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Published on: June 8, 2014
Role of Bcl2 in osteoclastogenesis and PTH anabolic actions in bone
Junro Yamashita1, Nabanita S Datta, Yong-Hee P Chun
1Department of Biologic and Materials Sciences, University of Michigan School of Dentistry, Ann Arbor, Michigan, USA.
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 muCT. 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 microg/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.
Insights
Bcl2 (B-cell leukemia/lymphoma 2) is crucial for osteoclast survival, not osteoblasts. Its absence increases bone mass by reducing osteoclast numbers and is not required for parathyroid hormone (PTH) anabolic actions.
Area of Science:
- Skeletal Biology
- Cell Death Regulation
- Bone Metabolism
Background:
- Bcl2 (B-cell leukemia/lymphoma 2) is a proto-oncogene known for suppressing apoptosis.
- Its role in skeletal homeostasis and osteoblast function was previously uncharacterized.
- Hypothesized anti-apoptotic role in osteoblasts during parathyroid hormone (PTH) action lacked in vivo validation.
Purpose of the Study:
- To investigate the in vivo role of Bcl2 in bone metabolism.
- To determine the impact of Bcl2 ablation on osteoblast and osteoclast function.
- To clarify the influence of Bcl2 on the anabolic actions of PTH.
Main Methods:
- Analysis of bone phenotype in Bcl2 homozygous mutant (Bcl2(-/-)) mice using histomorphometry and micro-CT.
- Isolation and evaluation of calvarial osteoblasts for cellular activity.
- Induction and analysis of osteoclastogenesis from bone marrow cells.
- Administration of PTH(1-34) to assess its anabolic effects in Bcl2(-/-) mice.
- Western blotting and real-time PCR to assess Bcl2 expression.
Main Results:
- Bcl2(-/-) mice exhibited reduced osteoclast numbers and increased bone mass.
- Bcl2(-/-) osteoclasts were larger and short-lived, indicating a pro-apoptotic role for Bcl2 in these cells.
- Osteoblast proliferation, differentiation, and mineralization remained normal in Bcl2(-/-) mice.
- PTH administration similarly increased bone mass in both wildtype and Bcl2(-/-) mice.
- PTH did not induce Bcl2 expression in calvarial osteoblasts.
Conclusions:
- Bcl2 is essential for osteoclast survival and function, but not for osteoblasts.
- Reduced osteoclast activity in Bcl2(-/-) mice contributes to their increased bone mass.
- Bcl2 is dispensable for mediating the anabolic effects of PTH during bone growth.
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