Hypoxia-inducible factor-1α (HIF-1α) protein diminishes sodium glucose transport 1 (SGLT1) and SGLT2 protein expression in renal epithelial tubular cells (LLC-PK1) under hypoxia

Fetal Reprogramming of Nutrient Surplus Signaling, O-GlcNAcylation, and the Evolution of CKD

ABSTRACT

Fetal kidney development is characterized by increased uptake of glucose, ATP production by glycolysis, and upregulation of mammalian target of rapamycin (mTOR) and hypoxia-inducible factor-1 alpha (HIF-1 α ), which (acting in concert) promote nephrogenesis in a hypoxic low-tubular-workload environment. By contrast, the healthy adult kidney is characterized by upregulation of sirtuin-1 and adenosine monophosphate-activated protein kinase, which enhances ATP production through fatty acid oxidation to fulfill the needs of a normoxic high-tubular-workload environment. During stress or injury, the kidney reverts to a fetal signaling program, which is adaptive in the short term, but is deleterious if sustained for prolonged periods when both oxygen tension and tubular workload are heightened. Prolonged increases in glucose uptake in glomerular and proximal tubular cells lead to enhanced flux through the hexosamine biosynthesis pathway; its end product-uridine diphosphate N -acetylglucosamine-drives the rapid and reversible O-GlcNAcylation of thousands of intracellular proteins, typically those that are not membrane-bound or secreted. Both O-GlcNAcylation and phosphorylation act at serine/threonine residues, but whereas phosphorylation is regulated by hundreds of specific kinases and phosphatases, O-GlcNAcylation is regulated only by O-GlcNAc transferase and O-GlcNAcase, which adds or removes N-acetylglucosamine, respectively, from target proteins. Diabetic and nondiabetic CKD is characterized by fetal reprogramming (with upregulation of mTOR and HIF-1 α ) and increased O-GlcNAcylation, both experimentally and clinically. Augmentation of O-GlcNAcylation in the adult kidney enhances oxidative stress, cell cycle entry, apoptosis, and activation of proinflammatory and profibrotic pathways, and it inhibits megalin-mediated albumin endocytosis in glomerular mesangial and proximal tubular cells-effects that can be aggravated and attenuated by augmentation and muting of O-GlcNAcylation, respectively. In addition, drugs with known nephroprotective effects-angiotensin receptor blockers, mineralocorticoid receptor antagonists, and sodium-glucose cotransporter 2 inhibitors-are accompanied by diminished O-GlcNAcylation in the kidney, although the role of such suppression in mediating their benefits has not been explored. The available evidence supports further work on the role of uridine diphosphate N -acetylglucosamine as a critical nutrient surplus sensor (acting in concert with upregulated mTOR and HIF-1 α signaling) in the development of diabetic and nondiabetic CKD.

Knockdown of TMEM30A in renal tubular epithelial cells leads to reduced glucose absorption

The kidney reabsorbs large amounts of glucose through Na+-glucose cotransporter 2 (SGLT2). P4-ATPase acts together with the β-subunit TMEM30A to mediate the asymmetric distribution of phosphatidylserine (PS), phosphatidylethanolamine (PE), and other amino phospholipids, promoting plasma membrane and internal vesicle fusion, and facilitating vesicle protein transport. We observed reduced TMEM30A expression in renal tubules of DKD and IgA patients, suggesting a potential role of TMEM30A in renal tubular cells. To investigate the role of TMEM30A in renal tubules, we constructed a TMEM30A knockdown cell model by transfecting mouse kidney tubular epithelium cells (TCMK-1) with TMEM30A shRNA. Knockdown of TMEM30A in TCMK-1 cells attenuated vesicle transporter protein synthesis, resulting in reduced transport and expression of SGLT2, which in turn reduced glucose absorption. These data suggested that TMEM30A plays a crucial role in renal tubules.

Effects of pharmacological delay with roxadustat on multi-territory perforator flap survival in rats

Roxadustat (FG-4592) is a specific hypoxia-inducible factor (HIF) prolyl hydroxylase inhibitor. We investigated the effects of FG-4592 pretreatment on survival and second choke vessels of multi-territory perforator flaps in rats. In total, 72 rats were divided into two groups (n = 36 each): the experimental (FG-4592) group and the control group. FG-4592 was administered orally as a single dose of 60 mg/kg every other day; the first drug solution was administered to the animals 7 days before the surgical procedure. On postoperative day 7, the surviving flap area was calculated. At 12 h post-surgery, in the second choke zone in the flaps, macrovascular hinges were compared by angiography and imaging, and microvascular changes were assessed by histology. Laser Doppler imaging was used to evaluate flap perfusion at the second choke zone at 12 h and 7 days after surgery. At 7 days after surgery, the flap survival area and perfusion were significantly greater in rats given FG-4592 compared with controls. At 12 h after surgery, the diameter of macrovascular and microvascular vessels, nitric oxide content, perfusion, and the protein levels of HIF-1α and inducible nitric oxide synthase were also significantly greater in FG-4592-treated rats than controls. In conclusion, pretreatment with roxadustat may improve initial flap survival and dilate the second choke zone vessels in a multi-territory perforator flap.

Molecular Mechanisms Linking Empagliflozin to Renal Protection in the LLC-PK1 Model of Diabetic Nephropathy

Aims: Chronic diabetes complications, including diabetic nephropathy (DN), frequently result in end-stage renal failure. This study investigated empagliflozin (SGLT2i) effects on collagen synthesis, oxidative stress, cell survival, and protein expression in an LLC-PK1 model of DN. Methods: Combinations of high glucose (HG) and increasing empagliflozin concentrations (100 nM and 500 nM), as well as combinations of HG, H2O2, and empagliflozin, were used for cell culture treatment. The cell viability, glutathione (tGSH), ECM expression, and TGF-β1 concentration were measured. In addition, the protein expression of Akt, pAkt, GSK3, pGSK3, pSTAT3, and SMAD7 was determined. Results: The addition of both concentrations of empagliflozin to cells previously exposed to glucose and oxidative stress generally improved cell viability and increased GSH levels (p < 0.001, p < 0.05). In HG30/H2O2/Empa500-treated cells, significant increase in pSTAT3, pGSK3β, GSK3β, SMAD7, and pAKT levels (p < 0.001, p < 0.001, p < 0.05) was observed except for AKT. Lower drug concentrations did not affect the protein expression levels. Furthermore, empagliflozin treatment (100 nM and 500 nM) of HG30/H2O2-injured cells led to a decrease in TGF-β1 levels (p < 0.001). In cells exposed to oxidative stress and hyperglycemia, collagen production remained unchanged. Conclusion: Renoprotective effects of empagliflozin, in this LLC-PK1 cell model of DN, are mediated via activation of the Akt/GSK-3 signalling pathway, thus reducing oxidative stress-induced damage, as well as enhanced SMAD7 expression leading to downregulation of TGF-β1, one of the key mediators of inflammation and fibrosis.

Sodium-glucose co-transporter 2 inhibitors and hematopoiesis

Many patients with diabetes mellitus, especially those with chronic kidney disorders, have some degree of anemia due to a spectrum of causes and underlying pathophysiologic pathways. As such, enhancement in erythropoiesis is important in these patients. Sodium-glucose cotransporter 2 inhibitors (SGLT2i) are a relatively new class of antidiabetic drugs with confirmed protective effects in kidney and cardiovascular tissues. Recent evidence suggests that these drugs may provide additional benefits in enhancing hematopoietic processes in diabetic patients. Though the exact mediating pathways have not been fully elucidated, cellular mechanisms are likely involved. In the current study, we present the potential pathways by which SGLT2i may modulate hematopoiesis and stimulate erythropoiesis.

