Positive and negative regulations of the renal 25-hydroxyvitamin D3 1α-hydroxylase gene by parathyroid hormone, calcitonin, and 1α,25(OH)2D3 in intact animals

Reflecting the prime role of 1 alpha,25(OH)(2)D-3 in calcium homeostasis, the activity of 25-hydroxyvitamin D-3 1 alpha-hydroxylase, a key enzyme for 1 alpha,25(OH)(2)D-3 biosynthesis, is tightly regulated by 1 alpha,25(OH)(2)D-3, PTH and calcitonin. Its significant activity is found in kidney, though the enzymatic activity is also reported in extra-renal tissues. In the present study. we found that the 1 alpha-hydroxylase gene abundantly expresses in kidney, and at low levels in other tissues and in some cell lines. Positive and negative regulations of 1 alpha-hydroxylase gene by PTH, calcitonin, or 1 alpha,25(OH)(2)D-3 were observed at transcriptional levels in kidneys of animals and in a mouse proximal tubule cell line. Moreover, the protein kinase A inhibitor abrogated the PTH-mediated positive regulation. In mice lacking the vitamin D receptor, the 1 alpha-hydroxylase gene expression was overinduced, and the inducible effect of either PTH or calcitonin, but not the repression by 1 alpha,25(OH)(2)D-3, was evident. Thus, vitamin D receptor is essential for the negative regulation by 1 alpha,25(OH)(2)D-3. Moreover, we demonstrate that renal 1 alpha-hydroxylase gene expression in chronic renal failure model rats was decreased and the positive effect by PTH and calcitonin was diminished. The present study demonstrates that PTH and calcitonin positively regulate renal 1 alpha-hydroxylase gene expression via PKA-dependent and independent pathway, respectively, and that 1 alpha,25(OH)(2)D-3 negatively regulates it mediated by vitamin D receptor. Furthermore, in a moderate state of chronic renal failure, renal cells expressing the 1 alpha-hydroxylase gene appear to have diminished potential in response to PTH and calcitonin.