The Sweet Pee model for Sglt2 mutation

Megalin Knockout Reduces SGLT2 Expression and Sensitizes to Western Diet-induced Kidney Injury

Megalin (Lrp2) is a multiligand receptor that drives endocytic flux in the kidney proximal tubule (PT) and is necessary for the recovery of albumin and other filtered proteins that escape the glomerular filtration barrier. Studies in our lab have shown that knockout (KO) of Lrp2 in opossum PT cells leads to a dramatic reduction in sodium-glucose co-transporter 2 (SGLT2) transcript and protein levels, as well as differential expression of genes involved in mitochondrial and metabolic function. SGLT2 transcript levels are reduced more modestly in Lrp2 KO mice. Here, we investigated the effects of Lrp2 KO on kidney function and health in mice fed regular chow (RC) or a Western-style diet (WD) high in fat and refined sugar. Despite a modest reduction in SGLT2 expression, Lrp2 KO mice on either diet showed increased glucose tolerance compared to control mice. Moreover, Lrp2 KO mice were protected against WD-induced fat gain. Surprisingly, renal function in male Lrp2 KO mice on WD was compromised, and the mice exhibited significant kidney injury compared with control mice on WD. Female Lrp2 KO mice were less susceptible to WD-induced kidney injury than male Lrp2 KO. Together, our findings reveal both positive and negative contributions of megalin expression to metabolic health, and highlight a megalin-mediated sex-dependent response to injury following WD.

Sodium-glucose cotranspor ter 2 (SGLT2) inhibitors in nephrolithiasis: should we "gliflozin" patients with kidney stone disease?

The prevalence of nephrolithiasis is increasing worldwide. Despite advances in understanding the pathogenesis of lithiasis, few studies have demonstrated that specific clinical interventions reduce the recurrence of nephrolithiasis. The aim of this review is to analyze the current data and potential effects of iSGLT2 in lithogenesis and try to answer the question: Should we also "gliflozin" our patients with kidney stone disease?

Exploring the anti-cancer potential of SGLT2 inhibitors in breast cancer treatment in pre-clinical and clinical studies

The link between type 2 diabetes mellitus (T2DM) and an increased risk of breast cancer (BC) has prompted the exploration of novel therapeutic strategies targeting shared metabolic pathways. This review focuses on the emerging evidence surrounding the potential anti-cancer effects of sodium-glucose cotransporter-2 (SGLT2) inhibitors in the context of BC. Preclinical studies have demonstrated that various SGLT2 inhibitors, such as canagliflozin, dapagliflozin, ipragliflozin, and empagliflozin, can inhibit the proliferation of BC cells, induce apoptosis, and modulate key cellular signaling pathways. These mechanisms include the activation of AMP-activated protein kinase (AMPK), suppression of mammalian target of rapamycin (mTOR) signaling, and regulation of lipid metabolism and inflammatory mediators. The combination of SGLT2 inhibitors with conventional treatments, including chemotherapy and radiotherapy, as well as targeted therapies like phosphoinositide 3-kinases (PI3K) inhibitors, has shown promising results in enhancing the anti-cancer efficacy and potentially reducing treatment-related toxicities. The identification of specific biomarkers or genetic signatures that predict responsiveness to SGLT2 inhibitor therapy could enable more personalized treatment selection and optimization, particularly for challenging BC subtypes [e, g., triple negative BC (TNBC)]. Ongoing and future clinical trials investigating the use of SGLT2 inhibitors, both as monotherapy and in combination with other agents, will be crucial in elucidating their translational potential and guiding their integration into comprehensive BC care. Overall, SGLT2 inhibitors represent a novel and promising therapeutic approach with the potential to improve clinical outcomes for patients with various subtypes of BC, including the aggressive and chemo-resistant TNBC.

Mouse Models with SGLT2 Mutations: Toward Understanding the Role of SGLT2 beyond Glucose Reabsorption

The sodium–glucose cotransporter 2 (SGLT2) mainly carries out glucose reabsorption in the kidney. Familial renal glycosuria, which is a mutation of SGLT2, is known to excrete glucose in the urine, but blood glucose levels are almost normal. Therefore, SGLT2 inhibitors are attracting attention as a new therapeutic drug for diabetes, which is increasing worldwide. In fact, SGLT2 inhibitors not only suppress hyperglycemia but also reduce renal, heart, and cardiovascular diseases. However, whether long-term SGLT2 inhibition is completely harmless requires further investigation. In this context, mice with mutations in SGLT2 have been generated and detailed studies are being conducted, e.g., the SGLT2−/− mouse, Sweet Pee mouse, Jimbee mouse, and SAMP10-ΔSglt2 mouse. Biological changes associated with SGLT2 mutations have been reported in these model mice, suggesting that SGLT2 is not only responsible for sugar reabsorption but is also related to other functions, such as bone metabolism, longevity, and cognitive functions. In this review, we present the characteristics of these mutant mice. Moreover, because the relationship between diabetes and Alzheimer’s disease has been discussed, we examined the relationship between changes in glucose homeostasis and the amyloid precursor protein in SGLT2 mutant mice.

Mode of Action and Human Relevance Assessment of Male CD-1 Mouse Renal Adenocarcinoma Associated With Lifetime Exposure to Empagliflozin

Inhibition of sodium-glucose cotransporter-2 (SGLT2) has been shown to be a safe and efficacious approach to support managing Type 2 diabetes. In the 2-year carcinogenicity study with the SGLT2 inhibitor empagliflozin in CD-1 mice, an increased incidence of renal tubular adenomas and carcinomas was identified in the male high-dose group but was not observed in female mice. An integrated review of available nonclinical data was conducted to establish a mode-of-action hypothesis for male mouse-specific tumorigenesis. Five key events were identified through systematic analysis to form the proposed mode-of-action: (1) Background kidney pathology in CD-1 mice sensitizes the strain to (2) pharmacology-related diuretic effects associated with SGLT2 inhibition. (3) In male mice, metabolic demand increases with the formation of a sex- and species-specific empagliflozin metabolite. These features converge to (4) deplete oxidative stress handling reserve, driving (5) constitutive cellular proliferation in male CD-1 mice. The proposed mode of action requires all five key events for empagliflozin to present a carcinogenicity risk in the CD-1 mouse. Considering that empagliflozin is not genotoxic in the standard battery of genotoxicity tests, and not all five key events are present in the context of female mice, rats or humans, nor for other osmotic diuretics or other SGLT2 inhibitors, the observed male mouse renal tumors are not considered relevant to humans.

Sodium Glucose Cotransporter-2 Inhibitors: Spotlight on Favorable Effects on Clinical Outcomes beyond Diabetes

Sodium glucose transporter type 2 (SGLT2) molecules are found in proximal tubules of the kidney, and perhaps in the brain or intestine, but rarely in any other tissue. However, their inhibitors, intended to improve diabetes compensation, have many more beneficial effects. They improve kidney and cardiovascular outcomes and decrease mortality. These benefits are not limited to diabetics but were also found in non-diabetic individuals. The pathophysiological pathways underlying the treatment success have been investigated in both clinical and experimental studies. There have been numerous excellent reviews, but these were mostly restricted to limited aspects of the knowledge. The aim of this review is to summarize the known experimental and clinical evidence of SGLT2 inhibitors' effects on individual organs (kidney, heart, liver, etc.), as well as the systemic changes that lead to an improvement in clinical outcomes.

rhADAMTS13 reduces oxidative stress by cleaving VWF in ischaemia/reperfusion-induced acute kidney injury

AIMS

Acute kidney injury (AKI), a major health burden, lacks effective therapy. Anti-inflammatory actions of a disintegrin and metalloproteinase with a thrombospondin type 1 motif member 13 (ADAMTS13) may provide a new treatment option for AKI. Along with inflammation, oxidative stress is critical for AKI development, yet the impact of ADAMTS13 on oxidative stress in AKI remains to be fully elucidated.

METHODS

We assess recombinant human ADAMTS13 (rhADAMTS13) actions on oxidative stress in a murine ischaemia/reperfusion (IR) model. Antioxidant stress-enzyme activities, renal morphology, kidney function markers and vascular function of isolated afferent arterioles are quantified.

