Src tyrosine kinase is a novel direct effector of G proteins

Adrenoreceptor phylogeny and novel functions of nitric oxide in ascidian immune cells

Nitric oxide (NO) is a simple molecule involved in many biological processes and functions in the cardiovascular, neural, and immune systems. In recent years, NO has also been recognized as a crucial messenger in communication between the nervous and immune systems. Together with NO, catecholamines are the main group of neurotransmitters involved in cross-talk between the nervous and immune systems. Catecholamines such as noradrenaline, can act on immune cells through adrenoreceptors (ARs) present on the cell surface, and NO can cross the cell membrane and interact with secondary messengers, modulating catecholamine production. Here, we analyzed the mutual modulation by noradrenaline and NO in Phallusia nigra immune cells for specific subtypes of ARs. We also investigated the involvement of protein kinases A and C as secondary messengers to these specific subtypes of ARs in the adrenergic signaling pathway that culminates in NO modulation, and the phylogenetic distribution of ARs in deuterostome genomes. This analysis provided evidence for single-copy orthologs of α1, α2 and β-AR in ascidian genomes, suggesting that NO and NA act on a less diverse set of ARs in urochordates. Pharmacological assays showed that high levels of NO can induce ascidian immune cells to produce catecholamines. We also observed that protein kinases A and C are the secondary messengers involved in downstream modulation of NO production through an ancestral β-AR. Taken together, these results provide new information on NO as a modulator of immune cells, and reveal the molecules involved in the signaling pathway of ARs. The results also indicate that ARs may participate in NO modulation. Finally, our results suggest that the common ancestor of urochordates possessed a less complex system of ARs required for immune action and diverse pharmacological responses, since the α-ARs are phylogenetically more related to D1-receptors than are the β-ARs.

Kisspeptin-10 binding to Gpr54 in osteoclasts prevents bone loss by activating Dusp18-mediated dephosphorylation of Src

Osteoclasts are over-activated as we age, which results in bone loss. Src deficiency in mice leads to severe osteopetrosis due to a functional defect in osteoclasts, indicating that Src function is essential in osteoclasts. G-protein-coupled receptors (GPCRs) are the targets for ∼35% of approved drugs but it is still unclear how GPCRs regulate Src kinase activity. Here, we reveal that GPR54 activation by its natural ligand Kisspeptin-10 (Kp-10) causes Dusp18 to dephosphorylate Src at Tyr 416. Mechanistically, Gpr54 recruits both active Src and the Dusp18 phosphatase at its proline/arginine-rich motif in its C terminus. We show that Kp-10 binding to Gpr54 leads to the up-regulation of Dusp18. Kiss1, Gpr54 and Dusp18 knockout mice all exhibit osteoclast hyperactivation and bone loss, and Kp-10 abrogated bone loss by suppressing osteoclast activity in vivo. Therefore, Kp-10/Gpr54 is a promising therapeutic target to abrogate bone resorption by Dusp18-mediated Src dephosphorylation.

Signalling of Adrenoceptors: Canonical Pathways and New Paradigms

The concept of G protein-coupled receptors initially arose from studies of the β-adrenoceptor, adenylyl cyclase, and cAMP signalling pathway. Since then both canonical G protein-coupled receptor signalling pathways and emerging paradigms in receptor signalling have been defined by experiments focused on adrenoceptors. Here, we discuss the evidence for G protein coupling specificity of the nine adrenoceptor subtypes. We summarise the ability of each of the adrenoceptors to activate proximal signalling mediators including cAMP, calcium, mitogen-activated protein kinases, and protein kinase C pathways. Finally, we highlight the importance of precise spatial and temporal control of adrenoceptor signalling that is controlled by the localisation of receptors at intracellular membranes and in larger protein complexes.

The role of genetic and epigenetic GNAS alterations in the development of early-onset obesity

BACKGROUND

GNAS (guanine nucleotide-binding protein, alpha stimulating) is an imprinted gene that encodes Gsα, the α subunit of the heterotrimeric stimulatory G protein. This subunit mediates the signalling of a diverse array of G protein-coupled receptors (GPCRs), including the melanocortin 4 receptor (MC4R) that serves a pivotal role in regulating food intake, energy homoeostasis, and body weight. Genetic or epigenetic alterations in GNAS are known to cause pseudohypoparathyroidism in its different subtypes and have been recently associated with isolated, early-onset, severe obesity. Given the diverse biological functions that Gsα serves, multiple molecular mechanisms involving various GPCRs, such as MC4R, β2- and β3-adrenoceptors, and corticotropin-releasing hormone receptor, have been implicated in the pathophysiology of severe, early-onset obesity that results from genetic or epigenetic GNAS changes.

SCOPE OF REVIEW

This review examines the structure and function of GNAS and provides an overview of the disorders that are caused by defects in this gene and may feature early-onset obesity. Moreover, it elucidates the potential molecular mechanisms underlying Gsα deficiency-induced early-onset obesity, highlighting some of their implications for the diagnosis, management, and treatment of this complex condition.

MAJOR CONCLUSIONS

Gsα deficiency is an underappreciated cause of early-onset, severe obesity. Therefore, screening children with unexplained, severe obesity for GNAS defects is recommended, to enhance the molecular diagnosis and management of this condition.

Active Gαi/o Mutants Accelerate Breast Tumor Metastasis via the c-Src Pathway

Constitutively active mutations in the Gαi2 and GαoA subunits of heterotrimeric G proteins have been found in various human cancers, including breast cancer, but their precise roles in tumor formation, progression, and metastasis remain poorly understood. This study focused on GαoAR243H and Gαi2R179C mutants in breast cancer. These mutants alone were insufficient to initiate mammary tumor formation in mice. However, when introduced into transgenic mouse models of breast cancer induced by Neu expression or PTEN loss, the Gαi2R179C mutant notably enhanced spontaneous lung metastasis, without affecting primary tumor initiation and growth. Ectopic expression of the GαoAR243H and Gαi2R179C mutants in tumor cells promoted cell migration in vitro and dissemination into multiple organs in vivo by activating the c-Src signaling pathway. These mutants activate c-Src through direct interaction, involving specific residues in the switch domains II of Gαi subunits, which only partially overlap with those involved in inhibiting adenylyl cyclases. This study uncovers a critical role of Gαi/o signaling in accelerating breast cancer metastasis through the c-Src pathway. These findings hold clinical significance as they may pave the way for personalized therapies targeting c-Src to inhibit breast cancer metastasis in patients with active Gαi/o mutations or elevated Gαi/o signaling.

Mitigating aberrant Cdk5 activation alleviates mitochondrial defects and motor neuron disease symptoms in spinal muscular atrophy

Spinal muscular atrophy (SMA), the top genetic cause of infant mortality, is characterized by motor neuron degeneration. Mechanisms underlying SMA pathogenesis remain largely unknown. Here, we report that the activity of cyclin-dependent kinase 5 (Cdk5) and the conversion of its activating subunit p35 to the more potent activator p25 are significantly up-regulated in mouse models and human induced pluripotent stem cell (iPSC) models of SMA. The increase of Cdk5 activity occurs before the onset of SMA phenotypes, suggesting that it may be an initiator of the disease. Importantly, aberrant Cdk5 activation causes mitochondrial defects and motor neuron degeneration, as the genetic knockout of p35 in an SMA mouse model rescues mitochondrial transport and fragmentation defects, and alleviates SMA phenotypes including motor neuron hyperexcitability, loss of excitatory synapses, neuromuscular junction denervation, and motor neuron degeneration. Inhibition of the Cdk5 signaling pathway reduces the degeneration of motor neurons derived from SMA mice and human SMA iPSCs. Altogether, our studies reveal a critical role for the aberrant activation of Cdk5 in SMA pathogenesis and suggest a potential target for therapeutic intervention.

Receptor-specific contributions of caveolae, PKC, and Src tyrosine kinase to serotonergic and adrenergic regulation of Kv channels and vasoconstriction

AIMS

Caveolae are invaginated, Ω-shaped membrane structures. They are now recognized as portals for signal transduction of multiple chemical and mechanical stimuli. Notably, the contribution of caveolae has been reported to be receptor-specific. However, details of how they differentially contribute to receptor signaling remain unclear.

MAIN METHODS

Using isometric tension measurements, patch-clamping, and western blotting, we examined the contribution of caveolae and their related signaling pathways to serotonergic (5-HT2A receptor-mediated) and adrenergic (α1-adrenoceptor-mediated) signaling in rat mesenteric arteries.

