Equivalent regulation of wild type and an epitope-tagged variant of Gs alpha by the IP prostanoid receptor following expression in neuroblastoma x glioma hybrid, NG108-15, cells

Inflammatory and contractile agents sensitize specific adenylyl cyclase isoforms in human airway smooth muscle

Beta-agonists, through activation of the beta(2)-adrenergic receptor (beta(2)AR)-G(s)-adenylyl cyclase (AC) pathway, promote bronchodilation via functional antagonism of airway smooth muscle (ASM) spasmogens associated with the asthmatic state. Although previous studies have demonstrated that beta(2)AR signaling in ASM is subject to homologous (beta-agonist-induced) beta(2)AR desensitization, the potential for inflammatory and contractile agents to impact beta(2)AR signaling in ASM through heterologous mechanisms has not been defined. Here we report that chronic exposure of human ASM (HASM) to carbachol, serotonin, the thromboxane analogue U46619, or histamine induced little change or a small increase in isoproterenol-stimulated cyclic adenosine monophosphate (cAMP) formation, but significantly increased cAMP formation elicited by stimulation with forskolin. This latter increase in intrinsic AC activity was largely reversed by pertussis toxin pretreatment, and was unaffected by protein kinase C inhibition. Analysis of both AC function and isoform expression supports a dominant role of AC VI in HASM, and points to important differences in ASM AC isoform expression among species. Additional studies identify AC as the limiting component in beta(2)AR-G(s)-AC signaling in HASM, and thus a potentially important target of therapeutic strategies designed to influence airway contractile state.

Selective activation of a chimeric Gi1/Gs G protein alpha subunit by the human IP prostanoid receptor: analysis using agonist stimulation of high affinity GTPase activity and [35S]guanosine-5'-O-(3-thio)triphosphate binding

A FLAG-tagged form of the human IP prostanoid receptor was expressed stably in HEK 293 cells. This bound [3H]iloprost with high affinity and stimulated cAMP production when exposed to agonist. Iloprost produced weak stimulation of GTPase activity and [35S]guanosine-5'-O-(3-thio)triphosphate binding in membranes of these cells. Pretreatment of cells with pertussis toxin did not modify iloprost-mediated stimulation, but this was blocked by cholera toxin. The effects of iloprost were not increased by coexpression of either Gsalpha or Gi1alpha. In contrast, coexpression of a chimeric G protein alpha subunit in which the carboxyl-terminal six amino acids of Gi1alpha were altered to those of Gsalpha resulted in robust stimulation by iloprost. Because the chimeric G protein alpha subunit (Gi1/Gs6alpha) is not a substrate for either pertussis or cholera toxin, pretreatment of cells coexpressing the IP prostanoid receptor and Gi1/Gs6alpha with a mixture of these toxins resulted in resolution of the signal derived from activation of the chimeric G protein. Agonist-stimulated [35S]guanosine-5'-O-(3-thio)triphosphate binding and GTPase activity assays are the most commonly used strategies to examine interactions between G protein-coupled receptors and G proteins. These usually are not appropriate for receptors such as the IP prostanoid receptor that interact with G proteins with low rates of guanine nucleotide exchange and hydrolysis. Chimeric G proteins such as Gi1/Gs6alpha that allow appropriate receptor contacts to be converted to the higher nucleotide turnover rates typical of the Gi family G proteins can overcome this and offer a novel means to examine agonist function at such receptors.

Overexpression of G(s)alpha in NG108-15, neuroblastomaXglioma cells: effects on receptor regulation of the stimulatory adenylyl cyclase cascade

NeuroblastomaXglioma hybrid, NG108-15 cells were stably transfected with an epitope tagged variant of G(s)alpha (HA-G(s)alpha). The introduced HA-G(s)alpha was able to interact with the IP prostanoid receptor and was able to stimulate adenylyl cyclase activity as measured by an enhanced capacity of membrane extracts to reconstitute NaF-dependent adenylyl cyclase activity to membranes of S49 lymphoma cyc- cells. Despite this, neither the maximal stimulation nor the potency of agonist ligands at the IP prostanoid, A2 adenosine or secretin receptors was altered substantially compared to the parental cells although the basal adenylyl cyclase activity was increased. These data indicate that cellular levels of G(s)alpha do not limit signal transduction capacity in NG108-15 cells, whereas enhanced expression of adenylyl cyclase allows greater maximal cAMP generation following receptor activation (MacEwan, D.J., Kim, G.D. and Milligan, G. (1996) Biochem. J. 318, 1033-1039).

Agonist regulation of adenylate cyclase activity in neuroblastoma x glioma hybrid NG108-15 cells transfected to co-express adenylate cyclase type II and the beta 2-adrenoceptor. Evidence that adenylate cyclase is the limiting component for receptor-mediated stimulation of adenylate cyclase activity

