Effect of Osteoblast‐Targeted Expression of Bcl‐2 in Bone: Differential Response in Male and Female Mice

Transgenic mice (Col2.3Bcl‐2) with osteoblast‐targeted human Bcl‐2 expression were established. Phenotypically, these mice were smaller than their wildtype littermates and showed differential effects of the transgene on bone parameters and osteoblast activity dependent on sex. The net effect was an abrogation of sex differences normally observed in wildtype mice and an inhibition of bone loss with age. Ex vivo osteoblast cultures showed that the transgene had no effect on osteoblast proliferation, but decreased bone formation. Estrogen was shown to stimulate endogenous Bcl‐2 message levels. These studies suggest a link between Bcl‐2 and sex regulation of bone development and age‐related bone loss. Introduction: Whereas Bcl‐2 has been shown to be an important regulator of apoptosis in development, differentiation, and disease, its role in bone homeostasis and development is not well understood. We have previously showed that the induction of glucocorticoid‐induced apoptosis occurred through a dose‐dependent decrease in Bcl‐2. Estrogen prevented glucocorticoid‐induced osteoblast apoptosis in vivo and in vitro by preventing the decrease in Bcl‐2 in osteoblasts. Therefore, Bcl‐2 may be an important regulator of bone growth through mechanisms that control osteoblast longevity and function. Materials and Methods: Col2.3Bcl‐2 mice were developed carrying a 2.3‐kb region of the type I collagen promoter driving 1.8 kb of human Bcl‐2 (hBcl‐2). Tissue specific expression of hBcl‐2 in immunoassays validated the transgenic animal model. Histomorphometry and DXA were performed. Proliferation, mineralization, and glucocorticoid‐induced apoptosis were examined in ex vivo cultures of osteoblasts. The effect of estrogen on mouse Bcl‐2 in ex vivo osteoblast cultures was assayed by RT‐PCR and Q‐PCR. Results and Conclusions: Two Col2.3Bcl‐2 (tg/+) founder lines were established and appeared normal except that they were smaller than their nontransgenic wildtype (+/+) littermates at 1, 2, and 6 months of age, with the greatest differences at 2 months. Immunohistochemistry showed hBcl‐2 in osteoblasts at the growth plate and cortical surfaces. Nontransgenic littermates were negative. Western blots revealed hBcl‐2 only in type I collagen‐expressing tissues. Histomorphometry of 2‐month‐old mice showed a significant decrease in tg/+ calvaria width with no significant differences in femoral trabecular area or cortical width compared with +/+. However, tg/+ males had significantly mor