Identification of a Translational Control Element Involved in KLF4 Posttranscriptional Regulation

Abstract

Krüppel‐like factor 4 (KLF4) is a transcription factor regulating cell growth and differentiation. An important feature of KLF4 is its induction upon growth‐arrest in normal cells. In cancer cells this regulation is disrupted, as proliferating cancer cells show increased levels of KLF4 compared with primary epithelial cells. However, the mechanisms regulating the induction of KLF4 upon growth arrest in normal cells or its upregulation in cancer cells remain largely unclear. We recapitulated the proliferation‐dependent regulation of KLF4 using E1A‐immortalized rat kidney epithelial cells, RK3E cells. In these cells, the endogenous Klf4 RNA and protein was low in proliferating cells but high in post‐confluent cells that underwent a process similar to epithelial differentiation, with upregulation of cell cycle inhibitors such as p21/Waf1 and p27/Kip1. To map potential regulatory elements in KLF4 transcripts, we established an assay system by inserting full‐length or partial‐length fragments of human KLF4 cDNA into the 3′ UTR of firefly luciferase. By measurement of luciferase mRNA and protein levels, we successfully identified a novel, cell‐cycle‐dependent translational‐control element (TCE) in KLF4. This element mediates translational inhibition in proliferating cells. Current studies are focused on possible factors that can bind to TCE and regulate KLF4 expression.