The MIF receptor CD74 in diabetic podocyte injury

Crosstalk between protective autophagy and NF-κB signal in high glucose-induced podocytes

Despite a great deal of recent studies focused on the pivotal role of autophagy in maintaining podocyte energy homeostasis, the mechanisms of autophagy in regulating transcriptional factors under high glucose (HG) condition are not fully understood. Here, we evaluated the effect of HG on nuclear factor-kappa B (NF-κB) signaling and autophagic process. The results showed that HG promoted autophagy in podocytes. Bafilomycin A1 (Baf A1) further enhanced this effect, but 3-methyadenine (3-MA) inhibited it. The proautophagic effects of HG manifested in the form of enhanced podocyte expression of light chain 3 (LC3)-II. In these cells, blockade of NF-κB signal by ammonium pyrrolidinethiocarbamate constrained in effectively reducing LC3-II up-regulation and increasing podocyte apoptosis. Furthermore, the autophagy inhibitors, such as Baf A1 and 3-MA, significantly enhanced HG-induced NF-κB activation and increased apoptosis. Thus, we conclude that the accumulation of autophagosomes results from enhancement of the autophagic flux, but not the blockage of autophagosome-lysosome fusion by HG. We also prove that HG-induced apoptosis, autophagy, and NF-κB signal are in a close crosstalk through a yet undetermined mechanism in podocytes.

Molecular mechanisms of diabetic kidney disease

Diabetic kidney disease (DKD) is the leading cause of kidney failure worldwide and the single strongest predictor of mortality in patients with diabetes. DKD is a prototypical disease of gene and environmental interactions. Tight glucose control significantly decreases DKD incidence, indicating that hyperglycemia-induced metabolic alterations, including changes in energy utilization and mitochondrial dysfunction, play critical roles in disease initiation. Blood pressure control, especially with medications that inhibit the angiotensin system, is the only effective way to slow disease progression. While DKD is considered a microvascular complication of diabetes, growing evidence indicates that podocyte loss and epithelial dysfunction play important roles. Inflammation, cell hypertrophy, and dedifferentiation by the activation of classic pathways of regeneration further contribute to disease progression. Concerted clinical and basic research efforts will be needed to understand DKD pathogenesis and to identify novel drug targets.

Suppressor of cytokine signaling (SOCS) 2 attenuates renal lesions in rats with diabetic nephropathy

Diabetic nephropathy (DN) involves damage to the kidney caused by diabetes. It is characterized by renal hypertrophy, tubular atrophy/dilation and glomerular hyperfiltration. Suppressor of cytokine signaling (SOCS) 2 has recently been indicated to be involved in the pathogenesis of DN, however, the exact regulatory mechanisms remain unclear. This study was conducted to explore the role of SOCS2 in the development and progress of DN in a rat model of streptozocin (STZ)-induced diabetes. Recombinant adenoviruses expressing SOCS2 were used to upregulate the expression of SOCS2 in the kidneys of diabetic rats. Our results demonstrated that intrarenal injection of SOCS2 adenoviruses reduced STZ-induced renal lesions, including renal/glomerular hypertrophy, glomerular hyperfiltration, aberrant inflammation and fibrosis. Increased expression levels of proinflammatory proteins (monocyte chemoattractant protein-1, tumor necrotic factor-alpha and interleukin-6) and profibrotic proteins (transforming growth factor-beta, collagen IV and fibronectin) in the diabetic kidneys were decreased after SOCS2 gene delivery. Additionally, adenovirus-mediated upregulation of renal SOCS2 markedly inhibited STZ-induced phosphorylation increases of Janus kinase (JAK) 2, signal transducer and activator of transcription (STAT) 3, STAT5 and extracellular receptor-activated kinase (ERK) 1/2. In summary, the present research demonstrates that SOCS2 reduces renal lesions associated with diabetes in rats.

TWEAK promotes peritoneal inflammation

Peritoneal dialysis (PD) is complicated by peritonitis episodes that cause loss of mesothelium and eventually sclerosing peritonitis. An improved understanding of the molecular contributors to peritoneal injury and defense may increase the therapeutic armamentarium to optimize peritoneal defenses while minimizing peritoneal injury. There is no information on the expression and function of the cytokine TWEAK and its receptor Fn14 during peritoneal injury. Fn14 expression and soluble TWEAK levels were measured in human PD peritoneal effluent cells or fluids with or without peritonitis. Fn14 expression was also analyzed in peritoneal biopsies from PD patients. Actions of intraperitoneal TWEAK were studied in mice in vivo. sTWEAK levels were increased in peritoneal effluent in PD peritonitis. Effluent sTWEAK levels correlated with the number of peritoneal macrophages (r = 0.491, p = 0.002). Potential TWEAK targets that express the receptor Fn14 include mesothelial cells and macrophages, as demonstrated by flow cytometry of peritoneal effluents and by analysis of peritoneal biopsies. Peritoneal biopsy Fn14 correlated with mesothelial injury, fibrosis and inflammation, suggesting a potential deleterious effect of TWEAK/Fn14. In this regard, intraperitoneal TWEAK administration to mice promoted peritoneal inflammation characterized by increased peritoneal effluent MCP-1, Fn14 and Gr1⁺ macrophages, increased mesothelial Fn14, MCP-1 and CCL21 expression and submesothelial tissue macrophage recruitment. Taken together these data suggest that the TWEAK/Fn14 system may promote inflammation and tissue injury during peritonitis and PD.

