Deletion of the insulin receptor in the proximal tubule promotes hyperglycemia

Absence of renal enlargement in fructose-fed proximal-tubule-select insulin receptor (IR), insulin-like-growth factor receptor (IGF1R) double knockout mice

The major site of fructose metabolism in the kidney is the proximal tubule (PT). To test whether insulin and/or IGF1 signaling in the PT is involved in renal structural/functional responses to dietary fructose, we bred mice with dual knockout (KO) of the insulin receptor (IR) and the IGF1 receptor (IGF1R) in PT by Cre-lox recombination, using a γ-glutamyl transferase promoter. KO mice had slightly (~10%) reduced body and kidney weights, as well as, a reduction in mean protein-to-DNA ratio in kidney cortex suggesting smaller cell size. Under control diet, IR and IGF1R protein band densities were 30-50% (P < 0.05) lower than WT, and the relative difference was greater in male animals. Male, but not female KO, also had significantly reduced band densities for Akt (protein kinase B), phosphorylated Akt^T308 and IR^Y1162/1163 A high-fructose diet (1-month) led to a significant increase in kidney weight in WT males (12%), but not in KO males or in either genotype of female mice. Kidney enlargement in the WT males was accompanied by a small, insignificant fall in protein-to-DNA ratio, supporting hyperplasia rather than hypertrophy. Fructose feeding of male WT mice led to significantly higher sodium bicarbonate exchanger (NBCe1), sodium hydrogen exchanger (NHE3), sodium phosphate co-transporter (NaPi-2), and transforming growth factor-β (TGF-β) abundances, as compared to male KO, suggesting elevated transport capacity and an early feature of fibrosis may have accompanied the renal enlargement. Overall, IR and/or IGF1R appear to have a role in PT cell size and enlargement in response to high-fructose diet.

(-)-Epicatechin and the Colonic 2,3-Dihydroxybenzoic Acid Metabolite Regulate Glucose Uptake, Glucose Production, and Improve Insulin Signaling in Renal NRK-52E Cells

SCOPE

(-)-Epicatechin (EC) and main colonic phenolic acids derived from flavonoid intake, such as 2,3-dihydroxybenzoic acid (DHBA), 3,4-dihydroxyphenylacetic acid (DHPAA), 3-hydroxyphenylpropionic acid (HPPA), and vanillic acid (VA), have been suggested to exert beneficial effects in diabetes, although the mechanism for their actions remains unknown. In this study, the modulation of glucose homeostasis and insulin signaling by the mentioned compounds on renal proximal tubular NRK-52E cells is investigated.

METHODS AND RESULTS

Levels of the glucose transporters SGLT-2 and GLUT-2, as well as glucose uptake, glucose production, and key proteins of the insulin pathways, namely insulin receptor (IR), insulin receptor substrate-1 (IRS-1), and PI3K/AKT pathway are analyzed. EC (5-20 μm) and DHBA (20 μm) reduced both renal glucose uptake and production. Interestingly, EC and DHBA did not modify the levels of SGLT-2 and GLUT-2, and modulated the expression of phosphoenolpyruvate carboxykinase via AKT leading to a diminished glucose production. EC and DHBA also enhanced the tyrosine phosphorylation and total IR and IRS-1 levels, and activated the PI3K/AKT pathway in NRK-52E cells.

CONCLUSION

EC and DHBA regulate the renal glucose homeostasis by modulating both glucose uptake and production, and strengthen the insulin signaling by activating key proteins of that pathway in NRK-52E cells.

The Evolving Importance of Insulin Signaling in Podocyte Health and Disease

Diabetic kidney disease (DKD) is the leading cause of end-stage renal disease worldwide, occuring in approximately one-third of diabetic patients. One of the earliest hallmarks of DKD is albuminuria, often occurring following disruptions to the glomerular filtration barrier. Podocytes are highly specialized cells with a central role in filtration barrier maintenance; hence, podocyte dysfunction is a major cause of albuminuria in many settings, including DKD. Numerous studies over the last decade have highlighted the importance of intact podocyte insulin responses in the maintenance of podocyte function. This review summarizes our current perspectives on podocyte insulin signaling, highlighting evidence to support the notion that dysregulated podocyte insulin responses contribute toward podocyte damage, particularly during the pathogenesis of DKD.

