REDUCED INSULIN-RECEPTOR EXPRESSION AND FUNCTION IN HUMAN COLONIC CACO-2 CELLS BY RAS AND POLYOMA MIDDLE-T ONCOGENES

Taking advantage of the potent mitogenic effect exerted by insulin in human colonic cells, we used Caco-2 cells transfected with an activate d (Val-12) human Ha-ras gene or the polyoma middle T (PyMT) oncogene, a constitutive activator of pp60(c-src)tyrosine kinase activity, to in vestigate the effect of oncogenic p21(ras) and PyMT/ pp60(c-src) on in sulin mitogenic signaling. As compared to vector control Caco-2 cells, both oncogene-transfected cells exhibited: 1) a loss of response to i nsulin's stimulatory effect on mitogen-activated protein (MAP) kinase activity and cell proliferation, both of which were constitutively inc reased; 2) a decrease in insulin receptor (IR) affinity and insulin-st imulated exogenous tyrosine kinase activity, which resulted from incre ased protein kinase C (PKC) activity (Delage, S., Chastre, E., Empereu r, S., Wicek, D., Veissiere, D., Capeau, J., Gespach, C., and Cherqui, G. (1993) Cancer Res. 53, 2762-2770), since IR alterations were corre cted by PKC down-regulation; and 3) a decrease in both IR mRNA level a nd IR number, which was independent of PKC since it persisted after PK C down-regulation. In conclusion, this is the first evidence that onco genic p21(ras) and PyMT/ pp60(c-src) abolish insulin mitogenic signali ng in human colonic cells through mechanisms involving (i) constitutiv e activation of MAP kinase and (ii) marked decreases in both IR functi on and expression which are mediated by PKC-dependent and PKC-independ ent pathways, respectively.