Endocrine Abstracts

The active form of vitamin D, 1,25-dihydroxyvitamin D₃ (1,25D₃), is vital for calcium/phosphate homeostasis, bone cell differentiation and modification of immune responses. Synthesis of 1,25D₃ is catalysed by the mitochondrial cytochrome P450 enzyme 25-hydroxyvitamin D₃-1α-hydroxylase (1α-OHase; CYP27B1). Although 1α-OHase has been detected at several extra-renal sites, circulating levels of 1,25D₃ appear to reflect the renal activity of this enzyme. Previous studies have shown that parathyroid hormone (PTH) enhances 1α-OHase activity, whereas 1,25D₃ inhibits it. Serum calcium and phosphate also modulate the production of 1,25D₃, in part, by stimulation of PTH secretion by the parathyroid glands. In studies using a human proximal tubule (PCT) cell line, HKC-8, we have shown that in addition to regulation by 1,25D₃ and PTH, 1α-OHase is directly regulated by extra-cellular calcium, probably mediated via the calcium sensing receptor. This suggests that integrated signalling between calcium, PTH and 1α-OHase may occur independent of parathyroid responses. In recent studies we have shown that expression of 1α-OHase occurs throughout the nephron and is not restricted to the PCT. To study the regulation of this enzyme in the distal nephron, we used a human collecting duct cell line, HCD. 1α-OHase activity in HCD cells was also sensitive to PTH, 1,25D₃ and extra-cellular calcium. However, the greatest stimulation of 1α-OHase activity occurred in HCD cells after incubation with lipopolysaccharide (LPS), which was coincident with the expression of mRNA for both CD14 and Toll-like receptor 4 (Tlr4). In contrast, HKC-8 cells showed minimal response to LPS and did not appear to express Tlr4 mRNA. As yet the precise function of 1α-OHase in different regions of the nephron remains to be determined. However, more sensitive feedback regulation and immune induction of 1α-OHase in the HCD cells suggests a more localised role for 1,25D₃ production in the distal nephron.