Apelin promotes diabetic nephropathy by inducing podocyte dysfunction via inhibiting proteasome activities

Iridoids modulate inflammation in diabetic kidney disease: A review

In recent years, preclinical research on diabetic kidney disease (DKD) has surged to the forefront of scientific and clinical attention. DKD has become a pervasive complication of type 2 diabetes. Given the complexity of its etiology and pathological mechanisms, current interventions, including drugs, dietary modifications, exercise, hypoglycemic treatments and lipid-lowering methods, often fall short in achieving desired therapeutic outcomes. Iridoids, primarily derived from the potent components of traditional herbs, have been the subject of long-standing research. Preclinical data suggest that iridoids possess notable renal protective properties; however, there has been no summary of the research on their efficacy in the management and treatment of DKD. This article consolidates findings from in vivo and in vitro research on iridoids in the context of DKD and highlights their shared anti-inflammatory activities in treating this condition. Additionally, it explores how certain iridoid components modify their chemical structures through the regulation of intestinal flora, potentially bolstering their therapeutic effects. This review provides a focused examination of the mechanisms through which iridoids may prevent or treat DKD, offering valuable insights for future research endeavors. Please cite this article as: Zhou TY, Tian N, Li L, Yu R. Iridoids modulate inflammation in diabetic kidney disease: A review. J Integr Med. 2024; 22(3): 210-222.

Advances in the study of ELABELA in renal physiological functions and related diseases

ELABELA (ELA), also known as Toddler or Apela, is a novel endogenous ligand of the angiotensin receptor AT1-related receptor protein (APJ). ELA is highly expressed in human embryonic, cardiac, and renal tissues and involves various biological functions, such as embryonic development, blood circulation regulation, and maintaining body fluid homeostasis. ELA is also closely related to the occurrence and development of acute kidney injury, hypertensive kidney damage, diabetic nephropathy, renal tumors, and other diseases. Understanding the physiological role of ELA and its mechanism of action in kidney-related diseases would provide new targets and directions for the clinical treatment of kidney diseases.

An evaluation of both serum Klotho/FGF-23 and apelin-13 for detection of diabetic nephropathy

PURPOSE

The aim of our study is to evaluate whether serum Klotho/FGF-23 and apelin-13 can be used as new biomarkers for detection of development of nephropathy.

METHODS

In this cross-sectional study, 88 type 2 diabetes mellitus (T2DM) patients and 38 healthy controls were included. The mean duration of T2DM was 11.4 ± 9.7 years. T2DM individuals were categorized into two groups as group 1 with e-GFR < 60 mL/min/1.73 m2 and group 2 with e-GFR > 60 mL/min/1.73 m2. They were also divided into two groups according to their 24 h urine albumin levels, classifying them as follows: normoalbuminuria if less than 30 mg/day and albuminuria if more than 30 mg/day.

RESULTS

Mean serum Klotho levels in the T2DM group were observed to be significantly higher than in the control group. Serum apelin-13 levels were observed to be significantly lower in the T2DM group compared to the control group (p < 0.001). In the diabetic group, apelin-13 levels were positively correlated with age, waist circumference, and albuminuria while they were negatively correlated with e-GFR. Apelin-13 levels were seen to be significantly higher in group 1 (p < 0.001).

CONCLUSION

Apelin-13 levels were found to be significantly higher in individuals with diabetic nephropathy than in those without diabetic nephropathy. In the diabetic group, a significant relationship was detected between apelin-13 levels and albumin excretion. Based on these findings, we consider that serum Klotho and apelin-13 levels may have a protective effect on diabetic nephropathy and can additionally be used as a biomarker to predict diabetic nephropathy.

Structure-function relationship and physiological role of apelin and its G protein coupled receptor

Apelin receptor (APJR) is a class A peptide (apelin) binding G protein-coupled receptor (GPCR) that plays a significant role in regulating blood pressure, cardiac output, and maintenance of fluid homeostasis. It is activated by a wide range of endogenous peptide isoforms of apelin and elabela. The apelin peptide isoforms contain distinct structural features that aid in ligand recognition and activation of the receptor. Site-directed mutagenesis and structure-based studies have revealed the involvement of extracellular and transmembrane regions of the receptor in binding to the peptide isoforms. The structural features of APJR activation of the receptor as well as mediating G-protein and β-arrestin-mediated signaling are delineated by multiple mutagenesis studies. There is increasing evidence that the structural requirements of APJR to activate G-proteins and β-arrestins are different, leading to biased signaling. APJR also responds to mechanical stimuli in a ligand-independent manner. A multitude of studies has focused on developing both peptide and non-peptide agonists and antagonists specific to APJR. Apelin/elabela-activated APJR orchestrates major signaling pathways such as extracellular signal-regulated kinase (ERKs), protein kinase B (PKB/Akt), and p70S. This review focuses on the structural and functional characteristics of apelin, elabela, APJR, and their interactions involved in the binding and activation of the downstream signaling cascade. We also focus on the diverse signaling profile of APJR and its ligands and their involvement in various physiological systems.

