Cholera Toxin Induces cAMP-independent Degradation of Gs

HDAC11 inhibition triggers bimodal thermogenic pathways to circumvent adipocyte catecholamine resistance

Stimulation of adipocyte β-adrenergic receptors (β-ARs) induces expression of uncoupling protein 1 (UCP1), promoting nonshivering thermogenesis. Association of β-ARs with a lysine-myristoylated form of A kinase-anchoring protein 12 (AKAP12, also known as gravin-α) is required for downstream signaling that culminates in UCP1 induction. Conversely, demyristoylation of gravin-α by histone deacetylase 11 (HDAC11) suppresses this pathway. Whether inhibition of HDAC11 in adipocytes is sufficient to drive UCP1 expression independently of β-ARs is not known. Here, we demonstrate that adipocyte-specific deletion of HDAC11 in mice leads to robust induction of UCP1 in adipose tissue (AT), resulting in increased body temperature. These effects are mimicked by treating mice in vivo or human AT ex vivo with an HDAC11-selective inhibitor, FT895. FT895 triggers biphasic, gravin-α myristoylation-dependent induction of UCP1 protein expression, with a noncanonical acute response that is posttranscriptional and independent of protein kinase A (PKA), and a delayed response requiring PKA activity and new Ucp1 mRNA synthesis. Remarkably, HDAC11 inhibition promotes UCP1 expression even in models of adipocyte catecholamine resistance where β-AR signaling is blocked. These findings define cell-autonomous, multimodal roles for HDAC11 as a suppressor of thermogenesis, and highlight the potential of inhibiting HDAC11 to therapeutically alter AT phenotype independently of β-AR stimulation.

HDAC11 inhibition triggers bimodal thermogenic pathways to circumvent adipocyte catecholamine resistance

Stimulation of adipocyte β-adrenergic receptors (β-ARs) induces expression of uncoupling protein 1 (UCP1), promoting non-shivering thermogenesis. Association of β-ARs with a lysine myristoylated form of A-kinase anchoring protein 12 (AKAP12)/gravin-α is required for downstream signaling that culminates in UCP1 induction. Conversely, demyristoylation of gravin-α by histone deacetylase 11 (HDAC11) suppresses this pathway. Whether inhibition of HDAC11 in adipocytes is sufficient to drive UCP1 expression independently of β-ARs is not known. Here, we demonstrate that adipocyte-specific deletion of HDAC11 in mice leads to robust induction of UCP1 in adipose tissue (AT), resulting in increased body temperature. These effects are mimicked by treating mice in vivo or human AT ex vivo with an HDAC11-selective inhibitor, FT895. FT895 triggers biphasic, gravin-α myristoylation-dependent induction of UCP1 protein expression, with a non-canonical acute response that is post-transcriptional and independent of protein kinase A (PKA), and a delayed response requiring PKA activity and new Ucp1 mRNA synthesis. Remarkably, HDAC11 inhibition promotes UCP1 expression even in models of adipocyte catecholamine resistance where β-AR signaling is blocked. These findings define cell autonomous, multi-modal roles for HDAC11 as a suppressor of thermogenesis, and highlight the potential of inhibiting HDAC11 to therapeutically alter AT phenotype independently of β-AR stimulation.

The cytopathic activity of cholera toxin requires a threshold quantity of cytosolic toxin

After binding to the surface of a target cell, cholera toxin (CT) moves to the endoplasmic reticulum (ER) by retrograde transport. In the ER, the catalytic CTA1 subunit dissociates from the rest of the toxin and is transferred to the cytosol where it is degraded by a ubiquitin-independent proteasomal mechanism. However, CTA1 persists long enough to induce excessive cAMP production through the activation of Gsα. It is generally believed that only one or a few molecules of cytosolic CTA1 are necessary to elicit a cytopathic effect, yet no study has directly correlated the levels of cytosolic toxin to the extent of intoxication. Here, we used the technology of surface plasmon resonance to quantify the cytosolic pool of CTA1. Our data demonstrate that only 4% of surface-bound CTA1 is found in the cytosol after 2 h of intoxication. This represented around 2600 molecules of cytosolic toxin per cell, and it was sufficient to produce a robust cAMP response. However, we did not detect elevated cAMP levels in cells containing less than 700 molecules of cytosolic toxin. Thus, a threshold quantity of cytosolic CTA1 is required to elicit a cytopathic effect. When translocation to the cytosol was blocked soon after toxin exposure, the pool of CTA1 already in the cytosol was degraded and was not replenished. The cytosolic pool of CTA1 thus remained below its functional threshold, preventing the initiation of a cAMP response. These observations challenge the paradigm that extremely low levels of cytosolic toxin are sufficient for toxicity, and they provide experimental support for the development of post-intoxication therapeutic strategies.

The manipulation of cell signaling and host cell biology by cholera toxin

Vibrio cholerae colonizes the small intestine and releases cholera toxin into the extracellular space. The toxin binds to the apical surface of the epithelium, is internalized into the host endomembrane system, and escapes into the cytosol where it activates the stimulatory alpha subunit of the heterotrimeric G protein by ADP-ribosylation. This initiates a cAMP-dependent signaling pathway that stimulates chloride efflux into the gut, with diarrhea resulting from the accompanying osmotic movement of water into the intestinal lumen. G protein signaling is not the only host system manipulated by cholera toxin, however. Other cellular mechanisms and signaling pathways active in the intoxication process include endocytosis through lipid rafts, retrograde transport to the endoplasmic reticulum, the endoplasmic reticulum-associated degradation system for protein delivery to the cytosol, the unfolded protein response, and G protein de-activation through degradation or the function of ADP-ribosyl hydrolases. Although toxin-induced chloride efflux is thought to be an irreversible event, alterations to these processes could facilitate cellular recovery from intoxication. This review will highlight how cholera toxin exploits signaling pathways and other cell biology events to elicit a diarrheal response from the host.

State-selective modulation of heterotrimeric Gαs signaling with macrocyclic peptides

The G protein-coupled receptor cascade leading to production of the second messenger cAMP is replete with pharmacologically targetable proteins, with the exception of the Gα subunit, Gαs. GTPases remain largely undruggable given the difficulty of displacing high-affinity guanine nucleotides and the lack of other drug binding sites. We explored a chemical library of 1012 cyclic peptides to expand the chemical search for inhibitors of this enzyme class. We identified two macrocyclic peptides, GN13 and GD20, that antagonize the active and inactive states of Gαs, respectively. Both macrocyclic peptides fine-tune Gαs activity with high nucleotide-binding-state selectivity and G protein class-specificity. Co-crystal structures reveal that GN13 and GD20 distinguish the conformational differences within the switch II/α3 pocket. Cell-permeable analogs of GN13 and GD20 modulate Gαs/Gβγ signaling in cells through binding to crystallographically defined pockets. The discovery of cyclic peptide inhibitors targeting Gαs provides a path for further development of state-dependent GTPase inhibitors.

Spatial intensity distribution analysis quantifies the extent and regulation of homodimerization of the secretin receptor

Previous studies have indicated that the G-protein-coupled secretin receptor is present as a homodimer, organized through symmetrical contacts in transmembrane domain IV, and that receptor dimerization is critical for high-potency signalling by secretin. However, whether all of the receptor exists in the dimeric form or if this is regulated is unclear. We used measures of quantal brightness of the secretin receptor tagged with monomeric enhanced green fluorescent protein (mEGFP) and spatial intensity distribution analysis to assess this. Calibration using cells expressing plasma membrane-anchored forms of mEGFP initially allowed us to demonstrate that the epidermal growth factor receptor is predominantly monomeric in the absence of ligand and while wild-type receptor was rapidly converted into a dimeric form by ligand, a mutated form of this receptor remained monomeric. Equivalent studies showed that, at moderate expression levels, the secretin receptor exists as a mixture of monomeric and dimeric forms, with little evidence of higher-order complexity. However, sodium butyrate-induced up-regulation of the receptor resulted in a shift from monomeric towards oligomeric organization. In contrast, a form of the secretin receptor containing a pair of mutations on the lipid-facing side of transmembrane domain IV was almost entirely monomeric. Down-regulation of the secretin receptor-interacting G-protein Gα_s did not alter receptor organization, indicating that dimerization is defined specifically by direct protein–protein interactions between copies of the receptor polypeptide, while short-term treatment with secretin had no effect on organization of the wild-type receptor but increased the dimeric proportion of the mutated receptor variant.

