Background: Recent studies have shown that renal expression of 25-hydroxyvitamin D3-1alpha-hydroxylase (1alpha-OHase) is not restricted to proximal tubules. To investigate the significance of this expression, we characterized the regulation of 1alpha-OHase expression and activity in a human cortical collecting duct cell line (HCD).
Methods: Expression of 1alpha-OHase mRNA and protein was assessed by reverse transcription-polymerase chain reaction (RT-PCR) and Western blot analyses. Enzyme activity was quantified using 25-hydroxyvitamin D3 as the substrate; conversion to 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] and 24,25-dihydroxyvitamin D3 was then determined by thin-layer chromatography.
Results: HCD cells expressed mRNA and protein for 1alpha-OHase. However, basal 1,25(OH)2D3 production was lower than that observed in proximal tubule HKC-8 cells. In both cell lines, synthesis of 1,25(OH)2D3 was increased by forskolin, parathyroid hormone, and low calcium medium. Conversely, treatment with 1,25(OH)2D3 itself decreased 1alpha-OHase activity. This effect was more pronounced in HCD cells, which also demonstrated significantly higher levels of 24-hydroxylase activity. The most striking induction of 1alpha-OHase activity was observed in the HCD cells following incubation with lipopolysaccharide, which was coincident with the expression of mRNA for both CD14 and Toll-like receptor 4.
Conclusions: These results highlight the capacity for synthesis of 1,25(OH)2D3 in cells from more distal areas of the nephron. However, more sensitive feedback regulation and immune induction of 1alpha-OHase in the HCD cells suggest a more localized role for 1,25(OH)2D3 production in the distal nephron.