Evaluation of Apelin-13 levels in patients with diabetic nephropathy

PURPOSE

There is no clear information about the level of Apelin-13 in patients with diabetic nephropathy (DN). In this study, we investigated whether there is a relationship between Apelin-13 level and the severity of the disease in patients with DN.

METHODS

In our case-control study, we included patients who applied to the endocrinology outpatient clinic in 2019. Patients without a history of diabetes were determined as the healthy group (group 1). The patients were divided into 4 groups according to their microalbumin and creatinine levels. Venous blood samples were obtained from all patients for routine laboratory parameters and Apelin-13 levels. Homeostatic Model Assessment-Insulin Resistance (HOMA-IR) for insulin resistance was calculated using the formula: plasma glucose X insulin level/405.

RESULTS

Albumin was found to be significantly lower in group 5 (p = 0.032), hemoglobin A1c, microalbumin/creatinine and HOMA-IR values were found to be significantly lower in group 1 (p < 0.001 for each). Apelin-13 level was found to be significantly higher in group 4 and group 5 (p < 0.001). A negative correlation was found between Apelin-13 and GFR (r = - 0.286, p = 0.003). A positive correlation was found between Apelin-13 and HOMA-IR (r = 0.309, p = 0.009) and microalbumin/creatinine (r = 0.296, p < 0.001).

CONCLUSION

In patients with DN, Apelin-13 level increases with the severity of the disease and can be used as a biomarker for staging of DN.

The Role of Apelin-APJ System in Diabetes and Obesity

Nowadays, diabetes and obesity are two main health-threatening metabolic disorders in the world, which increase the risk for many chronic diseases. Apelin, a peptide hormone, exerts its effect by binding with angiotensin II protein J receptor (APJ) and is considered to be linked with diabetes and obesity. Apelin and its receptor are widely present in the body and are involved in many physiological processes, such as glucose and lipid metabolism, homeostasis, endocrine response to stress, and angiogenesis. In this review, we summarize the literatures on the role of the Apelin-APJ system in diabetes and obesity for a better understanding of the mechanism and function of apelin and its receptor in the pathophysiology of diseases that may contribute to the development of new therapies.

Apelin pathway in cardiovascular, kidney, and metabolic diseases: Therapeutic role of apelin analogs and apelin receptor agonists

The apelin/apelin receptor (ApelinR) signal transduction pathway exerts essential biological roles, particularly in the cardiovascular system. Disturbances in the apelin/ApelinR axis are linked to vascular, heart, kidney, and metabolic disorders. Therefore, the apelinergic system has surfaced as a critical therapeutic strategy for cardiovascular diseases (including pulmonary arterial hypertension), kidney disease, insulin resistance, hyponatremia, preeclampsia, and erectile dysfunction. However, apelin peptides are susceptible to rapid degradation through endogenous peptidases, limiting their use as therapeutic tools and translational potential. These proteases include angiotensin converting enzyme 2, neutral endopeptidase, and kallikrein thereby linking the apelin pathway with other peptide systems. In this context, apelin analogs with enhanced proteolytic stability and synthetic ApelinR agonists emerged as promising pharmacological alternatives. In this review, we focus on discussing the putative roles of the apelin pathway in various physiological systems from function to dysfunction, and emphasizing the therapeutic potential of newly generated metabolically stable apelin analogs and non-peptide ApelinR agonists.

The therapeutic potential of apelin in kidney disease

Chronic kidney disease (CKD) is a leading cause of global morbidity and mortality and is independently associated with cardiovascular disease. The mainstay of treatment for CKD is blockade of the renin-angiotensin-aldosterone system (RAAS), which reduces blood pressure and proteinuria and slows kidney function decline. Despite this treatment, many patients progress to kidney failure, which requires dialysis or kidney transplantation, and/or die as a result of cardiovascular disease. The apelin system is an endogenous physiological regulator that is emerging as a potential therapeutic target for many diseases. This system comprises the apelin receptor and its two families of endogenous ligands, apelin and elabela/toddler. Preclinical and clinical studies show that apelin receptor ligands are endothelium-dependent vasodilators and potent inotropes, and the apelin system has a reciprocal relationship with the RAAS. In preclinical studies, apelin regulates glomerular haemodynamics and acts on the tubule to promote aquaresis. In addition, apelin is protective in several kidney injury models. Although the apelin system has not yet been studied in patients with CKD, the available data suggest that apelin is a promising potential therapeutic target for kidney disease.

