Genetically altered expression of polyamine acetylation affects fat metabolism and body fat accumulation in mice
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The polyamine acetylating enzyme, spermidine/spermine N 1 -acetyltransferase (SSAT), catalyzes the transfer of acetyl groups from acetyl-coenzyme A (acetyl-CoA) onto intracellular polyamines thereby rendering them susceptible to export and/or catabolism. Previously, we reported that over-expression of SSAT in both cultured cells and mice activates metabolic flux through the polyamine pathway and depletes the co-enzyme, acetyl-CoA (Kee et al. , 2004). Given that acetyl-CoA is a fatty acid synthesis precursor and key to the production of malonyl-CoA, a major regulator of fatty acid oxidation, the above finding represents a possible linkage between polyamine acetylation and fat metabolism. This study focuses the tenent that SSAT plays a modulatory role in fat metabolism by regulating acetyl- and malonyl-CoA pools. To investigate this hypothesis, we characterized two animal models: SSAT over-expressing (SSAT-tg) mice which have a lean phenotype and SSAT-knock-out (SSAT-ko) mice which tend to accumulate fat as indicated by whole-body MRI analysis and epididymal fat-pad weights. White adipose tissue (WAT) of SSAT-tg animals mimics the metabolic flux seen in cultured cells, being characterized by compensatory increases in polyamine biosynthetic enzyme activities, which together with elevated SSAT, lead to heightened metabolic flux through the polyamine pathway, a 70% decrease in the SSAT co-enzyme acetyl-CoA and a 50% decrease in its downstream metabolite malonyl-CoA. The latter is accompanied by 20-fold regulatory increases in both glucose metabolism and fatty acid (palmitate) oxidation thus, linking polyamines and fat metabolism. These animals were further characterized by an absense of stored liver glycogen, significant decreases in serum glucose, cholesterol, leptin and free fatty acids as well as significantly increase energy expenditure as measure by indirect calorimetry despite increased dietary food intake. In addition, SSAT-tg mice given ad libitum access to a high-fat diet failed to gain body weight or body fat while SSAT-wt animals gained >20% total body fat signifying the ability of SSAT-tg mice to resist fat accumulation despite a significant increase in dietary fat intake. To further validate these findings, we cross-bred our SSAT-tg animals with the leptin obese mouse, which is characterized by a non-functional leptin protein, deregulated fat homeostasis and a huge gain in body weight. SSAT-tg mice resisted the weight gain observed in leptin obese mice by 40%. In SSAT-ko mice a more obvious relationship between acetyl-CoA and fat metabolism was observed. Relative to wild-type mice, there was no N 1 -acetylation of polyamines, no compensatory increase in polyamine biosynthetic activity and no apparent rise in metabolic flux in white adipose tissue. Consistent with expectations based on SSAT-tg mice, there was a 30% accumulation of acetyl-CoA and a 40% increase in malonyl-CoA coupled with a 75% decrease in glucose metabolism. The phenotype was further exaggerated under a high fat diet where SSAT-ko mice gained significantly more weight (34%) than wild-type mice over a 20 wk period which could not be attributed to increased dietary food intake or decreased energy expenditure. Thus, in the absence of polyamine acetylation, SSAT-ko mice tend to accumulate acetyl- and malonyl-CoA, thereby promoting fat storage via decreased glucose utilization and fatty acid oxidation. Overall, the data obtained with these novel murine models reveal a previously unrecognized regulatory linkage between polyamine and fat metabolism with mechanistic implications for understanding and addressing obesity. More particularly, SSAT regulation of metabolic flux modulates acetyl- and malonyl-CoA pools giving rise to profound effects on fatty-acid and glucose oxidation as well as altered body-fat accumulation. The strength of these findings derives from their origin in two unique mouse models differing genetically in the expression of a single gene, SSAT. The balanced phenotypic differences associated with differential expression of SSAT lend strong support for the proposed metabolic mechanisms and models.