Increased ketogenesis related to insulin deficiency in isolated hepatocytes from NIDDM model rats

Host Cells Reduce Glucose Uptake and Glycogen Deposition in Response to Hepatic Insulin Gene Therapy

BACKGROUND

Hepatic insulin gene therapy (HIGT) restores weight gain and near-normal glycemia in rodent models of insulin-deficient diabetes mellitus. However, the effect of transgenic insulin on endogenous genes and recipient cell function is relatively unexplored. To investigate hepatocellular effects of transgenic insulin expression, we evaluated intermediary glucose metabolism in primary cultured hepatocytes treated with HIGT.

METHODS

Rat hepatocytes were transduced with adenovirus expressing a glucose-responsive human insulin transgene and cultured in high-glucose and high-insulin conditions. We determined glycogen content in cell cultures and intact liver directly. Glycogenolysis was compared using glucose production of cultured cells. Glucose uptake, oxidative, and glycolytic processing were determined by radiotracer analysis or direct end-product assessment. Quantitative real-time reverse transcriptase polymerase chain reaction was used to determine expression of glucose transporter 2 (GLUT2) and glucokinase genes. GLUT2 protein abundance was determined by Western blot analysis.

RESULTS

HIGT-treated hepatocytes contained significantly less glycogen than either untreated hepatocytes or those treated with an empty virus. Glucose release owing to glycogenolysis remained normal. However, HIGT treatment significantly impaired glucose uptake and processing. Metabolic synthetic processes were not generally inhibited, as indicated by enhanced beta-hydroxybutyrate secretion. While preserving cell viability, HIGT treatment diminished expression of both glucokinase and GLUT2. In HIGT-treated streptozocin-treated diabetic rats, total liver glycogen was intermediate between diabetic animals and normal controls.

CONCLUSIONS

These results suggest gene-specific effects in recipient hepatocytes following HIGT treatment and underscore the need for expanded studies examining host cell responses to the transfer of metabolically active transgenes.

Physical training reverses defect in 3-ketoacid CoA-transferase activity in skeletal muscle of diabetic rats

To investigate one potential mechanism whereby physical training improves the plasma concentration of ketone bodies in experimental diabetes mellitus, we measured the activity of 3-ketoacid CoA-transferase, the key enzyme in the peripheral utilization of ketone bodies. Diabetes was induced with streptozotocin (50 mg/kg) and training carried out on a treadmill with a progressive 10-wk program. Diabetes resulted in an increase (P < 0.001) in plasma concentration of beta-hydroxybutyric acid in sedentary rats, which was partly reversed by training (P < 0.001). Diabetes was also associated with a decreased activity of 3-ketoacid CoA-transferase in gastrocnemius muscle. When expressed per total gastrocnemius, training increased the activity of 3-ketoacid CoA-transferase by 66% in nondiabetic rats (P < 0.001) and by 150% in diabetic rats (P < 0.001), the decrease present in diabetic rats being fully reversed by training. Simple linear regression between the log of 3-ketoacid CoA-transferase activity and the log of plasma beta-hydroxybutyric acid levels showed a statistically significant (r = 0.563, P < 0.001) negative correlation. The beneficial effects of training on plasma ketone bodies in diabetic rats are probably explained, at least in part, by an increase in ketone body utilization, mediated by an increase in skeletal muscle 3-ketoacid CoA-transferase activity.