Svetlana Slavic

Vitamin D Is a Regulator of Endothelial Nitric Oxide Synthase and Arterial Stiffness in Mice

The vitamin D hormone 1α,25-dihydroxyvitamin D3 [1,25(OH)2D3] is essential for the preservation of serum calcium and phosphate levels but may also be important for the regulation of cardiovascular function. Epidemiological data in humans have shown that vitamin D insufficiency is associated with hypertension, left ventricular hypertrophy, increased arterial stiffness, and endothelial dysfunction in normal subjects and in patients with chronic kidney disease and type 2 diabetes. However, the pathophysiological mechanisms underlying these associations remain largely unexplained. In this study, we aimed to decipher the mechanisms by which 1,25(OH)2D3 may regulate systemic vascular tone and cardiac function, using mice carrying a mutant, functionally inactive vitamin D receptor (VDR). To normalize calcium homeostasis in VDR mutant mice, we fed the mice lifelong with the so-called rescue diet enriched with calcium, phosphate, and lactose. Here, we report that VDR mutant mice are characterized by lower bioavailability of the vasodilator nitric oxide (NO) due to reduced expression of the key NO synthesizing enzyme, endothelial NO synthase, leading to endothelial dysfunction, increased arterial stiffness, increased aortic impedance, structural remodeling of the aorta, and impaired systolic and diastolic heart function at later ages, independent of changes in the renin-angiotensin system. We further demonstrate that 1,25(OH)2D3 is a direct transcriptional regulator of endothelial NO synthase. Our data demonstrate the importance of intact VDR signaling in the preservation of vascular function and may provide a mechanistic explanation for epidemiological data in humans showing that vitamin D insufficiency is associated with hypertension and endothelial dysfunction.

Experimental Myocardial Infarction Upregulates Circulating Fibroblast Growth Factor-23

Myocardial infarction (MI) is a major cause of death worldwide. Epidemiological studies have linked vitamin D deficiency to MI incidence. Because fibroblast growth factor‐23 (FGF23) is a master regulator of vitamin D hormone production and has been shown to be associated with cardiac hypertrophy per se, we explored the hypothesis that FGF23 may be a previously unrecognized pathophysiological factor causally linked to progression of cardiac dysfunction post‐MI. Here, we show that circulating intact Fgf23 was profoundly elevated, whereas serum vitamin D hormone levels were suppressed, after induction of experimental MI in rat and mouse models, independent of changes in serum soluble Klotho or serum parathyroid hormone. Both skeletal and cardiac expression of Fgf23 was increased after MI. Although the molecular link between the cardiac lesion and circulating Fgf23 concentrations remains to be identified, our study has uncovered a novel heart–bone–kidney axis that may have important clinical implications and may inaugurate the new field of cardio‐osteology.

Estrogen Regulates Bone Turnover by Targeting RANKL Expression in Bone Lining Cells

Estrogen is critical for skeletal homeostasis and regulates bone remodeling, in part, by modulating the expression of receptor activator of NF-κB ligand (RANKL), an essential cytokine for bone resorption by osteoclasts. RANKL can be produced by a variety of hematopoietic (e.g. T and B-cell) and mesenchymal (osteoblast lineage, chondrocyte) cell types. The cellular mechanisms by which estrogen acts on bone are still a matter of controversy. By using murine reconstitution models that allow for selective deletion of estrogen receptor-alpha (ERα) or selective inhibition of RANKL in hematopoietic vs. mesenchymal cells, in conjunction with in situ expression profiling in bone cells, we identified bone lining cells as important gatekeepers of estrogen-controlled bone resorption. Our data indicate that the increase in bone resorption observed in states of estrogen deficiency in mice is mainly caused by lack of ERα-mediated suppression of RANKL expression in bone lining cells.

Genetic Ablation of Fgf23 or Klotho Does not Modulate Experimental Heart Hypertrophy Induced by Pressure Overload.

Left ventricular hypertrophy (LVH) ultimately leads to heart failure in conditions of increased cardiac pre- or afterload. The bone-derived phosphaturic and sodium-conserving hormone fibroblast growth factor-23 (FGF23) and its co-receptor Klotho have been implicated in the development of uremic LVH. Using transverse aortic constriction (TAC) in gene-targeted mouse models, we examine the role of Fgf23 and Klotho in cardiac hypertrophy and dysfunction induced by pressure overload. TAC profoundly increases serum intact Fgf23 due to increased cardiac and bony Fgf23 transcription and downregulation of Fgf23 cleavage. Aldosterone receptor blocker spironolactone normalizes serum intact Fgf23 levels after TAC by reducing bony Fgf23 transcription. Notably, genetic Fgf23 or Klotho deficiency does not influence TAC-induced hypertrophic remodelling, LV functional impairment, or LV fibrosis. Despite the profound, aldosterone-mediated increase in circulating intact Fgf23 after TAC, our data do not support an essential role of Fgf23 or Klotho in the pathophysiology of pressure overload-induced cardiac hypertrophy.

