AIM: To investigate the regulatory effects of nuclear factor-κB ( NF-κB) and activator protein-1 (AP-1) on the expression of ectopic trypsin and proinflammatory cytokines in influenza A virus (IAV)-induced myocardi-tis.METHODS:Male BALB/c mice of 8 weeks old ( n=40) were randomly divided into 4 groups:normal control group ( NC) , infection control group ( IC) , NF-κB inhibitor group ( NI) and AP-1 inhibitor group ( AI) .The mice in NC group and IC group were instilled intranasally with 15μL saline and 40 plaque forming units ( PFU) IAV, respectively.The mice in NI group and AI group were infected intranasally with 40 PFU IAV and injected intraperitoneally with 10 mg/kg NF-κB inhibitor pyrrolidine dithiocarbamate ( PDTC) or 2.5 mg/kg AP-1 inhibitor nordihydroguaiaretic acid ( NDGA) once daily. The mice were euthanized at day 9 after instillation, and the hearts were removed for pathological and biochemical analysis. RESULTS:IAV infection induced significant up-regulation of ectopic trypsin, and proinflammatory cytokines interleukin 6 (IL-6), IL-1βand tumor necrosis factor-α(TNF-α) in the myocardium, and triggered acute myocarditis.PDTC signifi-cantly inhibited NF-κB activation and up-regulation of ectopic trypsin and proinflammatory cytokines, and effectively sup-pressed IAV replication and myocardial inflammatory response (P<0.01).NDGA effectively inhibited AP-1 activity (P<0.01) and mildly suppressed up-regulation of proinflammatory cytokines ( P<0.05) , but had no effects on the expression of ectopic trypsin, IAV replication and the extent of myocarditis ( P>0.05) .CONCLUSION:IAV infection induces up-regulation of ectopic trypsin and proinflammatory cytokines in myocardium predominantly by the activation of NF-κB.AP-1 signaling pathway might be only partially involved in the regulation of proinflammatory cytokines.