Protein tyrosine kinase inhibitors increase cytosolic calcium and inhibit actin organization as resorbing activity in rat osteoclasts.
Although there is evidence that protein tyrosine kinase inhibitors (PTKIs) suppress bone resorption activity, the mechanism of action of these compounds on osteoclastic bone resorption remains obscure. In the present study, we investigated the effect of PTKIs on cytosolic Ca(2+) concentration ([Ca(2+)](i)) and on the cytoskeleton in rat osteoclasts. The PTKIs, genistein and herbimycin A, reversibly elevated [Ca(2+)](i) measured by fura-2 microfluorimetry. The PTKI-induced increase was abolished by omission of extracellular Ca(2+), but was not attenuated by depletion of Ca(2+) stores. The PTKI-induced increase was inhibited by addition of La(3+) and Ni(2+), but not abolished by dihydropyridine (DHP) Ca(2+) channel blockers. Genistin, an inactive analogue of genistein, had no effect on [Ca(2+)](i). In the cytoskeleton assay, genistein rapidly disrupted the actin ring formation that serves as a marker for the resorbing state of osteoclasts. Disruption of the actin ring formation was also diminished in Ca(2+)-free extracellular solution. These results suggest that PTKIs in rat osteoclasts elevate [Ca(2+)](i) via activation of a DHP-insensitive, nonspecific Ca(2+) entry pathway and disrupt the formation of actin rings, resulting in suppression of bone resorption activity. The regulation of this Ca(2+)-influx by PTKIs is likely to contribute to inhibition of bone resorption by these compounds.