Expression of 25-hydroxyvitamin D3-1α-hydroxylase in the human kidney

Abstract

The secosteroid hormone 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) plays a vital role in calcium metabolism, tissue differentiation, and normal bone growth. Biosynthesis of 1,25(OH)2D3 is catalyzed by the mitochondrial cytochrome P450 enzyme 25-hydroxyvitamin D3 1α-hydroxylase (1α- hydroxylase). Although activity of this enzyme has been described in several tissues, the kidneys are recognized to be the principal site of 1,25(OH)2D3 production. To date, enzyme activity studies using vitamin D-deficient animals have suggested that 1α-hydroxylase is expressed exclusively in proximal convoluted tubules. With the recent cloning of 1α-hydroxylase, specific cRNA probes and in-house polyclonal antiserum have been used to determine the distribution of 1α-hydroxylase along the human nephron. Immunohistochemistry and in situ hybridization studies indicated strong expression of 1α-hydroxylase protein and mRNA in the distal convoluted tubule, the cortical and medullary part of the collecting ducts, and the papillary epithelia. Lower expression was observed along the thick ascending limb of the loop of Henle and Bowman's capsule. Weaker and more variable expression of 1α-hydroxylase protein and mRNA was seen in proximal convoluted tubules, and no expression was observed in glomeruli or vascular structures. These data show for the first time the distribution of 1α- hydroxylase expression in normal human kidney. In contrast to earlier enzyme activity studies conducted in vitamin D-deficient animals, our data indicate that the distal nephron is the predominant site of 1α-hydroxylase expression under conditions of vitamin D sufficiency.