Differential regulation of the uridine nucleotide-activated P2Y4 and P2Y6 receptors. SER-333 and SER-334 in the carboxyl terminus are involved in agonist-dependent phosphorylation desensitization and internalization of the P2Y4 receptor.

Agonist-promoted regulation of the uridine nucleotide-activated human P2Y4 receptor (P2Y4-R) and P2Y6 receptor (P2Y6-R) was studied. Incubation of P2Y4-R-expressing 1321N1 human astrocytoma cells with the cognate agonist UTP resulted in rapid desensitization of the inositol phosphate response and a 50% loss of cell surface receptors. In contrast, incubation of P2Y6-R-expressing cells with the cognate agonist UDP caused neither rapid desensitization nor rapid loss of cell surface receptors. Removal of UTP from the medium of UTP-pretreated cells resulted in rapid and complete recovery of surface P2Y4-R even after 12 h of agonist treatment. Although extended incubation with UDP also caused a loss of surface P2Y6-R, rapid recovery of surface P2Y6-R did not occur following removal of agonist. Pharmacological studies indicated that neither protein kinase C nor other Ca(2+)-activated kinases were involved in agonist-promoted desensitization or loss of surface P2Y4-R or P2Y6-R. Mutational analyses were carried out to identify domains involved in agonist-dependent regulation of P2Y4-R. Sequential truncation of the carboxyl-terminal domain revealed that sequence between amino acids 332 and 343 was necessary for UTP-promoted desensitization and internalization. Further mutational analyses of the three serines in this domain confirmed that Ser-333 and Ser-334 play a major role in these agonist-promoted changes in P2Y4-R. Experiments were carried out with [(32)P]P(i)-labeled cells to ascertain the role of phosphorylation in regulation of P2Y4-R. Incubation with UTP for 2 min caused a marked increase in phosphorylation of both the wild-type P2Y4-R and the P2Y4-343 truncation mutant. In contrast, no UTP-promoted phosphorylation of the P2Y4-332 truncation mutant was observed. Taken together, these results demonstrate differential regulation of uridine nucleotide-activated P2Y4-R and P2Y6-R and indicate that Ser-333 and Ser-334 in the carboxyl terminus of P2Y4-R are important for UTP-dependent phosphorylation, desensitization, and loss of surface receptors.

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, p125FAK homologue, CADTK, has been detected in neural, epithelial, or hematopoietic cells but not in fibroblasts. We now demonstrate CADTK expression in a mesenchymal cell, rat aortic smooth muscle cells (RSMC). Angiotensin II (Ang II) or platelet-derived growth factor (PDGF-BB and PDGF-AA) markedly stimulated CADTK tyrosine phosphorylation in RSMC but did not affect p125FAK phosphorylation. The PDGF-depedent CADTK tyrosine phosphorylation was slower and more prolonged than that of Ang II, correlating well with the differential effects of these agonists on cytosolic calcium ([Ca2+]i) signaling. An intracellular calcium chelator inhibited both the rapid and sustained activation of CADTK by Ang II and PDGF. Extracellular calcium chelation inhibited the PDGF-stimulated increase in CADTK tyrosine phosphorylation as well as the sustained (but not the early) activation by Ang II. In contrast, p125FAK tyrosine phosphorylation was maximal in quiescent, adherent RSMC and was not affected by incubation with EGTA. Depletion of protein kinase C activity partially inhibited both the Ang II- and PDGF-induced CADTK tyrosine phosphorylation. Additional results confirm a relation between CADTK and the cytoskeleton. First, the tyrosine phosphorylation of paxilin correlated with activation of CADTK; this increase was inhibited by EGTA. Second, cytochalasin D blocked the PDGF- or Ang II-stimulated tyrosine phosphorylation of CADTK, suggesting a role for the cytoskeleton in agonist-dependent CADTK activation. Third, immunofluorescence analysis of CADTK localization demonstrated actin-like cytoskeleton staining extending into focal contacts. These results suggest that in mesenchymal cells, CADTK is localized to and activated by an actin cytoskeleton-dependent mechanism; a mechanism that is regulated in a calcium and protein kinase C-dependent manner independently of p125FAK.