The combined effects of tamoxifen, a competitive inhibitor of estrogen, and type I or II interferons on the proliferation of several human breast cancer cell lines in vitro were examined. Additive antiproliferative effects were observed with interferons and tamoxifen, in two estrogen receptor positive cell lines, MCF-7 and T-47D. In MCF-7 cells neither βser interferon, γ interferon, nor α6 interferon were able to significantly alter estrogen receptor levels. Antigrowth activities of βser interferon and γ interferon in an estrogen receptor negative cell line, HS578T, were equivalent to those in estrogen receptor-positive cell lines. Consistent with the antiproliferative effects of interferons, high affinity βser interferon receptors and interferon inducible 2′5′-oligoadenylate synthetase were present in both MCF-7 and HS578T cell lines. Thus steroid hormone receptor status did not influence the antiproliferative effects of interferons on breast carcinoma cells in vitro.

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Supported in part by Triton Biosciences and NIH/NCI CA 20432.

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