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

Fluoroquinolones suppress gluconeogenesis by inhibiting fructose 1,6-bisphosphatase in primary monkey hepatocytes

Dysglycemia is one of the most serious adverse events associated with the clinical use of certain fluoroquinolones. The purpose of this study was to investigate the effects of the representative fluoroquinolones moxifloxacin and gatifloxacin on hepatic gluconeogenesis using primary monkey hepatocytes. Glucose production was induced after the cells were incubated for 4 h with 10 mM sodium lactate and 1 mM sodium pyruvate as gluconeogenic substrates. Under these conditions, moxifloxacin and gatifloxacin dose-dependently suppressed gluconeogenesis at concentrations of 100 μM or higher. Transcriptome analysis of rate-limiting enzymes involved in hepatic gluconeogenesis revealed that moxifloxacin and gatifloxacin at a concentration of 1000 μM did not affect the expression of key gluconeogenic enzymes such as phosphoenolpyruvate carboxykinase, glucose 6-phosphatase, and fructose 1,6-bisphosphatase. Furthermore, metabolome analysis, in vitro glucose production assay using additional gluconeogenic substrates, and fructose 1,6-bisphosphatase assay using the cell extracts showed that fluoroquinolones enzymatically suppressed hepatic gluconeogenesis by inhibiting fructose 1,6-bisphosphatase. These inhibitory effects may involve in the clinically relevant dysglycemia associated with fluoroquinolones in human.

CCAAT-enhancer binding protein-α (C/EBPα) and hepatocyte nuclear factor 4α (HNF4α) regulate expression of the human fructose-1,6-bisphosphatase 1 (FBP1) gene in human hepatocellular carcinoma HepG2 cells

Fructose-1,6-bisphosphatase (FBP1) plays an essential role in gluconeogenesis. Here we report that the human FBP1 gene is regulated by two liver-enriched transcription factors, CCAAT-enhancer binding protein-α (C/EBPα) and hepatocyte nuclear factor 4α (HNF4α) in human hepatoma HepG2 cells. C/EBPα regulates transcription of FBP1 gene via binding to the two overlapping C/EBPα sites located at nucleotide -228/-208 while HNF4α regulates FBP1 gene through binding to the classical H4-SBM site and direct repeat 3 (DR3) located at nucleotides -566/-554 and -212/-198, respectively. Mutations of these transcription factor binding sites result in marked decrease of C/EBPα- or HNF4α-mediated transcription activation of FBP1 promoter-luciferase reporter expression. Electrophoretic mobility shift assays of -228/-208 C/EBPα or -566/-554 and -212/-198 HNF4α sites with nuclear extract of HepG2 cells overexpressing C/EBPα or HNF4α confirms binding of these two transcription factors to these sites. Finally, we showed that siRNA-mediated suppression of C/EBPα or HNF4α expression in HepG2 cells lowers expression of FBP1 in parallel with down-regulation of expression of other gluconeogenic enzymes. Our results suggest that an overall gluconeogenic program is regulated by these two transcription factors, enabling transcription to occur in a liver-specific manner.

Insights into obesity and diabetes at the intersection of mouse and human genetics

Many of our insights into obesity and diabetes come from studies in mice carrying natural or induced mutations. In parallel, genome-wide association studies (GWAS) in humans have identified numerous genes that are causally associated with obesity and diabetes, but discovering the underlying mechanisms required in-depth studies in mice. We discuss the advantages of studying natural variation in mice and summarize several examples where the combination of human and mouse genetics opened windows into fundamental physiological pathways. A noteworthy example is the melanocortin-4 receptor (MC4R) and its role in energy balance. The pathway was delineated by discovering the gene responsible for the Agouti mutation in mice. With more targeted phenotyping, we predict that additional pathways relevant to human pathophysiology will be discovered.