KEY FINDINGS

Disruption of caveolae by methyl-β-cyclodextrin effectively blocked vasoconstriction mediated by the 5-HT2A receptor (5-HT2AR), but not by the α1-adrenoceptor. Caveolar disruption selectively impaired 5-HT2AR-mediated voltage-dependent K+ channel (Kv) inhibition, but not α1-adrenoceptor-mediated Kv inhibition. In contrast, both serotonergic and α1-adrenergic effects on vasoconstriction, as well as Kv currents, were similarly blocked by the Src tyrosine kinase inhibitor PP2. However, inhibition of protein kinase C (PKC) by either GO6976 or chelerythrine selectively attenuated the effects mediated by the α1-adrenoceptor, but not by 5-HT2AR. Disruption of caveolae decreased 5-HT2AR-mediated Src phosphorylation, but not α1-adrenoceptor-mediated Src phosphorylation. Finally, the PKC inhibitor GO6976 blocked Src phosphorylation by the α1-adrenoceptor, but not by 5-HT2AR.

SIGNIFICANCE

5-HT2AR-mediated Kv inhibition and vasoconstriction are dependent on caveolar integrity and Src tyrosine kinase, but not on PKC. In contrast, α1-adrenoceptor-mediated Kv inhibition and vasoconstriction are not dependent on caveolar integrity, but rather on PKC and Src tyrosine kinase. Caveolae-independent PKC is upstream of Src activation for α1-adrenoceptor-mediated Kv inhibition and vasoconstriction.

Senescent cells in the brain and where to find them

Cellular senescence is a process in which cells change their characteristic phenotype in response to stress and enter a state of prolonged cell cycle arrest accompanied by a distinct secretory phenotype. Cellular senescence has both beneficial and detrimental outcomes. With age, senescent cells progressively accumulate in tissues and might be the bridge connecting ageing to many age-related pathologies. In recent years, evidence emerged supporting the accumulation of brain senescent cells during neurological disorders and ageing. Here, we will discuss the different brain cell populations that exhibit a senescent phenotype. Subsequently, we will explore several senolytic strategies which have been developed to eliminate senescent cells. Finally, we will examine their potential to directly eliminate these senescent brain cells.

Refinement of Singer-Nicolson fluid-mosaic model by microscopy imaging: Lipid rafts and actin-induced membrane compartmentalization

This year celebrates the 50th anniversary of the Singer-Nicolson fluid mosaic model for biological membranes. The next level of sophistication we have achieved for understanding plasma membrane (PM) structures, dynamics, and functions during these 50 years includes the PM interactions with cortical actin filaments and the partial demixing of membrane constituent molecules in the PM, particularly raft domains. Here, first, we summarize our current knowledge of these two structures and emphasize that they are interrelated. Second, we review the structure, molecular dynamics, and function of raft domains, with main focuses on raftophilic glycosylphosphatidylinositol-anchored proteins (GPI-APs) and their signal transduction mechanisms. We pay special attention to the results obtained by single-molecule imaging techniques and other advanced microscopy methods. We also clarify the limitations of present optical microscopy methods for visualizing raft domains, but emphasize that single-molecule imaging techniques can "detect" raft domains associated with molecules of interest in the PM.

Active Gαi/o mutants accelerate breast tumor metastasis via the c-Src pathway

Constitutively active mutations in the Gαi2 and GαoA subunits of heterotrimeric G proteins have been identified in several human cancers including breast cancer, but their functional significance in tumorigenesis and metastasis has not been well characterized. In this study, we show that expression of the constitutively active GαoAR243H and Gαi2R179C mutants alone was insufficient to induce mammary tumor formation in mice. However, in transgenic mouse models of breast cancer induced by Neu expression or PTEN loss, we found that the Gαi2R179C mutant enhanced spontaneous lung metastasis while having no effect on primary tumor initiation and growth. Additionally, we observed that ectopic expression of the GαoAR243H and Gαi2R179C mutants in tumor cells promote cell migration in vitro as well as dissemination into multiple organs in vivo by activating c-Src signaling. Thus, our study uncovers a critical function of Gαi/o signaling in accelerating breast cancer metastasis via the c-Src pathway. This work is clinically significant, as it can potentially pave the way to personalized therapies for patients who present with active Gαi/o mutations or elevated Gαi/o signaling by targeting c-Src to inhibit breast cancer metastasis.

TULA-Family Regulators of Platelet Activation

The two members of the UBASH3/TULA/STS-protein family have been shown to critically regulate cellular processes in multiple biological systems. The regulatory function of TULA-2 (also known as UBASH3B or STS-1) in platelets is one of the best examples of the involvement of UBASH3/TULA/STS proteins in cellular regulation. TULA-2 negatively regulates platelet signaling mediated by ITAM- and hemITAM-containing membrane receptors that are dependent on the protein tyrosine kinase Syk, which currently represents the best-known dephosphorylation target of TULA-2. The biological responses of platelets to collagen and other physiological agonists are significantly downregulated as a result. The protein structure, enzymatic activity and regulatory functions of UBASH3/TULA/STS proteins in the context of platelet responses and their regulation are discussed in this review.

Vasoactive intestinal peptide–VIPR2 signaling regulates tumor cell migration

Phosphoinositide metabolism is critically involved in human cancer cell migration and metastatic growth. The formation of lamellipodia at the leading edge of migrating cells is regulated by metabolism of the inositol phospholipid PI(4,5)P₂ into PI(3,4,5)P₃. The synthesized PI(3,4,5)P₃ promotes the translocation of WASP family verprolin homologous protein 2 (WAVE2) to the plasma membrane and regulates guanine nucleotide exchange factor Rac-mediated actin filament remodeling. Here, we investigated if VIPR2, a receptor for vasoactive intestinal peptide (VIP), has a potential role in regulating cell migration via this pathway. We found that silencing of VIPR2 in MDA-MB-231 and MCF-7 human breast cancer cells inhibited VIP-induced cell migration. In contrast, stable expression of exogenous VIPR2 promoted VIP-induced tumor cell migration, an effect that was inhibited by the addition of a PI3-kinase (PI3K)γ inhibitor or a VIPR2-selective antagonist. VIPR2 stably-expressing cells exhibited increased PI3K activity. Membrane localization of PI(3,4,5)P₃ was significantly attenuated by VIPR2-silencing. VIPR2-silencing in MDA-MB-231 cells suppressed lamellipodium extension; in VIPR2-overexpressing cells, VIPR2 accumulated in the cell membrane on lamellipodia and co-localized with WAVE2. Conversely, VIPR2-silencing reduced WAVE2 level on the cell membrane and inhibited the interaction between WAVE2, actin-related protein 3, and actin. These findings suggest that VIP–VIPR2 signaling controls cancer migration by regulating WAVE2-mediated actin nucleation and elongation for lamellipodium formation through the synthesis of PI(3,4,5)P₃.

Gαi protein subunit: A step toward understanding its non-canonical mechanisms

The heterotrimeric G protein family plays essential roles during a varied array of cellular events; thus, its deregulation can seriously alter signaling events and the overall state of the cell. Heterotrimeric G-proteins have three subunits (α, β, γ) and are subdivided into four families, Gαi, Gα12/13, Gαq, and Gαs. These proteins cycle between an inactive Gα-GDP state and active Gα-GTP state, triggered canonically by the G-protein coupled receptor (GPCR) and by other accessory proteins receptors independent also known as AGS (Activators of G-protein Signaling). In this review, we summarize research data specific for the Gαi family. This family has the largest number of individual members, including Gαi1, Gαi2, Gαi3, Gαo, Gαt, Gαg, and Gαz, and constitutes the majority of G proteins α subunits expressed in a tissue or cell. Gαi was initially described by its inhibitory function on adenylyl cyclase activity, decreasing cAMP levels. Interestingly, today Gi family G-protein have been reported to be importantly involved in the immune system function. Here, we discuss the impact of Gαi on non-canonical effector proteins, such as c-Src, ERK1/2, phospholipase-C (PLC), and proteins from the Rho GTPase family members, all of them essential signaling pathways regulating a wide range of physiological processes.

