Activation of multiple mitogen-activated protein kinase signal transduction pathways by the endothelin B receptor requires the cytoplasmic tail

Identification of genes related to growth traits from transcriptome profiles of duck breast muscle tissue

The growth and development of duck skeletal muscle is an important economic trait that is genetically regulated. The internal mechanism underlying the regulation of skeletal muscle growth and development in ducks remains unclear. The purpose of this study was to identify candidate genes related to the growth of duck skeletal muscle. RNA-sequencing technology was used to compare the transcriptome of duck breast muscles in an F2 population with the high breast muscle rate (HB) and the low breast muscle rate (LB). A total of 14,522 genes were confirmed to be expressed in the breast muscle, and 173 differentially expressed genes (DEGs) were identified between the HB and LB groups. Functional analysis showed that these DEGs were mainly involved in biological processes and pathways of fat metabolism and muscle growth, especially the FABP3 and MYL4 involved in the PPAR signaling pathway and cardiac muscle contraction pathway. These findings deepened our understanding of the molecular mechanisms involved in muscle growth in ducks and provided a theoretical basis for improving duck production and breeding of ducks.

Loss of endothelin type B receptor function improves insulin sensitivity in rats

High salt intake (HS) is associated with obesity and insulin resistance. ET-1, a peptide released in response to HS, inhibits the actions of insulin on cultured adipocytes through ET-1 type B (ETB) receptors; however, the in vivo implications of ETB receptor activation on lipid metabolism and insulin resistance is unknown. We hypothesized that activation of ETB receptors in response to HS intake promotes dyslipidemia and insulin resistance. In normal salt (NS) fed rats, no significant difference in body mass or epididymal fat mass was observed between control and ETB deficient rats. After 2 weeks of HS, ETB-deficient rats had significantly lower body mass and epididymal fat mass compared to controls. Nonfasting plasma glucose was not different between genotypes; however, plasma insulin concentration was significantly lower in ETB-deficient rats compared to controls, suggesting improved insulin sensitivity. In addition, ETB-deficient rats had higher circulating free fatty acids in both NS and HS groups, with no difference in plasma triglycerides between genotypes. In a separate experiment, ETB-deficient rats had significantly lower fasting blood glucose and improved glucose and insulin tolerance compared to controls. These data suggest that ET-1 promotes adipose deposition and insulin resistance via the ETB receptor.

Endothelins (EDN1, EDN2, EDN3) and their receptors (EDNRA, EDNRB, EDNRB2) in chickens: Functional analysis and tissue distribution

Endothelins (EDNs) and their receptors (EDNRs) are reported to be involved in the regulation of many physiological/pathological processes, such as cardiovascular development and functions, pulmonary hypertension, neural crest cell proliferation, differentiation and migration, pigmentation, and plumage in chickens. However, the functionality, signaling, and tissue expression of avian EDN-EDNRs have not been fully characterized, thus impeding our comprehensive understanding of their roles in this model vertebrate species. Here, we reported the cDNAs of three EDN genes (EDN1, EDN2, EDN3) and examined the functionality and expression of the three EDNs and their receptors (EDNRA, EDNRB and EDNRB2) in chickens. The results showed that: 1) chicken (c-) EDN1, EDN2, and EDN3 cDNAs were predicted to encode bioactive EDN peptides of 21 amino acids, which show remarkable degree of amino acid sequence identities (91-95%) to their respective mammalian orthologs; 2) chicken (c-) EDNRA expressed in HEK293 cells could be preferentially activated by chicken EDN1 and EDN2, monitored by the three cell-based luciferase reporter assays, indicating that cEDNRA is a functional receptor common for both cEDN1 and cEDN2. In contrast, both cEDNRB and cEDNRB2 could be activated by all three EDN peptides with similar potencies, indicating that both receptors can function as common receptors for the three EDNs and share functional similarity. Moreover, activation of three EDNRs could stimulate intracellular calcium, MAPK/ERK, and cAMP/PKA signaling pathways. 3) qPCR assay revealed that cEDNs and cEDNRs are widely, but differentially, expressed in adult chicken tissues. Taken together, our data establishes a clear molecular basis to uncover the physiological/pathological roles of EDN-EDNR system in birds and helps to reveal the conserved actions of EDN-EDNR signaling across vertebrates.

Endothelin-1 protects human melanocytes from UV-induced DNA damage by activating JNK and p38 signalling pathways

Endothelin-1 is a paracrine factor with mitogenic, melanogenic and survival effects on cultured human melanocytes. We report that endothelin-1 signalling reduced the generation and enhanced the repair of ultraviolet radiation (UV)-induced DNA photoproducts, and inhibited apoptosis of human melanocytes, without increasing cAMP levels, melanin content or proliferation. Treatment with endothelin-1 activated the MAP kinases JNK and p38, as evidenced by phosphorylation of their target, activating transcription factor-2 (ATF-2). Endothelin-1 also enhanced the phosphorylation of JNK, p38 and ATF-2 by UV. The effects of endothelin-1 were dependent on increasing intracellular calcium mobilization by endothelin B receptor signalling. Activation of both JNK and p38 was required for reducing DNA photoproducts, but only JNK partially contributed to the survival effect of endothelin-1. ATF-2 activation depended mainly on JNK, yet was not sufficient for the effect of endothelin-1 on UV-induced DNA damage, suggesting the requirement for other JNK and p38 targets for this effect. Our results underscore the significance of endothelin-1 and endothelin B receptor signalling in reducing the genotoxic effects of UV via activating JNK and p38, hence restoring genomic stability of melanocytes.

Endothelin-1 activation of ETB receptors leads to a reduced cellular proliferative rate and an increased cellular footprint

Endothelin-1 (ET-1) is a vasoactive peptide which signals through two G-protein coupled receptors, endothelin receptor A (ETA) and B (ETB). We determined that ET-1 activation of its ETB receptor in stably cDNA transfected CHO cells leads to a 55% reduction in cell number by end-point cell counting and a 35% decrease in cell growth by a real-time cell-substrate impedance-based assay after 24h of cell growth. When CHO ETB cells were synchronized in the late G1 cell cycle phase, ET-1 delayed their S phase progression compared to control by 30% as determined by [(3)H]-thymidine incorporation. On the other hand, no such delay was observed during late G2/M to G1 transit when cells were treated with ET-1 after release from mitotic arrest. Using the cell-substrate impedance-based assay, we observed that ET-1 induces opposing morphological changes in CHO ETA and CHO ETB cells with ETB causing an increase in the cell footprint and ETA a decrease. Likewise, in pulmonary artery smooth muscle cells, which express both ETA and ETB receptors, ET-1 induces an ETA-dependent contraction and an ETB dependent dilation. These results are shedding light on a possible beneficial role for ETB in diseases involving ET-1 dysfunction such as pulmonary hypertension.

