PROTEIN-KINASE C-DELTA 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 tran sformation, an oncogenic gag-IGF-IR beta-fusion receptor lacking the e ntire extracellular domain, which was designated NM1, and a full-lengt h IGF-IR mere coexpressed with either wild-type PKC-delta (PKC-delta W T) 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, expres sion of PKC-delta K376R severely impaired these events. In contrast, N M1-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-expres sing cells and were associated with them in an IGF-I-independent manne r. Activated IGF-IR was able to phosphorylate purified PKC-delta in vi tro and stimulated its kinase activity. Furthermore, the level of endo genous PKC-delta protein was up-regulated through transcriptional acti vation 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 t yrosine phosphorylation. Competition with endogenous PKC-delta for NM1 and IGF-IR association by PKC-delta K376R is probably an important me chanism underlying: the PKC-delta K376R-mediated inhibition of cell tr ansformation by NM1 and IGF-IR.