Alterations in macrophage G proteins are associated with endotoxin tolerance

Endotoxin-induced cross-tolerance to Gram-positive sepsis

The manifestations of Gram-positive sepsis and Gram-negative sepsis share some common clinical features suggesting common pathways of activation. The goal of this study was to assess whether lipopolysaccharide (LPS) can produce cross-tolerance to Gram-positive sepsis induced by group B streptococcus (GBS). Thromboxane (TxB2), tumor necrosis factor (TNFα), and nitric oxide (NO) production by in vitro LPS- and heat killed GBS-stimulated rat peritoneal macrophages were measured. Since our previous studies have demonstrated altered macrophage activation of extracellular signal-regulated kinases 1 and 2 (ERK 1/2) in tolerance, we also examined the effect of LPS and killed GBS on ERK 1/2 activation in normal and LPS tolerant macrophages. Tolerance was induced in rats by intraperitoneal injection of Salmonella enteritidis LPS or vehicle for two consecutive days at doses of 0.1 and 0.5 mg/kg body weight. Three days after the second LPS dose, rats were injected intravenously with viable GBS (5 x l09 cfu/kg) and D-galactosamine (1 g/kg). LPS tolerance significantly prolonged ( P <0.05) mean survival time to severe GBS sepsis in D-galactosamine sensitized rats from 12.9 ± 1.7 h in control rats to 44.0 ± 8.9 h in tolerant rats. Peritoneal macrophages from LPS tolerant rats exhibited suppressed LPS induced in vitro TxB2 and TNFα production ( P <0.05). Tolerance also decreased in vitro heat killed GBS-induced TNFα production, but did not significantly affect TxB2 production. NO production stimulated by LPS (10 µg/ml was not impaired in LPS tolerance; however at lower doses (0.02—1.25 µg/ml), NO production was significantly decreased ( P <0.05). NO production was augmented ( P <0.05) in response to stimulation with GBS (10 µg/ml) and unaltered at lower doses (0.02—1.25 µg/ml) in tolerant cells. LPS activated ERK 1/2 in control macrophages, but activation of ERK 1/2 was suppressed in LPS tolerance. GBS did not significantly affect ERK 1/2 activity in control or tolerant macrophages. Nevertheless, the selective mitogen-activated kinase (MAPK)/ERK kinase (MEK) inhibitor, PD 98059 blocked ( P <0.05) both GBS- and LPS-induced TNFα and TxB2 production, but not NO production. Thus, some level of ERK 1/2 activity appears essential for GBS- and LPS-induced macrophage activation. In conclusion, LPS tolerance induces partial cross-tolerance to Gram-positive sepsis induced lethality, and alters LPS- and GBS-induced in vitro peritoneal macrophage mediator production. This suggests common pathways of cellular activation for GBS and LPS that are altered by LPS tolerance.

Endotoxin-induced desensitization of THP-1 cells is not associated with altered G protein binding or content

In rats endotoxin tolerance is characterized by decreased endotoxin-stimulated peritoneal macrophage arachidonic acid metabolism and decreased GTP binding protein function. The hypothesis that THP-1 cells can be altered in a similar manner by pretreatment with endotoxin was tested. These studies examined endotoxin's ability to stimulate eicosanoid and tumor necrosis factor α (TNFα) in control and desensitized THP 1 cells. Additionally, membrane GTPγ 35S binding and Western blot analyses with specific antisera to G i1,2α, Gi3a, Gαcommon, and the β subunit of G in control and endotoxin-desensitized THP-1 cells were assessed. Endotoxin (10 μg/ml) stimulated thromboxane (Tx) B2 production in THP-1 cells. Pretreatment with pertussis toxin (PT), resulted in significant inhibition of TxB2 production at concentrations not inhibited by equimolar concentrations of PT-B protomer. The latter observations suggest a role of PT-sensitive G protein in endotoxin activation of THP-1 cells. Pre-exposure to endotoxin (1 μg/ml) for 18 h desensitized THP-1 cells to endotoxin-stimulated TxB2 production and endotoxin-stimulated TNFα. To determine if endotoxin pretreatment affects G protein function, THP-1 cell membranes were isolated from endotoxin pretreated and control cells for equilibrium binding with GTPγ35S, a nonhydrolyzable analog of GTP. Neither the total number of binding sites (Bmax) nor the dissociation constant (Kd) for GTPγ35S in desensitized THP-1 cells were significantly different from those of control cells. PT-catalyzed ADP-ribosylation of G proteins in control and LPS-desensitized THP-1 cells demonstrated no difference in the quantity of G protein labelled versus desensitized cells. Immunoblots also showed no difference between control and desensitized cells in the membrane content of specific heterotrimeric G proteins. The data demonstrate that pre-exposure to endotoxin desensitizes the cells subsequent endotoxin stimulation of mediator production. However, unlike the in vivo rat model, this is not associated with a decrease in G protein binding or content.

