5-hydroxytryptamine1A receptor-mediated increases in receptor expression and activation of nuclear factor-kappaB in transfected Chinese hamster ovary cells

Functional Selectivity and Antidepressant Activity of Serotonin 1A Receptor Ligands

Serotonin (5-HT) is a monoamine neurotransmitter that plays an important role in physiological functions. 5-HT has been implicated in sleep, feeding, sexual behavior, temperature regulation, pain, and cognition as well as in pathological states including disorders connected to mood, anxiety, psychosis and pain. 5-HT1A receptors have for a long time been considered as an interesting target for the action of antidepressant drugs. It was postulated that postsynaptic 5-HT1A agonists could form a new class of antidepressant drugs, and mixed 5-HT1A receptor ligands/serotonin transporter (SERT) inhibitors seem to possess an interesting pharmacological profile. It should, however, be noted that 5-HT1A receptors can activate several different biochemical pathways and signal through both G protein-dependent and G protein-independent pathways. The variables that affect the multiplicity of 5-HT1A receptor signaling pathways would thus result from the summation of effects specific to the host cell milieu. Moreover, receptor trafficking appears different at pre- and postsynaptic sites. It should also be noted that the 5-HT1A receptor cooperates with other signal transduction systems (like the 5-HT1B or 5-HT2A/2B/2C receptors, the GABAergic and the glutaminergic systems), which also contribute to its antidepressant and/or anxiolytic activity. Thus identifying brain specific molecular targets for 5-HT1A receptor ligands may result in a better targeting, raising a hope for more effective medicines for various pathologies.

Psychedelics and Immunomodulation: Novel Approaches and Therapeutic Opportunities

Classical psychedelics are psychoactive substances, which, besides their psychopharmacological activity, have also been shown to exert significant modulatory effects on immune responses by altering signaling pathways involved in inflammation, cellular proliferation, and cell survival via activating NF-κB and mitogen-activated protein kinases. Recently, several neurotransmitter receptors involved in the pharmacology of psychedelics, such as serotonin and sigma-1 receptors, have also been shown to play crucial roles in numerous immunological processes. This emerging field also offers promising treatment modalities in the therapy of various diseases including autoimmune and chronic inflammatory conditions, infections, and cancer. However, the scarcity of available review literature renders the topic unclear and obscure, mostly posing psychedelics as illicit drugs of abuse and not as physiologically relevant molecules or as possible agents of future pharmacotherapies. In this paper, the immunomodulatory potential of classical serotonergic psychedelics, including N,N-dimethyltryptamine (DMT), 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT), lysergic acid diethylamide (LSD), 2,5-dimethoxy-4-iodoamphetamine, and 3,4-methylenedioxy-methamphetamine will be discussed from a perspective of molecular immunology and pharmacology. Special attention will be given to the functional interaction of serotonin and sigma-1 receptors and their cross-talk with toll-like and RIG-I-like pattern-recognition receptor-mediated signaling. Furthermore, novel approaches will be suggested feasible for the treatment of diseases with chronic inflammatory etiology and pathology, such as atherosclerosis, rheumatoid arthritis, multiple sclerosis, schizophrenia, depression, and Alzheimer’s disease.

Neuronal phenotype dependency of agonist-induced internalization of the 5-HT(1A) serotonin receptor

Selective serotonin reuptake inhibitors (SSRI) are aimed at increasing brain 5-HT tone; however, this expected effect has a slow onset after starting SSRI treatment because of initial activation of 5-HT(1A) autoreceptor-mediated negative feedback of 5-HT release. After chronic SSRI treatment, 5-HT(1A) autoreceptors desensitize, which allows 5-HT tone elevation. Because 5-HT(1A) receptor (5-HT(1A)R) internalization has been proposed as a possible mechanism underlying 5-HT(1A) autoreceptor desensitization, we examined whether this receptor could internalize under well controlled in vitro conditions in the LLC-CPK1 cell line and in raphe or hippocampal neurons from rat embryos. To this goal, cells were transfected with recombinant lentiviral vectors encoding N-terminal tagged 5-HT(1A)R, and exposed to various pharmacological conditions. Constitutive endocytosis and plasma membrane recycling of tagged-5-HT(1A)R was observed in LLC-PK1 cells as well as in neurons. Acute exposure (for 1 h) to the full 5-HT(1A)R agonists, 5-HT and 5-carboxamido-tryptamine, but not the partial agonist 8-OH-DPAT, triggered internalization of tagged 5-HT(1A)R in serotonergic neurons only. In contrast, sustained exposure (for 24 h) to all agonists induced tagged-5-HT(1A)R endocytosis in raphe serotonergic neurons and a portion of hippocampal neurons, but not LLC-PK1 cells and partial agonist displayed an effect only in serotonergic neurons. In all cases, agonist-induced tagged 5-HT(1A)R endocytosis was prevented by the 5-HT(1A)R antagonist, WAY-100635, which was inactive on its own. These data showed that agonist-induced 5-HT(1A)R internalization does exist in neurons and depends on agonist efficacy and neuronal phenotype. Its differential occurrence in serotonergic neurons supports the idea that 5-HT(1A)R internalization might underlie 5-HT(1A) autoreceptor desensitization under SSRI antidepressant therapy.

CAP37 activation of PKC promotes human corneal epithelial cell chemotaxis

PURPOSE

The objective of this study was to elucidate the signaling pathway through which cationic antimicrobial protein of 37 kDa (CAP37) mediates human corneal epithelial cell (HCEC) chemotaxis.

METHODS

Immortalized HCECs were treated with pertussis toxin (10 and 1000 ng/mL), protein kinase C (PKC) inhibitors (calphostin c, 50 nM and Ro-31-8220, 100 nM), phorbol esters (phorbol 12,13-dibutyrate, 200 nM and phorbol 12-myristate 13-acetate, 1 μM) known to deplete PKC isoforms, and siRNAs (400 nM) before a modified Boyden chamber assay was used to determine the effect of these inhibitors and siRNAs on CAP37-directed HCEC migration. PKCδ protein levels, PKCδ-Thr(505) phosphorylation, and PKCδ kinase activity was assessed in CAP37-treated HCECs using immunohistochemistry, Western blotting, and a kinase activity assay, respectively.

RESULTS

Chemotaxis studies revealed that treatment with pertussis toxin, PKC inhibitors, phorbol esters, and siRNAs significantly inhibited CAP37-mediated chemotaxis compared with untreated controls. CAP37 treatment increased PKCδ protein levels and led to PKCδ phosphorylation on residue Thr(505). Direct activation of PKCδ by CAP37 was demonstrated using a kinase activity assay.

CONCLUSIONS

These findings lead us to conclude that CAP37 is an important regulator of corneal epithelial cell migration and mediates its effects through PKCδ.

