MicroRNA and cancer II

We haven't recently discussed the role of microRNA in cancer. Last time (February 2008) is here. There have been some relatively recent research announcements, so let's have a look.

If you want a refresher on the subject, here's 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 let's jump into a few summaries of recent research.

What's 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 cell's 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. There's 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

FoxO transcription factors

Transcription factors are proteins that help regulate genes. This regulation may involve either enabling the expression of a gene or preventing expression. In the first case, the transcription factor is an "activator", and in the second case a "repressor".

Transcription factors perform their function by binding to a particular portion of DNA that is specific to a given gene. When bound to the appropriate DNA segment, a transcription factor affects gene expression by either facilitating (activator) or inhibiting (repressor) the operation of RNA polymerase in transcribing the affected gene into messenger RNA. Usually more than one transcription factor must be present to affect gene transcription, and additional proteins (called "cofactors") may also be required.

To make things even more interesting, transcription factors usually affect multiple genes, which may be otherwise unrelated to each other.

A particularly important family of related transcriptions factors comprises what are called "forkhead box" proteins, or Fox proteins, for short. (The name refers to a sequence of 80 to 100 amino acids that are part of the protein and bind to DNA, and which was originally discovered in fruit flies (Drosophila).)

Among the genes that Fox proteins are involved with are genes related to cell growth, proliferation, differentiation, longevity, and embryonic development. So there are Fox proteins that are important for things like cancer and stem cells – and thus it's quite useful to know about them.

An important subfamily of Fox proteins are the FoxO proteins, and we'll discuss some recent examples in this note.

To begin with, perhaps the most recent example is this:

Molecular Signal That Helps Muscle Regenerate Discovered (12/19/07)

Muscle regeneration after injury is complex and requires a coordinated interplay between many different processes. Key players in regeneration are muscle stem cells, so-called satellite cells. They divide and produce many new muscle cells to fix the damage incurred by injury. A crucial regulator of muscle function and repair is a signalling molecule called calcineurin. It is activated by injury and controls the activity of other key proteins involved in differentiation and the response to damage.

It turns out that calcineurin works by inhibiting FoxO.

Using sophisticated molecular techniques, the scientists revealed that calcineurin accomplishes its effect on muscle by inhibiting another protein called FoxO. FoxO is a transcription factor, a protein that plays a crucial role in skeletal muscle atrophy through the induction of genes involved in cell cycle repression and protein degradation. Suppressing the effects of FoxO, calcineurin ensures that proliferating cells stay alive and keep dividing to produce enough cells to repair muscle damage.

In this case, the normal function of FoxO is to inhibit cell proliferation (as a check on cancer), but this needs to be bypassed (temporarily) to enable muscle regeneration.

This result follows the discovery a few months earlier of the way a specific FoxO protein (FoxO1) cooperates with another important developmental protein (Notch) to control muscle cell differentiation:

Building Muscle Requires Foxo1 (8/25/07)

The mechanisms by which Foxo proteins regulate metabolism are relatively well characterized. However, little was known about the mechanisms by which these same proteins regulate cellular differentiation.

New data generated by Domenico Accili and colleagues at Columbia University, New York, now indicates that Foxo1 cooperates with Notch to control muscle cell differentiation in vitro.

Overexpression of either a constitutively active form of Foxo1 or a constitutively active form of Notch was found to inhibit the in vitro differentiation of a mouse myoblast cell line.

Note that the preceding alludes to the involvement of FoxO proteins in regulation of metabolism. This comes about because they affect the insulin signaling pathway, and hence also glucose and lipid metabolism.

This function is what allows yet another well-known protein, mTOR, to play a role in "metabolic syndrome" – a group of disorders that includes insulin resistance, heart disease and high lipid levels. (mTOR is short for "mammalian target of rapamycin". It's a protein kinase that modifies other proteins by phosphorylation.) The same mechanism appears relevant also to the "Atkins diet" and the effects of calorie restriction.

Fly Genetics Reveal Key Workings Of Atkins Diet (8/8/06)

Using fruit flies bred with a newly created mutant form of the gene TOR (short for target of rapamycin), Oldham and his colleagues were able to determine how the TOR pathway interacted with other important regulators of insulin, glucose and lipid metabolism.

TOR is an ancient gene, found in nearly all animal and plant cells. The researchers discovered that their new mutant fly reduced TOR function, allowing them to observe what happens when TOR's influence is removed.

Reductions in TOR function lowered glucose and lipid levels in the body. They also blocked the function of another important insulin regulator, a factor called FOXO, which is known to be a critical mediator of insulin signals and therefore glucose and lipid metabolism.

As if all that weren't enough, FoxO proteins are also involved with cancer and stem cells:

Gene Knockouts Reveal FoxOs' Vital Functions In Cancer Defense, Health Of Stem Cells (1/25/07)

In an elegant, multiple-gene knockout experiment, a team of Boston scientists has discovered that a trio of molecules, called FoxOs, are fundamentally critical in preventing some cancers, maintaining blood vessel stability, and in keeping blood-forming stem cells healthy. ...

The researchers at Brigham and Women's found that mice engineered to lack genes for the FoxO1, FoxO3, and FoxO4 molecules had serious blood abnormalities. Without the FoxO gene-regulating molecules, the rodents' blood stem cells -- master cells that give birth to working blood cells while also renewing themselves -- divided too fast and "burned out." ...

In the companion paper, lead author Ji-Hye Paik, PhD, of Dana-Farber and colleagues from the DePinho lab report that the three FoxO molecules, known as transcription factors, normally function as tumor suppressors that override maverick cells threatening to grow too fast and form tumors. When FoxOs are eliminated, it may allow cancer to develop.

And even that's not the end of it. FoxO proteins are also involved in the increased levels of inflammation often associated with the aging process. (This phenomenon has been tagged with the neologism "inflammaging".) It has been hypothesized that inflammaging results from the effect of phosphorylated FoxO on another notorious transcription factor, NF-κB (which is heavily involved in inflammation). Some of the effects of calorie restriction may also be due to FOXO phosphorylation. Reference: Restricting inflammaging (11/12/07)

FoxO is also regulated (as is P53) by SIRT1 – so this is yet another relationship to calorie restriction. Reference: Unlocking the Secrets of Longevity Genes

Additional references (for the seriously interested):

An AMPK-FOXO pathway mediates longevity induced by a novel method of dietary restriction in C. elegans.

Ageing: When Less Is More

Tags: Fox proteins, transcription factor, longevity, aging, calorie restriction