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Scientists have identified a number of genes that seem to have some effect on an animals longevity. Mostly they have been found in small, short-lived creatures whose longevity is easily studied, such as mice, fruit flies, or roundworms (C. elegans), though they frequently have analogues in humans. See here for an earlier discussion.

Of course, any gene which is important for inhibiting cancer, such as the well-known p53, will tend to improve longevity, for obvious reasons. But surprisingly, there are some longevity genes which dont have such an obvious relation to cancer, and may lengthen expected life span even when cancer is present.

Longevity genes fight cancer at its source

Over the years, biologists have discovered a handful of genes in roundworms, mice and flies that bestow a dramatic increase in lifespan on the organism that carries it – sometimes up to twice their normal life expectancy.

These genes are involved in diverse biochemical pathways including those for growth hormones, insulin, food intake and caloric restriction. But it is thought that they are all have a role in how the body responds to stress.

Julie Pinkston at the University of California in San Francisco, US, and colleagues, wondered if these longevity genes had something else in common: the power to fight cancer – a notoriously age-related disease.

Pinkston manipulated a C. elegans gene to make the worm more susceptible to cancer, and she also introduced a mutated version of the daf-2 insulin-like receptor gene, known to be longevity-enhancing. Worms with both mutations, even though they developed tumors, still lived twice as long as unmutated worms. Apparently the mutated daf-2 was doing something in addition to preventing tumors from forming.

The something else seems to be related to apoptosis:

Daf-2 seemed to protect against the lethal cancer by stimulating apoptosis – programmed cell death – which tumour cells usually avoid, the researchers say.

Its understandable that a gene which stimulates apoptosis helps fight cancer. The question is whether stimulating apoptosis also has harmful side effects. Apparently not so much in this case, if longevity is doubled anyhow.

But theres more to it than that:

One hallmark of cancerous growth is a rapid acceleration of cell division. Daf-2 also decreased the number of cell divisions in the roundworms by 50% compared to what was expected for those with the gld-1 gene, Pinkston says.

Other longevity-releated gene mutations are known in C. elegans, and when these mutations were present, the longevity effect also occurred:

The team then used the same process to test three other known longevity genes in turn against the life-shortening gld-1 gene. These three double-mutant worms also lived longer than normal roundworms. Each of the three genes (eat-2, isp-1 and clk-1) suppressed cell division, even though they did not appear to increase apoptosis.

Again, it would seem that suppressing cell division with these mutations is a net benefit for longevity, despite the need for some cell division outside of tumors. Perhaps they simply cause an animals life cycle to proceed at a slower pace.

But roundworms are rather simple animals. One wonders how such an effect would play out in a human...

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Other references:

Longevity genes fight back at cancer - subscription required

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Tags: cancer, longevity, lifespan, medicine, aging

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 Alzheimers 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

Adiponectin is a hormone that is made exclusively in adipose (fat) tissue and secreted into the blood stream. It modulates a number of metabolic processes, such as glucose regulation and production of energy from fatty acids.

We had a long note on adiponectin last September (here), which has turned out to be very popular. That article summarized a number of research results concerning adiponectin that have appeared in the last few years. Undoubtedly, much of the interest in adiponectin is a result of its relevance to such things as weight control, diabetes, inflammation, cardiovascular disease, and kidney disease.

Some research that was reported in April had more to say about the relation to kidney disease:

Fat-cell Hormone Linked To Kidney Disease (4/22/08)

Reduced levels of a hormone produced by fat cells and linked to the development of insulin resistance may also be related to a higher risk of kidney disease, according to a study led by researchers at the University of California, San Diego School of Medicine and Thomas Jefferson University. ...

The new findings show that the hormone, adiponectin, produced by fat cells, circulates in the blood and acts to both suppress inflammation -- known to be a contributor to diabetes and cardiovascular disease -- and to reduce protein in the urine.

