The protective role of vitamin D on the heart and the kidney

Introduction

Recent investigations have deeply changed our notions regarding vitamin D involvement on fundamental metabolic functions. Indeed, this was a compound seen at the beginning of the last century as a miracle substance that could reverse rickets, an ominous and mysterious disease, rampant among poor children living in polluted cities around the world. In fact, because early notions saw vitamin D as a compound essential to prevent the bony abnormalities seen in animal models and in clinical situations, the concept was tightly attached to calcium and bone metabolism. Certainly, without vitamin D, only 10–15% of the calcium would be absorbed in the gastrointestinal tract. It is the interaction of vitamin D with its receptor that increases intestinal calcium absorption by about 30–40% and phosphorus absorption by nearly 80% [Holick and Garabedian, 2006; Bouillon, 2001; de Luca, 2004; Heaney et al. 2003]. Unfortunately, despite the remarkable advances in the knowledge and understanding of the nature and mechanisms of action of vitamin D, rickets continues to be diagnosed. Moreover, the US Food and Drug Administration, recommends the addition of vitamin D to milk and cereals hoping for a reduction in the incidence of rickets in children and osteoporosis in adults.

Be that as it may, modern literature shows that vitamin D also regulates gene expression associated with autoimmunity, tumours and infections and it does so by controlling the activation of a specific receptor, the vitamin D receptor (VDR), a type 1 nuclear receptor and a DNA transcription factor. Moreover, the traditional notion limiting the conversion of circulating 25-hydroxy vitamin D to the active 1,25-dihydroxy vitamin D only in the kidney, has been changed as most tissues and cells in the body have not only VDRs but also the enzymatic arrangement to generate the active form. In addition, newer investigations are showing an assortment of critical functions played by the vitamin D–VDR interactions. Certainly the VDR is present in intestinal cells, the central nervous system, prostate, and other tissues [de Luca, 2004; Holick, 2006a; Dusso et al. 2005].

Thus, the previous notion limiting vitamin D actions purely to calcium and phosphate metabolism has been changed. For instance, individuals with 25-hydroxy vitamin D blood levels lower than 20 ng/ml have a 30–50% higher incidence of breast, prostate, colon and other malignancies [Luscombe et al. 2001; Gorham et al. 2005; Giovannucci et al. 2006; Ahonen et al. 2000; Holick 2006b; Nagpal and Rathnachalam, 2005].

VDR effects unrelated to calcium-phosphorus metabolism

Associations between circulating vitamin D levels and disorders as diverse as osteoarthritis, rheumatoid arthritis, type 1 diabetes, inadequate insulin synthesis and schizophrenia have been described. Although much work is needed to confirm and understand all these associations, current evidences seem to indicate that vitamin D exerts crucial cellular functions. Indeed, directly or indirectly, 1,25-dihydroxy vitamin D regulates many genes, including those responsible for the regulation of cellular proliferation, differentiation, apoptosis, and angiogenesis [Holick, 2006a; Krause et al. 1998; Thomas et al. 1998]. Moreover, detecting the VDR in most body cells suggested a role for vitamin D in many body systems and thereby, not limiting its impact only to calcium metabolism. In this regard, we recently showed that changes in the expression pattern of the WT-1 transcription factor, a key mediator of nephrogenesis, could contribute to anatomical and functional kidney disorders linked to hypertension development. These changes could respond to VDR modulation (Figure 1) [Mazzei et al. 2016].

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Figure 1.

Histological sections of neonatal SHR group and the control group Wistar Kyoto rats (WKYs) kidney cortices.

Cardiovascular effects

The vitamin D cell effects recently described may explain the increased risk of hypertension and cardiovascular illness as described in individuals living in areas where sun exposure is lower. In these populations, hypertensive patients were exposed to ultraviolet B radiation three times a week for 3 months. As a result, 25-hydroxy vitamin D levels rose by about 180%, and blood pressure normalized [Krause et al. 1998; Thomas et al. 1998]. This is consistent with epidemiological studies linking lower levels of cholecalciferol to cardiovascular diseases.

