The association of smoking status with glycemic control, metabolic profile and diabetic complications- Results of the Australian National Diabetes Audit (ANDA)

BACKGROUND

Tobacco smoking and diabetes mellitus contribute significantly to the overall health burden and mortality of Australians. We aimed to assess the relationship of smoking with glycemic control, metabolic profile and complications in Australian patients living with diabetes.

METHODS

We analysed the 2011-2017 biennial Australian National Diabetes Audit cross-sectional data. Patients were classified as current, past or never smokers. Linear (or quantile) and logistic regression models were used to assess for associations.

RESULTS

Data from 15,352 patients were analysed, including 72.2% with type 2 diabetes. Current smokers comprised 13.5% of the study population. Current and past smokers had a median HbA_1c that was 0.49% and 0.14% higher than never smokers, respectively, as well as higher triglyceride and lower HDL levels (all p values < .0001). Compared to never smokers, current smokers had higher odds of severe hypoglycemia and current and past smokers had higher odds of myocardial infarction, stroke, peripheral vascular disease, lower limb amputation, erectile dysfunction and peripheral neuropathy (all p values ≤.001), with no significant change over time.

CONCLUSION

When compared to never smokers, current and past smokers had poorer glycemic and lipid control and higher odds of macrovascular and microvascular complications. Despite this, current smoking remains prevalent among Australians with diabetes.

Younger people with Type 2 diabetes have poorer self-care practices compared with older people: results from the Australian National Diabetes Audit

AIM

This cross-sectional study compares the self-care practices of younger and older people with Type 2 diabetes.

METHODS

Data were analysed from the Australian National Diabetes Audit (ANDA) including 2552 adults with Type 2 diabetes from Australian Diabetes Centres. Pre-specified demographic and clinical variables were obtained. Self-care variables (physical activity, following dietary recommendations, medication adherence and monitoring blood glucose levels) were compared in people ≤ 64 and > 64 years of age.

RESULTS

Mean age (± sd) of participants was 63 ± 13 years overall, 53 ± 9 years for the younger group and 73 ± 6 years for the older group. A greater proportion of younger people had HbA_1c levels > 53 mmol/mol (> 7.0%) (76% vs. 68%), reported difficulty following dietary recommendations (50% vs. 32%) and forgetting medications (37% vs. 22%) compared with older people (all P-values <0.001). A smaller proportion of younger compared with older people reported monitoring their blood glucose levels as often as recommended (60% vs. 70%, P < 0.001). Similar proportions of people aged ≤ 64 and > 64 years required insulin therapy (59% vs. 57%, P = 0.200). Younger age was associated with a twofold increase in the odds of not following the recommended self-care practices after adjustment for gender, smoking, insulin therapy, depression and allied health attendance (all P < 0.001).

CONCLUSIONS

Despite shorter diabetes duration, younger age was associated with worse glycaemic control and poorer diabetes self-care practices among people with Type 2 diabetes. Targeted strategies are required to optimize diabetes self-care practices and thereby glycaemic control.

A low-carbohydrate high-fat diet increases weight gain and does not improve glucose tolerance, insulin secretion or β-cell mass in NZO mice

Background/Objectives:

Dietary guidelines for the past 20 years have recommended that dietary fat should be minimized. In contrast, recent studies have suggested that there could be some potential benefits for reducing carbohydrate intake in favor of increased fat. It has also been suggested that low-carbohydrate diets be recommended for people with type 2 diabetes. However, whether such diets can improve glycemic control will likely depend on their ability to improve β-cell function, which has not been studied. The objective of the study was to assess whether a low-carbohydrate and therefore high-fat diet (LCHFD) is beneficial for improving the endogenous insulin secretory response to glucose in prediabetic New Zealand Obese (NZO) mice.

Methods:

NZO mice were maintained on either standard rodent chow or an LCHFD from 6 to 15 weeks of age. Body weight, food intake and blood glucose were assessed weekly. Blood glucose and insulin levels were also assessed after fasting and re-feeding and during an oral glucose tolerance test. The capacity of pancreatic β-cells to secrete insulin was assessed in vivo with an intravenous glucose tolerance test. β-Cell mass was assessed in histological sections of pancreata collected at the end of the study.

Results:

In NZO mice, an LCHFD reduced plasma triglycerides (P=0.001) but increased weight gain (P<0.0001), adipose tissue mass (P=0.0015), high-density lipoprotein cholesterol (P=0.044) and exacerbated glucose intolerance (P=0.013). Although fasting insulin levels tended to be higher (P=0.08), insulin secretory function in LCHFD-fed mice was not improved (P=0.93) nor was β-cell mass (P=0.75).

Conclusions:

An LCHFD is unlikely to be of benefit for preventing the decline in β-cell function associated with the progression of hyperglycemia in type 2 diabetes.

Improving glucose tolerance by reducing weight gain in a polygenic obese mouse model: use of a high protein diet

Diets to decrease body weight have limited success in achieving and importantly maintaining this weight loss long-term. It has recently been suggested that energy intake can be regulated by the amount of protein ingested, termed the protein leverage hypothesis. In this study, we determined whether a high protein diet would be effective in achieving and maintaining weight loss in a genetically obese model, the New Zealand Obese (NZO) mouse. NZO and C57BL/6J (C57) control mice were fed a high protein or chow diet for 5 weeks from weaning (3 weeks of age). Body weight and food intake were determined. Mice on the same diet were bred to produce offspring that were fed either a chow or high protein diet. Body weight, food intake, and glucose tolerance were determined. Feeding NZO and C57 mice a high protein diet for 5 weeks resulted in reduced food intake and consequently energy intake and body weight gain compared with mice on a chow diet. NZO mice fed a high protein diet showed a significant improvement in glucose tolerance compared with their chow-fed counterparts, while no difference was seen in C57 mice fed chow or protein diet. The offspring of NZO mice that were fed a high protein diet during gestation and weaning were also lighter and displayed improved glucose tolerance compared with chow fed animals. We conclude that a high protein diet is a reasonable strategy to reduce body weight gain and improve glucose tolerance in the NZO mouse, a polygenic model of obesity.

Damage to enteric neurons occurs in mice that develop fatty liver disease but not diabetes in response to a high-fat diet

BACKGROUND

Disorders of gastrointestinal functions that are controlled by enteric neurons commonly accompany fatty liver disease. Established fatty liver disease is associated with diabetes, which itself induces enteric neuron damage. Here, we investigate the relationship between fatty liver disease and enteric neuropathy, in animals fed a high-fat, high-cholesterol diet in the absence of diabetes.

METHODS

Mice were fed a high-fat, high-cholesterol diet (21% fat, 2% cholesterol) or normal chow for 33 weeks. Liver injury was assessed by hematoxylin and eosin, picrosirius red staining, and measurement of plasma alanine aminotransaminase (ALT). Quantitative immunohistochemistry was performed for different types of enteric neurons.

KEY RESULTS

The mice developed steatosis, steatohepatitis, fibrosis, and a 10-fold increase in plasma ALT, indicative of liver disease. Oral glucose tolerance was unchanged. Loss and damage to enteric neurons occurred in the myenteric plexus of ileum, cecum, and colon. Total numbers of neurons were reduced by 15-30% and neurons expressing nitric oxide synthase were reduced by 20-40%. The RNA regulating protein, Hu, became more concentrated in the nuclei of enteric neurons after high-fat feeding, which is an indication of stress on the enteric nervous system. There was also disruption of the neuronal cytoskeletal protein, neurofilament medium.

