Tyr-94 Phosphorylation, PDP, and the Warburg Effect
ChemBio Spotlight Week 6
Proper bodily functioning requires energy, and cellular energy is almost always in the form of ATP. Glycolysis is followed by either oxidative phosphorylation (OXPHOS) or lactic acid fermentation to form ATP, with the latter process being less efficient than the prior, but with the ability to be performed in hypoxic conditions. The Warburg Effect refers to the observation that cancer cells undergo rapid glycolysis followed by lactic acid fermentation rather than the more efficient oxidative phosphorylation pathway – regardless of the availability of oxygen. OXPHOS starts with acetyl-CoA production: pyruvate dehydrogenase (PDH) converts pyruvate into acetyl-CoA, pyruvate dehydrogenase kinase (PDK) and phosphatase (PDP) phosphorylate and dephosphorylate PDH, respectively, and all three enzymes are needed in a balance of sorts. PDH is inactivated via phosphorylation, so up-regulated PDK or inactivated PDP may be possible causes of the Warburg Effect by only allowing lactic acid fermentation to take place. In prior studies by the authors, certain kinases and acetyltransferases were correlated with cancer cell growth.1 In this study, the authors studied the effects of phosphorylation of PDP1 at Tyr-94 and the mechanism of action.
The authors performed several experiments to get at different levels of findings – from big-picture effects down to mechanistic details. Initially, PDP1 was found to be inhibited via Tyr-94 by FGFR1 (Fibroblast growth factor receptor 1), but not at other Tyr residues. Next, structural analysis showed that Tyr-94 is ~20Å from the catalytic cleft, which is likely too far to directly affect it, but it is closer to the location of lipoic acid binding to PDP1 (Figure 1). PDP1 activity is mediated by lipoic acid binding to E2 (an acetyltransferase). Thus, it was hypothesized that Tyr-94 phosphorylation inhibits PDP binding to lipoic acid and, consequently, to E2. To test this hypothesis, Shan et al. found that incubation of recombinant rPDP1 with rFGR1 resulted in decreased 3H-labeled lipoic acid binding to rPDP1 WT but not Y94F mutant. To test for the prevalence of this Tyr-94 phopshorylation among several cancer cell types, a specific phosphorylated-PDP1 antibody was used to find it was prevalent among several leukemia, breast, and lung cancer cells. Lastly and with great impact, it was shown that expression of a PDP1 Y94F mutant in cancerous cells increased oxidative phosphorylation under normoxic conditions, and decreased ATP production under hypoxic conditions, which was correlated to the idea that Y94F mutant relies more on OXPHOS as compared to WT cells.
The Warburg Effect has remained a mysterious phenomenon for many years, and only recently has it been shown that cancer cells undergo fermentation despite normoxic conditions. The mechanism by which cancerous cells switch to this metabolic fate has been mostly unknown. This paper fills in one of the gaps of this mystery with the finding that Tyr-94 phosphorylation of PDP1 inhibits its ability to de-phosphorylate and re-activate PDH, which may be integral to the shift towards lactic acid fermentation and the nature of the Warburg Effect in cancers. With a mechanism developed, more studies may explore PDP1 activators as possible anticancer agents.
- Kroemer, G. and Pouyssegur, J. (2008) Tumor cell metabolism: cancer’s Achilles heel. Cancer Cell 13, 472-482