Coregulation Analysis of Mechanistic Biomarkers in Autosomal Dominant Polycystic Kidney Disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary kidney disorder leading to deterioration of kidney function and end stage kidney disease (ESKD). A number of molecular processes are dysregulated in ADPKD but the exact mechanism of disease progression is not fully understood. We measured protein biomarkers being linked to ADPKD-associated molecular processes via ELISA in urine and serum in a cohort of ADPKD patients as well as age, gender and eGFR matched CKD patients and healthy controls. ANOVA and t-tests were used to determine differences between cohorts. Spearman correlation coefficient analysis was performed to assess coregulation patterns of individual biomarkers and renal function. Urinary epidermal growth factor (EGF) and serum apelin (APLN) levels were significantly downregulated in ADPKD patients. Serum vascular endothelial growth factor alpha (VEGFA) and urinary angiotensinogen (AGT) were significantly upregulated in ADPKD patients as compared with healthy controls. Arginine vasopressin (AVP) was significantly upregulated in ADPKD patients as compared with CKD patients. Serum VEGFA and VIM concentrations were positively correlated and urinary EGF levels were negatively correlated with urinary AGT levels. Urinary EGF and AGT levels were furthermore significantly associated with estimated glomerular filtration rate (eGFR) in ADPKD patients. In summary, altered protein concentrations in body fluids of ADPKD patients were found for the mechanistic markers EGF, APLN, VEGFA, AGT, AVP, and VIM. In particular, the connection between EGF and AGT during progression of ADPKD warrants further investigation.

Efficacy and Mechanisms of Oleuropein in Mitigating Diabetes and Diabetes Complications

The global pandemic of diabetes and diabetes complications confers heavy pressure on public health. Novel antidiabetes strategies with negligible unwanted effects are urgently needed. Currently, the anti-hyperglycemic potential of plant-based functional ingredients has been explored to provide alternative strategies. As a kind of dietary bioactive compound, oleuropein has aroused the growing interest of researchers in diabetes and diabetes complications management. This review reveals the research progress of oleuropein in treating diabetes and diabetes complications and summarizes the molecular mechanisms involved in these beneficial effects of oleuropein. Oleuropein achieves amelioration of diabetes, the mechanisms of which include the modulation of insulin secretion, the repairment islet morphology, the activation of hepatic AMP-activated protein kinase singling, and the improvement of glucose tolerance and insulin resistance. Oleuropein also can relieve diabetes complications including diabetic nephropathy, diabetes cardiovascular complications, diabetic retinopathy, poor wound healing, diabetic neuropathy, and diabetic testicular dysfunction. Oleuropein reverses cell apoptosis, regenerates tissues, restores the histological organization, and decreases oxidative stress in treating diabetes complications. Taken together, oleuropein is a promising compound for diabetes and diabetes complications management and can be used as a nutraceutical to fight against these diseases.

Impairment of Proteasome Function in Podocytes Leads to CKD

BACKGROUND

The ubiquitin-proteasome system (UPS) and the autophagy-lysosomal system (APLS) are major intracellular degradation procedures. The importance of the APLS in podocytes is established, but the role of the UPS is not well understood.

METHODS

To investigate the role of the UPS in podocytes, mice were generated that had deletion of Rpt3 (Rpt3^pdKO), which encodes an essential regulatory subunit required for construction of the 26S proteasome and its deubiquitinating function.

RESULTS

Rpt3^pdKO mice showed albuminuria and glomerulosclerosis, leading to CKD. Impairment of proteasome function caused accumulation of ubiquitinated proteins and of oxidative modified proteins, and it induced podocyte apoptosis. Although impairment of proteasome function normally induces autophagic activity, the number of autophagosomes was lower in podocytes of Rpt3^pdKO mice than in control mice, suggesting the autophagic activity was suppressed in podocytes with impairment of proteasome function. In an in vitro study, antioxidant apocynin and autophagy activator rapamycin suppressed podocyte apoptosis induced by proteasome inhibition. Moreover, rapamycin ameliorated the glomerular injury in the Rpt3^pdKO mice. The accumulation of ubiquitinated proteins and of oxidative modified proteins, which were detected in the podocytes of Rpt3^pdKO mice, is a characteristic feature of aging. An aging marker was increased in the podocytes of Rpt3^pdKO mice, suggesting that impairment of proteasome function promoted signs of aging in podocytes.

CONCLUSIONS

Impairment of proteasome function in podocytes led to CKD, and antioxidants and autophagy activators can be therapeutic agents for age-dependent CKD.

Inhibition of endoplasmic reticulum stress mediates the ameliorative effect of apelin on vascular calcification

AIMS

Apelin is the endogenous ligand of G protein-coupled receptor APJ and play an important role in the regulation of cardiovascular homeostasis. We aimed to investigate whether apelin ameliorates vascular calcification (VC) by inhibition of endoplasmic reticulum stress (ERS).

