Altered glycolysis triggers impaired mitochondrial metabolism and mTORC1 activation in diabetic β-cells

Chronic hyperglycaemia causes a dramatic decrease in mitochondrial metabolism and insulin content in pancreatic β-cells. This underlies the progressive decline in β-cell function in diabetes. However, the molecular mechanisms by which hyperglycaemia produces these effects remain unresolved. Using isolated islets and INS-1 cells, we show here that one or more glycolytic metabolites downstream of phosphofructokinase and upstream of GAPDH mediates the effects of chronic hyperglycemia. This metabolite stimulates marked upregulation of mTORC1 and concomitant downregulation of AMPK. Increased mTORC1 activity causes inhibition of pyruvate dehydrogenase which reduces pyruvate entry into the tricarboxylic acid cycle and partially accounts for the hyperglycaemia-induced reduction in oxidative phosphorylation and insulin secretion. In addition, hyperglycaemia (or diabetes) dramatically inhibits GAPDH activity, thereby impairing glucose metabolism. Our data also reveal that restricting glucose metabolism during hyperglycaemia prevents these changes and thus may be of therapeutic benefit. In summary, we have identified a pathway by which chronic hyperglycaemia reduces β-cell function.

The KCNJ11-E23K Gene Variant Hastens Diabetes Progression by Impairing Glucose-Induced Insulin Secretion

The ATP-sensitive K⁺ (K_ATP) channel controls blood glucose levels by coupling glucose metabolism to insulin secretion in pancreatic β-cells. E23K, a common polymorphism in the pore-forming K_ATP channel subunit (KCNJ11) gene, has been linked to increased risk of type 2 diabetes. Understanding the risk-allele-specific pathogenesis has the potential to improve personalized diabetes treatment, but the underlying mechanism has remained elusive. Using a genetically engineered mouse model, we now show that the K23 variant impairs glucose-induced insulin secretion and increases diabetes risk when combined with a high-fat diet (HFD) and obesity. K_ATP-channels in β-cells with two K23 risk alleles (KK) showed decreased ATP inhibition, and the threshold for glucose-stimulated insulin secretion from KK islets was increased. Consequently, the insulin response to glucose and glycemic control was impaired in KK mice fed a standard diet. On an HFD, the effects of the KK genotype were exacerbated, accelerating diet-induced diabetes progression and causing β-cell failure. We conclude that the K23 variant increases diabetes risk by impairing insulin secretion at threshold glucose levels, thus accelerating loss of β-cell function in the early stages of diabetes progression.

Phenotype of a transient neonatal diabetes point mutation (SUR1-R1183W) in mice

Background: The K ATP channel plays a key role in glucose homeostasis by coupling metabolically generated changes in ATP to insulin secretion from pancreatic beta-cells.  Gain-of-function mutations in either the pore-forming (Kir6.2) or regulatory (SUR1) subunit of this channel are a common cause of transient neonatal diabetes mellitus (TNDM), in which diabetes presents shortly after birth but remits within the first few years of life, only to return in later life. The reasons behind this time dependence are unclear. Methods: In an attempt to understand the mechanism behind diabetes remission and relapse, we generated mice expressing the common TNDM mutation SUR1-R1183W. We employed Cre/LoxP technology for both inducible and constitutive expression of SUR1-R1183W specifically in mouse beta-cells, followed by investigation of their phenotype using glucose tolerance tests and insulin secretion from isolated islets.  Results: We found that the R1183W mutation impaired inhibition of K ATP channels by ATP when heterologously expressed in human embryonic kidney cells. However, neither induced nor constitutive expression of SUR1-R1183W in mice resulted in changes in blood glucose homeostasis, compared to littermate controls. When challenged with a high fat diet, female mice expressing SUR1-R1183W showed increased weight gain, elevated blood glucose and impaired glycaemic control, but glucose-stimulated insulin secretion from pancreatic islets appeared unchanged. Conclusions: The mouse model of TNDM did not recapitulate the human phenotype. We discuss multiple potential reasons why this might be the case. Based on our findings, we recommend future TNDM mouse models employing a gain-of-function SUR1 mutation should be created using the minimally invasive CRISPR/Cas technology, which avoids many potential pitfalls associated with the Cre/LoxP system.

