Tonicity-responsive MicroRNAs Contribute to the Maximal Induction of Osmoregulatory Transcription Factor OREBP in Response to High-NaCl Hypertonicity

Osmotic response element binding protein (OREBP) is a Rel-like transcription factor critical for cellular osmoresponses. Previous studies suggest that hypertonicity-induced accumulation of OREBP protein might be mediated by transcription activation as well as posttranscriptional mRNA stabilization or increased translation. However, the underlying mechanisms remain incompletely elucidated. Here, we report that microRNAs (miRNAs) play critical regulatory roles in hypertonicity-induced induction of OREBP. In renal medullary epithelial mIMCD3 cells, hypertonicity greatly stimulates the activity of the 3'-untranslated region of OREBP (OREBP-3'UTR). Furthermore, overexpression of OREBP-3'UTR or depletion of miRNAs by knocking-down Dicer greatly increases OREBP protein expression. On the other hand, significant alterations in miRNA expression occur rapidly in response to high NaCl exposure, with miR-200b and miR-717 being most significantly down-regulated. Moreover, increased miR-200b or miR-717 causes significant down-regulation of mRNA, protein and transcription activity of OREBP, whereas inhibition of miRNAs or disruption of the miRNA-3'UTR interactions abrogates the silencing effects. In vivo in mouse renal medulla, miR-200b and miR-717 are found to function to tune OREBP in response to renal tonicity alterations. Together, our results support the notion that miRNAs contribute to the maximal induction of OREBP to participate in cellular responses to osmotic stress in mammalian renal cells.