Effects of insulin and the glucagon-like peptide 1 receptor agonist liraglutide on the kidney proteome in db/db mice

Diabetes mellitus (DM) is a worldwide disease that affects 9% of the adult world population and type 2 DM accounts for 90% of those. A common consequence of DM is kidney complications, which could lead to kidney failure. We studied the potential effects of treatment with insulin and the glucagon-like peptide 1 receptor (GLP-1R) agonist liraglutide on the diabetic kidney proteome through the use of the db/db mouse model system and mass spectrometry (MS). Multivariate analyses revealed distinct effects of insulin and liraglutide on the db/db kidney proteome, which was seen on the protein levels of, for example, pterin-4 α-carbinolamine dehydratase/dimerization cofactor of hepatocyte nuclear factor-1α (PCBD1), neural precursor cell expressed developmentally down-regulated-8 (NEDD8), transcription elongation factor-B polypeptide-1 (ELOC) and hepcidin (HEPC). Furthermore, the separation of the insulin, liraglutide and vehicle db/db mouse groups in multivariate analyses was not mainly related to the albumin excretion rate (AER) or the level of glycated hemoglobin A_1c (HbA_1c%) in the mice. In summary, we show that insulin and liraglutide give rise to separate protein profiles in the db/db mouse kidney.

Angiotensin II type 2 receptor inhibits expression and function of insulin receptor in rat renal proximal tubule cells

Both renin-angiotensin systems and insulin participate in kidney-involved blood pressure regulation. Activation of angiotensin II type 2 receptor (AT₂R) decreases sodium reabsorption in renal proximal tubule (RPT) cells, whereas insulin produces the opposite effect. We presume that AT₂R has an inhibitory effect on insulin receptor expression in RPT cells, which may affect renal sodium transport and therefore be of physiological or pathological significance. Our present study found that activation of AT₂R inhibited insulin receptor expression in a concentration and time-dependent manner in RPT cells from Wistar-Kyoto (WKY) rats. In the presence of a protein kinase C (PKC) inhibitor (PKC inhibitor peptide 19-31, 10^-6 mol/L) or a phosphatidylinositol 3 kinase inhibitor (wortmannin, 10^-6 mol/L), the inhibitory effect of AT₂R on insulin receptor was blocked, indicating that both PKC and phosphatidylinositol 3 kinase were involved in the signaling pathway. There was a linkage between AT₂R and insulin receptor which was determined by both laser confocal microscopy and coimmunoprecipitation. However, the effect of AT₂R activation on insulin receptor expression was different in RPT cells from spontaneously hypertensive rats (SHRs). Being contrary to the effect in WKY RPT cells, AT₂R stimulation increased insulin receptor in SHR RPT cells. Insulin (10^-7 mol/L, 15 minutes) enhanced Na⁺-K⁺-ATPase activity in both WKY and SHR RPT cells. Pretreatment with CGP42112 decreased the stimulatory effect of insulin on Na⁺-K⁺-ATPase activity in WKY RPT cells, whereas pretreatment with CGP42112 increased it in SHR RPT cells. It is suggested that activation of AT₂R inhibits insulin receptor expression and function in RPT cells. The lost inhibitory effect of AT₂R on insulin receptor expression may contribute to the pathophysiology of hypertension.