"A deficiency in adiponectin could be the major reason why obese patients develop the initial signs of kidney disease," said principal investigator Kumar Sharma.

The research showed that adiponectin promotes proper function of kidney cells called podocytes:

A network of fine capillaries in the kidney acts as a filter to prevent proteins in the blood from being secreted into the urine. This filter is made up of three components, one of which -- the podocyte cell -- serves to regulate albuminuria.

"We discovered that the hormone adiponectin, produced by fat cells, is directly linked to the healthy function of podocytes," said Sharma.

While thats interesting, its not clear that this activity has much to do with adiponectins effect on metabolism through favoring the use of fats as a source of energy instead of glucose. This may be a case where an important hormone really does have unrelated effects on different physiological systems.

Earlier research on adiponectin suggested that it served as a signal of low levels of available food calories, and hence caused the body to favor metabolism of stored fat as an energy source. This could well be related to the known effects of calorie restriction on longevity. Indeed, some research from last November suggested that longevity is promoted because metabolism of fat generates a lower level of reactive oxygen species than does metabolism of glucose:

Fat Hormone May Contribute To Longevity (11/21/07)

Using a mouse model of longevity, Terry Combs and colleagues report that changes in metabolism can indeed increase longevity. They demonstrated that long-lived Snell dwarf mice burn less glucose and more fatty acids during periods of fasting, and as a result produce fewer free radicals.

The key to this switch may be adiponectin, a hormone produced by fat cells that helps lower glucose production and stimulates cells to use fat for energy instead. The researchers found that Snell mice had three times as much adiponectin in their blood as control mice; Snell mice also had fewer triglycerides in their cells, indicative of higher fat metabolism.

The benefit of burning fats instead of glucose for energy is that it produces fewer oxygen radicals which can damage cells and exacerbate the effects of aging. Confirming this, Combs and colleagues found far less free radical damage.

Given that reactive oxygen species are also linked to increased inflammatory response and DNA damage, and that both of these effects are linked to cancer, its not too surprising to find that variations in the gene for adiponectin may affect cancer risk:

Gene Variations May Predict Risk Of Breast Cancer In Women (5/2/08)

According to a recent study, led by Virginia Kaklamani, MD, an oncologist at Northwestern Memorial Hospital and assistant professor of medicine, Northwestern University Feinberg School of Medicine, variations of the adiponectin gene, which regulates a number of metabolic processes, may increase a woman’s risk of developing breast cancer. ...

Dr. Kaklamani’s research, which is published in the May 1 issue of Cancer Research, suggests some women are born with different characteristics in the adiponectin gene which can alter its function and increase the risk of breast cancer. This finding, coupled with previous studies that have found a correlation between low levels of adiponectin in the body and cancer risk, suggest adiponectin may be the third gene linked to breast cancer among women with no previous family history of breast cancer. If confirmed through additional studies, adiponectin could be used along with TGF-beta and CHEK2, genes that have already been linked to breast cancer, to create a genetic testing model that will allow clinicians to more accurately predict breast cancer risk.

Further reading:

Happy fat: Calorie restriction modulates adipocyte gene expression – 7/17/07 blog article that discusses research relating calorie restriction to adiponectin

Adipogenic signaling in rat white adipose tissue: Modulation by aging and calorie restriction – abstract of the research discussed in the preceding item.

Tags: adiponectin, longevity, cancer, reactive oxygen species

We havent recently discussed the role of microRNA in cancer. Last time (February 2008) is here. There have been some relatively recent research announcements, so lets have a look.

If you want a refresher on the subject, heres a good introductory overview from Cancer Research UK: Micro RNAs and cancer. Although this piece is fairly elementary, it does have many good links to actual research papers.

Now lets jump into a few summaries of recent research.