3,4-DGE is cytotoxic and decreases HSP27/HSPB1 in podocytes

Hyperglycemia is the key driver of diabetic complications and increased concentrations of glucose degradation products. The study of peritoneal dialysis solution biocompatibility has highlighted the adverse biological effects of glucose degradation products. Recently, 3,4-dideoxyglucosone-3-ene (3,4-DGE) was identified as the most toxic glucose degradation product in peritoneal dialysis fluids. In addition, 3,4-DGE is present in high-fructose corn syrup, and its precursor 3-deoxyglucosone is increased in diabetes. The role of 3,4-DGE in glomerular injury had not been addressed. We studied the effects of 3,4-DGE on cultured human podocytes and in vivo in mice. 3,4-DGE induced apoptosis in podocytes in a dose- and time-dependent manner. 3,4-DGE promoted the release of cytochrome c from mitochondria and activation of caspase-3. While high glucose concentrations increased the levels of the podocyte intracellular antiapoptotic protein HSP27/HSPB1, 3,4-DGE decreased the expression of podocyte HSP27/HSPB1. Apoptosis induced by 3,4-DGE was caspase-dependent and could be prevented by the broad-spectrum caspase inhibitor zVAD-fmk. Antagonism of Bax by a Ku-70-derived peptide also prevented apoptosis. Intravenous administration of 3,4-DGE to healthy mice resulted in a decreased expression of HSP27/HSPB1 and caspase-3 activation in whole kidney and in podocytes in vivo. In conclusion, 3,4-DGE induces apoptotic cell death in cultured human podocytes, suggesting a potential role in glomerular injury resulting from metabolic disorders.

Endogenous NAMPT dampens chemokine expression and apoptotic responses in stressed tubular cells

Diabetic nephropathy (DN) is the most common cause of end-stage renal disease and identification of new therapeutic targets is needed. Nicotinamide phosphoribosyltransferase (NAMPT) is both an extracellular and intracellular protein. Circulating NAMPT is increased in diabetics and in chronic kidney disease patients. The role of NAMPT in renal cell biology is poorly understood. NAMPT mRNA and protein were increased in the kidneys of rats with streptozotocin-induced diabetes. Immunohistochemistry localized NAMPT to glomerular and tubular cells in diabetic rats. The inflammatory cytokine TNFα increased NAMPT mRNA, protein and NAD production in cultured kidney human tubular cells. Exogenous NAMPT increased the mRNA expression of chemokines MCP-1 and RANTES. The NAMPT enzymatic activity inhibitor FK866 prevented these effects. By contrast, FK866 boosted TNFα-induced expression of MCP-1 and RANTES mRNA and endogenous NAMPT targeting by siRNA also had a proinflammatory effect. Furthermore, FK866 promoted tubular cell apoptosis in an inflammatory milieu containing the cytokines TNFα/IFNγ. In an inflammatory environment FK866 promoted tubular cell expression of the lethal cytokine TRAIL. These data are consistent with a role of endogenous NAMPT activity as an adaptive, protective response to an inflammatory milieu that differs from the proinflammatory activity of exogenous NAMPT. Thus, disruption of endogenous NAMPT function in stressed cells promotes tubular cell death and chemokine expression. This information may be relevant for the design of novel therapeutic strategies in DN.

Gene expression analysis and urinary biomarker assays reveal activation of tubulointerstitial injury pathways in a rodent model of chronic proteinuria (Doxorubicin nephropathy)

BACKGROUND

Tubular atrophy and interstitial fibrosis are well-recognized sequelae of chronic proteinuria; however, little is known regarding the molecular pathways activated within tubulointerstitium in chronic proteinuric nephropathies.

METHODS

To investigate the molecular mechanisms of proteinuria-associated tubulointerstitial (TI) disease, doxorubicin nephropathy was induced in rats. Progression of disease was monitored with weekly urinary biomarker assays. Because histopathology revealed multifocal TI injury, immunodirected laser capture microdissection was used to identify and isolate injured proximal tubules, as indicated by kidney injury molecule-1 immunolabeling. Adjacent interstitial cells were harvested separately. Gene expression microarray, manual annotation of gene lists, and Gene Set Enrichment Analysis were performed. A subset of the regulated transcripts was validated by quantitative PCR and immunohistochemistry.

RESULTS

Severe proteinuria preceded tubular injury biomarkers by 1 week. Histology revealed multifocal, mild TI damage at 3 weeks, which progressed in severity at 5 weeks. Affymetrix microarray analysis revealed tissue-specific regulation of gene expression. Manual annotation of gene lists, gene set enrichment analysis, and urinary biomarker assays revealed similarities to pathways activated in direct TI injuries. This suggests commonalities amongst the molecular mechanisms of TI injury secondary to proteinuria, ischemia-reperfusion, and nephrotoxicity. © 2013 S. Karger AG, Basel.

Chemokine profile in the sera and urine of patients with schistosomal glomerulopathy

We investigated the serum and urine chemokine levels of patients with schistosomal mansoni glomerulonephritis. This cross-sectional study was conducted in the Southeast of Brazil. Overall, 160 subjects were enrolled and divided into five groups: 1) hepatosplenic schistosomiasis with renal disease (N = 12); 2) hepatosplenic schistosomiasis without renal disease (N = 68); 3) hepatointestinal schistosomiasis (N = 27); 4) glomerulopathy caused by other diseases (N = 22); and 5) healthy controls (N = 31). The patients with microalbuminuria > 30 mg in 24 hours were considered to have renal disease. The sera and urine chemokines CCL2, CCL3, CCL5, CCL11, and CXCL8 were measured using an enzyme-linked immunosorbent assay test. A similar profile was observed between the patients with schistosomal glomerulopathy and the patients with glomerulopathy caused by other diseases, with the exception of serum CCL2 ≤ 634.3 pg/mL. In cases with sera CCL2 > 634.3 pg/mL, the diagnosis of schistosomal glomerulopathy should be considered.

