Cytoplasmic Retinoid-Binding Proteins and Retinoid Effects on Insulin Release in RINm5F β-Cells

Vitamin A (retinol) is required for insulin secretion, and retinoic acid substitutes for retinol in this function. To determine if retinol acts at the β-cell level, we assayed β-cells of the rat insulinoma (RINm5F) line for cytosolic retinol- and retinoic acid-binding proteins (CRBP and CRABP) by radioimmunoassay (RIA) and [³H]retinol and [³H]retinoic acid binding to cytosol extracts. Furthermore, we tested whether insulin release from cells was affected by addition of retinol or retinoic acid to culture medium. RINm5F cells were grown to near confluence before assay of CRBP and CRABP. Scatchard analysis showed the K_d for retinol to be ∼6 nM at a level of 4.5 pmol/mg protein or 300,000 sites/cell. Sucrose density-gradient assay showed single discrete peaks migrating at 2S for both retinol and retinoic acid. RIA of whole-cell extracts showed CRBP and CRABP levels of 5.27 ± 0.41 and 2.95 ± 0.75 pmol/mg protein, respectively. Retinol (1.75 μM) and retinoic acid (0.175 and 1.75 μM) increased KCI-induced insulin release. Considered together, the presence of CRBP and CRABP in a β-cell line and the increase in KCI-induced insulin release by retinol and retinoic acid are consistent with the idea that retinol has a functional role in insulin secretion and suggest a potential mechanism of action at the β-cell level similar to that observed in other retinoid-responsive cells.