Dissociation of IFN-γ from IL-12 and IL-18 production during endotoxin tolerance

Endotoxin tolerance was induced in mice following one, two or three injections of low amounts of lipopolysaccharide (LPS) before a further LPS injection, and circulating cytokines were analyzed 1.5 h and 3 h after LPS challenge. Three different patterns of cytokine production were obtained. In a first group of cytokines, including tumor necrosis factor (TNF), interleukin-6 (IL-6) and gamma interferon (IFN-γ), the reduction of plasma peak levels was already significantly pronounced after one tolerizing injection of LPS. The second group of cytokines includes the CC chemokine KC, the CXC chemokine monocyte-chemo-attractant protein-1 (MCP-1) and IL-12. The plasma levels of these cytokines were modestly reduced, and the reduction was more pronounced with increasing numbers of tolerizing injections of LPS. The third group of cytokines includes IL-1β and IL-18, the levels of which 3 h after LPS challenge (i.e. at the peak timing) remained essentially similar to those of control mice and after 1.5 h were even enhanced. Altogether, these data illustrate that, in tolerized animals, in vivo regulation of cytokine production differs greatly among different mediators and that immunoparalysis is not a general state. Furthermore, despite the presence of large amounts of IL-12 and IL-18, IFN-γ was essentially suppressed in tolerized animals.

Review: Molecular mechanisms of endotoxin tolerance

The phenomenon of endotoxin tolerance has been widely investigated, but to date, the molecular mechanisms of endotoxin tolerance remain to be resolved clearly. The discovery of the Toll-like receptor (TLR) family as the major receptors for lipopolysaccharide (LPS) and other bacterial products has prompted a resurgence of interest in endotoxin tolerance mechanisms. Changes of cell surface molecules, signaling proteins, pro-inflammatory and anti -inflammatory cytokines and other mediators have been examined. During tolerance expression of LPS-binding protein (LBP), CD14, myeloid differentiation protein-2 (MD-2) and TLR2 are unchanged or up-regulated, whereas TLR4 is transiently suppressed or unchanged. Proximal post-receptor signaling proteins that are altered in tolerance include augmented degradation of interleukin-1 receptor-associated kinase (IRAK), and decreased TLR4-myeloid differentiation factor 88 (MyD88) and IRAK-MyD88 association. Tolerance has also been shown to be associated with decreased Gi protein content and activity, decreased protein kinase C (PKC) activity, reduction in mitogen-activated protein kinase (MAP kinase) activity, and reduced activator protein-1 (AP-1) and nuclear factor kappa B (NF-κB) induced gene transactivation. However, not all signaling proteins and pathways are suppressed in tolerance and induction of specific anti-inflammatory proteins and signaling pathways may serve important counter inflammatory functions. The latter include induction of IRAK-M and suppressor of cytokine-signaling-1 (SOCS-1), phosphoinositide-3-kinase (PI3K) signaling, and increased or maintained expression of inhibitor-κB (IκB) isoforms. Also at the nuclear level, increase in the NFκB subunit p50 homodimer expression and increased activation of peroxisome-proliferatoractivated receptors-γ (PPARγ) have been linked to tolerance phenotype. Although there are species and cellular variations in manifestation of the LPS tolerant phenotype, it is clear that the tolerance phenomena have evolved as a complex orchestrated counter regulatory response to inflammation.