Strategies for Investigating G-Protein Modulation of Voltage-Gated Ca2+ Channels

G-protein-coupled receptor modulation of voltage-gated ion channels is a common means of fine-tuning the response of channels to changes in membrane potential. Such modulation impacts physiological processes such as synaptic transmission, and hence therapeutic strategies often directly or indirectly target these pathways. As an exemplar of channel modulation, we examine strategies for investigating G-protein modulation of CaV2.2 or N-type voltage-gated Ca(2+) channels. We focus on biochemical and genetic tools for defining the molecular mechanisms underlying the various forms of CaV2.2 channel modulation initiated following ligand binding to G-protein-coupled receptors.

Dynamic mass redistribution analysis of endogenous β-adrenergic receptor signaling in neonatal rat cardiac fibroblasts

Label-free systems for the agnostic assessment of cellular responses to receptor stimulation have been shown to provide a sensitive method to dissect receptor signaling. β-adenergic receptors (βAR) are important regulators of normal and pathologic cardiac function and are expressed in cardiomyocytes as well as cardiac fibroblasts, where relatively fewer studies have explored their signaling responses. Using label-free whole cell dynamic mass redistribution (DMR) assays we investigated the response patterns to stimulation of endogenous βAR in primary neonatal rat cardiac fibroblasts (NRCF). The EPIC-BT by Corning was used to measure DMR responses in primary isolated NRCF treated with various βAR and EGFR ligands. Additional molecular assays for cAMP generation and receptor internalization responses were used to correlate the DMR findings with established βAR signaling pathways. Catecholamine stimulation of NRCF induced a concentration-dependent negative DMR deflection that was competitively blocked by βAR blockade and non-competitively blocked by irreversible uncoupling of Gs proteins. Subtype-selective βAR ligand profiling revealed a dominant role for β2AR in mediating the DMR responses, consistent with the relative expression levels of β2AR and β1AR in NRCF. βAR-mediated cAMP generation profiles revealed similar kinetics to DMR responses, each of which were enhanced via inhibition of cAMP degradation, as well as dynamin-mediated receptor internalization. Finally, G protein-independent βAR signaling through epidermal growth factor receptor (EGFR) was assessed, revealing a smaller but significant contribution of this pathway to the DMR response to βAR stimulation. Measurement of DMR responses in primary cardiac fibroblasts provides a sensitive readout for investigating endogenous βAR signaling via both G protein-dependent and –independent pathways.

A therapeutic chemical chaperone inhibits cholera intoxication and unfolding/translocation of the cholera toxin A1 subunit

Cholera toxin (CT) travels as an intact AB(5) protein toxin from the cell surface to the endoplasmic reticulum (ER) of an intoxicated cell. In the ER, the catalytic A1 subunit dissociates from the rest of the toxin. Translocation of CTA1 from the ER to the cytosol is then facilitated by the quality control mechanism of ER-associated degradation (ERAD). Thermal instability in the isolated CTA1 subunit generates an unfolded toxin conformation that acts as the trigger for ERAD-mediated translocation to the cytosol. In this work, we show by circular dichroism and fluorescence spectroscopy that exposure to 4-phenylbutyric acid (PBA) inhibited the thermal unfolding of CTA1. This, in turn, blocked the ER-to-cytosol export of CTA1 and productive intoxication of either cultured cells or rat ileal loops. In cell culture studies PBA did not affect CT trafficking to the ER, CTA1 dissociation from the holotoxin, or functioning of the ERAD system. PBA is currently used as a therapeutic agent to treat urea cycle disorders. Our data suggest PBA could also be used in a new application to prevent or possibly treat cholera.

Hsp90 is required for transfer of the cholera toxin A1 subunit from the endoplasmic reticulum to the cytosol

Cholera toxin (CT) is an AB(5) toxin that moves from the cell surface to the endoplasmic reticulum (ER) by retrograde vesicular transport. In the ER, the catalytic A1 subunit dissociates from the rest of the toxin and enters the cytosol by exploiting the quality control system of ER-associated degradation (ERAD). The driving force for CTA1 dislocation into the cytosol is unknown. Here, we demonstrate that the cytosolic chaperone Hsp90 is required for CTA1 passage into the cytosol. Hsp90 bound to CTA1 in an ATP-dependent manner that was blocked by geldanamycin (GA), an established Hsp90 inhibitor. CT activity against cultured cells and ileal loops was also blocked by GA, as was the ER-to-cytosol export of CTA1. Experiments using RNA interference or N-ethylcarboxamidoadenosine, a drug that inhibits ER-localized GRP94 but not cytosolic Hsp90, confirmed that the inhibitory effects of GA resulted specifically from the loss of Hsp90 activity. This work establishes a functional role for Hsp90 in the ERAD-mediated dislocation of CTA1.

Ric-8B stabilizes the alpha subunit of stimulatory G protein by inhibiting its ubiquitination

The alpha subunit of stimulatory G protein (G alpha(s)) activates adenylyl cyclase, which catalyzes cAMP production, and regulates many physiological aspects, such as cardiac regulation and endocrine systems. Ric-8B (resistance to inhibitors of cholinesterase 8B) has been identified as the G alpha(s)-binding protein; however, its role in G(s) signaling remains obscure. In this study, we present evidence that Ric-8B specifically and positively regulates G(s) signaling by stabilizing the G alpha(s) protein. An in vitro biochemical study suggested that Ric-8B does not possess guanine nucleotide exchange factor activity. However, knockdown of Ric-8B attenuated beta-adrenergic agonist-induced cAMP accumulation, indicating that Ric-8B positively regulates G(s) signaling. Interestingly, overexpression and knockdown of Ric-8B resulted in an increase and a decrease in the G alpha(s) protein, respectively, without affecting the G alpha(s) mRNA level. We found that the G alpha(s) protein is ubiquitinated and that this ubiquitination is inhibited by Ric-8B. This Ric-8B-mediated inhibition of G alpha(s) ubiquitination requires interaction between Ric-8B and G alpha(s) because Ric-8B splicing variants, which are defective for G alpha(s) binding, failed to inhibit the ubiquitination. Taken together, these results suggest that Ric-8B plays a critical and specific role in the control of G alpha(s) protein levels by modulating G alpha(s) ubiquitination and positively regulates G(s) signaling.

Molecular mechanisms of corticotropin-releasing factor receptor-induced calcium signaling

The molecular mechanisms governing calcium signal transduction of corticotropin-releasing factor (CRF) receptors CRF(1) and CRF(2(a)) stably expressed in human embryonic kidney (HEK) 293 cells were investigated. Calcium signaling strictly depended on intracellular calcium sources, and this is the first study to establish a prominent contribution of the three major G-protein families to CRF receptor-mediated calcium signaling. Overexpression of Galpha(q/11) and Galpha(16) led to leftward shifts of the agonist concentration-response curves. Blockade of Galpha(q/11) proteins by the small interfering RNA (siRNA) technology partially reduced agonist-mediated calcium responses in CRF(1)- and CRF(2(a))-expressing HEK293 cells, thereby proving a contribution of the G(q) protein family. A small but significant inhibition of calcium signaling was recorded by pharmacological inhibition of G(i/o) proteins with pertussis toxin treatment. This effect was mediated by direct binding of Gbetagamma subunits to phospholipase C. G(i/o) inhibition also elevated cAMP responses in CRF receptor-overexpressing HEK293 cells and in Y79 retinoblastoma cells endogenously expressing human CRF(1) and CRF(2(a)) receptors, thereby demonstrating natural coupling of G(i) proteins to both CRF receptors. The strongest reduction of CRF receptor-mediated calcium mobilization was noted when blocking the G(s) signaling protein either by cholera toxin or by siRNA. It is noteworthy that simultaneous inhibition of two G-proteins shed light on the additive effects of G(s) and G(q) on the calcium signaling and, hence, that they act in parallel. On the other hand, G(i) coupling required prior G(s) activation.

