Acetoacetyl-CoA synthetase specific activity and concentration in rat tissues

Cloning and functional characterization of porcine AACS revealing the regulative roles for fat deposition in pigs

Fat deposition is a quantitative trait controlled by multiple genes in pigs. Using transcriptome sequencing, we previously reported that AACS is differentially expressed in the subcutaneous fat tissue of Dingyuan pigs with divergent backfat thickness. Therefore, with the aim of further characterizing this gene and its protein, we cloned the entire 3286-bp mRNA sequence of the porcine AACS, and the encoded AACS protein is a hydrophilic protein without a signal peptide or transmembrane sequence. Our findings suggested that among various tissues and pig breeds, AACS was highly expressed in subcutaneous fat. We have identified three completely linked SNP loci in the AACS gene: A-1759C, C-1683T, and A-1664G. The double luciferase activity test in the 5' flanking region indicated that the flanking region of AACS contained several active regulatory elements. The three linked SNPs that were identified in one of the critical active elements, and might serve as important molecular markers regulating backfat thickness. Finally, we observed that AACS overexpression inhibited the proliferation and differentiation of subcutaneous preadipocytes. Collectively, our results suggest that AACS inhibits subcutaneous fat deposition in pigs. This study provides a new molecular marker for understanding the mechanism of porcine fat deposition.

The lipogenic enzyme acetoacetyl-CoA synthetase and ketone body utilization for denovo lipid synthesis, a review

Acetoacetyl-CoA synthetase (AACS) is the key enzyme in the anabolic utilization of ketone bodies (KBs) for denovo lipid synthesis, a process that bypasses citrate and ATP citrate lyase. This review shows that AACS is a highly regulated, cytosolic, and lipogenic enzyme and that many tissues can readily use KBs for denovo lipid synthesis. AACS has a low micromolar Km for acetoacetate, and supply of acetoacetate should not limit its activity in the fed state. In many tissues, AACS appears to be regulated in conjunction with the need for cholesterol, but in adipose tissue, it seems tied to fatty acid synthesis. KBs are readily utilized as substrates for lipid synthesis in lipogenic tissues, including liver, adipose tissue, lactating mammary gland, skin, intestinal mucosa, adrenals, and developing brain. In numerous studied cases, KBs served several-fold better than glucose as substrates for lipid synthesis, and when present, KBs suppressed the utilization of glucose for lipid synthesis. Here, it is hypothesized that a physiological role for the utilization of KBs for lipid synthesis is a metabolic process of lipid interconversion. Fatty acids are converted to KBs in liver, and then, the KBs are utilized to synthesize cholesterol and other long-chain fatty acids in liver and nonhepatic tissues. The conversion of fatty acids to cholesterol via the KBs may be a particularly important example of lipid interconversion. Utilizing KBs for lipid synthesis is glucose sparing and probably is important with low carbohydrate diets. Metabolic situations and tissues where this pathway may be important are discussed.

Site-specific cleavage of acetoacetyl-CoA synthetase by legumain

Acetoacetyl-CoA synthetase (AACS) is a ketone body-utilizing enzyme and is responsible for the synthesis of cholesterol and fatty acids. We have previously shown that AACS is cleaved by legumain, a lysosomal asparaginyl endopeptidase. In this study, we attempted to determine the cleavage site of AACS. Mutagenesis analysis of AACS revealed that Asn547 is the specific cleavage site of AACS in mouse livers. The cleaved form of AACS (1-547) lost the ability to convert acetoacetate to acetoacetyl-CoA. Moreover, hydrodynamics-based gene transduction showed that overexpression of AACS (1-547) increases the protein expression of caveolin-1, the principal component of the caveolae. These results suggest that cleavage of AACS by legumain is critical for the regulation of enzymatic activity and results in gain-of-function changes.