Gαi1 and Gαi3 regulate macrophage polarization by forming a complex containing CD14 and Gab1

Heterotrimeric G proteins have been implicated in Toll-like receptor 4 (TLR4) signaling in macrophages and endothelial cells. However, whether guanine nucleotide-binding protein G(i) subunit alpha-1 and alpha-3 (Gαi1/3) are required for LPS responses remains unclear, and if so, the underlying mechanisms need to be studied. In this study, we demonstrated that, in response to LPS, Gαi1/3 form complexes containing the pattern recognition receptor (PRR) CD14 and growth factor receptor binding 2 (Grb2)-associated binding protein (Gab1), which are required for activation of PI3K-Akt signaling. Gαi1/3 deficiency decreased LPS-induced TLR4 endocytosis, which was associated with decreased phosphorylation of IFN regulatory factor 3 (IRF3). Gαi1/3 knockdown in bone marrow-derived macrophage cells (Gαi1/3 KD BMDMs) exhibited an M2-like phenotype with significantly suppressed production of TNF-α, IL-6, IL-12, and NO in response to LPS. The altered polarization coincided with decreased Akt activation. Further, Gαi1/3 deficiency caused LPS tolerance in mice. In vitro studies revealed that, in LPS-tolerant macrophages, Gαi1/3 were down-regulated partially by the proteasome pathway. Collectively, the present findings demonstrated that Gαi1/3 can interact with CD14/Gab1, which modulates macrophage polarization in vitro and in vivo.

Modulation of the cAMP response by Gαi and Gβγ: a computational study of G protein signaling in immune cells

Cyclic AMP is important for the resolution of inflammation, as it promotes anti-inflammatory signaling in several immune cell lines. In this paper, we present an immune cell specific model of the cAMP signaling cascade, paying close attention to the specific isoforms of adenylyl cyclase (AC) and phosphodiesterase that control cAMP production and degradation, respectively, in these cells. The model describes the role that G protein subunits, including Gαs, Gαi, and Gβγ, have in regulating cAMP production. Previously, Gαi activation has been shown to increase the level of cAMP in certain immune cell types. This increase in cAMP is thought to be mediated by βγ subunits which are released upon Gα activation and can directly stimulate specific isoforms of AC. We conduct numerical experiments in order to explore the mechanisms through which Gαi activation can increase cAMP production. An important conclusion of our analysis is that the relative abundance of different G protein subunits is an essential determinant of the cAMP profile in immune cells. In particular, our model predicts that limited availability of βγ subunits may both (i) enable immune cells to link inflammatory Gαi signaling to anti-inflammatory cAMP production thereby creating a balanced immune response to stimulation with low concentrations of PGE2, and (ii) prohibit robust anti-inflammatory cAMP signaling in response to stimulation with high concentrations of PGE2.

Chronic morphine acts via a protein kinase Cgamma-G(beta)-adenylyl cyclase complex to augment phosphorylation of G(beta) and G(betagamma) stimulatory adenylyl cyclase signaling

Chronic morphine augments protein kinase C (PKC) phosphorylation of G(beta), which enhances the potency of G(betagamma) to stimulate adenylyl cyclase II (ACII) activity. The present study demonstrates an in vivo association between phosphorylated G(beta) and a specific PKC isoform, PKCgamma. We investigated the association of G(beta) and PKCgamma by assessing the ability of anti-PKCgamma antibodies to co-immunoprecipitate G(beta) from (32)P-radiolabeled Chinese Hamster Ovary cells stably transfected with a mu-opioid receptor (MOR-CHO). PKCgamma immunoprecipitate (IP) obtained from MOR-CHO membranes contained radiolabeled signals of approximately equals 33 and 36--38 kDa that were subsequently identified as G(beta)(s). Chronic morphine significantly increased ( approximately equals 75%) the magnitude of (32)P incorporated into G(beta) present in PKCgamma IP. This suggests that G(beta) is an in vivo substrate for PKCgamma, which mediates the chronic morphine-induced increment in G(beta) phosphorylation. In order to evaluate AC as a putative effector for phosphorylated G(betagamma), its presence in IP obtained using anti-AC antibodies was evaluated. Autoradiographic analyses of AC IP also revealed the presence of phosphorylated G(beta)(s), the magnitude of which was significantly enhanced ( approximately equals 60%) following chronic morphine treatment. This indicates that phosphorylated G(betagamma) associates and presumably interacts in vivo with AC, indicating that it is a target for the enhanced phosphorylated G(betagamma) that is generated following chronic morphine treatment. This would contribute to the previously observed shift from predominantly G(ialpha) inhibitory to G(betagamma) stimulatory AC signaling following chronic morphine. The PKCgamma-G(beta)-AC complex identified in this study provides an organizational framework for understanding the well-documented participation of PKCgamma in opioid tolerance-producing mechanisms.

