Mitochondrial pyruvate carrier blockade results in decreased osteoclastogenesis and bone resorption via regulating mitochondrial energy production

It's widely accepted that increasing mitochondrial respiration plays a pivotal role during osteoclastogenesis. Mitochondrial pyruvate carrier (MPC) is the key transporter that links glycolysis to mitochondrial respiration but little is known about its role during osteoclastogenesis. Our goal was to determine the effects of its blockade on osteoclastogenesis and bone resorption in vivo and in vitro. To address this issue, we performed gene knockdown or pharmacologically inhibited MPC in primary bone marrow-derived monocytes (BMMs) or in an ovariectomized mouse model. We also studied the metabolic changes in RANKL-induced differentiating BMMs with MPC blockade and performed rescue experiments. We found that MPC blockade resulted in decreased osteoclastogenesis both in vivo and in vitro and inhibiting MPC significantly alleviated ovariectomy-induced trabecular bone loss. Further investigations showed that MPC blockade significantly reversed the metabolic profile related to RANK activation, including decreased intermediates involved in citric acid cycle and glutamine metabolism. Moreover, metabolic flux analysis verified that MPC blockade decreased pyruvate flux into TCA cycle with no significant effect on glycolysis. Besides, MPC blockade resulted in suppressed mitochondrial biogenesis in addition to oxidative phosphorylation. Rescue experiments revealed that inhibiting pyruvate dehydrogenase kinase (PDK) via sodium dichloroacetate (DCA), but not targeting glutamine metabolism, could reverse the effects of MPC blockade on osteoclastogenesis. These implied that the effects of MPC blockade were mediated by reduced pyruvate fuel into citric acid cycle in multiple aspects. Taken together, our data demonstrated the inhibitory effects of MPC blockade on osteoclastogenesis, which was mediated by decreased mitochondrial energy production.