Since both the release and de novo biosynthesis of insulin are severely decreased by vitamin D3 deficiency and improved by 1, 25-dihydroxyvitamin D3 (1,25(OH)2D3) repletion following a 6-h delay in the rat, the present experiments investigated the effects of vitamin D3 deficiency on the biosynthesis of heavier molecular weight proteins using electrophoretic separation. Gel protein staining by Coomassie blue showed very different profiles for islets protein production from 4-week vitamin D3-deficient rats compared with normal islets. The pattern was characterised by a decrease in high molecular weight proteins, concomitantly accompanied by an increase in low molecular weight proteins. This tendency was partially reversed in vivo by 1,25(OH)2D3 repletion treatment for 7 days and was evident after only 16 h of treatment. In parallel with these in vivo observations, which represent a static index of islets protein production, a kinetic study was performed in vitro by a double-labelling method allowing us to measure the de novo synthesis of proteins in islets during a strong 16.7 mM glucose stimulation. Comparison of 3H and 14C labelled samples was achieved via coelectrophoresis to avoid experimental artefacts. The study of the ratio of d.p.m. 3H/d.p.m. 14C for each molecular weight protein in islets stimulated by 16.7 mM glucose (versus basal 4.2 mM glucose) showed an increase in the height of certain peaks: 150, 130 and 8.5 kDa. Under the same conditions, islets from 4-week vitamin D3-deficient rats (versus normal islets) presented a large deficit of numerous newly synthesised proteins and particularly those implicated in the response to glucose stimulation. In vitro repletion of 1,25(OH)2D3 tended to reverse, at least in part, the deleterious effect of vitamin D3 deficiency on the de novo protein synthesis of islets but these effects were gradual. Indeed, there was no detectable effect at 2 h incubation, but 1,25(OH)2D3 increased the 60 to 65 kDa, 55 kDa, and 9 to 8 kDa molecular mass proteins at 4 h, and increased the level of most newly synthesised proteins at 6 h. These data support the hypothesis of a beneficial genomic influence of 1,25(OH)2D3 that occurs progressively within the islets of Langerhans and which may prepare the beta cells for an enhanced response to glucose stimulation.
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