Phenotype of a transient neonatal diabetes point mutation (SUR1-R1183W) in mice

Background: The K ATP channel plays a key role in glucose homeostasis by coupling metabolically generated changes in ATP to insulin secretion from pancreatic beta-cells.  Gain-of-function mutations in either the pore-forming (Kir6.2) or regulatory (SUR1) subunit of this channel are a common cause of transient neonatal diabetes mellitus (TNDM), in which diabetes presents shortly after birth but remits within the first few years of life, only to return in later life. The reasons behind this time dependence are unclear. Methods: In an attempt to understand the mechanism behind diabetes remission and relapse, we generated mice expressing the common TNDM mutation SUR1-R1183W. We employed Cre/LoxP technology for both inducible and constitutive expression of SUR1-R1183W specifically in mouse beta-cells, followed by investigation of their phenotype using glucose tolerance tests and insulin secretion from isolated islets.  Results: We found that the R1183W mutation impaired inhibition of K ATP channels by ATP when heterologously expressed in human embryonic kidney cells. However, neither induced nor constitutive expression of SUR1-R1183W in mice resulted in changes in blood glucose homeostasis, compared to littermate controls. When challenged with a high fat diet, female mice expressing SUR1-R1183W showed increased weight gain, elevated blood glucose and impaired glycaemic control, but glucose-stimulated insulin secretion from pancreatic islets appeared unchanged. Conclusions: The mouse model of TNDM did not recapitulate the human phenotype. We discuss multiple potential reasons why this might be the case. Based on our findings, we recommend future TNDM mouse models employing a gain-of-function SUR1 mutation should be created using the minimally invasive CRISPR/Cas technology, which avoids many potential pitfalls associated with the Cre/LoxP system.

Diabetes causes marked inhibition of mitochondrial metabolism in pancreatic β-cells

Diabetes is a global health problem caused primarily by the inability of pancreatic β-cells to secrete adequate levels of insulin. The molecular mechanisms underlying the progressive failure of β-cells to respond to glucose in type-2 diabetes remain unresolved. Using a combination of transcriptomics and proteomics, we find significant dysregulation of major metabolic pathways in islets of diabetic βV59M mice, a non-obese, eulipidaemic diabetes model. Multiple genes/proteins involved in glycolysis/gluconeogenesis are upregulated, whereas those involved in oxidative phosphorylation are downregulated. In isolated islets, glucose-induced increases in NADH and ATP are impaired and both oxidative and glycolytic glucose metabolism are reduced. INS-1 β-cells cultured chronically at high glucose show similar changes in protein expression and reduced glucose-stimulated oxygen consumption: targeted metabolomics reveals impaired metabolism. These data indicate hyperglycaemia induces metabolic changes in β-cells that markedly reduce mitochondrial metabolism and ATP synthesis. We propose this underlies the progressive failure of β-cells in diabetes.

FTO demethylase activity is essential for normal bone growth and bone mineralization in mice

The Fto gene locus has been linked to increased body weight and obesity in human population studies, but the role of the actual FTO protein in adiposity has remained controversial. Complete loss of FTO protein in mouse and of FTO function in human patients has multiple and variable effects. To determine which effects are due to the ability of FTO to demethylate mRNA, we genetically engineered a mouse with a catalytically inactive form of FTO. Our results demonstrate that FTO catalytic activity is not required for normal body composition although it is required for normal body size and viability. Strikingly, it is also essential for normal bone growth and mineralization, a previously unreported FTO function.

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A mouse model for a human lethal FTO catalytic null mutation was established.

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Lean/fat body composition and energy metabolism parameters were unaffected.

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FTO catalytic activity was required for normal body size and viability.

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Lack of FTO enzymatic activity caused substantial bone demineralization.

