Galpha and Gbeta gamma require distinct Src-dependent pathways to activate Rap1 and Ras

Data resource: vasopressin-regulated protein phosphorylation sites in the collecting duct

Vasopressin controls renal water excretion through actions to regulate aquaporin-2 (AQP2) trafficking, transcription, and degradation. These actions are in part dependent on vasopressin-induced phosphorylation changes in collecting duct cells. Although most efforts have focused on the phosphorylation of AQP2 itself, phosphoproteomic studies have identified many vasopressin-regulated phosphorylation sites in proteins other than AQP2. The goal of this bioinformatics-based review is to create a compendium of vasopressin-regulated phosphorylation sites with a focus on those that are seen in both native rat inner medullary collecting ducts and cultured collecting duct cells from the mouse (mpkCCD), arguing that these sites are the best candidates for roles in AQP2 regulation. This analysis identified 51 vasopressin-regulated phosphorylation sites in 45 proteins. We provide resource web pages at https://esbl.nhlbi.nih.gov/Databases/AVP-Phos/ and https://esbl.nhlbi.nih.gov/AVP-Network/, listing the phosphorylation sites and describing annotated functions of each of the vasopressin-targeted phosphoproteins. Among these sites are 23 consensus protein kinase A (PKA) sites that are increased in response to vasopressin, consistent with a central role for PKA in vasopressin signaling. The remaining sites are predicted to be phosphorylated by other kinases, most notably ERK1/2, which accounts for decreased phosphorylation at sites with a X-p(S/T)-P-X motif. Additional protein kinases that undergo vasopressin-induced changes in phosphorylation are Camkk2, Cdk18, Erbb3, Mink1, and Src, which also may be activated directly or indirectly by PKA. The regulated phosphoproteins are mapped to processes that hypothetically can account for vasopressin-mediated control of AQP2 trafficking, cytoskeletal alterations, and Aqp2 gene expression, providing grist for future studies.NEW & NOTEWORTHY Vasopressin regulates renal water excretion through control of the aquaporin-2 water channel in collecting duct cells. Studies of vasopressin-induced protein phosphorylation have focused mainly on the phosphorylation of aquaporin-2. This study describes 44 phosphoproteins other than aquaporin-2 that undergo vasopressin-mediated phosphorylation changes and summarizes potential physiological roles of each.

Mechanistic insight into how gonadotropin hormone receptor complexes direct signaling†

Gonadotropin hormones and their receptors play a central role in the control of male and female reproduction. In recent years, there has been growing evidence surrounding the complexity of gonadotropin hormone/receptor signaling, with it increasingly apparent that the Gαs/cAMP/PKA pathway is not the sole signaling pathway that confers their biological actions. Here we review recent literature on the different receptor-receptor, receptor-scaffold, and receptor-signaling molecule complexes formed and how these modulate and direct gonadotropin hormone-dependent intracellular signal activation. We will touch upon the more controversial issue of extragonadal expression of FSHR and the differential signal pathways activated in these tissues, and lastly, highlight the open questions surrounding the role these gonadotropin hormone receptor complexes and how this will shape future research directions.

Temporal pattern and synergy influence activity of ERK signaling pathways during L-LTP induction

Long-lasting long-term potentiation (L-LTP) is a cellular mechanism of learning and memory storage. Studies have demonstrated a requirement for extracellular signal-regulated kinase (ERK) activation in L-LTP produced by a diversity of temporal stimulation patterns. Multiple signaling pathways converge to activate ERK, with different pathways being required for different stimulation patterns. To answer whether and how different temporal patterns select different signaling pathways for ERK activation, we developed a computational model of five signaling pathways (including two novel pathways) leading to ERK activation during L-LTP induction. We show that calcium and cAMP work synergistically to activate ERK and that stimuli given with large intertrial intervals activate more ERK than shorter intervals. Furthermore, these pathways contribute to different dynamics of ERK activation. These results suggest that signaling pathways with different temporal sensitivities facilitate ERK activation to diversity of temporal patterns.

Feeding of yeast cell wall extracts during a necrotic enteritis challenge enhances cell growth, survival and immune signaling in the jejunum of broiler chickens

Necrotic enteritis (NE) is one of the most common and costly diseases in the modern broiler industry, having an estimated economic impact of $6 billion dollars annually. Increasing incidents of NE have resulted from restrictions on the use of antibiotic feed additives throughout the broiler industry. As such, finding effective antibiotic alternatives has become a priority. In this study, an experimental model of NE was used, comprising a commercial infectious bursal disease virus vaccine and Clostridium perfringens (C. perfringens) inoculation. Yeast cells wall (YCW) components, β-glucan (BG), and mannoproteins (MPTs) were evaluated for their effects on disease development. Chicken-specific immunometabolic kinome peptide arrays were used to measure differential phosphorylation between control (uninfected), challenged (infected), and challenged and treated birds in duodenal, jejunal, and ileal tissues. Treatment groups included crude YCW preparation, BG, MPT, or BG+MPT as feed additives. Data analysis revealed kinome profiles cluster predominantly by tissue, with duodenum showing the greatest relative signaling and jejunum showing the greatest response to treatment. BG, MPT, and BG+MPT cluster together, separate from controls and challenge birds in each tissue. Changes in signaling resulting from the treatments were observed in cell growth and survival responses as well as immune responses. None of the treatments of disease challenge returned the profiles to control-like. This is attributable to immune modulation and metabolic effects of the treatments generating distinct profiles from control. Importantly, all the treatments are distinct from the challenge group despite being challenged themselves. Only BG+MPT treatment had a significant effect on bird weight gain compared with the NE challenge group, and this treatment had the greatest impact on gut tissue signaling in all segments. The signaling changes elicited by BG+MPT during an NE challenge were increased cell growth and survival signaling, reducing cell death, apoptosis and innate inflammatory responses, and generating compensatory signaling to reduce disease severity.

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.

Protein Kinase A-independent Ras Protein Activation Cooperates with Rap1 Protein to Mediate Activation of the Extracellular Signal-regulated Kinases (ERK) by cAMP

Cyclic adenosine monophosphate (cAMP) is an important mediator of hormonal stimulation of cell growth and differentiation through its activation of the extracellular signal-regulated kinase (ERK) cascade. Two small G proteins, Ras and Rap1, have been proposed to mediate this activation, with either Ras or Rap1 acting in distinct cell types. Using Hek293 cells, we show that both Ras and Rap1 are required for cAMP signaling to ERKs. The roles of Ras and Rap1 were distinguished by their mechanism of activation, dependence on the cAMP-dependent protein kinase (PKA), and the magnitude and kinetics of their effects on ERKs. Ras was required for the early portion of ERK activation by cAMP and was activated independently of PKA. Ras activation required the Ras/Rap guanine nucleotide exchange factor (GEF) PDZ-GEF1. Importantly, this action of PDZ-GEF1 was disrupted by mutation within its putative cyclic nucleotide-binding domain within PDZ-GEF1. Compared with Ras, Rap1 activation of ERKs was of longer duration. Rap1 activation was dependent on PKA and required Src family kinases and the Rap1 exchanger C3G. This is the first report of a mechanism for the cooperative actions of Ras and Rap1 in cAMP activation of ERKs. One physiological role for the sustained activation of ERKs is the transcription and stabilization of a range of transcription factors, including c-FOS. We show that the induction of c-FOS by cAMP required both the early and sustained phases of ERK activation, requiring Ras and Rap1, as well as for each of the Raf isoforms, B-Raf and C-Raf.

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.