Endothelin-1 and transforming growth factor-beta1 independently induce fibroblast resistance to apoptosis via AKT activation

Myofibroblast apoptosis is critical for the normal resolution of wound repair responses, and impaired myofibroblast apoptosis is associated with tissue fibrosis. Lung expression of endothelin (ET)-1, a soluble peptide implicated in fibrogenesis, is increased in murine models of pulmonary fibrosis and in the lungs of humans with pulmonary fibrosis. Mechanistically, ET-1 has been shown to induce fibroblast proliferation, differentiation, contraction, and collagen synthesis. In this study, we examined the role ET-1 in the regulation of lung fibroblast survival and apoptosis. ET-1 rapidly activates the prosurvival phosphatidylinositol 3'-OH kinase (PI3K)/AKT signaling pathway in normal and fibrotic human lung fibroblasts. ET-1-induced activation of PI3K/AKT is dependent on p38 mitogen-activated protein kinase (MAPK), but not extracellular signal-regulated kinase (ERK) 1/2, JNK, or transforming growth factor (TGF)-beta1. Activation of the PI3K/AKT pathway by ET-1 inhibits fibroblast apoptosis, and this inhibition is reversed by blockade of p38 MAPK or PI3K. TGF-beta1 has been shown to attenuate myofibroblast apoptosis through the p38 MAPK-dependent secretion of a soluble factor, which activates PI3K/AKT. In this study, we show that, although TGF-beta1 induces fibroblast synthesis and secretion of ET-1, TGF-beta1 activation of PI3K/AKT is not dependent on ET-1. We conclude that ET-1 and TGF-beta1 independently promote fibroblast resistance to apoptosis through signaling pathways involving p38 MAPK and PI3K/AKT. These findings suggest the potential for novel therapies targeting the convergence of prosurvival signaling pathways activated by these two profibrotic mediators.

Reactive Oxygen Species Generation Is Involved in Epidermal Growth Factor Receptor Transactivation through the Transient Oxidization of Src Homology 2-Containing Tyrosine Phosphatase in Endothelin-1 Signaling Pathway in Rat Cardiac Fibroblasts

Endothelin-1 (ET-1) is implicated in fibroblast proliferation, which results in cardiac fibrosis. Both reactive oxygen species (ROS) generation and epidermal growth factor receptor (EGFR) transactivation play critical roles in ET-1 signal transduction. In this study, we used rat cardiac fibroblasts treated with ET-1 to investigate the connection between ROS generation and EGFR transactivation. ET-1 treatment was found to stimulate the phosphorylation of EGFR and ROS generation, which were abolished by ETA receptor antagonist N-(N-(N-((hexahydro-1H-azepin-1-yl)carbonyl)-L-leucyl)-D-tryptophyl)-D-tryptophan (BQ485). NADPH oxidase inhibitor diphenyleneiodonium chloride (DPI), ROS scavenger N-acetyl cysteine (NAC), and p47phox small interfering RNA knockdown all inhibited the EGFR transactivation induced by ET-1. In contrast, EGFR inhibitor 4-(3'-chloroanilino)-6,7-dimethoxyquinazoline (AG-1478) cannot inhibit intracellular ROS generation induced by ET-1. Src homology 2-containing tyrosine phosphatase (SHP-2) was shown to be associated with EGFR during ET-1 treatment by EGFR coimmunoprecipitation. ROS have been reported to transiently oxidize the catalytic cysteine of phosphotyrosine phosphatases to inhibit their activity. We examined the effect of ROS on SHP-2 in cardiac fibroblasts using a modified malachite green phosphatase assay. SHP-2 was transiently oxidized during ET-1 treatment, and this transient oxidization could be repressed by DPI or NAC treatment. In SHP-2 knockdown cells, ET-1-induced phosphorylation of EGFR was dramatically elevated and is not influenced by NAC and DPI. However, this elevation was suppressed by GM6001 [a matrix metalloproteinase (MMP) inhibitor] and heparin binding (HB)-epidermal growth factor (EGF) neutralizing antibody. Our data suggest that ET-1-ETA-mediated ROS generation can transiently inhibit SHP-2 activity to facilitate the MMP-dependent and HB-EGF-stimulated EGFR transactivation and mitogenic signal transduction in rat cardiac fibroblasts.

Subdepressor dose of benidipine ameliorates diabetic cardiac remodeling accompanied by normalization of upregulated endothelin system in rats

We investigated whether benidipine, a long-acting calcium channel blocker (CCB), can normalize cardiac expression profiles of the endothelin (ET)-1 system in insulin-resistant diabetes. Otsuka Long-Evans Tokushima Fatty (OLETF) rats, a model of human Type 2 diabetes, were treated for 12 wk with vehicle or benidipine (3 mg.kg(-1).day(-1)). OLETF rats exhibited a significant increase in ET-1 in plasma and left ventricular (LV) tissues compared with nondiabetic controls. Expression of prepro-ET-1, ET-converting enzyme, and ET(A) and ET(B) receptors in LV tissues was also significantly higher in OLETF rats. The two MAPKs, JNK and p38MAPK, both of which are activated by ET-1, were more abundantly expressed in OLETF rat LV tissues. All these alterations were reversed to nondiabetic levels when OLETF rats were treated with the subdepressor dose of benidipine. Furthermore, benidipine therapy resulted in hindering cardiomyocyte hypertrophy and cardiac perivascular fibrosis in OLETF rats. The beneficial actions of benidipine at the subdepressor dose on cardiac remodeling in insulin-resistant diabetes may involve normalization of the upregulated ET-1 system.

Inhibition of T cell MAPKs (Erk 1/2, p38) with thermal injury is related to down-regulation of Ca2+ signaling

We evaluated MAPK (Erk 1/2 and p38) signaling mechanisms of altered T-cell-mediated immune responses in thermal injury condition. Rats were subjected to 30% body surface scald burn, and their mesenteric lymph node (MLN) and Peyer's patch (PP) T cells were purified using nylon wool method. Activation of MAPKs, Erk 1/2 and p38 was assessed in T cells by determining its phosphorylation using immunoblot analysis, intracellular immunostaining and confocal microscopy. The results showed a down-regulation of Erk 1/2 and p38 activation in anti-CD3-stimulated T cells from thermally injured animals, compared to Erk 1/2 and p38 in sham rat T cells. The down-regulation of MAPKs in T cells was reversed by treatment of T cells with calcium agonist, ionomycin. These data indicate that attenuated MAPKs (Erk 1/2, p38) activation in thermally injured animals' T cells could result from derangement of Ca(2+) mobilization. This finding suggests that T cell signaling derangements with thermal injury involve an altered cross-talk between Ca(2+) mobilization and MAPK signaling mechanisms.