CONCLUSIONS & INFERENCES

Enteric neuron loss and damage occurs in animals with fatty liver disease in the absence of glucose intolerance. The enteric neuron damage may contribute to the gastrointestinal complications of fatty liver disease.

A novel mechanism regulating insulin secretion involving Herpud1 in mice

AIMS/HYPOTHESIS

Type 2 diabetes results from beta cell dysfunction after prolonged physiological stress, which causes oversecretion of insulin. We recently found that insulin hypersecretion is mediated by at least two genes. Among mouse models of type 2 diabetes, the DBA/2 mouse strain is more susceptible to diabetes than is the C57BL/6J (B6J) strain. One distinctive feature of the DBA/2 mouse is that it hypersecretes insulin, independent of changes in insulin sensitivity; we identified Nnt as a gene responsible for this trait.

METHODS

To identify the other gene(s) affecting insulin hypersecretion, we tested a panel of recombinant inbred BXD strains, which have different combinations of B6 and DBA/2 alleles.

RESULTS

We found that 25% of the BXD strains hypersecreted insulin in response to glucose. Microarray profiling of islets from high- and low-secretor strains showed that at least four genes were differentially expressed. One gene was consistently underexpressed in islets from both DBA/2 and the high-secretor BXD strains. This gene (Herpud1 or Herp) encodes the 54 kDa endoplasmic reticulum stress-inducible protein (HERP) that resides in the integral endoplasmic reticulum membrane. To test directly whether Herpud1 can interact with Nnt, Herpud1 was either knocked down or overexpressed in MIN6 cells. These results showed that when Herpud1 was suppressed, Nnt expression was reduced, while overexpression of Herpud1 led to increased Nnt expression. Furthermore, Herpud1 suppression resulted in significantly decreased glucose-stimulated insulin secretion in the DBA/2 islets but not B6J islets.

CONCLUSIONS/INTERPRETATION

We conclude that Herpud1 regulates insulin secretion via control of Nnt expression.

First phase insulin secretion and type 2 diabetes

Type 2 diabetes (T2D) is a metabolic disorder characterised by the inability of β-cells to secrete enough insulin to maintain glucose homeostasis. Pancreatic β-cells secrete insulin in a biphasic manner, first and second phase insulin secretion, and loss of first phase insulin secretion is an independent predictor of T2D onset. Restoration of first phase insulin secretion has been shown to improve blood glucose in T2D by suppressing hepatic glucose production and priming insulin sensitive tissue to more readily take up glucose and has thus prompted numerous studies into its regulation. First phase insulin secretion is initiated primarily by the classical triggering pathway, a complex system comprised of multiple stimulatory signals. Recent studies have identified a number of novel regulatory factors that are crucial for first phase insulin secretion and glucose homeostasis. These include, among others, hypoxia inducible factor 1α, von Hippel-Lindau, factor inhibiting HIF, nicotinamide phospho-ribosyl-transferase, and the sirtuin family. This review will outline how first phase insulin secretion is initiated and detail some of the recent findings in its regulation.

Receptor for advanced glycation end-products (RAGE) provides a link between genetic susceptibility and environmental factors in type 1 diabetes

AIMS/HYPOTHESIS

This group of studies examines human genetic susceptibility conferred by the receptor for advanced glycation end-products (RAGE) in type 1 diabetes and investigates how this may interact with a western environment.

METHODS

We analysed the AGER gene, using 13 tag SNPs, in 3,624 Finnish individuals from the FinnDiane study, followed by AGER associations with a high risk HLA genotype (DR3)-DQA1*05-DQB1*02/DRB1*0401-DQB1*0302 (n = 546; HLA-DR3/DR4), matched in healthy newborn infants from the Finnish Type 1 Diabetes Prediction and Prevention (DIPP) Study (n = 373) using allelic analysis. We also studied islets and circulating RAGE in NODLt mice.

RESULTS

The rs2070600 and rs17493811 polymorphisms predicted increased risk of type 1 diabetes, whereas the rs9469089 SNP was related to decreased risk, on a high risk HLA background. Children from the DIPP study also showed a decline in circulating soluble RAGE levels, at seroconversion to positivity for type 1 diabetes-associated autoantibodies. Islet RAGE and circulating soluble RAGE levels in prediabetic NODLt mice decreased over time and were prevented by the AGE lowering therapy alagebrium chloride. Alagebrium chloride also decreased the incidence of autoimmune diabetes and restored islet RAGE levels.

CONCLUSIONS/INTERPRETATION

These studies suggest that inherited AGER gene polymorphisms may confer susceptibility to environmental insults. Declining circulating levels of soluble RAGE, before the development of overt diabetes, may also be predictive of clinical disease in children with high to medium risk HLA II backgrounds and this possibility warrants further investigation in a larger cohort.

Obesity and type 2 diabetes: slow down!--Can metabolic deceleration protect the islet beta cell from excess nutrient-induced damage?

Islet beta-cell dysfunction is a characteristic and the main cause of hyperglycaemia of Type 2 diabetes. Understanding the mechanisms that cause beta-cell dysfunction will lead to better therapeutic outcomes for patients with Type 2 diabetes. Chronic fatty acid exposure of susceptible islet beta-cells causes dysfunction and death and this is associated with increased reactive oxygen species production leading to oxidative stress and increased endoplasmic reticulum stress. We present the hypothesis that metabolic deceleration can reduce both oxidative and endoplasmic reticulum stress and lead to improved beta-cell function and viability when exposed to a deleterious fat milieu. This is illustrated by the C57BL/6J mouse which is characterised by reduced insulin secretion and glucose intolerance associated with a mutation in nicotinamide nucleotide transhydrogenase (Nnt) but is resistant to obesity induced diabetes. On the other hand the DBA/2 mouse has comparatively higher insulin secretion and better glucose tolerance associated with increased Nnt activity but is susceptible to obesity-induced diabetes, possibly as a result of increased oxidative stress. We therefore suggest that in states of excess nutrient load, a reduced ability to metabolise this load may protect both the function and viability of beta-cells. Strategies that reduce metabolic flux when beta-cells are exposed to nutrient excess need to be considered when treating Type 2 diabetes.

Identification of the Alzheimer's disease amyloid precursor protein (APP) and its homologue APLP2 as essential modulators of glucose and insulin homeostasis and growth

The amyloid precursor protein (APP), the source of the neurotoxic amyloid beta (A beta) peptide involved in Alzheimer's disease (AD), belongs to a conserved family of related proteins. In mammals, the APP family contains amyloid precursor-like protein 1 (APLP1) and amyloid precursor-like protein 2 (APLP2). Whilst a number of activities have been attributed to the APP family, an overall function has not been definitively established. While ablating either the APP or APLP2 gene in mice produces minimal phenotypic change, the combined knockout of these genes in mice causes postnatal mortality. Postnatal survival therefore requires a shared but unknown function of APP and APLP2. To investigate the biochemical basis for the postnatal lethality, plasma was analysed from double knockout mice (APP-/- APLP2-/-) 2 days before birth, at gestational day E17, and from mice at 12-16 h after birth. The postnatal double knockouts had 66% lower plasma glucose levels than their wild-type controls and 50% lower than their single knockout counterparts. Interestingly, the postnatal double knockouts displayed hyperinsulinaemia, as shown by inappropriate plasma insulin levels, given their degree of hypoglycaemia. The single knockout mice also showed hyperinsulinaemia and had 31% lower plasma glucose than the wild-types. While the double knockouts did not survive more than 24 h after birth, the single knockouts reached adulthood and their hypoglycaemia continued. Therefore, APP and APLP2 expression modulates plasma insulin and glucose concentrations. Plasma calcium, magnesium and phosphate were also significantly reduced in the double knockouts compared to the wild-types, and they showed distinctive growth restriction, suggesting the involvement of a metabolic impairment. These results link the expression of the APP and APLP2 genes with glucose homeostasis and growth and therefore identify a novel function for the APP family.