The Gβγ-Src signaling pathway regulates TNF-induced necroptosis via control of necrosome translocation

Formation of multi-component signaling complex necrosomes is essential for tumor necrosis factor α (TNF)-induced programmed necrosis (also called necroptosis). However, the mechanisms of necroptosis are still largely unknown. We isolated a TNF-resistant L929 mutant cell line generated by retrovirus insertion and identified that disruption of the guanine nucleotide-binding protein γ 10 (Gγ10) gene is responsible for this phenotype. We further show that Gγ10 is involved in TNF-induced necroptosis and Gβ2 is the partner of Gγ10. Src is the downstream effector of Gβ2γ10 in TNF-induced necroptosis because TNF-induced Src activation was impaired upon Gγ10 knockdown. Gγ10 does not affect TNF-induced activation of NF-κB and MAPKs and the formation of necrosomes, but is required for trafficking of necrosomes to their potential functioning site, an unidentified subcellular organelle that can be fractionated into heterotypic membrane fractions. The TNF-induced Gβγ-Src signaling pathway is independent of RIP1/RIP3 kinase activity and necrosome formation, but is required for the necrosome to function.

Activation state-dependent interaction between Gαq subunits and the Fhit tumor suppressor

Background

The FHIT tumor suppressor gene is arguably the most commonly altered gene in cancer since it is inactivated in about 60% of human tumors. The Fhit protein is a member of the ubiquitous histidine triad proteins which hydrolyze dinucleoside polyphosphates such as Ap₃A. Despite the fact that Fhit functions as a tumor suppressor, the pathway through which Fhit inhibits growth of cancer cells remains largely unknown. Phosphorylation by Src tyrosine kinases provides a linkage between Fhit and growth factor signaling. Since many G proteins can regulate cell proliferation through multiple signaling components including Src, we explored the relationship between Gα subunits and Fhit.

Results

Several members of the Gα_q subfamily (Gα₁₆, Gα₁₄, and Gα_q) were found to co-immunoprecipitate with Fhit in their GTP-bound active state in HEK293 cells. The binding of activated Gα_q members to Fhit appeared to be direct and was detectable in native DLD-1 colon carcinoma cells. The use of Gα_16/z chimeras further enabled the mapping of the Fhit-interacting domain to the α2-β4 region of Gα₁₆. However, Gα_q/Fhit did not affect either Ap₃A binding and hydrolysis by Fhit, or the ability of Gα_q/16 to regulate downstream effectors including phospholipase Cβ, Ras, ERK, STAT3, and IKK. Functional mutants of Fhit including the H96D, Y114F, L25W and L25W/I10W showed comparable abilities to associate with Gα_q. Despite the lack of functional regulation of G_q signaling by Fhit, stimulation of G_q-coupled receptors in HEK293 and H1299 cells stably overexpressing Fhit led to reduced cell proliferation, as opposed to an enhanced cell proliferation typically seen with parental cells.

Conclusions

Activated Gα_q members interact with Fhit through their α2-β4 region which may result in enhancement of the growth inhibitory effect of Fhit, thus providing a possible avenue for G protein-coupled receptors to modulate tumor suppression.

P2Y2 receptor inhibits EGF-induced MAPK pathway to stabilise keratinocyte hemidesmosomes

α6β4 integrin is the main component of hemidesmosomes (HD) that stably anchor the epithelium to the underlying basement membrane. Epithelial cell migration requires HD remodelling, which can be promoted by epidermal growth factor (EGF). We previously showed that extracellular nucleotides inhibit growth factor-induced keratinocyte migration. Here, we investigate the effect of extracellular nucleotides on α6β4 integrin localisation in HD during EGF-induced cell migration. Using a combination of pharmacological inhibition and gene silencing approaches, we found that UTP activates the P2Y2 purinergic receptor and Gαq protein to inhibit EGF/ERK1/2-induced cell migration in keratinocytes. Using a keratinocyte cell line expressing an inducible form of the Raf kinase, we show that UTP inhibits the EGF-induced ERK1/2 pathway activation downstream of Raf. Moreover, we established that ERK1/2 activation by EGF leads to the mobilisation of α6β4 integrin from HD. Importantly, activation of P2Y2R and Gαq by UTP promotes HD formation and protects these structures from EGF-triggered dissolution as revealed by confocal analysis of the distribution of α6β4 integrin, plectin, BPAG1, BPAG2 and CD151 in keratinocytes. Finally, we demonstrated that the activation of p90RSK, downstream of ERK1/2, is sufficient to promote EGF-mediated HD dismantling and that UTP does not stabilise HD in cells expressing an activated form of p90RSK. Our data underline an unexpected role of P2Y2R and Gαq in the inhibition of the ERK1/2 signalling pathway and in the modulation of hemidesmosome dynamics and keratinocyte migration.

Cigarette smoke extract stimulates epithelial-mesenchymal transition through Src activation

Epithelial-mesenchymal transition (EMT) is implicated in the pathogenesis of lung fibrosis and cancer metastasis, two conditions associated with cigarette smoke (CS). CS has been reported to promote the EMT process. CS is the major cause of lung cancer and nearly half of lung cancer patients are active smokers. Nonetheless, the mechanism whereby CS induces EMT remains largely unknown. In this study we investigated the induction of EMT by CS and explored the underlying mechanisms in the human non-small-cell lung carcinoma (H358) cell line. We demonstrate that exposure to an extract of CS (CSE) decreases E-cadherin and increases N-cadherin and vimentin, markers of EMT, in H358 cells cultured in RPMI 1640 medium with 1% fetal bovine serum. Pretreatment with N-acetylcysteine (NAC), a potent antioxidant and precursor of glutathione, abrogated changes in these EMT markers. In addition, CSE activated Src kinase (shown as increased phosphorylation of Src at Tyr418), and the Src kinase inhibitor PP2 inhibited CS-stimulated EMT changes, suggesting that Src is critical in CSE-stimulated EMT induction. Furthermore, NAC treatment abrogated CSE-stimulated Src activation. However, co-incubation with catalase had no effect on CSE-mediated Src activation. Finally, acrolein, an unsaturated aldehyde present in CSE, caused Src activation. Taken together, these data suggest that CSE initiates EMT through Src, which is activated by CS through redox modification.

Very-low-density lipoprotein mediates transcriptional regulation of aldosterone synthase in human adrenocortical cells through multiple signaling pathways

Diabetic dyslipidemia is characterized by increased circulatory very-low-density lipoprotein (VLDL) levels. Aldosterone, apart from its role in fluid and electrolyte homeostasis, has also been implicated in insulin resistance and myocardial fibrosis. The impact of VLDL as a potential risk factor for aldosterone-mediated cardiovascular injury in diabetes mellitus, however, remains to be investigated. We have therefore studied native and modified VLDL-mediated steroidogenesis and its underlying molecular mechanisms in human adrenocortical carcinoma cells, NCI H295R. Native VLDL (natVLDL), isolated from healthy volunteers, was subjected to in vitro modification with glucose (200 mmol/l) or sodium hypochlorite (1.5 mmol/l) for preparation of glycoxidized and oxidized VLDL, respectively. VLDL treatment induced steroidogenesis in both a concentration- and time-dependent manner. Native and glycoxidized VLDL (50 μg/ml) were almost two-fold more potent in adrenocortical aldosterone release than angiotensin II (100 nmol/l). These forms of VLDL significantly augmented transcriptional regulation of aldosterone synthase (Cyp11B2), partially through scavenger receptor class B type I, as evident from the effect of BLT-1. In contrast to glycoxidized VLDL, oxidized VLDL significantly attenuated the stimulatory effect of natVLDL on adrenocortical hormone synthesis. Moreover, treatment with specific pharmacological inhibitors (H89, U0126, AG490) provided supporting evidence that VLDL, irrespective of modification, presumably recruited PKA, ERK1/2 and Jak-2 for steroid hormone release through modulation of Cyp11B2 mRNA level. In conclusion, this study demonstrates a novel insight into intracellular mechanism of VLDL-mediated aldosterone synthesis through transcriptional regulation of steroidogenic acute regulatory protein (StAR) and Cyp11B2 expression in human adrenocortical carcinoma cell line.

