Reduced hepatic synthesis of calcidiol in uremia

Abstract

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.

Figure 1.

Vitamin D₃ biotransformation pathway.

Figure 2.

Increases in serum creatinine and PTH correlate with reduction in serum 25(OH)D₃ concentration. Analyses are performed to determine the correlation between the serum concentrations of creatinine and 25(OH)D₃ (r² = 0.78, P < 0.001; A) and the correlation between the serum concentrations of intact PTH and 25(OH)D₃ (r² = 0.40, P < 0.05; B) in rats with CRF.

Figure 3.

In vivo 25(OH)D₃ production decreases in rats with CRF. (A and B) The in vivo C-25-hydroxylation of vitamin D₃ is evaluated in control rats (□) and (■) rats with CRF after the injection of either vitamin D₃ (A) or 1α-hydroxyvitamin D₃ (B)_. (A) The production of 25(OH)D₃ is measured at the time of injection (T = 0) and 48 hours later. (Insert) The amount of 25(OH)D₃ produced in 48 hours. *P < 0.05 versus control rats. (B) Serum concentrations of 1,25(OH)₂D₃ are compared at the time of injection (T = 0) and 3 hours later. *P < 0.05 when compared with control rats.

Figure 4.

Uremic serum contains factors reducing 25-hydroxylation of vitamin D₃ by hepatocytes. Concentrations of 25(OH)D₃ in supernatant of cultured hepatocytes incubated with serum of control rats (□) or rats with CRF (■) in the absence or presence of vitamin D₃. (Insert) The increment in 25(OH)D₃ production when vitamin D₃ is added. *P < 0.05 versus control rats.

Figure 5.

CRF downregulates the protein and mRNA expression of various CYP450s implicated in 25(OH)D₃ hydroxylation and this downregulation can be prevented by PTX. (A and B) Impact of CRF on CYP450 protein expression (A) and mRNA expression (B) with or without previous PTX. Protein and mRNA encoding for various cytochrome isoforms are compared in control rats (□), rats with CRF (■), control rats with PTX (▨), and rats with CRF and PTX () after quantification by Western blot or quantitative PCR in the liver. (C) Protein and mRNA encoding for CYP24A1 are compared in control rats (□ and rats with CRF (■) after quantification by Western blot or quantitative PCR in the kidney. *P < 0.05 versus control rats; ^†P < 0.05 versus rats with CRF; ^‡P < 0.05 versus control rats with PTX.

Figure 6.

Serum PTH concentration regulates 25-hydroxylation of vitamin D₃. (A) Effect of PTH infusion on the in vivo production of 25(OH)D₃ in rats. Serum 25(OH)D₃ concentrations are measured at T = 0 (□) and T = 48 hours (■) on either control rats (CTL) or rats with PTX. Infusion pumps deliver either rat PTH (1-34) in 2% cysteine-HCl at 0.06 nmol/kg per h or 2% cysteine-HCl (vehicle) intravenously in rats with PTX. *P < 0.05 versus T = 0. (B and C) Effect of PTH infusion on CYP450 protein (B) and mRNA expression (C) in CTL rats (□), rats with PTX and vehicle (■), or rats with PTX and PTH pumps () after quantification by Western blot or quantitative PCR in the liver. *P < 0.05 versus CTL rats.