ERK signaling mediates the induction of inflammatory cytokines by bufalin in human monocytic cells

Treatment of human leukemia THP-1 cellswith bufalin, a specific inhibitor ofNa⁺-K⁺-ATPase, sequentially inducesc-fos and inflammatory cytokines interleukin-1(IL-1) and tumor necrosis factor- (TNF-) gene expressionsbefore the appearance of mature phenotypes of monocytic cells. In thisstudy we examined the signal transduction leading to bufalin-inducedgene expressions. Bufalin selectively activated extracellularsignal-regulated kinase (ERK), compared with other mitogen-activatedprotein (MAP) kinase family members. Pretreatment of THP-1 cells withPD-98059, an inhibitor of the ERK-kinase cascade, abolishedbufalin-induced c-fos and IL-1 gene expressions, indicating that the ERK-kinase cascade mediates the induction of inflammatory cytokines by bufalin. Inhibition of theNa⁺/Ca²⁺ exchanger by KB-R7943 and of proteinkinase C (PKC) by Ro-31-8220 suppressed ERK activation and geneexpressions of c-fos and IL-1. These findings suggest thatNa⁺-K⁺-ATPase inhibition by bufalin inducescalcium influx and thereby activates PKC and ERK. In cells treated withan inhibitor of p38 MAP kinases, SB-203580, bufalin-mediated ERKactivation became persistent and the induction of IL-1 and TNF-expressions was significantly augmented. These results suggest thatcross talk in bufalin-mediated ERK activation is negatively regulatedby endogenous p38 MAP kinase activations.