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We have produced a panel of monoclonal antibodies which bind cell surface domains of the 185 Kd tumor antigen (p185) encoded by the neu oncogene. All of these antibodies stain neu-transformed cells in immunofluorescence assays and immunoprecipitate p185 from metabolically labeled cell lysates. All of the anti-p185 monoclonal antibodies, regardless of isotype, exert a selective cytostatic effect on the growth of neu-transformed cells suspended in soft agar, demonstrating their ability to directly inhibit the transformed phenotype. Anti-p185 antibodies of the IgM, IgG2a, and IgG2b isotypes exert a cytolytic effect on neu-transformed cells in the presence of complement. Only one IgG2a monoclonal antibody is also able to mediate minimal levels of antibody-dependent cellular cytotoxicity (ADCC) (Roussel et al., 1984) in the presence of non-immune spleen cells. In vivo administration of anti-p185 antibodies of the IgG1, IgG2a, and IgG2b isotypes exerts a profound inhibitory effect on the tumorigenic growth of neu-transformed cells. This tumor inhibitory effect is unaffected by depleting tumor bearing animals of complement, and is only minimally affected by depleting tumor bearing animals of macrophages. This suggests that neither complement-mediated killing nor ADCC are necessary for the anti-tumor effects of p185-specific monoclonal antibodies. The results presented here demonstrate that monoclonal antibodies reactive with cell surface domains of an oncogene-encoded protein can directly inhibit tumor growth in vitro and in vivo. Such antibodies may prove useful in the therapy of certain malignancies.

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Several different protease inhibitors have the ability to suppress transformation in vitro and carcinogenesis in vivo. The mechanism(s) by which protease inhibitors suppress carcinogenesis, however, is not fully understood. Presumably, these agents inhibit one or more intracellular proteases whose functions are essential for the induction and/or expression of the transformed phenotype. We have isolated an endopeptidase activity capable of hydrolyzing the substrate Boc-Val-Pro-Arg-MCA (Boc = butoxycarbonyl; MCA = 7-amino-4-methylcoumarin) from C3H/10T1/2 mouse embryo fibroblast cells. This intracellular protease was inhibited by the soybean-derived Bowman-Birk inhibitor (BBI), chymostatin, and L-1-tosylamido-2-phenylethyl chloromethyl ketone, all of which have anticarcinogenic activity, but was unaffected by soybean trypsin inhibitor, which lacks anticarcinogenic activity. Other protease inhibitors affected the proteolytic activity to an extent that correlates with their relative ability to suppress transformation in vitro. The enzyme has a mass of about 70 kDa, contains a single subunit, and exhibits maximal activity at pH 7.0. Diisopropyl fluorophosphate covalently binds to this enzyme and blocks its activity, indicating that the enzyme is a serine protease. We have previously demonstrated that several protease inhibitors are effective suppressors of radiation-induced transformation of C3H/10T1/2 cells. Since these agents reduce the Boc-Val-Pro-Arg-MCA-hydrolyzing activity to an extent that correlates with their ability to inhibit malignant transformation in vitro, this endopeptidase activity may be a cellular target of the anticarcinogenic protease inhibitors.

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A clone of 3-methylcholanthrene-treated 10T1/2 cells has been isolated which possesses basic characteristics expected of "initiated" cells. In the presence of retinyl acetate, this clone exhibits contact inhibited growth control and is morphologically indistinguishable from the parental 10T1/2 cell line. Removal of retinyl acetate in vitro results in neoplastic transformation after a latent period of 3 weeks. The classical 10T1/2 transformation system was reconstructed by coculturing normal and "initiated" 10T1/2 cells formed either Type II or Type III foci after a latent period of 3-4 weeks, and an additional 22% formed Type I foci. Treatment of "initiated" 10T/2 cells with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate accelerated the formation of transformed foci in the coculture system by reducing the length of the latent period to less than 3 weeks. Injection of 10(6) "initiated" cells/mouse s.c. into nude mice resulted in the appearance of progressively growing fibrosarcomas after a latent period of 5-7 weeks. Dietary supplementation with 4-hydroxyphenyl-retinamide prevented tumor formation; after drug withdrawal, tumors developed in all surviving mice after 6 weeks. We believe this cell line possesses all characteristics expected of "initiated" cells. With this new cell line, designated INIT/10T1/2, we can now study the early biochemical changes in growth control mechanisms resulting in neoplastic transformation and the mechanism(s) of chemoprevention of cancer by vitamin A.

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