Degradation of acetoacetyl-CoA synthetase, a ketone body-utilizing enzyme, by legumain in the mouse kidney

Acetoacetyl-CoA synthetase (AACS) is a ketone body-utilizing enzyme, which is responsible for the synthesis of cholesterol and fatty acids from ketone bodies in lipogenic tissues, such as the liver and adipocytes. To explore the possibility of AACS regulation at the protein-processing level, we investigated the proteolytic degradation of AACS. Western blot analysis showed that the 75.1kDa AACS was cleaved to form a protein of approximately 55kDa in the kidney, which has considerable high activity of legumain, a lysosomal asparaginyl endopeptidase. Co-expression of AACS and legumain in HEK 293 cells generated the 55kDa product from AACS. Moreover, incubation of recombinant AACS with recombinant legumain resulted in the degradation of AACS. Knockdown of legumain with short-hairpin RNA against legumain using the hydrodynamics method led to a decrease in the 55kDa band of AACS in mouse kidney. These results suggest that legumain is involved in the processing of AACS through the lysosomal degradation pathway in the kidney.

Acetoacetyl-CoA synthetase, a ketone body-utilizing enzyme, is controlled by SREBP-2 and affects serum cholesterol levels

Ketone bodies have been regarded as an energy source that is mainly produced in the liver, and exported to extrahepatic tissues. However, ketone bodies have also been suggested to be used during the lipogenesis by the ketone body-utilizing enzyme, acetoacetyl-CoA synthetase (AACS). To elucidate the physiological role of AACS in the liver, we investigated the mechanism of transcription of the AACS gene and performed knockdown experiments. We showed that SREBP-2 regulates the expression of AACS and that knockdown of AACS in vivo, by the hydrodynamics method, resulted in the reduction of total blood cholesterol. These results suggest that ketone body metabolism via AACS activity plays an important role in cholesterol homeostasis.

Transcriptional regulation of the human acetoacetyl-CoA synthetase gene by PPARgamma

In the cytosol of lipogenic tissue, ketone bodies are activated by AACS (acetoacetyl-CoA synthetase) and incorporated into cholesterol and fatty acids. AACS gene expression is particularly abundant in white adipose tissue, as it is induced during adipocyte differentiation. In order to elucidate the mechanism controlling the gene expression of human AACS and to clarify its physiological role, we isolated the human promoter, characterized the elements required to initiate transcription and analysed the expression of the gene in response to PPARgamma (peroxisome-proliferator-activated receptor gamma), an inducer of adipogenesis. We show that the human AACS promoter is a PPARgamma target gene and that this nuclear receptor is recruited to the AACS promoter by direct interaction with Sp1 (stimulating protein-1).

Feasibility of pathways for transfer of acyl groups from mitochondria to the cytosol to form short chain acyl-CoAs in the pancreatic beta cell

The mitochondria of pancreatic beta cells are believed to convert insulin secretagogues into products that are translocated to the cytosol where they participate in insulin secretion. We studied the hypothesis that short chain acyl-CoA (SC-CoAs) might be some of these products by discerning the pathways of SC-CoA formation in beta cells. Insulin secretagogues acutely stimulated 1.5-5-fold increases in acetoacetyl-CoA, succinyl-CoA, malonyl-CoA, hydroxymethylglutaryl-CoA (HMG-CoA), and acetyl-CoA in INS-1 832/13 cells as judged from liquid chromatography-tandem mass spectrometry measurements. Studies of 12 relevant enzymes in rat and human pancreatic islets and INS-1 832/13 cells showed the feasibility of at least two redundant pathways, one involving acetoacetate and the other citrate, for the synthesis SC-CoAs from secretagogue carbon in mitochondria and the transfer of their acyl groups to the cytosol where the acyl groups are converted to SC-CoAs. Knockdown of two key cytosolic enzymes in INS-1 832/13 cells with short hairpin RNA supported the proposed scheme. Lowering ATP citrate lyase 88% did not inhibit glucose-induced insulin release indicating citrate is not the only carrier of acyl groups to the cytosol. However, lowering acetoacetyl-CoA synthetase 80% partially inhibited glucose-induced insulin release indicating formation of SC-CoAs from acetoacetate in the cytosol is important for insulin secretion. The results indicate beta cells possess enzyme pathways that can incorporate carbon from glucose into acetyl-CoA, acetoacetyl-CoA, and succinyl-CoA and carbon from leucine into these three SC-CoAs plus HMG-CoA in their mitochondria and enzymes that can form acetyl-CoA, acetoacetyl-CoA, malonyl-CoA, and HMG-CoA in their cytosol.