Dapagliflozin Attenuates Contrast-induced Acute Kidney Injury by Regulating the HIF-1α/HE4/NF-κB Pathway

ABSTRACT

Contrast-induced acute kidney injury (CI-AKI) causes clinically acquired nephropathy in patients who undergo coronary interventions. Hypoxic injury to proximal tubular epithelial cells is a pathological mechanism of CI-AKI. Previous studies have shown that hypoxia activates HIF-1α/HE4/NF-κB to enhance renal fibrosis, and the SGLT-2 inhibitor luseogliflozin inhibits hypoxia-inducible factor (HIF)-1α expression to reduce the progression of diabetic nephropathy. However, the therapeutic effects and mechanisms of SGLT-2 inhibitors on CI-AKI are unclear. We explored the role of the HIF-1α/HE4/NF-κB pathway in CI-AKI and how dapagliflozin effectively treats CI-AKI by inhibiting this pathway. In vitro, cells were divided into the control, hypoxia, hypoxia + dapagliflozin, and hypoxia + pSilencer-HIF-1α groups. Cellular hypoxia, apoptosis, and related protein expression were evaluated by immunofluorescence, western blotting, and flow cytometry, respectively. Dapagliflozin significantly decreased oxygen consumption, HIF-1α, human epididymis protein 4 (HE4), NF-κB expression, and apoptotic cells compared with the control (P < 0.01). In vivo, rats were divided into the control (C), diabetes (D), diabetes + contrast media, and diabetes + contrast media + dapagliflozin groups. Rats in the latter 2 groups were treated with dapagliflozin for 2 days. CI-AKI was induced by intravenously injecting indomethacin, N-nitro-l-arginine methyl ester, and iohexol. The effects of dapagliflozin on CI-AKI rats were elucidated by assessing renal function, H&E staining, and immunohistochemistry. Serum creatinine, urea nitrogen, TUNEL-positive tubular cells, HIF-1α, HE4, NF-κB expression, and histopathological scores were increased in diabetes + contrast media rats compared with C, D, and diabetes + dapagliflozin + contrast media rats (P < 0.01). Thus, dapagliflozin may ameliorate CI-AKI through suppression of HIF-1α/HE4/NF-κB signaling in vitro and in vivo.

The Pathophysiological Basis of Diabetic Kidney Protection by Inhibition of SGLT2 and SGLT1

SGLT2 inhibitors can protect the kidneys of patients with and without type 2 diabetes mellitus and slow the progression towards end-stage kidney disease. Blocking tubular SGLT2 and spilling glucose into the urine, which triggers a metabolic counter-regulation similar to fasting, provides unique benefits, not only as an anti-hyperglycemic strategy. These include a low hypoglycemia risk and a shift from carbohydrate to lipid utilization and mild ketogenesis, thereby reducing body weight and providing an additional energy source. SGLT2 inhibitors counteract hyperreabsorption in the early proximal tubule, which acutely lowers glomerular pressure and filtration and thereby reduces the physical stress on the filtration barrier, the filtration of tubule-toxic compounds, and the oxygen demand for tubular reabsorption. This improves cortical oxygenation, which, together with lesser tubular gluco-toxicity and improved mitochondrial function and autophagy, can reduce pro-inflammatory, pro-senescence, and pro-fibrotic signaling and preserve tubular function and GFR in the long-term. By shifting transport downstream, SGLT2 inhibitors more equally distribute the transport burden along the nephron and may mimic systemic hypoxia to stimulate erythropoiesis, which improves oxygen delivery to the kidney and other organs. SGLT1 inhibition improves glucose homeostasis by delaying intestinal glucose absorption and by increasing the release of gastrointestinal incretins. Combined SGLT1 and SGLT2 inhibition has additive effects on renal glucose excretion and blood glucose control. SGLT1 in the macula densa senses luminal glucose, which affects glomerular hemodynamics and has implications for blood pressure control. More studies are needed to better define the therapeutic potential of SGLT1 inhibition to protect the kidney, alone or in combination with SGLT2 inhibition.

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.

Acquired Decline in Ultrafiltration in Peritoneal Dialysis: The Role of Glucose

Ultrafiltration is essential in peritoneal dialysis (PD) for maintenance of euvolemia, making ultrafiltration insufficiency-preferably called ultrafiltration failure-an important complication. The mechanisms of ultrafiltration and ultrafiltration failure are more complex than generally assumed, especially after long-term treatment. Initially, ultrafiltration failure is mainly explained by a large number of perfused peritoneal microvessels, leading to a rapid decline of the crystalloid osmotic gradient, thereby decreasing aquaporin-mediated free water transport. The contribution of peritoneal interstitial tissue to ultrafiltration failure is limited during the first few years of PD, but becomes more important in long-term PD due to the development of interstitial fibrosis, which mainly consists of myofibroblasts. A dual hypothesis has been developed to explain why the continuous exposure of peritoneal tissues to the extremely high dialysate glucose concentrations causes progressive ultrafiltration decline. First, glucose absorption causes an increase of the intracellular NADH/NAD⁺ ratio, also called pseudohypoxia. Intracellular hypoxia stimulates myofibroblasts to produce profibrotic and angiogenetic factors, and the glucose transporter GLUT-1. Second, the increased GLUT-1 expression by myofibroblasts increases glucose uptake in these cells, leading to a reduction of the osmotic gradient for ultrafiltration. Reduction of peritoneal glucose exposure to prevent this vicious circle is essential for high-quality, long-term PD.

A small-molecule inhibitor of hypoxia-inducible factor prolyl hydroxylase improves obesity, nephropathy and cardiomyopathy in obese ZSF1 rats

Prolyl hydroxylase (PH) enzymes control the degradation of hypoxia-inducible factor (HIF), a transcription factor known to regulate erythropoiesis, angiogenesis, glucose metabolism, cell proliferation, and apoptosis. HIF-PH inhibitors (HIF-PHIs) correct anemia in patients with renal disease and in animal models of anemia and kidney disease. However, the effects of HIF-PHIs on comorbidities associated with kidney disease remain largely unknown. We evaluated the effects of the HIF-PHI FG-2216 in obese ZSF1 (Ob-ZSF1) rats, an established model of kidney failure with metabolic syndrome. Following unilateral nephrectomy (Nx) at 8 weeks of age, rats were treated with 40 mg/kg FG-2216 or vehicle by oral gavage three times per week for up to 18 weeks. FG-2216 corrected blood hemoglobin levels and improved kidney function and histopathology in Nx-Ob-ZSF1 rats by increasing the glomerular filtration rate, decreasing proteinuria, and reducing peritubular fibrosis, tubular damage, glomerulosclerosis and mesangial expansion. FG-2216 increased renal glucose excretion and decreased body weight, fat pad weight, and serum cholesterol in Nx-Ob-ZSF1 rats. Additionally, FG-2216 corrected hypertension, improved diastolic and systolic heart function, and reduced cardiac hypertrophy and fibrosis. In conclusion, the HIF-PHI FG-2216 improved renal and cardiovascular outcomes, and reduced obesity in a rat model of kidney disease with metabolic syndrome. Thus, in addition to correcting anemia, HIF-PHIs may provide renal and cardiac protection to patients suffering from kidney disease with metabolic syndrome.

