Hormonal regulation of pp60c-src expression during osteoclast formation in vitro

Recent studies have shown that the protooncogene c-src is required for normal osteoclastic bone resorption. In this study we examined the expression and regulation of pp60c-src in murine bone marrow cultures in which bone-resorbing multinucleated osteoclasts form over 6 days of culture. We found that both pp60c-src protein expression and pp60c-src tyrosine kinase activity correlated closely with the numbers of active osteoclasts in the cultures. PTH increased the numbers of osteoclasts, pp60c-src tyrosine kinase activity, and src protein, whereas calcitonin decreased the numbers of osteoclasts, src protein, and tyrosine kinase activity. However, when calcitonin was incubated for short periods (< 2 h) with the active osteoclasts present after 6 days of culture, there was a decrease in pp60c-src tyrosine kinase activity and the phosphorylation state, but not in total pp60c-src protein. These data suggest that pp60c-src is expressed in cultures of osteoclasts in parallel with the number of active bone-resorbing osteoclasts. They indicate that pp60c-src activity in osteoclast cultures depends on the activity and numbers of osteoclasts and is hormone regulated. As calcitonin receptors are detectable only in osteoclasts in these cultures, the inhibitory effects of calcitonin suggest that the critical site for pp60c-src expression in these cultures is in osteoclasts.