Alterations in receptor expression or agonist concentration change the pathways gastrin-releasing peptide receptor uses to regulate extracellular signal-regulated kinase

G protein-coupled receptors activate extracellular signal-regulated kinases (ERKs) via different pathways in different cell types. In this study, we demonstrate that gastrin-releasing peptide receptor (GRPr) regulates ERK through multiple pathways in a single cell type depending upon receptor expression and agonist concentration. We examined stably transfected BALB/c 3T3 fibroblasts expressing GRPr constructs at different levels and treated the cells with several concentrations of bombesin (BN, a GRPr agonist) to activate a variable number of GRPr per cell. GRPr induced two waves of ERK activation and one wave of ERK inhibition. One wave of activation required an intact GRPr carboxyl-terminal domain (CTD). It peaked 6 min after addition of high BN concentration ([BN]) in cells with high GRPr expression. Another wave of activation was CTD-independent. It peaked 2 to 4 min after BN addition in cells when [BN] and/or GRPr expression were lower. The early wave of ERK activation was more sensitive than the later one to pretreatment with Bisindolylmaleimide I (GF 109203X) (a protein kinase C inhibitor) or hypertonic sucrose. Because these two waves of activation differ in time course, dose-response curve, requirement for GRPr CTD, and sensitivity to inhibitors, they result from different signaling pathways. A third pathway in these cells inhibited ERK phosphorylation 2 min after addition of high [BN] in cells with high GRPr expression. Furthermore, a GRPr-expressing human duodenal cancer cell line showed differential sensitivity to GF 109203X throughout BN-induced ERK activation, indicating that GRPr may activate ERK via multiple pathways in cells expressing endogenous GRPr.

Galpha protein selectivity determinant specified by a viral chemokine receptor-conserved region in the C tail of the human herpesvirus 8 g protein-coupled receptor

The viral G-protein coupled receptor (vGPCR) specified by human herpesvirus 8 (HHV-8) open reading frame 74 (ORF74) is a ligand-independent chemokine receptor that has structural and functional homologues among other characterized gammaherpesviruses and related receptors in the betaherpesviruses. Sequence comparisons of the gammaherpesvirus vGPCRs revealed a highly conserved region in the C tail, just distal to the seventh transmembrane domain. Mutagenesis of the corresponding codons of HHV-8 ORF74 was carried out to provide C-tail-altered proteins for functional analyses. By measuring receptor-activated vascular endothelial growth factor promoter induction and NF-kappaB, mitogen-activated protein kinase, and Ca(2+) signaling, we found that while some altered receptors showed general signaling deficiencies, others had distinguishable activation profiles, suggestive of selective Galpha protein coupling. This was supported by the finding that vGPCR and representative functionally altered variants, vGPCR.8 (R322W) and vGPCR.15 (M325S), were affected differently by inhibitors of Galpha(i) (pertussis toxin), protein kinase C (GF109203X), and phosphatidylinositol 3-kinase (wortmannin). Consistent with the signaling data, [(35)S]GTPgammaS incorporation assays revealed preferential coupling of vGPCR.15 to Galpha(q) and an inability of vGPCR.8 to couple functionally to Galpha(q). However, both variants, wild-type vGPCR, and a C-tail deletion version of the receptor were equally able to associate physically with Galpha(q). Combined, our data demonstrate that HHV-8 vGPCR contains discrete sites of Galpha interaction and that receptor residues in the proximal region of the cytoplasmic tail are determinants of Galpha protein coupling specificity.

Role of calcium in E-selectin induced phenotype of T84 colon carcinoma cells

The adhesion of cancer cells to the endothelium during the metastatic process involves the interaction of specific cell-cell adhesion receptors on the cell surface. E-selectin on endothelial cells and sialyl Lewis X carbohydrate component on tumor cells are mainly implicated in the adhesion of colon carcinoma cells to the endothelium of target organ. In this paper we show that binding of E-selectin to T84 colon tumor cells causes approximately a twofold increase in intracellular calcium concentration. In particular, using two inhibitors of receptor operated calcium channels, CAI and SK&F 96365, we present evidences that the augmentation in cytoplasmic calcium originates from ionic influx from extracellular sources. Furthermore, we demonstrated that modulation of [Ca2+]i by engagement of E-selectin receptor starts signal transduction pathways that affect cell spreading, tyrosine phosphorylation signaling, and cancer cell motility.

The first inner loop of endothelin receptor type B is necessary for specific coupling to Galpha 13

Endothelin (EDN) receptor type B (EDNRB) activates serum response factor (SRF) via G(q/11) and G(12/13) G proteins. In this study, we investigated the involvement of intracellular loop sequences of EDNRB in coupling to these G proteins. EDNRB mutants were generated and tested for their abilities to activate SRF in NIH3T3 cells and in the mouse embryonic fibroblast cell line (F(q/11)) lacking both Galpha(q) and Galpha(11). EDNRB can activate SRF in NIH3T3 cells via G(q/11), although it can only activate SRF through G(12/13) in F(q/11) cells. Mutants with mutations in the second and third inner loops of EDNRB functioned in the same manner in both cell lines, either able or unable to activate SRF. This finding suggests that the second and third inner loops of EDNRB either participate or not in coupling to both G(q/11) and G(12/13) but are not specific for either one. However, in the first inner loop, a substitution of three Ala residues for Met(128)-Arg(129)-Asn(130) abolished the ability to activate SRF only in F(q/11) cells, suggesting that this mutation might specifically disrupt the coupling to G(12/13) rather than to G(q/11). Further characterization of this first inner loop mutant revealed that exogenous expression of Galpha(12) or Galpha(q) could restore SRF activation, whereas the expression of Galpha(13) did not. Therefore, we conclude that although the three intracellular loops of EDNRB may be involved in coupling to G proteins, residues Met(128)-Arg(129)-Asn(130) in the first intracellular loop are specifically required for activation of Galpha(13).

Endothelin receptor antagonists prevent parathyroid cell proliferation caused by hypocalcemia in rats

PURPOSE OF REVIEW

Secondary hyperparathyroidism in chronic renal failure is one of the most frequently encountered disorders of calcium homeostasis, characterized by parathyroid hypertrophy and hyperplasia. It is important to understand the pathogenesis of parathyroid epithelial cell hyperplasia and to determine the factors involved in the parathyroid cell proliferation for the development of the new therapeutic strategy for the secondary hyperparathyroidism. The existence of the endothelin system in the parathyroid gland and the involvement of the endothelin system in the proliferation of parathyroid cells have recently been demonstrated. The purpose of this review is to summarize the recent findings on the role of the endothelin system in the parathyroid cell proliferation, and to discuss the effect of the endothelin receptor antagonists for the treatment of the secondary hyperparathyroidism.

RECENT FINDINGS

It was recently demonstrated that the inhibitory effect of endothelin receptor antagonist, bosentan, on the proliferation of parathyroid cells in secondary hyperparathyroidism rats induced by the low calcium diet. The treatment of the low calcium diet rats with bosentan was found to substantially reduce the number of PCNA-positive parathyroid cells, wet weight of parathyroid gland, and serum PTH level.

SUMMARY

The effectiveness of the endothelin receptor antagonists for the prevention of the low calcium diet-induced secondary hyperparathyroidism in rats has been clearly demonstrated. These findings suggest the possibility that endothelin receptor antagonist might become one of the important therapeutic strategies for preventing secondary hyperparathyroidism in humans.

