RETINOIC ACID DECREASES NUCLEAR TRIIODOTHYRONINE RECEPTOR EXPRESSION AND IMPAIRS AN EARLY STEP OF ADIPOSE DIFFERENTIATION IN THE THYROID HORMONE-SENSITIVE MOUSE OB-17 PREADIPOCYTE CELL-LINE

In the murine preadipocyte cell line Ob 17, T3 is known to be necessary at an early step of adipose differentiation for the expression of late phenotypes [lipogenic enzymes such as malic enzyme, glycerol-3-phosphate dehydrogenase (GPDH), etc.] and not necessary for the expression of lipoprotein lipase (LPL), which emerges earlier, at growth arrest. These cells contain nuclear T3 receptors, which mainly belong to products of the c-erbA-alpha gene and are down-regulated by T3. In this work, retinoic acid (RA) added to Ob 17 cells at growth arrest impaired morphological differentiation and the development of both late (malic enzyme and GPDH) and early (LPL) phenotypes regardless of whether T3 was added. T3 sensitized the cells to the inhibitory action of RA; the ED50 for GPDH activity was shifted from 0.5-mu-M to 3 nM in cells cultured with 1.5 nM T3. Later addition of RA (6 days after growth arrest) did not inhibit the differentiation. RA also brought out a marked and fast decrease in nuclear T3 receptors. This was observed whatever the stage of cell development and related to both a rapid decrease in the relative abundance of c-erbA-alpha-related mRNA species and an increased disapperance rate, suggesting the involvement of pre- and posttranslational events. RA and T3 acted additively in decreasing the T3 receptor and c-erbA-alpha mRNA levels. The effects of RA on T3 receptors were rapidly reversed after RA withdrawal; the reversal was large (75%) when RA was introduced at growth arrest and total when introduced later. The cell sensitivity to RA, considering the T3 receptors, was higher at growth arrest (ED50 for RA, 0.2 and 1.5-mu-M in assays with RA added at growth arrest and 5 days later, respectively). The results suggest intricated regulatory pathways between RA and T3 at an early step of adipose differentiation and also suggest that among different mechanisms through which RA may impair this differentiation, a decreased level of nuclear T3 receptors at an early period should play a role.