CEP biomarkers as potential tools for monitoring therapeutics

BACKGROUND

Carboxyethylpyrrole (CEP) adducts are oxidative modifications derived from docosahexaenoate-containing lipids that are elevated in ocular tissues and plasma in age-related macular degeneration (AMD) and in rodents exposed to intense light. The goal of this study was to determine whether light-induced CEP adducts and autoantibodies are modulated by pretreatment with AL-8309A under conditions that prevent photo-oxidative damage of rat retina. AL-8309A is a serotonin 5-HT1A receptor agonist.

METHODS

Albino rats were dark adapted prior to blue light exposure. Control rats were maintained in normal cyclic light. Rats were injected subcutaneously 3x with 10 mg/kg AL-8309A (2 days, 1 day and 0 hours) before light exposure for 6 h (3.1 mW/cm(2), λ=450 nm). Animals were sacrificed immediately following light exposure and eyes, retinas and plasma were collected. CEP adducts and autoantibodies were quantified by Western analysis or ELISA.

RESULTS

ANOVA supported significant differences in mean amounts of CEP adducts and autoantibodies among the light + vehicle, light + drug and dark control groups from both retina and plasma. Light-induced CEP adducts in retina were reduced ~20% following pretreatment with AL-8309A (n = 62 rats, p = 0.006) and retinal CEP immunoreactivity was less intense by immunohistochemistry. Plasma levels of light-induced CEP adducts were reduced at least 30% (n = 15 rats, p = 0.004) by drug pretreatment. Following drug treatment, average CEP autoantibody titer in light exposed rats (n = 22) was unchanged from dark control levels, and ~20% (p = 0.046) lower than in vehicle-treated rats.

CONCLUSIONS

Light-induced CEP adducts in rat retina and plasma were significantly decreased by pretreatment with AL-8309A. These results are consistent with and extend previous studies showing AL-8309A reduces light-induced retinal lesions in rats and support CEP biomarkers as possible tools for monitoring the efficacy of select therapeutics.

Role of G protein-coupled receptors in inflammation

G protein-coupled receptors (GPCRs) play important roles in inflammation. Inflammatory cells such as polymorphonuclear leukocytes (PMN), monocytes and macrophages express a large number of GPCRs for classic chemoattractants and chemokines. These receptors are critical to the migration of phagocytes and their accumulation at sites of inflammation, where these cells can exacerbate inflammation but also contribute to its resolution. Besides chemoattractant GPCRs, protease activated receptors (PARs) such as PAR1 are involved in the regulation of vascular endothelial permeability. Prostaglandin receptors play different roles in inflammatory cell activation, and can mediate both proinflammatory and anti-inflammatory functions. Many GPCRs present in inflammatory cells also mediate transcription factor activation, resulting in the synthesis and secretion of inflammatory factors and, in some cases, molecules that suppress inflammation. An understanding of the signaling paradigms of GPCRs in inflammatory cells is likely to facilitate translational research and development of improved anti-inflammatory therapies.

Transcriptional dysregulation of 5-HT1A autoreceptors in mental illness

The serotonin-1A (5-HT1A) receptor is among the most abundant and widely distributed 5-HT receptors in the brain, but is also expressed on serotonin neurons as an autoreceptor where it plays a critical role in regulating the activity of the entire serotonin system. Over-expression of the 5-HT1A autoreceptor has been implicated in reducing serotonergic neurotransmission, and is associated with major depression and suicide. Extensive characterization of the transcriptional regulation of the 5-HT1A gene (HTR1A) using cell culture systems has revealed a GC-rich "housekeeping" promoter that non-selectively drives its expression; this is flanked by a series of upstream repressor elements for REST, Freud-1/CC2D1A and Freud-2/CC2D1B factors that not only restrict its expression to neurons, but may also regulate the level of expression of 5-HT1A receptors in various subsets of neurons, including serotonergic neurons. A separate set of allele-specific factors, including Deaf1, Hes1 and Hes5 repress at the HTR1A C(-1019)G (rs6295) polymorphism in serotonergic neurons in culture, as well as in vivo. Pet1, an obligatory enhancer for serotonergic differentiation, has been identified as a potent activator of 5-HT1A autoreceptor expression. Taken together, these results highlight an integrated regulation of 5-HT1A autoreceptors that differs in several aspects from regulation of post-synaptic 5-HT1A receptors, and could be selectively targeted to enhance serotonergic neurotransmission.

The interaction between the mu opioid receptor and filamin A

Our laboratory embarked on research to discover proteins the interaction of which with the mu opioid receptor (MOPr) is required for its function and regulation. We performed yeast two-hybrid screens, using the carboxy tail of the human MOPr as bait and a human brain library. This yielded a number of proteins that seemed to bind to the MOPr C-tail. The one we chose to study in detail was filamin A (FLNA). Evidence was obtained that there was indeed protein-protein binding between the C-tail of MOPr and FLNA. A human melanoma cell line (M2) lacking the gene for FLNA and a control cell line (A7) which differed from M2 only in having been transfected with the gene for FLNA and expressing the FLNA protein were made available to us. We transfected these cell lines with the gene for MOPr and used them in our studies. The absence of FLNA strongly reduced MOPr downregulation as well as desensitization of adenylyl cyclase inhibition and G protein activation. A recent finding, published here for the first time, is that FLNA is required for the activation by mu opioid agonists of the MAP kinase p38. Deletion studies indicated that the MOPr binding site on FLNA is in the 24th repeat, close to its C-terminal. It was further found that FLNA lacking the N-terminal actin binding domain is as capable as full length FLNA to restore cells to control status, suggesting that actin binding is not required. A surprising finding was that upregulation of MOPr by morphine and some agonist analogs occurs in M2 cells lacking FLNA, whereas normal receptor downregulation takes place in A7 cells.

Chronic morphine treatment up-regulates mu opioid receptor binding in cells lacking filamin A

We investigated the effects of morphine and other agonists on the human mu opioid receptor (MOP) expressed in M2 melanoma cells, lacking the actin cytoskeleton protein filamin A and in A7, a subclone of the M2 melanoma cells, stably transfected with filamin A cDNA. The results of binding experiments showed that after chronic morphine treatment (24 h) of A7 cells, MOP-binding sites were down-regulated to 63% of control, whereas, unexpectedly, in M2 cells, MOP binding was up-regulated to 188% of control naive cells. Similar up-regulation was observed with the agonists methadone and levorphanol. The presence of antagonists (naloxone or CTAP) during chronic morphine treatment inhibited MOP down-regulation in A7 cells. In contrast, morphine-induced up-regulation of MOP in M2 cells was further increased by these antagonists. Chronic morphine desensitized MOP in A7 cells, i.e., it decreased DAMGO-induced stimulation of GTPgammaS binding. In M2 cells DAMGO stimulation of GTPgammaS binding was significantly greater than in A7 cells and was not desensitized by chronic morphine. Pertussis toxin treatment abolished morphine-induced receptor up-regulation in M2 cells, whereas it had no effect on morphine-induced down-regulation in A7 cells. These results indicate that, in the absence of filamin A, chronic treatment with morphine, methadone or levorphanol leads to up-regulation of MOP, to our knowledge, the first instance of opioid receptor up-regulation by agonists in cell culture.

