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/epidermal growth factor receptor autocrine loop (L. W. Lee et al., Cancer Res., 51: 5238-5244, 1991). In the present study, we have used constitutive sense and antisense TGF-alpha expression vectors to modify TGF-alpha production in carcinogen-transformed clonal derivatives of a rat liver epithelial cell line, WB-F344, that variably express c-myc, endogenous TGF-alpha, and tumorigenicity. Transgene-mediated TGF-alpha protein production was elevated 2- to 9-fold in derivatives of a low c-myc-expressing transformed cell line, GN4, and 35-fold in a derivative of a high c-myc-expressing cell line, GN6. Although the GN4- and GN6-derived cell lines expressed functional EGF receptor and steady-state c-myc mRNA levels that were comparable to their respective parental cell lines, 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 were 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-expressing, highly tumorigenic clonal line, GP9; this suppression resulted in lowered steady-state c-myc levels and attenuated in vitro growth. Antisense-mediated suppression of all of these in vitro phenotypes in GP9 was reversed by exogenous TGF-alpha. The latency of tumor formation by the antisense derivative of cell line GP9 was significantly lengthened (> 3-fold) relative to the time required for tumor formation by its parental cell line. These results demonstrate that a TGF-alpha/epidermal growth factor receptor autocrine loop may be necessary for exaggerated in vitro and in vivo growth of some transformed rat liver epithelial cells (e.g., GP9); however, the autocrine loop is not generally sufficient to support tumorigenicity, even in transformed clonal lines expressing elevated levels of c-myc.
Duddy
S K
SK
Department of Pathology, School of Medicine, University of North Carolina, Chapel Hill 27599, USA.
Earp
H S
HS
Russell
W E
WE
Smith
G J
GJ
Grisham
J W
JW
D000199:Actins / Q000235:genetics; D000818:Animals; D015152:Blotting, Northern; D015153:Blotting, Western; D015197:Carcinogenicity Tests; D002452:Cell Count; D002455:Cell Division; D002461:Cell Line, Transformed; D002471:Cell Transformation, Neoplastic / Q000139:chemically induced / Q000378:metabolism*; D066246:ErbB Receptors / Q000096:biosynthesis*; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D051379:Mice; D016271:Proto-Oncogene Proteins c-myc / Q000096:biosynthesis* / Q000235:genetics; D016372:RNA, Antisense / Q000378:metabolism; D012333:RNA, Messenger / Q000032:analysis; D051381:Rats; D011916:Rats, Inbred F344; D014162:Transfection; D016211:Transforming Growth Factor alpha / Q000096:biosynthesis* / Q000235:genetics
D000199:Actins; D016271:Proto-Oncogene Proteins c-myc; D016372:RNA, Antisense; D012333:RNA, Messenger; D016211:Transforming Growth Factor alpha; D066246:ErbB Receptors