Galpha13 induces preproET-1 gene expression via JNK

The endothelin B receptor (ETBR) has been shown to mediate autoinduction of endothelin-1 (ET-1). We previously reported that the ET(B)R interacts with Galpha13, a member of the heterotrimeric GTP-binding protein family. In the present study, we examined whether Galpha13 induces preproET-1 (ppET-1) gene transcription, which could result in ET-1 autoinduction in a renal epithelial cell line. We generated a reporter gene construct under control of the ppET-1 promoter region. The construct was transiently expressed in COS-7 cells. Transient expression of ETBR increased the promoter activity of ppET-1 following treatment with 100 nmol/l of ET-1. Expression of Galpha13Q226L or Galpha9209L, constitutively active forms of Galpha13 and Galpha9, also activated the ppET-1 promoter. ETBR-stimulated ppET-1 promoter activity was partially diminished by the expression of dominant negative forms of c-Jun N-terminal kinase (JNK1APF) or MAPK/ERK kinase (MEKK97M). Expression of JNK1APF also inhibited Galpha13Q226L-induced ppET-1 promoter activation. These findings indicate that Galpha13 can induce ppET-1 gene expression through a JNK-mediated pathway. Our results also suggest that this Galpha13-coupled signaling pathway may play an important role in a sustained ET-1 autoinduction loop in various pathophysiological conditions.

Coupling of endothelin receptors to the ERK/MAP kinase pathway. Roles of palmitoylation and G(alpha)q

Endothelins are potent mitogens that stimulate extracellular signal-regulated kinases (ERK/MAP kinases) through their cognate G-protein-coupled receptors, ET(A) and ET(B). To address the role of post-translational ET receptor modifications such as acylation on ERK activation and to identify relevant downstream effectors coupling the ET receptor to the ERK signaling cascades we have constructed a panel of palmitoylation-deficient ET receptor mutants with differential G(alpha) protein binding capacity. Endothelin-1 stimulation of wild-type ET(A) or ET(B) induced a fivefold to sixfold increase in ERK in COS-7 and CHO cells whereas full-length nonpalmitoylated ET(A) and ET(B) mutants failed to stimulate ERK. A truncated ET(B) lacking the C-terminal tail domain including putative phosphorylation and arrestin binding site(s) but retaining the critical palmitoylation site(s) was still able to fully stimulate ERK activation. Using mutated ET receptors with selective G-protein-coupling we found that endothelin-induced stimulation of G(alpha)q, but not of G(alpha)i or G(alpha)s, is essential for endothelin-mediated ERK activation. Inhibition of protein kinases A and C or epidermal growth factor receptor kinase failed to prevent ET(A)- and ET(B)-mediated ERK activation whereas blockage of phospholipase C-beta completely abrogated endothelin-promoted ERK activation through ET(A) and ET(B) in recombinant COS-7 and native C6 cells. Complex formation of Ca2+ or inhibition of Src family tyrosine kinases prevented ET-1-induced ERK-2 activation in C6-cells. Our results indicate that endothelin-promoted ERK/MAPK activation criticially depends on palmitoylation but not on phosphorylation of ET receptors, and that the G(alpha)q/phospholipase C-beta/Ca2+/Src signaling cascade is necessary for efficient coupling of ET receptors to the ERK/MAPK pathway.

The Usf-1 transcription factor is a novel target for the stress-responsive p38 kinase and mediates UV-induced Tyrosinase expression

The stress-activated signalling cascade leading to phosphorylation of the p38 family of kinases plays a crucial role during development and in the cellular response to a wide variety of stress-inducing agents. Although alterations in gene expression characteristic of the stress response require the regulation of key transcription factors by the p38 family, few downstream targets for this signalling pathway have been identified. By examining the ability of pigment cells to respond to UV irradiation as part of the UV-induced tanning response, we show that while the microphthalmia-associated transcription factor Mitf regulates basal Tyrosinase expression, it is the ubiquitous basic helix-loop-helix-leucine zipper transcription factor Usf-1 that is required for the UV activation of the Tyrosinase promoter. Consistent with this we demonstrate that Usf-1 is phosphorylated and activated by the stress-responsive p38 kinase. The results suggest that activation of Usf-1 by p38 at a wide variety of viral and cellular promoters will provide a link between stimuli as diverse as UV irradiation, glucose, viral infection and pro-inflammatory cytokines, and the changes in gene expression associated with the stress response.

The expression, regulation and signal transduction pathways of the mammalian gonadotropin-releasing hormone receptor

Normal mammalian sexual maturation and reproductive functions require the integration and precise coordination of hormones at the hypothalamic, pituitary, and gonadal levels. Hypothalamic gonadotropin-releasing hormone (GnRH) is a key regulator in this system; after binding to its receptor (GnRHR), it stimulates de novo synthesis and release of gonadotropins in anterior pituitary gonadotropes. Since the isolation of the GnRHR cDNA, the expression of GnRHR mRNA has been detected not only in the pituitary, but also in extrapituitary tissues, including the ovary and placenta. It has been shown that change in GnRHR mRNA is one of the mechanisms for regulating the expression of the GnRHR. To help understand the molecular mechanism(s) involved in transcriptional regulation of the GnRHR gene, the 5' flanking region of the GnRHR gene has recently been isolated. Initial characterization studies have identified several DNA regions in the GnRHR 5' flanking region which are responsible for both basal expression and GnRH-mediated homologous regulation of this gene in pituitary cells. The mammalian GnRHR lacks a C-terminus and possesses a relatively short third intracellular loop; both features are important in desensitization of many others G-protein coupled receptors (GPCRs), Homologous desensitization of GnRHR has been shown to be regulated by various serine-threonine protein kinases including protein kinase A (PKA) and protein kinase C (PKC), as well as by G-protein coupled receptor kinases (GRKs). Furthermore, GnRHR was demonstrated to couple with multiple G proteins (Gq/11, Gs, and Gi), and to activate cascades that involved the PKC, PKA, and mitogen-activator protein kinases. These results suggest the diversity of GnRHR-G protein coupling and signal transduction systems. The identification of second form of GnRH (GnRH-II) in mammals adds to the complexity of the GnRH-GnRHR system. This review summaries our recent progress in understanding the regulation of GnRHR gene expression and the GnRHR signal transduction pathways.Key words: gonadotropin-releasing hormone receptor, transcriptional regulation, desensitization, signal transduction.

