Serum response factor (SRF) plays vital roles in numerous cellular processes; however, the physiological function of SRF in skeletal tissue
remains unknown. In several organ systems, SRF regulates the expression of insulin-like growth factor-1 (IGF-1), which is crucial for
normal development of mineralized skeleton and bone remodeling throughout life. Here, we show that conditional deletion of SRF in
osteoblasts by osteocalcin-Cre generated viable mice with normal body size and body weight. Compared with normal siblings,
osteoblast-specific SRF-deficient adult mice exhibited a marked decrease in bone mineral density and bone formation rate. Deletion of
SRF in primary mouse calvarial osteoblasts reduced cell differentiation and mineralization in vitro. This was accompanied by a decrease in
IGF-1 expression and secretion. Addition of IGF-1 in the culture media enhanced osteoblast differentiation in control cells and partially
restored the mineralization defect of SRF-deficient cells, supporting an important role of SRF in regulating IGF-1 and IGF-1–mediated
osteoblast differentiation. IGF-1–induced Akt activation was inhibited in SRF-deficient calvarial cells and enhanced in the SRF
overexpressed cells. In addition, SRF deficiency decreased the transcriptional activity of Runx2, the key transcription factor for
osteogenesis. Overexpression of SRF induced Runx2 transactivity in control cells and restored Runx2 transactivity in the SRF-deficient
cells. Taken together, we conclude that SRF is important for IGF-1–induced osteoblast differentiation and mineralization via regulating
IGF-1 expression and Runx2 transactivity.
2012 American Society for Bone and Mineral Research.