Plasticity of 5‐HT1Areceptor‐mediated signaling during early postnatal brain development

The presence of serotonin 1A receptor (5-HT(1A)-R) in the hippocampus, amygdala, and most regions of the frontal cortex is essential between postnatal day-5-21 (P5-21) for the expression of normal anxiety levels in adult mice. Thus, the 5-HT(1A)-R plays a crucial role in this time window of brain development. We show that the 5-HT(1A)-R-mediated stimulation of extracellular signal-regulated kinases 1 and 2 (Erk1/2) in the hippocampus undergoes a transition between P6 and P15. At P6, a protein kinase C (PKC) isozyme is required for the 5-HT(1A)-R -->Erk1/2 cascade, which causes increased cell division in the dentate gyrus. By contrast, at P15, PKC alpha participates downstream of Erk1/2 to augment synaptic transmission through the Schaffer Collateral pathway but does not cause increased cell division. Our data demonstrate that the 5-HT(1A)-R -->Erk1/2 cascade uses PKC isozymes differentially, first boosting the cell division to form new hippocampal neurons at P6 and then undergoing a plastic change in mechanism to strengthen synaptic connections in the hippocampus at P15.

WAY-100635 antagonist-induced plasticity of 5-HT receptors: regulatory differences between a stable cell line and an in vivo native system

We present evidence that the 5-hydroxytryptamine(1A) (5-HT(1A)) receptor antagonist, N-{2-[4-(2-methoxyphenyl)-1-piperazinyl]-ethyl}-N-(2-pyridinyl)cyclohexanecarboxamide (WAY-100635), can induce receptor internalization in a human (h)5-HT(1A) receptor Chinese hamster ovary (CHO-K1) cell system. Exposure of h5-HT(1A) CHO cells to WAY-100635 decreased the cell-surface h5-HT(1A) receptor density in a way that was both time (24-72 h) and concentration (1-100 nm) dependent.[(3)H]WAY-100635 and [(3)H]8-hydroxy-dipropylaminotetralin ([(3)H]8-OH-DPAT) saturation analyses demonstrated a significant reduction (50-60%) in total h5-HT(1A) receptor number in the WAY-100635-treated (100 nm; 72 h) compared with control cells. In WAY-100635-treated cells, the 8-OH-DPAT-mediated inhibition of forskolin (FSK)-stimulated cAMP accumulation was right-shifted and the maximal inhibitory response of 8-OH-DPAT was impaired compared with control cells. Similar results were obtained for 8-OH-DPAT-mediated Ca(2+) mobilization after WAY-100635 treatment. h5-HT(1A) receptors labeled with [(3)H]WAY-100635, as well as [(3)H]4-(2'-Methoxy)-phenyl-1-[2'-(N-2''-pyridinyl)-p-fluorobenzamido]ethyl-piperazine (MPPF), exhibited a time-dependent rate of cellular internalization that was blocked by endocytotic suppressors and was pertussis-toxin insensitive. In contrast, quantitative autoradiographic studies demonstrated that chronic treatment of rats with WAY-100635 for two weeks produced a region-specific increase in the 5-HT(1A) receptor density. In conclusion, prolonged exposure of an h5-HT(1A) cell-based system to the 5-HT(1A) antagonist, WAY-100635, induced a paradoxical internalization of cell surface receptor resulting in depressed functional activity. This suggests that an antagonist can influence 5-HT(1A) receptor recycling in vitro differently to in vivo regulatory conditions.

The effects of ethanol and the serotonin(1A) agonist ipsapirone on the expression of the serotonin(1A) receptor and several antiapoptotic proteins in fetal rhombencephalic neurons

Previously, this laboratory demonstrated that ethanol reduces the number of developing serotonin (5-HT)-containing neurons by increasing apoptosis. We also found that 5-HT(1A) agonists attenuate the proapoptotic effects of ethanol and the ethanol-mediated reduction of fetal 5-HT neurons. These neuroprotective effects are mediated in part by the ability of 5-HT(1A) agonists to activate the phosphatidyl 3'-kinase (PI-3K) prosurvival pathway. NF-kappaB is one of the downstream effectors activated by this pathway. In the present study, we used quantitative real-time reverse transcriptase-polymerase chain reaction (RT-PCR) to determine the effects of 50mM ethanol and 100nM of ipsapirone, a 5-HT(1A) agonist, on the expression of several NF-kappaB-dependent antiapoptotic genes: X-linked inhibitor of apoptosis protein (XIAP), cIAP1, cIAP2, Bcl-2, and Bcl-xl. We also investigated the effects of ethanol and ipsapirone on the expression of the gene encoding the 5-HT(1A) receptor. The results demonstrate that ethanol reduces the expression of several prosurvival genes: XIAP, cIAP1, cIAP2, Bcl-2, and Bcl-xl. Importantly, the ethanol-mediated reduction in the expression of XIAP and Bcl-xl was prevented by co-treatment with ipsapirone. Thus, the damaging effects of ethanol are likely to involve a reduction in several prosurvival proteins. Moreover, the protective effects of ipsapirone on ethanol-treated neurons might involve their ability to prevent the reduction of XIAP and Bcl-xl. Although ipsapirone treatment decreased the expression of cIAP1, Bcl-2, and Bcl-xl in control neurons, our prior studies suggest that their survival is not reduced by ipsapirone. We also observed an increased expression of the 5-HT(1A) receptor in ipsapirone-treated control neurons.

Prolonged treatment with ligands affects ligand binding to the human serotonin(1A) receptor in Chinese hamster ovary cells

1. The serotonin(1A) receptors are members of a superfamily of seven transmembrane domain receptors that couple to G-proteins, and appear to be involved in several behavioral and cognitive functions. 2. We monitored the effect of prolonged treatment of the human serotonin(1A) receptor expressed in Chinese hamster ovary (CHO) cells with pharmacologically well-characterized ligands on its binding to the agonist 8-hydroxy-2(di-N-propylamino)tetralin (8-OH-DPAT) and antagonist 4-(2'-methoxy)-phenyl-1-[2'-(N-2''-pyridinyl)-p-fluorodobenzamido]ethyl-piperazine (p-MPPF). 3. Our results indicate that prolonged treatment with the specific agonist (8-OH-DPAT) differentially affects subsequent binding of the agonist and antagonist to the receptor in a manner independent of receptor-G-protein coupling. Importantly, our results show that prolonged treatment with the commonly used antagonist p-MPPF, and its iodinated analogue 4-(2'-methoxy)-phenyl-1-[2'-(N-2''-pyridinyl)-p-iodobenzamido]ethyl-piperazine (p-MPPI), which have earlier been reported to display similar binding properties to serotonin(1A) receptors, induces significantly different effects on the ligand binding function of serotonin(1A) receptors.