Lack of vitamin D signalling per se does not aggravate cardiac functional impairment induced by myocardial infarction in mice

Epidemiological studies have linked vitamin D deficiency to an increased incidence of myocardial infarction and support a role for vitamin D signalling in the pathophysiology of myocardial infarction. Vitamin D deficiency results in the development of secondary hyperparathyroidism, however, the role of secondary hyperparathyroidism in the pathophysiology of myocardial infarction is not known. Here, we aimed to explore further the secondary hyperparathyroidism independent role of vitamin D signalling in the pathophysiology of myocardial infarction by inducing experimental myocardial infarction in 3-month-old, male, wild-type mice and in mice lacking a functioning vitamin D receptor. In order to prevent secondary hyperparathyroidism in vitamin D receptor mutant mice, all mice were maintained on a rescue diet enriched with calcium, phosphorus, and lactose. Surprisingly, survival rate, cardiac function as measured by echocardiography and intra-cardiac catheterisation and cardiomyocyte size were indistinguishable between normocalcaemic vitamin D receptor mutant mice and wild-type controls, 2 and 8 weeks post-myocardial infarction. In addition, the myocardial infarction-induced inflammatory response was similar in vitamin D receptor mutants and wild-type mice, as evidenced by a comparable upregulation in cardiac interleukin-1-β and tumor-necrosis-factor-α mRNA abundance and similar elevations in circulating interleukin-1-β and tumor-necrosis-factor-α. Our data suggest that the lack of vitamin D signalling in normocalcaemic vitamin D receptor mutants has no major detrimental effect on cardiac function and outcome post-myocardial infarction. Our study may have important clinical implications because it suggests that the secondary hyperparathyroidism induced by vitamin D deficiency, rather than the lack of vitamin D signalling per se, may negatively impact cardiac function post-myocardial infarction.

193 Lack of Fibroblast Growth factor-23 (FGF23) Preserves Cardiac Function in a Murine Model of Acute Myocardial Infarction

Myocardial infarction (MI) is a major cause of death worldwide. We recently showed that intact circulating fibroblast growth factor-23 (FGF23) is profoundly up-regulated after experimental MI in mice and rats. FGF23 is a bone-derived hormone involved in systemic phosphate homeostasis and vitamin D metabolism. Although the pathophysiological mechanisms remain to be identified, clinical studies have shown a strong association between FGF23 and left ventricular hypertrophy, atrial fibrillation and cardiac systolic dysfunction. Here, we explored the hypothesis that FGF23 may be causally linked to progression of cardiac dysfunction post-MI, using a mouse model lacking both Fgf23 and a functioning vitamin D receptor (VDR). Surgery was performed on 3-month-old, male, wild-type (WT), VDR and Fgf23-/- /VDRΔ/Δ(Fgf23/VDR) compound mutant mice on a C57BL/6 background. To normalise mineral homeostasis in VDR-ablated mice, all mice were kept lifelong on a rescue diet enriched with calcium, phosphorous and lactose. MI was induced by permanent ligation of the left descending coronary artery. Sham-operated (Sham) mice served as a control. One week after MI, cardiac function was assessed by echocardiography and electrocardiography (ECG). Intracardial pressure monitoring was performed by catheterization as a terminal procedure, 2 weeks post-MI. Echocardiography confirmed left ventricular infarction, and ECG recordings revealed comparable ST depression in MI mice of all genotypes. However, Fgf23/VDR compound mutant MI mice showed improved fractional shortening, relative to WT and VDR MI controls. In addition, Fgf23/VDR compound mutant MImice were more resistant against the impairment of ventricular contraction and relaxation observed in WT and VDR MI mice, as measured by left ventricular dP/dtmax, dP/dtmin, and relaxation time tau, 2 weeks post-MI surgery. Our data indicate that lack of Fgf23 improves cardiac contractile function following experimental MI. This finding underscores the potential importance of the heart-bone-axis and of the new field of cardio-osteology because the levels of circulating intact FGF23 may influence cardiac recovery after MI.