Dual Regulation of Gluconeogenesis by Insulin and Glucose in the Proximal Tubules of the Kidney

Growing attention has been focused on the roles of the proximal tubules (PTs) of the kidney in glucose metabolism, including the mechanism of regulation of gluconeogenesis. In this study, we found that PT-specific insulin receptor substrate 1/2 double-knockout mice, established by using the newly generated sodium-glucose cotransporter 2 (SGLT2)-Cre transgenic mice, exhibited impaired insulin signaling and upregulated gluconeogenic gene expression and renal gluconeogenesis, resulting in systemic insulin resistance. In contrast, in streptozotocin-treated mice, although insulin action was impaired in the PTs, the gluconeogenic gene expression was unexpectedly downregulated in the renal cortex, which was restored by administration of an SGLT1/2 inhibitor. In the HK-2 cells, the gluconeogenic gene expression was suppressed by insulin, accompanied by phosphorylation and inactivation of forkhead box transcription factor 1 (FoxO1). In contrast, glucose deacetylated peroxisome proliferator-activated receptor γ coactivator 1-α (PGC1α), a coactivator of FoxO1, via sirtuin 1, suppressing the gluconeogenic gene expression, which was reversed by inhibition of glucose reabsorption. These data suggest that both insulin signaling and glucose reabsorption suppress the gluconeogenic gene expression by inactivation of FoxO1 and PGC1α, respectively, providing insight into novel mechanisms underlying the regulation of gluconeogenesis in the PTs.

Greater efficacy of atorvastatin versus a non-statin lipid-lowering agent against renal injury: potential role as a histone deacetylase inhibitor

Statins, 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase inhibitors have been shown to improve diabetic nephropathy. However, whether they provide protection via Histone deacetylases (HDAC) inhibition is not clear. We conducted a comparative evaluation of Atorvastatin (AT) versus the non-statin cholesterol-lowering drug, Ezetimibe (EZT) on severity of diabetic nephropathy. Streptozotocin-treated male Wistar rats were fed a cholesterol-supplemented diet and gavaged daily with vehicle, AT or EZT. Control rats received normal diet and gavaged vehicle (n = 8-9/group). Diabetes increased blood glucose, urine albumin-to-creatinine ratio (ACR), kidney pathology and HDAC activity, and reduced renal E-cadherin levels. Both AT and EZT reduced circulating cholesterol, attenuated renal pathology, and did not lower blood glucose. However, AT was significantly more effective than EZT at reducing kidney pathology and HDAC activity. Chromatin immunoprecipitation revealed a significantly higher association of acetylated H3 and H4 with the E-cadherin promoter in kidneys from AT-, relative to EZT- or vehicle-treated rats. Moreover, we demonstrated a direct effect of AT, but not EZT, on HDAC-inhibition and, H3 and H4- acetylation in primary glomerular mesangial cells. Overall, both AT and EZT attenuated diabetic nephropathy; however, AT exhibited greater efficacy despite a similar reduction in circulating cholesterol. HDAC-inhibition may underlie greater efficacy of statins in attenuating kidney injury.

Diabetic Nephropathy: Perspective on Novel Molecular Mechanisms

Diabetes mellitus (DM) is the major cause of end-stage renal disease (ESRD) globally, and novel treatments are urgently needed. Current therapeutic approaches for diabetic nephropathy (DN) are focussing on blood pressure control with inhibitors of the renin-angiotensin-aldosterone system, on glycaemic and lipid control, and life-style changes. In this review, we highlight new molecular insights aiding our understanding of the initiation and progression of DN, including glomerular insulin resistance, dysregulation of cellular substrate utilisation, podocyte-endothelial communication, and inhibition of tubular sodium coupled glucose reabsorption. We believe that these mechanisms offer new therapeutic targets that can be exploited to develop important renoprotective treatments for DN over the next decade.

