p21Ras downstream effectors are increased in activity or expression in mouse liver tumors but do not differ between ras-mutated and ras-wild-type lesions.
Mouse liver tumors frequently harbor activating ras gene mutations. Downstream effector molecules of p21Ras include Raf-1 kinase which mediates external signals via kinase signaling pathways to nuclear transcription factors including c-Fos and c-Jun. Mouse liver tumors with differing ras-mutational status were analyzed for alterations in Ras/Raf-1 signal transduction. Tumors were characterized with respect to the presence of base substitutions in the 3 known hot-spot positions at codons 12, 13, and 61 of Ha-ras, Ki-ras, and N-ras. Ha-ras codon 61 or Ki-ras codon 13 mutations, but no N-ras mutations, were detected in 23 out of 33 tumors analyzed, while no ras-mutations were found in 10 of the tumors. There was no significant difference in the expression of p21RaS proteins between ras-mutated tumors and tumors without detectable ras mutations. To allow for determination of Raf-1 kinase activity in tumors, a sensitive and specific assay was developed for measurements with tissue homogenates. Raf-1 kinase activity was increased about four-fold in liver tumors as compared with normal liver tissue. No significant differences in kinase activity, however, were evident between ras-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 significant differences in cell division (5-bromo-2'-deoxyuridine-labeling indices) of hepatocytes from ras-mutated and ras-wild-type tumors. The similar degree of constitutive activation of the Ras/Raf-1 signaling pathway in liver tumors, with and without detectable ras mutations, suggests that other molecules within the signaling pathway may substitute for ras-mutations during oncogenic conversion of ras-wild-type hepatocytes.
Kalkuhl
A
A
Institute of Toxicology, University of Tübingen, Germany.
Troppmair
J
J
Buchmann
A
A
Stinchcombe
S
S
Buenemann
C L
CL
Rapp
U R
UR
Kaestner
K
K
Schwarz
M
M
D000818:Animals; D002455:Cell Division; D016762:Genes, fos; D016758:Genes, jun; D011905:Genes, ras; D008114:Liver Neoplasms, Experimental / Q000235:genetics* / Q000473:pathology; D008297:Male; D051379:Mice; D008809:Mice, Inbred C3H; D008810:Mice, Inbred C57BL; D009154:Mutation; D019908:Proto-Oncogene Proteins c-raf / Q000032:analysis; D016283:Proto-Oncogene Proteins p21(ras) / Q000032:analysis / Q000235:genetics; D012333:RNA, Messenger / Q000032:analysis
D012333:RNA, Messenger; D019908:Proto-Oncogene Proteins c-raf; D016283:Proto-Oncogene Proteins p21(ras)