Biochemical demonstration of mu-opioid receptor association with Gsalpha: enhancement following morphine exposure

Biochemical data indicate mu-opioid receptor (MOR) coupling predominantly to the G(i) and G(o) family. Additionally, MOR coupling to G(s) is suggested by pharmacological assessments that have revealed excitatory MOR effects, which are resistant to pertussis toxin and sensitive to cholera toxin. However, biochemical evidence for such interactions remains elusive; G(salpha) has not been shown to be present in immunoprecipitate obtained using anti-MOR antibodies. In the current study, the presence of MOR in immunoprecipitate obtained with anti-G(salpha ) antibodies was investigated using Chinese hamster ovary cells stably transfected with MOR (MOR-CHO). MOR Western analyses of opioid naive MOR-CHO membranes immunoprecipitated using anti-G(salpha) antibodies reveal the presence of an approximately 75-80 kDa MOR species. Interestingly, acute and chronic morphine treatment markedly enhances the magnitude of MOR that co-immunoprecipitates with G(salpha), despite the concomitant down-regulation of membrane MOR protein. Enhanced co-precipitation of MOR with G(salpha) occurs without a concomitant increase in the immunoprecipitated G(salpha) protein indicating their increased association. In contrast, chronic morphine diminishes the co-immunoprecipitation of MOR with G(ialpha). Moreover, although only a single MOR species co-immunoprecipitated with G(salpha), MOR Western analysis of MOR-CHO membranes as well as immunoprecipitate obtained with either anti-MOR or anti-G(ialpha) antibodies reveals the presence of multiple molecular mass species of MOR. These data reveal the existence of a subset of MORs whose association with G(salpha) can be enhanced by morphine exposure. Notably, the regulation by chronic morphine of MOR association with G(salpha) and G(ialpha) is reciprocal. The relevance of MOR-Gs(alpha) coupling to opioid tolerance formation is discussed.

AGS3 and Signal Integration by Gαs- and Gαi-coupled Receptors

AGS3-LONG and AGS3-SHORT contain G-protein regulatory motifs that interact with and stabilize the GDP-bound conformation of Galpha(i) > Galpha(o). AGS3 and related proteins may influence signal strength or duration as well as the adaptation of the signaling system associated with sustained stimulation. To address these issues, we determined the effect of AGS3 on the integration of stimulatory (Galpha(s)-mediated vasoactive intestinal peptide receptor) and inhibitory (Galpha(i)-mediated alpha(2)-adrenergic receptor (alpha(2)-AR)) signals to adenylyl cyclase in Chinese hamster ovary cells. AGS3-SHORT and AGS3-LONG did not alter the VIP-induced increase in cAMP or the inhibitory effect of alpha(2)-AR activation. System adaptation was addressed by determining the influence of AGS3 on the sensitization of adenylyl cyclase that occurs following prolonged activation of a Galpha(i)-coupled receptor. Incubation of cells with the alpha(2)-AR agonist UK14304 (1 microm) for 18 h resulted in a approximately 1.8-fold increase in the vasoactive intestinal peptide-induced activation of adenylyl cyclase, and this was associated with a decrease in membrane-associated Galpha(i3). Both effects were blocked by AGS3-SHORT. AGS3-SHORT also decreased the rate of Galpha(i3) decay. A mutant AGS3-SHORT incapable of binding G-protein was inactive. These data suggest that AGS3 and perhaps other G-protein regulatory motif-containing proteins increase the stability of Galpha(i) in the membrane, which influences the adaptation of the cell to prolonged activation of Galpha(i)-coupled receptors.

Macrophages exposed continuously to lipopolysaccharide and other agonists that act via toll-like receptors exhibit a sustained and additive activation state

BACKGROUND

Macrophages sense microorganisms through activation of members of the Toll-like receptor family, which initiate signals linked to transcription of many inflammation associated genes. In this paper we examine whether the signal from Toll-like receptors [TLRs] is sustained for as long as the ligand is present, and whether responses to different TLR agonists are additive.

