Protein kinase C-δ is an important signaling molecule in insulin-like growth factor I receptor-mediated cell transformation

To investigate the potential role of protein kinase C-delta (PKC-delta) in insulin-like growth factor I receptor (IGF-IR)-mediated cell transformation, an oncogenic gag-IGF-IR beta-fusion receptor lacking the entire extracellular domain, which was designated NM1, and a full-length IGF-IR mere coexpressed with either wild-type PKC-delta (PKC-delta WT) or an ATP-binding mutant of PKC-delta (PKC-delta K376R) in NIH 3T3 fibroblasts. While overexpression of PKC-delta WT did not affect NM1- and IGF-IR-induced focus and colony formation of NIH 3T3 cells, expression of PKC-delta K376R severely impaired these events. In contrast, NM1-mtrdiated cell growth in monolayer was not affected by coexpressing PKC-delta K376R. PKC-delta WT and PKC-delta K376R were constitutively phosphorylated on a tyrosine residue(s) in the NM1- and IGF-IR-expressing cells and were associated with them in an IGF-I-independent manner. Activated IGF-IR was able to phosphorylate purified PKC-delta in vitro and stimulated its kinase activity. Furthermore, the level of endogenous PKC-delta protein was up-regulated through transcriptional activation in response to long-term IGF-IR activation. Taken together, our results demonstrate that PKC-delta plays an important role in IGF-IR-mediated cell transformation, probably via association of the receptor with PKC-delta and its activation through protein up-regulation and tyrosine phosphorylation. Competition with endogenous PKC-delta for NM1 and IGF-IR association by PKC-delta K376R is probably an important mechanism underlying: the PKC-delta K376R-mediated inhibition of cell transformation by NM1 and IGF-IR.