RESULTS

rhADAMTS13 provided after IR, reduces blood urea nitrogen (BUN) by 33% and serum creatinine (Scr) by 73% in 24 hours post-IR. rhADAMTS13 reduces BUN (40.03 ± 20.34 mmol/L vs 72.35 ± 18.74 mmol/L, P < .01), Scr (75.67 ± 51.19 μmol/L vs 176.17 ± 55.38 μmol/L, P < .01) and proteinuria by 41% in 48 hours post-IR as well. Moreover, rhADAMTS13 administration decreases malondialdehyde (MDA) and increases the activity of antioxidant stress enzymes, and attenuates reactive oxygen species production. rhADAMTS13 also upregulates nuclear factor-erythroid-2-related factor 2/haem oxygenase-1, enhances antioxidant enzymes activity and alleviates endothelial dysfunction. Finally, treatment with rhADAMTS13 mitigates severe functional and morphological injury present in IR mice. Extracellular signal-regulated kinase (ERK) phosphorylation is limited by rhADAMTS13 and PPARγ expression is partly restored in ischaemic kidneys. Co-administration of von Willebrand factor (VWF) impairs rhADAMTS13's antioxidant capacity and its protective role in IR.

CONCLUSION

rhADAMTS13 alleviates renal IR injury through antioxidant effects by cleaving VWF.

Renal Tubular Handling of Glucose and Fructose in Health and Disease

The proximal tubule of the kidney is programmed to reabsorb all filtered glucose and fructose. Glucose is taken up by apical sodium-glucose cotransporters SGLT2 and SGLT1 whereas SGLT5 and potentially SGLT4 and GLUT5 have been implicated in apical fructose uptake. The glucose taken up by the proximal tubule is typically not metabolized but leaves via the basolateral facilitative glucose transporter GLUT2 and is returned to the systemic circulation or used as an energy source by distal tubular segments after basolateral uptake via GLUT1. The proximal tubule generates new glucose in metabolic acidosis and the postabsorptive phase, and fructose serves as an important substrate. In fact, under physiological conditions and intake, fructose taken up by proximal tubules is primarily utilized for gluconeogenesis. In the diabetic kidney, glucose is retained and gluconeogenesis enhanced, the latter in part driven by fructose. This is maladaptive as it sustains hyperglycemia. Moreover, renal glucose retention is coupled to sodium retention through SGLT2 and SGLT1, which induces secondary deleterious effects. SGLT2 inhibitors are new anti-hyperglycemic drugs that can protect the kidneys and heart from failing independent of kidney function and diabetes. Dietary excess of fructose also induces tubular injury. This can be magnified by kidney formation of fructose under pathological conditions. Fructose metabolism is linked to urate formation, which partially accounts for fructose-induced tubular injury, inflammation, and hemodynamic alterations. Fructose metabolism favors glycolysis over mitochondrial respiration as urate suppresses aconitase in the tricarboxylic acid cycle, and has been linked to potentially detrimental aerobic glycolysis (Warburg effect). © 2022 American Physiological Society. Compr Physiol 12:2995-3044, 2022.

Long-Term Effects of Sglt2 Deletion on Bone and Mineral Metabolism in Mice

Sodium‐glucose cotransporter 2 (SGLT2) inhibitors improve kidney and cardiovascular outcomes in patients with type 2 diabetes mellitus (T2DM). However, bone fragility has emerged as a side effect in some but not in all human studies. Because use of SGLT2 inhibitors in humans affects mineral metabolism, we investigated the long‐term effects of genetic loss of Sglt2 function on bone and mineral metabolism in mice. Slc5a2 nonsense mutation in Sweet Pee (SP) mice results in total loss of Sglt2 function. We collected urine, serum, and bone samples from 15‐week‐old and 25‐week‐old wild‐type (WT) and SP mice fasted from food overnight. We measured parameters of renal function and mineral metabolism and we assessed bone growth, microarchitecture, and mineralization. As expected, 15‐week‐old and 25‐week‐old SP mice showed increased glucosuria, and normal kidney function compared to age‐matched WT mice. At 15 weeks, SP mice did not show alterations in mineral metabolism parameters. At 25 weeks, SP mice showed reduced fasting 24‐hour urinary calcium excretion and increased fractional excretion of phosphate, but normal serum calcium and phosphate, parathyroid hormone (PTH), vitamin D (1,25(OH)₂D), and fibroblast growth factor (FGF23) levels. At 25 weeks, but not at 15 weeks, SP mice showed reduced body weight compared to WT. This was associated with reduced femur length at 25 weeks, suggesting impaired skeletal growth. SP mice did not show trabecular or cortical bone microarchitectural modifications but showed reduced cortical bone mineral density compared to WT mice at 25 weeks. These results suggest that loss of Sglt2 function in mice in the absence of T2DM does not alter regulatory hormones FGF23, PTH, and 1,25(OH)₂D, but may contribute to bone fragility over the long term. Future studies are required to determine how loss of Sglt2 function impacts bone fragility in T2DM. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Impact of an SGLT2-loss of function mutation on renal architecture, histology, and glucose homeostasis

Inhibitors of sodium/glucose co-transporter 2 (SGLT2) are currently in clinical use for type 2 diabetes (T2D) treatment due to their anti-hyperglycemic effect exerted by the inhibition of glucose reabsorption in the kidney. Inhibition of SGLT2 is associated with improvement of renal outcomes in chronic kidney disease associated with T2D. Our study aimed to describe the renal-specific phenotypic consequences of the SGLT2-loss of function "Jimbee" mutation within the Slc5a2 mouse gene in a non-diabetic/non-obese background. The Jimbee mice displayed reduced body weight, glucosuria, polyuria, polydipsia, and hyperphagia but were normoglycemic, with no signs of baseline insulin resistance or renal dysfunction. Histomorphological analysis of the kidneys revealed a normal architecture and morphology of the renal cortex, but shrinkage of the glomerular and tubular apparatus, including Bowman's space, glomerular tuft, mesangial matrix fraction, and proximal convoluted tubule (PCT). Immunofluorescent analysis of renal sections showed that SGLT2 was absent from the apical membrane of PCT of the Jimbee mice but remnant positive vesicles were detected within the cytosol or at the perinuclear interface. Renal localization and abundance of GLUT1, GLUT2, and SGLT1 were unchanged in the Jimbee genotype. Intriguingly, the mutation did not induce hepatic gluconeogenic gene expression in overnight fasted mice despite a high glucose excretion rate. The Jimbee phenotype is remarkably similar to humans with SLC5A2 mutations and provides a useful model for the study of SGLT2-loss of function effects on renal architecture and physiology, as well as for identifying possible novel roles for the kidneys in glucose homeostasis and metabolic reprogramming.

Genetic ablation of SGLT2 function in mice impairs tissue mineral density but does not affect fracture resistance of bone

Selective sodium-dependent glucose co-transporter 2 inhibitors (SGLT2Is) are oral hypoglycemic medications utilized increasingly in the medical management of hyperglycemia among persons with type 2 diabetes (T2D). Despite favorable effects on cardiovascular events, specific SGLT2Is have been associated with an increased risk for atypical fracture and amputation in subgroups of the T2D population, a population that already has a higher risk for typical fragility fractures than the general population. To better understand the effect of SGLT2 blockade on skeletal integrity, independent of diabetes and its co-morbidities, we utilized the "Jimbee" mouse model of slc5a2 gene mutation to investigate the impact of lifelong SGLT2 loss-of-function on metabolic and skeletal phenotype. Jimbee mice maintained normal glucose homeostasis, but exhibited chronic polyuria, glucosuria and hypercalciuria. The Jimbee mutation negatively impacted appendicular growth of the femur and resulted in lower tissue mineral density of both cortical and trabecular bone of the femur mid-shaft and distal femur metaphysis, respectively. Several components of the Jimbee phenotype were characteristic only of male mice compared with female mice, including reductions: in body weight; in cortical area of the mid-shaft; and in trabecular thickness within the metaphysis. Despite these decrements, the strength of femur diaphysis in bending (cortical bone), which increased with age, and the strength of L6 vertebra in compression (primarily trabecular bone), which decreased with age, were not affected by the mutation. Moreover, the age-related decline in bone toughness was less for Jimbee mice, compared with control mice, such that by 49-50 weeks of age, Jimbee mice had significantly tougher femurs in bending than C57BL/6J mice. These results suggest that chronic blockade of SGLT2 in this model reduces the mineralization of bone but does not reduce its fracture resistance.

Hypomagnesaemia and its determinants in a contemporary primary care cohort of persons with type 2 diabetes

AIMS

Among persons with type 2 diabetes mellitus (T2DM) hypomagnesaemia has been reported in 14-48% of patients. This may be of significance given the emerging associations of hypomagnesaemia with glucometabolic disturbances and possibly even complications. We assessed the prevalence of hypomagnesaemia and its determinants, in a well-defined cohort of persons with T2DM treated in primary care.

