Role of Bcl2 in osteoclastogenesis and PTH anabolic actions in bone

Introduction: B-cell leukemia/lymphoma 2 (Bcl2) is a proto-oncogene best known for its ability to suppress cell death. However, the role of Bcl2 in the skeletal system is unknown. Bcl2, has been hypothesized to play an important anti-apoptotic role in osteoblasts during anabolic actions of PTH. Although rational, this has not been validated in vivo; hence, the impact of Bcl2 in bone remains unknown. Materials and Methods: The bone phenotype of Bcl2 homozygous mutant (Bcl2^-/-) mice was analyzed with histomorphometry and μCT. Calvarial osteoblasts were isolated and evaluated for their cellular activity. Osteoclastogenesis was induced from bone marrow cells using RANKL and macrophage-colony stimulating factor (M-CSF), and their differentiation was analyzed. PTH(1-34) (50 μg/kg) or vehicle was administered daily to Bcl2 ^+/+ and Bcl2^-/- mice (4 days old) for 9 days to clarify the influence of Bcl2 ablation on PTH anabolic actions. Western blotting and real-time PCR were performed to detect Bcl2 expression in calvarial osteoblasts in response to PTH ex vivo. Results: There were reduced numbers of osteoclasts in Bcl2^-/- mice, with a resultant increase in bone mass. Bcl2 ^-/- bone marrow-derived osteoclasts ex vivo were significantly larger in size and short-lived compared with wildtype, suggesting a pro-apoptotic nature of Bcl2^-/- osteoclasts. In contrast, osteoblasts were entirely normal in their proliferation, differentiation, and mineralization. Intermittent administration of PTH increased bone mass similarly in Bcl2 ^+/+ and Bcl2^-/- mice. Finally, Western blotting and real-time PCR showed that Bcl2 levels were not induced in response to PTH in calvarial osteoblasts. Conclusions: Bcl2 is critical in osteoclasts but not osteoblasts. Osteoclast suppression is at least in part responsible for increased bone mass of Bcl2^-/- mice, and Bcl2 is dispensable in PTH anabolic actions during bone growth.