Apelin protects against acute renal injury by inhibiting TGF-β1

Apelinergic system in acute kidney injury: Mechanistic insights and therapeutic potential

Acute kidney injury (AKI) has emerged as a global health crisis, surpassing mortality rates associated with several cancers and heart failure. The lack of effective therapies, coupled with challenges in diagnosis and the high cost of kidney transplantation, underscores the urgent need to explore novel therapeutic targets and strategies for AKI. Understanding the intricate pathophysiology of AKI is paramount in this endeavor. The components of the apelinergic system-namely, apelin and elabela/toddler, along with their receptor-are prominently expressed in various kidney cells and have garnered significant attention in renal research. Recent studies have highlighted the renoprotective role of the apelinergic system in AKI. This system exerts its protective effects by modulating several pathophysiological processes, including reducing endoplasmic reticulum (ER) stress, improving mitochondrial dynamics, inhibiting inflammation and apoptosis, promoting diuresis through vasodilation of renal vasculature, and counteracting the effects of reactive oxygen species (ROS). Despite these advancements, the precise involvement of the apelinergic system in the progression of AKI remains unclear. Furthermore, the therapeutic potential of apelin-13 in AKI is not fully understood. This review aims to elucidate the role of the apelinergic system in AKI and its interactions with key pathomechanisms involved in the progression of AKI. Additionally, we discuss the current clinical status of exogenous apelin-13 therapy, providing insights that will guide future research on apelin against AKI.

Unraveling the Complex Molecular Interplay and Vascular Adaptive Changes in Hypertension-Induced Kidney Disease

Angiogenesis, the natural mechanism by which fresh blood vessels develop from preexisting ones, is altered in arterial hypertension (AH), impacting renal function. Studies have shown that hypertension-induced renal damage involves changes in capillary density (CD), indicating alterations in vascularization. We aimed to elucidate the role of the apelin receptor (APLNR), neuronal nitric oxide synthase (nNOS), and vascular endothelial growth factor (VEGF) in hypertension-induced renal damage. We used two groups of spontaneously hypertensive rats aged 6 and 12 months, representing different stages of AH, and compared them to age-matched normotensive controls. The kidney tissue samples were prepared through a well-established protocol. All data analysis was conducted with a dedicated software program. APLNR was localized in tubular epithelial cells and the endothelial cells of the glomeruli, with higher expression in older SHRs. The localization of nNOS and VEGF was similar. The expression of APLNR and nNOS increased with AH progression, while VEGF levels decreased. CD was lower in young SHRs compared to controls and decreased significantly in older SHRs in comparison to age-matched controls. Our statistical analysis revealed significant differences in molecule expression between age groups and varying correlations between the expression of the three molecules and CD.

RUNX3-activated apelin signaling inhibits cell proliferation and fibrosis in diabetic nephropathy by regulation of the SIRT1/FOXO pathway

BACKGROUND

Diabetic nephropathy is a major secondary cause of end-stage renal disease. Apelin plays an important role in the development of DN. Understanding the exact mechanism of Apelin can help expand the means of treating DN.

METHODS

Male C57BL/6 mice was used and STZ treatment was implemented for DN model establishment. Lentivirus systems including Lv-sh-RUNX3 and Lv-Apelin were obtained to knockdown RUNX3 and overexpress Apelin, respectively. A total of 36 mice were divided into 6 groups (n = 6 in each group): control, DN, DN + LV-Vector, DN + Lv-Apelin, DN + LV-Apelin + LV-sh-NC and DN + Lv-Apelin + Lv-sh-RUNX3 group. In vitro studies were performed using mesangial cells. Cell viability and proliferation was assessed through CCK8 and EDU analysis. Hematoxylin and eosin staining as well as Masson staining was implemented for histological evaluation. RT-qPCR was conducted for measuring relative mRNA levels, and protein expression was detected by western blotting. The interaction between SIRT1 and FOXO were verified by co-immunoprecipitations, and relations between RUNX3 and Apelin were demonstrated by dual luciferase report and chromatin immunoprecipitation.

RESULTS

The DN group exhibited significantly lower Apelin expression compared to control (p < 0.05). Apelin overexpression markedly improved blood glucose, renal function indicators, ameliorated renal fibrosis and reduced fibrotic factor expression (p < 0.05) in the DN group, accompanied by elevated sirt1 levels and diminished acetylated FOXO1/FOXO3a (p < 0.05). However, RUNX3 knockdown combined with Apelin overexpression abrogated these beneficial effects, leading to impaired renal function, exacerbated fibrosis, increased fibrotic factor expression and acetylated FOXO1/FOXO3a versus Apelin overexpression alone (p < 0.05). In mesangial cells under high glucose, Apelin overexpression significantly inhibited cell proliferation and fibrotic factor production (p < 0.05). Conversely, RUNX3 interference enhanced cell proliferation and the secretion of fibrotic factors. (p < 0.05). Remarkably, combining Apelin overexpression with RUNX3 interference reversed the proliferation and fibrosis induced by RUNX3 interference (p < 0.05). Mechanistic studies revealed RUNX3 binds to the Apelin promoter, with the 467-489 bp site1 as the primary binding region, and SIRT1 physically interacts with FOXO1 and FOXO3a in mesangial cells.

CONCLUSION

RUNX3 activated Apelin and regulated the SIRT1/FOXO signaling pathway, resulting in the suppressed cell proliferation and fibrosis in diabetic nephropathy. Apelin is a promising endogenous therapeutic target for anti-renal injury and anti-fibrosis in diabetic nephropathy. RUNX3 may serve as an endogenous intervention target for diseases related to Apelin deficiency.

Effects of conditioned media derived from human Wharton's jelly mesenchymal stem cells on diabetic nephropathy and hepatopathy via modulating TGF-β and apelin signaling pathways in male rats

BACKGROUND

Diabetic nephropathy and hepatopathy are health problems described by specific renal and hepatic structure and function disturbances. The protective effects of the stem cell secretome have been shown in several kidney and liver diseases. The current study aims to evaluate the capability of conditioned media derived from human Wharton's jelly mesenchymal stem cells (hWJ-MSCs-CM) to alleviate diabetic complications.

METHODS

Twenty Sprague Dawley rats were made diabetic through injection of STZ (60 mg/kg, i.p.). At week 8, diabetic rats were divided into two groups: treated [DM + hWJ-MSCs-CM (500 µl/rat for three weeks, i.p.)] and not treated (DM). At the 11th week, three groups (control, DM, and DM + hWJ-MSCs-CM) were kept in metabolic cages, and urine was collected for 24 h. The serum samples were maintained for measuring fasting blood glucose (FBG) and kidney and liver functional analysis. The left kidney and liver parts were kept at -80 °C to assess apelin and transforming growth factor-beta (TGF-β) expression. The right kidney, pancreas, and liver parts were used for histopathologic evaluation.

RESULTS

DM was detected by higher FBG, microalbuminuria, increased albumin/creatinine ratio, and pancreas, renal, and hepatic structural disturbances. Diabetic hepatopathy was determined by increasing liver enzymes and decreasing total bilirubin. The TGF-β gene expression was significantly upregulated in the diabetic kidney and liver tissues. Apelin gene expression was significantly downregulated in the diabetic liver tissue but did not change in kidney tissue. Administration of hWJ-MSCs-CM improved renal and hepatic functional and structural disturbances. Moreover, CM therapy significantly decreased TGF-β expression and enhanced apelin expression in the kidney and liver tissues.

CONCLUSION

Human WJ-MSCs-CM may have protective effects on diabetic renal and hepatic complications. These effects may happen through the regulation of TGF-β and apelin signaling pathways.