Histamine H2 receptor mediated dual signaling: mapping of structural requirements using beta2 adrenergic chimeric receptors

Previously we demonstrated that the histamine H2 receptor can activate both the adenylate cyclase and phosphoinositide/protein kinase (PKC) signaling pathways. Although dual coupling occurs via separate GTP-dependent mechanisms the structural components of the H2 receptor directing differential signaling have not been established. We explored this question by attempting to confer to the beta2-adrenergic receptor (betaAR), which is known to stimulate cAMP formation, the ability to activate PKC through the construction of beta2/H2 chimeric receptors. Intracytoplasmic domains of the human beta2 adrenergic receptor were substituted with the corresponding sequences of the human H2 receptor and stably expressed in HEK-293 cells. Binding of [(3)H]-CGP to chimeric wild type beta2 receptors was comparable. Substitution of the second intracellular loop (2i) of the betaAR led to a significant decrease in coupling to adenylate cyclase while leading to a 139.5 +/- 9.4% control increase in epinephrine mediated PKC activation. Introduction of the H2 receptor 3i also led to a decrease in betaAR mediated cAMP generation but provided the latter with the ability to stimulate PKC (182.2 +/- 8% of control). Concomitant expression of both 2i and 3i led to a substantial increase in epinephrine mediated PKC activation (201.8 +/- 10.5% of control). Addition of the carboxyl terminal tail did not facilitate stimulation of PKC. In summary, the third intracellular loop of the H2 receptor plays an essential role in activating PKC with maximal efficiency conferred by the second intracellular domain.

Regulation of mitogen-activated protein kinase signaling networks by G protein-coupled receptors

The family of receptors that transmit signals through the activation of heterotrimeric GTP-binding proteins (G proteins) constitutes the largest group of cell surface proteins involved in signal transduction. These receptors participate in a broad range of important biological functions and are implicated in a number of disease states. More than half of all drugs currently available influence G protein-coupled receptors (GPCRs). These receptors affect the generation of small molecules that act as intracellular mediators or second messengers, and can regulate a highly interconnected network of biochemical routes controlling the activity of several members of the mitogen-activated protein kinase (MAPK) superfamily. They include extracellular signal-regulated kinase 1 (ERK1) and ERK2 (or p44(MAPK) and p42(MAPK)), c-Jun NH(2)-terminal kinases (JNKs), ERK5 (or BMK), and p38 MAPKs, including p38alpha (or CSBP-1), p38beta, p38gamma (or SAPK3 or ERK6), and p38delta?(or SAPK4). This review will focus on the molecular mechanisms by which GPCRs signal to the nucleus through this intricate network of second messenger-generating systems and MAPK signaling pathways, thereby affecting the expression of genes whose products influence many biological processes, including normal and aberrant cell growth.

Functional coupling of G proteins to endothelin receptors is ligand and receptor subtype specific

1. The aims of the present study were (a) to determine the identity of the G proteins with which the endothelin receptor interacts and whether this interaction is subtype specific and (b) to determine whether agonist exposure can result in specific coupling between the endothelin receptor and G proteins. 2. Coupling between endothelin A (ET(A)) or endothelin B (ET(B)) receptors and G proteins was assessed in two fibroblast cell lines, each expressing one receptor subtype. Four ligands, ET-1, ET-3, SRTXb, and SRTXc, were used for receptor stimulation. The G protein alpha-subunit coupled to the receptor was identified by immunoprecipitation with an antibody against the endothelin receptor and immunoblotting with specific antibodies against different G protein alpha-subunits. 3. Unstimulated ET(A) and ET(B) receptors (ET(A)R and ET(B)R, respectively) were barely coupled to Go(alpha). The unstimulated ET(A)R coimmunoprecipitated with Gi3alpha, whereas the unstimulated ETBR was much less strongly coupled to Gi3alpha. The coupling of ETBR to Gi1Gi2 alpha-subunits was much stronger than the coupling of ET(A)R to these alpha-subunits. Stimulation with the different ET agonists also resulted in differential coupling of G proteins to the receptor subtypes. All four ligands caused a strong increase in coupling of the ET(B)R to Gi3alpha, whereas coupling of the ET(A)R to this subunit was not affected by ET-1 and was even decreased by SRTXc. On the other hand, all four ligands caused a much greater increase in the coupling of ET(A)R to G(q)alpha/G11alpha than in the coupling of ET(B)R to these alpha-subunits. Ligand-induced coupling between the receptors and the Gi1 and Gi2 alpha-subunits is similar for the two receptor subtypes. The same was true for ligand-induced coupling of the receptors to Go(alpha), except that ET-3 increased the coupling of this alpha-subunit to ET(B)R and decreased the coupling to ET(A)R. Taken together, the results of this study show that coupling between ET receptors and G proteins is ligand and receptor subtype specific. 4. It remains to be established whether this diversity of receptor-G protein coupling is of relevance for the various endothelin signaling pathways and/or pathological states.

Divergent signaling pathways requiring discrete calcium signals mediate concurrent activation of two mitogen-activated protein kinases by gonadotropin-releasing hormone

Receptors coupled to heterotrimeric G proteins are linked to activation of mitogen-activated protein kinases (MAPKs) via receptor- and cell-specific mechanisms. We have demonstrated recently that gonadotropin-releasing hormone (GnRH) receptor occupancy results in activation of extracellular signal-regulated kinase (ERK) through a mechanism requiring calcium influx through L-type calcium channels in alphaT3-1 cells and primary rat gonadotropes. Further studies were undertaken to explore the signaling mechanisms by which the GnRH receptor is coupled to activation of another member of the MAPK family, c-Jun N-terminal kinase (JNK). GnRH induces activation of the JNK cascade in a dose-, time-, and receptor-dependent manner in clonal alphaT3-1 cells and primary rat pituitary gonadotrophs. Coexpression of dominant negative Cdc42 and kinase-defective p21-activated kinase 1 and MAPK kinase 7 with JNK and ERK indicated that specific activation of JNK by GnRH appears to involve these signaling molecules. Unlike ERK activation, GnRH-stimulated JNK activity does not require activation of protein kinase C and is not blocked after chelation of extracellular calcium with EGTA. GnRH-induced JNK activity was reduced after treatment with the intracellular calcium chelator BAPTA-AM (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester), whereas activation of ERK was not affected. Chelation of intracellular calcium also reduced GnRH-induced activation of JNK in rat pituitary cells in primary culture. GnRH-induced induction and activation of the JNK target c-Jun was inhibited after chelation of intracellular calcium, whereas induction of c-Fos, a known target of ERK, was unaffected. Therefore, although activation of ERK by GnRH requires a specific influx of calcium through L-type calcium channels, JNK activation is independent of extracellular calcium but sensitive to chelation of intracellular calcium. Our results provide novel evidence that GnRH activates two MAPK superfamily members via strikingly divergent signaling pathways with differential sensitivity to activation of protein kinase C and mobilization of discrete pools of calcium.

