Renal Mitochondrial ATP Transporter Ablation Ameliorates Obesity-Induced CKD

SIGNIFICANCE STATEMENT

This study sheds light on the central role of adenine nucleotide translocase 2 (ANT2) in the pathogenesis of obesity-induced CKD. Our data demonstrate that ANT2 depletion in renal proximal tubule cells (RPTCs) leads to a shift in their primary metabolic program from fatty acid oxidation to aerobic glycolysis, resulting in mitochondrial protection, cellular survival, and preservation of renal function. These findings provide new insights into the underlying mechanisms of obesity-induced CKD and have the potential to be translated toward the development of targeted therapeutic strategies for this debilitating condition.

BACKGROUND

The impairment in ATP production and transport in RPTCs has been linked to the pathogenesis of obesity-induced CKD. This condition is characterized by kidney dysfunction, inflammation, lipotoxicity, and fibrosis. In this study, we investigated the role of ANT2, which serves as the primary regulator of cellular ATP content in RPTCs, in the development of obesity-induced CKD.

METHODS

We generated RPTC-specific ANT2 knockout ( RPTC-ANT2-/- ) mice, which were then subjected to a 24-week high-fat diet-feeding regimen. We conducted comprehensive assessment of renal morphology, function, and metabolic alterations of these mice. In addition, we used large-scale transcriptomics, proteomics, and metabolomics analyses to gain insights into the role of ANT2 in regulating mitochondrial function, RPTC physiology, and overall renal health.

RESULTS

Our findings revealed that obese RPTC-ANT2-/- mice displayed preserved renal morphology and function, along with a notable absence of kidney lipotoxicity and fibrosis. The depletion of Ant2 in RPTCs led to a fundamental rewiring of their primary metabolic program. Specifically, these cells shifted from oxidizing fatty acids as their primary energy source to favoring aerobic glycolysis, a phenomenon mediated by the testis-selective Ant4.

CONCLUSIONS

We propose a significant role for RPTC-Ant2 in the development of obesity-induced CKD. The nullification of RPTC-Ant2 triggers a cascade of cellular mechanisms, including mitochondrial protection, enhanced RPTC survival, and ultimately the preservation of kidney function. These findings shed new light on the complex metabolic pathways contributing to CKD development and suggest potential therapeutic targets for this condition.

Hyperglucagonaemia in diabetes: altered amino acid metabolism triggers mTORC1 activation, which drives glucagon production

AIM/HYPOTHESIS

Hyperglycaemia is associated with alpha cell dysfunction, leading to dysregulated glucagon secretion in type 1 and type 2 diabetes; however, the mechanisms involved are still elusive. The nutrient sensor mammalian target of rapamycin complex 1 (mTORC1) plays a major role in the maintenance of alpha cell mass and function. We studied the regulation of alpha cell mTORC1 by nutrients and its role in the development of hyperglucagonaemia in diabetes.

METHODS

Alpha cell mTORC1 activity was assessed by immunostaining for phosphorylation of its downstream target, the ribosomal protein S6, and glucagon, followed by confocal microscopy on pancreatic sections and flow cytometry on dispersed human and mouse islets and the alpha cell line, αTC1-6. Metabolomics and metabolic flux were studied by 13C glucose labelling in 2.8 or 16.7 mmol/l glucose followed by LC-MS analysis. To study the role of mTORC1 in mediating hyperglucagonaemia in diabetes, we generated an inducible alpha cell-specific Rptor knockout in the Akita mouse model of diabetes and tested the effects on glucose tolerance by IPGTT and on glucagon secretion.

RESULTS

mTORC1 activity was increased in alpha cells from diabetic Akita mice in parallel to the development of hyperglycaemia and hyperglucagonaemia (two- to eightfold increase). Acute exposure of mouse and human islets to amino acids stimulated alpha cell mTORC1 (3.5-fold increase), whereas high glucose concentrations inhibited mTORC1 (1.4-fold decrease). The mTORC1 response to glucose was abolished in human and mouse diabetic alpha cells following prolonged islet exposure to high glucose levels, resulting in sustained activation of mTORC1, along with increased glucagon secretion. Metabolomics and metabolic flux analysis showed that exposure to high glucose levels enhanced glycolysis, glucose oxidation and the synthesis of glucose-derived amino acids. In addition, chronic exposure to high glucose levels increased the expression of Slc7a2 and Slc38a4, which encode amino acid transporters, as well as the levels of branched-chain amino acids and methionine cycle metabolites (~1.3-fold increase for both). Finally, conditional Rptor knockout in alpha cells from adult diabetic mice inhibited mTORC1, thereby inhibiting glucagon secretion (~sixfold decrease) and improving diabetes, despite persistent insulin deficiency.

CONCLUSIONS/INTERPRETATION

Alpha cell exposure to hyperglycaemia enhances amino acid synthesis and transport, resulting in sustained activation of mTORC1, thereby increasing glucagon secretion. mTORC1 therefore plays a major role in mediating alpha cell dysfunction in diabetes.

DATA AVAILABILITY

All sequencing data are available from the Gene Expression Omnibus (GEO) repository (accession no. GSE154126; https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE154126 ).

Raptor levels are critical for β-cell adaptation to a high-fat diet in male mice

OBJECTIVE

The essential role of raptor/mTORC1 signaling in β-cell survival and insulin processing has been recently demonstrated using raptor knock-out models. Our aim was to evaluate the role of mTORC1 function in adaptation of β-cells to insulin resistant state.

METHOD

Here, we use mice with heterozygous deletion of raptor in β-cells (βraHet) to assess whether reduced mTORC1 function is critical for β-cell function in normal conditions or during β-cell adaptation to high-fat diet (HFD).

RESULTS

Deletion of a raptor allele in β-cells showed no differences at the metabolic level, islets morphology, or β-cell function in mice fed regular chow. Surprisingly, deletion of only one allele of raptor increases apoptosis without altering proliferation rate and is sufficient to impair insulin secretion when fed a HFD. This is accompanied by reduced levels of critical β-cell genes like Ins1, MafA, Ucn3, Glut2, Glp1r, and specially PDX1 suggesting an improper β-cell adaptation to HFD.

CONCLUSION

This study identifies that raptor levels play a key role in maintaining PDX1 levels and β-cell function during the adaptation of β-cell to HFD. Finally, we identified that Raptor levels regulate PDX1 levels and β-cell function during β-cell adaptation to HFD by reduction of the mTORC1-mediated negative feedback and activation of the AKT/FOXA2/PDX1 axis. We suggest that Raptor levels are critical to maintaining PDX1 levels and β-cell function in conditions of insulin resistance in male mice.

Time-dependent effects of endogenous hyperglucagonemia on glucose homeostasis and hepatic glucagon action

Elevation of glucagon levels and increase in α cell proliferation is associated with states of hyperglycemia in diabetes. A better understanding of the molecular mechanisms governing glucagon secretion could have major implications for understanding abnormal responses to hypoglycemia in patients with diabetes and provide novel avenues for diabetes management. Using mice with inducible induction of Rheb1 in α cells (αRhebTg mice), we showed that short-term activation of mTORC1 signaling is sufficient to induce hyperglucagonemia through increased glucagon secretion. Hyperglucagonemia in αRhebTg mice was also associated with an increase in α cell size and mass expansion. This model allowed us to identify the effects of chronic and short-term hyperglucagonemia on glucose homeostasis by regulating glucagon signaling in the liver. Short-term hyperglucagonemia impaired glucose tolerance, which was reversible over time. Liver glucagon resistance in αRhebTg mice was associated with reduced expression of the glucagon receptor and genes involved in gluconeogenesis, amino acid metabolism, and urea production. However, only genes regulating gluconeogenesis returned to baseline upon improvement of glycemia. Overall, these studies demonstrate that hyperglucagonemia exerts a biphasic response on glucose metabolism: Short-term hyperglucagonemia lead to glucose intolerance, whereas chronic exposure to glucagon reduced hepatic glucagon action and improved glucose tolerance.

