Orphan G protein-coupled receptor, GPR41, induces apoptosis via a p53/Bax pathway during ischemic hypoxia and reoxygenation

Gut microbial metabolites SCFAs and chronic kidney disease

The global incidence of Chronic Kidney Disease (CKD) is steadily escalating, with discernible linkage to the intricate terrain of intestinal microecology. The intestinal microbiota orchestrates a dynamic equilibrium in the organism, metabolizing dietary-derived compounds, a process which profoundly impacts human health. Among these compounds, short-chain fatty acids (SCFAs), which result from microbial metabolic processes, play a versatile role in influencing host energy homeostasis, immune function, and intermicrobial signaling, etc. SCFAs emerge as pivotal risk factors influencing CKD's development and prognosis. This paper review elucidates the impact of gut microbial metabolites, specifically SCFAs, on CKD, highlighting their role in modulating host inflammatory responses, oxidative stress, cellular autophagy, the immune milieu, and signaling cascades. An in-depth comprehension of the interplay between SCFAs and kidney disease pathogenesis may pave the way for their utilization as biomarkers for CKD progression and prognosis or as novel adjunctive therapeutic strategies.

The G Protein-Coupled Estrogen Receptor (GPER): A Critical Therapeutic Target for Cancer

Estrogens have been implicated in the pathogenesis of various cancers, with increasing concern regarding the overall rising incidence of disease and exposure to environmental estrogens. Estrogens, both endogenous and environmental, manifest their actions through intracellular and plasma membrane receptors, named ERα, ERβ, and GPER. Collectively, they act to promote a broad transcriptional response that is mediated through multiple regulatory enhancers, including estrogen response elements (EREs), serum response elements (SREs), and cyclic AMP response elements (CREs). Yet, the design and rational assignment of antiestrogen therapy for breast cancer has strictly relied upon an endogenous estrogen-ER binary rubric that does not account for environmental estrogens or GPER. New endocrine therapies have focused on the development of drugs that degrade ER via ER complex destabilization or direct enzymatic ubiquitination. However, these new approaches do not broadly treat all cancer-involved receptors, including GPER. The latter is concerning since GPER is directly associated with tumor size, distant metastases, cancer stem cell activity, and endocrine resistance, indicating the importance of targeting this receptor to achieve a more complete therapeutic response. This review focuses on the critical importance and value of GPER-targeted therapeutics as part of a more holistic approach to the treatment of estrogen-driven malignancies.

GPER: An Estrogen Receptor Key in Metastasis and Tumoral Microenvironments

Estrogens and their role in cancer are well-studied, and some cancer types are classified in terms of their response to them. In recent years, a G protein-coupled estrogen receptor (GPER) has been described with relevance in cancer. GPER is a pleiotropic receptor with tissue-specific activity; in normal tissues, its activation is related to correct development and homeostasis, while in cancer cells, it can be pro- or anti-tumorigenic. Also, GPER replaces estrogen responsiveness in estrogen receptor alpha (ERα)-lacking cancer cell lines. One of the most outstanding activities of GPER is its role in epithelial-mesenchymal transition (EMT), which is relevant for metastasis development. In addition, the presence of this receptor in tumor microenvironment cells contributes to the phenotypic plasticity required for the dissemination and maintenance of tumors. These characteristics suggest that GPER could be a promising therapeutic target for regulating cancer development. This review focuses on the role of GPER in EMT in tumorigenic and associated cells, highlighting its role in relation to the main hallmarks of cancer and possible therapeutic options.

LncRNA CEBPA-AS1 knockdown prevents neuronal apoptosis against oxygen glucose deprivation/reoxygenation by regulating the miR-455/GPER1 axis

Ischemic stroke (IS) is a common nervous system disease, which is a major cause of disability and death in the world. In present study, we demonstrated a regulatory mechanism of CCAAT/enhancer binding protein-alpha antisense 1 (CEBPA-AS1) in oxygen glucose deprivation/reoxygenation (OGD/R)-induced SH-SY5Y cells, with a focus on neuronal apoptosis. CEBPA-AS1, miR-455, and GPER1 expressions were evaluated by using qRT-PCR and Western blotting. The binding relationship among CEBPA-AS1, miR-455, and GPER1 was determined by a dual luciferase reporter assay. Neuronal viability and apoptosis were examined using MTT and flow cytometry assays, followed by determination of apoptosis-related factors (caspase 3, caspase 8, caspase 9, Bax, and Bcl-2). CEBPA-AS1 and GPER1 levels were upregulated, and miR-455 level was downregulated in the cell model of OGD/R induced. CEBPA-AS1 knockdown increased SH-SY5Y viability and reduced OGD/R-induced apoptosis. CEBPA-AS1 could act as a sponge of miR-455, and CEBPA-AS1 knockdown was found to elevate miR-455 expression. miR-455 overexpression also promoted SH-SY5Y cell viability and rescued them from OGD/R-induced apoptosis by binding to GPER1. GPER1 overexpression or miR-455 inhibition reversed the anti-apoptotic effect of CEBPA-AS1 knockdown. These findings suggest a regulatory network of CEBPA-AS1/miR-455/GPER1 that mediates neuronal cell apoptosis in the OGD model, providing a better understanding of pathogenic mechanisms after IS.

Comprehensive Analysis of ceRNA Regulation Network Involved in the Development of Coronary Artery Disease

Background

Coronary artery disease (CAD) is one of the most common causes of sudden death with high morbidity in recent years. This paper is aimed at exploring the early peripheral blood biomarkers of sudden death and providing a new perspective for clinical diagnosis and forensic pathology identification by integrated bioinformatics analysis.

Methods

Two microarray expression profiling datasets (GSE113079 and GSE31568) were downloaded from the Gene Expression Omnibus (GEO) database, and we identified differentially expressed lncRNAs, miRNAs, and mRNAs in CAD. Gene Ontology (GO) and KEGG pathway analyses of DEmRNAs were executed. A protein-protein interaction (PPI) network was constructed, and hub genes were identified. Finally, we constructed a competitive endogenous RNA (ceRNA) regulation network among lncRNAs, miRNAs, and mRNAs. Also, the 5 miRNAs of the ceRNA network were verified by RT-PCR.

Results

In total, 86 DElncRNAs, 148 DEmiRNAs, and 294 DEmRNAs were dysregulated in CAD. We received 12 GO terms and 5 pathways of DEmRNAs. 31 nodes and 78 edges were revealed in the PPI network. The top 10 genes calculated by degree method were identified as hub genes. Moreover, there were a total of 26 DElncRNAs, 5 DEmiRNAs, and 13 DEmRNAs in the ceRNA regulation network. We validated the 5 miRNAs of the ceRNA network by RT-PCR, which were consistent with the results of the microarray.

Conclusions

In this paper, a CAD-specific ceRNA network was successfully constructed, contributing to the understanding of the relationship among lncRNAs, miRNAs, and mRNAs. We identified potential peripheral blood biomarkers in CAD and provided novel insights into the development and progress of CAD.

Free Fatty Acid Receptors 2 and 3 as Microbial Metabolite Sensors to Shape Host Health: Pharmacophysiological View

The role of the gut microbiome in human health is becoming apparent. The major functional impact of the gut microbiome is transmitted through the microbial metabolites that are produced in the gut and interact with host cells either in the local gut environment or are absorbed into circulation to impact distant cells/organs. Short-chain fatty acids (SCFAs) are the major microbial metabolites that are produced in the gut through the fermentation of non-digestible fibers. SCFAs are known to function through various mechanisms, however, their signaling through free fatty acid receptors 2 and 3 (FFAR2/3; type of G-coupled protein receptors) is a new therapeutic approach. FFAR2/3 are widely expressed in diverse cell types in human and mice, and function as sensors of SCFAs to change several physiological and cellular functions. FFAR2/3 modulate neurological signaling, energy metabolism, intestinal cellular homeostasis, immune response, and hormone synthesis. FFAR2/3 function through Gi and/or Gq signaling, that is mediated through specific structural features of SCFAs-FFAR2/3 bindings and modulating specific signaling pathway. In this review, we discuss the wide-spread expression and structural homologies between human and mice FFAR2/3, and their role in different human health conditions. This information can unlock opportunities to weigh the potential of FFAR2/3 as a drug target to prevent human diseases.