METHODS

Observational cohort study among persons with T2DM treated in primary care in the Northeast of the Netherlands. Magnesium was measured using a colorimetric endpoint assay (Roche). Hypomagnesaemia was defined as a serum magnesium level <0.70 mmol/L. Pearson correlations were performed to correlate variables with serum magnesium. Next, a stepwise backward regression model was made.

RESULTS

Data of 929 persons (55% male) with a mean age of 65 (± 10) years, diabetes duration 6.5 [3.0-10.1] years, and HbA1c concentration 6.7 (± 0.7)% (50 (± 9) mmol/mol) were analysed. Serum magnesium was 0.79 (± 0.08) mmol/L. The percentage of persons with magnesium deficiency was 9.6%. Age, diabetes duration, BMI, HbA1c, use of metformin, sulfonylurea derivatives, and DPP4 inhibitors were negatively associated with magnesium concentrations. In contrast, LDL cholesterol and serum creatinine were positively associated serum magnesium.

CONCLUSIONS

Hypomagnesaemia was present in 9.6% of T2DM patients treated in primary care. This percentage is remarkably lower than reported previously, possibly due to the unselected nature of our population. Concerning T2DM-related factors, only BMI, HbA1c and the use of metformin, sulfonylurea derivatives and DPP4 inhibitors correlated negatively with magnesium concentrations.

Mineral and Electrolyte Disorders With SGLT2i Therapy

The newly developed sodium‐glucose cotransporter 2 inhibitors (SGLT2is) effectively modulate glucose metabolism in diabetes. Although clinical data suggest that SGLT2is (empagliflozin, dapagliflozin, ertugliflozin, canagliflozin, ipragliflozin) are safe and protect against renal and cardiovascular events, very little attention has been dedicated to the effects of these compounds on different electrolytes. As with other antidiabetic compounds, some effects on water and electrolytes balance have been documented. Although the natriuretic effect and osmotic diuresis are expected with SGLT2is, these compounds may also modulate urinary potassium, magnesium, phosphate, and calcium excretion. Notably, they have had no effect on plasma sodium levels and promoted only small increases in serum potassium and magnesium concentrations in clinical trials. Moreover, SGLT2is may induce an increase in serum phosphate, FGF‐23, and PTH; reduce 1,25‐dihydroxyvitamin D; and generate normal serum calcium. Some published and preliminary reports, as well as unconfirmed reports have suggested an association with bone fractures. Some homeostasis perturbations are transient, whereas others may persist, suggesting that the administration of SGLT2is may affect electrolyte balances in exposed subjects. Although current evidence supports their safety, additional efforts are needed to elucidate the long‐term impact of these compounds on chronic kidney disease, mineral metabolism, and bone health. Indeed, the limited follow‐up studies and the heterogeneity of the case‐mix of different randomized controlled trials preclude a definitive answer on the impact of these compounds on long‐term outcomes such as the risk of bone fracture. Here we review the current understanding of the mechanisms involved in electrolyte handling and the available data on the clinical implications of electrolytes and mineral metabolism perturbations induced by SGLT2i administration. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Tubular Deficiency of Heterogeneous Nuclear Ribonucleoprotein F Elevates Systolic Blood Pressure and Induces Glycosuria in Mice

We reported previously that overexpression of heterogeneous nuclear ribonucleoprotein F (Hnrnpf) in renal proximal tubular cells (RPTCs) suppresses angiotensinogen (Agt) expression, and attenuates systemic hypertension and renal injury in diabetic Hnrnpf-transgenic (Tg) mice. We thus hypothesized that deletion of Hnrnpf in the renal proximal tubules (RPT) of mice would worsen systemic hypertension and kidney injury, perhaps revealing novel mechanism(s). Tubule-specific Hnrnpf knockout (KO) mice were generated by crossbreeding Pax8-Cre mice with floxed Hnrnpf mice on a C57BL/6 background. Both male and female KO mice exhibited elevated systolic blood pressure, increased urinary albumin/creatinine ratio, tubulo-interstitial fibrosis and glycosuria without changes in blood glucose or glomerular filtration rate compared with control littermates. However, glycosuria disappeared in male KO mice at the age of 12 weeks, while female KO mice had persistent glycosuria. Agt expression was elevated, whereas sodium-glucose co-transporter 2 (Sglt2) expression was down-regulated in RPTs of both male and female KO mice as compared to control littermates. In vitro, KO of HNRNPF in human RPTCs (HK-2) by CRISPR gRNA up-regulated AGT and down-regulated SGLT2 expression. The Sglt2 inhibitor canagliflozin treatment had no effect on Agt and Sglt2 expression in HK-2 and in RPTCs of wild-type mice but induced glycosuria. Our results demonstrate that Hnrnpf plays a role in the development of hypertension and glycosuria through modulation of renal Agt and Sglt2 expression in mice, respectively.

Diabetes pharmacotherapy and effects on the musculoskeletal system

Persons with type 1 or type 2 diabetes have a significantly higher fracture risk than age-matched persons without diabetes, attributed to disease-specific deficits in the microarchitecture and material properties of bone tissue. Therefore, independent effects of diabetes drugs on skeletal integrity are vitally important. Studies of incretin-based therapies have shown divergent effects of different agents on fracture risk, including detrimental, beneficial, and neutral effects. The sulfonylurea class of drugs, owing to its hypoglycemic potential, is thought to amplify the risk of fall-related fractures, particularly in the elderly. Other agents such as the biguanides may, in fact, be osteo-anabolic. In contrast, despite similarly expected anabolic properties of insulin, data suggests that insulin pharmacotherapy itself, particularly in type 2 diabetes, may be a risk factor for fracture, negatively associated with determinants of bone quality and bone strength. Finally, sodium-dependent glucose co-transporter 2 inhibitors have been associated with an increased risk of atypical fractures in select populations, and possibly with an increase in lower extremity amputation with specific SGLT2I drugs. The role of skeletal muscle, as a potential mediator and determinant of bone quality, is also a relevant area of exploration. Currently, data regarding the impact of glucose lowering medications on diabetes-related muscle atrophy is more limited, although preclinical studies suggest that various hypoglycemic agents may have either aggravating (sulfonylureas, glinides) or repairing (thiazolidinediones, biguanides, incretins) effects on skeletal muscle atrophy, thereby influencing bone quality. Hence, the therapeutic efficacy of each hypoglycemic agent must also be evaluated in light of its impact, alone or in combination, on musculoskeletal health, when determining an individualized treatment approach. Moreover, the effect of newer medications (potentially seeking expanded clinical indication into the pediatric age range) on the growing skeleton is largely unknown. Herein, we review the available literature regarding effects of diabetes pharmacotherapy, by drug class and/or by clinical indication, on the musculoskeletal health of persons with diabetes.

New Therapies for the Treatment of Renal Fibrosis

Renal fibrosis is the common pathway for progression of chronic kidney disease (CKD) to end stage of renal disease. It is now widely accepted that the degree of renal fibrosis correlates with kidney function and CKD stages. The key cellular basis of renal fibrosis includes activation of myofibroblasts, excessive production of extracellular matrix components, and infiltration of inflammatory cells. Many cellular mechanisms responsible for renal fibrosis have been identified, and some antifibrotic agents show a greater promise in slowing down and even reversing fibrosis in animal models; however, translating basic findings into effective antifibrotic therapies in human has been limited. In this chapter, we will discuss the effects and mechanisms of some novel antifibrotic agents in both preclinical studies and clinical trials.

Dapagliflozin attenuates early markers of diabetic nephropathy in fructose-streptozotocin-induced diabetes in rats

Early detection and clinical interference are major challenges for the prevention of diabetic nephropathy (DN) progression. This study investigated the effects of dapagliflozin, a sodium glucose co-transporter 2 inhibitor, on some early markers for DN in fructose-streptozotocin (Fr-STZ)-induced diabetes in rats. Fr-STZ rats were treated with either dapagliflozin (1 mg/kg p.o. daily), metformin (350 mg/kg p.o. daily), or their combination for 6 weeks. Fr-STZ rats displayed marked early tubular renal damage and glomerular podocyte injury as evidenced by renal KIM-1, NGAL, cystatin C, and vanin-1 mRNA, as well as urinary NAG elevation and nephrin mRNA suppression, associated with the development of marked renal interstitial fibrosis and glomerulosclerosis despite the presence of normoalbuminuria. Propagation of oxidative, inflammatory, fibrotic, and apoptotic reactions was obvious in the setting of renal glucose overload. Dapagliflozin significantly attenuated the renal tubular injury makers namely KIM-1, NGAL, vanin-1 and urinary NAG. In addition, it restored glomerular nephrin expression and reversed renal histopathological changes. Oxidative, inflammatory, and fibrotic processes were also alleviated. This study suggests that dapagliflozin exerts a renoprotective effect against early features of DN in rats presumably by inhibition of diabetes-induced renal tubular and glomerular injury thereby modulating oxidative, inflammatory, and fibrotic as well as apoptotic mechanisms elicited during hyperglycemia.

