pp60 c-src expression and activity in MG-63 osteoblastic cells modulated by PTH but not required for PTH-mediated...

A role for pp60c-src) tyrosine kinase in bone resorption was recently demonstrated in vivo. However, it is not known whether expression of this ubiquitous tyrosine kinase is essential in osteoblasts or osteoclasts. In this work, we have examined the expression of c-src in osteoblast-like cells. We found that c-src was expressed in the MG-63 osteoblastic osteosarcoma cell line as assessed by immunocytochemistry. When MG-63 cells were treated for 30 minutes with parathyroid hormone (PTH), the levels of tyrosine phosphorylation of c-src were increased in comparison with the untreated control. On the other hand, no changes in total c-src protein were observed after PTH treatment. The increase in c-src tyrosine phosphorylation due to PTH treatment was paralleled by an increase in c-src tyrosine kinase activity. Calcitonin had no effect on c-src phosphorylation, c-src protein level, or tyrosine kinase activity. To determine if c-src tyrosine kinase was required for normal bone cell function and PTH responsiveness, osteoblastic cells were isolated from the calvaria of a src-deficient mouse. The cyclic AMP response to PTH in this cell was identical to that seen in freshly isolated calvarial cells from normal mice. These results suggest that the activity of c-src in MG-63 cells is regulated by PTH as a result of tyrosine phosphorylation. Expression of src is not required for PTH responsiveness in osteoblastic cells as assessed by adenylate cyclase activation. The mode of signal transduction from the PTH receptor to nonreceptor c-src tyrosine kinase is not known at present.