The role of glycogen synthase kinase-3 (GSK-3) protein in the development of myocardial hypertrophy in a rat model of diet induced obesity and insulin resistance

[摘要] ENGLISH ABSTRACT: Introduction: The worldwide escalation in the incidence of obesity and its strong associationwith insulin resistance, type 2 diabetes and the cardiovascular complications that accompanythese disease states have elicited interest in the underlying mechanisms of these pathologies.Preliminary data generated in our laboratory showed that obesity is associated withabnormalities in the insulin signalling pathway. Specifically, we found a down-regulation ofprotein kinase B (PKB/Akt), which is known to mediate the metabolic effects of insulin. Oneof the downstream targets of PKB/Akt is glycogen synthase kinase-3 (GSK-3), which isinhibited by this phosphorylation. Detrimental effects of unopposed activity of GSK-3 haverecently been described. This may play a pivotal role in some of the adverse consequences ofinsulin resistance in the heart.Hypothesis: Chronic inhibition of GSK-3 will induce myocardial hypertrophy or exacerbatethe development of existing hypertrophy in a pre-diabetic model of diet induced obesity andinsulin resistance.Objectives: (1) Assess the extent of the development of myocardial hypertrophy in a ratmodel of diet induced obesity (DIO) and insulin resistance. (2) Assess the effect of inhibitionof GSK-3 protein on the development of myocardial hypertrophy.Methods: Two groups of age-matched male Wistar rats were used. Control animals receivedstandard rat chow, while obese animals received a high caloric diet for 20 weeks. After 12weeks, half of the animals in both groups received GSK-3 inhibitor treatment (CHIR118637,30mg/kg/day, Novartis). At the end of 20 weeks, three series of experiments were conducted.(i) The animals were subjected to echocardiography to determine in vivo myocardial function,and biometric, metabolic and biochemical parameters were evaluated.(ii) The ability of the cardiomyocytes to accumulate deoxy-glucose after stimulation withinsulin was determined, and (iii) the localization of key proteins was monitored usingfluorescence microscopy and cell size was determined using light microscopy and flowactivated cell sorter analysis.Results and discussion: The high caloric diet increased body weight (p<0.005) and intraperitonealfat mass (p<0.01) when compared to controls. Complications associated withobesity, such as impaired glucose tolerance (p<0.05), hyperinsulinemia (p<0.0005) and anincreased HOMA-IR index (p<0.01) were observed. Additionally, cardiomyocytes from theDIO animals had a significantly impaired response to insulin, specifically when 10nM(p<0.05) and 100nM (p<0.05) of insulin were used as stimulus. We also found adysregulation in PKB/Akt, indicated by a down-regulation of phosphorylated PKB/Akt(p<0.01). The diet promoted the development of myocardial hypertrophy, since theventricular weight (p<0.05) and ventricular weight to tibia length ratio were increased(p<0.01). Echocardiography experiments showed an increase in end diastolic diameter in theDIO animals (p<0.05). Additionally, there was an increase in the cardiomyocyte cell width inthe DIO rats (p<0.0001) and a tendency for peri-nuclear localization of NFATc3. GSK-3inhibition promoted the development of insulin resistance in control animals, as indicated byan increase in the body weight (p<0.05), serum insulin levels (p<0.01) and HOMA-IR index(p<0.01). In the DIO animals, the GSK-3 inhibitor treatment improved insulin resistance, as adecrease in serum insulin concentration (p<0.05) was observed. The cardiomyocytes from thetreated DIO animals also showed an increase in glucose uptake (p<0.05) when stimulatedwith 100nM of insulin. The GSK-3 inhibitor promoted the development of myocardialhypertrophy in the control animals, indicated by an increase in ventricular weight (p<0.05)and cardiomyocyte cell width (p<0.0001), but did not exacerbate hypertrophy in the DIO animals.Conclusion: Both the high caloric diet and the GSK-3 inhibitor promoted the development ofinsulin resistance and myocardial hypertrophy in the rats. In the DIO animals the GSK-3inhibitor treatment ameliorated insulin resistance and did not promote the furtherdevelopment of myocardial hypertrophy.

[发布机构] Stellenbosch University