Cyclic AMP decreases the production of NO and CCL2 by macrophages stimulated with Trypanosoma cruzi GPI-mucins

Glycosylphosphatidylinositol-anchored mucin-like glycoproteins (tGPI-mucin) present on the surface of the cellular membrane of Trypanosoma cruzi forms activate toll-like receptors 2 (TLR2) on the surface of immune cells and induce the release of several mediators of inflammation which may be relevant in the context of Chagas disease. Here, we evaluated the ability of tGPI-mucins to activate murine peritoneal macrophages to induce nitric oxide (NO) and monocyte chemoattractant protein-1 (MCP-1/CCL2). We also investigated the ability of compounds which increase or mimic cyclic adenosine monophosphate (AMP) to modulate the production of NO and CCL2. Our data show that elevation of intracellular levels of cyclic AMP prevents the release of NO and CCL2 induced by tGPI-mucins in macrophages. Overall, the release of CCL2 was decreased to a greater extent and at lower concentrations of cyclic AMP-modifying agents than the production of NO. It is suggested that the elevation of cyclic AMP during T. cruzi infection may modify the release of pro-inflammatory mediators and alter significantly the course of T. cruzi infection.

Enhanced sensitivity to cholera toxin in ADP-ribosylarginine hydrolase-deficient mice

Cholera toxin (CT) produced by Vibrio cholerae causes the devastating diarrhea of cholera by catalyzing the ADP-ribosylation of the alpha subunit of the intestinal Gs protein (Gsalpha), leading to characteristic water and electrolyte losses. Mammalian cells contain ADP-ribosyltransferases similar to CT and an ADP-ribosyl(arginine)protein hydrolase (ADPRH), which cleaves the ADP-ribose-(arginine)protein bond, regenerating native protein and completing an ADP-ribosylation cycle. We hypothesized that ADPRH might counteract intoxication by reversing the ADP-ribosylation of Gsalpha. Effects of intoxication on murine ADPRH-/- cells were greater than those on wild-type cells and were significantly reduced by overexpression of wild-type ADPRH in ADPRH-/- cells, as evidenced by both ADP-ribose-arginine content and Gsalpha modification. Similarly, intestinal loops in the ADPRH-/- mouse were more sensitive than their wild-type counterparts to toxin effects on fluid accumulation, Gsalpha modification, and ADP-ribosylarginine content. Thus, CT-catalyzed ADP-ribosylation of cell proteins can be counteracted by ADPRH, which could function as a modifier gene in disease. Further, our study demonstrates that enzymatic cross talk exists between bacterial toxin ADP-ribosyltransferases and host ADP-ribosylation cycles. In disease, toxin-catalyzed ADP-ribosylation overwhelms this potential host defense system, resulting in persistence of ADP-ribosylation and intoxication of the cell.

Galpha(s) sensitizes human SH-SY5Y cells to apoptosis independently of the protein kinase A pathway

Disturbances in Galpha(s-L) levels and function have been implicated in the pathophysiology of bipolar disorder, but the role of these changes in the development of the illness is not clear. In view of the critical role of Galpha(s)-mediated cAMP signaling in regulating cell survival, we investigated the potential role of Galpha(s-L) in modulating susceptibility to cellular stressors in human SH-SY5Y neuroblastoma cells. Overexpression of Galpha(s-L) to a level twice that of the vector-transfected cells did not directly affect cell viability but significantly increased the sensitivity to induction of cell death by serum deprivation and other apoptotic stimuli, including staurosporine, H(2)O(2), and tunicamycin. This enhanced sensitivity was associated with increased caspase-3 activation and appearance of fragmented nuclei (Hoechst 33342 staining). The broad-spectrum caspase inhibitor z-VAD-fmk completely suppressed cell death evoked by these apoptotic insults in both vector-transfected and Galpha(s-L)-overexpressing cells. The increased vulnerability conferred by increased Galpha(s-L) expression was neither mimicked by cAMP analogs 8-Br-cAMP, 8-CPT-cAMP, and 8-CPT-2Me-cAMP nor attenuated by PKA inhibitors Rp-cAMPS and KT5720. These data indicate that Galpha(s-L) may modulate apoptotic processes in a caspase-dependent manner through a signaling cascade that is independent of the cAMP/PKA or cAMP/Epac pathway. These results suggest that enhanced Galpha(s-L) expression, as was observed in post-mortem brain of bipolar patients, may impair cellular resilience in response to intracellular stress signals resulting from mitochondrial and/or endoplasmic reticulum dysfunction implicated in this disorder.

Bcl-2 down modulation in WEHI-3B/CTRES cells resistant to Cholera Toxin (CT)-induced apoptosis

The very different effects of Cholera Toxin (CT) on cell growth and proliferation may depend on the type of ganglioside receptors in cell membranes and different signal transduction mechanisms triggered, but other functions related to the drug resistance mechanisms can not be excluded. The effect of CT treatment on the "in vitro" clonogenicity, the Population Doubling Time (PDT), apoptosis, PKA activation and Bax and Bcl-2 expression was evaluated in WEHI-3B cell line and its CT-resistant subclone (WEHI-3B/CTRES). In WEHI-3B parental cells the dramatic accumulation of cAMP induced by CT correlated well with PKA activation, increased PDT value, inhibition of clonogenicity and apoptosis. H-89 treatment inhibited PKA activation by CT but did not protect the cells from apoptosis and growth inhibition. In WEHI-3B/CTRES no significant CT-dependent accumulation of cAMP occurred with any increase of PKA activity and PDT. In CT resistant cells (WEHI-3B/CTRES), Bcl-2 expression was down regulated by both CT or drug treatment (eg., ciprofloxacin, CPX) although these cells were protected from CT-dependent apoptosis but not from drug-induced apoptosis. Differently from other cell models described, down regulation of Bcl-2 is proved to be independent on cAMP accumulation and PKA activation. Our observations support the implication of cAMP dependent kinase (PKA) in the inhibition of WEHI-3B cells growth and suggest that, in WEHI-3B/CTRES, Bcl-2 expression could be modulated by CT in the absence of cAMP accumulation. Also in consideration of many contradictory data reported in literature, our cell models (of one sensitive parental cell strain and two clones with different uncrossed specific resistance to CT and CPX) provides a new and interesting tool for better investigating the relationship between the CT signal transduction mechanisms and Bcl-2 expression and function.

Molecular mechanisms underlying the modulation of exocytotic noradrenaline release via presynaptic receptors

The release of noradrenaline from nerve terminals is modulated by a variety of presynaptic receptors. These receptors belong to one of the following three receptor superfamilies: transmitter-gated ion channels, G protein-coupled receptors (GPCR), and membrane receptors with intracellular enzymatic activities. For representatives of each of these three superfamilies, receptor activation has been reported to cause either an enhancement or a reduction of noradrenaline release. As these receptor classes display greatly diverging structures and functions, a multitude of different molecular mechanisms are involved in the regulation of noradrenaline release via presynaptic receptors. This review gives a short overview of the presynaptic receptors on noradrenergic nerve terminals and summarizes the events involved in vesicle exocytosis in order to finally delineate the most important signaling cascades that mediate the modulation via presynaptic receptors. In addition, the interactions between the various presynaptic receptors are described and the underlying molecular mechanisms are elucidated. Together, these presynaptic signaling mechanisms form a sophisticated network that precisely adapts the amount of noradrenaline being released to a given situation.

Specificity of TRH receptor coupling to G-proteins for regulation of ERG K+ channels in GH3 rat anterior pituitary cells

The identity of the G-protein coupling thyrotropin-releasing hormone (TRH) receptors to rat ether-à-go-go related gene (r-ERG) K+ channel modulation was studied in situ using perforated-patch clamped adenohypophysial GH(3) cells and dominant-negative variants (Galpha-QL/DN) of G-protein alpha subunits. Expression of dominant-negative Galpha(q/11) that minimizes the TRH-induced Ca2+ signal had no effect on r-ERG current inhibition elicited by the hormone. In contrast, the introduction of dominant-negative variants of Galpha13 and the small G-protein Rho caused a significant loss of the inhibitory effect of TRH on r-ERG. A strong reduction of this TRH effect was also obtained in cells expressing either dominant-negative Galpha(s) or transducin alpha subunits, an agent known to sequester free G-protein betagamma dimers. As a further indication of specificity of the dominant-negative effects, only the dominant-negative variants of Galpha13 and Rho (but not Galpha(s)-QL/DN or Galpha(t)) were able to reduce the TRH-induced shifts of human ERG (HERG) activation voltage dependence in HEK293 cells permanently expressing HERG channels and TRH receptors. Our results demonstrate that whereas the TRH receptor uses a G(q/11) protein for transducing the Ca2+ signal during the initial response to TRH, this G-protein is not involved in the TRH-induced inhibition of endogenous r-ERG currents in pituitary cells. They also identify G(s) (or a G(s)-like protein) and G13 as important contributors to the hormonal effect in these cells and suggest that betagamma dimers released from these proteins may participate in modulation of ERG currents triggered by TRH.