The impact of insulin resistance on the kidney and vasculature

Insulin resistance is a systemic disorder that affects many organs and insulin-regulated pathways. The disorder is characterized by a reduced action of insulin despite increased insulin concentrations (hyperinsulinaemia). The effects of insulin on the kidney and vasculature differ in part from the effects on classical insulin target organs. Insulin causes vasodilation by enhancing endothelial nitric oxide production through activation of the phosphatidylinositol 3-kinase pathway. In insulin-resistant states, this pathway is impaired and the mitogen-activated protein kinase pathway stimulates vasoconstriction. The action of insulin on perivascular fat tissue and the subsequent effects on the vascular wall are not fully understood, but the hepatokine fetuin-A, which is released by fatty liver, might promote the proinflammatory effects of perivascular fat. The strong association of salt-sensitive arterial hypertension with insulin resistance indicates an involvement of the kidney in the insulin resistance syndrome. The insulin receptor is expressed on renal tubular cells and podocytes and insulin signalling has important roles in podocyte viability and tubular function. Renal sodium transport is preserved in insulin resistance and contributes to the salt-sensitivity of blood pressure in hyperinsulinaemia. Therapeutically, renal and vascular insulin resistance can be improved by an integrated holistic approach aimed at restoring overall insulin sensitivity and improving insulin signalling.

Reduced Insulin Receptor Expression Enhances Proximal Tubule Gluconeogenesis

Reduced insulin receptor protein levels have been reported in the kidney cortex from diabetic humans and animals. We recently reported that, targeted deletion of insulin receptor (IR) from proximal tubules (PT) resulted in hyperglycemia in non-obese mice. To elucidate the mechanism, we examined human proximal tubule cells (hPTC) and C57BL/6 mice fed with high-fat diet (HFD, 60% fat for 20 weeks). Immunoblotting revealed a significantly lower protein level of IR in HFD compare to normal chow diet (NCD). Furthermore, a blunted rise in p-AKT³⁰⁸ levels in the kidney cortex of HFD mice was observed in response to acute insulin (0.75 IU/kg body weight, i.p) relative to NCD n = 8/group, P < 0.05). Moreover, we found significantly higher transcript levels of phosphoenolpyruvate carboxykinase (PEPCK, a key gluconeogenic enzyme) in the kidney cortex from HFD, relative to mice on NCD. The higher level of PEPCK in HFD was confirmed by immunoblotting. However, no significant differences were observed in cortical glucose-6-phosphatase (G6Pase) or fructose-1,6, bisphosphosphatase (FBPase) enzyme transcript levels. Furthermore, we demonstrated insulin inhibited glucose production in hPTC treated with cyclic AMP and dexamethasone (cAMP/DEXA) to stimulate gluconeogenesis. Transcript levels of the gluconeogenic enzyme PEPCK were significantly increased in cAMP/DEXA-stimulated hPTC cells (n = 3, P < 0.05), and insulin attenuated this upregulation Furthermore, the effect of insulin on cAMP/DEXA-induced gluconeogenesis and PEPCK induction was significantly attenuated in IR (siRNA) silenced hPTC (n = 3, P < 0.05). Overall the above data indicate a direct role for IR expression as a determinant of PT-gluconeogenesis. Thus reduced insulin signaling of the proximal tubule may contribute to hyperglycemia in the metabolic syndrome via elevated gluconeogenesis. J. Cell. Biochem. 118: 276-285, 2017. © 2016 Wiley Periodicals, Inc.

Obesity-related glomerulopathy: clinical and pathologic characteristics and pathogenesis

The prevalence of obesity-related glomerulopathy is increasing in parallel with the worldwide obesity epidemic. Glomerular hypertrophy and adaptive focal segmental glomerulosclerosis define the condition pathologically. The glomerulus enlarges in response to obesity-induced increases in glomerular filtration rate, renal plasma flow, filtration fraction and tubular sodium reabsorption. Normal insulin/phosphatidylinositol 3-kinase/Akt and mTOR signalling are critical for podocyte hypertrophy and adaptation. Adipokines and ectopic lipid accumulation in the kidney promote insulin resistance of podocytes and maladaptive responses to cope with the mechanical forces of renal hyperfiltration. Although most patients have stable or slowly progressive proteinuria, up to one-third develop progressive renal failure and end-stage renal disease. Renin-angiotensin-aldosterone blockade is effective in the short-term but weight loss by hypocaloric diet or bariatric surgery has induced more consistent and dramatic antiproteinuric effects and reversal of hyperfiltration. Altered fatty acid and cholesterol metabolism are increasingly recognized as key mediators of renal lipid accumulation, inflammation, oxidative stress and fibrosis. Newer therapies directed to lipid metabolism, including SREBP antagonists, PPARα agonists, FXR and TGR5 agonists, and LXR agonists, hold therapeutic promise.