RESULTS

RAW264 macrophage cells were doubly-transfected with reporter genes in which the IL-12p40, ELAM or IL-6 promoter controls firefly luciferase, and the human IL-1beta promoter drives renilla luciferase. The resultant stable lines provide robust assays of macrophage activation by TLR stimuli including LPS [TLR4], lipopeptide [TLR2], and bacterial DNA [TLR9], with each promoter demonstrating its own intrinsic characteristics. With each of the promoters, luciferase activity was induced over an 8 hr period, and thereafter reached a new steady state. Elevated expression required the continued presence of agonist. Sustained responses to different classes of agonist were perfectly additive. This pattern was confirmed by measuring inducible cytokine production in the same cells. While homodimerization of TLR4 mediates responses to LPS, TLR2 appears to require heterodimerization with another receptor such as TLR6. Transient expression of constitutively active forms of TLR4 or TLR2 plus TLR6 stimulated IL-12 promoter activity. The effect of LPS, a TLR4 agonist, was additive with that of TLR2/6 but not TLR4, whilst that of lipopeptide, a TLR2 agonist, was additive with TLR4 but not TLR2/6. Actions of bacterial DNA were additive with either TLR4 or TLR2/6.

CONCLUSIONS

These findings indicate that maximal activation by any one TLR pathway does not preclude further activation by another, suggesting that common downstream regulatory components are not limiting. Upon exposure to a TLR agonist, macrophages enter a state of sustained activation in which they continuously sense the presence of a microbial challenge.

Phosphorylation of Gβ is augmented by chronic morphine and enhances Gβγ stimulation of adenylyl cyclase activity

We previously demonstrated (Chakrabarti, et al., 2001) that in vivo phosphorylation of the Gbeta subunit of G proteins, via protein kinase A (PKA) and protein kinase C (PKC), is dramatically increased following chronic morphine. The present study investigates the PKC isoform selectivity of Gbeta phosphorylation and the consequences thereof on the ability of Gbetagamma to stimulate adenylyl cyclase II (ACII). The catalytic subunit of PKC and PKA, as well as the conventional PKC isoform PKCgamma, was effective in phosphorylating Gbeta. In contrast, Gbeta was only minimally phosphorylated by another conventional isoform, PKCalpha or the atypical isoform PKCzeta. In the presence of activated recombinant Gsalpha, ACII activity was dose dependently stimulated by G(betagamma), the magnitude of which was dependent upon its phosphorylation state. The increment in ACII activity produced by Gbetagamma was increased approximately 2-fold following in vitro phosphorylation by the catalytic subunit of either PKA or PKC. In contrast, the concomitant or sequential phosphorylation of Gbetagamma by PKA and PKC catalytic subunits did not result in an additive enhancement of its ability to stimulate ACII and, in fact, negated the observed enhancing effect of each kinase, individually. Threonine phosphorylated G(beta) occurs naturally in the spinal cord, the levels of which are augmented (approximately 60%) by chronic morphine. The natural occurrence of phosphorylated Gbeta in spinal cord, its up-regulation following chronic morphine and the augmented ability of phosphorylated Gbetagamma to stimulate ACII activity, in the aggregate, indicate that phosphorylation of Gbeta could be a regulatory mechanism causally associated with altered cellular signaling.

Different modes of IL-10 and TGF-beta to inhibit cytokine-dependent IFN-gamma production: consequences for reversal of lipopolysaccharide desensitization

LPS hyporesponsiveness is characterized by a diminished production of proinflammatory cytokines which can be caused by pretreatment with either LPS (=LPS desensitization) or the combination of the anti-inflammatory cytokines IL-10 and TGF-beta. However, the resulting hyporesponsive states differ regarding their reversibility by the IFN-gamma-inducing cytokine IL-12. Therefore, we aimed at studying the reasons for this differential IL-12 responsiveness of IFN-gamma-producing cells and its consequences for LPS hyporesponsiveness in more detail. In an in vitro IL-12/IL-18 responsiveness model, we demonstrated that IL-10, if permanently present, does not directly inhibit IL-12/IL-18 responsiveness in T/NK cells but indirectly interferes with IFN-gamma production in the presence of monocytes. In contrast, TGF-beta acted directly on IFN-gamma-producing cells by interfering with IL-12/IL-18 responsiveness. After removal of IL-10 but not of TGF-beta, LPS hyporesponsiveness can be reverted by IL-12/IL-18. Consequently, the addition of recombinant TGF-beta during LPS desensitization rendered PBMCs hyporesponsive to a reversal by IL-12/IL-18. Our data suggest that the persistence of IL-10 and the presence of TGF-beta determine the level of IFN-gamma inhibition and may result in different functional phenotypes of LPS desensitization and LPS hyporesponsiveness in vitro and in vivo.