A model of calcium transport and regulation in the proximal tubule

The objective of this study was to examine theoretically how Ca²⁺ reabsorption in the proximal tubule (PT) is modulated by Na⁺ and water fluxes, parathyroid hormone (PTH), Na⁺-glucose cotransporter (SGLT2) inhibitors, and acetazolamide. We expanded a previously published mathematical model of water and solute transport in the rat PT (Layton AT, Vallon V, Edwards A. Am J Physiol Renal Physiol 308: F1343–F1357, 2015) that did not include Ca²⁺. Our results indicate that Ca²⁺ reabsorption in the PT is primarily driven by the transepithelial Ca²⁺ concentration gradient that stems from water reabsorption, which is itself coupled to Na⁺ reabsorption. Simulated variations in permeability or transporter activity elicit opposite changes in paracellular and transcellular Ca²⁺ fluxes, whereas a simulated decrease in filtration rate lowers both fluxes. The model predicts that PTH-mediated inhibition of the apical Na⁺/H⁺ exchanger NHE3 reduces Na⁺ and Ca²⁺ transport to a similar extent. It also suggests that acetazolamide- and SGLT2 inhibitor-induced calciuria at least partly stems from reduced Ca²⁺ reabsorption in the PT. In addition, backleak of phosphate (PO₄) across tight junctions is predicted to reduce net PO₄ reabsorption by ~20% under normal conditions. When transcellular PO₄ transport is substantially reduced by PTH, paracellular PO₄ flux is reversed and contributes significantly to PO₄ reabsorption. Furthermore, PTH is predicted to exert an indirect impact on PO₄ reabsorption via its inhibitory action on NHE3. This model thus provides greater insight into the mechanisms that modulate Ca²⁺ and PO₄ reabsorption in the PT.

Urine: Waste product or biologically active tissue?

AIMS

Historically, urine has been viewed primarily as a waste product with little biological role in the overall health of an individual. Increasingly, data suggest that urine plays a role in human health beyond waste excretion. For example, urine might act as an irritant and contribute to symptoms through interaction with-and potential compromise of-the urothelium.

METHODS

To explore the concept that urine may be a vehicle for agents with potential or occult bioactivity and to discuss existing evidence and novel research questions that may yield insight into such a role, the National Institute of Diabetes and Digestive and Kidney Disease invited experts in the fields of comparative evolutionary physiology, basic science, nephrology, urology, pediatrics, metabolomics, and proteomics (among others) to a Urinology Think Tank meeting on February 9, 2015.

RESULTS

This report reflects ideas that evolved from this meeting and current literature, including the concept of urine quality, the biological, chemical, and physical characteristics of urine, including the microbiota, cells, exosomes, pH, metabolites, proteins, and specific gravity (among others). Additionally, the manuscript presents speculative, and hopefully testable, ideas about the functional roles of urine constituents in health and disease.

CONCLUSION

Moving forward, there are several questions that need further understanding and pursuit. There were suggestions to consider actively using various animal models and their biological specimens to elaborate on basic mechanistic information regarding human bladder dysfunction.

Interaction Between the Sodium-Glucose-Linked Transporter 2 Inhibitor Dapagliflozin and the Loop Diuretic Bumetanide in Normal Human Subjects

Background

Dapagliflozin inhibits the sodium‐glucose–linked transporter 2 in the renal proximal tubule, thereby promoting glycosuria to reduce hyperglycemia in type 2 diabetes mellitus. Because these patients may require loop diuretics, and sodium‐glucose–linked transporter 2 inhibition causes an osmotic diuresis, we evaluated the diuretic interaction between dapagliflozin and bumetanide.

Methods and Results

Healthy subjects (n=42) receiving a fixed diet with ≈110 mmol·d⁻¹ of Na⁺ were randomized to bumetanide (1 mg·d⁻¹), dapagliflozin (10 mg·d⁻¹), or both for 7 days, followed by 7 days of both. There were no meaningful pharmacokinetic interactions. Na⁺ excretion increased modestly with the first dose of dapagliflozin (22±6 mmol·d⁻¹; P<0.005) but by more (P<0.005) with the first dose of bumetanide (74±7 mmol·d⁻¹; P<0.005), which was not significantly different from both diuretics together (80±5 mmol·d⁻¹; P<0.005). However, Na⁺ excretion with dapagliflozin was 190% greater (P<0.005) when added after 1 week of bumetanide (64±6 mmol·d⁻¹), and Na⁺ excretion with bumetanide was 36% greater (P<0.005) when added after 1 week of dapagliflozin (101±8 mmol·d⁻¹). Serum urate was increased 4% by bumetanide but reduced 40% by dapagliflozin or 20% by combined therapy (P<0.05).

Conclusions

First‐dose Na⁺ excretion with bumetanide and dapagliflozin is not additive, but the weekly administration of one diuretic enhances the initial Na⁺ excretion with the other, thereby demonstrating mutual adaptive natriuretic synergy. Combined therapy reverses bumetanide‐induced hyperuricemia. This requires further study in diabetic patients with hyperglycemia who have enhanced glycosuria and natriuresis with dapagliflozin.

Clinical Trial Registration

URL: http://www.clinicaltrials.gov. Unique identifier: NCT00930865.

Modelling diabetic nephropathy in mice

Diabetic nephropathy (DN) is a leading cause of end-stage renal disease in the developed world. Accordingly, an urgent need exists for new, curative treatments as well as for biomarkers to stratify risk of DN among individuals with diabetes mellitus. A barrier to progress in these areas has been a lack of animal models that faithfully replicate the main features of human DN. Such models could be used to define the pathogenesis, identify drug targets and test new therapies. Owing to their tractability for genetic manipulation, mice are widely used to model human diseases, including DN. Questions have been raised, however, about the general utility of mouse models in human drug discovery. Standard mouse models of diabetes typically manifest only modest kidney abnormalities, whereas accelerated models, induced by superimposing genetic stressors, recapitulate key features of human DN. Incorporation of systems biology approaches and emerging data from genomics and metabolomics studies should enable further model refinement. Here, we discuss the current status of mouse models for DN, their limitations and opportunities for improvement. We emphasize that future efforts should focus on generating robust models that reproduce the major clinical and molecular phenotypes of human DN.

Effect of diuretics on renal tubular transport of calcium and magnesium

Calcium (Ca2+) and Magnesium (Mg2+) reabsorption along the renal tubule is dependent on distinct trans- and paracellular pathways. Our understanding of the molecular machinery involved is increasing. Ca2+ and Mg2+ reclamation in kidney is dependent on a diverse array of proteins, which are important for both forming divalent cation-permeable pores and channels, but also for generating the necessary driving forces for Ca2+ and Mg2+ transport. Alterations in these molecular constituents can have profound effects on tubular Ca2+ and Mg2+ handling. Diuretics are used to treat a large range of clinical conditions, but most commonly for the management of blood pressure and fluid balance. The pharmacological targets of diuretics generally directly facilitate sodium (Na+) transport, but also indirectly affect renal Ca2+ and Mg2+ handling, i.e., by establishing a prerequisite electrochemical gradient. It is therefore not surprising that substantial alterations in divalent cation handling can be observed following diuretic treatment. The effects of diuretics on renal Ca2+ and Mg2+ handling are reviewed in the context of the present understanding of basal molecular mechanisms of Ca2+ and Mg2+ transport. Acetazolamide, osmotic diuretics, Na+/H+ exchanger (NHE3) inhibitors, and antidiabetic Na+/glucose cotransporter type 2 (SGLT) blocking compounds, target the proximal tubule, where paracellular Ca2+ transport predominates. Loop diuretics and renal outer medullary K+ (ROMK) inhibitors block thick ascending limb transport, a segment with significant paracellular Ca2+ and Mg2+ transport. Thiazides target the distal convoluted tubule; however, their effect on divalent cation transport is not limited to that segment. Finally, potassium-sparing diuretics, which inhibit electrogenic Na+ transport at distal sites, can also affect divalent cation transport.