Whats Feeding Cancer Cells? (2/17/09)

Cancer cells grow and multiply rapidly, so they need lots of nutrients. Much is already known about how cancer cells use blood sugar, but other nutrients are also needed. One of these is the amino acid glutamine. This research found that the transcription factor Myc is able to enhance the expression of the enzyme glutaminase (GLS) in cellular mitochondria. GLS is the first enzyme that processes glutamine to produce energy in mitochondria. (Overexpression of Myc is frequently found in cancer – see here.)

The research found that depriving cancer cells of GLS slowed their growth significantly. It was suspected that Myc could directly up-regulate the GLS gene, but it was not that simple. Instead, it appears that Myc down-regulates genes for two types of microRNA: mi-R23a and mi-R23b. Since these mircoRNAs interfere with the GLS messenger RNA, the net effect of Myc is to enhance GLS production.

Research abstract: c-Myc suppression of miR-23a/b enhances mitochondrial glutaminase expression and glutamine metabolism

A new discovered mutation can hold the key to treat a large number of different cancers (2/17/09)

Since microRNA normally inhibits production of certain proteins, if the proteins affected promote cancer, the inhibitory miRNA will counteract this. This research examined cells of twelve different cancer types.

The basic finding was that mutations of the gene TARBP2 disrupts a pathway that produces anti-oncogenic microRNAs. Mutated TARBP2 diminishes TRBP protein expression, resulting in a defect in the processing of miRNAs. Specifically, the DICER1 protein, which is necessary for miRNA production, is adversely affected.

Research abstract: A TARBP2 mutation in human cancer impairs microRNA processing and DICER1 function

Micro RNA Plays A Key Role In Melanoma Metastasis (2/15/09)

Metastasis is the main process by which cancer becomes deadly, and it is especially problematic in melanoma. In order for cancer cells to metastasize (spread to another body location) they must become able to migrate and establish themselves in the new location. This research finds that the microRNA miR-182 assists in this process.

MiR-182 is frequently up-regulated in human melanoma, usually because melanoma cellular DNA contains extra copies of the miR-182 gene. This up-regulation was shown to assist metastasis. Conversely, down-regulation impedes invasion and triggers apoptosis. Over-expressed miR-182 is shown to repress the expression of two tumor suppressors, FOXO3 and MITF, which are both transcription factors. (For more on FOXO3, see here.)

Research abstract: Aberrant miR-182 expression promotes melanoma metastasis by repressing FOXO3 and microphthalmia-associated transcription factor

New Genes Involved In Acute Lymphoblastic Leukemia Play Fundamental Role In Prognosis Of The Disease (2/6/09)

This investigation found that 13 microRNAs were epigenetically regulated in an abnormal way in many patients with acute lymphoblastic leukaemia (ALL). This means that instead of having actual gene mutations, certain parts of the DNA were methylated in an unusual way, so that the underlying genes, which coded for microRNAs, were down-regulated. More precisely, certain histones of the cells chromatin were methylated, so that genes located on the DNA wrapped around those histones would not be expressed. The genes involved coded for microRNAs that, evidently, are important for suppressing cancer. When approriate steps were taken to reverse abnormal epigenetic regulation of the affected genes, expression levels rose, confirming that the abnormal methylation patterns were responsible for down-regulation.

65% of 352 ALL patients had one or more methylation abnormalities affecting microRNA under investigation. There was a highly significant positive correlation between patient survival at 14 years after diagnosis and absence of such abnormalities. Consequently, tests for methylation problems with the appropriate microRNA genes should be good predictors of survival prospects.

Research abstract: Epigenetic regulation of microRNAs in acute lymphoblastic leukemia

Researchers Identify Another Potential Biomarker For Lung Cancer (1/13/09)

The research showed that smoking impacts bronchial airway gene expression. Various miRNAs were found that were differently expressed in bronchial airway epithelial cells, mostly down-regulated. Messenger RNAs were also identified, whose expression was inversely correlated to the miRNA expression (so that the corresponding genes appear to be down-regulated by the miRNA.)