METHODS AND RESULTS

VC model in rats was induced by nicotine plus vitamin D, while calcification of vascular smooth muscle cell (VSMC) was induced by beta-glycerophosphate. Alizarin Red S staining showed dramatic calcium deposition in the aorta of rats with VC, while calcium contents and ALP activity also increased in calcified aorta. Protein levels of apelin and APJ were decreased in the calcified aorta. In rats with VC, apelin treatment significantly ameliorated aortic calcification, compliance and stimulation of ERS. The ameliorative effect of apelin on VC and ERS was also observed in calcified VSMCs. ERS stimulator (tunicamycin or DTT) blocked the beneficial effect of apelin. Apelin treatment activated the PI3K/Akt signaling, blockage of which by wortmannin or inhibitor IV prevented the ameliorative effect of apelin, while ERS inhibitor 4-PBA rescued the blockade effect of wortmannin. Akt-induced GSK inhibition prevented the phosphorylation of PERK and IRE1, and the activation of these two major ERS branches. F13A blocked the ameliorative effect of apelin on VC and ERS, which was reversed by treatment with 4-PBA or Akt activator SC79 CONCLUSIONS: Apelin ameliorated VC by binding to APJ and then prevented ERS activation by stimulating Akt signaling. These results might provide new target for therapy and prevention of VC.

Glomerular endothelial cell heterogeneity in Alport syndrome

Glomerular endothelial cells (GEC) are a crucial component of the glomerular physiology and their damage contributes to the progression of chronic kidney diseases. How GEC affect the pathology of Alport syndrome (AS) however, is unclear. We characterized GEC from wild type (WT) and col4α5 knockout AS mice, a hereditary disorder characterized by progressive renal failure. We used endothelial-specific Tek-tdTomato reporter mice to isolate GEC by FACS and performed transcriptome analysis on them from WT and AS mice, followed by in vitro functional assays and confocal and intravital imaging studies. Biopsies from patients with chronic kidney disease, including AS were compared with our findings in mice. We identified two subpopulations of GEC (dim^tdT and bright^tdT) based on the fluorescence intensity of the Tek^tdT signal. In AS mice, the bright^tdT cell number increased and presented differential expression of endothelial markers compared to WT. RNA-seq analysis revealed differences in the immune and metabolic signaling pathways. In AS mice, dim^tdT and bright^tdT cells had different expression profiles of matrix-associated genes (Svep1, Itgβ6), metabolic activity (Apom, Pgc1α) and immune modulation (Apelin, Icam1) compared to WT mice. We confirmed a new pro-inflammatory role of Apelin in AS mice and in cultured human GEC. Gene modulations were identified comparable to the biopsies from patients with AS and focal segmental glomerulosclerosis, possibly indicating that the same mechanisms apply to humans. We report the presence of two GEC subpopulations that differ between AS and healthy mice or humans. This finding paves the way to a better understanding of the pathogenic role of GEC in AS progression and could lead to novel therapeutic targets.

Apelin-13 attenuates high glucose-induced calcification of MOVAS cells by regulating MAPKs and PI3K/AKT pathways and ROS-mediated signals

Vascular calcification (VC) is an inducement of many cardiovascular diseases. Clinic evidences have confirmed that diabetes was the independent risk factor for VC, and the mechanism has not been well explored. Apelin as a ligand molecule is widely found in the cardiovascular system and showed potential in inhibiting VC, but the inhibitory effect and mechanism of apelin-13 against high glucose-induced VC have not been investigated yet. Herein, apelin-13 was employed to inhibit high glucose-induced VC in mouse aortic vascular smooth muscle cells (MOVAS), and the underlying mechanism was explored. The results showed that apelin-13 significantly inhibited high glucose-induced cells proliferation, migration and invasion of MOVAS cells. Apelin-13 also effectively attenuated high glucose-induced calcification by inhibiting alkaline phosphatase (ALP) activity and expression. Further investigation revealed that apelin-13 dramatically suppressed high glucose-induced DNA damage through inhibiting reactive oxide species (ROS) generation. Moreover, apelin-13 also effectively improved high glucose-induced dysfunction of MAPKs and PI3K/AKT. Inhibition of ERK by inhibitor (U0126) significantly blocked high glucose-induced calcification, which further confirmed the significance of MAPKs. Taken together, these results suggested that apelin-13 had the potential to attenuate high glucose-induced calcification of MOVAS cells by inhibiting ROS-mediated DNA damage and regulating MAPKs and PI3K/AKT pathways. Our findings validated the strategy of using apelin-13 maybe a novel way in treating high glucose-mediated VC.

