Expression of 25-hydroxyvitamin D3-1alpha-hydroxylase in pancreatic islets.

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 1alpha,25(OH)(2)D(3), but local production of 1alpha,25(OH)(2)D(3), catalysed by the enzyme 25-hydroxyvitamin D(3)-1alpha-hydroxylase (1alpha-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 beta cells) expressed 1alpha-OHase mRNA. The transcript in both cell types was similar to that seen in HKC-8 cells (a renal cell line, which expresses 1alpha-OHase). Western blot analysis and immunolocalisation identified 1alpha-OHase protein in MIN6 cells and human pancreatic tissue. In addition, suspensions of rat islets were able to convert [3H]-25-hydroxyvitamin D(3) to [3H]-1alpha,25(OH)(2)D(3), demonstrating 1alpha-OHase activity. Both cell systems expressed the Vitamin D receptor and 1alpha,25(OH)(2)D(3) (50nM) evoked a rapid rise in [Ca(2+)](i) in MIN6 cells. This data clearly demonstrates islets are able to produce 1alpha,25(OH)(2)D(3) and respond rapidly to treatment with 1alpha,25(OH)(2)D(3). Therefore, we would postulate that local production of 1alpha,25(OH)(2)D(3) maybe an important autocrine link between Vitamin D status and pancreatic function.