Decreased c-Src expression enhances osteoblast differentiation and bone formation

c-src deletion in mice leads to osteopetrosis as a result of reduced bone r esorption due to an alteration of the osteoclast. We report that deletion/r eduction of Src expression enhances osteoblast differentiation and bone for mation, contributing to the increase in bone mass. Bone histomorphometry sh owed that bone formation was increased in Src null compared with wild-type mice. In vitro, alkaline phosphatase (ALP) activity and nodule mineralizati on were increased in primary calvarial cells and in SV40-immortalized osteo blasts from Src(-/-) relative to Src(+/+) mice. Src-antisense oligodeoxynuc leotides (AS-src) reduced Src levels by similar to 60% and caused a similar increase in ALP activity and nodule mineralization in primary osteoblasts in vitro. Reduction in cell proliferation was observed in primary and immor talized Src-/- osteoblasts and in normal osteoblasts incubated with the AS- src, Semiquantitative reverse transcriptase-PCR revealed upregulation of AL P, Osf2/Cbfa1 transcription factor, PTH/PTHrP receptor, osteocalcin, and pr o-alpha 2(I) collagen in Src-deficient osteoblasts. The expression of the b one matrix protein osteopontin remained unchanged. Based on these results, we conclude that the reduction of Src expression not only inhibits bone res orption, but also stimulates osteoblast differentiation and bone formation, suggesting that the osteogenic cells may contribute to the development of the osteopetrotic phenotype in Src-deficient mice.