Absence of the Vitamin D Receptor Inhibits Atherosclerotic Plaque Calcification in Female Hypercholesterolemic Mice

Potential impact of the steroid hormone, vitamin D, on the vasculature

The role of vitamin D in the cardiovascular system is complex because it regulates expression of genes involved in diverse metabolic processes. Although referred to as a vitamin, it is more accurately considered a steroid hormone, because it is produced endogenously in the presence of ultraviolet light. It occurs as a series of sequentially activated forms, here referred to as vitamin D-hormones. A little-known phenomenon, based on pre-clinical data, is that its biodistribution and potential effects on vascular disease likely depend on whether it is derived from diet or sunlight. Diet-derived vitamin D-hormones are carried in the blood, at least in part, in chylomicrons and lipoprotein particles, including low-density lipoprotein. Since low-density lipoprotein is known to accumulate in the artery wall and atherosclerotic plaque, diet-derived vitamin D-hormones may also collect there, and possibly promote the osteochondrogenic mineralization associated with plaque. Also, little known is the fact that the body stores vitamin D-hormones in adipose tissue with a half-life on the order of months, raising doubts about whether the use of the term "daily requirement" is appropriate. Cardiovascular effects of vitamin D-hormones are controversial, and risk appears to increase with both low and high blood levels. Since low serum vitamin D-hormone concentration is reportedly associated with increased cardiovascular and orthopedic risk, oral supplementation is widely used, often together with calcium supplements. However, meta-analyses show that oral vitamin D-hormone supplementation does not protect against cardiovascular events, findings that are also supported by a randomized controlled trial. These considerations suggest that prevalent recommendations for vitamin D-hormone supplementation for the purpose of cardiovascular protection should be carefully reconsidered.

Roles of Nuclear Receptors in Vascular Calcification

Vascular calcification is defined as an inappropriate accumulation of calcium depots occurring in soft tissues, including the vascular wall. Growing evidence suggests that vascular calcification is an actively regulated process, sharing similar mechanisms with bone formation, implicating both inhibitory and inducible factors, mediated by osteoclast-like and osteoblast-like cells, respectively. This process, which occurs in nearly all the arterial beds and in both the medial and intimal layers, mainly involves vascular smooth muscle cells. In the vascular wall, calcification can have different clinical consequences, depending on the pattern, localization and nature of calcium deposition. Nuclear receptors are transcription factors widely expressed, activated by specific ligands that control the expression of target genes involved in a multitude of pathophysiological processes, including metabolism, cancer, inflammation and cell differentiation. Some of them act as drug targets. In this review we describe and discuss the role of different nuclear receptors in the control of vascular calcification.

Vascular Calcification in Rodent Models-Keeping Track with an Extented Method Assortment

Simple Summary

Arterial vessel diseases are the leading cause of death in the elderly and their accelerated pathogenesis is responsible for premature death in patients with chronic renal failure. Since no functioning therapy concepts exist so far, the identification of the main signaling pathways is of current research interest. To develop therapeutic concepts, different experimental rodent models are needed, which should be subject to the 3R principle of Russel and Burch: “Replace, Reduce and Refine”. This review aims to summarize the current available experimental rodent models for studying vascular calcification and their quantification methods.

Abstract

Vascular calcification is a multifaceted disease and a significant contributor to cardiovascular morbidity and mortality. The calcification deposits in the vessel wall can vary in size and localization. Various pathophysiological pathways may be involved in disease progression. With respect to the calcification diversity, a great number of research models and detection methods have been established in basic research, relying mostly on rodent models. The aim of this review is to provide an overview of the currently available rodent models and quantification methods for vascular calcification, emphasizing animal burden and assessing prospects to use available methods in a way to address the 3R principles of Russel and Burch: “Replace, Reduce and Refine”.

Expansive Vascular Remodeling and Increased Vascular Calcification Response to Cholecalciferol in a Murine Model of Obesity and Insulin Resistance

