Expression of 25-hydroxyvitamin D3-1$alpha;-hydroxylase in pancreatic islets*1

Abstract

A number of studies have suggested that Vitamin D has a potential role in the development/treatment of diabetes. These effects may be mediated by circulating levels of 1α,25(OH) 2 D 3 , but local production of 1α,25(OH) 2 D 3 , catalysed by the enzyme 25-hydroxyvitamin D 3 -1α-hydroxylase (1α-OHase), is also likely to be important. RT-PCR analyses demonstrated that both isolated rat islets and MIN6 cells (mouse insulin-secreting cell line, characteristic of β cells) expressed 1α-OHase mRNA. The transcript in both cell types was similar to that seen in HKC-8 cells (a renal cell line, which expresses 1α-OHase). Western blot analysis and immunolocalisation identified 1α-OHase protein in MIN6 cells and human pancreatic tissue. In addition, suspensions of rat islets were able to convert [ 3 H ]-25-hydroxyvitamin D 3 to [ 3 H ]-1α,25(OH) 2 D 3 , demonstrating 1α-OHase activity. Both cell systems expressed the Vitamin D receptor and 1α,25(OH) 2 D 3 (50 nM) evoked a rapid rise in [Ca 2+ ] i in MIN6 cells. This data clearly demonstrates islets are able to produce 1α,25(OH) 2 D 3 and respond rapidly to treatment with 1α,25(OH) 2 D 3 . Therefore, we would postulate that local production of 1α,25(OH) 2 D 3 maybe an important autocrine link between Vitamin D status and pancreatic function.