Cross talk between beta-adrenergic and bradykinin B(2) receptors results in cooperative regulation of cyclic AMP accumulation and mitogen-activated protein kinase activity

MAPK activation patterns of AT1R and CB1R in SHR versus Wistar astrocytes: Evidence of CB1R hypofunction and crosstalk between AT1R and CB1R

BACKGROUND

Angiotensin (Ang) II and cannabinoids regulate physiologically relevant astroglial functions via receptor-mediated activation of Mitogen-activated protein kinases (MAPKs). In this study, we investigated the consequences of astroglial Ang II type 1 receptor (AT1R) and Cannabinoid type 1 receptor (CB1R) activation, alone and in combination, on MAPK activation in the presence and absence of hypertensive states. In addition, we also investigated a novel unidirectional crosstalk mechanism between AT1R and CB1R, that involves PKC-mediated phosphorylation of CB1R.

METHODS

Astrocytes were isolated from the brainstem and cerebellum of Spontaneously hypertensive rats (SHRs) and normotensive Wistar rats. The cells were treated with either 100nM Ang II or 10nM Arachidonyl-2'-chloroethylamide (ACEA), both alone and in combination, for varying time periods, and the extent of phosphorylation of MAPKs, ERK and p38, and the phosphorylated forms of CB1R (p-CB1R), were measured using western blotting.

RESULTS

Ang II treatment resulted in a greater activation of MAPKs in SHR brainstem astrocytes, but not SHR cerebellar astrocytes when compared to Wistar rats. ACEA-mediated MAPK activation was significantly lower in brainstem astrocytes of SHRs when compared to Wistar rats. ACEA negatively modulates AT1R-mediated MAPK activation in both cerebellar and brainstem astrocytes of both models. The effect however was diminished in brainstem astrocytes. Ang II caused a significant increase in phosphorylation of CB1R in cerebellar astrocytes, while its effect was diminished in brainstem astrocytes of both models.

CONCLUSION

Both Ang II and ACEA-induced MAPK activation were significantly altered in SHR astrocytes when compared to Wistar astrocytes. A possible reduction in CB1R functionality, coupled with a hyperfunctional AT1R in the brainstem, could well be significant factors in the development of hypertensive states. AT1R-mediated phosphorylation of CB1R could be critical for impaired cerebellar development characterized by a hyperactive RAS.

Activation of β-adrenergic receptor promotes cellular proliferation in human glioblastoma

Glioblastoma multiforme is the most common and aggressive form of primary malignant brain tumor. Previous evidence demonstrates that β-adrenergic receptors (β-ARs) are closely associated with the occurrence and development of brain tumors. However, the functional role of β-ARs in human glioblastoma and the underlying mechanisms are not fully understood. In the present study, by using the MTT assay, western blotting, and the reverse transcription polymerase chain reaction, it was revealed that isoproterenol (ISO), an agonist of β-ARs, promoted the proliferation of U251 cells but not U87-MG cells, and that this effect was blocked by the β-ARs antagonist propranolol. It was also demonstrated that ISO transiently induced extracellular signal-related kinase 1/2 (ERK1/2) phosphorylation, and that blocking the mitogen-activated protein kinase pathway by U0126 inhibited ERK1/2 phosphorylation and suppressed U251 cell proliferation. In addition, β-ARs activation increased the expression of matrix metalloproteinase (MMP) family members MMP-2 and MMP-9 mRNA through ERK1/2 activation. In conclusion, these data suggest that β-ARs induce ERK1/2 phosphorylation, which may in turn increase MMPs expression to promote U251 cell proliferation. These results provide additional insight into the specific roles of β-ARs in glioblastoma.

The association of heart failure-related microRNAs with neurohormonal signaling

Heart failure (HF) is a widely prevalent syndrome imposing a significant burden of morbidity and mortality world-wide. Differential circulating microRNA profiles observed in HF cohorts suggest the diagnostic utility of microRNAs as biomarkers. Given their function in fine tuning gene expression, alternations in microRNA landscape could reflecting the underlying mechanisms of disease and present potential therapeutic targets. Using multiple computational target predicting algorithms together with the luciferase-based reporting platform, the interactions between HF-related microRNAs and the 3' untranslated regions (3'UTRs) of neurohormone associated genes were examined and compared. Our results indicate that although in silico prediction provides an overview of possible microRNA-mRNA target pairs, less than half of the predicted interactions were experimentally confirmed by reporter assays in HeLa cells. Thus, the establishment of microRNA/3'UTR reporters is essential to systemically evaluate the roles of microRNAs for signaling cascades of interest, including cardiovascular neurohormonal signaling. The physiological relevance of HF-related microRNAs on the expression of putative gene targets was further established by using gain-of-function assays in two human cardiac-derived cells. Our findings, for the first time, provide direct evidence of the regulatory effects of HF-related microRNAs on the neurohormonal signaling in cardiac cells. More importantly, our study presents a rational approach to further exploring microRNA profiling data in deciphering the role of microRNA in complex syndromes such as HF. This article is part of a Special Issue entitled: Genetic and epigenetic control of heart failure - edited by Jun Ren & Megan Yingmei Zhang.

