GPCR-jacking: from a new route in RTK signalling to a new concept in GPCR activation

Research approaches for exploring the hidden conversations of G protein-coupled receptor transactivation

G protein-coupled receptor (GPCR) signaling is a crucial physiological mechanism that encompasses a wide range of signaling phenomena. Although traditional GPCR signaling involves G protein or arrestin-related activation, other modes such as biphasic activation, dimer or oligomeric activation, and transactivation have also been observed. Herein, we focus on the increasingly recognized process of GPCR-transactivation. Transactivation refers to the ability of GPCRs to activate other receptor types, especially receptor tyrosine kinases, without engaging their own specific ligands. This cross-talk between GPCRs and other receptors facilitates the integration of multiple signaling pathways, thereby regulating diverse cellular responses, which underscores its physiological significance. In this review, we provide a comprehensive overview of the role of GPCR-transactivation in physiology. We also discuss the growing interest in this field and examine the various tools available for studying transactivation. Additionally, we highlight recent advancements in emerging tools and their application to GPCR-transactivation research. Finally, we propose future research directions and consider the potential impact of new technologies in this rapidly evolving field. SIGNIFICANCE STATEMENT: G protein-coupled receptor transactivation plays a key role in integrating multiple signaling pathways by activating other proteins, like receptor tyrosine kinases, without binding their specific ligands. Here, we focus on the significance of transactivation and the various approaches used to study this phenomenon.

Linsitinib Decreases Thyrotropin-Induced Thyroid Hormone Synthesis by Inhibiting Crosstalk Between Thyroid-Stimulating Hormone and Insulin-Like Growth Factor 1 Receptors in Human Thyrocytes In Vitro and In Vivo in Mice

Background: Thyrotropin receptor (TSHR) and insulin-like growth factor 1 receptor (IGF-1R) have been shown to crosstalk in primary cultures of human thyrocytes (hThyros) and Graves' orbital fibroblasts. The phenomenon of TSHR/IGF-1R crosstalk has been largely studied in the pathogenesis of thyroid eye disease (TED) in human orbital fibroblasts. Here, we investigated the effects of inhibiting the IGF-1R-mediated contribution to crosstalk by linsitinib (Lins), a small-molecule IGF-1R kinase inhibitor, on TSH-induced regulation of thyroperoxidase (TPO) and thyroglobulin (TG) mRNAs and proteins in hThyros in vitro, and on TPO and TG mRNAs and free thyroxine (fT4) levels in vivo in mice. Methods: Steady-state levels of mRNAs of TPO and TG in hThyros in vitro and mouse thyroid glands were measured by RT-qPCR. Human TG (hTG) and human TPO (hTPO) proteins in human thyroid cell cultures were measured by Western blot or ELISA. Translation rates of hTG were quantified by stable isotope labeling by amino acids method (SILAC). Thyroidal mouse Tpo (mTpo) and Tg (mTg) mRNAs and fT4 in mice were assessed after Lins administration on 3 consecutive days followed by an intraperitoneal dose of bovine TSH (bTSH) 3 hours prior to drawing blood. Results: In primary cultures of hThyros, Lins inhibited bTSH-induced upregulation of hTPO mRNA by 61.5%, and hTPO protein was inhibited by 42.4%. There was no effect of Lins on hTG mRNA, but Lins inhibited the upregulation of secreted and cell-associated hTG protein by 50.1% and 42.2%, respectively, by inhibiting hTG mRNA translation. mTpo mRNA measured in thyroid glands after treatment with Lins was reduced by 31.5%. There was no effect of Lins on mTg mRNA, however, Lins decreased fT4 levels in mice under basal (endogenous mTSH levels) and bTSH-treated conditions. Conclusions: The IGF-1R antagonist Lins inhibited bTSH-stimulated hTG and hTPO protein expression in primary cultures of hThyros and fT4 levels in mice. We suggest that thyroid function studies be monitored when Lins is administered to humans, for example, if it is used to treat TED.

An Alternative Mode of GPCR Transactivation: Activation of GPCRs by Adhesion GPCRs

G protein-coupled receptors (GPCRs), critical for cellular communication and signaling, represent the largest cell surface protein family and play important roles in numerous pathophysiological processes. Consequently, GPCRs have become a primary focus in drug discovery efforts. Beyond their traditional G protein-dependent signaling pathways, GPCRs are also capable of activating alternative signaling mechanisms, including G protein-independent signaling, biased signaling, and signaling crosstalk. A particularly novel signaling mode employed by these receptors is GPCR transactivation, which enables cross-communication between GPCRs and other receptor types. Intriguingly, GPCR transactivation by distinct GPCRs has also been identified. In this review, I provide an overview of the known GPCR transactivation mechanisms and explore recently uncovered GPCR transactivation mediated by adhesion-class GPCRs (aGPCRs). These aGPCR-GPCR transactivation processes regulate unique cell type-specific functions, offering an exciting opportunity to develop therapies that precisely modulate specific GPCR-mediated biological effects.

GHRH and the prostate

In the late 1960s and early 1970s, hypothalamic regulatory hormones were isolated, characterized and sequenced. Later, it was demonstrated hypothalamic and ectopic production of growth hormone-releasing hormone (GHRH) in normal and tumor tissues, of both humans and animals. Pituitary-type GHRH receptors (pGHRH-R) had been demonstrated to be expressed predominantly in the anterior pituitary gland but also found in other somatic cells, and significantly present in various human cancers; in addition, the expression of splice variants (SVs) of GHRH receptor (GHRH-R) has been found not only in the pituitary but in extrapituitary tissues, including human neoplasms. In relation to the prostate, besides the pGHRH-R, it has been detected the presence of truncated splice variants of GHRH-R (SV1-SV4) in normal human prostate and human prostate cancer (PCa) specimens; lastly, a novel SV of GHRH-R has been detected in human PCa. Signaling pathways activated by GHRH include AC/cAMP/PKA, Ras/Raf/ERK, PI3K/Akt/mTOR and JAK2/STAT3, which are involved in processes such as cell survival, proliferation and cytokine secretion. The neuropeptide GHRH can also transactivate the epidermal growth factor receptor (EGFR) and human epidermal growth factor receptor (HER)-2. Thus, GHRH-Rs have become drug targets for several types of clinical conditions, including prostate-related conditions such as prostatitis, benign hyperplasia and cancer. Over the last fifty years, the development of GHRH-R receptor antagonists has been unstoppable, improving their potency, stability and affinity for the receptor. The last series of GHRH-R antagonists, AVR, exhibits superior anticancer and anti-inflammatory activities in both in vivo and in vitro assays.

Artificial and Natural Sweeteners Biased T1R2/T1R3 Taste Receptors Transactivate Glycosylated Receptors on Cancer Cells to Induce Epithelial-Mesenchymal Transition of Metastatic Phenotype

Understanding the role of biased taste T1R2/T1R3 G protein-coupled receptors (GPCR) agonists on glycosylated receptor signaling may provide insights into the opposing effects mediated by artificial and natural sweeteners, particularly in cancer and metastasis. Sweetener-taste GPCRs can be activated by several active states involving either biased agonism, functional selectivity, or ligand-directed signaling. However, there are increasing arrays of sweetener ligands with different degrees of allosteric biased modulation that can vary dramatically in binding- and signaling-specific manners. Here, emerging evidence proposes the involvement of taste GPCRs in a biased GPCR signaling crosstalk involving matrix metalloproteinase-9 (MMP-9) and neuraminidase-1 (Neu-1) activating glycosylated receptors by modifying sialic acids. The findings revealed that most natural and artificial sweeteners significantly activate Neu-1 sialidase in a dose-dependent fashion in RAW-Blue and PANC-1 cells. To confirm this biased GPCR signaling crosstalk, BIM-23127 (neuromedin B receptor inhibitor, MMP-9i (specific MMP-9 inhibitor), and oseltamivir phosphate (specific Neu-1 inhibitor) significantly block sweetener agonist-induced Neu-1 sialidase activity. To assess the effect of artificial and natural sweeteners on the key survival pathways critical for pancreatic cancer progression, we analyzed the expression of epithelial-mesenchymal markers, CD24, ADLH-1, E-cadherin, and N-cadherin in PANC-1 cells, and assess the cellular migration invasiveness in a scratch wound closure assay, and the tunneling nanotubes (TNTs) in staging the migratory intercellular communication. The artificial and natural sweeteners induced metastatic phenotype of PANC-1 pancreatic cancer cells to promote migratory intercellular communication and invasion. The sweeteners also induced the downstream NFκB activation using the secretory alkaline phosphatase (SEAP) assay. These findings elucidate a novel taste T1R2/T1R3 GPCR functional selectivity of a signaling platform in which sweeteners activate downstream signaling, contributing to tumorigenesis and metastasis via a proposed NFκB-induced epigenetic reprogramming modeling.