Chronic morphine induces the concomitant phosphorylation and altered association of multiple signaling proteins: a novel mechanism for modulating cell signaling

Traditional mechanisms thought to underlie opioid tolerance include receptor phosphorylation/down-regulation, G-protein uncoupling, and adenylyl cyclase superactivation. A parallel line of investigation also indicates that opioid tolerance development results from a switch from predominantly opioid receptor G(i alpha) inhibitory to G(beta gamma) stimulatory signaling. As described previously, this results, in part, from the increased relative abundance of G(beta gamma)-stimulated adenylyl cyclase isoforms as well as from a profound increase in their phosphorylation [Chakrabarti, S., Rivera, M., Yan, S.-Z., Tang, W.-J. & Gintzler, A. R. (1998) Mol. Pharmacol. 54, 655-662; Chakrabarti, S., Wang, L., Tang, W.-J. & Gintzler, A. R. (1998) Mol. Pharmacol. 54, 949--953]. The present study demonstrates that chronic morphine administration results in the concomitant phosphorylation of three key signaling proteins, G protein receptor kinase (GRK) 2/3, beta-arrestin, and G(beta), in the guinea pig longitudinal muscle myenteric plexus tissue. Augmented phosphorylation of all three proteins is evident in immunoprecipitate obtained by using either anti-GRK2/3 or G(beta) antibodies, but the phosphorylation increment is greater in immunoprecipitate obtained with G(beta) antibodies. Analyses of coimmunoprecipitated proteins indicate that phosphorylation of GRK2/3, beta-arrestin, and G(beta) has varying consequences on their ability to associate. As a result, increased availability of and signaling via G(beta gamma) could occur without compromising the membrane content (and presumably activity) of GRK2/3. Induction of the concomitant phosphorylation of multiple proteins in a multimolecular complex with attendant modulation of their association represents a novel mechanism for increasing G(beta gamma) signaling and opioid tolerance formation.

Selective interaction of AGS3 with G-proteins and the influence of AGS3 on the activation state of G-proteins

AGS3 (activator of G-protein signaling 3) was isolated in a yeast-based functional screen for receptor-independent activators of heterotrimeric G-proteins. As an initial approach to define the role of AGS3 in mammalian signal processing, we defined the AGS3 subdomains involved in G-protein interaction, its selectivity for G-proteins, and its influence on the activation state of G-protein. Immunoblot analysis with AGS3 antisera indicated expression in rat brain, the neuronal-like cell lines PC12 and NG108-15, as well as the smooth muscle cell line DDT(1)-MF2. Immunofluorescence studies and confocal imaging indicated that AGS3 was predominantly cytoplasmic and enriched in microdomains of the cell. AGS3 coimmunoprecipitated with Galpha(i3) from cell and tissue lysates, indicating that a subpopulation of AGS3 and Galpha(i) exist as a complex in the cell. The coimmunoprecipitation of AGS3 and Galpha(i) was dependent upon the conformation of Galpha(i3) (GDP GTPgammaS (guanosine 5'-3-O-(thio)triphosphate)). The regions of AGS3 that bound Galpha(i) were localized to four amino acid repeats (G-protein regulatory motif (GPR)) in the carboxyl terminus (Pro(463)-Ser(650)), each of which were capable of binding Galpha(i). AGS3-GPR domains selectively interacted with Galpha(i) in tissue and cell lysates and with purified Galpha(i)/Galpha(t). Subsequent experiments with purified Galpha(i2) and Galpha(i3) indicated that the carboxyl-terminal region containing the four GPR motifs actually bound more than one Galpha(i) subunit at the same time. The AGS3-GPR domains effectively competed with Gbetagamma for binding to Galpha(t(GDP)) and blocked GTPgammaS binding to Galpha(i1). AGS3 and related proteins provide unexpected mechanisms for coordination of G-protein signaling pathways.