Close association of B2 bradykinin receptors with P2Y2 ATP receptors

Two G-protein-coupled receptors (GPCRs) that couple with Gαq/11, B2 bradykinin (BK) receptor (B2R) and ATP/UTP receptor P2Y2 (P2Y2R), are ubiquitously expressed and responsible for vascular tone, inflammation, and pain. We analysed the cellular signalling of P2Y2Rs in cells that express B2Rs. B2R desensitization induced by BK or B2R internalization-inducing glycans cross-desensitized the P2Y2R response to ATP/UTP. Fluorescence resonance energy transfer from P2Y2R-AcGFP to B2R-DsRed was detected in the cells and on the cell surfaces, showing the close association of these GPCRs. BK- and ATP-induced cross-internalization of P2Y2R and B2R, respectively, was shown in a β-galactosidase complementation assay using P2Y2R or B2R fused to the H31R substituted α donor peptide of a β-galactosidase reporter enzyme (P2Y2R-α or B2R-α) with coexpression of the FYVE domain of endofin, an early endosome protein, fused to the M15 acceptor deletion mutant of β-galactosidase (the ω peptide, FYVE-ω). Arrestin recruitment to the GPCRs by cross-activation was also shown with the similar way. Coimmunoprecipitation showed that B2R and P2Y2R were closely associated in the cotransfected cells. These results indicate that B2R couples with P2Y2R and that these GPCRs act together to fine-tune cellular responsiveness. The collaboration between these receptors may permit rapid onset and turning off of biological events.

β-blockade abolishes the augmented cardiac tPA release induced by transactivation of heterodimerised bradykinin receptor-2 and β2-adrenergic receptor in vivo

Bradykinin (BK) receptor-2 (B2R) and β2-adrenergic receptor (β2AR) have been shown to form heterodimers in vitro. However, in vivo proofs of the functional effects of B2R-β2AR heterodimerisation are missing. Both BK and adrenergic stimulation are known inducers of tPA release. Our goal was to demonstrate the existence of B2R-β2AR heterodimerisation in myocardium and to define its functional effect on cardiac release of tPA in vivo. We further investigated the effects of a non-selective β-blocker on this receptor interplay. To investigate functional effects of B2R-β2AR heterodimerisation (i. e. BK transactivation of β2AR) in vivo, we induced serial electrical stimulation of cardiac sympathetic nerves (SS) in normal pigs that underwent concomitant BK infusion. Both SS and BK alone induced increases in cardiac tPA release. Importantly, despite B2R desensitisation, simultaneous BK infusion and SS (BK+SS) was characterised by 2.3 ± 0.3-fold enhanced tPA release compared to SS alone. When β-blockade (propranolol) was introduced prior to BK+SS, tPA release was inhibited. A persistent B2R-β2AR heterodimer was confirmed in BK-stimulated and non-stimulated left ventricular myocardium by immunoprecipitation studies and under non-reducing gel conditions. All together, these results strongly suggest BK transactivation of β2AR leading to enhanced β2AR-mediated release of tPA. Importantly, non-selective β-blockade inhibits both SS-induced release of tPA and the functional effects of B2R-β2AR heterodimerisation in vivo, which may have important clinical implications.

