1,25(OH)2D3 Activates Autophagy to Protect against Oxidative Damage of INS-1 Pancreatic Beta Cells

1,25(OH)2D3 inhibits pancreatic stellate cells activation and promotes insulin secretion in T2DM

PURPOSE

To evaluate the effect and mechanism of 1,25(OH)2D3 on pancreatic stellate cells (PSCs) in type 2 diabetes mellitus (T2DM).

METHODS

A mouse model of T2DM was successfully established by high-fat diet (HFD) /streptozotocin (STZ) and administered 1,25(OH)2D3 for 3 weeks. Fasting blood glucose (FBG), glycated hemoglobin A1c (GHbA1c), insulin (INS) and glucose tolerance were measured. Histopathology changes and fibrosis of pancreas were examined by hematoxylin and eosin staining and Masson staining. Mouse PSCs were extracted, co-cultured with mouse insulinoma β cells (MIN6 cells) and treated with 1,25(OH)2D3. ELISA detection of inflammatory factor expression. Tissue reactive oxygen species (ROS) levels were also measured. Immunofluorescence or Western blotting were used to measure fibrosis and inflammation-related protein expression.

RESULTS

PSCs activation and islets fibrosis in T2DM mice. Elevated blood glucose was accompanied by significant increases in serum inflammatory cytokines and tissue ROS levels. 1,25(OH)2D3 attenuated islet fibrosis by reducing hyperglycemia, ROS levels, and inflammatory factors expression. Additionally, the co-culture system confirmed that 1,25(OH)2D3 inhibited PSCs activation, reduced the secretion of pro-inflammatory cytokines, down-regulated the expression of fibrosis and inflammation-related proteins, and promoted insulin secretion.

CONCLUSION

Our findings identify that PSCs activation contributes to islet fibrosis and β-cell dysfunction. 1,25(OH)2D3 exerts beneficial effects on T2DM potentially by inhibiting PSCs activation and inflammatory response, highlighting promising control strategies of T2DM by vitamin D.

Vitamin D, Cellular Senescence and Chronic Kidney Diseases: What Is Missing in the Equation?

As life expectancy increases in many countries, the prevalence of age-related diseases also rises. Among these conditions, chronic kidney disease is predicted to become the second cause of death in some countries before the end of the century. An important problem with kidney diseases is the lack of biomarkers to detect early damage or to predict the progression to renal failure. In addition, current treatments only retard kidney disease progression, and better tools are needed. Preclinical research has shown the involvement of the activation of cellular senescence-related mechanisms in natural aging and kidney injury. Intensive research is searching for novel treatments for kidney diseases as well as for anti-aging therapies. In this sense, many experimental shreds of evidence support that treatment with vitamin D or its analogs can exert pleiotropic protective effects in kidney injury. Moreover, vitamin D deficiency has been described in patients with kidney diseases. Here, we review recent evidence about the relationship between vitamin D and kidney diseases, explaining the underlying mechanisms of the effect of vitamin D actions, with particular attention to the modulation of cellular senescence mechanisms.

Effect of Vitamin D and/or Calcium Supplementation on Pancreatic β-Cell Function in Subjects with Prediabetes: A Randomized, Controlled Trial

So far, the potential role of vitamin D in β-cell function remains a matter of debate. Therefore, a randomized, placebo-controlled trial (RCT) was conducted to evaluate the effect of a vitamin D supplement with or without calcium on β-cell function in a Chinese population with prediabetes. Two hundred and forty-three subjects were randomly assigned in a 2-by-2 factorial-design RCT to receive either 1600 IU/day vitamin D₃ with/or 500 mg/day calcium for 24 weeks. The results showed that oral administration of vitamin D and calcium could increase the secretion of insulin. Vitamin D-insufficient individuals displayed an increment in the disposition index (adjusted change = 0.31, 95%CI: 0.07, 0.56) after treatment by vitamin D + calcium. It illustrated that supplementation with vitamin D and calcium might improve the function of pancreatic β-cell in prediabetes with low serum 25(OH)D levels. However, further studies are needed to confirm the findings. Given the low vitamin D content in natural foods, it is necessary to fortify processed foods with vitamin D.

Changes in Metabolic Parameters in Patients with Diabetic Kidney Disease Depending on the Status of D3

BACKGROUND

Type 2 diabetes is a metabolic disease characterized by hyperglycemia as a result of insulin resistance and decreased insulin secretion. A relatively large number of patients with this type of diabetes have abdominal obesity, which also affects insulin resistance development. Chronic hyperglycemia can lead to damage and dysfunction of various organs, and a striking example is diabetic nephropathy. Diabetic nephropathy is a specific kind of kidney damage that occurs due to complications of diabetes and is accompanied by the formation of diffuse or nodular glomerulosclerosis, which can lead to terminal renal failure and requires immediate substitution through renal therapy or renal transplantation. Diabetic nephropathy is diagnosed with albuminuria and a decrease in the rate of glomerular filtration.