Fn14 in podocytes and proteinuric kidney disease

Non-proliferative proteinuric diseases are the most common primary glomerular disorders causing end-stage renal disease. These disorders may associate low level glomerular inflammation and podocyte expression of inflammatory mediators. However, the factors regulating podocyte expression of inflammatory mediators in vivo in non-immune disorders are poorly understood. We have now explored the regulation and role of TWEAK receptor Fn14 in mediating glomerular inflammation in cultured podocytes and in experimental and human non-immune proteinuria. Transcriptomics disclosed Fn14 and MCP-1 mRNA upregulation in glomeruli from patients with focal segmental glomerulosclerosis, as well as a correlation between the expression of both transcripts. Increased glomerular Fn14 and MCP-1 mRNA was confirmed in a second focal segmental glomerulosclerosis cohort and was also observed in membranous nephropathy. In human non-proliferative proteinuric kidney diseases podocytes displayed Fn14 and MCP-1 expression and NFκB activation. Podocyte Fn14 was increased in murine protein overload-induced proteinuria. In Fn14 knock-out mice with protein overload-induced proteinuria, glomerular and periglomerular macrophage infiltrates were reduced, as were MCP-1 mRNA and podocyte MCP-1 staining and podocyte numbers preserved as compared to wild-type counterparts. Adenovirus-mediated overexpression of TWEAK increased periglomerular macrophage infiltration in mice without prior kidney injury. In cultured podocytes inflammatory cytokines increased Fn14 mRNA and protein levels. TWEAK activated NFκB and increased MCP-1 mRNA and protein, an effect prevented by the NFκB inhibitor parthenolide. In conclusion, Fn14 activation results in NFκB-mediated pro-inflammatory effects on podocytes that may be relevant for the pathogenesis of non-proliferative proteinuric kidney disease of non-immune origin.

Osteoprotegerin in exosome-like vesicles from human cultured tubular cells and urine

Urinary exosomes have been proposed as potential diagnostic tools. TNF superfamily cytokines and receptors may be present in exosomes and are expressed by proximal tubular cells. We have now studied the expression of selected TNF superfamily proteins in exosome-like vesicles from cultured human proximal tubular cells and human urine and have identified additional proteins in these vesicles by LC-MS/MS proteomics. Human proximal tubular cells constitutively released exosome-like vesicles that did not contain the TNF superfamily cytokines TRAIL or TWEAK. However, exosome-like vesicles contained osteoprotegerin (OPG), a TNF receptor superfamily protein, as assessed by Western blot, ELISA or selected reaction monitoring by nLC-(QQQ)MS/MS. Twenty-one additional proteins were identified in tubular cell exosome-like vesicles, including one (vitamin D binding protein) that had not been previously reported in exosome-like vesicles. Twelve were extracellular matrix proteins, including the basement membrane proteins type IV collagen, nidogen-1, agrin and fibulin-1. Urine from chronic kidney disease patients contained a higher amount of exosomal protein and exosomal OPG than urine from healthy volunteers. Specifically OPG was increased in autosomal dominant polycystic kidney disease urinary exosome-like vesicles and expressed by cystic epithelium in vivo. In conclusion, OPG is present in exosome-like vesicles secreted by proximal tubular epithelial cells and isolated from Chronic Kidney Disease urine.

Transcriptional profiling reveals progeroid Ercc1(-/Δ) mice as a model system for glomerular aging

Background

Aging-related kidney diseases are a major health concern. Currently, models to study renal aging are lacking. Due to a reduced life-span progeroid models hold the promise to facilitate aging studies and allow examination of tissue-specific changes. Defects in genome maintenance in the Ercc1^-/Δ progeroid mouse model result in premature aging and typical age-related pathologies. Here, we compared the glomerular transcriptome of young and aged Ercc1-deficient mice to young and aged WT mice in order to establish a novel model for research of aging-related kidney disease.

Results

In a principal component analysis, age and genotype emerged as first and second principal components. Hierarchical clustering of all 521 genes differentially regulated between young and old WT and young and old Ercc1^-/Δ mice showed cluster formation between young WT and Ercc1^-/Δ as well as old WT and Ercc1^-/Δ samples. An unexpectedly high number of 77 genes were differentially regulated in both WT and Ercc1^-/Δ mice (p < 0.0001). GO term enrichment analysis revealed these genes to be involved in immune and inflammatory response, cell death, and chemotaxis. In a network analysis, these genes were part of insulin signaling, chemokine and cytokine signaling and extracellular matrix pathways.

Conclusion

Beyond insulin signaling, we find chemokine and cytokine signaling as well as modifiers of extracellular matrix composition to be subject to major changes in the aging glomerulus. At the level of the transcriptome, the pattern of gene activities is similar in the progeroid Ercc1^-/Δ mouse model constituting a valuable tool for future studies of aging-associated glomerular pathologies.

Fibrosis: a key feature of Fabry disease with potential therapeutic implications

Fabry disease is a rare X-linked hereditary disease caused by mutations in the AGAL gene encoding the lysosomal enzyme alpha-galactosidase A. Enzyme replacement therapy (ERT) is the current cornerstone of Fabry disease management. Involvement of kidney, heart and the central nervous system shortens life span, and fibrosis of these organs is a hallmark of the disease. Fibrosis was initially thought to result from tissue ischemia secondary to endothelial accumulation of glycosphingolipids in the microvasculature. However, despite ready clearance of endothelial deposits, ERT is less effective in patients who have already developed fibrosis. Several potential explanations of this clinical observation may impact on the future management of Fabry disease. Alternative molecular pathways linking glycosphingolipids and fibrosis may be operative; tissue injury may recruit secondary molecular mediators of fibrosis that are unresponsive to ERT, or fibrosis may represent irreversible tissue injury that limits the therapeutic response to ERT. We provide an overview of Fabry disease, with a focus on the assessment of fibrosis, the clinical consequences of fibrosis, and recent advances in understanding the cellular and molecular mechanisms of fibrosis that may suggest novel therapeutic approaches to Fabry disease.