Natriuretic peptides inhibit G protein activation. Mediation through cross-talk between cyclic GMP-dependent protein kinase and regulators of G protein-signaling proteins

Atrial natriuretic peptide (ANP) inhibits the proliferation of many cells, in part through interfering with signal transduction enacted by G protein-coupled growth factor receptors. Signaling interactions between ANP and the G protein-coupled growth factor receptor ligand, endothelin-3 (ET-3), regulate astrocyte proliferation at a very proximal but undefined point. Here, we find that ANP inhibits the ability of ET-3 to activate Galpha(q) and Galpha(i) in these cells. ANP stimulated the translocation of endogenous regulators of G protein-signaling (RGS) proteins 3 and 4 from the cytosol to the cell membrane, and enhanced their association with Galpha(q) and Galpha(i). ANP effects were significantly blocked by HS-142-1, an inhibitor of guanylate cyclase activation, or by ET-3. KT5823, an inhibitor of cyclic GMP-dependent protein kinase (PKG) reversed the RGS translocation induced by ANP; conversely, expression of an active catalytic subunit of PKG-I, or 8-bromo-cyclic GMP stimulated RGS translocation. ANP caused the phosphorylation of both RGS proteins in a PKG-dependent fashion, and the expressed PKG (in the absence of ANP) also stimulated RGS phosphorylation. A novel cross-talk between PKG and RGS proteins is stimulated by ANP and leads to the increased translocation and association of RGS proteins with Galpha. The rapid inactivation of G proteins provides a mechanism by which ANP inhibits downstream signaling to the cell proliferation program.

W276 mutation in the endothelin receptor subtype B impairs Gq coupling but not Gi or Go coupling

The mutation of W276 to cysteine within the human endothelin receptor subtype B (ET(B)R) is associated with Hirschsprung's disease, a congenital intestinal disease. The sequence surrounding W276 is highly conserved between the endothelin receptor subtypes A and B. We have introduced sets of mutations into W275 and W276 of the ET(B)R gene, and the corresponding W257 and W258 of the ET(A)R gene, and studied their coupling properties with G(i), G(o), and G(q) in reconstituted phospholipid vesicles. The prepared mutants all showed a similar affinity for endothelin-1. The W276C/ET(B)R and W276A/ET(B)R mutants had reduced activities in G(q) coupling but not in G(i)/G(o) coupling, while the W275A/ET(B)R displayed reduced activities in G(i)/G(q) coupling, with normal G(o) coupling. On the other hand, W257A/ET(A)R and W258A/ET(A)R exhibited wild-type activities in all examined G protein couplings. These results suggest that the defects in the G(q) signaling pathway by the ET(B)R are connected with Hirschsprung's disease and that the two conserved tryptophans play distinct roles in signal transduction by the two receptor subtypes. In addition, W275 and W276, which are thought to be located near the extracellular side of the transmembrane helix 5, play important roles in forming the active structure of ET(B)R.

Calcium influx through L-type channels is required for selective activation of extracellular signal-regulated kinase by gonadotropin-releasing hormone

The hypothalamic decapeptide gonadotropin-releasing hormone stimulates mobilization of two discrete pools of calcium in clonal (alphaT3-1) and primary pituitary gonadotropes. A multidisciplinary approach was implemented to investigate the effects of discrete calcium fluctuations on the signaling pathways linking the gonadotropin-releasing hormone receptor to activation of mitogen-activated protein kinases and immediate early genes. Blockade of calcium influx through nifedipine-sensitive voltage-gated calcium channels reduced buserelin-induced activation of extracellular signal-regulated kinase (ERK) and c-Fos while activation of c-Jun N-terminal kinase and c-Jun was unaffected. Inhibition of buserelin-stimulated ERK activity by nifedipine was also observed in rat pituitary cells in primary culture. Direct activation of alphaT3-1 cell L-type calcium channels with the agonist Bay-K 8644 resulted in phosphorylation of ERK and induction of c-Fos. However, simple voltage-induced channel activation did not produce a sufficient calcium signal, since depolarization with 35 mM KCl failed to induce activation of ERK. Depletion of intracellular calcium stores with thapsigargin did not affect buserelin-induced ERK activation. An inhibitor of protein kinase C decreased calcium influx through nifedipine-sensitive calcium channels and phosphorylation of ERK induced by buserelin. Pharmacological inhibition of protein kinase C did not block Bay-K 8644-induced ERK activation. These observations suggest that calcium influx through L-type channels is required for GnRH-induced activation of ERK and c-Fos and that the influence of calcium lies downstream of protein kinase C.

The proximal portion of the COOH terminus of the oxytocin receptor is required for coupling to g(q), but not g(i). Independent mechanisms for elevating intracellular calcium concentrations from intracellular stores

As the oxytocin receptor plays a key role in parturition and lactation, there is considerable interest in defining its structure/functional relationships. We previously showed that the rat oxytocin receptor transfected into Chinese hamster ovary cells was coupled to both G(q/11) and G(i/o), and that oxytocin stimulated ERK-2 phosphorylation and prostaglandin E(2) synthesis via protein kinase C activity. In this study, we show that deletion of 51 amino acid residues from the carboxyl terminus resulted in reduced affinity for oxytocin and a corresponding rightward shift in the dose-response curve for oxytocin-stimulated [Ca(2+)](i). However, oxytocin-stimulated ERK-2 phosphorylation and prostaglandin E(2) synthesis did not occur in cells expressing the truncated receptor. Oxytocin also failed to increase phospholipase A activity or activate protein kinase C, indicating that the mutant receptor is uncoupled from G(q)-mediated pathways. The Delta51 receptor is coupled to G(i), as oxytocin-stimulated Ca(2+) transients were inhibited by pertussis toxin, and a Gbetagamma sequestrant. Preincubation of Delta51 cells with the tyrosine kinase inhibitor, genistein, also blocked the oxytocin effect. A Delta39 mutant had all the activities of the wild type oxytocin receptor. These results show that the portion between 39 and 51 residues from the COOH terminus of the rat oxytocin receptor is required for interaction with G(q/11), but not G(i/o). Furthermore, an increase in intracellular calcium was generated via a G(i)betagamma-tyrosine kinase pathway from intracellular stores that are distinct from G(q)-mediated inositol trisphosphate-regulated stores.

Endothelin-1 activates p38 mitogen-activated protein kinase via endothelin-A receptor in rat myocardial cells

In myocardial cells (MCs), endothelin-1 (ET-1) exerts various effects such as hypertrophy, and causes cellular injury. Long-term treatment with an endothelin-A (ET(A)) receptor antagonist improves the survival of rats with heart failure, suggesting that myocardial endothelin system contributes to the progression of heart failure. p38 mitogen-activated kinase (MAPK) is a member of the MAPK family and activated by several forms of environmental stresses. We show here the effect of ET-1 on p38 MAPK activation and the role of ET-1-activated p38 MAPK on morphological changes in MCs. ET-1-stimulated p38 MAPK phosphorylation was detectable within 2 min and maximal at 5 min and was concentration dependent. The maximum effect was obtained at 10 nM. An ET(A) receptor antagonist, BQ-123, but not an endothelin-B receptor antagonist, BQ-788, inhibited these reactions. A p38 MAPK inhibitor, SB203580, failed to inhibit the morphological changes associated with ET-1-induced myocardial cell hypertrophy. These results indicate that p38 MAPK is activated by ET-1 but does not contribute to the development of ET-1-induced myocardial cell hypertrophy.