Targeting the apelin system for the treatment of cardiovascular diseases

Cardiovascular disease is the leading cause of death worldwide. Its prevalence is rising due to ageing populations and the increasing incidence of diseases such as chronic kidney disease, obesity, and diabetes that are associated with elevated cardiovascular risk. Despite currently available treatments, there remains a huge burden of cardiovascular disease-associated morbidity for patients and healthcare systems, and newer treatments are needed. The apelin system, comprising the apelin receptor and its two endogenous ligands apelin and elabela, is a broad regulator of physiology that opposes the actions of the renin-angiotensin and vasopressin systems. Activation of the apelin receptor promotes endothelium-dependent vasodilatation and inotropy, lowers blood pressure, and promotes angiogenesis. The apelin system appears to protect against arrhythmias, inhibits thrombosis, and has broad anti-inflammatory and anti-fibrotic actions. It also promotes aqueous diuresis through direct and indirect (central) effects in the kidney. Thus, the apelin system offers therapeutic promise for a range of cardiovascular, kidney, and metabolic diseases. This review will discuss current cardiovascular disease targets of the apelin system and future clinical utility of apelin receptor agonism.

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.

A renal YY1-KIM1-DR5 axis regulates the progression of acute kidney injury

Acute kidney injury (AKI) exhibits high morbidity and mortality. Kidney injury molecule-1 (KIM1) is dramatically upregulated in renal tubules upon injury, and acts as a biomarker for various renal diseases. However, the exact role and underlying mechanism of KIM1 in the progression of AKI remain elusive. Herein, we report that renal tubular specific knockout of Kim1 attenuates cisplatin- or ischemia/reperfusion-induced AKI in male mice. Mechanistically, transcription factor Yin Yang 1 (YY1), which is downregulated upon AKI, binds to the promoter of KIM1 and represses its expression. Injury-induced KIM1 binds to the ECD domain of death receptor 5 (DR5), which activates DR5 and the following caspase cascade by promoting its multimerization, thus induces renal cell apoptosis and exacerbates AKI. Blocking the KIM1-DR5 interaction with rationally designed peptides exhibit reno-protective effects against AKI. Here, we reveal a YY1-KIM1-DR5 axis in the progression of AKI, which warrants future exploration as therapeutic targets.

Loss of renal tubular G9a benefits acute kidney injury by lowering focal lipid accumulation via CES1

Surgery-induced renal ischemia and reperfusion (I/R) injury and nephrotoxic drugs like cisplatin can cause acute kidney injury (AKI), for which there is no effective therapy. Lipid accumulation is evident following AKI in renal tubules although the mechanisms and pathological effects are unclear. Here, we report that Ehmt2-encoded histone methyltransferase G9a is upregulated in patients and mouse kidneys after AKI. Renal tubular specific knockout of G9a (Ehmt2Ksp ) or pharmacological inhibition of G9a alleviates lipid accumulation associated with AKI. Mechanistically, G9a suppresses transcription of the lipolytic enzyme Ces1; moreover, G9a and farnesoid X receptor (FXR) competitively bind to the same promoter regions of Ces1. Ces1 is consistently observed to be downregulated in the kidney of AKI patients. Pharmacological inhibition of Ces1 increases lipid accumulation, exacerbates renal I/R-injury and eliminates the beneficial effects on AKI observed in Ehmt2Ksp mice. Furthermore, lipid-lowering atorvastatin and an FXR agonist alleviate AKI by activating Ces1 and reducing renal lipid accumulation. Together, our results reveal a G9a/FXR-Ces1 axis that affects the AKI outcome via regulating renal lipid accumulation.

Apelin is expressed throughout the human kidney, is elevated in chronic kidney disease & associates independently with decline in kidney function

AIMS

Chronic kidney disease (CKD) is common and cardiovascular disease (CVD) is its commonest complication. The apelin system is a potential therapeutic target for CVD but data relating to apelin in CKD are limited. We examined expression of the apelin system in human kidney, and investigated apelin and Elabela/Toddler (ELA), the endogenous ligands for the apelin receptor, in patients with CKD.

METHODS

Using autoradiography, immunohistochemistry and enzyme-linked immunosorbent assay, we assessed expression of apelin, ELA and the apelin receptor in healthy human kidney, and measured plasma apelin and ELA in 155 subjects (128 patients with CKD, 27 matched controls) followed up for 5 years. Cardiovascular assessments included blood pressure, arterial stiffness (pulse wave velocity) and brachial artery flow-mediated dilation. Surrogate markers of endothelial function (plasma asymmetric dimethylarginine and endothelin-1) and inflammation (C-reactive protein and interleukin-6) were measured.

RESULTS

The apelin system was expressed in healthy human kidney, throughout the nephron. Plasma apelin concentrations were 60% higher in women than men (6.48 [3.62-9.89] vs. 3.95 [2.02-5.85] pg/mL; P < .0001), and increased as glomerular filtration rate declined (R = -0.41, P < .0001), and albuminuria rose (R = 0.52, P < .0001). Plasma apelin and ELA were associated with vascular dysfunction. Plasma apelin associated independently with a 50% decline in glomerular filtration rate at 5 years.

CONCLUSION

We show for the first time that the apelin system is expressed in healthy human kidney. Plasma apelin is elevated in CKD and may be a potential biomarker of risk of decline in kidney function. Clinical studies exploring the therapeutic potential of apelin agonism in CKD are warranted.

Renal UTX-PHGDH-serine axis regulates metabolic disorders in the kidney and liver

Global obesity epidemics impacts human health and causes obesity-related illnesses, including the obesity-related kidney and liver diseases. UTX, a histone H3K27 demethylase, plays important roles in development and differentiation. Here we show that kidney-specific knockout Utx inhibits high-fat diet induced lipid accumulation in the kidney and liver via upregulating circulating serine levels. Mechanistically, UTX recruits E3 ligase RNF114 to ubiquitinate phosphoglycerate dehydrogenase, the rate limiting enzyme for de novo serine synthesis, at Lys³¹⁰ and Lys³³⁰, which leads to its degradation, and thus suppresses renal and circulating serine levels. Consistently, phosphoglycerate dehydrogenase and serine levels are markedly downregulated in human subjects with diabetic kidney disease or obesity-related renal dysfunction. Notably, oral administration of serine ameliorates high-fat diet induced fatty liver and renal dysfunction, suggesting a potential approach against obesity related metabolic disorders. Together, our results reveal a metabolic homeostasis regulation mediated by a renal UTX-PHGDH-serine axis.

Epigenetic Regulation in Kidney Transplantation

Kidney transplantation is a standard care for end stage renal disease, but it is also associated with a complex pathogenesis including ischemia-reperfusion injury, inflammation, and development of fibrosis. Over the past decade, accumulating evidence has suggested a role of epigenetic regulation in kidney transplantation, involving DNA methylation, histone modification, and various kinds of non-coding RNAs. Here, we analyze these recent studies supporting the role of epigenetic regulation in different pathological processes of kidney transplantation, i.e., ischemia-reperfusion injury, acute rejection, and chronic graft pathologies including renal interstitial fibrosis. Further investigation of epigenetic alterations, their pathological roles and underlying mechanisms in kidney transplantation may lead to new strategies for the discovery of novel diagnostic biomarkers and therapeutic interventions.

