Suppression of Programmed Cell Death by Intracellular cAMP Is not Mediated by Expression of Genes Encoding an Inhibitor of Apoptosis

The elevation of intracellular cAMP content is accompanied by expression of genes whose promoter contains a Ca²⁺-cAMP responsive element. In vascular smooth muscle cells (VSMC), activation of cAMP signaling blocks apoptosis triggered by serum deprivation. In the present study we investigated the role of gene expression in the inhibition of apoptosis by cAMP. In VSMC transfected with E1A adenovirus, incubation in the absence of serum for 6 h led to 20-fold elevation of chromatin fragmentation and 10-fold activation of caspase-3 activity, these being employed as markers of apoptosis. Forskolin induced activation of cAMP signaling was accompanied by 50% elevation of RNA synthesis and completely abolished the development of apoptosis during the initial 6 h incubation in growth factor-free medium. In 12 h apoptosis in forskolin-treated VSMC was slowly developed and after 24 h the content of chromatin fragments was 2-fold less than in control cells. Addition of actinomycin D and cycloheximide completely blocked RNA synthesis and decreased protein synthesis by 80%, respectively. Neither compound affected baseline apoptosis or its inhibition by forskolin. More than 70 newly phosphorylated proteins were observed by 2D-electrophoresis of VSMC after incubation with forskolin for 3 h; in 24 h the number of phosphoproteins triggered by forskolin was decreased by 2-3-fold. These results show that suppression of VSMC apoptosis under activation of cAMP signaling is mediated via posttranslational modification of pre-existing intermediates of the apoptotic machinery rather than by de novo synthesis of inhibitors of programmed cell death.     

Characterization of an orphan G protein-coupled receptor, GPR20, that constitutively activates Gi proteins

GPR20 was isolated as an orphan G protein-coupled receptor from genomic DNA by PCR amplification. Although GPR20 was closely related to nucleotide or lipid receptors, the functional role of this receptor, as well as its endogenous ligand, remains unclear. Here we demonstrate that GPR20 is constitutively active in the absence of ligand, leading to continuous activation of its coupled G proteins. When GPR20 was exogenously expressed in HEK293 cells, both the basal level and the prostaglandin E(2)-induced production of cAMP were significantly decreased. A remarkable increase in [(35)S]guanosine 5'-(gamma-thio)triphosphate (GTPgammaS) binding to membrane preparations was also observed in GPR20-expressing cells. These effects of GPR20 overexpression were diminished in cells treated with pertussis toxin, suggesting that the expression of GPR20 results in the activation of G(i/o) proteins. Involvement of GPR20 in the activation of G(i/o) proteins was also supported by evidence that the disruption of a conserved DRY motif in GPR20 attenuated both [(35)S]GTPgammaS incorporation and inhibition of the prostaglandin E(2)-induced cAMP production. Knockdown of GPR20 in PC12h cells resulted in an elevation of the basal cAMP level, suggesting that the endogenous GPR20 achieves a constitutively or spontaneously active conformation. Furthermore, enhancement of [(3)H]thymidine incorporation was also observed in the GPR20-silencing cells, implying that the GPR20 expression seems to attenuate PC12h cell growth. Taken together, these data indicate that GPR20 constitutively activates G(i) proteins without ligand stimulation. The receptor may be involved in cellular processes, including control of intracellular cAMP levels and mitogenic signaling.     

Neural expression of G protein-coupled receptors GPR3, GPR6, and GPR12 up-regulates cyclic AMP levels and promotes neurite outgrowth

Cyclic AMP regulates multiple neuronal functions, including neurite outgrowth and axonal regeneration. GPR3, GPR6, and GPR12 make up a family of constitutively active G protein-coupled receptors (GPCRs) that share greater than 50% identity and 65% similarity at the amino acid level. They are highly expressed in the central nervous system, and their expression in various cell lines results in constitutive stimulation of cAMP production. When the constitutively active GPCRs were overexpressed in rat cerebellar granule neurons in culture, the transfected neurons exhibited significantly enhanced neurite outgrowth and overcame growth inhibition caused by myelin-associated glycoprotein. GPR12-mediated neurite outgrowth was the most prominent and was shown to depend on G(s) and cAMP-dependent protein kinase. Moreover, the GPR12-mediated rescue from myelin-associated glycoprotein inhibition was attributable to cAMP-dependent protein kinase-mediated inhibition of the small GTPase, RhoA. Among the three receptors, GPR3 was revealed to be enriched in the developing rat cerebellar granule neurons. When the endogenous GPR3 was knocked down, significant reduction of neurite growth was observed, which was reversed by expression of either GPR3 or GPR12. Taken together, our results indicate that expression of the constitutively active GPCRs up-regulates cAMP production in neurons, stimulates neurite outgrowth, and counteracts myelin inhibition. Further characterization of the GPCRs in developing and injured mammalian neurons should provide insights into how basal cAMP levels are regulated in neurons and could establish a firm scientific foundation for applying receptor biology to treatment of various neurological disorders.     

