Receptors

The steroid/thyroid hormone or nuclear receptor superfamily is quickly approaching its 40th anniversary. During this period, we have seen tremendous progress being made in our understanding of the mechanisms of action of these physiologically important proteins in the field of health and disease. Critical to this has been the insight provided by ever more detailed structural examination of nuclear receptor proteins and the complexes they are responsible for assembling on DNA. In this article, I will focus on the contributions made by Jan-Åke Gustafsson and colleagues at the Karolinska Institute (Sweden) and, more recently, the University of Houston (USA), to this area of nuclear receptor research. Full article

The cycle number (n_c) of a recycling receptor is defined as the average number of round trips (cell surface–endosome–cell surface) the receptor can make before it is degraded. This characteristic parameter of recycling receptors can be easily determined from the receptor’s half-life (t_½, the time in which 50% of the receptor is degraded) and cycling time (T_c, the time a receptor needs to complete a round trip). Relationship analyses revealed that n_c increases linearly with increasing t_½ and decreases exponentially with increasing T_c. For commonly observed t_½ and T_c values, it was calculated that recycling receptors have n_c values of <300. In addition, it was found that recycling receptors in cancer cells have generally smaller n_c values (<100), whereas recycling receptors in normal cells have larger n_c values (>100). Based on this latter finding, the cycle number n_c may be a useful criterion for distinguishing between cancer and normal cells. Full article

Communication between cells is essential in maintaining homeostasis. The persistent disruption of cell–cell communication by environmental contaminants contributes to progressive disease and toxicity. In this study, single-nuclei RNA sequencing (snRNAseq) data was used to examine dose-dependent cell-specific changes in cell–cell communication associated with the development of liver pathologies following the persistent activation of the aryl hydrocarbon receptor (AHR) by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Published hepatic snRNAseq data from male mice gavaged with sesame-oil vehicle or TCDD every 4 days for 28 days was used to assess the AHR-mediated disruption of ligand–receptor interactions. Analysis identified that portal fibroblasts and liver sinusoidal endothelial cells contributed the most ligand–receptor pairs at doses < 0.3μg/kg TCDD. Doses ≥ 0.3 μg/kg TCDD increased the putative intercellular communication between hepatocytes and hepatic stellate cells. In control livers, interactions primarily consisted of protease-activated receptor (PAR) signaling. TCDD treatment increased the number of active signaling pathways. Within hepatocytes, neuregulin signaling was induced, activating the NRG1–ERBB4 ligand axis, consistent with AHR genomic enrichment at dioxin response elements in a published chromatin immunoprecipitation sequencing (ChIP-seq) dataset, which suggested a direct regulation. Collectively, the results suggest that the disruption of cell signaling may play a central role in TCDD-elicited liver pathologies. Full article

Melatonin, the hormone of darkness, is secreted in minute amounts during the night and is virtually undetectable during the day. Melatonin mainly acts on high-affinity G protein-coupled receptors. The present review will trace the path of the discovery of melatonin receptors from their cloning, expression and purification to the development of recent radioactive and fluorescent tracers. We will then report on the state-of-the-art of melatonin receptor functional properties, including ligand bias and system bias due to receptor-associated proteins and receptor heteromers. Currently available antibodies raised against melatonin receptors will be critically reviewed here for the first time. The review will close with future perspectives in terms of the discovery of allosteric ligands and the in vivo validation of a range of melatonin receptor-associated signaling complexes to improve future drug development. Full article

Estrogens play a crucial role in sexual development and fertility as well as many other physiological processes, and it is estrogen receptors that mediate the physiological responses. To study the role of the estrogen receptors in these processes, several genetic mouse models have been developed using different strategies, which also in some cases yield different results. Here, we summarize the models that have been made and their impact on fertility in relation to known cases of human estrogen receptor mutations. Full article