Functional Selectivity of Cannabinoid Type 1 G Protein-Coupled Receptor Agonists in Transactivating Glycosylated Receptors on Cancer Cells to Induce Epithelial-Mesenchymal Transition Metastatic Phenotype

Understanding the role of biased G protein-coupled receptor (GPCR) agonism in receptor signaling may provide novel insights into the opposing effects mediated by cannabinoids, particularly in cancer and cancer metastasis. GPCRs can have more than one active state, a phenomenon called either 'biased agonism', 'functional selectivity', or 'ligand-directed signaling'. However, there are increasing arrays of cannabinoid allosteric ligands with different degrees of modulation, called 'biased modulation', that can vary dramatically in a probe- and pathway-specific manner, not from simple differences in orthosteric ligand efficacy or stimulus-response coupling. Here, emerging evidence proposes the involvement of CB1 GPCRs in a novel biased GPCR signaling paradigm involving the crosstalk between neuraminidase-1 (Neu-1) and matrix metalloproteinase-9 (MMP-9) in the activation of glycosylated receptors through the modification of the receptor glycosylation state. The study findings highlighted the role of CB1 agonists AM-404, Aravnil, and Olvanil in significantly inducing Neu-1 sialidase activity in a dose-dependent fashion in RAW-Blue, PANC-1, and SW-620 cells. This approach was further substantiated by findings that the neuromedin B receptor inhibitor, BIM-23127, MMP-9 inhibitor, MMP9i, and Neu-1 inhibitor, oseltamivir phosphate, could specifically block CB1 agonist-induced Neu-1 sialidase activity. Additionally, we found that CB1 receptors exist in a multimeric receptor complex with Neu-1 in naïve, unstimulated RAW-Blue, PANC-1, and SW-620 cells. This complex implies a molecular link that regulates the interaction and signaling mechanism among these molecules present on the cell surface. Moreover, the study results demonstrate that CB1 agonists induce NFκB-dependent secretory alkaline phosphatase (SEAP) activity in influencing the expression of epithelial-mesenchymal markers, E-cadherin, and vimentin in SW-620 cells, albeit the impact on E-cadherin expression is less pronounced compared to vimentin. In essence, this innovative research begins to elucidate an entirely new molecular mechanism involving a GPCR signaling paradigm in which cannabinoids, as epigenetic stimuli, may traverse to influence gene expression and contribute to cancer and cancer metastasis.

TrkB receptor interacts with mGlu2 receptor and mediates antipsychotic-like effects of mGlu2 receptor activation in the mouse

Metabotropic glutamate receptor 2 (mGlu2) attracts particular attention as a possible target for a new class of antipsychotics. However, the signaling pathways transducing the effects of mGlu2 in the brain remain poorly characterized. Here, we addressed this issue by identifying native mGlu2 interactome in mouse prefrontal cortex. Nanobody-based affinity purification and mass spectrometry identified 149 candidate mGlu2 partners, including the neurotrophin receptor TrkB. The later interaction was confirmed both in cultured cells and prefrontal cortex. mGlu2 activation triggers phosphorylation of TrkB on Tyr816 in primary cortical neurons and prefrontal cortex. Reciprocally, TrkB stimulation enhances mGlu2-operated Gi/o protein activation. Furthermore, TrkB inhibition prevents the rescue of behavioral deficits by glutamatergic antipsychotics in phencyclidine-treated mice. Collectively, these results reveal a cross-talk between TrkB and mGlu2, which is key to the behavioral response to glutamatergic antipsychotics.

An Assessment of the Therapeutic Landscape for the Treatment of Heart Disease in the RASopathies

The RAS/mitogen-activated protein kinase (MAPK) pathway controls a plethora of developmental and post-developmental processes. It is now clear that mutations in the RAS-MAPK pathway cause developmental diseases collectively referred to as the RASopathies. The RASopathies include Noonan syndrome, Noonan syndrome with multiple lentigines, cardiofaciocutaneous syndrome, neurofibromatosis type 1, and Costello syndrome. RASopathy patients exhibit a wide spectrum of congenital heart defects (CHD), such as valvular abnormalities and hypertrophic cardiomyopathy (HCM). Since the cardiovascular defects are the most serious and recurrent cause of mortality in RASopathy patients, it is critical to understand the pathological signaling mechanisms that drive the disease. Therapies for the treatment of HCM and other RASopathy-associated comorbidities have yet to be fully realized. Recent developments have shown promise for the use of repurposed antineoplastic drugs that target the RAS-MAPK pathway for the treatment of RASopathy-associated HCM. However, given the impact of the RAS-MAPK pathway in post-developmental physiology, establishing safety and evaluating risk when treating children will be paramount. As such insight provided by preclinical and clinical information will be critical. This review will highlight the cardiovascular manifestations caused by the RASopathies and will discuss the emerging therapies for treatment.

IGF-1R targeting in cancer - does sub-cellular localization matter?

The insulin-like growth factor receptor (IGF-1R) was among the most intensively pursued kinase targets in oncology. However, even after a slew of small-molecule and antibody therapeutics reached clinical trials for a range of solid tumors, the initial promise remains unfulfilled. Mechanisms of resistance to, and toxicities resulting from, IGF-1R-targeted drugs are well-catalogued, and there is general appreciation of the fact that a lack of biomarker-based patient stratification was a limitation of previous clinical trials. But no next-generation therapeutic strategies have yet successfully exploited this understanding in the clinic.Currently there is emerging interest in re-visiting IGF-1R targeted therapeutics in combination-treatment protocols with predictive biomarker-driven patient-stratification. One such biomarker that emerged from early clinical trials is the sub-cellular localization of IGF-1R. After providing some background on IGF-1R, its drugging history, and the trials that led to the termination of drug development for this target, we look more deeply into the correlation between sub-cellular localization of IGF-1R and susceptibility to various classes of IGF-1R - targeted agents.

Unexpected Crosslinking Effects of a Human Thyroid Stimulating Monoclonal Autoantibody, M22, with IGF1 on Adipogenesis in 3T3L-1 Cells

To study the effects of the crosslinking of IGF1 and/or the human thyroid-stimulating monoclonal autoantibody (TSmAb), M22 on mouse adipocytes, two- and three-dimensional (2D or 3D) cultures of 3T3-L1 cells were prepared. Each sample was then subjected to the following analyses: (1) lipid staining, (2) a real-time cellular metabolic analysis, (3) analysis of the mRNA expression of adipogenesis-related genes and extracellular matrix (ECM) molecules including collagen (Col) 1, 4 and 6, and fibronectin (Fn), and (4) measurement of the size and physical properties of the 3D spheroids with a micro-squeezer. Upon adipogenic differentiation (DIF+), lipid staining and the mRNA expression of adipogenesis-related genes in the 2D- or 3D-cultured 3T3-L1 cells substantially increased. On adding IGF1 but not M22 to DIF+ cells, a significant enhancement in lipid staining and gene expressions of adipogenesis-related genes was detected in the 2D-cultured 3T3-L1 cells, although some simultaneous suppression or enhancement effects by IGF1 and M22 against lipid staining or Fabp4 expression, respectively, were detected in the 3D 3T3-L1 spheroids. Real-time metabolic analyses indicated that monotherapy with IGF1 or M22 shifted cellular metabolism toward energetic states in the 2D 3T3-L1 cells upon DIF+, although no significant metabolic changes were induced by DIF+ alone in 2D cultures. In addition, some synergistical effects on cellular metabolism by IGF1 and M22 were also observed in the 2D 3T3-L1 cells as well as in cultured non-Graves' orbitopathy-related human orbital fibroblasts (n-HOFs), but not in Graves' orbitopathy-related HOFs (GHOFs). In terms of the physical properties of the 3D 3T3-L1 spheroids, (1) their sizes significantly increased upon DIF+, and this increase was significantly enhanced by the presence of both IGF1 and M22 despite downsizing by monotreatment, and (2) their stiffness increased substantially, and no significant effects by IGF-1 and/or M22 were observed. Regarding the expression of ECM molecules, (1) upon DIF+, significant downregulation or upregulation of Col1 and Fn (3D), or Col4 and 6 (2D and 3D) were observed, and (2) in the presence of IGF-1 and/or M22, the mRNA expression of Col4 was significantly downregulated by M22 (2D and 3D), but the expression of Col1 was modulated in different manners by monotreatment (upregulation) or the combined treatment (downregulation) (3D). These collective data suggest that the human-specific TSmAb M22 induced some unexpected simultaneous crosslinking effects with IGF-1 with respect to the adipogenesis of 2D-cultured 3T3-L1 cells and the physical properties of 3D 3T3-L1 spheroids.