Moreover, vitamin D deficiency has been associated with congestive heart failure and increased levels of inflammatory factors such as interleukin and C-reactive protein [Kong and Li, 2003]. This seems particularly important, as vitamin D could then be a natural vascular protector factor. In fact, experimental vitamin D deficiency is accompanied by increased renin expression and angiotensin II concentration, high blood pressure and cardiac hypertrophy [Li et al. 2002; Xiang et al. 2005; Kong et al. 2010; Ferder et al. 2013]. In agreement with these findings, VDR-knockout mice develop high blood pressure and left ventricular hypertrophy [Xiang et al. 2005], whereas vitamin D analogues improve left ventricular hypertrophy and diastolic function in the spontaneously hypertensive rat (SHR) [Ferder et al. 2013].

Moreover, combined paricalcitol and enalapril therapy ameliorated the cardiac oxidative injury occurring in a rat uremic model [Husain et al. 2009], and protected from the inflammatory and oxidative endothelial damage observed during the development of atherosclerosis [Husain et al. 2010]. In this respect, atherosclerosis is regarded as a chronic inflammatory condition that precedes clinical situations such as cardiovascular dysfunction, myocardial infarction, unstable angina, sudden cardiac death, stroke and peripheral thromboses. Moreover, it has been proposed that the renin-angiotensin system (RAS), through its main mediator, angiotensin II, also has a direct influence on the progression of the atherosclerotic process via effects on endothelial function, inflammation, fibrinolytic balance, and plaque stability [Husain et al. 2015]. Preclinical as well as clinical studies have provided evidence clearly demonstrating that vitamin D triggers vascular benefits. Indeed, vitamin D reduces the progression of coronary atherosclerosis in patients with stable angina pectoris, decreases vascular inflammatory markers and improves common carotid intima-media thickness and plaque volume in patients with diagnosed atherosclerosis [Artaza et al. 2009; Gunta et al. 2013; Norman and Powell, 2014; Panizo et al. 2013]. Moreover, a recent study on an older Australian population not only showed the beneficial effects of vitamin D on systemic arterial inflammation but also on circulating lipids levels and on biomarkers of endothelial cell activation. The authors concluded that higher vitamin D status might protect the endothelium [Alyami et al. 2016].

Interestingly, a recent investigation in an Italian population showed that a genetic variation of the VDR was associated with type 2 diabetes and impaired insulin secretion in adults, and increased cardio-metabolic risk in children [Sentinelli et al. 2016]. Notwithstanding a continued appreciation of the potential role of vitamin D in the prevention of cardiovascular disease, we are still missing a whole large-scale, randomized trial testing this agent for the primary prevention of cardiovascular disease. In this line of investigation, the VITamin D and OmegA-3 TriaL (VITAL), a randomized, double-blind, placebo-controlled trial is currently searching for benefits and risks of vitamin D and marine omega-3 fatty acids in the primary prevention of cancer and cardiovascular disease among 25,875 patients [Pradhan and Manson, 2016; Bassuk et al. 2016]. Be that as it may, current data are inconclusive as to whether supplementation with these agents reduces cardiovascular disease risks, and other nonskeletal illnesses in the general population. The results of VITAL are expected to help in individual decisions, in clinical recommendations, and in public health guidelines.

These novel notions on the beneficial cardiovascular effects of vitamin D are being expanded by several lines of investigation that show that the vitamin D protecting actions could be extended not only to the heart and the brain but also to the kidney [Levey et al. 2003].

Renal effects of vitamin D

This area of information is particularly important as, regardless its nature, chronic kidney disease increases cardiovascular risk [Sarnak et al. 2003; Holick, 2007]. Thus, vitamin D-induced renal protection could indirectly reduce cardiovascular risk by reducing renal disease progression. If so, protecting the diseased kidney with vitamin D could become a therapeutic goal not only to prevent cardiovascular or renal disease progression, but also to satisfy the multiple vitamin D needs. Indeed, the kidney, although not the sole regulator of 1,25-dihydroxy cholecalciferol, continues to be a major producer of this most active vitamin D metabolite and as a result, advanced kidney injury could have significant consequences not only on bone metabolism (rickets, osteomalacia, secondary hyperparathyroidism) but also in other organ systems, including cellular proliferation, differentiation and angiogenesis [Holick,2007; Branisteanu et al. 1993].

This is why much attention is being placed on the vitamin D effects on the kidney. Indeed, vitamin D has been shown to decrease protein excretion rate in Heymann nephritis, an experimental model that replicates membranous nephropathy in the human [Lemire et al. 1992], and in other nephropathy models [Lillevang et al. 1992; Schwarz et al. 1998]. These studies in animal models suggest that vitamin D exerts a protective effect on the diseased kidney as suggested by the reduction in albumin excretion rate.