Stable cell lines, derived from NG108-15 cells and transfected to express both the beta 2-adrenoceptor and adenylate cyclase type II, were produced and examined. The absence of adenylate cyclase type II in the parental cells and its presence in these clones was demonstrated by reverse transcriptase-PCR. Total cellular levels of adenylate cyclase were increased in a number of clones between 3- and 8-fold, as assessed by guanine nucleotide-stimulated specific high-affinity binding of [3H]forskolin to cellular membranes. Basal adenylate cyclase activity was markedly elevated compared with a clone expressing similar levels of the beta 2-adrenoceptor in the absence of adenylate cyclase type II. Each of NaF, forskolin and guanosine 5'-[beta, gamma-imido]triphosphate (a poorly hydrolysed analogue of GTP) produced substantially higher levels of adenylate cyclase activity in membranes of the clones positive for expression of adenylate cyclase type II than was achieved with the parental cells. Both isoprenaline, acting at the introduced beta 2-adrenoceptor, and iloprost, acting at the endogenously expressed IP prostanoid receptor, stimulated adenylate cyclase activity to much higher levels in the clones expressing adenylate cyclase type II compared with the clone lacking this adenylate cyclase; however, the concentration-effect curves for adenylate cyclase stimulation by these two agonists were not different between parental cells and clones over-expressing adenylate cyclase type II. A maximally effective concentration of the beta-adrenoceptor partial agonist ephedrine displayed similar intrinsic activity and potency to stimulate adenylate cyclase in membranes of clones both with and without adenylate cyclase type II. Both secretin and 5'-N-ethylcarbox-amidoadenosine (acting at an endogenous A2 adenosine receptor) were also able to produce substantially greater maximal activations of adenylate cyclase in the clones expressing excess adenylate cyclase type II, without alterations in agonist intrinsic activity or potency. These results demonstrate that the maximal output of the stimulatory arm of the adenylate cyclase cascade can be increased by increasing total levels of adenylate cyclase in the genetic background of NG108-15 cells.

The stoichiometry of expression of protein components of the stimulatory adenylyl cyclase cascade and the regulation of information transfer

Quantitative analysis of the proteins which compromise the stimulatory arm of the adenylyl cyclase cascade indicate that the adenylyl cyclase catalytic component is usually the least highly expressed. The effects on both potency of agonist ligands and maximal output resulting from targetted alterations in expression levels of each element of this cascade are discussed.

Regulation of cellular Gs alpha levels and basal adenylyl cyclase activity by expression of the beta 2-adrenoceptor in neuroblastoma cell lines

Mouse neuroblastoma x rat glioma hybrid NG 108-15 and mouse neuroblastoma x embryonic hamster brain NCB20 cells were transfected with a construct containing a human beta 2 adrenoceptor cDNA under the control of the beta actin promoter. Clones were selected on the basis of resistance to geneticin sulphate and those expressing a range of levels of the receptor expanded for further study. Membranes from a clone of NG108-15 cells expressing high levels of the receptor (beta N22) but not one expressing only low levels of the receptor (beta N17) exhibited a markedly elevated adenylyl cyclase activity when measured in the presence of Mg2+ compared to wild type cells. This was not due to elevated levels of the adenylyl cyclase catalytic moiety however as there was no difference in these membranes when the adenylyl cyclase activity was measured in the presence of Mn2+. The elevated basal activity was partially reversed by addition of the beta-adrenoceptor antagonist propranolol. Agonist activation of beta N22 but not beta N17 cells led to a large selective down-regulation of cellular Gs alpha levels which was independent of the generation of cyclic AMP. Isoprenaline stimulation of adenylyl cyclase activity and of the specific high affinity binding of [3H] forskolin was achieved with substantially greater potency (some 30 fold) in beta N22 cell membranes than in beta N17. By contrast agonist activation of the endogenously expressed IP prostanoid receptor caused stimulation of adenylyl cyclase and stimulation of high affinity [3H] forskolin binding which was equipotent in each of beta N22, beta N17 and wild type NG108-15 cells. Agonist activation of the IP prostanoid receptor caused an equivalent degree of Gs alpha down-regulation in each cell type. Expression of an epitope tagged variant of Gs alpha in NG108-15 cells resulted in prostanoid agonist-induced down-regulation of this polypeptide in a manner indistinguishable from that of wild type Gs alpha. Isolation of clones of NCB20 cells expressing high levels of the beta 2 adrenoceptor also resulted in a specific agonist-induced down-regulation of Gs alpha.

Degradation of G11 alpha/Gq alpha is accelerated by agonist occupancy of alpha 1A/D, alpha 1B, and alpha 1C adrenergic receptors

Cells of clones of rat 1 fibroblasts transfected to express the molecularly defined alpha 1A/D, alpha 1B, or alpha 1C adrenoreceptors and prelabeled with myo-[3H]inositol were each shown to generate high levels of inositol phosphates when exposed to the alpha 1 adrenoreceptor agonist phenylephrine. Maintained exposure of each of these cells to phenylephrine resulted in a large down-regulation of the receptors and also a marked down-regulation of cellular levels of both of the phosphoinositidase C-linked G-proteins Gq alpha and G11 alpha. To examine the mechanism of phenylephrine-induced down-regulation of Gq alpha and G11 alpha, pulse-chase 35S-amino acid labeling experiments were performed with each of the alpha 1A/D, alpha 1B, and alpha 1C adrenoreceptor-expressing cell lines. The rate of degradation of G11 alpha/Gq alpha, which was adequately modeled by a monoexponential with half-life between 33 and 40 h in each of the cell lines in the absence of agonist, was accelerated substantially (some 4-fold) in the presence of phenylephrine. By contrast, the rate of degradation of the G-protein Gi2 alpha, which would not be anticipated to be activated by members of the alpha 1 adrenoreceptor family, was unaltered by the presence of phenylephrine. Levels of mRNA encoding Gq alpha and G11 alpha were not substantially altered by exposure of the cells to phenylephrine in any of the cell lines studied.