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

Treatment of human leukemia THP-1 cells with bufalin, a specific inhibitor of Na+-K+-ATPase, sequentially induces c-fos and inflammatory cytokines interleukin-1 beta (IL-1 beta) and tumor necrosis factor-alpha (TNF-alpha) gene expressions before the appearance of mature phenotypes of monocytic cells. In this study we examined the signal transduction leading to bufalin-induced gene expressions. Bufalin selectively activated extracellular signal-regulated kinase (ERK), compared with other mitogen-activated protein (MAP) kinase family members. Pretreatment of THP-1 cells with PD-98059, an inhibitor of the ERK-kinase cascade, abolished bufalin-induced c-fos and IL-1 beta gene expressions, indicating that the ERK-kinase cascade mediates the induction of inflammatory cytokines by bufalin. Inhibition of the Na+/Ca2+ exchanger by KB-R7943 and of protein kinase C (PKC) by Ro-31-8220 suppressed ERK activation and gene expressions of c-fos and IL-1 beta. These findings suggest that Na+-K+-ATPase inhibition by bufalin induces calcium influx and thereby activates PKC and ERK. In cells treated with an inhibitor of p38 MAP kinases, SB-203580, bufalin-mediated ERK activation became persistent and the induction of IL-1 beta and TNF-alpha expressions was significantly augmented. These results suggest that cross talk in bufalin-mediated ERK activation is negatively regulated by endogenous p38 MAP kinase activations.