Association between Renal Podocalyxin Expression and Renal Dysfunction in Patients with Diabetic Nephropathy: A Single-Center, Retrospective Case-Control Study

This retrospective study investigated whether podocalyxin expression in renal biopsies and urine of patients with diabetic nephropathy (DN) is associated with renal function. This retrospective study included 32 patients with nephropathy, secondary to type 2 diabetes treated at the First Hospital of Lanzhou University (January 2010 to January 2015). Compared with the control group, the DN group had a significantly lower renal expression of podocalyxin and higher urinary podocalyxin/creatinine ratio. Patients with DN were divided into the high and low expression groups according to podocalyxin expression in renal tissues. Patients in the low expression group had longer diabetes duration, lower plasma albumin and eGFR, higher glycated hemoglobin (HbA1c), 24 h urinary protein, serum creatinine, and urinary podocalyxin/creatinine ratio, and more severe glomerular, tubulointerstitial, and renal interstitial inflammation than patients in the high expression group (all P < 0.01). The renal survival rate was significantly lower in the low expression group than in the high expression group (P < 0.01). Single-factor Cox regression analysis showed that reduced podocalyxin expression and increased urinary podocalyxin excretion were associated with poor renal outcome. Measuring podocalyxin levels in renal tissues and urine could help evaluate the progression of DN.

New Insights into Adipokines as Potential Biomarkers for Type-2 Diabetes Mellitus

A large number of studies have been focused on investigating serum biomarkers associated with risk or diagnosis of type-2 diabetes mellitus. In the last decade, promising studies have shown that circulating levels of adipokines could be used as a relevant biomarker for diabetes mellitus progression as well as therapeutic future targets. Here, we discuss the possible use of recently described adipokines, including apelin, omentin-1, resistin, FGF-21, neuregulin-4 and visfatin, as early biomarkers for diabetes. In addition, we also include recent findings of other well known adipokines such as leptin and adiponectin. In conclusion, further studies are needed to clarify the pathophysiological significance and clinical value of these biological factors as potential biomarkers in type-2 diabetes and related dysfunctions.

International Union of Basic and Clinical Pharmacology. CVII. Structure and Pharmacology of the Apelin Receptor with a Recommendation that Elabela/Toddler Is a Second Endogenous Peptide Ligand

The predicted protein encoded by the APJ gene discovered in 1993 was originally classified as a class A G protein-coupled orphan receptor but was subsequently paired with a novel peptide ligand, apelin-36 in 1998. Substantial research identified a family of shorter peptides activating the apelin receptor, including apelin-17, apelin-13, and [Pyr1]apelin-13, with the latter peptide predominating in human plasma and cardiovascular system. A range of pharmacological tools have been developed, including radiolabeled ligands, analogs with improved plasma stability, peptides, and small molecules including biased agonists and antagonists, leading to the recommendation that the APJ gene be renamed APLNR and encode the apelin receptor protein. Recently, a second endogenous ligand has been identified and called Elabela/Toddler, a 54-amino acid peptide originally identified in the genomes of fish and humans but misclassified as noncoding. This precursor is also able to be cleaved to shorter sequences (32, 21, and 11 amino acids), and all are able to activate the apelin receptor and are blocked by apelin receptor antagonists. This review summarizes the pharmacology of these ligands and the apelin receptor, highlights the emerging physiologic and pathophysiological roles in a number of diseases, and recommends that Elabela/Toddler is a second endogenous peptide ligand of the apelin receptor protein.

Angiotensin-(1-7), Adipokines and Inflammation

Nowadays the adipose tissue is recognized as one of the most critical endocrine organs releasing many adipokines that regulate metabolism, inflammation and body homeostasis. There are several described adipokines, including the renin-angiotensin system (RAS) components that are especially activated in some diseases with increased production of angiotensin II and several pro-inflammatory hormones. On the other hand, RAS also expresses angiotensin-(1-7), which is now recognized as the main peptide on counteracting Ang II effects. New studies have shown that increased activation of ACE2/Ang-(1-7)/MasR arm can revert and prevent local and systemic dysfunctions improving lipid profile and insulin resistance by modulating insulin actions, and reducing inflammation. In this context, the present review shows the interaction and relevance of Ang-(1-7) effects on regulating adipokines, and as one adipokine itself, modulating body homeostasis, with emphasis on its anti-inflammatory properties, especially in the context of metabolic disorders with focus on obesity and type 2 diabetes mellitus pandemic.

Apelin and chemerin as promising adipokines in children with type 1 diabetes mellitus

BACKGROUND

Type 1 diabetes mellitus (T1DM) is one of the most common chronic diseases in children that may be due to micro or macrovascular complications. Diabetic renal disease or nephropathy is a common complication of DM, clinically silent and the only detectable abnormality due to the presence of microalbuminuria.

SUBJECTS AND METHODS

This study was a case-control study. Participants were classified into two groups. The first group included 40 children with T1DM and the second group included 30 matched healthy controls. Serum apelin (APLN), chemerin, cholesterol, and triglycerides (TG) levels were measured for each case. Also, albumin/creatinine ratio was analyzed in random urine sample.