Transmembrane signaling in the brain by serotonin, a key regulator of physiology and emotion

Serotonin (5-HT) is an ancient chemical that plays a crucial functional role in almost every living organism. It regulates platelet aggregation, activation of immune cells, and contraction of stomach and intestinal muscles. In addition, serotonin acts as a neurotransmitter in the brain and the peripheral nervous system. These activities are initiated by the binding of serotonin to 15 or more receptors that are pharmacologically classified into seven groups, 5-HT1 through 5-HT7. Each group is further divided into subgroups of receptors that are homologous but are encoded by discrete genes. With the exception of the 5-HT3 receptor--a cation channel--all of the others are G protein-coupled receptors that potentially activate or inhibit a large number of biochemical cascades. This review will endeavor to compare and contrast such signaling pathways with special attention to their tissue-specific occurrence, their possible role in immediate effects on covalent modification of other proteins, and relatively slower effects on gene expression, physiology and behavior.

Roles of extracellular signal-regulated kinase and Akt signaling in coordinating nuclear transcription factor-kappaB-dependent cell survival after serotonin 1A receptor activation

To investigate the functional consequences of cross-talk between multiple effectors of serotonin (5-HT) 1A receptor, we employed transfected Chinese hamster ovary cells. Activation of 5-HT 1A receptor stimulated extracellular signal-regulated kinase (ERK)1/2, Akt and nuclear transcription factor-kappaB (NF-kappaB). Stimulation of cells with 5-HT 1A receptor agonist induced a rapid but transient ERK1/2 phosphorylation followed by increased phosphorylation of Akt. Elevated Akt activity in turn suppressed Raf activity and induced a decline in ERK activation. The activation of ERK and Akt downstream of 5-HT 1A receptor was sensitive to inhibitors of Ras, Raf and phosphatidylinositol 3-kinase (PI3K). Stimulation of 5-HT 1A receptor also resulted in activation of NF-kappaB through a decrease in inhibitor of nuclear transcription factor-kappaB. In support of the importance of 5-HT 1A receptor signaling for cell survival, inhibition of NF-kappaB facilitated caspase 3 activation and cleavage of poly (ADP-ribose) polymerase, while treatment of cells with agonist inhibited caspase 3, DNA fragmentation and cell death. Both agonist-dependent NF-kappaB activation and cell survival were decreased by Akt Inhibitor II or by overexpression of dominant-negative Akt. These findings suggest a role for 5-HT 1A receptor signaling in the Ras/Raf-dependent regulation of multiple intracellular signaling pathways that include ERK and PI3K/Akt. Of these, only PI3K/Akt and NF-kappaB activation were required for 5-HT 1A receptor-dependent cell survival, implying that the relative distribution of signals between competing transduction pathways determines the functional outcome of 5-HT 1A receptor activation.

Homogeneous Assays for Cellular Protein Degradation Usingβ-Galactosidase Complementation: NF-κB/IκB Pathway Signaling

Activation of cells by the tumor necrosis factor-alpha (TNF-alpha) and interleukin-1 (IL-1) cytokines results in activation of the nuclear factor-kappaB (NF-kappaB) via proteasomal degradation of an associated IkappaB molecule. To monitor cellular IkappaB, the protein was recombinantly expressed as a fusion protein with a novel enzymatic tag, ProLabel (PL). ProLabel is a small 5.5-kDa sequence from the amino-terminal amino acids of beta-galactosidase, possesses a simple ribbon structure, and can be fused to many proteins via the amino or carboxyl terminus. Expression of this construct allows quantitative detection of the recombinant protein in crude lysates by using a method based on beta-galactosidase enzyme fragment complementation (EFC). Transient transfection of IkappaB-PL in HeLa cells generated an EFC signal that was highly correlated with a western analysis of the protein construct. ProLabel expressed alone in the cells did not show any EFC activity, due to rapid proteolytic degradation, indicating a very low background signal from the protein tag. TNF-alpha and IL-1 treatment induced a concentration-dependent degradation of IkappaB-PL, with potency values similar to those reported using other methods. IkappaBM-PL (mutant of IkappaB-PL), in contrast, did not undergo degradation for concentrations up to and including 10 ng/ml TNF-alpha or IL-1, demonstrating that degradation of IkappaB-PL was specific to the NF-kappaB pathway activation. TNF-alpha and IL-1 induced maximal IkappaB-PL degradation within 30 min of induction. This was reversed by several agents that ablate this pathway, including anti-TNF-alpha antibodies and the proteasome inhibitor, MG-132. The assay was amenable to HTS systems, with good precision and reproducibility. Z' values and coefficients of variance for IkappaB-PL degradation were 0.6 and <9%, respectively.

Up-regulation of alpha1B-adrenergic receptors with defects in G protein coupling: ligand-induced protection from receptor instability

The biochemical basis for the unexpected agonist-induced up-regulation of the number of radioligand binding sites for two mutated alpha1B-adrenergic receptors reported previously was investigated. Up-regulation was independent of the expression vector used and was not prevented by cycloheximide or actinomycin D, eliminating several potential transcriptional mechanisms and new receptor protein synthesis. Antagonists were also able to induce up-regulation, suggesting that ligand occupancy without signal generation was sufficient to induce the increase in binding sites. Accordingly, we hypothesized that up-regulation results from ligand-induced protection from inherent instability of these mutated receptors. Studies with receptors in isolated membranes revealed that the two mutated receptors that exhibited up-regulation in intact cells also exhibited an inherent instability of their ligand binding capacity, and binding of either agonists or antagonists to these receptors could protect against the loss of binding. In contrast, the wild-type receptor and other mutated receptors that did not exhibit up-regulation in intact cells did not exhibit instability or ligand-induced protection in isolated membranes. The occurrence of instability and protection in isolated membranes for only those mutated receptors and ligands that exhibit up-regulation in intact cells provides compelling evidence that the apparent up-regulation of binding sites in intact cells results from ligand-induced protection from an inherent instability of these G protein coupling-defective receptors. Inclusion of protease inhibitors markedly reduced the loss of binding in isolated membranes, implicating membrane-localized proteolysis as the likely mechanism for the instability.