Adiponectin, Leptin, and Fatty Acids in the Maintenance of Metabolic Homeostasis through Adipose Tissue Crosstalk

Metabolism research has made tremendous progress over the last several decades in establishing the adipocyte as a central rheostat in the regulation of systemic nutrient and energy homeostasis. Operating at multiple levels of control, the adipocyte communicates with organ systems to adjust gene expression, glucoregulatory hormone exocytosis, enzymatic reactions, and nutrient flux to equilibrate the metabolic demands of a positive or negative energy balance. The identification of these mechanisms has great potential to identify novel targets for the treatment of diabetes and related metabolic disorders. Herein, we review the central role of the adipocyte in the maintenance of metabolic homeostasis, highlighting three critical mediators: adiponectin, leptin, and fatty acids.

Urinary Exosomal microRNA-451-5p Is a Potential Early Biomarker of Diabetic Nephropathy in Rats

Non-invasive renal signatures can help in serial monitoring of diabetic patients. We tested whether urinary exosomal (UE) microRNA (miR) analysis could non-invasively predict renal pathology in diabetic rats during the course of diabetes. Diabetes mellitus (DM) was induced in male Wistar rats by a single intraperitoneal injection of streptozotocin (STZ, 50 mg/kg body weight). Non-diabetic control (CTRL) rats were injected with vehicle. Insulin (INS) treatment (5U/d, s.c.) was provided to 50% of the DM rats. Urine samples were collected at weeks 3, 6, and 9 following injections and UE prepared. An increase in miR-451-5p and miR-16, observed by pilot small RNA sequencing of UE RNA, was confirmed by quantitative real-time polymerase chain reaction (qPCR) and selected for further study. Subsets of rats were euthanized after 3, 6, and 9 weeks of diabetes for renal pathology analysis, including determination of the tubulointerstitial fibrotic index (TFI) and glomerulosclerotic index (GI) scores. qPCR showed a substantial rise in miR-451-5p in UE from DM rats during the course of diabetes, with a significant rise (median fold change >1000) between 3 and 6 weeks. Moreover, UE miR-451-5p at 6 weeks predicted urine albumin at 9 weeks (r = 0.76). A delayed but significant rise was also observed for miR-16. In contrast, mean urine albumin only increased 21% between 3 and 6 weeks (non-significant rise), and renal TFI and GI were unchanged till 9 weeks. Renal expression of miR-451-5p and miR-16 (at 10 weeks) did not correlate with urine levels, and moreover, was negatively associated with indices of renal pathology (r≥-0.70, p = 0.005 for TFI and r≥-0.6, p≤0.02 for GI). Overall, a relative elevation in renal miR-451-5p and miR-16 in diabetes appeared protective against diabetes-induced kidney fibrosis; while UE miR-451-5p may hold prognostic value as an early and sensitive non-invasive indicator of renal disease.

