Endothelial cell and podocyte autophagy synergistically protect from diabetes-induced glomerulosclerosis

Berberine enhances the AMPK activation and autophagy and mitigates high glucose-induced apoptosis of mouse podocytes

High glucose concentration can induce injury of podocytes and berberine has a potent activity against diabetic nephropathy. However, whether and how berberine can inhibit high glucose-mediated injury of podocytes have not been clarified. This study tested the effect of berberine on high glucose-mediated apoptosis and the AMP-activated protein kinase (AMPK), mammalian target of rapamycin (mTOR) activation and autophagy in podocytes. The results indicated that berberine significantly mitigated high glucose-decreased cell viability, and nephrin and podocin expression as well as apoptosis in mouse podocytes. Berberine significantly increased the AMPK activation and mitigated high glucose and/or the AMPK inhibitor, compound C-mediated mTOR activation and apoptosis in podocytes. Berberine significantly enhanced the AMPK activation and protected from high glucose-induced apoptosis in the AMPK-silencing podocytes. Furthermore, berberine significantly increased the high glucose-elevated Unc-51-like autophagy-activating kinase 1 (ULK1) S317/S555 phosphorylation, Beclin-1 expression, the ratios of LC3II to LC3I expression and the numbers of autophagosomes, but reduced ULK1 S757 phosphorylation in podocytes. In addition, berberine significantly attenuated compound C-mediated inhibition of autophagy in podocytes. The protective effect of berberine on high glucose-induced podocyte apoptosis was significantly mitigated by pre-treatment with 3-methyladenine or bafilomycin A1. Collectively, berberine enhanced autophagy and protected from high glucose-induced injury in podocytes by promoting the AMPK activation. Our findings may provide new insights into the molecular mechanisms underlying the anti-diabetic nephropathy effect of berberine and may aid in design of new therapies for intervention of diabetic nephropathy.

Pericytes in diabetes-associated vascular disease

Pericytes are mural cells that support and stabilise the microvasculature, and are present in all vascular beds. Pericyte-endothelial cell crosstalk is essential in both remodelling and quiescent vasculature, and this complex interaction is often disrupted in disease states. Pericyte loss is believed to be an early hallmark of diabetes-associated microvascular disease, including retinopathy and nephropathy. Here we review the current literature defining pericyte biology in the context of diabetes-associated vascular disease, with a particular focus on whether pericytes contribute actively to disease progression. We also speculate regarding the role of pericytes in the recovery from macrovascular complications, such as critical limb ischaemia. It becomes clear that dysfunctional pericytes are likely to actively induce disease progression by causing vasoconstriction and basement membrane thickening, resulting in tissue ischaemia. Moreover, their altered interactions with endothelial cells are likely to cause abnormal and inadequate neovascularisation in diverse vascular beds. Further research is needed to identify mechanisms by which pericyte function is altered by diabetes, with a view to developing therapeutic approaches that normalise vascular function and remodelling.

Mst1 inhibits CMECs autophagy and participates in the development of diabetic coronary microvascular dysfunction

Cardiovascular complications account for a substantial proportion of morbidity and mortality in diabetic patients. Abnormalities of cardiac microvascular endothelial cells (CMECs) lead to impaired cardiac microvascular vessel integrity and subsequent cardiac dysfunction, underlining the importance of coronary microvascular dysfunction. In this study, experimental diabetes models were constructed using Mst1 transgenic, Mst1 knockout and sirt1 knockout mice. Diabetic Mst1 transgenic mice exhibited impaired cardiac microvessel integrity and decreased cardiac function. Mst1 overexpression deceased CMECs autophagy as evidenced by decreased LC3 expression and enhanced protein aggregation when subjected to high glucose culture. Mst1 knockout improved cardiac microvessel integrity and enhanced cardiac functions in diabetic mice. Mst1 knockdown up-regulated autophagy as indicated by more typical autophagosomes and increased LC3 expression in CMECs subjected to high glucose cultures. Mst1 knockdown also promoted autophagic flux in the presence of bafilomycin A1. Mst1 overexpression increased CMECs apoptosis, whereas Mst1 knockout decreased CMECs apoptosis. Sirt1 knockout abolished the effects of Mst1 overexpression in cardiac microvascular injury and cardiac dysfunction. In conclusion, Mst1 knockout preserved cardiac microvessel integrity and improved cardiac functions in diabetic mice. Mst1 decreased sirt1 activity, inhibited autophagy and enhanced apoptosis in CMECs, thus participating in the pathogenesis of diabetic coronary microvascular dysfunction.

