Embryonic Lethal Abnormal Vision-like HuR-dependent mRNA Stability Regulates Post-transcriptional Expression of Cyclin-dependent Kinase Inhibitor p27Kip

The cyclin-dependent kinase inhibitor p27Kip¹ plays a critical role in regulating entry into and exit from the cell cycle. Post-transcriptional regulation of p27Kip¹ expression is of significant interest. The embryonic lethal abnormal vision (ELAV)-like RNA-binding protein HuR is thought be important for the translation of p27Kip¹, however, different reports attributed diametrically opposite roles to HuR. We report here an alternative mechanism wherein HuR regulates stability of the p27Kip¹ mRNA. Specifically, human and mouse p27Kip¹ mRNAs interact with HuR protein through multiple U-rich elements in both 5' and 3' untranslated regions (UTR). These interactions, which occur in vitro and in vivo, stabilize p27Kip¹ mRNA and play a critical role in its accumulation. Deleting HuR binding sites or knocking down HuR expression destabilizes p27Kip¹ mRNA and reduces its accumulation. We also identified a CT repeat in the 5' UTR of full-length p27Kip¹ mRNA isoforms that interact with a ~41-kDa protein and represses p27Kip¹ expression. This CT-rich element and diffuse elements in the 3' UTR regulate post-transcriptional expression of p27Kip¹ at the level of translation. This is the first demonstration that HuR-dependent mRNA stability and HuR-independent mRNA translation plays a critical role in the regulation of post-transcriptional p27Kip¹ expression.