Vitamin D in Vascular Calcification: A Double-Edged Sword?

Regulation of calcific vascular and valvular disease by nuclear receptors

PURPOSE OF REVIEW

This review addresses recent developments in studies of lipid regulation of calcific disease of arteries and cardiac valves, including the role of nuclear receptors. The role of lipid-soluble signals and their receptors is timely given the recent evidence and concerns that lipid-lowering treatment may increase the rate of progression of coronary artery calcification, which has been long associated with increased cardiovascular risk. Understanding the mechanisms will be important for interpreting such clinical information.

RECENT FINDINGS

New findings support regulation of calcific vascular and valvular disease by nuclear receptors, including the vitamin D receptor, glucocorticoid receptor, nutrient-sensing nuclear receptors (liver X receptor, farnesoid X receptor, and peroxisome proliferator-activated receptors), and sex hormone (estrogen and androgen) receptors. There were two major unexpected findings: first, vitamin D supplementation, which was previously believed to prevent or reduce vascular calcification, showed no cardiovascular benefit in large randomized, controlled trials. Second, both epidemiological studies and coronary intravascular ultrasound studies suggest that treatment with HMG-CoA reductase inhibitors increases progression of coronary artery calcification, raising a question of whether there are mechanically stable and unstable forms of coronary calcification.

SUMMARY

For clinical practice and research, these new findings offer new fundamental mechanisms for vascular calcification and provide new cautionary insights for therapeutic avenues.

The vitamin D positive feedback hypothesis of inflammatory bowel diseases

While it appears that there are a variety of factors that exacerbate IBD, it is frustrating that symptoms can persist and worsen even when environmental insults are removed. We suggest that there may be a positive feedback loop which perpetuates the inflammatory response in IBD patients. The loop is triggered by vitamin D deficiency which reduces calcium uptake. Lowered vitamin D and calcium interfere with anti-inflammatory pathways. Inflammation of the mucosa inhibits absorption of calcium and thus perpetuates the reduced anti-inflammatory response. A number of predictions follow from this hypothesis and are supported by geographic and lifestyle patterns in IBD incidence and prevalence.

Vitamin D deficiency and electrocardiographic subclinical myocardial injury: Results from National Health and Nutrition Examination Survey-III

BACKGROUND

Association of cardiovascular disease (CVD) with nontraditional risk factors such as vitamin D deficiency has been examined previously. An investigation of the association of vitamin D with subclinical myocardial injury (SC-MI) based on an electrocardiographic score is a simple, cost-effective and innovative way to explore this relationship.

HYPOTHESIS

We hypothesize that low vitamin D levels will be associated with prevalent SC-MI independent of traditional CVD risk factors, lifestyle factors, and socioeconomic status.

METHODS

This analysis included 6079 participants (58.3 ± 13.1 years; 54.1% women) without CVD from the third National Health and Nutrition Examination Survey. A multivariable logistic regression model was used to examine the association between vitamin D categories (<20, 20-29, and >30 ng/mL (reference) and cardiac injury score.

RESULTS

There was an incremental increase in the prevalence of SC-MI across vitamin D categories with the highest prevalence in <20 ng/mL, followed by 20 to 29 ng/mL and then >30 ng/mL (trend P-value <0.0001). There was a statistically significant association between vitamin D deficiency (<20 ng/mL) and SC-MI (odds ratio [OR] (95% confidence interval [CI]): 1.27 (1.04-1.55), P = 0.04). This association was stronger in men than women (OR (95% CI): 1.74 (1.32-2.30) vs 0.94 (0.70-1.25) respectively; interaction P-value 0.002).

CONCLUSIONS

Vitamin D deficiency is associated with SC-MI, especially in men. These findings may further highlight the role of nontraditional risk factors in the development of CVD. The value of vitamin D supplementation in the prevention of myocardial ischemia and injury may warrant investigation.

Vitamin D Deficiency in Chronic Kidney Disease: Recent Evidence and Controversies

Vitamin D (VD) is a pro-hormone essential for life in higher animals. It is present in few types of foods and is produced endogenously in the skin by a photochemical reaction. The final step of VD activation occurs in the kidneys involving a second hydroxylation reaction to generate the biologically active metabolite 1,25(OH)₂-VD. Extrarenal 1α-hydroxylation has also been described to have an important role in autocrine and paracrine signaling. Vitamin D deficiency (VDD) has been in the spotlight as a major public healthcare issue with an estimated prevalence of more than a billion people worldwide. Among individuals with chronic kidney disease (CKD), VDD prevalence has been reported to be as high as 80%. Classically, VD plays a pivotal role in calcium and phosphorus homeostasis. Nevertheless, there is a growing body of evidence supporting the importance of VD in many vital non-skeletal biological processes such as endothelial function, renin-angiotensin-aldosterone system modulation, redox balance and innate and adaptive immunity. In individuals with CKD, VDD has been associated with albuminuria, faster progression of kidney disease and increased all-cause mortality. Recent guidelines support VD supplementation in CKD based on extrapolation from cohorts conducted in the general population. In this review, we discuss new insights on the multifactorial pathophysiology of VDD in CKD as well as how it may negatively modulate different organs and systems. We also critically review the latest evidence and controversies of VD monitoring and supplementation in CKD patients.

Role of PKB/SGK-dependent phosphorylation of GSK-3α/β in vascular calcification during cholecalciferol overload in mice

Medial vascular calcification is a highly regulated process involving osteo-/chondrogenic transdifferentiation of vascular smooth muscle cells. Both, protein kinase B (PKB) and serum- and glucocorticoid-inducible kinase 1 (SGK1) are involved in the intracellular signaling of vascular calcification and both phosphorylate and inactivate glycogen synthase kinase 3 (GSK-3). The present study explored whether PKB/SGK-dependent phosphorylation of GSK-3α/β is involved in vascular calcification. Experiments were performed in Gsk-3α/β double knockin mice lacking functional PKB/SGK phosphorylation sites (gsk-3^KI) and corresponding wild-type mice (gsk-3^WT) following high-dosed cholecalciferol treatment as well as ex vivo in aortic ring explants from gsk-3^KI and gsk-3^WT mice treated without and with phosphate. In gsk-3^WT mice, high-dosed cholecalciferol induced vascular calcification and aortic osteo-/chondrogenic signaling, shown by increased expression of osteogenic markers Msx2, Cbfa1 and tissue-nonspecific alkaline phosphatase (Alpl). All these effects were suppressed in aortic tissue from gsk-3^KI mice. Cholecalciferol decreased aortic Gsk-3α/β phosphorylation (Ser^21/9) in gsk-3^WT mice, while no phosphorylation was observed in gsk-3^KI mice. Moreover, the mRNA expression of type III sodium-dependent phosphate transporter (Pit1) and plasminogen activator inhibitor 1 (Pai1) was increased following cholecalciferol treatment in aortic tissue of gsk-3^WT mice, effects again blunted in gsk-3^KI mice. In addition, phosphate treatment induced mineral deposition and osteogenic markers expression in aortic ring explants from gsk-3^WT mice, effects reduced in aortic ring explants from gsk-3^KI mice. In conclusion, vascular PKB/SGK-dependent phosphorylation of GSK-3α/β contributes to the osteoinductive signaling leading to vascular calcification.