The G protein-coupled 5-HT1A receptor causes suppression of caspase-3 through MAPK and protein kinase Calpha

The 5-HT(1A) agonist 8-hydroxy-2 (di-n-propylamino) tetralin (8-OH-DPAT) causes inhibition of caspase-3 and apoptosis via the extracellular signal-regulated kinases (ERK1/2) in hippocampal HN2-5 cells. Two 5-HT(1A) agonists, Repinotan hydrochloride (BAY x 3702) and 8-OH-DPAT, block caspase-3 activation and apoptosis caused by anoxia/reoxygenation and H(2)O(2) treatment. This is reversed upon transient expression of dominant negative Ras (N17Ras) and Raf-1 (Raf301), confirming the involvement of Ras and Raf-1 in this 5-HT(1A)-R-->ERK1/2-->caspase-3 pathway. A selective inhibitor of phospholipase Cbeta (PLCbeta) (U73122) but not a general protein kinase C (PKC) inhibitor (GFX) reversed the 5-HT(1A)-R-mediated ERK1/2 stimulation. However, both GFX and the PKCalpha and PKCbeta(1) inhibitor Gö6976 reversed the ERK1/2-mediated inhibition of caspase-3. ERK-dependent activation of only PKCalpha was observed in immunoprecipitates obtained from 5-HT(1A) agonist-treated HN2-5 cells. Finally, transient expression of kinase-negative PKCalpha eliminated the 8-OH-DPAT-evoked block on the H(2)O(2)-triggered caspase-3 stimulation, establishing PKCalpha as a link between ERK and caspase-3 (5-HT(1A)-R-->PLC-->ERK1/2-->PKCalpha-->caspase-3). Our results elucidate a novel yet general, neuroprotective pathway through which G protein-coupled receptors could cause inhibition of effector caspases, such as caspase-3.

Positive coupling of atypical adenosine A3 receptors on human eosinophils to adenylyl cyclase

Adenosine A(3) receptors are reported to couple negatively to adenylyl cyclase (AC) but their mediation of anti-inflammatory effects in human eosinophils prompted us to investigate their coupling to AC. The A(3)-selective agonists IB-MECA and Cl-IB-MECA evoked a concentration-dependent generation of cAMP (EC(50), 3.2 and 1.8 microM, respectively) and were more potent than the A(2A) agonist CGS 21680 (EC(50)=15.4 microM) and adenosine (EC(50)=19.2 microM). The cAMP response was additive to that produced by forskolin (10 microM). The effect of IB-MECA was insensitive to A(1) and A(2A) receptor antagonists, but was antagonized by the A(3)-selective antagonist MRS 1220 (0.1-2.5 microM) in a competitive manner. The estimated K(B) of 190 nM was, however, atypical. The cyclo-oxygenase inhibitor, indomethacin, had no effect on the cAMP response. A general inverse relationship between cAMP generation and inhibition of degranulation was seen. We conclude that in human eosinophils, an atypical form of A(3) receptors positively coupled to AC may exist. The resulting cAMP generation may underlie the anti-inflammatory actions of A(3) agonists in eosinophils.

Regulatory properties of alpha(1B)-adrenergic receptors defective in coupling to phosphoinositide hydrolysis

Previous studies have suggested that G protein coupling, phospholipase C activation, phosphoinositide hydrolysis, and protein kinase C activation may be required for alpha(1B)-adrenergic receptor regulation, particularly for their endocytosis into intracellular vesicles. Accordingly, the internalization and down-regulation properties of mutated receptors with defects in G protein coupling and second messenger generation were investigated. The Delta12 and Delta5 receptors, previously shown to be defective in G protein coupling, exhibited greater agonist-induced losses of cell surface accessibility assessed by radioligand binding to intact cells on ice than for the wild-type receptor; however, these receptors were completely defective in endocytosis into intracellular vesicles assessed by sucrose density gradient centrifugation. These receptors also did not undergo down-regulation with long-term agonist exposure as did the wild-type receptor; instead, a prominent up-regulation was observed. The Y348A receptor, previously shown to be defective in phosphoinositide hydrolysis and endocytosis was also defective in down-regulation but did not exhibit significant up-regulation. In contrast, a receptor construct with amino acid residues 246 to 261 deleted (Delta[246-261]) was also defective in stimulation of phosphoinositide hydrolysis but exhibited internalization and down-regulation properties essentially identical to those for the wild-type receptor. Together, these results suggest that stimulation of phosphoinositide hydrolysis by alpha(1B)-adrenergic receptors is not required for their endocytosis or down-regulation but that similar and overlapping receptor structural domains are involved in mediating these processes.

Sphingosine 1-phosphate activates nuclear factor-kappa B through Edg receptors. Activation through Edg-3 and Edg-5, but not Edg-1, in human embryonic kidney 293 cells

Sphingosine 1-phosphate (S1P) exerts a variety of actions as a second messenger or as an agonist that binds to one or more members of the Edg family of G protein-coupled receptors. By using human embryonic kidney 293 cells, we show that S1P activates nuclear factor-kappa B (NF-kappa B) in a receptor-dependent fashion. Edg-3 and Edg-5, which are coupled to G(i), G(q), and G(13), affect activation of NF-kappa B, whereas Edg-1, which is coupled to G(i) alone, does not. We find that the activation of NF-kappa B requires protein kinase C and Ca(2+), probably downstream of G(q), but that the activation of Rho alone by S1P, whether through G(q) or G(13), does not translate into the activation of NF-kappa B. G beta gamma has little effect of its own but potentiates the activation of NF-kappa B achieved through other G proteins. We conclude that the activation of NF-kappa B by S1P is a receptor-mediated process that relies primarily on the activation of a phospholipase C by G(q) and secondarily on effector regulation through other G proteins.

Smad7 is induced by norepinephrine and protects rat hepatocytes from activin A-induced growth inhibition

Activin A induces growth arrest of rat hepatocytes in vitro and in vivo. The alpha(1)-adrenergic agonist, norepinephrine (NE), enhances epidermal growth factor-stimulated DNA synthesis and inhibits activin A-induced growth inhibition, but the mechanisms of these actions are unclear. Smad proteins have recently been identified as intracellular signaling mediators of transforming growth factor-beta family members. In the present study, we explored how NE modulates the Smad signaling pathway in rat cultured hepatocytes. We demonstrate that NE inhibits activin A-induced nuclear accumulation of Smad2/3 and that NE rapidly induces inhibitory Smad7 mRNA expression. Infection of Smad7 adenovirus into rat hepatocytes inhibited activin A-induced nuclear accumulation of Smad2/3, enhanced epidermal growth factor-stimulated DNA synthesis, and abolished the growth inhibitory effect of activin A. We also demonstrated that the induction of Smad7 by NE is dependent on nuclear factor-kappa B (NF-kappa B). The amount of active NF-kappa B complex rapidly increased after NE treatment. Preincubation of the cells with an NF-kappa B pathway inhibitor N-tosyl-l-phenylalanine chloromethyl ketone or infection of the cells with an adenovirus expressing an I kappa B super-repressor (Ad5I kappa B) abolished the NE-induced Smad7 expression. These results indicate a mechanism of transmodulation between the Smad and trimeric G protein signaling pathways in rat hepatocytes.

