Mitogenic regulation of p27Kip1 gene is mediated by AP-1 transcription factors

Abstract

The abundance of cyclin-dependent kinase inhibitor p27Kip1 during the cell cycle determines whether cells will proliferate or become quiescent. Although the post-translational regulation of p27Kip1 is well established, its transcriptional regulation is poorly understood. Here, we report that mitogenic stimulation of quiescent HEK293 and Huh7 cells showed a rapid decline in the levels of p27Kip1 transcript by 2.4 ± 0.1-fold. Inhibition of the p27Kip1 gene in response to mitogens involved transcriptional down-regulation and required newly synthesized protein(s). Mutation of the AP-1 element at position -469 in the human p27 Kip1 promoter abrogated the effect of mitogens. The recruitment of the AP-1 complex to the p27Kip1 promoter was confirmed by in vitro DNA binding and chromatin immunoprecipitation studies. Reporter gene analysis combined with enforced expression of Jun/Fos proteins suggested the involvement of Jun/Fos heterodimer in the transrepression process. Both MAPK and phosphatidylinositol 3-kinase signaling pathways appeared to mediate p27 Kip1 transcription. Furthermore, hepatitis B virus X protein-mediated down-regulation of p27Kip1 in a transgenic environment correlated with an increase in c-Fos levels, reiterating the physiological relevance of AP-1 in the transcriptional regulation of p27Kip1. Collectively, our studies present the first evidence demonstrating the role of the AP-1 complex in transcriptional down-regulation of the p27Kip1 gene following mitogenic stimulation. © 2010 by The American Society for Biochemistry and Molecular Biology, Inc.