Mu-opioid receptor and receptor tyrosine kinase crosstalk: Implications in mechanisms of opioid tolerance, reduced analgesia to neuropathic pain, dependence, and reward

Despite the prevalence of opioid misuse, opioids remain the frontline treatment regimen for severe pain. However, opioid safety is hampered by side-effects such as analgesic tolerance, reduced analgesia to neuropathic pain, physical dependence, or reward. These side effects promote development of opioid use disorders and ultimately cause overdose deaths due to opioid-induced respiratory depression. The intertwined nature of signaling via μ-opioid receptors (MOR), the primary target of prescription opioids, with signaling pathways responsible for opioid side-effects presents important challenges. Therefore, a critical objective is to uncouple cellular and molecular mechanisms that selectively modulate analgesia from those that mediate side-effects. One such mechanism could be the transactivation of receptor tyrosine kinases (RTKs) via MOR. Notably, MOR-mediated side-effects can be uncoupled from analgesia signaling via targeting RTK family receptors, highlighting physiological relevance of MOR-RTKs crosstalk. This review focuses on the current state of knowledge surrounding the basic pharmacology of RTKs and bidirectional regulation of MOR signaling, as well as how MOR-RTK signaling may modulate undesirable effects of chronic opioid use, including opioid analgesic tolerance, reduced analgesia to neuropathic pain, physical dependence, and reward. Further research is needed to better understand RTK-MOR transactivation signaling pathways, and to determine if RTKs are a plausible therapeutic target for mitigating opioid side effects.

Melatonin: A Neurotrophic Factor?

Melatonin, N-acetyl-5-hydroxytryptamine, is a hormone that synchronizes the internal environment with the photoperiod. It is synthesized in the pineal gland and greatly depends on the endogenous circadian clock located in the suprachiasmatic nucleus and the retina's exposure to different light intensities. Among its most studied functions are the regulation of the waking-sleep rhythm and body temperature. Furthermore, melatonin has pleiotropic actions, which affect, for instance, the modulation of the immune and the cardiovascular systems, as well as the neuroprotection achieved by scavenging free radicals. Recent research has supported that melatonin contributes to neuronal survival, proliferation, and differentiation, such as dendritogenesis and axogenesis, and its processes are similar to those caused by Nerve Growth Factor, Brain-Derived Neurotrophic Factor, Neurotrophin-3, and Neurotrophin-4/5. Furthermore, this indolamine has apoptotic and anti-inflammatory actions in specific brain regions akin to those exerted by neurotrophic factors. This review presents evidence suggesting melatonin's role as a neurotrophic factor, describes the signaling pathways involved in these processes, and, lastly, highlights the therapeutic implications involved.

Ang II (Angiotensin II)-Induced FGFR1 (Fibroblast Growth Factor Receptor 1) Activation in Tubular Epithelial Cells Promotes Hypertensive Kidney Fibrosis and Injury

BACKGROUND

Elevated Ang II (angiotensin II) level leads to a range of conditions, including hypertensive kidney disease. Recent evidences indicate that FGFR1 (fibroblast growth factor receptor 1) signaling may be involved in kidney injuries. In this study, we determined whether Ang II alters FGFR1 signaling to mediate renal dysfunction.

METHODS

Human archival kidney samples from patients with or without hypertension were examined. Multiple genetic and pharmacological approaches were used to investigate FGFR1-mediated signaling in tubular epithelial NRK-52E cells in response to Ang II stimulation. C57BL/6 mice were infused with Ang II for 28 days to develop hypertensive kidney disease. Mice were treated with either adeno-associated virus expressing FGFR1 shRNA or FGFR1 inhibitor AZD4547.

RESULTS

Kidney specimens from subjects with hypertension and mice challenged with Ang II have increased FGFR1 activity in renal epithelial cells. Renal epithelial cells in culture initiate extracellular matrix programming in response to Ang II, through the activation of FGFR1, which is independent of both AT1R (angiotensin II receptor type 1) and AT2R (angiotensin II receptor type 2). The RNA sequencing analysis indicated that disrupting FGFR1 suppresses Ang II-induced fibrogenic responses in epithelial cells. Mechanistically, Ang II-activated FGFR1 leads to STAT3 (signal transducer and activator of transcription 3) activation, which is responsible for fibrogenic factor expression in kidneys. In the mouse model of hypertensive kidney disease, genetic knockdown of FGFR1 or pharmacological inhibition of its activity protected kidneys from dysfunction and fibrosis upon Ang II challenge.

CONCLUSIONS

Our studies uncover a novel mechanism causing renal fibrosis in hypertension and indicate FGFR1 as a potential target to preserve renal function and integrity.

It Takes Two to Tango: IGF-I and TSH Receptors in Thyroid Eye Disease

CONTEXT

Thyroid eye disease (TED) is a complex autoimmune disease process. Orbital fibroblasts represent the central orbital immune target. Involvement of the TSH receptor (TSHR) in TED is not fully understood. IGF-I receptor (IGF-IR) is overexpressed in several cell types in TED, including fibrocytes and orbital fibroblasts. IGF-IR may form a physical and functional complex with TSHR.

OBJECTIVE

Review literature relevant to autoantibody generation in TED and whether these induce orbital fibroblast responses directly through TSHR, IGF-IR, or both.

EVIDENCE

IGF-IR has traditionally been considered a typical tyrosine kinase receptor in which tyrosine residues become phosphorylated following IGF-I binding. Evidence has emerged that IGF-IR possesses kinase-independent activities and can be considered a functional receptor tyrosine kinase/G-protein-coupled receptor hybrid, using the G-protein receptor kinase/β-arrestin system. Teprotumumab, a monoclonal IGF-IR antibody, effectively reduces TED disease activity, proptosis, and diplopia. In addition, the drug attenuates in vitro actions of both IGF-I and TSH in fibrocytes and orbital fibroblasts, including induction of proinflammatory cytokines by TSH and TED IgGs.

CONCLUSIONS

Although teprotumumab has been proven effective and relatively safe in the treatment of TED, many questions remain pertaining to IGF-IR, its relationship with TSHR, and how the drug might be disrupting these receptor protein/protein interactions. Here, we propose 4 possible IGF-IR activation models that could underlie clinical responses to teprotumumab observed in patients with TED. Teprotumumab is associated with several adverse events, including hyperglycemia and hearing abnormalities. Underpinning mechanisms of these are being investigated. Patients undergoing treatment with drug must be monitored for these and managed with best medical practices.

The Elastin Receptor Complex: An Emerging Therapeutic Target Against Age-Related Vascular Diseases

The incidence of cardiovascular diseases is increasing worldwide with the growing aging of the population. Biological aging has major influence on the vascular tree and is associated with critical changes in the morphology and function of the arterial wall together with an extensive remodeling of the vascular extracellular matrix. Elastic fibers fragmentation and release of elastin degradation products, also known as elastin-derived peptides (EDPs), are typical hallmarks of aged conduit arteries. Along with the direct consequences of elastin fragmentation on the mechanical properties of arteries, the release of EDPs has been shown to modulate the development and/or progression of diverse vascular and metabolic diseases including atherosclerosis, thrombosis, type 2 diabetes and nonalcoholic steatohepatitis. Most of the biological effects mediated by these bioactive peptides are due to a peculiar membrane receptor called elastin receptor complex (ERC). This heterotrimeric receptor contains a peripheral protein called elastin-binding protein, the protective protein/cathepsin A, and a transmembrane sialidase, the neuraminidase-1 (NEU1). In this review, after an introductive part on the consequences of aging on the vasculature and the release of EDPs, we describe the composition of the ERC, the signaling pathways triggered by this receptor, and the current pharmacological strategies targeting ERC activation. Finally, we present and discuss new regulatory functions that have emerged over the last few years for the ERC through desialylation of membrane glycoproteins by NEU1, and its potential implication in receptor transactivation.

