IRE1-mediated degradation of pre-miR-301a promotes apoptosis through upregulation of GADD45A

The unfolded protein response is a survival signaling pathway that is induced during various types of ER stress. Here we focus on the IRE1 pathway to determine IRE1s role in miRNA regulation during ER stress. During induction of ER stress in human bronchial epithelial cells, we utilized next generation sequencing to demonstrate that pre-miR-301a and pre-miR-106b, were significantly increased in the presence of an IRE1 inhibitor. Conversely, using nuclear-cytosolic fractionation on ER stressed cells, we found that these three pre-miRNAs were decreased in the nuclear fractions without the IRE1 inhibitor. We also found that miR-301a-3p targets the proapoptotic UPR factor, growth arrest and DNA-damage-inducible alpha (GADD45A). Inhibiting miR-301a-3p levels or blocking its predicted miRNA binding site in GADD45As 3 UTR with a target protector increased GADD45A mRNA expression. An elevation of XBP1s expression had no effect on GADD45A mRNA expression. We also demonstrated that the introduction of a target protector for the miR-301a-3p binding site in GADD45A mRNA during ER stress promoted cell death in the airway epithelial cells. These results indicated that IRE1s endonuclease activity is a two-edged sword that splices XBP1 mRNA for survival and degrades pre-miR-301a to elevate the mRNA expression of a pro-apoptotic gene, GADD45A.