Dual effect of beta-adrenergic receptors on mitogen-activated protein kinase. Evidence for a beta gamma-dependent activation and a G alpha s-cAMP-mediated inhibition

CRHR1 endocytosis: Spatiotemporal regulation of receptor signaling

Corticotropin releasing hormone (CRH) is crucial for basal and stress-initiated reactions in the hypothalamic-pituitary-adrenal axis (HPA) and extrahypothalamic brain circuits, where it acts as a neuromodulator to organize behavioral and humoral responses to stress. We review and describe cellular components and molecular mechanisms involved in CRH system signaling through G protein-coupled receptors (GPCRs) CRHR1 and CRHR2, under the current view of GPCR signaling from the plasma membrane but also from intracellular compartments, which establish the bases of signal resolution in space and time. Focus is placed on latest studies of CRHR1 signaling in physiologically significant contexts of the neurohormone function that disclosed new mechanistic features of cAMP production and ERK1/2 activation. We also introduce in a brief overview the pathophysiological function of the CRH system, underlining the need for a complete characterization of CRHRs signaling to design new and specific therapies for stress-related disorders.

β2-Adrenoceptor Agonists Promote ERK1/2 Dephosphorylation in Human Airway Epithelial Cells by Canonical, cAMP-driven Signaling Independently of β-Arrestin 2

Chronic use of β₂-adrenoceptor agonists as a monotherapy in asthma is associated with a loss of disease control and an increased risk of mortality. Herein, we tested the hypothesis that β₂-adrenoceptor agonists, including formoterol, promote biased, β-arrestin 2 (βArr2)-dependent activation of the mitogen-activated protein (MAP) kinases, ERK1/2, in human airway epithelial cells and, thereby, effect changes in gene expression that could contribute to their adverse clinical outcomes. Three airway epithelial cell models were used: the BEAS-2B cell line, human primary bronchial epithelial cells (HBEC) grown in submersion culture and HBEC that were highly differentiated at an air-liquid interface. Unexpectedly, treatment of all epithelial cell models with formoterol decreased basal ERK1/2 phosphorylation. This was mediated by cAMP-dependent protein kinase and involved the inactivation of C-rapidly-activated fibrosarcoma, which attenuated down-stream ERK1/2 activity, and the induction of dual-specificity phosphatase-1. Formoterol also inhibited the basal expression of early growth response-1, an ERK1/2-regulated gene that controls cell growth and repair in the airways. Neither carvedilol, a β₂-adrenoceptor agonist biased towards βArr2, nor formoterol promoted ERK1/2 phosphorylation in BEAS-2B cells although β₂-adrenoceptor desensitization was compromised in ARRB2-deficient cells. Collectively, these results contest the hypothesis that formoterol activates ERK1/2 in airway epithelia by nucleating a βArr2 signaling complex; instead, they indicate that β₂-adrenoceptor agonists inhibit constitutive ERK1/2 activity in a cAMP-dependent manner. These findings are the antithesis of results obtained using acutely challenged native and engineered HEK293 cells, which have been used extensively to study mechanisms of ERK1/2 activation, and highlight the cell-type-dependence of β₂-adrenoceptor-mediated signaling. Significance Statement It has been proposed that the adverse-effects of β₂-adrenoceptor agonist monotherapy in asthma are mediated by genomic mechanisms that occur principally in airway epithelial cells and are the result of β-arrestin 2-dependent activation of ERK1/2. This study shows that β₂-adrenoceptor agonists, paradoxically, reduced ERK1/2 phosphorylation in airway epithelia by disrupting upstream Ras-C-Raf complex formation and inducing DUSP1. Moreover, these effects were PKA-dependent suggesting that β₂-adrenoceptor agonists were not biased toward β-arrestin 2 and acted via canonical, cAMP-dependent signaling.

In utero exposure to ritodrine during pregnancy and risk of autism in their offspring until 8 years of age

Beta-2 adrenergic receptor (B2AR) agonists, used as asthma treatments and tocolytics during pregnancy, have recently been reported to be associated with autism in their offspring. However, the particular link between autism and ritodrine, a common type of B2AR agonist used solely as tocolytics, has never been substantiated with any nationwide database. Thus, we aimed to examine the association between in utero exposure of ritodrine and the risk of autism in their offspring using a national database. This population-based cohort study was conducted by merging the Korea National Health Insurance claims database and National Health Screening Program for Infants and Children database. These databases included all women who had delivered singleton between January 2007 and December 2008 in Korea. Out of the total 770,016 mothers, 30,959 (4.02%) were exposed to ritodrine during pregnancy, and 5583 (0.73%) of their children were identified as having autism, defined until 8 years of age. According to our analysis, the overall cumulative incidence of autism up to 8 years was 1.37% in ritodrine exposure group and 0.70% in ritodrine non-exposure group (p < 0.05, log-rank test). By Cox proportional hazard analysis, use of ritodrine in preterm birth was associated with significantly higher hazard of autism [adjusted hazard ratio: 1.23, 95% CI 1.04-1.47], after adjusting for confounding variables including maternal age, parity, cesarean section, preterm labor, steroid use, birth weight, gender, and preeclampsia. Thus, in utero exposure to ritodrine was associated with an increased risk of autism in their offspring.

Anabolic and Pro-metabolic Functions of CREB-CRTC in Skeletal Muscle: Advantages and Obstacles for Type 2 Diabetes and Cancer Cachexia

cAMP is one of the earliest described mediators of hormone action in response to physiologic stress that allows acute stress responses and adaptation in every tissue. The classic role of cAMP signaling in metabolic tissues is to regulate nutrient partitioning. In response to acute stress, such as epinephrine released during strenuous exercise or fasting, intramuscular cAMP liberates glucose from glycogen and fatty acids from triglycerides. In the long-term, activation of Gs-coupled GPCRs stimulates muscle growth (hypertrophy) and metabolic adaptation through multiple pathways that culminate in a net increase of protein synthesis, mitochondrial biogenesis, and improved metabolic efficiency. This review focuses on regulation, function, and transcriptional targets of CREB (cAMP response element binding protein) and CRTCs (CREB regulated transcriptional coactivators) in skeletal muscle and the potential for targeting this pathway to sustain muscle mass and metabolic function in type 2 diabetes and cancer. Although the muscle-autonomous roles of these proteins might render them excellent targets for both conditions, pharmacologic targeting must be approached with caution. Gain of CREB-CRTC function is associated with excess liver glucose output in type 2 diabetes, and growing evidence implicates CREB-CRTC activation in proliferation and invasion of different types of cancer cells. We conclude that deeper investigation to identify skeletal muscle specific regulatory mechanisms that govern CREB-CRTC transcriptional activity is needed to safely take advantage of their potent effects to invigorate skeletal muscle to potentially improve health in people with type 2 diabetes and cancer.

Alpha 1 adrenergic receptor-mediated inflammatory responses in human testicular peritubular cells

Stress activates the sympathetic nervous system and is linked to impaired fertility in man. We hypothesized that catecholamines by acting on testicular cells have a role in these events, possibly by fostering an inflammatory environment. The cells of the wall of seminiferous tubules, human testicular peritubular cells (HTPCs), express adrenergic receptors (ADRs) α1B, α1D, β1 and β2. A selective α1-ADR agonist, phenylephrine, increased intracellular Ca²⁺-levels in cultured HTPCs and induced COX-2, IL-6 and MCP-1 mRNA expression without affecting IL-1β mRNA. These changes were paralleled by a significant increase in the secretion of IL-6 and MCP-1. Epinephrine was also effective, but salbutamol, a selective β2-ADR agonist was not. Our results suggest that stress-associated elevation of catecholamines may be able to promote inflammatory events by targeting peritubular cells in the human testis. Blockage of α1-ADRs may therefore be a novel way to interfere with stress-related impairment of male reproductive functions.