PDGF Promotes Dermal Fibroblast Activation via a Novel Mechanism Mediated by Signaling Through MCHR1

Systemic sclerosis (SSc) is an autoimmune disease characterized by vasculopathy and excessive fibrosis of the skin and internal organs. To this day, no effective treatments to prevent the progression of fibrosis exist, and SSc patients have disabilities and reduced life expectancy. The need to better understand pathways that drive SSc and to find therapeutic targets is urgent. RNA sequencing data from SSc dermal fibroblasts suggested that melanin-concentrating hormone receptor 1 (MCHR1), one of the G protein-coupled receptors regulating emotion and energy metabolism, is abnormally deregulated in SSc. Platelet-derived growth factor (PDGF)-BB stimulation upregulated MCHR1 mRNA and protein levels in normal human dermal fibroblasts (NHDF), and MCHR1 silencing prevented the PDGF-BB-induced expression of the profibrotic factors transforming growth factor beta 1 (TGFβ1) and connective tissue growth factor (CTGF). PDGF-BB bound MCHR1 in membrane fractions of NHDF, and the binding was confirmed using surface plasmon resonance (SPR). MCHR1 inhibition blocked PDGF-BB modulation of intracellular cyclic adenosine monophosphate (cAMP). MCHR1 silencing in NHDF reduced PDGF-BB signaling. In summary, MCHR1 promoted the fibrotic response in NHDF through modulation of TGFβ1 and CTGF production, intracellular cAMP levels, and PDGF-BB-induced signaling pathways, suggesting that MCHR1 plays an important role in mediating the response to PDGF-BB and in the pathogenesis of SSc. Inhibition of MCHR1 should be considered as a novel therapeutic strategy in SSc-associated fibrosis.

Reactivities of a Prostanoid EP2 Agonist, Omidenepag, Are Useful for Distinguishing between 3D Spheroids of Human Orbital Fibroblasts without or with Graves' Orbitopathy

Background. To obtain new insights into the activation of the thyroid-stimulating hormone (TSH) and insulin-like growth factor 1 (IGF-1) receptors in human orbital fibroblasts (n-HOFs), the effects of the prostanoid EP2 agonist, omidenepag (OMD), and a rho-associated coiled-coil-containing protein kinase (ROCK) inhibitor, ripasudil (Rip) were evaluated using three-dimension (3D) n-HOFs spheroids in the absence and presence of the recombinant human TSH receptor antibodies, M22 and IGF-1. Methods. The effects of 100 nM OMD or 10 μM Rip on the physical properties, size, stiffness, and mRNA expression of several extracellular matrix (ECM) molecules, their regulator, inflammatory cytokines, and endoplasmic reticulum (ER) stress-related factors were examined and compared among 3D spheroids of n-HOFs, M22-/IGF-1-activated n-HOFs and GO-related human orbital fibroblasts (GHOFs). Results. The physical properties and mRNA expressions of several genes of the 3D n-HOFs spheroids were significantly and diversely modulated by the presence of OMD or Rip. The OMD-induced effects on M22-/IGF-1-activated n-HOFs were similar to the effects caused by GHOHs, but quite different from those of n-HOFs. Conclusions. The findings presented herein indicate that the changes induced by OMD may be useful in distinguishing between n-HOFs and GHOFs.

The Complex Formed by Group I Metabotropic Glutamate Receptor (mGluR) and Homer1a Plays a Central Role in Metaplasticity and Homeostatic Synaptic Scaling

G-protein-coupled receptors can be constitutively activated following physical interaction with intracellular proteins. The first example described was the constitutive activation of Group I metabotropic glutamate receptors (mGluR: mGluR1,5) following their interaction with Homer1a, an activity-inducible early-termination variant of the scaffolding protein Homer that lacks dimerization capacity (Ango et al., 2001). Homer1a disrupts the links, maintained by the long form of Homer (cross-linking Homers), between mGluR1,5 and the Shank-GKAP-PSD-95-ionotropic glutamate receptor network. Two characteristics of the constitutive activation of the Group I mGluR-Homer1a complex are particularly interesting: (1) it affects a large number of synapses in which Homer1a is upregulated following enhanced, long-lasting neuronal activity; and (2) it mainly depends on Homer1a protein turnover. The constitutively active Group I mGluR-Homer1a complex is involved in the two main forms of non-Hebbian neuronal plasticity: "metaplasticity" and "homeostatic synaptic scaling," which are implicated in a large series of physiological and pathologic processes. Those include non-Hebbian plasticity observed in visual system, synapses modulated by addictive drugs (rewarded synapses), chronically overactivated synaptic networks, normal sleep, and sleep deprivation.

Morphine promotes tumorigenesis and cetuximab resistance via EGFR signaling activation in human colorectal cancer

Morphine, a mu-opioid receptor (MOR) agonist, has been extensively used to treat advanced cancer pain. In particular, in patients with cancer metastasis, both morphine and anticancer drugs are given simultaneously. However, evidence showed that morphine might be a risk factor in promoting the tumor's malignant potential. In this study, we report that treatment with morphine could activate MOR and lead to the promotion of proliferation, migration, and invasion in HCT116 and DLD1 colorectal cancer (CRC) cells with time-concentration dependence. Moreover, morphine can also contribute to cetuximab's drug resistance, a targeted drug widely used to treat advanced CRC by inducing the activation of epidermal growth factor receptor (EGFR). The cell phenotype includes proliferation, migration, invasion, and drug resistance, which may be reversed by MOR knockdown or adding nalmefene, the MOR receptor antagonist. Receptor tyrosine kinase array analysis revealed that morphine selectively induced the transactivation of EGFR. EGFR transactivation resulted in the activation of ERK1/2 and AKT. In conclusion, morphine induces the transactivation of EGFR via MOR. It activates the downstream signal pathway AKT-MTOR and RAS-MAPK, increases proliferation, migration, and invasion, and promotes resistance to EGFR inhibitors in a CRC cell line. Furthermore, we verified that EGFR inhibition by cetuximab strongly reversed the protumoral effects of morphine in vitro and in vivo. Collectively, we provide evidence that morphine-EGFR signaling might be a promising therapeutic target for CRC patients, especially for cetuximab-resistant CRC patients.

ELTD1-An Emerging Silent Actor in Cancer Drama Play

The epidermal growth factor, latrophilin, and seven transmembrane domain–containing protein 1 (ELTD1), is a member of the G–protein coupled receptors (GPCRs) superfamily. Although discovered in 2001, ELTD1 has been investigated only by a few research groups, and important data about its role in normal and tumor cells is still missing. Even though its functions and structure are not yet fully understood, recent studies show that ELTD1 has a role in both physiological and pathological angiogenesis, and it appears to be a very important biomarker and a molecular target in cancer diseases. Upregulation of ELTD1 in malignant cells has been reported, and correlated with poor cancer prognosis. This review article aims to compile the existing data and to discuss the current knowledge on ELTD1 structure and signaling, and its role in physiological and neoplastic conditions.

Epidermal growth factor receptor as a molecular determinant of glioblastoma response to dopamine receptor D2 inhibitors

BACKGROUND

There are ongoing clinical trials exploring the efficacy of dopamine receptor D2 (DRD2) inhibition against glioblastomas, the most common primary brain tumor. Here we examine potential molecular determinants of this efficacy.

METHODS

The Cancer Genome Atlas glioblastoma database and other published mRNA profiles were used to analyze the DRD2 and epidermal growth factor receptor (EGFR) expression pattern. In vitro and in vivo responses to DRD2 inhibitors were determined using patient-derived xenograft (PDX) glioblastoma models. Immunohistochemical studies were performed on clinically annotated glioblastoma samples derived from patients treated with ONC201.

RESULTS

Analysis of clinical glioblastoma specimens derived from independent patient cohorts revealed an inverse correlation between EGFR and DRD2 mRNA expression, with implication that signaling mediated by these proteins shares overlapping functions. In independent panels of PDX glioblastoma lines, high EGFR expression was associated with poor in vitro and in vivo response to DRD2 inhibitors, including haloperidol and ONC201. Moreover, ectopic expression of a constitutively active EGFR, variant (v)III, suppressed glioblastoma sensitivity to ONC201. DRD2 expression positively correlated with expression of rate-limiting enzymes for dopamine synthesis as well as dopamine secretion, suggesting contribution of autocrine DRD2 signaling. Analysis of specimens from patients treated with ONC201 (n = 15) showed an inverse correlation between the intensity of EGFR staining and clinical response. The median overall survival for patients with high and low EGFR staining was 162 and 373 days, respectively (0.037).

CONCLUSIONS

High EGFR expression is a determinant of poor glioblastoma response to DRD2. This finding should inform future clinical trial designs.

ERK1/2: An Integrator of Signals That Alters Cardiac Homeostasis and Growth

Integration of cellular responses to extracellular cues is essential for cell survival and adaptation to stress. Extracellular signal-regulated kinase (ERK) 1 and 2 serve an evolutionarily conserved role for intracellular signal transduction that proved critical for cardiomyocyte homeostasis and cardiac stress responses. Considering the importance of ERK1/2 in the heart, understanding how these kinases operate in both normal and disease states is critical. Here, we review the complexity of upstream and downstream signals that govern ERK1/2-dependent regulation of cardiac structure and function. Particular emphasis is given to cardiomyocyte hypertrophy as an outcome of ERK1/2 activation regulation in the heart.