Recent evidence suggests that the environment-sensing transcription factor aryl hydrocarbon receptor (AHR) is an important regulator of hematopoiesis. Yet, the mechanisms and extent of AHR-mediated regulation within the most primitive hematopoietic cells, hematopoietic stem and progenitor cells (HSPCs), are poorly understood. Through a combination of transcriptomic and flow cytometric approaches, this study provides new insight into how the AHR influences hematopoietic stem and progenitor cells. Comparative analysis of intraphenotypic transcriptomes of hematopoietic stem cells (HSCs) and multipotent progenitor (MPP) cells from AHR knockout (AHR KO) and wild type mice revealed significant differences in gene expression patterns. Notable among these were differences in expression of cell cycle regulators, specifically an enrichment of G2/M checkpoint genes when Ahr was absent. This included the regulator Aurora A kinase (Aurka, AurA). Analysis of AurA protein levels in HSPC subsets using flow cytometry, in combination with inducible AHR KO or in vivo AHR antagonism, showed that attenuation of AHR increased levels of AurA in HSCs and lineage-biased MPP cells. Overall, these data highlight a potential novel mechanism by which AHR controls HSC homeostasis and HSPC differentiation. These findings advance the understanding of how AHR influences and regulates primitive hematopoiesis. Full article

The aryl hydrocarbon receptor (AhR) is overexpressed in many tumor types and exhibits tumor-specific tumor promoter and tumor suppressor-like activity. In colon cancer, most but not all studies suggest that the AhR exhibits tumor suppressor activity which is enhanced by AhR ligands acting as agonists. Our studies investigated the role of the AhR in colon tumorigenesis using wild-type and AhR-knockout mice, the inflammation model of colon tumorigenesis using mice treated with azoxymethane (AOM)/dextran sodium sulfate (DSS) and APC^S580/+; Kras^G12D/+ mice all of which form intestinal tumors. The effects of tissue-specific AhR loss in the intestine of the tumor-forming mice on colonic stem cells, organoid-initiating capacity, colon tumor formation and mechanisms of AhR-mediated effects were investigated. Loss of AhR enhanced stem cell and tumor growth and in the AOM/DSS model AhR-dependent suppression of FOXM1 and downstream genes was important for AhR-dependent anticancer activity. Furthermore, the effectiveness of interleukin-22 (IL22) in colonic epithelial cells was also dependent on AhR expression. IL22 induced phosphorylation of STAT3, inhibited colonic organoid growth, promoted colonic cell proliferation in vivo and enhanced DNA repair in AOM/DSS-induced tumors. In this mouse model, the AhR suppressed SOCS3 expression and enhanced IL22-mediated activation of STAT3, whereas the loss of the AhR increased levels of SOCS3 which in turn inhibited IL22-induced STAT3 activation. In the APC^S580/+; Kras^G12D/+ mouse model, the loss of the AhR enhanced Wnt signaling and colon carcinogenesis. Results in both mouse models of colon carcinogenesis were complemented by single cell transcriptomics on colonic intestinal crypts which also showed that AhR deletion promoted expression of FOXM1-regulated genes in multiple colonic cell subtypes. These results support the role of the AhR as a tumor suppressor-like gene in the colon. Full article

In an effort to provide an overview of the biophysical approaches used to study G-protein-coupled receptors, we chose to consider the adenosine A_2A receptor as a model, as it is widely reported in the literature to explore the way GPCRs are studied nowadays. After a brief introduction of the receptor, we gathered descriptions of the various tools used to investigate the pharmacology and structure of the A_2A receptor. We began by describing the key developments which have led to successful studies of GPCRs including the cloning, expression and purification of A_2A, and the subsequent characterizations including quality control, binding and functional studies that have been necessary for the further understanding of the receptor. Then, we reviewed the reconstitution of A_2A into nanodiscs as well as the use of this biological material in structural mass spectrometry, NMR, calorimetry and various other approaches to gain not only information about the structure and function of A_2A, but also the dynamics of the receptor and the tools necessary to pursue such investigations. The body of techniques presented herein are applicable to all GPCRs amenable to purification. Full article