Mapping the metabolic reprogramming induced by sodium-glucose cotransporter 2 inhibition

Diabetes is associated with increased risk for kidney disease, heart failure, and mortality. Sodium-glucose cotransporter 2 inhibitors (SGLT2i) prevent these adverse outcomes; however, the mechanisms involved are not clear. We generated a roadmap of the metabolic alterations that occur in different organs in diabetes and in response to SGLT2i. In vivo metabolic labeling with 13C-glucose in normoglycemic and diabetic mice treated with or without dapagliflozin, followed by metabolomics and metabolic flux analyses, showed that, in diabetes, glycolysis and glucose oxidation are impaired in the kidney, liver, and heart. Treatment with dapagliflozin failed to rescue glycolysis. SGLT2 inhibition increased glucose oxidation in all organs; in the kidney, this was associated with modulation of the redox state. Diabetes was associated with altered methionine cycle metabolism, evident by decreased betaine and methionine levels, whereas treatment with SGLT2i increased hepatic betaine along with decreased homocysteine levels. mTORC1 activity was inhibited by SGLT2i along with stimulation of AMPK in both normoglycemic and diabetic animals, possibly explaining the protective effects against kidney, liver, and heart diseases. Collectively, our findings suggest that SGLT2i induces metabolic reprogramming orchestrated by AMPK-mTORC1 signaling with common and distinct effects in various tissues, with implications for diabetes and aging.

The effect of steroid injection into the shoulder on glycemia in patients with type 2 diabetes

Background

Injections of corticosteroids into or around joints have been reported to increase blood glucose in patients with diabetes due to corticosteroid absorption into the bloodstream. However, the magnitude, duration, and clinical implications of local corticosteroid injections on glycemic control are not clear. The purpose of this study was to evaluate the effects of corticosteroid injection to the shoulder on glycemia in patients with type 2 diabetes mellitus using a continuous glucose monitoring device.

Methods

Twenty-five patients with symptomatic shoulder problems and type 2 diabetes mellitus, not treated with insulin, prescribed a corticosteroid injection into the shoulder, were investigated. The patients were connected to a flash glucose monitoring system, which continuously monitored interstitial glucose levels. Data were collected 3 days before injection and for additional 11 days after corticosteroid injection. We analyzed glucose levels in the first 3 days (early postinjection) and on days 4-11 (late postinjection) after the injection and compared them to the preinjection period. The outcome measures included change in the average glucose levels, per patient, between the preinjection and postinjection periods and the differences in the time spent at glucose >180 mg/dL, >250 mg/dL, and >350 mg/dL, per patient, between the preinjection and postinjection periods.

Results

The increase in the mean glucose level per patient was statistically significant from 136 mg/dL in the preinjection period to 159 mg/dL in the first 3 days after the injection and returned to normal thereafter. Time at blood glucose >250 mg/dL increased from 4.3% in the preinjection period to 9.5% on the first day after the injection. It then decreased to 7% on day 2, 3.8% on day 3, and 1.4% in the late postinjection period. New onset of glucose levels >350 mg/dL was found in 4 of 25 patients during the early postinjection period. In all 4 patients, the exposure to severe hyperglycemia (>350 mg/dL) was short. None of the patients required intensification of the antidiabetic treatment or insulin injections.

Conclusion

Local corticosteroid injection to the shoulder can create a significant, short-term increase in systemic glucose levels in patients with D2DM not treated with insulin. Some of these patients may have periods with glucose above 350 mg %. However, these glycemic changes are short lived and are mostly limited to the 2-3 days after the injection. In addition, none of the patients in our study needed any change in antidiabetic treatment or any medical care after the injection.

Novel roles of mTORC2 in regulation of insulin secretion by actin filament remodeling

Mammalian target of rapamycin (mTOR) kinase is an essential hub where nutrients and growth factors converge to control cellular metabolism. mTOR interacts with different accessory proteins to form complexes 1 and 2 (mTORC), and each complex has different intracellular targets. Although mTORC1's role in β-cells has been extensively studied, less is known about mTORC2's function in β-cells. Here, we show that mice with constitutive and inducible β-cell-specific deletion of RICTOR (βRicKO and iβRicKO mice, respectively) are glucose intolerant due to impaired insulin secretion when glucose is injected intraperitoneally. Decreased insulin secretion in βRicKO islets was caused by abnormal actin polymerization. Interestingly, when glucose was administered orally, no difference in glucose homeostasis and insulin secretion were observed, suggesting that incretins are counteracting the mTORC2 deficiency. Mechanistically, glucagon-like peptide-1 (GLP-1), but not gastric inhibitory polypeptide (GIP), rescued insulin secretion in vivo and in vitro by improving actin polymerization in βRicKO islets. In conclusion, mTORC2 regulates glucose-stimulated insulin secretion by promoting actin filament remodeling.NEW & NOTEWORTHY The current studies uncover a novel mechanism linking mTORC2 signaling to glucose-stimulated insulin secretion by modulation of the actin filaments. This work also underscores the important role of GLP-1 in rescuing defects in insulin secretion by modulating actin polymerization and suggests that this effect is independent of mTORC2 signaling.

FC024: Renal Proximal Tubules CB1 Receptor Modulates MTORC1 Signalling in Health and Disease

BACKGROUND AND AIMS

Diabetic kidney disease (DKD), the renal manifestation of diabetes, contributes to increased morbidity and mortality of patients with diabetes. Activation of the cannabinoid-1 receptor (CB1R) and the mammalian target of rapamycin complex 1 (mTORC1) in the renal proximal tubular cells (RPTCs) contributes to the development of DKD. However, whether these two signalling molecules interact with each other to regulate kidney function during health and disease has not been described yet.

METHOD

By using a multidisciplinary approach that includes pharmacological and genetic manipulations of CB1R and mTORC1, we assessed the effect of CB1R activation/inhibition on mTORC1 activity under conditions of acute and chronic hyperglycaemia or normoglycaemia in RPTCs and mice.

RESULTS

We showed hyperglycaemia-induced endocannabinoid/CB1R stimulation. This stimulation increased mTORC1 activity and resulted in enhancing the expression of sterol regulatory element-binding protein 1c (SREBP1c) and its nuclear translocation, which in turn, induced the transcription of the facilitative glucose transporter 2 (GLUT2), thus leading to the development of DKD in mice. This effect was ameliorated by CB1R or mTORC1 nullification specifically in RPTCs. However, in non-diabetic conditions, CB1R maintained normal activation of mTORC1 by preventing the cellular excess of various amino acids via regulating their transporters, megalin, SLC6A19 and SLC7A5.

CONCLUSION

Our findings highlight two novel molecular mechanisms by which the activation of mTORC1 in RPTCs is tightly controlled by CB1R, either by enhancing the reabsorption of glucose and inducing renal dysfunction in diabetes or by preventing amino acid uptake and maintaining normal kidney function in healthy conditions. Moreover, this work highlights the therapeutic potential of targeting peripheral CB1Rs for the treatment of DKD, and on the other hand, avoiding its use in non-diabetic patients due to their possible effect on enhancing mTORC1 signalling.

MO067: Role of ANT2 in the Development of Obesity-Induced Chronic Kidney Disease

BACKGROUND AND AIMS

Obesity, a widespread condition with an ever-increasing prevalence, imposes a great medical concern since it serves as a risk factor for a wide range of conditions, including the development of chronic kidney disease (CKD). Lipid accumulation and impaired fatty acid β-oxidation in the renal proximal tubular cells (RPTCs) as well as an overall mitochondrial dysfunction have been shown to induce kidney injury, inflammation and fibrosis, leading to the development of CKD. However, the molecular mechanisms linking mitochondrial dysfunction in RPTCs to obesity-induced CKD are not fully understood and require further examination. Adenine nucleotide translocases (ANTs), which transport ADP and ATP through the mitochondrial inner membrane, play an essential role in energy metabolism of eukaryotic cells. ANT2 (Slc25a5) mediates fatty acid-induced uncoupled respiration, and its complete deletion is embryonic lethal. Here we explore its specific role in RPTCs in the development of obesity-induced CKD.

METHOD

ANT2 protein levels were measured using western blot and immunohistochemistry analyses. RPTC-ANT2-/- animals were generated using the Cre/loxP system. The null mice and their wild-type littermate controls were fed with either a high-fat diet (HFD) or a standard diet (STD) for 24 weeks. Kidney function and lipid content were assessed by biochemical analysis. Histological techniques were used to visualize the morphological changes in the kidney as well as fibrogenesis. Kidney injury and inflammatory markers were measured via qPCR and ELISAs. Mitochondrial function was tested in primary mouse RPTCs extracted from the null and WT animals by measuring oxygen consumption rate via Seahorse as well as reactive oxygen species (ROS) production by DCFDA staining.

