All ETDs from UAB

Advisory Committee Chair

Anath Shalev

Advisory Committee Members

David Chaplin

Joanne Murphy-Ullrich

Jeonga Kim

Martin Young

Document Type

Dissertation

Date of Award

2016

Degree Name by School

Doctor of Philosophy (PhD) Heersink School of Medicine

Abstract

Pro-inflammatory cytokines, such as Interleukin (IL)-1β, Tumor necrosis factors (TNF) α, and Interferon (IFN)γ, have been implicated as critical mediators of β-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 β-cell apoptosis and β-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-1β/TNFα/IFNγ cytokine cocktail mildly up-regulated TXNIP expression and further tested the effects of individual cytokines on TXNIP expression. We first found that TNFα did not change TXNIP expression, but surprisingly IL-1β down-regulated TXNIP mRNA, whereas IFNγ up-regulated TXNIP mRNA in INS-1 β-cells and primary islets. In particular, human TXNIP promoter deletion analysis displayed a significant reduction in TXNIP promoter activity in response to IL-1β, 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-1β may inhibit TXNIP expression through ChREBP. Indeed, IL-1β treatment resulted in a decrease in ChREBP nuclear localization and ChREBP binding to the TXNIP promoter. Moreover, IL-1β down-regulated liver-type kinase (L-PK), which is another downstream target of ChREBP signaling, indicating that the effect of IL-1β is not restricted to TXNIP expression. Surprisingly, we found that despite the observed increase in TXNIP mRNA, IFNγ decreased TXNIP promoter activity, suggesting that IFNγ may modulate TXNIP expression post-transcriptionally. Indeed, we found that IFNγ up-regulated TXNIP expression via suppression of microRNA (miR)-17, which is cleaved by activation of Inositol-requiring enzyme (IRE) 1α. Therefore, our results demonstrate that proinflammatory cytokines TNFα, IL-1β, and IFNγ have differential effects on β-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.

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