Regulation of Immune Responses and Autoimmune Encephalomyelitis by PPARs

FKBP5 activates mitophagy by ablating PPAR-γ to shape a benign remyelination environment

Multiple sclerosis (MS) is an autoimmune and neurodegenerative disease of the central nervous system (CNS) that is characterized by myelin damage, followed by axonal and ultimately neuronal loss, which has been found to be associated with mitophagy. The etiology and pathology of MS remain elusive. However, the role of FK506 binding protein 5 (FKBP5, also called FKBP51), a newly identified gene associated with MS, in the progression of the disease has not been well defined. Here, we observed that the progress of myelin loss and regeneration in Fkbp5ko mice treated with demyelination for the same amount of time was significantly slower than that in wild-type mice, and that mitophagy plays an important regulatory role in this process. To investigate the mechanism, we discovered that the levels of FKBP5 protein were greatly enhanced in the CNS of cuprizone (CPZ) mice and the myelin-denuded environment stimulates significant activation of the PINK1/Parkin-mediated mitophagy, in which the important regulator, PPAR-γ, is critically regulated by FKBP5. This study reveals the role of FKBP5 in regulating a dynamic pathway of natural restorative regulation of mitophagy through PPAR-γ in pathological demyelinating settings, which may provide potential targets for the treatment of demyelinating diseases.

Characterization of Kruppel-like factor 6 in Epinephelus coioides: The role in viral infection and the transcriptional regulation on Peroxisome proliferator-activated receptor δ

The Kruppel-like factor 6 (KLF6) is a member of Kruppel-like factor family, which belong to the Zinc finger family of transcription factors that mediates various cellular processes, such as proliferation, differentiation, development, and programmed cell death. Peroxisome proliferator-activated receptors (PPARs) are a family of transcription factors belonging to the nuclear receptor superfamily and they regulate numerous genes through ligand-dependent transcriptional activation and repression. In this study, we focus on the role of KLF6 gene in virus infection and the regulation of KLF6 on PPAR-δ in orange-spotted grouper (Epinephelus coioides). The ORF sequence of EcKLF6 was 846 bp, encoding a polypeptide of 282 amino acids with three conserved Zinc finger (type Cys₂-His₂) domain in the C-terminal region. Basing on the detection of the mRNA levels of viral genes, western blotting of MCP protein, and morphological CPEs, we found that the overexpression of EcKLF6 suppressed the replication of Singapore grouper iridovirus (SGIV), exerting its antiviral activity against fish virus. Moreover, promoter analysis was performed to investigate whether EcKLF6 was a regulator of EcPPAR-δ. The luciferase reporter assay and real time PCR results indicated a negative regulatory role of EcKLF6 on EcPPAR-δ transcription in grouper. Further experimental analysis shows that the potential EcKLF6 binding sites may locate in the EcPPAR-δ-4-M3 (+133 to +154) and EcPPAR-δ-4-M4 (+354 to +368) region of the EcPPAR-δ promoter. Electrophoretic mobile shift assays (EMSAs) verified that EcKLF6 interacted with the binding site of the EcPPAR-δ-4-M4 promoter region. In addition, we also found that KLF6 promotes inflammatory responses in GS cells. Considering that KLF6 and PPAR-δ play opposite roles in regulating inflammatory responses, we speculated the promoting effect of KLF6 on inflammatory response may be related to its negative regulation on EcPPAR-δ. In conclusion, the present study provides the first evidence of the negative regulation of EcPPAR-δ transcription by EcKLF6 and contributes to a better understanding of the transcriptional mechanisms of EcKLF6 in fish.