Characterization of a novel proinflammatory effect mediated by BK and the kinin B₂ receptor in human preadipocytes

Obesity and adipose tissue contribute to local and systemic inflammation. However the role of the inflammatory mediator bradykinin (BK) in this context is not known. We therefore evaluated the effect of BK on adipokines secretion in human preadipocytes during the course of differentiation and characterized the receptors involved. Results obtained from antibody array and ELISA experiments showed that several adipokines are released by human preadipocytes under basal conditions while BK specifically stimulated the production of interleukin(IL)-6 and IL-8. The effect of BK diminished with the progression of differentiation, being almost inactive on adipocytes. In preadipocytes, BK also induced a rapid and transient [Ca²⁺](i) mobilization, a rapid and sustained increase in ERK1/2 activation and enhanced forskolin-stimulated cAMP accumulation. BK was without effect on cell proliferation and viability as assessed by bromodeoxyuridine incorporation, WST-1 conversion, or lactate dehydrogenase leakage and was without effect on adipogenesis as measured by triglyceride accumulation, GPDH activity and leptin release. The B₁ receptor agonist, Lys-[des-Arg⁹]-BK, displayed poor activity or was without effect while overall BK effects were prevented by the selective B₂ receptor antagonist, fasitibant chloride, but not by the B₁ selective antagonist, Lys-[Leu⁸][des-Arg⁹]-BK. Immunoblot analysis and immunofluorescence studies showed that the kinin B₂ receptor was essentially expressed at the beginning of the differentiation program. In conclusion, human preadipocytes expressed kinin B₂ receptors linked to multiple signaling pathways, IL-6 and IL-8 production, and BK proinflammatory response in adipose tissue could be prevented by fasitibant chloride.

G protein-coupled receptor oligomerization for what?

Although the G protein-coupled receptor (GPCR) oligomerization has been questioned during the last decade, under some premises the existence of a supramolecular organization of these receptors begins now to be widely accepted by the scientific community. Indeed, GPCR oligomers may enhance the diversity and performance by which extracellular signals are transferred to the G proteins in the process of receptor transduction, although the mechanism that underlie this phenomenon remains still unexplained. Recently, a trans-conformational switching model has been proposed as a mechanism allowing direct inhibition of receptor activation. Thus, heterotropic receptor-receptor allosteric regulations are behind the GPCR oligomeric function. Accordingly, we revise here how GPCR oligomerization impinge in several important receptor functions like biosynthesis, plasma membrane diffusion or velocity, pharmacology and signaling. Overall, the rationale of receptor oligomerization might lie in the cellular need of sensing complex extracellular signals and to translate into a simple computational mode.

Bradykinin inhibits the transient outward K+ current in mouse Schwann cells via the cAMP/PKA pathway

Bradykinin (BK) is an endogenous peptide with diverse biological actions and is considered to be an important mediator of the inflammatory response in both the peripheral and the central nervous systems. BK has attracted recent interest as a potential mediator of K(+) conductance, Cl(-) channels, and Ca(2+)-activated K(+) channels. However, few reports have associated BK with the voltage-gated K(+) current. In this study, we demonstrated that BK suppressed the transient outward potassium current (I(A)) in mouse Schwann cells using whole cell recording techniques. At a concentration of 0.1 muM to 5 muM, BK reversibly inhibited I(A) in a dose-dependent manner with the modulation of steady-state activation and inactivation properties. The effect of BK on I(A) current was abolished after preincubation with a B(2) receptor antagonist but could not be eliminated by B(1) receptor antagonist. Intracellular application of GTP-gammaS induced an irreversible decrease in I(A), and the inhibition of G(s) using NF449 provoked a gradual augmentation in I(A) and eliminated the BK-induced effect on I(A,) while the G(i)/(o) antagonist NF023 did not. The application of forskolin or dibutyryl-cAMP mimicked the inhibitory effect of BK on I(A) and abolished the BK-induced effect on I(A). H-89, an inhibitor of PKA, augmented I(A) amplitude and completely eliminated the BK-induced inhibitory effect on I(A). In contrast, activation of PKC by PMA augmented I(A) amplitude. A cAMP assay revealed that BK significantly increased intracellular cAMP level. It is therefore concluded that BK inhibits the I(A) current in Schwann cells by cAMP/PKA-dependent pathways via activation of the B(2) receptor.

Membrane-type 1 matrix metalloproteinase stimulates cell migration through epidermal growth factor receptor transactivation

