Immediate early-gene induction in rat osteoblastic cells after mechanical deformation

Previous studies have reported changes in proliferation, second- messenger generation and activation of various cellular processes when osteoblasts have been mechanically stimulated. Recent evidence suggests that mechanical loading of long bones induces immediate early-gene expression. Immediate early genes, such as Egr-1, are genes that control cell proliferation, are involved in signal transduction, and share properties of transcription factors. The purpose of this study was to examine how mechanical deformation of osteoblasts affects cellular proliferation and Egr- 1 mRNA induction. Osteoblasts were isolated from collagenase digestion of newborn rat calvariae, cultured in Petri dishes with flexible bottoms and then constantly stretched, producing an increase of 3 or 7% in surface area. A mechanical stretch of 7% for 0.5 or 24 h resulted in a doubling of [³H]thymidine incorporation, while 50 nM of epidermal growth factor resulted in a 4-fold increase. A time-course experiment showed that a 7% stretch induced Egr-1 mRNA as early as 15 min, reaching maximum levels by 60 rain and returning to baseline by 120 min. Epidermal growth factor at 50 nM for 60 min resulted in a 3.8-fold Egr-1 mRNA induction. A mechanical stretch of 3% for 30 min also produced an Egr-1 mRNA induction. No induction of Egr-1 mRNA was seen in osteoblasts that were exposed to conditioned media from deformed cells. It is concluded that the immediate early gene, Egr-1, may be directly involved in the signal-transduction pathway of mechanical stimuli in osteoblasts.