We have recently shown that macrophage-stimulating protein (MSP) promotes the invasion of recepteur d'origine nantais (RON), a tyrosine kinase receptor-positive MDA-MB-231, MDA-MB-468 breast cancer cells, and also identified the regulatory elements required for RON gene expression. In this report, we have analyzed the efficacy of a chemopreventive agent, curcumin, in blocking RON tyrosine kinase-mediated invasion of breast cancer cells. Reverse transcription-PCR and Western analysis indicated the down-regulation of the RON message and protein, respectively, in MDA-MB-231 and MDA-MB-468 cells. Significantly, curcumin-mediated inhibition of RON expression resulted in the blockade of RON ligand, MSP-induced invasion of breast cancer cells. We have identified two putative nuclear factor-kappaB p65 subunit binding sites on the RON promoter. Using chromatin immunoprecipitation analysis and site-directed mutagenesis of the RON promoter, we have confirmed the binding of p65 to the RON promoter. Our data show that curcumin reduces RON expression by affecting p65 protein expression and transcriptional activity. Treatment of MDA-MB-231 cells with pyrrolidine dithiocarbamate, an inhibitor of p65, or small interfering RNA knockdown of p65, blocked RON gene expression and MSP-mediated invasion of MDA-MB-231 cells. This is the first report showing the regulation of human RON gene expression by nuclear factor-kappaB and suggests a potential therapeutic role for curcumin in blocking RON tyrosine kinase-mediated invasion of carcinoma cells.