The nuclear hormone receptor peroxisome proliferator-activated receptor beta/delta potentiates cell chemotactism, polarization, and migration

Is the peroxisome proliferator-activated receptor gamma a putative target for epilepsy treatment? Current evidence and future perspectives

The peroxisome proliferator-activated receptor gamma (PPARγ), which belongs to the family of nuclear receptors, has been mainly studied as an important factor in metabolic disorders. However, in recent years the potential role of PPARγ in different neurological diseases has been increasingly investigated. Especially, in the search of therapeutic targets for patients with epilepsy the question of the involvement of PPARγ in seizure control has been raised. Epilepsy is a chronic neurological disorder causing a major impact on the psychological, social, and economic conditions of patients and their families, besides the problems of the disease itself. Considering that the world prevalence of epilepsy ranges between 0.5% - 1.0%, this condition is the fourth for importance among the other neurological disorders, following migraine, stroke, and dementia. Among others, temporal lobe epilepsy (TLE) is the most common form of epilepsy in adult patients. About 65% of individuals who receive antiseizure medications (ASMs) experience seizure independence. For those in whom seizures still recur, investigating PPARγ could lead to the development of novel ASMs. This review focuses on the most important findings from recent investigations about the potential intracellular PPARγ-dependent processes behind different compounds that exhibited anti-seizure effects. Additionally, recent clinical investigations are discussed along with the promising results found for PPARγ agonists and the ketogenic diet (KD) in various rodent models of epilepsy.

Perspectives for the application of neurogenetic research in programming Neurorehabilitation

Certain genetic variants underlie the proper functioning of the nervous system. They affect the nervous system in all aspects - molecular, systemic, cognitive, computational and sensorimotor. The greatest changes in the nervous system take place in the process of its maturation in the period of psychomotor development, as well as during neurorehabilitation, the task of which is to rebuild damaged neuronal pathways, e.g. by facilitating movement or training cognitive functions. Certain genetic polymorphisms affect the effectiveness of the processes of reconstruction or restoration of neural structures, which is clearly reflected in the effects of neurorehabilitation. This review presents the perspectives for the application of neurogenetic research in programming neurorehabilitation by determining the relationship of as many as 16 different genetic polymorphisms with specific functions of importance in rehabilitation. Thanks to this broad view, it may be possible to predict the effectiveness of rehabilitation on the basis of genetic testing, which would significantly contribute to the development of personalized medicine and to the optimal management of medical services in healthcare systems.

PPARs and Tumor Microenvironment: The Emerging Roles of the Metabolic Master Regulators in Tumor Stromal-Epithelial Crosstalk and Carcinogenesis

Peroxisome proliferator-activated receptors (PPARs) have been extensively studied for more than three decades. Consisting of three isotypes, PPARα, γ, and β/δ, these nuclear receptors are regarded as the master metabolic regulators which govern many aspects of the body energy homeostasis and cell fate. Their roles in malignancy are also increasingly recognized. With the growing interest in crosstalk between tumor stroma and epithelium, this review aims to highlight the current knowledge on the implications of PPARs in the tumor microenvironment. PPARγ plays a crucial role in the metabolic reprogramming of cancer-associated fibroblasts and adipocytes, coercing the two stromal cells to become substrate donors for cancer growth. Fibroblast PPARβ/δ can modify the risk of tumor initiation and cancer susceptibility. In endothelial cells, PPARβ/δ and PPARα are pro- and anti-angiogenic, respectively. Although the angiogenic role of PPARγ remains ambiguous, it is a crucial regulator in autocrine and paracrine signaling of cancer-associated fibroblasts and tumor-associated macrophages/immune cells. Of note, angiopoietin-like 4 (ANGPTL4), a secretory protein encoded by a target gene of PPARs, triggers critical oncogenic processes such as inflammatory signaling, extracellular matrix derangement, anoikis resistance and metastasis, making it a potential drug target for cancer treatment. To conclude, PPARs in the tumor microenvironment exhibit oncogenic activities which are highly controversial and dependent on many factors such as stromal cell types, cancer types, and oncogenesis stages. Thus, the success of PPAR-based anticancer treatment potentially relies on innovative strategies to modulate PPAR activity in a cell type-specific manner.

GSK3787-Loaded Poly(Ester Amide) Particles for Intra-Articular Drug Delivery

Osteoarthritis (OA) is a debilitating joint disorder affecting more than 240 million people. There is no disease modifying therapeutic, and drugs that are used to alleviate OA symptoms result in side effects. Recent research indicates that inhibition of peroxisome proliferator-activated receptor δ (PPARδ) in cartilage may attenuate the development or progression of OA. PPARδ antagonists such as GSK3787 exist, but would benefit from delivery to joints to avoid side effects. Described here is the loading of GSK3787 into poly(ester amide) (PEA) particles. The particles contained 8 wt.% drug and had mean diameters of about 600 nm. Differential scanning calorimetry indicated the drug was in crystalline domains in the particles. Atomic force microscopy was used to measure the Young's moduli of individual particles as 2.8 MPa. In vitro drug release studies showed 11% GSK3787 was released over 30 days. Studies in immature murine articular cartilage (IMAC) cells indicated low toxicity from the drug, empty particles, and drug-loaded particles and that the particles were not taken up by the cells. Ex vivo studies on murine joints showed that the particles could be injected into the joint space and resided there for at least 7 days. Overall, these results indicate that GSK3787-loaded PEA particles warrant further investigation as a delivery system for potential OA therapy.

The Impact of PPARD and PPARG Polymorphisms on Glioma Risk and Prognosis

Recent studies showed that peroxisome proliferator-activated receptors (PPARs) had effects on the progression of multiple tumors, but the role of PPARD and PPARG in glioma remains poorly understand. We conducted a case-control study to investigate the association of polymorphisms in PPARD and PPARG with glioma risk and prognosis in the Chinese Han population. Seven polymorphisms (PPARD: rs2016520, rs67056409, rs1053049 and rs2206030; PPARG: rs2920503, rs4073770 and rs1151988) were genotyped using the Agena MassARRAY system in 568 glioma patients and 509 healthy controls. The odd ratios (OR) and 95% confidence interval (CI) were calculated to assess the association of PPARD and PPARG polymorphisms with glioma risk. The Multifactor dimensionality reduction (MDR) method was used to analysis interactions of genetic polymorphisms on glioma risk. Then, we conducted log-rank test, Kaplan-Meier analysis and Cox regression model to evaluate the relationship of PPARD and PPARG polymorphisms with glioma prognosis. We found PPARD polymorphisms (rs2016520, rs67056409, rs1053049) were significantly associated with glioma risk in multiple models (P < 0.05). Stratified analysis showed rs2016520, rs67056409, rs1053049 of PPARD significantly decreased risk of glioma in the subgroup of age > 40 and astrocytoma (P < 0.05). For male, PPARD rs1053049 had a strong relationship with glioma risk in allele (P = 0.041), dominant (P = 0.040) and additive (P = 0.040) models. The effect of PPARG rs2920503 on glioma risk was related to glioma grade (P < 0.05). MDR showed that a seven-locus model was the best polymorphisms interaction pattern. Moreover, surgery and chemotherapy had strongly impact on overall survival and progression free survival of glioma patients. Our findings suggested that PPARD and PPARG polymorphisms were associated with glioma risk and prognosis in the Chinese Han population, and further studies are need to confirm our results.

