The antiapoptotic protein Bcl-xL negatively regulates the bone-resorbing activity of osteoclasts in mice
Mitsuyasu Iwasawa, … , Kozo Nakamura, Sakae Tanaka
Mitsuyasu Iwasawa, … , Kozo Nakamura, Sakae Tanaka
Published September 14, 2009
Citation Information: J Clin Invest. 2009;119(10):3149-3159. https://doi.org/10.1172/JCI39819.
View: Text | PDF
Research Article Bone biology

The antiapoptotic protein Bcl-xL negatively regulates the bone-resorbing activity of osteoclasts in mice

Abstract

The B cell lymphoma 2 (Bcl-2) family member Bcl-xL has a well-characterized antiapoptotic function in lymphoid cells. However, its functions in other cells — including osteoclasts, which are of hematopoietic origin — and other cellular processes remain unknown. Here we report an unexpected function of Bcl-xL in attenuating the bone-resorbing activity of osteoclasts in mice. To investigate the role of Bcl-xL in osteoclasts, we generated mice with osteoclast-specific conditional deletion of Bcl-x (referred to herein as Bcl-x cKO mice) by mating Bcl-xfl/fl mice with mice in which the gene encoding the Cre recombinase has been knocked into the cathepsin K locus and specifically expressed in mature osteoclasts. Although the Bcl-x cKO mice grew normally with no apparent morphological abnormalities, they developed substantial osteopenia at 1 year of age, which was caused by increased bone resorption. Bcl-x deficiency increased the bone-resorbing activity of osteoclasts despite their high susceptibility to apoptosis, whereas Bcl-xL overexpression produced the opposite effect. In addition, Bcl-x cKO osteoclasts displayed increased c-Src activity, which was linked to increased levels of vitronectin and fibronectin expression. These results suggest that Bcl-xL attenuates osteoclastic bone-resorbing activity through the decreased production of ECM proteins, such as vitronectin and fibronectin, and thus provide evidence for what we believe to be a novel cellular function of Bcl-xL.

Authors

Mitsuyasu Iwasawa, Tsuyoshi Miyazaki, Yuichi Nagase, Toru Akiyama, Yuho Kadono, Masaki Nakamura, Yasushi Oshima, Tetsuro Yasui, Takumi Matsumoto, Takashi Nakamura, Shigeaki Kato, Lothar Hennighausen, Kozo Nakamura, Sakae Tanaka

×

Figure 7

Bcl-xL reduced c-Src activity in osteoclasts by suppressing the expression of ECM proteins.

Options: View larger image (or click on image) Download as PowerPoint
Bcl-xL reduced c-Src activity in osteoclasts by suppressing the expressi...
(A) Western blotting with anti–phospho–c-Src antibody and anti–phospho-Akt antibody. Osteoclasts generated from Bcl-xfl/fl mouse bone marrow cells were infected with AxGFP, AxCre, or AxBcl-xL. After 24 h of infection, the lysates were subjected to Western blotting. c-Src was activated in Bcl-x cKO osteoclasts, while no difference in Akt activation was observed. The amount of total c-Src or Akt did not appear to differ. (B) mRNA expression of osteopontin, vitronectin, and fibronectin by real-time RT-PCR. Vitronectin and fibronectin expression increased in Bcl-x cKO osteoclasts and decreased in Bcl-xL–overexpressing osteoclasts. Results are mean ± SD of 6 samples. *P < 0.01, **P < 0.05 versus AxGFP-infected cells. (C) Effect of ECM protein coating on bone-resorbing activity of AxBcl-xL–infected osteoclasts. When AxBcl-xL–infected osteoclasts were cultured on vitronectin- or fibronectin-coated dentine slices, the negative effect of Bcl-xL overexpression on bone resorption was partially reversed, and vitronectin-coated dentine slices showed a significant increase in pit area. Results are mean ± SD of 4 cultures. **P < 0.05 versus AxBcl-xL–infected osteoclasts cultured on uncoated control dentine slices. (D) Western blotting with anti–phospho–c-Src antibody and anti-Src antibody. c-Src activity suppressed by AxBcl-xL expression in osteoclasts was partially restored by plating the cells onto vitronectin- or fibronectin-coated dishes. The total amount of c-Src did not appear to differ.