Apelin protects against acute renal injury by inhibiting TGF-β1

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.

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.

ELABELA and an ELABELA Fragment Protect against AKI

Renal ischemia-reperfusion (I/R) injury is the most common cause of AKI, which associates with high mortality and has no effective therapy. ELABELA (ELA) is a newly identified 32-residue hormone peptide highly expressed in adult kidney. To investigate whether ELA has protective effects on renal I/R injury, we administered the mature peptide (ELA32) or the 11-residue furin-cleaved fragment (ELA11) to hypoxia-reperfusion (H/R)-injured or adriamycin-treated renal tubular cells in vitro ELA32 and ELA11 significantly inhibited the elevation of the DNA damage response, apoptosis, and inflammation in H/R-injured renal tubular cells and suppressed adriamycin-induced DNA damage response. Similarly, overexpression of ELA32 or ELA11 significantly inhibited H/R-induced cell death, DNA damage response, and inflammation. Notably, treatment of mice with ELA32 or ELA11 but not an ELA11 mutant with a cysteine to alanine substitution at the N terminus (AE11C) inhibited I/R injury-induced renal fibrosis, inflammation, apoptosis, and the DNA damage response and markedly reduced the renal tubular lesions and renal dysfunction. Together, our results suggest that ELA32 and ELA11 may be therapeutic candidates for treating AKI.

Novel pathogenesis: regulation of apoptosis by Apelin/APJ system

Apelin is the endogenous peptide APJ receptor, while APJ is a member of the G protein-coupled receptors family. Recent evidence strongly suggests that Apelin/APJ system influences apoptosis in various diseases through different signal pathways. In this review, we discuss the possible mechanisms by which the Apelin/APJ system inhibits apoptosis, including the phosphatidylinositol-3-kinase (PI3K)/Akt, ERK1/2, caspase signaling, and autophagy pathway. We also summarize the role of Apelin/APJ system in apoptosis in myocardial ischemia-reperfusion (I/R) injury, pulmonary artery hypertension, retinal neovascular disease, acute renal injury, skeletal homeostasis, and gastrointestinal diseases. Apelin/APJ system decreases myocardial infarction size and alleviates myocardial I/R injury by inhibiting cardiomyocytes apoptosis. However, Apelin/APJ system improves pulmonary artery hypertension via increasing apoptosis. Apelin/APJ system exerts neuroprotective effect by blocking apoptosis and participates in the recovery of retinal neovascular disease by suppressing apoptosis. Apelin/APJ system also shows anti-apoptotic effect against acute renal injury and plays a role in regulating skeletal homeostasis. In gastrointestinal disease, Apelin/APJ system plays a potential physiological role in gastrointestinal cytoprotection by regulating apoptosis. We hope that a better understanding of the Apelin/APJ system will help to discover new disease pathogenesis and find possible therapeutic targets of the Apelin/APJ system essential for various diseases.

Epigenetics of kidney disease

DNA methylation and histone modifications determine renal programming and the development and progression of renal disease. The identification of the way in which the renal cell epigenome is altered by environmental modifiers driving the onset and progression of renal diseases has extended our understanding of the pathophysiology of kidney disease progression. In this review, we focus on current knowledge concerning the implications of epigenetic modifications during renal disease from early development to chronic kidney disease progression including renal fibrosis, diabetic nephropathy and the translational potential of identifying new biomarkers and treatments for the prevention and therapy of chronic kidney disease and end-stage kidney disease.

Histone HIST1H1C/H1.2 regulates autophagy in the development of diabetic retinopathy

Autophagy plays critical and complex roles in many human diseases, including diabetes and its complications. However, the role of autophagy in the development of diabetic retinopathy remains uncertain. Core histone modifications have been reported involved in the development of diabetic retinopathy, but little is known about the histone variants. Here, we observed increased autophagy and histone HIST1H1C/H1.2, an important variant of the linker histone H1, in the retinas of type 1 diabetic rodents. Overexpression of histone HIST1H1C upregulates SIRT1 and HDAC1 to maintain the deacetylation status of H4K16, leads to upregulation of ATG proteins, then promotes autophagy in cultured retinal cell line. Histone HIST1H1C overexpression also promotes inflammation and cell toxicity in vitro. Knockdown of histone HIST1H1C reduces both the basal and stresses (including high glucose)-induced autophagy, and inhibits high glucose induced inflammation and cell toxicity. Importantly, AAV-mediated histone HIST1H1C overexpression in the retinas leads to increased autophagy, inflammation, glial activation and neuron loss, similar to the pathological changes identified in the early stage of diabetic retinopathy. Furthermore, knockdown of histone Hist1h1c by siRNA in the retinas of diabetic mice significantly attenuated the diabetes-induced autophagy, inflammation, glial activation and neuron loss. These results indicate that histone HIST1H1C may offer a novel therapeutic target for preventing diabetic retinopathy.