Mechanisms of the antiangiogenic effects of aspirin in cancer

Aspirin is an old drug extracted from willow bark and is widely used for the prevention and treatment of cardiovascular diseases. Accumulating evidence has shown that aspirin use may significantly reduce the angiogenesis of cancer; however, the mechanism of the association between angiogenesis and aspirin is complex. Although COX-1 is widely known as a target of aspirin, several studies reveal other antiangiogenic targets of aspirin, such as angiotensin II, glucose transporter 1, heparanase, and matrix metalloproteinase. In addition, some data indicates that aspirin may produce antiangiogenic effects after acting in different cell types, such as endothelial cells, platelets, pericytes, and macrophages. In this review, we concentrate on research regarding the antiangiogenic effects of aspirin in cancer, and we discuss the molecular mechanisms of aspirin and its metabolites. Moreover, we discuss some mechanisms through which aspirin treatment may normalize existing blood vessels, including preventing the disintegration of endothelial adheres junctions and the recruitment of pericytes. We also address the antiangiogenic effects and the underlying mechanisms of aspirin derivatives, which are aimed at improving safety and efficacy.

Sodium/glucose Co-Transporter 2 Inhibitor, Empagliflozin, Alleviated Transient Expression of SGLT2 after Myocardial Infarction

Background and Objectives

Large clinical studies of sodium/glucose cotransporter 2 (SGLT2) inhibitors have shown a significant beneficial effect on heart failure-associated hospitalization and cardiovascular events. As SGLT2 is known to be absent in heart cells, improved cardiovascular outcomes are thought to be accounted for by the indirect effects of the drug. We sought to confirm whether such benefits were mediated through SGLT2 expressed in the heart using myocardial infarction (MI) model.

Methods

Mice pre-treated with empagliflozin (EMPA), an SGLT2 inhibitor, showed a significantly reduced infarct size compared with the vehicle group three days post-MI. Interestingly, we confirmed SGLT2 localized in the infarct zone. The sequential changes of SGLT2 expression after MI were also evaluated.

Results

One day after MI, SGLT2 transiently appeared in the ischemic areas in the vehicle group and increased until 72 hours. The appearance of SGLT2 was delayed and less in amount compared with the vehicle group. Additionally, there was a significant difference in metabolites, including glucose and amino acids in the ¹H nuclear magnetic resonance analysis between groups.

Conclusions

Our work demonstrates that SGLT2 is transiently expressed in heart tissue early after MI and EMPA may directly operate on SGLT2 to facilitate metabolic substrates shifts.

Expression of glucose transporters in duodenal mucosa of patients with type 1 diabetes

AIMS

A higher SGLT1 and GLUT2 gene expression was shown in the intestine of subjects with type 2 diabetes, while no data have been reported in type 1 diabetes (T1D). The purpose of our study was to evaluate the expression of glucose transporters in duodenal mucosa of subjects with T1D, compared to healthy controls (CTRL) and to patients with celiac disease (CD), as gut inflammatory disease control group.

MATERIALS AND METHODS

Gene expression of GLUT1, GLUT2, SGLT1 and SGLT2 was quantified on duodenal mucosa biopsies of subjects with T1D (n = 19), CD (n = 16), T1D and CD (n = 6) and CTRL (n = 12), recruited at San Raffaele Hospital (Milan, Italy), between 2009 and 2018. SGLT2 expression was further evaluated by immunohistochemical and immunofluorescence staining.

RESULTS

The expression of all four glucose transporters was detected in duodenal mucosa of all groups. A reduced GLUT2, SGLT1 and SGLT2 expression was observed in CD in comparison with T1D and CTRL, as expected; GLUT1 was significantly more expressed in T1D compared to CTRL. SGLT2 expression was quantified at much lower levels than other transporters, with no differences between groups. SGLT2 expression was confirmed by immunohistochemistry in a restricted number of enterocytes lining in the mucosa of intestinal villi, also shown on immunofluorescence.

CONCLUSIONS

Our results show that glucose transporters expression in duodenal mucosa of subjects with T1D, except an increased GLUT1, is not different from that observed in healthy controls. The expression of SGLT2 in human duodenal mucosa, although at low intensity, represents a novel finding.

Perfluorocarbon@Porphyrin Nanoparticles for Tumor Hypoxia Relief to Enhance Photodynamic Therapy against Liver Metastasis of Colon Cancer

Photodynamic therapy (PDT) shows great promise for the treatment of colon cancer. However, practically, it is a great challenge to use a nanocarrier for the codelivery of both the photosensitizer and oxygen to improve PDT against PDT-induced hypoxia, which is closely related to tumor metastasis. Hence, an effective strategy was proposed to develop an oxygen self-supplemented PDT nanocarrier based on the ultrasonic dispersion of perfluorooctyl bromide (PFOB) liquid into the preformed porphyrin grafted lipid (PGL) nanoparticles (NPs) with high porphyrin loading content of 38.5%, followed by entrapping oxygen. Interestingly, the orderly arranging mode of porphyrins and alkyl chains in PGL NPs not only guarantees a high efficacy of singlet oxygen generation but also reduces fluorescence loss of porphyrins to enable PGL NPs to be highly fluorescent. More importantly, PFOB liquid was stabilized inside PGL NPs with an ultrahigh loading content of 98.15% due to the strong hydrophobic interaction between PGL and PFOB molecules, facilitating efficient oxygen delivery. Both in vitro and in vivo results demonstrated that the obtained O₂@PFOB@PGL NPs could act as a prominent oxygen reservoir and effectively replenish oxygen into the hypoxic tumors with no need for external stimulation, conducive to augmented singlet oxygen generation, hypoxia relief, and subsequent downregulation of COX-2 expression. As a result, the use of O₂@PFOB@PGL NPs for hypoxia relief dramatically inhibits tumor growth and liver metastasis in an HT-29 colon cancer mouse model. In addition, the O₂@PFOB@PGL NPs could serve as a bimodal contrast agent to enhance fluorescence and CT imaging, visualizing nanoparticle accumulation to guide the subsequent laser irradiation for precise PDT.

Glucose transporters in the kidney in health and disease

The kidneys filter large amounts of glucose. To prevent the loss of this valuable fuel, the tubular system of the kidney, particularly the proximal tubule, has been programmed to reabsorb all filtered glucose. The machinery involves the sodium-glucose cotransporters SGLT2 and SGLT1 on the apical membrane and the facilitative glucose transporter GLUT2 on the basolateral membrane. The proximal tubule also generates new glucose, particularly in the post-absorptive phase but also to enhance bicarbonate formation and maintain acid-base balance. The glucose reabsorbed or formed by the proximal tubule is primarily taken up into peritubular capillaries and returned to the systemic circulation or provided as an energy source to further distal tubular segments that take up glucose by basolateral GLUT1. Recent studies provided insights on the coordination of renal glucose reabsorption, formation, and usage. Moreover, a better understanding of renal glucose transport in disease states is emerging. This includes the kidney in diabetes mellitus, when renal glucose retention becomes maladaptive and contributes to hyperglycemia. Furthermore, enhanced glucose reabsorption is coupled to sodium retention through the sodium-glucose cotransporter SGLT2, which induces secondary deleterious effects. As a consequence, SGLT2 inhibitors are new anti-hyperglycemic drugs that can protect the kidneys and heart from failing. Recent studies discovered unique roles for SGLT1 with implications in acute kidney injury and glucose sensing at the macula densa. This review discusses established and emerging concepts of renal glucose transport, and outlines the need for a better understanding of renal glucose handling in health and disease.