Tissue distribution and functional analyses of the constitutively active orphan G protein coupled receptors, GPR26 and GPR78

GPR26 and GPR78 are orphan GPCRs (oGPCRs) that share 51% amino acid sequence identity and are widely expressed in selected tissues of the human brain as well as the developing and adult mouse brain. Investigation of the functional activity of GPR26 and GPR78 via expression in HEK293 cells showed that both proteins are constitutively active and coupled to elevated cAMP production. Accordingly, in yeast, GPR26 demonstrated apparent agonist-independent coupling to a chimeric Gpa1 protein in which the 5 C-terminal amino acids were from Galphas. A comparison of the proteins revealed an atypical glutamine residue in GPR78 in place of the conserved arginine residue (R3.50) in the so-called DRY box. Site-directed mutants R3.50 in GPR26 were constructed and retained their constitutive activity suggesting that these 2 receptors activate G proteins in a manner that is distinct from other group 1 GPCRs.     

Termination of immediate-early gene expression after stimulation by parathyroid hormone or isoproterenol

cAMP/PKA signaling transientlystimulates mRNA expression of immediate-early genes, including IL-6 andc-fos. We confirmed that these mRNAs are transientlystimulated by parathyroid hormone (PTH) in ROS 17/2.8 osteoblasticcells. Consistent with the role for cAMP/PKA signaling in thisresponse, PTH induces transient cAMP elevation, PKA activation, andcAMP-responsive element-binding protein (CREB) phosphorylation. Ourgoal was to determine whether termination of immediate-early geneexpression is due to receptor desensitization or cAMP degradation. Theapproaches used were 1) inhibition of PTH receptordesensitization with G protein-coupled receptor kinase 2 (GRK2)antisense oligonucleotides or antisense plasmids, 2)sustained activation of adenyl cyclase with forskolin, and3) inhibition of cAMP degradation with3-isobutyl-1-methylxanthine. These experiments show that mechanismsdownstream of receptor desensitization and cAMP degradation areprimarily responsible for termination of PKA activity, CREBphosphorylation, and immediate-early gene expression. Similarconclusions were also obtained in response to PTH in a secondosteoblastic cell line (MC3T3-E1) and in response to isoproterenol inNIH3T3 fibroblasts. This conclusion may therefore reflect a generalmechanism for termination of immediate-early gene expression afterinduction by cAMP/PKA.

     

Obestatin does not activate orphan G protein-coupled receptor GPR39

Recently, the ligand of the orphan G protein-coupled receptor GPR39 has been identified as obestatin, a 23-amino acid peptide derived from the ghrelin precursor protein. We used two methods to study the possible activation of GPR39 by obestatin: cAMP measurements based on a luminescent reporter gene and a fluorometric Ca(2+) flux method. The former was similar to that reported in the original publication of Zhang et al. [J.V. Zhang, P.G. Ren, O. Avsian-Kretchmer, C.W. Luo, R. Rauch, C. Klein, Obestatin, a peptide encoded by the ghrelin gene, opposes ghrelin's effects on food intake, Science 310 (2005) 996-999]. The latter method used promiscuous as well as chimaeric G-proteins commonly used to couple orphan G protein-coupled receptors to the phospholipase C pathway, that leads to intracellular Ca(2+) rise. We could, however, not demonstrate activation of the GPR39 receptor by obestatin via any of these signal transduction pathways. We could activate GPR39 by high concentrations of Zn(2+), demonstrating cell surface expression of a functional receptor that could elicit a Ca(2+) response. The Zn(2+) response was not affected by obestatin. The identity of the native ligand for GPR39 remains to be elucidated.     