Transmembrane helix 7 of the endothelin B receptor regulates downstream signaling

Endothelin is a 21-amino acid peptide with a striking diversity of important biological responses, including, vasoconstriction, bronchoconstriction, and mitogenesis. Endothelin-1 binding to the endothelin B receptor (ETB), a member of the superfamily of G-protein-coupled receptors, was associated with catalytic activation of the extracellular-regulated kinase 2 (ERK2) and stimulation of AP-1 transcriptional reporter activity. A panel of single point mutations in transmembrane helix 6 (TM6), intracellular loop 3, and transmembrane helix 7 (TM7) were developed to study the structural requirements for ETB activation. Point mutations within highly conserved regions of TM6 and intracellular loop 3 were without effect on agonist-stimulated ERK activation. However, mutations within TM7 of the ETB significantly impacted ligand-stimulated downstream signaling. For example, nine point mutations within TM7 of the ETB were identified that prevented endothelin-stimulated ERK activation. Interestingly, the TM7 mutants fell into two classes; several exhibited greatly decreased AP-1 activity, relative to wild type ETB, whereas others displayed augmented endothelin-stimulated AP-1 transcriptional activity relative to wild type ETB. Our results suggest that TM7 of the ETB is involved in its activation mechanism and regulates agonist-stimulated ERK activation.

Thyrotropin regulates c-Jun N-terminal kinase (JNK) activity through two distinct signal pathways in human thyroid cells

c-Jun N-terminal kinases (JNK) participate in cellular responses to mitogenic stimuli and environmental stresses. We investigated whether and how TSH, which promotes the proliferation and differentiation of thyroid cells, regulates JNK activity in primary cultured human thyroid cells. TSH stimulated JNK activity in cytosolic fractions of thyroid cells measured by in vitro kinase assay. A low concentration of TSH (10(-11) M) stimulated JNK activity but at a higher dose (10(-8)-10(-7) M), TSH suppressed JNK activity without any change of JNK protein level. Activation of JNK by TSH was also observed in CHO cells stably transfected with TSH receptor complementary DNA (cDNA), suggesting a ligand-receptor specific interaction. TSH stimulated JNK activity through a pertussis toxin-sensitive pathway. We next elucidated the signal transduction pathways in TSH-induced JNK activation by examining the involvement of four distinct intracellular signal molecules; protein kinase C (PKC), cAMP, Ca2+, and PI3-kinase. The stimulation of JNK by TSH was blocked by two PKC inhibitors and suppressed by 8-bromo-cAMP or forskolin. These findings demonstrate that TSH regulates JNK activity biphasically in human thyroid cells through an interaction between Gi-PKC and cAMP-PKA pathways.

Endothelin-B receptors activate Galpha13

Endothelin (ET) receptors activate heterotrimeric G proteins that are members of the Gi, Gq, and Gs families but may also activate members of other families such as Galpha12/13. Galpha13 has multiple complex cellular effects that are similar to those of ET. We studied the ability of ET receptors to activate Galpha13 using an assay for G protein alpha-chain activation that is based on the fact that an activated (GTP-bound) alpha-chain is resistant to trypsinization compared with an inactive (GDP-bound) alpha-chain. Nonhydrolyzable guanine nucleotides and AlMgF protected Galpha13 from degradation by trypsin. In membranes from human embryonic kidney 293 cells that coexpress ETB receptors and alpha13, ET-3 and 5'-guanylylimidodiphosphate [Gpp(NH)p] increased the protection of alpha13 compared with Gpp(NH)p alone. The specificity of ETB receptor-alpha13 coupling was documented by showing that beta2 receptors and isoproterenol or ETA receptors and ET-1 did not activate alpha13 and that a specific antagonist for ETB receptors blocked ET-3-dependent activation of alpha13.

Activation of the p38 mitogen-activated protein kinase pathway by gonadotropin-releasing hormone

Previous studies have shown that interaction of GnRH with its serpentine, G protein-coupled receptor results in activation of the extracellular signal regulated protein kinase (ERK) and the Jun N-terminal protein kinase (JNK) pathways in pituitary gonadotropes. In the present study, we examined GnRH-stimulated activation of an additional member of the mitogen-activated protein kinase (MAPK) superfamily, p38 MAPK GnRH treatment of alphaT3-1 cells resulted in tyrosine phosphorylation of several intracellular proteins. Separation of phosphorylated proteins by ion exchange chromatography suggested that GnRH receptor stimulation can activate the p38 MAPK pathway. Immunoprecipitation studies using a phospho-tyrosine antibody resulted in increased amounts of immunoprecipitable p38 MAPK from alphaT3-1 cells treated with GnRH. Immunoblot analysis of whole cell lysates using a phospho-specific antibody directed against dual phosphorylated p38 kinase revealed that GnRH-induced phosphorylation of p38 kinase was dose and time dependent and was correlated with increased p38 kinase activity in vitro. Activation of p38 kinase was blocked by chronic phorbol ester treatment, which depletes protein kinase C isozymes alpha and epsilon. Overexpression of p38 MAPK and an activated form of MAPK kinase 6 resulted in activation of c-jun and c-fos reporter genes, but did not alter the expression of the glycoprotein hormone alpha-subunit reporter. Inhibition of p38 activity with SB203580 resulted in attenuation of GnRH-induced c-fos reporter gene expression, but was not sufficient to reduce GnRH-induced c-jun or glycoprotein hormone alpha-subunit promoter activity. These studies provide evidence that the GnRH signaling pathway in alphaT3-1 cells includes protein kinase C-dependent activation of the p38 MAPK pathway. GnRH integration of c-fos promoter activity may include regulation by p38 MAPK.

Endothelin-stimulated ERK activation in airway smooth-muscle cells requires calcium influx and Raf activation

Endothelin (ET)-1 is a 21-amino-acid peptide that is a potent vasoconstrictor and mitogen. By binding to its G-protein coupled receptor, ET-1 stimulates the proliferation of airway smooth-muscle (ASM) cells, which may be involved in the pathogenesis of asthma. The ETB receptor stimulates activation of the extracellular regulated kinase 2 (ERK2), which is thought to be required for proliferation of ASM cells. Our findings reveal that ET rapidly activates Raf, and that dominant-negative Raf interferes with ET-induced ERK activation in ASM cells. Expression of the amino-terminal Ras-binding domain of Raf inhibited ET-induced ERK activation, suggesting that ET-stimulated Raf activation is a Ras-dependent process. Furthermore, ET-stimulated ERK and Raf activation in ASM cells require calcium influx; chelating extracellular calcium or preventing calcium influx through calcium channels inhibited ET-stimulated, but not phorbol ester-stimulated, ERK and Raf activation.

Molecular basis of receptor/G-protein-coupling selectivity

Molecular cloning studies have shown that G-protein-coupled receptors form one of the largest protein families found in nature, and it is estimated that approximately 1000 different such receptors exist in mammals. Characteristically, when activated by the appropriate ligand, an individual receptor can recognize and activate only a limited set of the many structurally closely related heterotrimeric G-proteins expressed within a cell. To understand how this selectivity is achieved at a molecular level has become the focus of an ever increasing number of laboratories. This review provides an overview of recent structural, molecular genetic, biochemical, and biophysical studies that have led to novel insights into the molecular mechanisms governing receptor-mediated G-protein activation and receptor/G-protein coupling selectivity.