An experimental strategy to probe Gq contribution to signal transduction in living cells

Heterotrimeric G protein subunits Gαq and Gα11 are inhibited by two cyclic depsipeptides, FR900359 (FR) and YM-254890 (YM), both of which are being used widely to implicate Gq/11 proteins in the regulation of diverse biological processes. An emerging major research question therefore is whether the cellular effects of both inhibitors are on-target, that is, mediated via specific inhibition of Gq/11 proteins, or off-target, that is, the result of nonspecific interactions with other proteins. Here we introduce a versatile experimental strategy to discriminate between these possibilities. We developed a Gαq variant with preserved catalytic activity, but refractory to FR/YM inhibition. A minimum of two amino acid changes were required and sufficient to achieve complete inhibitor resistance. We characterized the novel mutant in HEK293 cells depleted by CRISPR–Cas9 of endogenous Gαq and Gα11 to ensure precise control over the Gα-dependent cellular signaling route. Using a battery of cellular outcomes with known and concealed Gq contribution, we found that FR/YM specifically inhibited cellular signals after Gαq introduction via transient transfection. Conversely, both inhibitors were inert across all assays in cells expressing the drug-resistant variant. These findings eliminate the possibility that inhibition of non-Gq proteins contributes to the cellular effects of the two depsipeptides. We conclude that combined application of FR or YM along with the drug-resistant Gαq variant is a powerful in vitro strategy to discern on-target Gq against off-target non-Gq action. Consequently, it should be of high value for uncovering Gq input to complex biological processes with high accuracy and the requisite specificity.

Receptor Tyrosine Kinase Signaling and Targeting in Glioblastoma Multiforme

Glioblastoma multiforme (GBM) is amongst the deadliest of human cancers, with a median survival rate of just over one year following diagnosis. Characterized by rapid proliferation and diffuse infiltration into the brain, GBM is notoriously difficult to treat, with tumor cells showing limited response to existing therapies and eventually developing resistance to these interventions. As such, there is intense interest in better understanding the molecular alterations in GBM to guide the development of more efficient targeted therapies. GBM tumors can be classified into several molecular subtypes which have distinct genetic signatures, and they show aberrant activation of numerous signal transduction pathways, particularly those connected to receptor tyrosine kinases (RTKs) which control glioma cell growth, survival, migration, invasion, and angiogenesis. There are also non-canonical modes of RTK signaling found in GBM, which involve G-protein-coupled receptors and calcium channels. This review uses The Cancer Genome Atlas (TCGA) GBM dataset in combination with a data-mining approach to summarize disease characteristics, with a focus on select molecular pathways that drive GBM pathogenesis. We also present a unique genomic survey of RTKs that are frequently altered in GBM subtypes, as well as catalog the GBM disease association scores for all RTKs. Lastly, we discuss current RTK targeted therapies and highlight emerging directions in GBM research.

Endothelial sprouting, proliferation, or senescence: tipping the balance from physiology to pathology

Therapeutic modulation of vascular cell proliferation and migration is essential for the effective inhibition of angiogenesis in cancer or its induction in cardiovascular disease. The general view is that an increase in vascular growth factor levels or mitogenic stimulation is beneficial for angiogenesis, since it leads to an increase in both endothelial proliferation and sprouting. However, several recent studies showed that an increase in mitogenic stimuli can also lead to the arrest of angiogenesis. This is due to the existence of intrinsic signaling feedback loops and cell cycle checkpoints that work in synchrony to maintain a balance between endothelial proliferation and sprouting. This balance is tightly and effectively regulated during tissue growth and is often deregulated or impaired in disease. Most therapeutic strategies used so far to promote vascular growth simply increase mitogenic stimuli, without taking into account its deleterious effects on this balance and on vascular cells. Here, we review the main findings on the mechanisms controlling physiological vascular sprouting, proliferation, and senescence and how those mechanisms are often deregulated in acquired or congenital cardiovascular disease leading to a diverse range of pathologies. We also discuss alternative approaches to increase the effectiveness of pro-angiogenic therapies in cardiovascular regenerative medicine.

Spinal Opioid Tolerance Depends upon Platelet-Derived Growth Factor Receptor-β Signaling, Not μ-Opioid Receptor Internalization

Opioids are some of the most potent analgesics available. However, their effectiveness is limited by the development of analgesic tolerance. Traditionally, tolerance was thought to occur by termination of μ-opioid receptor (MOR) signaling via desensitization and internalization. Contradictory findings led to a more recent proposal that sustained MOR signaling caused analgesic tolerance. However, this view has also been called into question. We recently discovered that the platelet-derived growth factor receptor(PDGFR)-β signaling system is both necessary and sufficient to cause opioid tolerance. We therefore propose a completely new hypothesis: that opioid tolerance is mediated by selective cellular signals and is independent of MOR internalization. To test this hypothesis, we developed an automated software-based method to perform unbiased analyses of opioid-induced MOR internalization in the rat substantia gelatinosa. We induced tolerance with either morphine, which did not cause MOR internalization, or fentanyl, which did. We also blocked tolerance by administering morphine or fentanyl with the PDGFR-β inhibitor imatinib. We found that imatinib blocked tolerance without altering receptor internalization induced by either morphine or fentanyl. We also showed that imatinib blocked tolerance to other clinically used opioids. Our findings indicate that opioid tolerance is not dependent upon MOR internalization and support the novel hypothesis that opioid tolerance is mediated by intracellular signaling that can be selectively targeted. This suggests the exciting possibility that undesirable opioid side effects can be selectively eliminated, dramatically improving the safety and efficacy of opioids. SIGNIFICANCE STATEMENT: Classically, it was thought that analgesic tolerance to opioids was caused by desensitization and internalization of μ-opioid receptors (MORs). More recently, it was proposed that sustained, rather than reduced, MOR signaling caused tolerance. Here, we present conclusive evidence that opioid tolerance occurs independently of MOR internalization and that it is selectively mediated by platelet-derived growth factor receptor signaling. This novel hypothesis suggests that dangerous opioid side effects can be selectively targeted and blocked, improving the safety and efficacy of opioids.

BDNF belongs to the nurse-like cell secretome and supports survival of B chronic lymphocytic leukemia cells

Evading apoptosis and sustained survival signaling pathways are two central hallmarks of B-cell chronic lymphocytic leukemia (B-CLL) cells. In this regard, nurse-like cells (NLC), the monocyte-derived type 2 macrophages, deliver stimulatory signals via B-cell activating factor (BAFF), a proliferation-inducing ligand (APRIL), and the C-X-C Motif Chemokine Ligand 12 (CXCL12). Previously, we demonstrated that brain-derived neurotrophic factor (BDNF) protects B-CLL cells from spontaneous apoptosis by activating the oncogenic complex NTSR2-TrkB (neurotensin receptor 2-tropomyosin-related kinase receptor B), only overexpressed in B-CLL cells, inducing anti-apoptotic protein Bcl-2 (B-cell lymphoma 2) expression and Src kinase survival signaling pathways. Herein, we demonstrate that BDNF belongs to the NLC secretome and promotes B-CLL survival. This was demonstrated in primary B-CLL co-cultured with their autologous NLC, compared to B-CLL cells cultured alone. Inhibition of BDNF in co-cultures, enhances B-CLL apoptosis, whereas its exogenous recombinant activates pro-survival pathways in B-CLL cultured alone (i.e. Src activation and Bcl-2 expression), at a higher level than those obtained by the exogenous recombinant cytokines BAFF, APRIL and CXCL12, the known pro-survival cytokines secreted by NLC. Together, these results showed that BDNF release from NLC trigger B-CLL survival. Blocking BDNF would support research strategies against pro-survival cytokines to limit sustained B-CLL cell survival.

Modulation of CXCR4-Mediated Gi1 Activation by EGF Receptor and GRK2

The CXCL12 chemokine receptor CXCR4 belongs to the GPCR superfamily and is often overexpressed in cancer, being involved in tumor progression and metastasis. How CXCR4 signaling integrates with other relevant oncogenic transduction pathways and the role of GPCR regulatory mechanisms in such contexts are not well-understood. Recent data indicate concurrent upregulation in certain tumors of CXCR4, EGF receptor (EGFR), and G protein-coupled receptor kinase 2 (GRK2), a signaling node functionally linked to both receptor types. We have investigated in a model system the effect of the EGFR and GRK2 status on CXCL12/CXCR4-mediated activation of Gi, the earliest step downstream of receptor activation. We find that overexpressed and activated EGFR reduces CXCR4-mediated Gi1 activation and that GRK2 phosphorylation at tyrosine residues is required to exert its inhibitory actions on CXCR4–Gi stimulation, suggesting a shared path of modulation. Our data point to a role for GRK2 in the crosstalk of the CXCR4 and EGFR signal transduction pathways in pathological contexts characterized by concurrent overactivation of these proteins.