Proteolysis of extracellular matrix proteins by membrane-type 1 matrix metalloproteinase (MT1-MMP) plays a pivotal role in tumor and endothelial cell migration. In addition to its proteolytic activity, several studies indicate that the proinvasive properties of MT1-MMP also involve its short cytoplasmic domain, but the specific mechanisms mediating this function have yet to be fully elucidated. Having previously shown that the serum factor sphingosine 1-phosphate stimulates MT1-MMP promigratory function through a process that involves its cytoplasmic domain, we now extend these findings to show that this cooperative interaction is permissive to cellular migration through MT1-MMP-dependent transactivation of the epidermal growth factor receptor (EGFR). In the presence of sphingosine 1-phosphate, MT1-MMP stimulates EGFR transactivation through a process that is dependent upon the cytoplasmic domain of the enzyme but not its catalytic activity. The MT1-MMP-induced EGFR transactivation also involves G(i) protein signaling and Src activities and leads to enhanced cellular migration through downstream extracellular signal-regulated kinase activation. The present study, thus, elucidates a novel role of MT1-MMP in signaling events mediating EGFR transactivation and provides the first evidence of a crucial role of this receptor activity in MT1-MMP promigratory function. Taken together, our results suggest that the inhibition of EGFR may represent a novel target to inhibit MT1-MMP-dependent processes associated with tumor cell invasion and angiogenesis.

Activation of TRPM7 Channels by Phospholipase C-coupled Receptor Agonists

TRPM7 is a ubiquitously expressed nonspecific cation channel that has been implicated in cellular Mg(2+) homeostasis. We have recently shown that moderate overexpression of TRPM7 in neuroblastoma N1E-115 cells elevates cytosolic Ca(2+) levels and enhances cell-matrix adhesion. Furthermore, activation of TRPM7 by phospholipase C (PLC)-coupled receptor agonists caused a further increase in intracellular Ca(2+) levels and augmented cell adhesion and spreading in a Ca(2+)-dependent manner (1). Regulation of the TRPM7 channel is not well understood, although it has been reported that PIP(2) hydrolysis closes the channel. Here we have examined the regulation of TRPM7 by PLC-coupled receptor agonists such as bradykinin, lysophosphatidic acid, and thrombin. Using FRET assays for second messengers, we have shown that the TRPM7-dependent Ca(2+) increase closely correlates with activation of PLC. Under non-invasive "perforated patch clamp" conditions, we have found similar activation of TRPM7 by PLC-coupled receptor agonists. Although we could confirm that, under whole-cell conditions, the TRPM7 currents were significantly inhibited following PLC activation, this PLC-dependent inhibition was only observed when [Mg(2+)](i) was reduced below physiological levels. Thus, under physiological ionic conditions, TRPM7 currents were activated rather than inhibited by PLC-activating receptor agonists.

The thyroid disruptor 1,1,1-trichloro-2,2-bis(p-chlorophenyl)-ethane appears to be an uncompetitive inverse agonist for the thyrotropin receptor

In this study, we aimed at establishing whether two previously identified thyroid disruptors, the insecticide 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) and Aroclor 1254 (a complex mixture of polychlorinated water), may inhibit thyrotropin (TSH) receptor (TSHr) activity. DDT and Aroclor 1254 were shown to inhibit both the basal and bovine TSH (bTSH)-stimulated accumulation of cAMP in Chinese hamster ovary (CHO)-K1 cells stably transfected with the TSHr. Furthermore, both DDT and Aroclor 1254 did indeed prevent cAMP accumulation, as induced by the constitutive activity of a point mutant TSHr(I486M) transiently transfected in African green monkey kidney fibroblast (COS)-7 cells. Neither trypsin digestion of the extracellular domain (ECD) nor deletion of the ECD in a mutant TSHr trunk transiently transfected in COS-7 cells counteracted the inhibitory activity of DDT and Aroclor 1254. DDT exerted a weak inhibitory activity against forskolin in both CHO-K1 and COS-7 cells, whereas it was nil against the agonists dopamine and 5'-(N-ethyl-carboxamido)-adenosine (NECA) in CHO cells stably transfected with the dopamine D1 receptor and in COS-7 cells transiently transfected with the adenosine type 2a receptor (A2a) receptor. Furthermore, DDT was inactive against the stimulation by isoproterenol of the endogenously expressed beta2 adrenergic receptor in COS-7 cells. Conversely, Aroclor 1254 inhibited completely forskolin activity in CHO-K1 cells but not in COS-7 cells. Furthermore, it did not prevent accumulation of cAMP as induced by NECA in A2a transfected cells. The analog of DDT, diphenylethylene, was inactive against bTSH-induced increase in cAMP in CHO-K1 cells stably transfected with the TSHr. We interpreted these results as indicating that DDT and possibly Aroclor 1254 may have an uncompetitive inverse agonist activity for the TSHr.