Histone demethylase UTX aggravates acetaminophen overdose induced hepatotoxicity through dual mechanisms

Acetaminophen (APAP) overdose is a major cause of acute liver failure, while the underlying mechanisms of APAP hepatotoxicity are not fully understood. Recently, emerging evidence suggests that epigenetic enzymes play roles in APAP-induced liver injury. Here, we found that Utx (ubiquitously transcribed tetratricopeptide repeat, X chromosome, also known as KDM6A), a X-linked histone demethylase which removes the di- and tri-methyl groups from histone H3K27, was markedly induced in the liver of APAP-overdosed female mice. Hepatic deletion of Utx suppressed APAP overdose-induced hepatotoxicity in female but not male mice. RNA-sequencing analysis suggested that Utx deficiency in female mice upregulated antitoxic phase II conjugating enzymes, including sulfotransferase family 2 A member 1 (Sult2a1), thus reduces the amount of toxic APAP metabolites in injured liver; while Utx deficiency also alleviated ER stress through downregulating transcription of ER stress genes including Atf4, Atf3, and Chop. Mechanistically, Utx promoted transcription of ER stress related genes in a demethylase activity-dependent manner, while repressed Sult2a1 expression through mediating H3K27ac levels independent of its demethylase activity. Moreover, overexpression of Sult2a1 in the liver of female mice rescued APAP-overdose induced liver injury. Together, our results indicated a novel UTX-Sult2a1 axis for the prevention or treatment of APAP-induced liver injury.

Effect of Fc-Elabela-21 on renal ischemia/reperfusion injury in mice: Mediation of anti-apoptotic effect via Akt phosphorylation

INTRODUCTION

Renal ischemia/reperfusion injury (IRI) is the most common cause of acute kidney injury (AKI), and patients with AKI have a high rate of mortality. Apelin is a therapeutic candidate for treatment of IRI and Elabela (ELA) is a recently discovered hormone that also activates the apelin receptor (APJ). We examined the use of ELA as a preventive treatment for IRI using in vitro and in vivo models.

METHODS

Male mice were subjected to renal IRI, with or without administration of a stabilized form of ELA (Fc-ELA-21) for 4 days. Renal tubular lesions were measured using H&E staining, reactive oxygen species (ROS) were measured using a dihydroethidium stain assay, and renal cell apoptosis was measured using the TUNEL assay and flow cytometry. Immortalized human proximal tubular epithelial (HK-2) cells were pretreated with or without LY294002 and/or ELA-32, maintained at normoxic or hypoxic conditions, and then returned to normal culture conditions to mimic IRI. Cell apoptosis was determined using the TUNEL assay and cell proliferation was determined using the MTT assay. The levels of Akt, p-Akt, ERK1/2, p- ERK1/2, Bcl-2, Bax, caspase-3 and cleaved caspase-3 were measured using western blotting.

RESULTS

Fc-ELA-21 administration reduced renal tissue damage, ROS production, and apoptosis in mice that had renal IRI. ELA-32 reduced HK-2 cell apoptosis and restored the proliferation of cells subjected to IRI. Akt phosphorylation had a role in the anti-apoptotic effect of ELA.

CONCLUSION

This study of in vitro and in vivo models of IRI indicated that the preventive and anti-apoptotic effects of ELA were mediated via the PI3K/Akt signaling pathway.

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.

Protective effect of Apelin-13 in a cyclophosphamide-induced cardiorenal toxicity model in rats

Cyclophosphamide is a chemotherapeutic drug that is widely used in the clinic and can cause multi-organ toxicity. Apelin-13 is an endogenous adipocytokine with antioxidant properties. Therefore, this study aimed to investigate the possibility of apelin-13 being a potential therapeutic agent on cardiac toxicity and nephrotoxicity caused by cyclophosphamide. In this study, a total of 4 groups were formed, including 8 rats in each group. Group 1: The control group was administered only saline (ip). Group 2: Cyclophosphamide, a single dose of 200 mg/kg (ip) on day 7. Group 3: Apelin-13 (15 μg/kg), for 7 days (ip). Group 4: Administering apelin-13 (15 μg/kg) (ip) for 7 days and a single dose of cyclophosphamide (200 mg/kg) (ip) on day 7, the rats were sacrificed on day 8. LDH, cTn1, cK-Mb, AST, ALT, ALP, MDA, creatinine, and BUN were found to be high in the cyclophosphamide group, however, these values were reduced with apelin-13 administration. Antioxidant enzymes such as SOD, GPx, CAT, and GSH decreased in the cyclophosphamide group, apelin-13 increased these enzyme activities. In addition, histopathological examinations also supported the results obtained. The findings of this study showed that apelin-13 has a protective effect against cardiorenal toxicity caused by cyclophosphamide.

Apela inhibits systemic and renal inflammatory reactions in mice with type I cardiorenal syndrome

This study investigated the effect of apela on renal function and anti-inflammatory effect on whole body and kidney tissue in mice with type I cardiorenal syndrome (CRS). The murine type I CRS model was established and apela was subcutaneously infused for two weeks. Cardiac and renal functions were evaluated by echocardiography and blood biochemistry, respectively. The systemic and renal inflammatory responses were examined with molecular biological and histological methods. Human renal glomerular endothelial cells (RGECs) were used to evaluate the adhesion effect of monocytes in vitro. Compared to mice from the control group (CRS + vehicle), the plasma levels of N-terminal pro-brain natriuretic peptide, blood urea nitrogen and creatinine were significantly decreased, while the mean left ventricular ejection fraction was increased in apela-treated CRS mice at the 4th week. The expression of monocyte chemoattractant protein-1 (MCP-1) and tumor necrosis factor-α (TNF-α) in the circulation and kidney was decreased in apela-treated mice compared with control mice, and apela improved cardio-renal pathology in mice with type I CRS. Additionally, Apela significantly suppressed the expression of MCP-1, TNF-α, intercellular adhesion molecule-1 and vascular intercellular adhesion molecule-1 in RGECs induced by angiotensin II (Ang II), and inhibited the promoting effect of Ang II on the adhesion of THP-1 cells to RGECs. Western blot results showed that the expression of phosphorylated nuclear factor kappa B (phospho-NFκB) in CRS mice was increased, but the expression of phospho-NFκB was down-regulated after apela treatment. Furthermore, apela significantly inhibited the Ang II-mediated increase in phospho-NFκB expression in RGECs in vitro, but the administration of an apelin peptide jejunum receptor (APJ) inhibitor blocked the inhibitory effect of apela. This study revealed that apela improves cardiorenal function and reduces systemic and renal inflammatory response in type I CRS mice and the apela/APJ system may alleviate renal inflammatory responses by inhibiting the NFκB signalling pathway.

Epigenetics in kidney diseases

Epigenetics examines heritable changes in DNA and its associated proteins except mutations in gene sequence. Epigenetic regulation plays fundamental roles in kidney cell biology through the action of DNA methylation, chromatin modification via epigenetic regulators and non-coding RNA species. Kidney diseases, including acute kidney injury, chronic kidney disease, diabetic kidney disease and renal fibrosis are multistep processes associated with numerous molecular alterations even in individual kidney cells. Epigenetic alterations, including anomalous DNA methylation, aberrant histone alterations and changes of microRNA expression all contribute to kidney pathogenesis. These changes alter the genome-wide epigenetic signatures and disrupt essential pathways that protect renal cells from uncontrolled growth, apoptosis and development of other renal associated syndromes. Molecular changes impact cellular function within kidney cells and its microenvironment to drive and maintain disease phenotype. In this chapter, we briefly summarize epigenetic mechanisms in four kidney diseases including acute kidney injury, chronic kidney disease, diabetic kidney disease and renal fibrosis. We primarily focus on current knowledge about the genome-wide profiling of DNA methylation and histone modification, and epigenetic regulation on specific gene(s) in the pathophysiology of these diseases and the translational potential of identifying new biomarkers and treatment for prevention and therapy. Incorporating epigenomic testing into clinical research is essential to elucidate novel epigenetic biomarkers and develop precision medicine using emerging therapies.