RESULTS

Comparison between T1DM patients and controls revealed that serum apelin, chemerin, cholesterol, TG levels, and albuminuria were significantly increased in cases compared to their controls. Significant positive correlations were found between HbA1c% and albuminuria for APLN and chemerin in the diabetic group. Whereas significant negative correlations were found between apelin and glomerular filtration rate (GFR).

CONCLUSION

Increased levels of serum apelin and chemerin in T1DM patients may be considered as promising adipokines for the development of diabetic complication.

Apelin inhibited epithelial-mesenchymal transition of podocytes in diabetic mice through downregulating immunoproteasome subunits β5i

The epithelial−mesenchymal transition (EMT) of podocytes had been reported to be involved in the glomerular fibrosis in diabetic kidney diseases, which was regulated by TGFβ and NFκB pathways. And apelin, an adipokine which is upregulated in diabetic kidney diseases, was reported to be negatively correlated to TGFβ in polycystic kidney disease and attenuate EMT in renal tubular cells. Therefore, it is hypothesized that apelin might inhibit the EMT of podocytes through downregulating the expression and activation of TGFβ/Smad pathway in diabetic kidney diseases. The results showed that apelin in glomeruli of diabetic mice were increased and exogenous apelin inhibited the EMT of podocytes in diabetic mice, which were accompanied with the decreased expression of proteasome subunits β5i. The results from β5iKO mice confirmed that the inhibiting effects of apelin on EMT of podocytes in diabetic mice were dependent on β5i. The results from culture podocytes showed that apelin decreased the degradation of pIκB and promoted the translocation of IκB into nucleus through decreasing the expression of β5i, which would inhibit the promoting effects of NFκB on expression of TGFβ and followed by decreased activation of Smad pathway and EMT in podocytes. In conclusion, apelin might act as an EMT suppressor for podocytes to decrease the process of glomerular fibrosis in diabetic mice.

Serum Apelin: A New Marker of Early Atherosclerosis in Children with Type 1 Diabetes Mellitus

INTRODUCTION:

Type 1 diabetes mellitus (T1DM) is one of the most common chronic diseases in children that may be complicated by micro or macrovascular complications. Measurement of the carotid intima-media thickness (CIMT) allows the early detection of atherosclerotic alterations of blood vessels that may complicate T1DM.

SUBJECTS AND METHODS:

This study is a case-control study. Participants were classified into two groups. The first group included 40 children with T1DM and the second group included 30 matched healthy controls. The studied cases were recruited from Endocrinology and Diabetology Unit, Pediatric Hospital, Ain Shams University. Serum apelin, cholesterol, TG, LDL were measured for every case. Also, albumin level was analyzed in urine. Measurement of the carotid intima-media thickness (CIMT) was done for all cases.

RESULTS:

Comparison between T1DM patients and controls revealed that serum apelin, cholesterol, TG, LDL and albuminuria were significantly increased in cases compared to controls. Significant positive correlations were detected between HbA1C, albuminuria and lipid profile with apelin in the diabetic group (p < 0.05). CIMT has significant positive correlation with serum apelin levels (r = 0.36, p = 0.05). Also, this study found positive correlations between CIMT and some variables as LDL, SBP z-score and duration of the illness.

CONCLUSION:

Increased levels of serum apelin in T1DM patients may be considered as predicting factor for the ongoing development of vascular sequels. This study highlighted the possible validity of apelin assay as an early predictor of atherosclerosis in T1DM children. Evaluating CIMT in these patients is of at most important for early detection of subclinical atherosclerosis.

Apelin impairs myogenic response to induce diabetic nephropathy in mice

The cause of the invalid reaction of smooth muscle cells to mechanical stimulation that results in a dysfunctional myogenic response that mediates the disruption of renal blood flow (RBF) in patients with diabetes is debatable. The present study revealed that increased apelin concentration in serum of diabetic mice neutralized the myogenic response mediated by apelin receptor (APJ) and resulted in increased RBF, which promoted the progression of diabetic nephropathy. The results showed that apelin concentration, RBF, and albuminuria:creatinine ratio were all increased in kkAy mice, and increased RBF correlated positively with serum apelin both in C57 and diabetic mice. The increased RBF was accompanied by decreased phosphorylation of myosin light chain (MLC), β-arrestin, and increased endothelial NOS in glomeruli. Meanwhile, calcium, phosphorylation of MLC, and β-arrestin were decreased by high glucose and apelin treatment in cultured smooth muscle cells, as well. eNOS was increased by high glucose and increased by apelin in cultured endothelial cells (ECs). Knockdown of β-arrestin expression in smooth muscle cells cancelled phosphorylation of MLC induced by apelin. Therefore, apelin may induce the progression of diabetic nephropathy by counteracting the myogenic response in smooth muscle cells.-Zhang, J., Yin, J., Wang, Y., Li, B., Zeng, X. Apelin impairs myogenic response to induce diabetic nephropathy in mice.

