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

Abstract

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.

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

“…Furthermore transgenic mice overexpressing liver PEPCK showed increased glucose production, hyperglycaemia, hyperinsulinaemia and glucose intolerance [34,35]. The transgenic rats used in this study over-express PEPCK predominantly in the kidney under the control of a non-insulin responsive promoter [23] and resulted in impaired suppression of EGP during an hyperinsulinaemic clamp. Thus the same concentration of plasma insulin that resulted in complete suppression in control rats was unable to suppress EGP from basal values in the PEPCK transgenic rats.…”

“…PEPCK transgenic rats expressing the PEPCK gene predominantly in the kidney under the control of the metallothionein promoter were produced as previously described on the PVG/c background strain. We have previously shown that there is no PEPCK expression in fat, heart, muscle, lung and brain of transgenic rats [23]. The rats were housed in the University of Melbourne, Department of Medicine Animal Research Facility at the Royal Melbourne Hospital.…”

“…Total RNA was extracted from kidney and liver using the guanidinium thiocyanate procedure as previously described [23]. RNA was then reverse transcribed using random priming with an AMV reverse transcriptase kit (Promega, Annandale, NSW, Australia).…”

“…To determine the consequences of chronically impaired suppression of gluconeogenesis we have produced transgenic rats that overexpress the regulated enzyme of gluconeogenesis, phosphoenolpyruvate carboxykinase (PEPCK) predominantly in the kidney under the control of a non-insulin responsive promoter [23]. These rats developed a number of features associated with the metabolic syndrome including obesity, mild hyperglycaemia, hyperinsulinaemia, and dyslipidaemia [24].…”

De novo lipogenesis in humans: metabolic and regulatory aspects

“…Pagliassotti et al (1997a,b) have shown that non-suppressability of HGP by insulin is the best predictor of obesity in rats fed ad libitum high-fat diets. Over-expression of the GNG enzyme PEP-CK in the liver of transgenic mice (Valera et al, 1994) and rats (Rosella et al, 1995) results in hyperinsulinemia, obesity and ultimately hyperglycemia. Therefore, a liver enzyme can be responsible for obesity and its sequelae.…”