Anti-apoptotic molecule Bcl-2 regulates the differentiation, activation, and survival of both osteoblasts and osteoclasts

The anti-apoptotic molecule Bcl-2 inhibits apoptosis by preventing cytochrome c release from mitochondria. Although several studies have indicated the importance of Bcl-2 in maintaining skeletal integrity, the detailed cellular and molecular mechanisms remain elusive. Bcl-2^-/- mice are growth-retarded and exhibit increased bone volume of the primary spongiosa, mainly due to the decreased number and dysfunction of osteoclasts. Osteoblast function is also impaired in Bcl-2^-/- mice. Ex vivo studies on osteoblasts and osteoclasts showed that Bcl-2 promoted the differentiation, activation, and survival of both cell types. Because Bcl-2^-/- mice die before 6 weeks of age due to renal failure and cannot be compared with adult wild type mice, we generated Bcl-2^-/-Bim^{+/_ m}ice, in which a single Bim allele was inactivated, and compared them with their Bcl-2^+/-Bim^+/- littermates. Loss of a single Bim allele restored normal osteoclast function in Bcl-2^-/- mice but did not restore the impaired function of osteoblasts, and the mice exhibited osteopenia. These data demonstrate that Bcl-2 promotes the differentiation, activity, and survival of both osteoblasts and osteoclasts. The balance between Bcl-2 and Bim regulates osteoclast apoptosis and function, whereas other pro-apoptotic members are important for osteoblasts.