Running out of time: the decline of channel activity and nucleotide activation in adenosine triphosphate-sensitive K-channels

K_ATP channels act as key regulators of electrical excitability by coupling metabolic cues—mainly intracellular adenine nucleotide concentrations—to cellular potassium ion efflux. However, their study has been hindered by their rapid loss of activity in excised membrane patches (rundown), and by a second phenomenon, the decline of activation by Mg-nucleotides (DAMN). Degradation of PI(4,5)P₂ and other phosphoinositides is the strongest candidate for the molecular cause of rundown. Broad evidence indicates that most other determinants of rundown (e.g. phosphorylation, intracellular calcium, channel mutations that affect rundown) also act by influencing K_ATP channel regulation by phosphoinositides. Unfortunately, experimental conditions that reproducibly prevent rundown have remained elusive, necessitating post hoc data compensation. Rundown is clearly distinct from DAMN. While the former is associated with pore-forming Kir6.2 subunits, DAMN is generally a slower process involving the regulatory sulfonylurea receptor (SUR) subunits. We speculate that it arises when SUR subunits enter non-physiological conformational states associated with the loss of SUR nucleotide-binding domain dimerization following prolonged exposure to nucleotide-free conditions. This review presents new information on both rundown and DAMN, summarizes our current understanding of these processes and considers their physiological roles.

This article is part of the themed issue ‘Evolution brings Ca²⁺ and ATP together to control life and death’.

The value of in vitro studies in a case of neonatal diabetes with a novel Kir6.2-W68G mutation

In infants, especially with novel previously undescribed mutations of the K_ATP channel causing neonatal diabetes, in vitro studies can be used to both predict the response to sulphonylurea treatment and support a second trial of glibenclamide at higher than standard doses if the expected response is not observed.

FTO influences adipogenesis by regulating mitotic clonal expansion

The fat mass and obesity-associated (FTO) gene plays a pivotal role in regulating body weight and fat mass; however, the underlying mechanisms are poorly understood. Here we show that primary adipocytes and mouse embryonic fibroblasts (MEFs) derived from FTO overexpression (FTO-4) mice exhibit increased potential for adipogenic differentiation, while MEFs derived from FTO knockout (FTO-KO) mice show reduced adipogenesis. As predicted from these findings, fat pads from FTO-4 mice fed a high-fat diet show more numerous adipocytes. FTO influences adipogenesis by regulating events early in adipogenesis, during the process of mitotic clonal expansion. The effect of FTO on adipogenesis appears to be mediated via enhanced expression of the pro-adipogenic short isoform of RUNX1T1, which enhanced adipocyte proliferation, and is increased in FTO-4 MEFs and reduced in FTO-KO MEFs. Our findings provide novel mechanistic insight into how upregulation of FTO leads to obesity.

Mutations in the FTO gene have been linked to obesity. Here, Merkestein et al. provide in vitro and in vivo evidence that FTO directly regulates adipogenesis in mice at the stage of mitotic clonal expansion, likely by modulating the expression of the transcription factor RUNX1T1.

Smooth muscle BK channel activity influences blood pressure independent of vascular tone in mice

The large conductance voltage- and Ca(2+)-activated K(+) (BK) channel is an important determinant of vascular tone and contributes to blood pressure regulation. Both activities depend on the ancillary BKβ1 subunit. To determine the significance of smooth muscle BK channel activity for blood pressure regulation, we investigated the potential link between changes in arterial tone and altered blood pressure in BKβ1 knockout (BKβ1(-/-)) mice from three different genetically defined strains. While vascular tone was consistently increased in all BKβ1(-/-) mice independent of genetic background, BKβ1(-/-) strains exhibited increased (strain A), unaltered (strain B) or decreased (strain C) mean arterial blood pressures compared to their corresponding BKβ1(+/+) controls. In agreement with previous data on aldosterone regulation by renal/adrenal BK channel function, BKβ1(-/-) strain A mice have increased plasma aldosterone and increased blood pressure. Consistently, blockade of mineralocorticoid receptors by spironolactone treatment reversibly restored the elevated blood pressure to the BKβ1(+/+) strain A level. In contrast, loss of BKβ1 did not affect plasma aldosterone in strain C mice. Smooth muscle-restricted restoration of BKβ1 expression increased blood pressure in BKβ1(-/-) strain C mice, implying that impaired smooth muscle BK channel activity lowers blood pressure in these animals. We conclude that BK channel activity directly affects vascular tone but influences blood pressure independent of this effect via different pathways.

