ACTIVATED RAS AND RET ONCOGENES INDUCE OVER-EXPRESSION OF C-MET (HEPATOCYTE GROWTH-FACTOR RECEPTOR) IN HUMAN THYROID EPITHELIAL-CELLS

Hepatocyte Growth Factor (HGF) receptor, encoded by the protooncogene c-met, is overexpressed in many human tumours, including those of thyr oid epithelium. The absence in most cases of any primary structural ab normality of the met gene suggests that overexpression is secondary to mutation of other gene(s). To test this hypothesis we investigated th e effect on met expression of two activated oncogenes known to play a major role in thyroid oncogenesis, ras and ret. To minimize the possib ility of unknown co-operating events, we introduced these genes direct ly into normal human thyrocytes in primary culture using amphotropic r etroviral vectors and assessed met expression as early as possible in the resulting epithelial colonies. Double immunofluorescence revealed expression of met protein, strictly localized to cells expressing the mutant ras and ret vectors, expression in background normal cells bein g barely detectable. In contrast, colonies induced to proliferate at a comparable rate by a vector expressing SV40 T showed no increase in m et expression. To permit quantitation by Western blotting we also exte nded these findings to a thyroid cell line (R18) containing a zinc-ind ucible mutant ras gene. Induction of the oncogene led to a fourfold in crease in met protein expression. We conclude that overexpression of m et is induced by activation of the ras or ret signalling pathway and n ot simply by deregulation of the cell cycle per se. The data suggest t hat the proliferative advantage conferred by these oncogenes may be in part due to the resulting sensitization of tumour epithelium to parac rine HGF secreted by stromal cells.