METHODS

This review was based on a literature search for the most important evidence of vitamin D as a possible method of prevention for obesity, type 2 diabetes, and diabetic nephropathy. Collected published articles were summarized according to their overall themes.

RESULTS

In this review, we considered vitamin D as a possible method of treatment for type 2 diabetes, as well as its complications, including diabetic nephropathy.

CONCLUSION

Studies show that vitamin D inhibits the renin-angiotensin-aldosterone system, resulting in improved renal function in diabetic nephropathy. Vitamin D also has antiinflammatory, antiproliferative, and anti-metastatic effects, which improve endothelial function.

Vitamin D: A Potential Star for Treating Chronic Pancreatitis

Chronic pancreatitis (CP) is a chronic inflammatory and fibrotic disease of the pancreas. The incidence of CP is increasing worldwide but the effective therapies are lacking. Hence, it is necessary to identify economical and effective agents for the treatment of CP patients. Vitamin D (VD) and its analogues have been confirmed as pleiotropic regulators of cell proliferation, apoptosis, differentiation and autophagy. Clinical studies show that VD deficiency is prevalent in CP patients. However, the correlation between VD level and the risk of CP remains controversial. VD and its analogues have been demonstrated to inhibit pancreatic fibrosis by suppressing the activation of pancreatic stellate cells and the production of extracellular matrix. Limited clinical trials have shown that the supplement of VD can improve VD deficiency in patients with CP, suggesting a potential therapeutic value of VD in CP. However, the mechanisms by which VD and its analogues inhibit pancreatic fibrosis have not been fully elucidated. We are reviewing the current literature concerning the risk factors for developing CP, prevalence of VD deficiency in CP, mechanisms of VD action in PSC-mediated fibrogenesis during the development of CP and potential therapeutic applications of VD and its analogues in the treatment of CP.

Conundrum of vitamin D on glucose and fuel homeostasis

As an endocrine hormone, vitamin D plays an important role in bone health and calcium homeostasis. Over the past two decades, the non-calcemic effects of vitamin D were extensively examined. Although the effect of vitamin D on beta cell function were known for some time, the effect of vitamin D on glucose and fuel homeostasis has attracted new interest among researchers. Yet, to date, studies remain inconclusive and controversial, in part, due to a lack of understanding of the threshold effects of vitamin D. In this review, a critical examination of interventional trials of vitamin D in prevention of diabetes is provided. Like use of vitamin D for bone loss, the benefits of vitamin D supplementation in diabetes prevention were observed in vitamin D-deficient subjects with serum 25-hydroxyvitamin D < 50 nmol/L (20 ng/mL). The beneficial effect from vitamin D supplementation was not apparent in subjects with serum 25-hydroxyvitamin D > 75 nmol/L (30 ng/mL). Furthermore, no benefit was noted in subjects that achieved serum 25-hydroxyvitamin D > 100 nmol/L (40 ng/mL). Further studies are required to confirm these observations.

Vitamin D-VDR (vitamin D receptor) regulates defective autophagy in renal tubular epithelial cell in streptozotocin-induced diabetic mice via the AMPK pathway

Diabetic nephropathy (DN) has become a major cause of end-stage renal disease, and autophagy disorder is implicated in the pathogenesis of DN. Our previous studies found that vitamin D (VD) and VDR (vitamin D receptor) played a renoprotective role by inhibiting inflammation and fibrosis. However, whether VD-VDR regulates autophagy disorders in DN remains unclear. In this study, we established a streptozotocin (STZ)-induced diabetic model in vdr knockout (vdr-KO) mice and VDR specifically overexpressed in renal proximal tubular epithelial cells (Vdr-OE) mice. Our results showed that paricalcitol (an activated vitamin D analog) or Vdr-OE could alleviate STZ-induced ALB (albumin) excretion, renal tubule injury and inflammation, while these were worsened in vdr-KO mice. Defective autophagy was observed in the kidneys of STZ mice, which was more pronounced in vdr-KO mice and could be partially restored by paricalcitol or Vdr-OE. In high glucose-induced HK-2 cells, defective autophagy and decreased PRKAA1/AMPK phosphorylation was observed, which could be partially restored by paricalcitol in a VDR-dependent manner. AMPK inhibitor abolished paricalcitol-induced autophagy activation, and AMPK activator restored the defective autophagy in high glucose-induced HK-2 cells. Furthermore, paricalcitol-mediated AMPK activation was abrogated by CAMKK2/CaMKKβ inhibition, but not by STK11/LKB1 knockout. Meanwhile, paricalcitol rescued the decreased Ca²⁺ concentration induced by high glucose. In conclusion, VD-VDR can restore defective autophagy in the kidney of STZ-induced diabetic mice, which could be attributed to the activation of the Ca²⁺-CAMKK2-AMPK pathway in renal tubular epithelial cells.

