Regulation of KLF5 involves the Sp1 transcription factor in human epithelial cells

Human Kruppel-like factor 5 (hKLF5) is a transcription factor with a potential tumor suppressor function in prostate and breast cancers. In the majority of cancer samples examined, a significant loss of expression for KLF5 has been detected. Whereas hemizygous deletion appears to be responsible for KLF5's reduced expression in about half of the cases, the mechanism for reduction is unknown in the remaining half; gene promoter methylation does not appear to be involved. In this report, we studied the regulation of KLF5 and cloned and functionally characterized a 1944-bp fragment of the 5′-flanking region of the hKLF5 gene. Several mitogens as well as global demethylation induced the expression of KLF5, implicating multiple factors in the regulation of KLF5. KLF5's promoter lacks a TATA box and has a GC-rich region. Deletion mapping in combination with promoter activity assay showed that multiple cis-elements are involved in the transcriptional regulation of KLF5, some of which may play a repressor role whereas some others play an enhancer role. The Sp1 site between position -239 and -219 is essential for a basal promoter activity. Deletion or mutations of this Sp1 site significantly reduced promoter activity in several epithelial cell lines. Electrophoretic mobility shift assays (EMSAs) revealed that the Sp1 site binds Sp1 protein in nucleic extracts of different cell lines. In addition, overexpression of Sp1 protein transactivates KLF5 promoter activity. These findings suggest that Sp1 is a key transcription factor in KLF5's dynamic transcriptional regulation.