Progesterone activates GPR126 to promote breast cancer development via the Gi pathway

The steroid hormone progesterone is highly involved in different physiological–pathophysiological processes, including bone formation and cancer progression. Understanding the working mechanisms, especially identifying the receptors of progesterone hormones, is of great value. In the present study, we identified GPR126 as a membrane receptor for both progesterone and 17-hydroxyprogesterone and triggered its downstream G protein signaling. We further characterized the residues of GPR126 that interact with these two ligands and found that progesterone promoted the progression of a triple-negative breast cancer model through GPR126-dependent Gi-SRC signaling. Therefore, developing antagonists targeting GPR126-Gi may provide an alternative therapeutic option for patients with triple-negative breast cancer.

GPR126 is a member of the adhesion G protein-coupled receptors (aGPCRs) that is essential for the normal development of diverse tissues, and its mutations are implicated in various pathological processes. Here, through screening 34 steroid hormones and their derivatives for cAMP production, we found that progesterone (P4) and 17-hydroxyprogesterone (17OHP) could specifically activate GPR126 and trigger its downstream Gi signaling by binding to the ligand pocket in the seven-transmembrane domain of the C-terminal fragment of GPR126. A detailed mutagenesis screening according to a computational simulated structure model indicated that K1001^ECL2 and F1012^ECL2 are key residues that specifically recognize 17OHP but not progesterone. Finally, functional analysis revealed that progesterone-triggered GPR126 activation promoted cell growth in vitro and tumorigenesis in vivo, which involved Gi-SRC pathways in a triple-negative breast cancer model. Collectively, our work identified a membrane receptor for progesterone/17OHP and delineated the mechanisms by which GPR126 participated in potential tumor progression in triple-negative breast cancer, which will enrich our understanding of the functions and working mechanisms of both the aGPCR member GPR126 and the steroid hormone progesterone.

Coronary Large Conductance Ca2+-Activated K+ Channel Dysfunction in Diabetes Mellitus

Diabetes mellitus (DM) is an independent risk of macrovascular and microvascular complications, while cardiovascular diseases remain a leading cause of death in both men and women with diabetes. Large conductance Ca2+-activated K+ (BK) channels are abundantly expressed in arteries and are the key ionic determinant of vascular tone and organ perfusion. It is well established that the downregulation of vascular BK channel function with reduced BK channel protein expression and altered intrinsic BK channel biophysical properties is associated with diabetic vasculopathy. Recent efforts also showed that diabetes-associated changes in signaling pathways and transcriptional factors contribute to the downregulation of BK channel expression. This manuscript will review our current understandings on the molecular, physiological, and biophysical mechanisms that underlie coronary BK channelopathy in diabetes mellitus.

Targeting Gi/o protein-coupled receptor signaling blocks HER2-induced breast cancer development and enhances HER2-targeted therapy

GPCRs are highly desirable drug targets for human disease. Although GPCR dysfunction drives development and progression of many tumors, including breast cancer (BC), targeting individual GPCRs has limited efficacy as a cancer therapy because numerous GPCRs are activated. Here, we sought a new way of blocking GPCR activation in HER2⁺ BC by targeting a subgroup of GPCRs that couple to G_i/o proteins (G_i/o-GPCRs). In mammary epithelial cells of transgenic mouse models, and BC cell lines, HER2 hyperactivation altered GPCR expression, particularly, G_i/o-GPCR expression. G_i/o-GPCR stimulation transactivated EGFR and HER2 and activated the PI3K/AKT and Src pathways. If we uncoupled G_i/o-GPCRs from their cognate G_i/o proteins by pertussis toxin (PTx), then BC cell proliferation and migration was inhibited in vitro and HER2-driven tumor formation and metastasis were suppressed in vivo. Moreover, targeting G_i/o-GPCR signaling via PTx, PI3K, or Src inhibitors enhanced HER2-targeted therapy. These results indicate that, in BC cells, HER2 hyperactivation drives aberrant G_i/o-GPCR signaling and G_i/o-GPCR signals converge on the PI3K/AKT and Src signaling pathways to promote cancer progression and resistance to HER2-targeted therapy. Our findings point to a way to pharmacologically deactivate GPCR signaling to block tumor growth and enhance therapeutic efficacy.

New Structural Perspectives in G Protein-Coupled Receptor-Mediated Src Family Kinase Activation

Src family kinases (SFKs) are key regulators of cell proliferation, differentiation, and survival. The expression of these non-receptor tyrosine kinases is strongly correlated with cancer development and tumor progression. Thus, this family of proteins serves as an attractive drug target. The activation of SFKs can occur via multiple signaling pathways, yet many of them are poorly understood. Here, we summarize the current knowledge on G protein-coupled receptor (GPCR)-mediated regulation of SFKs, which is of considerable interest because GPCRs are among the most widely used pharmaceutical targets. This type of activation can occur through a direct interaction between the two proteins or be allosterically regulated by arrestins and G proteins. We postulate that a rearrangement of binding motifs within the active conformation of arrestin-3 mediates Src regulation by comparison of available crystal structures. Therefore, we hypothesize a potentially different activation mechanism compared to arrestin-2. Furthermore, we discuss the probable direct regulation of SFK by GPCRs and investigate the intracellular domains of exemplary GPCRs with conserved polyproline binding motifs that might serve as scaffolding domains to allow such a direct interaction. Large intracellular domains in GPCRs are often understudied and, in general, not much is known of their contribution to different signaling pathways. The suggested direct interaction between a GPCR and a SFK could allow for a potential immediate allosteric regulation of SFKs by GPCRs and thereby unravel a novel mechanism of SFK signaling. This overview will help to identify new GPCR-SFK interactions, which could serve to explain biological functions or be used to modulate downstream effectors.

Biological characterization of ligands targeting the human CC chemokine receptor 8 (CCR8) reveals the biased signaling properties of small molecule agonists

The human CC chemokine receptor 8 (CCR8) is a promising drug target for cancer immunotherapy and autoimmune disease. Besides human and viral chemokines, previous studies revealed diverse classes of CCR8-targeting small molecules. We characterized a selection of these CCR8 ligands (hCCL1, vCCL1, ZK756326, AZ6; CCR8 agonists and a naphthalene-sulfonamide-based CCR8 antagonist), in in vitro cell-based assays (hCCL1^AF647 binding, calcium mobilization, cellular impedance, cell migration, β-arrestin 1/2 recruitment), and used pharmacological tools to determine G protein-dependent and -independent signaling pathways elicited by these ligands. Our data reveal differences in CCR8-mediated signaling induced by chemokines versus small molecules, which was most pronounced in cell migration studies. Human CCL1 most efficiently induced cell migration whereby Gβγ signaling was indispensable. In contrast, Gβγ signaling did not contribute to cell migration induced by other CCR8 ligands (vCCL1, ZK756326, AZ6). Although all tested CCR8 agonists were full agonists for calcium mobilization, a significant contribution for Gβγ signaling herein was only apparent for human and viral CCL1. Despite both Gα_i- and Gα_q-signaling regulate intracellular Ca²⁺-release, cellular impedance experiments showed that CCR8 agonists predominantly induce Gα_i-dependent signaling. Finally, small molecule agonists displayed higher efficacy in β-arrestin 1 recruitment, which occurred independently of Gα_i signaling. Also in this latter assay, only hCCL1-induced activity was dependent on Gβγ-signaling. Our study provides insight into CCR8 signaling and function and demonstrates differential CCR8 activation by different classes of ligands. This reflects the ability of CCR8 small molecules to evoke different subsets of the receptor's signaling repertoire, which categorizes them as biased agonists.

Reelin signaling modulates GABAB receptor function in the neocortex

Reelin is a protein that is best known for its role in controlling neuronal layer formation in the developing cortex. Here, we studied its role for post-natal cortical network function, which is poorly explored. To preclude early cortical migration defects caused by Reelin deficiency, we used a conditional Reelin knock-out (RelncKO ) mouse, and induced Reelin deficiency post-natally. Induced Reelin deficiency caused hyperexcitability of the neocortical network in vitro and ex vivo. Blocking Reelin binding to its receptors ApoER2 and VLDLR resulted in a similar effect. Hyperexcitability in RelncKO organotypic slice cultures could be rescued by co-culture with wild-type organotypic slice cultures. Moreover, the GABAB receptor (GABAB R) agonist baclofen failed to activate and the antagonist CGP35348 failed to block GABAB Rs in RelncKO mice. Immunolabeling of RelncKO cortical slices revealed a reduction in GABAB R1 and GABAB R2 surface expression at the plasma membrane and western blot of RelncKO cortical tissue revealed decreased phosphorylation of the GABAB R2 subunit at serine 892 and increased phosphorylation at serine 783, reflecting receptor deactivation and proteolysis. These data show a role of Reelin in controlling early network activity, by modulating GABAB R function. Cover Image for this issue: https://doi.org/10.1111/jnc.15054.