EGFR Signaling Causes Morphine Tolerance and Mechanical Sensitization in Rats

The safety and efficacy of opioids are compromised as analgesic tolerance develops. Opioids are also ineffective against neuropathic pain. Recent reports have suggested that inhibitors of the epidermal growth factor receptor (EGFR), a receptor tyrosine kinase (RTK), may have analgesic effects in cancer patients suffering from neuropathic pain. It has been shown that the platelet-derived growth factor receptor-β (PDGFR-β), an RTK that has been shown to interact with the EGFR, mediates opioid tolerance but does not induce analgesia. Therefore, we sought to determine whether EGFR signaling was involved in opioid tolerance and whether EGFR and PDGFR signaling could induce pain in rats. We found that gefitinib, an EGFR antagonist, eliminated morphine tolerance. In addition, repeated EGF administration rendered animals unresponsive to subsequent analgesic doses of morphine, a phenomenon we call "pre-tolerance." Using a nerve injury model, we found that gefitinib alone was not analgesic. Rather, it reversed insensitivity to morphine analgesia (pre-tolerance) caused by the release of EGF by injured nerves. We also showed that repeated, but not acute EGF or PDGF-BB administration induced mechanical hypersensitivity in rats. EGFR and PDGFR-β signaling interacted to produce this sensitization. EGFR was widely expressed in primary sensory afferent cell bodies, demonstrating a neuroanatomical substrate for our findings. Taken together, our results suggest a direct mechanistic link between opioid tolerance and mechanical sensitization. EGFR antagonism could eventually play an important clinical role in the treatment of opioid tolerance and neuropathic pain that is refractory to opioid treatment.

TSH/IGF1 receptor crosstalk: Mechanism and clinical implications

Increasing evidence of interdependence between G protein-coupled receptors and receptor tyrosine kinase signaling pathways has prompted reevaluation of crosstalk between these receptors in disease and therapy. Investigations into thyroid-stimulating hormone (TSH) and insulin-like growth factor 1 (IGF1) receptor crosstalk, and its application to the clinic have in particular shown recent progress. In this review, we summarize current insights into the mechanism of TSH/IGF1 receptor crosstalk. We discuss evidence that crosstalk is one of the underlying causes of TSHR-based disease and the feasibility of using combinations of TSH receptor and IGF1 receptor antagonists to increase the therapeutic index for the treatment of Graves' hyperthyroidism and Graves' ophthalmopathy.

β-Arrestin 1 in Thyrotropin Receptor Signaling in Bone: Studies in Osteoblast-Like Cells

A direct action of thyrotropin (TSH, thyroid-stimulating hormone) on bone precursors in humans is controversial. Studies in rodent models have provided conflicting findings. We used cells derived from a moderately differentiated osteosarcoma stably overexpressing human TSH receptors (TSHRs) as a model of osteoblast precursors (U2OS-TSHR cells) to investigate TSHR-mediated effects in bone differentiation in human cells. We review our findings that (1) TSHR couples to several different G proteins to induce upregulation of genes associated with osteoblast activity-interleukin 11 (IL-11), osteopontin (OPN), and alkaline phosphatase (ALPL) and that the kinetics of the induction and the G protein-mediated signaling pathways involved were different for these genes; (2) TSH can stimulate β-arrestin-mediated signal transduction and that β-arrestin 1 in part mediates TSH-induced pre-osteoblast differentiation; and (3) TSHR/insulin-like growth factor 1 (IGF1) receptor (IGF1R) synergistically increased OPN secretion by TSH and IGF1 and that this crosstalk was mediated by physical association of these receptors in a signaling complex that uses β-arrestin 1 as a scaffold. These findings were complemented using a novel β-arrestin 1-biased agonist of TSHR. We conclude that TSHR can signal via several transduction pathways leading to differentiation of this model system of human pre-osteoblast cells and, therefore, that TSH can directly regulate these bone cells.

CXCR4/ACKR3 Phosphorylation and Recruitment of Interacting Proteins: Key Mechanisms Regulating Their Functional Status

The C-X-C motif chemokine receptor type 4 (CXCR4) and the atypical chemokine receptor 3 (ACKR3/CXCR7) are class A G protein-coupled receptors (GPCRs). Accumulating evidence indicates that GPCR subcellular localization, trafficking, transduction properties, and ultimately their pathophysiological functions are regulated by both interacting proteins and post-translational modifications. This has encouraged the development of novel techniques to characterize the GPCR interactome and to identify residues subjected to post-translational modifications, with a special focus on phosphorylation. This review first describes state-of-the-art methods for the identification of GPCR-interacting proteins and GPCR phosphorylated sites. In addition, we provide an overview of the current knowledge of CXCR4 and ACKR3 post-translational modifications and an exhaustive list of previously identified CXCR4- or ACKR3-interacting proteins. We then describe studies highlighting the importance of the reciprocal influence of CXCR4/ACKR3 interactomes and phosphorylation states. We also discuss their impact on the functional status of each receptor. These studies suggest that deeper knowledge of the CXCR4/ACKR3 interactomes along with their phosphorylation and ubiquitination status would shed new light on their regulation and pathophysiological functions.

PI3Kβ is selectively required for growth factor-stimulated macropinocytosis

Macropinocytosis is an actin-dependent but clathrin-independent endocytic process by which cells nonselectively take up large aliquots of extracellular material. Macropinocytosis is used for immune surveillance by dendritic cells, as a route of infection by viruses and protozoa, and as a nutrient uptake pathway in tumor cells. In this study, we explore the role of class I phosphoinositide 3-kinases (PI3Ks) during ligand-stimulated macropinocytosis. We find that macropinocytosis in response to receptor tyrosine kinase activation is strikingly dependent on a single class I PI3K isoform, namely PI3Kβ (containing the p110β catalytic subunit encoded by PIK3CB). Loss of PI3Kβ expression or activity blocks macropinocytosis at early steps, before the formation of circular dorsal ruffles, but also plays a role in later steps, downstream from Rac1 activation. PI3Kβ is also required for the elevated levels of constitutive macropinocytosis found in tumor cells that are defective for the PTEN tumor suppressor. Our data shed new light on PI3K signaling during macropinocytosis, and suggest new therapeutic uses for pharmacological inhibitors of PI3Kβ.

Arrestin-β-1 Physically Scaffolds TSH and IGF1 Receptors to Enable Crosstalk

Endogenously expressed TSH receptors (TSHRs) on orbital fibroblasts of patients with Graves ophthalmopathy (GO) use crosstalk with IGF1 receptors (IGF1R) to synergistically stimulate secretion of hyaluronan (HA), a major component of GO pathology. We previously showed crosstalk occurred upstream of mitogen-activated protein kinase (ERK) phosphorylation. Because other G protein-coupled receptors engage arrestin-β-1 (ARRB1) and ERK, we tested whether ARRB1 was a necessary component of TSHR/IGF1R crosstalk. HA secretion was stimulated by the TSHR-stimulating monoclonal antibodies M22 and KSAb1, or immunoglobulins from patients with GO (GO-Igs). Treatment with M22, as previously shown, resulted in biphasic dose-response stimulation of HA secretion. The high-potency phase was IGF1R dependent, and the low-potency phase was partly IGF1R independent. KSAb1 produced a monophasic dose-response stimulation of HA secretion, whose potency was lowered >20-fold after IGF1R knockdown. ARRB1 knockdown abolished M22's high-potency phase and lowered KSAb1's potency and efficacy. ARRB1 knockdown inhibited GO-Ig stimulation of HA secretion and of ERK phosphorylation. Last, ARRB1 was shown to be necessary for TSHR/IGF1R proximity. In contrast, ARRB2 knockdowns did not show these effects. Thus, TSHR must neighbor IGF1R for crosstalk in GO fibroblasts to occur, and this depends on ARRB1 acting as a scaffold. Similar scaffolding of TSHR and IGF1R by ARRB1 was found in human osteoblast-like cells and human thyrocytes. These findings support a model of TSHR/IGF1R crosstalk that may be a general mechanism for G-protein-coupled receptor/receptor tyrosine kinase crosstalk dependent on ARRB1.