Protective Effects of Glucagon-Like Peptide-1 Analog on Renal Tubular Injury in Mice on High-Fat Diet

Aims: The study aimed to investigate the renoprotective effect of glucagon-like peptide-1 (GLP-1) against renal tubular injury in C57BL/6 mice induced by a high-fat diet (HFD). Methods: Twenty C57BL/6 mice were fed HFD for 12 weeks. Ten of these mice were treated with GLP-1 at 200 µg/kg subcutaneously twice daily for 4 weeks (HFDG group), and the other ten mice received vehicle only (HFD group). Ten mice fed standard rodent chow served as controls (Con group). Body weight, kidney weight, food intake, and systolic blood pressure were measured. The expression of endoplasmic reticulum stress (ERS) markers (BIP, p-eIF2α, ATF4, and CHOP) and apoptosis in the kidney were examined utilizing western blotting, immunohistochemistry and TUNEL, respectively. Angiotensin II and angiotensin II type 1 receptor (AT1R) were examined by ELISA. Human proximal tubule epithelial cells (HK2) were treated with GLP-1(150 nM) followed by treatment with palmitic acid (500 nM [PA]) for 24 h. HK2 cells treated with BSA were used as controls. The protein levels of ERS markers, apoptosis-associated protein, and AT1R were measured by western blotting. Results: Increase of body weight, food intake, and systolic blood pressure was less pronounced in GLP-1 treated HFDG mice compared to HFD mice. The levels of ERS markers (BIP, p-eIF2α, ATF4, and CHOP) and apoptosis decreased following GLP-1 treatment in vivo and in vitro (p<0.05). Increased AT1R induced by HFD and PA were blocked with GLP-1 treatment. In contrast, the level of angiotensin II after GLP-1 treatment was not significantly different between the HFD and HFDG mice. Conclusion: The study indicated that saturated fatty acids induced ERS and apoptosis in the kidney and increased AT1R expression. GLP-1 treatment exerted renoprotective effects against saturated fatty acid-induced kidney tubular cell ERS and apoptosis together with inhibition of AT1R expression in vivo and in vitro.

The next generation of therapeutics for chronic kidney disease

Chronic kidney disease (CKD) represents a leading cause of death in the United States. There is no cure for this disease, with current treatment strategies relying on blood pressure control through blockade of the renin-angiotensin system. Such approaches only delay the development of end-stage kidney disease and can be associated with serious side effects. Recent identification of several novel mechanisms contributing to CKD development - including vascular changes, loss of podocytes and renal epithelial cells, matrix deposition, inflammation and metabolic dysregulation - has revealed new potential therapeutic approaches for CKD. This Review assesses emerging strategies and agents for CKD treatment, highlighting the associated challenges in their clinical development.

Sodium-glucose cotransporter 2 inhibition: cardioprotection by treating diabetes-a translational viewpoint explaining its potential salutary effects

Diabetes is a growing epidemic worldwide characterized by an elevated concentration of blood glucose, associated with a high incidence of cardiovascular disease and mortality. Although in general reduction of hyperglycaemia is considered a therapeutic goal, hypoglycaemic therapies do not necessarily reduce cardiovascular mortality and may even aggravate cardiovascular risk factors, such as body weight. A new class of antidiabetic drugs acts by inhibition of the sodium-glucose cotransporter 2 (SGLT2), which (partially) prevents reabsorption of glucose from the renal filtrate. The induction of glucose excretion via the urine (glycosuria) was turned into an effective strategy to reduce blood glucose. Ancillary advantages are the caloric and volumetric loss and thereby the reduction of body weight and blood pressure. Additionally, SGLT2 inhibition has been suggested to exert direct cardioprotective effects by the reduction of cardiac fibrosis, inflammation, and oxidative stress. This article summarizes the functional consequences of SGLT2 inhibition on the diabetic and hyperglycaemic organism. We especially focused on the effects on the kidney and the cardiovascular system as described in experimental studies. The interesting observations in experimental studies may extend to clinical medicine, as a recent trial reported a decrease in heart failure outcomes in patients at high cardiovascular risk. In conclusion, SGLT2 inhibition represents a novel treatment, which might be a promising target not only to (further) reduce blood glucose but also to target other cardiovascular risk factors. More research and long-term follow-ups will reveal the specific influence of SGLT2 inhibition on the circulatory system and cardiovascular outcomes.

Hypomagnesemia in Type 2 Diabetes: A Vicious Circle?

Over the past decades, hypomagnesemia (serum Mg(2+) <0.7 mmol/L) has been strongly associated with type 2 diabetes mellitus (T2DM). Patients with hypomagnesemia show a more rapid disease progression and have an increased risk for diabetes complications. Clinical studies demonstrate that T2DM patients with hypomagnesemia have reduced pancreatic β-cell activity and are more insulin resistant. Moreover, dietary Mg(2+) supplementation for patients with T2DM improves glucose metabolism and insulin sensitivity. Intracellular Mg(2+) regulates glucokinase, KATP channels, and L-type Ca(2+) channels in pancreatic β-cells, preceding insulin secretion. Moreover, insulin receptor autophosphorylation is dependent on intracellular Mg(2+) concentrations, making Mg(2+) a direct factor in the development of insulin resistance. Conversely, insulin is an important regulator of Mg(2+) homeostasis. In the kidney, insulin activates the renal Mg(2+) channel transient receptor potential melastatin type 6 that determines the final urinary Mg(2+) excretion. Consequently, patients with T2DM and hypomagnesemia enter a vicious circle in which hypomagnesemia causes insulin resistance and insulin resistance reduces serum Mg(2+) concentrations. This Perspective provides a systematic overview of the molecular mechanisms underlying the effects of Mg(2+) on insulin secretion and insulin signaling. In addition to providing a review of current knowledge, we provide novel directions for future research and identify previously neglected contributors to hypomagnesemia in T2DM.

A comprehensive review of the pharmacodynamics of the SGLT2 inhibitor empagliflozin in animals and humans

Empagliflozin (formerly known as BI 10773) is a potent, competitive, and selective inhibitor of the sodium glucose transporter SGLT2, which mediates glucose reabsorption in the early proximal tubule and most of the glucose reabsorption by the kidney, overall. Accordingly, empagliflozin treatment increased urinary glucose excretion. This has been observed across multiple species including humans and was reported under euglycemic conditions, in obesity and, most importantly, in type 2 diabetic patients and multiple animal models of type 2 diabetes and of type 1 diabetes. This led to a reduction in blood glucose, smaller blood glucose excursions during oral glucose tolerance tests, and, upon chronic treatment, a reduction in HbA1c in animal models and patients. In rodents, such effects were observed in early and late phases of experimental diabetes and were associated with preservation of pancreatic β-cell function. Combination studies in animals demonstrated that beneficial metabolic effects of empagliflozin may also manifest when added to other types of anti-hyperglycemic treatments including linagliptin and pioglitazone. While some anti-hyperglycemic drugs lead to weight gain, empagliflozin treatment was associated with reduced body weight in normoglycemic obese and non-obese animals despite an increased food intake, largely due to a loss of adipose tissue; on the other hand, empagliflozin preserved body weight in models of type 1 diabetes. Empagliflozin improved endothelial dysfunction in diabetic rats and arterial stiffness, reduced blood pressure in diabetic patients, and attenuated early signs of nephropathy in diabetic animal models. Taken together, the SGLT2 inhibitor empagliflozin improves glucose metabolism by enhancing urinary glucose excretion; upon chronic administration, at least in animal models, the reductions in blood glucose levels are associated with beneficial effects on cardiovascular and renal complications of diabetes.

Adverse effects and safety of SGLT-2 inhibitors

In type 2 diabetes (T2DM), glycaemic control delays the development and slows the progression of complications. Although there are numerous glucose-lowering agents in clinical use, only approximately half of T2DM patients achieve glycaemic control, while undesirable side-effects, such as hypoglycaemia and body weight gain, often impede treatment in those taking these medications. Thus, there is a need for novel agents and treatment options. Sodium-glucose cotransporter-2 inhibitors (SGLT-2-i) have recently been developed for the treatment of T2DM. The available data suggest a good tolerability profile for the three available drugs - canagliflozin, dapagliflozin and empagliflozin - approved by the US Food and Drug Administration (FDA) for the American market as well as in other countries. The most frequently reported adverse events with SGLT-2-i are female genital mycotic infections, urinary tract infections and increased urination. The pharmacodynamic response to SGLT-2-i declines with increasing severity of renal impairment, requiring dosage adjustments or restrictions with moderate-to-severe renal dysfunction. Most patients treated with SGLT-2-i also have a modest reduction in blood pressure and modest effects on serum lipid profiles, some of which are beneficial (increased high-density lipoprotein cholesterol and decreased triglycerides) and others which are not (increased low-density lipoprotein cholesterol, LDL-C). A number of large-scale and longer-term cardiovascular trials are now ongoing. In patients treated with dapagliflozin, a non-significant excess number of breast and bladder cancers has been reported; considered as due to a bias, this is nevertheless being followed in the ongoing trials. No other significant safety issues have been reported so far. Although there is some benefit for several cardiovascular risk factors such as HbA1c, high blood pressure, obesity and increases in LDL-C, adequately powered trials are still required to determine the effects of SGLT-2-i on macrovascular outcomes.

