POSITIVE AND NEGATIVE SELECTION FOR TUMOR-NECROSIS-FACTOR RESPONSIVENESS REVEALS AN INHIBITORY ROLE FOR EGF RECEPTOR IN TNF-INDUCED ANTIPROLIFERATION

Tumor necrosis factor (TNF) induces dose-dependent, but incomplete cyt otoxicity in ME-180 cervical carcinoma cells resulting in a significan t reduction in cell viability. In this cell line there exists a charac teristic residual tumor cell population that appears to be resistant t o TNF. In order to investigate tumor cell heterogeneity and characteri stics that correlate with their escape from TNF-induced cytotoxicity, TNF-resistant ME-180 cell variants (ME-180R) were isolated from a popu lation of ME-180 cervical carcinoma cells (ME-180 parental). Incubatio n of ME-180 parental cells with TNF resulted in measurable changes in tumor cell DNA structural integrity and dose-dependent cytotoxicity, w hereas ME-180R cell growth and DNA integrity were not effected by incu bation with TNF. Binding of I-125-labeled TNF to a TNF-specific cell-s urface receptor was measurable and equivalent on both ME-180R and ME-1 80 parental cells and both cell lines predominantly expressed the p55 form of the TNF receptor based upon flow cytometric analysis. Although both cell lines shared similar doubling times, intrinsic EGF receptor tyrosine kinase activity in ME-180R cells was found to be >3-fold hig her than that isolated from ME-180 parental cells. These results sugge st that TNF-responsiveness may be mediated at a point subsequent to TN F binding and may be regulated, in part, by the expression of tyrosine kinase activity. To further explore this hypothesis, A431 vulvular ca rcinoma cells that express resistance to TNF were cloned and variants were isolated that escaped EGF-induced growth inhibition. These varian ts were all shown to express 2- to 4-fold lower levels of EGF receptor protein or intrinsic tyrosine kinase activity when compared to the pa rental A431 population and coordinately expressed sensitivity to the g rowth inhibitory effects of TNF. These results suggest that EGF recept or may function to suppress TNF-induced growth inhibition or cytotoxic ity in squamous carcinoma cells. Modulation of EGF receptor levels or intrinsic tyrosine kinase activity may represent one mechanism whereby tumor cells escape TNF-induced antiproliferation.