RESULTS

As evident from the double-stained immunofluorescence staining of mouse kidney, high levels of ANT2 were found in RPTCs. Interestingly, its protein expression was downregulated in HFD-fed WT mice. Nullification of ANT2 specifically in RPTCs prevented the obesity-induced kidney dysfunction as reflected by lowered albumin-to-creatinine ratio. This was associated with the improved renal morphology in HFD-fed RPTC-ANT2-/- mice, evident by the reduction in obesity-related increased glomerular and Bowman's space areas and mesangial expansion. Compared with their littermate controls, obese RPTC-ANT2-/- mice showed reduced renal KIM-1 and NGAL protein levels and downregulation in the expression levels of Mcp1 and Lcn2, markers of renal injury and inflammation, respectively. The null mice were also protected from obesity-induced fibrosis, measured by trichrome staining, as well as accumulation of triglycerides and cholesterol in the kidney. In addition, preserved mitochondrial function was found in primary mouse RPTCs lacking ANT2, measured by increased spare respiratory capacity and decreased ROS production. Lastly, downregulated Hif1α gene and protein expression levels were found in RPTCs and kidney tissue lacking ANT2.

CONCLUSION

Our findings introduce ANT2 in the RPTCs as a key player in the development of obesity-induced CKD. Deletion of RPTC-ANT2 protects the kidney from the deleterious effects of lipotoxicity. Therefore, targeted manipulation of renal mitochondrial metabolism, particularly via inhibiting ANT2, may represent a novel approach for the treatment of obesity-induced CKD.

Opposite physiological and pathological mTORC1-mediated roles of the CB1 receptor in regulating renal tubular function

Activation of the cannabinoid-1 receptor (CB₁R) and the mammalian target of rapamycin complex 1 (mTORC1) in the renal proximal tubular cells (RPTCs) contributes to the development of diabetic kidney disease (DKD). However, the CB₁R/mTORC1 signaling axis in the kidney has not been described yet. We show here that hyperglycemia-induced endocannabinoid/CB₁R stimulation increased mTORC1 activity, enhancing the transcription of the facilitative glucose transporter 2 (GLUT2) and leading to the development of DKD in mice; this effect was ameliorated by specific RPTCs ablation of GLUT2. Conversely, CB₁R maintained the normal activity of mTORC1 by preventing the cellular excess of amino acids during normoglycemia. Our findings highlight a novel molecular mechanism by which the activation of mTORC1 in RPTCs is tightly controlled by CB₁R, either by enhancing the reabsorption of glucose and inducing kidney dysfunction in diabetes or by preventing amino acid uptake and maintaining normal kidney function in healthy conditions.

Renal proximal tubules modulate whole-body homeostasis by sensing various nutrients. Here the authors describe the existence and importance of a unique CB1/mTORC1/GLUT2 signaling axis in regulating nutrient homeostasis in healthy and diseased kidney.

The second-generation antipsychotic drug aripiprazole modulates the serotonergic system in pancreatic islets and induces beta cell dysfunction in female mice

AIMS/HYPOTHESIS

Second-generation antipsychotic (SGA) drugs have been associated with the development of type 2 diabetes and the metabolic syndrome in patients with schizophrenia. In this study, we aimed to investigate the effects of two different SGA drugs, olanzapine and aripiprazole, on metabolic state and islet function and plasticity.

METHODS

We analysed the functional adaptation of beta cells in 12-week-old B6;129 female mice fed an olanzapine- or aripiprazole-supplemented diet (5.5-6.0 mg kg^-1 day^-1) for 6 months. Glucose and insulin tolerance tests, in vivo glucose-stimulated insulin secretion and indirect calorimetry were performed at the end of the study. The effects of SGAs on beta cell plasticity and islet serotonin levels were assessed by transcriptomic analysis and immunofluorescence. Insulin secretion was assessed by static incubations and Ca²⁺ fluxes by imaging techniques.

RESULTS

Treatment of female mice with olanzapine or aripiprazole for 6 months induced weight gain (p<0.01 and p<0.05, respectively), glucose intolerance (p<0.01) and impaired insulin secretion (p<0.05) vs mice fed a control chow diet. Aripiprazole, but not olanzapine, induced serotonin production in beta cells vs controls, likely by increasing tryptophan hydroxylase 1 (TPH1) expression, and inhibited Ca²⁺ flux. Of note, aripiprazole increased beta cell size (p<0.05) and mass (p<0.01) vs mice fed a control chow diet, along with activation of mechanistic target of rapamycin complex 1 (mTORC1)/S6 signalling, without preventing beta cell dysfunction.

CONCLUSIONS/INTERPRETATION

Both SGAs induced weight gain and beta cell dysfunction, leading to glucose intolerance; however, aripiprazole had a more potent effect in terms of metabolic alterations, which was likely a result of its ability to modulate the serotonergic system. The deleterious metabolic effects of SGAs on islet function should be considered while treating patients as these drugs may increase the risk for development of the metabolic syndrome and diabetes.

Nutrient Sensor mTORC1 Regulates Insulin Secretion by Modulating β-Cell Autophagy

The dynamic regulation of autophagy in β-cells by cycles of fasting-feeding and its effects on insulin secretion are unknown. In β-cells, mechanistic target of rapamycin complex 1 (mTORC1) is inhibited while fasting and is rapidly stimulated during refeeding by a single amino acid, leucine, and glucose. Stimulation of mTORC1 by nutrients inhibited the autophagy initiator ULK1 and the transcription factor TFEB, thereby preventing autophagy when β-cells were continuously exposed to nutrients. Inhibition of mTORC1 by Raptor knockout mimicked the effects of fasting and stimulated autophagy while inhibiting insulin secretion, whereas moderate inhibition of autophagy under these conditions rescued insulin secretion. These results show that mTORC1 regulates insulin secretion through modulation of autophagy under different nutritional situations. In the fasting state, autophagy is regulated in an mTORC1-dependent manner, and its stimulation is required to keep insulin levels low, thereby preventing hypoglycemia. Reciprocally, stimulation of mTORC1 by elevated leucine and glucose, which is common in obesity, may promote hyperinsulinemia by inhibiting autophagy.

A Combined Drug Treatment That Reduces Mitochondrial Iron and Reactive Oxygen Levels Recovers Insulin Secretion in NAF-1-Deficient Pancreatic Cells

Decreased insulin secretion, associated with pancreatic β-cell failure, plays a critical role in many human diseases including diabetes, obesity, and cancer. While numerous studies linked β-cell failure with enhanced levels of reactive oxygen species (ROS), the development of diabetes associated with hereditary conditions that result in iron overload, e.g., hemochromatosis, Friedreich's ataxia, and Wolfram syndrome type 2 (WFS-T2; a mutation in CISD2, encoding the [2Fe-2S] protein NAF-1), underscores an additional link between iron metabolism and β-cell failure. Here, using NAF-1-repressed INS-1E pancreatic cells, we observed that NAF-1 repression inhibited insulin secretion, as well as impaired mitochondrial and ER structure and function. Importantly, we found that a combined treatment with the cell permeant iron chelator deferiprone and the glutathione precursor N-acetyl cysteine promoted the structural repair of mitochondria and ER, decreased mitochondrial labile iron and ROS levels, and restored glucose-stimulated insulin secretion. Additionally, treatment with the ferroptosis inhibitor ferrostatin-1 decreased cellular ROS formation and improved cellular growth of NAF-1 repressed pancreatic cells. Our findings reveal that suppressed expression of NAF-1 is associated with the development of ferroptosis-like features in pancreatic cells, and that reducing the levels of mitochondrial iron and ROS levels could be used as a therapeutic avenue for WFS-T2 patients.

Kidney Outcomes With Glucagon-Like Peptide-1 Receptor Agonists in Patients With Type 2 Diabetes

Glucagon-like peptide-1 receptor agonists (GLP-1 RAs) are highly effective in reducing glycemia in patients with type 2 diabetes (T2D). These medications effectively reduce cardiovascular (CV) risk in patients with T2D and established CV disease or with multiple risk factors. In addition, treatment with GLP-1 RA may exert protective effects on the diabetic kidney. Herein, we summarize the findings regarding the kidney safety and efficacy of GLP-1 RAs in patients with T2D. We review data from GLP-1 RAs phase 3 kidney studies, CV outcome trials, as well as real-world evidence. The accumulating data show that treatment with GLP-1 RAs is safe, well-tolerated, and effective in patients with different levels of kidney dysfunction. Furthermore, CV outcome trials suggest that GLP-1 RAs reduce albuminuria and may attenuate the decline in kidney function over time. The ongoing FLOW trial studying the effects of semaglutide in patients with diabetic kidney disease is expected to shed light on the effects of GLP-1 RAs on kidney outcomes and clarify their role in the management of patients with T2D and kidney disease.