Pancreatic ductal adenocarcinoma (PDAC) is an aggressive disease with a high mortality rate and few effective treatments. A growing area of cancer therapeutics seeks to exploit the metabolic dysregulation of cancer cells, such as glucose, amino acid, and fatty acid metabolism, to selectively target malignant cells. As ligand-dependent transcription factors and critical regulators of metabolism, liver X receptors (LXRs) are amenable to small-molecule targeting for such purposes. We have profiled the transcriptomic, metabolomic, and cytotoxic effects of a newly discovered small-molecule LXR modulator, GAC0003A4 (3A4), in PDAC cell lines. On the transcriptomic level, marked changes in gene expression were observed, including downregulation of LXR target genes and pathways. Gene set enrichment analysis determined downregulation of several metabolic pathways, such as fatty acid and cholesterol metabolism, while upregulated pathways involved TNFα/NF-κB and other stress-induced processes. Metabolomic analyses revealed altered metabolites in several pathways, the most enriched categories being lipids and amino acid metabolites, while phospholipids and sphingolipids, including ceramides, were also found to be significantly altered. Insights from transcriptomic and metabolomic studies helped guide the determination of alterations in cholesterol and ceramides as integral to the antiproliferative mechanisms of 3A4. Additionally, a concurrent programmed cell death mechanism involving apoptosis and necroptosis was shown to be activated. These studies provide novel insights into the effects of LXR modulation on gene expression, metabolism, and cell death induction in PDAC cells. The metabolic and cytotoxic effects of LXR modulation on the PDAC cell lines used in this study could also aid in the design and application of drugs to target other refractory cancers. Full article

Intuitively, it is easy to understand why we search for G protein-coupled receptor (GPCR) antagonists. It is obviously to block a functionality of a specific receptor potentially linked to some aspects of disease. Whether by focused research or by serendipity, many drugs were discovered in the last century that function as antagonist at a precise receptor. A current idea is that at least half of the drugs on the market are antagonist ligands of GPCRs. Then, why are we searching for alternative receptor agonists while the endogenous activating molecule is known? In the present commentary we try to rationalize these fields of research, since they proved to be very successful over the years, with receptor pharmacology populated with dozens of alternative agonists, particularly to bioaminergic receptors, and to a lesser extent to peptidergic ones. However, the action of such compounds is not well-characterized: are they surrogates to the endogenous agonist, and if yes in which context and for which purpose? The present essay is a reflection on this subject that leads to fundamental interrogations of our understanding of GPCR roles and functions. Full article

Numerous nuclear receptors including farnesoid X receptor, liver X receptor, peroxisome proliferator-activated receptors, pregnane X receptor, hepatic nuclear factors have been extensively studied within the context of non-alcoholic fatty liver disease (NAFLD). Following the first description of the Aryl hydrocarbon Receptor (AhR) in the 1970s and decades of research which unveiled its role in toxicity and pathophysiological processes, the functional significance of AhR in NAFLD has not been completely decoded. Recently, multiple research groups have utilized a plethora of in vitro and in vivo models that mimic NAFLD pathology to investigate the functional significance of AhR in fatty liver disease. This review provides a comprehensive account of studies describing both the beneficial and possible detrimental role of AhR in NAFLD. A plausible reconciliation for the paradox indicating AhR as a ‘double-edged sword’ in NAFLD is discussed. Finally, understanding AhR ligands and their signaling in NAFLD will facilitate us to probe AhR as a potential drug target to design innovative therapeutics against NAFLD in the near future. Full article

Neurokinin-1 receptor (NK1R) signaling pathways play a crucial role in a number of biological processes in the eye. Specifically, in the ocular surface, their activity modulates epithelial integrity, inflammation, and generation of pain, while they have a role in visual processing in the retina. The NK1R is broadly expressed in the eye, in both ocular and non-ocular cells, such as leukocytes and neurons. In this review, we will discuss the roles of neurokinin-1 receptors and substance P (SP) in the physiopathology of eye disorders. Finally, we will review and highlight the therapeutic benefits of NK1R antagonists in the treatment of ocular diseases. Full article

The important role played by the substance P (SP)/neurokinin-1 receptor (NK-1R) system in cancer is reviewed: this includes tumor cell proliferation and migration, anti-apoptotic mechanisms, and angiogenesis. SP, through the NK-1R, behaves as a universal mitogen in cancer cells. The NK-1R is overexpressed in tumor cells and, in addition, affects the viability of cancer cells. NK-1R antagonists counteract all the previous actions mediated by SP through NK-1R. In a concentration-dependent manner, these antagonists promote tumor cell death by apoptosis. Therefore, NK-1R is a potential and promising therapeutic target for cancer treatment by using NK-1R antagonists (e.g., aprepitant) alone or in combination therapy with chemotherapy or radiotherapy. Full article