PPARβ/δ: Linking Metabolism to Regeneration

In contrast to the general belief that regeneration is a rare event, mainly occurring in simple organisms, the ability of regeneration is widely distributed in the animal kingdom. Yet, the efficiency and extent of regeneration varies greatly. Humans can recover from blood loss as well as damage to tissues like bone and liver. Yet damage to the heart and brain cannot be reversed, resulting in scaring. Thus, there is a great interest in understanding the molecular mechanisms of naturally occurring regeneration and to apply this knowledge to repair human organs. During regeneration, injury-activated immune cells induce wound healing, extracellular matrix remodeling, migration, dedifferentiation and/or proliferation with subsequent differentiation of somatic or stem cells. An anti-inflammatory response stops the regenerative process, which ends with tissue remodeling to achieve the original functional state. Notably, many of these processes are associated with enhanced glycolysis. Therefore, peroxisome proliferator-activated receptor (PPAR) β/δ—which is known to be involved for example in lipid catabolism, glucose homeostasis, inflammation, survival, proliferation, differentiation, as well as mammalian regeneration of the skin, bone and liver—appears to be a promising target to promote mammalian regeneration. This review summarizes our current knowledge of PPARβ/δ in processes associated with wound healing and regeneration.

Selective deletion of PPARβ/δ in fibroblasts causes dermal fibrosis by attenuated LRG1 expression

Connective tissue diseases of the skin are characterized by excessive collagen deposition in the skin and internal organs. Fibroblasts play a pivotal role in the clinical presentation of these conditions. Nuclear receptor peroxisome-proliferator activated receptors (PPARs) are therapeutic targets for dermal fibrosis, but the contribution of the different PPAR subtypes are poorly understood. Particularly, the role of fibroblast PPARβ/δ in dermal fibrosis has not been elucidated. Thus, we generated a mouse strain with selective deletion of PPARβ/δ in the fibroblast (FSPCre-Pparb/d-/-) and interrogated its epidermal and dermal transcriptome profiles. We uncovered a downregulated gene, leucine-rich alpha-2-glycoprotein-1 (Lrg1), of previously unknown function in skin development and architecture. Our findings suggest that the regulation of Lrg1 by PPARβ/δ in fibroblasts is an important signaling conduit integrating PPARβ/δ and TGFβ1-signaling networks in skin health and disease. Thus, the FSPCre-Pparb/d-/- mouse model could serve as a novel tool in the current gunnery of animal models to better understand dermal fibrosis.

PPARs: Key Regulators of Airway Inflammation and Potential Therapeutic Targets in Asthma

Asthma affects approximately 300 million people worldwide, significantly impacting quality of life and healthcare costs. While current therapies are effective in controlling many patients' symptoms, a large number continue to experience exacerbations or treatment-related adverse effects. Alternative therapies are thus urgently needed. Accumulating evidence has shown that the peroxisome proliferator-activated receptor (PPAR) family of nuclear hormone receptors, comprising PPARα, PPARβ/δ, and PPARγ, is involved in asthma pathogenesis and that ligand-induced activation of these receptors suppresses asthma pathology. PPAR agonists exert their anti-inflammatory effects primarily by suppressing pro-inflammatory mediators and antagonizing the pro-inflammatory functions of various cell types relevant to asthma pathophysiology. Experimental findings strongly support the potential clinical benefits of PPAR agonists in the treatment of asthma. We review current literature, highlighting PPARs' key role in asthma pathogenesis and their agonists' therapeutic potential. With additional research and rigorous clinical studies, PPARs may become attractive therapeutic targets in this disease.

4-Hydroxybenzaldehyde accelerates acute wound healing through activation of focal adhesion signalling in keratinocytes

4-Hydroxybenzaldehyde (4-HBA) is a naturally occurring benzaldehyde and the major active constituent of Gastrodia elata. While recent studies have demonstrated metabolic effects of 4-HBA, little is known about the physiological role of 4-HBA in acute wound healing. Here, we investigated the effects and mechanisms of 4-HBA on acute wound healing. Using an in vitro approach, we found that 4-HBA significantly promoted keratinocyte cell migration and invasion by increasing focal adhesion kinase and Src activity. In addition, 4-HBA treatment also promoted wound healing and re-epithelialization in an in vivo excision wound animal model. Combination treatment with 4-HBA and platelet-derived growth factor subunit B homodimer showed synergistic effects in promoting wound healing. Taken together, our results demonstrated that treatment with 4-HBA promoted keratinocyte migration and wound healing in mouse skin through the Src/mitogen-activated protein kinase pathway. Therefore, 4-HBA could be a candidate therapeutic agent with the potential to promote acute wound healing.

PPARδ as a Metabolic Initiator of Mammary Neoplasia and Immune Tolerance

PPARδ is a ligand-activated nuclear receptor that regulates the transcription of genes associated with proliferation, metabolism, inflammation, and immunity. Within this transcription factor family, PPARδ is unique in that it initiates oncogenesis in a metabolic and tissue-specific context, especially in mammary epithelium, and can regulate autoimmunity in some tissues. This review discusses its role in these processes and how it ultimately impacts breast cancer.

Non-classical Transcriptional Activity of Retinoic Acid

It has long been established that the transcriptional activity of retinoic acid (RA) is mediated by members of the nuclear receptor family of ligand-activated transcription factors termed RA receptors (RARs). More recent observations have established that RA also activates an additional nuclear receptor, PPARβ/δ. Partitioning RA between RARs and PPARβ/δ is governed by different intracellular lipid-binding proteins: cellular RA binding protein 2 (CRABP2) delivers RA to nuclear RARs and a fatty acid binding protein (FABP5) delivers the hormone from the cytosol to nuclear PPARβ/δ. Consequently, RA signals through RARs in CRABP2-expressing cells, but activates PPARβ/δ in cells that express a high level of FABP5. RA elicits different and sometimes opposing responses in cells that express different FABP5/CRABP2 ratios because PPARβ/δ and RARs regulate the expression of distinct sets of genes. An overview of the observations that led to the discovery of this non-classical activity of RA are presented here, along with a discussion of evidence demonstrating the involvement of the dual transcriptional activities of RA in regulating energy homeostasis, insulin responses, and adipocyte and neuron differentiation.

