Inhibitory role of polyamines in dexamethasone-induced apoptosis of mouse thymocytes

It has been well known that polyamines ensure the stability of chromatin structure and the fidelity of DNA transcription. This study was carried out to evaluate the effect of polyamines on the apoptosis of mouse thymocytes induced by dexamethasone and polyamine synthesis inhibitors. 1) In the histological death findings of thymocytes double-stained with acridine orange and ethidium bromide, the apoptotic and the necrotic fractions (AF; NF) in the control group were 9.4 ± 4.2% and 4.5 ± 5.3%, respectively. Dexamethasone (3 x 10^-8 M: DX) increased AF upto 52.0 ± 8.1% and did not change NF, but A23187 (5 x 10^-7 M: A2) increased AF and NF upto 45.0 ± 8.9% and 20.5 ± 10.6%, respectively. 2) The thymocyte viability was significantly reduced by DX, DHEA (1 x 10^-4 M), A2, DFMO (1 x 10^-4 M), and MGBG (1 x 10^-4 M), respectively. It was, however, little affected by aminoguanidine (1 x 10^-4 M: AG), putrescine (1 x 10^-5 M: PT), spermidine (1 x 10^-5 M: SD), and spermine (1 x 10^-5 M: SM). 3) The genomic DNA of mouse thymocyte was markedly fragmented by DX and A2, respectively, and to a lesser extent, by DHEA, but was little affected by MGBG, DFMO, AG, and each of the polyamines. 4) The DX induced reduction of thymocyte viability was moderately attenuated by DHEA, but little affected by DFMO, MGBG, and AG. However, SM significantly attenuated the viability reduction induced by A2 as well as DX. 5) The thymocyte viability reduction by MGBG and DFMO was significantly attenuated by only SM among three polyamines applied in this study. 6) The thymocyte viability reduction by combined treatments of DX with DFMO and MGBG, respectively, was significantly attenuated by SM, and moderately by PT. But the viability reduction by combined treatment of DX with AG or DHEA was not affected by polyamines. These results suggest that polyamines, particularly spermine, might play the inhibitory role in thymocyte apoptosis and the inhibitory effect can be ascribed in part to the increase of polyamine uptake by thymocytes pretreated with DFMO and MGBG.