Different localization in rat brain of the novel cytosolic ketone body-utilizing enzyme, acetoacetyl-CoA synthetase, as compared to succinyl-CoA:3-oxoacid CoA-transferase

In lipogenic tissue cytosol, ketone bodies are known to be activated by acetoacetyl-CoA synthetase (AACS) and incorporated into cholesterol and fatty acids. In order to investigate the physiological role of AACS in the brain, we examined the localization of AACS mRNA in rat brain by in situ hybridization using a labeled probe. High labeling was observed in the midbrain, pons/medulla, cerebral cortex, hippocampus and cerebellum, and the localization profile of AACS mRNA was different from that of succinyl-CoA:3-oxoacid CoA-transferase (SCOT), a mitochondrial ketone body-activating enzyme. In addition, the expression of AACS mRNA in the cerebellum was restricted primarily to glial cells, while in the cerebral cortex, it was restricted to neuronal cells. Streptozotocin treatment caused remarkable decreases in AACS mRNA levels in all regions where expression was observed, but changes in SCOT mRNA levels were not observed. These results suggest that the physiological role of AACS is different from that of SCOT and varies depending upon its localization in the brain.

Expression of acetoacetyl-CoA synthetase, a novel cytosolic ketone body-utilizing enzyme, in human brain

Acetoacetyl-CoA synthetase (AACS, acetoacetate-CoA ligase, EC 6.2.1.16) is a ketone body-utilizing enzyme, the physiological role of which remains unclear yet in mammals, particularly has never been studied in human. In order to investigate the tissue distribution of AACS in human, cDNA encoding AACS was isolated from HepG2 cells. Amino acid sequence of human AACS deduced from the open reading frame showed high homology (89.3%) with that of rat AACS and much less homology (43.7%) with that of bacterial AACS. The expression level of the AACS mRNA was high in kidney, heart and brain, but low in liver, and the expression profile of AACS in the human brain was quite similar to that of 3-hydroxy-3-methylglutaryl-CoA reductase.

cDNA-derived amino acid sequence of acetoacetyl-CoA synthetase from rat liver

In order to examine the primary structure of acetoacetyl-CoA synthetase (acetoacetate-CoA ligase, EC 6.2.1.16; AA-CoA synthetase), the cDNA clone encoding this enzyme has been isolated from the cDNA library which was prepared from the liver of rat fed a diet supplemented with 4% cholestyramine and 0.4% pravastatin for 4 days. Nucleotide sequence analysis of cloned cDNA revealed that AA-CoA synthetase of rat liver contains an open reading frame of 2019 nucleotides, and the deduced amino acid sequence (672 amino acid residues) bears 25.0 and 38.9% homologies with acetyl-CoA synthetases of Saccharomyces cerevisiae and Archaeoglobus fulgidus, respectively.

Immunotitration analysis of cytosolic acetoacetyl-coenzyme A thiolase activity in human fibroblasts

There are five known thiolases in human fibroblasts, and all but mitochondrial trifunctional protein (TFP) have thiolase activity toward acetoacetyl-CoA (AACoA). We investigated the contribution of mitochondrial acetoacetyl-CoA thiolase (AACoAT) (T2), cytosolic AACoAT (CT), and mitochondrial 3-ketoacyl-CoA thiolase (T1) to the total AACoAT activity in control human fibroblasts. Immunotitration of AACoAT activity with antibodies against T2, CT, or T1 was carried out in control fibroblasts, with the following results. In the case of AACoAT activity in the absence of potassium ion, 26-38%, 40-47%, and 11-20% of the total activity derived from CT, T1, and T2, respectively. The residual 6-9% total activity was not immunotitrated when three antibodies were used in combination. Hence, the contribution of peroxisomal 3-ketoacyl-CoA thiolase to the total AACoAT activity in the absence of potassium ion was at least less than 6-9%. Because the normal range of total AACoAT activity is relatively wide, it is difficult to evaluate CT defects based on a decrease of total AACoAT activity. Immunotitration with anti-CT antibody in six control fibroblasts revealed that CT activity ranges between 1.3 and 2.4 nmol/min/mg of protein. Immunotitration proved to be an accurate method to evaluate CT activity. The two cell lines from patients with CT deficiency have become extinct.