Differential transcriptional expression of PPARalpha, PPARgamma1, and PPARgamma2 in the peritoneal macrophages and T-cell subsets of non-obese diabetic mice

Nuclear receptors, the aryl hydrocarbon receptor, and macrophage function

Nuclear receptors (NRs) are key regulators of innate immune responses and tissue homeostasis. Evidence indicates that NRs significantly impact steady-state immune regulation, uptake and processing of apoptotic cells, tolerance induction, and control of inflammatory immunity. In this review, we describe our current understanding of the NR activity for balancing inflammation and tolerance, the signaling cascade inducing the NR activation and functional responses, and different mechanisms of the NR-driven immune effects in the context of autoimmune diseases. We further describe the ligand-activated transcription factor the aryl hydrocarbon receptor (AhR) that exhibits analogous functionality. Moreover, we will discuss the putative role of NRs and AhR in immune regulation and disease pathogenesis providing a rationale for therapeutic targeting as a unique opportunities in the clinical management of autoimmune diseases.

PPARs and the Development of Type 1 Diabetes

Peroxisome proliferator-activated receptors (PPARs) are a family of transcription factors with a key role in glucose and lipid metabolism. PPARs are expressed in many cell types including pancreatic beta cells and immune cells, where they regulate insulin secretion and T cell differentiation, respectively. Moreover, various PPAR agonists prevent diabetes in the non-obese diabetic (NOD) mouse model of type 1 diabetes. PPARs are thus of interest in type 1 diabetes (T1D) as they represent a novel approach targeting both the pancreas and the immune system. In this review, we examine the role of PPARs in immune responses and beta cell biology and their potential as targets for treatment of T1D.

Anti-Inflammatory Effects of Rosiglitazone in Obesity-Impaired Wound Healing Depend on Adipocyte Differentiation

Combined diabetes-obesity syndromes severely impair regeneration of acute skin wounds in mouse models. This study assessed the contribution of subcutaneous adipose tissue to exacerbated wound inflammatory conditions. Genetically obese (ob/ob) mice showed an increased expression of positive transcriptional effectors of adipocyte differentiation such as Krüppel-like factor (KLF)-5 and peroxisome proliferator-activated receptor (PPAR)-γ and an associated expression of leptin and fatty acid-binding protein (FABP)-4, but also CXCL2 in isolated subcutaneous fat. This observation in obese mice is in keeping with differentially elevated levels of KLF-5, PPAR-γ, leptin, FABP-4 and CXCL2 in in vitro-differentiated 3T3-L1 adipocytes. Notably, CXCL2 expression restrictively appeared upon cytokine (IL-1β/TNF-α) stimulation only in mature, but not immature 3T3-L1 adipocytes. Of importance, the critical regulator of adipocyte maturation, PPAR-γ, was merely expressed in the final phase of in-vitro induced adipocyte differentiation from 3T3-L1 pre-adipocytes. Consistently, the PPAR-γ agonist rosiglitazone suppressed cytokine-induced CXCL2 release from mature adipocytes, but not from early 3T3-L1 adipocyte stages. The inhibitory effect of PPAR-γ activation on CXCL2 release appeared to be a general anti-inflammatory effect in mature adipocytes, as cytokine-induced cyclooxygenase (Cox)-2 was simultaneously repressed by rosiglitazone. In accordance with these findings, oral administration of rosiglitazone to wounded obese mice significantly changed subcutaneous adipocyte morphology, reduced wound CXCL2 and Cox-2 expression and improved tissue regeneration. Thus, our data suggest that PPAR-γ might provide a target to suppress inflammatory signals from mature adipocytes, which add to the prolonged wound inflammation observed in diabetes-obesity conditions.

Influence of Tyrphostin AG490 on the expression of diabetes-associated markers in human adipocytes

Tyrosine kinase inhibitors (TKi) hold promise as a treatment for a variety of disorders ranging from those in oncology to diseases thought as immune mediated. Tyrphostin AG490 is a potent Jak-Stat TKi shown effective in the prevention of allograft transplant rejection, experimental autoimmune disease, as well as the treatment of cancer. However, given its ability to modulate this important but pleiotropic intracellular pathway, we thought that it is important to examine its effects on glucose metabolism and expression of major transcription factors and adipokines associated with insulin insensitivity and diabetes. We investigated the metabolic effects of AG490 on glucose levels in vivo using an animal model of diabetes, nonobese diabetic (NOD) mice, and transcription factor expression through assessment of human adipocytes. AG490 treatment of young nondiabetic NOD mice significantly reduced blood glucose levels (p = 0.002). In vitro, treatment of adipocytes with rosiglitazone, an insulin sensitizer that binds to peroxisome proliferator-activated receptor (PPAR) receptors and increases the adipocyte response to insulin, significantly increased the expression of the antidiabetic adipokine adiponectin. Importantly, the combination of rosiglitazone plus Tyrphostin AG490 further increased this effect and was specifically associated with significant upregulation of C-enhanced binding protein (C/EBP) (p < 0.0001). In terms of the mechanism underlying this action, regulatory regions of the PPARγ, ADIPOQ, and C/EBP contain the Stat5 DNA-binding sequences and were demonstrated, by gel shift experiments in vitro. These data suggest that blocking Jak-Stat signaling with AG490 reduces blood glucose levels and modulates the expression of transcription factors previously associated with diabetes, thereby supporting its potential as a therapy for this disease.