On the G protein-coupling selectivity of the native A2B adenosine receptor

A2B adenosine receptor (A2BAR) activation induces Gs-dependent cyclic AMP accumulation. However, A2BAR G protein-coupling to other signaling events, e.g. ERK1/2 and calcium, is not well documented. We explored Gi, Gq/11 and Gs coupling in 1321 N1 astrocytoma, HEK293, and T24 bladder cancer cells endogenously expressing human A2BAR, using NECA or nonnucleoside BAY60-6583 as agonist, selective Gi, Gs and Gq/11 blockers, and CRISPR/Cas9-based Gq- and Gs-null HEK293 cells. In HEK293 cells, A2BAR-mediated ERK1/2 activity occurred via both Gi and Gs, but not Gq/11. However, HEK293 cell calcium mobilization was completely blocked by Gq/11 inhibitor UBO-QIC and by Gq/11 knockout. In T24 cells, Gi was solely responsible for A2BAR-mediated ERK1/2 stimulation, and Gs suppressed ERK1/2 activity. A2BAR-mediated intracellular calcium mobilization in T24 cells was mainly via Gi, although Gs may also play a role, but Gq/11 is not involved. In 1321 N1 astrocytoma cells A2BAR activation suppressed rather than stimulated ERK1/2 activity. The ERK1/2 activity decrease was reversed by Gs downregulation using cholera toxin, but potentiated by Gi inhibitor pertussis toxin, and UBO-QIC had no effect. EPACs played an important role in A2BAR-mediated ERK1/2 signaling in all three cells. Thus, A2BAR may: couple to the same downstream pathway via different G proteins in different cell types; activate different downstream events via different G proteins in the same cell type; activate Gi and Gs, which have opposing or synergistic roles in different cell types/signaling pathways. The findings, relevant to drug discovery, address some reported controversial roles of A2BAR and could apply to signaling mechanisms in other GPCRs.

ERK Activation Pathways Downstream of GPCRs

GPCRs, the 7-TM receptors, represent a class of cell surface receptors which modulate a variety of physiological responses. The serpentine structure in addition to contributing the diversity of stimuli these receptors can sense also provides flexibility to the extracellular and intracellular regions where other proteins can interact with and can form functionally active multimeric entities. The range in signaling and physiological responses generated by these receptors can be attributed to a large repertoire of the receptor subtypes as well as their differential coupling to various classes of G-protein subunits and other proteins which facilitate multistate activation. A multistate GPCR can engage diverse signaling molecules, thereby modulating not only the canonical cellular responses but also noncanonical responses typically associated with activation of other cascades such as RTK and MAPK/ERK signaling. Given the crucial involvement of MAP kinase/ERK signaling in cell fate determination specially with respect to regulating cell proliferation, cellular apoptosis, and survival, GPCR-mediated cross-activation of MAPK has been explored in various systems and shown to involve functional integration of multiple pathways. This review describes the present knowledge of the different mechanisms of ERK activation downstream of GPCRs and our present understanding of receptor-dependent and -independent MAPK activation cascades.

Gαi is required for carvedilol-induced β1 adrenergic receptor β-arrestin biased signaling

The β₁ adrenergic receptor (β₁AR) is recognized as a classical Gα_s-coupled receptor. Agonist binding not only initiates G protein-mediated signaling but also signaling through the multifunctional adapter protein β-arrestin. Some βAR ligands, such as carvedilol, stimulate βAR signaling preferentially through β-arrestin, a concept known as β-arrestin-biased agonism. Here, we identify a signaling mechanism, unlike that previously known for any Gα_s-coupled receptor, whereby carvedilol induces the transition of the β₁AR from a classical Gα_s-coupled receptor to a Gα_i-coupled receptor stabilizing a distinct receptor conformation to initiate β-arrestin-mediated signaling. Recruitment of Gα_i is not induced by any other βAR ligand screened, nor is it required for β-arrestin-bias activated by the β₂AR subtype of the βAR family. Our findings demonstrate a previously unrecognized role for Gα_i in β₁AR signaling and suggest that the concept of β-arrestin-bias may need to be refined to incorporate the selective bias of receptors towards distinct G protein subtypes.

Membrane Androgen Receptor ZIP9 Induces Croaker Ovarian Cell Apoptosis via Stimulatory G Protein Alpha Subunit and MAP Kinase Signaling

Recent studies show that androgen-induced apoptosis in Atlantic croaker primary granulosa and theca (G/T) cells and in human breast and prostate cancer cell lines is mediated by the membrane androgen receptor ZIP9, which belongs to the SLC39A zinc transporter family. However, the apoptotic signaling pathways remain unclear because ZIP9 activates an inhibitory G protein in human cancer cells, whereas recombinant croaker ZIP9 activates a stimulatory G protein (Gs) in transfected cancer cells. We investigated androgen-dependent apoptotic pathways to identify the signaling pathways regulated through wild-type croaker ZIP9 in ovarian follicle cells. We show that the ZIP9-mediated apoptotic signaling pathway in croaker G/T cells shares several proapoptotic members with those in human cancer cells, but is activated through a Gsα subunit-dependent pathway. Testosterone treatment of croaker G/T cells increased intracellular zinc levels, mitogen-activated protein (MAP) kinase activity, caspase 3 activity, messenger RNA levels of proapoptotic members Bax, p53, and c-Jun N-terminal kinase, and the incidence of apoptosis, similar to findings in mammalian cancer cells, but also increased cyclic adenosine monophosphate concentrations. Transfection with small interfering RNA targeting croaker ZIP9 blocked testosterone-induced increase in bax, p53, and jnk expression. Testosterone-induced apoptosis and caspase 3 activation depended on the presence of extracellular zinc and were effectively blocked with cotreatment of inhibitors of the Gsα subunit, adenylyl cyclase, protein kinase A, and MAP kinase (Erk1/2) activation. These results indicate that ZIP9-mediated testosterone signaling in croaker G/T cells involves multiple pathways, some of which differ from those activated through ZIP9 in human cancer cells even though a similar apoptotic response is observed.

Endocrinology and the brain: corticotropin-releasing hormone signaling

Corticotropin-releasing hormone (CRH) is a key player of basal and stress-activated responses in the hypothalamic-pituitary-adrenal axis (HPA) and in extrahypothalamic circuits, where it functions as a neuromodulator to orchestrate humoral and behavioral adaptive responses to stress. This review describes molecular components and cellular mechanisms involved in CRH signaling downstream of its G protein-coupled receptors (GPCRs) CRHR1 and CRHR2 and summarizes recent findings that challenge the classical view of GPCR signaling and impact on our understanding of CRHRs function. Special emphasis is placed on recent studies of CRH signaling that revealed new mechanistic aspects of cAMP generation and ERK1/2 activation in physiologically relevant contexts of the neurohormone action. In addition, we present an overview of the pathophysiological role of the CRH system, which highlights the need for a precise definition of CRHRs signaling at molecular level to identify novel targets for pharmacological intervention in neuroendocrine tissues and specific brain areas involved in CRH-related disorders.

Advances in the role of oxytocin receptors in human parturition

Oxytocin (OT) is a neurohypophysial hormone which has been found to play a central role in the regulation of human parturition. The most established role of oxytocin/oxytocin receptor (OT/OTR) system in human parturition is the initiation of uterine contractions, however, recent evidence have demonstrated that it may have a more complex role including initiation of inflammation, regulation of miRNA expression, as well as mediation of other non-classical oxytocin actions via receptor crosstalk with other G protein-coupled receptors (GPCRs). In this review we highlight both established and newly emerging roles of OT/OTR system in human parturition and discuss the expanding potential for OTRs as pharmacological targets in the management of preterm labour.

Exogenous ceramide-1-phosphate (C1P) and phospho-ceramide analogue-1 (PCERA-1) regulate key macrophage activities via distinct receptors

Inflammation is an ensemble of tightly regulated steps, in which macrophages play an essential role. Previous reports showed that the natural sphingolipid ceramide 1-phosphate (C1P) stimulates macrophages migration, while the synthetic C1P mimic, phospho-ceramide analogue-1 (PCERA-1), suppresses production of the key pro-inflammatory cytokine TNFα and amplifies production of the key anti-inflammatory cytokine IL-10 in LPS-stimulated macrophages, via one or more unidentified G-protein coupled receptors. We show that C1P stimulated RAW264.7 macrophages migration via the NFκB pathway and MCP-1 induction, while PCERA-1 neither mimicked nor antagonized these activities. Conversely, PCERA-1 synergistically elevated LPS-dependent IL-10 expression in RAW264.7 macrophages via the cAMP-PKA-CREB signaling pathway, while C1P neither mimicked nor antagonized these activities. Interestingly, both compounds have the capacity to additively inhibit TNFα secretion; PCERA-1, but not C1P, suppressed LPS-induced TNFα expression in macrophages in a CREB-dependent manner, while C1P, but not PCERA-1, directly inhibited recombinant TNFα converting enzyme (TACE). Finally, PCERA-1 failed to interfere with binding of C1P to either the cell surface receptor or to TACE. These results thus indicate that the natural sphingolipid C1P and its synthetic analog PCERA-1 bind and activate distinct receptors expressed in RAW264.7 macrophages. Identification of these receptors will be instrumental for elucidation of novel activities of extra-cellular sphingolipids, and may pave the way for the design of new sphingolipid mimics for the treatment of inflammatory diseases, and pathologies which depend on cell migration, as in metastatic tumors.

