APOLIPOPROTEIN-D GENE INDUCTION BY RETINOIC ACID IS CONCOMITANT WITH GROWTH ARREST AND CELL-DIFFERENTIATION IN HUMAN BREAST-CANCER CELLS

We have examined the regulation by retinoic acid of the gene encoding apolipoprotein D (apoD), a human plasma protein belonging to the super family of the lipocalins that is produced by a specific subtype of hig hly differentiated breast carcinomas. Northern blot analysis revealed that all-trans-retinoic acid (RA) strongly induced the accumulation of apoD mRNA in T-47D and ZR-75-1 estrogen receptor-positive human breas t cancer cells in a time- and dose-dependent manner, while no inductiv e effect was observed in estrogen receptor-negative cell lines, includ ing MDA-MB-231 and MDA-MB-435. The effect of RA on apoD expression by T-47D cells was at least 12-fold more potent than the effect of the st eroids dihydrotestosterone and dexamethasone, which had been previousl y described as hormonal up-regulators of apoD expression in these cell s. A time course study demonstrated that the induction of apoD mRNA re ached a level of 15-fold over the untreated control cells after 48 h o f incubation in the presence of a 10(-7) M concentration of RA. A dose -response analysis showed that as little as 10(-13) M RA produced an a ccumulation of 5-fold over the control, while incubation of the cells in the presence of 10(-5) M RA induced a maximal accumulation of 24-fo ld over the control untreated cells. The induction of apoD mRNA was in dependent of the synthesis of proteins de novo, as demonstrated by the fact that the induction was also detected in the presence of cyclohex imide. The incubation of the cells in the presence of RA did not affec t significantly the stability of apoD mRNA By contrast, treatment of t he T-47D cells with RA produced an increase of approximately 8-fold in the rate of transcription of the apoD gene. Furthermore, treatment of the T-47D cells with RA induced the synthesis and secretion to the cu lture medium of apoD. This increased expression of apoD was accompanie d by an inhibition of cell proliferation and a progression through a m ore differentiated phenotype, suggesting that the mechanisms controlli ng RA-induced growth arrest, cell differentiation, and apoD synthesis may be directly coordinated in human breast cancer cells.