C1 inhibitor (C1 INH) is the major inhibitor of the proteolytically active subcomponents of C1, kallikrein, activated forms of factor XII, and factor XIa in plasma. We determined the mechanism(s) how interferon- gamma (IFN-gamma) regulates C1 INH mRNA expression in HepG2 cells. Cycloheximide or anisomycin treatment alone did not increase C1 INH mRNA nor did it potentiate C1 INH mRNA expression after IFN-gamma stimulation. C1 INH mRNA levels on Northern blot from untreated and IFN- gamma-treated cells did not change for more than 20 hours after actinomycin D treatment. Actinomycin D and 5,6-dichloro-1-beta- ribofuranosylbenzimidazole abolished IFN-gamma-induced C1 INH mRNA expression. Relatively more C1 INH mRNA precursor (heterogeneous nuclear RNA [hnRNA]) was detected in total RNA from IFN-gamma-treated HepG2 cells than unstimulated cells. Treatment of HepG2 cells with the phosphatase 1 and 2A inhibitors, okadaic acid (> or = 50 nmol/L) and calyculin (> or = 25 nmol/L), decreased IFN-gamma's ability to upregulated C1 INH mRNA. The phosphatase 2A inhibitor, cantharidin (> or = 10 micromol/L), also blocked the IFN-gamma induction of the C1 INH gene. In HepG2 cells total phosphatase 2A activity was significantly increased by C6 ceramide but not IFN-gamma. However, C6 ceramide itself did not increase C1 INH mRNA expression. These data indicate that phosphatase 2A is required to dephosphorylate a substrate in order for IFN-gamma to induce the transcriptional upregulation of C1 INH mRNA, but phosphatase 2A is not a direct stimulator of C1 INH gene expression.