Somatostatin activates Ras and ERK1/2 via a G protein βγ-subunit-initiated pathway in thyroid cells

Somatostatin (SST) is one of the main regulators of thyroid function. It acts by binding to its receptors, which lead to the dissociation of G proteins into Gαi and Gβγ subunits. However, much less is known about the function of Gβγ in thyroid cells. Here, we studied the role of SST and Gβγ dimers released upon SST stimulation on the Ras-ERK1/2 pathway in FTRL-5 thyroid cells. We demonstrate that SST activates Ras through Gi proteins, since SST-induced Ras activation is inhibited by pertussis toxin. Moreover, the specific sequestration of Gβγ dimers decreases Ras-GTP and phosphorylated ERK1/2 levels, and overexpression of Gβγ increases ERK1/2 phosphorylation induced by SST, indicating that Gβγ dimers released after SST treatment mediate activation of Ras and ERK1/2. On the other hand, SST treatment does not modify the expression of the thyroid differentiation marker sodium/iodide symporter (NIS) through ERK1/2 activation. However, SST increases AKT activation and the inhibition of the Src/PI3K/AKT pathway increases NIS levels in SST-treated cells. Thus, we conclude that, in thyroid cells, signalling from SST receptors to ERK1/2 involves a Gβγ-mediated signal acting on a Ras-dependent pathway. Moreover, we demonstrate that SST might regulates NIS expression through a Src/PI3K/AKT-dependent mechanism, but not through ERK1/2 signalling, showing the main role of this hormone in thyroid function.

N-n-butyl haloperidol iodide ameliorates cardiomyocytes hypoxia/reoxygenation injury by extracellular calcium-dependent and -independent mechanisms

N-n-butyl haloperidol iodide (F2) has been shown to antagonize myocardial ischemia/reperfusion injury by blocking calcium channels. This study explores the biological functions of ERK pathway in cardiomyocytes hypoxia/reoxygenation injury and clarifies the mechanisms by which F2 ameliorates cardiomyocytes hypoxia/reoxygenation injury through the extracellular-calcium-dependent and -independent ERK1/2-related pathways. In extracellularcalcium-containing hypoxia/reoxygenation cardiomyocytes, PKCα and ERK1/2 were activated, Egr-1 protein level and cTnI leakage increased, and cell viability decreased. The ERK1/2 inhibitors suppressed extracellular-calcium-containing-hypoxia/reoxygenation-induced Egr-1 overexpression and cardiomyocytes injury. PKCα inhibitor downregulated extracellularcalcium-containing-hypoxia/reoxygenation-induced increase in p-ERK1/2 and Egr-1 expression. F2 downregulated hypoxia/reoxygenation-induced elevation of p-PKCα, p-ERK1/2, and Egr-1 expression and inhibited cardiomyocytes damage. The ERK1/2 and PKCα activators antagonized F2's effects. In extracellular-calcium-free-hypoxia/reoxygenation cardiomyocytes, ERK1/2 was activated, LDH and cTnI leakage increased, and cell viability decreased. F2 and ERK1/2 inhibitors antagonized extracellular-calcium-free-hypoxia/reoxygenation-induced ERK1/2 activation and suppressed cardiomyocytes damage. The ERK1/2 activator antagonized F2's above effects. F2 had no effect on cardiomyocyte cAMP content or PKA and Egr-1 expression. Altogether, ERK activation in extracellular-calcium-containing and extracellular-calcium-free hypoxia/reoxygenation leads to cardiomyocytes damage. F2 may ameliorate cardiomyocytes hypoxia/reoxygenation injury by regulating the extracellular-calcium-dependent PKCα/ERK1/2/Egr-1 pathway and through the extracellular-calcium-independent ERK1/2 activation independently of the cAMP/PKA pathway or Egr-1 overexpression.

Sodium tungstate activates glycogen synthesis through a non-canonical mechanism involving G-proteins

Tungstate treatment ameliorates experimental diabetes by increasing liver glycogen deposition through an as yet unidentified mechanism. The signalling mechanism of tungstate was studied in CHOIR cells and primary cultured hepatocytes. This compound exerted its pro-glycogenic effects through a new G-protein-dependent and Tyr-Kinase Receptor-independent mechanism. Chemical or genetic disruption of G-protein signalling prevented the activation of the Ras/ERK cascade and the downstream induction of glycogen synthesis caused by tungstate. Thus, these findings unveil a novel non-canonical signalling pathway that leads to the activation of glycogen synthesis and that could be exploited as an approach to treat diabetes.

Activation of extracellular-signal regulated kinase by epidermal growth factor is potentiated by cAMP-elevating agents in primary cultures of adult rat hepatocytes

We investigated the effects of α- and β-adrenergic agonists on epidermal growth factor (EGF)-stimulated extracellular-signal regulated kinase (ERK) isoforms in primary cultures of adult rat hepatocytes. Hepatocytes were isolated and cultured with EGF (20 ng/ml) and/or α(1)-, α(2)- and β(2)-adrenergic agonists. Phosphorylated ERK isoforms (ERK1; p44 mitogen-activated protein kinase (MAPK) and ERK2; p42 MAPK) were detected by Western blotting analysis using anti-phospho-ERK1/2 antibody. The results show that EGF induced a 2.5-fold increase in ERK2-, but not ERK1-, phosphorylation within 3 min. This EGF-induced ERK2 activation was abolished by treatment with the EGF-receptor kinase inhibitor AG1478 (10(-7) M) or the MEK (MAPK kinase) inhibitor PD98059 (10(-6) M). The α(2)-adrenergic and β(2)-adrenergic agonists, UK14304 (10(-6) M) and metaproterenol (10(-6) M), respectively, had no effect in the absence of EGF, but metaproterenol significantly potentiated EGF-induced ERK2 phosphorylation. Moreover, the cell-permeable cAMP analog 8-bromo cAMP (10(-7) M), also potentiated EGF-induced ERK2 phosphorylation. The effects of these analogs were antagonized by the protein kinase A (PKA) inhibitor H-89 (10(-7) M). These results suggest that direct or indirect activation of PKA represents a positive regulatory mechanism for EGF stimulation of ERK2 induction.

OX1 orexin/hypocretin receptor activation of phospholipase D

BACKGROUND AND PURPOSE

Orexin receptors potently signal to lipid messenger systems, and our previous studies have suggested that PLD would be one of these. We thus wanted to verify this by direct measurements and clarify the molecular mechanism of the coupling.

EXPERIMENTAL APPROACH

Orexin receptor-mediated PLD activation was investigated in CHO cells stably expressing human OX(1) orexin receptors using [(14) C]-oleic acid-prelabelling and the transphosphatidylation assay.

KEY RESULTS

Orexin stimulation strongly increased PLD activity - even more so than the phorbol ester TPA (12-O-tetradecanoyl-phorbol-13-acetate), a highly potent activator of PLD. Both orexin and TPA responses were mediated by PLD1. Orexin-A and -B showed approximately 10-fold difference in potency, and the concentration-response curves were biphasic. Using pharmacological inhibitors and activators, both orexin and TPA were shown to signal to PLD1 via the novel PKC isoform, PKCδ. In contrast, pharmacological or molecular biological inhibitors of Rho family proteins RhoA/B/C, cdc42 and Rac did not inhibit the orexin (or the TPA) response, nor did the molecular biological inhibitors of PKD. In addition, neither cAMP elevation, Gα(i/o) nor Gβγ seemed to play an important role in the orexin response.

CONCLUSIONS AND IMPLICATIONS

Stimulation of OX(1) receptors potently activates PLD (probably PLD1) in CHO cells and this is mediated by PKCδ but not other PKC isoforms, PKDs or Rho family G-proteins. At present, the physiological significance of orexin-induced PLD activation is unknown, but this is not the first time we have identified PKCδ in orexin signalling, and thus some specific signalling cascade may exist between orexin receptors and PKCδ.

Dopamine D(2) Receptor-Mediated Heterologous Sensitization of AC5 Requires Signalosome Assembly

Chronic dopamine receptor activation is implicated in several central nervous system disorders. Although acute activation of Gα_i-coupled D₂ dopamine receptors inhibits adenylyl cyclase, persistent activation enhances adenylyl cyclase activity, a phenomenon called heterologous sensitization. Previous work revealed a requirement for Gα_s in D₂-induced heterologous sensitization of AC5. To elucidate the mechanism of Gα_s dependency, we expressed Gα_s mutants in Gα_s-deficient Gnas^E2−/E2− cells. Neither Gα_s-palmitoylation nor Gα_s-Gβγ interactions were required for sensitization of AC5. Moreover, we found that coexpressing βARKct-CD8 or Sar1(H79G) blocked heterologous sensitization. These studies are consistent with a role for Gα_s-AC5 interactions in sensitization however, Gβγ appears to have an indirect role in heterologous sensitization of AC5, possibly by promoting proper signalosome assembly.