Structure-Activity Relationship Studies of Tetrahydroquinolone Free Fatty Acid Receptor 3 Modulators

Free fatty acid receptor 3 (FFA3, previously GPR41) is activated by short-chain fatty acids, mediates health effects of the gut microbiota, and is a therapeutic target for metabolic and inflammatory diseases. The shortage of well-characterized tool compounds has however impeded progress. Herein, we report structure-activity relationship of an allosteric modulator series and characterization of physicochemical and pharmacokinetic properties of selected compounds, including previous and new tools. Two representatives, 57 (TUG-1907) and 63 (TUG-2015), showed improved solubility and preserved potency. Of these, 57, with EC50 = 145 nM and a solubility of 33 μM, showed high clearance in vivo but is a preferred tool in vitro. In contrast, 63, with EC50 = 162 nM and a solubility of 9 μM, showed lower clearance and seems better suited for in vivo studies. Using 57, we demonstrate for the first time that FFA3 activation leads to calcium mobilization in murine dorsal root ganglia.

GPER Overexpression in Cervical Cancer Versus Premalignant Lesions: Its Activation Induces Different Forms of Cell Death

BACKGROUND

The effect of estrogen has been traditionally studied through the modulation of its alpha and beta nuclear receptors; however, the G Protein-Coupled Estrogen Receptor (GPER) has been recently involved in the pathology of numerous tumors. Although the study of GPER in cervical cancer has begun, its contribution still remains to be completely evaluated.

OBJECTIVE

The purpose of this work was to determine the expression of this receptor in different degrees of cervical lesions and whether the stimulation with its specific agonist (G-1) modulated mechanisms of cell survival or cell death in cervical cancer cells.

METHODS

Sections of 44 formalin-fixed paraffin-embedded blocks from patients were analyzed by automated immunohistochemistry. After the stimulation with G-1, proliferation was evaluated by the xCELLigence technology, the integrity of the mitochondrial membrane permeability by MitoCaptureTM fluorescence staining, apoptosis by flow cytometry, and senescence by the senescence-associated β-galactosidase kit.

RESULTS

GPER was widely expressed in cervical cancer but not in its precursor lesions. The staining was predominantly cytoplasmic, although it was also important in the nucleus of the epithelial cells. G-1 inhibited proliferation, decreased the mitochondrial permeability, and increased the percentage of apoptosis in SiHa, HeLa, and C-33A. Only in C-33A, an increase of the cells in necrosis was observed, whereas SiHa was the only cell line in which senescence was evidenced.

CONCLUSION

GPER is a receptor associated with cervical cancer that inhibits the growth and induces different mechanisms of death in cells derived from uterine cervical cancer. It suggests that GPER can be considered a pharmacological target that prevents the development of cervical carcinogenesis.

Free fatty acid receptor 3 differentially contributes to β-cell compensation under high-fat diet and streptozotocin stress

The free fatty acid receptor 3 (FFA3) is a nutrient sensor of gut microbiota-generated nutrients, the short-chain fatty acids. Previously, we have shown that FFA3 is expressed in β-cells and inhibits islet insulin secretion ex vivo. Here, we determined the physiological relevance of the above observation by challenging wild-type (WT) and FFA3 knockout (KO) male mice with 1) hyperglycemia and monitoring insulin response via highly sensitive hyperglycemic clamps, 2) dietary high fat (HF), and 3) chemical-induced diabetes. As expected, FFA3 KO mice exhibited significantly higher insulin secretion and glucose infusion rate in hyperglycemic clamps. Predictably, under metabolic stress induced by HF-diet feeding, FFA3 KO mice exhibited less glucose intolerance compared with the WT mice. Moreover, similar islet architecture and β-cell area in HF diet-fed FFA3 KO and WT mice was observed. Upon challenge with streptozotocin (STZ), FFA3 KO mice initially exhibited a tendency for an accelerated incidence of diabetes compared with the WT mice. However, this difference was not maintained. Similar glycemia and β-cell mass loss was observed in both genotypes 10 days post-STZ challenge. Higher resistance to STZ-induced diabetes in WT mice could be due to higher basal islet autophagy. However, this difference was not protective because in response to STZ, similar autophagy induction was observed in both WT and FFA3 KO islets. These data demonstrate that FFA3 plays a role in modulating insulin secretion and β-cell response to stressors. The β-cell FFA3 and autophagy link warrant further research.

EDNRB isoform 3 confers Temozolomide resistance in A375 melanoma cells by modulating membrane potential, reactive oxygen species and mitochondrial Ca2

Background

The role of endothelin receptor type B (EDNRB) isoform 3 involved in Temozolomide (TMZ)-induced melanoma cell death has not yet been elucidated.

Methods

The subcellular localization of EDNRB isoform 3 was determined by confocal and immunoblotting assays. Silencing EDNRB isoform 3 was performed by CRISPR/Cas9. Apoptosis was assessed by annexin V/propium iodide staining and caspases 3/7/9 activity. Mitochondrial membrane potential, reactive oxygen species and mitochondrial Ca²⁺ were measured by flow cytometry. Apoptosis protein array was applied.

Results

Confocal and immunoblot analyses indicate mitochondrial localization of EDNRB isoform 3 and the first N-terminal (1–22) amino acids are sufficient for its mitochondrial targeting. EDNRB isoform 3 depleted A375 cells significantly confers chemoresistance with mitochondrial depolarization, reduced reactive oxygen species, enhanced mitochondrial Ca²⁺ uptake and decreased caspase 9 activation. Additionally, apoptosis array shows that lack of EDNRB isoform 3 has relatively lower expression of phosphorylation of p53 at S392 and a slightly higher expression of Paraoxonase 2.

Conclusion

Our findings raise the possibility of targeting EDNRB isoform 3 as a new therapeutic strategy in combination with TMZ for melanoma treatment.

Effects of Post-translational Modifications on Membrane Localization and Signaling of Prostanoid GPCR-G Protein Complexes and the Role of Hypoxia

G protein-coupled receptors (GPCRs) play a pivotal role in the adaptive responses to cellular stresses such as hypoxia. In addition to influencing cellular gene expression profiles, hypoxic microenvironments can perturb membrane protein localization, altering GPCR effector scaffolding and altering downstream signaling. Studies using proteomics approaches have revealed significant regulation of GPCR and G proteins by their state of post-translational modification. The aim of this review is to examine the effects of post-translational modifications on membrane localization and signaling of GPCR-G protein complexes, with an emphasis on vascular prostanoid receptors, and to highlight what is known about the effect of cellular hypoxia on these mechanisms. Understanding post-translational modifications of protein targets will help to define GPCR targets in treatment of disease, and to inform research into mechanisms of hypoxic cellular responses.

Gut Microbiota-Kidney Cross-Talk in Acute Kidney Injury

The recent surge in research on the intestinal microbiota has greatly changed our understanding of human biology. Significant technical advances in DNA sequencing analysis and its application to metagenomics and metatranscriptomics has profoundly enhanced our ability to quantify and track complex microbial communities and to begin understanding their impact on human health and disease. This has led to a better understanding of the relationships between the intestinal microbiome and renal physiology/pathophysiology. In this review, we discuss the interactions between intestinal microbiota and kidney. We focus on select aspects including the intestinal barrier, immunologic and soluble mediators of microbiome effects, and effects of dysbiosis on acute kidney injury. Relevant studies on microbiome changes in other renal diseases are highlighted. We also introduce potential mechanisms of intervention with regard to gut microbiota in renal diseases.