Probing SGLT2 as a therapeutic target for diabetes: basic physiology and consequences

Traditional treatments for type 1 and type 2 diabetes are often associated with side effects, including weight gain and hypoglycaemia that may offset the benefits of blood glucose lowering. The kidneys filter and reabsorb large amounts of glucose, and urine is almost free of glucose in normoglycaemia. The sodium-dependent glucose transporter (SGLT)-2 in the early proximal tubule reabsorbs the majority of filtered glucose. Remaining glucose is reabsorbed by SGLT1 in the late proximal tubule. Diabetes enhances renal glucose reabsorption by increasing the tubular glucose load and the expression of SGLT2 (as shown in mice), which maintains hyperglycaemia. Inhibitors of SGLT2 enhance urinary glucose excretion and thereby lower blood glucose levels in type 1 and type 2 diabetes. The load-dependent increase in SGLT1-mediated glucose reabsorption explains why SGLT2 inhibitors in normoglycaemic conditions enhance urinary glucose excretion to only ~50% of the filtered glucose. The role of SGLT1 in both renal and intestinal glucose reabsorption provides a rationale for the development of dual SGLT1/2 inhibitors. SGLT2 inhibitors lower blood glucose levels independent of insulin and induce pleiotropic actions that may be relevant in the context of lowering cardiovascular risk. Ongoing long-term clinical studies will determine whether SGLT2 inhibitors have a safety profile and exert cardiovascular benefits that are superior to traditional agents.

The mechanisms and therapeutic potential of SGLT2 inhibitors in diabetes mellitus

The kidneys in normoglycemic humans filter 160-180 g of glucose per day (∼30% of daily calorie intake), which is reabsorbed and returned to the systemic circulation by the proximal tubule. Hyperglycemia increases the filtered and reabsorbed glucose up to two- to three-fold. The sodium glucose cotransporter SGLT2 in the early proximal tubule is the major pathway for renal glucose reabsorption. Inhibition of SGLT2 increases urinary glucose and calorie excretion, thereby reducing plasma glucose levels and body weight. The first SGLT2 inhibitors have been approved as a new class of antidiabetic drugs in type 2 diabetes mellitus, and studies are under way to investigate their use in type 1 diabetes mellitus. These compounds work independent of insulin, improve glycemic control in all stages of diabetes mellitus in the absence of clinically relevant hypoglycemia, and can be combined with other antidiabetic agents. By lowering blood pressure and diabetic glomerular hyperfiltration, SGLT2 inhibitors may induce protective effects on the kidney and cardiovascular system beyond blood glucose control.

Sodium-glucose transporter 2 inhibitors in type 2 diabetes mellitus: navigating between Scylla and Charybdis

Despite the extensive pharmacopeia for type 2 diabetes mellitus treatment, long-term glycemic control is far from being optimal, and morbimortality has increased. This demonstrates the importance of developing drugs with new mechanisms of actions, such as sodium-glucose transporter (SGLT) inhibitors (Charybdis). Since the beginning of 2000, numerous SGLT2-inhibitors have been developed and have started to be tested (Scylla) by the pharmaceutical companies that are engaged in this race. Although reductions in hemoglobin A1c have been shown in clinical trials, several issues related to the use of SGLT2 inhibitors deserve further investigation, rendering some aspects of their true safety still uncertain.

Increase in SGLT1-mediated transport explains renal glucose reabsorption during genetic and pharmacological SGLT2 inhibition in euglycemia

In the kidney, the sodium-glucose cotransporters SGLT2 and SGLT1 are thought to account for >90 and ∼3% of fractional glucose reabsorption (FGR), respectively. However, euglycemic humans treated with an SGLT2 inhibitor maintain an FGR of 40-50%, mimicking values in Sglt2 knockout mice. Here, we show that oral gavage with a selective SGLT2 inhibitor (SGLT2-I) dose dependently increased urinary glucose excretion (UGE) in wild-type (WT) mice. The dose-response curve was shifted leftward and the maximum response doubled in Sglt1 knockout (Sglt1-/-) mice. Treatment in diet with the SGLT2-I for 3 wk maintained 1.5- to 2-fold higher urine glucose/creatinine ratios in Sglt1-/- vs. WT mice, associated with a temporarily greater reduction in blood glucose in Sglt1-/- vs. WT after 24 h (-33 vs. -11%). Subsequent inulin clearance studies under anesthesia revealed free plasma concentrations of the SGLT2-I (corresponding to early proximal concentration) close to the reported IC50 for SGLT2 in mice, which were associated with FGR of 64 ± 2% in WT and 17 ± 2% in Sglt1-/-. Additional intraperitoneal application of the SGLT2-I (maximum effective dose in metabolic cages) increased free plasma concentrations ∼10-fold and reduced FGR to 44 ± 3% in WT and to -1 ± 3% in Sglt1-/-. The absence of renal glucose reabsorption was confirmed in male and female Sglt1/Sglt2 double knockout mice. In conclusion, SGLT2 and SGLT1 account for renal glucose reabsorption in euglycemia, with 97 and 3% being reabsorbed by SGLT2 and SGLT1, respectively. When SGLT2 is fully inhibited by SGLT2-I, the increase in SGLT1-mediated glucose reabsorption explains why only 50-60% of filtered glucose is excreted.

SGLT2 inhibitor empagliflozin reduces renal growth and albuminuria in proportion to hyperglycemia and prevents glomerular hyperfiltration in diabetic Akita mice

Our previous work has shown that gene knockout of the sodium-glucose cotransporter SGLT2 modestly lowered blood glucose in streptozotocin-diabetic mice (BG; from 470 to 300 mg/dl) and prevented glomerular hyperfiltration but did not attenuate albuminuria or renal growth and inflammation. Here we determined effects of the SGLT2 inhibitor empagliflozin (300 mg/kg of diet for 15 wk; corresponding to 60-80 mg·kg(-1)·day(-1)) in type 1 diabetic Akita mice that, opposite to streptozotocin-diabetes, upregulate renal SGLT2 expression. Akita diabetes, empagliflozin, and Akita + empagliflozin similarly increased renal membrane SGLT2 expression (by 38-56%) and reduced the expression of SGLT1 (by 33-37%) vs. vehicle-treated wild-type controls (WT). The diabetes-induced changes in SGLT2/SGLT1 protein expression are expected to enhance the BG-lowering potential of SGLT2 inhibition, and empagliflozin strongly lowered BG in Akita (means of 187-237 vs. 517-535 mg/dl in vehicle group; 100-140 mg/dl in WT). Empagliflozin modestly reduced GFR in WT (250 vs. 306 μl/min) and completely prevented the diabetes-induced increase in glomerular filtration rate (GFR) (255 vs. 397 μl/min). Empagliflozin attenuated increases in kidney weight and urinary albumin/creatinine ratio in Akita in proportion to hyperglycemia. Empagliflozin did not increase urinary glucose/creatinine ratios in Akita, indicating the reduction in filtered glucose balanced the inhibition of glucose reabsorption. Empagliflozin attenuated/prevented the increase in systolic blood pressure, glomerular size, and molecular markers of kidney growth, inflammation, and gluconeogenesis in Akita. We propose that SGLT2 inhibition can lower GFR independent of reducing BG (consistent with the tubular hypothesis of diabetic glomerular hyperfiltration), while attenuation of albuminuria, kidney growth, and inflammation in the early diabetic kidney may mostly be secondary to lower BG.

Investigational anti-hyperglycemic agents: the future of type 2 diabetes therapy?

