We next used heterologous reporter systems to test the consequence of miR-17 reduction. We constructed a mCherry, red fluorescent protein (RFP) reporter that contains tandem miR-17 seed sequences in its 3′-UTR; the reporter is designed to express RFP when cellular miR-17 levels drop. Upon transfection into wild-type MEFs, the reporter becomes derepressed under ER stress to produce RFP, indicating reduction in endogenous miR-17 (Figure 3G). The reporter remains silenced in Ire1α−/− MEFs, indicating that IRE1α is necessary for reduction of miR-17 under ER stress. To further investigate whether IRE1α is sufficient for miR-17-dependent control of TXNIP, we constructed a luciferase reporter containing the entire TXNIP 3′-UTR, and a version mutated in the miR-17 seed sequences. Upon transfection of these reporters into dox-inducible WT-IRE1α cells (Figure 2C), induction with Dox increases baseline luciferase activity driven from the wild-type—but not the miR-17-mutant—TXNIP 3′-UTR reporter (Figure 3H). Together these results argue that activation of IRE1α increases TXNIP mRNA levels post-transcriptionally by reducing its inhibitory micro-RNA, miR-17. Rationalizing our results, a mathematical model (Figure S5J and K) shows that a combination of transcriptional and