Pathogenesis of diabesity-induced kidney disease: role of kidney nutrient sensing

Diabetes kidney disease (DKD) is a major healthcare problem associated with increased risk for developing end-stage kidney disease and high mortality. It is widely accepted that DKD is primarily a glomerular disease. Recent findings however suggest that kidney proximal tubule cells (KPTCs) may play a central role in the pathophysiology of DKD. In diabetes and obesity, KPTCs are exposed to nutrient overload, including glucose, free-fatty acids and amino acids, which dysregulate nutrient and energy sensing by mechanistic target of rapamycin complex 1 and AMP-activated protein kinase, with subsequent induction of tubular injury, inflammation, and fibrosis. Pharmacological treatments that modulate nutrient sensing and signaling in KPTCs, including cannabinoid-1 receptor antagonists and sodium glucose transporter 2 inhibitors, exert robust kidney protective effects. Shedding light on how nutrients are sensed and metabolized in KPTCs and in other kidney domains, and on their effects on signal transduction pathways that mediate kidney injury, is important for understanding the pathophysiology of DKD and for the development of novel therapeutic approaches in DKD and probably also in other forms of kidney disease.

Inhibition of mTORC1 by ER stress impairs neonatal β-cell expansion and predisposes to diabetes in the Akita mouse

Unresolved ER stress followed by cell death is recognized as the main cause of a multitude of pathologies including neonatal diabetes. A systematic analysis of the mechanisms of β-cell loss and dysfunction in Akita mice, in which a mutation in the proinsulin gene causes a severe form of permanent neonatal diabetes, showed no increase in β-cell apoptosis throughout life. Surprisingly, we found that the main mechanism leading to β-cell dysfunction is marked impairment of β-cell growth during the early postnatal life due to transient inhibition of mTORC1, which governs postnatal β-cell growth and differentiation. Importantly, restoration of mTORC1 activity in neonate β-cells was sufficient to rescue postnatal β-cell growth, and to improve diabetes. We propose a scenario for the development of permanent neonatal diabetes, possibly also common forms of diabetes, where early-life events inducing ER stress affect β-cell mass expansion due to mTOR inhibition.

Effects of proinsulin misfolding on β-cell dynamics, differentiation and function in diabetes

ER stress due to proinsulin misfolding has an important role in the pathophysiology of rare forms of permanent neonatal diabetes (PNDM) and probably also of common type 1 (T1D) and type 2 diabetes (T2D). Accumulation of misfolded proinsulin in the ER stimulates the unfolded protein response (UPR) that may eventually lead to apoptosis through a process called the terminal UPR. However, the β-cell ER has an incredible ability to cope with accumulation of misfolded proteins; therefore, it is not clear whether in common forms of diabetes the accumulation of misfolded proinsulin exceeds the point of no return in which terminal UPR is activated. Many studies showed that the UPR is altered in both T1D and T2D; however, the observed changes in the expression of different UPR markers are inconsistent and it is not clear whether they reflect an adaptive response to stress or indeed mediate the β-cell dysfunction of diabetes. Herein, we critically review the literature on the effects of proinsulin misfolding and ER stress on β-cell dysfunction and loss in diabetes with emphasis on β-cell dynamics, and discuss the gaps in understanding the role of proinsulin misfolding in the pathophysiology of diabetes.

Opposing effects of intracellular versus extracellular adenine nucleotides on autophagy: implications for β-cell function

AMPK-mTORC1 signaling senses nutrient availability, thereby regulating autophagy. Surprisingly, we found that, in β-cells, the AMPK activator 5-amino-4-imidazolecarboxamide ribofuranoside (AICAR) inhibited, rather than stimulated, autophagy. AICAR is an intermediate in the generation of inosine monophosphate, with subsequent conversion to other purine nucleotides. Adenosine regulated autophagy in a concentration-dependent manner: at high concentrations, it mimicked the AICAR effect on autophagy, whereas at low concentrations it stimulated autophagy through its cognate A₁ receptor. Adenosine regulation of autophagy was independent of AMPK or mTORC1 activity. Adenosine kinase (ADK) is the principal enzyme for metabolic adenosine clearance. ADK knockdown and pharmacological inhibition of the enzyme markedly stimulated autophagy in an adenosine A₁ receptor-dependent manner. High-concentration adenosine increased insulin secretion in a manner sensitive to treatment with the autophagy inducer Tat-beclin1, and inhibition of autophagy augmented secretion. In conclusion, high concentrations of AICAR or adenosine inhibit autophagy, whereas physiological concentrations of adenosine or inhibition of adenosine clearance by ADK stimulate autophagy via the adenosine receptor. Adenosine might thus be an autocrine regulator of autophagy, independent of AMPK-mTORC1 signaling. Adenosine regulates insulin secretion, in part, through modulation of autophagy.

Combining chloroquine with RAD001 inhibits tumor growth in a NEN mouse model

Patients with neuroendocrine neoplasms (NENs) often require systemic treatment, which is frequently limited by the emergence of drug resistance. mTOR inhibitors (mTORi), such as RAD001 (everolimus), have been shown to inhibit neoplasm progression. mTORi stimulates autophagy, a degradation pathway that might promote the survival of neoplasm cells that are exposed to anti-cancer therapy. Chloroquine (CQ), a well-known anti-malarial and anti-rheumatic drug, suppresses autophagy. Based on our previous results, we hypothesized that CQ may enhance the anti-tumorigenic effects of mTORi by inhibiting autophagy and we aimed to examine the anti-tumorigenic effect of CQ, alone or in combination with RAD001. We established a NEN subcutaneous xenograft mouse model and evaluated the effect of the drugs on tumor growth, mTOR pathway, autophagy and apoptosis. CQ alone and in combination with RAD001 significantly decreased neoplasm volume. Histopathological analysis revealed that the combination of CQ and RAD001 markedly inhibited mTOR activity and neoplasm cell growth, along with accumulation of autophagosomes and increased apoptosis. In conclusion, CQ enhances the anti-tumorigenic effect of RAD001 in vivo by inhibiting autophagy. Clinical trials addressing the effects of CQ therapy on neoplasm progression in patients with NENs, mainly in those treated with mTORi, are warranted.

mTORC1 Signaling: A Double-Edged Sword in Diabetic β Cells

The mechanistic target of rapamycin complex 1 (mTORC1) is a central regulator of metabolic and nutrient cues that integrates environmental inputs into downstream signaling pathways to control cellular metabolism, growth, and survival. While numerous in vitro and in vivo studies reported the positive functions of mTORC1 in the regulation of β cell survival and proliferation under physiological conditions, more recent work demonstrates the opposite in the long term; this is exemplified by the constitutive inappropriate hyper-activation of mTORC1 in diabetic islets or β cells under conditions of increased β cell stress and metabolic demands. These recent findings uncover mTORC1's importance as an emerging significant player in the development and progression of β cell failure in type 2 diabetes and suggest that mTORC1 may act as a "double edge sword" in the regulation of β cell mass and function in response to metabolic stress such as nutrient overload and insulin resistance.

Modulation of Renal GLUT2 by the Cannabinoid-1 Receptor: Implications for the Treatment of Diabetic Nephropathy

Altered glucose reabsorption via the facilitative glucose transporter 2 (GLUT2) during diabetes may lead to renal proximal tubule cell (RPTC) injury, inflammation, and interstitial fibrosis. These pathologies are also triggered by activating the cannabinoid-1 receptor (CB₁R), which contributes to the development of diabetic nephropathy (DN). However, the link between CB₁R and GLUT2 remains to be determined. Here, we show that chronic peripheral CB₁R blockade or genetically inactivating CB₁Rs in the RPTCs ameliorated diabetes-induced renal structural and functional changes, kidney inflammation, and tubulointerstitial fibrosis in mice. Inhibition of CB₁R also downregulated GLUT2 expression, affected the dynamic translocation of GLUT2 to the brush border membrane of RPTCs, and reduced glucose reabsorption. Thus, targeting peripheral CB₁R or inhibiting GLUT2 dynamics in RPTCs has the potential to treat and ameliorate DN. These findings may support the rationale for the clinical testing of peripherally restricted CB₁R antagonists or the development of novel renal-specific GLUT2 inhibitors against DN.

Should Patients With Frozen Shoulder Be Screened for Diabetes Mellitus?

Background:

Idiopathic frozen shoulder (nontraumatic) is commonly encountered in patients between the ages of 35 and 60 years in general orthopaedic practice. While the prevalence of frozen shoulder among the general population is estimated to be between 2% and 4%, a significantly higher prevalence of 10% to 22% has been reported in patients with diabetes mellitus. Since diabetic patients are more prone to develop frozen shoulder than nondiabetics, the question arises as to whether patients diagnosed as having idiopathic frozen shoulder are at greater risk to develop diabetes mellitus and should be routinely screened for this condition.

