3-aminobenzamide protects the mouse thymocytes in vitro from dexamethasone-mediated apoptotic cell death and cytolysis without changing DNA strand breakage.

Exposure of mouse thymocytes to 1 microM dexamethasone (Dex) resulted in a dramatic increase in the degree of DNA strand breakage up to 80% between 4 to 6 h postincubation. During incubation a marked decrease in the number of total and viable cells as well as an increase in the release of lactate dehydrogenase into medium were detectable, indicating a strong cytotoxicity of Dex on the mouse thymocytes. Agarose gel electrophoresis of DNA from cells exposed to Dex for 6 h clearly demonstrated an increased laddering of DNA fragments multiple of approx. 200 base pairs as a characteristic feature of an apoptosis or programmed cell death. The cytotoxicity of Dex, as judged by the decrease in the viability and increase in the cell lysis, was effectively prevented by 3-aminobenzamide, a potent inhibitor of poly(ADP-ribose) synthesis. Furthermore, the lowering of intracellular NAD levels, which was observable in the present study most probably as a result of activation of poly(ADP-ribose) synthesis due to Dex-mediated DNA strand breakage, was also specifically prevented by the inhibitor, although the DNA strand breakage itself was not affected under these conditions. Our present results indicate that the Dex-mediated thymocyte death and cytolysis and probably intrathymic apoptotic thymocytolysis could be attributable primarily to the loss of intracellular NAD.