Methyl B12 protects PC12 cells against cytotoxicity induced by Aβ25-35

Alzheimer's disease (AD) is the most common aging-associated dementia. The population of AD patients is increasing as the world age grows. Currently, there is no cure for AD. Given that methyl vitamin B12 (methylcobalamin) deficiency is related to AD and Aβ-induced oxidative damage and that methylcobalamin can scavenge reactive oxygen species (ROS) by direct or indirect ways, we studied the effect of methylcobalamin on the cytotoxicity of Aβ. PC12 cells were chronically exposed (24 hours) to Aβ_25-35 (25 μM) to establish an AD cell model. The cells were pretreated with or without methylcobalamin (1-100 μM) to investigate the role of methylcobalamin. Cell viability and apoptosis were tested, followed by testing of mitochondrial damage, oxidative stress, and mitochondrial calcium concentration. We observed that methylcobalamin improved the cell viability by decreasing the ratio of apoptosis cells in this AD cell model. Further experiments suggested that methylcobalamin functioned as an antioxidant to scavenge ROS, reducing the endoplasmic reticulum-mitochondria calcium flux through IP3R, preventing mitochondria dysfunction, ultimately protecting cells against apoptosis and cell death. Taken together, our results presented, for the first time, that methyl vitamin B12 can protect cells from Aβ-induced cytotoxicity and the mechanism was mainly relevant to the antioxidative function of methyl B12.

A novel somatic mutation of SIN3A detected in breast cancer by whole-exome sequencing enhances cell proliferation through ERα expression

Breast cancer is the most frequent tumor in women, and in nearly two-thirds of cases, the tumors express estrogen receptor α (ERα, encoded by ESR1). Here, we performed whole-exome sequencing of 16 breast cancer tissues classified according to ESR1 expression and 12 samples of whole blood, and detected 310 somatic mutations in cancer tissues with high levels of ESR1 expression. Of the somatic mutations validated by a different deep sequencer, a novel nonsense somatic mutation, c.2830 C>T; p.Gln944*, in transcriptional regulator switch-independent 3 family member A (SIN3A) was detected in breast cancer of a patient. Part of the mutant protein localized in the cytoplasm in contrast to the nuclear localization of ERα, and induced a significant increase in ESR1 mRNA. The SIN3A mutation obviously enhanced MCF7 cell proliferation. In tissue sections from the breast cancer patient with the SIN3A c.2830 C>T mutation, cytoplasmic SIN3A localization was detected within the tumor regions where nuclear enlargement was observed. The reduction in SIN3A mRNA correlates with the recurrence of ER-positive breast cancers on Kaplan-Meier plots. These observations reveal that the SIN3A mutation has lost its transcriptional repression function due to its cytoplasmic localization, and that this repression may contribute to the progression of breast cancer.

A role for G-protein coupled estrogen receptor (GPER) in estrogen-induced carcinogenesis: Dysregulated glandular homeostasis, survival and metastasis

Mechanisms of carcinogenesis by estrogen center on its mitogenic and genotoxic potential on tumor target cells. These models suggest that estrogen receptor (ER) signaling promotes expansion of the transformed population and that subsequent accumulation of somatic mutations that drive cancer progression occur via metabolic activation of cathecol estrogens or by epigenetic mechanisms. Recent findings that GPER is linked to obesity, vascular pathology and immunosuppression, key events in the development of metabolic syndrome and intra-tissular estrogen synthesis, provides an alternate view of estrogen-induced carcinogenesis. Consistent with this concept, GPER is directly associated with clinicopathological indices that predict cancer progression and poor survival in breast and gynecological cancers. Moreover, GPER manifests cell biological responses and a microenvironment conducive for tumor development and cancer progression, regulating cellular responses associated with glandular homeostasis and survival, invading surrounding tissue and attracting a vascular supply. Thus, the cellular actions attributed to GPER fit well with the known molecular mechanisms of G-protein coupled receptors, GPCRs, namely, their ability to transactivate integrins and EGF receptors and alter the interaction between glandular epithelia and their extracellular environment, affecting epithelial-to-mesenchymal transition (EMT) and allowing for tumor cell survival and dissemination. This perspective reviews the molecular and cellular responses manifested by GPER and evaluates its contribution to female reproductive cancers as diseases that progress as a result of dysregulated glandular homeostasis resulting in chronic inflammation and metastasis. This review is organized in sections as follows: I) a brief synopsis of the current state of knowledge regarding estrogen-induced carcinogenesis, II) a review of evidence from clinical and animal-based studies that support a role for GPER in cancer progression, and III) a mechanistic framework describing how GPER-mediated estrogen action may influence the tumor and its microenvironment.

Free Fatty Acid Receptors and Cancer: From Nutrition to Pharmacology

The effects of fatty acids on cancer cells have been studied for decades. The roles of dietary long-chain n-3 polyunsaturated fatty acids, and of microbiome-generated short-chain butyric acid, have been of particular interest over the years. However, the roles of free fatty acid receptors (FFARs) in mediating effects of fatty acids in tumor cells have only recently been examined. In reviewing the literature, the data obtained to date indicate that the long-chain FFARs (FFA1 and FFA4) play different roles than the short-chain FFARs (FFA2 and FFA3). Moreover, FFA1 and FFA4 can in some cases mediate opposing actions in the same cell type. Another conclusion is that different types of cancer cells respond differently to FFAR activation. Currently, the best-studied models are prostate, breast, and colon cancer. FFA1 and FFA4 agonists can inhibit proliferation and migration of prostate and breast cancer cells, but enhance growth of colon cancer cells. In contrast, FFA2 activation can in some cases inhibit proliferation of colon cancer cells. Although the available data are sometimes contradictory, there are several examples in which FFAR agonists inhibit proliferation of cancer cells. This is a unique response to GPCR activation that will benefit from a mechanistic explanation as the field progresses. The development of more selective FFAR agonists and antagonists, combined with gene knockout approaches, will be important for unraveling FFAR-mediated inhibitory effects. These inhibitory actions, mediated by druggable GPCRs, hold promise for cancer prevention and/or therapy.

The membrane-type estrogen receptor G-protein-coupled estrogen receptor suppresses lipopolysaccharide-induced interleukin 6 via inhibition of nuclear factor-kappa B pathway in murine macrophage cells

The female sex hormone estrogen exerts anti-inflammatory effects. The G-protein-coupled estrogen receptor (GPER) has been recently identified as a novel membrane-type estrogen receptor that can mediate non-genomic estrogenic effects on many cell types. We previously demonstrated that GPER inhibits tumor necrosis factor alpha-induced expression of interleukin 6 (IL-6) through repression of nuclear factor-kappa B (NF-κB) promoter activity using human breast cancer cells. Although several reports have indicated that GPER suppresses Toll-like receptor-induced inflammatory cytokine expression in macrophages, the molecular mechanisms of the inhibition of cytokine production via GPER remain poorly understood. In the present study, we examined GPER-mediated inhibition of IL-6 expression induced by lipopolysaccharide (LPS) stimulation in a mouse macrophage cell line. We found that the GPER agonist G-1 inhibited LPS-induced IL-6 expression in macrophage cells, and this inhibition was due to the repression of NF-κB promoter activity by GPER. G-1 treatment also decreased the phosphorylation of inhibitor of κB kinases. Among the mitogen-activated protein kinases, the phosphorylation of c-jun N-terminal kinase (JNK) was increased by G-1. These findings delineate the novel mechanism of the inhibition of LPS-induced IL-6 through GPER-activated JNK-mediated negative regulation of the NF-κB pathway in murine macrophage cells, which links anti-inflammatory effects to estrogen.

Not lost in translation: Emerging clinical importance of the G protein-coupled estrogen receptor GPER

It has been 20years that the G protein-coupled estrogen receptor (GPER) was cloned as the orphan receptor GPR30 from multiple cellular sources, including vascular endothelial cells. Here, I will provide an overview of estrogen biology and the historical background leading to the discovery of rapid vascular estrogen signaling. I will also review the recent advances in the understanding of the mechanisms underlying GPER function, its role in physiology and disease, some of the currently available GPER-targeting drugs approved for clinical use such as SERMs (selective estrogen receptor modulators) and SERDs (selective estrogen receptor downregulators). Many of currently used drugs such as tamoxifen, raloxifene, or faslodex™/fulvestrant were discovered targeting GPER many years after they had been introduced to the clinics for entirely different purposes. This has important implications for the clinical use of these drugs and their modes of action, which I have termed 'reverse translational medicine'. In addition, environmental pollutants known as 'endocrine disruptors' have been found to bind to GPER. This article also discusses recent evidence in these areas as well as opportunities in translational clinical medicine and GPER research, including medical genetics, personalized medicine, prevention, and its theranostic use.

