U0126 reverses Ki-ras-mediated transformation by blocking both mitogen-activated protein kinase and p70 S6 kinase pathways

U0126, a recently introduced mitogen-activated protein kinase kinase (MAPK) /extracellular signal-regulated kinase kinase inhibitor reversed morphology and inhibited anchorage-independent growth of Ki-ras-transformed rat fibro blasts, Immunoblot analyses with phosphospecific antibodies indicated that in addition to MAPK, U0126 suppressed activation of p70(S6K), but not Akt, at concentrations at which it normalized the transformed phenotypes, Anothe r MAPK/extracellular signal-regulated kinase kinase inhibitor, PD98059, sho wed only marginal effects on p70(S6K) phosphorylation and did not effective ly block Ki-ras-induced transformation, However, simultaneous inhibition of the MAPK pathway and the p70(S6K) pathway by PD98059 in conjunction with t he p70(S6K) inhibitor rapamycin essentially restored the normal phenotype. U0126 or the combination of PD98059 and rapamycin flattened morphology of v -src-transformed cells, but did not reverse anchorage independence, althoug h activation of both MAPK and p70(S6K) was blocked. The results suggest tha t normalization of Ki-ras-induced transformed phenotypes by U0126 is a cons equence of concurrent inhibition of the MAPK and p70(S6K) pathways. Interve ntion of other pathway(s) appears to be required to completely antagonize t ransformation by v-src, Simultaneous blockade of more than one signal trans duction pathway by combining selective inhibitors might be effective in sup pressing uncontrolled tumorigenic growth.