Apelinergic System Structure and Function

Apelin and apela (ELABELA/ELA/Toddler) are two peptide ligands for a class A G-protein-coupled receptor named the apelin receptor (AR/APJ/APLNR). Ligand-AR interactions have been implicated in regulation of the adipoinsular axis, cardiovascular system, and central nervous system alongside pathological processes. Each ligand may be processed into a variety of bioactive isoforms endogenously, with apelin ranging from 13 to 55 amino acids and apela from 11 to 32, typically being cleaved C-terminal to dibasic proprotein convertase cleavage sites. The C-terminal region of the respective precursor protein is retained and is responsible for receptor binding and subsequent activation. Interestingly, both apelin and apela exhibit isoform-dependent variability in potency and efficacy under various physiological and pathological conditions, but most studies focus on a single isoform. Biophysical behavior and structural properties of apelin and apela isoforms show strong correlations with functional studies, with key motifs now well determined for apelin. Unlike its ligands, the AR has been relatively difficult to characterize by biophysical techniques, with most characterization to date being focused on effects of mutagenesis. This situation may improve following a recently reported AR crystal structure, but there are still barriers to overcome in terms of comprehensive biophysical study. In this review, we summarize the three components of the apelinergic system in terms of structure-function correlation, with a particular focus on isoform-dependent properties, underlining the potential for regulation of the system through multiple endogenous ligands and isoforms, isoform-dependent pharmacological properties, and biological membrane-mediated receptor interaction. © 2018 American Physiological Society. Compr Physiol 8:407-450, 2018.

An overview on biological functions and emerging therapeutic roles of apelin in diabetes mellitus

Type 2 diabetes mellitus is a common type of diabetes and considered as multifactorial disease. Apelin is an adipokine which secreted from white adipose tissue and involved in various functions such as insulin sensitivity and food intake. Many studies showed that apelin has a crucial role in diabetes and its concentration will change in relation with insulin resistance. In this review, we will discuss the roles of apelin in energy metabolism and pathogenesis of diabetes and explain why apelin can be a good candidate adipokine to promoting insulin sensitivity.

Apelin/APJ system: A novel potential therapy target for kidney disease

Apelin is an endogenous ligand of seven-transmembrane G protein-coupled receptor APJ. Apelin and APJ are distributed in various tissues, including the heart, lung, kidney, and even in tumor tissues. Studies show that apelin mRNA is highly expressed in the inner stripe of kidney outer medulla, which plays an important role in process of water and sodium balance. Additionally, more studies also indicate that apelin/APJ system exerts a broad range of activities in kidney. Therefore, we review the role of apelin/APJ system in kidney diseases such as renal fibrosis, renal ischemia/reperfusion injury, diabetic nephropathy, polycystic kidney disease, and hemodialysis (HD). Apelin/APJ system can improve renal interstitial fibrosis by reducing the deposition of extracellular matrix. Apelin/APJ system significantly reduces renal ischemia/reperfusion injury by inhibiting renal cell death. Apelin/APJ system involves the progression of diabetic nephropathy (DN). Apelin/APJ system also predicts the process of polycystic kidney disease. Besides, apelin/APJ system prevents some dialysis complications in HD patients. And apelin/APJ system alleviates chronic kidney disease (CKD) by inhibiting vascular calcification (VC). Overall, apelin/APJ system plays diversified roles in kidney disease and may be a potential target for the treatment of kidney disease.

Functional characterization of zebrafish orthologs of the human Beta 3-Glucosyltransferase B3GLCT gene mutated in Peters Plus Syndrome

Peters Plus Syndrome (PPS) is a rare autosomal recessive disease characterized by ocular defects, short stature, brachydactyly, characteristic facial features, developmental delay and other highly variable systemic defects. Classic PPS is caused by loss-of-function mutations in the B3GLCT gene encoding for a β3-glucosyltransferase that catalyzes the attachment of glucose via a β1–3 glycosidic linkage to O-linked fucose on thrombospondin type 1 repeats (TSRs). B3GLCT was shown to participate in a non-canonical ER quality control mechanism; however, the exact molecular processes affected in PPS are not well understood. Here we report the identification and characterization of two zebrafish orthologs of the human B3GLCT gene, b3glcta and b3glctb. The b3glcta and b3glctb genes encode for 496-aa and 493-aa proteins with 65% and 57% identity to human B3GLCT, respectively. Expression studies demonstrate that both orthologs are widely expressed with strong presence in embryonic tissues affected in PPS. In vitro glucosylation assays demonstrated that extracts from wildtype embryos contain active b3glct enzyme capable of transferring glucose from UDP-glucose to an O-fucosylated TSR, indicating functional conservation with human B3GLCT. To determine the developmental role of the zebrafish genes, single and double b3glct knockouts were generated using TALEN-induced genome editing. Extracts from double homozygous b3glct^-/- embryos demonstrated complete loss of in vitro b3glct activity. Surprisingly, b3glct^-/- homozygous fish developed normally. Transcriptome analyses of head and trunk tissues of b3glct^-/- 24-hpf embryos identified 483 shared differentially regulated transcripts that may be involved in compensation for b3glct function in these embryos. The presented data show that both sequence and function of B3GLCT/b3glct genes is conserved in vertebrates. At the same time, complete b3glct deficiency in zebrafish appears to be inconsequential and possibly compensated for by a yet unknown mechanism.

