Fasting hyperglycemia is not associated with increased expression of PEPCK or G6Pc in patients with Type 2 Diabetes

Abstract

Fasting hyperglycemia in patients with type 2 diabetes mellitus (T2DM) is attributed to increased hepatic gluconeogenesis, which has been ascribed to increased transcriptional expression of phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase, catalytic (G6Pc). To test this hypothesis, we examined hepatic expression of these 2 key gluconeogenic enzymes in 2 rodent models of fasting hyperglycemia and in patients with T2DM. In rats, high-fat feeding (HFF) induces insulin resistance but a robust beta-cell response prevents hyperglycemia. Fasting hyperglycemia was induced in the first rat model by using nicotinamide and streptozotocin to prevent beta-cell compensation, in combination with HFF (STZ/HFF). In a second model, control and HFF rats were infused with somatostatin, followed by portal vein infusion of insulin and glucagon. Finally, the expression of these enzymes was measured in liver biopsy samples obtained from insulin sensitive, insulin resistant, and untreated T2DM patients undergoing bariatric surgery. Rats treated with STZ/HFF developed modest fasting hyperglycemia (119 +/- 4 vs. 153 +/- 6 mg/dL, P < 0.001) and increased rates of endogenous glucose production (EGP) (4.6 +/- 0.6 vs. 6.9 +/- 0.6 mg/kg/min, P = 0.02). Surprisingly, the expression of PEPCK or G6Pc was not increased. Matching plasma insulin and glucagon with portal infusions led to higher plasma glucoses in the HFF rats (147 +/- 4 vs. 161 +/- 4 mg/dL, P = 0.05) with higher rates of EGP and gluconeogenesis. However, PEPCK and G6Pc expression remained unchanged. Finally, in patients with T2DM, hepatic expression of PEPCK or G6Pc was not increased. Thus, in contrast to current dogma, these data demonstrate that increased transcriptional expression of PEPCK1 and G6Pc does not account for increased gluconeogenesis and fasting hyperglycemia in patients with T2DM.

Fig. 1.

Basal data for control and diabetic rats. Rats were either untreated or treated with a combination of Streptozocin and nicotinic acid followed by feeding with either a control chow or high-fat chow for 5 or 6 days. (A) Fasting plasma glucose. (B) Fasting plasma insulin. (C) Glucagon. (D) Ra glucose in control and STZ/HFF rats. (E) Rates of hepatic glucose production vs. plasma glucose concentration, including data from the very diabetic STZ/HFF rats. *, P < 0.05 vs. control; †, P < 0.01 vs. HFF; ‡, P < 0.001 vs. control, §, P < 0.01 vs. STZ.

Fig. 2.

PEPCK and G6Pc expression in diabetic rats. (A–C) Plasma glucose (A), PEPCK1 (B), and G6Pc (C) expression from a representative cohort of control and moderately diabetic rats. (D–F) Plasma glucose (D), PEPCK1 (E), and G6Pc (F) expression from a cohort of control and very diabetic rats. (G–I) PEPCK 1 and 2 Western blots for control and very diabetic rats. Plasma glucose for the rats from which the samples were taken is shown. †, P < 0.01 vs. control.

Fig. 3.

UPortal infusion studies in control and HFF rats. Triply catheterized rats (portal vein, jugular vein, and carotid artery) were subjected to either 3 days of control chow or high-fat feeding before studies. (A) Infusion protocol and time-course of plasma glucose. (B) Plasma insulin at basal and steady states. (C) Plasma glucagon at basal and steady state conditions. (D) Gluconeogenesis. (E) PEPCK1 expression. (F) G6Pc expression. *, P < 0.05.

Fig. 4.

Gluconeogenic gene expression in livers of additional human subjects. (A) Plasma glucose. (B) PEPCK1 expression. (C) G6Pc expression. ‡, P < 0.0001 vs. control; *, P < 0.05 vs. control