Too much of a good thing: why it is bad to stimulate the beta cell to secrete insulin

In many countries, first- or second-line pharmacological treatment of patients with type 2 diabetes consists of sulfonylureas (such as glibenclamide [known as glyburide in the USA and Canada]), which stimulate the beta cell to secrete insulin. However, emerging evidence suggests that forcing the beta cell to secrete insulin at a time when it is struggling to cope with the demands of obesity and insulin resistance may accelerate its demise. Studies on families with persistent hyperinsulinaemic hypoglycaemia of infancy (PHHI), the primary defect of which is hypersecretion of insulin, have shown that overt diabetes can develop later in life despite normal insulin sensitivity. In addition, in vitro experiments have suggested that reducing insulin secretion from islets isolated from patients with diabetes can restore insulin pulsatility and improve function. This article will explore the hypothesis that forcing the beta cell to hypersecrete insulin may be counterproductive and lead to dysfunction and death via mechanisms that may involve the endoplasmic reticulum and oxidative stress. We suggest that, in diabetes, therapeutic approaches should be targeted towards relieving the demand on the beta cell to secrete insulin.

Increased nicotinamide nucleotide transhydrogenase levels predispose to insulin hypersecretion in a mouse strain susceptible to diabetes

AIMS/HYPOTHESIS

Insulin hypersecretion may be an independent predictor of progression to type 2 diabetes. Identifying genes affecting insulin hypersecretion are important in understanding disease progression. We have previously shown that diabetes-susceptible DBA/2 mice congenitally display high insulin secretion. We studied this model to map and identify the gene(s) responsible for this trait.

METHODS

Intravenous glucose tolerance tests followed by a genome-wide scan were performed on 171 (C57BL/6 x DBA/2) x C57BL/6 backcross mice.

RESULTS

A quantitative trait locus, designated hyperinsulin production-1 (Hip1), was mapped with a logarithm of odds score of 7.7 to a region on chromosome 13. Production of congenic mice confirmed that Hip1 influenced the insulin hypersecretion trait. By studying appropriate recombinant inbred mouse strains, the Hip1 locus was further localised to a 2 Mb interval, which contained only nine genes. Expression analysis showed that the only gene differentially expressed in islets isolated from the parental strains was Nnt, which encodes the mitochondrial proton pump, nicotinamide nucleotide transhydrogenase (NNT). We also found in five mouse strains a positive correlation (r2 = 0.90, p < 0.01) between NNT activity and first-phase insulin secretion, emphasising the importance of this enzyme in beta cell function. Furthermore, of these five strains, only those with high NNT activity are known to exhibit severe diabetes after becoming obese.

CONCLUSIONS/INTERPRETATION

Insulin hypersecretion is associated with increased Nnt expression. We suggest that NNT must play an important role in beta cell function and that its effect on the high insulin secretory capacity of the DBA/2 mouse may predispose beta cells of these mice to failure.

Modulation of central leptin sensitivity and energy balance in a rat model of diet-induced obesity

AIM

The aim of this study was to further explore the time-dependent changes in leptin sensitivity using a rat model of dietary fat-induced obesity and to investigate the potential mechanisms governing these changes.

METHODS

We used male, adult Sprague-Dawley rats that were fed either a standard laboratory chow diet (3% fat) or a high-saturated fat (HF) diet (60% fat) for 2 or 5 weeks. Energy balance (body weight, energy intake and energy expenditure); sensitivity to central leptin and central alpha-melanin stimulating hormone (alpha-MSH) administration and expression levels of hypothalamic ObRb, signal transducers and activators of transcription factor (STAT)-3 phosphorylation, suppressor of cytokine signalling-3 (SOCS-3), proopiomelanocortin (POMC) processing hormones (prohormone convertase-1 and prohormone convertase-2) and neuropeptide Y (NPY) were measured.

RESULTS

After 2 weeks of feeding HF diet, there was an increase in total energy intake (TEI) but a reduction in food intake as measured by the mass of food ingested. Body weight at this time was not significantly different between the two diet groups; however, white adipose tissue (WAT) weight was significantly greater in the HF-fed rats than in the chow-fed rats. In addition, spontaneous physical activity levels were increased, but no changes were observed in resting energy expenditure. Furthermore, chow-fed lean rats responded to central leptin administration by reducing the energy intake by approximately 67 kJ compared with saline treatment (p < 0.05), while the HF-fed diet-induced obese (DIO) rats responded by reducing their energy intake by approximately 197 kJ compared with saline treatment (p < 0.05). After 5 weeks of feeding HF diet, TEI remained significantly higher, body weight was significantly increased by 5% in the HF-fed rats and WAT weight was significantly heavier in HF-fed rats than in the chow-fed lean rats. After leptin treatment, the chow-fed lean rats reduced their energy intake by approximately 97 kJ (p < 0.05); yet, leptin had no significant effect in the HF-fed DIO rats. ObRb protein expression, STAT-3 phosphorylation levels, content and messenger RNA (mRNA) expression of NPY, SOCS-3 mRNA and protein expression and energy intake response to central alpha-MSH administration were not altered after HF diet feeding.

CONCLUSION

These results suggest that early in the course of HF diet-induced weight gain, there was a period of central leptin hypersensitivity, and as the obesity progresses, central leptin insensitivity develops. This insensitivity does not appear to be explained by a downregulation of ObRb protein levels, reduced leptin signalling, an increase in either SOCS-3 or NPY expression or reduced function of the melanocortin system. The effect of an HF diet on other actions of leptin such as its effect on the endocannabinoid system should be investigated.

The influence of genetic background on the induction of oxidative stress and impaired insulin secretion in mouse islets

AIMS/HYPOTHESIS

We determined whether high-glucose-induced beta cell dysfunction is associated with oxidative stress in the DBA/2 mouse, a mouse strain susceptible to islet failure.

MATERIALS AND METHODS

Glucose- and non-glucose-mediated insulin secretion from the islets of DBA/2 and control C57BL/6 mice was determined following a 48-h exposure to high glucose. Flux via the hexosamine biosynthesis pathway was assessed by determining O-glycosylated protein levels. Oxidative stress was determined by measuring hydrogen peroxide levels and the expression of anti-oxidant enzymes.

RESULTS

Exposure to high glucose levels impaired glucose-stimulated insulin secretion in DBA/2 islets but not C57BL/6 islets, and this was associated with reduced islet insulin content and lower ATP levels than in C57BL/6 islets. Exposure of islets to glucosamine for 48 h mimicked the effects of high glucose on insulin secretion in the DBA/2 islets. High glucose exposure elevated O-glycosylated proteins; however, this occurred in islets from both strains, excluding a role for O-glycosylation in the impairment of DBA/2 insulin secretion. Additionally, both glucosamine and high glucose caused an increase in hydrogen peroxide in DBA/2 islets but not in C57BL/6 islets, an effect prevented by the antioxidant N-acetyl-L: -cysteine. Interestingly, while glutathione peroxidase and catalase expression was comparable between the two strains, the antioxidant enzyme manganese superoxide dismutase, which converts superoxide to hydrogen peroxide, was increased in DBA/2 islets, possibly explaining the increase in hydrogen peroxide levels.

