Selected contribution: Regulatory pathways involved in mechanical induction of c-fos gene expression in bone cells

The regulatory pathways involved in the rapid response of the AP-1 transcri ption factor, c-fos, to mechanical load in human primary osteoblast-like (H OB) cells and the human MG-63 bone cell line were investigated using a four -point bending model. HOB and MG-63 cells showed upregulation of c-fos expr ession on fibronectin and collagen type I substrates; however, MG-63 cells did not respond on laminin YIGSR substrates. Addition of cytochalasin D and Arg-Gly-Asp peptides during loading did not inhibit the response, whereas addition of beta (1)-integrin antibodies inhibited the load response. The r ole of Ca2+ signaling has been demonstrated by blocking upregulation with a ddition of 2 mM EGTA, which chelates extracellular Ca2+, and gadolinium (10 muM), which inhibits stretch-activated channels. Addition of the Ca2+ iono phore A-23187 induced upregulation without loading; however, addition of ni fedipine (10 muM), the L-type channel blocker, failed to prevent the load r esponse. Inhibitors of downstream pathways indicated the involvement of pro tein kinase C. Our results demonstrate a key involvement of Ca2+ signaling pathways and integrin binding in the c-fos response to mechanical strain.