Enhancing liver mitochondrial fatty acid oxidation capacity in obese mice improves insulin sensitivity independently of hepatic steatosis.

BACKGROUND & AIMS: Despite major public health concern, therapy for non-alcoholic fatty liver, the liver manifestation of the metabolic syndrome often associated with insulin resistance (IR), remains elusive. Strategies aiming to decrease liver lipogenesis effectively corrected hepatic steatosis and IR in obese animals. However, they also indirectly increased mitochondrial long-chain fatty acid oxidation (mFAO) by decreasing malonyl-CoA, a lipogenic intermediate, which is the allosteric inhibitor of carnitine palmitoyltransferase 1 (CPT1A), the key enzyme of mFAO. We thus addressed whether enhancing hepatic mFAO capacity, through a direct modulation of liver CPT1A/malonyl-CoA partnership, can reverse an already established hepatic steatosis and IR in obese mice. METHODS: Adenovirus-mediated liver expression of a malonyl-CoA-insensitive CPT1A (CPT1mt) in high-fat/high-sucrose (HF/HS) diet-induced or genetically (ob/ob) obese mice was followed by metabolic and physiological investigations. RESULTS: In association with increased hepatic mFAO capacity, liver CPT1mt expression improved glucose tolerance and insulin response to a glucose load in HF/HS and ob/ob mice, showing increased insulin sensitivity, and corrected IR in ob/ob mice. Surprisingly, hepatic steatosis was not affected in CPT1mt-expressing obese mice, indicating a clear dissociation between hepatic steatosis and IR. Moreover, liver CPT1mt expression rescued HF/HS-induced impaired hepatic insulin signaling at the level of IRS-1, IRS-2, Akt, and GSK-3ß, most likely through the observed decrease in the HF/HS-induced accumulation of lipotoxic lipids, oxidative stress, and JNK activation. CONCLUSIONS: Enhancing hepatic mFAO capacity is sufficient to reverse a state of IR and glucose intolerance in obese mice independently of hepatic steatosis.

Modulation of the hepatic malonyl-CoA-carnitine palmitoyltransferase 1A partnership creates a metabolic switch allowing oxidation of de novo fatty acids.

Liver mitochondrial beta-oxidation of LCFAs (long-chain fatty acids) is tightly regulated through inhibition of CPT1A (carnitine palmitoyltransferase 1A) by malonyl-CoA, an intermediate of lipogenesis stimulated by glucose and insulin. Moreover, CPT1A sensitivity to malonyl-CoA inhibition varies markedly depending on the physiopathological state of the animal. In the present study, we asked whether an increase in CPT1A activity solely or in association with a decreased malonyl-CoA sensitivity could, even in the presence of high glucose and insulin concentrations, maintain a sustained LCFA beta-oxidation and/or protect from triacylglycerol (triglyceride) accumulation in hepatocytes. We have shown that adenovirus-mediated expression of rat CPT1wt (wild-type CPT1A) and malonyl-CoA-insensitive CPT1mt (CPT1AM593S mutant) in cultured fed rat hepatocytes counteracted the inhibition of oleate beta-oxidation induced by 20 mM glucose/10 nM insulin. Interestingly, the glucose/insulin-induced cellular triacylglycerol accumulation was prevented, both in the presence and absence of exogenous oleate. This resulted from the generation of a metabolic switch allowing beta-oxidation of de novo synthesized LCFAs, which occurred without alteration in glucose oxidation and glycogen synthesis. Moreover, CPT1mt expression was more effective than CPT1wt overexpression to counteract glucose/insulin effects, demonstrating that control of CPT1A activity by malonyl-CoA is an essential driving force for hepatic LCFA metabolic fate. In conclusion, the present study highlights that CPT1A is a prime target to increase hepatic LCFA beta-oxidation and that acting directly on the degree of its malonyl-CoA sensitivity may be a relevant strategy to prevent and/or correct hepatic steatosis.