Cellular Responses to Mechanical Stress: Selected Contribution: Regulatory pathways involved in mechanical induction of c-fos gene expression in bone cells

Centre for Science and Technology in Medicine, School of Postgraduate Medicine, Keele University, North Staffordshire Hospital, Stoke on Trent ST4 7QB, United Kingdom The regulatory pathways involved in the rapid response of the AP-1 transcription factor, c- fos , to mechanical load in human primary osteoblast-like (HOB) 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 expression 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 1 -integrin antibodies inhibited the load response. The role of Ca 2+ signaling has been demonstrated by blocking upregulation with addition of 2 mM EGTA, which chelates extracellular Ca 2+ , and gadolinium (10 µM), which inhibits stretch-activated channels. Addition of the Ca 2+ ionophore A-23187 induced upregulation without loading; however, addition of nifedipine (10 µM), the L-type channel blocker, failed to prevent the load response. Inhibitors of downstream pathways indicated the involvement of protein kinase C. Our results demonstrate a key involvement of Ca 2+ signaling pathways and integrin binding in the c- fos response to mechanical strain. secondary messenger; mechanical loading; gene regulation; calcium channels; integrins