Ligand-directed signalling at beta-adrenoceptors

beta-Adrenoceptors (ARs) classically mediate responses to the endogenous ligands adrenaline and noradrenaline by coupling to Gsalpha and stimulating cAMP production; however, drugs designed as beta-AR agonists or antagonists can activate alternative cell signalling pathways, with the potential to influence clinical efficacy. Furthermore, drugs acting at beta-ARs have differential capacity for pathway activation, described as stimulus trafficking, biased agonism, functional selectivity or ligand-directed signalling. These terms refer to responses where drug A has higher efficacy than drug B for one signalling pathway, but a lower efficacy than drug B for a second pathway. The accepted explanation for such responses is that drugs A and B have the capacity to induce or stabilize distinct active conformations of the receptor that in turn display altered coupling efficiency to different effectors. This is consistent with biophysical studies showing that drugs can indeed promote distinct conformational states. Agonists acting at beta-ARs display ligand-directed signalling, but many drugs acting as cAMP antagonists are also able to activate signalling pathways central to cell survival and proliferation or cell death. The observed complexity of drug activity at beta-ARs, prototypical G protein-coupled receptors, necessitates rethinking of the approaches used for screening and characterization of novel therapeutic agents. Most studies of ligand-directed signalling employ recombinant cell systems with high receptor abundance. While such systems are valid for examining upstream signalling events, such as receptor conformational changes and G protein activation, they are less robust when comparing downstream signalling outputs as these are likely to be affected by complex pathway interactions.

Betagamma subunits of G(i/o) suppress EGF-induced ERK5 phosphorylation, whereas ERK1/2 phosphorylation is enhanced

Extracellular signal-regulated kinases (ERKs) play important physiological roles in proliferation, differentiation and gene expression. ERK5 is twice the size of ERK1/2, the amino-terminal half contains the kinase domain that shares the homology with ERK1/2 and TEY activation motif, whereas the carboxy-terminal half is unique. In this study, we examined the cross-talk mechanism between G-protein-coupled receptors (GPCRs) and receptor tyrosine kinases, focusing on ERK1/2 and 5. The pretreatment of rat pheochromocytoma cells (PC12) with pertussis toxin (PTX) specifically enhanced epidermal growth factor (EGF)-induced ERK5 phosphorylation. In addition, lysophosphatidic acid (LPA) attenuated the EGF-induced ERK5 phosphorylation in LPA(1) receptor- and G(i/o)-dependent manners. On the other hand, LPA alone activated ERK1/2 via Gbetagamma subunits and Ras and potentiated EGF-induced ERK1/2 phosphorylation at late time points. These results suggest G(i/o) negatively regulates ERK5, while it positively regulates ERK1/2. LPA did not affect cAMP levels after EGF treatment, and the reagents promoting cAMP production such as forskolin and cholera toxin also attenuated the EGF-induced ERK5 phosphorylation, indicating that the inhibitory effect of LPA on ERK5 inhibition via G(i/o) is not due to inhibition of adenylyl cyclase by Galpha(i/o). However, the inhibitory effect of LPA on ERK5 was abolished in PC12 cells stably overexpressing C-terminus of GPCR kinase2 (GRK2), and overexpression of Gbeta(1) and gamma(2) subunits also suppressed ERK5 phosphorylation by EGF. In response to LPA, Gbetagamma subunits interacted with EGF receptor in a time-dependent manner. These results strongly suggest that LPA negatively regulates the EGF-induced ERK5 phosphorylation through Gbetagamma subunits.

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.

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.

Cyclic AMP-dependent, protein kinase A-independent activation of extracellular signal-regulated kinase 1/2 following adenosine receptor stimulation in human umbilical vein endothelial cells: role of exchange protein activated by cAMP 1 (Epac1)

A critical process in angiogenesis is endothelial cell proliferation, which requires activation of extracellular signal-regulated kinase (ERK)1/2. This study analyzed the pathway responsible for adenosine-induced ERK1/2 phosphorylation in human umbilical vein endothelial cells (HUVEC). Characterization with adenosine receptor (AR) agonists and antagonists and the AR mRNA profile demonstrated that stimulation of the A(2B)AR can mediate ERK1/2 phosphorylation in HUVEC. The lack of sensitivity of A(2B)AR-mediated ERK1/2 phosphorylation to 3-[1-[3-(dimethylaminopropyl]-1H-indol-3-yl]-4-(1H-indol-3-yl)-1H-pyrrole-2,5-dione monohydrochloride (GF109203X) and 3-[1-[3-(amidinothio)propyl]-1H-in-dol-3-yl]-3-(1-methyl-1H-indol-3-yl) maleimide (bisindolylmaleimide IX) (Ro31-8220) indicated that protein kinase C stimulation is not required. The response did not involve transactivation of receptors for epidermal growth factor or vascular endothelial growth factor (VEGF). The A(2B)AR-mediated response required functional G(alphas) and was mimicked by forskolin and 8-bromoadenosine 3',5'-cyclic monophosphate. However, ERK1/2 phosphorylation induced by A(2B)AR stimulation and forskolin was insensitive to protein kinase A inhibitors. It was hypothesized that the A(2B)AR-mediated ERK1/2 activation may involve exchange protein activated by cAMP (Epac), a cAMP-activated guanine nucleotide exchange factor for Rap GTPases. Reverse Transcription-polymerase chain reaction analysis detected Epac1 but not Epac2 in HUVEC. 8-(p-Chlorophenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate (8CPT-2Me-cAMP), an Epac activator, stimulated ERK1/2 phosphorylation. Overexpression of Epac1 enhanced A(2B)AR-mediated and forskolin-induced ERK1/2 phosphorylation, whereas response to VEGF was unaffected. Inhibition of Epac1 expression with small interfering RNA substantially reduced A(2B)AR-mediated and forskolin-induced ERK1/2 phosphorylation and abolished that by 8CPT-2Me-cAMP. A(2B)AR stimulation and forskolin activated Rap1. Expression of a dominant-negative Ras protein did not affect either forskolin-induced or A(2B)AR-mediated ERK1/2 phosphorylation. In summary, Epac1 activation in HUVEC results in ERK1/2 activation, and this protein, at least in part, mediates response to the physiologically relevant event of A(2B)AR stimulation.

The requirement of Ras and Rap1 for the activation of ERKs by cAMP, PACAP, and KCl in cerebellar granule cells

In cerebellar granule cells, the mitogen-activated protein kinase (MAPK) or extracellular signal-regulated kinase (ERK) cascade mediates multiple functions, including proliferation, differentiation, and survival. In these cells, ERKs are activated by diverse stimuli, including cyclic adenosine monophosphate (cAMP), pituitary adenylate cyclase activating protein (PACAP), depolarization induced by elevated extracellular potassium (KCl), and the neurotrophin brain-derived neurotrophic factor. Extensive studies in neuronal cell lines have implicated the small G proteins Ras and Rap1 in the activation of ERKs by cAMP, PACAP, and KCl. However, the requirement of Ras and Rap1 in these pathways in cerebellar granule cells has not been addressed. In this study, we utilize multiple biochemical assays to determine the mechanisms of action and requirement of Ras and Rap1 in cultured cerebellar granule cells. We show that both Ras and Rap1 can be activated by cAMP or PACAP via protein kinase (PKA)-dependent mechanisms. KCl activation of Ras also required PKA. Using both adenoviral and transgenic approaches, we show that Ras plays a major role in ERK activation by cAMP, PACAP, and KCl, while Rap1 also mediates activation of a selective membrane-associated pool of ERKs. Furthermore, Rap1, but not Ras, activation by PKA appears to require the action of Src family kinases.