Ser/ Thr residues at α3/β5 loop of Gαs are important in morphine-induced adenylyl cyclase sensitization but not mitogen-activated protein kinase phosphorylation

The signaling switch of β2-adrenergic and μ(1) -opioid receptors from stimulatory G-protein (G(αs) ) to inhibitory G-protein (G(αi) ) (and vice versa) influences adenylyl cyclase (AC) and extracellular-regulated kinase (ERK)1/2 activation. Post-translational modifications, including dephosphorylation of G(αs) , enhance opioid receptor coupling to G(αs) . In the present study, we substituted the Ser/Thr residues of G(αs) at the α3/β5 and α4/β6 loops aiming to study the role of G(αs) lacking Ser/Thr phosphorylation with respect to AC sensitization and mitogen-activated protein kinase activation. Isoproterenol increased the cAMP concentration (EC(50) = 22.8 ± 3.4 μm) in G(αs) -transfected S49 cyc- cells but not in nontransfected cells. However, there was no significant difference between the G(αs) -wild-type (wt) and mutants. Morphine (10 μm) inhibited AC activity more efficiently in cyc- compared to G(αs) -wt introduced cells (P < 0.05); however, we did not find a notable difference between G(αs) -wt and mutants. Interestingly, G(αs) -wt transfected cells showed more sensitization with respect to AC after chronic morphine compared to nontransfected cells (101 ± 12% versus 34 ± 6%; P < 0.001); μ1-opioid receptor interacted with G(αs) , and both co-immunoprecipitated after chronic morphine exposure. Furthermore, mutation of T270A and S272A (P < 0.01), as well as T270A, S272A and S261A (P < 0.05), in α3/β5, resulted in a higher level of AC supersensitization. ERK1/2 phosphorylation was rapidly induced by isoproterenol (by 9.5 ± 2.4-fold) and morphine (22 ± 2.2-fold) in G(αs) -transfected cells; mutations of α3/β5 and α4/β6 did not affect the pattern or extent of mitogen-activated protein kinase activation. The findings of the present study show that G(αs) interacts with the μ1-opioid receptor, and the Ser/Thr mutation to Ala at the α3/β5 loop of G(αs) enhances morphine-induced AC sensitization. In addition, G(αs) was required for the rapid phosphorylation of ERK1/2 by isoproterenol but not morphine.

Cell contact-dependent functional selectivity of β2-adrenergic receptor ligands in stimulating cAMP accumulation and extracellular signal-regulated kinase phosphorylation

Activation of β(2)-adrenegic receptor (β(2)-AR) leads to an increase in intracellular cAMP and activation of ERK. These two signals are activated by the interaction of the receptor with different transducer partners. We showed that the intrinsic activities of β(2)-AR ligands for stimulating cAMP production and ERK phosphorylation responses in HEK-293 cells were not correlated. The lack of correlation resulted mainly from the discrepancy between the intrinsic activities of two groups of ligands for these two responses: The first group consisted of clenbuterol, cimaterol, procaterol, and terbutaline which acted as full agonists for cAMP production but displayed very weak effect on ERK phosphorylation. The second group comprised adrenaline and noradrenaline which displayed higher intrinsic activity for the ERK phosphorylation than for the cAMP response. Thus, both groups behaved as functionally selective ligands. The functional selectivity of the first group was observable only in adherent cells when confluence was approximately 100%. When cell-cell contact was minimized either by decreasing the density of the adherent cells or by bringing the cells into suspension, the first group of ligands gained the ability to stimulate ERK phosphorylation without a change in their effect on cAMP production. In contrast, selectivity of the second group was independent of the adherence state of the cells. Our results show that the inherent "bias" of ligands in coupling a G protein-coupled receptor to different transducers may not always be revealed as functional selectivity when there is a "cross-talk" between the signaling pathways activated by the same receptor.

Identification of novel competing β2AR phospho-extracellular signal regulated kinase 1/2 signaling pathways in human trabecular meshwork cells

PURPOSE

β-Adrenergic receptor (βAR) agonists reduce intraocular pressure (IOP) by increasing aqueous outflow through the trabecular meshwork (TM). However, although this effect is well established, the specific signaling mechanisms responsible are less clear. To address this, the current study examined βAR signaling in primary human trabecular meshwork cells (HTMCs), specifically, focusing on the effect of βAR activation on the extracellular signal regulated kinases 1/2 (ERK).

METHODS

HTMCs were cultured and assessed for βAR expression by both immunofluorescence and reverse-transcription-polymerase chain reaction. The effect of βAR activation on ERK phosphorylation was examined in these cells by In-Cell Western™ analysis. Pharmacological approaches were used to characterize the mechanism of the βAR effect on ERK.

RESULTS

Treatment of HTMCs with the nonselective βAR agonist, isoproterenol (ISO), decreased the basal phospho-ERK (pERK) level, through actions at the β2AR. The response was insensitive to pertussis toxin (PTx), but pretreatment with cholera toxin (CTx) resulted in a reversal of the response, such that ISO treatment instead increased pERK, thus implicating Gα(s) in the inhibitory pERK response. The adenylyl cyclase activator, forskolin, also decreased pERK, suggesting the involvement of adenylyl cyclase and cAMP, whereas a protein kinase A (PKA) inhibitor, H-89, blocked both ISO and forskolin-mediated pERK inhibition in HTMCs. Finally, a closer examination of the pERK increase generated in the presence of CTx demonstrated that it was also insensitive to PTx, and appeared to have differing rank orders of efficacy for various βAR agonists compared with the inhibitory pERK pathway in HTMCs.

CONCLUSION

A novel β(2)AR-signaling pathway leading to a decrease in pERK, which was dependent on Gα(s), cAMP, and PKA, was identified in HTMCs. A competing β(2)AR signaling pathway resulting in increased pERK was also identified. Since βAR effects on aqueous humor (AH) outflow have been linked to cAMP, and inhibition of ERK in TM cells has recently been suggested as increasing AH outflow, our findings suggest that the inhibitory β(2)AR-pERK pathway likely represents the mechanism by which βAR agonists decrease IOP. The presence of a competing β(2)AR-ERK activation pathway in the same cells suggests that this is an ideal system for the development and validation of functionally selective β(2)AR agonists.

Ablation of p21-activated kinase-1 in mice promotes isoproterenol-induced cardiac hypertrophy in association with activation of Erk1/2 and inhibition of protein phosphatase 2A

Earlier investigations in our lab indicated an anti-adrenergic effect induced by activation of p21-activated kinase (Pak-1) and protein phosphatase 2A (PP2A). Our objective was to test the hypothesis that Pak-1/PP2A is a signaling cascade controlling stress-induced cardiac growth. We determined the effects of ablation of the Pak-1 gene on the response of the myocardium to chronic stress of isoproterenol (ISO) administration. Wild-type (WT) and Pak-1-knockout (Pak-1-KO) mice were randomized into six groups to receive either ISO, saline (CTRL), or ISO and FR180204, a selective inhibitor of Erk1/2. Echocardiography revealed that hearts of the Pak-1-KO/ISO group had increased LV fractional shortening, reduced LV chamber volume in diastole and systole, increased cardiac hypertrophy, and enhanced transmitral early filling deceleration time, compared to all other groups. The changes were associated with an increase in relative Erk1/2 activation in Pak-1-KO/ISO mice versus all other groups. ISO-induced cardiac hypertrophy and Erk1/2 activation in Pak-1-KO/ISO were attenuated when the selective Erk1/2 inhibitor FR180204 was administered. Immunoprecipitation showed an association between Pak-1, PP2A, and Erk1/2. Cardiac myocytes infected with an adenoviral vector expressing constitutively active Pak-1 showed a repression of Erk1/2 activation. p38 MAPK phosphorylation was decreased in Pak-1-KO/ISO and Pak-1-KO/CTRL mice compared to WT. Levels of phosphorylated PP2A were increased in ISO-treated Pak-1-KO mice, indicating reduced phosphatase activity. Maximum Ca(2+)-activated tension in detergent-extracted bundles of papillary fibers from ISO-treated Pak-1-KO mice was higher than in all other groups. Analysis of cTnI phosphorylation indicated that compared to WT, ISO-induced phosphorylation of cTnI was blunted in Pak-1-KO mice. Active Pak-1 is a natural inhibitor of Erk1/2 and a novel anti-hypertrophic signaling molecule upstream of PP2A.