The use of animal models in diabetes research

Diabetes is a disease characterized by a relative or absolute lack of insulin, leading to hyperglycaemia. There are two main types of diabetes: type 1 diabetes and type 2 diabetes. Type 1 diabetes is due to an autoimmune destruction of the insulin-producing pancreatic beta cells, and type 2 diabetes is caused by insulin resistance coupled by a failure of the beta cell to compensate. Animal models for type 1 diabetes range from animals with spontaneously developing autoimmune diabetes to chemical ablation of the pancreatic beta cells. Type 2 diabetes is modelled in both obese and non-obese animal models with varying degrees of insulin resistance and beta cell failure. This review outlines some of the models currently used in diabetes research. In addition, the use of transgenic and knock-out mouse models is discussed. Ideally, more than one animal model should be used to represent the diversity seen in human diabetic patients.

The liver: Key in regulating appetite and body weight

Liver fructose-1,6-bisphosphatase (FBPase) is a regulatory enzyme in gluconeogenesis that is elevated by obesity and dietary fat intake. Whether FBPase functions only in glucose metabolism or has other metabolic roles is currently unclear. In our recently published study, we examined the importance of liver FBPase in body weight regulation by performing a series of comprehensive physiological and biochemical assessments of energy balance and specific intervention studies in our transgenic mouse line that overexpresses FBPase specifically in the liver. Compared with negative littermates, these FBPase transgenic mice weighed 10% less, had 50% less adiposity, ate 15% less food but did not have altered energy expenditure. Increased circulating leptin and cholecystokinin levels, elevated fatty acid oxidation and reduced appetite stimulating neuropeptides, neuropeptide Y (NPY) and agouti-related peptide (AGRP), underpinned this phenotype. Blocking the action of FBPase returned food intake and body weight to those of the negative littermates. Our study is the first to identify liver FBPase as a previously unknown regulator of appetite and adiposity. Importantly, this work recognizes the liver as an important organ in appetite and body weight regulation. This commentary will provide further insight and expand on this novel concept that the liver does in fact play an important role in adiposity.

The role of liver fructose-1,6-bisphosphatase in regulating appetite and adiposity

Liver fructose-1,6-bisphosphatase (FBPase) is a regulatory enzyme in gluconeogenesis that is elevated by obesity and dietary fat intake. Whether FBPase functions only to regulate glucose or has other metabolic consequences is not clear; therefore, the aim of this study was to determine the importance of liver FBPase in body weight regulation. To this end we performed comprehensive physiologic and biochemical assessments of energy balance in liver-specific transgenic FBPase mice and negative control littermates of both sexes. In addition, hepatic branch vagotomies and pharmacologic inhibition studies were performed to confirm the role of FBPase. Compared with negative littermates, liver-specific FBPase transgenic mice had 50% less adiposity and ate 15% less food but did not have altered energy expenditure. The reduced food consumption was associated with increased circulating leptin and cholecystokinin, elevated fatty acid oxidation, and 3-β-hydroxybutyrate ketone levels, and reduced appetite-stimulating neuropeptides, neuropeptide Y and Agouti-related peptide. Hepatic branch vagotomy and direct pharmacologic inhibition of FBPase in transgenic mice both returned food intake and body weight to the negative littermates. This is the first study to identify liver FBPase as a previously unknown regulator of appetite and adiposity and describes a novel process by which the liver participates in body weight regulation.

Effect of Rebaudioside A, a diterpenoid on glucose homeostasis in STZ-induced diabetic rats

Rebaudioside A (Reb A), a major constituent of Stevia rebaudiana, was recently proposed as an insulinotropic agent. The aim of this investigation was to evaluate the antihyperglycemic effect of Reb A on the activities of hepatic enzymes of carbohydrate metabolism in streptozotocin (STZ)-induced diabetic rats. Diabetes was induced in adult male Albino Wistar rats, weighing 180-200 g, by a single intraperitoneal injection at a dose of STZ (40 mg/kg body weight). Diabetic rats showed significant (P<0.05) increase in the levels of plasma glucose and glycosylated hemoglobin and significant (P<0.05) decrease in the levels of plasma insulin and hemoglobin. Activities of gluconeogenic enzymes such as glucose-6-phosphatase and fructose-1,6-bisphosphatase were significantly (P<0.05) increased while hexokinase and glucose-6-phosphate dehydrogenase were significantly (P<0.05) decreased in the liver along with glycogen. Oral treatment with Reb A to diabetic rats significantly (P<0.05) decreased blood glucose and reversed these hepatic carbohydrate metabolizing enzymes in a significant manner. Histopathology changes of pancreas confirmed the protective effects of Reb A in diabetic rats. Thus, the results show that Reb A possesses an antihyperglycemic activity and provide evidence for its traditional usage in the control of diabetes.