Ghrelin Amplifies Dopamine Signaling by Cross Talk Involving Formation of Growth Hormone Secretagogue Receptor/Dopamine Receptor Subtype 1 Heterodimers

Our objective is to determine the neuromodulatory role of ghrelin in the brain. To identify neurons that express the ghrelin receptor [GH secretagogue receptor (GHS-R)], we generated GHS-R-IRES-tauGFP mice by gene targeting. Neurons expressing the GHS-R exhibit green fluorescence and are clearly evident in the hypothalamus, hippocampus, cortex, and midbrain. Using immunohistochemistry in combination with green fluorescent protein fluorescence, we identified neurons that coexpress the dopamine receptor subtype 1 (D1R) and GHS-R. The potential physiological relevance of coexpression of these two receptors and the direct effect of ghrelin on dopamine signaling was investigated in vitro. Activation of GHS-R by ghrelin amplifies dopamine/D1R-induced cAMP accumulation. Intriguingly, amplification involves a switch in G protein coupling of the GHS-R from Gα11/q to Gαi/o by a mechanism consistent with agonist-dependent formation of GHS-R/D1R heterodimers. Most importantly, these results indicate that ghrelin has the potential to amplify dopamine signaling selectively in neurons that coexpress D1R and GHS-R.

Activated EGF receptor may balance ERK-inhibitory network signalling pathways

In the COS-7 cell signalling network high levels of cAMP produced, for example, by co-stimulation of beta2-adrenergic receptor (beta2-AR) and bradykinin B2 receptor (BKR) may affect epidermal growth factor receptor (EGFR)-mediated activation of extracellular signal-stimulated kinase (ERK). In contrast, co-stimulation of either beta2-AR or B2R with EGFR leads to synergistic activation of ERK. Due to triple stimulation of these receptors the synergistic effects on ERK activation as well as cAMP accumulation are diminished. Here we demonstrate that EGF is capable of inducing Src-mediated phosphorylation of the tyrosine residues 177 and 347 of BKR. Their replacement by phenylalanine led to BKR mutants which are unable to activate the cAMP pathway. Using these mutants we can show that EGF attenuates but does not completely inhibit the BKR/cAMP pathway which is counteracting the EGFR signalling to ERK. Our findings suggest that the EGFR may control the cellular network rather by balancing mechanisms then by switch on/off reactions.

Involvement of leukotriene B4 receptor 1 signaling in platelet-activating factor-mediated neutrophil degranulation and chemotaxis

Platelet-activating factor (PAF) is a potent lipid mediator of inflammation that can act on human neutrophils. When neutrophils are stimulated with PAF at concentrations greater than 10 nM, a double peak of intracellular calcium mobilization is observed. The second calcium peak observed in PAF-treated neutrophils has already been suggested to come from the production of endogenous leukotriene B4 (LTB4). Here we demonstrate the involvement of endogenous LTB4 production and subsequent activation of the high affinity LTB4 receptor (BLT1) in this second calcium mobilization peak observed after stimulation with PAF. We also show that the second, but not the first peak, could be desensitized by prior exposure to LTB4. Moreover, when neutrophils were pre-treated with pharmacological inhibitors of LTB4 production or with the specific BLT1 antagonist, U75302, PAF-mediated neutrophil degranulation was inhibited by more than 50%. On the other hand, pre-treating neutrophils with the PAF receptor specific antagonist (WEB2086) did not prevent any LTB4-induced degranulation. Also, when human neutrophils were pre-treated with U75302, PAF-mediated chemotaxis was reduced by more than 60%. These data indicate the involvement of BLT1 signaling in PAF-mediated neutrophil activities.

Adenosine signaling promotes neuronal, catecholaminergic differentiation of primary neural crest cells and CNS-derived CAD cells

In neural crest (NC) cultures cAMP signaling is an instructive signal in catecholaminergic, sympathoadrenal cell development. However, the extracellular signals activating the cAMP pathway during NC cell development have not been identified. We demonstrate that in avian NC cultures, evidenced by tyrosine hydroxylase expression and catecholamine biosynthesis, adenosine and not adrenergic signaling, together with BMP2, promotes sympathoadrenal cell development. In NC cultures, addition of the adenosine receptor agonist NECA in the presence of BMP2 promotes sympathoadrenal cell development, whereas the antagonist CGS 15943 or the adenosine degrading enzyme adenosine deaminase (ADA) suppresses TH expression. Importantly, NC cells express A2A and A2B receptors which couple with Gsalpha increasing intracellular cAMP. Employing the CNS-derived catecholaminergic CAD cell line, we also demonstrate that neuronal differentiation mediated by serum withdrawal is further enhanced by treatment with IBMX, a cAMP-elevating agent, or the adenosine receptor agonist NECA, acting via cAMP. By contrast, the adenosine receptor antagonist CGS 15943 or the adenosine degrading enzyme ADA inhibits CAD cell neuronal differentiation mediated by serum withdrawal. These results support that adenosine is a physiological signal in neuronal differentiation of the CNS-derived catecholaminergic CAD cell line and suggest that adenosine signaling is involved in NC cell development in vivo.