Multiplicity of mechanisms of serotonin receptor signal transduction

The serotonin (5-hydroxytryptamine, 5-HT) receptors have been divided into 7 subfamilies by convention, 6 of which include 13 different genes for G-protein-coupled receptors. Those subfamilies have been characterized by overlapping pharmacological properties, amino acid sequences, gene organization, and second messenger coupling pathways. Post-genomic modifications, such as alternative mRNA splicing or mRNA editing, creates at least 20 more G-protein-coupled 5-HT receptors, such that there are at least 30 distinct 5-HT receptors that signal through G-proteins. This review will focus on what is known about the signaling linkages of the G-protein-linked 5-HT receptors, and will highlight some fascinating new insights into 5-HT receptor signaling.

Inhibition of NF-kappaB-mediated gene transcription by the human A2B adenosine receptor in Chinese hamster ovary cells

NF-kappaB is a transcription factor that plays a vital role in regulating inducible gene expression in immune and inflammatory responses. In view of the well documented effects of adenosine on immune and inflammatory responses, we have explored whether adenosine A1, A2B and A3 receptors regulate NF-kappaB activity in transfected Chinese hamster ovary (CHO) cells using a luciferase reporter gene construct. No increases in NF-kappaB activity were observed in CHO-A1, -A2B and -A3 cells stimulated with the non-selective adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine. Elevating intracellular cyclic AMP (cAMP) levels using forskolin (direct activator of adenylyl cyclase) and rolipram (type IV, cAMP-specific phosphodiesterase inhibitor), inhibited NF-kappaB activity in CHO cells. Adenosine A2B receptor stimulation also inhibited NF-kappaB activity, whereas adenosine A1 and A3 receptor activation had no effect. These data reflect the known coupling of adenosine A2B receptors to increases in cAMP. In conclusion, adenosine A1, A2B and A3 receptors do not directly activate NF-kappaB in CHO cells. However, adenosine A2B receptor activation significantly inhibited NF-kappaB activity. Inhibition of NF-kappaB activity by the adenosine A2B receptor may contribute to the anti-inflammatory effects of adenosine.

Heterodimerization of mineralocorticoid and glucocorticoid receptors at a novel negative response element of the 5-HT1A receptor gene

Negative regulation of neuronal serotonin (5-HT1A) receptor levels by glucocorticoids in vivo may contribute to depression. Both types I (mineralocorticoid) and II (glucocorticoid) receptors (MR and GR, respectively) participate in corticosteroid-induced transcriptional repression of the 5-HT1A gene; however, the precise mechanism is unclear. A direct repeat 6-base pair glucocorticoid response element (GRE) half-site 5'-TGTCCT separated by 6 nucleotides was conserved in human, mouse, and rat 5-HT1A receptor promoters. In SN-48 neuronal cells that express MR, GR, and 5-HT1A receptors, deletion or inactivation of the nGRE (negative GRE) eliminated negative regulation of the rat 5-HT1A or heterologous promoters by corticosteroids, whereas its inclusion conferred corticosteroid-induced inhibition to a heterologous promoter. Bacterially expressed recombinant MR and GR preferentially bound to the nGRE as a heterodimer, as identified in nuclear extracts of MR/GR-transfected COS-7 cells, and with higher affinity than MR or GR homodimers. In SN48 and COS-7 cells, concentration-dependent coactivation of MR and GR was required for maximal inhibitory action by corticosteroids and was abrogated in the L501P-GR mutant lacking DNA binding activity. Corticosteroid-mediated transcriptional inhibition was greater for MR/GR in combination than for MR or GR alone. These data represent the first identification of an nMRE/GRE and indicate that heterodimerization of MR and GR mediates direct corticosteroid-induced transrepression of the 5-HT1A receptor promoter.

Nicotine regulates 5-HT(1A) receptor gene expression in the cerebral cortex and dorsal hippocampus

The 5-HT(1A) receptor has previously been shown to be important in mediating the behavioural effects of nicotine. It is possible that nicotine administration might regulate the levels of 5-HT receptors in limbic and cortical regions, and such regulations may underlie adaptive responses to nicotine in the central nervous system. The effects of acute and chronic systemic (--)-nicotine administration on 5-HT(1A) receptor gene expression were measured by in situ hybridization, in the rat cerebral cortex, dorsal hippocampus and lateral septum. In the cortex, acute nicotine (0.5 mg/kg i.p.) significantly increased the expression of 5-HT(1A) receptor mRNA 2 h and 24 h after injection. Similarly, acute nicotine significantly increased 5-HT(1A) receptor mRNA in the dentate gyrus (DG), CA3 and CA1 regions of the dorsal hippocampus 2 h and 24 h after injection. Acute nicotine was without effect in the lateral septum. Chronic nicotine (0.5 mg/kg i.p; twice daily for 7 days) significantly decreased 5-HT(1A) receptor mRNA in the cortex 2 h after the final injection, but was without effect at 24 h or 72 h. Chronic nicotine caused no changes in 5-HT(1A) mRNA in the lateral septum or dorsal hippocampus. These data demonstrate that nicotine regulates 5-HT(1A) receptor gene expression in the cortex and hippocampus. The rapid regulation of expression of 5-HT(1A) receptor mRNA leads to the hypothesis that nicotine-induced 5-HT release may alter the postsynaptic sensitivity to 5-HT.

Transcriptional mechanisms for induction of 5-HT1A receptor mRNA and protein in activated B and T lymphocytes

Serotonin (5-HT) up-regulates B and T lymphocyte proliferation by activating mitogen-induced cell surface 5-HT(1A) receptors. The mechanism of 5-HT(1A) receptor induction by B and T cell mitogens at the mRNA and protein levels in mouse splenocytes was addressed. Quantitation by RNase protection assay showed maximal increases of 3.4-, 3.0-, 3.8-, and 4.9-fold in relative 5-HT(1A) mRNA levels after 48 h of stimulation of splenocytes with lipopolysaccharide, phytohemagglutinin, concanavalin A, or phorbol 12-myristate 13-acetate plus ionomycin, respectively, as compared with unstimulated cells. Mitogens did not alter 5-HT(1A) mRNA stability (t(12) = 26 h), but induction of 5-HT(1A) mRNA was blocked by the transcriptional inhibitor actinomycin D (10 microgram/ml) and by inhibition of nuclear factor-kappaB signaling. Additionally, mitogenic stimulation of transcription was paralleled by increased cell surface 5-HT(1A) receptor immunoreactivity in splenocytes. Thus, mitogen-induced 5-HT(1A) receptor expression appears to involve transcriptional regulation by the nuclear factor-kappaB signaling cascade. Increased expression of the 5-HT(1A) receptor in activated B and T lymphocytes may enhance the immune response and provide therapeutic target for tissue inflammation and immune stimulation.

