Retinoic acid (RA) inhibits proliferation of numerous breast carcinoma cells and prevents estrogen stimulation of growth of several estrogen receptor (ER)-positive cell lines. RA inhibition of human breast carcinoma cell proliferation is associated with marked inhibition of the synthesis of a Mr 39,000 protein in the ER-positive human breast carcinoma cell lines investigated. Inhibition of the synthesis of the Mr 39,000 protein occurred within 24 h of RA addition and coincides with the onset of inhibition of cellular proliferation. Increasing the dose of RA results in increasing inhibition of Mr 39,000 synthesis. RA does not inhibit the proliferation of the ER-negative human breast carcinoma cell line MDA MB-231; synthesis and inhibition of the Mr 39,000 protein is not noted in this cell line. Tamoxifen, which inhibits ER-positive breast carcinoma proliferation, moderately inhibits Mr 39,000 synthesis, while a concentration of difluoromethylornithine which inhibits cellular proliferation by greater than 50% does not affect Mr 39,000 protein synthesis. Thus, inhibition of the Mr 39,000 protein appears not to be simply related to the cessation of cellular proliferation.

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This research was supported by a grant from the Veterans Administration.

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