RETINOIC ACID-INDUCED EXPRESSION OF APOLIPOPROTEIN-D AND CONCOMITANT GROWTH ARREST IN HUMAN BREAST-CANCER CELLS ARE MEDIATED THROUGH A RETINOIC ACID RECEPTOR RAR-ALPHA-DEPENDENT SIGNALING PATHWAY

Apolipoprotein D (apoD) is a human plasma protein, belonging to the li pocalin superfamily, that is produced by a specific subtype of highly differentiated breast carcinomas and that is strongly up-regulated by retinoic acid (RA) in breast cancer cells. Ln this work, we have exami ned the molecular mechanisms mediating the induction of apoD gene expr ession by retinoids in T-47D human breast cancer cells, Northern blot analysis revealed that Ro40-6055, a synthetic retinoid that selectivel y binds and activates the retinoic acid receptor RAR alpha induced the accumulation of apoD mRNA in breast cancer cells in a time- and dose dependent manner. The time course analysis demonstrated that apoD mRNA was induced 14-fold over control cells after 48 h of incubation with 10(-8) M Ro40-6055. As little as 10(-11) M of this retinoid induced ap oD mRNA 8-fold over the control, whereas incubation with 10(-7) RI Ro4 0-6055 induced maximally 15-fold over control cells, RAR alpha-selecti ve antagonists counteracted the inductive effects of all-trans-RA, 9-c is-RA, and Ro40-6055 on the expression of apoD, when present at the sa me concentration as the retinoid agonists. By contrast, RAP beta-, RAR gamma-, and RXR-selective retinoids did not affect apoD gene expressi on. The retinoid agonist Ro40-6055 had an antiproliferative effect on T-47D cells, with maximal growth inhibition of approximately 60% obtai ned after 7 days of incubation with 10(-7) RI, This antiproliferative effect could be counteracted by a 100-fold excess of the antagonist Ro 41-5253. Treatment of the cells with retinoids that do not bind the nu clear retinoic acid receptors did not affect apoD expression, despite the fact that they did have a strong antiproliferative effect on T-47D cells. On the basis of these results, a role for RAR alpha on apoD ge ne expression induction by retinoids in breast cancer cells is propose d.