Inorganic Pyrophosphate Promotes Osteoclastogenic Commitment and Survival of Bone Marrow Derived Monocytes mediated by Egr-1 up-regulation and MITF phosphorylation

Several reports emphasized the importance of inorganic pyrophosphate (PPi) in hindering osteoblast differentiation and bone matrix mineralization. Its ubiquitous presence is thought to prevent “soft” tissue calcification, whereas its degradation to Pi in bones and teeth by alkaline phosphatase (ALP) may facilitate crystal growth. While the inhibiting role of PPi on osteoblast differentiation and function is largely understood, less is known about its effects on osteoclast determination and activity. In this study, we investigated the role of PPi in bone resorption using calverial organ cultures ex vivo . We present an evidence that PPi stimulated calvarial bone resorption marked by calcium (Ca2+) release in the condition media (CM). We then examined PPi effects on osteoclast differentiation using mouse bone marrow-derived monocytes (BMMs). Our results revealed that PPi enhanced osteoclast differentiation ex vivo , marked by increased number and size of TRAP-stained mature osteoclasts. Moreover, PPi stimulated osteoclastogenesis in BMMs co-cultured with osteoblasts. These data supported the increased osteoclast activity in bone resorption using functional osteo-assays. The finding of PU.1-Egr-1 dependent up-regulation of c-FMS and RANK receptors in BMMs supported the enhanced pre-osteoclast commitment and differentiation. Moreover, osteoclast survival was enhanced by activation of MITF-BCL-2 pathway that was mediated by MAPK-ERK1/2 signaling. Last, our data showed that PPi up-regulated ANK; PPi transporter, during osteoclast differentiation through ERK1/2 phosphorylation whereas mutation of ANK inhibited osteoclastogenesis. Collectively, our data suggest that PPi promotes osteoclast differentiation, survival, and function through PU.1 up-regulation and MITF phosphorylation whereas ANK loss-of-function inhibited osteoclastogenesis.