Integrative and systemic approaches for evaluating PPARβ/δ (PPARD) function

The peroxisome proliferator-activated receptors (PPARs) are a group of nuclear receptors that function as transcription factors regulating the expression of genes involved in cellular differentiation, development, metabolism and also tumorigenesis. Three PPAR isotypes (α, β/δ and γ) have been identified, among which PPARβ/δ is the most difficult to functionally examine due to its tissue-specific diversity in cell fate determination, energy metabolism and housekeeping activities. PPARβ/δ acts both in a ligand-dependent and -independent manner. The specific type of regulation, activation or repression, is determined by many factors, among which the type of ligand, the presence/absence of PPARβ/δ-interacting corepressor or coactivator complexes and PPARβ/δ protein post-translational modifications play major roles. Recently, new global approaches to the study of nuclear receptors have made it possible to evaluate their molecular activity in a more systemic fashion, rather than deeply digging into a single pathway/function. This systemic approach is ideally suited for studying PPARβ/δ, due to its ubiquitous expression in various organs and its overlapping and tissue-specific transcriptomic signatures. The aim of the present review is to present in detail the diversity of PPARβ/δ function, focusing on the different information gained at the systemic level, and describing the global and unbiased approaches that combine a systems view with molecular understanding.

Glycyrrhizin Attenuates Proinflammatory Cytokines through a Peroxisome Proliferator-Activated Receptor-γ-Dependent Mechanism and Experimental Vasospasm in a Rat Model

The peroxisome proliferator-activated receptor (PPAR) is downregulated in the cortex of experimental subarachnoid hemorrhage (SAH) animals. This study is to examine the effect of glycyrrhizin on the alternation of PPARs and proinflammatory cytokines in a rodent SAH model. CSF cytokines were evaluated by RT-PCR. Basilar arteries (BAs) were harvested to examine PPARs (RT-PCR and Western blot), and a morphological examination was conducted. Deformed endothelium and tortuous elastic lamina were observed in the BAs of the SAH groups, but they were absent in the glycyrrhizin groups or the healthy controls. The PPAR-γ and -δ protein levels were reduced in the SAH groups (p < 0.01). Glycyrrhizin significantly increased the expressed PPAR-γ protein and mRNA (preconditioning) and PPAR-δ mRNA (both treatment and preconditioning), which corresponded to the reduced IL-1β and TNF-α levels. The administration of a PPAR-γ inhibitor, BADGE, halted the reduction of IL-1β and TNF-α in the glycyrrhizin groups. Conclusively, glycyrrhizin exerts anti-inflammatory effects on SAH-induced vasospasm and attenuates the expression of PPARs, especially PPAR-γ, which corresponds to the severity of SAH-related inflammation. These findings also offer credit to the antivasospastic effect of glycyrrhizin and its vasculoprotective effect in animals subjected to SAH.

PPARD rs2016520 polymorphism and circulating lipid levels connect with brain diseases in Han Chinese and suggest sex-dependent effects

The PPARD polymorphisms were shown to be associated with circulating lipoprotein metabolism in various diseases. We aimed to check the contribution of PPARD rs2016520 and lipid concentration to the risk of intracerebral hemorrhages (ICH) and brain tumors (BT) in Han Chinese. A total of 864 participants were included in the case-control study. The melting temperature shift (Tm-shift) method was used for rs2016520 genotyping. Under the recessive model, PPARD rs2016520 was shown to be associated with the risk of ICH (P=0.029, odds ratio (OR)=2.72), specifically in males (P=0.045, OR=3.98). Additionally, we also found that the levels of TC and LDL-C were significantly higher in participants with brain diseases than in the controls (TC: P<0.0001; LDL-C: P<0.0001). Significantly higher HDL-C and lower ApoA-I levels were observed in the male patients with brain diseases (HDL-C: P<0.0001; ApoA-I: P=0.008), in contrast of a higher TG level in female ICH (P=0.023). Subsequent interaction analysis between PPARD rs2016520 and lipoprotein metabolism showed that the LDL-C level was positively correlated with ICH in the rs2016520-AA carriers (P<0.0001), but not in the other genotype carriers (AG or GG, P=0.300). Our results showed that PPARD rs2016520 displayed a strong relationship with ICH risk in the male Han Chinese. The TC and LDL-C levels were positively higher in the patients with brain diseases than in the controls. The levels of TG, HDL-C and ApoA-I were shown to affect brain disease in a gender-dependent model. The genotype rs2016520-AA showed significant interaction with the circulating LDL-C levels in ICH.

Peroxisome proliferator activator receptor gamma (PPARG) regulates conceptus elongation in sheep

The ovine blastocyst hatches from the zona pellucida by Day 8 and develops into an ovoid or tubular conceptus (embryo and associated extraembryonic membranes) that grows and elongates into a filamentous form between Days 12 and 16. The trophectoderm of the elongating conceptus synthesizes and secretes interferon tau (IFNT) as well as prostaglandins (PGs) via prostaglandin synthase two (PTGS2). Intrauterine infusion of a PTGS2 inhibitor prevents conceptus elongation in sheep. Although many PGs are secreted, PGI2 and PGJ2 can activate nuclear peroxisome proliferator activator receptors (PPARs) that heterodimerize with retinoic X receptors (RXRs) to regulate gene expression and cellular function. Expression of PPARD, PPARG, RXRA, RXRB, and RXRG is detected in the elongating ovine conceptus, and nuclear PPARD and PPARG are present in the trophectoderm. Consequently, PPARD and PPARG are hypothesized to have essential roles in conceptus elongation in ruminants. In utero loss-of-function studies of PPARD and PPARG in the ovine conceptus trophectoderm were conducted using morpholino antisense oligonucleotides (MAOs) that inhibit mRNA translation. Elongating, filamentous-type conceptuses were recovered from ewes infused with a control morpholino or PPARD MAO. In contrast, PPARG MAO resulted in severely growth-retarded conceptuses or conceptus fragments with apoptotic trophectoderm. In order to identify PPARG-regulated genes, PPARG chromatin immunoprecipitation sequencing and RNA sequencing were conducted using Day 14 ovine conceptuses. These analyses revealed candidate PPARG-regulated genes involved in biological pathways, including lipid and glucose uptake, transport, and metabolism. Collectively, results support the hypothesis that PTGS2-derived PGs and PPARG are essential regulators of conceptus elongation, with specific roles in trophectoderm survival and proliferation.

Promising role of ANGPTL4 gene in diabetic wound healing

Diabetes mellitus (DM) is one of the severe metabolic disorders of carbohydrate metabolism worldwide. Developing countries are at higher risk of DM, and there is significant evidence that it is epidemic in many economically developing and newly industrialized countries. Among all other complications associated with DM, delayed wound healing is a major concern in diabetic patients. Wound healing is a natural healing process that starts immediately after injury. This involves interaction of a complex cascade of cellular events that generates resurfacing, reconstitution, and restoration of the tensile strength of injured skin. There are multiple factors responsible for delayed wound healing among which the contribution of DM has been well documented. The wound healing process is also delayed by the metabolic, vascular, neurological, and inflammatory alterations, which are well known in both type 1 and type 2 diabetes. Keratinocytes are crucial for wound re-epithelialization, and defects in directed migration of keratinocytes due to DM are associated with the delayed wound healing process. Many factors responsible for re-epithelialization have been identified, characterized, and well described; however, the genes responsible for the healing process have only partially been illustrated. This article will therefore focus on the efficacy of ANGPTL4 (angiopoietin-like 4) gene, which plays a novel role in keratinocyte migration during wound healing.