Mitochondrial Fission Increases Apoptosis and Decreases Autophagy in Renal Proximal Tubular Epithelial Cells Treated with High Glucose

The aim of this study was to examine the effect of mitochondrial morphogenesis changes on apoptosis and autophagy of high-glucose-treated proximal tubular epithelial cells (HK2). Cell viability, apoptosis, and mitochondrial morphogenesis were examined using crystal violet, terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL), and mitotracker staining, respectively. High glucose inhibited cell viability and induced mitochondrial fission in HK2 cells. After depleting mitofusin 1 (MFN1), the MFN1(-) HK2 cells (fission type) became more susceptible to high-glucose-induced apoptosis and mitochondrial fragmentation observed by TUNEL and mitotracker assays. In siMFN2 HK2 cells (fission type), mitochondria were highly fragmented (>80% fission rate) with or without high-glucose treatment; however, siFIS1 (mitochondrial fission protein 1) HK2 cells (fusion type) exhibited little fragmentation (<13%). High-glucose treatment induced autophagy, characterized by the formation of autophagosome and microtubule-associated protein light chain 3 (LC3) B-II, as observed by transmission electron microscopy and western blotting, respectively. LC3B-II levels decreased in both MFN1(-) and siMFN2 HK2 cells, but increased in siFIS1 HK2 cells. Moreover, autophagy displays a protective role against high-glucose-induced cell death based on cotreatment with autophagy inhibitors (3-methyladenine and chloroquine). Mitochondrial fission may increase apoptosis and decrease autophagy of high-glucose-treated HK2 cells.

GSK-3β inhibitor attenuates urinary albumin excretion in type 2 diabetic db/db mice, and delays epithelial-to-mesenchymal transition in mouse kidneys and podocytes

The mechanism underlying epithelial‑to‑mesenchymal transition (EMT) caused by high glucose (HG) stimulation in diabetic nephropathy (DN) remains to be fully elucidated. The present study investigated the effects of HG on EMT and the activity of glycogen synthase kinase 3β (GSK‑3β) in podocytes and the kidneys of db/db mice, and assessed the effects of (2'Z, 3'E)‑6‑bromoindirubin‑3'‑oxime (BIO), an inhibitor of GSK‑3β, on EMT and glomerular injury. The resulting data showed that the activity of GSK‑3β was upregulated by HG and downregulated by BIO in the podocytes and the renal cortex. The expression levels of epithelial markers, including nephrin, podocin and synaptopodin, were decreased by HG and increased by BIO, whereas the reverse were true for mesenchymal markers, including α‑smooth muscle actin (α‑SMA) and fibronectin. The expression levels of β‑catenin and Snail, in contrast to current understanding of the Wnt signaling pathway, were increased by HG and decreased by BIO. In addition, expression of the vitamin D receptor (VDR) was decreased by HG and increased by BIO. In conclusion, the present study revealed that the mechanism by which BIO inhibited HG‑mediated EMT in podocytes and the renal cortex was primarily due to the VDR. Treatment with BIO protected renal function by maintaining the integrity of the filtration membrane and decreasing UAE, but not by regulating blood glucose. Therefore, GSK‑3β may be used as a sensitive biomarker of DN, and its inhibition by BIO may be effective in the treatment of DN.

Podocytes: the Weakest Link in Diabetic Kidney Disease?

Diabetes is increasing in prevalence and is the leading cause of end-stage renal disease in the United States. Diabetic kidney disease is considered a proteinuric glomerular disease. Although the glomerulus is composed of various cell types, research suggests that podocytes are critical to overall glomerular health. Podocyte injury has been identified as a pivotal event resulting in proteinuric kidney disease, glomerulosclerosis, and loss of renal function. Thus, understanding the signaling mechanisms that trigger podocyte injury in diabetic kidney disease might allow for the development of targeted therapeutics to prevent or ameliorate progression to end-stage renal failure. This review focuses on the role of podocytes in diabetic kidney disease.

β-Arrestins promote podocyte injury by inhibition of autophagy in diabetic nephropathy

β-Arrestins are multifunctional proteins originally identified as negative adaptors of G protein-coupled receptors (GPCRs). Emerging evidence has also indicated that β-arrestins can activate signaling pathways independent of GPCR activation. This study was to elucidate the role of β-arrestins in diabetic nephropathy (DN) and hypothesized that β-arrestins contribute to diabetic renal injury by mediating podocyte autophagic process. We first found that both β-arrestin-1 and β-arrestin-2 were upregulated in the kidney from streptozotocin-induced diabetic mice, diabetic db/db mice and kidney biopsies from diabetic patients. We further revealed that either β-arrestin-1 or β-arrestin-2 deficiency (Arrb1^−/− or Arrb2^−/−) ameliorated renal injury in diabetic mice. In vitro, we observed that podocytes increased both β-arrestin-1 and β-arrestin-2 expression levels under hyperglycemia condition and further demonstrated that β-arrestin-1 and β-arrestin-2 shared common mechanisms to suppress podocyte autophagy by negative regulation of ATG12–ATG5 conjugation. Collectively, this study for the first time demonstrates that β-arrestin-1 and β-arrestin-2 mediate podocyte autophagic activity, indicating that β-arrestins are critical components of signal transduction pathways that link renal injury to reduce autophagy in DN. Modulation of these pathways may be an innovative therapeutic strategy for treating patients with DN.

