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In this issue of Molecular Cell, Lv et al. (2011) identify a novel feedback mechanism in which increased glycolysis induces the acetylation and chaperone-mediated autophagic degradation of the glycolytic regulator PKM2, revealing a novel metabolic feedback loop that drives tumor growth.
Fifty years prior to the discovery of the first oncogene, Otto Warburg and colleagues demonstrated that many tumors were highly glycolytic compared to noncancerous tissue (Warburg et al., 1927). This elevation in glucose consumption is now appreciated to support the energetic requirements for proliferation and provide catabolic intermediates to fuel lipid and nucleic acid biosynthesis. While not mutually exclusive, these processes must be carefully balanced to allow the production of both ATP and biosynthetic precursors in all growing cells, including cancer cells. Therefore, investigating this process may lead to the identification of new targets for cancer treatment.
A major finding in recent years that has provided insight into the cancer metabolic program has been the identification of the M2 isoform of pyruvate kinase (PKM2) as a driver of aerobic glycolysis (Christofk et al., 2008). Pyruvate kinase (PK) catalyzes the last stage of glycolysis and transfers phosphate from phosphoenol pyruvate (PEP) to ADP to generate ATP. The resultant pyruvate is then converted either to lactate or acetyl-CoA. In common with other highly proliferative cells, the majority of tumors express PK isozyme M2 rather than the M1 form. Despite the glycolytic character of cancer cells, however, PKM2 is enzymatically slower than PKM1. Thus, reduced PK activity appears to favor cell growth, possibly by allowing glycolytic intermediates to be siphoned off for biosynthesis, or potentially enhancing an alternate glycolytic pathway that bypasses PK altogether (Vander Heiden et al., 2010). This alternate pathway converts PEP to pyruvate without phosphorylating ADP, thus providing biosynthetic intermediates while avoiding potential feedback inhibition of glycolysis from the accumulation of ATP.
Because PK activity acts as a central node to determine if glycolysis favors ATP generation or biosynthesis, understanding how this enzyme is regulated is critical. PKM2 can alternate between a dimeric form that exhibits low catalytic activity, and a highly active tetrameric form that is driven by allosteric binding of fructose-1,6-bisphosphate, an upstream glycolytic intermediate (Mazurek et al., 2005). The report by Lv and colleagues in this issue (Lv et al., 2011) identifies a potentially important new regulatory mechanism to reduce PK activity. They provide evidence that a conserved lysine residue on PKM2 (K305) is acetylated when glucose is abundant to trigger a three-pronged suppression of PKM2. Acetylation leads to reduced PKM2 enzymatic activity, decreased PEP affinity, and, interestingly, PKM2 degradation through chaperone-mediated autophagy (CMA).
Decreased overall PK activity is proposed to increase the availability of glycolytic intermediates for cell growth. To address the role of PKM2 acetylation in biosynthesis, the authors knock down expression of endogenous PKM2 and replace it with a K305Q acetyl-mimetic of PKM2. This resulted in the accumulation of glycolytic intermediates. Replacement with a nonacetylizable PKM2, however, reduced intermediate availability. Glucose-dependent acetylation of PKM2 K305 therefore represents a new mechanism for the regulation of glycolytic flux and for determining whether glucose is used for ATP generation or biosynthesis.
Posttranslational modifications such as those described by Lv et al. (2011) are becoming increasingly apparent as a mechanism to control cell function in direct response to levels of available nutrients. Protein acetylation plays an established role in the regulation of gene expression, and mass spectrometry profiling studies have demonstrated that acetylation is also highly prevalent on enzymes that regulate energy metabolism (Wang et al., 2010; Zhao et al., 2010). Importantly, acetylation profiles change in response to glucose availability, suggesting that protein acetylation may represent a mechanism to coordinate global responses to nutrient levels. Acetyl-CoA levels have now been found to control acetylation of histones as well as a variety of metabolic enzymes (Wang et al., 2010; Wellen et al., 2009; Zhao et al., 2010). In the case of PKM2, cancer cells will be stimulated to grow if glucose is abundant, as excess glucose will increase acetyl-CoA and acetylated PKM2 levels, leading to PKM2 degradation. This will further slow glycolysis and promote the alternate glycolytic pathway to maximize biosynthesis (Figure 1).
An intriguing further connection between metabolism and PKM2 lies in the role of autophagy in PKM2 degradation. During CMA, the chaperone Hsc70 binds target proteins with an exposed hydrophobic motif, directing them to lysosomes for breakdown. Given that CMA has previously been associated with the breakdown of glycolytic enzymes during prolonged nutrient deprivation (Aniento et al., 1993), this may be the first indication that CMA plays a broader role in regulating ATP production. The use of targeted CMA to limit energy production and maximize biosynthesis is also in striking contrast to the previously described role of macroautophagy during nutrient starvation, where the broad-spectrum breakdown of cell components fuels energy generation rather than biosynthesis (Mizushima, 2005).
A key step now will be to extend these studies further in animal models to determine the role of PKM2 acetylation in tumor progression. In the current study, the authors generated a lung carcinoma cell line in which PKM2 is knocked down and replaced with either PKM2 or the K305Q acetyl mimetic of PKM2, which consistently led to a larger tumor burden in xenografts. However, there are a number of caveats to this result. First, PKM2 expression was not restored entirely to endogenous levels, which may affect overall PK activity and thus impact the ATP versus biosynthesis balance. Second, the inverse experiment, generating tumors containing nonacetylizable PKM2, is predicted to repress tumor growth, and this model as well as spontaneous cancer models of different types should also be analyzed to establish the regulation of metabolism and growth by PKM2 acetylation.
Together, the picture is now emerging that PKM2 is required to fine-tune glycolysis and promote biosynthesis when nutrients are available, and to respond to periods of nutrient stress or when growth signals are absent to promote ATP generation. This suggests that PKM2 may be a target to disrupt tumor metabolism, but also that targeting of PKM2 may be problematic—inhibiting the enzyme may actually worsen a cancer by promoting growth. Lv and colleagues’ data suggest an alternative treatment strategy: manipulation of acetylation to indirectly alter PKM2 levels. In this way, disruption of the acetylation event could increase PKM2 levels to push cancer cells toward the ATP generating mode rather than the biosynthetic mode. This, however, may also be a double-edged sword—blocking biosynthesis may suppress proliferation, but the impact of elevating cellular energy levels on the susceptibility of tumors to existing chemotherapeutic approaches is unclear given other findings showing links between bioenergetics and apoptosis. Nevertheless, the novel feedback pathway described by Lv et al. (2011) opens new avenues for manipulating cancer metabolism and potential ways to target this fundamental process.