THE ROLE OF POLYAMINE ACETYLATION IN REGULATING ADIPOSE TISSUE METABOLISM

Abstract

Because excessive body weight is a major health issue, there is an urgent need to understand all physiological mechanisms relating to control of fat deposition/mobilization. Here we investigated the linkage between polyamine metabolism and fat homeostasis that we recently discovered to operate in mice. Our previous data show that the expression level of spermine/spermidine acetyltransferase (SSAT), a polyamine catabolic enzyme, potently modulates body fat content of mice. In particular, our data indicated that SSAT overexpressing mice (SSAT-Tg) have reduced acetyl CoA levels and are lean while SSAT null mice (SSAT-ko) are obese. Since the acetyl CoA/malonyl CoA levels are critical for control of free fatty acid synthesis and oxidation via malonyl CoA regulation of CPT-1 (carnitine palmitoyltransferase-1), we hypothesized that genetic manipulation of SSAT alters body fat accumulation by activating of AMP-activated protein kinase pathway and thus has a global effect on energy metabolism. To test this hypothesis, we performed a combination of proteomics and antibody based expression studies in white adipose tissue (WAT) of SSAT-ko, SSAT-wt and SSAT-tg: We identified 9 proteins in WAT that show an increasing gradient across SSAT-ko, SSAT-wt and SSAT-tg, all of which have a connection with acetyl-CoA consumption. These include: a) glycolytic enzymes (aldolase, enolase, pyruvate dehydrogenase); b) TCA cycle enzymes (aconitate hydratase, malate dehydrogenase); c) fatty acid lipolysis and beta oxidation enzymes (hormone-sensitive lipase, monoglyceride lipase, 3-hydroxyacyl CoA dehydrogenase). Additional expression studies using Western blots indicated that acetyl CoA regulates metabolism by AMP-activated protein kinase pathway. Furthermore, to determine how tissue-specific changes in SSAT expression will impact fat accumulation and the precise role of SSAT expression status in fat homeostasis and obesity, we generated adipose-specific SAT1 knockout (FSAT1KO) mice using the Cre-Lox method. On 27-week-old, FSAT1KO mice have higher average body weight than wild type mice (WT: 45.13 ± 2.23 g vs. FSAT1KO: 52.28 ± 1.62 g, p<0.05) when fed a high-fat diet. Larger lipid droplets and lipid accumulation were present in FSAT1KO mouse livers compared to the control WT mice. Several proteins involved in fat metabolism were found to be up-regulated in FSAT1KO mice using GeLC-MS proteomics approach. These data indicated that the lack of SSAT activity in adipose tissue, but not liver or muscle, drives the phenotypic changes in SSAT-ko obese mice. Our interpretation of these results is that genetic modulation of SSAT causes a combination of changes in WAT that involve lipolysis, energy metabolism and calorie loss resulting from polyamine export. In summary, the data indicate that modulation of SSAT activity affects fat metabolism and calorie balance.