In this respect, we have addressed this notion in the obstructive uropathy rat model. Indeed, VDR activation through paricalcitol prevents fibrosis and decreases the number of TUNEL-positive apoptotic cells in this ureteral obstruction rat model. By electron microscopy, these changes are attended by mitochondrial expansion, dilated crests and wider gaps. Interestingly, all these changes are inhibited by paricalcitol. Moreover, the increased AT₁-receptor mRNA and nicotinamide adenine dinucleotide phosphate (NADPH) activity observed in untreated rats with neonatal obstructive uropathy are reversed in mitochondrial fractions from paricalcitol-treated animals in the same rat model. This notion strongly suggests that the paricalcitol-induced rise in VDR expression exerts beneficial effects that are mediated at least in part by protective effects on the mitochondria (Figure 2) [García et al. 2012].

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Figure 2.

Kidney cortices histological sections following unilateral obstruction for 15 days. The cytoprotective effect of paricalcitol is shown.

Remarkable, previous studies by Khong and colleagues have shown that low levels of vitamin D are associated with elevated activity of the RAS and hypertension [Kong et al. 2010]. For these reasons, SHRs were treated with paricalcitol. Although blood pressure was not strikingly reduced in the paricalcitol-treated group, fibrosis, apoptosis, mitochondrial damage, and NADPH oxidase activity were reduced in the paricalcitol-treated SHRs. Additionally, high AT₁ receptor expression, like low heat shock protein 70 (Hsp70) expression, were reversed in the renal cortices of paricalcitol-treated animals. The recovery parameters were consistent with an improvement in VDR expression. These data suggested that VDR-modulated Hsp70/AT₁ is involved in the mechanism by which paricalcitol provides renal protection in SHRs [García et al. 2014; Manucha, 2014].

This is particularly relevant because it is known that Hsp70 regulates signalling pathways for cellular oxidative stress responses. Hsp70 has been shown to protect against angiotensin II-induced hypertension and to exert a cytoprotective effect [Molina et al. 2016; Rodríguez-Iturbe et al. 2012; Bocanegra et al. 2010; Nishiyama and Hitomi, 2010].

Notably, cardiovascular disease is often associated with chronic kidney disease and vice versa. Thus, it seems feasible that myocardial VDRs are among the likely links between the two disorders. Unfortunately, the structural and electrophysiological effects of myocardial VDR modification and its impact on the response to ischaemia-reperfusion are currently unknown. In this regard, we have studied the myocardial changes linked to VDR deficiency in Langendorff-perfused hearts in the obstructive nephropathy rat model. Ureteral obstructed rats showed a reduction in VDR and increase angiotensin II type 1 receptor expression. These rats showed fibrosis and myofibril reduction with an increase in mitochondrial size and dilated crests. These changes were reversed by paricalcitol. In addition, during ischemia-reperfusion, paricalcitol-treated ureteral-obstructed rats had a lower incidence of ventricular fibrillation compared with untreated ureteral-obstructed rats. The action potential duration was prolonged throughout the experiment in paricalcitol-treated rats. Therefore, the reduction in myocardial VDR expression in this rat model seems linked to myocardial remodelling and increase arrhythmogenesis, as paricalcitol protects against these changes by restoring myocardial VDR levels and prolonging action potentials [Diez et al. 2015].

In agreement with these concepts, we have recently expanded these notions in 5/6 nephrectomy rats using calcitriol (1,25-dihydroxy vitamin D3). Indeed, calcitriol not only lowered blood pressure in this salt-sensitive hypertension model but also lessened renal function changes, left ventricular hypertrophy, and myocardial AT₁ receptor-mediated oxidative stress [Bordcoch et al. 2014].

In brief, activation of the VDR seems to protect the kidney from disease progression through intracellular effects that affect mitochondrial structure and function. These effects seem to extend to the heart where hypertrophy and remodelling are inhibited by activation of the VDR, either from paricalcitol or from calcitriol. Whether the beneficial events on the heart are direct effects from receptor activation or secondary to renal protection is yet to be defined. Nevertheless, clinical trials are needed to confirm the vitamin D beneficial effects on the heart and the kidney. Up to now, randomized clinical trials have yielded encouraging though indefinite results [Shoben et al. 2008; Thadhani et al. 2011; Agarwal et al. 2011]. Certainly, conclusions such as these should open up a novel and interesting therapeutic approach for the treatment of chronic kidney disease and its associated cardiovascular risk.