Activation and inhibition of G proteins by lipoamines

Strategies towards Targeting Gαi/s Proteins: Scanning of Protein-Protein Interaction Sites To Overcome Inaccessibility

Heterotrimeric G proteins are classified into four subfamilies and play a key role in signal transduction. They transmit extracellular signals to intracellular effectors subsequent to the activation of G protein-coupled receptors (GPCRs), which are targeted by over 30 % of FDA-approved drugs. However, addressing G proteins as drug targets represents a compelling alternative, for example, when G proteins act independently of the corresponding GPCRs, or in cases of complex multifunctional diseases, when a large number of different GPCRs are involved. In contrast to Gαq, efforts to target Gαi/s by suitable chemical compounds has not been successful so far. Here, a comprehensive analysis was conducted examining the most important interface regions of Gαi/s with its upstream and downstream interaction partners. By assigning the existing compounds and the performed approaches to the respective interfaces, the druggability of the individual interfaces was ranked to provide perspectives for selective targeting of Gαi/s in the future.

Ligand-based peptide design and combinatorial peptide libraries to target G protein-coupled receptors

G protein-coupled receptors (GPCRs) are considered to represent the most promising drug targets; it has been repeatedly said that a large fraction of the currently marketed drugs elicit their actions by binding to GPCRs (with cited numbers varying from 30-50%). Closer scrutiny, however, shows that only a modest fraction of (≈60) GPCRs are, in fact, exploited as drug targets, only ≈20 of which are peptide-binding receptors. The vast majority of receptors in the humane genome have not yet been explored as sites of action for drugs. Given the drugability of this receptor class, it appears that opportunities for drug discovery abound. In addition, GPCRs provide for binding sites other than the ligand binding sites (referred to as the "orthosteric site"). These additional sites include (i) binding sites for ligands (referred to as "allosteric ligands") that modulate the affinity and efficacy of orthosteric ligands, (ii) the interaction surface that recruits G proteins and arrestins, (iii) the interaction sites of additional proteins (GIPs, GPCR interacting proteins that regulate G protein signaling or give rise to G protein-independent signals). These sites can also be targeted by peptides. Combinatorial and natural peptide libraries are therefore likely to play a major role in identifying new GPCR ligands at each of these sites. In particular the diverse natural peptide libraries such as the venom peptides from marine cone-snails and plant cyclotides have been established as a rich source of drug leads. High-throughput screening and combinatorial chemistry approaches allow for progressing from these starting points to potential drug candidates. This will be illustrated by focusing on the ligand-based drug design of oxytocin (OT) and vasopressin (AVP) receptor ligands using natural peptide leads as starting points.

Inhibition of heterotrimeric G protein signaling by a small molecule acting on Galpha subunit

The simultaneous activation of many distinct G protein-coupled receptors (GPCRs) and heterotrimeric G proteins play a major role in various pathological conditions. Pan-inhibition of GPCR signaling by small molecules thus represents a novel strategy to treat various diseases. To better understand such therapeutic approach, we have characterized the biomolecular target of BIM-46187, a small molecule pan-inhibitor of GPCR signaling. Combining bioluminescence and fluorescence resonance energy transfer techniques in living cells as well as in reconstituted receptor-G protein complexes, we observed that, by direct binding to the Galpha subunit, BIM-46187 prevents the conformational changes of the receptor-G protein complex associated with GPCR activation. Such a binding prevents the proper interaction of receptors with the G protein heterotrimer and inhibits the agonist-promoted GDP/GTP exchange. These observations bring further evidence that inhibiting G protein activation through direct binding to the Galpha subunit is feasible and should constitute a new strategy for therapeutic intervention.

[35S]GTP gamma S binding studies of amphiphilic drugs-activated Gi proteins: a caveat

This paper documents a serious problem met during the testing of Gi protein-activating properties of a new series of synthetic compounds by measuring the induced binding of [(35)S]GTPgammaS to different subtypes of Gi protein. The problem arose from the strong affinity between [(35)S]GTPgammaS and the tested compounds, that are characterized by several (2-4) positive charges and high lipophilicity. Apparently, such affinity yields insoluble, labelled complexes that, also in the absence of Gi protein, are retained on the filters and give rise to false positive results.

