Down-regulation of c-Fos is a key event for the inhibition of osteoclastogenesis in the presence of GM-CSF.

The c-Kit⁺c-Fms⁺RANK⁻ cells were cultured in the presence of M-CSF (100 ng/mL) alone; M-CSF and sRANKL (25 ng/mL); and M-CSF, sRANKL, and GM-CSF (1 ng/mL) for 6 days. The effect of retroviral infection with full-length c-Fos (c-Fos–full), truncated c-Fos (c-Fos–δ) or mock-infected cells was examined in cultures containing M-CSF and sRANKL and GM-CSF. Expression of c-Fos, Fra-1, and G3PDH was examined by RT-PCR on day 6 (A). NC indicates no template control. Expression of Fra-1 was induced by c-Fos–full but not by c-Fos–δ. Retroviral infection could be detected by GFP expression visualized with a fluorescence microscope (B). Cell spreading and disappearance of cell clusters were observed following infection with c-Fos–full or c-Fos–δ but not in mock-infected cells (arrow indicates cell cluster). Significant increases in TRAP-positive cells were detected in the c-Fos–full infection (B,C). Bar = 25 μm.