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.

Flagellin and lipopolysaccharide stimulate the MEK-ERK signaling pathway in chicken heterophils through differential activation of the small GTPases, Ras and Rap1

The TLR agonists, flagellin (FLG) and lipopolysaccharide (LPS) stimulate functional activation and cytokine gene expression via the extracellular signal regulated kinase 1/2 (ERK1/2) MAP kinase cascade. However, the upstream mechanisms of these signaling events remain unknown. In mammals, the small GTP-binding protein Ras mediates ERK1/2 activation through activation of downstream effectors Raf-1-MEK1/2-ERK1/2 in response to a variety of stimuli. It is not clear whether this classic Ras cascade plays a role in TLR signaling in avian cells. In the present study, we investigated the role of Ras in FLG- and LPS-mediated signaling in ERK activation in chicken heterophils. Treatment of heterophils with LPS caused a rapid (within 5min) activation of Ras-GTP. The role of Ras activation in LPS-induced stimulation of ERK1/2 was corroborated when the specific Ras inhibitor, FTI-277, inhibited ERK1/2 activation. The classic Ras-mediated pathway of ERK1/2 activation by LPS was confirmed when the specific Raf-1 inhibitor, GW 5074, and the MEK1/2 inhibitor, U0126, both reduced ERK activation by 51-60%. Of more interest was that treatment of the heterophils with FLG did not activate Ras-GTP. Likewise, neither FTI-277 nor GW 5074 had any effect on FLG-mediated activation of ERK1/2. Another small GTPase, Rap1, has been shown to play a role in mammalian neutrophil function. Using a Rap1-GTP pull-down assay, we found that FLG stimulation, but not LPS, of avian heterophils induced a rapid and transient Rap1 activation. Rap1 has been shown to activate the ERK1/2 via a different Raf family member B-Raf whose downstream effector is MEK1/2. We show here that FLG stimulation of heterophils induces the phosphorylation of Rap1. The FLG induction of the Rap1-->B-Raf-->MEK1/2-->ERK1/2 cascade was confirmed by the reduction of ERK1/2 activation by the specific Rap1 inhibitor (GGTI-298) and U0126. The results demonstrate that for the first time that the small GTPase Ras family is involved in TLR signaling of avian heterophils with the TLR agonists LPS (Ras) and FLG (Rap1) inducing differential signaling cascades to activate the downstream ERK MAP kinase.

Identification of the proteins present in the bull sperm cytosolic fraction enriched in tyrosine kinase activity: a proteomic approach

Numerous sperm proteins have been identified on the basis of their increase in tyrosine phosphorylation during capacitation. However, the tyrosine kinases present in spermatozoa that are responsible for this phosphorylation remain unknown. As spermatozoa are devoid of transcriptional and translational activities, molecular biology approaches might not reflect the transcriptional pattern in mature spermatozoa. Working directly with the proteins present in ejaculated spermatozoa is the most reliable approach to identify the tyrosine kinases potentially involved in the capacitation-associated increase in protein tyrosine phosphorylation. A combination of tyrosine kinase assays and proteomic identification tools were used as an approach to identify sperm protein tyrosine kinases. Fractionation by nitrogen cavitation showed that the majority of tyrosine kinase activity is present in the cytosolic fraction of bovine spermatozoa. By the use of Poly-Glu:Tyr(4:1)-agarose affinity chromatography, we isolated a fraction enriched in tyrosine kinase activity. Proteomics approaches permitted the identification of tyrosine kinases from three families: Src (Lyn), Csk, and Tec (Bmx, Btk). We also identified proteins implicated in different cellular events associated with sperm capacitation and acrosome reaction. These results confirm the implication of tyrosine phosphorylation in some aspects of capacitation/acrosome reaction and reveal the identity of new players potentially involved in these processes.

Ligand-independent and EGF receptor-supported transactivation: Lessons from β2-adrenergic receptor signalling

Transactivation of epidermal growth factor receptor (EGFR) by G protein-coupled receptors (GPCRs) is currently understood to be mediated by matrix metalloproteases (MMPs) and the release of EGF-like ligands. This ligand-mediated process also suggests that downstream of EGFR the signalling in response to GPCR ligands or EGF appears to be indistinguishable. Here we provide evidence that transactivation of EGFR by the beta2-adrenergic receptor (beta2-AR) is independent of MMPs and results in an incomplete downstream signalling involving extracellular signal-activated kinase (ERK) but not PLCgamma1 and Akt. In contrast, beta2-AR has the ability to activate PLCgamma1 when the EGFR is primed either by co-stimulation with EGF or by increased basal activity due to over-expression. In that way but not via the beta2-AR-mediated transactivation the EGFR docking sites pY992 and pY1173 may be generated which are critical for PLCgamma1. This EGFR-supported transactivation is strongly dependent on EGFR tyrosine kinase, c-Src, and the c-Src-specific EGFR tyrosine residue 845 and represents a novel paradigm of EGFR transactivation.

P2Y12 receptor signalling towards PKB proceeds through IGF-I receptor cross-talk and requires activation of Src, Pyk2 and Rap1

Previously it was shown that stimulation of the P2Y12 receptor activates PKB signalling in C6 glioma cells [K. Van Kolen and H. Slegers, J. Neurochem. 89, 442.]. In the present study, the mechanisms involved in this response were further elucidated. In cells transfected with the Gbetagamma-scavenger beta-ARK1/GRK2 or Rap1GAPII, stimulation with 2MeSADP failed to enhance PKB phosphorylation demonstrating that the signalling proceeds through Gbetagamma-subunits and Rap1. Moreover, Rap1-GTP pull-down assays revealed that P2Y12 receptor stimulation induced a rapid activation of Rap1. Treatment of cells with the Ca2+ chelator BAPTA-AM and inhibition of Src and PLD2 with PP2 or 1-butanol, respectively, abrogated P2Y12 receptor-mediated activation of Rap1 and PKB. In addition inhibition of PKCzeta decreased basal and 2MeSADP-stimulated phosphorylation of PKB indicating a role for this PKC isoform in PKB signalling. Although the increased PKB phosphorylation was abolished in the presence of the IGF-I receptor tyrosine kinase inhibitor AG 1024, 2MeSADP did not significantly increase receptor phosphorylation. Nevertheless, phosphorylation of a 120 kDa IGF-I receptor-associated protein was observed. The latter protein was identified by MALDI-TOF/TOF-MS as the proline-rich tyrosine kinase 2 (Pyk2) that co-operates with Src in a PLD2-dependent manner. Consistent with the signalling towards Rap1 and PKB, activation of Pyk2 was abrogated by Ca2+ chelation, inhibition of PLD2 and IGF-I receptor tyrosine kinase activity. In conclusion, the data reveal a novel type of cross-talk between P2Y12 and IGF-I receptors that proceeds through Gbetagamma-, Ca2+-and PLD2-dependent activation of the Pyk2/Src pathway resulting in GTP-loading of Rap1 required for an increased PKB phosphorylation.