Insulin-like Growth Factor-I Receptor and Thyroid-Associated Ophthalmopathy

Thyroid-associated ophthalmopathy (TAO) is a complex disease process presumed to emerge from autoimmunity occurring in the thyroid gland, most frequently in Graves disease (GD). It is disfiguring and potentially blinding, culminating in orbital tissue remodeling and disruption of function of structures adjacent to the eye. There are currently no medical therapies proven capable of altering the clinical outcome of TAO in randomized, placebo-controlled multicenter trials. The orbital fibroblast represents the central target for immune reactivity. Recent identification of fibroblasts that putatively originate in the bone marrow as monocyte progenitors provides a plausible explanation for why antigens, the expressions of which were once considered restricted to the thyroid, are detected in the TAO orbit. These cells, known as fibrocytes, express relatively high levels of functional TSH receptor (TSHR) through which they can be activated by TSH and the GD-specific pathogenic antibodies that underpin thyroid overactivity. Fibrocytes also express insulin-like growth factor I receptor (IGF-IR) with which TSHR forms a physical and functional signaling complex. Notably, inhibition of IGF-IR activity results in the attenuation of signaling initiated at either receptor. Some studies suggest that IGF-IR-activating antibodies are generated in GD, whereas others refute this concept. These observations served as the rationale for implementing a recently completed therapeutic trial of teprotumumab, a monoclonal inhibitory antibody targeting IGF-IR in TAO. Results of that trial in active, moderate to severe disease revealed dramatic and rapid reductions in disease activity and severity. The targeting of IGF-IR with specific biologic agents may represent a paradigm shift in the therapy of TAO.

Investigating Cell Signaling with Gene Expression Datasets

Modern molecular biology is a data- and computationally-intensive field with few instructional resources for introducing undergraduate students to the requisite skills and techniques for analyzing large data sets. This Lesson helps students: (i) build an understanding of the role of signal transduction in the control of gene expression; (ii) improve written scientific communication skills through engagement in literature searches, data analysis, and writing reports; and (iii) develop an awareness of the procedures and protocols for analyzing and making inferences from high-content quantitative molecular biology data. The Lesson is most suited to upper level biology courses because it requires foundational knowledge on cellular organization, protein structure and function, and the tenets of information flow from DNA to proteins. The first step lays the foundation for understanding cell signaling, which can be accomplished through assigned readings and presentations. In subsequent active learning sessions, data analysis is integrated with exercises that provide insight into the structure of scientific papers. The Lesson emphasizes the role of quantitative methods in research and helps students gain experience with functional genomics databases and data analysis, which are important skills for molecular biologists. Assessment is conducted through mini-reports designed to gauge students' perceptions of the purpose of each step, their awareness of the possible limitations of the methods utilized, and the ability to identify opportunities for further investigation. Summative assessment is conducted through a final report. The modules are suitable for complementing wet-laboratory experiments and can be adapted for different courses that use molecular biology data.

GPCRs in Autocrine and Paracrine Regulations

G protein-coupled receptors (GPCRs) constitute the largest superfamily of integral membrane protein receptors. As signal detectors, the several 100 known GPCRs are responsible for sensing the plethora of endogenous ligands that are critical for the functioning of our endocrine system. Although GPCRs are typically considered as detectors for first messengers in classical signal transduction pathways, they seldom operate in isolation in complex biological systems. Intercellular communication between identical or different cell types is often mediated by autocrine or paracrine signals that are generated upon activation of specific GPCRs. In the context of energy homeostasis, the distinct complement of GPCRs in each cell type bridges the autocrine and paracrine communication within an organ, and the various downstream signaling mechanisms regulated by GPCRs can be integrated in a cell to produce an ultimate output. GPCRs thus act as gatekeepers that coordinate and fine-tune a response. By examining the role of GPCRs in activating and receiving autocrine and paracrine signals, one may have a better understanding of endocrine diseases that are associated with GPCR mutations, thereby providing new insights for treatment regimes.

Crosstalk between receptor tyrosine kinases (RTKs) and G protein-coupled receptors (GPCR) in the brain: Focus on heteroreceptor complexes and related functional neurotrophic effects

Neuronal events are regulated by the integration of several complex signaling networks in which G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs) are considered key players of an intense bidirectional cross-communication in the cell, generating signaling mechanisms that, at the same time, connect and diversify the traditional signal transduction pathways activated by the single receptor. For this receptor-receptor crosstalk, the two classes of receptors form heteroreceptor complexes resulting in RTKs transactivation and in growth-promoting signals. In this review, we describe heteroreceptor complexes between GPCR and RTKs in the central nervous system (CNS) and their functional effects in controlling a variety of neuronal effects, ranging from development, proliferation, differentiation and migration, to survival, repair, synaptic transmission and plasticity. In this interaction, RTKs can also recruit components of the G protein signaling cascade, creating a bidirectional intricate interplay that provides complex control over multiple cellular events. These heteroreceptor complexes, by the integration of different signals, have recently attracted a growing interest as novel molecular target for depressive disorders. This article is part of the Special Issue entitled 'Receptor heteromers and their allosteric receptor-receptor interactions'.

Thyroid Stimulating Hormone (TSH)/Insulin-like Growth Factor 1 (IGF1) Receptor Cross-talk in Human Cells

Thyroid stimulating hormone and insulin-like growth factor 1 receptors (TSHRs and IGF1Rs, respectively) interact leading to additive or synergistic stimulation of cellular responses. Recent findings provide evidence that the interaction between TSHRs and IGF1Rs is similar to that described for other G protein-coupled receptors and receptor tyrosine kinases. These types of interactions occur at or proximal to the receptors and are designated "receptor cross-talk." Herein, we describe our studies in human thyrocytes, human retro-orbital fibroblasts from Graves' orbitopathy patients and a model cell line that support the concept of TSHR/IGF1R cross-talk. We also discuss how receptor cross-talk is involved in stimulation by a monoclonal TSHR-stimulating antibody and how targeting both receptors may lead to novel treatments of Graves' orbitopathy.

Epidermal Growth Factor (EGF) Autocrine Activation of Human Platelets Promotes EGF Receptor-Dependent Oral Squamous Cell Carcinoma Invasion, Migration, and Epithelial Mesenchymal Transition

Activated platelets release functional, high m.w. epidermal growth factor (HMW-EGF). In this study, we show platelets also express epidermal growth factor (EGF) receptor (EGFR) protein, but not ErbB2 or ErbB4 coreceptors, and so might respond to HMW-EGF. We found HMW-EGF stimulated platelet EGFR autophosphorylation, PI3 kinase-dependent AKT phosphorylation, and a Ca²⁺ transient that were blocked by EGFR tyrosine kinase inhibition. Strong (thrombin) and weak (ADP, platelet-activating factor) G protein-coupled receptor agonists and non-G protein-coupled receptor collagen recruited EGFR tyrosine kinase activity that contributed to platelet activation because EGFR kinase inhibition reduced signal transduction and aggregation induced by each agonist. EGF stimulated ex vivo adhesion of platelets to collagen-coated microfluidic channels, whereas systemic EGF injection increased initial platelet deposition in FeCl₃-damaged murine carotid arteries. EGFR signaling contributes to oral squamous cell carcinoma (OSCC) tumorigenesis, but the source of its ligand is not established. We find individual platelets were intercalated within OSCC tumors. A portion of these platelets expressed stimulation-dependent Bcl-3 and IL-1β and so had been activated. Stimulated platelets bound OSCC cells, and material released from stimulated platelets induced OSCC epithelial-mesenchymal transition and stimulated their migration and invasion through Matrigel barriers. Anti-EGF Ab or EGFR inhibitors abolished platelet-induced tumor cell phenotype transition, migration, and invasion; so the only factor released from activated platelets necessary for OSCC metastatic activity was HMW-EGF. These results establish HMW-EGF in platelet function and elucidate a previously unsuspected connection between activated platelets and tumorigenesis through rapid, and prolonged, autocrine-stimulated release of HMW-EGF by tumor-associated platelets.

Is the membrane estrogen receptor, GPER1, a promiscuous receptor that modulates nuclear estrogen receptor-mediated functions in the brain?

Contribution to Special Issue on Fast effects of steroids. Estrogen signals both slowly to regulate transcription and rapidly to activate kinases and regulate calcium levels. Both rapid, non-genomic signaling as well as genomic transcriptional signaling via intracellular estrogen receptors (ER)s can change behavior. Rapid non-genomic signaling is initiated from the plasma membrane by a G-protein coupled receptor called GPER1 that binds 17β-estradiol. GPER1 or GPR30 is one of the candidates for a membrane ER (mER) that is not only highly expressed in pathology i.e. cancers but also in several behaviorally-relevant brain regions. In the brain, GPER1 signaling, in response to estrogen, facilitates neuroprotection, social behaviors and cognition. In this review, we describe several notable characteristics of GPER1 such as the ability of several endogenous steroids as well as artificially synthesized molecules to bind the GPER1. In addition, GPER1 is localized to the plasma membrane in breast cancer cell lines but may be present in the endoplasmic reticulum or the Golgi apparatus in the hippocampus. Unusually, GPER1 can also translocate to the perinuclear space from the plasma membrane. We explore the idea that subcellular localization and ligand promiscuity may determine the varied downstream signaling cascades of the activated GPER1. Lastly, we suggest that GPER1 can act as a modulator of ERα-mediated action on a convergent target, spinogenesis, in neurons that in turn drives female social behaviors such as lordosis and social learning.