Abbreviations: ACTB/β-actin: actin beta;AGE: advanced glycation end-products;AMPK: AMP-activated protein kinase;CAMKK2/CaMKKβ: calcium-calmodulin dependent protein kinase kinase 2;CQ: chloroquine;DN: diabetic nephropathy;HG: high levels of glucose;KO: knockout;LG: low levels of glucose;MAP1LC3/LC3: microtubule associated protein 1 light chain 3;NOD2: nucleotide binding oligomerization domain containing 2;OE: overexpression;PAS: periodic acid Schiff; Pari: paricalcitol;PTECs: proximal renal tubule epithelial cells;RT: room temperature;SQSTM1/p62: sequestosome 1;STK11/LKB1: serine/threonine kinase 11;STZ: streptozotocin;TEM: transmission electron microscopy;VD: vitamin D;VDR: vitamin D receptor;WT: wild-type

Vitamin D in autophagy signaling for health and diseases: Insights on potential mechanisms and future perspectives

Vitamin D regulates the pleiotropic effect to maintain cellular homeostasis and epidemiological evidence establishes an association between vitamin D deficiency and various human diseases. Here, the role of autophagy, the cellular self-degradation process, in vitamin D-dependent function is documented in different cellular settings and discussed the molecular aspects for treating chronic inflammatory, infectious diseases, and cancer. Vitamin D activates autophagy through a genomic and non-genomic signaling pathway to influence a wide variety of physiological functions of different body organs along with bone health and calcium metabolism. Moreover, it induces autophagy as a protective mechanism to inhibit oxidative stress and apoptosis to regulate cell proliferation, differentiation, and immune modulation. Furthermore, vitamin D and its receptor regulate autophagy signaling to control inflammation and host immunity by activating antimicrobial defense mechanisms. Vitamin D has been revealed as a potent anticancer agent and induces autophagy to increase the response to radiation and chemotherapeutic drugs for potential cancer therapy. Increasing vitamin D levels in the human body through timely exposure to sunlight or vitamin D supplements could activate autophagy as part of the homeostasis mechanism to prevent multiple human diseases and aging-associated dysfunctions.

1,25-(OH)2D3 protects pancreatic beta cells against H2O2-induced apoptosis through inhibiting the PERK-ATF4-CHOP pathway

Endoplasmic reticulum (ER) stress plays a critical role in pancreatic β cell destruction which leads to the pathogenesis of type 1 diabetes mellitus (T1DM). Vitamin D (VD) has been reported to reduce the risk of T1DM; however, it remains unknown whether VD affects ER stress in pancreatic β cells. In this study, we investigated the role of the active form of VD, 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], in ER stress-induced β cell apoptosis and explored its potential mechanism in mouse insulinoma cell line mouse insulinoma 6 (MIN6). The results of cell counting kit-8 (CCK8) and flow cytometric analyses showed that 1,25-(OH)2D3 caused a significant increase in the viability of MIN6 cells injured by H2O2. The protein kinase like ER kinase (PERK) signal pathway, one of the most conserved branches of ER stress, was found to be involved in this process. H2O2 activated the phosphorylation of PERK, upregulated the activating transcription factor 4 (ATF4) and C/EBP homologous protein (CHOP) expression, and subsequently initiated cell apoptosis, which were significantly reversed by 1,25-(OH)2D3 pretreatment. In addition, GSK2606414, a specific inhibitor of PERK, suppressed PERK phosphorylation and reduced the expressions of ATF4 and CHOP, leading to a significant decrease in β cell apoptosis induced by H2O2. Taken together, the present findings firstly demonstrated that 1,25-(OH)2D3 could prevent MIN6 cells against ER stress-associated apoptosis by inhibiting the PERK-ATF4-CHOP pathway. Therefore, our results suggested that 1,25-(OH)2D3 might serve as a potential therapeutic target for preventing pancreatic β cell destruction in T1DM.