Cloning and characterization of an mRNA encoding a novel G protein alpha-subunit abundant in mantle and gill of pearl oyster Pinctada fucata

Nacre formation is an ideal model to study biomineralization processes. Although much has been done about biomineralization mechanism of nacre, little is known as to how cellular signaling regulates this process. We are interested in whether G protein signaling plays a role in mineralization. Degenerate primers against conserved amino acid regions of G proteins were employed to amplify cDNA from the pearl oyster Pinctada fucata. As a result, the cDNA encoding a novel G(s)alpha (pfG(s)alpha) from the pearl oyster was isolated. The G(s)alpha cDNA encodes a polypeptide of 377 amino acid residues, which shares high similarity to the octopus (Octopus vulgaris) G(s)alpha. The well-conserved A, C, G (switch I), switch II functional domains and the carboxyl terminus that is a critical site for interaction with receptors are completely identical to those from other mollusks. However, pfG(s)alpha has a unique amino acid sequence, which encodes switch III and interaction sites of adenylyl cyclase respectively. In situ hybridization and Northern blotting analysis revealed that the oyster G(s)alpha mRNA is widely expressed in a variety of tissues, with highest levels in the outer fold of mantle and epithelia of gill, the regions essential for biomineralization. We also show that overexpression of the pfG(s)alpha in mammalian MC3T3-E1 cells resulted in increased cAMP levels. Mutant pfG(s)alpha that has impaired CTX substrate diminished its ability to induce cAMP production. Furthermore, the alkaline phosphatase (ALP) activity, an indicator for mineralization, is induced by the G(s)alpha in MC3T3-E1 cells. These results indicated that G(s)alpha may be involved in regulation of physiological function, particularly in biological biomineralization.

PTH-dependent adenylyl cyclase activation in SaOS-2 cells: passage dependent effects on G protein interactions

Parathyroid hormone (PTH) sensitive adenylyl cyclase activity (ACA) in SaOS-2 cells varies as a function of cell passage. In early passage (EP) cells (< 6), ACA in response to PTH and forskolin (FOR) was relatively low and equivalent, whereas in late passage (LP) cells (> 22), PTH exceeded FOR dependent ACA. Potential biochemical mechanisms for this passage dependent change in ACA were considered. In EP, prolonged exposure to pertussis toxin (PT) markedly enhanced ACA activity in response to PTH, Isoproterenol and Gpp(NH)p, whereas ACA in response to FOR was decreased. In contrast, the identical treatment of LP with PT diminished all ACA in response to PTH, Gpp(NH)p, and FOR. The dose dependent effects of PT on subsequent [(32)P]ADP-ribosylation of its substrates, GTPase activity, as well as FOR-dependent ACA, were equivalent in EP and LP. The relative amounts of G(alpha)i and G(alpha)s proteins, as determined both by Western blot, PT and cholera toxin (CT) dependent [(32)P]ADP-ribosylation, were quantitatively similar in EP and LP. Western blot levels of G(alpha)s and G(alpha)i proteins were not influenced by prior exposure to PT. Both PT and CT dependent [(32)P]ADP-ribosylation were dose-dependently decreased following exposure to PT. However, the PT-dependent decline in CT-dependent [(32)P]ADP-ribosylation occurred with enhanced sensitivity in LP. The protein synthesis inhibitor cycloheximide partially reversed the PT associated decrease in FOR dependent ACA in EP. In contrast, cycloheximide completely reversed the PT associated decrease in FOR and as well as PTH dependent ACA in LP. G(alpha)s activity, revealed by cyc(-) reconstitution, was not altered either by cell passage or exposure to PT. The results suggest that the coupling between the components of the complex may be pivotally important in the differential responsiveness of early and late passage SaOS-2 cells to PTH.

Stimulation of beta(3)-adrenoceptors causes phosphorylation of p38 mitogen-activated protein kinase via a stimulatory G protein-dependent pathway in 3T3-L1 adipocytes

1. This study deals with phosphorylation and activation of p38 mitogen-activated protein kinase (MAPK) via beta(3)-adrenoceptor (AR) and the signal transduction pathway in 3T3-L1 adipocytes. 2. beta(3)-AR agonist BRL37344A (10 nM) caused phosphorylation and activation of p38 MAPK in 3T3-L1 adipocytes but not in fibroblasts. BRL37344A and also the other beta(3)-AR agonists, CGP12177A and SR58611A, caused p38 MAPK phosphorylation in dose-dependent manners. 3. The p38 MAPK phosphorylations by BRL37344A (10 nM), CGP12177A (100 nM), and SR58611A (10 nM) were not antagonized by beta(1)- and beta(2)-ARs antagonist 1-propranolol (100 nM) but blocked by beta(3)-AR antagonist SR59230A (10 microM), suggesting the phosphorylation was caused via beta(3)-AR. 4. The phosphorylations of p38 MAPK were completely abolished by treatment with cholera toxin (CTX) but not pertussis toxin (100 ng ml(-1), 24 h). Activation of Gs by CTX (100 ng ml(-1)) and adenylyl cyclase by forskolin mimicked p38 MAPK phosphorylation. 5. p38 MAPK phosphorylation by BRL37344A was reduced to almost 50% by cyclic AMP-dependent protein kinase (PKA) inhibitors such as H89 (10 microM) and PKI (10 microM). A src-family tyrosine kinases inhibitor PP2 (1 microM) also halved the p38 MAPK phosphorylation. Combined use of H89 (10 microM) and PP2 (10 microM) did not bring about further inhibition. 6. These results suggest that beta(3)-AR caused phosphorylation of p38 MAPK via Gs protein and partly through a pathway involving PKA and src-family kinase(s), although the contribution of the unidentified pathway remains to be clarified.

Prostaglandin F(2alpha) receptor-dependent regulation of prostaglandin transport

Prostaglandin (PG) F(2alpha) may act on its G protein-coupled receptor (FP) or be imported intracellularly via a transporter, which has high affinity for PGF(2alpha) and PGE(2), but not prostacyclin (PGI(2)). In cells overexpressing the epitope-tagged FP together with the human prostaglandin transporter (hPGT), stimulation of the FP with PGF(2alpha) (1 nM-1 microM), or the less potent FP agonist, the isoprostane 8,12-iso-iPF(2alpha)-III, inhibited prostaglandin uptake via the hPGT. This effect was abolished by pretreatment of the cells with cholera toxin, but not with pertussis toxin. Furthermore, two dominant negative constructs directed against Galpha(s) partially blocked FP-mediated regulation of hPGT function, also suggesting Galpha(s) involvement in this phenomenon. Surprisingly, neither an activator (dibutyryl cyclic AMP) nor an inhibitor (H89) of cyclic AMP-dependent protein kinase had any effect on FP-mediated inhibition of hPGT activity. Furthermore, although PGF(2alpha) increases intracellular cyclic AMP via Galpha(s) activation, it does not induce phosphorylation of the transporter, excluding a role of cyclic AMP-dependent protein kinase in hPGT regulation. Activation of the PGI(2) receptor, which is also coupled to Galpha(s), does not regulate hPGT activity, despite markedly augmenting adenylate cyclase activation. In conclusion, activation of the FP reduces intracellular import of prostaglandins for metabolic inactivation, increasing prostanoid availability for membrane receptor activation. This effect seems to be mediated via Galpha(s), independent of adenylate cyclase and cyclic AMP-dependent protein kinase activation.