High-level activation of cyclic AMP signaling attenuates bone morphogenetic protein 2-induced sympathoadrenal lineage development and promotes melanogenesis in neural crest cultures

The intensity of cyclic AMP (cAMP) signaling is a differential instructive signal in neural crest (NC) cell specification. By an unknown mechanism, sympathoadrenal lineage specification is suppressed by high-level activation of cAMP signaling. In NC cultures, high-level activation of cAMP signaling mediates protein kinase A (PKA)-dependent Rap1-B-Raf-ERK1/2 activation, leading to cytoplasmic accumulation of phospho-Smad1, thus terminating bone morphogenetic protein 2 (BMP2)-induced sympathoadrenal cell development. Concurrently, cAMP signaling induces transcription of the melanocyte-determining transcription factor Mitf and melanogenesis. dnACREB and E1A inhibit Mitf expression and melanogenesis, supporting the notion that CREB activation is necessary for melanogenesis. However, constitutively active CREB(DIEDML) without PKA activation is insufficient for Mitf expression and melanogenesis, indicating PKA regulates additional aspects of Mitf transcription. Thus, high-level activation of cAMP signaling plays a dual role in NC cell differentiation: attenuation of BMP2-induced sympathoadrenal cell development and induction of melanogenesis. We conclude the intensity of activation of signal transduction cascades determines cell lineage segregation mechanisms.

Bradykinin B2 and GPR100 receptors: a paradigm for receptor signal transduction pharmacology

The aim of the present report was to investigate the ligand selectivity of the human orphan G-protein-coupled receptor GPR100 (hGPR100), recently identified as a novel bradykinin (BK) receptor, as compared with that of the human B(2) receptor (hB(2)R) stably transfected in Chinese hamster ovary cells. BK was able to inhibit the cAMP production induced by forskolin with a potency 100-fold lower at the hGPR100 (pEC(50) = 6.6) than that measured at the hB(2)R (pEC(50) = 8.6). Both effects were inhibited by the B(2) receptor antagonist Icatibant (1 microM). The nonpeptide B(2) receptor agonist FR190997 (8-[2,6-dichloro-3-[N-methylcarbamoyl)cinnamidoacetyl]-N-methylamino]benzyloxy]-2-methyl-4-(2-pyridylmethoxy)quinoline) did inhibit the forskolin-induced cAMP production (pEC(50) = 7.7) at the hB(2)R, whereas it was not able to exert any effect at the hGPR100. The human insulin-like peptide relaxin 3 did inhibit the cAMP production at the hGPR100 (pEC(50) = 7.3) at a greater extent than BK, and was devoid of any effect at the hB(2)R. FR190997 and relaxin 3 responses at the hB(2)R and hGPR100, respectively, were not inhibited by Icatibant (1 microM). These data indicate FR190997 and relaxin 3 as selective agonists for hB(2)R and hGPR100, respectively, and support the concept that different agonists may specifically bias the conformational states of a receptor to result in a final common G protein coupling, which is differentially recognized by antagonists.

Inhibiting cytokines of the interleukin-12 family: recent advances and novel challenges

Interleukin-12 (IL-12) and the more recently discovered IL-23 and IL-27 constitute a unique family of structurally related, heterodimeric cytokines that regulate cell-mediated immune responses and T helper 1 (Th1)-type inflammatory reactions. Not surprisingly, the potentiality of treating conditions such as multiple sclerosis (MS) and rheumatoid arthritis (RA) through pharmacological interference with IL-12 pathways has received widespread attention. In this review we have examined over 50 substances with reported IL-12 inhibitory effects. We demonstrate that a majority of these belong to a limited number of major functional classes, each of which targets discrete events in the IL-12 biological pathway. Thus, most IL-12 inhibitory substances appear to work either through inhibition of transcription factor NF-kappa B activation, up-regulation of intracellular cAMP, blockage of posttranslational processing or interference with signal transduction pathways. In addition, cyclophilin-binding drugs, and generic inhibitors of nuclear histone deacetylases, and of ion channels, pumps and antiporters are emerging as potential leads to novel targets for interference with IL-12 production. Many inhibitors of NF-kappa B and of IL-12 signal transduction have been proven effective in limiting or preventing disease in experimental autoimmune encephalomyelitis (EAE) models of MS. The sharing of the p40 subunit, the IL-12R beta 1 and components of the signal transduction pathways between IL-12 and IL-23 raises the question as to whether the beneficial effects of various drugs previously ascribed to inhibition of IL-12 may, in fact, have been due to concurrent blockage of both cytokines, or of IL-23, rather than IL-12. Moreover, the homodimeric beta(2)-form of IL-12, though originally considered to display only antagonistic effects, is now emerging as a pronounced agonist in a variety of inflammatory processes. Reassessment of IL-12 inhibitory compounds is therefore needed to scrutinize their effects on IL-12 alpha beta, beta(2) and IL-23 formation. This is likely to open exciting perspectives to the identification of drugs that target these cytokines either indiscriminately or selectively. The functional diversity of presently available inhibitors should facilitate an unprecedented flexibility in designing future trials for the treatment of IL-12- and IL-23-mediated disorders.

