Vitamin D Receptor: A Novel Therapeutic Target for Kidney Diseases

Activation of vitamin D receptor attenuates high glucose-induced cellular injury partially dependent on CYP2J5 in murine renal tubule epithelial cell

AIMS

Vitamin D and its receptor, vitamin D receptor (VDR), have renoprotection effect against diabetic nephropathy (DN). But the exact mechanism has not been fully elucidated. Epoxyeicosatrienoic acids (EETs) are cytochrome P450 (CYP) epoxygenase-derived metabolites of arachidonic acid, protecting against diabetes and DN. Herein, we hypothesized that activation of VDR attenuated high glucose-induced cellular injury in renal tubular epithelial cells partially through up-regulating CYP2J5 expression.

MAIN METHODS

Streptozotocin (STZ) was injected to induce diabetic in wild type and Vdr^-/- mice. The effects of VDR knockout and an activator of VDR, paricalcitol, on the renal injury were detected. In vitro, a murine kidney proximal tubule epithelial cell line BU.MPT induced by high glucose were treated with or without paricalcitol (30 mM) for 12 h or 24 h.

KEY FINDINGS

The expression of CYP2J5 was significantly decreased both in wild type and Vdr^-/- diabetic mice induced by STZ. The STZ-induced kidney architecture damage and apoptosis rate in Vdr^-/- mice were more severe. In vitro, high glucose treatment strongly reduced the CYP2J5 expression and the synthesis of 14,15-EET in BU.MPT cells. Supplement of 14,15-EET significantly reduced the lactate dehydrogenase (LDH) release induced by high glucose in BU.MPT cells. Furthermore, treatment with paricalcitol attenuated cellular injury and restored the expression of CYP2J5 reduced by high glucose in BU.MPT cells.

SIGNIFICANCE

We conclude that activation of VDR attenuates high glucose-induced cellular injury partially dependent on CYP2J5 in murine renal tubule epithelial cells and paricalcitol may represent a potential therapy for DN.

ASSOCIATIONS BETWEEN VITAMIN D AND MICROVASCULAR COMPLICATIONS IN MIDDLE-AGED AND ELDERLY DIABETIC PATIENTS

Objective: The objective of this cross-sectional study was to investigate the association of serum 25-hydroxyvitamin D (25[OH]D) levels with estimated glomerular filtration rate (eGFR), albumin/creatinine ratio (ACR), and the prevalence of diabetic retinopathy (DR) in Chinese diabetic adults. Methods: A total of 4,767 diabetic participants were enrolled from seven communities in Shanghai, China, in 2018. Participants underwent several examinations, which included the measurement of anthropometric parameters, blood pressure, glucose, lipid profiles, 25(OH)D, and ACR. DR was detected based on high-quality fundus photographs and remotely read by ophthalmologists. Results: Compared with the first 25(OH)D quartile, participants in the fourth quartile had a lower prevalence of high ACR (odds ratio [OR], 0.77; 95% confidence interval [CI], 0.61 to 0.96) (P for trend <.01). No association was found between 25(OH)D levels and eGFR. For DR, the OR (95% CI) for DR ranging from 0 to 4 in ordinal logistic regression associated with 25(OH)D was 0.62 (0.47 to 0.82) for the fourth 25(OH)D quartile (P for trend <.01) compared with the first quartile. These associations were all fully adjusted for confounding factors. Conclusion: Lower serum 25(OH)D concentration is significantly associated with increased ACR and higher prevalence of DR in middle-aged and elderly diabetic adults. However, the possibility of a causal relationship between 25(OH)D deficiency and diabetic microvascular complications remains to be demonstrated. Abbreviations: 25(OH)D = 25-hydroxyvitamin D; ACR = albumin/creatinine ratio; BMI = body mass index; CI = confidence interval; DKD = diabetic kidney disease; DR = diabetic retinopathy; eGFR = estimated glomerular filtration rate; HbA1c = glycated hemoglobin; HDL = high-density lipoprotein; LDL = low-density lipoprotein; OR = odds ratio; T2DM = type 2 diabetes mellitus.