C-Src is Activated by the EGF Receptor in a Pathway that Mediates JNK and ERK Activation by Gonadotropin-Releasing Hormone in COS7 Cells

The key participants in G-protein-coupled receptor (GPCR) signaling are the mitogen-activated protein kinase (MAPK) signaling cascades. The mechanisms involved in the activation of the above cascades by GPCRs are not fully elucidated. The prototypical GPCR is the receptor for gonadotropin-releasing hormone (GnRHR), which serves as a key regulator of the reproductive system. Here, we expressed GnRHR in COS7 cells and found that GnRHR transmits its signals to MAPKs mainly via Gαi and the EGF receptor, without the involvement of Hb-EGF or PKCs. The main pathway that leads to JNK activation downstream of the EGF receptor involves a sequential activation of c-Src and PI3K. ERK activation by GnRHR is mediated by the EGF receptor, which activates Ras either directly or via c-Src. Beside the main pathway, the dissociated Gβγ and β-arrestin may initiate additional (albeit minor) pathways that lead to MAPK activation in the transfected COS7 cells. The pathways detected are significantly different from those in other GnRHR-bearing cells, indicating that GnRH can utilize various signaling mechanisms for MAPK activation. The unique pathway elucidated here, in which c-Src and PI3K are sequentially activated downstream of the EGF receptor, may serve as a prototype of signaling mechanisms by GnRHR and additional GPCRs in various cell types.

Biased signaling of G protein coupled receptors (GPCRs): Molecular determinants of GPCR/transducer selectivity and therapeutic potential

G protein coupled receptors (GPCRs) convey signals across membranes via interaction with G proteins. Originally, an individual GPCR was thought to signal through one G protein family, comprising cognate G proteins that mediate canonical receptor signaling. However, several deviations from canonical signaling pathways for GPCRs have been described. It is now clear that GPCRs can engage with multiple G proteins and the line between cognate and non-cognate signaling is increasingly blurred. Furthermore, GPCRs couple to non-G protein transducers, including β-arrestins or other scaffold proteins, to initiate additional signaling cascades. Receptor/transducer selectivity is dictated by agonist-induced receptor conformations as well as by collateral factors. In particular, ligands stabilize distinct receptor conformations to preferentially activate certain pathways, designated 'biased signaling'. In this regard, receptor sequence alignment and mutagenesis have helped to identify key receptor domains for receptor/transducer specificity. Furthermore, molecular structures of GPCRs bound to different ligands or transducers have provided detailed insights into mechanisms of coupling selectivity. However, receptor dimerization, compartmentalization, and trafficking, receptor-transducer-effector stoichiometry, and ligand residence and exposure times can each affect GPCR coupling. Extrinsic factors including cell type or assay conditions can also influence receptor signaling. Understanding these factors may lead to the development of improved biased ligands with the potential to enhance therapeutic benefit, while minimizing adverse effects. In this review, evidence for ligand-specific GPCR signaling toward different transducers or pathways is elaborated. Furthermore, molecular determinants of biased signaling toward these pathways and relevant examples of the potential clinical benefits and pitfalls of biased ligands are discussed.

Focal Adhesion Kinases in Platelet Function and Thrombosis

The focal adhesion kinase family includes 2 homolog members, FAK and Pyk2 (proline-rich tyrosine kinase 2), primarily known for their roles in nucleated cells as regulators of cytoskeletal dynamics and cell adhesion. FAK and Pyk2 are also expressed in megakaryocytes and platelets and are activated by soluble agonists and on adhesion to the subendothelial matrix. Despite high sequence homology and similar molecular organization, FAK and Pyk2 play different roles in platelet function. Whereas FAK serves mostly as a traditional focal adhesion kinase activated downstream of integrins, Pyk2 coordinates multiple signals from different receptors. FAK, but not Pyk2, is involved in megakaryocyte maturation and platelet production. In circulating platelets, FAK is recruited by integrin αIIbβ3 to regulate hemostasis, whereas it plays minimal roles in thrombosis. By contrast, Pyk2 is implicated in platelet activation and is an important regulator of thrombosis. The direct activation of Pyk2 by calcium ions provides a connection between GPCRs (G-protein coupled receptors) and Src family kinases. In this review, we provide the comprehensive overview of >20 years of investigations on the role and regulation of focal adhesion kinases in blood platelets, highlighting common and distinctive features of FAK and Pyk2 in hemostasis and thrombosis.

Antidepressant amitriptyline-induced matrix metalloproteinase-9 activation is mediated by Src family tyrosine kinase, which leads to glial cell line-derived neurotrophic factor mRNA expression in rat astroglial cells

Background

Astrocytes have been implicated in the pathophysiology of mood disorders and in the mechanism of the pharmacological effects of antidepressant drugs by the production of neurotrophic/growth factors. Previous studies have identified astrocyte‐expressed Gα_i/o‐coupled lysophosphatidic acid receptor 1 (LPAR1), as being involved in antidepressant‐induced production of glial cell line‐derived neurotrophic factor (GDNF) and matrix metalloproteinase‐9 (MMP‐9) activation, an important step in the production of GNDF. However, the precise mechanism of MMP‐9 activation by antidepressants has yet to be identified, in particular the intracellular signaling pathway between LPAR1/Gα_i/o and MMP‐9.

Methods and Results

Treatment of rat C6 astroglial cells (C6 cells) with amitriptyline increased Src family tyrosine kinase phosphorylation in a time and concentration‐dependent manner. Amitriptyline‐induced GDNF mRNA expression was blocked by Src family tyrosine kinase inhibitors. In addition, inhibiting Src family tyrosine kinase blocked amitriptyline‐induced zymographic MMP‐9 activation in C6 cells. The amitriptyline‐induced zymographic MMP‐9 activity was completely blocked by selective inhibition of Gα_i/o protein and LPAR1. Furthermore, the amitriptyline‐induced Src family tyrosine kinase phosphorylation was blocked by LPAR1, but not MMP‐9 inhibition, indicating that Src family tyrosine kinase involvement is downstream of LPAR1.

Conclusions

The current findings suggest that the pharmacological effect of antidepressant such as amitriptyline is mediated through an intracellular signaling pathway via the LPAR1/Gα_i/o/Src family tyrosine kinase, which leads to MMP‐9 activation and GDNF production.

Treatment of rat C6 astroglial cells (C6 cells) with amitriptyline increased Src family tyrosine kinase phosphorylation in a time‐ and concentration‐dependent manner. The current findings suggest that the pharmacological effect of antidepressant such as amitriptyline is mediated through an intracellular signaling pathway via the LPAR1/Gαi/o/Src family tyrosine kinase, which leads to MMP‐9 activation and glial cell line‐derived neurotrophic factor production.

Constitutive SRC-mediated phosphorylation of pannexin 1 at tyrosine 198 occurs at the plasma membrane

Pannexin 1 (PANX1)-mediated ATP release in vascular smooth muscle coordinates α1-adrenergic receptor (α1-AR) vasoconstriction and blood pressure homeostasis. We recently identified amino acids 198-200 (YLK) on the PANX1 intracellular loop that are critical for α1-AR-mediated vasoconstriction and PANX1 channel function. We report herein that the YLK motif is contained within an SRC homology 2 domain and is directly phosphorylated by SRC proto-oncogene, nonreceptor tyrosine kinase (SRC) at Tyr¹⁹⁸ We demonstrate that PANX1-mediated ATP release occurs independently of intracellular calcium but is sensitive to SRC family kinase (SFK) inhibition, suggestive of channel regulation by tyrosine phosphorylation. Using a PANX1 Tyr¹⁹⁸-specific antibody, SFK inhibitors, SRC knockdown, temperature-dependent SRC cells, and kinase assays, we found that PANX1-mediated ATP release and vasoconstriction involves constitutive phosphorylation of PANX1 Tyr¹⁹⁸ by SRC. We specifically detected SRC-mediated Tyr¹⁹⁸ phosphorylation at the plasma membrane and observed that it is not enhanced or induced by α1-AR activation. Last, we show that PANX1 immunostaining is enriched in the smooth muscle layer of arteries from hypertensive humans and that Tyr¹⁹⁸ phosphorylation is detectable in these samples, indicative of a role for membrane-associated PANX1 in small arteries of hypertensive humans. Our discovery adds insight into the regulation of PANX1 by post-translational modifications and connects a significant purinergic vasoconstriction pathway with a previously identified, yet unexplored, tyrosine kinase-based α1-AR constriction mechanism. This work implicates SRC-mediated PANX1 function in normal vascular hemodynamics and suggests that Tyr¹⁹⁸-phosphorylated PANX1 is involved in hypertensive vascular pathology.