Elabela Protects Spontaneously Hypertensive Rats From Hypertension and Cardiorenal Dysfunctions Exacerbated by Dietary High-Salt Intake

Objectives: Arterial hypertension, when exacerbated by excessive dietary salt intake, worsens the morbidity and mortality rates associated with cardiovascular and renal diseases. Stimulation of the apelinergic system appears to protect against several circulatory system diseases, but it remains unknown if such beneficial effects are conserved in severe hypertension. Therefore, we aimed at determining whether continuous infusion of apelinergic ligands (i.e., Apelin-13 and Elabela) exerted cardiorenal protective effects in spontaneously hypertensive (SHR) rats receiving high-salt diet.

Methods: A combination of echocardiography, binding assay, histology, and biochemical approaches were used to investigate the cardiovascular and renal effects of Apelin-13 or Elabela infusion over 6 weeks in SHR fed with normal-salt or high-salt chow.

Results: High-salt intake upregulated the cardiac and renal expression of APJ receptor in SHR. Importantly, Elabela was more effective than Apelin-13 in reducing high blood pressure, cardiovascular and renal dysfunctions, fibrosis and hypertrophy in high-salt fed SHR. Unlike Apelin-13, the beneficial effects of Elabela were associated with a counter-regulatory role of the ACE/ACE2/neprilysin axis of the renin-angiotensin-aldosterone system (RAAS) in heart and kidneys of salt-loaded SHR. Interestingly, Elabela also displayed higher affinity for APJ in the presence of high salt concentration and better resistance to RAAS enzymes known to cleave Apelin-13.

Conclusion: These findings highlight the protective action of the apelinergic system against salt-induced severe hypertension and cardiorenal failure. As compared with Apelin-13, Elabela displays superior pharmacodynamic and pharmacokinetic properties that warrant further investigation of its therapeutic use in cardiovascular and kidney diseases.

Kidney injury molecule-1 is a potential receptor for SARS-CoV-2

COVID-19 patients present high incidence of kidney abnormalities, which are associated with poor prognosis and mortality. The identification of SARS-CoV-2 in the kidney of COVID-19 patients suggests renal tropism of SARS-CoV-2. However, whether there is a specific target of SARS-CoV-2 in the kidney remains unclear. Herein, by using in silico simulation, coimmunoprecipitation, fluorescence resonance energy transfer, fluorescein isothiocyanate labeling, and rational design of antagonist peptides, we demonstrate that kidney injury molecule-1 (KIM1), a molecule dramatically upregulated upon kidney injury, binds with the receptor-binding domain (RBD) of SARS-CoV-2 and facilitates its attachment to cell membrane, with the immunoglobulin variable Ig-like (Ig V) domain of KIM1 playing a key role in this recognition. The interaction between SARS-CoV-2 RBD and KIM1 is potently blockaded by a rationally designed KIM1-derived polypeptide AP2. In addition, our results also suggest interactions between KIM1 Ig V domain and the RBDs of SARS-CoV and MERS-CoV, pathogens of two severe infectious respiratory diseases. Together, these findings suggest KIM1 as a novel receptor for SARS-CoV-2 and other coronaviruses. We propose that KIM1 may thus mediate and exacerbate the renal infection of SARS-CoV-2 in a ‘vicious cycle’, and KIM1 could be further explored as a therapeutic target.

Bioactive peptide apelin rescues acute kidney injury by protecting the function of renal tubular mitochondria

Mitochondrial dysfunction in proximal tubular epithelial cells is a key event in acute kidney injury (AKI), which is a risk factor for the development of chronic kidney disease (CKD). Apelin is a bioactive peptide that protects against AKI by alleviating inflammation, inhibiting apoptosis, and preventing lipid oxidation, but its role in protecting against mitochondrial damage remains unknown. Herein, we examined the protective effects of apelin on mitochondria in cisplatin-stimulated human renal proximal tubular epithelial cells and evaluated its therapeutic efficacy in cisplatin-induced AKI mice. In vitro, apelin inhibited the cisplatin-induced mitochondrial fission factor (MFF) upregulation and the fusion-promoting protein optic atrophy 1 (OPA1) downregulation. Apelin co-treatment reversed the decreased levels of the deacetylase, Sirt3, and the increased levels of protein acetylation in mitochondria of cisplatin-stimulated cells. Overall, apelin improved the mitochondrial morphology and membrane potential in vitro. In the AKI model, apelin administration significantly attenuated mitochondrial damage, as evidenced by longer mitochondrial profiles and increased ATP levels in the renal cortex. Suppression of MFF expression, and maintenance of Sirt3 and OPA1 expression in apelin-treated AKI mice was also observed. Finally, exogenous administration of apelin normalized the serum level of creatinine and urea nitrogen and the urine levels of NGAL and Kim-1. We also confirmed a regulatory pathway that drives mitochondrial homeostasis including PGC-1α, ERRα and Sirt3. In conclusion, we demonstrated that apelin ameliorates renal functions by protecting tubular mitochondria through Sirt3 upregulation, which is a novel protective mechanism of apelin in AKI. These results suggest that apelin has potential renoprotective effects and may be an effective agent for AKI treatment to significantly retard CKD progression.

Apelin-13 Protects Corpus Cavernosum Against Fibrosis Induced by High-Fat Diet in an MMP-Dependent Mechanism

BACKGROUND

An increased fibrosis of the corpora cavernosa is a prevalent process that underlies most cases of erectile dysfunction. Apelin, an endogenous circulating peptide, has been documented as an important effector on cardiovascular homeostasis, controlling vascular function and reducing fibrosis in multiple pathological conditions. Recently, initial studies have shown that Apelin, acting through the APJ receptor, also modulates penile erection, however, the role of this system on penile structure and intracorporal collagen remodeling has not been investigated yet.

AIMS

Here we sought to investigate the effect of chronic Apelin treatment on the corpus cavernosum structure of hyperchOlesterolemic mice.

METHODS

Apolipoprotein gene-deleted (ApoE^-/-) mice were fed with a Western diet for 11 weeks and received Apelin-13 (2 mg/kg/day) or vehicle during the last 3 weeks. Penile samples were obtained for histological and biochemical analyses to assess the intracorporal collagen content and key proteins expression. Furthermore, the effect of Apelin-13 was evaluated in cultured NIH3T3 mouse fibroblasts stimulated with TGF-β.

OUTCOME

Local expression of Apelin-13 in mouse corpus cavernosum and its protective effect against fibrosis.

RESULTS

Apelin and APJ receptor were expressed (gene and protein) within the corpus cavernosum of ApoE^-/- mice, indicating a local modulation of the Apelin system. Interestingly, 3 weeks of Apelin-13 treatment strongly reduced intracavernosal collagen content. In addition, Apelin-13 enhanced total matrix metalloproteinase (MMP) activity in the mice penis, which was associated with an increased protein expression of MMP-1, MMP-3, MMP-8, and MMP-9, while tissue inhibitor of metalloproteinase were unaltered. These beneficial actions were not associated with changes in nNOS or eNOS protein expression, intracavernosal reactive oxygen species content, or atherosclerotic plaque deposition. Additionally, in cultured fibroblast, Apelin-13 inhibited TGF-β-induced fibroblast to myofibroblast differentiation and collagen production, possibly through the activation of ERK1/2 kinase.