The stimulation of beta(3)-adrenoceptor causes phosphorylation of extracellular signal-regulated kinases 1 and 2 through a G(s)- but not G(i)-dependent pathway in 3T3-L1 adipocytes

The treatment of 3T3-L1 adipocytes with three beta(3)-adrenoceptor agonists, (+/-)-(R*, R*)-(4-[2-([2-(3-chlorophenyl)-2-hydroxyethyl]amino)propyl]phenoxy)ac etic acid (BRL37344), 4-[3-[(1, 1-dimethylethyl)amino]-2-hydroxypropoxy]-1, 3-dihydro-2H-benzimidazol-2-one (CGP12177) and [(7S)7-¿(2R)2-(3-chlorophenyl)-2-hydroxyethyl-amino¿-5,6,7, 8-tetrahydronapht-2-yl]ethyl oxyacetate, hydrochloride (SR58611) induces phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2). The phosphorylations were not affected by pretreatment of the adipocytes with pertussis toxin, whereas the same treatment completely abolished lisophosphatidic acid-induced phosphorylation of ERK1/2, suggesting the role of pertussis toxin-insensitive G protein in the ERK1/2 phosphorylation by stimulation with the beta(3)-adrenoceptor agonists. The phosphorylation of ERK1/2 was mimicked by treating the adipocytes with cholera toxin, a direct activator of stimulatory G (G(s)) protein. In addition, the ERK1/2 phosphorylations by the beta(3)-adrenoceptor agonists were completely diminished by long-term treatment of the adipocytes with cholera toxin (100 ng/ml, 24 h), whereas that obtained with lisophosphatidic acid stimulation was not. Our findings strongly suggest that the three beta(3)-adrenoceptor agonists induce ERK1/2 phosphorylation in 3T3-L1 adipocytes through a G(s) protein-dependent cascade.

Activating mutations of the Gs alpha gene are associated with low levels of Gs alpha protein in growth hormone-secreting tumors

Evidence suggests the existence of a direct relationship between cellular Gs alpha content and activation of the adenylyl cyclase system. Data on Gs alpha levels in endocrine tumors that depend on cAMP for growth, particularly pituitary adenomas, are still limited. The levels of Gs alpha protein were evaluated in 11 GH-secreting adenomas with Gs alpha mutations (gsp+) and 15 without (gsp). Complementary DNAs from gsp+ tumors contained very low amounts of wild-type Gs alpha sequences, indicating a preponderance of the mutant Gs alpha transcripts in these tumors. Immunoblotting of Gs alpha protein showed that the two isoforms were present at high levels in all gsp-, but were undetectable or barely detectable in gsp+. The low Gs alpha content in gsp+ tumors was not due to a reduction in ribonucleic acid synthesis or stability, as Gs alpha messenger ribonucleic acid levels were similar in wild-type and mutant tissues. Treatment of gsp- cells with cholera toxin caused a marked reduction of Gs alpha levels. As in other cell systems cholera toxin increases Gs alpha degradation, our data are consistent with an accelerated removal of mutant Gs alpha. This may represent an additional mechanism of feedback response to the constitutive activation of cAMP signaling in pituitary tumors with mutations in the Gs alpha gene.

Involvement of Ca2+ signalling in the vasoactive intestinal peptide and 8-Br-cAMP induction of c-fos mRNA expression

Vasoactive intestinal peptide (VIP) is known to signal via Gs mediated pathways. VIP stimulated c-fos mRNA expression in a clonal GH3 pituitary tumour cell line, GH3Ca, whereas 8-Br-cAMP only moderately induced c-fos expression. The VIP-induced c-fos expression was inhibited in the presence of EGTA, or the L-type Ca2+ channel blockers verapamil and nifedipine. Measurement of intracellular Ca2+ concentration ([Ca2+]i) by Fura-2 indicates that VIP gradually elevates [Ca2+]i, with the maximum level attained at 4 min following hormone addition. No [Ca2+]i increase could be detected in Ca2+ free buffer or in buffer containing nifedipine or verapamil, which suggests that VIP induced Ca2+ entry from L-type Ca2+ channels. 8-Br-cAMP rapidly increased [Ca2+]i, with a maximum concentration attained within 1 min of its addition and the elevated level maintained for 15 min. In the absence of external Ca2+ or in the presence of verapamil or nifedipine, the sustained Ca2+ increase was abolished whereas the transient Ca2+ peak was unaffected. Depletion of the internal calcium pools by thapsigargin (1 microM, 30 min), on the other hand, blocked the rapid transient [Ca2+], rise, suggesting the biphasic [Ca2+]i elicited by 8-Br-cAMP was due to mobilization from internal Ca2+ pool followed by extracellular flow. Interestingly, pretreatment with thapsigargin greatly potentiated the 8-Br-cAMP-stimulated c-fos expression. Pretreatment of cells with cholera toxin (1 microg/ml, 9 h) to deplete Gs proteins abolished VIP stimulated-[Ca2+] elevation, while it had little effect on the 8-Br-cAMP induced [Ca2+]i rise. Our results show that VIP increased Ca2+ influx from L-type channel through a Gs-mediated mechanism and this Ca2+ entry across the plasma membrane plays a major role in the hormone induced c-fos mRNA expression.

Cyclic AMP-independent activation of neutrophil-like HL-60 cells by prostaglandin E2

The role of cAMP in mediating prostaglandin E2 (PGE2)-stimulated aggregation of neutrophil-like HL-60 cells has been investigated. Although the EP2 receptors appear to couple to Gs-proteins, PGE2 stimulated HL-60 cell aggregation appears to be a cAMP-independent process. This response to PGE2 in independent of calcium and tyrosine kinase activity, appears to involve activation of phosphatidylinositol 3-kinase which is negatively regulated by phosphatidic acid generated from phospholipase D activity, and is partially dependent on protein kinase C activity. In contrast, although the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (FMLP) produces a similar aggregation response to PGE2, FMLP uses a distinct intracellular signalling pathway. The aggregation response to FMLP involves activation of Gi-proteins, is partially dependent on extracellular calcium, is negatively regulated by protein kinase C, and is independent of phosphatidylinositol 3-kinase, phospholipase D and tyrosine kinase activity. The possibility exists that EP2 receptor activation leads to Gs-dependent, but cAMP-independent, stimulation of phosphatidylinositol 3-kinase activity in HL-60 cells.

Chronic ethanol promotes the neuronal differentiation of NG108-15 cells independently of toxin-sensitive G-proteins

The ability of ethanol to promote neuronal differentiation of NG108-15 mouse neuroblastoma x rat glioma hybrid cells was investigated using morphological, biochemical and electrophysiological techniques. Ethanol concentration-(10-200 mM) and time-(1 h-3 days) dependently reduced cell proliferation, but increased acetylcholinesterase (AChE) activity and cell protein content. Chronic ethanol (200 mM) also time-dependently increased voltage-sensitive Ca(2+) currents in the cells. Similar effects were obtained with chronic treatment of the cells with the standard differentiating agents sodium butyrate or forskolin. Chronic treatment of NG108-15 cells with primary alcohols (0.1-200 mM) of varying chain length all reduced cell proliferation and increased cell protein content and AChE activity with the potency order butanol>propanol>ethanol>methanol. Chronic treatment of NG108-15 cells with cholera toxin (50 ng ml(-1)) or pertussis toxin (50 ng ml(-1)) did not induce differentiation of the cells, nor did it modify the effects of 50 or 200 mM ethanol on cell proliferation, AChE activity or cellular protein content. Chronic cholera toxin did however abolish agonist-stimulated adenylyl cyclase activity in the cells, whereas pertussis toxin abolished receptor-mediated inhibition of adenylyl cyclase activity. Furthermore, inhibitors of protein kinase C (GF 109203X, 5 μM), protein kinase A (H-89, 10 μM) or Ca(2+)/calmodulin-dependent protein kinase II (KN-62, 3 μM) all failed to modify the effects of 200 mM ethanol on cell proliferation, AChE activity and cellular protein content. These experiments indicate that chronic ethanol is able to promote neuronal differentiation of NG108-15 cells independently of toxin-sensitive G-proteins and some protein kinases.

