CONSTITUTIVE ACTIVATION OF MITOGEN-ACTIVATED PROTEIN (MAP) KINASES INHUMAN RENAL-CELL CARCINOMA

Mitogen-activated protein kinases (MAPKs) play a pivotal role in the m itogenic signal transduction pathway and are essential components of t he MAPK cascade, which includes MEK (also known as MAP kinase kinase), Raf-1, and Ras. In this study, we examined whether constitutive activ ation of the MAPK cascade was associated with the carcinogenesis of hu man renal cell carcinomas in a series of 25 tumors and in correspondin g normal kidneys. Constitutive activation of MAPKs in tumor tissue, as determined by the appearance of phosphorylated forms, was found in 12 cases (48%), and this activation was confirmed by a direct in vitro k inase assay of immunoprecipitate using myelin basic protein as the sub strate. The phosphorylation of MEK and of Raf-1, as monitored by a mob ility shift in SDS-PAGE, which is reportedly associated with the activ ation of these kinases, occurred in 9 of 18 cases (50%) and in 6 of 11 cases (55%) respectively. The activation of MAPKs was correlated with MEK activation (P = 0.0045) and with Raf-1 activation (P = 0.067). Fu rthermore, overexpression of MEK was found in 13 of 25 cases (52%) by Western blot analysis, and this overexpression was associated signific antly with MAPK activation (P = 0.034). No mutations were noted in H-, K-, or N-ras genes by PCR direct sequencing in any of the 25 tumor sam ples. Of the patients studied, 8 of 18 (44%) stage pT(2) patients and four of six (67%) stage pT(3) patients showed MAPK activation. The sin gle stage pT(1) patient did not evidence MAPK activation. Furthermore, one of seven (14%) grade 1 patients, 9 of 13 (69%) grade 2 patients, and two of five (40%) grade 3 patients showed MAPK activation (grade 1 versus grades 2 and 3, P = 0.046). Our results suggest that constitut ive activation of MAPK may be associated with the carcinogenesis of hu man RCCs.