Design, synthesis, molecular modeling and anti-hyperglycemic evaluation of phthalimide-sulfonylurea hybrids as PPARγ and SUR agonists

New series of phthalimide-sulfonylurea hybrids were prepared and examined for their in vivo anti-hyperglycemic activities in STZ-induced hyperglycemic rats using glibenclamide as a reference drug. Compounds 6_c, 6_d, 6_g, 6_h, 6_j and 6_k induced significant reduction in the blood glucose levels of diabetic rats ranging from 24.43 to 21.43%. Moreover, molecular docking and pharmacophore approaches were carried out to examine binding modes and fit values of the prepared compounds against PPARγ and SUR, respectively. Compounds 6_c, 6_d, 6_j and 6_m exhibited the highest binding free energies against PPARγ. Compounds 6_c, 6_j, 6_k, 6_l, and 6_n showed the highest fit values against the generated pharmacophore model. Also, QSAR technique was carried out to estimate the proposed PPARγ binding affinities and insulin-secreting abilities. The synthesized compounds showed promising estimated activities. In-silico ADMET studies were performed to investigate pharmacokinetics of the synthesized compounds. They showed considerable human intestinal absorption with low BBB penetration.

Metabolism in Immune Cell Differentiation and Function

The immune system is a central determinant of organismal health. Functional immune responses require quiescent immune cells to rapidly grow, proliferate, and acquire effector functions when they sense infectious agents or other insults. Specialized metabolic programs are critical regulators of immune responses, and alterations in immune metabolism can cause immunological disorders. There has thus been growing interest in understanding how metabolic processes control immune cell functions under normal and pathophysiological conditions. In this chapter, we summarize how metabolic programs are tuned and what the physiological consequences of metabolic reprogramming are as they relate to immune cell homeostasis, differentiation, and function.

Exploiting the Therapeutic Potential of Endogenous Immunomodulatory Systems in Multiple Sclerosis-Special Focus on the Peroxisome Proliferator-Activated Receptors (PPARs) and the Kynurenines

Multiple sclerosis (MS) is a progressive neurodegenerative disease, characterized by autoimmune central nervous system (CNS) demyelination attributable to a disturbed balance between encephalitic T helper 1 (Th1) and T helper 17 (Th17) and immunomodulatory regulatory T cell (Treg) and T helper 2 (Th2) cells, and an alternatively activated macrophage (M2) excess. Endogenous molecular systems regulating these inflammatory processes have recently been investigated to identify molecules that can potentially influence the course of the disease. These include the peroxisome proliferator-activated receptors (PPARs), PPARγ coactivator-1alpha (PGC-1α), and kynurenine pathway metabolites. Although all PPARs ameliorate experimental autoimmune encephalomyelitis (EAE), recent evidence suggests that PPARα, PPARβ/δ agonists have less pronounced immunomodulatory effects and, along with PGC-1α, are not biomarkers of neuroinflammation in contrast to PPARγ. Small clinical trials with PPARγ agonists have been published with positive results. Proposed as immunomodulatory and neuroprotective, the therapeutic use of PGC-1α activation needs to be assessed in EAE/MS. The activation of indolamine 2,3-dioxygenase (IDO), the rate-limiting step of the kynurenine pathway of tryptophan (Trp) metabolism, plays crucial immunomodulatory roles. Indeed, Trp metabolites have therapeutic relevance in EAE and drugs with structural analogy to kynurenines, such as teriflunomide, are already approved for MS. Further studies are required to gain deeper knowledge of such endogenous immunomodulatory pathways with potential therapeutic implications in MS.

Demyelination in Multiple Sclerosis: Reprogramming Energy Metabolism and Potential PPARγ Agonist Treatment Approaches

Demyelination in multiple sclerosis (MS) cells is the site of several energy metabolic abnormalities driven by dysregulation between the opposed interplay of peroxisome proliferator-activated receptor γ (PPARγ) and WNT/β-catenin pathways. We focus our review on the opposing interactions observed in demyelinating processes in MS between the canonical WNT/β-catenin pathway and PPARγ and their reprogramming energy metabolism implications. Demyelination in MS is associated with chronic inflammation, which is itself associated with the release of cytokines by CD4⁺ Th17 cells, and downregulation of PPARγ expression leading to the upregulation of the WNT/β-catenin pathway. Upregulation of WNT/β-catenin signaling induces activation of glycolytic enzymes that modify their energy metabolic behavior. Then, in MS cells, a large portion of cytosolic pyruvate is converted into lactate. This phenomenon is called the Warburg effect, despite the availability of oxygen. The Warburg effect is the shift of an energy transfer production from mitochondrial oxidative phosphorylation to aerobic glycolysis. Lactate production is correlated with increased WNT/β-catenin signaling and demyelinating processes by inducing dysfunction of CD4⁺ T cells leading to axonal and neuronal damage. In MS, downregulation of PPARγ decreases insulin sensitivity and increases neuroinflammation. PPARγ agonists inhibit Th17 differentiation in CD4⁺ T cells and then diminish release of cytokines. In MS, abnormalities in the regulation of circadian rhythms stimulate the WNT pathway to initiate the demyelination process. Moreover, PPARγ contributes to the regulation of some key circadian genes. Thus, PPARγ agonists interfere with reprogramming energy metabolism by directly inhibiting the WNT/β-catenin pathway and circadian rhythms and could appear as promising treatments in MS due to these interactions.

