Herbimycin A, a pp60^c-src tyrosine kinase inhibitor, inhibits osteoclastic bone resorption in vitro and hypercalcemia in vivo

Since absence of expression of the c-src gene product in mice indicates that the pp60^c-src tyrosine kinase is required and essential for osteoclastic bone resorption, we tested the effects of the antibiotic herbimycin A, which is an inhibitor of pp60^c-src on osteoclastic bone resorption in vitro and on hypercalcemia in vivo. We examined the effects of herbimycin A on the formation of bone resorbing osteoclasts in mouse long-term marrow cultures, on isolated rodent osteoclasts and on bone resorption in organ cultures of fetal rat long bones stimulated by parathyroid hormone. We found that herbimycin A in concentrations of 1-100 ng/ml inhibited bone resorption in each of these systems. We determined the effects of herbimycin A ( 100 ng/ml) on src tyrosine kinase activity in mouse marrow cultures and found that it was decreased. Herbimycin A also decreased elevated blood calcium levels that were induced either by repeated subcutaneous injections of recombinant human interleukin-1α or by a human tumor. There was no evidence for toxicity in any of these culture systems or in mice treated with herbimycin A. A different tyrosine kinase inhibitor that does not inhibit pp60^c-src was used as a control and caused none of these effects. These data suggest that pp60^c-src tyrosine kinase inhibitors may be useful pharmacologic inhibitors of osteoclastic bone resorption and hypercalcemia.