Acyl-CoA thioesterases belong to a novel gene family of peroxisome proliferator-regulated enzymes involved in lipid metabolism.

Acyl-CoA thioesterases hydrolyze acyl-CoAs to the corresponding free fatty acid plus coenzyme A. The activity is strongly induced in rat and mouse liver after feeding the animals peroxisome proliferators (PPs). To elucidate the role of these enzymes in lipid metabolism, the authors have cloned the cDNAs corresponding to the inducible cytosolic and mitochondrial type I enzymes (CTE-I and MTE-I), and studied tissue expression and nutritional regulation of expression of the mRNAs in mice. The constitutive expression of both mRNAs was low in liver, with CTE-I expressed mainly in kidney and brown adipose tissue, and MTE-I expressed in brown adipose tissue and heart. As expected, the expression in liver of both the CTE-I and MTE-I mRNAs were strongly induced (> 50-fold) by treatment with clofibrate. A similar level of induction was observed by fasting and a time-course study showed that the CTE-I and MTE-I mRNAs were increased already at 6 h after removal of the diet. Refeeding normal chow diet to mice fasted for 24 h normalized the mRNA levels with a T1/2 of about 3-4 h. Feeding mice a fat-free diet further decreased the expression, possibly indicating repression of expression. The strong expression of MTE-I and CTE-I in the heart was increased about 10-fold by fasting. To further characterize these highly regulated enzymes, the authors have cloned the corresponding genes and promoter regions. The structures of the two genes were found to be very similar, consisting of three exons and two introns. Exon-intron borders conform to general consensus sequences, and, especially, the first exon appears to be highly conserved. The promoter regions of both the CTE-I and MTE-I genes contain putative PP response elements, suggesting an involvement of PP-activated receptors in the regulation of these genes.

Cloning and regulation of peroxisome proliferator-induced acyl-CoA thioesterases from mouse liver.

1.1. Acyl-CoA thioesterases hydrolyze acyl-CoAs to the corresponding free fatty acid plus CoASH. The activity is strongly induced in rat and mouse liver after feeding the animals peroxisome proliferators. To elucidate the role of these enzymes in lipid metabolism, we have cloned the cDNAs corresponding to the inducible cytosolic and mitochondrial type I enzymes (CTE-I and MTE-I) and studied tissue expression and nutritional regulation of expression of the mRNAs in mice. The constitutive expression of both mRNAs was low in liver, with CTE-I being expressed mainly in kidney and brown adipose tissue and MTE-I expressed in brown adipose tissue and heart. As expected, the expression in liver of both the CTE-I and MTE-I mRNAs was strongly induced (> 50-fold) by treatment with clofibrate. A similar level of induction was observed by fasting and a time-course study showed that both mRNAs were increased already at 6 hours after removal of the diet. Refeeding normal chow diet to mice fasted for 24 hours normalized the mRNA levels with a T1/2 of about 3-4 hours. Feeding mice a fat-free diet further decreased the expression, possibly indicating repression of expression. The strong expression of MTE-I and CTE-I in the heart was increased about 10-fold by fasting. To further characterize these highly regulated enzymes, we have cloned the corresponding genes and promoter regions. The structures of the two genes were found to be very similar, consisting of three exons and two introns. Exon-intron borders conform to general consensus sequences and especially the first exon appears to be highly conserved. The promoter regions of both the CTE-I and MTE-I genes contain putative peroxisome proliferator response elements (PPREs), suggesting an involvement of peroxisome proliferator-activated receptors in the regulation of these genes.

Peroxisome proliferator-induced long chain acyl-CoA thioesterases comprise a highly conserved novel multi-gene family involved in lipid metabolism.

Long chain acyl-CoA esters are important intermediates in degradation and synthesis of fatty acids, as well as having important functions in regulation of intermediary metabolism and gene expression. Although the physiological functions for most acyl-CoA thioesterases have not yet been elucidated, previous data suggest that these enzymes may be involved in lipid metabolism by modulation of cellular concentrations of acyl-CoAs and fatty acids. In line with this, we have cloned four highly homologous acyl-CoA thioesterase genes from mouse, showing multiple compartmental localizations. The nomenclature for these genes has tentatively been assigned as CTE-I (cytosolic), MTE-I (mitochondrial), and PTE-Ia and Ib (peroxisomal), based on the identification of putative targeting signals. Although the various isoenzymes show between 67% and 94% identity at amino acid level, each individual enzyme shows a specific tissue expression. Our data suggest that all four genes are located within a very narrow cluster on chromosome 12 in mouse, similar to a sequence cluster on human chromosome 14, which identified four genes homologous to the mouse thioesterase genes. Four related genes were also identified in Caenorhabditis elegans, all containing putative PTS1 targeting signals, suggesting that the ancestral type I thioesterase gene(s) is/are of peroxisomal origin. All four thioesterases are differentially expressed in tissues examined, but all are inducible at mRNA level by treatment with the peroxisome proliferator clofibrate, or during the physiological condition of fasting, both of which conditions cause a perturbation in overall lipid homeostasis. These results strongly support the existence of a novel multi-gene family cluster of mouse acyl-CoA thioesterases, each with a distinct function in lipid metabolism.