Purpose:

To compare the prevalence of diabetes mellitus and prediabetes among patients diagnosed with idiopathic frozen shoulder who are not known to have either diabetes mellitus or prediabetic conditions with that of an age-matched group from the general population.

Study Design:

Case series; Level of evidence, 4.

Methods:

Patients at a shoulder clinic with a diagnosis of idiopathic frozen shoulder were asked to participate in the study if they were aged between 35 to 60 years and had no known previous diagnosis of diabetes mellitus or prediabetic conditions. These patients underwent a 2-hour oral glucose tolerance test. According to their fasting and 2-hour plasma glucose levels, patients were diagnosed as normal glucose tolerance, prediabetic, or diabetic. Findings were matched with the prevalence in an age-matched general population.

Results:

Fifty patients completed the test. Four patients with idiopathic frozen shoulder (8%) were found to be prediabetic. No patient was found to be diabetic. All 4 patients reported a history of diabetes in their parents or siblings.

Conclusion:

Patients diagnosed with idiopathic frozen shoulder who are 60 years or younger and are not known diabetics have a similar probability of having diabetes or prediabetes to an age-matched population. No routine diabetic workup is warranted specifically for these patients.

Hypoglycaemia manifestations and recurrent events: Lessons from the SAVOR-TIMI 53 outcome study

Hypoglycaemia is a well-known risk associated with the use of sulphonylureas and insulin, often limiting achievement of glycaemic goals. Recognizing the precipitants and recurrence patterns of hypoglycaemic events, particularly major events, is therefore clinically important. The SAVOR-TIMI-53 trial was a cardiovascular outcome study of 16 492 patients allocated to saxagliptin vs placebo added to conventional care for a median of 2.1 years. Hypoglycaemic events were a prespecified outcome in the study and were defined as a symptomatic episode that recovered with carbohydrates or any recorded blood glucose <3.0 mmol/l (<54 mg/dL). A major event was defined as one that required third-party assistance. Analysis of the features of the first hypoglycaemic event for each patient showed that a precipitant for the event was recognized by fewer than half of the patients, with the precipitant most often being a missed meal. In 40% of patients reporting major hypoglycaemic events, no precipitating factor was recognized, and in >60%, no previous hypoglycaemic event was reported during the timespan of the study, underscoring the lack of predictability of such an event.

Pancreatic β-Cells Express the Fetal Islet Hormone Gastrin in Rodent and Human Diabetes

β-Cell failure in type 2 diabetes (T2D) was recently proposed to involve dedifferentiation of β-cells and ectopic expression of other islet hormones, including somatostatin and glucagon. Here we show that gastrin, a stomach hormone typically expressed in the pancreas only during embryogenesis, is expressed in islets of diabetic rodents and humans with T2D. Although gastrin in mice is expressed in insulin⁺ cells, gastrin expression in humans with T2D occurs in both insulin⁺ and somatostatin⁺ cells. Genetic lineage tracing in mice indicates that gastrin expression is turned on in a subset of differentiated β-cells after exposure to severe hyperglycemia. Gastrin expression in adult β-cells does not involve the endocrine progenitor cell regulator neurogenin3 but requires membrane depolarization, calcium influx, and calcineurin signaling. In vivo and in vitro experiments show that gastrin expression is rapidly eliminated upon exposure of β-cells to normal glucose levels. These results reveal the fetal hormone gastrin as a novel marker for reversible human β-cell reprogramming in diabetes.

Effect of Saxagliptin on Renal Outcomes in the SAVOR-TIMI 53 Trial

OBJECTIVE

Dipeptidyl peptidase 4 inhibitors may have a protective effect in diabetic nephropathy.

RESEARCH DESIGN AND METHODS

We studied renal outcomes of 16,492 patients with type 2 diabetes, randomized to saxagliptin versus placebo and followed for a median of 2.1 years in the Saxagliptin Assessment of Vascular Outcomes Recorded in Patients with Diabetes Mellitus-Thrombolysis in Myocardial Infarction 53 (SAVOR-TIMI 53) trial.

RESULTS

At baseline, 9,696 (58.8%) subjects had normoalbuminuria (albumin/creatinine ratio [ACR] <30 mg/g), 4,426 (26.8%) had microalbuminuria (ACR 30-300 mg/g), and 1,638 (9.9%) had macroalbuminuria (ACR >300 mg/g). Treatment with saxagliptin was associated with improvement in and/or less deterioration in ACR categories from baseline to end of trial (EOT) (P = 0.021, P < 0.001, and P = 0.049 for individuals with baseline normoalbuminuria, microalbuminuria, and macroalbuminuria, respectively). At 2 years, the difference in mean ACR change between saxagliptin and placebo arms was -19.3 mg/g (P = 0.033) for estimated glomerular filtration rate (eGFR) >50 mL/min/body surface area per 1.73 m² (BSA), -105 mg/g (P = 0.011) for 50 ≥ eGFR ≥ 30 mL/min/BSA, and -245.2 mg/g (P = 0.086) for eGFR <30 mL/min/BSA. Analyzing ACR as a continuous variable showed reduction in ACR with saxagliptin (1 year, P < 0.0001; 2 years, P = 0.0143; and EOT, P = 0.0158). The change in ACR did not correlate with that in HbA_1c (r = 0.041, 0.052, and 0.036; 1 year, 2 years, and EOT, respectively). The change in eGFR was similar in the saxagliptin and placebo groups. Safety renal outcomes, including doubling of serum creatinine, initiation of chronic dialysis, renal transplantation, or serum creatinine >6.0 mg/dL, were similar as well.

CONCLUSIONS

Treatment with saxagliptin improved ACR, even in the normoalbuminuric range, without affecting eGFR. The beneficial effect of saxagliptin on albuminuria could not be explained by its effect on glycemic control.

Upregulation of Mitochondrial Content in Cytochrome c Oxidase Deficient Fibroblasts

Cytochrome-c-oxidase (COX) deficiency is a frequent cause of mitochondrial disease and is associated with a wide spectrum of clinical phenotypes. We studied mitochondrial function and biogenesis in fibroblasts derived from the Cohen (CDs) rat, an animal model of COX deficiency. COX activity in CDs-fibroblasts was 50% reduced compared to control rat fibroblasts (P<0.01). ROS-production in CDs fibroblasts increased, along with marked mitochondrial fragmentation and decreased mitochondrial membrane-potential, indicating mitochondrial dysfunction. Surprisingly, cellular ATP content, oxygen consumption rate (OCR) and the extracellular acidification rate (ECAR) were unchanged. To clarify the discrepancy between mitochondrial dysfunction and ATP production, we studied mitochondrial biogenesis and turnover. The content of mitochondria was higher in CDs-fibroblasts. Consistently, AMPK activity and the expression of NRF1-target genes, NRF2 and PGC1-α that mediate mitochondrial biogenesis were increased (P<0.01 vs control fibroblast). In CDs-fibrobalsts, the number of autophagosomes (LC3+ puncta) containing mitochondria in CDs fibroblasts was similar to that in control fibroblasts, suggesting that mitophagy was intact. Altogether, our findings demonstrate that mitochondrial dysfunction and oxidative stress are associated with an increase in mitochondrial biogenesis, resulting in preservation of ATP generation.

GLP-1-RA Corrects Mitochondrial Labile Iron Accumulation and Improves β-Cell Function in Type 2 Wolfram Syndrome

CONTEXT

Type 2 Wolfram syndrome (T2-WFS) is a neuronal and β-cell degenerative disorder caused by mutations in the CISD2 gene. The mechanisms underlying β-cell dysfunction in T2-WFS are not known, and treatments that effectively improve diabetes in this context are lacking.

OBJECTIVE

Unraveling the mechanisms of β-cell dysfunction in T2-WFS and the effects of treatment with GLP-1 receptor agonist (GLP-1-RA).

DESIGN AND SETTING

A case report and in vitro mechanistic studies.

PATIENT AND METHODS

We treated an insulin-dependent T2-WFS patient with the GLP-1-RA exenatide for 9 weeks. An iv glucose/glucagon/arginine stimulation test was performed off-drug before and after intervention. We generated a cellular model of T2-WFS by shRNA knockdown of CISD2 (nutrient-deprivation autophagy factor-1 [NAF-1]) in rat insulinoma cells and studied the mechanisms of β-cell dysfunction and the effects of GLP-1-RA.