CLINICAL TRANSLATION

These results point out Apelin/APJ system as a potential target to treat intracavernosal fibrosis-related disorders.

STRENGTH & LIMITATIONS

These results provide the first evidence of the Apelin system's positive role on erectile tissue structure/remodeling. Nevertheless, additional functional study addressing erectile response would bring extended validation regarding the relevance of such effect.

CONCLUSION

These results suggest a local modulation of the Apelin system within the corpus cavernosum. Remarkably, Apelin-13 reduced intracavernosal fibrosis in hypercholesterolemic mice by: (i) enhancing MMPs expression and activity; and (ii) inhibiting fibroblast differentiation into myofibroblast. Altogether, these results suggest an essential protective role of Apelin, indicating Apelin/APJ system as a promising candidate for the development of fibrosis-associated erectile dysfunction treatments. Sturny M, Anguenot L Costa-Fraga FP, et al. Apelin-13 Protects Corpus Cavernosum Against Fibrosis Induced by High-Fat Diet in an MMP-Dependent Mechanism. J Sex Med 2021;18:875-888.

Apelin/APJ-Manipulated CaMKK/AMPK/GSK3β Signaling Works as an Endogenous Counterinjury Mechanism in Promoting the Vitality of Random-Pattern Skin Flaps

A random-pattern skin flap plays an important role in the field of wound repair; the mechanisms that influence the survival of random-pattern skin flaps have been extensively studied but little attention has been paid to endogenous counterinjury substances and mechanism. Previous reports reveal that the apelin-APJ axis is an endogenous counterinjury mechanism that has considerable function in protecting against infection, inflammation, oxidative stress, necrosis, and apoptosis in various organs. As an in vivo study, our study proved that the apelin/APJ axis protected the skin flap by alleviating vascular oxidative stress and the apelin/APJ axis works as an antioxidant stress factor dependent on CaMKK/AMPK/GSK3β signaling. In addition, the apelin/APJ-manipulated CaMKK/AMPK/GSK3β-dependent mechanism improves HUVECs' resistance to oxygen and glucose deprivation/reperfusion (OGD/R), reduces ROS production and accumulation, maintained the normal mitochondrial membrane potential, and suppresses oxidative stress in vitro. Besides, activation of the apelin/APJ axis promotes vascular migration and angiogenesis under relative hypoxia condition through CaMKK/AMPK/GSK3β signaling. In a word, we provide new evidence that the apelin/APJ axis is an effective antioxidant and can significantly improve the vitality of random flaps, so it has potential be a promising clinical treatment.

PEGylated and Acylated Elabela Analogues Show Enhanced Receptor Binding, Prolonged Stability, and Remedy of Acute Kidney Injury

Acute kidney injury (AKI), mostly caused by renal ischemia-reperfusion (I/R) injury and nephrotoxins, is characterized by rapid deterioration in renal-functions without effective drug treatment available. Through activation of a G protein-coupled receptor APJ, a furin-cleaved fragment of Elabela (ELA[22-32], E11), an endogenous APJ ligand, protects against renal I/R injury. However, the poor plasma stability and relatively weak APJ-binding ability of E11 limit its application. To address these issues, we rationally designed and synthesized a set of E11 analogues modified by palmitic acid (Pal) or polyethylene glycol; improved plasma stability and APJ-binding capacity of these analogues were achieved. In cultured renal tubular cells, these analogues protected against hypoxia-reperfusion or cisplatin-caused injury. For renal I/R-injured mice, these analogues showed improved reno-protective effects than E11; notably, Pal-E11 showed therapeutic effects at 24 h post I/R injury. These results present ELA analogues as potential therapeutic options in managing AKI.

The interaction partners of (pro)renin receptor in the distal nephron

The (pro)renin receptor (PRR), a key regulator of intrarenal renin-angiotensin system (RAS), is predominantly presented in podocytes, proximal tubules, distal convoluted tubules, and the apical membrane of collecting duct A-type intercalated cells, and plays a crucial role in hypertension, cardiovascular disease, kidney disease, and fluid homeostasis. In addition to its well-known renin-regulatory function, increasing evidence suggests PRR can also act in a variety of intracellular signaling cascades independently of RAS in the renal medulla, including Wnt/β-catenin signaling, cyclooxygenase-2 (COX-2)/prostaglandin E₂ (PGE₂ ) signaling, and the apelinergic system, and work as a component of the vacuolar H⁺ -ATPase. PRR and these pathways regulate the expression/activity of each other that controlling blood pressure and renal functions. In this review, we highlight recent findings regarding the antagonistic interaction between PRR and ELABELA/apelin, the mutually stimulatory relationship between PRR and COX-2/PGE₂ or Wnt/β-catenin signaling in the renal medulla, and their involvement in the regulation of intrarenal RAS thereby control blood pressure, renal injury, and urine concentrating ability in health and patho-physiological conditions. We also highlight the latest progress in the involvement of PRR for the vacuolar H⁺ -ATPase activity.

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.

Role of SET7/9 in the progression of ischemic renal injury in diabetic and non-diabetic rats

SET domain with lysine methyltransferase 7/9 (Set7/9), a histone lysine methyltransferase (HMT), recently suggested to exert a critical role among kidney disorders, whereas its role in diabetes associated IRI co-morbidity remains complete elusive. The present study aimed to understand the role of SET7/9 and histone methylation in regulation of inflammatory signaling under IRI in diabetes mellitus and non-diabetic rats. Our results demonstrated that IRI caused renal dysfunction via increased blood urea nitrogen (BUN) levels in ND and DM rats. The NF-κB mediated inflammatory cascade like increased p-NF-κB, reduced IκBα levels followed by enhanced leukocyte infiltration as shown by increased MCP-1 expressions. IRI results in increased histone H3 methylation at lysine 4 and 36 (H3K4Me2, H3K36Me2), and decreased histone H3 methylation at lysine 9. Additionally, IRI increased the protein and mRNA expression of H3K4Me2 specific histone methyltransferase-SET7/9 in DM and ND rats. The abovementioned results remain prominent in DM rats compared to ND rats followed by IRI. Further, treatment with a novel SET7/9 inhibitor; cyproheptadine, significantly improved renal functioning via reducing the BUN levels in ND and DM rats. Hence, this study demonstrated the role of SET7/9 in mediating active transcription via H3K4Me2, ultimately regulated the NFκB-mediated inflammatory cascade. Therefore, SET7/9 can be explored as novel target for drug development against IRI under DM and ND conditions.

