Role of Local Versus Systemic Vitamin D Receptors in Vascular Calcification

Abstract

Objective-Calcitriol and various analogs are commonly used to suppress secondary hyperparathyroidism in chronic kidney disease but may also exacerbate vascular calcification. Although this could be because of increased intestinal calcium and phosphate absorption, direct effects through vitamin D receptors (VDRs) on vascular smooth muscle have also been proposed. Approach and Results-The role of these receptors was investigated by examining gene regulation in rat aortas treated with calcitriol ex vivo and in vivo and by transplanting aortas from VDR-null (VDR −/− ) mice into wild-type mice before induction of uremia and treatment with calcitriol. In cultured rat aortas, calcitriol increased the expression of mRNA for CYP24A1 but not mRNA for any bone-related or calcification-related genes. Gene expression in aortas in vivo was not altered by doses of calcitriol that promote calcification. Calcitriol markedly increased aortic calcification in uremic mice and this did not differ between VDR −/− aortic allografts and VDR +/+ recipient aortas. Conclusions-Calcitriol Lomashvili et al Calcitriol and Vascular Calcification 1476-fold increase in mRNA for CYP24A1, an enzyme that is induced by and metabolizes calcitriol 24 and used as a positive control. None of the genes, including CYP24A1, were upregulated in aortas from normal rats treated with calcitriol (100 ng/kg every 48 hours) in vivo ( Figure 1B). There was no aortic calcification. Although there was increased expression of some of these genes in uremic rats, there was no change with calcitriol treatment ( Figure 1C). The rats were fed a normal phosphate diet (to avoid any calcification that could have influenced the results) and received 4 injections of calcitriol 1, 3, 5, and 7 days before euthanization No aortic calcification occurred in vivo. Adenine-Induced Renal Failure in MiceInitial studies were performed to determine the optimal dosing of adenine, which differed with age. Mice were like rats in that weight loss is a reliable method to follow the degree of uremia. All animals have an initial weight loss as a result of decreased food intake, presumably because of palatability. Animals that do not become uremic regain this weight within 2 weeks, whereas subsequent weight loss is dependent on the degree of uremia. The adenine content is reduced as necessary to keep the weight >75% of baseline, the threshold for euthanasia required by the Institutional Animal Care and Use Committee. A dietary content of 0.75%, which is routinely used in rats, resulted in 100% mortality in mice within 2 weeks, regardless of age. A reduction to 0.6% still resulted in 100% mortality in the younger mice, whereas most of the older mice survived but required a dose reduction in >50%. At 0.45% adenine, 70% to 80% of the younger mice survived and only 20% required a dose reduction; older mice did not develop uremia. At contents of ≤0.3%, mice did not develop uremia and did not develop aortic calcification, despite a high-phosphorus diet (2%) and high doses of calcitriol (1000 ng/kg)....

Persistence of Vascular Calcification after Reversal of Uremia

“…This model has been used previously to distinguish between local and systemic factors in vascular calcification, also without any signs of cell-mediated rejection. 17,25 Calcification of the aortas renders the transplantation more challenging; however, despite that, the surgical survival rate was greater than 90%.…”

“…Injury associated with transplantation does not contribute to the persistence of the calcifications because we previously have shown no calcification in aortic allografts from normal mice. 17,25 No evidence was found for active resorption of the calcifications. Apatite is highly insoluble under physiological conditions and requires an acid environment for dissolution that, in bone, is created by osteoclasts.…”

“…25 Powdered adenine was mixed with standard rodent chow at 0.45% (wt/wt). Vascular calcification was induced by adding sodium phosphate (neutral mixture of Na 2 HPO 4 and NaH 2 PO 4 ) to the diet at a final content of 2% and by s.c. injections of calcitriol (1 mg/kg) 3 times per week.…”

Chronic kidney disease and bone metabolism

“…One major strategy to protect bone mechanical strength would be to maintain high circulating mineral levels. However, the frequent occurrence of vascular calcification is regarded as a more serious problem than bone metabolic disorders in CKD patients; therefore, the amount of exogenous 1,25D administration is often limited to avoid hypercalcemia [19].…”

Recent Highlights of ATVB

“…54 Similarly, vitamin D receptor antagonists show increased calcification in uremic animals and is thought to be because of an increase in circulating calcium or phosphate levels. However, Lomashvili et al 55 used vitamin D receptor-null mice and showed that treatment with calcitriol that activates vitamin D receptor promoted calcification through a systemic effect and not through a direct vascular action Epidemiological studies of asymptomatic individuals with type 2 diabetes mellitus have shown that an increase in CAC score by CT (Agatston score >100 U) and an abnormal myocardial perfusion scintigraphy were potent predictors of coronary events within 2.2 years compared with those with CAC score <100 AU. 56 In 2009, the MESA study determined the correlation of hemoglobin A1C (HbA1C) and CAC by CT and showed that although a small but positive association exists between HbA1C and carotid intimal medial thickness, this association between HbA1C and CAC was only observed in asymptomatic women.…”