Cannabinoid receptor 2 selective agonists stimulate osteoclast formation in vitro but act as anabolic agents in vivo by stimulating bone formation

Abstract

Glucocorticoids are used as powerful drugs in the treatment of various diseases. One of the most severe side effects of this treatment is bone loss. Glucocorticoids are known to strongly inhibit and kill osteoblasts, whereas glucocorticoid-treated osteoclasts remain active and possibly show even increased activity. This imbalance causes a rapid and pronounced bone loss. However, in many cases there are no alternatives to glucocorticoid therapy. Thus, it would be optimal to find a treatment that minimizes the adverse effects of glucocorticoids while still keeping its beneficial effects. From investigations on other tissues it is known that high doses of glucocorticoids stimulate degradation of essential factors through the proteasome. We have therefore combined high dose glucocorticoid treatment with bortezomib, a proteasome inhibitor used for treating multiple myeloma, and investigated the effect of this combination on osteoblasts and osteoclasts in vitro. We found that culturing osteoblast-like cells (MC3T3) with therapeutic doses of prednisolone (1.6 microM) had strong toxic effects as expected, but that this was partly overcome by pre-treating the cells with bortezomib. Furthermore, the expression of key osteoblast genes was significantly up-regulated in the bortezomib pretreated osteoblasts as compared to prednisolone alone. This was found to be true for the vitamin D3 receptor, collagen type 1, osteocalcin, CD56 and osteopontin genes. In addition, we found that osteoclast survival was unaffected by prednisolone (0.1 and 1.6 microM) or bortezomib alone, whereas release of TRACP activity by the osteoclasts was strongly reduced by either drug alone. However, when the osteoclasts were pre-treated with bortezomib followed by prednisolone, survival was compromised and TRACP release was severely affected. Thus, bortezomib reduces the anti-osteoblastic activity of glucocorticoids and turns them into strong anti-osteoclastic drugs. The molecular mechanism explaining how proteasome inhibition counteracts the adverse effects of glucocorticoids on bone cells remains to be elucidated.