Conceptus elongation in ruminants: roles of progesterone, prostaglandin, interferon tau and cortisol

The majority of pregnancy loss in ruminants occurs during the first three weeks after conception, particularly during the period of conceptus elongation that occurs prior to pregnancy recognition and implantation. This review integrates established and new information on the biological role of ovarian progesterone (P4), prostaglandins (PGs), interferon tau (IFNT) and cortisol in endometrial function and conceptus elongation. Progesterone is secreted by the ovarian corpus luteum (CL) and is the unequivocal hormone of pregnancy. Prostaglandins (PGs) and cortisol are produced by both the epithelial cells of the endometrium and the trophectoderm of the elongating conceptus. In contrast, IFNT is produced solely by the conceptus trophectoderm and is the maternal recognition of pregnancy signal that inhibits production of luteolytic pulses of PGF_2α by the endometrium to maintain the CL and thus production of P4. Available results in sheep support the idea that the individual, interactive, and coordinated actions of P4, PGs, IFNT and cortisol regulate conceptus elongation and implantation by controlling expression of genes in the endometrium and/or trophectoderm. An increased knowledge of conceptus-endometrial interactions during early pregnancy in ruminants is necessary to understand and elucidate the causes of infertility and recurrent early pregnancy loss and provide new strategies to improve fertility and thus reproductive efficiency.

Polymorphisms of peroxisome proliferator-activated receptors and survival of lung cancer and upper aero-digestive tract cancers

BACKGROUND

Peroxisome proliferator-activated receptors (PPARs) are transcriptional factors involved in several biological processes such as inflammation, cancer growth, progression and apoptosis that are important in lung and upper aero-digestive tract (UADT) cancer outcomes. Nonetheless, there are no published studies of the relationship between PPARs gene polymorphisms and survival of patients with lung cancer or UADT cancers.

METHODS

1212 cancer patients (611 lung, 303 oral, 100 pharyngeal, 90 laryngeal, and 108 esophageal) were followed for a median duration of 11 years. We genotyped three potentially functional single nucleotide polymorphisms (SNPs) using Taqman - rs3734254 of the gene PPARD and rs10865710 and rs1801282 of the gene PPARG - and investigated their associations with lung and UADT cancer survival using Cox regression. A semi-Bayesian shrinkage approach was used to reduce the potential for false positive findings when examining multiple associations.

RESULTS

The variant homozygote CC (vs. TT) of PPARD rs3734254 was inversely associated with mortality of both lung cancer (adjusted hazard ratio [aHR]=0.63, 95% confidence interval [CI]=0.42, 0.96) and UADT cancers (aHR=0.51, 95% CI=0.27, 0.99). Use of the semi-Bayesian shrinkage approach yielded a posterior aHR for lung cancer of 0.66 (95% posterior limits=0.44, 0.98) and a posterior aHR for UADT cancers of 0.58 (95% posterior limits=0.33, 1.03).

CONCLUSION

Our findings suggest that lung-cancer patients with the CC variant of PPARD rs3734254 may have a survival advantage over lung-cancer patients with other gene variants.

Src is activated by the nuclear receptor peroxisome proliferator-activated receptor β/δ in ultraviolet radiation-induced skin cancer

Although non-melanoma skin cancer (NMSC) is the most common human cancer and its incidence continues to rise worldwide, the mechanisms underlying its development remain incompletely understood. Here, we unveil a cascade of events involving peroxisome proliferator-activated receptor (PPAR) β/δ and the oncogene Src, which promotes the development of ultraviolet (UV)-induced skin cancer in mice. UV-induced PPARβ/δ activity, which directly stimulated Src expression, increased Src kinase activity and enhanced the EGFR/Erk1/2 signalling pathway, resulting in increased epithelial-to-mesenchymal transition (EMT) marker expression. Consistent with these observations, PPARβ/δ-null mice developed fewer and smaller skin tumours, and a PPARβ/δ antagonist prevented UV-dependent Src stimulation. Furthermore, the expression of PPARβ/δ positively correlated with the expression of SRC and EMT markers in human skin squamous cell carcinoma (SCC), and critically, linear models applied to several human epithelial cancers revealed an interaction between PPARβ/δ and SRC and TGFβ1 transcriptional levels. Taken together, these observations motivate the future evaluation of PPARβ/δ modulators to attenuate the development of several epithelial cancers.

PPAR-alpha and PPAR-beta expression changes in the hippocampus of rats undergoing global cerebral ischemia/reperfusion due to PPAR-gamma status

Background

Peroxisome proliferator-activated receptors (PPARs, including alpha, beta and gamma subtypes) and their agonists have a protective role in treatment of central nervous system (CNS) diseases. The present study was designed to investigate the expression changes of PPAR-alpha, -beta, -gamma and NF-kappa B in the hippocampus of rats with global cerebral ischemia/reperfusion injury (GCIRI) after treatment with agonists or antagonists of PPAR-gamma.

Methods

A rat GCIRI model was established by occlusion of bilateral common carotid arteries and cervical vena retransfusion. GW9662 (5 μg), a selective PPAR- gamma antagonist, was intraventricularly injected at 0.5 h before GCIR; Rosiglitazone (0.8, 2.4 and 7.2 mg/kg), a selective PPAR- gamma agonist, was injected intraperitoneally at 1 h before GCIRI. The expression changes of PPAR-alpha, -beta and -gamma at mRNA and protein levels were detected by RT-PCR and western blotting. The changes of spatial learning and memory (SLM) functions were assessed by using a Morris water maze; the pathohistological changes of hippocampal neurons were evaluated by hematoxylin-eosin (HE) staining; the contents of IL-1, IL-6, IL-10 and TNF-alpha, and the NF- kappa B expression were measured by enzyme-linked immunosorbent assay (ELISA) and immunohistochemical staining. The superoxide dismutase (SOD) activity and malondialdehyde (MDA) content were also detected.

Results

The SLM function and hippocampal neurons were significantly impaired after the occurrence of GCIRI. The MDA, IL-1, IL-6, IL-10, TNF-alpha content and expression of PPARs increased significantly, but the SOD activity and NF-kappa B expression were weakened in the hippocampus. Rosiglitazone treatment significantly protected rats from SLM function impairment and neuron death, and resulted in higher expressions of SOD activity and NF-kappa B, but lower contents of MDA and inflammatory factors. After treatment with rosiglitazone or GW9662, no significant change in PPAR-alpha or -beta expression was detected.

