Convergent signaling by acidosis and receptor activator of NF-κB ligand (RANKL) on the calcium/calcineurin/NFAT pathway in osteoclasts

Systemic acidosis has detrimental effects on the skeleton, and local acidosis coincides with bone destruction in inflammatory and metastatic diseases. Acidification dramatically enhances osteoclastic resorption, although the underlying mechanism has remained elusive. We investigated the effect of acidosis on the osteoclastogenic transcription factor NFATc1, which upon dephosphorylation translocates from the cytoplasm to nuclei. Lowering extracellular pH dramatically increased accumulation of NFATc1 in nuclei of rat and rabbit osteoclasts to levels comparable with those induced by the proresorptive cytokine receptor activator of NF-κB ligand (RANKL). Activation of NFATc1 by RANKL was mediated by means of prolonged stimulation of the Ca²⁺/calmodulin-dependent protein phosphatase, calcineurin. In contrast, NFATc1 activation by acidosis involved stimulation of calcineurin and suppression of NFATc1 inactivation. Acidosis, like RANKL, induced transient elevation of cytosolic free Ca²⁺ concentration ([Ca ²⁺]_i), which persisted in Ca²⁺-free media and was abolished by inhibition of phospholipase C or depletion of intracellular Ca²⁺ stores. Real-time-PCR of osteoclast-like cells generated from RAW 264.7 cells revealed high levels of expression of ovarian cancer G protein-coupled receptor 1, which links extracellular acidification to elevation of [Ca²⁺]_i. In addition, the calcineurin inhibitor cyclosporin A suppressed the stimulatory effect of acidification on resorption, implicating NFAT in mediating the actions of acidosis on osteoclast activity. In summary, acidification and RANKL induce signals in osteoclasts that converge on the Ca²⁺/calcineurin/NFAT pathway. Acidosis acts directly on osteoclasts to activate NFATc1 and stimulate resorption.