Functional interactions between the oxytocin receptor and the β2-adrenergic receptor: implications for ERK1/2 activation in human myometrial cells

The Gq-coupled oxytocin receptor (OTR) and the Gs-coupled β(2)-adrenergic receptor (β(2)AR) are both expressed in myometrial cells and mediate uterine contraction and relaxation, respectively. The two receptors represent important pharmacological targets as OTR antagonists and β(2)AR agonists are used to control pre-term uterine contractions. Despite their physiologically antagonistic effects, both receptors activate the MAP kinases ERK1/2, which has been implicated in uterine contraction and the onset of labor. To determine the signalling pathways involved in mediating the ERK1/2 response, we assessed the effect of blockers of specific G protein-associated pathways. In human myometrial hTERT-C3 cells, inhibition of Gαi as well as inhibition of the Gαq/PKC pathway led to a reduction of both OTR- and β(2)AR-mediated ERK1/2 activation. The involvement of Gαq/PKC in β(2)AR-mediated ERK1/2 induction was unexpected. To test whether the emergence of this novel signalling mechanism was dependent on OTR expression in the same cell, we conducted experiments in HEK 293 cells that were transfected with the β(2)AR alone or co-transfected with the OTR. Using this approach, we found that β(2)AR-mediated ERK1/2 responses became sensitive to PKC inhibition only in cells co-transfected with the OTR. Inhibitor studies indicated the involvement of an atypical PKC isoform in this process. We confirmed the specific involvement of PKCζ in this pathway by assessing PKCζ translocation to the cell membrane. Consistent with our inhibitor studies, we found that β(2)AR-mediated PKCζ translocation was dependent on co-expression of OTR. The present demonstration of a novel β(2)AR-coupled signalling pathway that is dependent on OTR co-expression is suggestive of a molecular interaction between the two receptors.

Contribution of Kv1.2 voltage-gated potassium channel to D2 autoreceptor regulation of axonal dopamine overflow

Impairments in axonal dopamine release are associated with neurological disorders such as schizophrenia and attention deficit hyperactivity disorder and pathophysiological conditions promoting drug abuse and obesity. The D2 dopamine autoreceptor (D2-AR) exerts tight regulatory control of axonal dopamine (DA) release through a mechanism suggested to involve K(+) channels. To evaluate the contribution of Kv1 voltage-gated potassium channels of the Shaker gene family to the regulation of axonal DA release by the D2-AR, the present study employed expression analyses, real time measurements of striatal DA overflow, K(+) current measurements and immunoprecipitation assays. Kv1.1, -1.2, -1.3, and -1.6 mRNA and protein were detected in midbrain DA neurons purified by fluorescence-activated cell sorting and in primary DA neuron cultures. In addition, Kv1.1, -1.2, and -1.6 were localized to DA axonal processes in the dorsal striatum. By means of fast scan cyclic voltammetry in striatal slice preparations, we found that the inhibition of stimulation-evoked DA overflow by a D2 agonist was attenuated by Kv1.1, -1.2, and -1.6 toxin blockers. A particular role for the Kv1.2 subunit in the process whereby axonal D2-AR inhibits DA overflow was established with the use of a selective Kv1.2 blocker and Kv1.2 knock-out mice. Moreover, we demonstrate the ability of D2-AR activation to increase Kv1.2 currents in co-transfected cells and its reliance on Gβγ subunit signaling along with the physical coupling of D2-AR and Kv1.2-containing channels in striatal tissue. These findings underline the contribution of Kv1.2 in the regulation of nigrostriatal DA release by the D2-AR and thereby offer a novel mechanism by which DA release is regulated.

GnRH signaling, the gonadotrope and endocrine control of fertility

Mammalian reproductive cycles are controlled by an intricate interplay between the hypothalamus, pituitary and gonads. Central to the function of this axis is the ability of the pituitary gonadotrope to appropriately respond to stimulation by gonadotropin-releasing hormone (GnRH). This review focuses on the role of cell signaling and in particular, mitogen-activated protein kinase (MAPK) activities regulated by GnRH that are necessary for normal fertility. Recently, new mouse models making use of conditional gene deletion have shed new light on the relationships between GnRH signaling and fertility in both male and female mice. Within the reproductive axis, GnRH signaling is initiated through discrete membrane compartments in which the receptor resides leading to the activation of the extracellular signal-regulated kinases (ERKs 1/2). As defined by gonadotrope-derived cellular models, the ERKs appear to play a central role in the regulation of a cohort of immediate early genes that regulate the expression of late genes that, in part, define the differentiated character of the gonadotrope. Recent data would suggest that in vivo, conditional, pituitary-specific disruption of ERK signaling by GnRH leads to a gender-specific perturbation of fertility. Double ERK knockout in the anterior pituitary leads to female infertility due to LH biosynthesis deficiency and a failure in ovulation. In contrast, male mice are modestly LH deficient; however, this does not have an appreciable impact on fertility.

In utero beta 2 adrenergic agonist exposure and adverse neurophysiologic and behavioral outcomes

Beta 2 adrenergic receptor overstimulation during critical periods of prenatal development can induce a permanent shift in the balance of sympathetic-to-parasympathetic tone. This is a biologically plausible mechanism whereby beta 2 adrenergic agonists can induce functional and behavioral teratogenesis, which explains their association with increases in autism spectrum disorders, psychiatric disorders, poor cognitive, motor function and school performance, and changes in blood pressure in the offspring. The use of beta 2 adrenergic agonists should be limited to proven indications when alternate drugs are ineffective or unavailable; the risks of untreated disease to the mother and fetus are greater than the risk of the beta 2 adrenergic agonist.

Kappa opioids promote the proliferation of astrocytes via Gbetagamma and beta-arrestin 2-dependent MAPK-mediated pathways

GTP binding regulatory protein (G protein)-coupled receptors can activate MAPK pathways via G protein-dependent and -independent mechanisms. However, the physiological outcomes correlated with the cellular signaling events are not as well characterized. In this study, we examine the involvement of G protein and beta-arrestin 2 pathways in kappa opioid receptor-induced, extracellular signal-regulated kinase 1/2 (ERK1/2)-mediated proliferation of both immortalized and primary astrocyte cultures. As different agonists induce different cellular signaling pathways, we tested the prototypic kappa agonist, U69593 as well as the structurally distinct, non-nitrogenous agonist, C(2)-methoxymethyl salvinorin B (MOM-Sal-B). In immortalized astrocytes, U69593, activated ERK1/2 by a rapid (min) initial stimulation that was sustained over 2 h and increased proliferation. Sequestration of activated Gbetagamma subunits attenuated U69593 stimulation of ERK1/2 and suppressed proliferation in these cells. Furthermore, small interfering RNA silencing of beta-arrestin 2 diminished sustained ERK activation induced by U69593. In contrast, MOM-Sal-B induced only the early phase of ERK1/2 phosphorylation and did not affect proliferation of immortalized astrocytes. In primary astrocytes, U69593 produced the same effects as seen in immortalized astrocytes. MOM-Sal-B elicited sustained ERK1/2 activation which was correlated with increased primary astrocyte proliferation. Proliferative actions of both agonists were abolished by either inhibition of ERK1/2, Gbetagamma subunits or beta-arrestin 2, suggesting that both G protein-dependent and -independent ERK pathways are required for this outcome.

Epinephrine increases DNA synthesis via ERK1/2s through cAMP, Ca(2+)/PKC, and PI3K/Akt signaling pathways in mouse embryonic stem cells

Epinephrine is a catecholamine that plays important roles in regulating a wide variety of physiological systems by acting through the adrenergic receptors (ARs). The cellular responses to AR stimulation are mediated through various signaling pathways. Therefore, this study examined the effects of epinephrine on DNA synthesis and related signaling molecules in mouse embryonic stem cells (ESCs). Epinephrine increased DNA synthesis in a dose- and time-dependent manner, as determined by the level of [(3)H]-thymidine incorporation. AR subtypes (alpha1(A), alpha2(A), beta1, beta2, and beta3) were expressed in mouse ESCs and their expression levels were increased by epinephrine. In this experiment, epinephrine increased cAMP levels, intracellular Ca(2+) concentration ([Ca(2+)](i)), and translocation of protein kinase C (PKC) from the cytosol to the membrane compartment. In addition, we observed Akt phosphorylation in response to epinephrine; this was stimulated by phosphorylation of the epidermal growth factor receptor (EGFR). Epinephrine also induced phosphorylation of ERK1/2 (p44/42 MAPKs), while inhibition of PKC or Akt blocked this phosphorylation. Epinephrine increased the mRNA levels of proto-oncogenes (c-fos, c-jun, c-myc), while inhibition of ERK1/2 decreased these mRNA levels. In experiments aimed at examining the involvement of cell cycle regulatory proteins, epinephrine increased the levels of cyclin E/cyclin-dependent kinase 2 (CDK2) and cyclin D1/cyclin-dependent kinase 4 (CDK4). In conclusion, epinephrine stimulates DNA synthesis via ERK1/2 through cAMP, Ca(2+)/PKC, and PI3K/Akt signaling pathways in mouse ESCs.