CONCLUSIONS/INTERPRETATION

Chronic high glucose culture caused an impairment in glucose-stimulated insulin secretion in DBA/2 islets, which have a genetic predisposition to failure, and this may be the result of oxidative stress.

Threshold effects of glucose transporter-4 (GLUT4) deficiency on cardiac glucose uptake and development of hypertrophy

The aim of this study was to investigate the metabolic and structural consequences of a decrease in glucose transporter-4 (GLUT4) levels on the heart. The CreLoxP system was utilised to delete GLUT4 in muscle tIssue including heart. The presence of the PGK-neoR cassette in the GLUT4-Lox mice resulted in reduced expression in all tIssues to levels 15-30% of wild-type control mice. In mice expressing Cre recombinase, there was a further reduction of GLUT4 in cardiac tIssue to almost undetectable levels. Cardiac glucose uptake was measured basally and during a euglycaemic/hyperinsulinaemic clamp using 2-deoxy-[1-(14)C]glucose. Insulin-stimulated glucose uptake was normal in hearts expressing 15% of normal GLUT4 levels but markedly reduced in mice with more profound reduction in GLUT4. Cardiac enlargement occurred only when GLUT4 levels were less than 5% of normal values. In heart there is a threshold level of GLUT4 above which insulin-stimulated glucose uptake is maintained. As little as 5% of normal GLUT4 levels expressed in heart is sufficient to prevent the development of cardiac hypertrophy.

Peripheral insulin resistance develops in transgenic rats overexpressing phosphoenolpyruvate carboxykinase in the kidney

AIMS/HYPOTHESIS

To study the secondary consequences of impaired suppression of endogenous glucose production (EGP) we have created a transgenic rat overexpressing the gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PEPCK) in the kidney. The aim of this study was to determine whether peripheral insulin resistance develops in these transgenic rats.

METHODS

Whole body rate of glucose disappearance (R(d)) and endogenous glucose production were measured basally and during a euglycaemic/hyperinsulinaemic clamp in phosphoenolpyruvate carboxykinase transgenic and control rats using [6-(3)H]-glucose. Glucose uptake into individual tissues was measured in vivo using 2-[1-(14)C]-deoxyglucose.

RESULTS

Phosphoenolpyruvate carboxykinase transgenic rats were heavier and had increased gonadal and infrarenal fat pad weights. Under basal conditions, endogenous glucose production was similar in phosphoenolpyruvate carboxykinase transgenic and control rats (37.4+/-1.1 vs 34.6+/-2.6 micromol/kg/min). Moderate hyperinsulinaemia (810 pmol/l) completely suppressed EGP in control rats (-0.6+/-5.5 micromol/kg/min, p<0.05) while there was no suppression in phosphoenolpyruvate carboxykinase rats (45.2+/-7.9 micromol/kg/min). Basal R(d) was comparable between PEPCK transgenic and control rats (37.4+/-1.1 vs 34.6+/-2.6 micromol/kg/min) but under insulin-stimulated conditions the increase in R(d) was greater in control compared to phosphoenolpyruvate carboxykinase transgenic rats indicative of insulin resistance (73.4+/-11.2 vs 112.0+/-8.0 micromol/kg/min, p<0.05). Basal glucose uptake was reduced in white and brown adipose tissue, heart and soleus while insulin-stimulated transport was reduced in white and brown adipose tissue, white quadriceps, white gastrocnemius and soleus in phosphoenolpyruvate carboxykinase transgenic compared to control rats. The impairment in both white and brown adipose tissue glucose uptake in phosphoenolpyruvate carboxykinase transgenic rats was associated with a decrease in GLUT4 protein content. In contrast, muscle GLUT4 protein, triglyceride and long-chain acylCoA levels were comparable between PEPCK transgenic and control rats.

CONCLUSIONS/INTERPRETATION

A primary defect in suppression of EGP caused adipose tissue and muscle insulin resistance.

Interleukin-6 gene expression is increased in insulin-resistant rat skeletal muscle following insulin stimulation

IL-6 expression in skeletal muscle is stimulated by contractions. We sought to examine whether hyperinsulinaemia increases IL-6 mRNA in skeletal muscle and whether any increase is modified in insulin resistant muscle. We hypothesized that intramuscular IL-6 mRNA would be increased in response to insulin, but such an affect would be unaffected by insulin resistance because the primary insulin sensitive signalling protein responsible for activating IL-6 functions normally in insulin resistant muscle. Transgenic rats over-expressing the gluconeogenic regulatory enzyme phosphoenolpyruvate carboxykinase (PEPCK) were studied. White gastrocnemius muscle samples were obtained under hyperinsulinaemic, euglycaemic clamp (4 mU kg(-1)min(-1) insulin, plasma glucose concentration 4-6 mmol L(-1)) and basal conditions in both PEPCK (basal n=4; insulin n=5) and wild-type (CON) (basal n=5; insulin n=4) rats, which were previously injected with a bolus of 2-[1-14C]deoxyglucose (2-DG) into the carotid artery. Muscle samples were assayed for 2-DG uptake and IL-6 mRNA. No differences in 2-DG uptake or IL-6 mRNA were observed when comparing groups under basal conditions. Under clamp conditions, 2-DG uptake was lower (P<0.05) in PEPCK compared with CON. Insulin stimulation in CON did not change IL-6 mRNA compared with basal levels. In contrast, there was an approximately 8-fold increase (P<0.05) in IL-6 mRNA in insulin-stimulated PEPCK compared with CON basal levels. Insulin stimulation increases IL-6 gene expression in insulin resistant, but not healthy, skeletal muscle, suggesting that IL-6 expression in skeletal muscle is sensitive to changes in insulin in circumstances of insulin resistance. It is likely that the differences observed when comparing healthy with insulin resistant muscle are due to the differential activation of insulin sensitive signalling proteins responsible for activating IL-6.

The effect of apolipoprotein E deficiency on islet amyloid deposition in human islet amyloid polypeptide transgenic mice

AIMS/HYPOTHESIS

Islet amyloid deposits are present in over 85% of Type 2 diabetic patients and have been suggested to be pathogenic. The mechanism that converts islet amyloid polypeptide (IAPP), the unique component of these deposits, into amyloid fibrils in vivo is not known. The amino acid sequence of IAPP is critical but insufficient for beta-pleated sheet formation. As apolipoprotein E (apoE), another component of islet amyloid deposits, plays a critical role in amyloid formation in Alzheimer's disease, we hypothesised that apoE could play an important role in islet amyloid formation.

METHODS

Transgenic mice expressing the human form of IAPP ( hIAPP (+/0)) were crossbred with apoE deficient ( apoE (-/-)) mice and followed for 12 months, at which time the prevalence and severity of islet amyloid, as well as plasma glucose, hIAPP, immunoreactive insulin (IRI) and lipid concentrations were measured.

RESULTS

The prevalence and severity of islet amyloid after one year of follow up were comparable among hIAPP (+/0) mice that were apoE (+/+), apoE (+/-) or apoE (-/-). Differences in glucose tolerance, lipid abnormalities or changes in pancreatic content or plasma concentrations of hIAPP and/or IRI did not account for these findings.

