1,25-DIHYDROXYVITAMIN-D(3) AND PANCREATIC BETA-CELL FUNCTION - VITAMIN-D RECEPTORS, GENE-EXPRESSION, AND INSULIN-SECRETION

Previous studies have indicated that the pancreas has receptors specif ic for 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] and that 1,25(OH)2D3 in creases insulin secretion in vitamin D-deficient rats. In this study w e report that in vitamin D-replete, but calcium-deficient, rats in whi ch 1,25-(OH)2D3 levels are elevated, insulin secretion is not altered. In addition, in in vitro studies 1,25-(OH)2D3 at concentrations of 10 (-10)-10(-7) M was consistently found to inhibit insulin secretion fro m islets of vitamin D-replete rats or from the rat insulinoma beta-cel l line RIN 1046-38. The RIN cell line was found to contain both vitami n D receptors and calbindin-D28k (CaBP-D28k) protein and mRNA. In RIN cells, treatment with sodium butyrate (2 mm for 3 days) induces a more islet phenotype, as indicated by increased insulin content and secret ion and increased insulin gene expression. 1,25-(OH)2D3 treatment (50- 100 nm for 48 or 72 h) had no effect on the enhanced levels of insulin secreted in the presence of butyrate. However, 2 mm sodium butyrate i nduced CaBP-D28k protein (4-fold; control, 0.8 +/- 0.2; sodium butyrat e, 3.5 +/- 0.1 mug/mg protein) and mRNA (3-fold) in the RIN cell line, in accord with the induction by butyrate of insulin content and secre tion and beta-cell differentiation, suggesting a possible role for CaB P-D28k in these processes. Although 1,25-(OH)2D3, unlike butyrate, did not enhance insulin secretion, both 1,25-(OH)2D3 (100 nm) and butyrat e (2 mM) inhibited RIN cell growth (to 69% and 28% of the control, res pectively), and butyrate and 1,25-(OH)2D, in combination led to a furt her inhibition of cell growth (to 13% of the control). In response to 1,25-(OH)2D3 (10 nM for 72 h), vitamin D receptors were up-regulated 3 13% in RIN cells [control, 37 +/- 2; 1,25-(OH)2D3 treated, 115 +/- 5 f mol/mg protein]. In conclusion, 1) contrary to previous studies in the vitamin D-deficient rat, our findings indicate that 1,25-(OH)2D3 acti on does not necessarily result in enhanced insulin secretion; 2) inhib ition of cell growth and up-regulation of vitamin D receptors by 1,25- (OH)2D3 suggest that parameters in addition to insulin secretion can b e affected by 1,25-(OH)2D3 in the beta-cell; 3) the RIN beta-cell line provides a novel in vitro system for studying the effect of the vitam in D endocrine system on pancreatic islet physiology.