RETINOIC ACID RECEPTOR, CYTOSOLIC RETINOL-BINDING AND RETINOIC ACID-BINDING PROTEIN MESSENGER-RNA TRANSCRIPTS AND PROTEINS IN RAT INSULIN-SECRETING CELLS

To define the mechanism of vitamin A action at the beta-cell level, we tested for the presence of messenger RNA for retinoic acid receptors alpha, beta, and gamma; cytosolic retinol-binding protein; and cytosol ic retinoic acid-binding protein in RINm5F cells, an insulin-secreting cell line, and determined whether cytosolic retinol-binding protein a nd cytosolic retinoic acid-binding protein are present in isolated pur ified normal rat beta-cells. Northern blot analyses showed two transcr ipts of retinoic acid receptor alpha messenger RNA (3.8 and 2.4 kb), o ne transcript of retinoic acid receptor messenger RNA (3.8 kb), and on e transcript of cytosolic retinol-binding protein (0.9 kb) in RINm5F c ells. Ribonuclease protection assays also showed the presence of cytos olic retinol-binding protein and cytosolic retinoic acid-binding prote in in RINm5F cells. Quantitatively, cytosolic retinol-binding protein levels were 0.10 +/- 0.02 pg/mug total RNA. Using specific radioimmuno assays, normal isolated purified rat 13-cells contained CRBP (19.2 +/- 2.38) and cytosolic retinoic acid-binding protein (16 +/- 0.53 ng/10( 6) cells). The presence of message for retinoic acid receptors alpha a nd gamma, cytosolic retinol-binding protein, cytosolic retinoic acid-b inding protein, and the gene products of cytosolic retinol-binding pro tein and cytosolic retinoic acid-binding protein in insulin-secreting cells support a mechanism of vitamin A action and role for cytosolic a nd nuclear receptors at the beta-cell level similar to that suggested in nonendocrine cells. The presence of nuclear retinoic acid receptor alpha and gamma suggests that vitamin A may affect insulin secretion t hrough gene expression in the beta-cell.