Identification and Validation of Potential Biomarkers and Their Functions in Acute Kidney Injury

Acute kidney injury (AKI) is a global public health concern associated with high morbidity, mortality, and health-care costs, and the therapeutic measures are still limited. This study aims to investigate crucial genes correlated with AKI, and their potential functions, which might contribute to a better understanding of AKI pathogenesis. The high-throughput data GSE52004 and GSE98622 were downloaded from Gene Expression Omnibus; four group sets were extracted and integrated. Differentially expressed genes (DEGs) in the four group sets were identified by limma package in R software. The overlapping DEGs among four group sets were further analyzed by the VennDiagram package, and their potential functions were analyzed by the GO and KEGG pathway enrichment analyses using the DAVID database. Furthermore, the protein-protein interaction (PPI) network was constructed by STRING, and the functional modules of the PPI network were filtered by MCODE and ClusterOne in Cytoscape. Hub genes of overlapping DEGs were identified by Cyto-Hubba and cytoNCA. The expression of 35 key genes was validated by quantitative real-time PCR (qRT-PCR). Western blot and immunofluorescence were performed to validate an important gene Egr1. A total of 722 overlapping DEGs were differentially expressed in at least three group sets. These genes mainly enriched in cell proliferation and fibroblast proliferation. Additionally, 5 significant modules and 21 hub genes, such as Havcr1, Krt20, Sox9, Egr1, Timp1, Serpine1, Edn1, and Apln were screened by analyzing the PPI networks. The 5 significant modules were mainly enriched in complement and coagulation cascades and Metabolic pathways, and the top 21 hub genes were mainly enriched in positive regulation of cell proliferation. Through validation, Krt20 were identified as the top 1 upregulated genes with a log2 (fold change) larger than 10 in all these 35 genes, and 21 genes were validated as significantly upregulated; Egr1 was validated as an upregulated gene in AKI in both RNA and protein level. In conclusion, by integrated analysis of different high-throughput data and validation by experiment, several crucial genes were identified in AKI, such as Havcr1, Krt20, Sox9, Egr1, Timp1, Serpine1, Edn1, and Apln. These genes were very important in the process of AKI, which could be further utilized to explore novel diagnostic and therapeutic strategies.

Non-polyphenolic natural inhibitors of amyloid aggregation

Protein misfolding diseases (PMDs) are chronic and progressive, with no effective therapy so far. Aggregation and misfolding of amyloidogenic proteins are closely associated with the onset and progression of PMDs, such as amyloid-β (Aβ) in Alzheimer's disease, α-Synuclein (α-Syn) in Parkinson's disease and human islet amyloid polypeptide (hIAPP) in type 2 diabetes. Inhibiting toxic aggregation of amyloidogenic proteins is regarded as a promising therapeutic approach in PMDs. The past decade has witnessed the rapid progresses of this field, dozens of inhibitors have been screened and verified in vitro and in vivo, demonstrating inhibitory effects against the aggregation and misfolding of amyloidogenic proteins, together with beneficial effects. Natural products are major sources of small molecule amyloid inhibitors, a number of natural derived compounds have been identified with great bioactivities and translational prospects. Here, we review the non-polyphenolic natural inhibitors that potentially applicable for PMDs treatment, along with their working mechanisms. Future directions are proposed for the development and clinical applications of these inhibitors.

Activation of Nrf2 Signaling by Apelin Attenuates Renal Ischemia Reperfusion Injury in Diabetic Rats

OBJECTIVE

Renal ischemia/reperfusion (I/R) injury is commonly seen in diabetic patients. Apelin has been demonstrated to protect against renal I/R injury, whereas detailed modulatory mechanisms by which Apelin exerts its role in renal I/R injury in diabetic patients remain unclarified. This research aimed to probe the functional molecules under the regulation of Apelin in renal I/R injury in diabetic rats.

MATERIALS AND METHODS

First, animal models were established for subsequent assays. Biochemical kits measured the serum levels of blood urea nitrogen (BUN) and serum creatinine (SCR), and hematoxylin and eosin (H&E) staining examined the histopathological changes of kidney tissues. Inflammatory factors containing tumor necrosis factor alpha (TNF-α), interleukin 1β (IL-1β), interleukin 6 (IL-6) and monocyte chemoattractant protein-1 (MCP-1) were tested through enzyme-linked immunosorbent assay (ELISA) and reverse transcription-quantitative polymerase chain reaction (RT-qPCR), respectively. Reactive oxygen species (ROS) levels in the serum and kidney tissues were separately assessed by specific ROS kits. Cell apoptosis was further estimated through terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) and Western blot analysis. Eventually, the influences of Apelin on nuclear factor erythroid 2-related factor (Nrf2) and its downstream genes were explored via Western blot analysis and immunohistochemistry (IHC).

RESULTS

In the present study, Apelin ameliorated the damage to renal function and histological structure, decreased levels of inflammatory factors and ROS, and hampered cell apoptosis in renal I/R injury of diabetic rats. Moreover, Apelin could elevate the levels of Nrf2 and downstream genes which were decreased under renal I/R injury.

CONCLUSION

These data indicated that Apelin inhibited renal I/R injury through regulating Nrf2 signaling in diabetic rats, which might shed new light on the treatment of renal I/R injury in diabetic patients.

Fat-Specific Knockout of Mecp2 Upregulates Slpi to Reduce Obesity by Enhancing Browning

Abnormalities of methyl-CpG binding protein 2 (Mecp2) cause neurological disorders with metabolic dysfunction; however, its role in adipose tissues remains unclear. Here, we report upregulated Mecp2 in white adipose tissues (WAT) of obese humans, as well as in obese mice and during in vitro adipogenesis. Normal chow-fed adipocyte-specific Mecp2 knockout mice (Mecp2 ^Adi KO mice) showed a lean phenotype, with downregulated lipogenic genes and upregulated thermogenic genes that were identified using RNA sequencing. Consistently, the deficiency of Mecp2 in adipocytes protected mice from high-fat diet (HFD)-induced obesity and inhibited in vitro adipogenesis. Furthermore, Mecp2 ^Adi KO mice showed increased browning under different stimuli, including cold treatment. Mechanistically, Mecp2 bound to the promoter of secretory leukocyte protease inhibitor (Slpi) and negatively regulated its expression. Knockdown of Slpi in inguinal WAT of Mecp2 ^Adi KO mice prevented cold-induced browning. Moreover, recombinant SLPI treatment reduced the HFD-induced obesity via enhancing browning. Together, our results suggest a novel non-central nervous system function of Mecp2 in obesity by suppressing browning, at least partially, through regulating adipokine Slpi.

The effect of experimental varicocele on the apelin and APJ expressions in rat testis tissue

Varicocele, which is one of the causes of infertility in men, can be defined as the expansion of spermatic cord veins. The presence of apelin and apelin receptor (APJ) in many tissues and the effects of apelin have been reported in several studies. There is no study showing apelin and APJ protein expressions in normal and varicocele-induced testicular tissues. In this study, we aimed to demonstrate varicocele-induced changes in apelin and APJ expressions in testicular tissue by immunohistochemical and western blotting techniques. In our study, Wistar male rats were randomly divided into three groups as control, varicocele, and sham. While the control group rats were not subjected to any treatment, the unilateral varicocele model was created under anesthesia in the varicocele group. In the sham group, the left abdominal region was opened and closed to exclude the effect of the surgical procedure. At the 13th postoperative week, the left testes were obtained under anesthesia in all groups, and the immunohistochemistry and Western blotting techniques were used to detect apelin and APJ expressions. In our study; apelin and APJ were significantly expressed in control group's testicular tissue; apelin in testicular tissues of varicocele groups increased compared to the control group, whereas APJ expression decreased. In conclusion, the presence of apelin/APJ system in normal testis and the increased expression of apelin in varicocele-induced testicular tissue suggested that apelin may have a role in the varicocele etiopathogenesis.

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.

