Influence of vitamin D3 deficiency and 1,25 dihydroxyvitamin D3 on de novo insulin biosynthesis in the islets of the rat endocrine pancreas

because 1,25 dihydroxyvitamin D 3 (1,25(OH) 2 D 3 ) is known to activate the biosynthesis of numerous proteins in various tissues, experiments were undertaken to compare the influence of 1,25(OH) 2 D 3 n vitro on both the secretion and biosynthesis of insulin in islets of Langerhans from both 4-week vitamin D 3 -deficient rats and normal rats. Islets were either incubated or perifused after a 6-h induction period in the presence of various concentrations of 1,25(OH) 2 D 3 from 10 -12 M, which was inactive in controls, to 10 -6 M. Experiments were performed in the presence of a non-labelled amino acid mixture, to favour protein synthesis. Tritiated tyrosine as added as tracer during glucose stimulation. The newly synthesised proteins, labelled with [ 3 H]tyrosine ere extracted by an acid-alcohol method and separated by gel chromatography adapted for low-molecular-weight proteins. Even in the presence ofthe amino acid mixture, the insulin response of the islets to 16.7 mM glucose was decreased by vitamin D 3 deficiency and improved by 1,25(OH) 2 D 3 . This beneficial effect did not occur in basal conditions, but only during glucose stimulation, and was observed in both phases of insulin release. Moreover, these effects disappeared in the presence of 5 x 10 -4 M cycloheximide, a protein biosynthesis inhibitor. Islets from vitamin D 3 -deficient rats exhibited a general decrease in the amount of de novo biosynthesised proteins and of [ 3 H]tyrosine-labelled insulin and proinsulin fractions. A 6-h period of 1,25(OH) 2 D 3 induction significantly improved the amount of de novo biosynthesised proteins, and particularly of newly synthesised insulin in response to a 2-h glucose stimulation. Calculation of the rate of conversion of newly synthesised proinsulin-like material to insuli as the [ 3 H]insulin/ [ 3 H]proinsulin-like material ratio provided evidence for a dose-dependent increase, induced by 1,25(OH) 2 D 3 , that could exceed that of normal islets. These data support the hypothesis that 1,25(OH) 2 D 3 in vitro not only facilitated the biosynthetic capacity of the β cell - which was highly induced during a 16.7-mM glucose stimulation, via a global activation of islets protein biosynthesis - but also produced an acceleration ofthe conversion of proinsulin to insulin.