Christian Demers

Reduced Hepatic Synthesis of Calcidiol in Uremia

Calcidiol insufficiency is highly prevalent in chronic kidney disease (CKD), but the reasons for this are incompletely understood. CKD associates with a decrease in liver cytochrome P450 (CYP450) enzymes, and specific CYP450 isoforms mediate vitamin D(3) C-25-hydroxylation, which forms calcidiol. Abnormal levels of parathyroid hormone (PTH), which also modulates liver CYP450, could also contribute to the decrease in liver CYP450 associated with CKD. Here, we evaluated the effects of PTH and uremia on liver CYP450 isoforms involved in calcidiol synthesis in rats. Uremic rats had 52% lower concentrations of serum calcidiol than control rats (P < 0.002). Compared with controls, uremic rats produced 71% less calcidiol and 48% less calcitriol after the administration of vitamin D(3) or 1alpha-hydroxyvitamin D(3), respectively, suggesting impaired C-25-hydroxylation of vitamin D(3). Furthermore, uremia associated with a reduction of liver CYP2C11, 2J3, 3A2, and 27A1. Parathyroidectomy prevented the uremia-associated decreases in calcidiol and liver CYP450 isoforms. In conclusion, these data suggest that uremia decreases calcidiol synthesis secondary to a PTH-mediated reduction in liver CYP450 isoforms.

Calcitriol regulates the expression of the genes encoding the three key vitamin D3 hydroxylases and the drug‐metabolizing enzyme CYP3A4 in the human fetal intestine

background and aims The human fetal jejunum has been shown to harbour the vitamin D3 (D3) nuclear receptor (VDRn) and to be responsive to calcitriol/1,25‐dihydroxyvitamin D3[1,25(OH)2D3] through modulation of proliferation and differentiation processes. The aim of the study was to evaluate the presence as well as the effect of 1,25(OH)2D3 exposure on the expression levels of the three key D3‐hydroxylase gene transcripts (25‐hydroxylase, CYP27A; 24‐hydroxylase, CYP24; 1α‐hydroxylase, CYP27B1) as well as that of the 1,25(OH)2D3‐responsive endobiotic/xenobiotic metabolizing enzyme CYP3A4 (which is also considered a major detoxifiying enzyme) in the human proximal and distal intestine.methods Specimens from normal fetuses ranging from 15 to 20 weeks of gestation were obtained following elective termination of normal pregnancies. Intestinal explants were cultured for a period of 24 h or 48 h with 10−7 m 1,25(OH)2D3. All data were compared to paired‐control cultures without 1,25(OH)2D3. Total RNA was extracted and cDNA synthesized by RT‐PCR. The cDNA obtained was amplified by radioactive PCR, the signal intensity evaluated by densitometric analyses and expressed in relation to the levels of GAPDH.

Influence of the in vivo calcium status on cellular calcium homeostasis and the level of the calcium-binding protein calreticulin in rat hepatocytes.

Little attention has been given to the consequences of the in vivo calcium status on intracellular calcium homeostasis despite several pathological states induced by perturbations of the in vivo calcium balance. The aim of these studies was to probe the influence of an in vivo calcium deficiency on the resting cytoplasmic Ca2+ concentration and the inositol-1,4,5-trisphosphate-sensitive Ca2+ pools. Studies were conducted in hepatocytes (a cell type well characterized for its cellular Ca2+ response) isolated from normal and calcium-deficient rats secondary to vitamin D depletion. Both resting cytoplasmic Ca2+ concentration and Ca2+ mobilization from inositol-1,4,5-trisphosphate-sensitive cellular pools were significantly lowered by calcium depletion. In addition, Ca deficiency was shown to significantly reduce calreticulin messenger RNA and protein levels but calcium entry through store-operated calcium channels remained unaffected, indicating that the Ca2+ entry mechanisms are still fully operational in calcium deficiency. The effects of calcium deficiency on cellular calcium homeostasis were reversible by repletion with oral calcium feeding alone or by the administration of the calcium-regulating hormone 1,25-dihydroxyvitamin D3, further strengthening the tight link between extra- and intracellular calcium. These data, therefore, challenge the currently prevailing hypothesis that extracellular Ca2+ has no significant impact on cellular Ca2+ by demonstrating that despite the large Ca2+ gradient between extra- and intracellular Ca2+ concentrations, calcium deficiency in vivo significantly alters the hormone-sensitive cellular calcium homeostasis.