Role of Glucose Transporters in Drug Membrane Transport

BACKGROUND

Glucose is the main energy component of cellular activities. However, as a polar molecule, glucose cannot freely pass through the phospholipid bilayer structure of the cell membrane. Thus, glucose must rely on specific transporters in the membrane. Drugs with a similar chemical structure to glucose may also be transported through this pathway.

METHODS

This review describes the structure, distribution, action mechanism and influencing factors of glucose transporters and introduces the natural drugs mediated by these transporters and drug design strategies on the basis of this pathway.

RESULTS

The glucose transporters involved in glucose transport are of two major types, namely, Na+-dependent and Na+-independent transporters. Glucose transporters can help some glycoside drugs cross the biological membrane. The transmembrane potential is influenced by the chemical structure of drugs. Glucose can be used to modify drugs and improve their ability to cross biological barriers.

CONCLUSION

The membrane transport mechanism of some glycoside drugs may be related to glucose transporters. Glucose modification may improve the oral bioavailability of drugs or achieve targeted drug delivery.

Effects of Ischemia-Reperfusion on Tubular Cell Membrane Transporters and Consequences in Kidney Transplantation

Ischemia-reperfusion (IR)-induced acute kidney injury (IRI) is an inevitable event in kidney transplantation. It is a complex pathophysiological process associated with numerous structural and metabolic changes that have a profound influence on the early and the late function of the transplanted kidney. Proximal tubular cells are particularly sensitive to IRI. These cells are involved in renal and whole-body homeostasis, detoxification processes and drugs elimination by a transporter-dependent, transcellular transport system involving Solute Carriers (SLCs) and ATP Binding Cassettes (ABCs) transporters. Numerous studies conducted mainly in animal models suggested that IRI causes decreased expression and activity of some major tubular transporters. This could favor uremic toxins accumulation and renal metabolic alterations or impact the pharmacokinetic/toxicity of drugs used in transplantation. It is of particular importance to understand the underlying mechanisms and effects of IR on tubular transporters in order to improve the mechanistic understanding of IRI pathophysiology, identify biomarkers of graft function or promote the design and development of novel and effective therapies. Modulation of transporters’ activity could thus be a new therapeutic opportunity to attenuate kidney injury during IR.

Mechanisms Leading to Differential Hypoxia-Inducible Factor Signaling in the Diabetic Kidney: Modulation by SGLT2 Inhibitors and Hypoxia Mimetics

Sodium/glucose cotransporter 2 (SGLT2) inhibitors exert important renoprotective effects in the diabetic kidney, which cannot be readily explained by their actions to lower blood glucose, blood pressure, or glomerular filtration pressures. Their effects to promote erythrocytosis suggest that these drugs act on hypoxia-inducible factors (HIFs; specifically, HIF-1α and HIF-2α), which may underlie their ability to reduce the progression of nephropathy. Type 2 diabetes is characterized by renal hypoxia, oxidative and endoplasmic reticulum stress, and defective nutrient deprivation signaling, which (acting in concert) are poised to cause both activation of HIF-1α and suppression of HIF-2α. This shift in the balance of HIF-1α/HIF-2α activities promotes proinflammatory and profibrotic pathways in glomerular and renal tubular cells. SGLT2 inhibitors alleviate renal hypoxia and cellular stress and enhance nutrient deprivation signaling, which collectively may explain their actions to suppress HIF-1α and activate HIF-2α and thereby augment erythropoiesis, while muting organellar dysfunction, inflammation, and fibrosis. Cobalt chloride, a drug conventionally classified as a hypoxia mimetic, has a profile of molecular and cellular actions in the kidney that is similar to those of SGLT2 inhibitors. Therefore, many renoprotective benefits of SGLT2 inhibitors may be related to their effect to promote oxygen deprivation signaling in the diabetic kidney.

Role of Deranged Energy Deprivation Signaling in the Pathogenesis of Cardiac and Renal Disease in States of Perceived Nutrient Overabundance

Sodium-glucose cotransporter 2 inhibitors reduce the risk of serious heart failure and adverse renal events, but the mechanisms that underlie this benefit are not understood. Treatment with SGLT2 inhibitors is distinguished by 2 intriguing features: ketogenesis and erythrocytosis. Both reflect the induction of a fasting-like and hypoxia-like transcriptional paradigm that is capable of restoring and maintaining cellular homeostasis and survival. In the face of perceived nutrient and oxygen deprivation, cells activate low-energy sensors, which include sirtuin-1 (SIRT1), AMP-activated protein kinase (AMPK), and hypoxia inducible factors (HIFs; especially HIF-2α); these enzymes and transcription factors are master regulators of hundreds of genes and proteins that maintain cellular homeostasis. The activation of SIRT1 (through its effects to promote gluconeogenesis and fatty acid oxidation) drives ketogenesis, and working in concert with AMPK, it can directly inhibit inflammasome activation and maintain mitochondrial capacity and stability. HIFs act to promote oxygen delivery (by stimulating erythropoietin and erythrocytosis) and decrease oxygen consumption. The activation of SIRT1, AMPK, and HIF-2α enhances autophagy, a lysosome-dependent degradative pathway that removes dangerous constituents, particularly damaged mitochondria and peroxisomes, which are major sources of oxidative stress and triggers of cellular dysfunction and death. SIRT1 and AMPK also act on sodium transport mechanisms to reduce intracellular sodium concentrations. It is interesting that type 2 diabetes mellitus, obesity, chronic heart failure, and chronic kidney failure are characterized by the accumulation of intracellular glucose and lipid intermediates that are perceived by cells as indicators of energy overabundance. The cells respond by downregulating SIRT1, AMPK, and HIF-2α, thus leading to an impairment of autophagic flux and acceleration of cardiomyopathy and nephropathy. SGLT2 inhibitors reverse this maladaptive signaling by triggering a state of fasting and hypoxia mimicry, which includes activation of SIRT1, AMPK, and HIF-2α, enhanced autophagic flux, reduced cellular stress, decreased sodium influx into cells, and restoration of mitochondrial homeostasis. This mechanistic framework clarifies the findings of large-scale randomized trials and the close association of ketogenesis and erythrocytosis with the cardioprotective and renoprotective benefits of these drugs.

Empagliflozin alleviates neuronal apoptosis induced by cerebral ischemia/reperfusion injury through HIF-1α/VEGF signaling pathway

Ischemic stroke is a serious condition associated with severe functional disability and high mortality, however; effective therapy remains elusive. Empagliflozin, a sodium-glucose cotransporter 2 inhibitor, has been shown to exert additional non-glycemic benefits including anti-apoptotic effects in different disease settings. Thereby, this study was designed to investigate the ameliorative effect of empagliflozin on the neuronal apoptosis exhibited in cerebral ischemia/reperfusion (I/R) in a rat model targeting HIF-1α/VEGF signaling which is involved in this insult. Global cerebral I/R injury was induced in male Wistar rats through occlusion of the bilateral common carotid arteries for 30 min followed by one-hour reperfusion. Empagliflozin doses of 1 and 10 mg/kg were administered 1 and 24 h after reperfusion. In I/R-injured rats, empagliflozin treatments significantly reduced infarct size and enhanced neurobehavioral functions in a dose-dependent manner. The drug alleviated neuronal death and cerebral injury inflicted by global ischemia as it suppressed neuronal caspase-3 protein expression. In parallel, protein expressions of HIF-1α and its downstream mediator VEGF were upregulated in the ischemic brain following empagliflozin treatment. The results indicated that empagliflozin attenuates cerebral I/R-induced neuronal death via the HIF-1α/VEGF cascade.

