Abstract
Calcitonin induces the association and tyrosine phosphorylation of focal adhesion kinase (FAK), paxillin, and HEF1 in HEK-293 cells that overexpress the calcitonin receptor (C1a-HEK), but the hormone's effect on these adhesion-related proteins in osteoclasts is not known. We therefore studied the effect of calcitonin on the tyrosine phosphorylation and subcellular distribution of paxillin, HEF1, FAK, and Pyk2, a FAK-related tyrosine kinase, in osteoclasts. Osteoclasts expressed both Pyk2 and FAK, with Pyk2 much more highly expressed. The two tyrosine kinases and paxillin were prominently associated with small punctate structures that were most densely clustered in the region of the peripheral F-actin-rich ring. Some of the punctate structures stained either for Pyk2 alone or FAK alone. Treatment with calcitonin disrupted the actin ring and induced the loss of the peripheral staining of paxillin, Pyk2, and FAK. In calcitonin-treated osteoclast-like cells, the tyrosine phosphorylation of paxillin and FAK increased, whereas the tyrosine phosphorylation of Pyk2 decreased. Calcitonin also induced increased phosphorylation of Erk1 and Erk2 in osteoclasts, as it did in the C1a-HEK cells. The unexpected dephosphorylation of Pyk2 correlated with decreased phosphorylation of Tyr(402), the autophosphorylation site of Pyk2. The calcitonin-induced dephosphorylation of Pyk2 was not observed in C1a-HEK cells transfected with Pyk2, suggesting that the reduced phosphorylation seen in osteoclasts may be specific to these cells. Treatment of osteoclast-like cells with 12-phorbol 13-myristate acetate increased the tyrosine phosphorylation of both Pyk2 and FAK, and calphostin C, an inhibitor of protein kinase C, blocked calcitonin-stimulated FAK phosphorylation. Increasing intracellular calcium with ionomycin caused a decrease in the tyrosine phosphorylation of Pyk2 and the loss of the actin ring in a manner similar to the effect of calcitonin. Ionomycin had no effect on FAK tyrosine phosphorylation. Calcitonin (CT)-induced changes in Pyk2, FAK, and Erk1/2 phosphorylation were independent of c-Src.
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