RESULTS

Treatment with exenatide resulted in a 70% reduction in daily insulin dose with improved glycemic control, as well as an off-drug 7-fold increase in maximal insulin secretion. NAF-1 repression in INS-1 cells decreased insulin content and glucose-stimulated insulin secretion, while maintaining the response to cAMP, and enhanced the accumulation of labile iron and reactive oxygen species in mitochondria. Remarkably, treatment with GLP-1-RA and/or the iron chelator deferiprone reversed these defects.

CONCLUSION

NAF-1 deficiency leads to mitochondrial labile iron accumulation and oxidative stress, which may contribute to β-cell dysfunction in T2-WFS. Treatment with GLP-1-RA and/or iron chelation improves mitochondrial function and restores β-cell function. Treatment with GLP-1-RA, probably aided by iron chelation, should be considered in WFS and other forms of diabetes associated with iron dysregulation.

Predisposing Factors for Any and Major Hypoglycemia With Saxagliptin Versus Placebo and Overall: Analysis From the SAVOR-TIMI 53 Trial

OBJECTIVE

To analyze the impact of adding saxagliptin versus placebo on the risk for hypoglycemia and to identify predictors of any and major hypoglycemia in patients with type 2 diabetes included in the Saxagliptin Assessment of Vascular Outcomes Recorded in Patients with Diabetes Mellitus-Thrombolysis in Myocardial Infarction 53 (SAVOR-TIMI 53) study.

RESEARCH DESIGN AND METHODS

Patients with type 2 diabetes (n = 16,492) were randomized to saxagliptin or placebo and followed for a median of 2.1 years. Associations between any hypoglycemia (symptomatic or glucose measurement <54 mg/dL) or major hypoglycemia (requiring extended assistance) and patient characteristics overall and by treatment allocation were studied.

RESULTS

At least one hypoglycemic event was reported in 16.6% of patients, and 1.9% reported at least one major event. Patients allocated to saxagliptin versus placebo experienced higher rates of any (hazard ratio [HR] 1.16 [95% CI 1.08, 1.25]; P < 0.001) or major (HR 1.26 [1.01, 1.58]; P = 0.038) hypoglycemia. Hypoglycemia rates (any or major) were increased with saxagliptin in patients taking sulfonylureas (SURs) but not in those taking insulin. Rates were increased with saxagliptin in those with baseline HbA1c ≤7.0% and not in those with baseline HbA1c >7.0%. Multivariate analysis of the overall population revealed that independent predictors of any hypoglycemia were as follows: allocation to saxagliptin, long duration of diabetes, increased updated HbA1c, macroalbuminuria, moderate renal failure, SUR use, and insulin use. Predictors of major hypoglycemia were allocation to saxagliptin, advanced age, black race, reduced BMI, long duration of diabetes, declining renal function, microalbuminuria, and use of short-acting insulin. Among SURs, glibenclamide was associated with increased risk of major but not any hypoglycemia.

CONCLUSIONS

The identification of patients at risk for hypoglycemia can guide physicians to better tailor antidiabetic therapy.

Anti-inflammatory Agents in the Treatment of Diabetes and Its Vascular Complications

The association between hyperglycemia and inflammation and vascular complications in diabetes is now well established. Antidiabetes drugs may alleviate inflammation by reducing hyperglycemia; however, the anti-inflammatory effects of these medications are inconsistent and it is unknown whether their beneficial metabolic effects are mediated via modulation of chronic inflammation. Recent data suggest that immunomodulatory treatments may have beneficial effects on glycemia, β-cell function, and insulin resistance. However, the mechanisms underlying their beneficial metabolic effects are not always clear, and there are concerns regarding the specificity, safety, and efficacy of immune-based therapies. Herein, we review the anti-inflammatory and metabolic effects of current antidiabetes drugs and of anti-inflammatory therapies that were studied in patients with type 2 diabetes. We discuss the potential benefit of using anti-inflammatory treatments in diabetes and important issues that should be addressed prior to implementation of such therapeutic approaches.

Autophagy is a major regulator of beta cell insulin homeostasis

AIMS/HYPOTHESIS

We studied the role of protein degradation pathways in the regulation of insulin production and secretion and hypothesised that autophagy regulates proinsulin degradation, thereby modulating beta cell function.

METHODS

Proinsulin localisation in autophagosomes was demonstrated by confocal and electron microscopy. Autophagy was inhibited by knockdown of autophagy-related (ATG) proteins and using the H(+)-ATPase inhibitor bafilomycin-A1. Proinsulin and insulin content and secretion were assessed in static incubations by ELISA and RIA.

RESULTS

Confocal and electron microscopy showed proinsulin localised in autophagosomes and lysosomes. Beta-Atg7 (-/-) mice had proinsulin-containing sequestosome 1 (p62 [also known as SQSTM1])(+) aggregates in beta cells, indicating proinsulin is regulated by autophagy in vivo. Short-term bafilomycin-A1 treatment and ATG5/7 knockdown increased steady-state proinsulin and hormone precursor chromogranin A content. ATG5/7 knockdown also increased glucose- and non-fuel-stimulated insulin secretion. Finally, mutated forms of proinsulin that are irreparably misfolded and trapped in the endoplasmic reticulum are more resistant to degradation by autophagy.

CONCLUSIONS/INTERPRETATION

In the beta cell, transport-competent secretory peptide precursors, including proinsulin, are regulated by autophagy, whereas efficient clearance of transport-incompetent mutated forms of proinsulin by alternative degradative pathways may be necessary to avoid beta cell proteotoxicity. Reduction of autophagic degradation of proinsulin increases its residency in the secretory pathway, followed by enhanced secretion in response to stimuli.

Abrogation of Autophagy by Chloroquine Alone or in Combination with mTOR Inhibitors Induces Apoptosis in Neuroendocrine Tumor Cells

BACKGROUND

Everolimus (RAD001), an mTORC1 inhibitor, demonstrated promising, but limited, anticancer effects in neuroendocrine tumors (NETs). Torin1 (a global mTOR inhibitor) and NVP-BEZ235 (a PI3K/mTOR inhibitor) seem to be more effective than RAD001. Autophagy, a degradation pathway that may promote tumor growth, is regulated by mTOR; mTOR inhibition results in stimulation of autophagy. Chloroquine (CQ) inhibits autophagy.

AIM

To explore the effect of CQ alone or in combination with RAD001, Torin1 or NVP-BEZ235 on autophagy and on NET cell viability, proliferation and apoptosis.

METHODS

The NET cell line BON1 was treated with CQ with or without different mTOR inhibitors. siRNA against ATG5/7 was used to genetically inhibit autophagy. Cellular viability was examined by XTT, proliferation by Ki-67 staining and cell cycles by flow cytometry. Apoptosis was analyzed by Western blotting for cleaved caspase 3 and staining for annexin V; autophagy was evaluated by Western blotting and immunostaining for LC3.

RESULTS

RAD001, Torin1, NVP-BEZ235 and CQ all decreased BON1 cell viability. The effect of RAD001 was smaller than that of the other mTOR inhibitors or CQ. Torin1 and NVP-BEZ235 markedly inhibited cell proliferation, without inducing apoptosis. CQ similarly decreased cell proliferation, while robustly increasing apoptosis. Treatment with Torin1 or NVP-BEZ235 together with CQ was additive on viability, without increasing CQ-induced apoptosis. Inhibition of autophagy by ATG5/7 knockdown increased apoptosis in the presence or absence of mTOR inhibitors, mimicking the CQ effects.

CONCLUSION

CQ inhibits NET growth by inducing apoptosis and by inhibiting cell proliferation, probably via inhibition of autophagy. CQ may potentiate the antitumor effect of mTOR inhibitors.

Loss of Liver Kinase B1 (LKB1) in Beta Cells Enhances Glucose-stimulated Insulin Secretion Despite Profound Mitochondrial Defects

The tumor suppressor liver kinase B1 (LKB1) is an important regulator of pancreatic β cell biology. LKB1-dependent phosphorylation of distinct AMPK (adenosine monophosphate-activated protein kinase) family members determines proper β cell polarity and restricts β cell size, total β cell mass, and glucose-stimulated insulin secretion (GSIS). However, the full spectrum of LKB1 effects and the mechanisms involved in the secretory phenotype remain incompletely understood. We report here that in the absence of LKB1 in β cells, GSIS is dramatically and persistently improved. The enhancement is seen both in vivo and in vitro and cannot be explained by altered cell polarity, increased β cell number, or increased insulin content. Increased secretion does require membrane depolarization and calcium influx but appears to rely mostly on a distal step in the secretion pathway. Surprisingly, enhanced GSIS is seen despite profound defects in mitochondrial structure and function in LKB1-deficient β cells, expected to greatly diminish insulin secretion via the classic triggering pathway. Thus LKB1 is essential for mitochondrial homeostasis in β cells and in parallel is a powerful negative regulator of insulin secretion. This study shows that β cells can be manipulated to enhance GSIS to supra-normal levels even in the face of defective mitochondria and without deterioration over months.

