Features of syndrome X develop in transgenic rats expressing a non-insulin responsive phosphoenolpyruvate carboxykinase gene

Aims/hypothesis. Obesity, glucose intolerance, dyslipidaemia and hypertension are a cluster of disorders (syndrome X) affecting many people. It has been hypothesised that these abnormalities are caused by insulin resistance, but definitive proof is lacking. We have developed transgenic rats in which the rate-limiting gluconeogenic enzyme, phosphoenolpyruvate carboxykinase, is non-insulin responsive. The aim of our study was to investigate whether syndrome X develops in these animals and if a high-fat diet interacts with this genetic defect. Methods. Chow-fed transgenic and control rats aged 1, 3, 6 and 17 months and a subgroup of transgenic and control rats fed chow plus cafeteria foods for 6 months were examined for features of syndrome X. Results. At 3 months, transgenic rats had fasting and postprandial hyperinsulinaemia, mild obesity (in abdominal and, to a lesser extent, peripheral regions) and fasting hypercholesterolaemia. Hypertriglyceridaemia was evident after 6 months while hyperglycaemia was apparent at 17 months. Hypertension had not developed by 17 months. The effect of a high-fat diet on insulin, glucose, body weight and body fat was more dramatic than the effect of the transgene alone while the effect of a high-fat diet on cholesterol and triglyceride was similar to the transgene. This illustrates that a high-fat diet is a potent catalyst for many abnormalities associated with syndrome X. There was no evidence of an additive effect of the high-fat diet plus transgene. Conclusion/interpretation. Therefore rats genetically-engineered with a non-insulin responsive gluconeogenic enzyme develop several aspects of syndrome X, supporting the hypothesis that insulin resistance initiates this cluster of disorders. [Diabetologia (1999) 42: 419–426] Received: 21 September 1998 and in final revised form: 23 November 1998     

Regulation of citrate synthase and phosphoenolpyruvate carboxykinase by hydrocortisone in the liver of aging rats

The activities and hormonal regulations of citrate synthase (CS) and phosphoenolpyruvate carboxykinase (PEPCK) in the liver of male rats of various ages were studied. It has been observed that the activity of CS increases gradually as a function of age of the rat. The activity of PEPCK, on the other hand is highest in the liver of adult rats. Adrenalectomy causes no significant change in the activity of CS of the liver of young, adult and old rats. However, this treatment decreases significantly the activity of PEPCK of the liver of rats of all the ages. Administration of hydrocortisone to adrenalectomized rats depresses and induces, respectively, the activity of CS and PEPCK in the liver of young and adult rats but not in the old rats. This hormone mediated effect of the enzymes decreases with increasing age of the rats. Treatment of actinomycin D prior to hydrocortisone administration tends to normalize the depressed level of CS. However, this inhibitor inhibits the PEPCK induction.     

Inhibition of phosphoenolpyruvate carboxykinase by streptonigrin

Streptonigrin, an antibiotic with antineoplastic activity, inhibited rat liver phosphoenolpyruvate carboxykinase with an I₅₀ of 0.3 μM when excess FeCl₂ was present. No inhibition occurred in the absence of added metal ion. Inhibition was partial and noncompetitive versus ITP and oxalacetic acid. The enzyme was more susceptible to inhibition by Streptonigrin in the absence of substrates. Fe²⁺ supported inhibition by Streptonigrin to a greater extent than did Fe³⁺, while Mn²⁺ activated the enzyme in the presence of Streptonigrin. For maximum inhibition, at least a 3-fold molar excess of iron over Streptonigrin was required. The methyl ester of Streptonigrin was also an inhibitor (I₅₀ = 4 μM) while the fragment containing the C and D rings was not, indicating that inhibition did not depend solely on the presence of the picolinic acid moiety. When oxalacetate synthesis was measured, Streptonigrin plus iron had no more effect on enzymatic activity than iron alone, and Mn²⁺ was capable of stimulating the streptonigrin-Fe²⁺ inhibited enzyme.     

Cytochrome C oxidase deficiency in two siblings with Leigh encephalomyelopathy

Two siblings with cytochrome c oxidase deficiency are described. One of them died of subacute necrotizing encephalomyelopathy which was proven by autopsy. The other was also suspected of having Leigh encephalomyelopathy by the findings on brain CT scans. The former, a younger brother, was in good health until the age of 10 months when progressive dysphagia, muscular hypotonia and abnormal eye movements became apparent. Six months later he suddenly died due to respiratory insufficiency. The latter, an elder brother, started to show nystagmus, abnormal eye movements and ataxia at the age of 5 years. A deficiency of cytochrome c oxidase in the younger brother was demonstrated in autopsied liver and brain, while such a deficiency in the elder brother was shown in biopsied peripheral muscle tissue and in cultured skin fibroblasts. Both patients showed a marked heat lability of cytochrome c oxidase. These results suggest that the biochemical defect observed in the siblings is due to a genetic defect. This seems to be the first case of a generalized defect in cytochrome c oxidase.     

The effects of a polychlorinated biphenyl mixture (Aroclor 1254) on liver gluconeogenic enzymes of normal and alloxan-diabetic rats.

Normal and alloxan-diabetic rats were fed ground Purina Laboratory Chow with or without 500 ppm of Aroclor 1254 (AR) ad lib for 2 weeks. In both normal and diabetic rats, AR administration decreased food consumption, weight gain and blood glucose concentration, and increased liver weight, liver:body weight ratio, total liver lipid, liver protein and malic enzyme (ME) activity. In the normal rat, AR increased the concentrations of acetoacetate and beta-hydroxybutyrate in blood, but in the diabetic rat the concentrations were markedly reduced. AR administration decreased the activity of phosphoenolpyruvate carboxykinase (PEPck) in normal liver and the activities of pyruvate carboxylase (PC), PEPck and glucose-6-phosphatase (G6Pase) in diabetic liver.     

High selenium impairs hepatic insulin sensitivity through opposite regulation of ROS

Insulin resistance is the hallmark of type 2 diabetes. As an essential trace element, selenium (Se) is recommended worldwide for supplementation to prevent Se-deficient pathological conditions, including diabetes and insulin resistance. However, recent evidence has shown that supra-nutritional Se intake is positively associated with the prevalence of diabetes. In the present research, we examined the effect of high Se on insulin sensitivity, and studied possible mechanisms in rats and in rat hepatocytes. Insulin sensitivity and glucose/lipid metabolism were determined by glucose/insulin tolerance test, western blot, immunofluorescence, specific probes and other biochemical assays. We show that high Se activates selenoproteins, including glutathione peroxidase and selenoprotein P, and depletes chromium, leading to a common metabolic intersection—lipolysis in adipose tissue and influx of fatty acids in liver. Fatty acid β-oxidation generates acetyl-CoA, which is metabolized in trichloroacetic acid cycle, supplying excessive electrons for mitochondrial oxidative phosphorylation and leading to increased “bad” reactive oxygen species (ROS) production in mitochondria and final disturbance of insulin signaling. Furthermore, high Se-activated selenoproteins also weaken insulin-stimulated “good” ROS signal generated by NAD(P)H oxidase, leading to attenuation of insulin signaling. Taken together, these data suggest that excessive intake of Se induces hepatic insulin resistance through opposite regulation of ROS.