Design, synthesis, molecular modeling and anti-hyperglycemic evaluation of quinazolin-4(3H)-one derivatives as potential PPARγ and SUR agonists

Peroxisome proliferator-activated receptor gamma (PPARγ) and sulfonylurea receptor (SUR) play crucial roles in management of type-2 diabetes mellitus. In this study, a series of novel quinazoline-4(3H)-one-sulfonylurea hybrids were designed and synthesized as dual PPARγ and SUR agonists. The synthesized compounds were evaluated for their in vivo anti-hyperglycemic activities against STZ-induced hyperglycemic rats. Four compounds (19_a, 19_d, 19_f and 25g) demonstrated potent activities with reduction in blood glucose levels of 40.43, 46.42, 41.23 and 42.50 %, respectively. The most active ten compounds were further evaluated in vitro for their PPARγ binding affinities and insulin-secreting abilities. Compounds 19_b, 19_d, 19_f, 25_f and 25g exhibited the highest affinities against PPARγ with IC₅₀ values of 0.371, 0.350, 0.369, 0.408 and 0.353µM, respectively. In addition, compounds 19_d, 19_f, and 25_d showed the highest insulin-secreting activities with EC₅₀ values of 0.97, 1.01 and 1.15µM, respectively. Furthermore, molecular docking and pharmacophore generation techniques were carried out to investigate binding patterns and fit values of the designed compounds with PPARγ and SUR, respectively. Also, two QSAR models were generated to explore the structural requirements controlling the different biological activities of the synthesized compounds against PPARγ and SUR.

Unlike PPARgamma, neither other PPARs nor PGC-1alpha is elevated in the cerebrospinal fluid of patients with multiple sclerosis

Corroborating with prior experimental findings, we recently reported the pronounced elevation of peroxisome proliferator-activated receptor gamma (PPARγ) protein concentration in the cerebrospinal fluid (CSF) of patients with multiple sclerosis (MS), in association with neuroinflammatory markers and clinical severity. Based on subsequent reports on the possible involvement of other PPARs and PPARγ coactivator-1alpha (PGC-1α) in neuroinflammation in MS, we analyzed the protein levels of PPARα, PPARβ/δ, and PGC-1α in a subset of CSF samples from the same cohort of relapsing-remitting MS patients. Unlike PPARγ, none of these proteins were found elevated in MS patients (n=25) compared to non-inflammatory controls (n=16), with the levels of PPARα and PPARβ/δ found generally below the limit of detection, and that of PGC-1α being detectable but comparable in both groups. The clinical and laboratory associations previously reported with PPARγ were however significant even in this smaller subset. The potential underlying causes of these differential alterations are discussed. The findings suggest that despite their proposed involvement in the regulation of inflammatory processes in MS, PPARα, PPARβ/δ, and PGC-1α proteins are not potential biomarkers of neuroinflammation in MS, and indicate a preferential role of PPARγ in the endogenous regulation of autoimmune response in the human CNS within its receptor family.

The skewed balance between Tregs and Th17 in chronic lymphocytic leukemia

While Tregs maintain self-tolerance and inhibit antitumor responses, T helper (Th)17 cells may enhance inflammatory and antitumor responses. The balance between these two important T-cell subsets has been skewed in many immunopathologic conditions such as autoimmune and cancer diseases. B-cell chronic lymphocytic leukemia (CLL) is the most common form of leukemia in the western world and is characterized with monoclonal expansion of B lymphocytes. There is evidence which implies that the progression of CLL is associated with expansion of Treg and downregulation of Th17 cells. In this review, we will discuss about immunobiology of Treg and Th17 cells and their role in immunopathogenesis of CLL as well as their reciprocal changes during disease progression.

