Expression of mdr-1 is complex and highly regulated. Several lines of evidence indirectly suggest that transcription factor c-Jun may negatively regulate human mdr-1 gene expression. We recently found that salvicine, a novel topoisomerase II inhibitor, is cytotoxic for multidrug resistance (MDR) tumor cells and down-regulates mdr-1 expression in MDR K562/A02 cells. Salvicine also stimulates a significant increase in the level of c-jun mRNA in HL60 cells. This study investigated the relationship between c-Jun activation and down-regulation of mdr-1 expression by salvicine in K562/A02 cells. Reverse-transcription PCR and Western blotting analyses revealed that salvicine suppressed mdr-1 expression in MDR cells and promoted c-jun expression in both MDR and parental K562 cells. Moreover, levels of c-jun expression were enhanced by salvicine before reduction of mdr-1 expression in K562/A02 cells. Furthermore, c-jun antisense oligodeoxynucleotides prevented salvicine-stimulated enhancement of c-Jun protein and reduction of mdr-1 gene expression, but did not affect the increase in c-jun mRNA levels. Salvicine promoted phosphorylation of c-Jun-N-terminal kinase and c-Jun protein in MDR K562/A02 and parental K562 cells. Electrophoretic mobility shift assay analysis showed that salvicine enhanced DNA binding activity of transcription factor activator protein 1. Additionally, c-jun antisense oligodeoxynucleotides also inhibited salvicine-induced apoptosis and cytotoxicity in MDR and parental K562 cells. A possible pathway emerges from these results: salvicine stimulates c-Jun-N-terminal kinase phosphorylation and activation, resulting in c-Jun phosphorylation and activation. Activated c-Jun promotes expression of c-jun itself, represses mdr-1 transcription, and triggers pro-apoptotic signals, resulting in low mdr-1 expression and cell death. The present results demonstrate that transcription factor c-Jun plays a principal role in down-regulation of mdr-1 expression and induction of apoptosis in salvicine-treated human MDR K562/A02 cells, providing new insights into the complicated mechanisms regulating mdr-1 expression. The findings also suggest that c-Jun might be a potential drug target for circumventing tumor MDR.