Functional Expression of Sodium-Dependent Glucose Transporter in Amelogenesis

Glucose is an essential source of energy for mammalian cells and is transported into the cells by glucose transporters. There are 2 types of glucose transporters: one is a passive glucose transporter, GLUT (SLC2A), and the other is a sodium-dependent active glucose transporter, SGLT (SLC5A). We previously reported that the expression of GLUTs during tooth development is precisely and spatiotemporally controlled and that the glucose uptake mediated by GLUT1 plays a crucial role in early tooth morphogenesis and tooth size determination. This study aimed to clarify the localization and roles of SGLT1 and SGLT2 in murine ameloblast differentiation by using immunohistochemistry, immunoelectron microscopy, an in vitro tooth organ culture experiment, and in vivo administration of an inhibitor of SGLT1/2, phloridzin. SGLT1, which has high affinity with glucose, was immunolocalized in the early secretory ameloblasts and the ruffle-ended ameloblasts in the maturation stage. However, SGLT2, which has high glucose transport capacity, was observed in the stratum intermedium, papillary layer, and ameloblasts at the maturation stage and colocalized with Na⁺-K⁺-ATPase. The inhibition of SGLT1/2 by phloridzin in the tooth germs induced the disturbance of ameloblast differentiation and enamel matrix formation both in vitro (organ culture) and in vivo (mouse model). The expression of SGLT1 and SGLT2 was significantly upregulated in hypoxic conditions in the ameloblast-lineage cells. These findings suggest that the active glucose uptake mediated by SGLT1 and SGLT2 is strictly regulated and dependent on the intra- and extracellular microenvironments during tooth morphogenesis and that the appropriate passive and active glucose transport is an essential event in amelogenesis.

Critical examination of mechanisms underlying the reduction in heart failure events with SGLT2 inhibitors: identification of a molecular link between their actions to stimulate erythrocytosis and to alleviate cellular stress

Sodium-glucose co-transporter 2 (SGLT2) inhibitors reduce the risk of serious heart failure events, even though SGLT2 is not expressed in the myocardium. This cardioprotective benefit is not related to an effect of these drugs to lower blood glucose, promote ketone body utilization or enhance natriuresis, but it is linked statistically with their action to increase haematocrit. SGLT2 inhibitors increase both erythropoietin and erythropoiesis, but the increase in red blood cell mass does not directly prevent heart failure events. Instead, erythrocytosis is a biomarker of a state of hypoxia mimicry, which is induced by SGLT2 inhibitors in manner akin to cobalt chloride. The primary mediators of the cellular response to states of energy depletion are sirtuin-1 and hypoxia-inducible factors (HIF-1α/HIF-2α). These master regulators promote the cellular adaptation to states of nutrient and oxygen deprivation, promoting mitochondrial capacity and minimizing the generation of oxidative stress. Activation of sirtuin-1 and HIF-1α/HIF-2α also stimulates autophagy, a lysosome-mediated degradative pathway that maintains cellular homoeostasis by removing dangerous constituents (particularly unhealthy mitochondria and peroxisomes), which are a major source of oxidative stress and cardiomyocyte dysfunction and demise. SGLT2 inhibitors can activate SIRT-1 and stimulate autophagy in the heart, and thereby, favourably influence the course of cardiomyopathy. Therefore, the linkage between erythrocytosis and the reduction in heart failure events with SGLT2 inhibitors may be related to a shared underlying molecular mechanism that is triggered by the action of these drugs to induce a perceived state of oxygen and nutrient deprivation.

The tubular hypothesis of nephron filtration and diabetic kidney disease

Kidney size and glomerular filtration rate (GFR) often increase with the onset of diabetes, and elevated GFR is a risk factor for the development of diabetic kidney disease. Hyperfiltration mainly occurs in response to signals passed from the tubule to the glomerulus: high levels of glucose in the glomerular filtrate drive increased reabsorption of glucose and sodium by the sodium-glucose cotransporters SGLT2 and SGLT1 in the proximal tubule. Passive reabsorption of chloride and water also increases. The overall capacity for proximal reabsorption is augmented by growth of the proximal tubule, which (alongside sodium-glucose cotransport) further limits urinary glucose loss. Hyperreabsorption of sodium and chloride induces tubuloglomerular feedback from the macula densa to increase GFR. In addition, sodium-glucose cotransport by SGLT1 on macula densa cells triggers the production of nitric oxide, which also contributes to glomerular hyperfiltration. Although hyperfiltration restores sodium and chloride excretion it imposes added physical stress on the filtration barrier and increases the oxygen demand to drive reabsorption. Tubular growth is associated with the development of a senescence-like molecular signature that sets the stage for inflammation and fibrosis. SGLT2 inhibitors attenuate the proximal reabsorption of sodium and glucose, normalize tubuloglomerular feedback signals and mitigate hyperfiltration. This tubule-centred model of diabetic kidney physiology predicts the salutary effect of SGLT2 inhibitors on hard renal outcomes, as shown in large-scale clinical trials.

Renal protective effect of nebivolol in rat models of acute renal injury: role of sodium glucose co-transporter 2

BACKGROUND

Upregulation of the sodium glucose co-transporter (SGLT2) is implicated in acute renal injury (ARI) progression and is regulated by extracellular signal-regulated kinase (ERK), hypoxia-inducible factor 1 alpha (HIF1α) or prostaglandin E2 (PGE2). This study aimed to assess the possible protective effect of nebivolol on renal ischemia/reperfusion (IR) and glycerol-induced ARI targeting SGLT2 via modulating the ERK-HIF1α pathway.

METHODS

Rats were divided into control, sham, IR or nebivolol-treated group, in which rats were treated with nebivolol (10 mg/kg) for 3 days prior to the induction of IR. The rats were subjected to renal ischemia by bilateral clamping of the pedicles for 45 min, followed by 24 h reperfusion. Another group of rats received the vehicle or nebivolol (10 mg/kg) for 3 days followed by injection of 50% glycerol (8 ml/kg, IM) or saline. Kidney function tests, systolic blood pressure (SBP), oxidative stress markers [malondialdehyde (MDA) and NADPH oxidase] and kidney levels of nitric oxide (NO), inducible nitric oxide synthase (iNOS), HIF1α, ERK phosphorylation and PGE2 were determined. Additionally, renal sections were used for histological grading of renal injury and immunological expression of SGLT2.

RESULTS

ARI rats showed significantly increased SBP, poor kidney function tests, increased oxidative stress, iNOS, NO, HIF1α levels, decreased PGE2 and ERK phosphorylation and upregulation of SGLT2 expression. Nebivolol treatment protected against the kidney damage both on the biochemical and histological levels.

CONCLUSION

Nebivolol has a direct renoprotective effect, at least in part, by down-regulating SGLT2 possibly via modulating HIF1α, ERK activity and PGE2 production.