The in vitro effects of H-89, a specific inhibitor of protein kinase A, in the human colonic carcinoma cell line Caco-2

SUMMARY

H-89 is a compound characterized in vitro as a potent and selective inhibitor of protein kinase A. In the present study, we observed that H-89 induced morphological transformation and caused growth inhibition of the human colon cancer cell line Caco-2 in a dose-dependent manner. However, another protein kinase A inhibitor, H-8, had no effect on Caco-2 cells. To evaluate the possible molecular mechanism of H-89-evoked effects in Caco-2 cells, we analysed the capacity of H-89 to regulate the protein kinase B (Akt/PKB) signalling pathway. H-89 treatment led to an activation of Akt/PKB in Caco-2 cells. This activation was phosphatidylinositol 3 (PI3)-kinase-dependent and promoted survival of Caco-2 cells because the PI3 kinase inhibitor LY294002 inhibited the Akt/PKB activation and induced apoptosis of Caco-2 cells. To test whether Akt/PKB activity promoted resistance to H-89-induced effects, LY294002 was added in combination with H-89. LY294002 greatly potentiated the H-89-induced growth inhibition and apoptosis of Caco-2 cells. These results suggest that the H-89-induced growth inhibition of Caco-2 cells is associated with phosphorylation of Akt/PKB protein and that the cells become more sensitive to H-89 and die by apoptosis upon inhibition of the PI3K/Akt pathway.

[Cooperation of two subtypes of PGE2 receptor, Gi coupled EP3 and Gs coupled EP2 or EP4 subtype]

Four prostaglandin E (EP) receptor subtypes have been identified and cloned, designated as EP1, EP2, EP3 and EP4. These EP receptors are members of the G-protein coupled receptor family. EP3 receptor signals are primarily involved in inhibition of adenylyl cyclase via Gi activation, while EP2 and EP4 receptor signals cause a stimulation of adenylyl cyclase via Gs activation. Immune cells, such as mast cells, express multiple EP subtypes on their cell membranes, but few studies have been conducted to understand exactly what signals the main flow for the multiple subtypes expressing immune cells. We previously demonstrated that activation of Gi-coupled EP3 receptor exhibited a cooperative effect on cAMP synthesis induced by Gs-coupled EP2 receptor in COS-7 cells. Here we report that a selective EP4 agonist-induced adenylyl cyclase activity was augmented by simultaneous addition of a selective EP3 agonist in mastocytoma P-815 cells, which express mRNAs for both EP3 and EP4 subtypes. The augmentation in cAMP synthesis was found to be pertussis toxin-sensitive. P-815 cells are demonstrated to bind to Pronectin-F, a proteolytic fragment of fibronectin, in adhesion protein of the extracellular matrix, by addition of PGE2, which is mediated by PKA. The binding of P-815 cells to Pronectin-F mediated by EP4 receptor was augmented by the EP3 receptor. These findings indicate that two subtypes of PGE2 receptors, EP3 and EP4, cooperatively activate the cAMP-mediated adhesion event through induction of fibronectin ligand elicited by PGE2 in P-815 cells. Furthermore, the PGE2-induced adhesion response may contribute to the mast cell recruitment function on extracellular matrix during inflammation.