Apelin affects the mouse aging urinary peptidome with minimal effects on kidney

Kidney function is altered by age together with a declined filtration capacity of 5–10% per decade after 35 years. Renal aging shares many characteristics with chronic kidney disease. Plasma levels of the bioactive peptide apelin also decline with age and apelin has been shown to be protective in chronic kidney disease. Therefore we evaluated whether apelin could also improve aging-induced renal lesions and function in mice. Since urine is for the major part composed of proteins and peptides originating from the kidney, we first studied apelin-induced changes, in the aging urinary peptidome. Despite the recently published age-associated plasma decrease of apelin, expression of the peptide and its receptor was increased in the kidneys of 24 months old mice. Twenty-eight days treatment with apelin significantly modified the urinary peptidome of 3 and 24 months old mice towards a signature suggesting more advanced age at 3 months, and a younger age at 24 months. The latter was accompanied by a decreased staining of collagen (Sirius red staining) in 24 months old apelin-treated mice, without changing aging-induced glomerular hypertrophy. In addition, apelin was without effect on aging-induced renal autophagy, apoptosis, inflammation and reduced renal function. In conclusion, treatment of aged mice with apelin had a limited effect on kidney lesions although modifying the urinary peptidome towards a younger signature. This supports evidence of apelin inducing more general beneficial effects on other aging organs, muscles in particular, as recently shown for sarcopenia, markers of which end up via the glomerular filtration in urine.

APLN/APJ pathway: The key regulator of macrophage functions

Macrophages play key roles during cardiovascular diseases (CVD) and their related complications. Apelin (APLN) is a key molecule, whose roles during CVD have been documented previously. Therefore, it has been hypothesized that APLN may perform its roles via modulation of macrophages. Additionally, due to the widespread distribution of the CVD, more effective therapeutic strategies need to be developed to overcome the related complications. This review article collected recent information regarding the roles of APLN on the macrophages and discusses its potential chance to be a target for molecular/cellular therapy of APLN and the APLN treated macrophages for CVD.

Rational Design of Hybrid Peptides: A Novel Drug Design Approach

Peptides play crucial roles in various physiological and pathological processes. Consequently, the investigation of peptide-based drugs is a highlight in the research and development of new drugs. However, natural peptides are not always ideal choices for clinical application due to their limited number and sometimes cytotoxicity to normal cells. Aiming to gain stronger or specific or novel biological effects and overcome the disadvantages of natural peptides, artificial hybrid peptides have been designed by combining the sequence of two or more different peptides with varied biological functions. Compared to natural peptides, hybrid peptides have shown better therapeutic potentials against bacteria, tumors, and metabolic diseases. In this review, design strategies, structure features and recent development of hybrid peptides are summarized; future directions for the research and development of hybrid peptide drugs are also discussed.

Histone demethylase UTX is a therapeutic target for diabetic kidney disease

KEY POINTS

Diabetic kidney disease (DKD) is a major complication of diabetes. We found that UTX (ubiquitously transcribed tetratricopeptide repeat on chromosome X, also known as KDM6A), a histone demethylase, was upregulated in the renal mesangial and tubular cells of diabetic mice and DKD patients. In cultured renal mesangial and tubular cells, UTX overexpression promoted palmitic acid-induced elevation of inflammation and DNA damage, whereas UTX knockdown or GSK-J4 treatment showed the opposite effects. We found that UTX demethylase activity-dependently regulated the transcription of inflammatory genes and apoptosis; moreover, UTX bound with p53 and p53-dependently exacerbated DNA damage. Administration of GSK-J4, an H3K27 demethylase inhibitor, ameliorated the diabetes-induced renal abnormalities in db/db mice, an animal model of type 2 diabetes. These results revealed the possible mechanisms underlying the regulation of histone methylation in DKD and suggest UTX as a potential therapeutic target for DKD.

ABSTRACT

Diabetic kidney disease (DKD) is a microvascular complication of diabetes and the leading cause of end-stage kidney disease worldwide without effective therapy available. UTX (ubiquitously transcribed tetratricopeptide repeat on chromosome X, also known as KDM6A), a histone demethylase that removes the di- and tri-methyl groups from histone H3K27, plays important biological roles in gene activation, cell fate control and life span regulation in Caenorhabditis elegans. In the present study, we report upregulated UTX in the kidneys of diabetic mice and DKD patients. Administration of GSK-J4, an H3K27 demethylase inhibitor, ameliorated the diabetes-induced renal dysfunction, abnormal morphology, inflammation, apoptosis and DNA damage in db/db mice, comprising an animal model of type 2 diabetes. In cultured renal mesanglial and tubular cells, UTX overexpression promoted palmitic acid induced elevation of inflammation and DNA damage, whereas UTX knockdown or GSK-J4 treatment showed the opposite effects. Mechanistically, we found that UTX demethylase activity-dependently regulated the transcription of inflammatory genes; moreover, UTX bound with p53 and p53-dependently exacerbated DNA damage. Collectively, our results suggest UTX as a potential therapeutic target for DKD.

Cardioprotective apelin effects and the cardiac-renal axis: review of existing science and potential therapeutic applications of synthetic and native regulated apelin

First described in 1998, apelin is one of the endogenous ligands of the apelinergic receptor. Since its discovery, its possible role in human physiology and disease has been intensively studied. Apelin is a native cardioprotective agent that the body synthesizes to create atheroprotective, antihypertensive, and regenerative effects in the body. By antagonizing the RAA system, apelin could play an important role in heart failure and hypertension. It is also involved in myocardial protection against ischemia/reperfusion injury, post-ischemic remodeling, and myocardial fibrosis. A small number of studies even suggest that serum apelin levels may be involved the development of life-threatening arrhythmias. All this information generated excitement about potential therapeutic effects in patients with heart failure and myocardial infarction. The therapeutic index of apelin is unknown but is anticipated to be favorable based on the small number of studies. In this review, we summarize the mechanisms by which apelin exerts its cardioprotective effects and its connection with the cardiorenal axis. Also, we report the potential therapeutic applications of synthetic and native regulated apelin. If larger studies can be performed, it is possible that apelin-mediated drug treatment may play a major role for a large number of patients worldwide in the future.

The biological function of ELABELA and APJ signaling in the cardiovascular system and pre-eclampsia

Pre-eclampsia (PE) is a pregnancy-specific syndrome that is characterized by hypertension and proteinuria. The etiology of PE is not completely understood but is believed to involve placental insufficiency and maternal vascular damage. Growing evidence supports an important role for the apelin receptor (APJ) system in regulating cardiovascular physiology. There are two vertebrate APJ ligands, APELIN and ELABELA, both of which mediate vasodilatory functions. A recent study linked deficient ELABELA signaling and the development of PE, though the molecular mechanism remains largely unknown. In this review, we summarize the biological function of the ELABELA and APJ system in cardiovascular homeostasis and discuss the potential mechanisms by which ELABELA and APJ regulate placenta trophoblast invasion and vascular functions and participate in the development of PE.

Apelinergic system in the kidney: implications for diabetic kidney disease

The bioactive peptides of the apelinergic system and its receptor APJ have been shown to play a protective role in experimental cardiovascular and diabetic kidney disease (DKD). Mechanisms of this renoprotective effect remain to be elucidated. In this study, we examined the localization of APJ within the normal kidney and its kidney expression in the db/db model of DKD. The effect of hyperglycemia and angiotensin II on APJ was examined in cultured podocytes. In the glomerulus, APJ colocalized with podocyte but not endothelial cell markers. In podocytes stimulated with Pyr1 Apelin-13, a change in the phosphorylation status of the signaling proteins, AKT, ERK, and p70S6K, was observed with an increase 15 min after stimulation. Apelin-13 decreased activity of Caspase-3 in podocytes after high glucose treatment reflecting an antiapoptotic effect of APJ stimulation. In podocytes, APJ mRNA was downregulated in high glucose, when compared to normal glucose conditions and exposure to angiotensin II led to a further significant decrease in APJ mRNA. APJ and preproapelin mRNA levels in kidneys from db/db mice were markedly decreased along with decreased tubular APJ protein by western blotting and immunostaining when compared to db/m controls. In conclusion, the apelinergic system is decreased in kidneys from db/db mice. Within the glomerulus, APJ is mainly localized in podocytes and in this cell type its activation by Apelin-13 abolishes the proapoptotic effect of high glucose, suggesting a potential therapeutic role of apelin and emerging agonists with extended half-life for therapy of DKD.

