The Warburg Effect, named for its discoverer, Otto Warburg, describes the altered metabolic state of cancer cells, specifically the phenomenon that cancer cells take up glucose at higher rates than normal cells, yet use less of the glucose for oxidative phosphorylation. This difference in metabolism appears to be a key factor contributing to tumorigenesis. Two related articles describe a relationship between the M2 isoform of pyruvate kinase (PKM2) and the metabolism of cancer cells. In the first paper, Christofk et al. reconstituted pyruvate kinase 1 (PKM1) or PKM2 in cancer cells in which the endogenous pyruvate kinase was knocked down with RNA interference. Although under high glucose and normoxia conditions the reconstituted cells had similar proliferation rates and glucose metabolism, under conditions more similar to a tumor environment with limited oxygen (0.5%) or in which oxidative phosphorylation was inhibited (with oligomycin), proliferation was greater in the PKM2-expressing cells. In a lung cancer cell line, expression of PKM2 resulted in higher lactate production, whereas expression of PKM1 resulted in higher oxygen consumption but lower lactate production. To verify that this difference in pyruvate kinase isoforms was important for tumorigenesis, the authors introduced the cells into nude mice and found that tumors arising from the PKM1 cells had delayed growth and that the tumors were consistently smaller than those arising from the PKM2 cells. Analysis of the tumors revealed that all of the tumors had the PKM2 isoform, suggesting that repression of the endogenous pyruvate kinase was lost in the tumors. When a mixture of PKM1 and PKM2 cells was injected, the tumors all had the PKM2 isoform. Thus, it appeared that tumorigenesis required the PKM2 isoform. In the second article, Christofk et al. identified PKM2 in a screen for proteins from cultured cells that bound to phosphotyrosine (pTyr)-peptides. The interaction between PKM2 and pTyr-peptides was confirmed in several assays and appeared unique to this isoform; the L, R, and M1 isoforms did not bind to pTyr-peptides. Fructose 1,6-bisphosphate (FBP) is an allosteric activator of pyruvate kinases; however, PKM2 appeared to bind to FBP much more tightly than did PKM1. The interaction of PKM2 with pTyr-peptides competed with FBP in vitro, and pTyr-peptides generated by exposure of cells to pervanadate to inhibit protein tyrosine phosphatases stimulated the release of labeled FBP from PKM2. Stimulation of cells with growth factors, overexpression of activated Src, or treatment of cells with pervanadate resulted in reduced pyruvate kinase activity. Analysis of the effect of pervanadate in cells in which only one isoform was present showed that PKM2 was required for the inhibition of pyruvate kinase activity. Cells in which PKM2 was knocked down exhibited decreased proliferation, and a mutant form of PKM2 (M2KE) that did not bind to pTyr-peptides failed to restore proliferation. M2KE-expressing cells showed less lactate production and more oxygen consumption than did cells expressing wild-type PKM2, thus confirming that phosphotyrosine binding to PKM2 plays a role in regulating metabolism. The authors propose that in response to growth factors, PKM2 may bind to phosphorylated lactate dehydrogenase or enolase, thus stimulating the release of FBP and inhibiting PKM2 activity, which shunts glucose from the oxidative phosphorylation pathway into an anabolic pathway.
H. R. Christofk, M. G. Vander Heiden, M. H. Harris, A. Ramanathan, R. E. Gerszten, R. Wei, M. D. Fleming, S. L. Schreiber, L. C. Cantley, The M2 splice isoform of pyruvate kinase is important for cancer metabolism and tumour growth. Nature 452, 230-233 (2008). [PubMed]
H. R. Christofk, M. G. Vander Heiden, N. Wu, J. M. Asara, L. C. Cantley, Pyruvate kinase M2 is a phosphotyrosine-binding protein. Nature 452, 181-186 (2008). [PubMed]