Impact of treatment with saxagliptin on glycaemic stability and β-cell function in the SAVOR-TIMI 53 study

AIMS

To study the effects of saxagliptin, a dipeptidyl peptidase-4 inhibitor, on glycaemic stability and β-cell function in the SAVOR-TIMI 53 trial.

METHODS

We randomized 16,492 patients with type 2 diabetes (T2D) to saxagliptin or placebo, added to current antidiabetic medications, and followed them for a median of 2.1 years. Glycaemic instability was defined by: (i) a glycated haemoglobin (HbA1c) increase of ≥ 0.5% post-randomization; (ii) the initiation of new antidiabetic medications for ≥ 3 months; or (iii) an increase in dose of oral antidiabetic medication or ≥ 25% increase in insulin dose for ≥ 3 months. β-cell function was assessed according to fasting homeostatic model 2 assessment of β-cell function (HOMA-2β) values at baseline and at year 2 in patients not treated with insulin.

RESULTS

Compared with placebo, participants treated with saxagliptin had a reduction in the development of glycaemic instability (hazard ratio 0.71; 95% confidence interval 0.68-0.74; p < 0.0001). In participants treated with saxagliptin compared with placebo, the occurrence of an HbA1c increase of ≥ 0.5% was reduced by 35.2%; initiation of insulin was decreased by 31.7% and the increases in doses of an oral antidiabetic drug or insulin were reduced by 19.5 and 23.5%, respectively (all p < 0.0001). At 2 years, HOMA-2β values decreased by 4.9% in participants treated with placebo, compared with an increase of 1.1% in those treated with saxagliptin (p < 0.0001).

CONCLUSIONS

Saxagliptin improved glycaemia and prevented the reduction in HOMA-2β values. Saxagliptin may reduce the usual decline in β-cell function in T2D, thereby slowing diabetes progression.

Foam cell-derived 4-hydroxynonenal induces endothelial cell senescence in a TXNIP-dependent manner

Vascular endothelial cell (VEC) senescence is considered an early event in the development of atherosclerotic lesions. Stressful stimuli, in particular oxidative stress, have been linked to premature senescence in the vasculature. Foam cells are a major source of reactive oxygen species and may play a role in the induction of VEC senescence; hence, we investigated their involvement in the induction of VEC senescence in a co-culture transwell system. Primary bovine aortic endothelial cells, exposed to the secretome of THP-1 monocyte-derived foam cells, were analysed for the induction of senescence. Senescence associated β-galactosidase activity and the expression of p16 and p21 were increased, whereas phosphorylated retinoblastoma protein was reduced. This senescent phenotype was mediated by 4-hydroxnonenal (4-HNE), a lipid peroxidation product secreted from foam cells; scavenging of 4-HNE in the co-culture medium blunted this effect. Furthermore, both foam cells and 4-HNE increased the expression of the pro-oxidant thioredoxin-interacting protein (TXNIP). Molecular manipulation of TXNIP expression confirmed its involvement in foam cell-induced senescence. Previous studies showed that peroxisome proliferator-activated receptor (PPAR)δ was activated by 4-hydroalkenals, such as 4-HNE. Pharmacological interventions supported the involvement of the 4-HNE-PPARδ axis in the induction of TXNIP and VEC senescence. The association of TXNIP with VEC senescence was further supported by immunofluorescent staining of human carotid plaques in which the expression of both TXNIP and p21 was augmented in endothelial cells. Collectively, these findings suggest that foam cell-released 4-HNE activates PPARδ in VEC, leading to increased TXNIP expression and consequently to senescence.

Autophagy in adipose tissue and the beta cell: implications for obesity and diabetes

Autophagy is a lysosomal degradation pathway recycling intracellular long-lived proteins and damaged organelles, thereby maintaining cellular homeostasis. In addition to inflammatory processes, autophagy has been implicated in the regulation of adipose tissue and beta cell functions. In obesity and type 2 diabetes autophagic activity is modulated in a tissue-dependent manner. In this review we discuss the regulation of autophagy in adipose tissue and beta cells, exemplifying tissue-specific dysregulation of autophagy and its implications for the pathophysiology of obesity and type 2 diabetes. We will highlight common themes and outstanding gaps in our understanding, which need to be addressed before autophagy could be envisioned as a therapeutic target for the treatment of obesity and diabetes.

Islet transplantation in type 1 diabetes: hype, hope and reality - a clinician's perspective

The β-cell replacement by islet transplantation is an attractive approach for normalizing blood glucose without hypoglycaemia in patient with type 1 diabetes mellitus (T1D). A pioneer study by the Edmonton group more than a decade ago showed that alloislet transplantation may result in insulin independence for at least 1 year after transplantation. This breakthrough excited researchers, physicians and patients, who felt that the ultimate goal of cure for T1D was at hand. Longer follow-up of patients who underwent islet transplantation showed less favourable results, with only approximately 10% of the patients remaining insulin-free 5 years after transplantation. In the last few years, progress has been made, and the success rate of islet transplantation has steadily increased. Important hurdles, however, related to limited tissue supply and need for life-long immunosuppressive drugs have yet to be overcome. Herein, we review recent achievements in islet transplantation and the challenges that still need to be addressed before this procedure can become a standard therapy for T1D.

AMPK regulates ER morphology and function in stressed pancreatic β-cells via phosphorylation of DRP1

Experimental lipotoxicity constitutes a model for β-cell demise induced by metabolic stress in obesity and type 2 diabetes. Fatty acid excess induces endoplasmic reticulum (ER) stress, which is accompanied by ER morphological changes whose mechanisms and relevance are unknown. We found that the GTPase dynamin-related protein 1 (DRP1), a key regulator of mitochondrial fission, is an ER resident regulating ER morphology in stressed β-cells. Inhibition of DRP1 activity using a GTP hydrolysis-defective mutant (Ad-K38A) attenuated fatty acid-induced ER expansion and mitochondrial fission. Strikingly, stimulating the key energy-sensor AMP-activated protein kinase (AMPK) increased the phosphorylation at the anti-fission site Serine 637 and largely prevented the alterations in ER and mitochondrial morphology. Expression of a DRP1 mutant resistant to phosphorylation at this position partially prevented the recovery of ER and mitochondrial morphology by AMPK. Fatty acid-induced ER enlargement was associated with proinsulin retention in the ER, together with increased proinsulin/insulin ratio. Stimulation of AMPK prevented these alterations, as well as mitochondrial fragmentation and apoptosis. In summary, DRP1 regulation by AMPK delineates a novel pathway controlling ER and mitochondrial morphology, thereby modulating the response of β-cells to metabolic stress. DRP1 may thus function as a node integrating signals from stress regulators, such as AMPK, to coordinate organelle shape and function.

Decreased serum concentrations of 25-hydroxycholecalciferol are associated with increased risk of progression to impaired fasting glucose and diabetes

OBJECTIVE

To study the association between vitamin D status and the risk of incident impaired fasting glucose (IFG) and diabetes in a population-based cohort of diabetes-free subjects.

RESEARCH DESIGN AND METHODS

In a historical prospective cohort study of subjects from the Clalit Health Services database, which includes information on nearly 4 million people, diabetes-free subjects aged 40-70 years with serum 25-hydroxycholecalciferol (25-OHD) measurements available were followed for 2 years to assess the development of IFG and diabetes in five 25-OHD subgroups: ≥25, 25.1-37.5, 37.6-50, 50.1-75, and >75 nmol/L.

RESULTS

The baseline cohort included 117,960 adults: 83,526 normoglycemic subjects and 34,434 subjects with IFG. During follow-up, 8,629 subjects (10.3% of the normoglycemic group) developed IFG, and 2,162 subjects (1.8% of the total cohort) progressed to diabetes. A multivariable model adjusted for age, sex, population group, immigrant status, BMI, season of vitamin D measurement, LDL and HDL cholesterol, triglycerides, estimated glomerular filtration rate, history of hypertension or cardiovascular disease, Charlson comorbidity index, smoking, and socioeconomic status revealed an inverse association between 25-OHD and the risk of progression to IFG and diabetes. The odds of transitioning from normoglycemia to IFG, from normoglycemia to diabetes, and from IFG to diabetes in subjects with a 25-OHD level ≤25 nmol/L were greater than those of subjects with a 25-OHD level >75 nmol/L [odds ratio 1.13 (95% CI 1.03-1.24), 1.77 (1.11-2.83), and 1.43 (1.16-1.76), respectively].

