Leptin attenuates gene expression for renal 25-hydroxyvitamin D3-1α-hydroxylase in mice via the long form of the leptin receptor

Leptin, the ob gene product secreted by adipocytes, controls overall energy balance. We previously showed that leptin administration to leptin-deficient obese (ob/ob) mice suppressed mRNA expression and activity of renal 25-hydroxyvitamin D3-1α-hydroxylase (CYP27B1). In leptin receptor-deficient (db/db) mice, we presently examined whether leptin affects 1α-hydroxylase expression in renal tubules through the active form of the leptin receptor (ObRb). Elevated serum concentrations of calcium and 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] in untreated ob/ob mice showed sharp reduction with leptin administration (4 mg/kg, i.p. every 12 h for 2 days); no such reduction of elevation occurred in db/db mice. ObRb mRNA was expressed in kidney, brain, fat, lung, and bone in wild-type and ob/ob mice, but not db/db mice. The ob/ob and db/db mice showed large increases in renal 1α-hydroxylase mRNA expression and activity. Leptin administration (4 mg/kg) completely abrogated these increases in ob/ob but not db/db mice. Renal 25-hydroxyvitamin D3-24-hydroxylase (CYP24) mRNA synthesis also was greatly elevated in ob/ob and db/db mice; excesses decreased significantly with leptin administration in ob/ob mice, but increased in db/db mice. Renal tubular cells in primary culture expressed mRNAs including proximal tubules markers (1α-hydroxylase and megalin), parathyroid hormone receptor, and vitamin D receptor. Calcitonin receptor mRNA, synthesized mainly in distal tubules, was scant, indicating that most cultured cells were from proximal tubules. Cells did not express ObRb mRNA. Forskolin exposure at 10−6 M for 3 or 6 h significantly increased 1α-hydroxylase mRNA. Leptin at 10−6 M did not change mRNA expression in either presence or absence of forskolin. Accordingly, leptin attenuates renal 1α-hydroxylase gene expression through ObRb. Furthermore, leptin appears to act indirectly on renal proximal tubules to regulate 1α-hydroxylase gene expression.