Transcriptional regulation of importin-α1 by JunD modulates subcellular localization of RNA-binding protein HuR in intestinal epithelial cells.

The RNA-binding protein HuR is crucial for normal intestinal mucosal regeneration by modulating the stability and translation of target mRNAs, but the exact mechanism underlying HuR trafficking between the cytoplasm and nucleus remains largely unknown. Here we report a novel function of transcription factor JunD in the regulation of HuR subcellular localization through the control of importin-α expression in intestinal epithelial cells (IECs). Ectopically expressed JunD specifically inhibited importin-α at the transcription level, and this repression is mediated via interaction with CREB-binding site that was located at the proximal region of importin-α promoter. Reduction in the levels of importin-α by JunD increased cytoplasmic levels of HuR, although it failed to alter whole cell HuR levels. Increased levels of endogenous JunD by depleting cellular polyamines also inhibited importin-α expression and increased cytoplasmic HuR levels, whereas JunD silencing rescued importin-α expression and enhanced HuR nuclear translocation in polyamine-deficient cells. Moreover, importin-α silencing protected IECs against apoptosis, which was prevented by HuR silencing. These results indicate that JunD regulates HuR subcellular distribution by downregulating importin-α, thus contributing to the maintenance of gut epithelium homeostasis.