F.2a and B.2, cell clones of the human squamous cell carcinoma SCC-12, were examined to characterize their interactions through the expression of growth factors. F.2a was nontumorigenic yet B.2 was fully tumorigenic when injected into the flanks of athymic nude mice. Combination injections of F.2a and a subtumorigenic level of B.2 produced tumors. F.2a and B.2 overexpressed the 4.8-kb transcript for transforming growth factor-α (TGF-α) as well as the 10.5- and 5.8kb transcripts for the epidermal growth factor receptor. Neither clone expressed the transcript for epidermal growth factor, while both expressed transcripts for insulin-like growth factor-I (IGF-I) of 8.15, 4.9, and 1.6 kb and transcripts for its receptor of 8.5 and 6.5 kb. Conditioned medium (CM) from either clone stimulated the growth of itself and the other clone in tissue culture. Both clones produced intracellular TGF-α detectable by immunohistochemical staining, but not detectable in CM by enzyme-linked immunosorption assay. IGF-I was detected at low levels in CM by radioimmunoassay. Neutralizing antibodies to TGF-α but not IGF-I partially inhibit the growth of both clones in tissue culture. These results suggest that (1) TGF-α is active in autocrine signaling, (2) IGF-I is not directly stimulatory, and (3) additional factors, as yet unidentified, are present in CM and enhance tumor growth. It is concluded that human squamous cell carcinoma SCC-12 is composed of tumorigenic and nontumorigenic clones which interact to enhance growth.
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