Interferon regulatory factor-1 (IRF-1), a transcription factor and tumor suppressor involved in cell growth regulation and immune responses, has been shown to be induced by all-trans retinoic acid (ATRA). However, the factors controlling the cellular location and activity of IRF-1 are not well understood. In this study, we examined the expression of IRF-1 and its nuclear localization, DNA-binding activity, and target gene expression in human mammary epithelial MCF10A cells, a model of breast epithelial cell differentiation and carcinogenesis. Following initial treatment with ATRA, IRF-1 mRNA and protein were induced within 2 hrs, reached a peak (>30-fold induction) at 8 hrs, and declined afterwards. IRF-1 protein was predominantly cytoplasmic during this treatment. Although a second dose of ATRA or Am580 (a related retinoid selective for retinoic acid receptor-alpha [RARalpha]), given 16 hrs after the first dose, restimulated IRF-1 mRNA and protein levels to a similar level to that obtained by the first dose, IRF-1 was predominantly concentrated in the nucleus after restimulation. ATRA and Am580 also increased nuclear RARalpha, whereas retinoid X receptor-alpha (RXRalpha)--a dimerization partner for RARalpha, was localized to the nucleus upon second exposure to ATRA. However, ATRA and Am580 did not regulate the expression or activation of signal transducer and activator of transcription-1 (STAT-1), a transcription factor capable of inducing the expression of IRF-1, indicating an STAT-1-independent mechanism of regulation by ATRA and Am580. The increase in nuclear IRF-1 after retinoid restimulation was accompanied by enhanced binding to an IRF-E DNA response element, and elevated expression of an IRF-1 target gene, 2',5'-oligoadenylate synthetase-2. The dual effect of retinoids in increasing IRF-1 mRNA and protein and in augmenting the nuclear localization of IRF-1 protein may be essential for maximizing the tumor suppressor activity and the immunosurveillance functions of IRF-1 in breast epithelial cells.