| Pro-inflammatory cytokines, such as Interleukin (IL)-1beta, Tumor necrosis factors (TNF) alpha, and Interferon (IFN)gamma, have been implicated as critical mediators of beta-cell destruction in diabetes. In addition, although a combination of these three cytokines has been used to mimic the inflammatory conditions of type 1 diabetes in vitro, the mechanisms underlying the effect are not fully understood. Previously, we discovered Thioredoxin-interacting protein (TXNIP) as a key regulator of glucotoxicity-induced beta-cell apoptosis and beta-cell dysfunction, while deletion of TXNIP prevented type 1 (T1D) and type 2 diabetes (T2D). However, the effects of proinflammatory cytokines on the regulation of TXNIP have not been fully elucidated.;We here show that the IL-1beta/TNFalpha/IFNgamma cytokine cocktail mildly up-regulated TXNIP expression and further tested the effects of individual cytokines on TXNIP expression. We first found that TNFalpha did not change TXNIP expression, but surprisingly IL-1beta down-regulated TXNIP mRNA, whereas IFNgamma up-regulated TXNIP mRNA in INS-1 beta-cells and primary islets. In particular, human TXNIP promoter deletion analysis displayed a significant reduction in TXNIP promoter activity in response to IL-1beta, but mutation of the E-box motif, which acts as the Carbohydrate responsive element binding protein (ChREBP) binding site, blunted this effect, indicating that IL-1beta may inhibit TXNIP expression through ChREBP. Indeed, IL-1beta treatment resulted in a decrease in ChREBP nuclear localization and ChREBP binding to the TXNIP promoter. Moreover, IL-1beta down-regulated liver-type kinase (L-PK), which is another downstream target of ChREBP signaling, indicating that the effect of IL-1beta is not restricted to TXNIP expression.;Surprisingly, we found that despite the observed increase in TXNIP mRNA, IFNgamma decreased TXNIP promoter activity, suggesting that IFNgamma may modulate TXNIP expression post-transcriptionally. Indeed, we found that IFNgamma up-regulated TXNIP expression via suppression of microRNA (miR)-17, which is cleaved by activation of Inositol-requiring enzyme (IRE) 1alpha.;Therefore, our results demonstrate that proinflammatory cytokines TNFalpha, IL-1beta, and IFNgamma have differential effects on beta-cell TXNIP expression and act via distinct molecular mechanisms. They further reveal for the first time the complexity of proinflammatory cytokine-mediated TXNIP regulation and providing a novel link between proinflammatory cytokines and ChREBP-mediated transcription of TXNIP and miR-17-mediated posttranscriptional TXNIP regulation. |
