Vascular calcification (VC) is a complication of chronic kidney disease that predicts morbidity and mortality. Uremic serum promotes VC, but the mechanism involved is unknown. A role for 1,25(OH) D in VC has been proposed, but the mechanism is unclear because both low and high levels have been shown to increase it. In this work we investigate the role of 1,25(OH) D produced in vascular smooth muscle cells (VSMCs) in VC. Rats with subtotal nephrectomy and kidney recipient patients showed increased arterial expression of 1α-hydroxylase in vivo. VSMCs exposed in vitro to serum obtained from uremic rats also showed increased 1α-hydroxylase expression. Those increases were parallel to an increase in VC. After 6 days with high phosphate media, VSMCs overexpressing 1α-hydroxylase show significantly higher calcium content and RUNX2 expression than control cells. 1α-hydroxylase null mice (KO) with subtotal nephrectomy and treated with calcitriol (400 ng/kg) for 2 weeks showed significantly lower levels of vascular calcium content, Alizarin red staining, and RUNX2 expression than wild-type (WT) littermates. Serum calcium, phosphorus, blood urea nitrogen (BUN), PTH, and 1,25(OH) D levels were similar in both calcitriol-treated groups. In vitro, WT VSMCs treated with uremic serum also showed a significant increase in 1α-hydroxylase expression and higher calcification that was not observed in KO cells. We conclude that local activation of 1α-hydroxylase in the artery mediates VC observed in uremia. © 2016 American Society for Bone and Mineral Research.

The final step in vitamin D activation is catalyzed by 1-alpha-hydroxylase (CYP27B1). Chronic kidney disease (CKD) is characterized by low levels of both 25(OH)D3 and 1,25(OH)2D3 provoking secondary hyperparathyroidism (2HPT). Therefore, treatments with active or native vitamin D compounds are common in CKD to restore 25(OH)D3 levels and also to decrease PTH. This study evaluates the dose of 25(OH)D3 that restores parathyroid hormone (PTH) and calcium levels in a model of CKD in CYP27B1-/- mice. Furthermore, we compare the safety and efficacy of the same dose in CYP27B1+/+ animals. The dose needed to decrease PTH levels in CYP27B1-/- mice with CKD was 50 ng/g. That dose restored blood calcium levels without modifying phosphate levels, and increased the expression of genes responsible for calcium absorption (TRPV5 and calbindinD- 28K in the kidney, TRPV6 and calbindinD-9k in the intestine). The same dose of 25(OH)D3 did not modify PTH in CYP27B1+/+ animals with CKD. Blood calcium remained normal, while phosphate increased significantly. Blood levels of 25(OH)D3 in CYP27B1-/- mice were extremely high compared to those in CYP27B1+/+ animals. CYP27B1+/+ animals with CKD showed increases in TRPV5, TRPV6, calbindinD-28K and calbindinD-9K, which were not further elevated with the treatment. Furthermore, CYP27B1+/+ animals displayed an increase in vascular calcification. We conclude that the dose of 25(OH)D3 effective in decreasing PTH levels in CYP27B1-/- mice with CKD, has a potentially toxic effect in CYP27B1+/+ animals with CKD.