate group attached at the right place on a protein is what enables the protein to take part in a signaling pathway, which is the basic communication mechanism in a cell responsible for making things happen. Therefore, phosphatases disrupt pathways, and stop things from happening. This can be beneficial, for example, if whats happening is the excessive cell division that occurs in cancer. Accordingly, SHIP1 has been found to be a tumor suppressing protein.

In the case of diffuse large B-cell lymphoma (DLBCL), it is found that SHIP1 levels are abnormally low. Its not that the SHIP1 is defective; theres just not enough of it. So the question is why. Is there some other defective gene thats responsible?

Apparently, there is not. Instead, its the presence of inflammation thats responsible, and in an interesting way. Inflammation is a perfectly normal product of the bodys immune system, and it exists to counteract harmful agents such as bacteria. The immune system initiates and regulates the process of inflammation by means of signaling molecules called cytokines. One of the more common and important of these cytokines is TNFα.

Now, TNFα normally goes about its business without causing cancer or other lasting ill effects. In fact, under the right conditions it can induce apoptosis or inhibit tumor formation in other ways. But for some reason, in DLBCL, TNFα suppresses SHIP1, and thus promotes cancer. The research in question also discovered the mechanism of SHIP1 suppression. It turns out that the real culprit here is a small piece of microRNA called miR-155. This little bugger was already known to be involved with leukemia in mice, and with other cancers. (See references in here.)

The resistant type of lymphoma cells also have elevated levels of miR-155, a specific example of a type of genetic material called microRNA, the team found. They demonstrated that miR-155 suppresses SHIP1 by sticking to the template for the protein, preventing its manufacture. ...

The final clue came from earlier reports that an inflammatory molecule called TNFα could boost levels of miR-155. Additional laboratory work confirmed the observation for this type of lymphoma cell.

Some anti-inflammatory drugs, used for diseases such as arthritis and inflammatory bowel disease, where inflammation gets out of hand, work by suppressing TNFα. So it was hypothesized that such a drug might be beneficial in treating DLBCL. And voilà:

The anti-inflammatory drugs etanercept and infliximab, which are currently used to treat arthritis and inflammatory bowel disease, work by suppressing TNFα, suggesting a new way to curb the malignancy of this type of lymphoma.

The team tested the idea in mice that had been injected with aggressive lymphoma cells and found that nascent tumors shrank in six days.

However, mice are not humans, so the drugs need to be tested in human DLBCL patients. Patients are already being recruited for clinical studies.

Now, there are plenty of questions remaining. More needs to be understood about just what pathways SHIP1 disrupts in order to suppress tumors. This should also help in understanding why inflammation and the resulting TNFα do not, fortunately, cause cancer more often. Baby steps. But perhaps significant ones.

Heres the research abstract:

Onco-miR-155 targets SHIP1 to promote TNFα-dependent growth of B cell lymphomas

Non-coding microRNAs (miRs) are a vital component of post-transcriptional modulation of protein expression and, like coding mRNAs harbour oncogenic properties. However, the mechanisms governing miR expression and the identity of the affected transcripts remain poorly understood. Here we identify the inositol phosphatase SHIP1 as a bonafide target of the oncogenic miR-155. We demonstrate that in diffuse large B cell lymphoma (DLBCL) elevated levels of miR-155, and consequent diminished SHIP1 expression are the result of autocrine stimulation by the pro-inflammatory cytokine tumour necrosis factor alpha (TNFα). Anti-TNFα regimen such as eternacept or infliximab were sufficient to reduce miR-155 levels and restored SHIP1 expression in DLBCL cells with an accompanying reduction in cell proliferation. Furthermore, we observed a substantial decrease in tumour burden in DLBCL xenografts in response to eternacept. These findings strongly support the concept that cytokine-regulated miRs can function as a crucial link between inflammation and cancer, and illustrate the feasibility of anti-TNFα therapy as a novel and immediately accessible (co)treatment for DLBCL.

Pedersen, I., Otero, D., Kao, E., Miletic, A., Hother, C., Ralfkiaer, E., Rickert, R., Gronbaek, K., & David, M. (2009). Onco-miR-155 targets SHIP1 to promote TNFα-dependent growth of B cell lymphomas EMBO Molecular Medicine, 1 (5), 288-295 DOI: 10.1002/emmm.200900028

Tags: cancer, inflammation

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Folate and cancer

I must admit I dont really understand all the hubbub about folate and cancer. First we have this strange business where some say that low levels of folate in ones diet either increase the risk of colon cancer, or else decrease it. Take your pick.

And now we read that, as far as breast cancer is concerned, it doesnt have anything to do with risk:

Dietary Folate Intake Not Associated With Breast Cancer Risk

Folate, a vitamin that is abundant in fruits and vegetables, helps maintain DNA integrity, and a lack of it has been associated with DNA strand breaks and disruptions in DNA repair. Previous studies have suggested that increased folate intake may be associated with a reduced risk of breast cancer, but this association was not replicated by large studies that followed study participants prospectively. In addition, a common genetic change in the gene encoding a key enzyme in folate metabolism, called MTHFR, can lead to low folate levels in the body and therefore could be associated with breast cancer risk.

OK, so folate somehow is good for DNA integrity in the petri dish. Fair enough. Evidently, however, theres more to the story when folate is ingested with ones food. Like, maybe, it has a hard time reaching ones cells where it can do some good. Looks like we have a drug delivery issue here.

Tags: folate, cancer

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