Effect of Modifiers of Polyamine Metabolism in the ob/ob Mouse, an Animal Model of Obesity and Type-2 Diabetes

Abstract

This thesis reports original research on the biology of polyamines and comprises studies in two related areas. Firstly, the hepatic metabolism of a clinically relevant polyamine analogue, triethylenetetramine (TETA), was studied with particular focus on its hepatic metabolism in three mammalian species. Using a newly developed method based on liquid chromatography/mass spectrometry, acetylation of TETA, and of its first acetylation product, N1- acetyltriethylenetetramine (ATETA) was measured in vitro using liver supernatant and liver microsomal preparations from humans, rats and mice. The results showed that all of the tissue preparations tested could catalyse the acetylation of both TETA and ATETA (at uniformly lower rates). Together with the results of immunoblotting for the enzyme anti-spermidine-spermine N1- acetyltransferase 1 (SSAT1), it was further concluded that SSAT1, which is one of the key enzymes controlling polyamine catabolism, was not the main enzyme involved in metabolism of TETA in the species studied. Secondly, the effect of SSAT1 inducers in a mouse model of obesity and type-2 diabetes was studied. Jell et al. (2007) and Pirien et al. (2007) reported that genetically-modified mice overexpressing SSAT1 showed a distinguishable anti-obesity phenotype, which included decreases in body-weight gain, body-fat mass gain, serum leptin, insulin resistance, and increased glucose tolerance. The underpinning molecular mechanism was thought to be an elevated polyamine catabolism caused by overexpression of SSAT1. Here, leptin-deficient mice (ob/ob) and corresponding background-control mice (C57) were treated with N1,N11-diethylnorspermine (DENS), aspirin, and TETA, all modifiers of polyamine metabolism, for periods of up to ~7 months after weaning. DENS and TETA are polyamine analogues; and aspirin is an activator of the SSAT1-gene promoter. All three induced polyamine catabolism in cell culture. The results of these experiments showed that DENS elevated polyamine catabolism through SSAT1 induction in both C57 and ob/ob mice, in which it lowered fat-mass gain by 22%. This was also associated with amelioration of obesity and type-2 diabetes phenotypes, especially in ob/ob mice. The other two drugs showed lesser effects on elevation of polyamine catabolism. TETA decreased the fat mass gain by 14% but increased the non-fat mass gain by 30%. TETA in C57 mice and aspirin in both C57 and ob/ob mice caused no changes in either body-weight or body composition.