CONCLUSIONS

Vitamin D deficiency appears to be an independent risk factor for the development of IFG and diabetes.

Stimulation of autophagy improves endoplasmic reticulum stress-induced diabetes

Accumulation of misfolded proinsulin in the β-cell leads to dysfunction induced by endoplasmic reticulum (ER) stress, with diabetes as a consequence. Autophagy helps cellular adaptation to stress via clearance of misfolded proteins and damaged organelles. We studied the effects of proinsulin misfolding on autophagy and the impact of stimulating autophagy on diabetes progression in Akita mice, which carry a mutation in proinsulin, leading to its severe misfolding. Treatment of female diabetic Akita mice with rapamycin improved diabetes, increased pancreatic insulin content, and prevented β-cell apoptosis. In vitro, autophagic flux was increased in Akita β-cells. Treatment with rapamycin further stimulated autophagy, evidenced by increased autophagosome formation and enhancement of autophagosome-lysosome fusion. This was associated with attenuation of cellular stress and apoptosis. The mammalian target of rapamycin (mTOR) kinase inhibitor Torin1 mimicked the rapamycin effects on autophagy and stress, indicating that the beneficial effects of rapamycin are indeed mediated via inhibition of mTOR. Finally, inhibition of autophagy exacerbated stress and abolished the anti-ER stress effects of rapamycin. In conclusion, rapamycin reduces ER stress induced by accumulation of misfolded proinsulin, thereby improving diabetes and preventing β-cell apoptosis. The beneficial effects of rapamycin in this context strictly depend on autophagy; therefore, stimulating autophagy may become a therapeutic approach for diabetes.

Identification of a SIRT1 mutation in a family with type 1 diabetes

Type 1 diabetes is caused by autoimmune-mediated β cell destruction leading to insulin deficiency. The histone deacetylase SIRT1 plays an essential role in modulating several age-related diseases. Here we describe a family carrying a mutation in the SIRT1 gene, in which all five affected members developed an autoimmune disorder: four developed type 1 diabetes, and one developed ulcerative colitis. Initially, a 26-year-old man was diagnosed with the typical features of type 1 diabetes, including lean body mass, autoantibodies, T cell reactivity to β cell antigens, and a rapid dependence on insulin. Direct and exome sequencing identified the presence of a T-to-C exchange in exon 1 of SIRT1, corresponding to a leucine-to-proline mutation at residue 107. Expression of SIRT1-L107P in insulin-producing cells resulted in overproduction of nitric oxide, cytokines, and chemokines. These observations identify a role for SIRT1 in human autoimmunity and unveil a monogenic form of type 1 diabetes.

Improvement of ER stress-induced diabetes by stimulating autophagy

Pancreatic β-cell dysfunction is central in diabetes. The diabetic milieu may impair proinsulin folding, leading to β-cell endoplasmic reticulum (ER) stress and apoptosis, and thus a worsening of the diabetes. Autophagy is crucial for the well-being of the β-cell; however, the impact of stimulating autophagy on β-cell adaptation to ER stress is unknown. We studied the crosstalk between ER stress and autophagy in a rodent model of diabetes, called Akita, in which proinsulin gene mutation leads to protein misfolding and β-cell demise. We found that proinsulin misfolding stimulates autophagy and, in symmetry, inhibition of autophagy induces β-cell stress and apoptosis. Under conditions of excessive proinsulin misfolding, stimulation of autophagy by inhibiting MTORC1 alleviates stress and prevents apoptosis. Moreover, treatment of diabetic Akita mice with the MTORC1 inhibitor rapamycin improves diabetes and prevents β-cell apoptosis. Thus, autophagy is a central adaptive mechanism in β-cell stress. Stimulation of autophagy may become a novel therapeutic strategy in diabetes.

Diet-induced diabetes in the sand rat (Psammomys obesus)

Insulin deficiency is the underlying cause of hyperglycemia in type 2 diabetes. The gerbil Psammomys obesus (P. obesus) is a naturally insulin resistant rodent with tendency to develop diet-induced hyperglycemia associated with obesity. P. obesus does not exhibit hyperglycemia in its natural desert habitat, feeding on low caloric vegetation. However, when fed regular laboratory chow containing higher caloric density, the animals develop moderate obesity and hyperglycemia. Diabetes development and progression is very fast in P. obesus. The animals reach the irreversible hypoinsulinemic stage of the disease, in which a marked reduction of β-cell mass is apparent, within 4-6 weeks of high caloric diet. The present review describes the P. obesus of the Hebrew University colony, with emphasis on its use for the study of β-cell dysfunction in type 2 diabetes.

AMP-activated protein kinase (AMPK) mediates nutrient regulation of thioredoxin-interacting protein (TXNIP) in pancreatic beta-cells

Thioredoxin-interacting protein (TXNIP) regulates critical biological processes including inflammation, stress and apoptosis. TXNIP is upregulated by glucose and is a critical mediator of hyperglycemia-induced beta-cell apoptosis in diabetes. In contrast, the saturated long-chain fatty acid palmitate, although toxic to the beta-cell, inhibits TXNIP expression. The mechanisms involved in the opposing effects of glucose and fatty acids on TXNIP expression are unknown. We found that both palmitate and oleate inhibited TXNIP in a rat beta-cell line and islets. Palmitate inhibition of TXNIP was independent of fatty acid beta-oxidation or esterification. AMP-activated protein kinase (AMPK) has an important role in cellular energy sensing and control of metabolic homeostasis; therefore we investigated its involvement in nutrient regulation of TXNIP. As expected, glucose inhibited whereas palmitate stimulated AMPK. Pharmacologic activators of AMPK mimicked fatty acids by inhibiting TXNIP. AMPK knockdown increased TXNIP expression in presence of high glucose with and without palmitate, indicating that nutrient (glucose and fatty acids) effects on TXNIP are mediated in part via modulation of AMPK activity. TXNIP is transcriptionally regulated by carbohydrate response element-binding protein (ChREBP). Palmitate inhibited glucose-stimulated ChREBP nuclear entry and recruitment to the Txnip promoter, thereby inhibiting Txnip transcription. We conclude that AMPK is an important regulator of Txnip transcription via modulation of ChREBP activity. The divergent effects of glucose and fatty acids on TXNIP expression result in part from their opposing effects on AMPK activity. In light of the important role of TXNIP in beta-cell apoptosis, its inhibition by fatty acids can be regarded as an adaptive/protective response to glucolipotoxicity. The finding that AMPK mediates nutrient regulation of TXNIP may have important implications for the pathophysiology and treatment of diabetes.

β-Cell failure in type 2 diabetes

Type 2 diabetic patients are insulin resistant as a result of obesity and a sedentary lifestyle. Nevertheless, it has been known for the past five decades that insulin response to nutrients is markedly diminished in type 2 diabetes. There is now a consensus that impaired glucose regulation cannot develop without insulin deficiency. First-phase insulin response to glucose is lost very early in the development of type 2 diabetes. Several prospective studies have shown that impaired insulin response to glucose is a predictor of future impaired glucose tolerance (IGT) and type 2 diabetes. Recently discovered type 2 diabetes-risk gene variants influence β-cell function, and might represent the molecular basis for the low insulin secretion that predicts future type 2 diabetes. We believe type 2 diabetes develops on the basis of normal but 'weak'β-cells unable to cope with excessive functional demands imposed by overnutrition and insulin resistance. Several laboratories have shown a reduction in β-cell mass in type 2 diabetes and IGT, whereas others have found modest reductions and most importantly, a large overlap between β-cell masses of diabetic and normoglycemic subjects. Therefore, at least initially, the β-cell dysfunction of type 2 diabetes seems more functional than structural. However, type 2 diabetes is a progressive disorder, and animal models of diabetes show β-cell apoptosis with prolonged hyperglycemia/hyperlipemia (glucolipotoxicity). β-Cells exposed in vitro to glucolipotoxic conditions show endoplasmic reticulum (ER) and oxidative stress. ER stress mechanisms might participate in the adaptation of β-cells to hyperglycemia, unless excessive. β-Cells are not deficient in anti-oxidant defense, thioredoxin playing a major role. Its inhibitor, thioredoxin-interacting protein (TXNIP), might be important in leading to β-cell apoptosis and type 2 diabetes. These topics are intensively investigated and might lead to novel therapeutic approaches. (J Diabetes Invest, doi: 10.1111/j.2040-1124.2010.00094.x, 2011).