As the pandemic of type 2 diabetes spreads globally, clinicians face many challenges in treating an increasingly diverse patient population varying in age, comorbidities, and socioeconomic status. Current therapies for type 2 diabetes are often unable to alter the natural course of the disease and provide durable glycemic control, and side effects in the context of individual patient characteristics often limit treatment choices. This often results in the progression to insulin use and complex regimens that are difficult to maintain. Therefore, a number of agents are being developed to better address the pathogenesis of type 2 diabetes and to overcome limitations of current therapies. The hope is to provide more options for glucose lowering and complication reduction with less risk for hypoglycemia and other adverse effects. These agents include newer incretin-based therapies and PPAR agonists, as well as new therapeutic classes such as sodium-coupled glucose cotransporter 2 inhibitors, free fatty acid receptor agonists, 11-β-hydroxysteroid dehydrogenase type 1 inhibitors, glucokinase activators, and several others that may enter clinical use over the next decade. Herein we review these agents that are advancing through clinical trials and describe the rationale behind their use, mechanisms of action, and potential for glucose lowering, as well as what is known of their limitations.

Proximal nephron

The kidney plays a fundamental role in maintaining body salt and fluid balance and blood pressure homeostasis through the actions of its proximal and distal tubular segments of nephrons. However, proximal tubules are well recognized to exert a more prominent role than distal counterparts. Proximal tubules are responsible for reabsorbing approximately 65% of filtered load and most, if not all, of filtered amino acids, glucose, solutes, and low molecular weight proteins. Proximal tubules also play a key role in regulating acid-base balance by reabsorbing approximately 80% of filtered bicarbonate. The purpose of this review article is to provide a comprehensive overview of new insights and perspectives into current understanding of proximal tubules of nephrons, with an emphasis on the ultrastructure, molecular biology, cellular and integrative physiology, and the underlying signaling transduction mechanisms. The review is divided into three closely related sections. The first section focuses on the classification of nephrons and recent perspectives on the potential role of nephron numbers in human health and diseases. The second section reviews recent research on the structural and biochemical basis of proximal tubular function. The final section provides a comprehensive overview of new insights and perspectives in the physiological regulation of proximal tubular transport by vasoactive hormones. In the latter section, attention is particularly paid to new insights and perspectives learnt from recent cloning of transporters, development of transgenic animals with knockout or knockin of a particular gene of interest, and mapping of signaling pathways using microarrays and/or physiological proteomic approaches.

Glucose metabolism in the Belgrade rat, a model of iron-loading anemia

The iron-diabetes hypothesis proposes an association between iron overload and glucose metabolism that is supported by a number of epidemiological studies. The prevalence of type 2 diabetes in patients with hereditary hemochromatosis and iron-loading thalassemia supports this hypothesis. The Belgrade rat carries a mutation in the iron transporter divalent metal transporter 1 (DMT1) resulting in iron-loading anemia. In this study, we characterized the glycometabolic status of the Belgrade rat. Belgrade rats displayed normal glycemic control. Insulin signaling and secretion were not impaired, and pancreatic tissue did not incur damage despite high levels of nonheme iron. These findings suggest that loss of DMT1 protects against oxidative damage to the pancreas and helps to maintain insulin sensitivity despite iron overload. Belgrade rats had lower body weight but increased food consumption compared with heterozygous littermates. The unexpected energy balance was associated with increased urinary glucose output. Increased urinary excretion of electrolytes, including iron, was also observed. Histopathological evidence suggests that altered renal function is secondary to changes in kidney morphology, including glomerulosclerosis. Thus, loss of DMT1 appears to protect the pancreas from injury but damages the integrity of kidney structure and function.

Sodium glucose cotransporter 2 and the diabetic kidney

PURPOSE OF REVIEW

Reabsorption of glucose in the proximal tubule occurs predominantly via the sodium glucose cotransporter 2 (SGLT2). There has been intense interest in this transporter as a number of SGLT2 inhibitors have entered clinical development. SGLT2 inhibitors act to lower plasma glucose by promoting glycosuria and this review aims to outline the effect on the diabetic kidney of this hypoglycaemic agent.

RECENT FINDINGS

This review provides an overview of recent findings in this area: the transcriptional control of SGLT2 expression in human proximal tubular cells implicates a number of cytokines in the alteration of SGLT2 expression; experimental data show that SGLT2 inhibition may correct early detrimental effects of diabetes by reducing proximal tubular sodium and glucose transport, suggesting a possible renoprotective effect independent of the glucose lowering effects of these agents; and the nonglycaemic effects of SGLT2 inhibitors may have an impact on renal outcomes.

SUMMARY

The available clinical evidence shows consistent reduction in glycaemic parameters and some evidence suggests additional effects including weight loss and mild blood pressure reduction. There are some side effects that warrant further investigation and establishing whether SGLT2 inhibition provides a renal benefit relies on future long-term studies with specific renal end-points.

Report on ISN Forefronts, Melbourne, Australia, 4-7 October 2012: tubulointerstitial disease in diabetic nephropathy

The mechanisms involved in expansion of the tubulointerstitial compartment of the kidney in individuals with diabetes are not well understood. Given that tubulointerstitial damage is an important predictor of progression to end-stage kidney disease in most forms of chronic kidney disease it is imperative to gain a greater understanding of the processes involved. With this in mind, a very clear objective for the scientific content of this meeting was to spend more than half the program outside the comfort zone of nephrology, gaining insights from sources such as neurodegenerative and mitochondrial diseases, stem cells, cancer and high-level computing to reconstruct organ systems. The meeting also aimed to place the new concepts presented in the context of current knowledge in diabetic kidney disease and the milestones achieved to date in this area. The presenters were all extremely generous, giving not only their time, but also showing a large proportion of unpublished data to stimulate discussions, questions and innovation.

Nephron filtration rate and proximal tubular fluid reabsorption in the Akita mouse model of type I diabetes mellitus

An increase of glomerular filtration rate (hyperfiltration) is an early functional change associated with type I or type II diabetes mellitus in patients and animal models. The causes underlying glomerular hyperfiltration are not entirely clear. There is evidence from studies in the streptozotocin model of diabetes in rats that an increase of proximal tubular reabsorption results in the withdrawal of a vasoconstrictor input exerted by the tubuloglomerular feedback (TGF) mechanism. In the present study, we have used micropuncture to assess single nephron function in wild type (WT) mice and in two strains of type I diabetic Ins2+/- mice in either a C57Bl/6 (Akita) or an A1AR-/- background (Akita/A1AR-/-) in which TGF is non-functional. Kidney glomerular filtration rate (GFR) of anesthetized mice was increased by 25% in Akita mice and by 52% in Akita/A1AR-/-, but did not differ between genotypes when corrected for kidney weight. Single nephron GFR (SNGFR) measured by end-proximal fluid collections averaged 11.8 ± 1 nl/min (n=17), 13.05 ± 1.1 nl/min (n=23; p=0.27), and 15.4 ± 0.84 nl/min (n=26; p=0.009 compared to WT; p=0.09 compared to Akita) in WT, Akita, and Akita/A1AR-/- mice respectively. Proximal tubular fluid reabsorption was not different between WT and diabetic mice and correlated with SNGFR in all genotypes. We conclude that glomerular hyperfiltration is a primary event in the Akita model of type I diabetes, perhaps driven by an increased filtering surface area, and that it is ameliorated by TGF to the extent that this regulatory system is functional.

Differentiating sodium-glucose co-transporter-2 inhibitors in development for the treatment of type 2 diabetes mellitus

INTRODUCTION

Sodium-glucose co-transporter-2 (SGLT2) inhibitors are a novel class of agents for the treatment of type 2 diabetes mellitus (T2DM). By inhibiting SGLT2, they prevent renal glucose reabsorption, resulting in glucosuria.

AREAS COVERED

The rationale for development of SGLT2 inhibitors is reviewed, with particular focus on the nine SGLT2 inhibitors currently in development. The authors compare the potency and SGLT2 selectivity of the agents, as well as the results from both animal and clinical studies, considering the potential implications they may have for clinical use.

EXPERT OPINION

Current evidence suggests that SGLT2 inhibitors have similar efficacy in terms of glycemic control and also demonstrate benefits beyond glycemic reductions, including reductions in body weight and modest reductions in blood pressure. Additionally, they appear to preserve beta-cell function and improve insulin sensitivity. Their mechanism of action allows for combination of SGLT2 inhibitors with other antidiabetic drugs and use across the treatment continuum for T2DM. Potential differences in safety and efficacy based on observed differences in potency and selectivity among the SGLT2 inhibitors, particularly versus SGLT1, remain to be seen. Further long-term data, including post-marketing surveillance, are required to fully determine the safety profile of SGLT2 inhibitors in large patient groups.

