Acyl-CoA synthetase isoforms 1, 4, and 5: Molecular characterization, cellular localization, and regulation

Acyl-CoA synthetase (ACS, EC 6.2.1.3) activates long-chain fatty acids to form acyl-CoAs, which are the substrates for numerous metabolic pathways including β-oxidation, the synthesis of glycerolipids, cholesterol esters, and sphingolipids, and the desaturation and elongation of fatty acids. After being activated by ACS, fatty acids encounter different metabolic fates. We hypothesized that ACS may play a critical role in determining the partitioning of acyl-CoA pools between synthetic and degradative pathways through independent regulation of different ACS isoenzymes. Further, we postulated that triacsin-sensitive ACS is related to triacylglycerol (TAG) synthesis than to oxidation because treatment of various cell types with triacsin, a well-established ACS inhibitor, more strongly inhibited TAG synthesis than the formation of other metabolic end products.; To distinguish the effects of triacsin on each ACS isoenzyme, we cloned, expressed and purified ACS1, 4, and 5 proteins. Consequently, we showed that triacsin inhibited ACS1 and ACS4, but had little effect on ACS5. The following support the hypothesis that ACS1 and ACS4 may be channeled toward TAG synthesis: the cellular location of ACS1 and ACS4 in the endoplasmic reticulum or mitochondrial-associated membrane, the sites for VLDL synthesis, together with the results that the expression of ACS1 and ACS4 proteins was upregulated in rat liver during lipogenic states. We also suggest that ACS5 preferentially supplies acyl-CoA for β-oxidation, based on its location in mitochondria and the increased protein expression after starvation. Such an independent regulation of ACS1, 4, and 5 under various nutritional states suggests that each ACS is linked to different metabolic pathways. In addition, purified ACS1, 4, and 5 differed in terms of kinetics for substrates, thermolability, pH optima, requirement for Triton X-100, and effects of several amino acid-reactive compounds.; Thiazolidinediones (TZD), oral antidiabetic drugs, and PPARγ ligands, strongly and specifically inhibited only ACS4, suggesting that a lipid-lowering effect of TZDs may partly be achieved by direct interaction with ACS4, in a PPARγ-independent manner. Therefore, the results of this research may possibly contribute to new treatments for diseases such as obesity and type 2 diabetes, by leading to the development of pharmacologic agents that specifically act on each ACS isoform.