Activation of NF-kappa B by bradykinin through a Galpha(q)- and Gbeta gamma-dependent pathway that involves phosphoinositide 3-kinase and Akt

Recent work has suggested a role for the serine/threonine kinase Akt and IkappaB kinases (IKKs) in nuclear factor (NF)-kappaB activation. In this study, the involvement of these components in NF-kappaB activation through a G protein-coupled pathway was examined using transfected HeLa cells that express the B2-type bradykinin (BK) receptor. The function of IKK2, and to a lesser extent, IKK1, was suggested by BK-induced activation of their kinase activities and by the ability of their dominant negative mutants to inhibit BK-induced NF-kappaB activation. BK-induced NF-kappaB activation and IKK2 activity were markedly inhibited by RGS3T, a regulator of G protein signaling that inhibits Galpha(q), and by two Gbetagamma scavengers. Co-expression of Galpha(q) potentiated BK-induced NF-kappaB activation, whereas co-expression of either an activated Galpha(q)(Q209L) or Gbeta(1)gamma(2) induced IKK2 activity and NF-kappaB activation without BK stimulation. BK-induced NF-kappaB activation was partially blocked by LY294002 and by a dominant negative mutant of phosphoinositide 3-kinase (PI3K), suggesting that PI3K is a downstream effector of Galpha(q) and Gbeta(1)gamma(2) for NF-kappaB activation. Furthermore, BK could activate the PI3K downstream kinase Akt, whereas a catalytically inactive mutant of Akt inhibited BK-induced NF-kappaB activation. Taken together, these findings suggest that BK utilizes a signaling pathway that involves Galpha(q), Gbeta(1)gamma(2), PI3K, Akt, and IKK for NF-kappaB activation.

Regulation of galpha i palmitoylation by activation of the 5-hydroxytryptamine-1A receptor

Nearly all alpha subunits of heterotrimeric GTP-binding regulatory proteins (G proteins) are palmitoylated at cysteine residues near the N terminus. A regulated cycle of palmitoylation could provide a mechanism for modulating G protein signaling by affecting protein interactions and localization of the subunit. In the present studies we utilized both [(3)H]palmitate incorporation and pulse-chase techniques to address the dynamics of alpha(i) palmitoylation in Chinese hamster ovary cells. Both techniques demonstrated a dose- and time-dependent change in [(3)H]palmitate labeling of alpha(i) upon activation of stably expressed 5-hydroxytryptamine-1A receptors by the agonist (+/-)-2-dipropylamino-8-hydroxy-1,2,3, 4-tetrahydronaphthalene hydrobromide (DPAT), with an EC(50) of approximately 10 nm. For the incorporation assay, DPAT elicited an approximate doubling in labeling at the earliest time point measured. For the pulse-chase assay, DPAT promoted a significant loss of radiolabel almost equally as fast. These data demonstrate that the exchange of palmitate on alpha(i) is increased upon stimulation of 5-hydroxytryptamine-1A receptors through the combined processes of depalmitoylation and palmitoylation. These results provide the basis for extending the concept of regulated exchange of palmitate beyond G(s) and provide a framework for exploring the specific functional attributes of the palmitoylated and depalmitoylated forms of subunit.

Effect of p-chloroamphetamine on 5-HT1A and 5-HT7 serotonin receptor expression in rat brain

The aim of this study was to investigate if p-chloroamphetamine (PCA), which is neurotoxic to serotonin (5-HT) nerve terminals, was able to induce, like 3,4-methylenedioxymethamphetamine, a region-specific regulation of 5-HT1A receptor mRNA expression. The effect of PCA on the expression of 5-HT7 receptors, which share some pharmacological properties with 5-HT1A receptors, was comparatively studied. PCA (2 x 5 mg/kg) produced a lasting depletion of 5-HT content in the rat frontal cortex and hippocampus. In the hippocampus, the maximal 5-HT depletion was found on day 21 (-70%), whereas in the cortex, the highest 5-HT depletion was found on day 14 (-73%), with a partial but significant recovery on day 21. At the latter time point, 5-HT1A receptor mRNA expression was increased by 80% in the cortex and decreased by 50% in the hippocampus. The 5-HT1A receptor mRNA expression was also enhanced after exposure to PCA of rat cortical but not of hippocampal primary cultures. In regard to 5-HT7 receptor mRNA expression, the most remarkable change after PCA was the great increase (+200%) in the brain-stem. Binding studies to 5-HT1A receptors matched the changes in receptor mRNA expression. Gel shift assays revealed enhanced nuclear protein binding to the KB sequence with use of cortical but not hippocampal extracts of PCA-treated rats. Overall, the data show region-specific changes in 5-HT receptor-type expression that may not be entirely dependent on the neurotoxic effect of PCA on 5-HT terminals.

Differential regulation of vascular smooth muscle nuclear factor kappa-B by G alpha q-coupled and cytokine receptors

NF kappaB has been implicated as a downstream effector of G alphaq-coupled receptor signaling, but whether these and cytokine receptors activate NF kappaB similarly remains unclear. Stimulation of rat vascular smooth muscle cell G alphaq-coupled P2Y nucleotide receptors with UTP induces luciferase transcription from a sensitive and specific NF kappaB dependent promoter. However, these responses are only;15% of that to the reference cytokine IL-1 beta. IL-1 beta is a powerful stimulator of I kappaB alpha degradation, RelA nuclear import, and isoform specific NF kappaB enhancer binding in vitro, responses that are not detectable after P2Y receptor stimulation. Expression of two trans -dominant NF kappaB polypeptides suppresses induction of the NF kappaB reporter and also IL-1 beta stimulated monocyte chemoattractant-1 mRNA, which is not induced by UTP. In contrast, UTP induces higher expression of the endogenous COX-2 and IL-6 mRNAs than does IL-1 beta, implying that G alphaq-coupled receptor evokes additional NF kappaB-independent transcription factors in regulating these two genes. P2Y receptors are as effective as the reference growth factor PDGF-BB at inducing CREB, AP-1, SRE and NFAT transcription, which are largely unaffected by IL-1 beta treatment. NF kappaB is less efficiently activated then several other transcriptional effectors of G alphaq-coupled receptor signaling in vascular smooth muscle cells, and is instead preferentially activated by inflammatory cytokines.

