INVESTIGATION OF THE COOPERATIVE EFFECTS OF TRANSFORMING GROWTH-FACTOR-ALPHA AND C-MYC OVEREXPRESSION IN RAT-LIVER EPITHELIAL-CELLS

Overexpression of both transforming growth factor (TGF)-alpha and c-my c is consistently reported in hepatic tumors. We transfected rat liver epithelial cells (RLECs) with expression vectors for TGF-alpha, c-myc , or both and analyzed the morphology, biological properties, and tumo rigenicity of clones that overexpressed these genes. The transfectants were morphologically indistinguishable from the parental RLECs, but t he overexpression of TGF-alpha resulted in changes in growth propertie s and an enhanced response to the mitogenic effects of hepatocyte grow th factor. The concomitant overexpression of c-myc decreased growth fa ctor requirements of the TCF-alpha/c-myc clones compared with RLEC and TGF-alpha clones. The TGF-alpha and TGF-alpha/c-myc clones were tumor igenic in nude mice at frequencies of 27% and 53%, respectively, indic ating that the genes cooperate in malignant transformation. However, t he untransformed nature and low tumorigenicity of the transfectants su ggest that transformation depends on other cellular events in addition to the overexpression of TGF-alpha or c-myc. Characterization of tumo r cell lines showed that in contrast to the transfectants, the tumor c lones were morphologically transformed, capable of autonomous growth a nd anchorage-independent growth, and aggressively tumorigenic with a f requency of 100%. Clearly, the tumor cells differed from the transfect ants and had undergone biological or genetic alterations (or both) as a consequence of the overexpression of TGF-alpha or c-myc. Our data su ggest that the overexpression of TGF-alpha leads to enhanced responsiv eness to hepatocyte growth factor, whereas the concomitant overexpress ion of c-myc confers growth-factor independence, providing a potential explanation of the mechanisms by which the overexpression of these ge nes results in transformation, (C) 1995 Wiley-Liss, Inc.