Toll‐like receptor 4 suppression leads to islet allograft survival

The synthetic liver X receptor agonist GW3965 reduces tissue factor production and inflammatory responses in human islets in vitro

AIMS/HYPOTHESIS

Optimising islet culture conditions may be one strategy for reducing islet loss prior to, and immediately after, islet transplantation. Liver X receptor (LXR) agonism has previously been shown to increase insulin release from pancreatic islets and reduce inflammation in leucocytes. Our aim was to investigate whether the synthetic LXR agonist GW3965 could modulate the inflammatory status of human pancreatic islets.

METHODS

Levels of pro-inflammatory cytokines and tissue factor in isolated human islets were determined by TaqMan low density array and/or real-time quantitative RT-PCR (mRNA levels) and enzyme immunoassay (EIA) (protein levels). Islet viability was measured using intracellular ATP content, ADP/ATP ratio, mitochondrial dehydrogenase activity (XTT assay) and insulin secretion in a dynamic glucose-challenge model. Apoptosis was determined by EIA measurement of histone-DNA complexes present in cytoplasm and by assaying caspase-3/-7 activity.

RESULTS

Treatment of LPS-stimulated human islets with the synthetic LXR agonist GW3965 (1 micromol/l) for 24 h reduced mRNA and protein levels of selected pro-inflammatory cytokines (IL-8, monocyte chemotactic protein-1 and tissue factor). Moreover, GW3965 had no adverse effect on insulin secretion, islet viability or apoptosis. No excess of lipid accumulation could be detected with the dosage and exposure time used.

CONCLUSIONS/INTERPRETATION

LXR activation suppresses inflammation in human islets in vitro without adverse effects on islet viability. Short-term moderate activation of LXR prior to islet transplantation may represent a possible strategy for improving post-transplant islet survival.

PTPN2, a candidate gene for type 1 diabetes, modulates interferon-gamma-induced pancreatic beta-cell apoptosis

OBJECTIVE

The pathogenesis of type 1 diabetes has a strong genetic component. Genome-wide association scans recently identified novel susceptibility genes including the phosphatases PTPN22 and PTPN2. We hypothesized that PTPN2 plays a direct role in beta-cell demise and assessed PTPN2 expression in human islets and rat primary and clonal beta-cells, besides evaluating its role in cytokine-induced signaling and beta-cell apoptosis.

RESEARCH DESIGN AND METHODS

PTPN2 mRNA and protein expression was evaluated by real-time PCR and Western blot. Small interfering (si)RNAs were used to inhibit the expression of PTPN2 and downstream STAT1 in beta-cells, allowing the assessment of cell death after cytokine treatment.

RESULTS

PTPN2 mRNA and protein are expressed in human islets and rat beta-cells and upregulated by cytokines. Transfection with PTPN2 siRNAs inhibited basal- and cytokine-induced PTPN2 expression in rat beta-cells and dispersed human islets cells. Decreased PTPN2 expression exacerbated interleukin (IL)-1beta + interferon (IFN)-gamma-induced beta-cell apoptosis and turned IFN-gamma alone into a proapoptotic signal. Inhibition of PTPN2 amplified IFN-gamma-induced STAT1 phosphorylation, whereas double knockdown of both PTPN2 and STAT1 protected beta-cells against cytokine-induced apoptosis, suggesting that STAT1 hyperactivation is responsible for the aggravation of cytokine-induced beta-cell death in PTPN2-deficient cells.

CONCLUSIONS

We identified a functional role for the type 1 diabetes candidate gene PTPN2 in modulating IFN-gamma signal transduction at the beta-cell level. PTPN2 regulates cytokine-induced apoptosis and may thereby contribute to the pathogenesis of type 1 diabetes.

The role of inflammation in insulitis and beta-cell loss in type 1 diabetes

Type 1 diabetes mellitus (T1DM) is a chronic autoimmune disease with a strong inflammatory component. The latest studies indicate that innate immunity and inflammatory mediators have a much broader role in T1DM than initially assumed. Inflammation might contribute to early induction and amplification of the immune assault against pancreatic beta cells and, at later stages, to the stabilization and maintenance of insulitis. Inflammatory mediators probably contribute to the suppression of beta-cell function and subsequent apoptosis; they may also inhibit or stimulate beta-cell regeneration and might cause peripheral insulin resistance. The different effects of inflammation take place in different phases of the course of T1DM, and should be considered in the context of a 'dialog' between invading immune cells and the target beta cells. This dialog is mediated both by cytokines and chemokines that are released by beta cells and immune cells, and by putative, immunogenic signals that are delivered by dying beta cells. In this Review, we divided the role of inflammation in T1DM into three arbitrary stages: induction, amplification and maintenance or resolution of insulitis. These stages, and their progression or resolution, might depend on a patient's genetic background, which contributes to disease heterogeneity.

CXCL10 impairs beta cell function and viability in diabetes through TLR4 signaling

In type 1 and type 2 diabetes (T1/T2DM), beta cell destruction by apoptosis results in decreased beta cell mass and progression of the disease. In this study, we found that the interferon gamma-inducible protein 10 plays an important role in triggering beta cell destruction. Islets isolated from patients with T2DM secreted CXCL10 and contained 33.5-fold more CXCL10 mRNA than islets from control patients. Pancreatic sections from obese nondiabetic individuals and patients with T2DM and T1DM expressed CXCL10 in beta cells. Treatment of human islets with CXCL10 decreased beta cell viability, impaired insulin secretion, and decreased insulin mRNA. CXCL10 induced sustained activation of Akt, JNK, and cleavage of p21-activated protein kinase 2 (PAK-2), switching Akt signals from proliferation to apoptosis. These effects were not mediated by the commonly known CXCL10 receptor CXCR3 but through TLR4. Our data suggest CXCL10 as a binding partner for TLR4 and as a signal toward beta cell failure in diabetes.