Toll-like receptor 4 on islet β cells senses expression changes in high-mobility group box 1 and contributes to the initiation of type 1 diabetes

Type 1 diabetes mellitus is caused by the autoimmune destruction of β cells within the islets. In recent years, innate immunity has been proposed to play a key role in this process. High-mobility group box 1 (HMGB1), an inflammatory trigger in a number of autoimmune diseases, activates proinflammatory responses following its release from necrotic cells. Our aim was to determine the significance of HMGB1 in the natural history of diabetes in non-obese diabetic (NOD) mice. We observed that the rate of HMGB1 expression in the cytoplasm of islets was much greater in diabetic mice compared with non-diabetic mice. The majority of cells positively stained for toll-like receptor 4 (TLR4) were β cells; few α cells were stained for TLR4. Thus, we examined the effects of anti-TLR4 antibodies on HMGB1 cell surface binding, which confirmed that HMGB1 interacts with TLR4 in isolated islets. Expression changes in HMGB1 and TLR4 were detected throughout the course of diabetes. Our findings indicate that TLR4 is the main receptor on β cells and that HMGB1 may signal via TLR4 to selectively damage β cells rather than α cells during the development of type 1 diabetes mellitus.

Activation of PPARγ by restores mast cell numbers and reactivity in alloxan-diabetic rats by reducing the systemic glucocorticoid levels

Mast cell function and survival have been shown to be down-regulated under diabetic conditions. This study investigates the role of the peroxisome proliferator-activated receptor (PPAR)-γ in reducing mast cell number and reactivity in diabetic rats. The effect of rosiglitazone on mast cell apoptosis was also evaluated. Diabetes was induced by intravenous injection of alloxan into fasted rats and PPARγ agonist rosiglitazone and/or specific antagonist 2-chloro-5-nitrobenzanilide (GW9662) were administered 3 day after diabetes induction, once daily for 18 consecutive days. Mast cell apoptosis and plasma corticosterone levels were evaluated by TUNEL and radioimmunoassay, respectively. Treatment with rosiglitazone restored mast cell numbers in the pleural cavity and mesenteric tissue of diabetic rats. Rosiglitazone also significantly reversed the diabetes-induced reduction of histamine release by mast cells, as measured by fluorescence, following activation with the antigen in vitro. Increased apoptosis in mast cells from diabetic rats were inhibited by rosiglitazone. Moreover, we noted that the increase in plasma corticosterone levels in diabetic rats was inhibited by rosiglitazone. In addition, GW9662 blocked the ability of rosiglitazone to restore baseline numbers of mast cells and plasma corticosterone in diabetic rats. In conclusion, our findings showed that rosiglitazone restored the number and reactivity of mast cells in diabetic rats, accompanied with a suppression of apoptosis, in parallel with impairment of diabetes hypercorticolism, indicating that PPARγ has an important role in these phenomena.

Tyrphostin AG490 agent modestly but significantly prevents onset of type 1 in NOD mouse; implication of immunologic and metabolic effects of a Jak-Stat pathway inhibitor

Previously, we have reported that the Jak-Stat signaling pathway is defective in NOD mice. In this study, prediabetic female NOD mice (4 weeks) were treated by intraperitoneal injection either with AG490 or DMSO three times per week for 4 consecutive weeks, followed by once a week for an additional 6 weeks. The onset of diabetes was attenuated in NOD mice treated with AG490 relative to DMSO treated control mice (p < 0.02). From an immunological standpoint, AG490 induced the expression of Foxp3 in CD4(+)CD25(-) T-cells and down-regulated expression of co-stimulatory molecules in dendritic cells (DC) both in vitro and in vivo. AG490 treated CD4+CD25- T-cells and DC in vitro, acquired regulatory functions; namely, the ability to suppress proliferation of a responding cell population in vitro. AG490 treatment resulted in significant reduction of blood glucose values and increased expression of PPARγ in splenocytes and markedly increased expression PPARγ2 but not PPARγ1 in adipocyte in vitro. Presence of multiple Stat5 DNA binding consensus sequences within the promoter region of the PPARγ gene in human and in mouse suggests that PPARγ is downstream to the Jak-Stat signaling pathway. This study highlights a critical role of the Jak-Stat signaling pathway in the pathogenesis of T1D and suggests that blocking the Jak-Stat signaling pathway by AG490 as a tyrosine kinase inhibitor may provide an effective means for preventing autoimmune T1D via both immunological and metabolic effects.

Male and female NOD mice differentially express peroxisome proliferator-activated receptors and pathogenic cytokines

The peroxisome proliferator-activated receptors (PPARs) have been implicated in regulating the immune response. We determined the relative changes in the transcriptional expression of PPAR isoforms (α, γ1 and γ2) and cytokines involved in the pathogenesis of type 1 diabetes (T1D) in the immune cells of 5 weeks, 10 weeks and diabetic male non-obese diabetic (NOD) mice compared to those of female NOD mice from our previous studies, "normalized" against their respective non-obese diabetic resistant (NOR) mice controls. Overall PPARα was significantly more elevated in the macrophages of female NOD mice of all age groups whereas PPARγ, particularly the PPARγ2 isoform was more depressed in the macrophages and CD4(+) lymphocytes of female NOD mice compared to their male counterparts. The pro-inflammatory cytokines, IL-1 and TNFα, as well as the Th1 cytokines, IL-2 and IFNγ were more elevated in female NOD mice whereas the Th2 cytokine, IL-4, was more depressed in these mice compared to their male counterparts. These findings suggest that the preponderance of T1D in female NOD mice may be influenced by the more pronounced changes in the expression of PPAR isoforms and pathogenic cytokines compared to those in male NOD mice.