Mechanisms of diabetic complications

It is increasingly apparent that not only is a cure for the current worldwide diabetes epidemic required, but also for its major complications, affecting both small and large blood vessels. These complications occur in the majority of individuals with both type 1 and type 2 diabetes. Among the most prevalent microvascular complications are kidney disease, blindness, and amputations, with current therapies only slowing disease progression. Impaired kidney function, exhibited as a reduced glomerular filtration rate, is also a major risk factor for macrovascular complications, such as heart attacks and strokes. There have been a large number of new therapies tested in clinical trials for diabetic complications, with, in general, rather disappointing results. Indeed, it remains to be fully defined as to which pathways in diabetic complications are essentially protective rather than pathological, in terms of their effects on the underlying disease process. Furthermore, seemingly independent pathways are also showing significant interactions with each other to exacerbate pathology. Interestingly, some of these pathways may not only play key roles in complications but also in the development of diabetes per se. This review aims to comprehensively discuss the well validated, as well as putative mechanisms involved in the development of diabetic complications. In addition, new fields of research, which warrant further investigation as potential therapeutic targets of the future, will be highlighted.

Coming full circle in diabetes mellitus: from complications to initiation

Glycaemic control, reduction of blood pressure using agents that block the renin-angiotensin system and control of dyslipidaemia are the major strategies used in the clinical management of patients with diabetes mellitus. Each of these approaches interrupts a number of pathological pathways, which directly contributes to the vascular complications of diabetes mellitus, including renal disease, blindness, neuropathy and cardiovascular disease. However, research published over the past few years has indicated that many of the pathological pathways important in the development of the vascular complications of diabetes mellitus are equally relevant to the initiation of diabetes mellitus itself. These pathways include insulin signalling, generation of cellular energy, post-translational modifications and redox imbalances. This Review will examine how the development of diabetes mellitus has come full circle from initiation to complications and suggests that the development of diabetes mellitus and the progression to chronic complications both require the same mechanistic triggers.

Improved glycemic control in mice lacking Sglt1 and Sglt2

Sodium-glucose cotransporter 2 (SGLT2) is the major, and SGLT1 the minor, transporter responsible for renal glucose reabsorption. Increasing urinary glucose excretion (UGE) by selectively inhibiting SGLT2 improves glycemic control in diabetic patients. We generated Sglt1 and Sglt2 knockout (KO) mice, Sglt1/Sglt2 double-KO (DKO) mice, and wild-type (WT) littermates to study their relative glycemic control and to determine contributions of SGLT1 and SGLT2 to UGE. Relative to WTs, Sglt2 KOs had improved oral glucose tolerance and were resistant to streptozotocin-induced diabetes. Sglt1 KOs fed glucose-free high-fat diet (G-free HFD) had improved oral glucose tolerance accompanied by delayed intestinal glucose absorption and increased circulating glucagon-like peptide-1 (GLP-1), but had normal intraperitoneal glucose tolerance. On G-free HFD, Sglt2 KOs had 30%, Sglt1 KOs 2%, and WTs <1% of the UGE of DKOs. Consistent with their increased UGE, DKOs had lower fasting blood glucose and improved intraperitoneal glucose tolerance than Sglt2 KOs. In conclusion, 1) Sglt2 is the major renal glucose transporter, but Sglt1 reabsorbs 70% of filtered glucose if Sglt2 is absent; 2) mice lacking Sglt2 display improved glucose tolerance despite UGE that is 30% of maximum; 3) Sglt1 KO mice respond to oral glucose with increased circulating GLP-1; and 4) DKO mice have improved glycemic control over mice lacking Sglt2 alone. These data suggest that, in patients with type 2 diabetes, combining pharmacological SGLT2 inhibition with complete renal and/or partial intestinal SGLT1 inhibition may improve glycemic control over that achieved by SGLT2 inhibition alone.

Knockout of Na-glucose transporter SGLT2 attenuates hyperglycemia and glomerular hyperfiltration but not kidney growth or injury in diabetes mellitus

The Na-glucose cotransporter SGLT2 mediates high-capacity glucose uptake in the early proximal tubule and SGLT2 inhibitors are developed as new antidiabetic drugs. We used gene-targeted Sglt2 knockout (Sglt2(-/-)) mice to elucidate the contribution of SGLT2 to blood glucose control, glomerular hyperfiltration, kidney growth, and markers of renal growth and injury at 5 wk and 4.5 mo after induction of low-dose streptozotocin (STZ) diabetes. The absence of SGLT2 did not affect renal mRNA expression of glucose transporters SGLT1, NaGLT1, GLUT1, or GLUT2 in response to STZ. Application of STZ increased blood glucose levels to a lesser extent in Sglt2(-/-) vs. wild-type (WT) mice (∼300 vs. 470 mg/dl) but increased glucosuria and food and fluid intake to similar levels in both genotypes. Lack of SGLT2 prevented STZ-induced glomerular hyperfiltration but not the increase in kidney weight. Knockout of SGLT2 attenuated the STZ-induced renal accumulation of p62/sequestosome, an indicator of impaired autophagy, but did not attenuate the rise in renal expression of markers of kidney growth (p27 and proliferating cell nuclear antigen), oxidative stress (NADPH oxidases 2 and 4 and heme oxygenase-1), inflammation (interleukin-6 and monocyte chemoattractant protein-1), fibrosis (fibronectin and Sirius red-sensitive tubulointerstitial collagen accumulation), or injury (renal/urinary neutrophil gelatinase-associated lipocalin). SGLT2 deficiency did not induce ascending urinary tract infection in nondiabetic or diabetic mice. The results indicate that SGLT2 is a determinant of hyperglycemia and glomerular hyperfiltration in STZ-induced diabetes mellitus but is not critical for the induction of renal growth and markers of renal injury, inflammation, and fibrosis.

Dapagliflozin: a novel sodium-glucose cotransporter type 2 inhibitor for the treatment of type 2 diabetes mellitus

The prevalence of diabetes mellitus has grown to staggering numbers, and its incidence is expected to rise in the next 2 decades. The need for novel approaches to treat hyperglycemia cannot be ignored. Current agents have been shown to modestly improve glycemia and in some cases prevent complications of diabetes, but they become less effective over time and are often accompanied by undesirable adverse effects. Dapagliflozin is the lead agent in a new class of oral antidiabetic agents known as sodium-glucose cotransporter type 2 (SGLT2) inhibitors, which represent a novel approach to the management of type 2 diabetes mellitus. By selectively and reversibly blocking the SGLT2 receptor, dapagliflozin prevents the reabsorption of glucose at the renal proximal tubule. Phase II and III clinical trials have demonstrated that dapagliflozin is a safe and effective method for treating type 2 diabetes. Dapagliflozin produces a sustained, dose-dependent reduction in plasma glucose levels while simultaneously improving insulin secretion and sensitivity. Over 12-24 weeks, reductions in hemoglobin A(1c) ranged from 0.54-0.89% when dapagliflozin was administered once/day (either as monotherapy or add-on therapy to oral antidiabetic drugs with or without insulin) to patients with type 2 diabetes. Therapy with dapagliflozin also results in a mild osmotic-diuretic effect that may account for decreases in total body weight (~2-3 kg) and blood pressure (systolic 2-5 mm Hg, diastolic 1.5-3 mm Hg), and increases in hematocrit (1-2%). Dapagliflozin has a favorable safety profile, with the rates of hypoglycemia similar to those of placebo. Genital and urinary tract infections were more commonly reported in patients taking dapagliflozin (2-13%) than those taking placebo (0-8%). Dapagliflozin does not appear to cause electrolyte disturbances, hepatotoxicity, or nephrotoxicity. Results from clinical trials have been promising, and well-designed clinical programs that address the long-term safety and efficacy of dapagliflozin are under way.

Renal function in diabetic disease models: the tubular system in the pathophysiology of the diabetic kidney

Diabetes mellitus affects the kidney in stages. At the onset of diabetes mellitus, in a subset of diabetic patients the kidneys grow large, and glomerular filtration rate (GFR) becomes supranormal, which are risk factors for developing diabetic nephropathy later in life. This review outlines a pathophysiological concept that focuses on the tubular system to explain these changes. The concept includes the tubular hypothesis of glomerular filtration, which states that early tubular growth and sodium-glucose cotransport enhance proximal tubule reabsorption and make the GFR supranormal through the physiology of tubuloglomerular feedback. The diabetic milieu triggers early tubular cell proliferation, but the induction of TGF-β and cyclin-dependent kinase inhibitors causes a cell cycle arrest and a switch to tubular hypertrophy and a senescence-like phenotype. Although this growth phenotype explains unusual responses like the salt paradox of the early diabetic kidney, the activated molecular pathways may set the stage for tubulointerstitial injury and diabetic nephropathy.