Endocytosis-dependent lysosomal degradation of Src induced by protease-activated receptor 1

Src plays a critical role in regulating cellular responses induced by protease-activated receptor 1 (PAR1). Here, we found that PAR1 activation induces lysosomal degradation of Src. Src is associated and trafficked together with activated PAR1 to early endosomes and then sorted to lysosomes for degradation. Blocking agonist-induced endocytosis of PAR1 by inhibition of dynamin activity suppresses PAR1-induced degradation of Src. However, Src activity is neither required for agonist-induced PAR1 internalization nor required for Src degradation upon PAR1 activation. We show that PAR1 activation triggers endocytosis-dependent lysosomal degradation of Src in both human embryonic kidney 293 and human umbilical vein endothelial cells. Our finding provides a new paradigm for how an irreversibly activated receptor regulates its downstream signalling.

CRF1 Receptor Signaling via the ERK1/2-MAP and Akt Kinase Cascades: Roles of Src, EGF Receptor, and PI3-Kinase Mechanisms

In the present study, we determined the cellular regulators of ERK1/2 and Akt signaling pathways in response to human CRF₁ receptor (CRF₁R) activation in transfected COS-7 cells. We found that Pertussis Toxin (PTX) treatment or sequestering Gβγ reduced CRF₁R-mediated activation of ERK1/2, suggesting the involvement of a G_i-linked cascade. Neither G_s/PKA nor G_q/PKC were associated with ERK1/2 activation. Besides, CRF induced EGF receptor (EGFR) phosphorylation at Tyr¹⁰⁶⁸, and selective inhibition of EGFR kinase activity by AG1478 strongly inhibited the CRF₁R-mediated phosphorylation of ERK1/2, indicating the participation of EGFR transactivation. Furthermore, CRF-induced ERK1/2 phosphorylation was not altered by pretreatment with batimastat, GM6001, or an HB-EGF antibody indicating that metalloproteinase processing of HB-EGF ligands is not required for the CRF-mediated EGFR transactivation. We also observed that CRF induced Src and PYK2 phosphorylation in a Gβγ-dependent manner. Additionally, using the specific Src kinase inhibitor PP2 and the dominant-negative-SrcYF-KM, it was revealed that CRF-stimulated ERK1/2 phosphorylation depends on Src activation. PP2 also blocked the effect of CRF on Src and EGFR (Tyr⁸⁴⁵) phosphorylation, further demonstrating the centrality of Src. We identified the formation of a protein complex consisting of CRF₁R, Src, and EGFR facilitates EGFR transactivation and CRF₁R-mediated signaling. CRF stimulated Akt phosphorylation, which was dependent on G_i/βγ subunits, and Src activation, however, was only slightly dependent on EGFR transactivation. Moreover, PI3K inhibitors were able to inhibit not only the CRF-induced phosphorylation of Akt, as expected, but also ERK1/2 activation by CRF suggesting a PI3K dependency in the CRF₁R ERK signaling. Finally, CRF-stimulated ERK1/2 activation was similar in the wild-type CRF₁R and the phosphorylation-deficient CRF₁R-Δ386 mutant, which has impaired agonist-dependent β-arrestin-2 recruitment; however, this situation may have resulted from the low β-arrestin expression in the COS-7 cells. When β-arrestin-2 was overexpressed in COS-7 cells, CRF-stimulated ERK1/2 phosphorylation was markedly upregulated. These findings indicate that on the base of a constitutive CRF₁R/EGFR interaction, the G_i/βγ subunits upstream activation of Src, PYK2, PI3K, and transactivation of the EGFR are required for CRF₁R signaling via the ERK1/2-MAP kinase pathway. In contrast, Akt activation via CRF₁R is mediated by the Src/PI3K pathway with little contribution of EGFR transactivation.

Targeting Purinergic Receptor P2Y2 Prevents the Growth of Pancreatic Ductal Adenocarcinoma by Inhibiting Cancer Cell Glycolysis

PURPOSE

Extensive research has reported that the tumor microenvironment components play crucial roles in tumor progression. Thus, blocking the supports of tumor microenvironment is a promising approach to prevent cancer progression. We aimed to determine whether blocking extracellular ATP-P2RY2 axis could be a potential therapeutic approach for PDAC treatment.

EXPERIMENTAL DESIGN

Expression of P2RY2 was determined in 264 human PDAC samples and correlated to patient survival. P2RY2 was inhibited in human PDAC cell lines by antagonist and shRNA, respectively, and cell viability, clonogenicity, and glycolysis were determined. RNA sequencing of PDAC cell line was applied to reveal underlying molecular mechanisms. Multiple PDAC mouse models were used to assess the effects of the P2RY2 inhibition on PDAC progression.

RESULTS

P2RY2 was upregulated and associated with poor prognosis in PDAC. Activated P2RY2 by increased extracellular ATP in tumor microenvironment promoted PDAC growth and glycolysis. Further studies showed that the agonist-activated P2RY2 triggered PI3K/AKT-mTOR signaling by crosstalk with PDGFR mediated by Yes1, resulting in elevated expression of c-Myc and HIF1α, which subsequently enhanced cancer cell glycolysis. Genetic and pharmacologic inhibition of P2RY2 impaired tumor cell growth in subcutaneous and orthotopic xenograft model, as well as delayed tumor progression in inflammation-driven PDAC model. In addition, synergy was observed when AR-C118925XX, the selective antagonist of P2RY2 receptor, and gemcitabine were combined, resulting in prolonged survival of xenografted PDAC mice.

CONCLUSIONS

These findings reveal the roles of the P2RY2 in PDAC metabolic reprogramming, suggesting that P2RY2 might be a potential metabolic therapeutic target for PDAC.

Investigating β-adrenergic-induced cardiac hypertrophy through computational approach: classical and non-classical pathways

The chronic stimulation of β-adrenergic receptors plays a crucial role in cardiac hypertrophy and its progression to heart failure. In β-adrenergic signaling, in addition to the well-established classical pathway, Gs/AC/cAMP/PKA, activation of non-classical pathways such as Gi/PI3K/Akt/GSK3β and Gi/Ras/Raf/MEK/ERK contribute in cardiac hypertrophy. The signaling network of β-adrenergic-induced hypertrophy is very complex and not fully understood. So, we use a computational approach to investigate the dynamic response and contribution of β-adrenergic mediators in cardiac hypertrophy. The proposed computational model provides insights into the effects of β-adrenergic classical and non-classical pathways on the activity of hypertrophic transcription factors CREB and GATA4. The results illustrate that the model captures the dynamics of the main signaling mediators and reproduces the experimental observations well. The results also show that despite the low portion of β2 receptors out of total cardiac β-adrenergic receptors, their contribution in the activation of hypertrophic mediators and regulation of β-adrenergic-induced hypertrophy is noticeable and variations in β1/β2 receptors ratio greatly affect the ISO-induced hypertrophic response. The model results illustrate that GSK3β deactivation after β-adrenergic receptor stimulation has a major influence on CREB and GATA4 activation and consequent cardiac hypertrophy. Also, it is found through sensitivity analysis that PKB (Akt) activation has both pro-hypertrophic and anti-hypertrophic effects in β-adrenergic signaling.

