Retinoic Acid Receptor, Cytosolic Retinol-Binding and Retinoic Acid-Binding Protein mRNA Transcripts and Proteins in Rat Insulin-Secreting Cells

To define the mechanism of vitamin A action at the β-cell level, we tested for the presence of messenger RNA for retinoic acid receptors α, β, and γ; cytosolic retinol-binding protein; and cytosolic retinoic acid-binding protein in RINm5F cells, an insulin-secreting cell line, and determined whether cytosolic retinol-binding protein and cytosolic retinoic acid-binding protein are present in isolated purified normal rat β-cells. Northern blot analyses showed two transcripts of retinoic acid receptor α messenger RNA (3.8 and 2.4 kb), one transcript of retinoic acid receptor messenger RNA (3.8 kb), and one transcript of cytosolic retinol-binding protein (0.9 kb) in RINm5F cells. Ribonuclease protection assays also showed the presence of cytosolic retinol-binding protein and cytosolic retinoic acid-binding protein in RINm5F cells. Quantitatively, cytosolic retinol-binding protein levels were 0.10 ± 0.02 pg/micrograms total RNA. Using specific radioimmunoassays, normal isolated purified rat β-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 α and γ, cytosolic retinol-binding protein, cytosolic retinoic acid-binding protein, and the gene products of cytosolic retinol-binding protein and cytosolic retinoic acid-binding protein in insulin-secreting cells support a mechanism of vitamin A action and role for cytosolic and nuclear receptors at the β-cell level similar to that suggested in nonendocrine cells. The presence of nuclear retinoic acid receptor a. and 7 suggests that vitamin A may affect insulin secretion through gene expression in the β-cell.