GPR88 Reveals a Discrete Function of Primary Cilia as Selective Insulators of GPCR Cross-Talk

A number of G protein-coupled receptors (GPCRs) localize to primary cilia but the functional significance of cilia to GPCR signaling remains incompletely understood. We investigated this question by focusing on the D1 dopamine receptor (D1R) and beta-2 adrenergic receptor (B2AR), closely related catecholamine receptors that signal by stimulating production of the diffusible second messenger cyclic AMP (cAMP) but differ in localization relative to cilia. D1Rs robustly concentrate on cilia of IMCD3 cells, as shown previously in other ciliated cell types, but disrupting cilia did not affect D1R surface expression or ability to mediate a concentration-dependent cAMP response. By developing a FRET-based biosensor suitable for resolving intra- from extra- ciliary cAMP changes, we found that the D1R-mediated cAMP response is not restricted to cilia and extends into the extra-ciliary cytoplasm. Conversely the B2AR, which we show here is effectively excluded from cilia, also generated a cAMP response in both ciliary and extra-ciliary compartments. We identified a distinct signaling effect of primary cilia through investigating GPR88, an orphan GPCR that is co-expressed with the D1R in brain, and which we show here is targeted to cilia similarly to the D1R. In ciliated cells, mutational activation of GPR88 strongly reduced the D1R-mediated cAMP response but did not affect the B2AR-mediated response. In marked contrast, in non-ciliated cells, GPR88 was distributed throughout the plasma membrane and inhibited the B2AR response. These results identify a discrete ‘insulating’ function of primary cilia in conferring selectivity on integrated catecholamine signaling through lateral segregation of receptors, and suggest a cellular activity of GPR88 that might underlie its effects on dopamine-dependent behaviors.     

The effect of ICER on screening methods involving CRE-mediated reporter gene expression

We describe a mechanism whereby increasing levels of cAMP in Chinese hamster ovary (CHO) and other cell lines lead to a significant repression in cAMP response element (CRE)-mediated luciferase reporter gene expression. This effect was shown to be mediated by a modulatory factor located downstream of cyclic AMP (cAMP), which displayed the temporal regulation pattern of an immediate early gene. The expression of this inducible cAMP early repressor (ICER) was shown to be coincident with the time and concentration dependency of the repression of CRE-mediated luciferase gene expression on the treatment of CHO cells with forskolin. Furthermore, this phenomenon was also observed in JEG and GH3 cell lines (both previously reported to express ICER), but not in COS-7 cells, which do not express ICER. These studies suggest that, in certain cell lines, expression of ICER can be induced at pharmacologically elevated cAMP levels, leading to a potent inhibition of CRE-mediated gene expression. We therefore conclude that screening methodologies employing such CRE-linked reporter genes (particularly in high-throughput screening assays) may produce false functional responses in certain cell lines. Moreover, such effects are likely to be exacerbated in screening assays in which receptors either are overexpressed or high concentrations of potent cAMP-elevating compounds are used.     

Elevated cAMP is required for stimulation of eicosanoid synthesis by interleukin 1 and bradykinin in BALB/c 3T3 fibroblasts.

In Swiss 3T3 murine fibroblasts, interleukin 1 (IL-1) and bradykinin stimulate prostaglandin E2 (PGE2) synthesis. However, in the present study, we found that neither agonist stimulated PGE2 synthesis in BALB/c 3T3 murine fibroblasts, this in spite of expression of similar numbers of receptors for each agonist compared to Swiss 3T3 cells. When BALB/c 3T3 cells were preincubated with cAMP analogs, both IL-1 and bradykinin stimulated PGE2 synthesis to levels similar to those observed in Swiss 3T3 cells. Similarly, when the cells were preincubated with forskolin, which activates the catalytic subunit of adenylate cyclase directly, or NECA, which stimulates cellular cAMP accumulation by activating adenosine receptors, IL-1 and bradykinin stimulated PGE2 synthesis. Rp-cAMPS, an inhibitor of cAMP-dependent protein kinase, blocked the ability of cAMP or NECA to render cells responsive to IL-1 and bradykinin. In basal BALB/c 3T3 cells, bradykinin and IL-1 stimulated arachidonate release in the absence of cAMP, but little conversion of released arachidonate to PGE2 occurred. cAMP, forskolin, and NECA all increased cyclooxygenase activity in the cells. SV-T2 is a clonal line originating from BALB/c 3T3 transformed with SV-40. In these cells, IL-1 and bradykinin stimulated PGE2 synthesis despite basal intracellular cAMP concentrations similar to BALB/c, and cAMP only modestly potentiated the response. In summary, cyclooxygenase expression appears to be regulated by cAMP in BALB/c 3T3 cells, and SV-40 transformation results in increased cyclooxygenase expression, apparently independent of cAMP.     