Autophagy stimulation and intracellular sodium reduction as mediators of the cardioprotective effect of sodium-glucose cotransporter 2 inhibitors

In five large-scale trials involving >40 000 patients, sodium-glucose cotransporter 2 (SGLT2) inhibitors decreased the risk of serious heart failure events by 25-40%. This effect cannot be explained by control of hyperglycaemia, since it is not observed with antidiabetic drugs with greater glucose-lowering effects. It cannot be attributed to ketogenesis, since it is not causally linked to ketone body production, and the benefit is not enhanced in patients with diabetes. The effect cannot be ascribed to a natriuretic action, since SGLT2 inhibitors decrease natriuretic peptides only modestly, and they reduce cardiovascular death, a benefit that diuretics do not possess. Although SGLT2 inhibitors increase red blood cell mass, enhanced erythropoiesis does not favourably influence the course of heart failure. By contrast, experimental studies suggest that SGLT2 inhibitors may reduce intracellular sodium, thereby preventing oxidative stress and cardiomyocyte death. Additionally, SGLT2 inhibitors induce a transcriptional paradigm that mimics nutrient and oxygen deprivation, which includes activation of adenosine monophosphate-activated protein kinase, sirtuin-1, and/or hypoxia-inducible factors-1α/2α. The interplay of these mediators stimulates autophagy, a lysosomally-mediated degradative pathway that maintains cellular homeostasis. Autophagy-mediated clearance of damaged organelles reduces inflammasome activation, thus mitigating cardiomyocyte dysfunction and coronary microvascular injury. Interestingly, the action of hypoxia-inducible factors-1α/2α to both stimulate erythropoietin and induce autophagy may explain why erythrocytosis is strongly correlated with the reduction in heart failure events. Therefore, the benefits of SGLT2 inhibitors on heart failure may be mediated by a direct cardioprotective action related to modulation of pathways responsible for cardiomyocyte homeostasis.

Empagliflozin Protects Against Proximal Renal Tubular Cell Injury Induced by High Glucose via Regulation of Hypoxia-Inducible Factor 1-Alpha

BACKGROUND

Evidence from both animal and human studies clearly supports the renal beneficial effects of empagliflozin (emp), a sodium glucose co-transporter 2 (SGLT2) inhibitor, but the mechanism in which it exerts its effect is not well understood. In this study, we investigated the capability of emp on reducing hyperglycemia-induced renal proximal tubular epithelial cells injury and we evaluated if the renoprotective effect of emp associates with hypoxia-inducible factor-1α (HIF-1α).

MATERIALS AND METHODS

Human kidney cell lines (HK-2 cells) were incubated in normoxia, high glucose with or without emp treatment for 72 hours to evaluate the induction of HIF-1α, glucose transporter-1, SGLT2, the fibrosis signal pathway and epithelial-mesenchymal transition (EMT) markers.

RESULTS

High glucose (HG) increased expression of Collagen IV, Fibronectin, transforming growth factor-beta1 (TGF-β1). However, emp treatment remarkably decreased expression of TGF-β1, accumulation of extracellular matrix proteins (Fibronectin, Collagen IV), as well as (phosphorylated-smad3) P-smad3. HG increased SGLT2 protein expression compared to normal glucose (NG) while emp significantly decreased SGLT2 expression. Furthermore, emp decreased high glucose-induced alpha-smooth muscle actin (α-SMA) expression and reversed epithelial marker (E-catherin) suppression induced by high glucose. In addition, emp treatment for 72 h increased expression of HIF-1α protein (95% CI: -0.5918 to -0.002338, at 100nM, P < 0.05, 95% CI -0.6631 to -0.07367 at 500nM, P < 0.05) in hyperglycemic normoxic HK-2 cells. Furthermore, we observed increased expression of GLUT-1 protein after emp treatment and remarkably decreased cell proliferation.

CONCLUSION

Emp treatment protected proximal renal tubular cells injury induced by high glucose. Induction of HIF-1α expression by emp may play an essential role in the protection of high glucose-induced proximal renal tubular epithelial cells injury.

Doxazosin down-regulates sodium-glucose cotransporter-2 and exerts a renoprotective effect in rat models of acute renal injury

Sodium-glucose cotransporter-2 (SGLT2) is known to be involved in the progression of acute renal injury (ARI) and is regulated by different mediators in the kidneys including extracellular signal-regulated kinase (ERK), hypoxia-inducible factor 1 alpha (HIF1α) and prostaglandin E2 (PGE2). In the present study, we investigated the possible protective effect of doxazosin on renal ischaemia/reperfusion (IR) and glycerol-induced ARI by determining its effect on SGLT2 via modifying ERK-HIF1α pathway and/or PGE2. Rats were divided into control, sham or IR where the rats received the vehicle, doxazosin (8 mg/kg) or the SGLT2 inhibitor, dapagliflozin (10 mg/kg) for 3 days followed by 45 minutes bilateral renal ischaemia then 24 hours reperfusion. Another group of rats received the vehicle, doxazosin or dapagliflozin for three days followed by injection of 50% glycerol (8 mL/kg, IM) or saline. Kidney function tests, systolic blood pressure (SBP), oxidative stress markers (malondialdehyde [MDA] and NADPH oxidase), nitric oxide (NO), inducible nitric oxide synthase (iNOS), HIF1α, ERK phosphorylation and PGE2 levels were determined. Additionally, renal sections were used for immunological expression of SGLT2. ARI rats showed significantly increased SBP; worsened kidney function tests; increased oxidative stress, iNOS, NO, HIF1α levels; and decreased PGE2 and ERK phosphorylation along with up-regulated SGLT2. Doxazosin treatment protected against the kidney damage and attenuated the associated biochemical changes. Doxazosin has a direct renoprotective effect possibly by down-regulating SGLT2.

Aspirin, a Potential GLUT1 Inhibitor in a Vascular Endothelial Cell Line

Recent epidemiological and preclinical studies have revealed that aspirin possesses antitumor properties; one of the mechanisms results from inhibition of angiogenesis. However, the underlying mechanisms of such action remain to be elucidated, in particular, the effect of aspirin on glucose metabolism of vascular endothelial cells (ECs) has not yet been reported. Herein, we demonstrate that glucose transporter 1 (GLUT1), a main glucose transporter in ECs, can be down-regulated by aspirin. Exposure to 4-mM aspirin significantly decreased GLUT1 at the mRNA and protein level, resulting in impaired glucose uptake capacity in vascular ECs. In addition, we also showed that exposure to 4-mM aspirin led to an inhibition of intracellular ATP and lactate synthesis in vascular ECs, and a down-regulation of the phosphorylation level of NF-κB p65 was observed. Taken together, these findings indicate 4-mM aspirin inhibits glucose uptake and glucose metabolism of vascular ECs through down-regulating GLUT1 expression and suggest that GLUT1 has potential to be a target for aspirin in vascular ECs.

Early-Life Iron Deficiency Alters Glucose Transporter-1 Expression in the Adult Rodent Hippocampus

BACKGROUND

Early-life iron deficiency (ID) impairs hippocampal energy production. Whether there are changes in glucose transporter (GLUT) expression is not known.

OBJECTIVE

The aim of this study was to investigate whether early-life ID and the treatment iron dose alter brain regional GLUT expression in adult rats and mice.