Akt Links Insulin Signaling to Albumin Endocytosis in Proximal Tubule Epithelial Cells

Diabetes mellitus (DM) has become an epidemic, causing a significant decline in quality of life of individuals due to its multisystem involvement. Kidney is an important target organ in DM accounting for the majority of patients requiring renal replacement therapy at dialysis units. Microalbuminuria (MA) has been a valuable tool to predict end-organ damage in DM but its low sensitivity has driven research efforts to seek other alternatives. Albumin is taken up by albumin receptors, megalin and cubilin in the proximal tubule epithelial cells. We demonstrated that insulin at physiological concentrations induce albumin endocytosis through activation of protein kinase B (Akt) in proximal tubule epithelial cells. Inhibition of Akt by a phosphorylation deficient construct abrogated insulin induced albumin endocytosis suggesting a role for Akt in insulin-induced albumin endocytosis. Furthermore we demonstrated a novel interaction between Akt substrate 160kDa (AS160) and cytoplasmic tail of megalin. Mice with type 1 DM (T1D) displayed decreased Akt, megalin, cubilin and AS160 expression in their kidneys in association with urinary cubilin shedding preceding significant MA. Patients with T1D who have developed MA in the EDC (The Pittsburgh Epidemiology of Diabetes Complications) study demonstrated urinary cubilin shedding prior to development of MA. We hypothesize that perturbed insulin-Akt cascade in DM leads to alterations in trafficking of megalin and cubilin, which results in urinary cubilin shedding as a prelude to MA in early diabetic nephropathy. We propose that utilization of urinary cubilin shedding, as a urinary biomarker, will allow us to detect and intervene in diabetic nephropathy (DN) at an earlier stage.

Paediatrics, insulin resistance and the kidney

Systemic insulin resistance is becoming more prevalent in the young due to modern lifestyles predisposing to the metabolic syndrome and obesity. There is also evidence that there are critical insulin-resistant phases for the developing child, including puberty, and that renal disease per se causes systemic insulin resistance. This review considers the factors that render children insulin resistant, as well as the accumulating evidence that the kidney is an insulin-responsive organ and could be affected by insulin resistance.

Insulin regulates nitric oxide production in the kidney collecting duct cells

The kidney is an important organ for arterial blood pressure (BP) maintenance. Reduced NO generation in the kidney is associated with hypertension in insulin resistance. NO is a critical regulator of vascular tone; however, whether insulin regulates NO production in the renal inner medullary collecting duct (IMCD), the segment with the greatest enzymatic activity for NO production in kidney, is not clear. Using an NO-sensitive 4-amino-5-methylamino-2',7'-difluorofluorescein (DAF-FM) fluorescent dye, we found that insulin increased NO production in mouse IMCD cells (mIMCD) in a time- and dose-dependent manner. A concomitant dose-dependent increase in the NO metabolite (NOx) was also observed in the medium from insulin-stimulated cells. NO production peaked in mIMCD cells at a dose of 100 nm insulin with simultaneously increased NOx levels in the medium. At this dose, insulin significantly increased p-eNOS(Ser1177) levels in mIMCD cells. Pretreatment of cells with a PI 3-kinase inhibitor or insulin receptor silencing with RNA interference abolished these effects of insulin, whereas insulin-like growth factor-1 receptor (IGF-1R) silencing had no effect. We also showed that chronic insulin infusion to normal C57BL/6J mice resulted in increased endothelial NOS (eNOS) protein levels and NO production in the inner medulla. However, insulin-infused IRKO mice, with targeted deletion of insulin receptor from tubule epithelial cells of the kidney, had ∼50% reduced eNOS protein levels in their inner medulla along with a significant rise in BP relative to WT littermates. We have previously reported increased baseline BP and reduced urine NOx in IRKO mice. Thus, reduced insulin receptor signaling in IMCD could contribute to hypertension in the insulin-resistant state.

Insulin Receptor and the Kidney: Nephrocalcinosis in Patients with Recessive INSR Mutations

BACKGROUND/AIMS

Donohue and Rabson-Mendenhall syndrome are rare autosomal recessive disorders caused by mutations in the insulin receptor gene, INSR. Phenotypic features include extreme insulin resistance, linear growth retardation, paucity of fat and muscle, and soft tissue overgrowth. The insulin receptor is also expressed in the kidney, where animal data suggest it plays a role in glomerular function and blood pressure (BP) regulation, yet such a role in the human kidney is untested. Patients with biallelic INSR mutations provide a rare opportunity to ascertain its role in man.