Conclusions

Rosiglitazone, a PPAR-gamma agonist, plays a protective role in hippocampal neuron damage of GCIRI rats by inhibiting the oxidative stress response and inflammation. The activation or antagonism of PPAR-gamma did not affect the expression of PPAR-alpha or -beta, indicating that the three subtypes of PPARs act in independent pathways in the CNS.

Tcf3 promotes cell migration and wound repair through regulation of lipocalin 2

Cell migration is an integral part of re-epithelialization during skin wound healing, a complex process involving molecular controls that are still largely unknown. Here we identify a novel role for Tcf3, an essential transcription factor regulating embryonic and adult skin stem cell functions, as a key effector of epidermal wound repair. We show that Tcf3 is upregulated in skin wounds and that Tcf3 overexpression accelerates keratinocyte migration and skin wound healing. We also identify Stat3 as an upstream regulator of Tcf3. We show that the pro-migration effects of Tcf3 are non-cell autonomous and occur independently of its ability to interact with β-catenin. Finally, we identify lipocalin-2 as the key secreted factor downstream of Tcf3 that promotes cell migration in vitro and wound healing in vivo. Our findings provide new insights into the molecular controls of wound-associated cell migration and identify potential therapeutic targets for the treatment of defective wound repair.

PPARβ interprets a chromatin signature of pluripotency to promote embryonic differentiation at gastrulation

Epigenetic post-transcriptional modifications of histone tails are thought to help in coordinating gene expression during development. An epigenetic signature is set in pluripotent cells and interpreted later at the onset of differentiation. In pluripotent cells, epigenetic marks normally associated with active genes (H3K4me3) and with silent genes (H3K27me3) atypically co-occupy chromatin regions surrounding the promoters of important developmental genes. However, it is unclear how these epigenetic marks are recognized when cell differentiation starts and what precise role they play. Here, we report the essential role of the nuclear receptor peroxisome proliferator-activated receptor β (PPARβ, NR1C2) in Xenopus laevis early development. By combining loss-of-function approaches, large throughput transcript expression analysis by the mean of RNA-seq and intensive chromatin immunoprecipitation experiments, we unveil an important cooperation between epigenetic marks and PPARβ. During Xenopus laevis gastrulation PPARβ recognizes H3K27me3 marks that have been deposited earlier at the pluripotent stage to activate early differentiation genes. Thus, PPARβis the first identified transcription factor that interprets an epigenetic signature of pluripotency, in vivo, during embryonic development. This work paves the way for a better mechanistic understanding of how the activation of hundreds of genes is coordinated during early development.

Genetic ablation of the fatty acid-binding protein FABP5 suppresses HER2-induced mammary tumorigenesis

The fatty acid-binding protein FABP5 shuttles ligands from the cytosol to the nuclear receptor PPARβ/δ (encoded for by Pparδ), thereby enhancing the transcriptional activity of the receptor. This FABP5/PPARδ pathway is critical for induction of proliferation of breast carcinoma cells by activated epidermal growth factor receptor (EGFR). In this study, we show that FABP5 is highly upregulated in human breast cancers and we provide genetic evidence of the pathophysiologic significance of FABP5 in mammary tumorigenesis. Ectopic expression of FABP5 was found to be oncogenic in 3T3 fibroblasts where it augmented the ability of PPARδ to enhance cell proliferation, migration, and invasion. To determine whether FABP5 is essential for EGFR-induced mammary tumor growth, we interbred FABP5-null mice with MMTV-ErbB2/HER2 oncomice, which spontaneously develop mammary tumors. FABP5 ablation relieved activation of EGFR downstream effector signals, decreased expression of PPARδ target genes that drive cell proliferation, and suppressed mammary tumor development. Our findings establish that FABP5 is critical for mammary tumor development, rationalizing the development of FABP5 inhibitors as novel anticarcinogenic drugs.

Nuclear control of the inflammatory response in mammals by peroxisome proliferator-activated receptors

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that play pivotal roles in the regulation of a very large number of biological processes including inflammation. Using specific examples, this paper focuses on the interplay between PPARs and innate immunity/inflammation and, when possible, compares it among species. We focus on recent discoveries establishing how inflammation and PPARs interact in the context of obesity-induced inflammation and type 2 diabetes, mostly in mouse and humans. We illustrate that PPAR γ ability to alleviate obesity-associated inflammation raises an interesting pharmacologic potential. In the light of recent findings, the protective role of PPAR α and PPAR β / δ against the hepatic inflammatory response is also addressed. While PPARs agonists are well-established agents that can treat numerous inflammatory issues in rodents and humans, surprisingly very little has been described in other species. We therefore also review the implication of PPARs in inflammatory bowel disease; acute-phase response; and central, cardiac, and endothelial inflammation and compare it along different species (mainly mouse, rat, human, and pig). In the light of the data available in the literature, there is no doubt that more studies concerning the impact of PPAR ligands in livestock should be undertaken because it may finally raise unconsidered health and sanitary benefits.

Inverse PPARβ/δ agonists suppress oncogenic signaling to the ANGPTL4 gene and inhibit cancer cell invasion

Besides its established functions in intermediary metabolism and developmental processes, the nuclear receptor peroxisome proliferator-activated receptor β/δ (PPARβ/δ) has a less defined role in tumorigenesis. In the present study, we have identified a function for PPARβ/δ in cancer cell invasion. We show that two structurally divergent inhibitory ligands for PPARβ/δ, the inverse agonists ST247 and DG172, strongly inhibit the serum- and transforming growth factor β (TGFβ)-induced invasion of MDA-MB-231 human breast cancer cells into a three-dimensional matrigel matrix. To elucidate the molecular basis of this finding, we performed chromatin immunoprecipitation sequencing (ChIP-Seq) and microarray analyses, which identified the gene encoding angiopoietin-like 4 (ANGPTL4) as the major transcriptional PPARβ/δ target in MDA-MB-231 cells, previously implicated in TGFβ-mediated tumor progression and metastatic dissemination. We show that the induction of ANGPTL4 by TGFβ and other oncogenic signals is strongly repressed by ST247 and DG172 in a PPARβ/δ-dependent fashion, resulting in the inhibition of ANGPTL4 secretion. This effect is attributable to these ligands' ability to induce a dominant transcriptional repressor complex at the site of transcription initiation that blocks preinitiation complex formation through an histone deacetylase-independent, non-canonical mechanism. Repression of ANGPTL4 transcription by inverse PPARβ/δ agonists is functionally linked to the inhibition of cancer cell invasion into a three-dimensional matrix, as (i) invasion of MDA-MB-231 cells is critically dependent on ANGPTL4 expression, (ii) recombinant ANGPTL4 stimulates invasion, and (iii) reverses the inhibitory effect of ST247 and DG172. These findings indicate that a PPARβ/δ-ANGPTL4 pathway is involved in the regulation of tumor cell invasion and that its pharmacological manipulation by inverse PPARβ/δ agonists is feasible.