Gαi Signaling Promotes Marginal Zone B Cell Development by Enabling Transitional B Cell ADAM10 Expression

The follicular (FO) versus marginal zone (MZ) B cell fate decision in the spleen depends upon BCR, BAFF, and Notch2 signaling. Whether or how G_i signaling affects this fate decision is unknown. Here, we show that direct contact with Notch ligand expressing stromal cells (OP9-Delta-like 1) cannot promote normal MZ B cell development when progenitor B cells lack Gα_i proteins, or if Gi signaling is disabled. Consistent with faulty ADAM10-dependent Notch2 processing, Gα_i-deficient transitional B cells had low ADAM10 membrane expression levels and reduced Notch2 target gene expression. Immunoblotting Gα_i-deficient B cell lysates revealed a reduction in mature, processed ADAM10. Suggesting that Gα_i signaling promotes ADAM10 membrane expression, stimulating normal transitional B cells with CXCL12 raised it, while inhibiting Gα_i nucleotide exchange blocked its upregulation. Surprisingly, inhibiting Gα_i nucleotide exchange in transitional B cells also impaired the upregulation of ADAM10 that occurs following antigen receptor crosslinking. These results indicate that Gα_i signaling supports ADAM10 maturation and activity in transitional B cells, and ultimately Notch2 signaling to promote MZ B cell development.

The Molecular Targets and Anti-Invasive Effects of 2,6-bis-(4-hydroxyl-3methoxybenzylidine) cyclohexanone or BHMC in MDA-MB-231 Human Breast Cancer Cells

In order to metastasize, tumor cells need to migrate and invade the surrounding tissues. It is important to identify compound(s) capable of disrupting the metastasis of invasive cancer cells, especially for hindering invadopodia formation, so as to provide anti-metastasis targeted therapy. Invadopodia are thought to be specialized actin-rich protrusions formed by highly invasive cancer cells to degrade the extracellular matrix (ECM). A curcuminoid analogue known as 2,6-bis-(4-hydroxy-3-methoxybenzylidine)cyclohexanone or BHMC has shown good potential in inhibiting inflammation and hyperalgesia. It also possesses an anti-tumor effects on 4T1 murine breast cancer cells in vivo. However, there is still a lack of empirical evidence on how BHMC works in preventing human breast cancer invasion. In this study, we investigated the effect of BHMC on MDA-MB-231 breast cancer cells and its underlying mechanism of action to prevent breast cancer invasion, especially during the formation of invadopodia. All MDA-MB-231 cells, which were exposed to the non-cytotoxic concentrations of BHMC, expressed the proliferating cell nuclear antigen (PCNA), which indicate that the anti-proliferative effects of BHMC did not interfere in the subsequent experiments. By using a scratch migration assay, transwell migration and invasion assays, we determined that BHMC reduces the percentage of migration and invasion of MDA-MB-231 cells. The gelatin degradation assay showed that BHMC reduced the number of cells with invadopodia. Analysis of the proteins involved in the invasion showed that there is a significant reduction in the expressions of Rho guanine nucleotide exchange factor 7 (β-PIX), matrix metalloproteinase-9 (MMP-9), and membrane type 1 matrix metalloproteinase (MT1-MMP) in the presence of BHMC treatment at 12.5 µM. Therefore, it can be postulated that BHMC at 12.5 µM is the optimal concentration for preventing breast cancer invasion.

G-Protein Gα13 Functions with Abl Kinase to Regulate Actin Cytoskeletal Reorganization

Heterotrimeric G-proteins are essential cellular signal transducers. One of the G-proteins, Gα₁₃, is critical for actin cytoskeletal reorganization, cell migration, cell proliferation, and apoptosis. Previously, we have shown that Gα₁₃ is essential for both G-protein-coupled receptor and receptor tyrosine kinase-induced actin cytoskeletal reorganization such as dynamic dorsal ruffle turnover and cell migration. However, the mechanism by which Gα₁₃ signals to actin cytoskeletal reorganization is not completely understood. Here we show that Gα₁₃ directly interacts with Abl tyrosine kinase, which is a critical regulator of actin cytoskeleton. This interaction is critical for Gα₁₃-induced dorsal ruffle turnover, endothelial cell remodeling, and cell migration. Our data uncover a new molecular signaling pathway by which Gα₁₃ controls actin cytoskeletal reorganization.

The free fatty acid receptor 1 promotes airway smooth muscle cell proliferation through MEK/ERK and PI3K/Akt signaling pathways

Obesity is a risk factor for asthma and influences airway hyperresponsiveness, which is in part modulated by airway smooth muscle proliferative remodeling. Plasma free fatty acids (FFAs) levels are elevated in obese individuals, and long-chain FFAs act as endogenous ligands for the free fatty acid receptor 1 (FFAR1), which couples to both G_q and G_i proteins. We examined whether stimulation of FFAR1 induces airway smooth muscle cell proliferation through classical MEK/ERK and/or phosphoinositide 3-kinase (PI3K)/Akt signaling pathways. The long-chain FFAs (oleic acid and linoleic acid) and a FFAR1 agonist (GW9508) induced human airway smooth muscle (HASM) cell proliferation, which was inhibited by the MEK inhibitor U0126 and the PI3K inhibitor LY294002 . The long-chain FFAs and GW9508 increased phosphorylation of ERK, Akt, and p70S6K in HASM cells and freshly isolated rat airway smooth muscle. Downregulation of FFAR1 in HASM cells by siRNA significantly attenuated oleic acid-induced phosphorylation of ERK and Akt. Oleic acid-induced ERK phosphorylation was blocked by either the Gα_i-protein inhibitor pertussis toxin or U0126 and was partially inhibited by either the Gα_q-specific inhibitor YM-254890 or the Gβγ signaling inhibitor gallein. Oleic acid significantly inhibited forskolin-stimulated cAMP activity, which was attenuated by pertussis toxin. Akt phosphorylation was inhibited by pertussis toxin, the ras inhibitor manumycin A, the Src inhibitor PP1, or LY294002 . Phosphorylation of p70S6K by oleic acid or GW9508 was significantly inhibited by LY294002 , U0126, and the mammalian target of rapamycin (mTOR) inhibitor rapamycin. In conclusion, the FFAR1 promoted airway smooth muscle cell proliferation and p70S6K phosphorylation through MEK/ERK and PI3K/Akt signaling pathways.

Prognostic Classifier Based on Genome-Wide DNA Methylation Profiling in Well-Differentiated Thyroid Tumors

Context

Even though the majority of well-differentiated thyroid carcinoma (WDTC) is indolent, a number of cases display an aggressive behavior. Cumulative evidence suggests that the deregulation of DNA methylation has the potential to point out molecular markers associated with worse prognosis.

Objective

To identify a prognostic epigenetic signature in thyroid cancer.

Design

Genome-wide DNA methylation assays (450k platform, Illumina) were performed in a cohort of 50 nonneoplastic thyroid tissues (NTs), 17 benign thyroid lesions (BTLs), and 74 thyroid carcinomas (60 papillary, 8 follicular, 2 Hürthle cell, 1 poorly differentiated, and 3 anaplastic). A prognostic classifier for WDTC was developed via diagonal linear discriminant analysis. The results were compared with The Cancer Genome Atlas (TCGA) database.

Results

A specific epigenetic profile was detected according to each histological subtype. BTLs and follicular carcinomas showed a greater number of methylated CpG in comparison with NTs, whereas hypomethylation was predominant in papillary and undifferentiated carcinomas. A prognostic classifier based on 21 DNA methylation probes was able to predict poor outcome in patients with WDTC (sensitivity 63%, specificity 92% for internal data; sensitivity 64%, specificity 88% for TCGA data). High-risk score based on the classifier was considered an independent factor of poor outcome (Cox regression, P < 0.001).

Conclusions

The methylation profile of thyroid lesions exhibited a specific signature according to the histological subtype. A meaningful algorithm composed of 21 probes was capable of predicting the recurrence in WDTC.

Genetic evidence that β-arrestins are dispensable for the initiation of β2-adrenergic receptor signaling to ERK

The β₂-adrenergic receptor (β₂AR) has provided a paradigm to elucidate how G protein-coupled receptors (GPCRs) control intracellular signaling, including the discovery that β-arrestins, which bind to ligand-activated GPCRs, are central for GPCR function. We used genome editing, conditional gene deletion, and small interfering RNAs (siRNAs) to determine the roles of β-arrestin 1 (β-arr1) and β-arr2 in β₂AR internalization, trafficking, and signaling to ERK. We found that only β-arr2 was essential for β₂AR internalization. Unexpectedly, β-arr1 and β-arr2 and receptor internalization were dispensable for ERK activation. Instead, β₂AR signaled through Gα_s and Gβγ subunits through a pathway that involved the tyrosine kinase SRC, the adaptor protein SHC, the guanine nucleotide exchange factor SOS, the small GTPase RAS, and the kinases RAF and MEK, which led to ERK activation. These findings provide a molecular framework for β₂AR signaling through β-arrestin-independent pathways in key physiological functions and under pathological conditions.