Deficiency in interferon-gamma results in reduced body weight and better glucose tolerance in mice

Obesity is a chronic low-grade inflammatory disease caused by increased energy intake and reduced energy expenditure. Studies using animal models with deletion of inflammatory cytokines have produced conflicting results with some showing increased weight gain and others showing no effect or even reduced body weights. Clearly, more work is necessary to understand the role of cytokines on body weight control. The aim of this study was to determine the effect of interferon-γ deletion (IFNγ(-/-)) on body weight regulation and glucose metabolism. Male IFNγ(-/-) and wild-type C57BL/6 mice were fed a low-fat chow diet, and body weight, food intake, and energy expenditure were monitored over 20 wk. At the end of the study, ip glucose tolerance test, insulin tolerance test, basal glucose turnover, and hyperinsulinemic/euglycemic clamps were performed. Expression levels of arcuate nucleus neuropeptide Y, Agouti-related peptide, and proopiomelanocortin mRNA as well as circulating leptin levels were also determined. IFNγ(-/-) mice had improved glucose tolerance with reduced rate of glucose appearance and increased insulin sensitivity due to greater suppression of endogenous glucose output, which was associated with decreased hepatic glucose-6-phosphatase activity. In addition, we also observed reduced body weight associated with decreased food intake and increased physical activity. Neuropeptide Y and Agouti-related peptide mRNA expression was reduced, whereas proopiomelanocortin mRNA expression was increased, as were plasma leptin levels. Global deletion of IFNγ in mice resulted in reduced body weight associated with negative energy balance, improved glucose tolerance, and hepatic insulin sensitivity. Our findings demonstrate that IFNγ plays a critical role in the regulation of body weight and glucose metabolism.

Modulatory effects of resveratrol on attenuating the key enzymes activities of carbohydrate metabolism in streptozotocin-nicotinamide-induced diabetic rats

Resveratrol, a ubiquitous stress-induced phytoalexin, has demonstrated a wide variety of biological activities which make it a good candidate for the treatment of diabetes mellitus. The present study was aimed to evaluate its therapeutic potential by assaying the activities of key enzymes of carbohydrate metabolism in streptozotocin-nicotinamide-induced diabetic rats. The daily oral treatment of resveratrol (5 mg/kg body weight) to diabetic rats for 30 days demonstrated a significant (p<0.05) decline in blood glucose and glycosylated hemoglobin levels and a significant (p<0.05) increase in plasma insulin level. The altered activities of the key enzymes of carbohydrate metabolism such as hexokinase, pyruvate kinase, lactate dehydrogenase, glucose-6-phosphatase, fructose-1,6-bisphosphatase, glucose-6-phosphate dehydrogenase, glycogen synthase and glycogen phosphorylase in liver and kidney tissues of diabetic rats were significantly (p<0.05) reverted to near normal levels by the administration of resveratrol. Further, resveratrol administration to diabetic rats improved hepatic glycogen content suggesting the antihyperglycemic potential of resveratrol in diabetic rats. The obtained results were compared with glyclazide, a standard oral hypoglycemic drug. Thus, the modulatory effects of resveratrol on attenuating these enzymes activities afford a promise for widespread use for treatment of diabetes in the future.

Increased glucose production in mice overexpressing human fructose-1,6-bisphosphatase in the liver

