Acute inhibition of hepatic β-oxidation in APOE*3Leiden mice does not affect hepatic VLDL secretion or insulin sensitivity

Abstract

Hepatic VLDL and glucose production is enhanced in type 2 diabetes and associated with hepatic steatosis. Whether the derangements in hepatic metabolism are attributable to steatosis or to the increased availability of FA metabolites is not known. We used methyl palmoxirate (MP), an inhibitor of carnitine palmitoyl transferase I, to acutely inhibit hepatic FA oxidation and investigated whether the FAs were rerouted into VLDL secretion and whether this would affect hepatic glucose production. After an overnight fast, male APOE3*Leiden transgenic mice received an oral dose of 10 mg/kg MP. Administration of MP led to an 83% reduction in plasma ␤ -hydroxybutyrate (ketone body) levels compared with vehicle-treated mice (0.47 ؎ 0.07 vs. 2.81 ؎ 0.16 mmol/l, respectively; P Ͻ 0.01), indicative of impaired ketogenesis. Plasma FFA levels were increased by 32% and cholesterol and insulin levels were decreased by 17% and 50%, respectively, in MP-treated mice compared with controls. MP treatment led to a 30% increase in liver triglyceride (TG) content. Surprisingly, no effect on hepatic VLDL-TG production was observed between the groups at 8 h after MP administration. In addition, the capacity of insulin to suppress endogenous glucose production was unaffected in MP-treated mice compared with controls.In conclusion, acute inhibition of FA oxidation increases hepatic lipid content but does not stimulate hepatic VLDL secretion or reduce insulin sensitivity. Hepatic VLDL-triglyceride (TG) secretion is an important determinant of plasma lipid levels. The rate of VLDL secretion is generally believed to be substrate-driven (i.e., the hepatic content of TG and FA determines VLDL assembly and the rate of VLDL secretion) (1-3). Indeed, addition of FA to hepatocytes in vitro leads to increased hepatic TG and eventually to enhanced VLDL secretion (4). However, the hypothesis that VLDL secretion is substratedriven in vivo has not been proven.Hepatic TG content is the result of uptake and synthesis on the one hand and ␤ -oxidation and VLDL-TG secretion on the other. The importance of ␤ -oxidation is illustrated by the severe phenotype of humans and mice with impaired ␤ -oxidation. Deficiency in hepatic ␤ -oxidation enzymes results in severe plasma hypoketosis and fatty liver (5, 6). In ␤ -oxidation, the rate-limiting enzyme is carnitine palmitoyl transferase 1 (CPT1), which couples long-chain FAs to carnitine for transportation into the mitochondria (7,8). Methyl palmoxirate (MP) is a specific and irreversible inhibitor of CPT1 and thereby inhibits ␤ -oxidation (9, 10).In this study, we tested the effect of acute inhibition of ␤ -oxidation in vivo by MP on hepatic VLDL production in fasted hyperlipidemic APOE3*Leiden transgenic mice.Abbreviations: acc1 , acetyl-coenzyme A carboxylase 1; apoB, apolipoprotein B; CPT1, carnitine palmitoyl transferase 1; dgat , acyl:diacylglycerol transferase; fas , fatty acid synthase; ␤ -HB, ␤ -hydroxybutyrate; hmgs , 3-hydroxy-3-methylglutaryl-coenzyme A synthase; mcad , mediumchain acyl coenzyme...

Selective Reversible Inhibition of Liver Carnitine Palmitoyl-Transferase 1 by Teglicar Reduces Gluconeogenesis and Improves Glucose Homeostasis

“…Even if increased hepatic triglyceride content is commonly thought to be associated with reduced insulin sensitivity (31), our evidence are in agreement with the results of studies in which liver steatosis induced by acute treatment with TDGA in C57BL/6J mice, or with methylpalmoxirate in hyperlipidemic APO3*Leiden mice, does not induce liver insulin resistance (32,33). In addition, Buettner et al (34) perfusing ex vivo livers from high-fat-fed Wistar rats showed that increased intracellular lipids per se do not induce defects in insulin action, reporting that fasting-induced hepatic steatosis is not associated with an impairment of liver insulin sensitivity in healthy rodents (35), suggesting alternative extrahepatic regulators of liver insulin sensitivity other than triglycerides.…”

Hepatic glucose production is more sensitive to insulin-mediated inhibition than hepatic VLDL-triglyceride production

“…Apparently, plasma FA levels and FA availability to the liver per se do not determine hepatic VLDL-TG production. In accord with this notion, we have previously shown that acute redirection of hepatic FA flux from ␤-oxidation to storage does not affect hepatic VLDL-TG production (8). We suggest that, under the conditions of our experiment, insulin exerts direct effects on hepatic VLDL-TG production that are apparently of greater importance than the indirect effects via suppression of FA release from adipose tissue or FA availability in general, at least under the conditions of our experiments.…”

Fenofibrate Increases Very Low Density Lipoprotein Triglyceride Production Despite Reducing Plasma Triglyceride Levels in APOE*3-Leiden.CETP Mice

“…First, it has been generally assumed that, via increased FA ␤-oxidation, fenofibrate reduces the amount of FA available for VLDL-TG output (5). However, we previously showed that specific inhibition of FA ␤-oxidation using methyl palmoxirate does not affect VLDL-TG production despite increasing hepatic TG content (45). Moreover, this simplified assumption does not take into account that fenofibrate in fact increases the liverdirected flux of FA/TG.…”