MiR-218 was especially noteworthy. It is known to be strongly affected by smoking. The conclusion is that miR-218 levels modulate airway epithelial gene expression response to cigarette smoke, suggesting a role for miRNAs in regulating response to environmental toxins.

Research abstract: MicroRNAs as modulators of smoking-induced gene expression changes in human airway epithelium

Molecule Linked To Muscle Maturation, Muscle Cancer (12/31/08)

The study clarified the role of MiR-29 in myogenesis (muscle cell formation) and found that its down-regulation is associated with rhabdomyosarcoma (RMS), a cancer caused by the proliferation of immature muscle cells. While miR-29 is required for maturation of myoblasts (immature muscle cells), it is also found to be mostly absent from RMS cells.

The study found, further, that the transcription factor NF-κB is responsible for down-regulating miR-29. (NF-κB is an old friend of ours. See here for a small part of the story about its role in inflammation. Theres also much more to be said about the role of NF-κB in cancer, where it provides an important connection between inflammation and cancer.)

NF-κB acts to repress miR-29 through another transcription factor, YY1, and Polycomb-group proteins (which remodel chromatin to block transcription factors from DNA promoter sequences).

During myogenesis, NK-κB and YY1 are down-regulated, permitting expression of miR-29, which then further down-regulates YY1 and accelerates cell differentiation. However, in RMS the NF-κB–YY1 pathway remains active, silencing miR-29 and inhibiting differentiation. But reconstitution of miR-29 in RMS in mice inhibits tumor growth and stimulates differentiation,

Research abstract: NF-κB–YY1–miR-29 Regulatory Circuitry in Skeletal Myogenesis and Rhabdomyosarcoma

Harnessing MiRNA Natural Gene Repressors For Anticancer Therapy (12/1/08)

This research investigates the potential therapeutic use of miR-181a through its ability to repress expression of selected genes. If successful, this would provide a very clever kind of immunotherapy for cancer and possibly other diseases.

In immune system T cells miR-181a is highly expressed in developing T cells, but is markedly down-regulated in mature T cells. Mouse bone marrow cells were engineered to express desired therapeutic genes only when miR-181a is down-regulated. These cells were transplanted into mice and allowed to develop into mature T cells. The proteins repressed by miR-181a would therefore not be found in the immature cells, but would show up in the mature T cells. And so when the genes repressed by miR-181a corresponded to proteins that direct T cells to attack tumor cells expressing the protein hCD19, mice with the engineered bone marrow cells were able to reject tumors expressing hCD19.

Research article (open access): Harnessing endogenous miR-181a to segregate transgenic antigen receptor expression in developing versus post-thymic T cells in murine hematopoietic chimeras

Molecule Linked To Aggressive Cancer Growth And Spread Identified (11/13/08)

EZH2 is a polycomb group protein, which helps maintain transcriptional repression of genes over successive cell generations. It contributes to the epigenetic silencing of target genes and enables the survival and metastasis of cancer. The research indicates that miR-101 inhibits the expression and function of EZH2 in cancer cells.

The researchers found that miR-101 is significantly underexpressed in a variety of cancers, including prostate and breast cancer. In human prostate tumors miR-101 expression decreases as cancer progresses and expression of EZH2 increases. MiR-101 is coded for at two locations in cell DNA. One or both of those locations is found to be defective in 37.5% of localized prostate cancer cells and in 66.7% of metastatic cells. This suggests that that underexpression of miR-101 is responsible for overexpression of EZH2 and consequent cancer progression.

More: here (11/13/08)

Research abstract: Genomic Loss of microRNA-101 Leads to Overexpression of Histone Methyltransferase EZH2 in Cancer

Further reading:

MicroRNA—implications for cancer – excellent open access review article

Tags: cancer, microRNA

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. Whats 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 doesnt "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 Occams 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 isnt 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.

Theres one more thing to note about p53s 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 thats 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. Weve 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. Heres 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 whats going on, its necessary to explain a few things about how miRNAs are produced. Its 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 isnt 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 – editors 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