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how to expliut/reverse the Warberg effect--changing glucose metabolism 2starve cancer
CSHL study identifies potential way to reverse cancer cell metabolism and tumor growth
[Cold Spring Harbor Laboratory]
Cold Spring Harbor, N.Y. - A team of scientists led by Professor Adrian Krainer, Ph.D., of Cold Spring Harbor Laboratory has discovered molecular factors in cancer cells that boost the production of an enzyme that helps alter the cells' glucose metabolism. The altered metabolic state, called the Warburg effect, promotes extremely rapid cell proliferation and tumor growth. Adrian Krainer, Ph.D.
Discovered eighty years ago by Nobel Prize-winning scientist Otto Warburg, this altered metabolism in cancer cells is most critically mediated by a protein called PK-M2 (pyruvate kinase M2). This is one of two versions - or isoforms - of the enzyme pyruvate kinase, whose other isoform, PK-M1, is harmless.
In a study published online ahead of print in the Proceedings of the National Academy of Sciences, Krainer and colleagues report their discovery of three factors that contribute to high levels of PK-M2 in cancer cells, in part by suppressing production of PK-M1.
"These findings suggest a new way in which cancer's altered glucose metabolism might be targeted for therapeutic benefit," explains Krainer. "Drugs that inhibit these factors and reverse the Warburg effect might work as anti-cancer agents." The study was performed in collaboration with Professor Lewis Cantley, Ph.D., and his colleagues at Harvard Medical School and The Broad Institute, in Cambridge, Mass.
Cancer cells consume glucose at a much higher rate than normal cells, but use very little glucose to produce energy, spending the rest instead on cell-building material. They also produce huge amounts of a byproduct called lactate. PK-M2, which facilitates this alternate metabolic lifestyle in cancer cells, was recently shown by the Cantley laboratory to be critical for tumor formation and growth.
This isoform and its non-cancerous counterpart PK-M1, which is found only in normal cells, both arise from the same gene, PK-M, via alternative splicing, a process that allows a single gene to produce multiple proteins. The initial RNA copy of a gene's DNA includes unnecessary pieces called introns that are first spliced out. The remaining bits, called exons, can be stitched back together in different ways by the cell's splicing machinery to form different RNAs that can then give rise to different proteins.
In the case of the PK-M gene, its RNA undergoes alternative splicing in a mutually exclusive fashion, giving rise to either the M1 or the M2 isoform. Krainer, an expert on alternative splicing, has been focused on understanding how the benign M1 isoform is switched off and the dangerous M2 isoform switched on in cancer cells. His team began by tracking down splicing factors and mechanisms that cause cancer cells to exclusively produce the M2 isoform.
By examining the levels of various splicing factors in numerous types of cancer cells, the scientists have narrowed the list of suspects to three proteins so far. All three are present at high levels in cancer cells, and repress the splicing of the harmless M1 isoform. This, by default, causes cells to produce only the M2 isoform.
The scientists could largely reverse this situation - restoring M1 production while decreasing M2 levels and lactate production - by forcing a reduction in the levels of the three splicing repressors. Whether this switch back to normal metabolism also impedes cancer cells' rapid growth remains to be tested.
OPEN ACCESS: The alternative splicing repressors hnRNP A1/A2 and PTB influence pyruvate kinase isoform expression and cell metabolism
[Proceedings of the National Academy of Sciences]
Cancer cells preferentially metabolize glucose by aerobic glycolysis, characterized by increased lactate production. This distinctive metabolism involves expression of the embryonic M2 isozyme of pyruvate kinase, in contrast to the M1 isozyme normally expressed in differentiated cells, and it confers a proliferative advantage to tumor cells. The M1 and M2 pyruvate-kinase isozymes are expressed from a single gene through alternative splicing of a pair of mutually exclusive exons. We measured the expression of M1 and M2 mRNA and protein isoforms in mouse tissues, tumor cell lines, and during terminal differentiation of muscle cells, and show that alternative splicing regulation is sufficient to account for the levels of expressed protein isoforms. We further show that the M1-specific exon is actively repressed in cancer-cell lines—although some M1 mRNA is expressed in cell lines derived from brain tumors—and demonstrate that the related splicing repressors hnRNP A1 and A2, as well as the polypyrimidine-tract-binding protein PTB, contribute to this control. Downregulation of these splicing repressors in cancer-cell lines using shRNAs rescues M1 isoform expression and decreases the extent of lactate production. These findings extend the links between alternative splicing and cancer, and begin to define some of the factors responsible for the switch to aerobic glycolysis.
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