ONCOGENIC ACTIVATION OF P21(RAS) AND PP60 (C-SRC) IN HUMAN COLONIC CACO-2 CELLS DECREASES INSULIN-RECEPTOR FUNCTION AND EXPRESSION THROUGH PROTEIN-KINASE C-DEPENDENT AND INDEPENDENT PATHWAYS

In view of the potent mitogenic effect exerted by insulin in human col onic cells, we used Caco-2 cells transfected with an activated (Val12) human Ha-ras gene or the polyoma middle T (PyMT) oncogene, a constitu tive activator of pp60(c-src) tyrosine kinase activity, to investigate the effect of oncogenic p21(ras) and PyMT/pp60(c-arc) on insulin mito genic signaling. As compared to vector control Caco-2 cells, both onco gene-transfected cells exhibited: ii a loss of response to insulin's s timulatory effect on mitogen-activated protein (MAP) kinase activity a nd cell proliferation, both of which were constitutively increased; 2) a decrease in insulin receptor (IR) affinity and insulin-stimulated e xogenous tyrosine kinase activity, which resulted, at least in part, f rom increased protein kinase C (PKC) activity (4), since both IR alter ations were partially corrected by PKC down-regulation; and 3) a decre ase in both insulin receptor mRNA level and insulin receptor number, w hich was independent of PKC since it persisted after PKC down-regulati on. In conclusion, oncogenic p21(ras) and PyMT/pp60(c-src) abolished i nsullin mitogenic signaling in Caco-2 cells through mechanisms involvi ng (ii constitutive activation of MAP kinase, and (ill marked decrease s in both insulin receptor function and expression which were mediated by PKC-dependent and PKC-independent pathways respectively. This is t he first evidence that, when oncogenically activated, p21(ras) and pp6 0(c-src) not only exert a negative control on insulin receptor functio n but also repress insulin receptor gene expression in human colonic c ells.