The human retinoic acid receptor β (RARβ) has three isoforms (β1, β2, and β4), which play important, distinct roles in mediating the effects of retinoic acid on cell growth and apoptosis. Whereas RARβ2 is a potent inhibitor of breast cancer cell proliferation, RARβ4 can act as a dominant-negative repressor of RARβ2-mediated growth suppression. In this study we investigated the effects of all-trans-retinoic acid (ATRA) on two clones derived from the breast cancer cell line MDA-MB-435: a non-metastatic clone (NM-2C5) and a metastatic clone (M-4A4). ATRA treatment of the NM-2C5 cells resulted in growth inhibition and apoptosis, whereas the M-4A4 cells were resistant to ATRA. Analyses of the expression of RARβ isoforms revealed that the sensitive NM-2C5 clone expressed only RARβ2, whereas the resistant M-4A4 cells expressed both RARβ2 and RARβ4 mRNA and protein. ATRA treatment increased RARβ2 mRNA level in NM-2C5 cells, whereas the same treatment of the M-4A4 cells resulted in an increase in RARβ4 and a decrease in RARβ2 mRNA. ATRA treatment of NM-2C5 cells increased the protein levels of the histone acetyl transferases p300 and CBP, suppressed the level of histone deacetylase and increased the level of acetylated histone H4. ATRA also decreased Bcl-2 and increased Bax and decreased VEGF. In contrast, the same treatment of the M-4A4 cells resulted in opposite effects. These results suggest that the effects of ATRA on the growth of the metastatic and non-metastatic breast cancer cell lines depend on the expression of RARβ isoforms and that the expression of RARβ4 may contribute to metastatic properties.

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