Autophagy: A Novel Therapeutic Target for Diabetic Nephropathy

Diabetic nephropathy is a leading cause of end stage renal disease and its occurance is increasing worldwide. The most effective treatment strategy for the condition is intensive treatment to strictly control glycemia and blood pressure using renin-angiotensin system inhibitors. However, a fraction of patients still go on to reach end stage renal disease even under such intensive care. New therapeutic targets for diabetic nephropathy are, therefore, urgently needed. Autophagy is a major catabolic pathway by which mammalian cells degrade macromolecules and organelles to maintain intracellular homeostasis. The accumulation of damaged proteins and organelles is associated with the pathogenesis of diabetic nephropathy. Autophagy in the kidney is activated under some stress conditions, such as oxidative stress and hypoxia in proximal tubular cells, and occurs even under normal conditions in podocytes. These and other accumulating findings have led to a hypothesis that autophagy is involved in the pathogenesis of diabetic nephropathy. Here, we review recent findings underpinning this hypothesis and discuss the advantages of targeting autophagy for the treatment of diabetic nephropathy.

The Role of Autophagy in Lupus Nephritis

Systemic lupus erythematosus (SLE) is a multifactorial autoimmune disease characterized by the generation of immune responses to self-antigens. Lupus nephritis is one of the most common and severe complications in SLE patients. Though the pathogenesis of lupus nephritis has been studied extensively, unresolved questions are still left and new therapeutic methods are needed for disease control. Autophagy is a conserved catabolic process through which cytoplasmic constituents can be degraded in lysosome and reused. Autophagy plays vital roles in maintaining cell homeostasis and is involved in the pathogenesis of many diseases. In particular, autophagy can affect almost all parts of the immune system and is involved in autoimmune diseases. Based on genetic analysis, cell biology, and mechanism studies of the classic and innovative therapeutic drugs, there are growing lines of evidence suggesting the relationship between autophagy and lupus nephritis. In the present review, we summarize the recent publications investigating the relationship between autophagy and lupus nephritis and provide a new perspective towards the pathogenesis of lupus nephritis.

MOF maintains transcriptional programs regulating cellular stress response

MOF (MYST1, KAT8) is the major H4K16 lysine acetyltransferase (KAT) in Drosophila and mammals and is essential for embryonic development. However, little is known regarding the role of MOF in specific cell lineages. Here we analyze the differential role of MOF in proliferating and terminally differentiated tissues at steady state and under stress conditions. In proliferating cells, MOF directly binds and maintains the expression of genes required for cell cycle progression. In contrast, MOF is dispensable for terminally differentiated, postmitotic glomerular podocytes under physiological conditions. However, in response to injury, MOF is absolutely critical for podocyte maintenance in vivo. Consistently, we detect defective nuclear, endoplasmic reticulum and Golgi structures, as well as presence of multivesicular bodies in vivo in podocytes lacking Mof following injury. Undertaking genome-wide expression analysis of podocytes, we uncover several MOF-regulated pathways required for stress response. We find that MOF, along with the members of the non-specific lethal but not the male-specific lethal complex, directly binds to genes encoding the lysosome, endocytosis and vacuole pathways, which are known regulators of podocyte maintenance. Thus, our work identifies MOF as a key regulator of cellular stress response in glomerular podocytes.

Ox-Lp(a) transiently induces HUVEC autophagy via an ROS-dependent PAPR-1-LKB1-AMPK-mTOR pathway

Oxidised lipoprotein(a) [oxLp(a)] is considered as a more potent arteriosclerotic factor than native Lp(a). However, the molecular mechanisms underlying this potency remain unclear. Reactive oxygen species (ROS) possibly act as intracellular second messengers that participate in autophagy stimulation. In this study, the effect of oxLp(a) on endothelial cell autophagy was determined. The mechanism and effect of autophagy on endothelial cells were also investigated. Results showed that oxLp(a) could induce autophagy depending on the generation of cellular ROS. Superoxide dismutase, an antioxidant, could inhibit oxLp(a)-induced autophagy in human umbilical vascular endothelial cells. Furthermore, poly(adenosine diphosphate-ribose) polymerase-1 (PARP-1)-liver kinase B1 (LKB1)-adenosine monophosphate-activated protein kinase (AMPK)-mammalian target of rapamycin (mTOR) and LKB1-AMPK-mTOR pathways are involved in oxLp(a)-induced autophagy. These pathways are also dependent on ROS. Thus, oxLp(a) induced autophagy via LKB1-AMPK-mTOR and PAPR-1-LKB1-AMPK-mTOR pathways, which are dependent on ROS generation.