Strontium ranelate promotes osteoblastic differentiation and mineralization of murine bone marrow stromal cells: Involvement of prostaglandins

Strontium ranelate is a new anti-osteoporosis treatment. This study showed that strontium ranelate stimulated PGE(2) production and osteoblastic differentiation in murine marrow stromal cells, which was markedly reduced by inhibition of COX-2 activity or disruption of COX-2 gene expression. Hence, some anabolic effects of strontium ranelate may be mediated by the induction of COX-2 and PGE2 production. Introduction: Strontium ranelate is an orally active drug that reduces vertebral and hip fracture risk by increasing bone formation and reducing bone resorption. Strontium ranelate effects on bone formation are the result of increased osteoblastic differentiation and activity, but the mechanisms governing these effects are unknown. Based on previous work, we hypothesized that strontium ranelate increases cyclooxygenase (COX)-2 expression and that, consequently, the prostaglandin E-2 (PGE2) produced could mediate some effects of strontium ranelate on osteoblasts. Materials and Methods: Marrow stromal cells (MSCs) from COX-2 wildtype (WT) and knockout (KO) mice were cultured with and without low-dose dexamethasone. Osteoblastic differentiation was characterized by alkaline phosphatase (ALP) activity, real-time PCR for ALP and osteocalcin (OCN) mRNA expression, and alizarin red staining for mineralization. Medium PGE2 was measured by radioimmunoassay or enzyme immunoassay. Results and Conclusions: In MSCs from COX-2 WT mice, strontium ranelate significantly increased ALP activity, ALP and OCN mRNA expression, and mineralization after 14 or 21 days of culture. A short treatment at the beginning of the culture (0-7 days) with strontium ranelate was as effective as continuous treatment. Strontium ranelate (1 and 3 mM Sr+2) dose-dependently increased PGE2 production, with maximum PGE2 production occurring during the first week of culture. NS-398, a selective COX-2 inhibitor, blocked the strontium ranelate stimulation of PGE2 production and significantly inhibited the strontium ranelate stimulation of ALP activity. In MSCs from COX-2 KO mice, the strontium ranelate stimulation of ALP and OCN mRNA expression and mineralization were markedly reduced compared with COX-2 WT cultures. Similar effects of strontium ranelate on osteoblastic markers and on PGE2 production were seen when MSCs were cultured with or without low-dose dexamethasone (10 nM). We conclude that PGE2 produced by the strontium ranelate induction of COX-2 expression plays a role in strontium ranelate-induced osteoblastic differentiation in MSCs in vitro.