2F), consistent with its known dominant-negative effects against endogenous IRE1α. Furthermore, induction of either IRE1α (I642G)—or forced expression of spliced XBP1 transcription factor (XBP1s)—cause minimal elevation of TXNIP mRNA, without discernible changes in TXNIP protein (Figure 2D–G). These results argue that robust TXNIP induction requires both functional kinase and RNase catalytic activities of IRE1α, and is largely independent of XBP1 transcription factor activity. This last point was confirmed using Xbp1 −/− MEFs, in which production of TXNIP under ER stress is intact (Figure S3D). Indeed, TXNIP protein is detectable in Xbp1 −/− MEFs even under basal conditions, consistent with a previous observation that in the absence of XBP1, IRE1α becomes partially activated even without ER stress (Lee et al., 2008). In contrast, induction of TXNIP under ER stress is abrogated in Jnk1,2 −/− MEFs (Figure S3E), arguing that TXNIP regulation by IRE1α occurs downstream of JNK.