CONCLUSION/INTERPRETATION

Our data shows that, unlike in the localized amyloidosis in the brain characteristic of Alzheimer's disease, apoE is not critical for islet amyloid formation in a transgenic mouse model of Type 2 diabetes mellitus. These results indicate that the mechanisms of localised amyloid formation probably vary among different amyloid-associated disorders. Therefore, therapeutic strategies targeting apoE might not apply equally to patients with different amyloid associated diseases.

Effects of free fatty acids on insulin secretion in obesity

The prevalence of obesity in Western society has reached epidemic proportions and its aetiological role in the development of type 2 diabetes has made finding an effective treatment for the condition of crucial importance. Of the many consequences of obesity, derangements in glucose metabolism present one of the greatest problems to health. While the role of obesity in causing insulin resistance has received much attention, the effect of obesity on beta-cell failure and the consequent development of type 2 diabetes requires re-emphasis. In this review, the current understanding of the effects of elevated free-fatty acids on beta-cell function will be examined, including a discussion of potential mechanisms. In particular, dysregulation of biochemical pathways and alterations in key enzymes, proteins and hormones will be considered as grounds for the progression to a diabetic phenotype.

Mechanism of fat-induced hepatic gluconeogenesis: effect of metformin

High-fat feeding has been shown to cause hepatic insulin resistance. The aims of this study were to investigate the biochemical steps responsible for enhanced gluconeogenesis as a result of increased dietary fat intake and the site or sites at which the antihyperglycemic agent metformin acts to inhibit this process. Male Hooded Wistar rats were fed either a standard chow diet (5% fat by weight) or a high-fat diet (60% fat by weight) for 14 days with or without metformin. Total endogenous glucose production and gluconeogenesis were determined using [6-(3)H]glucose and [U-(14)C]alanine, respectively. Gluconeogenic enzyme activity and, where appropriate, protein and mRNA levels were measured in liver tissues. The high-fat diet increased endogenous glucose production (21.9 +/- 4.4 vs. 32.2 +/- 4.8 micromol x kg(-1) x min(-1), P < 0.05) and alanine gluconeogenesis (4.5 +/- 0.9 vs. 9.6 +/- 1.9 micromol x kg(-1) x min(-1), P < 0.05). Metformin reduced both endogenous glucose production (32.2 +/- 4.8 vs. 16.1 +/- 2.1 micromol x kg(-1) x min(-1), P < 0.05) and alanine gluconeogenesis (9.6 +/- 1.9 vs. 4.7 +/- 0.8 micromol x kg(-1) x min(-1), P < 0.05) after high-fat feeding. These changes were reflected in liver fructose-1,6-bisphosphatase protein levels (4.5 +/- 0.9 vs. 9.6 +/- 1.9 arbitrary units, P < 0.05 chow vs. high-fat feeding; 9.5 +/- 1.9 vs. 4.7 +/- 0.8 arbitrary units, P < 0.05 high fat fed in the absence vs. presence of metformin) but not in changes to the activity of other gluconeogenic enzymes. There was a significant positive correlation between alanine gluconeogenesis and fructose-1,6-bisphosphatase protein levels (r = 0.56, P < 0.05). Therefore, excess supply of dietary fat stimulates alanine gluconeogenesis via an increase in fructose-1,6-bisphosphatase protein levels. Metformin predominantly inhibits alanine gluconeogenesis by preventing the fat-induced changes in fructose-1,6-bisphosphatase levels.

beta-cell glucokinase deficiency and hyperglycemia are associated with reduced islet amyloid deposition in a mouse model of type 2 diabetes

Type 2 diabetes is characterized by impaired beta-cell function, hyperglycemia, and islet amyloid deposition. The primary constituent of islet amyloid is the 37-amino acid beta-cell product called islet amyloid polypeptide (IAPP) or amylin. To study mechanisms of islet amyloid formation, we developed a transgenic mouse model that produces and secretes the amyloidogenic human IAPP (hIAPP) molecule and have shown that 81% of male transgenic mice develop islet amyloid after 14 months on a high-fat diet. To test whether impaired beta-cell function and hyperglycemia could enhance islet amyloid formation, we cross-bred our hIAPP transgenic mice with beta-cell glucokinase-knockout mice (GKKO) that have impaired glucose-mediated insulin secretion and fasting hyperglycemia. The resulting new (hIAPPxGKKO) line of mice had higher basal plasma glucose concentrations than the hIAPP transgenic mice at 3, 6, and 12 months of age (P < 0.05), as did GKKO mice compared with hIAPP transgenic mice at 6 and 12 months of age (P < 0.05). Basal plasma immunoreactive insulin (IRI) levels were lower in hIAPP x GKKO mice than in hIAPP transgenic mice at 6 months of age (P < 0.05). The area under the glucose curve in response to an intraperitoneal glucose challenge (1 g/kg body weight) was larger in hIAPPxGKKO mice than in hIAPP transgenic mice at 3, 6, and 12 months of age (P < 0.005) and in GKKO mice compared with hIAPP transgenic mice at 6 and 12 months of age (P < 0.005). The area under the IRI curve was lower in hIAPPxGKKO mice at 6 and 12 months of age (P < 0.05) than in hIAPP transgenic mice and in GKKO mice compared with hIAPP transgenic mice at 12 months of age (P < 0.05). Despite the presence of hyperglycemia, hIAPPxGKKO mice had a lower incidence (4 of 17 vs. 12 of 19, P < 0.05) and amount (0.40 +/- 0.24 vs. 1.2 +/- 0.3 arbitrary units, P < 0.05) of islet amyloid than hIAPP transgenic mice had. As expected, no islet amyloid was observed in GKKO mice lacking the hIAPP transgene (0 of 13). There was no difference in pancreatic content of IRI and hIAPP among the three groups of mice. Thus, despite the presence of impaired islet function and hyperglycemia, hIAPPxGKKO mice had a decreased incidence and quantity of islet amyloid. Therefore, our data suggest that impaired beta-cell glucose metabolism or hyperglycemia are not likely to contribute to islet amyloid formation in diabetes. Furthermore, this finding may explain the lack of progression of glycemia in patients with maturity-onset diabetes of the young.

Oophorectomy promotes islet amyloid formation in a transgenic mouse model of Type II diabetes

AIMS/HYPOTHESIS

In Type II (non-insulin-dependent) diabetes mellitus, amyloid depletes islet mass. We previously found that 81% of male human islet amyloid polypeptide (IAPP) transgenic mice but only 11% of female mice developed islet amyloid, suggesting that either testosterone promotes or ovarian products protect against amyloid deposition.

METHODS

We did a bilateral oophorectomy or sham procedure in female human IAPP transgenic mice (n = 11 and n = 8, respectively) and in female non-transgenic mice (n = 7 and n = 9, respectively) at 6-8 weeks of age. Animals were followed for 1 year on a 9% fat (w/w) diet. Before we killed them we measured, fasting plasma human IAPP and did an intraperitoneal glucose tolerance test. Pancreatic content of IAPP and immunoreactive insulin (IRI) were estimated and pancreata were analysed for islet amyloid.

RESULTS

No amyloid was detected in either the sham-operated transgenic mice or, as expected, in both groups of non-transgenic mice. In strong contrast, 7 of 11 (64%) oophorectomized mice developed islet amyloid (p < 0.05). Amyloid deposition in the oophorectomized transgenic mice was not associated with any differences in incremental body weight, fasting human IAPP concentrations or glucose tolerance between the groups. Furthermore, pancreatic content of mouse IAPP, human IAPP and immunoreactive insulin did not differ between groups.

