Excessive hepatic mitochondrial TCA cycle and gluconeogenesis in humans with nonalcoholic fatty liver disease

Abstract

Approximately one-third of the U.S. population has nonalcoholic fatty liver disease (NAFLD), a condition closely associated with insulin resistance and increased risk of liver injury. Dysregulated mitochondrial metabolism is central in these disorders, but the manner and degree of dysregulation are disputed. This study tested whether humans with NAFLD have abnormal in vivo hepatic mitochondrial metabolism. Subjects with low (3.0%) and high (17%) intrahepatic triglyceride (IHTG) were studied using (2)H and (13)C tracers to evaluate systemic lipolysis, hepatic glucose production, and mitochondrial pathways (TCA cycle, anaplerosis, and ketogenesis). Individuals with NAFLD had 50% higher rates of lipolysis and 30% higher rates of gluconeogenesis. There was a positive correlation between IHTG content and both mitochondrial oxidative and anaplerotic fluxes. These data indicate that mitochondrial oxidative metabolism is ~2-fold greater in those with NAFLD, providing a potential link between IHTG content, oxidative stress, and liver damage.

Figure 1. Glucose and Mitochondrial Fat Metabolism in Human Subjects with Low or High IHTG Content

²H and ¹³C isotopomer analysis of plasma glucose and β-hydroxybutyrate by NMR and FFA by GC-MS was used to determine hepatic flux in overnight fasted individuals. Shown are (A) endogenous glucose production and its contributions from gluconeogenesisand glycogenolysis in hexose units, (B) anaplerotic flux as an estimateofpyruvate carboxylase and PEPCK flux and its contribution to pyruvate cycling and gluconeogenesis, (C) hepatic TCA cyclefluxinacetyl-CoA units, (D) apparent β-hydroxybutyrate turnover as an estimate of ketogenesis; (E) correlation between TCA cycle flux and anaplerosis; and (F) FFA turnover as an estimate of systemic lipolysis. Data are presented as means ±SEM (n = 8) with significance declared at p ≤ 0.05 and p ≤ 0.1 considered a trend.

Figure 2. Hepatic Mitochondrial Metabolism Is Increased in Subjects with Nonalcoholic Fatty Liver Disease

Subjects with increased IHTG had elevated adipose lipolysis which contributed to increased lipid delivery to liver. Hepatic TCA cycle flux was increased, indicating upregulated mitochondrial respiration (at least via complex II) and suggesting increased flux of acetyl-CoA from β-oxidation. Mitochondrial anaplerosis was also increased and provided substrate for the increased rate of gluconeogenesis observed in subjects with high IHTG. Pathways that are increased during high IHTG are designated by ↑.