Focus on 1,25-Dihydroxyvitamin D3 in the Peripheral Nervous System

In this review, we draw attention to the roles of calcitriol (1,25-dihydroxyvitamin D3) in the trophicity of the peripheral nervous system. Calcitriol has long been known to be crucial in phosphocalcium homeostasis. However, recent discoveries concerning its involvement in the immune system, anti-cancer defenses, and central nervous system development suggest a more pleiotropic role than previously thought. Several studies have highlighted the impact of calcitriol deficiency as a promoting factor of various central neurological diseases, such as multiple sclerosis, amyotrophic lateral sclerosis, Parkinson's disease, and Alzheimer's disease. Based on these findings and recent publications, a greater role for calcitriol may be envisioned in the peripheral nervous system. Indeed, calcitriol is involved in myelination, axonal homogeneity of peripheral nerves, and neuronal-cell differentiation. This may have useful clinical consequences, as calcitriol supplementation may be a simple means to avoid the onset and/or development of peripheral nervous-system disorders.

Vitamin D receptor activation protects against lipopolysaccharide-induced acute kidney injury through suppression of tubular cell apoptosis

Acute kidney injury (AKI) is a common complication of sepsis characterized by a rapid degradation of renal function. The effect of vitamin D on AKI remains poorly understood. Here, we showed that vitamin D receptor (VDR) activation protects against lipopolysaccharide (LPS)-induced AKI by blocking renal tubular epithelial cell apoptosis. Mice lacking VDR developed more severe AKI than wild-type (WT) control mice after LPS treatment, which was manifested by marked increases in body weight loss and accumulation of serum blood urea nitrogen and creatinine as well as the magnitude of apoptosis of tubular epithelial cells. In the renal cortex, LPS treatment led to more dramatic downregulation of Bcl-2, more robust induction of p53-upregulated modulator of apoptosis (PUMA) and miR-155, and more severe caspase-3 activation in VDR knockout mice compared with WT control mice. Conversely, paricalcitol pretreatment markedly prevented LPS-induced AKI. Paricalcitol ameliorated body weight loss, attenuated serum blood urea nitrogen and creatinine accumulation, blocked tubular cell apoptosis, prevented the suppression of Bcl-2, and reversed PUMA and miR-155 induction and caspase-3 activation in LPS-treated WT mice. In HK2 cells, LPS induced PUMA and miR-155 by activating NF-κB, whereas 1,25(OH)₂D₃ blocked PUMA and miR-155 induction by repressing NF-κB activation. Both PUMA and miR-155 target Bcl-2 to promote apoptosis; namely, PUMA inhibits Bcl-2 activity, whereas miR-155 promotes Bcl-2 mRNA degradation and inhibits Bcl-2 protein translation. Collectively, these data provide strong evidence that LPS induces tubular cell apoptosis via upregulating PUMA and miR-155, whereas vitamin D/VDR signaling protects against AKI by blocking NF-κB-mediated PUMA and miR-155 upregulation.

Vitamin D protects against diabetic nephropathy: Evidence-based effectiveness and mechanism

Vitamin D has been suggested to harbor multiple biological activities, among them the potential of vitamin D in the protection of diabetic nephropathy (DN) has attracted special attention. Both animal studies and clinical trials have documented an inverse correlation between low vitamin D levels and DN risk, and supplementation with vitamin D or its active derivatives has been demonstrated to improve endothelial cell injury, reduce proteinuria, attenuate renal fibrosis, and resultantly retard DN progression. Vitamin D exerts its pharmacological effects primarily via vitamin D receptor, whose activation inhibits the renin-angiotensin system, a key culprit for DN under hyperglycemia. The anti-DN benefit of vitamin D can be enhanced when administrated in combination with angiotensin converting enzyme inhibitors or angiotensin II receptor blockers. Mechanistic studies reveal that pathways relevant to inflammation participate in the pathogenesis of DN, however, consumption of vitamin D-related products negatively regulates inflammatory response at multiple levels, indicated by inhibiting macrophage infiltration, nuclear factor-kappa B (NF-κB) activation, and production of such inflammatory mediators as transforming growth factor-β(TGF-β), monocyte chemoattractant protein 1(MCP-1), and regulated upon activation normal T cell expressed and secreted protein(RANTES). The robust anti-inflammatory property of vitamin D-related products allows them with a promising renoprotective therapeutic option for DN. This review summarizes new advances in our understanding of vitamin D-related products in the DN management.