METHODS

Retrospective review of patients with INSR mutations. Data for BP, renal imaging, plasma creatinine and electrolyte levels, as well as urine protein, albumin and calcium excretion were sought from the treating clinicians.

RESULTS

From 33 patients with INSR mutations, data were available for 17 patients. Plasma creatinine was low (mean ± SD: 25 ± 9 μmol/l) and mean plasma electrolyte concentrations were within the normal range (n = 13). Systolic BP ranged between the 18th and 91st percentile for age, sex, height and weight (n = 9; mean ± SD: 49 ± 24). Twenty-four-hour urinary calcium data were available from 10 patients and revealed hypercalciuria in all (mean ± SD: 0.32 ± 0.17 mmol/kg/day; normal <0.1). Nephrocalcinosis was present in all patients (n = 17). Urinary albumin excretion (n = 7) ranged from 4.3-122.5 μg/min (mean ± SD: 32.4 ± 41.0 μg/min; normal <20).

CONCLUSIONS

INSR dysfunction is associated with hypercalciuria and nephrocalcinosis. No other consistent abnormality of renal function was noted. Normotension and stable glomerular function with only moderate proteinuria is in contrast to genetically modified mice who have elevated BP and progressive diabetic nephropathy.

Stress in the kidney is the road to pERdition: is endoplasmic reticulum stress a pathogenic mediator of diabetic nephropathy?

The endoplasmic reticulum (ER) is an organelle that primarily functions to synthesise new proteins and degrade old proteins. Owing to the continual and variable nature of protein turnover, protein synthesis is inherently an error-prone process and is therefore tightly regulated. Fortunately, if this balance between synthesis and degradation is perturbed, an intrinsic response, the unfolded protein response (UPR) is activated to restore ER homoeostasis through the action of inositol-requiring protein 1, activating transcription factor 6 and PKR-like ER kinase transmembrane sensors. However, if the UPR is oversaturated and misfolded proteins accumulate, the ER can shift into a cytotoxic response, a physiological phenomenon known as ER stress. The mechanistic pathways of the UPR have been extensively explored; however, the role of this process in such a synthetic organ as the kidney requires further clarification. This review will focus on these aspects and will discuss the role of ER stress in specific resident kidney cells and how this may be integral in the pathogenesis and progression of diabetic nephropathy (DN). Given that diabetes is a perturbed state of protein turnover in most tissues, it is important to understand if ER stress is a secondary or tertiary response to other changes within the diabetic milieu or if it is an independent accelerator of kidney disease. Modulators of ER stress could provide a valuable tool for the treatment of DN and are under active investigation in other contexts.

PHEX mimetic (SPR4-peptide) corrects and improves HYP and wild type mice energy-metabolism

CONTEXT

PHEX or DMP1 mutations cause hypophosphatemic-rickets and altered energy metabolism. PHEX binds to DMP1-ASARM-motif to form a complex with α5β3 integrin that suppresses FGF23 expression. ASARM-peptides increase FGF23 by disrupting the PHEX-DMP1-Integrin complex. We used a 4.2 kDa peptide (SPR4) that binds to ASARM-peptide/motif to study the DMP1-PHEX interaction and to assess SPR4 for the treatment of energy metabolism defects in HYP and potentially other bone-mineral disorders.

DESIGN

Subcutaneously transplanted osmotic pumps were used to infuse SPR4-peptide or vehicle (VE) into wild-type mice (WT) and HYP-mice (PHEX mutation) for 4 weeks.

