Loss of Krüppel-like factor 4 expression contributes to Sp1 overexpression and human gastric cancer development and progression

Purpose: Increasing evidence indicates that the transcription factor, Sp1, regulates the expression of multiple genes involved in tumor development and progression. We have recently reported that Sp1 overexpression is directly correlated with the angiogenic potential of and poor prognosis for human gastric cancer. However, the underlying mechanisms that result in Sp1 overexpression remain unclear. Experimental Design: The expression of Sp1 and Krüppel-like factor 4 (KLF4), a potential tumor suppressor gene, in gastric cancer tissue was analyzed by immunohistochemistry and Western blot analysis. Alterations of Sp1 and KLF4 expression were achieved by gene transfer and verified by Northern and Western blot analyses. Furthermore, Sp1 promoter activity assay, electrophoretic mobility shift assay, and chromatin immunoprecipitation assay were done to identify the KLF4 binding sites on the Sp1 promoter. Results: Mutually exclusive expression of Sp1 and KLF4 was evident in gastric cancer and non-cancerous tissue. Specifically, strong Sp1 expression but loss of KLF4 expression was found in cancer tissue, whereas the adjacent noncancerous tissue showed negative Sp1 expression but strong KLF4 expression. Enforced KLF4 expression repressed Sp1 expression at the promoter activity, mRNA, and protein levels. Moreover, a region within the proximal Sp1 promoter was identified to have overlapping KLF4- and Sp1-binding sites, to which KLF4 and Sp1 compete for binding. Sp1 positively regulated its own promoter, whereas KLF4 did the opposite. Conclusions: Our data suggests that disruption of KLF4-mediated negative regulation contributes to the molecular events of Sp1 overexpression and to the development and progression of human gastric cancer.