Targeting Metabolic Enzyme Pyruvate Kinase M2: A Novel Strategy to Inhibit Platelet Function and Arterial Thrombosis

Background: The cellular responses initiated upon platelet activation are energy consuming. Activated platelets, in comparison to their resting state, exhibit a high level of aerobic glycolysis (conversion of glucose to lactate in the presence of oxygen) relative to oxidative phosphorylation (OXPHOS), suggesting that metabolic plasticity exists in platelets. Although aerobic glycolysis yields less total ATP when compared to OXPHOS, the rate of ATP generation is faster in aerobic glycolysis compared to OXPHOS, which we hypothesize is well suited for high-energy requirement during platelet activation. The glycolytic enzyme pyruvate kinases (PKs) catalyzes the final step of glycolysis and contributes to net ATP production. Four PK isoforms (L, R, M1 and M2) exist in mammals: L and R isoforms are expressed in the liver and red blood cells; the M1 isoform is expressed in most adult tissues that have high catabolic demands including muscle and brain; M2 is expressed in cells including activated platelets and leukocytes. Unlike other isoforms of PK that function only as tetramers, PKM2 can exist in either a tetrameric state or a dimeric state. PKM2 is allosterically regulated by the upstream metabolite fructose-1, 6 biphosphate. While PKM1 and tetrameric PKM2 favor ATP production from OXPHOS through the TCA cycle, dimeric PKM2 drives aerobic glycolysis. The glycolytic and non-glycolytic functions of PKM2 in platelets have not investigated yet.

Objective: We tested an innovative concept that whether targeting metabolic enzyme PKM2 will inhibit platelet function and arterial thrombosis.

Methods: Using a specific inhibitor of PKM2 (that prevents PKM2 dimerization and stabilizes tetramers) and a range of standardized platelet in vitro assays, we determined the mechanistic role of PKM2 in modulating platelet function in human and mice. To provide definitive evidence, we generated a megakaryocyte or platelet-specific PKM2^-/- mouse (PKM2^fl/flPF4Cre). Susceptibility to thrombosis was evaluated in vitro (microfluidics flow chamber) and in vivo (FeCl₃-induced carotid and laser-injury induced mesenteric artery thrombosis models) by utilizing intravital microscopy. Susceptibility to hemostasis was evaluated in tail bleeding assay.

Results: Human and mouse platelets pretreated with PKM2 inhibitor significantly decreased platelet aggregation to sub-optimal doses of collagen, convulxin, thrombin, and ADP. Consistent with this, inhibiting PKM2 dimerization reduced αIIbβ3 activation, alpha and dense granule secretion, clot retraction that was concomitant with decreased glucose uptake. Furthermore, treatment with PKM2 inhibitor reduced Akt and GSK3β phosphorylation, that are predominantly involved in PI3K/Akt signaling, suggesting a non-glycolytic role of the PKM2 in regulating platelet function. In microfluidics flow chamber assay, human and whole mouse blood pretreated with PKM2 inhibitor formed small thrombi when perfused over collagen for 5 minutes at an arterial shear rate of 1500s^-1 (P<0.05 vs. vehicle). In agreement with PKM2 inhibitor studies, platelets from PKM2^fl/flPF4Cre mice exhibited decreased agonist-induced platelet aggregation, which was in agreement with decreased alpha and dense granule secretion, αIIbβ3 activation, clot retraction, lactate production, and Akt and GSK3β phosphorylation (P<0.05 vs. PKM2^fl/fl littermate controls). Wild-type mice-treated with PKM2 inhibitor and/or PKM2^fl/flPF4Cre were less susceptible to thrombosis in the FeCl₃-induced carotid and laser injury-induced mesenteric artery thrombosis models. Lack of effect on tail bleeding time suggested normal hemostasis in PKM2^fl/flPF4Cre mice and PKM2 inhibitor-treated wild-type mice. No sex-based differences were observed. Currently, we are performing platelet metabolomics to determine the effect of targeting PKM2 on metabolic pathways.

Conclusions: Our results suggest that manipulating metabolic plasticity by targeting dimeric PKM2 may be explored as a novel strategy to inhibit platelet function and arterial thrombosis.