Integration of P2Y receptor-activated signal transduction pathways in G protein-dependent signalling networks

The role of nucleotides in intracellular energy provision and nucleic acid synthesis has been known for a long time. In the past decade, evidence has been presented that, in addition to these functions, nucleotides are also autocrine and paracrine messenger molecules that initiate and regulate a large number of biological processes. The actions of extracellular nucleotides are mediated by ionotropic P2X and metabotropic P2Y receptors, while hydrolysis by ecto-enzymes modulates the initial signal. An increasing number of studies have been performed to obtain information on the signal transduction pathways activated by nucleotide receptors. The development of specific and stable purinergic receptor agonists and antagonists with therapeutical potential largely contributed to the identification of receptors responsible for nucleotide-activated pathways. This article reviews the signal transduction pathways activated by P2Y receptors, the involved second messenger systems, GTPases and protein kinases, as well as recent findings concerning P2Y receptor signalling in C6 glioma cells. Besides vertical signal transduction, lateral cross-talks with pathways activated by other G protein-coupled receptors and growth factor receptors are discussed.

Protein 4.1B/differentially expressed in adenocarcinoma of the lung-1 functions as a growth suppressor in meningioma cells by activating Rac1-dependent c-Jun-NH(2)-kinase signaling

Meningiomas are the second most common brain tumor in adults, yet comparatively little is presently known about the dysregulated growth control pathways involved in their formation and progression. One of the most frequently observed genetic changes in benign meningioma involves loss of protein 4.1B expression. Previous studies from our laboratory have shown that protein 4.1B growth suppression in meningioma is associated with the activation of the c-Jun-NH(2)-kinase (JNK) pathway and requires localization of a small unique region (U2 domain) of protein 4.1B to the plasma membrane. To define the relationship between protein 4.1B expression and JNK activation, as well as to determine the mechanism of JNK activation by protein 4.1B, we used a combination of genetic and pharmacologic approaches. In this report, we show that protein 4.1B/differentially expressed in adenocarcinoma of the lung-1 (DAL-1) expression in meningioma cells in vitro results in JNK activation, which requires the sequential activation of Src, Rac1, and JNK. In addition, inhibition of Rac1 or JNK activation abrogates protein 4.1B/DAL-1 growth suppression and cyclin A regulation. Last, protein 4.1B/DAL-1 regulation of this critical growth control pathway in meningioma cells requires the presence of the U2 domain. Collectively, these observations provide the first mechanistic insights into the function of protein 4.1B as a growth regulator in meningioma cells.

Atypical PKCzeta is involved in RhoA-dependent mitogenic signaling by the P2Y12 receptor in C6 cells

When nucleotide hydrolysis is prevented, agonists of the P2Y(12) receptor enhance the proliferation of C6 glioma cells by RhoA-dependent, protein kinase C (PKC)-dependent activation of the extracellular signal-regulated kinase (ERK) pathway [Claes P, Grobben B, Van Kolen K, Roymans D & Slegers H (2001) Br J Pharmacol134, 402-408; Grobben B, Claes P, Van Kolen K, Roymans D, Fransen P, Sys SU & Slegers H (2001) J Neurochem78, 1325-1338]. In this study, we show that ERK1/2 phosphorylation was not affected by transfection of the cells with the Gbetagamma-subunit-scavenging adrenergic receptor kinase peptide [betaARK1-(495-689)] or with Rap1GAPII, indicating that P2Y(12) receptor stimulation enhances ERK1/2 phosphorylation by G(i)alpha subunit-mediated signaling independently of Rap1 activation. Inhibition of the RhoA downstream effector Rho-associated coiled-coil-containing kinase (ROCK) with Y-27632 did not affect the P2Y(12) receptor-induced increase in ERK1/2 phosphorylation but abrogated the mitogenic response. Involvement of growth factor receptor transactivation in the signaling towards ERK phosphorylation could be ruled out by the lack of an effect of PP2, AG1024, AG1296 or SU1498, inhibitors of Src, insulin-like growth factor receptor, platelet-derived growth factor receptor and vascular endothelial growth factor receptor kinase activity, respectively. Experiments with bisindolylmaleimide I and IX indicated the requirement of PKC activity. Classical and novel PKC isoforms could be excluded by treatment of the cells with Gö6976 and calphostin C, whereas addition of a myristoylated PKCzeta pseudosubstrate inhibitor completely abolished P2Y(12) receptor-induced ERK1/2 activation. Moreover, coimmunoprecipitation experiments revealed PKCzeta/Raf1 and PKCzeta/ERK association, indicating the involvement of PKCzeta. From the data presented, we can conclude that the P2Y(12) receptor enhances cell proliferation by a G(i)alpha-dependent, RhoA-dependent PKCzeta/Raf1/MEK/ERK pathway that requires activation of ROCK, which is not involved in ERK1/2 signaling.

Role of cyclic-AMP responsive element binding (CREB) proteins in cell proliferation in a rat model of hepatocellular carcinoma

The role of cyclic adenosine monophosphate (cAMP) is poorly understood in the regulation of normal and abnormal hepatic cell growth. In this study, we examined the regulation of intracellular cAMP levels and its effect on nuclear cAMP responsive elements (CREs) in a rat model of hepatocellular carcinoma (HCC). Tumorigenic liver cells were cultured from an in vivo model of HCC and the role of cAMP in cell mitogenesis determined. These data demonstrated agents that elevate intracellular cAMP ([cAMP]i) levels caused significant dose-dependent inhibition of serum-stimulated mitogenesis in HCC cells. Cells were next analyzed for transcription factor expression and activity following increased [cAMP]i. These data demonstrated time- and dose-dependent increases in CRE binding protein (pCREB) activity, a maximal response occurring after 10-20 min before returning to basal levels within 60 min. In contrast, increased [cAMP]i levels led to sustained inducible cAMP early repressor (ICER) II/IIgamma mRNA and protein induction. To understand these data in relation to the in vivo setting, HCC tumors were analyzed and compared to pair-matched normal liver (NL) samples. These studies demonstrated significantly elevated Gsalpha-protein expression in HCC versus NL in the absence of significant changes in basal cAMP levels. Analysis of total and active CREB demonstrated significantly increased total CREB/pCREB in HCC versus NL. Further analysis of CRE expression demonstrated significantly increased expression of ICER mRNA and protein in HCC versus sham operated (Sh). These data demonstrate cAMP, while capable of stimulating promitogenic CREB activation inhibits cell mitogenesis in HCC possibly via ICER induction.

Transmembrane signaling in the brain by serotonin, a key regulator of physiology and emotion

Serotonin (5-HT) is an ancient chemical that plays a crucial functional role in almost every living organism. It regulates platelet aggregation, activation of immune cells, and contraction of stomach and intestinal muscles. In addition, serotonin acts as a neurotransmitter in the brain and the peripheral nervous system. These activities are initiated by the binding of serotonin to 15 or more receptors that are pharmacologically classified into seven groups, 5-HT1 through 5-HT7. Each group is further divided into subgroups of receptors that are homologous but are encoded by discrete genes. With the exception of the 5-HT3 receptor--a cation channel--all of the others are G protein-coupled receptors that potentially activate or inhibit a large number of biochemical cascades. This review will endeavor to compare and contrast such signaling pathways with special attention to their tissue-specific occurrence, their possible role in immediate effects on covalent modification of other proteins, and relatively slower effects on gene expression, physiology and behavior.