The effects of orlistat on body weight and glycaemic control in overweight patients with type 2 diabetes: a randomized, placebo-controlled trial

AIM

To assess the long-term effects of orlistat on body weight, glycaemic control and cardiovascular risk factors in overweight patients with type 2 diabetes.

METHODS

This was a multicentre, randomized, placebo-controlled study with a 4-week placebo plus diet lead-in period and a 48-week, double-blind treatment period. Overweight or obese adults [body mass index (BMI) >or= 28 kg/m2] with HbA1c of 6.5-11% and clinical type 2 diabetes were randomized to orlistat (120 mg t.i.d. n = 189) or placebo (n = 180) in conjunction with a low-calorie diet. Patients had either received sulphonylurea therapy for at least 2 months before the study or were not receiving any antidiabetic medication (the majority of which were drug-naïve).

RESULTS

After 1 year, patients in the orlistat group lost significantly more weight than patients in the placebo group (-5.4% vs. -3.6%; p = 0.006). Moreover, significantly more patients achieved weight loss of >or= 5% with orlistat compared with placebo (51.3% vs. 31.6%; p = 0.0001). Patients treated with orlistat also had significantly greater improvements than placebo-treated patients in HbA1c (-0.9% vs. -0.4%; p < 0.001), fasting glucose (-1.6 vs.-0.7 mmol/l; p = 0.004) and post-prandial glucose (-1.8 vs. -0.5 mmol/l; p = 0.003). In addition, orlistat-treated patients had a significantly greater reduction in LDL cholesterol compared with placebo. Overall, orlistat had a similar safety profile to placebo, with the exception of a higher incidence of generally mild and transient gastrointestinal events known to be associated with the mode of action of orlistat.

CONCLUSIONS

Treatment with orlistat plus diet resulted in significant weight loss, improved glycaemic control and cardiovascular risk factor profile in overweight patients with type 2 diabetes.

Mutations in the hepatocyte nuclear factor-1alpha gene in MODY and early-onset NIDDM: evidence for a mutational hotspot in exon 4

We have recently shown that mutations in the gene encoding the transcription factor hepatocyte nuclear factor (HNF)-1alpha are the cause of one form of maturity-onset diabetes of the young (MODY3). Here, we report the exon-intron organization and partial sequence of the human HNF-1alpha gene. In addition, we have screened the ten exons and flanking introns of this gene for mutations in a group of 25 unrelated white subjects from Germany who presented with NIDDM before 35 years of age and had a first-degree relative with NIDDM. Mutations were identified in nine of these individuals, suggesting that mutations in the HNF-1alpha gene are a common cause of diabetes in German subjects with early-onset NIDDM and a family history of diabetes. Thus, screening for mutations in this gene may be indicated in subjects with early-onset NIDDM. Interestingly, three of the nine mutations occurred at the same site in exon 4 with insertion of a C in a polyC tract, centered around codon 290 (designated Pro291fsinsC), thereby resulting in a frameshift during translation and premature termination. Analyses of linked DNA polymorphisms in the HNF-1alpha gene indicated that the Pro291fsinsC mutation was present on a different haplotype in each subject, implying that the polyC tract represents a mutational hot spot. We have also identified the mutation in the HNF-1alpha gene in the Jutland pedigree, one of the original MODY pedigrees reported in the literature, as being a T-->G substitution in codon 241, resulting in the replacement of a conserved Cys by Gly (C241G). The information on the sequence of the HNF-1alpha gene and its promoter region will facilitate the search for mutations in other subjects and studies of the role of the gene in determining normal beta-cell functions.

[Differential therapy with oral antidiabetic drugs]

The more than 3 million type II diabetics in Germany constitute a true therapeutic challenge. Type II diabetes mellitus is part of the so-called metabolic syndrome characterized by the problem of insulin resistance/hyperinsulinemia. Treatment of type II diabetes aims at reducing insulin resistance. Oral antidiabetic management must be based on diabetic diet, in conjunction--if needed--with monotherapy with acarbose or metformin. Only after exhausting these principles of management, acarbose or metformin may be combined with sulfonylurea. Primary monotherapy with insulinotropically acting sulfonylureas is, in most cases, no longer appropriate as we are learning more about the pathophysiology of metabolic syndrome.