The transcriptional PPARβ/δ network in human macrophages defines a unique agonist-induced activation state

Peroxisome proliferator-activated receptor β/δ (PPARβ/δ) is a lipid ligand-inducible transcription factor with established metabolic functions, whereas its anti-inflammatory function is poorly understood. To address this issue, we determined the global PPARβ/δ-regulated signaling network in human monocyte-derived macrophages. Besides cell type-independent, canonical target genes with metabolic and immune regulatory functions we identified a large number of inflammation-associated NFκB and STAT1 target genes that are repressed by agonists. Accordingly, PPARβ/δ agonists inhibited the expression of multiple pro-inflammatory mediators and induced an anti-inflammatory, IL-4-like morphological phenotype. Surprisingly, bioinformatic analyses also identified immune stimulatory effects. Consistent with this prediction, PPARβ/δ agonists enhanced macrophage survival under hypoxic stress and stimulated CD8⁺ T cell activation, concomitantly with the repression of immune suppressive target genes and their encoded products CD274 (PD-1 ligand), CD32B (inhibitory Fcγ receptor IIB) and indoleamine 2,3-dioxygenase 1 (IDO-1), as well as a diminished release of the immune suppressive IDO-1 metabolite kynurenine. Comparison with published data revealed a significant overlap of the PPARβ/δ transcriptome with coexpression modules characteristic of both anti-inflammatory and pro-inflammatory cytokines. Our findings indicate that PPARβ/δ agonists induce a unique macrophage activation state with strong anti-inflammatory but also specific immune stimulatory components, pointing to a context-dependent function of PPARβ/δ in immune regulation.

Physiological functions of peroxisome proliferator-activated receptor β

The peroxisome proliferator-activated receptors, PPARα, PPARβ, and PPARγ, are a family of transcription factors activated by a diversity of molecules including fatty acids and fatty acid metabolites. PPARs regulate the transcription of a large variety of genes implicated in metabolism, inflammation, proliferation, and differentiation in different cell types. These transcriptional regulations involve both direct transactivation and interaction with other transcriptional regulatory pathways. The functions of PPARα and PPARγ have been extensively documented mainly because these isoforms are activated by molecules clinically used as hypolipidemic and antidiabetic compounds. The physiological functions of PPARβ remained for a while less investigated, but the finding that specific synthetic agonists exert beneficial actions in obese subjects uplifted the studies aimed to elucidate the roles of this PPAR isoform. Intensive work based on pharmacological and genetic approaches and on the use of both in vitro and in vivo models has considerably improved our knowledge on the physiological roles of PPARβ in various cell types. This review will summarize the accumulated evidence for the implication of PPARβ in the regulation of development, metabolism, and inflammation in several tissues, including skeletal muscle, heart, skin, and intestine. Some of these findings indicate that pharmacological activation of PPARβ could be envisioned as a therapeutic option for the correction of metabolic disorders and a variety of inflammatory conditions. However, other experimental data suggesting that activation of PPARβ could result in serious adverse effects, such as carcinogenesis and psoriasis, raise concerns about the clinical use of potent PPARβ agonists.

Elevated levels of PPAR-gamma in the cerebrospinal fluid of patients with multiple sclerosis

Peroxisome proliferator-activated receptor gamma (PPARγ), a ligand-activated transcriptional factor involved in the regulation of glucose and lipid metabolism, has gained interest as a potential therapeutic target in multiple sclerosis (MS) due to its potent immunoregulatory properties and the therapeutic efficacy of its ligands in experimental autoimmune encephalitis (EAE). Elevated expression of PPARγ has been observed in the spinal cord of EAE mice and in an in vitro model of antigen-induced demyelination; however, no reports have yet been available on the PPARγ status in the central nervous system of human individuals with MS. Aiming to identify a possible alteration, the present study assessed the levels of PPARγ protein in the cerebrospinal fluid (CSF) of MS patients via ELISA technique. We report a pronounced elevation in the CSF levels of PPARγ in MS patients (n=35) compared to non-inflammatory controls (n=22). This elevation was independent of blood-CSF barrier integrity, but correlated with CSF white blood cell count and IgG index, associating the observed elevation with neuroinflammation. Controlling for potential confounders, the CSF levels of PPARγ further displayed a moderate but significant association with clinical severity. Corroborating with prior experimental findings, these results may contribute to our understanding about the role of PPARγ in MS, and may implicate this protein as a potential CSF biomarker of the disease.