Objective- We hypothesized that ob/ob mice develop expansive vascular remodeling associated with calcification. Approach and Results- We quantified and investigated mechanisms of vascular remodeling and vascular calcification in ob/ob mice after vitamin D₃(VD) stimulation or PBS (control), compared with C57BL/6 mice. Both ob/ob (OBVD [VD-treated ob/ob mice]) and C57BL/6 (C57VD [VD-treated C57BL/6 mice]) received 8×10³ IU/day of intraperitoneal VD for 14 days. Control ob/ob (OBCT [PBS-treated ob/ob mice]) and C57BL/6 (C57CT [PBS-treated C57BL/6 mice]) received intraperitoneal PBS for 14 days. Hypervitaminosis D increased the external and internal elastic length in aortae from OBVD, resulting in increased total vascular area and lumen vascular area, respectively, which characterizes expansive vascular remodeling. OBVD decreased the aortic wall thickness, resulting in hypotrophic vascular remodeling. We demonstrated increased collagen deposition, elastolysis, and calcification in aortae from OBVD. Our results showed a positive correlation between expansive vascular remodeling and vascular calcification in OBVD. We demonstrated increased serum calcium levels, augmented Bmp (bone morphogenetic protein)-2 and osteochondrogenic proteins expression in OBVD aortae. Furthermore, aortae from OBVD increased oxidative stress, coincidently with augmented in situ MMP (matrix metalloproteinase) activity and exhibited no VDR (VD receptor) inhibition after VD. Conclusions- Our data provide evidence that obese and insulin-resistant mice (ob/ob) developed expansive hypotrophic vascular remodeling correlated directly with increased vascular calcification after chronic VD stimulation. Positive hypotrophic vascular remodeling and vascular calcification in this mouse model is possibly mediated by the convergence of absence VDR downregulation after VD stimulation, increased reactive oxygen species generation, and MMP activation.

Steroid Hormone Vitamin D: Implications for Cardiovascular Disease

Understanding of vitamin D physiology is important because about half of the population is being diagnosed with deficiency and treated with supplements. Clinical guidelines were developed based on observational studies showing an association between low serum levels and increased cardiovascular risk. However, new randomized controlled trials have failed to confirm any cardiovascular benefit from supplementation in the general population. A major concern is that excess vitamin D is known to cause calcific vasculopathy and valvulopathy in animal models. For decades, administration of vitamin D has been used in rodents as a reliable experimental model of vascular calcification. Technically, vitamin D is a misnomer. It is not a true vitamin because it can be synthesized endogenously through ultraviolet exposure of the skin. It is a steroid hormone that comes in 3 forms that are sequential metabolites produced by hydroxylases. As a fat-soluble hormone, the vitamin D-hormone metabolites must have special mechanisms for delivery in the aqueous bloodstream. Importantly, endogenously synthesized forms are carried by a binding protein, whereas dietary forms are carried within lipoprotein particles. This may result in distinct biodistributions for sunlight-derived versus supplement-derived vitamin D hormones. Because the cardiovascular effects of vitamin D hormones are not straightforward, both toxic and beneficial effects may result from current recommendations.

Deletion of a Distal RANKL Gene Enhancer Delays Progression of Atherosclerotic Plaque Calcification in Hypercholesterolemic Mice

Receptor activator of NF-κB ligand (RANKL) is a TNF-like cytokine which mediates diverse physiological functions including bone remodeling and immune regulation. RANKL has been identified in atherosclerotic lesions; however, its role in atherosclerotic plaque development remains elusive. An enhancer located 75 kb upstream of the murine Rankl gene's transcription start site designated D5 is important for its calciotropic hormone- and cytokine-mediated expression. Here, we determined the impact of RANKL levels in atherosclerotic plaque development in the D5 enhancer-null (D5^-/- ) mice in an atherogenic Apoe^-/- background fed a high-fat diet (HFD). Rankl mRNA transcripts were increased in aortic arches and thoracic aortae of Apoe^-/- mice; however, this increase was blunted in Apoe^-/- ;D5^-/- mice. Similarly, higher Rankl transcripts were identified in splenic T lymphocytes in Apoe^-/- mice, and their levels were reduced in Apoe^-/- ;D5^-/- mice. When analyzed by micro-computed tomography (µCT), atherosclerotic plaque calcification was identified in six out of eight Apoe^-/- mice, whereas only one out of eight Apoe^-/- ;D5^-/- mice developed plaque calcification after 12 weeks of HFD. However, following 18 weeks of HFD challenge, all of Apoe^-/- and Apoe^-/- ;D5^-/- animals developed atherosclerotic plaque calcification. Likewise, atherosclerotic lesion sizes were site-specifically reduced in the aortic arch of Apoe^-/- ;D5^-/- mice at initial stage of atherosclerosis and this effect was diminished as atherosclerosis proceeded to a more advanced stage. Our data suggest that deletion of the RANKL D5 enhancer delays the progression of atherosclerotic plaque development and plaque calcification in hypercholesterolemic mice. This work provides important insight into RANKL's regulatory role in atherosclerosis. J. Cell. Biochem. 118: 4240-4253, 2017. © 2017 Wiley Periodicals, Inc.