METHODS

In Study 1, ID was induced in male and female Sprague Dawley rat pups by feeding dams a 3-mg/kg iron diet during gestation and the first postnatal week, followed by treatment using low-iron [3-10 mg/kg; formerly iron-deficient (FID)-10 group], standard-iron (40-mg/kg; FID-40 group), or high-iron (400-mg/kg; FID-400 group) diets until weaning. The control group received the 40 mg/kg iron diet. GLUT1, GLUT3, hypoxia-inducible factor (HIF)-1α, and prolyl-hydroxylase-2 (PHD2) mRNA and protein expression in the cerebral cortex, hippocampus, striatum, cerebellum, and hypothalamus were determined at adulthood. In Study 2, the role of hippocampal ID in GLUT expression was examined by comparing the Glut1, Glut3, Hif1α, and Phd2 mRNA expression in adult male and female wild-type (WT) and nonanemic hippocampal iron-deficient and iron-replete dominant negative transferrin receptor 1 (DNTfR1-/-) transgenic mice.

RESULTS

In Study 1, Glut1, Glut3, and Hif1α mRNA, and GLUT1 55-kDa protein expression was upregulated 20-33% in the hippocampus of the FID-10 group but not the FID-40 group, relative to the control group. Hippocampal Glut1 mRNA (-39%) and GLUT1 protein (-30%) expression was suppressed in the FID-400 group, relative to the control group. Glut1 and Glut3 mRNA expression was not altered in the other brain regions in the 3 FID groups. In Study 2, hippocampal Glut1 (+14%) and Hif1α (+147%) expression was upregulated in the iron-deficient DNTfR1-/- mice, but not in the iron-replete DNTfR1-/- mice, relative to the WT mice (P < 0.05, all).

CONCLUSIONS

Early-life ID is associated with altered hippocampal GLUT1 expression in adult rodents. The mouse study suggests that tissue ID is potentially responsible.

Cardiac ischemia-reperfusion injury under insulin-resistant conditions: SGLT1 but not SGLT2 plays a compensatory protective role in diet-induced obesity

BACKGROUND

Recent large-scale clinical trials have shown that SGLT2-inhibitors reduce cardiovascular events in diabetic patients. However, the regulation and functional role of cardiac sodium-glucose cotransporter (SGLT1 is the dominant isoform) compared with those of other glucose transporters (insulin-dependent GLUT4 is the major isoform) remain incompletely understood. Given that glucose is an important preferential substrate for myocardial energy metabolism under conditions of ischemia-reperfusion injury (IRI), we hypothesized that SGLT1 contributes to cardioprotection during the acute phase of IRI via enhanced glucose transport, particularly in insulin-resistant phenotypes.

METHODS AND RESULTS

The hearts from mice fed a high-fat diet (HFD) for 12 weeks or a normal-fat diet (NFD) were perfused with either the non-selective SGLT-inhibitor phlorizin or selective SGLT2-inhibitors (tofogliflozin, ipragliflozin, canagliflozin) during IRI using Langendorff model. After ischemia-reperfusion, HFD impaired left ventricular developed pressure (LVDP) recovery compared with the findings in NFD. Although phlorizin-perfusion impaired LVDP recovery in NFD, a further impaired LVDP recovery and a dramatically increased infarct size were observed in HFD with phlorizin-perfusion. Meanwhile, none of the SGLT2-inhibitors significantly affected cardiac function or myocardial injury after ischemia-reperfusion under either diet condition. The plasma membrane expression of GLUT4 was significantly increased after IRI in NFD but was substantially attenuated in HFD, the latter of which was associated with a significant reduction in myocardial glucose uptake. In contrast, SGLT1 expression at the plasma membrane remained constant during IRI, regardless of the diet condition, whereas SGLT2 was not detected in the hearts of any mice. Of note, phlorizin considerably reduced myocardial glucose uptake after IRI, particularly in HFD.

CONCLUSIONS

Cardiac SGLT1 but not SGLT2 plays a compensatory protective role during the acute phase of IRI via enhanced glucose uptake, particularly under insulin-resistant conditions, in which IRI-induced GLUT4 upregulation is compromised.

Elevated glucose concentration in culture media decreases membrane trafficking of SGLT2 in LLC-PK1 cells via a cAMP/PKA-dependent pathway

Na⁺-dependent glucose reabsorption in the renal proximal tubule is dynamically regulated by changes in blood glucose levels. There is, however, a disparity in reports studying the relationship between hyperglycemia and Na⁺-glucose-linked transporter (SGLT) function and expression. Similarly, manipulation of the glucose content in growth media of cultured renal cells has been shown to influence SGLT activity. In this investigation, SGLT activity was significantly lower in proximal tubule LLC-PK₁ cells cultured in medium containing 17.5 than 5 mM glucose. α-Methyl d-glucopyranoside (AMG) transport kinetics showed reduced apparent V_max and K_m in cells grown in 17.5 mM glucose. SGLT2 was identified as the isoform responsible for glucose transport, and protein expression analyses showed decreased apical membrane localization of SGLT2 in cells grown in 17.5 mM glucose, explaining the reduced activity. Multiple signaling pathways have been implicated in regulation of SGLT activity and trafficking. Elevated media glucose decreased intracellular cAMP and PKA activation, leading to decreased SGLT2 trafficking into the plasma membrane, which was reversed after treatment with 1 µM forskolin. The effects of media glucose on SGLT activity were found to be dependent on p38 MAPK activation due to PKA-mediated signaling. Glucose-modulated AMG uptake is reversible and was associated with altered SGLT2 membrane trafficking and cAMP alterations. In summary, elevated glucose concentrations in culture medium decrease SGLT activity in LLC-PK₁ cells by reducing membrane trafficking of SGLT2 via decreasing intracellular cAMP, resulting in a lowered PKA-dependent phosphorylation of p38 MAPK.

Sodium glucose cotransporter 2 inhibition and renal ischemia: implications for future clinical trials

Sodium glucose cotransporter 2 inhibitors are increasingly being recognized for renal protective effects that are largely independent of hemoglobin A1c-lowering or glucosuria-related endpoints. Accordingly, there is growing interest in potential renal benefits with sodium glucose cotransporter 2 inhibitors in nondiabetic patients to take advantage of natriuresis-mediated effects on blood pressure, proteinuria, and renal function. In this issue of Kidney International, Zhang et al. report renoprotective effects with the sodium glucose cotransporter 2 inhibitor lusogliflozin in an ischemia-reperfusion injury model under nondiabetic conditions, thereby providing important mechanistic insights into the use of these agents in chronic kidney disease.

Hypoxia increases glucose transporter 1 expression in bovine corpus luteum at the early luteal stage

A major role of the corpus luteum (CL) is to produce progesterone (P4). The CL has immature vasculature shortly after ovulation, suggesting it exists under hypoxic conditions. Hypoxia-inducible factor-1 (HIF1) induces the expression of glucose transporter 1 (GLUT1). To clarify the physiological roles of GLUT1 in bovine CL, we examined GLUT1 mRNA expression in the CL under hypoxic conditions by quantitative RT-PCR. We also measured the effects of glucose (0-25 mM) and GLUT1 inhibitors (cytochalasin B, STF-31) on P4 production in bovine luteal cells. GLUT1 mRNA expression in bovine CL was higher at the early luteal stage compared to the other later stages. Hypoxia (3% O₂) increased GLUT1 mRNA expression in early luteal cells, but not in mid luteal cells. Glucose (0-25 mM) increased P4 production in early luteal cells, but not in mid luteal cells. Both GLUT1 inhibitors decreased P4 production in early and mid luteal cells. Overall, the results suggest that GLUT1 (possibly induced by hypoxic conditions in the early CL) plays a role in the establishment and development of bovine CL, especially in supporting luteal P4 synthesis at the early luteal stage.

