Nuclear pp64 is phosphorylated in both serine/threonine and tyrosine through complex pathways regulated by 12-O-tetradecanoylphorbol-13-acetate and platelet-derived growth factor.

Platelet-derived growth factor (PDGF) induces the time and dose dependent serine/threonine phosphorylation of pp64, a nuclear protein in normal rat kidney (NRK) cells. pp64 is phosphorylated additionally on tyrosine in SSV-transformed NRK cells. To further characterize the regulation of phosphorylation of pp64, other mitogens and inhibitors were studied. 12-O-tetradecanoylphorbol-13-acetate (TPA) but not epidermal growth factor (EGF) or insulin induced the phosphorylation of nuclear pp64. Addition of the inhibitor H7 to TPA-treated NRK cells resulted in a striking further increase in phosphorylation of pp64 and, to a lesser extent, in NRK cells treated with PDGF and H7. When cells were treated with PDGF and H7, pp64 was recognized by anti-phosphotyrosine antisera. The increased phosphorylation induced by H7 was inhibited when forskolin was included. This loss of phosphorylation in pp64 with forskolin treatment paralleled a loss of immunoreactivity of pp64 to anti-phosphosphotyrosine. Complex and independent pathways thus appear to signal the growth factor dependent nuclear phosphorylation of pp64, involving phosphorylations both on serine/threonine and on tyrosine.