Substance P (SP), the first isolated neuropeptide, belongs to the family of tachykinin peptides and is the natural ligand of neurokinin-1 receptors (NK-1R), also named SP receptors. The undecapeptide activates the receptor after specifically binding to the protein and triggers intracellular signals leading to different biochemical events and subsequent physiological responses. This study reviews the main architectural features of this receptor, its interaction with natural and synthetic ligands, and the functional conformational states adopted after interacting with ligands and effector G proteins. The analysis of the main intracellular signaling pathways turned on by the activation of NK-1 receptors reveals the participation of different proteins supporting metabolic changes and genetic and epigenetic regulations. Furthermore, the analysis of receptor occupancy and receptor downregulation and internalization represents a complex and estimable field for basic and clinical research focused on the role of SP in physiopathology. Profound knowledge of the structural dynamics of NK-1R may help develop and assay new selective synthetic non-peptide antagonists as potential therapeutic agents applied to various pathologies and symptoms. Full article

Here, our data provide the first evidence for the existence of a previously unknown receptive system formed by novel DNA- and RNA-based receptors in eukaryotes. This system, named the TR-system, is capable of recognizing and generating a response to different environmental factors and has been shown to orchestrate major vital functions of fungi, mammalian cells, and plants. Recently, we discovered the existence of a similar regulatory system in prokaryotes. These DNA- and RNA-based receptors are localized outside of the membrane forming a type of a network around cells that responds to a variety of chemical, biological, and physical factors and enabled the TR-system to regulate major aspects of eukaryotic cell life as follows: growth, including reproduction and development of multicellular structures; sensitivity to temperature, geomagnetic field, UV, light, and hormones; interaction with viruses; gene expression, recognition and utilization of nutrients. The TR-system was also implicated in cell-memory formation and was determined to be responsible for its maintenance and the forgetting of preceding events. This system is the most distant receptive and regulatory system of the cell that regulates interactions with the outer environment and governs the functions of other receptor-mediated signaling pathways. Full article

(1) Background: In addition to the major phytocannabinoids, trans-Δ⁹-tetrahydrocannabinol (Δ⁹-THC) and cannabidiol (CBD), the cannabis plant (Cannabis sativa L.) synthesizes over 120 additional cannabinoids that are known as minor cannabinoids. These minor cannabinoids have been proposed to act as agonists and antagonists at numerous targets including cannabinoid type 1 (CB₁) and type 2 (CB₂) receptors, transient receptor potential (TRP) channels and others. The goal of the present study was to determine the agonist effects of the minor cannabinoids: cannabinol (CBN), cannabigerol (CBG), cannabichromene (CBC), cannabitriol (CBT) and cannabidivarin (CBDV) at the CB₁ receptor. In addition, the CB₁ receptor antagonist effects of Δ⁹-tetrahydrocannabivarin (Δ⁹-THCV) were compared with its isomer Δ⁸-tetrahydrocannabivarin (Δ⁸-THCV). (2) Methods: CB₁ receptor activity was monitored by measuring cannabinoid activation of G protein-gated inward rectifier K⁺ (GIRK) channels in AtT20 pituitary cells using a membrane potential-sensitive fluorescent dye assay. (3) Results: When compared to the CB₁ receptor full agonist WIN 55,212-2 and the partial agonist Δ⁹-THC, none of the minor cannabinoids caused a significant activation of G_i/GIRK channel signaling. However, Δ⁹-THCV and Δ⁸-THCV antagonized the effect of WIN 55,212-2 with half-maximal inhibitory concentrations (IC₅₀s) of 434 nM and 757 nM, respectively. Δ⁹-THCV antagonism of the CB₁ receptor was “ligand-dependent”; Δ⁹-THCV was more potent in inhibiting WIN 55,212-2 and 2-arachidonoylglycerol (2-AG) than Δ⁹-THC. (4) Conclusions: While none of the minor cannabinoids caused G_i/GIRK channel activation, Δ⁹-THCV antagonized the CB₁ receptor in an isomer- and ligand-dependent manner. Full article