Receptor-independent modulation of reconstituted Galpha(i) protein mediated by liposomes

A cationic amphiphile, BC5 (N-pentadecylpiperidin-4-amine), was recently designed and tested for its ability to directly stimulate the activity of recombinant Galpha inhibitory subunits. However, amphiphilic drugs can self-associate and bind to plasma membranes, causing undesired side effects. In this contribution, we report on the incorporation of BC5 in 1,2-dipalmytoyl-sn-glycerophosphocoline (DPPC) liposomes and on the characterization of the mixed DPPC/BC5 systems at various lipid/drug mole ratios by means of dynamic light scattering, differential scanning calorimetry and fluorescence spectroscopy. The myristoylated Galpha(i) subunit (Galpha-mir) was reconstituted in 1,2-dimiristoyl-sn-glycerophosphocoline (DMPC) bilayers, as a mimic of the drug target. We compare several reconstitution procedures in liposomes and present for the first time a complete characterization of a Galpha subunit reconstitution in model membranes in terms of protein activity as a function of the reconstitution protocol. The incorporation of the drug in DPPC bilayers resulted in enhanced Gi-modulating efficiency (evaluated in terms of binding to GTPgammaS (guanosine-5'-(gamma-thio)-triphosphate)). A correlation of the physico-chemical features and binding activity of protein-containing membrane model is proposed.

Molecular mechanisms for the effect of mastoparan on G proteins in tissues of vertebrates and invertebrates

The peptide toxin mastoparan increased GTP-binding activity of heterotrimeric G proteins in tissues of vertebrate and invertebrate animals, the effect of mastoparan in mussel tissues being less pronounced. The stimulatory effect of mastoparan on GTP binding was not observed after treatment of membranes with pertussis toxin that selectively modulates function of Gi proteins. Activity of mastoparan decreased in the presence of C-terminal peptide 346-355 from the Gi protein alphai2-subunit. Mastoparan dose-dependently decreased the stimulatory effect of hormones on GTP binding in tissues of rats and mussels. The influence of these hormones on the cell is realized via Gi proteins. However, mastoparan did not modulate the effect of Gs protein-activating hormones.

Anticancer activity of BIM-46174, a new inhibitor of the heterotrimeric Galpha/Gbetagamma protein complex

A large number of hormones and local agonists activating guanine-binding protein-coupled receptors (GPCR) play a major role in cancer progression. Here, we characterize the new imidazo-pyrazine derivative BIM-46174, which acts as a selective inhibitor of heterotrimeric G-protein complex. BIM-46174 prevents the heterotrimeric G-protein signaling linked to several GPCRs mediating (a) cyclic AMP generation (Galphas), (b) calcium release (Galphaq), and (c) cancer cell invasion by Wnt-2 frizzled receptors and high-affinity neurotensin receptors (Galphao/i and Galphaq). BIM-46174 inhibits the growth of a large panel of human cancer cell lines, including anticancer drug-resistant cells. Exposure of cancer cells to BIM-46174 leads to caspase-3-dependent apoptosis and poly(ADP-ribose) polymerase cleavage. National Cancer Institute COMPARE analysis for BIM-46174 supports its novel pharmacologic profile compared with 12,000 anticancer agents. The growth rate of human tumor xenografts in athymic mice is significantly reduced after administration of BIM-46174 combined with either cisplatin, farnesyltransferase inhibitor, or topoisomerase inhibitors. Our data validate the feasibility of targeting heterotrimeric G-protein functions downstream the GPCRs to improve anticancer chemotherapy.