PPARβ/δ affects pancreatic β cell mass and insulin secretion in mice

PPARβ/δ protects against obesity by reducing dyslipidemia and insulin resistance via effects in muscle, adipose tissue, and liver. However, its function in pancreas remains ill defined. To gain insight into its hypothesized role in β cell function, we specifically deleted Pparb/d in the epithelial compartment of the mouse pancreas. Mutant animals presented increased numbers of islets and, more importantly, enhanced insulin secretion, causing hyperinsulinemia. Gene expression profiling of pancreatic β cells indicated a broad repressive function of PPARβ/δ affecting the vesicular and granular compartment as well as the actin cytoskeleton. Analyses of insulin release from isolated PPARβ/δ-deficient islets revealed an accelerated second phase of glucose-stimulated insulin secretion. These effects in PPARβ/δ-deficient islets correlated with increased filamentous actin (F-actin) disassembly and an elevation in protein kinase D activity that altered Golgi organization. Taken together, these results provide evidence for a repressive role for PPARβ/δ in β cell mass and insulin exocytosis, and shed a new light on PPARβ/δ metabolic action.

New insights into the role of peroxisome proliferator-activated receptors in regulating the inflammatory response after tissue injury

Major trauma results in a strong inflammatory response in injured tissue. This posttraumatic hyperinflammation has been implied in the adverse events leading to a breakdown of host defense mechanisms and ultimately to delayed organ failure. Ligands to peroxisome proliferator-activated receptors (PPARs) have recently been identified as potent modulators of inflammation in various acute and chronic inflammatory conditions. The main mechanism of action mediated by ligand binding to PPARs is the inhibition of the nuclear transcription factor NF-κB, leading to downregulation of downstream gene transcription, such as for genes encoding proinflammatory cytokines. Pharmacological PPAR agonists exert strong anti-inflammatory properties in various animal models of tissue injury, including central nervous system trauma, ischemia/reperfusion injury, sepsis, and shock. In addition, PPAR agonists have been shown to induce wound healing process after tissue trauma. The present review was designed to provide an up-to-date overview on the current understanding of the role of PPARs in the pathophysiology of the inflammatory response after major trauma. Therapeutic options for using recombinant PPAR agonists as pharmacological agents in the management of posttraumatic inflammation will be discussed.

GW0742, a high-affinity PPAR-δ agonist, mediates protection in an organotypic model of spinal cord damage

STUDY DESIGN

Experimental study of spinal cord injury (SCI) using an organotypic slice culture.

OBJECTIVE

To clarify the protective mechanism of PPAR-δ agonist GW0742 in the injured spinal cord using an in vitro model.

SUMMARY OF BACKGROUND DATA

In vivo data suggest that ligands of the δ isoform have activity in a number of disease models that are partly driven by the inflammatory response. Moreover, reports from in vivo studies using models of ischemia reperfusion and Parkinson disease also have shown neuroprotection conferred by PPAR-δ. The biological role and function of PPAR-δ remains relatively unclear.

METHODS

Spinal cord from 6-week-old mice was cut into transverse slices of 400-μm thickness to generate the organotypic slice cultures. The slices were injured using a weight dropped onto the center of the slice. PPAR-δ agonist was applied at 10 μM at 1 hour before injury.

RESULTS

Our study shows that GW0742 incubation (10 μM) at 1 hour before transverse lesion significantly reduced (1) p38 mitogen-activated protein kinase (MAPK), (2) c-Jun N-terminal kinase (JNK/SAP kinase), (3) NF-κB activation, (4) loss of neurotrophic factors (BDNF, GDNF), (5) COX-2 expression, and (6) cell death.

CONCLUSION

GW0742 reduces the cellular and molecular changes occurring in SCI by targeting different downstream pathways modulating PPAR-δ receptors.

Functional role of PPARδ in corneal epithelial wound healing

The peroxisome proliferator-activated receptor (PPAR) δ is involved in tissue repair. In this study, we investigated the functional role of PPARδ in corneal epithelial wound healing. In an in vivo corneal wound-healing model, the changes of PPARδ expression in corneal epithelia were examined by immunofluorescence microscopy, and the effect of topical administrations of a PPARδ agonist on corneal wound healing was also evaluated. The inhibitory effect of a PPARδ agonist on the cytokine-induced death of human corneal epithelial cells was evaluated using a DNA fragmentation assay kit. The changes of PPARδ expression and epithelial cell death were also investigated using human corneoscleral tissues ex vivo. Our findings showed that PPARδ expression was temporally up-regulated in corneal epithelial cells during experimental wound healing and that topical administration of a PPARδ agonist significantly promoted the healing of experimental corneal epithelial wounds. In human corneal epithelial cells, up-regulation of PPARδ and DNA fragmentation was demonstrated by stimulation with cytokines, and the DNA fragmentation was significantly inhibited by pretreatment with a PPARδ agonist. By using human corneoscleral tissues ex vivo, PPARδ was up-regulated in both healthy corneal epithelia (during re-epithelialization) and diseased corneal epithelia. Inflammatory stimulation-induced corneal epithelial cell death was inhibited by pretreatment with a PPARδ agonist. These results strongly suggest that PPARδ is involved in the corneal epithelial wound healing.

TNIP1 is a corepressor of agonist-bound PPARs

Nuclear receptor (NR) coregulators include coactivators, contributing to holoreceptor transcriptional activity, and corepressors, mediating NR target gene silencing in the absence of hormone. We identified an atypical NR coregulator, TNFα-induced protein 3-interacting protein 1 (TNIP1), from a peroxisome proliferator activated receptor (PPAR) α screen of a human keratinocyte cDNA library. TNIP1's complex nomenclature parallels its additional function as an NF-κB inhibitor. Here we show TNIP1 is an atypical NR corepressor using two-hybrid systems, biochemical studies, and receptor activity assays. The requirements for TNIP1-PPAR interaction are characteristic for coactivators; however, TNIP1 partially decreases PPAR activity. TNIP1 has separable transcriptional activation and repression domains suggesting a modular nature to its overall effect. It may provide a means of lowering receptor activity in the presence of ligand without total loss of receptor function. TNIP1's multiple roles and expression in several cell types suggest its regulatory effect depends on its expression level and the expression of other regulators in NR and/or NF-κB signaling pathways. As a NR coregulator, TNIP1 targeting agonist-bound PPAR and reducing transcriptional activity offers control of receptor signaling not available from typical corepressors and may contribute to combinatorial regulation of transcription.