Identification of Gbetagamma binding sites in the third intracellular loop of the M(3)-muscarinic receptor and their role in receptor regulation

Gbetagamma binds directly to the third intracellular (i3) loop subdomain of the M(3)-muscarinic receptor (MR). In this report, we identified the Gbetagamma binding motif and G-protein-coupled receptor kinase (GRK2) phosphorylation sites in the M(3)-MR i3 loop via a strategy of deletional and site-directed mutagenesis. The Gbetagamma binding domain was localized to Cys(289)-His(330) within the M(3)-MR-Arg(252)-Gln(490) i3 loop, and the binding properties (affinity, influence of ionic strength) of the M(3)-MR-Cys(289)-His(330) i3 loop subdomain were similar to those observed for the entire i3 loop. Site-directed mutagenesis of the M(3)-MR-Cys(289)-His(330) i3 loop subdomain indicated that Phe(312), Phe(314), and a negatively charged region (Glu(324)-Asp(329)) were required for interaction with Gbetagamma. Generation of the full-length M(3)-MR-Arg(252)-Gln(490) i3 peptides containing the F312A mutation were also deficient in Gbetagamma binding and exhibited a reduced capacity for phosphorylation by GRK2. A similar, parallel strategy resulted in identification of major residues ((331)SSS(333) and (348)SASS(351)) phosphorylated by GRK2, which were just downstream of the Gbetagamma binding motif. Full-length M(3)-MR constructs lacking the 42-amino acid Gbetagamma binding domain (Cys(289)-His(330)) or containing the F312A mutation exhibited ligand recognition properties similar to wild type receptor and also effectively mediated agonist-induced increases in intracellular calcium following receptor expression in Chinese hamster ovary and/or COS 7 cells. However, the M(3)-MRDeltaCys(289)-His(330) and M(3)-MR(F312A) constructs were deficient in agonist-induced sequestration, indicating a key role for the Gbetagamma-M(3)-MR i3 loop interaction in receptor regulation and signal processing.

Nucleoside diphosphate kinase: role in bacterial growth, virulence, cell signalling and polysaccharide synthesis

Nucleoside diphosphate kinase (Ndk) is an important enzyme that generates nucleoside triphosphates (NTPs) or their deoxy derivatives by terminal phosphotransfer from an NTP such as ATP or GTP to any nucleoside diphosphate or its deoxy derivative. As NTPs, particularly GTP, are important for cellular macromolecular synthesis and signalling mechanisms, Ndk plays an important role in bacterial growth, signal transduction and pathogenicity. Specific examples of the role of Ndk in regulating growth, NTP formation and cell surface polysaccharide synthesis in two respiratory tract pathogens, Pseudomonas aeruginosa and Mycobacterium tuberculosis, are discussed.

Increased prostacyclin and PGE2 stimulated cAMP production by macrophages from endotoxin-tolerant rats

Sublethal administration of lipopolysaccharide (LPS) renders rats tolerant to multiple lethal stimuli. Tolerant macrophages exhibit differential alterations in LPS-stimulated cytokine and inflammatory mediator release. Increased cAMP levels stimulated by PGE2 or prostacyclin (PGI2) result in differential effects on LPS-induced cytokine release and protect against the pathophysiological changes of endotoxemia. In the present studies, we sought to determine whether PGE2- and PGI2-stimulated cAMP levels are altered in tolerant macrophages. Incubation of macrophages with cicaprost or 11-deoxy-PGE1 in the presence of phosphodiesterase inhibitors resulted in significantly higher (2.5- to 6.5-fold) cAMP concentrations in tolerant macrophages compared with control. In contrast, isoproterenol-stimulated cAMP levels were not significantly different between control and tolerant cells. Also, incubation of tolerant macrophages with LPS did not result in significantly elevated cAMP levels. Prostacyclin (IP) receptor mRNA levels were significantly increased in tolerant cells compared with controls, whereas [3H]PGE2 binding and PGE2 EP4 receptor mRNA levels were not significantly changed. These studies suggest that LPS tolerance induces selective alterations in eicosanoid regulation of cAMP formation.