CONCLUSION/INTERPRETATION

Oophorectomy is associated with an enhancement of islet amyloid formation in the absence of changes in glucose tolerance, circulating IAPP or pancreatic content of IRI, mouse or human IAPP. Thus, the early stages of islet amyloidogenesis seem to be independent of glucose tolerance, with ovarian products having a protective role.

Two novel immortal pancreatic beta-cell lines expressing and secreting human islet amyloid polypeptide do not spontaneously develop islet amyloid

Type 2 diabetes is characterized by islet amyloid deposits, which are primarily composed of the amyloidogenic human form of islet amyloid polypeptide (IAPP, amylin). The mechanism of islet amyloido-genesis is not known, but other products (e.g., apolipoprotein E and perlecan) contained within islet amyloid may be necessary. Because rodent IAPP does not form islet amyloid, the currently available beta-cell lines are not useful for studying processes involved in amyloid formation. To develop a suitable in vitro cell system for the study of islet amyloid formation, we generated two new beta-cell lines that express the amyloidogenic human IAPP. We did this by crossbreeding human IAPP transgenic mice with RIP-Tag mice that develop islet tumors and then culturing one of these islet tumors from two separate offspring of this cross. The resultant 2350-2C0 and 2511 cell lines produce human as well as mouse IAPP-like immunoreactivity (IAPP-LI) and immunoreactive insulin (IRI). Incubation of both these cell lines with 16.7 mmol/l glucose resulted in a two- to fourfold increase in human IAPP-LI, mouse IAPP-LI, and IRI secretion compared with 1.67 mmol/l glucose and the combination of 16.7 mmol/l glucose and 10 mmol/l arginine, 0.1 mmol/l 3-isobutyl-1-methylxanthine (IBMX), and 5 micromol/l carbachol induced a >50-fold increase in the release of these peptides. The omission of calcium from the above secretagogue cocktail reduced secretion of all three peptides to only two- to sixfold higher than the 16.7 mmol/l glucose condition. Perifusion with 16.7 mmol/l glucose plus 0.1 mmol/l IBMX caused a biphasic secretion of human IAPP-LI and mouse IAPP-LI, as well as IRI, in both cell lines, with the peak of the first phase being five- to sixfold higher than the prestimulated 1.67 mmol/l glucose condition. Immunoelectron microscopic inspection of both 2350-2C0 and 2511 cells after 7 days of culture did not reveal the presence of amyloid fibrils, suggesting the need for other critical components. We conclude that we have established two novel beta-cell lines that produce and secrete human IAPP in a regulated manner. These cell lines will be a useful tool to investigate the secretion of human IAPP as well as the necessity of other components for islet amyloid formation.

Islet amyloid: a long-recognized but underappreciated pathological feature of type 2 diabetes

Islet amyloid has been recognized as a pathological entity in type 2 diabetes since the turn of the century. It has as its unique component the islet beta-cell peptide islet amyloid polypeptide (IAPP), or amylin, which is cosecreted with insulin. In addition to this unique component, islet amyloid contains other proteins, such as apolipoprotein E and the heparan sulfate proteoglycan perlecan, which are typically observed in other forms of generalized and localized amyloid. Islet amyloid is observed at pathological examination in the vast majority of individuals with type 2 diabetes but is rarely observed in humans without disturbances of glucose metabolism. In contrast to IAPP from rodents, human IAPP has been shown to form amyloid fibrils in vitro. Because all human subjects produce and secrete the amyloidogenic form of IAPP, yet not all develop islet amyloid, some other factor(s) must be involved in islet amyloid formation. One hypothesis is that an alteration in beta-cell function resulting in a change in the production, processing, and/or secretion of IAPP is critical to the initial formation of islet amyloid fibrils in human diabetes. This nidus of amyloid fibrils then allows the progressive accumulation of IAPP-containing fibrils and the eventual replacement of beta-cell mass by amyloid and contributes to the development of hyperglycemia. One factor that may be involved in producing the changes in the beta-cell that result in the initiation of amyloid formation is the consumption of increased dietary fat. Dietary fat is known to alter islet beta-cell peptide production, processing, and secretion, and studies in transgenic mice expressing human IAPP support the operation of this mechanism. Further investigation using this and other models should provide insight into the mechanism(s) involved in islet amyloidogenesis and allow the development of therapeutic agents that inhibit or reverse amyloid fibril formation, with the goal being to preserve beta-cell function and improve glucose control in type 2 diabetes.

Human aging is associated with parallel reductions in insulin and amylin release

Aging is associated with an increased risk of type 2 diabetes. To determine whether the insulin resistance of aging is associated with an increase in amylin release or whether amylin release parallels the reduction in insulin release, we studied 10 older (72 +/- 2 yr) and 9 young (25 +/- 1 yr) subjects. Insulin sensitivity was quantified as the insulin sensitivity index (SI) and B cell function as the acute insulin and amylin responses to iv glucose (AIRg and AARg, respectively) and iv arginine at a glucose level of >25 mM (AIRmax and AARmax). To account for the effect of SI to modulate B cell function, we calculated SI x B cell function. Older subjects were insulin resistant (SI: 4.6 +/- 0.8 vs. 8.6 +/- 1.4 x 10(-5) min-1/pM, P < 0.05). Acute responses to glucose [AIRg (older vs. young): 420 +/- 106 vs. 537 +/- 87 pM; AARg: 6.5 +/- 1.7 vs. 9.0 +/- 1.5 pM] and arginine (AIRmax: 1,096 +/- 203 vs. 1,572 +/- 307 pM; AARmax: 14.0 +/- 3.5 vs. 16.5 +/- 2.4 pM) did not differ despite the difference in SI. When adjusted for SI, insulin responses were reduced in older subjects (SI x AIRg: 1.54 +/- 0.29 vs. 4.10 +/- 0. 63 x 10(-2) min-1, P = 0.001; SI x AIRmax: 4.03 +/- 0.52 vs. 12.7 +/- 2.9 x 10(-2) min-1, P < 0.01), as was amylin release (SI x AARg: 2.46 +/- 0.59 vs. 6.85 +/- 0.95 x 10(-4) min-1, P < 0.001; SI x AARmax: 4.71 +/- 0.52 vs. 13.5 +/- 2.2 x 10(-4) min-1, P < 0.001). Amylin and insulin release was proportionate, with a molar ratio of 1.5% in older and 1.4% in young subjects. Thus aging is associated with parallel impairments in the adaptation of insulin and amylin release to insulin resistance. It is unlikely that an alteration in amylin release contributes to the increased risk of type 2 diabetes.