Apelin involved in progression of diabetic nephropathy by inhibiting autophagy in podocytes

Podocyte autophagy dysfunction has been reported to be responsible for the progression of diabetic nephropathy (DN), however, the factors contributed to autophagy dysfunction in type 2 diabetes are not fully understood. Among promoting factors in DN, an adipokine, apelin, had been showed to trigger podocyte dysfunction. Therefore, it is hypothesized that apelin, which is increased in plasma in type 2 diabetes, lead to podocyte apoptosis through inhibiting podocyte autophagy, which resulted in podocyte dysfunction followed by DN. KkAy mice (diabetic mice) and cultured podocytes (MPC5 cells and native podocytes) were treated with high glucose (HG) and apelin or its antagonist F13A. Renal function, podocyte autophagy, podocyte apoptosis and corresponding cell signaling pathways in podocytes were detected. The results showed that apelin aggravated the renal dysfunction and foot process injuries in kkAy mice, which is positively correlated to podocyte apoptosis and negatively correlated to podocyte autophagy. Apelin induced podocyte apoptosis and inhibited podocyte autophagy in both normal glucose and HG conditions while F13A reversed these effects. Investigations by western blot found that apelin inhibits podocyte autophagy through ERK-, Akt- and mTOR-dependent pathways. In conclusion, increased apelin concentration in plasma inhibited podocyte autophagy, which would lead to podocyte apoptosis and renal dysfunction in diabetes. These effects would contribute to the progression of DN.

Apelin/APJ system as a therapeutic target in diabetes and its complications

The G-protein-coupled receptor APJ and its endogenous ligand apelin are widely expressed in many peripheral tissues and central nervous system, including adipose tissue, skeletal muscles and hypothalamus. Apelin/APJ system, involved in numerous physiological functions like angiogenesis, fluid homeostasis and energy metabolism regulation, is notably implicated in the development of different pathologies such as diabetes and its complications. Increasing evidence suggests that apelin regulates insulin sensitivity, stimulates glucose utilization and enhances brown adipogenesis in different tissues associated with diabetes. Moreover, apelin is also involved in the regulation of diabetic complications via binding to APJ receptor. Apelin improves diabetes-induced kidney hypertrophia, normalizes obesity-associated cardiac hypertrophy and negatively promotes retinal angiogenesis in diabetic retinopathy. In this review, we provide a comprehensive overview about the role of apelin/APJ system in different tissues related with diabetes. Furthermore, we describe the pathogenesis of diabetic complications associated with apelin/APJ system. Finally, agonists and antagonists targeted to APJ receptor are described in the literature. Thus, we highlight apelin/APJ system as a novel therapeutic target for pharmacological intervention in treating diabetes and its complications.

1,4-Dihydropyridine derivatives without Ca2+-antagonist activity up-regulate Psma6 mRNA expression in kidneys of intact and diabetic rats

Impaired degradation of proteins by the ubiquitin-proteasome system (UPS) is observed in numerous pathologies including diabetes mellitus (DM) and its complications. Dysregulation of proteasomal degradation might be because of altered expression of genes and proteins involved in the UPS. The search for novel compounds able to normalize expression of the UPS appears to be a topical problem. A novel group of 1,4-dihydropyridine (1,4-DHP) derivatives lacking Ca2+-antagonists activities, but capable to produce antidiabetic, antioxidant and DNA repair enhancing effects, were tested for ability to modify Psma6 mRNA expression levels in rat kidneys and blood in healthy animals and in rats with streptozotocin (STZ) induced DM. Psma6 gene was chosen for the study, as polymorphisms of its human analogue are associated with DM and cardiovascular diseases. 1,4-DHP derivatives (metcarbatone, etcarbatone, glutapyrone, J-9-125 and AV-153-Na) were administered per os for three days (0.05 mg/kg and/or 0.5 mg/kg). Psma6 gene expression levels were evaluated by quantitative PCR. Psma6 expression was higher in kidneys compared to blood. Induction of diabetes caused increase of Psma6 expression in kidneys, although it was not changed in blood. Several 1,4-DHP derivatives increased expression of the gene both in kidneys and blood of control and model animals, but greater impact was observed in kidneys. The observed effect might reflect coupling of antioxidant and proteolysis-promoting activities of the compounds.