Angiopoietin-like 4 regulates epidermal differentiation

The nuclear hormone receptor PPARβ/δ is integral to efficient wound re-epithelialization and implicated in epidermal maturation. However, the mechanism underlying the latter process of epidermal differentiation remains unclear. We showed that ligand-activated PPARβ/δ indirectly stimulated keratinocyte differentiation, requiring de novo gene transcription and protein translation. Using organotypic skin cultures constructed from PPARβ/δ- and angiopoietin-like 4 (ANGPTL4)-knockdown human keratinocytes, we showed that the expression of ANGPTL4, a PPARβ/δ target gene, is essential for the receptor mediated epidermal differentiation. The pro-differentiation effect of PPARβ/δ agonist GW501516 was also abolished when keratinocytes were co-treated with PPARβ/δ antagonist GSK0660 and similarly in organotypic skin culture incubated with blocking ANGPTL4 monoclonal antibody targeted against the C-terminal fibrinogen-like domain. Our focused real-time PCR gene expression analysis comparing the skin biopsies from wildtype and ANGPTL4-knockout mice confirmed a consistent down-regulation of numerous genes involved in epidermal differentiation and proliferation in the ANGPTL4-knockout skin. We further showed that the deficiency of ANGPTL4 in human keratinocytes and mice skin have diminished expression of various protein kinase C isotypes and phosphorylated transcriptional factor activator protein-1, which are well-established for their roles in keratinocyte differentiation. Chromatin immunoprecipitation confirmed that ANGPTL4 stimulated the activation and binding of JUNB and c-JUN to the promoter region of human involucrin and transglutaminase type 1 genes, respectively. Taken together, we showed that PPARβ/δ regulates epidermal maturation via ANGPTL4-mediated signalling pathway.

Between death and survival: retinoic acid in regulation of apoptosis

The vitamin A metabolite all-trans-retinoic acid (RA) regulates multiple biological processes by virtue of its ability to regulate gene expression. It thus plays critical roles in embryonic development and is involved in regulating growth, remodeling, and metabolic responses in adult tissues. RA can also suppress carcinoma cell growth and is currently used in treatment of some cancers. Growth inhibition by RA may be exerted by induction of differentiation, cell cycle arrest, or apoptosis, or by a combination of these activities. Paradoxically, in the context of some cells, RA not only fails to inhibit growth but, instead, enhances proliferation and survival. This review focuses on the involvement of RA in regulating apoptotic responses. It includes brief overviews of transcriptional signaling by RA and of apoptotic pathways, and then addresses available information on the mechanisms by which RA induces apoptosis or, conversely, inhibits cell death and enhances survival.

Angiopoietin-like 4 interacts with matrix proteins to modulate wound healing

A dynamic cell-matrix interaction is crucial for a rapid cellular response to changes in the environment. Appropriate cell behavior in response to the changing wound environment is required for efficient wound closure. However, the way in which wound keratinocytes modify the wound environment to coordinate with such cellular responses remains less studied. We demonstrated that angiopoietin-like 4 (ANGPTL4) produced by wound keratinocytes coordinates cell-matrix communication. ANGPTL4 interacts with vitronectin and fibronectin in the wound bed, delaying their proteolytic degradation by metalloproteinases. This interaction does not interfere with integrin-matrix protein recognition and directly affects cell-matrix communication by altering the availability of intact matrix proteins. These interactions stimulate integrin- focal adhesion kinase, 14-3-3, and PKC-mediated signaling pathways essential for effective wound healing. The deficiency of ANGPTL4 in mice delays wound re-epithelialization. Further analysis revealed that cell migration was impaired in the ANGPTL4-deficient keratinocytes. Altogether, the findings provide molecular insight into a novel control of wound healing via ANGPTL4-dependent regulation of cell-matrix communication. Given the known role of ANGPTL4 in glucose and lipid homeostasis, it is a prime therapeutic candidate for the treatment of diabetic wounds. It also underscores the importance of cell-matrix communication during angiogenesis and cancer metastasis.

Fatty acid-binding protein 5 and PPARbeta/delta are critical mediators of epidermal growth factor receptor-induced carcinoma cell growth

Epidermal growth factors and their receptors (EGFRs) promote breast cancer cell proliferation and can drive tumorigenesis. However, the molecular mechanisms that mediate these effects are incompletely understood. We previously showed that mammary tumor development in the mouse model of breast cancer MMTV-neu, a model characterized by amplification of the EGFR ErbB2 in mammary tissue, correlates with a marked up-regulation of fatty acid-binding protein 5 (FABP5). FABP5 functions to deliver ligands to and enhance the transcriptional activity of the nuclear receptor peroxisome proliferator-activated receptor beta/delta (PPARbeta/delta), a receptor whose target genes include genes involved in cell growth and survival. We show here that in MCF-7 mammary carcinoma cells, EGFR signaling directly up-regulates the expression of FABP5. The data demonstrate that treatment of these cells with the EGFR ligand heregulin-beta1 signals through the ERK and the phophatidylinositol-3-kinase cascades, resulting in activation of the transcription factor NF-kappaB. In turn, NF-kappaB induces the expression of FABP5 through two cognate response elements in the promoter of this gene. The observations further demonstrate that FABP5 and PPARbeta/delta are critical mediators of the ability of EGFR to enhance cell proliferation, indicating that this transcriptional pathway plays a key role in EGFR-induced tumorigenesis. Additional observations indicate that the expression of FABP5 is down-regulated by the Krüppel-like factor KLF2, suggesting a tumor suppressor activity for this factor.

Chronic expression of PPAR-delta by oligodendrocyte lineage cells in the injured rat spinal cord

The transcription factor peroxisome proliferator-activated receptor (PPAR)-delta promotes oligodendrocyte differentiation and myelin formation in vitro and is prevalent throughout the brain and spinal cord. Its expression after injury, however, has not been examined. Thus, we used a spinal contusion model to examine the spatiotemporal expression of PPAR-delta in naïve and injured spinal cords from adult rats. As previously reported, PPAR-delta was expressed by neurons and oligodendrocytes in uninjured spinal cords; PPAR-delta was also detected in NG2 cells (potential oligodendrocyte progenitors) within the white matter and gray matter. After spinal cord injury (SCI), PPAR-delta mRNA and protein were present early and increased over time. Overall PPAR-delta+ cell numbers declined at 1 day post injury (dpi), likely reflecting neuron loss, and then rose through 14 dpi. A large proportion of NG2 cells expressed PPAR-delta after SCI, especially along lesion borders. PPAR-delta+ NG2 cell numbers were significantly higher than naive by 7 dpi and remained elevated through at least 28 dpi. PPAR-delta+ oligodendrocyte numbers declined at 1 dpi and then increased over time such that >20% of oligodendrocytes expressed PPAR-delta after SCI compared with approximately 10% in uninjured tissue. The most prominent increase in PPAR-delta+ oligodendrocytes was along lesion borders where at least a portion of newly generated oligodendrocytes (bromodeoxyuridine+) were PPAR-delta+. Consistent with its role in cellular differentiation, the early rise in PPAR-delta+ NG2 cells followed by an increase in new PPAR-delta+ oligodendrocytes suggests that this transcription factor may be involved in the robust oligodendrogenesis detected previously along SCI lesion borders.