High Glucose-Induced Hypomethylation Promotes Binding of Sp-1 to Myo-Inositol Oxygenase: Implication in the Pathobiology of Diabetic Tubulopathy

The catabolic enzyme myo-inositol oxygenase (MIOX) is expressed in proximal tubules and up-regulated in the diabetic state. Previously, we reported its transcriptional and translation regulation by high glucose (HG), osmolytes, and fatty acids. However, its epigenetic regulation is unknown. Bisulfite sequencing revealed that both human and mouse MIOX promoters, enriched with CpG sites, are hypomethylated and unmethylated under HG ambience and hyperglycemic states associated with increased MIOX expression. Eletrophoretic mobility shift assays revealed increased binding of unmethylated oligos with nucleoproteins of cells maintained under HG. In addition, a strong binding of specificity protein (Sp)-1 transcription factor with MIOX promoter was observed under HG, especially with unmethylated Sp-1 oligo. Specificity of binding was established by supershift assays and treatment with the Sp-1 inhibitor mithramycin. Promoter analysis revealed an increase in luciferase activity under HG, which was reduced after mutation of the Sp-1-binding site. Sp1 siRNA treatment reduced mRNA and protein expression of Sp-1 and MIOX and generation of reactive oxygen species derived from NADPH oxidase (NOX)-4 and mitochondrial sources. In addition, there was reduced expression of hypoxia-inducible factor-1α relevant in the pathogenesis of diabetic nephropathy. Sp1 siRNA treatment reduced fibronectin expression, an extracellular matrix protein that is increased in diabetic nephropathy and tubulopathy. HG-induced MIOX expression was also reduced with the treatment of apelin-13, which deacetylates histones. Overall, these findings highlight the epigenetic regulation of MIOX in the pathogenesis of diabetic tubulopathy.

Apelin/APJ system: A novel therapeutic target for oxidative stress-related inflammatory diseases (Review)

Apelin, the endogenous ligand of APJ which is a member of G protein-coupled receptors, has been shown to be expressed in a variety of tissues in vivo and to exert significant biological effects. Studies have indicated that the apelin/APJ system is involved in the regulation of a variety of physiological functions and pathological processes, and that it is associated with cardiovascular diseases (such as atherosclerosis, hypertension, heart failure and myocardial injury), diabetes with microvascular complications, ischemia reperfusion injury, tumors, pre-eclampsia, as well as others. The occurrence of these diseases is closely related to endothelial dysfunction and the local inflammatory response; however, the occurrence of oxidative stress is related to vascular injury, due to the excessive generation of reactive oxygen species (ROS) and can lead to vascular damage and a series of inflammatory reactions. Therefore, this review summarizes the association between apelin/APJ, oxidative stress and inflammation-related diseases. In addition, drugs targeting the apelin/APJ system are recommended, thus providing a novel therapeutic strategy for oxidative stress-related inflammatory diseases.

Restoration of Opa1-long isoform inhibits retinal injury-induced neurodegeneration

Optic atrophy 1 (Opa1) is a critical factor that regulates fusion and other important functions of mitochondria. In mitochondrion, the N-terminal mitochondrial targeting sequence of Opa1 precursors is removed to generate Opa1 long isoforms (L-Opa1), which are further cleaved into short isoforms (S-Opa1). In the present study, we found that retinal ischemia-reperfusion (I/R) injury and intravitreal injection of carbonylcyanide m-chlorophenyl hydrazone (CCCP) both dramatically induced Opa1 cleavage and caused loss of L-Opa1. In cultured neuronal cells under hypoxia-reoxygenation (H/R) injury, similar changes for Opa1 were also observed. In contrast, restoration of L-Opa1 level by overexpression of S1 cleavage site deletion Opa1 splice 1 (Opa1-ΔS1) not only normalized the H/R-induced mitochondrial morphology changes, but also inhibited the H/R-induced apoptosis, necrosis, and the intracellular ATP loss. Furthermore, recovering L-Opa1 level in the I/R-injured retina by intravitreal injection of genipin or overexpression of Opa1-ΔS1 inhibited apoptosis, necrosis, cell loss in the ganglion cell layer and retinal thickness reduction. Together, our data demonstrated the loss of L-Opa1 is involved in the development of retinal I/R injury, indicating restoring L-Opa1 level may be considered as a therapeutic target for I/R injury-related diseases, at least for the retina. Key messages: Retinal ischemia-reperfusion (I/R) or hypoxia-reoxygenation (H/R) injury induces L-Opa1 loss. Opa1-ΔS1 overexpression inhibits H/R-induced L-Opa1 loss. Opa1-ΔS1 overexpression inhibits H/R-induced mitochondria morphology change. Opa1-ΔS1 and genipin inhibit retinal I/R injury-induced necroptosis. Opa1-ΔS1 and genipin inhibit retinal I/R injury-induced neurodegeneration.