Estrogens and Coronary Artery Disease: New Clinical Perspectives

In premenopausal women, endogenous estrogens are associated with reduced prevalence of arterial hypertension, coronary artery disease, myocardial infarction, and stroke. Clinical trials conducted in the 1990s such as HERS, WHI, and WISDOM have shown that postmenopausal treatment with horse hormone mixtures (so-called conjugated equine estrogens) and synthetic progestins adversely affects female cardiovascular health. Our understanding of rapid (nongenomic) and chronic (genomic) estrogen signaling has since advanced considerably, including identification of a new G protein-coupled estrogen receptor (GPER), which like the "classical" receptors ERα and ERβ is highly abundant in the cardiovascular system. Here, we discuss the role of estrogen receptors in the pathogenesis of coronary artery disease and review natural and synthetic ligands of estrogen receptors as well as their effects in physiology, on cardiovascular risk factors, and atherosclerotic vascular disease. Data from preclinical and clinical studies using nonselective compounds activating GPER, which include selective estrogen receptor modulators such as tamoxifen or raloxifene, selective estrogen receptor downregulators such as Faslodex™ (fulvestrant/ICI 182,780), vitamin B3 (niacin), green tea catechins, and soy flavonoids such as genistein or resveratrol, strongly suggest that activation of GPER may afford therapeutic benefit for primary and secondary prevention in patients with or at risk for coronary artery disease. Evidence from preclinical studies suggest similar efficacy profiles for selective small molecule GPER agonists such as G-1 which are devoid of uterotrophic activity. Further clinical research in this area is warranted to provide opportunities for future cardiovascular drug development.

GPER negatively regulates TNFα-induced IL-6 production in human breast cancer cells via NF-κB pathway

Estrogen is known to have anti-inflammatory effects, that are thought to be mediated by the classical estrogen receptors (ERs), ERα and ERβ. G protein coupled estrogen receptor1 (GPER) is a novel membrane-type estrogen receptor that can mediate non-genomic estrogenic responses. Although there have been several reports asserting that the participation of GPER in anti-inflammatory effects is induced by estrogen, the role of GPER remains poorly understood. In this study, we investigated the involvement of GPER in the regulation of a representative inflammatory cytokine, IL-6. We first examined the expression of IL-6 mRNA by TNFα stimulation in the transfection of GPER-expression plasmid into HeLa cells. Exogenous GPER significantly inhibited TNFα-induced IL-6 expression, and blocked NF-κB promoter activity inducing the expression of IL-6 in a dose-dependent manner. The promoter activity was restored almost to control level by transfection with the C-terminal deletion mutant of GPER. Similar results have been observed in endogenous GPER using SKBR3 cells which do not express the classical ERs. The data have been validated by treatment of GPER with siRNA. These findings indicate that GPER negatively regulates TNFα-induced IL-6 expression, probably through inhibition of NF-κB promoter activity by a signal(s) derived from the C-terminal region of GPER.

Recent Advances on the Role of G Protein-Coupled Receptors in Hypoxia-Mediated Signaling

G protein-coupled receptors (GPCRs) are cell surface proteins mainly involved in signal transmission; however, they play a role also in several pathophysiological conditions. Chemically heterogeneous molecules like peptides, hormones, lipids, and neurotransmitters activate second messengers and induce several biological responses by binding to these seven transmembrane receptors, which are coupled to heterotrimeric G proteins. Recently, additional molecular mechanisms have been involved in GPCR-mediated signaling, leading to an intricate network of transduction pathways. In this regard, it should be mentioned that diverse GPCR family members contribute to the adaptive cell responses to low oxygen tension, which is a distinguishing feature of several illnesses like neoplastic and cardiovascular diseases. For instance, the G protein estrogen receptor, namely G protein estrogen receptor (GPER)/GPR30, has been shown to contribute to relevant biological effects induced by hypoxia via the hypoxia-inducible factor (HIF)-1α in diverse cell contexts, including cancer. Likewise, GPER has been found to modulate the biological outcome of hypoxic/ischemic stress in both cardiovascular and central nervous systems. Here, we describe the role exerted by GPCR-mediated signaling in low oxygen conditions, discussing, in particular, the involvement of GPER by a hypoxic microenvironment.

The G-protein coupled estrogen receptor, GPER: The inside and inside-out story

GPER possesses structural and functional characteristics shared by members of the G-protein-coupled receptor (GPCR) superfamily, the largest class of plasma membrane receptors. This newly appreciated estrogen receptor is localized predominately within intracellular membranes in most, but not all, cell types and its surface expression is modulated by steroid hormones and during tissue injury. An intracellular staining pattern is not unique among GPCRs, which employ a diverse array of molecular mechanisms that restrict cell surface expression and effectively regulating receptor binding and activation. The finding that GPER displays an intracellular predisposition has created some confusion as the estrogen-inducible transcription factors, ERα and ERβ, also reside intracellularly, and has led to complex suggestions of receptor interaction. GPER undergoes constitutive retrograde trafficking from the plasma membrane to the endoplasmic reticulum and recent studies indicate its interaction with PDZ binding proteins that sort transmembrane receptors to synaptosomes and endosomes. Genetic targeting and selective ligand approaches as well as cell models that express GPER in the absence of ERs clearly supports GPER as a bonafide "stand alone" receptor. Here, the molecular details that regulate GPER action, its cell biological activities and its implicated roles in physiological and pathological processes are reviewed.

From microbe to man: the role of microbial short chain fatty acid metabolites in host cell biology

Recent studies have highlighted a myriad of ways in which the activity and composition of the gut microbiota can affect the host organism. A primary way in which the gut microbiota affect host physiology is by the production of metabolites, such as short-chain fatty acids (SCFAs), which are subsequently absorbed into the bloodstream of the host. Although recent studies have begun to unravel the ways in which gut microbial SCFAs affect host physiology, less is understood regarding the underlying cell biological mechanisms. In this review, we will outline the known receptors and transporters for SCFAs, and review what is known about the cell biological effects of microbial SCFAs.

Sex-Dependent Effects of G Protein–Coupled Estrogen Receptor Activity on Outcome After Ischemic Stroke

Background and Purpose—

Experimental studies indicate that estrogen typically, but not universally, has a neuroprotective effect in stroke. Ischemic stroke increases membrane-bound G protein–coupled estrogen receptor (GPER) distribution and expression in the brain of male but not female mice. We hypothesized that GPER activation may have a greater neuroprotective effect in males than in females after stroke.

Methods—

Vehicle (dimethyl sulfoxide), a GPER agonist (G-1, 30 μg/kg), or a GPER antagonist (G-15, 300 μg/kg) were administered alone or in combination to young or aged male mice, or young intact or ovariectomized female mice, 1 hour before or 3 hours after cerebral ischemia-reperfusion. Some mice were treated with a combination of G-1 and the pan-caspase inhibitor, quinoline-Val-Asp(Ome)-CH2-O-phenoxy (Q-V D -OPh), 1 hour before stroke. We evaluated functional and histological end points of stroke outcome up to 72 hours after ischemia-reperfusion. In addition, apoptosis was examined using cleaved caspase-3 immunohistochemistry.

Results—

Surprisingly, G-1 worsened functional outcomes and increased infarct volume in males poststroke, in association with an increased expression of cleaved caspase-3 in peri-infarct neurons. These effects were blocked by G-15 or Q-V D -OPh. Conversely, G-15 improved functional outcomes and reduced infarct volume after stroke in males, whether given before or after stroke. In contrast to findings in males, G-1 reduced neurological deficit, apoptosis, and infarct volume in ovariectomized females, but had no significant effect in intact females.

Conclusions—

Future therapies for acute stroke could exploit the modulation of GPER activity in a sex-specific manner.

A novel SCFA receptor, the microbiota, and blood pressure regulation

The maintenance of blood pressure homeostasis is a complex process which is carefully regulated by a variety of inputs. We recently identified two sensory receptors (Olfactory receptor 78 and G protein couple receptor 41) as novel regulators of blood pressure. Both Olfr78 and Gpr41 are receptors for short chain fatty acids (SCFAs), and we showed that propionate (a SCFA) modifies blood pressure in a manner which is differentially modulated by the absence of either Olfr78 or Gpr41. In addition, propionate modifies renin release in an Olfr78-dependent manner. Our study also demonstrated that antibiotic treatment modulates blood pressure in Olfr78 null mice, indicating that SCFAs produced by the gut microbiota likely influence blood pressure regulation. In this addendum, we summarize the findings of our recent study and provide a perspective on the implications of the interactions between the gut microbiota and blood pressure control.