Increased endogenous glucose production (EGP) predominantly from the liver is a characteristic feature of type 2 diabetes, which positively correlates with fasting hyperglycemia. Gluconeogenesis is the biochemical pathway shown to significantly contribute to increased EGP in diabetes. Fructose-1,6-bisphosphatase (FBPase) is a regulated enzyme in gluconeogenesis that is increased in animal models of obesity and insulin resistance. However, whether a specific increase in liver FBPase can result in increased EGP has not been shown. The objective of this study was to determine the role of upregulated liver FBPase in glucose homeostasis. To achieve this goal, we generated human liver FBPase transgenic mice under the control of the transthyretin promoter, using insulator sequences to flank the transgene and protect it from site-of-integration effects. This resulted in a liver-specific model, as transgene expression was not detected in other tissues. Mice were studied under the following conditions: 1) at two ages (24 wk and 1 yr old), 2) after a 60% high-fat diet, and 3) when bred to homozygosity. Hemizygous transgenic mice had an approximately threefold increase in total liver FBPase mRNA with concomitant increases in FBPase protein and enzyme activity levels. After high-fat feeding, hemizygous transgenics were glucose intolerant compared with negative littermates (P < 0.02). Furthermore, when bred to homozygosity, chow-fed transgenic mice showed a 5.5-fold increase in liver FBPase levels and were glucose intolerant compared with negative littermates, with a significantly higher rate of EGP (P < 0.006). This is the first study to show that FBPase regulates EGP and whole body glucose homeostasis in a liver-specific transgenic model. Our homozygous transgenic model may be useful for testing human FBPase inhibitor compounds with the potential to treat patients with type 2 diabetes.

Fructose-1,6-bisphosphatase overexpression in pancreatic beta-cells results in reduced insulin secretion: a new mechanism for fat-induced impairment of beta-cell function

OBJECTIVE

Fructose-1,6-bisphosphatase (FBPase) is a gluconeogenic enzyme that is upregulated in islets or pancreatic beta-cell lines exposed to high fat. However, whether specific beta-cell upregulation of FBPase can impair insulin secretory function is not known. The objective of this study therefore is to determine whether a specific increase in islet beta-cell FBPase can result in reduced glucose-mediated insulin secretion.

RESEARCH DESIGN AND METHODS

To test this hypothesis, we have generated three transgenic mouse lines overexpressing the human FBPase (huFBPase) gene specifically in pancreatic islet beta-cells. In addition, to investigate the biochemical mechanism by which elevated FBPase affects insulin secretion, we made two pancreatic beta-cell lines (MIN6) stably overexpressing huFBPase.

RESULTS

FBPase transgenic mice showed reduced insulin secretion in response to an intravenous glucose bolus. Compared with the untransfected parental MIN6, FBPase-overexpressing cells showed a decreased cell proliferation rate and significantly depressed glucose-induced insulin secretion. These defects were associated with a decrease in the rate of glucose utilization, resulting in reduced cellular ATP levels.

CONCLUSIONS

Taken together, these results suggest that upregulation of FBPase in pancreatic islet beta-cells, as occurs in states of lipid oversupply and type 2 diabetes, contributes to insulin secretory dysfunction.

Gene expression profiles in the liver and kidney of metallothionein-null mice

It has been reported that the expression of certain genes was altered in rodent cells lacking metallothioneins (MTs). To further explore the effects of MT deficiency, we screened genes differentially expressed in the liver and kidney of MT-null mice by cDNA microarray analysis. In the liver, 29 of 8737 genes analyzed were altered in their expression levels: 19 and 10 genes were up-regulated and down-regulated, respectively. Particularly, 14 of the 29 genes were related to energy metabolism, and some of these suggested that loss of MTs might lead to obesity and irregular ATP synthesis. In the kidney, 41 differentially expressed genes were observed: 27 and 14 genes were up-regulated and down-regulated, respectively. Eleven of the 41 genes were also related to energy metabolism. Microarray results were confirmed by Northern blot analysis for five of the energy metabolism-related genes.

Molecular Mechanisms of Increased Glucose Production: Identifying Potential Therapeutic Targets

Many patients with type 2 diabetes go on to require insulin therapy to achieve adequate control. A need remains to develop new classes of oral hypoglycemic agents to complement those already in use. A useful target is the inappropriately elevated endogenous glucose production present in patients with type 2 diabetes. This review discusses mechanisms of increased glucose production and possible strategies and targets for its suppression. Several approaches are being investigated, including inhibitors of glycogenolysis and gluconeogenesis, inhibitors of stimulatory hormones or their receptors, metabolic modulators, and agents that alter gene expression. There is a high probability that one of these approaches will soon result in a safe and effective inhibitor of glucose production.

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.