Elevated cyclic-AMP represses expression of exchange protein activated by cAMP (EPAC1) by inhibiting YAP-TEAD activity and HDAC-mediated histone deacetylation

Ligand-induced activation of Exchange Protein Activated by cAMP-1 (EPAC1) is implicated in numerous physiological and pathological processes, including cardiac fibrosis where changes in EPAC1 expression have been detected. However, little is known about how EPAC1 expression is regulated. Therefore, we investigated regulation of EPAC1 expression by cAMP in cardiac fibroblasts.Elevation of cAMP using forskolin, cAMP-analogues or adenosine A2B-receptor activation significantly reduced EPAC1 mRNA and protein levels and inhibited formation of F-actin stress fibres. Inhibition of actin polymerisation with cytochalasin-D, latrunculin-B or the ROCK inhibitor, Y-27632, mimicked effects of cAMP on EPAC1 mRNA and protein levels. Elevated cAMP also inhibited activity of an EPAC1 promoter-reporter gene, which contained a consensus binding element for TEAD, which is a target for inhibition by cAMP. Inhibition of TEAD activity using siRNA-silencing of its co-factors YAP and TAZ, expression of dominant-negative TEAD or treatment with YAP-TEAD inhibitors, significantly inhibited EPAC1 expression. However, whereas expression of constitutively-active YAP completely reversed forskolin inhibition of EPAC1-promoter activity it did not rescue EPAC1 mRNA levels. Chromatin-immunoprecipitation detected a significant reduction in histone3-lysine27-acetylation at the EPAC1 proximal promoter in response to forskolin stimulation. HDAC1/3 inhibition partially reversed forskolin inhibition of EPAC1 expression, which was completely rescued by simultaneously expressing constitutively active YAP.Taken together, these data demonstrate that cAMP downregulates EPAC1 gene expression via disrupting the actin cytoskeleton, which inhibits YAP/TAZ-TEAD activity in concert with HDAC-mediated histone deacetylation at the EPAC1 proximal promoter. This represents a novel negative feedback mechanism controlling EPAC1 levels in response to cAMP elevation.     

Modulation of ICAM-1 expression by α-MSH in human melanoma cells and melanocytes

α-MSH, a proopiomelanocortin (POMC)-derived peptide, is known to be produced in the pituitary, the skin, and melanoma tumors and to possess many biological effects, mainly on melanocyte pigmentation and growth. Moreover, the melanocyte expresses adhesion molecules, including ICAM-1. The latter has been reported to play a role in melanoma spread and associated metastatic process. We conducted a study in order to evaluate the possible effect of MSH on ICAM-1 expression in human cultured malignant and normal melanocytes. Our data show that α-MSH inhibits ICAM-1 expression stimulated by TNF in a concentration-dependent manner, both at the protein and gene expression level. Ninety percent inhibition was obtained with 10 nM MSH, while 50% inhibition was achieved with 1 nM. Endogenous cAMP elevation with forskolin as well as an exogenous cAMP stable analogue (Sp-cAMPS) produced the same inhibitory effect. A screening of malignant melanocytes showed that inhibition of ICAM-1 expression could be achieved only in those cells expressing detectable MSH receptors and seemed to correlate with the number of binding sites. In conclusion, our data strongly suggest α-MSH as a potent inhibitor of ICAM-1 expression in malignant melanocytes acting through MSH receptor stimulation and subsequent cAMP increase. J. Cell. Physiol. 175:276–282, 1998. © 1998 Wiley-Liss, Inc.     

Toward the next step in G protein-coupled receptor research: a knowledge-driven analysis for the next potential targets in drug discovery

More than 800 G protein-coupled receptor (GPCR) genes have been discovered in the human genome. Towards the next step in GPCR research, we performed a knowledge-driven analysis of orphan class-A GPCRs that may serve as novel targets in drug discovery. We examined the relationship between 61 orphan class-A GPCR genes and diseases using the Online Mendelian Inheritance in Man (OMIM) database and the DDSS tool. The OMIM database contains data on disease-related variants of the genes. Particularly, the variants of GPR101, GPR161, and GPR88 are related to the genetic diseases: growth hormone-secreting pituitary adenoma 2, pituitary stalk interruption syndrome (not confirmed), and childhood-onset chorea with psychomotor retardation, respectively. On the other hand, the Drug Discovery and Diagnostic Support System (DDSS) tool suggests that 48 out of the 61 orphan receptor genes are related to diseases, judging from their co-occurrences in abstracts of biomedical literature. Notably, GPR50 and GPR3 are related to as many as 25 and 24 disease-associated keywords, respectively. GPR50 is related to 17 keywords of psychiatric disorders, whereas GPR3 is related to 11 keywords of neurological disorders. The aforementioned five orphan GPCRs were characterized genetically, structurally and functionally using the structural life science data cloud VaProS, so as to evaluate their potential as next targets in drug discovery.