Secretion of antileucoprotease from a human lung tumor cell line

Two human tumor cell lines were analyzed for the production of human antileucoprotease (ALP). One of them, a human squamous lung carcinoma cell line (HS-24) synthesized, as confirmed by Western blot analysis, high amounts of ALP in serum-free medium. The supernatant inhibited elastase, chymotrypsin and trypsin. Northern blot analysis with an 18-mer radiolabelled oligonucleotide, derived from an ALP specific cDNA clone, revealed a specific mRNA of about 700-800 nucleotides in HS-24 tumor cells. In contrast, a secondary human lung tumor cell line (SB-3), derived from the adrenal cortex, did not synthesize ALP when assayed under identical conditions. The supernatant inhibited only trypsin and chymotrypsin.

[Long-term therapy with human insulin. Clinical experiences]

During in-patient admission after metabolic stabilisation insulin treatment of 32 type-1-diabetics was changed from bovine insulin (Depot-Insulin Hoechst CR) to semisynthetic human insulin of the intermediary type (Depot-H-Insulin Hoechst). Out-patient follow-up over a period of 12 months showed constant satisfactory mean day blood sugar values and morning postprandial blood sugar. 14 patients, however, showed significantly increased fasting blood sugar values which could be adjusted after change of evening insulin injections to the longer acting NPH insulin without normal insulin admixture (Basal-H-insulin Hoechst). In addition, a slight but not significant reduction of daily insulin requirements could be noted. There was no change of the relation between morning and evening insulin dosage. Hypoglycaemias did not occur significantly more frequently and hypoglycaemic symptoms showed no alterations.

[Combination therapy with insulin and sulfonylurea in secondary failure of sulfonylurea therapy]

Among 16 type II diabetics who were secondary failures on sulphonylurea treatment alone, eight (group A) were changed to a combined regimen of insulin and sulphonylurea (glibenclamide), the other eight (group B) received insulin alone. Patients in group A were on the combined schedule for ten days, followed by ten days of insulin alone, followed by six months of combined treatment. In the first ten days those on combined treatment in group A required on average 30% less insulin that those of group B, with a comparable metabolic state, while insulin requirements significantly rose after ten days on insulin alone. After renewed combined treatment the insulin dose could once again be reduced, but after eight weeks there was a rise in the insulin requirement, as for group B patients. During the further observation period of three months there were no significant differences in the insulin requirement between the two groups. Combination treatment with sulphonylurea and insulin in secondary failures thus, in the short term, reduces insulin requirement; but in the long term it is not significantly different from insulin treatment alone.

[Des-phe-insulin-containing intermediary insulin compared with usual commercial preparations (author's transl)]

In 20 type I diabetics comparison were made between Optisulin -depot CS (25% dissolved porcine Des-Phe-insulin/insulin and 75% crystalline porcine insulin), an intermediary insulin without depot additives, and two depot-additive containing insulins (Depot Insulin Hoechst CS and insulin Leo Mixtard) during a period of hospitalization. Des-Phe-insulins are insulins in which phenylalanine has been split off from the B chain of insulin. Since mixtures of Des-Phe-insulins with crystalline insulins are stable, there is the possibility of manufacturing monospecies-intermediary insulins without depot additives. Continued surveillance of blood sugar during application of Optisulin depot CS showed good control of the diabetic metabolism when compared with the commercially available insulins. Optisulin depot CS in its action is comparable to the two commercially available depot insulins. Side effects and allergic skin changes did not occur. Thus Optisulin depot SC adds to the at present available range of Des-Phe-insulin-containing insulins.

[Are des-Phe-insulin-containing insulin combinations preferable to regular trade preparations?]

Des-phe-insulins are modified insulins in which phenylalanin is eliminated from the b-chain of the normal insulin. Preparations containing des-phe-insulin and cristalline insulin have a good stability and one can produce insulins with different profiles. 50 diabetics were treated with different des-phe-insulin containing preparations. The carbohydrate metabolism was well controlled during the application of the new preparations. There were no side effects nor skin allergies. Des-phe-insulin containing insulins present a new interesting aspect in the treatment of insulin dependent diabetes mellitus.