TNF-related weak inducer of apoptosis (TWEAK) promotes kidney fibrosis and Ras-dependent proliferation of cultured renal fibroblast

Tumor necrosis factor-like weak inducer of apoptosis (TWEAK) regulates apoptosis, proliferation and inflammation in renal epithelial cells and plays a role in acute kidney injury. However, there is little information on the chronic effects of TWEAK. We hypothesized that TWEAK may influence renal fibrosis and regulate kidney fibroblast biology, in part, through Ras pathway. We studied a chronic model of experimental unilateral ureteral obstruction in wild type and TWEAK deficient mice, and a murine model of systemic TWEAK overexpression. TWEAK actions were also explored in cultured renal and embryonic fibroblasts. TWEAK and TWEAK receptor expression was increased in the obstructed kidneys. The absence of TWEAK decreased early kidney tubular damage, inflammatory infiltrates and myofibroblast number. TWEAK deficient mice had decreased renal fibrosis 21days after obstruction, as assessed by extracellular matrix staining. In mice without prior underlying kidney disease, systemic overexpression of TWEAK induced kidney inflammation and fibrosis. In cultured fibroblasts, TWEAK induced proliferation through activation of the Ras/ERK pathway. TWEAK also activated nuclear factor κB (NFκB)-dependent inflammatory chemokine production in murine renal fibroblasts. In conclusion, lack of TWEAK reduces renal fibrosis in a model of persistent kidney insult and overexpression of TWEAK led to renal fibrosis. TWEAK actions on renal fibroblasts may contribute to the in vivo observations, as TWEAK promotes inflammatory activity and proliferation in fibroblast cultures.

Uropathogenic Escherichia coli causes cortical tubular necrotic cell death and the release of macrophage migration inhibitory factor

The macrophage migration inhibitory factor (MIF), a pro-inflammatory cytokine, is deregulated in acute kidney injury (AKI) through an unknown mechanism. In the present study, we used a previously described mouse model of ascending urinary tract infection in which uropathogenic Escherichia coli (UPEC) were transurethrally inoculated to induce kidney infections. Here, we show that urinary MIF was upregulated during AKI while MIF was abundantly expressed in the renal cortical tubules and that UPEC infection caused a decrease in tubular MIF. Infections with UPEC in vitro caused MIF release in a cell type-dependent manner, which was independent of receptor-mediated internalization, signal transduction, and transcription. Indeed, UPEC infection-induced necrotic cell death in vitro and in vivo correlated with extracellular acidification and processed MIF secretion. These data suggest that MIF is released by necrotic renal cortical tubular cells during UPEC infection.

Urinary macrophage migration inhibitory factor serves as a potential biomarker for acute kidney injury in patients with acute pyelonephritis

Conventional markers of kidney function that are familiar to clinicians, including the serum creatinine and blood urea nitrogen levels, are unable to reveal genuine injury to the kidney, and their use may delay treatment. Macrophage migration inhibitory factor (MIF) is a proinflammatory cytokine, and the predictive role and pathogenic mechanism of MIF deregulation during kidney infections involving acute kidney injury (AKI) are not currently known. In this study, we showed that elevated urinary MIF levels accompanied the development of AKI during kidney infection in patients with acute pyelonephritis (APN). In addition to the MIF level, the urinary levels of interleukin (IL)-1β and kidney injury molecule (KIM)-1 were also upregulated and were positively correlated with the levels of urinary MIF. An elevated urinary MIF level, along with elevated IL-1β and KIM-1 levels, is speculated to be a potential biomarker for the presence of AKI in APN patients.

MIF, CD74 and other partners in kidney disease: tales of a promiscuous couple

Macrophage migration inhibitory factor (MIF) is increased in kidney and urine during kidney disease. MIF binds to and activates CD74 and chemokine receptors CXCR2 and CXCR4. CD74 is a protein trafficking regulator and a cell membrane receptor for MIF, D-dopachrome tautomerase (D-DT/MIF-2) and bacterial proteins. MIF signaling through CD74 requires CD44. CD74, CD44 and CXCR4 are upregulated in renal cells in diseased kidneys and MIF activation of CD74 in kidney cells promotes an inflammatory response. MIF or CXCR2 targeting protects from experimental kidney injury, CD44 deficiency modulates kidney injury and CXCR4 activation promotes glomerular injury. However, the contribution of MIF or MIF-2 to these actions of MIF receptors has not been explored. The safety and efficacy of strategies targeting MIF, CD74, CD44 and CXCR4 are under study in humans.