Stimulation of the p38 mitogen-activated protein kinase pathway in neonatal rat ventricular myocytes by the G protein-coupled receptor agonists, endothelin-1 and phenylephrine: a role in cardiac myocyte hypertrophy?

We examined the activation of the p38 mitogen-activated protein kinase (p38-MAPK) pathway by the G protein-coupled receptor agonists, endothelin-1 and phenylephrine in primary cultures of cardiac myocytes from neonatal rat hearts. Both agonists increased the phosphorylation (activation) of p38-MAPK by approximately 12-fold. A p38-MAPK substrate, MAPK-activated protein kinase 2 (MAPKAPK2), was activated approximately fourfold and 10 microM SB203580, a p38-MAPK inhibitor, abolished this activation. Phosphorylation of the MAPKAPK2 substrate, heat shock protein 25/27, was also increased. Using selective inhibitors, activation of the p38-MAPK pathway by endothelin-1 was shown to involve protein kinase C but not Gi/Go nor the extracellularly responsive kinase (ERK) pathway. SB203580 failed to inhibit the morphological changes associated with cardiac myocyte hypertrophy induced by endothelin-1 or phenylephrine between 4 and 24 h. However, it decreased the myofibrillar organization and cell profile at 48 h. In contrast, inhibition of the ERK cascade with PD98059 prevented the increase in myofibrillar organization but not cell profile. These data are not consistent with a role for the p38-MAPK pathway in the immediate induction of the morphological changes of hypertrophy but suggest that it may be necessary over a longer period to maintain the response.

Activation of mitogen-activated protein kinases (p38-MAPKs, SAPKs/JNKs and ERKs) by the G-protein-coupled receptor agonist phenylephrine in the perfused rat heart

We investigated the ability of phenylephrine (PE), an alpha-adrenergic agonist and promoter of hypertrophic growth in the ventricular myocyte, to activate the three best-characterized mitogen-activated protein kinase (MAPK) subfamilies, namely p38-MAPKs, SAPKs/JNKs (i.e. stress-activated protein kinases/c-Jun N-terminal kinases) and ERKs (extracellularly responsive kinases), in perfused contracting rat hearts. Perfusion of hearts with 100 microM PE caused a rapid (maximal at 10 min) 12-fold activation of two p38-MAPK isoforms, as measured by subsequent phosphorylation of a p38-MAPK substrate, recombinant MAPK-activated protein kinase 2 (MAPKAPK2). This activation coincided with phosphorylation of p38-MAPK. Endogenous MAPKAPK2 was activated 4-5-fold in these perfusions and this was inhibited completely by the p38-MAPK inhibitor, SB203580 (10 microM). Activation of p38-MAPK and MAPKAPK2 was also detected in non-contracting hearts perfused with PE, indicating that the effects were not dependent on the positive inotropic/chronotropic properties of the agonist. Although SAPKs/JNKs were also rapidly activated, the activation (2-3-fold) was less than that of p38-MAPK. The ERKs were activated by perfusion with PE and the activation was at least 50% of that seen with 1 microM PMA, the most powerful activator of the ERKs yet identified in cardiac myocytes. These results indicate that, in addition to the ERKs, two MAPK subfamilies, whose activation is more usually associated with cellular stresses, are activated by the Gq/11-protein-coupled receptor (Gq/11PCR) agonist, PE, in whole hearts. These data indicate that Gq/11PCR agonists activate multiple MAPK signalling pathways in the heart, all of which may contribute to the overall response (e.g. the development of the hypertrophic phenotype).

Post-translational modifications of endothelin receptor B from bovine lungs analyzed by mass spectrometry

A new mild experimental approach for isolation of peptide membrane receptors and subsequent analysis of post-translational modifications is described. Endothelin receptors A and B were isolated on oligo(dT)-cellulose using N-(epsilon-maleimidocaproyloxy)succinimide endothelin coupled to a protected (dA)-30-mer. This allowed a one-step isolation of the receptor from oligo(dT)-cellulose via variation solely of salt concentration. The identity of the receptor was confirmed by direct amino acid sequencing of electroblotted samples or by using antibodies against ETA and ETB receptors. The method used here is very fast, requires only very mild elution conditions and, for the first time, gave both ETA and ETB receptors concurrently in very good yield. Following enzymatic in-gel digestion, MALDI, and electrospray ion trap mass spectrometric analysis of the isolated endothelin B receptor showed phosphorylation at Ser-304, -418, -438, -439, -440, and -441. Further phosphorylation at either Ser-434 or -435 was observed. The endothelin B receptor is also palmitoylated at Cys residues 402 and 404. Phosphorylation of Ser304 may play a role in Hirschsprung's disease.

Human serotonin 5-HT7 receptor: cloning and pharmacological characterisation of two receptor variants

Two splice variants of the 5-HT7 receptor were identified in human brain that differ in the lengths of their intracellular carboxy terminal tail. Identification of the variants of this receptor is of particular interest since the 5-HT7 receptor is known to have a high affinity for a number of antidepressants and is localized in brain regions thought to be implicated in depression. The two isoforms are expressed in roughly equal amounts in various regions of the human brain. When expressed in NIH-3T3 cells, both variants encode functional 5-HT7 receptors, positively coupled to adenylyl cyclase. We suggest that both variants are derived from a single gene by alternative mRNA splicing. Furthermore, our results from Southern blot analysis studies suggest that additional 5-HT7 receptor genes may exist in human.

Palmitoylation of human endothelinB. Its critical role in G protein coupling and a differential requirement for the cytoplasmic tail by G protein subtypes

By site-directed mutagenesis, three cysteine residues (amino acids 402, 403, and 405) in the carboxyl terminus of human endothelinB (ETB) were identified as potential palmitoylation sites. Substitutions of all of the three cysteine residues with serine gave an unpalmitoylated mutant, C2S/C3S/C5S. When expressed in Chinese hamster ovary cells, C2S/C3S/C5S was localized on the cell surface, retained high affinities to ET-1 and ET-3, and was rapidly internalized when bound to the ligand. However, unlike the wild-type ETB, C2S/C3S/C5S transmitted neither an inhibitory effect on adenylate cyclase nor a stimulatory effect on phospholipase C, indicating a critical role of palmitoylation in the coupling with G proteins, regardless of the G protein subtypes. Truncation of the carboxyl terminus including Cys403/Cys405 gave a deletion mutant Delta403 that was palmitoylated on Cys402 and lacked the carboxyl terminus downstream to the palmitoylation site. Delta403 did transmit a stimulatory effect on phospholipase C via a pertussis toxin-insensitive G protein but it failed to transmit an inhibitory effect on adenylate cyclase. These results indicated a differential requirement for the carboxyl terminus downstream to the palmitoylation site in the coupling with G protein subtypes, i.e. it is required for the coupling with Gi but not for that with Gq.