Receptors controlling transmitter release from sympathetic neurons in vitro

Primary cultures of postganglionic sympathetic neurons were established more than 30 years ago. More recently, these cultures have been used to characterize various neurotransmitter receptors that govern sympathetic transmitter release. These receptors may be categorized into at least three groups: (1) receptors which evoke transmitter release: (2) receptors which facilitate; (3) receptors which inhibit, depolarization-evoked release. Group (1) comprises nicotinic and muscarinic acetylcholine receptors, P2X purinoceptors and pyrimidinoceptors. Group (2) currently harbours beta-adrenoceptors, P2 purinoceptors, receptors for PACAP and VIP, as well as prostanoid EP1 receptors. In group (3), muscarinic cholinoceptors, alpha 2- and beta-adrenoceptors, P2 purinoceptors, and receptors for the neuropeptides NPY, somatostatin (SRIF1) and LHRH, as well as opioid (delta and kappa) receptors can be found. Receptors which regulate transmitter release from neurons in cell culture may be located either at the somatodendritic region or at the sites of exocytosis, i.e. the presynaptic specializations of axons. Most of the receptors that evoke release are located at the soma. There ionotropic receptors cause depolarizations to generate action potentials which then trigger Ca(2+)-dependent exocytosis at axon terminals. The signalling mechanisms of metabotropic receptors which evoke release still remain to be identified. Receptors which facilitate depolarization-evoked release appear to be located preferentially at presynaptic sites and presumably act via an increase in cyclic AMP. Receptors which inhibit stimulation evoked release are also presynaptic origin and most commonly rely on a G protein-mediated blockade of voltage-gated Ca2+ channels. Results obtained with primary cell cultures of postganglionic sympathetic neurons have now supplemented previous data about neurotransmitter receptors involved in the regulation of ganglionic as well as sympatho-effector transmission. In the future, this technique may prove useful to identify yet unrecognized receptors which control the output of the sympathetic nervous system and to elucidate underlying signalling mechanisms.

Enhanced degradation of stimulatory G-protein (Gs alpha) by cholera toxin is mediated by ADP-ribosylation of Gs alpha protein but not by increased cyclic AMP levels

Cholera toxin (CT) catalyses ADP-ribosylation of the alpha-subunit of stimulatory protein (Gs) leading to stimulation of adenylyl cyclase and elevated intracellular cAMP. Persistent treatment (24-48 h) of C6 glioma cells with cholera toxin (100 ng/ml) caused marked downregulation of Gs alpha (75-80%) which could not be mimicked by dibutyryl cAMP (1 mM) and forskolin (10 microM) over the same time periods suggesting that CT-mediated Gs alpha downregulation is independent of cAMP production. However, CT increased the expression of Gq/11 alpha proteins at 24 and 48 h of treatment. The increase in mRNA levels of Gq/11 alpha proteins preceded the increase in Gq/11 proteins. Such stimulatory effects of CT were mimicked by forskolin and dibutyryl-cAMP. These results suggest that CT-mediated downregulation of Gs alpha is independent of cAMP but CT upregulates the expression of Gq/11 alpha proteins in a cAMP-dependent manner.

ADP-ribosylation of G alpha i and G alpha o in pituitary cells enhances their recognition by antibodies directed against their carboxyl termini

Using antibodies raised against synthetic peptides of heterotrimeric GTP binding proteins, we demonstrate the presence of G alpha s, G alpha i1,2, G alpha i3, G alpha o2, and G beta subunits in pituitary cells. Pretreatment of pituitary cells with cholera toxin diminished the immunoreactivity of G alpha s and this decrease was kinetically coupled to the rate of G alpha s ADP-ribosylation. ADP-ribosylation by islet activating protein (IAP or Bordetella pertussis toxin) of G alpha i and G alpha o enhanced their immunoreactivities to antibodies raised against synthetic decapeptides that correspond to the G alpha carboxyl termini. Such enhancement was not observed when antibodies directed against the NH2-termini were used. These findings are consistent with the fact that ADP-ribosylation by IAP occurs on the cysteine located in the carboxyl terminal part of G alpha i and G alpha o. These observations mean that the kinetics and extent of Gi and Go ADP-ribosylation by IAP in whole pituitary cells and membrane preparations can be followed. It could be that ADP-ribosylation causes conformational changes in G alpha i and G alpha o. Indeed, we observed that ADP-ribosylated G alpha i was more sensitive to trypsin proteolysis and that the ADP-ribosylation rates of G alpha i and G alpha o in whole cells were comparable to the rate of loss of coupling between inhibitory neurohormone receptors and adenylyl cyclase.

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.

A comparison of ubiquitin-dependent proteolysis of rod outer segment proteins in reticulocyte lysate and a retinal pigment epithelial cell line

We compared ATP- and ubiquitin-dependent proteolysis in supernatants of rabbit reticulocyte lysate and a human retinal pigment epithelial (RPE) cell line. At pH 7.8, both preparations catalyzed the conjugation of [125I]ubiquitin to endogenous proteins, generating an equivalent amount of high mass (> 150 kDa) [125I]ubiquitin-protein adducts. Both preparations degraded exogenous histone 2A, oxRNase and beta-lactoglobulin in an ATP-dependent manner. Addition of ubiquitin (12 or 120 microM) to reticulocyte lysate stimulated (1.4-fold) ATP-dependent degradation only of histone 2A. Addition of 12 microM ubiquitin to RPE supernatant resulted in > or = 3-fold enhancement in degradation of all three substrates. Next, we compared the ability of the two proteolysis systems to degrade bovine rod outer segment (ROS) nonintegral membrane proteins. [125I]ROS protein degradation by reticulocyte lysate was almost exclusively ATP-dependent and was completely inhibited by hemin and vanadate, inhibitors of ATP- and ubiquitin-dependent proteolysis. RPE supernatant also degraded ROS proteins by an ATP-dependent mechanism, and, unlike results obtained in reticulocyte assays, this degradation increased (2-fold) upon ubiquitin supplementation. Both proteolysis systems degraded ROS proteins of molecular mass approximately 10, 30, 37, 40 and 60 kDa, with coincident formation of high mass species. Reticulocyte lysate also degraded 100 and 150 kDa ROS proteins, whereas RPE supernatant did not. The 10, 37 and 40 kDa species were identified by western blot as the gamma-, beta- and alpha- subunits of rod transducin (Gt), respectively. RPE supernatant generated (some) ROS proteolysis products that remained acid-precipitable. As compared with patterns of proteolysis in reticulocytes, RPE supernatant (1) degraded 100% more Gt beta gamma, (2) generated > 10-fold the amount of high mass (putative ubiquitin-ROS protein) conjugates and (3) preferentially degraded Gt beta gamma relative to G t alpha. The ubiquitin-dependent enhancement of ATP-dependent degradation of all proteins tested in RPE supernatant makes the RPE system a valuable experimental tool for the explicit demonstration of ubiquitin-dependent proteolysis.

Differential interaction with and regulation of multiple G-proteins by the rat A3 adenosine receptor

Interaction of the rat A3 adenosine receptor (A3AR) with G-proteins has been assessed using a stably transfected Chinese hamster ovary cell system. The non-selective AR agonist 5'-N-ethylcarboxamidoadenosine (NECA) increased the labeling of a 41-kDa membrane protein by 4-azidoanilido-[alpha-32P]guanosine 5'-triphosphate (AA-[32P]GTP), a photolabile GTP analogue. Subsequent immunoprecipitation of Gi alpha-subunits indicated that NECA stimulated incorporation of label into both Gi alpha-2 and Gi alpha-3. Additional experiments revealed an A3AR stimulation of label into Gq and/or G11 alpha-subunits, albeit to a lesser degree than that elicited by endogenous P2U purinergic receptors. No interaction with Gs could be detected. Sustained cellular exposure to NECA induced A3AR desensitization and specific down-regulation of Gi alpha-3 and G-protein beta-subunits without changing levels of Gi alpha-2, Gs alpha, or Gq+11 alpha-subunits. Therefore the A3AR can interact with Gi alpha-2, Gi alpha-3, and, to some extent, Gq-like proteins, but sustained agonist exposure down-regulates only one of the G-proteins with which it interacts. This is the first description of the differing specificities of A3AR/G-protein coupling versus down-regulation in situ and provides a potential mechanism by which the A3AR could elicit the heterologous desensitization of signaling events mediated by Gi3.

