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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)₂D₃, but local production of 1α,25(OH)₂D₃, catalysed by the enzyme 25-hydroxyvitamin D₃-1α-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 [³H ]-25-hydroxyvitamin D₃ to [³H ]-1α,25(OH)₂D₃, demonstrating 1α-OHase activity. Both cell systems expressed the Vitamin D receptor …