beta-Adrenergic receptor activation induces internalization of cardiac Cav1.2 channel complexes through a beta-arrestin 1-mediated pathway

Voltage-dependent calcium channels (VDCCs) play a pivotal role in normal excitation-contraction coupling in cardiac myocytes. These channels can be modulated through activation of beta-adrenergic receptors (beta-ARs), which leads to an increase in calcium current (I(Ca-L)) density through cardiac Ca(v)1 channels as a result of phosphorylation by cAMP-dependent protein kinase A. Changes in I(Ca-L) density and kinetics in heart failure often occur in the absence of changes in Ca(v)1 channel expression, arguing for the importance of post-translational modification of these channels in heart disease. The precise molecular mechanisms that govern the regulation of VDCCs and their cell surface localization remain unknown. Our data show that sustained beta-AR activation induces internalization of a cardiac macromolecular complex involving VDCC and beta-arrestin 1 (beta-Arr1) into clathrin-coated vesicles. Pretreatment of myocytes with pertussis toxin prevents the internalization of VDCCs, suggesting that G(i/o) mediates this response. A peptide that selectively disrupts the interaction between Ca(V)1.2 and beta-Arr1 and tyrosine kinase inhibitors readily prevent agonist-induced VDCC internalization. These observations suggest that VDCC trafficking is mediated by G protein switching to G(i) of the beta-AR, which plays a prominent role in various cardiac pathologies associated with a hyperadrenergic state, such as hypertrophy and heart failure.

Conformational rearrangements and signaling cascades involved in ligand-biased mitogen-activated protein kinase signaling through the beta1-adrenergic receptor

In recent years, several studies have demonstrated that different ligands can have distinct efficacy profiles toward various signaling pathways through a unique receptor. For example, beta1-adrenergic compounds that are inverse agonists toward the adenylyl cyclase (AC) can display agonist activity for the mitogen-activated protein kinase (MAPK) pathway. Such a phenomenon, often termed functional selectivity, has now been clearly established for many G protein-coupled receptors when considering distinct signaling output. However, the possibility that ligands could selectively engage distinct effectors to activate a single signaling output by promoting specific receptor conformations has not been extensively examined. Here, we took advantage of the fact that isoproterenol, bucindolol and propranolol (full, partial, and inverse agonists for the AC pathway, respectively) all activate MAPK through the beta1-adrenergic receptor (beta1AR) to probe such conformational-biased signaling. Although the three compounds stimulated MAPK in a src-dependent manner, isoproterenol acted through both Galpha(i)betagamma- and G protein-independent pathways, whereas bucindolol and propranolol promoted MAPK activation through the G protein-independent pathway only. The existence of such distinct signaling cascades linking beta1AR to MAPK activation was correlated with ligand-specific conformational rearrangements of receptor/G protein complexes measured by bioluminescence resonance energy transfer. Taken together, our data indicate that discrete local conformational changes can selectively promote the recruitment of distinct proximal signaling partners that can engage distinct signaling outputs and/or converge on the same signaling output.

Prolonged heterologous beta2-adrenoceptor desensitization promotes proasthmatic airway smooth muscle function via PKA/ERK1/2-mediated phosphodiesterase-4 induction

Beta2-adrenergic receptor (beta2AR) agonists acutely relieve bronchoconstriction via cAMP-mediated relaxation of airway smooth muscle (ASM). Airway constrictor responsiveness may be significantly heightened, however, following protracted exposure to these agents, presumably reflecting the effects of beta2AR desensitization in ASM accompanying prolonged cAMP signaling. Because cAMP phosphodiesterase (PDE) activity can significantly modulate ASM contractility, we investigated the mechanism regulating PDE expression and its potential role in mediating changes in agonist-induced constrictor and relaxation responsiveness in ASM following its heterologous beta2AR desensitization by prolonged exposure to cAMP-elevating agents. Isolated rabbit ASM tissues and cultured human ASM cells treated for 24 h with the receptor- or nonreceptor-coupled cAMP-stimulating agent, prostaglandin E(2) (PGE(2)) or forskolin, respectively, exhibited constrictor hyperresponsiveness to acetylcholine and impaired beta2AR-mediated relaxation and cAMP accumulation. These proasthmatic-like changes in ASM function were associated with upregulated PDE4 activity, reflective of increased transcription of the PDE4D5 isoform, and were prevented by pretreatment of the ASM with a PDE4 inhibitor. Extended studies using gene silencing and pharmacological approaches to inhibit specific intracellular signaling molecules demonstrated that the mechanism underlying PGE(2)-induced transcriptional upregulation of PDE4D5 involves PKA-dependent activation of G(i) protein signaling via the betagamma-subunits, the latter eliciting downstream activation of ERK1/2 and its consequent induction of PDE4D5 transcription. Collectively, these findings identify that beta2AR desensitization in ASM following prolonged exposure to cAMP-elevating agents is associated with proasthmatic-like changes in ASM responsiveness that are mediated by upregulated PDE4 expression induced by activated cross talk between the PKA and ERK1/2 signaling pathways.

Fetal mechanisms in neurodevelopmental disorders

Normal development of the central nervous system depends on complex, dynamic mechanisms with multiple spatial and temporal components during gestation. Neurodevelopmental disorders may originate during fetal life from genetic as well as intrauterine and extrauterine factors that affect the fetal-maternal environment. Fetal neurodevelopment depends on cell programs, developmental trajectories, synaptic plasticity, and oligodendrocyte maturation, which are variously modifiable by factors such as stress and endocrine disruption, exposure to pesticides such as chlorpyrifos and to drugs such as terbutaline, maternal teratogenic alleles, and premature birth. Current research illustrates how altered fetal mechanisms may affect long-term physiological and behavioral functions of the central nervous system more significantly than they affect its form, and these effects may be transgenerational. This research emphasizes the diversity of such prenatal mechanisms and the need to expand our understanding of how, when altered, they may lead to disordered development, the signs of which may not appear until long after birth.

Gbetagamma dimers released in response to thyrotropin activate phosphoinositide 3-kinase and regulate gene expression in thyroid cells

Signaling by TSH through its receptor leads to the dissociation of trimeric G proteins into Galpha and Gbetagamma. Galphas activates adenylyl cyclase, which increases cAMP levels that induce several effects in the thyroid cell, including transcription of the sodium-iodide symporter (NIS) gene through a mechanism involving Pax8 binding to the NIS promoter. Much less is known about the function of Gbetagamma in thyroid differentiation, and therefore we studied their role in TSH signaling. Gbetagamma overexpression inhibits NIS promoter activation and reduces NIS protein accumulation in response to TSH and forskolin. Conversely, inhibition of Gbetagamma-dependent pathways increases NIS promoter activity elicited by TSH but does not modify forskolin-induced activation. Gbetagamma dimers are being released from the Gs subfamily of proteins, because cholera toxin mimics the effects elicited by TSH, whereas pertussis toxin has no effect on NIS promoter activity. We also found that TSH stimulates Akt phosphorylation in a phosphoinositide 3-kinase (PI3K)-dependent and cAMP-independent manner. This is mediated by Gbetagamma, because its overexpression or specific sequestration, respectively, increased or reduced phosphorylated Akt levels upon TSH stimulation. Gbetagamma sequestration increases NIS protein levels induced by TSH and Pax8 binding to the NIS promoter, which is also increased by PI3K inhibition. This is, at least in part, caused by Gbetagamma-mediated Pax8 exclusion from the nucleus that is attenuated when PI3K activity is blocked. These data unequivocally demonstrate that Gbetagamma released by TSH action stimulate PI3K, inhibiting NIS gene expression in a cAMP-independent manner due to a decrease in Pax8 binding to the NIS promoter.

Multiple G-protein-coupling specificity of beta-adrenoceptor in macrophages

Adrenergic signalling of the immune system is one of the important modulator pathways of the inflammatory immune response realized via G protein-mediated pathways. The resulted signal depends on the type of the receptor-coupled G-protein (GPCR) that, according to the classical paradigm in the case of beta-adrenergic receptor (beta-AR), is Gs-type. Recently, alternate and/or multiple G protein coupling specificity of GPCRs have been demonstrated including a switch from Gs to Gi binding. The possibility of a Gs/Gi switch and its role in the immune response of macrophages has not been investigated yet. In this study, we demonstrate that beta-adrenergic stimulation itself is able to induce a transient mitogen-activated protein kinase phosphorylation in murine peritoneal macrophages in a pertussis toxin-sensitive manner, suggesting that the Gs/Gi switch also occurs in the immune system. Although this process is very rapid, it can influence different signalling pathways and can reprogramme effector functions suggesting that sympathetic modulation of the defence mechanism of the innate immune system has an additional, Gs/Gi switch-dependent component.