Emerging roles of peroxisome proliferator-activated receptor-beta/delta in inflammation

Peroxisome proliferator-activated receptor (PPAR)-beta/delta is a member of the PPAR nuclear hormone receptor family. The PPARs are a family of 3 ligand-activated transcription factors: PPARalpha (NR1C1), PPARbeta/delta (NR1C2), and PPARgamma (NR1C3). All the PPARs play important roles in the regulation of metabolic pathways, including those of lipid of biosynthesis and glucose metabolism, as well as in a variety of cell differentiation, proliferation, and apoptosis pathways. Recently, there has been a great deal of interest in the involvement of PPARs in the inflammatory processes. In particular, PPARalpha and PPARgamma inhibit the activation of inflammatory gene expression and can negatively interfere with pro-inflammatory transcription factor signalling pathways in vascular and inflammatory cells. In contrast, the roles of PPARbeta/delta regulating inflammation and immunity are only just emerging. This review will focus on these emerging roles of PPARbeta/delta in regulating inflammatory processes.

PPARdelta promotes wound healing by up-regulating TGF-beta1-dependent or -independent expression of extracellular matrix proteins

Although the peroxisome proliferator-activated receptor (PPAR) δ has been implicated in the wound healing process, its exact role and mechanism of action have not been fully elucidated. Our previous findings showed that PPARδ induces the expression of the transforming growth factor (TGF)-β1, which has been implicated in the deposit of extracellular matrix proteins. Here, we demonstrate that administration of GW501516, a specific PPARδ ligand, significantly promoted wound closure in the experimental mouse and had a profound effect on the expression of collagen types I and III, alpha-smooth muscle actin, pSmad3 and TGF-β1, which play a pivotal role in wound healing processes. Activation of PPARδ increased migration of human epidermal keratinocytes and dermal fibroblasts in in vitro scrape-wounding assays. Addition of a specific ALK5 receptor inhibitor SB431542 significantly suppressed GW501516-induced migration of human keratinocytes and fibroblasts. In these cells, activated PPARδ also induced the expression of collagen types I and III and fibronectin in a TGF-β1-dependent or -independent manner. The effect of PPARδ on the expression of type III collagen was dually regulated by the direct binding of PPARδ and Smad3 to a direct repeat-1 site and a Smad-binding element, respectively, of the type III gene promoter. Taken together, these results demonstrated that PPARδ plays an important role in skin wound healing in vivo and that it functions by accelerating extracellular matrix-mediated cellular interactions in a process mediated by the TGF-β1/Smad3 signaling-dependent or - independent pathway.

Regulation of cell proliferation and migration by TAK1 via transcriptional control of von Hippel-Lindau tumor suppressor

Skin maintenance and healing after wounding requires complex epithelial-mesenchymal interactions purportedly mediated by growth factors and cytokines. We show here that, for wound healing, transforming growth factor-beta-activated kinase 1 (TAK1) in keratinocytes activates von Hippel-Lindau tumor suppressor expression, which in turn represses the expression of platelet-derived growth factor-B (PDGF-B), integrin beta1, and integrin beta5 via inhibition of the Sp1-mediated signaling pathway in the keratinocytes. The reduced production of PDGF-B leads to a paracrine-decreased expression of hepatocyte growth factor in the underlying fibroblasts. This TAK1 regulation of the double paracrine PDGF/hepatocyte growth factor signaling can regulate keratinocyte cell proliferation and is required for proper wound healing. Strikingly, TAK1 deficiency enhances cell migration. TAK1-deficient keratinocytes displayed lamellipodia formation with distinct microspike protrusion, associated with an elevated expression of integrins beta1 and beta5 and sustained activation of cdc42, Rac1, and RhoA. Our findings provide evidence for a novel homeostatic control of keratinocyte proliferation and migration mediated via TAK1 regulation of von Hippel-Lindau tumor suppressor. Dysfunctional regulation of TAK1 may contribute to the pathology of non-healing chronic inflammatory wounds and psoriasis.

Regulation of epithelial-mesenchymal IL-1 signaling by PPARbeta/delta is essential for skin homeostasis and wound healing

Skin morphogenesis, maintenance, and healing after wounding require complex epithelial–mesenchymal interactions. In this study, we show that for skin homeostasis, interleukin-1 (IL-1) produced by keratinocytes activates peroxisome proliferator–activated receptor β/δ (PPARβ/δ) expression in underlying fibroblasts, which in turn inhibits the mitotic activity of keratinocytes via inhibition of the IL-1 signaling pathway. In fact, PPARβ/δ stimulates production of the secreted IL-1 receptor antagonist, which leads to an autocrine decrease in IL-1 signaling pathways and consequently decreases production of secreted mitogenic factors by the fibroblasts. This fibroblast PPARβ/δ regulation of the IL-1 signaling is required for proper wound healing and can regulate tumor as well as normal human keratinocyte cell proliferation. Together, these findings provide evidence for a novel homeostatic control of keratinocyte proliferation and differentiation mediated via PPARβ/δ regulation in dermal fibroblasts of IL-1 signaling. Given the ubiquitous expression of PPARβ/δ, other epithelial–mesenchymal interactions may also be regulated in a similar manner.

PPARs Mediate Lipid Signaling in Inflammation and Cancer

Lipid mediators can trigger physiological responses by activating nuclear hormone receptors, such as the peroxisome proliferator-activated receptors (PPARs). PPARs, in turn, control the expression of networks of genes encoding proteins involved in all aspects of lipid metabolism. In addition, PPARs are tumor growth modifiers, via the regulation of cancer cell apoptosis, proliferation, and differentiation, and through their action on the tumor cell environment, namely, angiogenesis, inflammation, and immune cell functions. Epidemiological studies have established that tumor progression may be exacerbated by chronic inflammation. Here, we describe the production of the lipids that act as activators of PPARs, and we review the roles of these receptors in inflammation and cancer. Finally, we consider emerging strategies for therapeutic intervention.

Peroxisome Proliferator-Activated Receptor beta/delta in the Brain: Facts and Hypothesis

peroxisome proliferator-activated receptors (PPARs) are nuclear receptors acting as lipid sensors. Besides its metabolic activity in peripheral organs, the PPAR beta/delta isotype is highly expressed in the brain and its deletion in mice induces a brain developmental defect. Nevertheless, exploration of PPARβ action in the central nervous system remains sketchy. The lipid content alteration observed in PPARβ null brains and the positive action of PPARβ agonists on oligodendrocyte differentiation, a process characterized by lipid accumulation, suggest that PPARβ acts on the fatty acids and/or cholesterol metabolisms in the brain. PPARβ could also regulate central inflammation and antioxidant mechanisms in the damaged brain. Even if not fully understood, the neuroprotective effect of PPARβ agonists highlights their potential benefit to treat various acute or chronic neurological disorders. In this perspective, we need to better understand the basic function of PPARβ in the brain. This review proposes different leads for future researches.