High extracellular Ca2+ enhances the adipocyte accumulation of bone marrow stromal cells through a decrease in cAMP.

Bone marrow stromal cells (BMSCs) are common progenitors of both adipocytes and osteoblasts. We recently suggested that increased [Ca2+]o caused by bone resorption might accelerate adipocyte accumulation in response to treatment with both insulin and dexamethasone. In this study, we investigated the mechanism by which high [Ca2+]o enhances adipocyte accumulation. We used primary mouse BMSCs and evaluated the levels of adipocyte accumulation by measuring Oil Red O staining. CaSR agonists (both Ca2+ and Sr2+) enhanced the accumulation of adipocytes among BMSCs in response to treatment with both insulin and dexamethasone. We showed that high [Ca2+]o decreases the concentration of cAMP using ELISA. Real-time RT-PCR revealed that increasing the intracellular concentration of cAMP (both chemical inducer (1µM forskolin and 200nM IBMX) and a cAMP analog (10µM pCPT-cAMP)) suppressed the expression of PPARγ and C/EBPα. In addition, forskolin, IBMX, and pCPT-cAMP inhibited the enhancement in adipocyte accumulation under high [Ca2+]o in BMSCs. However, this inhibited effect was not observed in BMSCs that were cultured in a basal concentration of [Ca2+]o. We next observed that the accumulation of adipocytes in the of bone marrow of middle-aged mice (25-40 weeks old) is higher than that of young mice (6 weeks old) based on micro CT. ELISA results revealed that the concentration of cAMP in the bone marrow mononuclear cells of middle-aged mice is lower than that of young mice. These data suggest that increased [Ca2+]o caused by bone resorption might accelerate adipocyte accumulation through CaSR following a decrease in cAMP.