Dipeptidyl peptidase-4 inhibition ameliorates Western diet-induced hepatic steatosis and insulin resistance through hepatic lipid remodeling and modulation of hepatic mitochondrial function

Abstract

Novel therapies are needed for treating the increasing prevalence of hepatic steatosis in Western populations. In this regard, dipeptidyl peptidase-4 (DPP-4) inhibitors have recently been reported to attenuate the development of hepatic steatosis, but the potential mechanisms remain poorly defined. In the current study, 4-week-old C57Bl/6 mice were fed a high-fat/high-fructose Western diet (WD) or a WD containing the DPP-4 inhibitor, MK0626, for 16 weeks. The DPP-4 inhibitor prevented WD-induced hepatic steatosis and reduced hepatic insulin resistance by enhancing insulin suppression of hepatic glucose output. WD-induced accumulation of hepatic triacylglycerol (TAG) and diacylglycerol (DAG) content was significantly attenuated with DPP-4 inhibitor treatment. In addition, MK0626 significantly reduced mitochondrial incomplete palmitate oxidation and increased indices of pyruvate dehydrogenase activity, TCA cycle flux, and hepatic TAG secretion. Furthermore, DPP-4 inhibition rescued WD-induced decreases in hepatic PGC-1α and CPT-1 mRNA expression and hepatic Sirt1 protein content. Moreover, plasma uric acid levels in mice fed the WD were decreased after MK0626 treatment. These studies suggest that DPP-4 inhibition ameliorates hepatic steatosis and insulin resistance by suppressing hepatic TAG and DAG accumulation through enhanced mitochondrial carbohydrate utilization and hepatic TAG secretion/export with a concomitant reduction of uric acid production.

Figure 1

DPP-4 inhibition attenuated WD-induced hepatic insulin resistance. Values are means ± SE. Blood glucose levels and glucose infusion rate to maintain euglycemia (A), glucose infusion rate (GIR) during the final 40 min (steady-state) of a hyperinsulinemic-euglycemic clamp (B), plasma insulin during basal and steady-state insulin clamp conditions (C), hepatic glucose production during the basal and insulin-stimulated condition and percent insulin suppression of hepatic glucose output during the clamp (D), and hepatic insulin signaling at the level of Akt (Ser473 phosphorylation) from acute insulin stimulation studies (E). n = 5 per group for clamp data and n = 6 per group for acute insulin stimulation studies. *P < 0.05, main effect of diet. #P < 0.05, main effect of MK compound.

Figure 2

DPP-4 inhibition reduces WD-induced hepatic steatosis and hepatic TAG accumulation. Values are means ± SE. A: Hematoxylin and eosin staining of hepatocytes for lipid droplets. B: Hepatic lipids were extracted and analyzed by liquid chromatography–mass spectrometry, and hepatic TAG content was determined as fatty acid content of TAG. C: Distribution of fatty acid species in TAG. n = 5 per group. *P < 0.01, main effect of diet. #P < 0.01, diet-by-drug interaction, WD-MK vs. WD.

Figure 3

DPP-4 inhibition lowers WD-induced hepatic DAG accumulation. Hepatic lipids were extracted and analyzed by liquid chromatography–mass spectrometry. Values are means ± SE. Fatty acid species and total fatty acid content in DAG (A), and fatty acid species (B) and total fatty acid (C) content in ceramides. n = 5 per group. *P < 0.01, main effect of diet. $P < 0.01, main effect for drug; #P < 0.01, diet-by-drug interaction, WD-MK vs. WD.

Figure 4

Effects of DPP-4 inhibition on hepatic de novo lipogenesis markers and hepatic TAG secretion. Values are means ± SE. Hepatic mRNA expression for ACC, FAS and SREBF (A), protein content for ACC, FAS and SREBP-1c (B), hepatic TAG secretion time course (C) and rate (D), mRNA expression for MTTP and apoB (E), and protein content for MTTP and apoB100 (F). n = 8–10 per group for gene expression and protein content. n = 6–7 per group for TAG secretion studies. *P < 0.01, main effect of diet. &P < 0.05 for interaction (significantly different than CD). $P < 0.05 for interaction (significantly different than WD).

Figure 5

Effects of DPP-4 inhibition on hepatic [1-¹⁴C]pyruvate oxidation to CO₂ (A), [2-¹⁴C]pyruvate oxidation to CO₂ (B), [1-¹⁴C]palmitate oxidation to CO₂ (C), incomplete [1-¹⁴C]palmitate oxidation (D), and mitochondrial respiration (E) in isolated mitochondria. n = 6–10 per group. Values are means ± SE. *P < 0.05, significant main effect for diet. #P < 0.05, significant main effect for MK compound.

Figure 6

Effects of DPP-4 inhibition on hepatic mitochondrial genes PGC-1α, TFAM, CPT-1, and PPAR-α (A), OXPHOS complex I-V protein content (B), Sirt1 protein content (C), and Sirt3 protein content (D) measured in isolated mitochondria. n = 6–10 per group for Western blot analyses and n = 8–10 per group for gene expression. Values are means ± SE. *P < 0.05, significant main effect for diet; #P < 0.05, significant main effect for MK compound; &P < 0.05 for interaction (significantly different than CD); $P < 0.05 for interaction (significantly different than WD).

Figure 7

Mechanistic schematic of DPP-4 inhibition with MK0626 on hepatic metabolism and hepatic insulin resistance. Improvement in hepatic insulin resistance by DPP-4 inhibitor MK0626 (1) resulted from decreased levels of DAG and TAG (2), increasing hepatic TAG secretion (3), enhancing select genes and proteins involved in mitochondrial content and function (4), suppressing incomplete oxidation of fatty acids (5), enhancing utilization of metabolites through enhanced PDH activity (6) and TCA cycle flux (6), presumably decreasing carbon intermediates available of lipogenesis and lowering uric acid levels (7) and thereby suppressing the lipogenic effects of uric acid.