Nitric oxide (NO) is emerging as a key regulator of gene expression, capable of playing either positive or negative roles. The results of this study indicate that NO exerts a dual effect on cyclooxygenase-2 (COX-2) expression in human mesangial cells (HMC). Treatment of HMC with NO synthase inhibitors attenuated interleukin-1beta (IL-1beta/tumor necrosis factor-alpha (TNF-alpha)-elicited COX-2 protein and mRNA expression, suggesting a positive role of endogenous NO on COX-2 induction. However, NO donors (sodium nitroprusside [SNP] and S-nitroso-N-acetylpenicillamine [SNAP]) amplified cytokine-elicited COX-2 expression at early time points of treatment (up to 8 h for mRNA and up to 24 h for protein expression), but were inhibitory at later times. Oligonucleotide decoy experiments confirmed the importance of nuclear factor kappaB (NF-kappaB) activation for COX-2 induction by IL-1beta/TNF-alpha. Treatment with N(G)-nitro-L-arginine methyl ester (L-NAME) did not affect initial activation of NF-kappaB by IL-1beta/TNF-alpha, but unveiled an inhibitory effect of NO generation on NF-kappaB activity after 4 h. In HMC supplemented with SNP, cytokine-induced NF-kappaB activation was potentiated at early times of induction (5 to 15 min), but inhibited at later times (1 to 4 h), suggesting a dual effect of NO donors on NF-kappaB activation. Interestingly, IkappaBalpha protein levels followed a reciprocal pattern of expression: IkappaBalpha levels were lower at early times of induction in NO donor-supplemented cells; however, after 1 h of treatment, IkappaBalpha levels became higher than in cells treated only with cytokines. In the presence of SNP, cytokine-elicited IkappaBalpha mRNA induction was initially delayed, but was amplified at later times. These changes in IkappaBalpha expression could contribute to the dual effects of NO donors on NF-kappaB activation and COX-2 expression in HMC.