Hypermethylation often leads to gene silencing; however, the mechanism responsible for the low expression resulting from hypermethylation of the tumor suppressor gene Kelch-like ECH-associating protein 1 (Keap1) in human lung cancer cell lines remains unclear. In this study, using promoter deletion and site mutagenesis assays, we determined that one transcription factor stimulating protein-1 (Sp1) regulatory element in the Keap1 promoter region was important for the transcription of Keap1 in A549 cells. We demonstrated that the transcription factor Sp1 can directly bind to this element in the normal bronchial epithelial BEAS-2B cell line but not in A549 cells, as assessed with chromatin immunoprecipitation (ChIP). EMSAs and supershift assays also showed that CpG island methylation could abrogate Sp1 binding to the Keap1 promoter. Moreover, Keap1 mRNA decreased by 50% after the knock-down of Sp1 with siRNA in BEAS-2B cells, whereas the over-expression of Sp1 led to a dramatic increase in Keap1 promoter activity. The treatment of A549 cells with 5-aza-2'-deoxycytidine restored the binding of Sp1 to the promoter and Keap1 expression. Our results indicate that Sp1 is essential for Keap1 expression and that promoter methylation blocks Sp1 binding in A549 cells. These results demonstrate that hypermethylation may act as an epigenetic gene silencing mechanism, i.e., the inhibition of Sp1 binding to the hypermethylated Keap1 promoter in lung cancer cells, which suggests new approaches to lung cancer treatment. (C) 2012 Elsevier Inc. All rights reserved.