G protein-coupled estrogen receptor is apoptotic and correlates with increased distant disease-free survival of estrogen receptor-positive breast cancer patients

PURPOSE

G protein-coupled estrogen receptor 1 (GPER1), previously named GPR30, is a membrane receptor reported to mediate nongenomic estrogen responses. We investigated if GPER1 expression correlates with any clinicopathologic variables and distant disease-free survival (DDFS) in patients with breast cancer, if any prognostic impact of the receptor is dependent on estrogen receptor-α (ER-α) status, and if the receptor impacts apoptotic signaling in ER-positive breast cancer cells.

EXPERIMENTAL DESIGN

GPER1 expression was analyzed by immunohistochemistry in breast tumors from 273 pre- and postmenopausal stage II patients, all treated with adjuvant tamoxifen for 2 years (cohort I) and from 208 premenopausal lymph node-negative patients, of which 87% were not subjected to any adjuvant systemic treatment (cohort II). GPER1-dependent proapoptotic signaling was analyzed in MCF7 cells with and without GPER1 knockdown, T47D cells, HEK293 cells (HEK), and HEK stably expressing GPER1 (HEK-R).

RESULTS

GPER1 positively correlates with ER and progesterone receptor expression. Multivariate analysis showed that GPER1 is an independent prognostic marker of increased 10-year DDFS in the ER-positive subgroup. HEK-R has higher basal proapoptotic signaling compared with HEK including increased cytochrome C release, caspase-3 cleavage, PARP cleavage, and decreased cell viability. Treating HEK-R with the proteasome inhibitor epoxomicin, to decrease GPER1 degradation, further increases receptor-dependent proapoptotic signaling. Also, GPER1 knockdown decreases basal and agonist-stimulated proapoptotic receptor signaling in MCF7 cells.

CONCLUSIONS

GPER1 is a prognostic indicator for increased DDFS in ER-positive breast cancer, which may be associated with constitutive GPER1-dependent proapoptotic signaling in ER-positive breast cancer cells.

Butyrate-induced GPR41 activation inhibits histone acetylation and cell growth

Butyrate has been recently identified as a natural ligand of the G-protein-coupled receptor 41 (GPR41). In addition, it is an inhibitor of histone deacetylase (HDAC). Butyrate treatment results in the hyperacetylation of histones, with resultant multiple biological effects including inhibition of proliferation, induction of cell cycle arrest, and apoptosis, in a variety of cultured mammalian cells. However, it is not clear whether GPR41 is actively involved in the above-mentioned processes. In this study, we generated a stable cell line expressing the hGPR41 receptor in order to investigate the involvement of GPR41 on butyrate-induced biochemical and physiologic processes. We found that GPR41 activation may be a compensatory mechanism to counter the increase in histone H3 acetylation levels induced by butyrate treatment. Moreover, GPR41 had an inhibitory effect on the anti-proliferative, pro-apoptotic effects of butyrate. GPR41 expression induced cell cycle arrest at the G1-stage, while its activation by butyrate can cause more cells to pass the G1 checkpoint. These results indicated that GPR41 was associated with histone acetylation and might be involved in the acetylation-related regulation of cell processes including proliferation, apoptosis, and the cell cycle.

GPR41 gene expression is mediated by internal ribosome entry site (IRES)-dependent translation of bicistronic mRNA encoding GPR40 and GPR41 proteins

GPR41 is a G protein-coupled receptor activated by short chain fatty acids. The gene encoding GPR41 is located immediately downstream of a related gene encoding GPR40, a receptor for long chain fatty acids. Expression of GPR41 has been reported in a small number of cell types, including gut enteroendocrine cells and sympathetic ganglia, where it may play a role in the maintenance of metabolic homeostasis. We now demonstrate that GPR41, like GPR40, is expressed in pancreatic beta cells. Surprisingly, we found no evidence for transcriptional control elements or transcriptional initiation in the intergenic GPR40-GPR41 region. Rather, using 5'-rapid amplification of cDNA ends analysis, we demonstrated that GPR41 is transcribed from the promoter of the GPR40 gene. We confirmed this finding by generating bicistronic luciferase reporter plasmids, and we were able to map a potential internal ribosome entry site-containing region to a 2474-nucleotide region of the intergenic sequence. Consistent with this, we observed m(7)G cap-independent reporter gene expression upon transfection of RNA containing this region. Thus, GPR41 expression is mediated via an internal ribosome entry site located in the intergenic region of a bicistronic mRNA. This novel sequence organization may be utilized to permit coordinated regulation of the fatty acid receptors GPR40 and GPR41.

Extracellular loop 2 of the free fatty acid receptor 2 mediates allosterism of a phenylacetamide ago-allosteric modulator

Allosteric agonists are powerful tools for exploring the pharmacology of closely related G protein-coupled receptors that have nonselective endogenous ligands, such as the short chain fatty acids at free fatty acid receptors 2 and 3 (FFA2/GPR43 and FFA3/GPR41, respectively). We explored the molecular mechanisms mediating the activity of 4-chloro-α-(1-methylethyl)-N-2-thiazolylbenzeneacetamide (4-CMTB), a recently described phenylacetamide allosteric agonist and allosteric modulator of endogenous ligand function at human FFA2, by combining our previous knowledge of the orthosteric binding site with targeted examination of 4-CMTB structure-activity relationships and mutagenesis and chimeric receptor generation. Here we show that 4-CMTB is a selective agonist for FFA2 that binds to a site distinct from the orthosteric site of the receptor. Ligand structure-activity relationship studies indicated that the N-thiazolyl amide is likely to provide hydrogen bond donor/acceptor interactions with the receptor. Substitution at Leu(173) or the exchange of the entire extracellular loop 2 of FFA2 with that of FFA3 was sufficient to reduce or ablate, respectively, allosteric communication between the endogenous and allosteric agonists. Thus, we conclude that extracellular loop 2 of human FFA2 is required for transduction of cooperative signaling between the orthosteric and an as-yet-undefined allosteric binding site of the FFA2 receptor that is occupied by 4-CMTB.

Fatty acid binding receptors in intestinal physiology and pathophysiology

Free fatty acids are essential dietary components and recognized as important molecules in the maintenance of cellular homeostasis. In the last decade, the molecular pathways for free fatty acid sensing in the gastrointestinal tract have been further elucidated by molecular identification and functional characterization of fatty acid binding receptors. These sensing molecules belong to the family of G protein-coupled receptors. In the intestine, four important receptors have been described so far. They differ in molecular structure, ligand specificity, expression pattern, and functional properties. In this review, an overview of intestinal fatty acid binding receptors and their role in intestinal physiology and pathophysiology is given.

In vivo functions of GPR30/GPER-1, a membrane receptor for estrogen: from discovery to functions in vivo

G protein-coupled receptor 30/G protein-coupled estrogen receptor-1 (GPR30/GPER-1) was reported as a novel membrane receptor for estrogen in 2005. However, the research on GPR30 has produced conflicting reports with regard to its intracellular localization, the tissue distribution of its expression, and some its functions. Recently, in addition to the finding of G-1, a GPR30 agonist, GPR30 KO mice have been produced in laboratories, and this has significantly increased the confidence in the data. In this review, the intrinsic appearance of GPR30 is approached based mainly on data obtained in vivo.

Butyrate and propionate induced activated or non-activated neutrophil apoptosis via HDAC inhibitor activity but without activating GPR-41/GPR-43 pathways

OBJECTIVE

Decreased neutrophil apoptosis is implicated in persistent inflammation resulting in systemic inflammatory response syndrome and multiple organ dysfunctions syndromes. Short-chain fatty acids (SCFAs) may be a candidate to control neutrophil apoptosis because SCFAs are normally produced in the gut and related products have been approved for human use. We investigated the effects of SCFAs on apoptosis of activated and non-activated neutrophils and their mechanisms.