RESULTS

SPR4 partially corrected HYP mice hypophosphatemia and increased serum 1.25(OH)2D3. Serum FGF23 remained high and PTH was unaffected. WT-SPR4 mice developed hypophosphatemia and hypercalcemia with increased PTH, FGF23 and 1.25(OH)2D3. SPR4 increased GAPDH HYP-bone expression 60× and corrected HYP-mice hyperglycemia and hypoinsulinemia. HYP-VE serum uric-acid (UA) levels were reduced and SPR4 infusion suppressed UA levels in WT-mice but not HYP-mice. SPR4 altered leptin, adiponectin, and sympathetic-tone and increased the fat mass/weight ratio for HYP and WT mice. Expression of perlipin-2 a gene involved in obesity was reduced in HYP-VE and WT-SPR4 mice but increased in HYP-SPR4 mice. Also, increased expression of two genes that inhibit insulin-signaling, ENPP1 and ESP, occurred with HYP-VE mice. In contrast, SPR4 reduced expression of both ENPP1 and ESP in WT mice and suppressed ENPP1 in HYP mice. Increased expression of FAM20C and sclerostin occurred with HYP-VE mice. SPR4 suppressed expression of FAM20C and sclerostin in HYP and WT mice.

CONCLUSIONS

ASARM peptides and motifs are physiological substrates for PHEX and modulate osteocyte PHEX-DMP1-α5β3-integrin interactions and thereby FGF23 expression. These interactions also provide a nexus that regulates bone and energy metabolism. SPR4 suppression of sclerostin and/or sequestration of ASARM-peptides improves energy metabolism and may have utility for treating familial rickets, osteoporosis, obesity and diabetes.

Fatty kidney: emerging role of ectopic lipid in obesity-related renal disease

The global increase in chronic kidney disease (CKD) parallels the obesity epidemic. Obesity conveys a gradual but independent risk of progression of CKD that seems irrespective of the underlying nephropathy. Obesity has been associated with a secondary focal segmental glomerulosclerosis coined obesity-related glomerulopathy (ORG). Pathways through which obesity might cause renal disease are not well understood, and early clinical biomarkers for incipient ORG or renal relevant obesity are currently lacking. Recent human and experimental studies have associated ectopic lipid accumulation in the kidney (fatty kidney) with obesity-related renal disease. There is enough growing insight that ectopic lipid--the accumulation of lipid in non-adipose tissue--is associated with structural and functional changes of mesangial cells, podocytes, and proximal tubular cells to propose the development of ORG as a maladaptive response to hyperfiltration and albuminuria. Recent advances in metabolic imaging might validate ectopic lipid as a biomarker and research aid, to help translate novel therapeutics from experimental models to patients.

Podocytes, signaling pathways, and vascular factors in diabetic kidney disease

Alterations and injury to glomerular podocytes play a key role in the initiation and progression of diabetic kidney disease (DKD). Multiple factors in diabetes cause abnormalities in podocyte signaling that lead to podocyte foot process effacement, hypertrophy, detachment, loss, and death. Alterations in insulin action and mammalian target of rapamycin activation have been well documented to lead to pathology. Reduced insulin action directly leads to albuminuria, increased glomerular matrix accumulation, thickening of the glomerular basement membrane, podocyte apoptosis, and glomerulosclerosis. In addition, podocytes generate factors that alter signaling in other glomerular cells. Prominent among these is vascular endothelial growth factor-A, which maintains glomerular endothelium viability but causes endothelial cell pathology when generated at too high a level. Finally, circulating vascular factors (eg, activated protein C) have a profound effect on podocyte stability and survival. This cytoprotective factor is critical for podocyte health, and its deficiency promotes podocyte injury and apoptosis. Thus, the podocyte sits in the center of a network of paracrine and hormonal signaling systems that in health keep the podocyte adaptable and viable, but in diabetes they can lead to pathologic changes, detachment, and death.

Recent advances in our understanding of insulin signalling to the podocyte

It is becoming increasingly clear that the insulin responses of a number of different cell types within the kidney are important in the maintenance of normal renal function. This review summarizes our current understanding of renal insulin signalling, with specific focus on the podocyte, presenting recent evidence that suggests these responses are altered in systemic insulin-resistant states and chronic kidney disease via a number of different mechanisms.