Molecular Characterization of Human Acetyl-CoA Synthetase, an Enzyme Regulated by Sterol Regulatory Element-binding Proteins

Abstract

Through suppressive subtractive hybridization, we identified a new gene whose transcription is induced by sterol regulatory element-binding proteins (SREBPs). The gene encodes acetyl-CoA synthetase (ACS), the cytosolic enzyme that activates acetate so that it can be used for lipid synthesis or for energy generation. ACS genes were isolated previously from yeast, but not from animal cells. Recombinant human ACS was produced by expressing the cloned cDNA transiently in human cells. After purification by nickel chromatography, the 701-amino acid cytosolic enzyme was shown to function as a monomer. The recombinant enzyme produced acetylCoA from acetate in a reaction that required ATP. As expected for a gene controlled by SREBPs, ACS mRNA was induced when cultured cells were deprived of sterols and repressed by sterol addition. The pattern of regulation resembled the regulation of enzymes of fatty acid synthesis. ACS mRNA was also elevated in livers of transgenic mice that express dominant-positive versions of all three isoforms of SREBP. We conclude that ACS mRNA, and hence the ability of cells to activate acetate, is regulated by SREBPs in parallel with fatty acid synthesis in animal cells.Sterol regulatory element-binding proteins (SREBPs) 1 are transcription factors that activate more than 20 genes that produce enzymes required for the synthesis of cholesterol and unsaturated fatty acids in animal cells (1-3). The SREBPs differ from other transcription factors because they are synthesized as membrane-bound proteins whose active fragments must be released by proteolysis in order to enter the nucleus and activate transcription. The proteolytic release of the nuclear fragments is controlled by the cholesterol content of the cell; release is rapid when cells are depleted of cholesterol, and it is blocked when cholesterol overaccumulates (1).In tissue culture cells and in livers of transgenic mice, nuclear SREBPs (nSREBPs) increase the levels of mRNAs encoding multiple enzymes in the cholesterol biosynthetic pathway, including 3-hydroxy-3-methylglutaryl coenzyme A synthase (HMG-CoA synthase), HMG-CoA reductase, farnesyl diphosphate synthase, squalene synthase, lanosterol demethylase, and others. In the fatty acid biosynthetic pathway, SREBPs increase the mRNAs encoding acetyl-CoA carboxylase, fatty acid synthetase, and stearoyl-CoA desaturase. As a result of these changes, there is a massive increase in the content of unsaturated fatty acids and cholesterol in livers of transgenic mice that overexpress either of two nuclear isoforms of SREBP (nSREBP-1a and nSREBP-2) (4, 5).In addition to increasing enzymes that participate directly in lipid biosynthesis, the SREBPs increase the mRNA encoding ATP-citrate lyase, which is the major source of the acetyl-CoA that is the ultimate building block for lipid synthesis (6, 7). The SREBPs also increase the mRNAs for three enzymes that supply the NADPH that is needed for lipogenesis (malic enzyme, glucose-6-phosphate dehydrogenase, and 6-phosphogluconate dehydrogenase) (8).In th...

Direct noninvasive estimation of myocardial tricarboxylic acid cycle flux in vivo using hyperpolarized 13C magnetic resonance

“…The signal decay of precursor and metabolites results from a combination of the effects of longitudinal relaxation (characterized by the time constant T 1 ), RF excitation, and biochemical conversion and all time courses were corrected for the effect of repeated RF excitations. The T 1 of [1-13 C]acetylcarnitine was set to 14.9 s, as previously determined in vivo [35], and that of [5][6][7][8][9][10][11][12][13] C]citrate to 20 s [46]. The acetate signal decay was treated as free parameter.…”

“…It is the shortest fatty acid and its uptake into the heart occurs through simple diffusion, its exchange between plasma and tissue only depending on the concentration gradient [5]. It is an effective oxidizable substrate formed endogenously that can be activated to acetylCoA via cytosolic or mitochondrial acetylCoA synthetase [6,7]. The cytosolic variant of acetylCoA synthetase is mainly present in the liver, while the mitochondrial variant is abundant in the heart and muscle and provides acetylCoA that is utilized mainly for oxidation under ketogenic conditions [7].…”

PET imaging with 11C-acetate in prostate cancer: a biochemical, radiochemical and clinical perspective

“…One of these acetyl kinases is a cytosolic acetyl-CoA synthetase 1 (AceCS1). It produces acetyl-CoA, which is active only in fatty acid synthesis, is highly expressed in the liver and is regulated by the same factors and patterns as FAS [14]. The other (AceCS2) is an enzyme bound to the mitochondrial matrix, found abundantly in hearts and kidneys, and synthesising acetyl-CoA active in the TCA cycle and producing [ 14 C]CO 2 from [ 14 C]acetate [15].…”

“…It is interesting to note that the authors who discovered human AceCS have found that from the screened 95 different cell lines AceCS1 was most abundant in human adenocarcinoma of the colon [14].…”

“…Each of the key lipogenic enzymes-AceCS, ATP citrate lyase, acetyl-CoA carboxylase, FAS-is essential for cancer cell survival and malignant transformation. In fact, their inhibition can induce apoptosis in many types of cancers [13,14,20,22,24].…”