METHODS

Purified neutrophils obtained from healthy volunteers were preincubated for 1 h with or without the G-protein receptor (GPR) inhibitor pertussis toxin (100 ng/mL) or U-73122 (50 ng/mL), extracellular signal-related protein kinase inhibitor PD98059 (10 microM), mitogen-activated protein kinase (MAPK) p38 inhibitor SB203580 (25 microM), Jun kinase inhibitor-I (2 microM), caspase-3 and -7 inhibitor Z-VAD-FMK (100 microM), caspase-8 inhibitor Z-IETD-FMK (50 microM), or caspase-9 inhibitor Z-LEHD-FMK (50 microM). The cells were then cultured with or without SCFAs or trichostatin A, a typical histone deacetylase inhibitor, in the presence or absence of lipopolysaccharide (1 microg/mL) or tumor necrosis factor-alpha (100 ng/mL). Neutrophil apoptosis was assessed by annexin V staining using flow cytometry. The GPR-41 and -43 and apoptosis-related proteins (bax, mcl-1, a1) mRNA were measured by quantitative real-time polymerase chain reaction and the expression of acetylated histone H3 was determined by western blot.

RESULTS

The caspase inhibitors inhibited butyrate- and propionate-induced neutrophil apoptosis treated or untreated with lipopolysaccharide or tumor necrosis factor-alpha, whereas GPR and MAPK inhibitors had no effect. The mRNA expressions of GPR-43 and a1 protein were reduced by butyrate and propionate. The expressions of acetylated histone H3 were induced by butyrate and propionate.

CONCLUSION

These results suggest that butyrate and propionate increase apoptosis of neutrophils irrespective of their activation state, by factors other than GPRs and MAPKs, and their mechanisms likely relate to their histone deacetylase inhibition activity, which may control a1 mRNA expression.

TPRA40/GPR175 regulates early mouse embryogenesis through functional membrane transport by Sjögren's syndrome-associated protein NA14

TPRA40/GPR175 is an orphan receptor whose physiological functions have not been found to date. In an attempt to generate transgenic mice that express an shRNA of TPRA40, we observed that the cell division of early mouse embryos that injected the short hairpin RNA expression vector was significantly accelerated compared with the control vector. The regulation of cell division by TPRA40 was also observed in HeLa cells. Since the C-terminal region of TPRA40 has been shown to be essential for the regulation of cell division, we performed yeast two-hybrid screening using the C-terminal region as bait. Nuclear antigen of 14 kDa (NA14), an autoantigen of Sjögren's syndrome, was identified as a binding protein to the C-terminal region of TPRA40. The binding of TPRA40 and NA14 was confirmed by GST pull-down assay and co-immunoprecipitation assay. FLAG-TPRA40 is transported from the cytosol to the plasma membrane in time-dependent manner and the translocation was inhibited by GFP-NA14DeltaN, an N-terminal deletion mutant that cannot bind to microtubules but binds to TPRA40. TPRA40DeltaC, which cannot bind to NA 14, shows impaired transport to the plasma membrane. Finally, we found that the effect of TPRA40 on mouse embryogenesis is strengthened by GFP-NA14, but not by GFP or GFP-NA14DeltaN. These observations indicate that the functional plasma membrane transport of TPRA40 that regulates cell division of mouse embryos is mediated by NA14.

Phosphoproteome analysis of isoflurane-protected heart mitochondria: phosphorylation of adenine nucleotide translocator-1 on Tyr194 regulates mitochondrial function

AIMS

Reversible phosphorylation of mitochondrial proteins is essential in the regulation of respiratory function, energy metabolism, and mitochondrion-mediated cell death. We hypothesized that mitochondrial protein phosphorylation plays a critical role in cardioprotection during pre and postconditioning, two of the most efficient anti-ischaemic therapies.

METHODS AND RESULTS

Using phosphoproteomic approaches, we investigated the profiles of phosphorylated proteins in Wistar rat heart mitochondria protected by pharmacological pre and postconditioning elicited by isoflurane. Sixty-one spots were detected by two-dimensional blue-native gel electrophoresis-coupled Western blotting using a phospho-Ser/Thr/Tyr-specific antibody, and 45 of these spots were identified by matrix-assisted laser desorption/ionization-time of flight mass spectrometry. Eleven protein spots related to oxidative phosphorylation, energy metabolism, chaperone, and carrier functions exhibited significant changes in their phosphorylation state when protected mitochondria were compared with unprotected. Using a phosphopeptide enrichment protocol followed by liquid chromatography-MS/MS, 26 potential phosphorylation sites were identified in 19 proteins. Among these, a novel phosphorylation site was detected in adenine nucleotide translocator-1 (ANT1) at residue Tyr(194). Changes in ANT phosphorylation between protected and unprotected mitochondria were confirmed by immunoprecipitation. The biological significance of ANT phosphorylation at Tyr(194) was further tested with site-directed mutagenesis in yeast. Substitution of Tyr(194) with Phe, mimicking the non-phosphorylated state, resulted in the inhibition of yeast growth on non-fermentable carbon sources, implying a critical role of phosphorylation at this residue in regulating ANT function and cellular respiration.

CONCLUSIONS

Our analysis emphasizes the regulatory functions of the phosphoproteome in heart mitochondria and reveals a novel, potential link between bioenergetics and cardioprotection.

Identification and physiological activity of survival factor released from cardiomyocytes during ischaemia and reperfusion

AIMS

We carried out a screening of survival factors released from cells exposed to simulated ischaemia and reperfusion (sI/R) using the embryonic rat heart-derived cell line, H9c2 cells, and examined the physiological role of the identified factor.

METHOD AND RESULTS

The culture medium supernatant of H9c2 cells exposed to sI/R was separated by column chromatography and the fractions examined for survival activity. The protein with survival activity was identified by mass spectrometry, and its physiological role was examined in the models of ischaemia. Cell survival activity was detected in at least three fractions of the cell supernatant collected during sI/R and subjected to a series of column chromatographic steps. Among the proteins measured by mass spectrometry and western blotting, a p36 protein identified as a glycolytic enzyme, lactate dehydrogenase muscle subunit (M-LDH), showed strong survival activity. H(2)O(2)-induced intracellular calcium overload in H9c2 cells and irregular Ca(2+) transients in adult rat cardiomyocytes were both found to be inhibited by pretreatment with M-LDH. M-LDH also lowered the frequency and amplitude of early afterdepolarizations induced by H(2)O(2) in adult rat cardiomyocytes and suppressed the ischaemia-reperfusion-induced reduction of cardiac output from mouse working heart preparations. M-LDH was found to increase the phosphorylation of extracellular signal-regulated kinase1/2 (ERK1/2), which plays a role in H9c2 cell survival.

CONCLUSION

M-LDH released from cardiomyocytes after hypoxia and reoxygenation has a role in protecting the heart from oxidative stress-induced injury through an intracellular signal transduction pathway involving ERK1/2.

GPR30 contributes to estrogen-induced thymic atrophy

The mechanisms by which prolonged estrogen exposures, such as estrogen therapy and pregnancy, reduce thymus weight, cellularity, and CD4 and CD8 phenotype expression, have not been well defined. In this study, the roles played by the membrane estrogen receptor, G protein-coupled receptor 30 (GPR30), and the intracellular estrogen receptors, estrogen receptor alpha (ERalpha) and beta (ERbeta), in 17beta-estradiol (E2)-induced thymic atrophy were distinguished by construction and the side-by-side comparison of GPR30-deficient mice with ERalpha and ERbeta gene-deficient mice. Our study shows that whereas ERalpha mediated exclusively the early developmental blockage of thymocytes, GPR30 was indispensable for thymocyte apoptosis that preferentially occurs in T cell receptor beta chain(-/low) double-positive thymocytes. Additionally, G1, a specific GPR30 agonist, induces thymic atrophy and thymocyte apoptosis, but not developmental blockage. Finally, E2 treatment attenuates the activation of nuclear factor-kappa B in CD25(-)CD4(-)CD8(-) double-negative thymocytes through an ERalpha-dependent yet ERbeta- and GPR30-independent pathway. Differential inhibition of nuclear factor-kappaB by ERalpha and GPR30 might underlie their disparate physiological effects on thymocytes. Our study distinguishes, for the first time, the respective contributions of nuclear and membrane E2 receptors in negative regulation of thymic development.

