Only fatty acids that are activated by acyl Co A synthase (ACS) can be converted to triglycerides or enter into the mitochondria for oxidation. Acyl Co A thioesterase can hydrolyse the activated fatty acids to their free fatty acid form in a tissue specific manner. These enzymes are thought to reduce lipotoxicity and may be upregulated to limit lipid oversupply in muscle tissues of high fat-fed rodents.
To assess the impact of increased thioesterase activity on muscle fatty acid metabolism we used in vivo electroporation to overexpress an Acot7 containing plasmid in the extensor digitorus longus (EDL) muscle of one leg of Wistar rats. An empty vector was electroporated into the other leg of the same rats as a control. Fatty acid oxidation was measured in EDL strips incubated in media with 0.5mM 14C- oleic acid and 5mM glucose for 1.5hr. Oleic acid can undergo complete oxidation to release 14CO2 and incomplete oxidation to release 14C acid soluble metabolites (ASM).
Electrporation of the ACOT7 plasmid resulted in a 3-fold increase in ACOT activity in the electroporated EDL compared to control muscles (1.228±0.28 versus 0.33±0.06 µmol/min/g respectively, p<0.01, paired t-test). This was accompanied by an increased conversion in 14C- oleate to in 14CO2 (38.3±5.9 versus 29.4±5.3 nmoles/h/g for test and control respectively, p<0.01). Conversion of 14C- oleate to ASM was also higher in the ACOT7 expressing muscle (26.4±4.4 versus 18.7±3 nmoles/h/g, p<0.01). The overexpression of ACOT7 did not result in any compensatory increase in ACS activity or mitochondrial content.
Increasing thioesterase activity in muscle might be expected to reduce fatty acid oxidation by limiting the availability of fatty acyl CoA substrate for carnitine palmitoyl transferase. However, our finding that increasing ACOT activity elevates fatty acid oxidation suggests a possible acceleration of substrate oxidation to supply ATP for a substrate (“futile”) cycle created between ACS and ACOT enzymes.