Metabolic control of the Treg/Th17 axis

The interplay of the immune system with other aspects of physiology is continually being revealed and in some cases studied in considerable mechanistic detail. A prime example is the influence of metabolic cues on immune responses. It is well appreciated that upon activation, T cells take on a metabolic profile profoundly distinct from that of their quiescent and anergic counterparts; however, a number of recent breakthroughs have greatly expanded our knowledge of how aspects of cellular metabolism can shape a T-cell response. Particularly important are findings that certain environmental cues can tilt the delicate balance between inflammation and immune tolerance by skewing T-cell fate decisions toward either the T-helper 17 (Th17) or T-regulatory (Treg) cell lineage. Recognizing the unappreciated immune-modifying potential of metabolic factors and particularly those involved in the generation of these functionally opposing T-cell subsets will likely add new and potent therapies to our repertoire for treating immune mediated pathologies. In this review, we summarize and discuss recent findings linking certain metabolic pathways, enzymes, and by-products to shifts in the balance between Th17 and Treg cell populations. These advances highlight numerous opportunities for immune modulation.

Inhibition of 12/15-lipoxygenase by baicalein induces microglia PPARβ/δ: a potential therapeutic role for CNS autoimmune disease

12/15-Lipoxygenase (12/15-LO) is an enzyme that converts polyunsaturated fatty acids into bioactive lipid derivatives. In this study, we showed that inhibition of 12/15-LO by baicalein (BA) significantly attenuated clinical severity of experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS). Inhibited migration of autoimmune T cells into the central nervous system (CNS) by BA treatment could be attributed to reduced activation of microglia, which was indicated by suppressed phagocytosis, and decreased production of proinflammatory cytokines and chemokines in the CNS. We further observed that inhibition of 12/15-LO with BA led to increased expression of peroxisome proliferator-activated receptor (PPAR)β/δ in microglia of EAE mice. This was confirmed in vitro in primary microglia and a microglia cell line, BV2. In addition, we demonstrated that BA did not affect 12/15-LO or 5-lipoxygenase (5-LO) expression in microglia, but significantly decreased 12/15-LO products without influencing the levels of 5-LO metabolites. Moreover, among these compounds only 12/15-LO metabolite 12-hydroxyeicosatetraenoic acid was able to reverse BA-mediated upregulation of PPARβ/δ in BV2 cells. We also showed that inhibition of microglia activation by PPARβ/δ was associated with repressed NF-κB and MAPK activities. Our findings indicate that inhibition of 12/15-LO induces PPARβ/δ, demonstrating important regulatory properties of 12/15-LO in CNS inflammation. This reveals potential therapeutic applications for MS.

The nuclear receptor PPARs as important regulators of T-cell functions and autoimmune diseases

Members of the nuclear receptor superfamily function as transcription factors involved in innate and adaptive immunity as well as lipid metabolism. These highly conserved proteins participate in ligand-dependent or -independent regulatory mechanisms that affect gene expression. Peroxisome proliferator-activated receptors (PPARs), which include PPARα, PPARβ/δ, and PPARΓ, are a group of nuclear receptor proteins that play diverse roles in cellular differentiation, development, and metabolism. Each PPAR subfamily is activated by different endogenous and synthetic ligands. Recent studies using specific ligand treatments and cell type-specific PPAR knockout mice have revealed important roles for these proteins in T-cell-related autoimmune diseases. Moreover, PPARs have been shown to regulate T-cell survival, activation, and CD4(+) T helper cell differentiation into the Th1, Th2, Th17, and Treg lineages. Here, we review the studies that provide insight into the important regulatory roles of PPARs in T-cell activation, survival, proliferation, differentiation, and autoimmune disease.