Magnesium tanshinoate B protects endothelial cells against oxidized lipoprotein-induced apoptosisThis article is one of a selection of papers published in this special issue (part 2 of 2) on the Safety and Efficacy of Natural Health Products

The activation of c-Jun N-terminal kinase (JNK) signaling pathway plays an important role in the induction of cell apoptosis. We previously reported that magnesium tanshinoate B (MTB), a compound purified from a Chinese herb danshen ( Salvia miltiorrhiza ), could inhibit ischemia/reperfusion-induced myocyte apoptosis in the heart. The objective of the present study was to investigate whether MTB can prevent oxidized lipoprotein-induced apoptosis in endothelial cells. Human umbilical vein endothelial cells (HUVECs) were incubated with copper-oxidized very low density lipoprotein (Cu-OxVLDL) or copper-oxidized low density lipoprotein (Cu-OxLDL). Treatment of cells with Cu-OxVLDL or Cu-OxLDL resulted in a 3-fold increase in the JNK activity. The amount of cytochrome c released and the activity of caspase-3 in cells treated with Cu-OxVLDL or Cu-OxLDL were significantly elevated, indicating the occurrence of apoptosis. The presence of MTB was able to abolish the JNK activation, cytochrome c release, and caspase-3 activation induced by Cu-OxVLDL or Cu-OxLDL, resulting in a marked reduction in apoptosis in endothelial cells. The data from this study indicate that oxidized lipoproteins induce apoptosis in endothelial cells. We postulate that the inhibition of the JNK signaling pathway by MTB is a key mechanism that protects these cells from oxidized lipoprotein-induced apoptosis.

Fatty acid receptors as new therapeutic targets for diabetes

G-protein-coupled receptors (GPCRs) are key regulators of several physiological functions. Their roles in cellular signal transduction have made them the target for majority of all currently prescribed drugs. Additionally, there are many orphan GPCRs that provide potential novel therapeutic targets. Several GPCRs are involved in metabolic regulation and glucose homeostasis such as GLP-1 receptor, glucagon receptor, adiponectin receptor and so on. Recently, free fatty acids (FFAs) have been demonstrated as ligands for orphan GPCRs and have been proposed to play a critical role in physiological glucose homeostasis. GPR40 and GPR120 are activated by medium and long-chain FFAs, whereas GPR41 and GPR43 can be activated by short-chain FFAs. GPR40, which is preferentially expressed in pancreatic beta-cells, mediates the majority of the effects of FFAs on insulin secretion. In this review, these findings and also critical analysis of these GPCRs as novel targets for diabetes are discussed.

Short-chain fatty acids induce acute phosphorylation of the p38 mitogen-activated protein kinase/heat shock protein 27 pathway via GPR43 in the MCF-7 human breast cancer cell line

The expression of GPR41 and 43, which have recently been identified as G-protein-coupled cell-surface receptors for short-chain fatty acids (SCFAs), was detected in a human breast cancer cell line (MCF-7) by RT-PCR. Acetate, propionate and butyrate induced an increase in intracellular Ca2+ in these cells that was not blocked by treatment with pertussis toxin (PTX). SCFAs significantly reduced forskolin-induced cAMP levels in these cells. The phosphorylation of mitogen-activated protein kinase (MAPK) p38 was selectively increased by SCFAs. The downstream substrate heat shock protein 27 (HSP27) was also phosphorylated by SCFAs at Ser-78 and-82, but not-15. Propionate induced elevations in intracellular Ca2+ and the phosphorylation of p38 were inhibited by the silencing of GPR43 using a specific siRNA. These results suggest that GPR41 and 43 mediate SCFA signaling in mammary epithelial cells and thereby play an important role in their stress management.

G protein-coupled receptor 30 is an estrogen receptor in the plasma membrane

Recently, GPR30 was reported to be a novel estrogen receptor; however, its intracellular localization has remained controversial. To investigate the intracellular localization of GPR30 in vivo, we produced four kinds of polyclonal antibodies for distinct epitopes on GPR30. Immunocytochemical observations using anti-GPR30 antibody and anti-FLAG antibody show that FLAG-GPR30 localizes to the plasma membrane 24 h after transfection. Treatment with estrogen (17beta-estradiol or E2) causes an elevation in the intracellular Ca2+ concentration ([Ca2+]i) within 10 s in HeLa cells expressing FLAG-GPR30. In addition, E2 induces the translocation of GPR30 from the plasma membrane to the cytoplasm by 1 h after stimulation. Immunohistochemical analysis shows that GPR30 exists on the cell surface of CA2 pyramidal neuronal cells. The images on transmission electron microscopy show that GPR30 is localized to a particular region associated with the plasma membranes of the pyramidal cells. These data indicate that GPR30, a transmembrane receptor for estrogen, is localized to the plasma membrane of CA2 pyramidal neuronal cells of the hippocampus in rat brain.

G protein-coupled receptor 30 down-regulates cofactor expression and interferes with the transcriptional activity of glucocorticoid

G protein-coupled receptor 30 (GPR30) has previously been described to be important in steroid-mediated growth and to inhibit cell proliferation. Here we investigated whether the effect of GPR30 on cell growth is dependent on steroid hormone receptors. We stably introduced GPR30 in immortalized normal mammary epithelial (HME) cells using retroviruses for gene delivery. GPR30 inhibited the growth and proliferation of the cells. They expressed glucocorticoid receptor, but not estrogen or progesterone receptor. GPR30 down-regulated the expression of cofactor transcription intermediary factor 2 (TIF2) analyzed using quantitative RT-PCR analysis, and also diminished the expression of TIF2 at protein level analyzed by Western blotting using nuclear extracts from mammary epithelial cells. When HME cells were transiently transfected with the glucocorticoid response element MMTV-luc reporter plasmid, stable expression of GPR30 resulted in the abolition of ligand-induced transactivation of the promoter. In COS cells, transient transfection of GPR30 with glucocorticoid receptor alpha resulted in an abrogation of the MMTV-luc and GRE-luc reporter activities induced by dexamethasone. The results suggest a novel mechanism by which membrane-initiated signaling interferes with steroid signaling.

ERK1/2 regulates intracellular ATP levels through alpha-enolase expression in cardiomyocytes exposed to ischemic hypoxia and reoxygenation

Extracellular signal-regulated kinase 1/2 (ERK1/2) is known to function in cell survival in response to various stresses; however, the mechanism of cell survival by ERK1/2 remains poorly elucidated in ischemic heart. Here we applied functional proteomics by two-dimensional electrophoresis to identify a cellular target of ERK1/2 in response to ischemic hypoxia. Approximately 1500 spots were detected by Coomassie Brilliant Blue staining of a sample from unstimulated cells. The staining intensities of at least 50 spots increased at 6-h reoxygenation after 2-h ischemic hypoxia. Of the 50 spots that increased, at least 4 spots were inhibited in the presence of PD98059, a MEK inhibitor. A protein with a molecular mass of 52 kDa that is strongly induced by ERK1/2 activation in response to ischemic hypoxia and reoxygenation was identified as alpha-enolase, a rate-limiting enzyme in the glycolytic pathway, by liquid chromatography-mass spectrometry and amino acid sequencing. The expressions of the alpha-enolase mRNA and protein are inhibited during reoxygenation after ischemic hypoxia in the cells containing a dominant negative mutant of MEK1 and treated with a MEK inhibitor, PD98059, leading to a decrease in ATP levels. alpha-Enolase expression is also observed in rat heart subjected to ischemia-reperfusion. The induction of alpha-enolase by ERK1/2 appears to be mediated by c-Myc. The introduction of the alpha-enolase protein into the cells restores ATP levels and prevents cell death during ischemic hypoxia and reoxygenation in these cells. These results show that alpha-enolase expression by ERK1/2 participates in the production of ATP during reoxygenation after ischemic hypoxia, and a decrease in ATP induces apoptotic cell death. Furthermore, alpha-enolase improves the contractility of cardiomyocytes impaired by ischemic hypoxia. Our results reveal that ERK1/2 plays a role in the contractility of cardiomyocytes and cell survival through alpha-enolase expression during ischemic hypoxia and reoxygenation.