A recombinant transductor–effector system: In vitro study of G inhibitory protein (G-alpha-i1) direct activators

Mutations and altered functionality of the inhibitory subfamily of G proteins (Gi) are involved in pathological states. Compounds able to activate Gi in a receptor-independent manner would be useful to treat these pathological conditions. Aimed to study Gi direct activation we have reconstituted a recombinant transductor-effector complex cloning both the mammalian Galpha(i1) subunit and adenylate cyclase (AC). The myristoylation of Galpha, fundamental for interaction with AC, was obtained in the procaryotic expression host Escherichia coli transformed with a single plasmid containing both the coding sequences for human Galpha(i1) and Saccharomyces cerevisiae myristoyl transferase. AC-V isoform was obtained by the expression of its cytosolic domains. A recent synthesized molecule, named BC5, was tested to evaluate its pharmacological profile in a Gi/AC cell-free complex model. In this functional transductor-effector system BC5 was able to activate Gi signalling, moreover providing a new tool to give a better insight into G-protein receptor-independent modulation.

Gi/o proteins: Expression for direct activation enquiry

G protein-mediated pathways are fundamental mechanisms of cell signaling. In this paper, the expression and the characterization of the alphai1, alphai3, alphao1, beta1, and gamma2 subunits of the human G protein are described. This approach was developed to evaluate the G protein activation profile of new compounds. pCR-TOPO T7 vectors, engineered to contain the target sequences, were used to transform Escherichia coli competent cells. Subunits were over-expressed in a preparative scale as fusion proteins with a six-histidine tag, and subsequently purified by metal chelate chromatography. Afterward, the His-tag was removed by enterokinase digestion, and the secondary structures of the recombinant subunits were analyzed by circular dichroism. To assess the functionality of the subunits, the rate of GTP hydrolysis and GTPgammaS binding were evaluated both in the absence and in the presence of two modulators: the peptidic activator Mastoparan and the non-peptidic activator N-dodecyl-lysinamide (ML250). Tests were conducted on isolated alpha-subunit and on heterotrimeric alphabetagamma complex, alone or reconstituted in phospholipidic vesicles. Our results show that recombinant subunits are stable, properly folded and, fully active, which makes them suitable candidates for functional studies.

Design, synthesis, and preliminary pharmacological evaluation of a set of small molecules that directly activate gi proteins

Heterotrimeric G proteins play a pivotal role in the communication of cells with the environment. G proteins are stimulated by cell surface receptors (GPCR) that catalyze the exchange of GDP, bound to Galpha subunit, with GTP and can per se be the target of drugs. Based on the structure of two nonpeptidic modulators of Gi proteins, a series of new molecules characterized by a long hydrophobic chain and at least two nitrogen atoms protonated at physiological pH was designed. The compounds were tested for their ability to stimulate binding of GTPgammaS to recombinant Gi proteins. Gi activation properties were also evaluated by inhibition of adenylyl cyclase activity in intact lymphocytes. Most compounds were able to stimulate GTPgammaS binding and to inhibit cAMP production at micromolar doses. Among the active compounds, 34 showed good efficacy and was the most potent compound studied, particularly on alpha(o) subtype; its regioisomer, 36, was the most efficacious one. Compound 7 showed also an interesting profile as it showed selectivity toward the alpha(o) subtype, in both efficacy and potency. Some of the compounds synthesized and found to be active may be useful leads to develop more potent and selective Gi protein modulators.

The carboxyl terminus of the Galpha-subunit is the latch for triggered activation of heterotrimeric G proteins

The receptor-mimetic peptide D2N, derived from the cytoplasmic domain of the D(2) dopamine receptor, activates G protein alpha-subunits (G(i) and G(o)) directly. Using D2N, we tested the current hypotheses on the mechanism of receptor-mediated G protein activation, which differ by the role assigned to the Gbetagamma-subunit: 1) a receptor-prompted movement of Gbetagamma is needed to open up the nucleotide exit pathway ("gear-shift" and "lever-arm" model) or 2) the receptor first engages Gbetagamma and then triggers GDP release by interacting with the carboxyl (C) terminus of Galpha (the "sequential-fit" model). Our results with D2N were compatible with the latter hypothesis. D2N bound to the extreme C terminus of the alpha-subunit and caused a conformational change that was transmitted to the switch regions. Hence, D2N led to a decline in the intrinsic tryptophan fluorescence, increased the guanine nucleotide exchange rate, and modulated the Mg(2+) control of nucleotide binding. A structural alteration in the outer portion of helix alpha5 (substitution of an isoleucine by proline) blunted the stimulatory action of D2N. This confirms that helix alpha5 links the guanine nucleotide binding pocket to the receptor contact site on the G protein. However, neither the alpha-subunit amino terminus (as a lever-arm) nor Gbetagamma was required for D2N-mediated activation; conversely, assembly of the Galphabetagamma heterotrimer stabilized the GDP-bound species and required an increased D2N concentration for activation. We propose that the receptor can engage the C terminus of the alpha-subunit to destabilize nucleotide binding from the "back side" of the nucleotide binding pocket.