When a G protein-coupled receptor does not couple to a G protein

Classically, G protein-coupled receptors (GPCRs) relay signals by directly activating heterotrimeric guanine nucleotide-binding proteins (G proteins). Increasing evidence indicates that GPCRs may also signal through G protein-independent pathways. JAK/STATs, Src-family tyrosine kinases, GRKs/beta-arrestins, and PDZ domain-containing proteins have been suggested to directly relay signals from GPCRs independent of G proteins. In addition, our laboratory recently reported that the beta(2) adrenergic receptor (beta(2)AR) could switch from G protein-coupled to G protein-independent ERK (extracellular signal-regulated kinase) activation in an agonist dosage-dependent manner. This finding provides a novel mechanism for G protein-independent GPCR signaling. This review focuses on recent progress in understanding the mechanisms by which G protein-independent GPCR signaling occurs.

Differential regulation on human skin fibroblast by alpha1 adrenergic receptor subtypes

Alpha 1 adrenoceptor (alpha1-AR) regulation of DNA synthesis was studied in human neonatal foreskin fibroblast. Saturation assay with a specific radioligand for alpha1 adrenergic [3H]-prazosin revealed two saturated and specific binding sites with high or low affinity. Competitive binding assay with different antagonist subtypes, defined pharmacologically three major types of alpha1-AR. The alpha1-AR agonists (from 1x10(-10) to 1x10(-4) M) triggered a biphasic action on DNA synthesis reaching maximal stimulation at 1x10(-9) M and maximal inhibition at 1x10(-6) M. Prazosin, abolished the stimulatory (pA2: 9.24) and inhibitory (pA2: 8.80) actions of alpha1-AR agonists. The alpha1-AR stimulation resulted in the activation of phosphoinositide turnover (InsP) via phospholipase C (PLC) involving calcium/calmodulin (CaM) and nitric oxide synthase (NOS) that correlates with the DNA synthesis increment; whereas the inhibition resulted in a decrease of cyclic AMP (cAMP) accumulation via adenylate cyclase inhibition. The potency displayed by the specific antagonists tested in binding, DNA synthesis, InsP and NOS at low agonist concentration suggests that they can be elicited by the activation of the same receptor (alpha1B-AR subtype); while the decrement in DNA synthesis and cAMP at high concentration account by the activation of alpha1D-AR coupled to Gi protein. Non-functional alpha1A-AR in neonatal human foreskin fibroblast was observed. Results suggest that the expression of alpha1-AR subtypes on human skin fibroblast may differentially activate signaling pathways that modulate physiological response of the cells.

G protein regulation of MAPK networks

G proteins provide signal-coupling mechanisms to heptahelical cell surface receptors and are critically involved in the regulation of different mitogen-activated protein kinase (MAPK) networks. The four classes of G proteins, defined by the G(s), G(i), G(q) and G(12) families, regulate ERK1/2, JNK, p38MAPK, ERK5 and ERK6 modules by different mechanisms. The alpha- as well as betagamma-subunits are involved in the regulation of these MAPK modules in a context-specific manner. While the alpha- and betagamma-subunits primarily regulate the MAPK pathways via their respective effector-mediated signaling pathways, recent studies have unraveled several novel signaling intermediates including receptor tyrosine kinases and small GTPases through which these G-protein subunits positively as well as negatively regulate specific MAPK modules. Multiple mechanisms together with specific scaffold proteins that can link G-protein-coupled receptors or G proteins to distinct MAPK modules contribute to the context-specific and spatio-temporal regulation of mitogen-activated protein signaling networks by G proteins.

Novel aspects of G-protein-coupled receptor signalling--different ways to achieve specificity

Our understanding of signal transduction via G-protein-coupled receptors (GPCR) has developed dramatically during the last decades. The initial idea of linear signalling pathways transferring information from the cell membrane to the nucleus has evolved into a complicated network of signalling pathways offering the possibility of crosstalk, fine tuning and specific regulation at multiple levels. During the recent meeting on GPCRs at the Karolinska Institutet, Stockholm novel aspects of GPCR signalling were presented and discussed. Here, we will discuss several possibilities for GPCRs to achieve specificity in signal transduction, such as the phenomenon of biased agonism, receptor multimerization, the role of co-receptors, the regulation of heterotrimeric G proteins as well as multiple G(s)-dependent pathways to extracellular single-regulated protein kinases.

Visualization and manipulation of phosphoinositide dynamics in live cells using engineered protein domains

There is hardly a membrane-associated molecular event that is not regulated by phosphoinositides, a minor but critically important class of phospholipids of cellular membranes. The rapid formation, elimination, and conversion of these lipids in specific membrane compartments are ensured by a wealthy number of inositol lipid kinases and phosphatases with unique localization and regulatory properties. The existence of multiple inositol lipid pools have been indicated by metabolic labeling studies, but the level of functional compartmentalization revealed by the identification of numerous protein effectors acted upon by phosphoinositides could not have been foreseen. The changing perception of inositides from just serving as lipid precursors of second messengers to becoming highly dynamic local membrane-bound regulators poses new challenges concerning the detection of their rapid localized changes. Moreover, it is increasingly evident that manipulation of lipids in highly defined compartments would be a highly superior approach to soaking the cells with a particular phosphoinositide when studying the local regulation of the lipid on any effectors. In this review, we will summarize our efforts to improve our tools in studying phosphoinositide dynamics and discuss our views on the values of these methods compared to other options currently used or being explored.

Prostaglandin F2alpha inhibits adipocyte differentiation via a G alpha q-calcium-calcineurin-dependent signaling pathway

Prostaglandin F2alpha (PGF2alpha) is a potent physiological inhibitor of adipocyte differentiation, however the specific signaling pathways and molecular mechanisms involved in mediating its anti-adipogenic effects are not well understood. In the current study, we now provide evidence that PGF2alpha inhibits adipocyte differentiation via a signaling pathway that requires heterotrimeric G-protein G alpha q subunits, the elevation of the intracellular calcium concentration ([Ca2+]i), and the activation of the Ca2+/calmodulin-regulated serine/threonine phosphatase calcineurin. We show that while this pathway acts to inhibit an early step in the adipogenic cascade, it does not interfere with the initial mitotic clonal expansion phase of adipogenesis, nor does it affect either the expression, DNA binding activity or differentiation-induced phosphorylation of the early transcription factor C/EBPbeta. Instead, we find that PGF2alpha inhibits adipocyte differentiation via a calcineurin-dependent mechanism that acts to prevent the expression of the critical pro-adipogenic transcription factors PPARgamma and C/EBPalpha. Furthermore, we demonstrate that the inhibitory effects of PGF2alpha on both the expression of PPARgamma and C/EBPalpha and subsequent adipogenesis can be attenuated by treatment of preadipocytes with the histone deacetylase (HDAC) inhibitor trichostatin A. Taken together, these results indicate that PGF2alpha inhibits adipocyte differentiation via a G alpha q-Ca2+-calcineurin-dependent signaling pathway that acts to block expression of PPARgamma and C/EBPalpha by a mechanism that appears to involves an HDAC-sensitive step.

Molecular mechanisms mediating the G protein-coupled receptor regulation of cell cycle progression

G protein-coupled receptors are key regulators of cellular communication, mediating the efficient coordination of a cell's responses to extracellular stimuli. When stimulated these receptors modulate the activity of a wide range of intracellular signalling pathways that facilitate the ordered development, growth and reproduction of the organism. There is now a growing body of evidence examining the mechanisms by which G protein-coupled receptors are able to regulate the expression, activity, localization and stability of cell cycle regulatory proteins that either promote or inhibit the initiation of DNA synthesis. In this review, we will detail the intracellular pathways that mediate the G protein-coupled receptor regulation of cellular proliferation, specifically the progression from the G1 phase to the S phase of the cell cycle.

Dosage-dependent switch from G protein-coupled to G protein-independent signaling by a GPCR

G-protein-coupled receptors (GPCRs) mostly signal through heterotrimeric G proteins. Increasing evidence suggests that GPCRs could function in a G-protein-independent manner. Here, we show that at low concentrations of an agonist, beta(2)-adrenergic receptors (beta(2)-ARs) signal through Galpha(s) to activate the mitogen-activated protein kinase pathway in mouse embryonic fibroblast cells. At high agonist concentrations, signals are also transduced through beta(2)-ARs via an additional pathway that is G-protein-independent but tyrosine kinase Src-dependent. This new dosage-dependent switch of signaling modes of GPCRs has significant implications for GPCR intrinsic properties and desensitization.