Beta-arrestin2 enhances beta2-adrenergic receptor-mediated nuclear translocation of ERK

Beta-arrestin mediates desensitization and internalization of beta-adrenergic receptors (betaARs), but also acts as a scaffold protein in extracellular signal-regulated kinase (ERK) cascade. Thus, we have examined the role of beta-arrestin2 in the betaAR-mediated ERK signaling pathways. Isoproterenol stimulation equally activated cytoplasmic and nuclear ERK in COS-7 cells expressing beta1AR or beta2AR. However, the activity of nuclear ERK was enhanced by co-expression of beta-arrestin2 in beta2AR-but not beta1AR-expressing cells. Pertussis toxin treatment and blockade of Gbetagamma action inhibited beta-arrestin2-enhanced nuclear activation of ERK, suggesting that beta-arrestin2 promotes nuclear ERK localization in a Gbetagamma dependent mechanism upon receptor stimulation. beta2AR containing the carboxyl terminal region of beta1AR lost the beta-arrestin2-promoted nuclear translocation. As the carboxyl terminal region is important for beta-arrestin binding, these results demonstrate that recruitment of beta-arrestin2 to carboxyl terminal region of beta2AR is important for ERK localization to the nucleus.

Carbohydrates that mimic schistosome surface coat components affect ERK and PKC signalling in Lymnaea stagnalis haemocytes

Molluscs are intermediate hosts for helminth parasites such as Schistosoma spp. that possess an immunogenic surface coat of high carbohydrate content, with fucose as the predominant saccharide. More than a decade ago, it was postulated that such components could block receptors on snail haemocytes thus preventing recognition of intra-molluscan schistosome stages. Although more recent studies have shown that carbohydrates can suppress processes such as phagocytosis by haemocytes, interference of the haemocyte cell signalling pathways that regulate immunity by saccharides has not yet been investigated. We have recently reported the presence of extracellular-signal regulated kinase and protein kinase C in Lymnaea stagnalis haemocytes. Here we show that extracellular-signal regulated kinase and protein kinase C activities are down-regulated when haemocytes are exposed to albumin-linked fucose and galactose in the absence of haemolymph. Moreover, we demonstrate that phagocytosis is reduced under these conditions. Interestingly, in the presence of haemolymph, only protein kinase C activity is down-regulated and only galactose suppresses phagocytosis, implying a role for serum factors in the preservation of haemocyte function following exposure. We therefore propose that the establishment of a compatible relationship between a schistosome and its snail host is at least in part due to down-regulation of cell signalling events in haemocytes.

Elastin Peptides Activate Extracellular Signal-Regulated Kinase 1/2 via a Ras-Independent Mechanism Requiring Both p110γ/Raf-1 and Protein Kinase A/B-Raf Signaling in Human Skin Fibroblasts

Elastin peptides (EPs) produced during cancer progression bind to the elastin binding protein (EBP) found at the surface of dermal fibroblasts, leading to the expression of collagenase-1 gene. The production of this enzyme involved in stromal reaction is caused by the sustained activation of the extracellular signal-regulated kinases 1/2 (ERK1/2) pathway via cAMP/protein kinase A (PKA) and phosphatidylinositol 3-kinase (PI3K). However, the mechanism of these signaling events remains unknown. We show that kappa-elastin (kappaE), a commonly used EP, induces maximum phosphorylation of mitogen-activated protein kinase/extracellular signal-regulated kinase (MEK)1/2 and ERK1/2 after 30 min. The simultaneous inhibition of PKA and PI3K, by N-(2-(p-bromocinnamylamino)ethyl)-5-isoquinolinesulfonamide (H89) and 2-(4-morpholynil)-8-phenyl-4H-1-bemzopyran-4-one (LY294002), respectively, blocked MEK1/2 and ERK1/2 phosphorylation, as did lactose, an EBP antagonist. kappaE induced Raf-1 phosphorylation and activation in a PI3K-dependent manner. In our system, the PI3K p110gamma is expressed and activated by betagamma-derived subunits from a pertussis toxin-sensitive G protein after fibroblast stimulation. Pertussis toxin also blocks the Raf-1/MEK1/2/ERK1/2 phosphorylation cascade. In addition, we found that B-Raf is expressed in dermal fibroblasts and activated in a PKA-dependent manner after kappaE treatment, thereby integrating PKA signals to MEK1/2. It is noteworthy that Ras involvement was excluded because ERK1/2 activation by kappaE was not blocked in RasN17-transfected fibroblasts. Together, our results identify a novel Ras-independent ERK1/2 activation system in which p110gamma/Raf-1/MEK1/2 and PKA/B-Raf/MEK1/2 cooperate to activate ERK1/2. Thus, p110gamma and B-Raf seem to be important modulators of dermal fibroblasts physiology and should now qualify as therapeutic targets in strategies aiming at limiting elastin degradation contribution to cancer progression.

Characterization of the Regulation and Functional Consequences of p21rasActivation in Neutrophils by Antineutrophil Cytoplasm Antibodies

Antineutrophil cytoplasm antibodies (ANCA) are implicated in the pathogenesis of systemic vasculitis. ANCA are directed against antigens expressed on the surface of cytokine-primed neutrophils. It was shown previously that whole IgG ANCA and its fraction antigen binding [F(ab')(2)] fragment can activate the GTPase p21(ras). This study shows a functional involvement of this molecule in the ANCA activation of neutrophils by inhibiting the production of superoxide with farnesylthiosalicylic acid. Using the ras activation assay, farnesylthiosalicylic acid inhibits p21(ras) binding to its substrate at comparable concentrations to those seen for superoxide inhibition. It is also shown that activation of p21(ras) by ANCA is transient, peaking at 5 to 10 min and returning to baseline by 30 min. The use of ras isoform-specific antibodies in Western blots established, for the first time, that Harvey-ras is not present in human neutrophils, but both Kirsten-ras (K-ras) and Neuronal-ras are. Stimulation with ANCA is able to differentially activate K-ras without effects on neuronal-ras. The activation of p21(ras) by ANCA and its F(ab')(2) is prevented by inhibition of both Src kinases and phosphatidylinositol-3-kinase, indicating a cooperative role for both molecules in the G protein pathway activated by ANCA F(ab')(2) upstream of p21(ras). It is concluded that ANCA selectively activates K-ras during induction of a respiratory burst via pathways involving multiple upstream kinases.

PKA phosphorylation of Src mediates Rap1 activation in NGF and cAMP signaling in PC12 cells

Recent studies suggest that the tyrosine kinase Src plays an important role in the hormonal regulation of extracellular signal-regulated kinases (ERKs) via cyclic AMP (cAMP). Src has also been proposed to mediate signals downstream of nerve growth factor (NGF). Here, we report that the cAMP-dependent protein kinase A (PKA) induced the phosphorylation of Src at residue serine17 (S17) in multiple cell types including PC12, Hek293, AtT-20 and CHO cells. In PC12 cells, Src phosphorylation on S17 participates in the activation of the small G protein Rap1 by both cAMP and NGF. In these cells, Rap1 is required for cAMP/PKA signaling to ERKs and also for the sustained activation of ERKs by NGF. The activation of Rap1 by both cAMP and NGF was blocked by PP2, an inhibitor of Src family kinases, and by a Src mutant incapable of being phosphorylated by PKA (SrcS17A), consistent with the requirement of PKA phosphorylation of Src at S17 in these actions. PP2 and SrcS17A also inhibited the Rap1-dependent activation of ERKs by both agents. These results strongly indicate that PKA phosphorylation of Src at S17 is essential for cAMP and NGF signaling in PC12 cells and identify PKA as an important downstream target of NGF. PKA phosphorylation of Src may therefore be required for Rap1 activation in PC12 cells.

