Metabolic targeting of platelets to combat thrombosis: dawn of a new paradigm?

Abstract

Current antithrombotic therapies used in clinical settings target either the coagulation pathways or platelet activation receptors (P2Y12 or GPIIb/IIIa), as well as the cyclooxygenase (COX) enzyme through aspirin. However, they are associated with bleeding risk and are not suitable for long-term use. Thus, novel strategies which provide broad protection against platelet activation with minimal bleeding risks are required. Regardless of the nature of agonist stimulation, platelet activation is an energy-intensive and ATP-driven process characterized by metabolic switching toward a high rate of aerobic glycolysis, relative to oxidative phosphorylation (OXPHOS). Consequently, there has been considerable interest in recent years in investigating whether targeting metabolic pathways in platelets, especially aerobic glycolysis and OXPHOS, can modulate their activation, thereby preventing thrombosis. This review briefly discusses the choices of metabolic substrates available to platelets that drive their metabolic flexibility. We have comprehensively elucidated the relevance of aerobic glycolysis in facilitating platelet activation and the underlying molecular mechanisms that trigger this switch from OXPHOS. We have provided a detailed account of the antiplatelet effects of targeting vital metabolic checkpoints such as pyruvate dehydrogenase kinases (PDKs) and pyruvate kinase M2 (PKM2) that preferentially drive the pyruvate flux to aerobic glycolysis. Furthermore, we discuss the role of fatty acids and glutamine oxidation in mitochondria and their subsequent role in driving OXPHOS and platelet activation. While the approach of targeting metabolic regulatory mechanisms in platelets to prevent their activation is still in a nascent stage, accumulating evidence highlights its beneficial effects as a potentially novel antithrombotic strategy.

Keywords: Platelets • Aerobic glycolysis • OXPHOS • Thrombosis.

Graphical Abstract

Figure 1

Schematic representation of the principal metabolic pathways fueling bioenergetic needs of activated platelets. Glucose undergoes glycolysis to form two pyruvate molecules along with ATP (2) and NADH (2). Pyruvate then enters the Krebs cycle to produce ATP (2), NADH (6), and FADH₂ (2). The reducing equivalents (NADH and FADH₂) are used by the electron transport chain to generate ATP through OXPHOS. Through aerobic glycolysis, pyruvate gets converted to lactate along with the regeneration of NAD⁺ (2) and ATP (2) synthesis. Platelets also metabolize fatty acids and glutamine via β-oxidation and glutaminolysis, respectively, to replenish the Krebs cycle and fuel OXPHOS. ADP, adenosine diphosphate; ATP, adenosine triphosphate; NAD, nicotinamide adenine dinucleotide; NADH, nicotinamide adenine dinucleotide (NAD) + hydrogen (H); FADH, flavin adenine dinucleotide; PDH, pyruvate dehydrogenase.

Figure 2

The Warburg effect in activated platelets. The Warburg effect or aerobic glycolysis involves the conversion of glucose/pyruvate to lactate in the presence of oxygen. It plays a highly significant role in platelet activation by ensuring a rapid production of ATP, regeneration of NAD⁺, regulation of platelet signaling, and biosynthesis of precursors such as nucleotides, lipids, and proteins. LDH, lactate dehydrogenase; NAD, nicotinamide adenine dinucleotide; NADH, nicotinamide adenine dinucleotide + hydrogen (H).

Figure 3

A schematic demonstrating the metabolic pathways and checkpoints that have been shown to modulate platelet activation. Activation of platelets is characterized by an increased rate of aerobic glycolysis relative to OXPHOS. Upon agonist stimulation, PDKs phosphorylate the E1α subunit of PDH to inhibit its activity, thereby diverting the pyruvate flux from the Krebs cycle to aerobic glycolysis, leading to lactate production. Treatment with DCA (small-molecule inhibitor of PDKs) or genetic deletion of PDK2/4 has been shown to inhibit platelet activation. The PKM2 catalyzes the conversion of phosphoenolpyruvate pyruvate to pyruvate. The dimeric PKM2 exhibits low enzymatic activity and provides a metabolic switch to regulate aerobic glycolysis. Treatment with ML265 or DASA (small-molecule activator that limits the formation of PKM2 dimers) or platelet-specific deletion of PKM2 has been reported to inhibit platelet functions. Additionally, the dimeric PKM2 form also facilitates the production of glycolytic intermediates such as glucose-6-P that enter the pentose phosphate pathway (PPP). The increased glucose-6-P flux generates NADPH, which is catalyzed by NOX to promote ROS generation, leading to platelet activation. Treatment with DASA or DHEA significantly reduced ROS generation and subsequent platelet functions. Treatment of platelets with the inhibitors of etomoxir (inhibitor of β-oxidation of fatty acids) has been demonstrated to attenuate platelet functions. Additionally, platelets treated with azaserine (glutaminase inhibitor) attenuated thrombin-stimulated OCR. Glucose-6-P, glucose-6-phosphate; G6PD, glucose-6-phosphate-dehyrogenase; DASA, diaryl sulfonamide; DHEA, dehydroepiandrosterone sulphate; DCA: dichloroacetate; PDK, pyruvate dehydrogenase kinase; PDH, pyruvate dehydrogenase; PKM2: pyruvate kinase M2; NOX, NADPH oxidase; ROS, reactive oxygen species; NADP, nicotinamide adenine dinucleotide phosphate; NADPH, nicotinamide adenine dinucleotide phosphate + hydrogen (H).