Retinoic acid induces expression of the interleukin‐1β gene in cultured normal human mammary epithelial cells and in human breast carcinoma lines

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Abstract
Retinoic acid (RA) and its derivatives inhibit the proliferation of normal human mammary epithelial cells (HMEC) and some breast carcinoma lines by mechanisms which are not fully understood. To identify genes that mediate RA‐induced cell growth arrest, an HMEC cDNA library was synthesized and subtractive screening was performed. We identified the interleukin‐1β (IL‐1β) gene as an RA induced gene in HMEC. Northern blot analyses showed that the IL‐1β gene was up‐regulated as early as 2 h after RA treatment. Results from the treatment of HMEC with cycloheximide and actinomycin D indicated that the regulation of the IL‐1β gene by RA occurred at the transcriptional level and that the IL‐1β gene is a direct, downstream target gene of RA. To evaluate the effects of IL‐1β on cell proliferation, the proliferation of HMEC was measured in the presence of RA or IL‐1β, or both. Either RA or IL‐1β could significantly inhibit the proliferation of HMEC. However, the addition of soluble IL‐1 receptor antagonist (sIL‐1ra) to the cell culture medium did not block RA‐induced HMEC growth inhibition, whereas sIL‐1ra did block the growth inhibition of HMEC by IL‐1β. IL‐1β expression was not observed in the three carcinoma cell lines, MCF‐7, MDA‐MB‐231, and MDA‐MB‐468, as compared to the HMEC. Growth curves of the breast carcinoma cell lines showed strong inhibitory effects of RA and IL‐1β on the growth of the estrogen receptor (ER) positive MCF‐7 cell line, but only a small effect on the ER negative MDA‐MB‐231 cells. The expression of the IL‐1β gene was also transcriptionally activated by RA in normal epithelial cells of prostate and oral cavity. Our results suggest that: (a) the IL‐1β gene is a primary target of RA receptors in HMEC; (b) the enhanced expression of the IL‐1β gene does not mediate the RA‐induced growth arrest of HMEC; and (c) the expression of the IL‐1β gene is low or absent in all three human breast carcinoma cell lines examined, but the defect in the IL‐1β signaling pathway may be different in ER positive versus ER negative carcinoma cells.