Constitutively active Gq/11-coupled receptors enable signaling by co-expressed G(i/o)-coupled receptors

Co-expression of guanine nucleotide-binding regulatory (G) protein-coupled receptors (GPCRs), such as the G(i/o)-coupled human 5-hydroxytryptamine receptor 1B (5-HT(1B)R), with the G(q/11)-coupled human histamine 1 receptor (H1R) results in an overall increase in agonist-independent signaling, which can be augmented by 5-HT(1B)R agonists and inhibited by a selective inverse 5-HT(1B)R agonist. Interestingly, inverse H1R agonists inhibit constitutively H1R-mediated as well as 5-HT(1B)R agonist-induced signaling in cells co-expressing both receptors. This phenomenon is not solely characteristic of 5-HT(1B)R; it is also evident with muscarinic M2 and adenosine A1 receptors and is mimicked by mastoparan-7, an activator of G(i/o) proteins, or by over-expression of Gbetagamma subunits. Likewise, expression of the G(q/11)-coupled human cytomegalovirus (HCMV)-encoded chemokine receptor US28 unmasks a functional coupling of G(i/o)-coupled CCR1 receptors that is mediated via the constitutive activity of receptor US28. Consequently, constitutively active G(q/11)-coupled receptors, such as the H1R and HCMV-encoded chemokine receptor US28, constitute a regulatory switch for signal transduction by G(i/o)-coupled receptors, which may have profound implications in understanding the role of both constitutive GPCR activity and GPCR cross-talk in physiology as well as in the observed pathophysiology upon HCMV infection.

Transactivation joins multiple tracks to the ERK/MAPK cascade

Many agonists of G-protein-coupled receptors (GPCRs) can stimulate receptor tyrosine kinases and the extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) pathway. A 'transactivation' mechanism, which links these events in one signalling chain, inspired many researchers, but inevitably raised new questions. A 'multi-track' model for GPCR signalling to the ERK/MAPK pathway might resolve some of the puzzles in the transactivation field.

Induction of adherent activity in mastocytoma P-815 cells by the cooperation of two prostaglandin E2 receptor subtypes, EP3 and EP4

In this study, we investigated the role of PGE(2) in mouse mastocytoma P-815 cell adhesion to extracellular matrix proteins (ECMs) in vitro. We report that PGE(2) accelerated ProNectin F(TM) (a proteolytic fragment of fibronectin)-mediated adhesion, which was abolished by addition of the GRGDS peptide, an inhibitor of the RDG binding site of ProNectin F(TM). We show that the cAMP level and cAMP-regulated protein kinase (PKA) activity are critical mediators of this PGE(2) effect, because the cell-permeable cAMP analogue 8-Br-cAMP accelerated P-815 cell adhesion to ProNectin F(TM) and the pharmacological inhibitor of PKA, H-89, blocked PGE(2)-mediated adhesion. Consistent with mRNA expression of the G(s)-coupled EP4- and G(i)-coupled EP3-PGE receptor subtypes, P-815 cell adhesion was accelerated by treatment with a selective EP4 agonist, ONO-AE1-329, but not a selective EP1/EP3 agonist, sulprostone. However, simultaneous treatment with ONO-AE1-329 and sulprostone resulted in augmentation of both the cAMP level and cell adhesion. The augmentation of EP3-mediated cAMP synthesis was dose-dependent, without affecting the half-maximal concentration for EP4-mediated G(s)-activity, which was inhibited by a G(i) inhibitor, pertussis toxin. In conclusion, these findings suggest that PGE(2) accelerates RGD-dependent adhesion via cooperative activation between EP3 and EP4 and contributes to the recruitment of mast cells to the ECM during inflammation.

Adenosine A1 receptor agonist treatment up-regulates rat brain metabotropic glutamate receptors

Chronic R-N(6)-phenylisopropiladenosine (R-PIA) subcutaneous injection for 6 days significantly increased total glutamate receptor number (180% of control) in rat brain synaptic plasma membranes (SPM), without affecting receptor affinity. A higher increase in metabotropic glutamate (mGlu) receptor number (258% of control) was also detected, indicating that mGlu is the main type of glutamate receptor affected by this treatment. On the other hand, the observed increase in basal and calcium- and Gpp(NH)p-stimulated phospholipase C (PLC) activity after treatment was associated with a significant increase in PLC beta(1) isoform, detected in SPM by immunoblotting assays. Moreover, an increase in PLC activity stimulation with trans-ACPD, in the absence and in the presence of Gpp(NH)p, was detected after R-PIA treatment. These results show that mGlu receptors and its effector system, PLC activity, are up-regulated by chronic exposure to an adenosine A(1) receptor agonist and suggest the existence of a cross-talk mechanism between both signal transduction pathways in rat brain.