Apelin, a promising target for Alzheimer disease prevention and treatment

Alzheimer's disease (AD) is a progressive neurodegenerative disease with high outbreak rates. It is estimated that about 35 million individuals around the world suffered from dementia in 2010. AD is expected to increase twofold every 20 years and, by 2030, approximately 65 million people could suffer from this illness. AD is determined clinically by a cognitive impairment and pathologically by the production of amyloid beta (Aβ), neurofibrillary tangles, toxic free radicals and inflammatory mediators in the brain. There is still no treatment to cure or even alter the progressive course of this disease; however, many new therapies are being investigated and are at various stages of clinical trials. Neuropeptides are signaling molecules used by neurons to communicate with each other. One of the important neuropeptides is apelin, which can be isolated from bovine stomach. Apelin and its receptor APJ have been shown to broadly disseminate in the neurons and oligodendrocytes of the central nervous system. Apelin-13 is known to be the predominant neuropeptide in neuroprotection. It is involved in the processes of memory and learning as well as the prevention of neuronal damage. Studies have shown that apelin can directly or indirectly prevent the production of Aβ and reduce its amounts by increasing its degradation. Phosphorylation and accumulation of tau protein may also be inhibited by apelin. Apelin is considered as an anti-inflammatory agent by preventing the production of inflammatory mediators such as interleukin-1β and tumor necrosis factor alpha. It has been shown that in vivo and in vitro anti-apoptotic effects of apelin have prevented the death of neurons. In this review, we describe the various functions of apelin associated with AD and present an integrated overview of recent findings that, in general, recommend apelin as a promising therapeutic agent in the treatment of this ailment.

The complex effects of adipokines in the patients with kidney disease

Kidney diseases are categorized as the highest prevalent ones with worldwide noticeable incidence. They cause accelerated cardiovascular diseases and noticeable mortalities. Adipose tissue and its messengers, adipokines, are reported to have the highest relationship with end-stage renal diseases or chronic kidney diseases. Over recent years, with shifting of scientists’ mindset from a simple overview of adipose tissue as a fat store to the complex paradigm of this issue as a multipotential secretory organ, the importance of studies on this tissue has emerged.

MacroH2A1.1 cooperates with EZH2 to promote adipogenesis by regulating Wnt signaling

White adipocytes play important roles in many physiological processes, including energy storage, endocrine signaling, and inflammatory responses. Understanding the molecular mechanisms of adipocyte formation (adipogenesis) provides insights into therapeutic approaches against obesity and its related diseases. Many transcriptional factors and epigenetic enzymes are known to regulate adipogenesis; however, whether histone variants play a role in this process is unknown. Here we found that macroH2A1.1 (mH2A1.1), a variant of histone H2A, was upregulated during adipocyte differentiation in 3T3-L1 cells and in the white adipose tissue of obese mice. Ablation of mH2A1.1 activated Wnt/β-catenin signaling pathway, while overexpression of mH2A1.1 showed opposite effects. We further found that mH2A1.1 regulated Wnt/β-catenin signaling pathway by cooperating with EZH2, a histone H3K27 methyltransferase, thus led to accumulation of H3K27me2 and H3K27me3 on the promoters of Wnt genes. Mutations in the macro-domain, mH2A1.1G224E, and mH2A1.1G314E, not only impaired adipogenesis, but also impaired the binding ability of mH2A1.1 to EZH2 and the enrichments of H3K27me2 and H3K27me3 on the promoters of Wnt genes. Together, our study reveals a novel regulatory role of mH2A1.1 in adipogenesis and obesity, which provides new insights in white fat development.

Review: Apelin in disease

Apelin, a regulatory peptide, is a ligand of the APJ receptor that belongs to the G protein-coupled receptor family. Apelin and APJ are widely distributed in the body and play potential physiological roles in the cytoprotection of many internal organs. This review article summarizes information about the roles of the apelin/APJ system in neurological, metabolic, hypertension, respiratory, gastrointestinal, hepatic, kidney and cancerous diseases. It is suggested that apelin positively affects the treatment of non-cancerous diseases and may be considered as a therapeutic drug in many illnesses. However, in cancers, apelin appears as a tumour growth stimulator, and its suggested role is as a marker in the diagnosis of tumour cancers in tissues. In summary, apelin has certain therapeutic abilities and can be useful in the treatment of, e.g., insulin resistance, hypertension, etc., but it also can sometimes serve as a negative factor.

Reducing protein regulator of cytokinesis 1 as a prospective therapy for hepatocellular carcinoma

Proteins that bind to microtubule are important for cell cycle, and some of these proteins show oncogenic characteristics with mechanisms not fully understood. Herein we demonstrate overexpression of protein regulator of cytokinesis 1 (PRC1), a microtubule-associated regulator of mitosis, in human hepatocellular carcinoma (HCC). Moreover, upregulated PRC1 is associated with lower survival rates of HCC patients. Mechanistically, reducing PRC1 blocks mitotic exit of HCC cells at telophase in a spindle assembly checkpoint independent manner, and acts synergistically with microtubule-associated agents (MTAs) to suppress p53-wt or p53-null HCC cells in a p53- or p14ARF-dependent manner; while overexpressing PRC1 increases the resistance of HCC to taxol. A combined treatment of taxol/shPRC1 results in 90% suppression of tumor growth in subcutaneous HCC xenograft models. In orthotopic xenograft mice, reducing PRC1 significantly alleviates HCC development and hepatic injury. Together, our results suggest a dual-mitotic suppression approach against HCC by combining MTAs with cytokinesis inhibition, which blocks mitosis at both metaphase and telophase.

Apelin attenuates TGF-β1-induced epithelial to mesenchymal transition via activation of PKC-ε in human renal tubular epithelial cells

Epithelial to mesenchymal transition (EMT), a process whereby fully differentiated epithelial cells transition to a mesenchymal phenotype, has been implicated in the pathogenesis of renal fibrosis. Apelin, a bioactive peptide, has recently been recognized to protect against renal profibrotic activity, but the underlying mechanism has not yet been elucidated. In this study, we investigated the regulation of EMT in the presence of apelin-13 in vitro. Expression of the mesenchymal marker alpha-smooth muscle actin (α-SMA) and the epithelial marker E-cadherin was examined by immunofluorescence and western blotting in transforming growth factor beta 1 (TGF-β1)-stimulated human proximal tubular epithelial cells. Expression of extracellular matrix, fibronectin and collagen-I was examined by quantitative real-time PCR and ELISA. F13A, an antagonist of the apelin receptor APJ, and small interfering RNA targeting protein kinase C epsilon (PKC-ε) were used to explore the relevant signaling pathways. Apelin attenuated TGF-β1-induced EMT, and inhibited the EMT-associated increase in α-SMA, loss of E-cadherin, and secretion of extracellular matrix. Moreover, apelin activated PKC-ε in tubular epithelial cells, which in turn decreased phospho-Smad2/3 levels and increased Smad-7 levels. APJ inhibition or PKC-ε deletion diminished apelin-induced modulation of Smad signaling and suppression of tubular EMT. Our findings identify a novel PKC-ε-dependent mechanism in which apelin suppresses TGF-β1-mediated activation of Smad signaling pathways and thereby inhibits tubular EMT. These results suggest that apelin may be a new agent that can suppress renal fibrosis and retard chronic kidney disease progression.