Receptor activator of NF-kappaB and podocytes: towards a function of a novel receptor-ligand pair in the survival response of podocyte injury

BACKGROUND

Glomerulosclerosis correlates with reduction in podocyte number that occurs through mechanisms which include apoptosis. Podocyte injury or podocyte loss in the renal glomerulus has been proposed as the crucial mechanism in the development of glomerulosclerosis. However, the mechanism by which podocytes respond to injury is poorly understood. TNF and TNF receptor superfamilies are important in the pathogenesis of podocyte injury and apoptosis. The ligand of receptor activator of NF-kappaB (RANKL) and receptor activator of NF-kappaB (RANK) are members of the TNF and receptor superfamilies. We investigated whether RANK-RANKL is a receptor-ligand complex for podocytes responding to injury.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, RANKL and RANK were examined in human podocyte diseases and a rat model of puromycin aminonucleoside nephrosis (PAN). Compared with controls, RANK and RANKL were increased in both human podocyte diseases and the rat PAN model; double immunofluorescence staining revealed that RANK protein expression was mainly attributed to podocytes. Immunoelectron microscopy showed that RANK was localized predominantly at the top of the foot process membrane and the cytoplasm of rat podocyte. In addition, RANK was upregulated in mouse podocytes in vitro after injury induced by puromycin aminonucleoside (PA). Knockdown of RANK expression by small interference RNA (siRNA) exacerbated podocyte apoptosis induced by PA. However, RANKL inhibited significantly the apoptosis of podocytes induced by PA.

CONCLUSIONS/SIGNIFICANCE

These findings suggest the increase in RANK-RANKL expression is a response to podocyte injury, and RANK-RANKL may be a novel receptor-ligand complex for the survival response during podocyte injury.

Acute and sustained actions of hyperglycaemia on endothelial and glomerular barrier permeability

Microalbuminuria is an established marker of systemic endothelial dysfunction, which for patients with diabetes signals an increased risk of both diabetic nephropathy and cardiovascular complications. A better understanding of the pathogenesis of microalbuminuria is important in the quest of finding new approaches to treat patients with diabetes. Direct acute effects of episodes of hyperglycaemia (HG) could have implications for the microalbuminuria seen in early diabetes before renal structural alterations have started, especially in those patients with poor glycaemic control. This review summarizes the literature evidence that acute or sustained HG may lead to an increased vascular or glomerular permeability. Special focus is on glomerular barrier permeability. There is evidence in the literature that HG increases systemic capillary and glomerular barrier permeability within 20-30 min in vivo in rats and mice. Furthermore, exposure of monolayers of cultured endothelial cells to HG has been shown to increase monolayer permeability rapidly and transiently (during 60-100 min). Instant cellular changes following F-actin cytoskeleton rearrangements, which could be abrogated by Rho-kinase (ROCK) inhibition, are implicated. Data in this review also suggest that activation of protein kinase C, the polyol pathway, and an increased release of reactive oxygen species (ROS) and cytokines could contribute to the increase in barrier permeability induced by HG. Recent in vitro data from cultured podocyte monolayers also designates a role of insulin in acute podocyte F-actin remodelling, underpinning the complexity of the mechanisms leading to glomerular and endothelial barrier alterations in diabetes mellitus.

The outcome of renal ischemia-reperfusion injury is unchanged in AMPK-β1 deficient mice

Aim

Activation of the master energy-regulator AMP-activated protein kinase (AMPK) in the heart reduces the severity of ischemia-reperfusion injury (IRI) but the role of AMPK in renal IRI is not known. The aim of this study was to determine whether activation of AMPK by acute renal ischemia influences the severity of renal IRI.

Methods

AMPK expression and activation and the severity of renal IRI was studied in mice lacking the AMPK β1 subunit and compared to wild type (WT) mice.

Results

Basal expression of activated AMPK, phosphorylayed at αThr¹⁷², was markedly reduced by 96% in AMPK-β1^−/− mice. Acute renal ischaemia caused a 3.2-fold increase in α1-AMPK activity and a 2.5-fold increase in α2-AMPK activity (P<0.001) that was associated with an increase in AMPK phosphorylation of the AMPK-α subunit at Thr¹⁷² and Ser⁴⁸⁵, and increased inhibitory phosphorylation of the AMPK substrate acetyl-CoA carboxylase. After acute renal ischemia AMPK activity was reduced by 66% in AMPK-β1^−/− mice compared with WT. There was no difference, however, in the severity of renal IRI at 24-hours between AMPK-β1^−/− and WT mice, as measured by serum urea and creatinine and histological injury score. In the heart, macrophage migration inhibitory factor (MIF) released during IRI contributes to AMPK activation and protects from injury. In the kidney, however, no difference in AMPK activation by acute ischemia was observed between MIF^−/− and WT mice. Compared with the heart, expression of the MIF receptor CD74 was found to be reduced in the kidney.

Conclusion

The failure of AMPK activation to influence the outcome of IRI in the kidney contrasts with what is reported in the heart. This difference might be due to a lack of effect of MIF on AMPK activation and lower CD74 expression in the kidney.

HSP27/HSPB1 as an adaptive podocyte antiapoptotic protein activated by high glucose and angiotensin II

Apoptosis is a driving force of diabetic end-organ damage, including diabetic nephropathy (DN). However, the mechanisms that modulate diabetes-induced cell death are not fully understood. Heat shock protein 27 (HSP27/HSPB1) is a cell stress protein that regulates apoptosis in extrarenal cells and is expressed by podocytes exposed to toxins causing nephrotic syndrome. We investigated the regulation of HSPB1 expression and its function in podocytes exposed to factors contributing to DN, such as high glucose and angiotensin (Ang) II. HSPB1 expression was assessed in renal biopsies from patients with DN, minimal change disease or focal segmental glomerulosclerosis (FSGS), in a rat model of diabetes induced by streptozotocin (STZ) and in Ang II-infused rats. The regulation of HSPB1 was studied in cultured human podocytes and the function of HSPB1 expressed in response to pathophysiologically relevant stimuli was explored by short interfering RNA knockdown. Total kidney HSPB1 mRNA and protein expression was increased in rats with STZ-induced diabetes and in rats infused with Ang II. Upregulation of HSPB1 protein was confirmed in isolated diabetic glomeruli. Immunohistochemistry showed increased glomerular expression of HSPB1 in both models and localized glomerular HSPB1 to podocytes. HSPB1 protein was increased in glomerular podocytes from patients with DN or FSGS. In cultured human podocytes HSPB1 mRNA and protein expression was upregulated by high glucose concentrations and Ang II. High glucose, but not Ang II, promoted podocyte apoptosis. HSPB1 short interfering RNA (siRNA) targeting increased apoptosis in a high-glucose milieu and sensitized to Ang II or TGFβ1-induced apoptosis by promoting caspase activation. In conclusion, both high glucose and Ang II contribute to HSPB1 upregulation. HSPB1 upregulation allows podocytes to better withstand an adverse high-glucose or Ang II-rich environment, such as can be found in DN.