Increased hepatic peroxisome proliferator-activated receptor-gamma coactivator-1 gene expression in a rat model of intrauterine growth retardation and subsequent insulin resistance

Uteroplacental insufficiency and subsequent intrauterine growth retardation (IUGR) increase the risk of type 2 diabetes in humans and rats. Unsuppressed endogenous hepatic glucose production is a common component of the insulin resistance associated with type 2 diabetes. Peroxisome proliferator-activated receptor-gamma coactivator-1 (PGC-1) mediates hepatic glucose production by controlling mRNA levels of glucose-6-phosphatase (G-6-Pase), phosphoenolpyruvate carboxykinase (PEPCK), and fructose-1,6-bisphosphatase (FBPase). We therefore hypothesized that gene expression of PGC-1 would be increased in juvenile IUGR rat livers, and this increase would directly correlate with hepatic mRNA levels of PEPCK, G-6-Pase, and FBPase, but not glucokinase. We found that IUGR hepatic PGC-1 protein levels were increased to 230 +/- 32% and 310 +/- 47% of control values at d 0 and d 21 of life, respectively. Similarly, IUGR hepatic PGC-1 mRNA levels were significantly elevated at both ages. Concurrent with the increased PGC-1 gene expression, IUGR hepatic mRNA levels of G-6-Pase, PEPCK, and FBPase were also significantly increased, whereas glucokinase mRNA levels were significantly decreased. These data suggest that increased PGC-1 expression and subsequent hepatic glucose production contribute to the insulin resistance observed in the IUGR juvenile rat.

Comparison of insulin secretory function in two mouse models with different susceptibility to beta-cell failure

Type 2 diabetes is characterized by a susceptibility to beta-cell failure. However, subjects at risk of developing type 2 diabetes, such as those with obesity or a family history of diabetes, have been shown to display hyperinsulinemia. Although this hyperinsulinemia may be an adaptive response to insulin resistance, the possibility that insulin hypersecretion may be a primary defect has not been thoroughly investigated. The DBA/2 mouse is a model of pancreatic islet susceptibility. Unlike the resistant C57BL/6 mouse strain, the DBA/2 mouse islet fails when stressed with insulin resistance or when exposed to chronic high glucose concentrations. The aim of this study was to compare insulin secretory function in the DBA/2 and C57BL/6 strains in the absence of insulin resistance or high glucose. Insulin secretion was assessed in vivo using the iv glucose tolerance test and in vitro using isolated islets in static incubations. It was shown that DBA/2 mice hypersecreted insulin in vivo, compared with C57BL/6 mice, at 1 d and at 4 and 10 wk of age. This hypersecretion was not attributable to insulin resistance (as assessed by the insulin tolerance test) or increased parasympathetic nervous system outflow. Insulin hypersecretion was also demonstrated in vitro. This was associated with higher glycolysis and glucose oxidation, and elevated activity (but not protein levels) of islet glucokinase and hexokinase. Furthermore, GLUT2 protein levels were higher, which may explain an increase in glucokinase activity in DBA/2 mouse islets. In summary, the DBA/2 mouse, a model of islet failure, has increased glucose-mediated insulin secretion from a very early age, which is associated with an increase in glucose utilization. Further studies will determine whether there is a link between insulin hypersecretion and subsequent beta-cell failure.

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.

Variable expression of hepatic glucokinase in mice is due to a regulational locus that cosegregates with the glucokinase gene

The Gk activity locus affects expression of hepatic glucokinase (GK) in mice. Analysis of microsatellites within the mouse GK gene locus revealed two major haplotypes in 19 of 22 inbred strains predictive of either high or low hepatic GK gene expression. C3H/HeJ mice, a high-activity strain, and two other wild-derived strains contain less common haplotypes. No coding sequence differences were found in hepatic GK-coding sequences from representative high and low Gk activity strains, thereby excluding kinetic abnormalities as the basis for hepatic GK activity differences. Screening of approximately 10 kb of potential regulatory DNA, including all eight known and three of four newly identified DNase I-hypersensitive sites, by restriction enzyme fingerprinting-single-strand conformation analysis revealed a tetranucleotide microsatellite, the length of which was also predictive of the Gk activity phenotype. This tetranucleotide repeat is located in the first intron of the hepatic transcription unit and lies close to a newly identified liver-specific DNase I-hypersensitive site. These results indicate that the Gk activity alleles are a regulational locus associated with the GK gene locus.

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.