Natural pathogens of laboratory mice, rats, and rabbits and their effects on research

Laboratory mice, rats, and rabbits may harbor a variety of viral, bacterial, parasitic, and fungal agents. Frequently, these organisms cause no overt signs of disease. However, many of the natural pathogens of these laboratory animals may alter host physiology, rendering the host unsuitable for many experimental uses. While the number and prevalence of these pathogens have declined considerably, many still turn up in laboratory animals and represent unwanted variables in research. Investigators using mice, rats, and rabbits in biomedical experimentation should be aware of the profound effects that many of these agents can have on research.

Receptor docking sites for G-protein betagamma subunits. Implications for signal regulation

We report the direct interaction of Gbetagamma with the third intracellular (i3) loop of the M2- and M3-muscarinic receptors (MR) and the importance of this interaction relative to effective phosphorylation of the receptor subdomain. The i3 loop of the M2- and the M3-MR were expressed in bacteria and purified as glutathione S-transferase fusion proteins for utilization as an affinity matrix and to generate substrate for receptor subdomain phosphorylation. In its inactive heterotrimeric state stabilized by GDP, brain G-protein did not associate with the i3 peptide affinity matrix. However, stimulation of subunit dissociation by GTPgammaS/Mg2+ resulted in the retention of Gbetagamma, but not the Galpha subunit, by the M2- and M3-MR i3 peptide resin. Purified Gbetagamma bound to the M3-MR i3 peptide with an apparent affinity similar to that observed for the Gbetagamma binding domain of the receptor kinase GRK2 and Bruton tyrosine kinase, whereas transducin betagamma was not recognized by the M3-MR i3 peptide. Effective phosphorylation of the M3-MR peptide by GRK2 required both Gbetagamma and lipid as is the case for the intact receptor. Incubation of purified GRK2 with the i3 peptide in the presence of Gbetagamma resulted in the formation of a functional ternary complex in which Gbetagamma served as an adapter protein. Such a complex provides a mechanism for specific spatial translocation of GRK2 within the cell positioning the enzyme on its substrate, the activated receptor. The apparent ability of Gbetagamma to act as a docking protein may also serve to provide an interface for this class of membrane-bound receptors to an expanded array of signaling pathways.

TNF-alpha tolerance blocks LPS-induced hypophagia but LPS tolerance fails to prevent TNF-alpha-induced hypophagia

To investigate the role of tumor necrosis factor-alpha (TNF-alpha) in bacterial lipopolysaccharide (LPS)-induced hypophagia, we tested whether a cross tolerance between LPS and TNF-alpha exists with respect to their anorectic effects. Only the first of three subsequent intraperitoneal injections of LPS (100 micrograms/kg body wt) given every second day at dark onset (12:12-h light-dark cycle) led to a significant reduction of food intake in male rats. Likewise, intraperitoneal injections of human recombinant TNF-alpha (150 micrograms > or = 3 x 10(6) U/kg body wt) also resulted in tolerance to its hypophagic effect. LPS tolerance did not alter the hypophagic response to subsequently injected TNF-alpha (n = 14). However, TNF-alpha pretreatment completely blocked the hypophagic response to LPS (n = 14). The results demonstrate that tolerance to the hypophagic effect of exogenous TNF-alpha is sufficient to eliminate LPS-induced hypophagia. This is consistent with the hypothesis that endogenous TNF-alpha plays a major role in LPS-induced hypophagia. The ineffectiveness of LPS tolerance to attenuate TNF-alpha-induced hypophagia is compatible with findings demonstrating that reduced TNF-alpha production is an important feature of LPS tolerance.

Tolerance to lipopolysaccharide is not related to the ability of the hypothalamus to produce prostaglandin E2

The effects of repeated administration of Escherichia coli lipopolysaccharide (LPS) on body temperature and hypothalamic prostaglandin E2 (PGE2) production was examined in male Sprague-Dawley rats to elucidate whether the development of endotoxin tolerance is related to the ability of the hypothalamus to produce PGE2. Initial injection of LPS resulted in hyperthermia, preceded by short-termed hypothermia, while no changes in body temperature were observed after the second injection (administered 48 h later). In contrast, LPS induced elevation in hypothalamic PGE2 production after both the first and second injections of the pyrogen. This led us to conclude that endotoxin tolerance is independent of the hypothalamic production of PGE2 in response to repeated administration of LPS.