Macrophage migration inhibitory factor activates cyclooxygenase 2-prostaglandin E2 in cultured spinal microglia

In our previous study, peripheral inflammatory stimulation evoked production of macrophage migration inhibitory factor (MIF) in the spinal cord and found spinal microglia are the major source of MIF in this context. Given the contribution of the activated-microglia to the inflammatory neuropathy plus the role for upregulated COX 2 expression and PGE(2) production in the severity of clinical manifestations of these neuroinflammatory conditions, we herein tested the hypothesis that in vitro MIF stimulation to spinal microglia could result in an activation of COX 2-PGE(2) system by MIF-CD74 interaction. We found MIF played roles in evoking COX 2 mRNA and protein expression in a dose-dependent manner correspondingly in changes in PGE(2) level in the cultured rat microglia, but these changes could be inhibited by genetic deletion of CD74. Finally, MIF-induced COX 2-PGE(2) activation could be blocked by selective inhibitors of p44/p42 and p38 MAPKs. These data highlight MIF/CD74 interaction induces upregulation of COX 2 expression and PGE(2) secretion in primary rodent microglia, and further this effect is associated with downstream activation of p38 and p44/p42 signaling cascades, and favors the role of MIF as a novel pathway for microglia-associated neuroinflammation.

The role of metabolic and haemodynamic factors in podocyte injury in diabetes

Podocyte loss is a common feature in human diabetes as well as in experimental diabetes in rodents. Almost all components of the diabetic milieu lead to serious podocyte stress, driving the cells towards cell cycle arrest and hypertrophy, detachment and apoptosis. Common pathway components induced by high glucose and advanced glycation end-products are reactive oxygen species, cyclin-dependent kinases (p27(Kip1)) and transforming growth factor-beta. In addition, mechanical stresses by stretch or shear forces, insulin deficiency or insulin resistance are independent components resulting in podocyte apoptosis and detachment. In this review, we discuss the common pathways leading to podocyte death as well as novel pathways and concepts of podocyte dedifferentiation and detachment that influence the progression of diabetic glomerulopathy.

TNFR2 interposes the proliferative and NF-κB-mediated inflammatory response by podocytes to TNF-α

The development of proliferative podocytopathies has been linked to ligation of tumor necrosis factor receptor 2 (TNFR2) expressed on the renal parenchyma; however, the TNFR2-positive cells within the kidney responsible for podocyte injury are unknown. We detected de novo expression of TNFR2 on podocytes before hyperplastic injury in crescentic glomerulonephritis of mice with nephrotoxic nephritis, and in collapsing glomerulopathy of Tg26(HIV/nl) mice, kd/kd mice, and human beings. We further found that serum levels of soluble TNF-α and TNFR2 correlated significantly with renal injury in Tg26(HIV/nl) mice. Thus, we asked whether ligand binding of TNFR2 on podocytes ex vivo precipitates the characteristic proliferative and pro-inflammatory diseased podocyte phenotypes. Soluble TNF-α activated NF-κB and dose-dependently induced podocyte proliferation, marked by the expression of the podocyte G(1) cyclin and NF-κB target gene, cyclin D1. Microarray gene and chemokine protein expression profiling showed a marked pro-inflammatory NF-κB signature, and activated podocytes secreting CCL2- and CCL5-induced macrophage migration in transwell assays. Neutralization of TNFR2 on podocytes with blocking antibodies abrogated NF-κB activation and the induction of cyclin D1 by TNF-α, and identified TNFR2 as the primary receptor that induced IκBα degradation, the initiating event in NF-κB activation. These results suggest that TNFR2 expressed on podocytes and its canonical NF-κB signaling may directly interpose the compound pathogenic responses by podocytes to TNF-α, in the absence of other TNFR2-positive renal cell types in proliferative podocytopathies.

Macrophage migration inhibitory factor regulates interleukin-6 production by facilitating nuclear factor-kappa B activation during Vibrio vulnificus infection

BACKGROUND

Patients infected with Vibrio vulnificus (V. vulnificus) show severe inflammatory responses characterised by the upregulation of proinflammatory cytokines. Macrophage migration inhibitory factor (MIF), an upstream proinflammatory regulator, increases the inflammation caused by sepsis. Whether MIF regulates responses to V. vulnificus infection and the actual mechanism by which V. vulnificus initiates these MIF-modulated proinflammatory cytokines remain unclear.

RESULTS

MIF increased inflammation during V. vulnificus infection in vivo. In V. vulnificus-infected mice, MIF was produced earlier than tumour necrosis factor (TNF)-α and interleukin (IL)-6 and was expressed in a time-dependent manner. ISO-1 ((S, R)-3-(4-hydroxyphenyl)-4,5-dihydro-5-isoxazole acetic acid methyl ester), a small-molecule inhibitor of MIF, significantly decreased IL-6, IL-8, and TNF-α production in a time- and dose-dependent manner in human peripheral blood cells infected with V. vulnificus. The induction of IL-6, IL-8, and TNF-α production by V. vulnificus infection was mediated via the NF-κB- and p38 MAPK-regulated pathways but not via the Akt pathway. ISO-1-treated human peripheral blood cells showed lower V. vulnificus-induced NF-κB activation, IL-6 mRNA expression, and IκB phosphorylation, but they did not show lower p38 MAPK activation.