An investigation into the potential role of brain angiogenesis inhibitor protein 3 (BAI3) in the tumorigenesis of small-cell carcinoma: a review of the surrounding literature

Brain angiogenesis inhibitor protein 3 (BAI3) is from the adhesion group of seven-transmembrane spanning G protein-coupled receptors (GPCRs) and has been identified via gene expression profiling as being upregulated in small-cell lung cancer (SCLC) tumors. It has subsequently been validated as a sensitive and specific immunohistochemical marker for SCLC, helping to differentiate these tumors from morphologically similar large-cell neuroendocrine (LCNEC) malignancies. It is, however, still unclear as to the role BAI3 proteins might play in SCLC and indeed how they might contribute to tumorigenesis. Interestingly, the pattern of staining observed on immunohistochemistry was in fact nuclear as opposed to the membranous staining pattern expected of transmembrane-bound molecules. This fact has lead the authors to believe that the protein receptor is structurally altered in SCLC and that this modification may confer different behavioral properties that contribute toward tumorigenesis. Nuclear localization is not unique to BAI3 and has been reported in a number of GPCRs and frequently correlates with survival outcomes. BAI3 has the potential to act as target for pharmaceutical intervention inline with developing trends in molecular pathology aiming to provide personalized, treatment regimes based on tumor-specific mutation profiles. The adhesion group of the GPCR superfamily is still poorly understood. We present a review of the existing literature regarding the role they play in both physiological and disease states and the mechanisms by which they influence a range of cellular processes.

Catecholamines are produced by ascidian immune cells: The involvement of PKA and PKC in the adrenergic signaling pathway

The stress response is a complex mechanism, which includes changes in the immune system to enable organisms to maintain homeostasis. The neurohormones dopamine, noradrenaline (NA) and adrenalin are responsible for the physiological modulations that occur during acute stress. In the present study, we analyzed the effects of NA on the immune system specific to nitric-oxide (NO) production by subpopulations of immune cells (hemocytes) of the ascidian Phallusia nigra. We also investigated the capability of immune cells to produce catecholamine (CA). Finally, we tested the involvement of protein kinase A (PKA) and C (PKC) in the NA downstream signaling pathway. The results revealed that NA can reduce NO production by P. nigra hemocytes threefold, and that signet-ring cells, univacuolar refractile granulocytes and morula cells are the cell types most involved in this event. A challenge effected with Zymosan A induced CA production, and co-incubation with both inhibitors of the second messengers PKA and PKC revealed the involvement of these molecules in the adrenergic pathway of P. nigra hemocytes. Taken together, these results suggest that NO production can be down-regulated by NA through α- and β-adrenoceptors via the second messengers PKA and PKC.

The Role of the Lysyl Oxidases in Tissue Repair and Remodeling: A Concise Review

Tissue injury provokes a series of events containing inflammation, new tissue formation and tissue remodeling which are regulated by the spatially and temporally coordinated organization. It is an evolutionarily conserved, multi-cellular, multi-molecular process via complex signalling network. Tissue injury disorders present grievous clinical problems and are likely to increase since they are generally associated with the prevailing diseases such as diabetes, hypertension and obesity. Although these dynamic responses vary not only for the different types of trauma but also for the different organs, a balancing act between the tissue degradation and tissue synthesis is the same. In this process, the degradation of old extracellular matrix (ECM) elements and new ones' synthesis and deposition play an essential role, especially collagens. Lysyl oxidase (LOX) and four lysyl oxidase-like proteins are a group of enzymes capable of catalyzing cross-linking reaction of collagen and elastin, thus initiating the formation of covalent cross-links that insolubilize ECM proteins. In this way, LOX facilitates ECM stabilization through ECM formation, development, maturation and remodeling. This ability determines its potential role in tissue repair and regeneration. In this review, based on the current in vitro, animal and human in vivo studies which have shown the significant role of the LOXs in tissue repair, e.g., tendon regeneration, ligament healing, cutaneous wound healing, and cartilage remodeling, we focused on the role of the LOXs in inflammation phase, proliferation phase, and tissue remodeling phase of the repair process. By summarizing its healing role, we hope to shed light on the understanding of its potential in tissue repair and provide up to date therapeutic strategies towards related injuries.

Heterotrimeric G protein Gαs subunit attenuates PLEKHG2, a Rho family-specific guanine nucleotide exchange factor, by direct interaction

PLEKHG2 is a Gβγ-dependent guanine nucleotide exchange factor (GEF) for the small GTPases Rac and Cdc42, and has been shown to mediate signalling pathways such as actin cytoskeleton reorganization and serum response element (SRE)-dependent gene transcription. Here we show that the constitutively active mutant of the Gαs subunit significantly attenuated PLEKHG2-induced SRE-mediated gene transcription. Strikingly, we observed that the constitutive activation of endogenous Gαs by treatment with CTx caused a similar inhibitory effect on PLEKHG2-induced activation of SRE. However, both the enforced expression of the catalytic subunit β of protein kinase A and the treatment with dibutyl-cyclic AMP failed to mimic the inhibitory effect of Gαs on PLEKHG2. Furthermore, the dominant negative mutant of protein kinase A had no effect on PLEKHG2-mediated SRE activation. Performing immunoprecipitation and an in vitro pulldown assay, we found that PLEKHG2 directly interacted with the active form of the Gαs subunit in cells. The interaction between PLEKHG2 and Gαs required the N-terminal region of PLEKHG2, which includes the DH domain, a functional domain of GEF, suggesting that Gαs directly masks the DH domain of PLEKHG2. In a previous study, we reported that Gβγ accelerates PLEKHG2-mediated SRE-dependent gene transcription. Interestingly, Gαs also inhibited the hyperactivation of SRE induced by the co-expression of Gβγ and PLEKHG2; however, Gαs and Gβγ bind to different regions of PLEKHG2. This is the first report to show that PLEKHG2 is a novel effector of Gαs, and is negatively regulated by the Gαs subunit through direct interaction.

Dipeptidyl Peptidase-4 Inhibitor Increases Vascular Leakage in Retina through VE-cadherin Phosphorylation

The inhibitors of CD26 (dipeptidyl peptidase-4; DPP4) have been widely prescribed to control glucose level in diabetic patients. DPP4-inhibitors, however, accumulate stromal cell-derived factor-1α (SDF-1α), a well-known inducer of vascular leakage and angiogenesis both of which are fundamental pathophysiology of diabetic retinopathy. The aim of this study was to investigate the effects of DPP4-inhibitors on vascular permeability and diabetic retinopathy. DPP4-inhibitor (diprotin A or sitagliptin) increased the phosphorylation of Src and vascular endothelial-cadherin (VE-cadherin) in human endothelial cells and disrupted endothelial cell-to-cell junctions, which were attenuated by CXCR4 (receptor of SDF-1α)-blocker or Src-inhibitor. Disruption of endothelial cell-to-cell junctions in the immuno-fluorescence images correlated with the actual leakage of the endothelial monolayer in the transwell endothelial permeability assay. In the Miles assay, vascular leakage was observed in the ears into which SDF-1α was injected, and this effect was aggravated by DPP4-inhibitor. In the model of retinopathy of prematurity, DPP4-inhibitor increased not only retinal vascularity but also leakage. Additionally, in the murine diabetic retinopathy model, DPP4-inhibitor increased the phosphorylation of Src and VE-cadherin and aggravated vascular leakage in the retinas. Collectively, DPP4-inhibitor induced vascular leakage by augmenting the SDF-1α/CXCR4/Src/VE-cadherin signaling pathway. These data highlight safety issues associated with the use of DPP4-inhibitors.

Critical Roles of STAT3 in β-Adrenergic Functions in the Heart

BACKGROUND

β-Adrenergic receptors (βARs) play paradoxical roles in the heart. On one hand, βARs augment cardiac performance to fulfill the physiological demands, but on the other hand, prolonged activations of βARs exert deleterious effects that result in heart failure. The signal transducer and activator of transcription 3 (STAT3) plays a dynamic role in integrating multiple cytokine signaling pathways in a number of tissues. Altered activation of STAT3 has been observed in failing hearts in both human patients and animal models. Our objective is to determine the potential regulatory roles of STAT3 in cardiac βAR-mediated signaling and function.

