P21(RAS) DOWNSTREAM EFFECTORS ARE INCREASED IN ACTIVITY OR EXPRESSIONIN MOUSE-LIVER TUMORS BUT DO NOT DIFFER BETWEEN RAS-MUTATED AND RAS-WILD-TYPE LESIONS

Mouse liver tumors frequently harbor activating ras gene mutations, Do wnstream effector molecules of p21(Ras) include Raf-l kinase which med iates external signals via kinase signaling pathways to nuclear transc ription factors including c-Fos and c-Jun, Mouse liver tumors with dif fering uas-mutational status were analyzed for alterations in Ras/Raf- 1 signal transduction. Tumors were characterized with respect to the p resence of base substitutions in the 3 known hot-spot positions at cod ons 12, 13, and 61 of Ha-ras, Ki-vas, and N-ras. Ha-uas codon 61 or Ki -ms codon 13 mutations, but no N-ras mutations, were detected in 23 ou t of 33 tumors analyzed, while no ras-mutations were found in 10 of th e tumors, There was no significant difference in the expression of p21 (Ras) proteins between ras-mutated tumors and tumors without detectabl e ras mutations, To allow for determination of Raf-l kinase activity i n tumors, a sensitive and specific assay was developed for measurement s with tissue homogenates, Raf-l kinase activity was increased about f our-fold in liver tumors as compared with normal liver tissue, No sign ificant differences in kinase activity however, were evident between u as-mutated and ras-wild-type tumors. The same was true with respect to the levels of c-fos and c-jun mRNAs, Moreover, there were no signific ant differences in cell division (5-bromo-2'-deoxyuridine-labeling ind ices) of hepatocytes from ras-mutated and ras-wild-type tumors, The si milar degree of constitutive activation of the Ras/Raf-1 signaling pat hway in liver tumors, with and without detectable vas mutations, sugge sts that other molecules within the signaling pathway may substitute f or vas-mutations during oncogenic conversion of ras-wild-type hepatocy tes.