Gβγ Activation of Src Induces Caveolae-mediated Endocytosis in Endothelial Cells

Caveolae-mediated endocytosis in endothelial cells is stimulated by the binding of albumin to gp60, a specific albumin-binding protein localized in caveolae. The activation of gp60 induces its cell surface clustering and association with caveolin-1, the caveolar-scaffolding protein. This interaction leads to G(i)-induced Src kinase activation, which in turn signals dynamin-2-mediated fission and directed migration of caveolae-derived vesicles from apical to basal membrane. In this study, we investigated the possible role of the Gbetagamma heterodimer in signaling G(i)-induced Src activation and subsequent caveolae-mediated endocytosis. We observed using rat lung microvascular endothelial cells that expression of the C terminus of beta-adrenergic receptor kinase (ct-betaARK), an inhibitor Gbetagamma signaling, prevented gp60-dependent Src activation as well as caveolae-mediated endocytosis and transcellular transport of albumin and uptake of cholera toxin subunit B, a specific marker of caveolae internalization. Expression of ct-betaARK also prevented Src-mediated tyrosine phosphorylation of caveolin-1 and dynamin-2 and the resultant phosphorylation-dependent association of dynamin-2 and caveolin-1. Also, the direct activation of Gbetagamma using a specific cell-permeant activating peptide (myristoylated-SIRKALNILGYPDYD) simulated the effects of gp60 in inducing Src activation, caveolin-1, and dynamin-2 phosphorylation as well as caveolae-mediated endocytosis of cholera toxin subunit B. The myristoylated-SIRKALNILGYPDYD peptide-induced responses were inhibited by the expression of ct-betaARK. Taken together, our results demonstrate that Gbetagamma activation of Src signals caveolae-mediated endocytosis and transendothelial albumin transport via transcytosis.

Epac- and Ca2+ -controlled activation of Ras and extracellular signal-regulated kinases by Gs-coupled receptors

We have recently reported that two typical Gs-coupled receptors, the beta2-adrenergic receptor and the receptor for prostaglandin E1, stimulate phospholipase C-epsilon (PLC-epsilon) and increase intracellular Ca2+ concentration ([Ca2+]i) in HEK-293 cells and N1E-115 neuroblastoma cells, respectively, by a pathway involving Epac1, a cAMP-activated and Rap-specific guanine nucleotide exchange factor (GEF), and the GTPase Rap2B. Here we have demonstrated that these Gs-coupled receptors use this pathway to activate H-Ras and the extracellular signal-regulated kinases 1 and 2 (ERK1/2). Specifically, agonist activation of the receptors resulted in activation of H-Ras and ERK1/2. The latter action was suppressed by dominant negative H-Ras, but not Rap1A. The receptor actions were independent of protein kinase A but fully mimicked by an Epac-specific cAMP analog as well as by a constitutively active Rap2B mutant. On the other hand, a cAMP-binding-deficient Epac1 mutant, the Rap GTPase-activating proteinII, and a dominant negative Rap2B mutant suppressed receptor- and Epac-mediated activation of H-Ras and ERK1/2. Finally, we have demonstrated that activation of H-Ras and ERK1/2 requires the lipase activity of PLC-epsilon and the subsequent [Ca2+]i increase, suggesting that H-Ras activation is mediated by a Ca2+ -activated GEF. In line with this hypothesis, receptor-mediated activation of H-Ras and ERK1/2 was strongly enhanced by expression of RasGRP1, a Ca2+ -regulated Ras-GEF. Collectively, our data indicated that Gs-coupled receptors can activate H-Ras and subsequently the mitogen-activated protein kinases ERK1/2 by a Ca2+ -activated Ras-GEF, possibly RasGRP1, mediated by cAMP-activated Epac proteins, which then lead via Rap2B and PLC-epsilon stimulation to [Ca2+]i increase.

JAK/STAT, Raf/MEK/ERK, PI3K/Akt and BCR-ABL in cell cycle progression and leukemogenesis

The roles of the JAK/STAT, Raf/MEK/ERK and PI3K/Akt signal transduction pathways and the BCR-ABL oncoprotein in leukemogenesis and their importance in the regulation of cell cycle progression and apoptosis are discussed in this review. These pathways have evolved regulatory proteins, which serve to limit their proliferative and antiapoptotic effects. Small molecular weight cell membrane-permeable drugs that target these pathways have been developed for leukemia therapy. One such example is imatinib mesylate, which targets the BCR-ABL kinase as well as a few structurally related kinases. This drug has proven to be effective in the treatment of CML patients. However, leukemic cells have evolved mechanisms to become resistant to this drug. A means to combat drug resistance is to target other prominent signaling components involved in the pathway or to inhibit BCR-ABL by other mechanisms. Treatment of imatinib-resistant leukemia cells with drugs that target Ras (farnysyl transferase inhibitors) or with the protein destabilizer geldanamycin has proven to be a means to inhibit the growth of resistant cells. This review will tie together three important signal transduction pathways involved in the regulation of hematopoietic cell growth and indicate how their expression is dysregulated by the BCR-ABL oncoprotein.

G Proteins in Cancer: The Prostate Cancer Paradigm

Signal transduction research investigating mechanisms of androgen-independent prostate cancer cell proliferation has historically focused on the role of androgen and peptide growth factor receptors. More recent work has raised the idea that intracellular signaling mechanisms triggered by extracellular hormonal factors acting through heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) can also mediate and sustain this pathologic process. Prostate cancer patients with advanced disease express elevated levels of GPCRs and GPCR ligands, suggesting that the GPCR system is activated in the cancerous gland and may contribute to tumor growth. Importantly, inhibition of G protein signaling attenuates prostate cancer cell growth in animal models. The nature of intracellular signaling pathways mediating mitogenic effects of GPCRs in prostate cancer is poorly defined, although the G protein-dependent activation of the Ras-to-mitogen-activated protein kinase pathway has emerged as a critical regulatory event. Activated GPCRs may also exert their mitogenic effects in the prostate by activating the androgen receptor.

Coupling of neuronal 5-HT7 receptors to activation of extracellular-regulated kinase through a protein kinase A-independent pathway that can utilize Epac

The roles of 3',5'-cyclic adenosine monophosphate (cAMP) and protein kinase A in 5-hydroxytryptamine (5-HT)7 receptor-mediated activation of extracellular-regulated kinase (ERK) were studied in cultured hippocampal neurons and transfected PC12 cells. Activation of ERK by neuronal Gs-coupled receptors has been thought to proceed through a protein kinase A-dependent pathway. In fact we identified coupling of 5-HT7 receptors to activation of adenylyl cyclase and protein kinase A. However, no inhibition of agonist-stimulated ERK activation was found when cells were treated with H-89 and KT5720 at concentrations sufficient to completely inhibit activation of protein kinase A. However, activation of ERK was found to be sensitive to the adenylyl cyclase inhibitor 9-(tetrahydrofuryl)-adenine, suggesting a possible role for a cAMP-guanine nucleotide exchange factor (cAMP-GEF). Co-treatment of cells with 8-(4-chlorophenylthio)-2'-O-methyladenosine 3',5'-cyclic monophosphate, a direct activator of the cAMP-GEFs Epac1 and 2, reversed the inhibition of agonist-stimulated ERK activation induced by adenylyl cyclase inhibition. Additionally, over-expression of Epac1 enhanced 5-HT7 receptor-mediated activation of ERK. These results demonstrate that the activation of ERK mediated by neuronal Gs-coupled receptors can proceed through cAMP-dependent pathways that utilize cAMP-GEFs rather than protein kinase A.

