DIFFERENTIAL DEPENDENCE OF THE TUMORIGENICITY OF CHEMICALLY TRANSFORMED RAT-LIVER EPITHELIAL-CELLS ON AUTOCRINE PRODUCTION OF TRANSFORMING GROWTH-FACTOR-ALPHA

The tumorigenic phenotype in rat liver epithelial cells overexpressing C-myc may depend on a transforming growth factor (TGF)-alpha/epiderma l growth factor receptor autocrine loop (L. W. Lee et al., Cancer Res. , 51: 5238-5244, 1991). In the present study, we have used constitutiv e sense and antisense TGF-alpha expression vectors to modify TGF-alpha production in carcinogen-transformed clonal derivatives of a rat live r epithelial cell line, WB-F344, that variably express c-myc, endogeno us TGF-alpha, and tumorigenicity. Transgene-mediated TGF-alpha protein production was elevated 2- to 9-fold in derivatives of a low c-myc-ex pressing transformed cell line, GN4, and 35-fold in a derivative of a high c-myc-expressing cell line, GN6. Although the GN4- and GN6-derive d cell lines expressed functional EGF receptor and steady-state c-myc mRNA levels that were comparable to their respective parental cell lin es, increased TGF-alpha expression did not increase the tumorigenicity of the derivatives relative to the parental cell lines. Similarly, in vitro growth characteristics of the GN4- and GN6-derived cell lines w ere not markedly altered by increased autocrine TGF-alpha production. Additionally, GN4, GN6, and their derivatives were, for the most part, unresponsive to exogenously applied TGF-alpha in vitro. In contrast, antisense TGF-alpha RNA expression significantly suppressed endogenous TGF-alpha production in a high c-myc-expressing, high TGF-alpha-expre ssing, highly tumorigenic clonal line, GP9; this suppression resulted in towered steady-state c-myc levels and attenuated in vitro growth. A ntisense-mediated suppression of all of these in vitro phenotypes in G P9 was reversed by exogenous TGF-alpha. The latency of tumor formation by the antisense derivative of cell line GP9 was significantly length ened (> 3-fold) relative to the time required for tumor formation by i ts parental cell line. These results demonstrate that a TGF-alpha/epid ermal growth factor receptor autocrine loop may be necessary for exagg erated in vitro and in vivo growth of some transformed Pat liver epith elial cells (e.g. GP9); however, the autocrine loop is not generally s ufficient to support tumorigenicity, even in transformed clonal lines expressing elevated levels of c-myc.