Constitutive expression of 25-hydroxyvitamin D3-1α-hydroxylase in a transformed human proximal tubule cell line:: Evidence for direct regulation of vitamin D metabolism by calcium

Circulating levels of the active form of vitamin D, 1,25-dihydroxyvitamin D-3 (1,25-(OH)(2)D-3) are dependent on activity of the renal mitochondrial cytochrome P450 enzyme, 25-hydroxyvitamin D-3-1 alpha-hydroxylase (1 alpha-hydroxylase). Production of 1,25-(OH)(2)D-3 occurs predominantly in the renal proximal tubule, with 1 alpha-hydroxylase activity being impaired in renal insufficiency and renal disease. The expression and activity of 1 alpha-hydroxylase are tightly regulated in response to serum levels of PTH, calcium, phosphate, and 1,25(OH)(2)D-3 itself. As a consequence of this, the characterization of 1 alpha-hydroxylase in human renal tissue has proved difficult. In this study we have characterized constitutive 1 alpha-hydroxylase expression in a simian virus 40-transformed human proximal tubule cell line, HKC-8. Initial analyses of [H-3]25-hydroxyvitamin D-3 (250HD(3)) metabolism in these cells using straight and reverse phase HPLC revealed product peaks that coincided with authentic 1,25-(OH)(2)D-3 as well as 24,25-dihydroxyvitamin D-3 (24,25-(OH)(2)D-3). Enzyme kinetic studies indicated that the K-m for synthesis of 1,25-(OH)(2)D-3 in HKC-8 cells was 120 nmol/liter 25OHD(3), with a maximum velocity of 21 pmol/h/mg protein. This activity was inhibited by treatment with ketoconazole, but not diphenyl phenylenediamine. RT-PCR analysis of RNA from HKC-8 cells revealed a transcript similar in size to that observed in keratinocytes and primary cultures of human proximal tubule cells, and protein was detected by Western blot analysis. Synthesis of 1,25(OH)(2)D-3 was up regulated by treatment with forskolin (10 mu mol/liter, 24 h) and was down-regulated by 1,25-(OH)(2)D-3 (10 nmol/liter, 24 h). 1 alpha-Hydroxylase activity in HKC-8 cells was also sensitive to the concentration of calcium. Cells grown in low calcium (0.5 mmol/liter) showed a 4.8-fold induction of 1 alpha-hydroxylase, whereas treatment with medium containing high levels of calcium (2 mmol/liter) significantly inhibited 1,25-(OH)(2)D-3 production. These data suggest that direct effects of calcium on proximal tubule cells may be an important feature of the regulation of renal 1,25-(OH)(2)D-3 production.