METHODS AND RESULTS

We observed that STAT3 can be directly activated in cardiomyocytes by β-adrenergic agonists. To follow up this finding, we analyzed βAR function in cardiomyocyte-restricted STAT3 knockouts and discovered that the conditional loss of STAT3 in cardiomyocytes markedly reduced the cardiac contractile response to acute βAR stimulation, and caused disengagement of calcium coupling and muscle contraction. Under chronic β-adrenergic stimulation, Stat3cKO hearts exhibited pronounced cardiomyocyte hypertrophy, cell death, and subsequent cardiac fibrosis. Biochemical and genetic data supported that Gαs and Src kinases are required for βAR-mediated activation of STAT3. Finally, we demonstrated that STAT3 transcriptionally regulates several key components of βAR pathway, including β1AR, protein kinase A, and T-type Ca(2+) channels.

CONCLUSIONS

Our data demonstrate for the first time that STAT3 has a fundamental role in βAR signaling and functions in the heart. STAT3 serves as a critical transcriptional regulator for βAR-mediated cardiac stress adaption, pathological remodeling, and heart failure.

AKT-independent Reelin signaling requires interactions of heterotrimeric Go and Src

Reelin, a large secreted extracellular matrix glycoprotein, plays a key role in neuronal migration during cortical development and promotes neuronal maturation. The signaling pathway regulating neuronal maturation in the postnatal period are relatively less well understood. In this study, we demonstrated that a heterotrimeric G protein, Go, is a novel target of Reelin-induced signaling to promote neurite outgrowth. In primary hippocampal neurons of Reelin-deficient reeler mice, neurite outgrowth was significantly reduced and rescued upon addition of Reelin. Pertussis toxin (PTX) treatment or transfection with Gαo-siRNA suppressed Reelin-mediated neurite outgrowth in wild-type neurons. Additionally, Reelin treatment led to increased phosphorylation of AKT, GSK3β, and JNK, which were all effectively blocked by the PI3K inhibitor, LY294002. By comparison, PTX specifically blocked JNK activation, but not AKT and GSK3β. Immunoprecipitation assays disclosed that Reelin increases the active forms of both Src and Gαo and promotes their direct association. Notably, Dab1, a cytoplasmic adaptor molecule that mediates Reelin signaling, did not interact with Gαo. Neurite outgrowth by Reelin was induced via activating Src kinase, which directly stimulated Gαo, activity, leading to JNK activation. Based on the collective findings, we suggest that Reelin-dependent signaling mechanisms may be split into Src-AKT-dependent and Src-Go-dependent pathways. Our results additionally provide evidence that Reelin receptors cross-communicate with heterologous G protein-coupled receptors (GPCR) independently of the cognate ligands of GPCR.

Prostaglandin E-mediated molecular mechanisms driving remodeling of the ductus arteriosus

The ductus arteriosus (DA), a fetal arterial connection between the pulmonary arteries and aorta, normally closes after birth. Persistent DA patency usually has life-threatening consequences. In certain DA-dependent congenital heart diseases, however, patient survival depends on maintaining DA patency. Complete closure of the DA involves both functional closure, induced by muscle contraction, and anatomical closure, achieved through morphological and molecular remodeling. Anatomical closure of the DA is associated with the formation of intimal thickening, which is characterized by deposition of extracellular matrix in the subendothelial region, sparse elastic fiber formation, and migration of medial smooth muscle cells into the subendothelial space. In addition, fetal molecular remodeling that is suitable for postnatal muscle contraction has been observed in the DA. After the second trimester, high concentration of prostaglandin E2 (PGE2) causes the DA to dilate through the remainder of the fetal period. Emerging evidence from studies using pharmacological approaches and genetically modified mice suggests that, in addition to its vasodilatory effect, this chronic exposure to PGE2 promotes DA-specific anatomical and molecular remodeling through EP4, one of four receptor subtypes for PGE2. Signals that are downstream of PGE2-EP4, such as cyclic AMP (cAMP)-protein kinase A (PKA), exchange protein activated by cAMP (Epac), phospholipase C, and Wnt/β-catenin, may be involved in the regulation of intimal thickening, elastogenesis, and contraction-related genes. Understanding the physiological role of PGE2 in DA remodeling could enable more effective regulation of PDA, both in isolation and in the context of congenital cardiac anomalies.

Melanocortin 4 receptor activates ERK-cFos pathway to increase brain-derived neurotrophic factor expression in rat astrocytes and hypothalamus

Melanocortins are neuropeptides with well recognized anti-inflammatory and anti-apoptotic effects in the brain. Of the five melanocortin receptors (MCR), MC4R is abundantly expressed in the brain and is the only MCR present in astrocytes. We have previously shown that MC4R activation by the α-melanocyte stimulating hormone (α-MSH) analog, NDP-MSH, increased brain-derived neurotrophic factor (BDNF) expression through the classic cAMP-Protein kinase A-cAMP responsive element binding protein pathway in rat astrocytes. Now, we examined the participation of the mitogen activated protein kinases pathway in MC4R signaling. Rat cultured astrocytes treated with NDP-MSH 1 µM for 1 h showed increased BDNF expression. Inhibition of extracellular signal-regulated kinase (ERK) and ribosomal p90 S6 kinase (RSK), an ERK substrate, but not of p38 or JNK, prevented the increase in BDNF expression induced by NDP-MSH. Activation of MC4R increased cFos expression, a target of both ERK and RSK. ERK activation by MC4R involves cAMP, phosphoinositide-3 kinase (PI3K) and the non receptor tyrosine kinase, Src. Both PI3K and Src inhibition abolished NDP-MSH-induced BDNF expression. Moreover, we found that intraperitoneal injection of α-MSH induces BDNF and MC4R expression and activates ERK and cFos in male rat hypothalamus. Our results show for the first time that MC4R-induced BDNF expression in astrocytes involves ERK-RSK-cFos pathway which is dependent on PI3K and Src, and that melanocortins induce BDNF expression and ERK-cFos activation in rat hypothalamus.

The focal adhesion kinase Pyk2 links Ca2+ signalling to Src family kinase activation and protein tyrosine phosphorylation in thrombin-stimulated platelets

We address the mechanism for Src family kinases activation downstream of G-protein-coupled receptors (GPCRs) in thrombin-stimulated blood platelets and we describe a novel interplay between Pyk2 and the Src kinases Fyn and Lyn in the regulation of Ca2+-dependent protein-tyrosine phosphorylation.

Gαs proteins activate p72(Syk) and p60-c-Src tyrosine kinases to mediate sickle red blood cell adhesion to endothelium via LW-αvβ3 and CD44-CD44 interactions

G protein-coupled receptors (GPCRs) have been suggested as new drug targets to treat a variety of diseases. In sickle cell disease (SCD), the LW erythrocyte adhesion receptor can be activated by stimulation of β2 adrenergic receptors (β2ARs), to mediate sickle erythrocyte (SSRBC) adhesion to endothelium. However, the involvement of tyrosine protein kinases in β2AR signaling to activate SSRBC adhesion to endothelium has not been thoroughly elucidated. Either direct activation with Cholera toxin of Gαs protein, which acts downstream of β2ARs, or inhibition with Pertussis toxin of Gαi, mediating suppression of adenylyl cyclase, increased SSRBC adhesion to endothelium over baseline adhesion. This effect involved the non-receptor tyrosine kinases, p72(Syk) and p60-c-Src, which were more abundant in SSRBCs than in normal erythrocytes. In contrast, Pertussis toxin and Cholera toxin failed to increase adhesion of normal erythrocytes. SSRBC Gαi inhibition also increased phosphorylation of p72(Syk) and p60-c-Src. Further, we investigated the relevance of activation of p72(Syk) and p60-c-Src, and identified LW (ICAM-4, CD242) and CD44 as the erythroid adhesion molecules both physically interacting with activated p60-c-Src. As a result, SSRBC LW underwent increased tyrosine phosphorylation, leading to SSRBC LW and CD44 binding to endothelial αvβ3 integrin and CD44, respectively. These data provide in vitro mechanistic evidence that p60-c-Src, which could act downstream of Gαs/p72(Syk), associates with LW and CD44 on SSRBCs leading to their interactions with endothelial αvβ3 and CD44, respectively. Thus, increased activation of these signaling mechanisms in SSRBCs could initiate or exacerbate vascular occlusion, the hallmark of SCD.