Reduced amylin release is a characteristic of impaired glucose tolerance and type 2 diabetes in Japanese Americans

Islet amyloid is a characteristic feature of type 2 diabetes. Its major component is the normal beta-cell secretory product amylin, or islet amyloid polypeptide (IAPP). To determine whether increased or disproportionate release of amylin may explain the propensity for amyloid deposition in type 2 diabetes, we measured plasma amylin-like immunoreactivity (ALI) and immunoreactive insulin (IRI) release in response to an oral glucose load in 94 Japanese-American subjects with normal glucose tolerance (NGT; n=56), impaired glucose tolerance (IGT; n=10), and type 2 diabetes (n=28) as defined by World Health Organization criteria. The incremental increase in ALI, IRI, and glucose (G) at 30 min after oral glucose ingestion was used to calculate deltaALI/deltaG and deltaIRI/deltaG as measures of beta-cell function. Overall glucose metabolism was assessed as the incremental glucose area (glucose AUC) during the 2 h of the oral glucose tolerance test. As expected, plasma glucose concentrations at both fasting (NGT, 5.0+/-0.4; IGT, 5.5+/-0.1; type 2 diabetes, 6.2+/-0.3 mmol/l; P < 0.0001) and 2 h (NGT, 6.7+/-0.1; IGT, 9.4+/-0.3; type 2 diabetes, 13.2 +/-0.5 mmol/l; P < 0.0001) were elevated in individuals with IGT and type 2 diabetes. In response to glucose ingestion, plasma IRI and ALI increased in all subjects, but these increments were lower in individuals with reduced glucose tolerance, as reflected in the deltaIRI/deltaG (NGT, 119+/-10.3; IGT, 60.7+/-7.1; type 2 diabetes, 49.7 +/-5.4 pmol/l; P < 0.0001) and deltaALI/deltaG (NGT, 2.6+/-0.2; IGT, 1.8+/-0.3; type 2 diabetes, 1.2+/-0.1 pmol/l; P < 0.0001). Moreover, these reductions in the 30-min incremental ALI and IRI responses were proportionate such that the molar ratio of ALI to IRI was not different among the three groups (NGT, 2.6+/-0.2; IGT, 2.9 +/-0.3; type 2 diabetes, 2.9+/-0.3%; NS). Further, the relationship between beta-cell function, measured as either deltaIRI/deltaG or deltaALI/deltaG, and glucose metabolism, assessed as glucose AUC, was nonlinear and inverse in nature, with r2 values of 0.38 (P < 0.0001) and 0.33 (P < 0.0001), respectively. We conclude that the reduced beta-cell function of IGT and type 2 diabetes includes proportionate reductions in both IRI and ALI release. Thus, it is unlikely that the development of islet amyloid in type 2 diabetes is the result of increased release of ALI.

Transgenic overproduction of islet amyloid polypeptide (amylin) is not sufficient for islet amyloid formation

Islet amyloid polypeptide forms islet amyloid deposits in non-insulin-dependent diabetes mellitus. We have generated transgenic mice which express human islet amyloid polypeptide in their pancreatic beta cells yet do not develop islet amyloid deposits despite producing levels of the amyloidogenic human peptide 2 - 3 fold higher than the native (mouse) peptide. To determine whether marked overproduction of islet amyloid polypeptide is a potential cause of islet amyloid formation, we increased expression of this transgene by producing homozygous transgenic animals and by making heterozygous mice experimentally insulin resistant with nicotinic acid. Pancreatic content of islet amyloid polypeptide-like immunoreactivity in homozygous and nicotinic acid-treated mice was 2-fold (25 +/- 7 fmol/microg; n = 6) and 3.5-fold (47 +/- 20 fmol/microg; n = 3) higher, respectively, than that of untreated heterozygous animals (13+/-2 fmol/microg; n = 11; both p < 0.05). Despite this marked increase in production of islet amyloid polypeptide, neither group of mice developed gross islet amyloid deposits even after 16 months of age. We conclude that overproduction of islet amyloid polypeptide, even as produced by extreme insulin resistance, is not in itself sufficient for islet amyloid formation.

Impaired regulation of hepatic fructose-1,6-biphosphatase in the New Zealand Obese mouse: an acquired defect

Increased hepatic glucose production, a feature of (non-insulin-dependent diabetes mellitus [NIDDM]), is present at an early age in the New Zealand Obese (NZO) mouse and is associated with impaired suppression of the gluconeogenic enzyme, fructose-1,6-bisphosphatase (FBPase). The aim of this study was to further characterize the abnormality in the regulation of hepatic FBPase in NZO mice versus New Zealand Chocolate (NZC) control mice. At 20 weeks of age, NZO mice have elevated FBPase activity (65.3 +/- 7.9 v 46.7 +/- 5.0 micromol/min/mg protein, P =.07) and protein levels (31.7 +/- 3.1 v 22.5 +/- 2.8 arbitrary units, P < .05), but not mRNA levels (0.18 +/- 0.03 v 0.16 +/- 0.03 arbitrary units). Elevated FBPase activity and protein levels in NZO mice were also shown at 4 to 6 weeks of age, but not in 1-day-old mice, suggesting that the increase occurs between birth and weaning. The Km of the enzyme was the same in NZO and NZC mice (3.7 +/- 0.5 v 5.0 +/- 0.9 micromol/L, NZO v NZC). The regulation of FBPase by the competitive inhibitor, fructose-2,6-bisphosphate ([Fru(2,6)Pz] 5 micromol/L) measured over a range of substrate concentrations (2.5 to 80 micromol/L) was similar between NZO and control mice (Km in the presence of Fru(2,6)Pz, 10.8 +/- v 1.9 v 13.2 +/- 3.3 micromol/L, NZO v NZC). It is concluded that increased FBPase activity in the NZO mouse is due to elevated protein levels, and that this appears to be due to a failure of the normal decrease that occurs following birth in control animals.

The biochemical basis of increased hepatic glucose production in a mouse model of type 2 (non-insulin-dependent) diabetes mellitus

The mechanism of increased hepatic glucose production in obese non-insulin-dependent diabetic (NIDDM) patients is unknown. The New Zealand Obese (NZO) mouse, a polygenic model of obesity and NIDDM shows increased hepatic glucose production. To determine the mechanism of this phenomenon, we measured gluconeogenesis from U-14C-glycerol and U-14C-alanine and relevant gluconeogenic enzymes. Gluconeogenesis from glycerol (0.07 +/- 0.01 vs 0.21 +/- 0.02 micromol.min-1.body mass index (BMI)-1, p < 0.005) and alanine (0.57 +/- 0.07 vs 0.99 +/- 0.07 micromol.min-1.BMI-1, p < 0.005) was elevated in control mice NZO vs as was glycerol turnover (0.25 +/- 0.02 vs 0.63 +/- 0.09 micromol.min-1.BMI-1, p < 0.05). Fructose 1,6-bisphosphatase activity (44.2 +/- 1.9 vs 55.7 +/- 4.1 nmol.min-1.mg protein-1, p < 0.05) and protein levels (6.9 +/- 1.1 vs 16.7 +/- 2.3 arbitrary units, p < 0.01) were increased in NZO mouse livers, as was the activity of pyruvate carboxylase (0.12 +/- 0.01 vs 0.17 +/- 0.02 nmol.min-1.mg protein-1, p < 0.05). To ascertain whether elevated lipid supply is responsible for these biochemical changes in NZO mice, we fed lean control mice a 60% fat diet for 2 weeks. Fat-fed mice were hyperinsulinaemic (76.37 +/- 4.06 vs 98.00 +/- 7.07 pmol/l, p = 0.05) and had elevated plasma non-esterified fatty acid levels (0.44 +/- 0.05 vs 0.59 +/- 0.03 mmol/l, p = 0.05). Fructose 1,6-bisphosphatase activity (43.86 +/- 2.54 vs 52.93 +/- 3.09 nmol.min-1.mg protein-1, p = 0.05) and protein levels (33.03 +/- 0.96 vs 40.04 +/- 1.26 arbitrary units, p = 0.005) and pyruvate carboxylase activity (0.10 +/- 0.003 vs 0.14 +/- 0.01 nmol.min-1.mg protein-1, p < 0.05) were elevated in fat-fed mice. We conclude that in NZO mice increased hepatic glucose production is due to elevated lipolysis resulting from obesity.

