Effects of dexamethasone on spermidine N1-acetyltransferase and ornithine decarboxylase activities in rat spleen

Treatment of rats with the glucocorticoid dexamethasone causes an increase in the activity of cytosolic spermidine N¹-acetyltransferase both in the spleen and thymus, but not, however, in liver, kidney or lung. The induced spermidine N¹-acetyltransferase activity in the spleen catalyses acetylation of spermidine as well as spermine and sym-norspermidine, but not of diamines and histones. The enzyme induction depends on the dose of dexamethasone, and is suppressed by cycloheximide, which suggests that de novo protein synthesis is required for the action of this glucocorticoid. N¹-acetylspermidine accumulates in the spleen after dexamethasone treatment, while spermidine progressively decreases and is partly converted into putrescine, the content of which transiently increases. In accordance with previous reports, dexamethasone was found to cause a rapid and large fall in the activity of spleen ornithine decarboxylase which was effected via the appearance of an inhibitor of the enzyme. Glucocorticoids exert large catabolic effects on lymphoid tissues, and further selectively affect the activities of spermidine N¹-acetyltransferase and ornithine decarboxylase in the thymus and spleen. These latter selective responses may represent an important early event in lymphoid tissue response to glucocorticoid hormones.