Enhancement of expression of inducible NO synthase and inhibition of DNA synthesis in rat thymocytes by in vivo hydrocortisone treatment

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Glucocorticoids (GC) are known to inhibit the mitogen-induced proliferation of T cells. Some of the effects of GC have been ascribed to the inhibition of nitrogen monoxide (NO) production, since NO is involved in the effecter function of phagocytic cells. Although the effects of GC in vitro on thymocytes are known, the effect of in vivo GC treatment on proliferation and NO synthesis in thymocytes has not been clarified. In this study, we investigated the effects of the administration of hydrocortisone succinate (HC), a potent anti-inflammatory GC, in Sprague–Dawley rats by s.c. injection (100 mg/kg). A substantial reduction of concanavalin A (Con A)-stimulated [ 3 H ]thymidine incorporation was observed in the thymocytes from HC-treated rats. This effect was accompanied by an increase in the Con A-stimulated expression of the inducible type of nitric oxide synthase (iNOS) and nitrite accumulation. The constitutive type of NOS (cNOS) in thymocytes did not change during the course of in vivo HC treatment. Addition of NO donors, which stimulated cyclic GMP accumulation, to rat thymocytes in vitro inhibited Con A-stimulated DNA synthesis. Addition of dibutyryl cyclic GMP, a membrane permeable analog, also inhibited DNA synthesis. Co-culture with N G-monomethyl- l-arginine, an inhibitor of NOS, recovered Con A-stimulated [ 3 H ]thymidine incorporation in the thymocytes from HC-treated rats. These findings suggest that NO and cyclic GMP inhibited DNA synthesis in rat thymocytes and that HC treatment in vivo inhibited DNA synthesis via the expression of the iNOS protein, and the accumulation of NO and cyclic GMP. Although it is known that GC regulate iNOS expression negatively in several types of cells in vitro, GC treatment in vivo regulates iNOS protein expression positively in rat thymocytes.