CONCLUSIONS

We conclude that MIF regulates V. vulnificus-induced IL-6 production via NF-κB activation and that p38 MAPK activation in V. vulnificus infection is not MIF dependent.

TNF superfamily: a growing saga of kidney injury modulators

Members of the TNF superfamily participate in kidney disease. Tumor necrosis factor (TNF) and Fas ligand regulate renal cell survival and inflammation, and therapeutic targeting improves the outcome of experimental renal injury. TNF-related apoptosis-inducing ligand (TRAIL and its potential decoy receptor osteoprotegerin are the two most upregulated death-related genes in human diabetic nephropathy. TRAIL activates NF-kappaB in tubular cells and promotes apoptosis in tubular cells and podocytes, especially in a high-glucose environment. By contrast, osteoprotegerin plays a protective role against TRAIL-induced apoptosis. Another family member, TNF-like weak inducer of apoptosis (TWEAK induces inflammation and tubular cell death or proliferation, depending on the microenvironment. While TNF only activates canonical NF-kappaB signaling, TWEAK promotes both canonical and noncanonical NF-kappaB activation in tubular cells, regulating different inflammatory responses. TWEAK promotes the secretion of MCP-1 and RANTES through NF-kappaB RelA-containing complexes and upregulates CCl21 and CCL19 expression through NF-kappaB inducing kinase (NIK-) dependent RelB/NF-kappaB2 complexes. In vivo TWEAK promotes postnephrectomy compensatory renal cell proliferation in a noninflammatory milieu. However, in the inflammatory milieu of acute kidney injury, TWEAK promotes tubular cell death and inflammation. Therapeutic targeting of TNF superfamily cytokines, including multipronged approaches targeting several cytokines should be further explored.

New paradigms in cell death in human diabetic nephropathy

Cell death is thought to contribute to progressive renal cell depletion in diabetic nephropathy. Unbiased gene expression profiling identified novel cell death molecules in human diabetic nephropathy. The expression of TNF-related apoptosis-inducing ligand (TRAIL), its decoy receptor osteoprotegerin, and receptors Fas (a Fas ligand receptor) and CD74 (a migration inhibitory factor (MIF) receptor) were induced in human diabetic nephropathy. Cell culture studies supported the functional relevance of this observation and the relationship to a high glucose environment. To define novel proapoptotic proteins upregulated in diabetic nephropathy, functional genomic screens for novel apoptosis mediators were integrated with genome-wide expression profiling and identified candidates for further functional analysis, including brain acid-soluble protein 1 (BASP1). Several lines of evidence point toward induction of endoplasmic reticulum stress response in human diabetic nephropathy. Functional studies defining an unequivocal contribution of endoplasmic reticulum stress to cell death in this setting are still needed. Further comparative studies will be required to define whether there is a specific aspect of apoptosis in progressive human diabetic nephropathy or whether the mechanisms are shared among all patients with chronic kidney disease. The next challenge will be to define the consequence of therapeutic interference of the apoptosis pathways in diabetic nephropathy and chronic kidney disease.

High glucose up-regulates angiotensin II subtype 2 receptors via interferon regulatory factor-1 in proximal tubule epithelial cells

Earlier studies have reported an increase in the proximal tubule AT(2) receptor (AT(2)R) expression in diabetes, with a beneficial role in kidney function and blood pressure regulation. Here, we demonstrate that the increase in AT(2)R protein expression is associated with an increased expression of transcription factor IRF-1 in hyperglycemic rat and in high glucose-treated HK2 cells. Knock-down of IRF-1 by siRNA in HK2 cells prevented high glucose-induced AT(2)R up-regulation. The data suggest that IRF-1 is a transcriptional regulator of AT(2)R expression in hyperglycemia, and warrant further studies to understand the physiological role of IRF-1 along with AT(2)R in diabetic kidney.

Globotriaosylsphingosine actions on human glomerular podocytes: implications for Fabry nephropathy

BACKGROUND

Transforming growth factor-β1 (TGF-β1) and the macrophage inhibitory factor receptor CD74 link the metabolic disorder with tissue injury in diabetic nephropathy. Fabry disease is an X-linked lysosomal glycosphingolipid storage disorder resulting from a deficient activity of α-galactosidase A that leads to proteinuric renal injury. However, the link between the metabolic abnormality and renal injury is poorly characterized. Globotriaosylsphingosine (lyso-Gb3) was recently identified as a bioactive molecule accumulating in Fabry disease. We hypothesized that lyso-Gb3 could modulate the release of secondary mediators of injury in glomerular podocytes and that recently described nephroprotective actions of vitamin D receptor activation in diabetic nephropathy may apply to lyso-Gb3.

METHODS

Real time RT-PCR, ELISA and Western blot were used to study the biological activity of lyso-Gb3 in cultured human podocytes and potential modulation by vitamin D receptor activation.

RESULTS

In human podocytes, lyso-Gb3 dose and time dependently increased the expression of TGF-β1, extracellular matrix proteins (fibronectin and type IV collagen) and CD74. TGF-β1 mediated lyso-Gb3 effects on extracellular matrix production. Vitamin D receptor activation with paricalcitol or calcitriol prevented the increase in TGF-β1, CD74 and extracellular matrix induced by lyso-Gb3.