The lutropin/choriogonadotropin receptor-induced phosphorylation of the extracellular signal-regulated kinases in leydig cells is mediated by a protein kinase a-dependent activation of ras

The pathways involved in activation of the ERK1/2 cascade in Leydig cells were examined in MA-10 cells expressing the recombinant human LH receptor (hLHR) and in primary cultures of rat Leydig cell precursors. In MA-10 cells expressing the recombinant hLHR, human choriogonadotropin-induced activation of ERK1/2 is effectively inhibited by overexpression of a cAMP phosphodiesterase (a manipulation that blunts the human choriogonadotropin-induced cAMP response), by addition of H89 (a selective inhibitor of protein kinase A), or by overexpression of the heat-stable protein kinase A inhibitor, but not by overexpression of an inactive mutant of this inhibitor. Stimulation of hLHR did not activate Rap1, but activated Ras in an H89-sensitive fashion. Addition of H89 to MA-10 cells that had been cotransfected with a guanosine triphosphatase-deficient mutant of Ras almost completely inhibited the hLHR-mediated activation of ERK1/2. We also show that 8-bromo-cAMP activates Ras and ERK1/2 in MA-10 cells and in primary cultures of rat Leydig cells precursors in an H89-sensitive fashion, whereas a cAMP analog 8-(4-chloro-phenylthio)-2'-O-methyl-cAMP (8CPT-2Me-cAMP) that is selective for cAMP-dependent guanine nucleotide exchange factor has no effect. Collectively, our results show that the hLHR-induced phosphorylation of ERK1/2 in Leydig cells is mediated by a protein kinase A-dependent activation of Ras.

sst2 Somatostatin receptor inhibits cell proliferation through Ras-, Rap1-, and B-Raf-dependent ERK2 activation

The G protein-coupled sst2 somatostatin receptor is a critical negative regulator of cell proliferation. sstII prevents growth factor-induced cell proliferation through activation of the tyrosine phosphatase SHP-1 leading to induction of the cyclin-dependent kinase inhibitor p27Kip1. Here, we investigate the signaling molecules linking sst2 to p27Kip1. In Chinese hamster ovary-DG-44 cells stably expressing sst2 (CHO/sst2), the somatostatin analogue RC-160 transiently stimulates ERK2 activity and potentiates insulin-stimulated ERK2 activity. RC-160 also stimulates ERK2 activity in pancreatic acini isolated from normal mice, which endogenously express sst2, but has no effect in pancreatic acini derived from sst2 knock-out mice. RC-160-induced p27Kip1 up-regulation and inhibition of insulin-dependent cell proliferation are both prevented by pretreatment of CHO/sst2 cells with the MEK1/2 inhibitor PD98059. In addition, using dominant negative mutants, we show that sst2-mediated ERK2 stimulation is dependent on the pertussis toxin-sensitive Gi/o protein, the tyrosine kinase Src, both small G proteins Ras and Rap1, and the MEK kinase B-Raf but is independent of Raf-1. Phosphatidylinositol 3-kinase (PI3K) and both tyrosine phosphatases, SHP-1 and SHP-2, are required upstream of Ras and Rap1. Taken together, our results identify a novel mechanism whereby a Gi/o protein-coupled receptor inhibits cell proliferation by stimulating ERK signaling via a SHP-1-SHP-2-PI3K/Ras-Rap1/B-Raf/MEK pathway.

Differential involvement of the Ras and Rap1 small GTPases in vasoactive intestinal and pituitary adenylyl cyclase activating polypeptides control of the prolactin gene

In pituitary cells, transcriptional regulation of the prolactin (PRL) gene and prolactin secretion are controlled by multiple transduction pathways through the activation of G protein coupled receptors and receptor tyrosine kinases. In the somatolactotrope GH4C1 cell line, we have previously identified crosstalk between the MAPKinase cascade ERK1/2 and the cAMP/protein kinase A pathway after the activation of the VPAC2 receptor by vasoactive intestinal polypeptide (VIP) or pituitary adenylyl cyclase-activating polypeptide (PACAP38). In the present study, we focus on the involvement of the GTPases Ras and Rap1 as downstream components of signal transmission initiated by activation of the VPAC2 receptor. By using pull-down experiments, we show that VIP and PACAP38 preferentially activate Rap1, whereas thyrotropin releasing hormone (TRH) and epidermal growth factor (EGF) mainly activate Ras GTPase. Experiments involving the expression of the dominant-negative mutants of Ras and Rap1 signaling (RasN17 or Rap1N17) indicate that both GTPases Ras and Rap1 are recruited for the ERK activation by VIP and PACAP38, whereas Rap1 is poorly involved in TRH or EGF-induced ERK activation. The use of U0126, a selective inhibitor of MAPKinase kinase, provides evidence that MAPKinase contributes to the regulation of the PRL gene. Moreover, cotransfection of RasN17 or Rap1N17 with the PRL proximal promoter luciferase reporter construct indicates that Rap1 may be responsible for VIP/PACAP-induced activation of the PRL promoter. Interestingly, Ras would be involved as a negative regulator of VIP/PACAP-induced PRL gene activation, in contrast to its stimulatory role in the regulation of the PRL promoter by TRH and EGF.

Activation of extracellular signal-regulated kinase mediates bombesin-induced mitogenic responses in prostate cancer cells

Bombesin and its mammalian homologue gastrin-releasing peptide have been shown to be highly expressed and secreted by neuroendocrine cells in prostate cancer, and are thought to be related to the carcinogenesis and progression of this disease. We found, in this study, bombesin specifically induced mitogen-activated protein (MAP) kinase activation as shown by increased extracellular regulated kinase (ERK) phosphorylation and epidermal growth factor (EGF) receptor transactivation in prostate cancer cells, which express functional gastrin-releasing peptide receptor. The transactivation of EGF receptor was required for bombesin-induced ERK phosphorylation. Furthermore, non-receptor tyrosine kinase Src and cellular Ca2+ were shown to be involved in bombesin-induced EGF receptor transactivation and ERK phosphorylation. Inhibition of either EGF receptor transactivation or ERK activation blocked bombesin-induced DNA synthesis in these cells. Taken together, these data suggest bombesin may act as a mitogen in prostate cancer by activating MAP kinase pathway via EGFR transactivation.

Melanin-concentrating hormone receptor 1 activates extracellular signal-regulated kinase and synergizes with G(s)-coupled pathways

Melanin-concentrating hormone (MCH) is a hypothalamic neuropeptide that plays a key role in energy homeostasis. Like many neuropeptides, it signals through two G protein-coupled receptors. MCH receptor 1 (MCHR1) is the sole receptor expressed in rodents and couples to G(i) and G(q) proteins. Little is known about the intracellular pathways engaged by MCH and its receptor. Using HEK293 cells stably expressing MCHR1, we demonstrate that MCH, acting through MCHR1, antagonizes the action of forskolin, an adenylate cyclase activator that increases intracellular levels of cAMP. MCH also inhibits cAMP induction by the G(s)-coupled beta-adrenergic receptor. Activation of either the G(i)- or G(s)-dependent pathway typically results in ERK phosphorylation in HEK293 cells. In contrast to opposing actions on cAMP synthesis, simultaneous MCH and forskolin treatment results in synergistic activation of ERK. This synergy proceeds through pertussis toxin-independent pathways and requires several enzymatic activities such as protein kinase A, protein kinase C, phospholipase C, and Src kinase. Finally, we provide evidence that such positive interactions are not limited to cell lines but can also be observed in the brain.

Does Rap1 deserve a bad Rap?

The Ras superfamily of small G proteins is remarkable for both its diversity and physiological functions. One member, Rap1, has been implicated in a particularly wide range of biological processes, from cell proliferation and differentiation to cell adhesion. But the diversity of Rap1 has lead to contradictory reports of its effects. Originally identified as an antagonist of Ras-induced transformation, Rap1 can oppose other actions of Ras including regulation of cell growth and differentiation, integrin-dependent responses and synaptic plasticity. Furthermore, recent evidence confirms that Rap1, like Ras, can activate the MAP kinase cascade (ERK) in several cell types. These diverse functions of Rap1 underscore that the activation and action of Rap1 are regulated by complex factors that are cell-type specific.