Conditional stimulation of type V and VI adenylyl cyclases by G protein betagamma subunits

In a yeast two-hybrid screen of mouse brain cDNA library, using the N-terminal region of human type V adenylyl cyclase (hACV) as bait, we identified G protein beta2 subunit as an interacting partner. Additional yeast two-hybrid assays showed that the Gbeta(1) subunit also interacts with the N-terminal segments of hACV and human type VI adenylyl cyclase (hACVI). In vitro adenylyl cyclase (AC) activity assays using membranes of Sf9 cells expressing hACV or hACVI showed that Gbetagamma subunits enhance the activity of these enzymes provided either Galpha(s) or forskolin is present. Deletion of residues 77-151, but not 1-76, in the N-terminal region of hACVI obliterated the ability of Gbetagamma subunits to conditionally stimulate the enzyme. Likewise, activities of the recombinant, engineered, soluble forms of ACV and ACVI, which lack the N termini, were not enhanced by Gbetagamma subunits. Transfection of the C terminus of G protein receptor kinase 2 to sequester endogenous Gbetagamma subunits attenuated the ability of isoproterenol to increase cAMP accumulation in COS-7 cells overexpressing hACVI even when G(i) was inactivated by pertussis toxin. Therefore, we conclude that the N termini of human hACV and hACVI are necessary for interactions with, and regulation by, Gbetagamma subunits both in vitro and in intact cells. Moreover, Gbetagamma subunits derived from a source(s) other than G(i) are necessary for the full activation of hACVI by isoproterenol in intact cells.

Heterotrimeric G proteins form stable complexes with adenylyl cyclase and Kir3.1 channels in living cells

Bioluminescence resonance energy transfer (BRET) and co-immunoprecipitation experiments revealed that heterotrimeric G proteins and their effectors were found in stable complexes that persisted during signal transduction. Adenylyl cyclase, Kir3.1 channel subunits and several G-protein subunits (Gαs, Gαi, Gβ1 and Gγ2) were tagged with luciferase (RLuc) or GFP, or the complementary fragments of YFP (specifically Gβ1-YFP1-158 and Gγ2-YFP159-238, which heterodimerize to produce fluorescent YFP-Gβ1γ2). BRET was observed between adenylyl-cyclase-RLuc or Kir3.1-RLuc and GFP-Gγ2, GFP-Gβ1 or YFP-Gβ1γ2. Gα subunits were also stably associated with both effectors regardless of whether or not signal transduction was initiated by a receptor agonist. Although BRET between effectors and Gβγ was increased by receptor stimulation, our data indicate that these changes are likely to be conformational in nature. Furthermore, receptor-sensitive G-protein-effector complexes could be detected before being transported to the plasma membrane, providing the first direct evidence for an intracellular site of assembly.

Arrestin serves as a molecular switch, linking endogenous alpha2-adrenergic receptor to SRC-dependent, but not SRC-independent, ERK activation

Our previous studies have demonstrated that neither receptor endocytosis nor arrestin is required for ERK activation by the alpha2-adrenergic receptor (Wang, Q., Zhao, J., Brady, A. E., Feng, J., Allen, P. B., Lefkowitz, R. J., Greengard, P., and Limbird, L. E. (2004) Science 304, 1940-1944). The present studies address whether arrestin plays a role in determining the route of alpha2AR-evoked ERK signaling activation, taking advantage of endogenous expression of the alpha(2A)AR subtype in mouse embryonic fibroblasts (MEFs) and the availability of MEFs without arrestin expression (derived from Arr2,3-/- mice). Our data demonstrate that the endogenous alpha(2A)AR evokes ERK phosphorylation through both a Src-dependent and a Src-independent pathway, both of which are G protein dependent and converge on the Ras-Raf-MEK pathway. Arrestin is essential to recruit Src to this process, as alpha(2A)AR-mediated ERK signaling in Arr2,3-/- MEFs does not involve Src. Stimulation of alpha(2A)AR enhances arrestin-Src interaction and promotes activation of Src. alpha2 agonists have similar potencies in stimulating Src-dependent and Src-independent ERK phosphorylation in wild-type and Arr2,3-/- cells, respectively. However, Src-independent alpha(2A)AR-mediated ERK stimulation has both a longer duration of activation and a more rapid translocation of pERK into the nucleus when compared with Src-dependent activation. These data not only affirm the role of arrestin as an escort for signaling molecules such as Src family kinases but also demonstrate the impact of arrestin-dependent modulation on both the temporal and spatial properties of ERK activation.

The V2 vasopressin receptor stimulates ERK1/2 activity independently of heterotrimeric G protein signalling

The V2 vasopressin receptor (V2R) activates the mitogen activated protein kinases (MAPK) ERK1/2 through a mechanism involving the scaffolding protein beta arrestin. Here we report that this activating pathway is independent of G alpha s, G alpha i, G alpha q or G betagamma and that the V2R-mediated activation of G alpha s inhibits ERK1/2 activity in a cAMP/PKA-dependent manner. In the HEK293 cells studied, the beta arrestin-promoted activation was found to dominate over the PKA-mediated inhibition of the pathway, leading to a strong vasopressin-stimulated ERK1/2 activation. Despite the strong MAPK activation and in contrast with other GPCR, V2R did not induce any significant increase in DNA synthesis, consistent with the notion that the stable interaction between V2R and beta arrestin prevents signal propagation to the nucleus. Beta arrestin was found to be essential for the ERK1/2 activation, indicating that the recruitment of the scaffolding protein is necessary and sufficient to initiate the signal in the absence of any other stimulatory cues. Based on the use of selective pharmacological inhibitors, dominant negative mutants and siRNA, we conclude that the beta arrestin-dependent activation of ERK1/2 by the V2R involves c-Src and a metalloproteinase-dependent trans-activation event. These findings demonstrate that beta arrestin is a genuine signalling initiator that can, on its own, engage a MAPK activation machinery upon stimulation of a GPCR by its natural ligand.

Novel, isotype-specific sensors for protein kinase A subunit interaction based on bioluminescence resonance energy transfer (BRET)

Homogeneous protein-protein interaction assays without the need of a separation step are an essential tool to unravel signal transduction events in live cells. We have established an isoform specific protein kinase A (PKA) subunit interaction assay based on bioluminescence resonance energy transfer (BRET). Tagging human Ralpha(I)-, Ralpha(II)-, as well as Calpha-subunits of PKA with Renilla luciferase (Rluc) as the bioluminescent donor or with green fluorescent protein (GFP2) as the energy acceptor, respectively, allows to directly probe PKA subunit interaction in living cells as well as in total cell extracts in order to study side by side PKA type I versus type II holoenzyme dynamics. Several novel, genetically encoded cAMP sensors and-for the first time PKA type I sensors-were generated. When C- and R-subunits are assembled to the respective holoenzyme complexes inside the cell, BRET occurs with a signal up to three times above the background. An increase of endogenous cAMP levels as well as treatment with the cAMP analog 8-Br-cAMP is reflected by a dose-dependent BRET signal reduction in cells expressing wild type proteins. In contrast to type II, the dissociation of the PKA type I holoenzyme complex was never complete in cells with maximally elevated cAMP levels. Both sensors dissociated completely upon treatment with 8-Br-cAMP after cell lysis, consistent with in vitro activation assays using holoenzymes assembled from purified PKA subunits. Interestingly, incubation of cells with the PKA antagonist Rp-8-Br-cAMPS leads to a significant BRET signal increase in cells expressing PKA type I or type II isoforms, indicating a stabilization of the holoenzyme complexes in vivo. Mutant RI subunits with reduced (hRIalpha-R210K) or abolished (hRIalpha-G200E/G324E) cAMP binding capability were studied to quantify maximal signal to noise ratios for the RI-BRET sensor. Utilizing BRET we demonstrate that PKA type II holoenzyme was rendered insensitive to beta-adrenergic receptor stimulation with isoproterenol when anchoring to the plasma membrane of COS-7 cells was disrupted by either using Ht31 peptide or by depletion of membrane cholesterol.

Muscarinic M2 receptors mediate transactivation of EGF receptor through Fyn kinase and without matrix metalloproteases

Transactivation of epidermal growth factor receptor (EGFR) by G protein-coupled receptors (GPCRs) has been attributed to the activation of matrix metalloproteases (MMPs) and the release of EGF family ligands such as HB-EGF. This mode of transactivation leads to signalling downstream of EGFR which is indistinguishable from that induced by the ligand. Here we provide evidence that in the COS-7 cell model EGFR transactivation via the muscarinic M2 receptor (M2R) is independent of MMPs and results in an incomplete EGFR signalling including ERK and Akt but not PLCgamma1. Using dominant-negative mutants of c-Src and Fyn and Src-deficient SYF cells as well as by co-immunoprecipitation studies, we can demonstrate that the M2R-mediated transactivation of EGFR specifically involves Fyn but not c-Src or Yes. This specific role of Fyn can be verified in SH-SY5Y human neuroblastoma cells with endogenously expressed M2 receptors.