Quantitative variation of proto-oncogene and cytokine gene expression in isolated breast fibroblasts

Transcripts coding for transcription factors (RB, P53, FOS, MYC, MYB, ERBA, REL), growth factors (FGF1, FGF2, INT2, TGFA, TGFB, PDGF, IGF1, IGF2), interleukins, (IL1, IL2, IL3, IL4, IL6, TNF), growth-factor receptors or cytosolic protein kinases (RAF, PIM, FES, MET, SRC, ROS, TRK, KIT, CSFR, IGFR, PDGFR, EGFR, NEU) were quantified in cultured human mammary fibroblasts from normal tissues, benign tumours, carcinomas and post-radiation fibrosis lesions by slot-blot autoradiography and image analysis. The effects of a differentiating agent (cholera toxin) and of a tumour promoter (12-O-tetradecanoyl-phorbol-13-acetate) were also examined. The drugs modulated the levels of the anti-oncogene transcripts (RB, P53) and of ERBA, REL, RAF, MET, ROS, TRK, CSFR, EGFR, NEU, FGF1, INT2, IGF1, IL1, IL2, IL4 and IL6. Apart from this variation, there were multiple differences in gene expression among normal and pathological cells (concerning all but P53, TGFB and interleukin transcripts) and between sub-types defined by the presence of alpha-sm-actin (myofibroblasts) or EDB-fibronectin (RAF, ROS, FES, KIT, IGFR, NEU, INT2, TGFB, PDGF, IGFs, ILs). It appears, therefore, that mammary stroma progress irreversibly along with the epithelium during tumoral development, and that breast cancer is not only a multi-gene but also a multi-tissue phenotype.

Cholera toxin-stimulated bone resorption in cultured mouse calvarial bones not inhibited by calcitonin: a possible interaction at the stimulatory G protein

We examined the effect of calcitonin in cultured mouse calvarial bones after prestimulation with different activators of adenylyl cyclase. Calcitonin (100 ng/ml), added after 48 h of culture, inhibited bone resorption (assessed as release of 45Ca from prelabeled bones cultured for 96-144 h) stimulated with parathyroid hormone (PTH, 10 nM; 0-144 h) or the adenylyl cyclase stimulator forskolin (2 microM; 0-144 h). However, no effect of calcitonin was demonstrated when bone resorption was prestimulated with the adenylyl cyclase stimulator cholera toxin, at and above 1 ng/ml, at any time point studied. In contrast, two other types of inhibitors of bone resorption in vitro, the carbonic anhydrase inhibitor acetazolamide (10 microM) and the aminobisphosphonate AHPrBP (10 microM), significantly inhibited cholera toxin-stimulated bone resorption. No cyclic AMP response to calcitonin was seen after preculture for 48 h with cholera toxin (0.1-100 ng/ml), although bones precultured in basic medium, in the absence or presence of forskolin, were still able to respond to calcitonin with elevation of cyclic AMP. Binding studies with [125I]calcitonin demonstrated that the preculture with cholera toxin did not affect the binding of calcitonin to the receptor. In summary, our data show that cholera toxin pretreatment makes calvarial bones insensitive to calcitonin-induced inhibition of bone resorption as a result of an interaction with cholera toxin at the level of calcitonin receptor-linked signal transduction. We suggest that the interaction, distal to the calcitonin receptor, is caused by the irreversible activation of Gs produced by cholera toxin.(ABSTRACT TRUNCATED AT 250 WORDS)

VIP inhibits N-type Ca2+ channels of sympathetic neurons via a pertussis toxin-insensitive but cholera toxin-sensitive pathway

The best characterized Ca2+ channel modulation in mammalian sympathetic neurons is an inhibition of N-type channels via a pertussis toxin (PTX)-sensitive heterotrimeric G protein. Here, we show that vasoactive intestinal polypeptide (VIP), an abundant neuropeptide in the PNS and CNS, inhibited N-type Ca2+ channels in rat sympathetic neurons in a voltage-dependent, membrane-delimited manner. The effect of VIP was insensitive to PTX but was attenuated by cholera toxin or anti-Gs alpha antibodies. VIP-mediated inhibition was independent of cAMP-dependent protein kinase A (PKA). The results provide evidence for a new signal transduction pathway in which N-type Ca2+ channel modulation requires activation of Gs alpha but is independent of PKA-mediated phosphorylation.

Gs alpha-dependent and -independent desensitisation of prostanoid IP receptor-activated adenylyl cyclase in NG108-15 cells

NG108-15 mouse neuroblastoma x rat glioma cells were treated with the prostanoid IP receptor agonist iloprost (1 microM) and the time course of changes in the levels of prostanoid IP receptors, adenylyl cyclase activity, and the alpha-subunit of the stimulatory guanine nucleotide binding regulatory protein, Gs, were measured. Incubation of cells with iloprost produced a biphasic time course of desensitisation of prostanoid IP receptor-activated adenylyl cyclase. Parallel analysis of iloprost-induced loss of membrane Gs alpha, NaF-stimulated adenylyl cyclase and [3H]iloprost binding suggested only monophasic curves, with t0.5 values similar to the initial phase of desensitisation of iloprost-stimulated adenylyl cyclase activity. This suggests that the loss of receptor and Gs alpha occur at the same time and account for the initial period of desensitisation due to iloprost pretreatment. Pretreatment of NG108-15 cells with cholera toxin produced a near complete loss of membrane-associated Gs alpha, but the loss of [3H]iloprost binding due to iloprost treatment was not affected by pretreatment with cholera toxin, suggesting that prostanoid IP receptors can be down-regulated in the absence of any coupling to Gs. The second phase of desensitisation of iloprost-stimulated adenylyl cyclase activity, during which there was no further change in NaF-stimulated adenylyl cyclase or in the membrane levels of Gs alpha, was not due to protein kinase A activation, since elevating intracellular cyclic AMP levels with forskolin did not subsequently decrease iloprost-stimulated adenylyl cyclase activity or [3H]iloprost binding. These results demonstrate that iloprost pretreatment of NG108-15 cells induces two distinct phases of desensitisation; an initial desensitisation due to concurrent loss of prostanoid IP receptors and Gs alpha, and then a further desensitisation by an as yet uncharacterized mechanism during which there is no further loss of Gs alpha.

Cholera toxin-induced Gs alpha down-regulation in neural tissue: studies on the pineal gland

Cholera toxin (CT) treatment (50 micrograms/ml) was used to down regulate the alpha subunit of the stimulatory guanine nucleotide binding protein (Gs alpha) in pineal glands in organ culture, as has been seen in non-neural tissue. A 15 h treatment reduces Gs alpha by approximately 75% as measured using semi-quantitative Western blot technology. In contrast, this treatment does not alter the abundance of G beta, Gi alpha or Go alpha. This effect on Gs alpha was still apparent following a 36-h washout period. The 48-h CT treatment increased cyclic AMP accumulation 10- to 17-fold but blocked the norepinephrine (NE)-induced increase in cyclic AMP accumulation, presumably reflecting the loss of Gs alpha. This treatment did not, however, inhibit protein synthesis or stimulation of arylalkylamine N-acetyltransferase (NAT) activity produced by treatment with either DB-cyclic AMP (N6,2'-O-dibutyryl adenosine 3',5' monophosphate) or 8 Br-cyclic AMP, stable cyclic AMP derivatives. This indicates that a 48-h CT treatment was not generally toxic. In contrast, this treatment blocked subsequent CT stimulation of NAT. The effects of CT treatment on the adrenergic stimulation of NAT was examined using treatments which selectively produced alpha- or beta-adrenergic stimulation. alpha 1-Adrenergic activation of the pineal gland elevates [Ca2+]i, which potentiates effects of cyclic AMP; in these studies the response to alpha-adrenergic activation was markedly increased in 48 h CT-treated glands, reflecting Ca2+ potentiation of the effects of elevated levels of cyclic AMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative stoichiometry of the proteins of the stimulatory arm of the adenylyl cyclase cascade in neuroblastoma x glioma hybrid, NG108-15 cells