Macrophages induce the inflammatory response in the pulmonary Arthus reaction through G alpha i2 activation that controls C5aR and Fc receptor cooperation

Complement and FcgammaR effector pathways are central triggers of immune inflammation; however, the exact mechanisms for their cooperation with effector cells and their nature remain elusive. In this study we show that in the lung Arthus reaction, the initial contact between immune complexes and alveolar macrophages (AM) results in plasma complement-independent C5a production that causes decreased levels of inhibitory FcgammaRIIB, increased levels of activating FcgammaRIII, and highly induced FcgammaR-mediated TNF-alpha and CXCR2 ligand production. Blockade of C5aR completely reversed such changes. Strikingly, studies of pertussis toxin inhibition show the essential role of G(i)-type G protein signaling in C5aR-mediated control of the regulatory FcgammaR system in vitro, and analysis of the various C5aR-, FcgammaR-, and G(i)-deficient mice verifies the importance of Galpha(i2)-associated C5aR and the FcgammaRIII-FcgammaRIIB receptor pair in lung inflammation in vivo. Moreover, adoptive transfer experiments of C5aR- and FcgammaRIII-positive cells into C5aR- and FcgammaRIII-deficient mice establish AM as responsible effector cells. AM lacking either C5aR or FcgammaRIII do not possess any such inducibility of immune complex disease, whereas reconstitution with FcgammaRIIB-negative AM results in an enhanced pathology. These data suggest that AM function as a cellular link of C5a production and C5aR activation that uses a Galpha(i2)-dependent signal for modulating the two opposing FcgammaR, FcgammaRIIB and FcgammaRIII, in the initiation of the inflammatory cascade in the lung Arthus reaction.

Coupling of boswellic acid-induced Ca2+ mobilisation and MAPK activation to lipid metabolism and peroxide formation in human leucocytes

1. We have previously shown that 11-keto boswellic acids (11-keto-BAs), the active principles of Boswellia serrata gum resins, activate p38 MAPK and p42/44(MAPK) and stimulate Ca(2+) mobilisation in human polymorphonuclear leucocytes (PMNL). 2. In this study, we attempted to connect the activation of MAPK and mobilisation of Ca(2+) to functional responses of PMNL, including the formation of reactive oxygen species (ROS), release of arachidonic acid (AA), and leukotriene (LT) biosynthesis. 3. We found that, in PMNL, 11-keto-BAs stimulate the formation of ROS and cause release of AA as well as its transformation to LTs via 5-lipoxygenase. 4. Based on inhibitor studies, 11-keto-BA-induced ROS formation is Ca(2+)-dependent and is mediated by NADPH oxidase involving PI 3-K and p42/44(MAPK) signalling pathways. Also, the release of AA depends on Ca(2+) and p42/44(MAPK), whereas the pathways stimulating 5-LO are not readily apparent. 5. Pertussis toxin, which inactivates G(i/0) protein subunits, prevents MAPK activation and Ca(2+) mobilisation induced by 11-keto-BAs, implying the involvement of a G(i/0) protein in BA signalling. 6. Expanding studies on differentiated haematopoietic cell lines (HL60, Mono Mac 6, BL41-E-95-A) demonstrate that the ability of BAs to activate MAPK and to mobilise Ca(2+) may depend on the cell type or the differentiation status. 7. In summary, we conclude that BAs act via G(i/0) protein(s) stimulating signalling pathways that control functional leucocyte responses, in a similar way as chemoattractants, that is, N-formyl-methionyl-leucyl-phenylalanine or platelet-activating factor.