Sodium-glucose co-transporter (SGLT)2 and SGLT1 renal expression in patients with type 2 diabetes

AIM

To quantify the expression of sodium-glucose co-transporter (SGLT)2 and SGLT1, their cognate basolateral transporters, GLUT2 and GLUT1, and the transcriptional regulator of SGLTs in renal tissue obtained from people with T2DM and a group of well-matched people without diabetes.

METHODS

We measured SGLT2 and SGLT1 expression in unaffected renal tissue from 19 people with T2DM and 20 people without diabetes, matched for age and estimated glomerular filtration rate (controls), undergoing unilateral nephrectomy. Expression of SGLT2 and SGLT1, as well as that of GLUT2 and GLUT1, was quantified using real-time and digital PCR; an affinity-purified antibody against human SGLT2 was used to localize SGLT2 by immunohistochemistry.

RESULTS

SGLT2 expression was higher in control than T2DM tissue (median [interquartile range] target/β-actin 1.62 [2.02] vs 0.67 [0.61]; P < .0001), and SGLT1 trended in the same direction (0.98 [1.19] vs 0.44 [0.48]; P = .08). Immunohistochemistry clearly localized SGLT2 to the tubular brush-border membranes, and was semi-quantitatively stronger in control than T2DM tissue (5.0 [1.0] vs 4.0 [1.0] score units; P = .043). GLUT2 (control vs T2DM: 1.00 [0.69] vs 0.49 [0.36]) and GLUT1 expression (control vs T2DM: 0.86 [0.73] vs 0.35 [0.30]; P = .0007 for both) were closely correlated with those of the respective SGLT partner. Hypoxia-inducible factor 1α, more abundant in control than T2DM tissue, might be a transcription factor involved in the modulation of SGLT2 expression.

CONCLUSIONS

In whole renal tissue, expressions of SGLT2/GLUT2 and SGLT1/GLUT1 are coupled and slightly lower in typical people with T2DM as compared with well-matched people without diabetes.

Dapagliflozin, SGLT2 Inhibitor, Attenuates Renal Ischemia-Reperfusion Injury

Dapagliflozin, a new type of drug used to treat diabetes mellitus (DM), is a sodium/glucose cotransporter 2 (SGLT2) inhibitor. Although some studies showed that SGLT2 inhibition attenuated reactive oxygen generation in diabetic kidney the role of SGLT2 inhibition is unknown. We evaluated whether SLT2 inhibition has renoprotective effects in ischemia-reperfusion (IR) models. We evaluated whether dapagliflozin reduces renal damage in IR mice model. In addition, hypoxic HK2 cells were treated with or without SGLT2 inhibitor to investigate cell survival, the apoptosis signal pathway, and the induction of hypoxia-inducible factor 1 (HIF1) and associated proteins. Dapagliflozin improved renal function. Dapagliflozin reduced renal expression of Bax, renal tubule injury and TUNEL-positive cells and increased renal expression of HIF1 in IR-injured mice. HIF1 inhibition by albendazole negated the renoprotective effects of dapagliflozin treatment in IR-injured mice. In vitro, dapagliflozin increased the expression of HIF1, AMP-activated protein kinase (AMPK), and ERK and increased cell survival of hypoxic HK2 cells in a dose-dependent manner. In conclusion, dapagliflozin attenuates renal IR injury. HIF1 induction by dapagliflozin may play a role in renoprotection against renal IR injury.

Sodium-glucose cotransport

PURPOSE OF REVIEW

Sodium-glucose cotransporters (SGLTs) are important mediators of glucose uptake across apical cell membranes. SGLT1 mediates almost all sodium-dependent glucose uptake in the small intestine, while in the kidney SGLT2, and to a lesser extent SGLT1, account for more than 90% and nearly 3%, respectively, of glucose reabsorption from the glomerular ultrafiltrate. Although the recent availability of SGLT2 inhibitors for the treatment of diabetes mellitus has increased the number of clinical studies, this review has a focus on mechanisms contributing to the cellular regulation of SGLTs.

RECENT FINDINGS

Studies have focused on the regulation of SGLT expression under different physiological/pathophysiological conditions, for example diet, age or diabetes mellitus. Several studies provide evidence of SGLT regulation via cyclic adenosine monophosphate/protein kinase A, protein kinase C, glucagon-like peptide 2, insulin, leptin, signal transducer and activator of transcription-3 (STAT3), phosphoinositide-3 kinase (PI3K)/Akt, mitogen-activated protein kinases (MAPKs), nuclear factor-kappaB (NF-kappaB), with-no-K[Lys] kinases/STE20/SPS1-related proline/alanine-rich kinase (Wnk/SPAK) and regulatory solute carrier protein 1 (RS1) pathways.

SUMMARY

SGLT inhibitors are important drugs for glycemic control in diabetes mellitus. Although the contribution of SGLT1 for absorption of glucose from the intestine as well as SGLT2/SGLT1 for renal glucose reabsorption has been comprehensively defined, this review provides an up-to-date outline for the mechanistic regulation of SGLT1/SGLT2.

Glucosamine-induced Sp1 O-GlcNAcylation ameliorates hypoxia-induced SGLT dysfunction in primary cultured renal proximal tubule cells

The aim of this study is to determine whether GlcN could recover the endoplasmic reticulum (ER) stress-induced dysfunction of Na(+) /glucose cotransporter (SGLT) in renal proximal tubule cells (PTCs) under hypoxia. With the rabbit model, the renal ischemia induced tubulointerstitial abnormalities and decreased SGLTs expression in tubular brush-border, which were recovered by GlcN. Thus, the protective mechanism of GlcN against renal ischemia was being examined by using PTCs. Hypoxia decreased the level of protein O-GlcNAc and the expression of O-GlcNAc transferase (OGT) while increased O-GlcNAcase (OGA) and these were reversed by GlcN. Hypoxia also decreased the expression of SGLTs (SGLT1 and 2) and [(14) C]-α-methyl-D-glucopyranoside (α-MG) uptake which were recovered by GlcN and PUGNAc (OGA inhibitor). Hypoxia enhanced reactive oxygen species (ROS) and then ER stress proteins, glucose-regulated protein 78 (GRP78), and C/EBP-homologous protein (CHOP). However, the expression of GRP78 increased till 6 h and then decreased whereas CHOP increased gradually. Moreover, decreased GRP78 and increased CHOP were reversed by NAC (antioxidant) and GlcN. GlcN ameliorated hypoxia-induced decrease of O-GlcNAc modification of Sp1 but OGT or Sp1 siRNAs blocked the recovery effect of GlcN on SGLT expression and α-MG uptake. In addition, hypoxia-decreased GRP78 and HIF-1α expression was reversed by GlcN but OGT siRNA or Sp1 siRNA ameliorated the effect of GlcN. When PTCs were transfected with GRP78 siRNA or HIF-1α siRNA, SGLT expression and α-MG uptake was decreased. Taken together, these data suggest that GlcN-induced O-GlcNAc modified Sp1 with stimulating GRP78 and HIF-1α activity ameliorate hypoxia-induced SGLT dysfunction in renal PTCs. J. Cell. Physiol. 229: 1557-1568, 2014. © 2014 Wiley Periodicals, Inc.