Understanding the pathogenesis of type 2 diabetes: can we get off the metabolic merry-go-rounds?

The aetiology of non-insulin-dependent diabetes mellitus (NIDDM) is not known. The concordance of NIDDM in identical twins and differences in the prevalence rate of NIDDM between different racial groups suggest a genetic cause. Hyperglycaemia in established diabetes is caused by a combination of hepatic insulin resistance, impaired peripheral (muscle and fat) glucose uptake and a defect in glucose-mediated insulin secretion. However, it is not known if these defects are all inherited or if one can cause the others. This uncertainty is due to the fact that hyperglycaemia per se can cause defects in insulin action and insulin secretion that resemble those found in NIDDM. Furthermore the elevated free fatty acid (FFA) levels found when NIDDM is associated with obesity are known to cause both peripheral and hepatic insulin resistance. Recently we have demonstrated the mechanism by which elevated FFA levels can cause hepatic insulin resistance. However, we also have evidence that the converse holds in that genetically engineered hepatic insulin resistance in a transgenic rat model leads to obesity. Thus an understanding of the pathogenesis of NIDDM is complicated by the fact that hyperglycaemia and obesity can be both causes and consequences of insulin resistance. To overcome these difficulties, studies in young euglycaemic diabetes-prone subjects have been conducted. Results suggest that there may be different causes for NIDDM in different racial groups.

Impaired glucose tolerance and increased weight gain in transgenic rats overexpressing a non-insulin-responsive phosphoenolpyruvate carboxykinase gene

The effects of an overexpressed, non-insulin-responsive gluconeogenic enzyme, phosphoenolpyruvate carboxykinase (GTP) (PEPCK; EC 4.1.1.32), on glucose homeostasis were investigated. Transgenic rats harboring a metallothionein-driven PEPCK gene (lacking the entire PEPCK upstream-regulatory region) expressed transgene PEPCK mRNA in the key gluconeogenic tissues, liver and kidney. Female transgenic rats, studied at 10 weeks of age, showed mild fasting hyperglycemia (6.9 +/- 0.2 vs. 5.9 +/- 0.1 mM P = 0.002 n = 6), hyperinsulinemia (92.2 +/- 4.0 vs. 54.0 +/- 6.6 pM, P = 0.001, n = 6), impaired glucose tolerance and increased weight gain (178.3 +/- 3.2 vs. 153.4 +/- 2.5 g, P = 0.001, n = 16 and n = 13 transgenic and control rats, respectively). Despite hyperinsulinemia at this age, kidneys of transgenic rats maintained a significant 20% elevation of total PEPCK enzyme activity, while total liver PEPCK activity was not reduced. This study suggests that an insulin-resistant step in the gluconeogenic pathway can lead to glucose intolerance and an increase in weight. These rats offer the unique opportunity to study the metabolic consequences of chronic, mild excess glucose supply, as seen in non-insulin-dependent diabetes.

Defects in liver and muscle glycogen metabolism in neonatal and adult New Zealand obese mice

Impaired glycogen synthesis is present in subjects at risk for developing non-insulin-dependent diabetes mellitus (NIDDM), suggesting that it is a primary defect in NIDDM. To examine whether defects in glycogen metabolism are present at birth in an animal model of NIDDM, glycogen synthase (GS), glycogen phosphorylase (GP), and total glycogen content were measured in liver and quadriceps muscle of 1-day- and 20-week-old insulin-resistant New Zealand Obese (NZO) mice and control (NZC) mice. In livers of both neonatal and adult NZO mice, active GS was reduced by 54% and 36%, respectively, as compared with that in NZC mice (P < .03). Total liver GS activity was the same in neonates, but was 65% higher in adult NZO as compared with NZC mice (P < .02). Liver glycogen was 28% lower at birth in NZO mice (P < .03), but was 49% higher at 20 weeks of age. Active and total GP were the same in NZO and NZC animals, despite hyperinsulinemia in 20-week-old NZO mice. In muscle, active GS was reduced by 41% in both 1-day- and 20-week-old NZO mice (P < .02). Total GS was also lower in NZC mice at 1 day of age (P < .01), but not at 20 weeks. No differences were detected in GP activity or in total glycogen content in muscle. Therefore, reduced GS activity is an early defect present at birth in the insulin-resistant NZO mouse in both liver and muscle. However, it is not the sole determinant of the amount of glycogen deposited in tissues.(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired regulation of hepatic fructose-1,6-bisphosphatase in the New Zealand obese mouse model of NIDDM

The New Zealand obese mouse, a model of NIDDM, is characterized by hyperglycemia, hyperinsulinemia, and hepatic and peripheral insulin resistance. The aim of this study was to investigate the biochemical basis of hepatic insulin resistance in NZO mice. Glycolytic and gluconeogenic enzyme activities were measured in fed and overnight fasted 19- to 20-wk-old NZO and control New Zealand chocolate mice. The NZO mice were twice as heavy as the NZC mice. The activity of the glycolytic enzymes glucokinase and pyruvate kinase was higher, whereas that of the gluconeogenic enzymes PEPCK and glucose-6-phosphatase was lower in fed and fasted NZO mice. These enzyme changes are consistent with a normal response to the hyperinsulinemia in NZO mice. In contrast, the activity of the third regulated gluconeogenic enzyme, fructose-1,6-bisphosphatase, was similar in fed and fasted NZO and NZC mice despite the higher insulin and glucose levels in the NZO mouse. This enzyme is primarily regulated by the powerful inhibitor fructose-2,6-bisphosphate. The levels of this metabolite were measured and found to be increased in both the fed and fasted states in the NZO mouse, suggesting that the activity of the bifunctional enzyme that regulates the level of inhibitor (6-phosphofructo-2-kinase/fructose-2,6- bisphosphatase) is normally regulated in the NZO mouse. We conclude that most insulin-responsive gluconeogenic and glycolytic enzymes are normally regulated in the NZO mouse, but an abnormality in the regulation of fructose-1,6-bisphosphatase may contribute to the increase hepatic glucose production in these mice.

Impaired suppression of gluconeogenesis induced by overexpression of a noninsulin-responsive phosphoenolpyruvate carboxykinase gene

Despite detailed knowledge of the regulation of individual steps in the gluconeogenic pathway, the relative importance of each step to the overall control of gluconeogenesis by insulin is not known. The aim of this study was to determine the role of phosphoenolpyruvate carboxykinase (PEPCK) in the regulation of gluconeogenesis by insulin. Clones of the rat hepatoma cell line H4IIE-C3 were produced, overexpressing a PEPCK gene, driven by a promoter not responsive to insulin. In these cells basal gluconeogenesis from 2-[14C]pyruvate was increased 2.1-fold compared to controls (4.63 +/- 0.49 nmol/10(5) cells vs. 2.21 +/- 0.24 nmol/10(5) cells after 3 h, P < 0.05, n = 5). Increased gluconeogenesis was associated with an increase in basal PEPCK mRNA levels (1.9-fold) and enzyme activity (2.8-fold). Insulin (10(-7) M) suppressed basal gluconeogenesis, PEPCK mRNA levels, and enzyme activity in control cells, but no detectable decrease was observed in PEPCK-transfected cells. These experiments provide direct evidence in intact cells that PEPCK is the rate-limiting enzyme in gluconeogenesis from pyruvate and show that insulin's action to inhibit gluconeogenesis is predominantly on the inhibition of PEPCK transcription.