On the Role of Polyamines in Apoptosis

Sammanfattning: Apoptosis is a kind of death that is best described as cellular suicide and it is important for normal tissue homeostasis. The process can be induced by a number of different stimuli and the dying cell goes through a defined pathway terminating in typical biochemical and morphological alterations of the cell. With my studies I wanted to investigate the role of the polyamines in the regulation of apoptosis. When inducing apoptosis in mouse thymus in vivo by treatment with the glucocorticoid dexamethasone, the activity of the polyamine biosynthetic enzyme S-adenosylmethionine decarboxylase decreased and the activity of the rate-limiting catabolic enzyme spermidine/spermine N¹-acetyltransferase (SSAT) was increased. A reduction in polyamine levels was only evident though several hours after the first signs of apoptosis. A decrease in the polyamine levels was also observed in dexamethasone-induced apoptosis in mouse thymocytes in vitro and in Fas-induced apoptosis of human leukaemic T cells. Exogenous addition of 1 mM spermine inhibited apoptosis induced by dexamethasone in primary cultures of mouse thymocytes. Spermine prevented DNA fragmentation as well as the activation of caspase-9, -8 and –3. The total inhibition of caspase-9 activity was due to the spermine-mediated inhibition of cytochrome c release from the mitochondria into the cytosol. When treating the human breast cancer cell line L56Br-C1 with the spermine analogue N¹, N¹¹-diethylnorspermine, SSAT activity was induced leading to an almost total depletion of the natural polyamines. The majority of the cells died within 48 hours. This rapid cell death was shown to be apoptotic with activation of caspases and fragmentation of the DNA. In conclusion, a decrease of intra-cellular levels of polyamines seems to be important for the progression of the apoptotic process and the addition of spermine protected mouse thymocytes from apoptosis induced by dexamethasone.