Carbachol activates IkappaB kinase in isolated canine gastric parietal cells

IkappaB kinase (IKK) is a recently discovered kinase complex composed of the kinases IKKalpha and beta, which plays a crucial role in the activation of NF-kappaB. In this study we examined the regulation of IKK by carbachol in isolated gastric parietal cells. IKKalpha and beta activities were measured by immune complex kinase assay. Carbachol induced both IKK alpha and beta in a time-dependent fashion, with a maximal stimulatory effect detected after 5 min of incubation. The action of carbachol was inhibited by the intracellular Ca(++) chelator BAPTA-AM, the PKC inhibitor GF109203X, and the NF-kappaB inhibitor PDTC. Carbachol also induced degradation of IkappaBalpha, which was reversed by addition of both GF109203X and PDTC and stimulated the activity of a NF-kappaB-luciferase reporter gene plasmid in COS-7 cells stably expressing the human M3 muscarinic receptor. In conclusion, carbachol induces IKK in the parietal cells via intracellular Ca(++)- and PKC-dependent signaling pathways. This observation represents a novel mechanism for the regulation of NF-kappaB through the activation of seven transmembrane G-protein-coupled receptors.

The recombinant 5-HT1A receptor: G protein coupling and signalling pathways

The 5-hydroxytryptamine 5-HT1A receptor was one of the first G protein coupled receptors whose cDNA and gene were isolated by molecular cloning methods. Transfection of the cDNA of this receptor into cells previously bearing no 5-HT receptors has resulted in the acquisition of large amounts of information regarding potential signal transduction pathways linked to the receptor, correlations of receptor structure to its various functions, and pharmacological properties of the receptor. Transfection studies with the 5-HT1A receptor have generated critical new information that might otherwise have been elusive. This information notably includes the discovery of unsuspected novel signalling linkages, the elucidation of the mechanisms of receptor desensitization, the refinement of models of the receptor pharmacophore, and the development of silent receptor antagonists, among others. The current review summarizes the most important studies of the recombinant 5-HT1A receptor in the decade since the identification of its cDNA.

Differential coupling of serotonin 5-HT1A and 5-HT1B receptors to activation of ERK2 and inhibition of adenylyl cyclase in transfected CHO cells

Although the subtypes of serotonin 5-HT1 receptors have distinct structure and pharmacology, it has not been clear if they also exhibit differences in coupling to cellular signals. We have sought to compare directly the coupling of 5-HT1A and 5-HT1B receptors to adenylyl cyclase and to the mitogen-activated protein kinase ERK2 (extracellular signal-regulated kinase-2). We found that 5-HT1B receptors couple better to activation of ERK2 and inhibition of adenylyl cyclase than do 5-HT1A receptors. 5-HT stimulated a maximal fourfold increase in ERK2 activity in nontransfected cells that express endogenous 5-HT1B receptors at a very low density and a maximal 13-fold increase in transfected cells expressing 230 fmol of 5-HT1B receptor/mg of membrane protein. In contrast, activation of 5-HT1A receptors stimulated only a 2.8-fold maximal activation of ERK2 in transfected cells expressing receptors at 300 fmol/mg of membrane protein but did stimulate a 12-fold increase in activity in cells expressing receptors at 3,000 fmol/mg of membrane protein. Similarly, 5-HT1A, but not 5-HT1B, receptors were found to cause significant inhibition of forskolin-stimulated cyclic AMP accumulation only when expressed at high densities. These findings demonstrate that although both 5-HT1A and 5-HT1B receptors have been shown to couple to G proteins of the Gi class, they exhibit differences in coupling to ERK2 and adenylyl cyclase.

Lysophosphatidic acid activates NF-kappaB in fibroblasts. A requirement for multiple inputs

Lysophosphatidic acid (LPA) is a growth factor that exerts a number of well characterized biological actions on fibroblasts and other cells. In the present study, we investigated the possibility that LPA activates the transcription factor NF-kappaB. NF-kappaB is a target of cytokines, but its activation by other classes of agonists has raised considerable interest in the control of processes such as inflammation and wound healing through varied mechanisms. We find that LPA causes a marked activation of NF-kappaB in Swiss 3T3 fibroblasts as determined by the degradation of IkappaB-alpha in the cytosol and the emergence of kappaB binding activity in nuclear extracts. The EC50 for activation of NF-kappaB is 1-5 microM, a range similar to that reported for reinitiation of DNA synthesis and activation of the serum response element. Activation of NF-kappaB is attenuated by pertussis toxin and inhibitors of protein kinase C, and it is completely blocked by the Ca2+ chelator BAPTA-AM. The combination of phorbol ester and thapsigargin promotes an activation comparable with that of LPA. Activation by LPA is additionally inhibited by tyrphostin A25 but not genistein or AG1478, indicating a selective utilization of protein-tyrosine kinases, and by certain antioxidants, implying a role for reactive oxygen species. The activation is also inhibited by tricyclodecan-9-yl-xanthogenate (D609), implying a requirement for hydrolysis of phosphatidylcholine. The data demonstrate the utilization of multiple pathways in the activation of NF-kappaB by LPA, not inconsistent with the relevance of several families of GTP-binding regulatory proteins.

Agonist-independent activation of Gz by the 5-hydroxytryptamine1A receptor co-expressed in Spodoptera frugiperda cells. Distinguishing inverse agonists from neutral antagonists

The human 5-hydroxytryptamine1A receptor, when expressed in Spodoptera frugiperda (Sf9) cells, facilitates the binding of [35S]GTPgammaS to a co-expressed GTP-binding regulatory protein, Gz, consistent with constitutive activity. The antagonists 4-(2'-methoxyphenyl)-1-[2'(n-2"-pyridinyl)-p-iodobenzamido]ethyl-p ipe razine (p-MPPI) and the related fluorobenzamido analogue p-MPPF had little (p-MPPI) or no (p-MPPF) effect on this activity. In contrast, a third antagonist, the neuroleptic spiperone, produced an almost complete suppression. Thus, using G protein activation as an index of receptor activity, p-MPPF was classified as a neutral antagonist, p-MPPI as a partial inverse agonist, and spiperone as essentially a full inverse agonist. As predicted, spiperone displayed properties consistent with a special form of noncompetitive antagonism when used to displace the agonist [125I]R-(+)-trans-8-hydroxy-2-[N-n-propyl-N-(3'-iodo-2'-propenyl)amin o]tetralin. Our data profile Sf9 cells as a unique vehicle for the characterization of inverse agonists, as these cells support a systematic pairing of mammalian receptors and G proteins, quantitative assays of G protein activation, and unambiguously labeled populations of coupled and uncoupled receptors.