Antigenic and catalytic disparity in the distribution of cytochrome P-450-dependent 25-hydroxyvitamin D3-1 alpha- and 24-hydroxylases

Protective Role of Vitamin D in Renal Tubulopathies

Vitamin D is tightly linked with renal tubular homeostasis: the mitochondria of proximal convoluted tubule cells are the production site of 1α,25-dihydroxyvitamin D3. Patients with renal impairment or tubular injury often suffer from chronic inflammation. This alteration comes from oxidative stress, acidosis, decreased clearance of inflammatory cytokines and stimulation of inflammatory factors. The challenge is to find the right formula for each patient to correctly modulate the landscape of treatment and preserve the essential functions of the organism without perturbating its homeostasis. The complexity of the counter-regulation mechanisms and the different axis involved in the Vitamin D equilibrium pose a major issue on Vitamin D as a potential effective anti-inflammatory drug. The therapeutic use of this compound should be able to inhibit the development of inflammation without interfering with normal homeostasis. Megalin-Cubilin-Amnionless and the FGF23-Klotho axis represent two Vitamin D-linked mechanisms that could modulate and ameliorate the damage response at the renal tubular level, balancing Vitamin D therapy with an effect potent enough to contrast the inflammatory cascades, but which avoids potential severe side effects.

1,25-Dihydroxyvitamin D-stimulated calmodulin binding proteins: a sustained effect on distal tubules

The tubular localization of 1,25-dihydroxyvitamin D[1,25(OH)(2)D(3)]-stimulated calmodulin binding proteins (CaMBP-Ds) in the rat kidney and the specificity of their induction were characterized to better understand renal responses to protracted 1,25(OH)(2)D(3) treatment in vivo. None of the other hormones tested (parathyroid hormone, calcitonin, estradiol-17beta, testosterone, progesterone, hydrocortisone, or dexamethasone) stimulated the CaMBP-Ds, whereas maximal 1,25(OH)(2)D(3) stimulation occurred after a 5- to 7-day treatment with 100 ng/day 1,25(OH)(2)D(3). With the exception of the more ubiquitously distributed CaMBP-D150, the CaMBP-Ds were localized in distal, but not proximal, tubule preparations. 1,25(OH)(2)D(3) induction of vitamin D receptors and the CaMBP-Ds was similar with respect to dose-response and time course. Finally, the CaMBP-Ds remained elevated for at least 4 wk after 1,25(OH)(2)D(3) withdrawal. Because the vitamin D-stimulated renal CaMBP-Ds are principally proteins of the distal tubule, they may be associated with renal regulation of Ca(2+) homeostasis. The sustained induction of CaMBP-Ds is important in addressing the question of whether their induction is a function of normal Ca(2+) homeostasis or a pathophysiological consequence of hypervitaminosis D and hypercalcemia.

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

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 1alpha-hydroxylase (1alpha-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 1alpha-hydroxylase is expressed exclusively in proximal convoluted tubules. With the recent cloning of 1alpha-hydroxylase, specific cRNA probes and in-house polyclonal antiserum have been used to determine the distribution of 1alpha-hydroxylase along the human nephron. Immunohistochemistry and in situ hybridization studies indicated strong expression of 1alpha-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 1alpha-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 alpha1-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 1alpha-hydroxylase expression under conditions of vitamin D sufficiency.