Induction of cytochrome CYP4F3A in all-trans-retinoic acid-treated HL60 cells

Cytochrome P-450 CYP4F3A catalyzes the inactivation of leukotriene B(4) by omega-hydroxylation, an activity of which is specifically expressed in human neutrophils. Here, we examined expression of the LTB(4) omega-hydroxylating activity during the differentiation of HL60 cells, an acute promyelocytic leukemia cell line, in the presence of various inducers. Among the inducers used, all-trans-retinoic acid (ATRA) most strongly induces the LTB(4) omega-hydroxylating activity in a dose-dependent manner. The time course of the induction of the omega-hydroxylating activity correlates well with that of the superoxide-generating activity, indicative of cell differentiation. ATRA-treated cell microsomes convert LTB(4) to its 20-hydroxyl derivative under aerobic conditions in the present of NADPH. The reaction is inhibited by carbon monoxide, an inhibitor of cytochrome P-450, and by antibodies raised against NADPH-P-450 reductase. CYP4F3A appears to be responsible for the LTB(4) omega-hydroxylase activity, based on the following observations: expression of the mRNA for CYP4F3A is observed together with the induction of LTB(4) omega-hydroxylating activity in ATRA-treated HL60 cells; and the apparent K(m) values for the omega-hydroxylation of LTB(4) and lipoxin B(4) by ATRA-treated cell microsomes are essentially the same as those of CYP4F3A in human neutrophil microsomes.

Activation of stimulatory heterotrimeric G proteins increases glutathione and protects neuronal cells against oxidative stress

Oxidative glutamate toxicity in the neuronal cell line HT22 is a model for cell death by oxidative stress, where an excess of extracellular glutamate inhibits import of cystine, a building block of the antioxidant glutathione. The subsequent decrease in glutathione then leads to the accumulation of reactive oxygen species (ROS) and programmed cell death. We used pharmacological compounds known to interact with heterotrimeric G-protein signalling and studied their effects on cell survival, morphology, and intracellular events that ultimately lead to cell death. Cholera toxin and phorbol esters were most effective and prevented cell death through independent pathways. Treating HT22 cells with cholera toxin attenuated the glutamate-induced accumulation of ROS and calcium influx. This was, at least in part, caused by an increase in glutathione due to improved uptake of cystine mediated by the induction of the glutamate/cystine-antiporter subunit xCT or, additionally, by the up-regulation of the antiapoptotic protein Bcl-2. Gs activation also protected HT22 cells from hydrogen peroxide or inhibition of glutathione synthesis by buthionine sulfoximine, and immature cortical neurones from oxidative glutamate toxicity. Thus, this pathway might be more generally implicated in protection from neuronal death by oxidative stress.

Functional characterization of human receptors for short chain fatty acids and their role in polymorphonuclear cell activation

Short chain fatty acids (SCFAs), including acetate, propionate, and butyrate, are produced at high concentration by bacteria in the gut and subsequently released in the bloodstream. Basal acetate concentrations in the blood (about 100 microm) can further increase to millimolar concentrations following alcohol intake. It was known previously that SCFAs can activate leukocytes, particularly neutrophils. In the present work, we have identified two previously orphan G protein-coupled receptors, GPR41 and GPR43, as receptors for SCFAs. Propionate was the most potent agonist for both GPR41 and GPR43. Acetate was more selective for GPR43, whereas butyrate and isobutyrate were more active on GPR41. The two receptors were coupled to inositol 1,4,5-trisphosphate formation, intracellular Ca2+ release, ERK1/2 activation, and inhibition of cAMP accumulation. They exhibited, however, a differential coupling to G proteins; GPR41 coupled exclusively though the Pertussis toxin-sensitive Gi/o family, whereas GPR43 displayed a dual coupling through Gi/o and Pertussis toxin-insensitive Gq protein families. The broad expression profile of GPR41 in a number of tissues does not allow us to infer clear hypotheses regarding its biological functions. In contrast, the highly selective expression of GPR43 in leukocytes, particularly polymorphonuclear cells, suggests a role in the recruitment of these cell populations toward sites of bacterial infection. The pharmacology of GPR43 matches indeed the effects of SCFAs on neutrophils, in terms of intracellular Ca2+ release and chemotaxis. Such a neutrophil-specific SCFA receptor is potentially involved in the development of a variety of diseases characterized by either excessive or inefficient neutrophil recruitment and activation, such as inflammatory bowel diseases or alcoholism-associated immune depression. GPR43 might therefore constitute a target allowing us to modulate immune responses in these pathological situations.

The Orphan G protein-coupled receptors GPR41 and GPR43 are activated by propionate and other short chain carboxylic acids

GPR41 and GPR43 are related members of a homologous family of orphan G protein-coupled receptors that are tandemly encoded at a single chromosomal locus in both humans and mice. We identified the acetate anion as an agonist of human GPR43 during routine ligand bank screening in yeast. This activity was confirmed after transient transfection of GPR43 into mammalian cells using Ca(2+) mobilization and [(35)S]guanosine 5'-O-(3-thiotriphosphate) binding assays and by coexpression with GIRK G protein-regulated potassium channels in Xenopus laevis oocytes. Other short chain carboxylic acid anions such as formate, propionate, butyrate, and pentanoate also had agonist activity. GPR41 is related to GPR43 (52% similarity; 43% identity) and was activated by similar ligands but with differing specificity for carbon chain length, with pentanoate being the most potent agonist. A third family member, GPR42, is most likely a recent gene duplication of GPR41 and may be a pseudogene. GPR41 was expressed primarily in adipose tissue, whereas the highest levels of GPR43 were found in immune cells. The identity of the cognate physiological ligands for these receptors is not clear, although propionate is known to occur in vivo at high concentrations under certain pathophysiological conditions.

Activated protein C blocks p53-mediated apoptosis in ischemic human brain endothelium and is neuroprotective

Activated protein C (APC) is a systemic anti-coagulant and anti-inflammatory factor. It reduces organ damage in animal models of sepsis, ischemic injury and stroke and substantially reduces mortality in patients with severe sepsis. It was not known whether APC acts as a direct cell survival factor or whether its neuroprotective effect is secondary to its anti-coagulant and anti-inflammatory effects. We report that APC directly prevents apoptosis in hypoxic human brain endothelium through transcriptionally dependent inhibition of tumor suppressor protein p53, normalization of the pro-apoptotic Bax/Bcl-2 ratio and reduction of caspase-3 signaling. These mechanisms are distinct from those involving upregulation of the genes encoding the anti-apoptotic Bcl-2 homolog A1 and inhibitor of apoptosis protein-1 (IAP-1) by APC in umbilical vein endothelial cells. Cytoprotection of brain endothelium by APC in vitro required endothelial protein C receptor (EPCR) and protease-activated receptor-1 (PAR-1), as did its in vivo neuroprotective activity in a stroke model of mice with a severe deficiency of EPCR. This is consistent with work showing the direct effects of APC on cultured cells via EPCR and PAR-1 (ref. 9). Moreover, the in vivo neuroprotective effects of low-dose mouse APC seemed to be independent of its anti-coagulant activity. Thus, APC protects the brain from ischemic injury by acting directly on brain cells.

Progestin upregulates G-protein-coupled receptor 30 in breast cancer cells

A differential display method was used to study genes the expression of which is altered during growth inhibition induced by medroxyprogesterone acetate (MPA). A transcript of G-protein-coupled receptor 30 (GPR30) was upregulated by MPA in estrogen-treated MCF-7 breast cancer cells. Northern-blot analysis showed a progestin-specific primary target gene, which was enhanced by progesterone and different progestins, but not by dihydrotestosterone or dexamethasone, and which was abrogated by antiprogestin RU486. The dose-dependent and time-dependent increase in GPR30 mRNA expression correlated with MPA-induced growth inhibition in MCF-7 cells. Additionally, GPR30 upregulation by progestin correlated with growth inhibition when a comparison was made between different breast cancer cell lines. The ERK1/ERK2 pathway is capable of inducing progesterone receptor-dependent and ligand-dependent transcription. Thus we sought to establish whether different MAPK pathway inhibitors affect progestin-induced GPR30 mRNA regulation. The regulation of GPR30 was independent of ERK pathway activation, but the p38 pathway inhibitor induced GPR30 expression, which suggested a potential gene regulation pathway. These data demonstrate a new progestin target gene, the expression of which correlates with growth inhibition.