CONCLUSIONS

Lyso-Gb3 may have a role in glomerular injury in Fabry disease by promoting the release of secondary mediators of glomerular injury common to diabetic nephropathy. These effects are prevented by paricalcitol, raising the issue of vitamin D receptor activation as potential adjunctive therapy in Fabry nephropathy.

Suppressors of cytokine signaling abrogate diabetic nephropathy

Activation of Janus kinase/signal transducers and activators of transcription (JAK/STAT) is an important mechanism by which hyperglycemia contributes to renal damage, suggesting that modulation of this pathway may prevent renal and vascular complications of diabetes. Here, we investigated the involvement of suppressors of cytokine signaling (SOCS) as intracellular negative regulators of JAK/STAT activation in diabetic nephropathy. In a rat model, inducing diabetes resulted in JAK/STAT activation and increased expression of SOCS1 and SOCS3. In humans, we observed increased expression of glomerular and tubulointerstitial SOCS proteins in biopsies of patients with diabetic nephropathy. In vitro, high concentrations of glucose activated JAK/STAT/SOCS in human mesangial and tubular cells. Overexpression of SOCS reversed the glucose-induced activation of the JAK/STAT pathway, expression of STAT-dependent genes (chemokines, growth factors, and extracellular matrix proteins), and cell proliferation. In vivo, intrarenal delivery of adenovirus expressing SOCS1 and SOCS3 to diabetic rats significantly improved renal function and reduced renal lesions associated with diabetes, such as mesangial expansion, fibrosis, and influx of macrophages. SOCS gene delivery also decreased the activation of STAT1 and STAT3 and the expression of proinflammatory and profibrotic proteins in the diabetic kidney. In summary, these results provide direct evidence for a link between the JAK/STAT/SOCS axis and hyperglycemia-induced cell responses in the kidney. Suppression of the JAK/STAT pathway by increasing intracellular SOCS proteins may have therapeutic potential in diabetic nephropathy.

Macrophage migration inhibitory factor: critical role in obesity, insulin resistance, and associated comorbidities

Obesity is associated with insulin resistance, disturbed glucose homeostasis, low grade inflammation, and comorbidities such as type 2 diabetes and cardiovascular disease. The cytokine macrophage migration inhibitory factor (MIF) is an ubiquitously expressed protein that plays a crucial role in many inflammatory and autoimmune disorders. Increasing evidence suggests that MIF also controls metabolic and inflammatory processes underlying the development of metabolic pathologies associated with obesity. This is a comprehensive summary of our current knowledge on the role of MIF in obesity and obesity-associated comorbidities, based on human clinical data as well as animal models of disease.

Diabetic angiopathy, the complement system and the tumor necrosis factor superfamily

Among the most serious consequences of diabetes mellitus is the development of diabetic angiopathy, of which the clinical features are cardiovascular disease, retinopathy, nephropathy and neuropathy. Diabetic kidney problems affect up to one third of all patients with diabetes mellitus and are a major cause of end-stage renal failure. Although a huge number of pharmaceutical interventions are available today, diabetic angiopathy remains a leading cause of mortality and morbidity in diabetes mellitus, therefore, an urgent need exists to develop new therapeutic strategies. Recent data support the hypothesis that dysregulation of the complement system and of members of the tumor necrosis factor (TNF) superfamily may be involved in the development of diabetic vascular complications. The mannose-binding lectin pathway-an overall regulatory component of the complement system-is a particularly promising biomarker as it is directly involved in the development of diabetic angiopathy. In addition, two components of the TNF superfamily, namely TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) and osteoprotegerin, may be involved in the pathogenesis of diabetic angiopathy. Several ways of specifically manipulating the complement and TNF superfamily systems already exist, but whether or not these drugs provide new targets for intervention for late diabetic complications is still to be revealed.

Elevated serum macrophage migration inhibitory factor (MIF) concentrations in chronic kidney disease (CKD) are associated with markers of oxidative stress and endothelial activation

Chronic kidney disease (CKD) carries an increased risk of cardiovascular disease (CVD). Macrophage migration inhibiting factor (MIF) is a proinflammatory cytokine implicated in the pathogenesis of sepsis, autoimmune disease, atherogenesis, and plaque instability, and is a known cardiac depressant. This post-hoc, cross-sectional study examined whether MIF serum concentrations are elevated in CKD patients. Our study included CKD 3-5 patients with moderate to severe renal dysfunction (n = 257) (mean age SD; 55 +/- 12 years) and 53 controls (60 +/- 12 years). Serum MIF concentrations, measured by enzyme-linked immunosorbent assay (ELISA), were studied in relation to glomerular filtration rate (GFR), presence of CVD, outcome and inflammatory and oxidative stress markers. MIF was significantly elevated in CKD patients compared with controls (CKD: median 676 [range 118-8275 pg/mL] controls: 433 [142-4707] pg/mL; P = 0.008). MIF was also associated with 8-hydroxy-2-deoxyguanosine (8-OH-dG) levels (rho = 0.26; P = 0.001), a marker of oxidative stress, and ICAM-1 levels (rho = 0.14; P = 0.02), a marker of endothelial activation. However, the elevated MIF concentrations were neither correlated with glomerular filtration rate (GFR) nor inflammatory markers such as CRP, IL-6, and TNF. When combining MIF and IL-6 as a marker of inflammation, a significant increase in risk for CVD was found, but when analyzing all-cause mortality, this did not differ significantly with regard to mortality from inflamed patients with low MIF levels. The data suggest that increased serum MIF levels found in CKD is not caused primarily by poor renal function, but is associated with markers of oxidative stress and endothelial activation and may play a role in vascular disease associated with CKD.