REGULATION OF A CALCIUM-DEPENDENT TYROSINE KINASE IN VASCULAR SMOOTH-MUSCLE CELLS BY ANGIOTENSIN-II AND PLATELET-DERIVED GROWTH-FACTOR - DEPENDENCE ON CALCIUM AND THE ACTIN CYTOSKELETON

A novel, p125(FAK) homologue, CADTK, has been detected in neural, epit helial, or hematopoietic cells but not in fibroblasts. We now demonstr ate CADTK expression in a mesenchymal cell, rat aortic smooth muscle c ells (RSMC). Angiotensin II (Ang II) or platelet-derived growth factor (PDGF-BB and PDGF-AA) markedly stimulated CADTK tyrosine phosphorylat ion in RSMC: but did not affect p125(FAK) phosphorylation. The PDGF-de pendent CADTK tyrosine phosphorylation was slower and more prolonged t han that of Ang II, correlating well with the differential effects of these agonists on cytosolic calcium ([Ca2+](i)) signaling. An intracel lular calcium chelator inhibited both the rapid and sustained activati on of CADTK by Ang II and PDGF. Extracellular calcium chelation inhibi ted the PDGF-stimulated increase in CADTK tyrosine phosphorylation as well as the sustained (but not the early) activation by Ang II, In con trast, p125(FAK) tyrosine phosphorylation was maximal in quiescent, ad herent RSMC and was not affected by incubation with EGTA. Depletion of protein kinase C activity partially inhibited both the Ang II- and PD GF-induced CADTK tyrosine phosphorylation. Additional results confirm a relation between CADTK and the cytoskeleton, First, the tyrosine pho sphorylation of paxillin correlated with activation of CADTK; this inc rease was inhibited by EGTA, Second, cytochalasin D blocked the PDGF- or Ang II stimulated tyrosine phosphorylation of CADTK, suggesting a r ole for the cytoskeleton in agonist-dependent CADTK activation. Third, immunofluorescence analysis of CADTK localization demonstrated actin- like cytoskeleton staining extending into focal contacts, These result s suggest that in mesenchymal cells, CADTK is localized to and activat ed by an actin cytoskeleton-dependent mechanism; a mechanism that is r egulated in a calcium and protein kinase C-dependent manner independen tly of p125(FAK).