To understand the details of regulation of guanine-nucleotide-binding-protein-linked transmembrane cellular-signalling cascades, it is important to know the absolute levels of each polypeptide component and the stoichiometry of their interactions. Amounts of the IP prostanoid receptor, the stimulatory G protein of the adenylyl cyclase cascade (Gs alpha) and the functional complex of Gs alpha with adenylyl cyclase, which acts as the cyclic AMP generator, were measured in membranes of neuroblastoma x glioma hybrid, NG108-15, cells. As measured by the specific binding of [3H]prostaglandin E1, the IP prostanoid receptor was present in some 100,000 copies/cell. Gs alpha assessed by quantitative immunoblotting with recombinantly expressed protein, was present in considerably higher levels (1,250,000 copies/cell). However, the maximal formation of a complex of Gs alpha and adenylyl cyclase represented only some 17,500 copies/cell. The previously established 8:1 stoichiometry of concurrent downregulation of Gs alpha and the IP prostanoid receptor in these cells [Adie, E. J., Mullaney, I., McKenzie, F. R. & Milligan, G. (1992) Biochem. J. 285, 529-536] indicates that full-agonist occupation of the receptor should be able to activate some 65% of the expressed Gs. Despite the potential 70-fold excess of Gs alpha over the Gs alpha/adenylyl cyclase complex, IP prostanoid-receptor-agonist-mediated reduction of Gs alpha levels by some 35% resulted in a 25% reduction in the maximal formation of the Gs alpha/adenylyl cyclase complex. Such results demonstrate that adenylyl cyclase is quantitatively the least highly expressed component of this signalling cascade and suggests that much of the cellular Gs alpha may not have access to adenylyl cyclase.

Agonist regulation of cellular G protein levels and distribution: mechanisms and functional implications

Exposure of cells to agonists of receptors linked to G proteins can result in downregulation of cellular levels or redistribution of G proteins from membranes to the cytosol. Agonist-induced reductions in G protein levels have been observed for members of each of the Gs, Gi and Gq families of G proteins, are likely to be dependent upon the level of receptor expression, and are generally restricted to the G protein(s) with which the receptor interacts. The mechanisms responsible, reviewed here by Graeme Milligan, vary with cell type and include both second messenger-dependent and -independent enhanced protein degradation. Agonist-induced reduction in cellular G protein levels can provide one mechanism for the development of sustained heterologous desensitization.

Vip-induced cross-talk between G-proteins in membranes from rat anterior pituitary cells

In order to study the activation mechanism of heterotrimeric G-proteins by agonist-liganded receptors, GTP gamma S binding to membranes was measured in rat adenohypophyseal cells after addition of dopamine (DA) or vasoactive intestinal peptide (VIP), which, respectively, inhibit and activate pituitary adenylyl cyclase. G-protein subunit present in anterior pituitary cells was characterized by either ADP-ribosylation catalysed by Bordetella pertussis and cholera toxins or by immunoblot using specific antisera. Binding of GTP gamma S was found to depend upon GTP gamma S and Mg2+ concentrations; it was sensitive to pretreatment of the cells with cholera and Bordetella pertussis toxins (IAP). DA increased binding of the nucleotide. Paradoxically, VIP decreased the rate of GTP gamma S binding; the effect was suppressed by prior treatment of the cells with either cholera toxin or IAP. VIP also increased [33P]ADPribose incorporation in Gi/Go-proteins catalysed by IAP. Forskolin was also able to decrease GTP gamma S binding, thus suggesting that the binding of forskolin with the adenylyl cyclase catalytic unit might activate Gs proteins through an increased interaction between Gs and adenylyl cyclase. Taken together, these results suggest that VIP, as well as forskolin, may both accelerate the activation of Gs and suppress the inhibitory effect of activated Gi/Go-proteins. Interactions between Gs and Gi/Go subunits mediated by beta gamma and/or adenylyl cyclase might thus result in a kinetic coupling of transduction pathways involving distinct G-proteins.

Guinea-pig treatment with pertussis toxin suppresses macrophage-dependent bronchoconstriction by fMLP and fails to inhibit the effects of PAF

1. Bronchoconstriction and thromboxane B2 (TxB2) release following the intra-tracheal administration of the secretagogue N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP) to lungs from pertussis toxin-treated guinea-pigs in vivo and in vitro were inhibited as compared to saline-treated animals, under conditions where the responses to PAF were modified less effectively. 2. The cell target accounting for bronchoconstriction by fMLP and for inhibition by pertussis toxin is located in the airways and is probably the alveolar macrophage. Indeed (a) fMLP-induced superoxide anions and TxB2 formation by alveolar macrophages were inhibited by pertussis toxin given in vivo; (b) Gi proteins of membranes from alveolar macrophages were ADP-ribosylated in vivo by pertussis toxin and (c) bronchoconstriction and TxB2 release in response to the intra-tracheal administration of fMLP to lungs from pertussis toxin-treated animals were restored when alveolar macrophages from control guinea-pigs were transferred into the airways of pertussis toxin-treated animals before lung isolation. 3. Pertussis toxin administered to guinea-pigs in vivo, reduced the subsequent TxB2 formation and superoxide anion release by alveolar macrophages stimulated with PAF, but failed to inhibit PAF-induced bronchoconstriction. 4. Formation of TxB2 by alveolar macrophages following the intra-tracheal administration of fMLP accounts for bronchoconstriction and requires pertussis toxin-sensitive Gi proteins. PAF operates via a different mechanism, which is independent of Gi-like protein and involves mediators other than TxB2 and superoxide anions.

Post-receptor defect accounts for phosphorylase hypersensitivity in cultured diabetic cardiomyocytes

The basis for the hypersensitive response of glycogen phosphorylase to epinephrine stimulation was investigated in adult rat cardiomyocytes isolated from normal and alloxan-diabetic animals. To assess potential G-protein involvement in the response, normal and diabetic derived myocytes were incubated with either cholera or pertussis toxin prior to hormonal stimulation. Pretreatment of cardiomyocytes with cholera toxin resulted in a potentiated response to epinephrine stimulation whereas pertussis toxin did not affect the activation of this signaling pathway. To determine if the enhanced response of phosphorylase activation resulted from an alteration in adenylate cyclase activation, the cells were challenged with forskolin. After 3 hr in primary culture, diabetic cardiomyocytes exhibited a hypersensitive response to forskolin stimulation relative to normal cells. However, after 24 hr in culture, both normal and diabetic myocytes responded identically to forskolin challenge. The present data suggest that a cholera toxin sensitive G-protein mediates the hypersensitive response of glycogen phosphorylase to catecholamine stimulation in diabetic cardiomyocytes and this response which is present in alloxan-diabetic cells and is induced in vitro in normal cardiomyocytes is primarily due to a defect at a post-receptor site.

Alpha-subunits of Gs and Gi in adipocyte plasma membranes of genetically diabetic (db/db) mice

The adipocyte membrane G protein pattern, beta-adrenergic receptor activity, and adenylyl cyclase were determined in adipocyte membranes of the db/db mouse, a mutant that is a model of diabetes preceded by hyperinsulinemia, hyperglycemia, and extreme obesity. These studies were undertaken to determine whether the alterations already noted in the ob/ob mouse and those in the db/db mouse are similar and related to the hormonal defects, particularly the hyperinsulinemia and hyperglycemia prevalent in these animals (cf. Ref. 11). The ADP ribosylation data show that Gs alpha was more highly labeled in the tissues of the db/db mutant than in the homozygous control, but there was no significant difference in the amount of ADP-ribose incorporated in the Gi alpha-subunits. Quantification of the proteins by immunodetection revealed that the long (46-kDa) form of Gs alpha was significantly less abundant in the db mutant than in its control, whereas there was no difference in the short (42-kDa) form. Gi alpha-peptides corresponding to Gi alpha 2 and Gi alpha 1 were both less abundant in the db mutant than in the homozygous control. These data contrasted with those obtained for ob mutants and their lean homozygous controls reported previously (4) and confirmed here. It is concluded on the basis of these studies that factors other than the hormonal status are responsible for the G protein patterns in the ob and db mutants. Differences in G protein patterns noted in between the control groups (B/Ks or B/6 homozygotes) correlated strongly with the quantitative differences in adenylyl cyclase response in the two strains.(ABSTRACT TRUNCATED AT 250 WORDS)