Differential requirements of the MAP kinase and PI3 kinase signaling pathways in Src- versus insulin and IGF-1 receptors-induced growth and transformation of rat intestinal epithelial cells

There have been few studies on the specific signaling pathways involved in the transformation of epithelial cells by oncogenic protein tyrosine kinase s. Here we investigate the requirement of MAP (MAPK) and phosphatidylinosit ol 3-(PI3K) kinases in the transformation of rat intestinal epithelial (RIE ) cells by oncogenic forms of insulin receptor (gag-IR), insulin-like growt h factor-1 receptor (gag-ICFR), and v-Src. MAPK is not significantly activa ted in cells transformed by gag-IR and gag-IGFR but is activated in v-Src t ransformed cells. Treatment with PD98059, a MEK inhibitor, at concentration s where MAPK activity was reduced below the basal level showed that MAPK is partially required for the monolayer growth of parental and transformed RI E cells. However, MAPK is not essential for the focus forming ability of th e three oncogene-transformed cells. It is also not necessary for the colony forming ability of gag-IR- and gag-IGFR-, but is partially required for v- Src-transformed cells. PI3K is significantly activated in all three oncogen e transformed RIE cells. LY294002, a PI3K inhibitor, potently inhibited mon olayer growth of all three oncogene-transformed cells. However, at concentr ations of LY294002 where activated forms of Akt, a downstream component of the PI3K pathway, were undetectable, colony and focus forming abilities of the v-Src-RIE cells were only slightly affected whereas those of gag-IR/IGF R-RIE cells were greatly inhibited. These results were confirmed using a di fferent pharmacological inhibitor, wortmannin, and a dominant negative form of P13K, Delta p85. Similarly, rapamycin, known to inhibit p70S6 kinase, a downstream component of the PI3K-Akt pathway, also inhibited gag-IR/IGFR-i nduced, but not v-Src-induced, focus and colony formation. We conclude that the MAPK and PI3K signaling pathways are differentially required for trans formation of RIE cells by oncogenic IR and IGFR versus Src and the pattern of requirements is different from that of fibroblast transformation.