Kim et al. uncovered a novel role for STAT1 (signal transducer and activator of transcription 1) as an inhibitor of differentiation of osteoblasts, the cells that deposit bone. STAT1 was previously known to inhibit differentiation of osteoclasts, the cells that degrade and resorb bone, through an interferon-dependent pathway. However, examination of the skeletons of STAT1–/– mice showed that despite having an increased number of osteoclasts, the mice actually exhibited increased bone density. The mutant mice had an increased bone-formation rate, and cultured precursor cells from the mutant mice exhibited enhanced osteoblast differentiation compared to the wild type. The skeletal phenotypes were different from those observed for mice with deficiencies in the interferon pathway (IFNAR–/– or IRF9–/– mice) that inhibits osteoclast differentiation, suggesting that STAT1 acted through a different pathway to inhibit osteoblast differentiation. Indeed, in culture, osteoblast differentiation was not affected by treatment with cytokines known to stimulate the interferon pathway. Instead, the STAT-deficient cells exhibited enhanced differentiation of osteoblasts in response to bone morphogenetic protein 2 (BMP-2), which stimulates Smad signaling that ultimately results in the interaction of Smads with the transcription factor Runx2. Coexpression of STAT1 and Runx2 showed that STAT1 inhibited Runx2-dependent expression of a reporter gene without affecting Smad-specific reporter-gene expression. Furthermore, DNA binding of Runx2 was increased in the STAT-deficient osteoblasts. STAT1 coimmunoprecipitated with Runx2 in cultured osteoblasts, an interaction that was decreased after BMP-2 treatment. Phosphorylation of STAT1 was not required for Runx2 interaction or inhibition of DNA binding; instead, treatment of the cells with interferon-γ, which promotes STAT1 phosphorylation and dimerization, decreased the interaction between STAT1 and Runx2. When STAT1 and Runx2 were coexpressed in 293T cells, STAT1 sequestered Runx2 in the cytosol. Thus, the authors propose that in the absence of cytokine stimulation, STAT1 interacts with Runx2, inhibiting its translocation to the nucleus and thereby inhibiting osteoblast differentiation.
S. Kim, T. Koga, M. Isobe, B. E. Kern, T. Yokochi, Y. E. Chin, G. Karsenty, T. Taniguchi, H. Takayangi, Stat1 functions as a cytoplasmic attenuator of Runx2 in the transcriptional program of osteoblast differentiation. Genes Dev. 17, 1979-1991 (2003). [Abstract] [Full Text]