GRK2 mediates TCR-induced transactivation of CXCR4 and TCR-CXCR4 complex formation that drives PI3Kγ/PREX1 signaling and T cell cytokine secretion

The immune system includes abundant examples of biologically-relevant cross-regulation of signaling pathways by the T cell antigen receptor (TCR) and the G protein-coupled chemokine receptor, CXCR4. TCR ligation induces transactivation of CXCR4 and TCR-CXCR4 complex formation, permitting the TCR to signal via CXCR4 to activate a phosphatidylinositol 3,4,5-trisphosphate-dependent Rac exchanger 1 protein (PREX1)-dependent signaling pathway that drives robust cytokine secretion by T cells. To understand this receptor heterodimer and its regulation, we characterized the molecular mechanisms required for TCR-mediated TCR-CXCR4 complex formation. We found that the cytoplasmic C-terminal domain of CXCR4 and specifically phosphorylation of Ser-339 within this region were required for TCR-CXCR4 complex formation. Interestingly, siRNA-mediated depletion of G protein-coupled receptor kinase-2 (GRK2) or inhibition by the GRK2-specific inhibitor, paroxetine, inhibited TCR-induced phosphorylation of CXCR4-Ser-339 and TCR-CXCR4 complex formation. Either GRK2 siRNA or paroxetine treatment of human T cells significantly reduced T cell cytokine production. Upstream, TCR-activated tyrosine kinases caused inducible tyrosine phosphorylation of GRK2 and were required for the GRK2-dependent events of CXCR4-Ser-339 phosphorylation and TCR-CXCR4 complex formation. Downstream of TCR-CXCR4 complex formation, we found that GRK2 and phosphatidylinositol 3-kinase γ (PI3Kγ) were required for TCR-stimulated membrane recruitment of PREX1 and for stabilization of cytokine mRNAs and robust cytokine secretion. Together, our results identify a novel role for GRK2 as a target of TCR signaling that is responsible for TCR-induced transactivation of CXCR4 and TCR-CXCR4 complex formation that signals via PI3Kγ/PREX1 to mediate cytokine production. Therapeutic regulation of GRK2 or PI3Kγ may therefore be useful for limiting cytokines produced by T cell malignancies or autoimmune diseases.

Inflammation-linked adaptations in dermal microvascular reactivity accompany the development of obesity and type 2 diabetes

Background/Objectives

The increased prevalence of obesity has prompted great strides in our understanding of specific adipose depots and their involvement in cardio-metabolic health. However, the impact of obesity on dermal white adipose tissue (dWAT) and dermal microvascular functionality remains unclear. This study aimed to investigate the temporal changes that occur in dWAT and dermal microvascular functionality during the development of diet-induced obesity and type 2 diabetes in mice.

Methods

Metabolic phenotyping of a murine model of hypercaloric diet (HCD)-induced obesity and type 2 diabetes was performed at three time points that reflected three distinct stages of disease development; 2 weeks of HCD-overweight-metabolically healthy, 4 weeks of HCD-obese-prediabetic and 12 weeks of HCD-obese-type 2 diabetic mice. Expansion of dWAT was characterized histologically, and changes in dermal microvascular reactivity were assessed in response to pressure and the vasodilators SNP and Ach.

Results

HCD resulted in a progressive expansion of dWAT and increased expression of pro-inflammatory markers (IL1β and COX-2). Impairments in pressure-induced (PIV) and Ach-induced (endothelium-dependent) vasodilation occurred early, in overweight-metabolically healthy mice. Residual vasodilatory responses were NOS-independent but sensitive to COX inhibition. These changes were associated with reductions in NO and adiponectin bioavailability, and rescued by exogenous adiponectin or hyperinsulinemia. Obese-prediabetic mice continued to exhibit impaired Ach-dependent vasodilation but PIV appeared normalized. This normalization coincided with elevated endogenous adiponectin and insulin levels, and was sensitive to NOS, COX and PI3K, inhibition. In obese-type 2 diabetic mice, both Ach-stimulated and pressure-induced vasodilatory responses were increased through enhanced COX-2-dependent prostaglandin response.

Conclusions

We demonstrate that the development of obesity, metabolic dysfunction and type 2 diabetes, in HCD-fed mice, is accompanied by increased dermal adiposity and associated metaflammation in dWAT. Importantly, these temporal changes are also linked to disease stage-specific dermal microvascular reactivity, which may reflect adaptive mechanisms driven by metaflammation.

Application of Nanoparticles for Targeting G Protein-Coupled Receptors

Nanoparticles (NPs) have attracted unequivocal attention in recent years due to their potential applications in therapeutics, bio-imaging and material sciences. For drug delivery, NP-based carrier systems offer several advantages over conventional methods. When conjugated with ligands and drugs (or other therapeutic molecules), administrated NPs are able to deliver cargo to targeted sites through ligand-receptor recognition. Such targeted delivery is especially important in cancer therapy. Through this targeted cancer nanotherapy, cancer cells are killed with higher specificity, while the healthy cells are spared. Furthermore, NP drug delivery leads to improved drug load, enhanced drug solubility and stability, and controlled drug release. G protein-coupled receptors (GPCRs) are a superfamily of cell transmembrane receptors. They regulate a plethora of physiological processes through ligand-receptor-binding-induced signaling transduction. With recent evidence unveiling their roles in cancer, GPCR agonists and antagonists have quickly become new targets in cancer therapy. This review focuses on the application of some notable nanomaterials, such as dendrimers, quantum dots, gold nanoparticles, and magnetic nanoparticles, in GPCR-related cancers.

Phospho-proteomic analyses of B-Raf protein complexes reveal new regulatory principles

B-Raf represents a critical physiological regulator of the Ras/RAF/MEK/ERK-pathway and a pharmacological target of growing clinical relevance, in particular in oncology. To understand how B-Raf itself is regulated, we combined mass spectrometry with genetic approaches to map its interactome in MCF-10A cells as well as in B-Raf deficient murine embryonic fibroblasts (MEFs) and B-Raf/Raf-1 double deficient DT40 lymphoma cells complemented with wildtype or mutant B-Raf expression vectors. Using a multi-protease digestion approach, we identified a novel ubiquitination site and provide a detailed B-Raf phospho-map. Importantly, we identify two evolutionary conserved phosphorylation clusters around T401 and S419 in the B-Raf hinge region. SILAC labelling and genetic/biochemical follow-up revealed that these clusters are phosphorylated in the contexts of oncogenic Ras, sorafenib induced Raf dimerization and in the background of the V600E mutation. We further show that the vemurafenib sensitive phosphorylation of the T401 cluster occurs in trans within a Raf dimer. Substitution of the Ser/Thr-residues of this cluster by alanine residues enhances the transforming potential of B-Raf, indicating that these phosphorylation sites suppress its signaling output. Moreover, several B-Raf phosphorylation sites, including T401 and S419, are somatically mutated in tumors, further illustrating the importance of phosphorylation for the regulation of this kinase.

Activation of Grm1 expression by mutated BRaf (V600E) in vitro and in vivo

Our laboratory previously showed that ectopic expression of Grm1 is sufficient to induce spontaneous melanoma formation with 100% penetrance in transgenic mouse model, TG-3, which harbors wild-type BRaf. Studies identified Grm1 expression in human melanoma cell lines and primary to secondary metastatic melanoma biopsies having wild-type or mutated BRaf, but not in normal melanocytes or benign nevi. Grm1 expression was detected in tissues from mice genetically engineered with inducible melanocyte-specific BRaf^V600E. Additionally, stable clones derived from introduction of exogenous BRaf^V600E in mouse melanocytes also showed Grm1 expression, which was not detected in the parental or empty vector-derived cells, suggesting that expression of BRaf^V600E could activate Grm1 expression. Despite aberrant Grm1 expression in the inducible, melanocyte-specific BRaf^V600E mice, no tumors formed. However, in older mice, the melanocytes underwent senescence, as demonstrated previously by others. It was proposed that upregulated p15 and TGFβ contributed to the senescence phenotype. In contrast, in older TG-3 mice the levels of p15 and TGFβ remained the same or lower. Taken together, these results suggest the temporal regulation on the expression of "oncogenes" such as Grm1 or BRaf^V600E is critical in the future fate of the cells. If BRaf^V600E is turned on first, Grm1 expression can be induced, but this is not sufficient to result in development of melanoma; the cells undergo senescence. In contrast, if ectopic expression of Grm1 is turned on first, then regardless of wild-type or mutated BRaf in the melanocytes melanoma development is the consequence.