Pharmacological characteristics and species-related variations of beta 3-adrenergic receptors

Beta-adrenergic receptors (beta-AR) belong to the large multigenic family of receptors coupled to GTP-binding proteins. Three subtypes have been identified: beta 1-, beta 2- and beta 3-AR. Much of the work delineating the precise pharmacological comparison of the three beta-ARs has come from investigations with stably transfected Chinese hamster ovary cells (CHO cells). This review discusses the structure and function of beta 3-AR in various species and presents new findings on a number of beta 3-AR ligands including carazolol, tertatolol and CL 316,243 which were found to be selective and potent beta 3-AR agonists and ZD 2079 and salmeterol which appear to display full but non-subtype selective agonistic activity. Species-related variations of the beta 3-AR pharmacology have been shown for propranolol and bupranolol. With the ongoing characterization of the beta 3-AR at the molecular and cellular level, and with the advent of computer-assisted molecular modelling to aid in the determination of the three-dimensional structure of the receptor, it is thought that novel beta 3-AR compounds will become available with improved selectivity and potency.

Long term phorbol ester treatment down-regulates the beta 3-adrenergic receptor in 3T3-F442A adipocytes

The role of protein kinase C (PKC) in the regulation of the beta 3-adrenergic receptor (beta 3-AR) gene was examined in murine 3T3-F442A adipocytes, which express this receptor subtype at a high level. We also investigated the involvement of this kinase in the modulation of beta 3-AR gene expression by insulin. Long term exposure of 3T3-F442A adipocytes to phorbol 12-myristate 13-acetate (PMA) decreased beta 3-AR mRNA content in a time- and concentration-dependent manner, with maximal changes observed at 6 h (6.5-fold decrease) and at 100 nM PMA. This inhibition was selective for beta 3-AR transcripts, since beta 1- and beta 2-AR mRNA content remained unchanged. Also, (-)-[125I]cyanopindolol saturation and competition binding experiments on adipocyte membranes indicated that PMA induced an approximately 2-fold decrease in beta 3-AR expression, while that of the two other subtypes was not affected. This correlated with a lower efficacy of beta 3-AR agonists to stimulate adenylyl cyclase. Conversely, long term exposure to PMA did not alter adenylyl cyclase activity in response to guanosine 5'-O-(3-thiotriphosphate) or forskolin. The inactive phorbol ester 4 alpha-phorbol 12,13-didecanoate did not repress beta 3-AR mRNA levels. Inhibition of beta 3-AR mRNA by PMA was suppressed by the PKC-selective inhibitor bisindolylmaleimide, and was not observed in PKC-depleted cells, indicating that PKC was involved in this response. mRNA turnover experiments showed that the half-life of beta 3-AR transcripts was not affected by long term PMA exposure. When 3T3-F442A adipocytes were pretreated with PMA for 24 h to down-regulate PKC, or with bisindolylmaleimide, the insulin-induced inhibition of beta 3-AR mRNA levels was reduced by 44-67%. These findings demonstrate that sustained PKC activation exerts a specific control of beta 3-AR gene expression and is involved, at least in part, in the modulation by insulin of this adrenergic receptor subtype.

Anergy induction in encephalitogenic T cells by brain microvessel endothelial cells is inhibited by interleukin-1

Experimental autoimmune encephalomyelitis (EAE) is an inflammatory disease of the central nervous system (CNS) which can be induced, in susceptible strains like Lewis rats, by transfer of activated myelin basic protein (MBP)-specific CD4+ T lymphocytes. The role of cerebral endothelium in the onset of EAE, with regard to adhesion, activation and infiltration in the CNS of encephalitogenic T lymphocytes, is not fully understood. When pretreated by interferon-gamma, the immortalized Lewis rat brain microvessel endothelial (RBE4) cells expressed major histocompatibility complex class II molecules and stimulated MBP-specific proliferation and cytolytic activity of the syngeneic encephalitogenic T cell line, designated PAS. However, RBE4-stimulated PAS lymphocytes subsequently entered an unresponsive state, known as anergy. When inoculated in syngeneic animals, anergic PAS cells, although still cytotoxic, failed to induce EAE, and no cell infiltration was detectable within CNS. The addition of interleukin-1 beta (IL-1 beta) during MBP presentation by RBE4 cells prevented T cell anergy induction, and maintained T cell encephalitogenicity, although PAS cells stimulated in these conditions caused delayed and attenuated clinical signs of EAE, with only discrete inflammatory lesions in the CNS, compared with EAE induced by PAS cells fully activated by thymic cells. Altogether, our results indicate that MBP presentation by brain microvessel endothelial cells to encephalitogenic T cells induces T cell anergy and loss of pathogenicity. In addition, IL-1 beta co-stimulation of T cells prevents anergy induction in vitro and at least partially maintains encephalitogenicity in vivo.

Infection of bovine brain microvessel endothelial cells with Cowdria ruminantium elicits IL-1 beta, -6, and -8 mRNA production and expression of an unusual MHC class II DQ alpha transcript

Cowdria ruminantium is a bacterial parasite that infects ruminants, causing an acute and often fatal disease. These obligate intracellular Gram-negative bacteria preferentially infect neutrophils and vascular endothelial cells, especially in the brain. The present study was performed with bovine brain microvessel endothelial cells in culture, infected by C. ruminantium in the presence or absence of IFN-gamma. Infection induced the production of IL-1 beta, -6, and -8 mRNAs, and this effect was potentiated by IFN-gamma. A semi-quantitative PCR analysis indicated that similar amounts of IL-1 beta and IL-6 mRNAs were produced in response to C. ruminantium infection and to treatment with 30 to 40 ng/ml LPS. In addition, although IFN-gamma induced the synthesis of an MHC class II DQ alpha transcript (1.3 kb), an unusual transcript (1.5 kb) was induced by infection and not after LPS treatment. Infection did not affect MHC class I, class II DQ beta, and invariant chain mRNA levels. The present results suggest that C. ruminantium infection raises the immune activity of brain endothelial cells in vitro and that only part of this response can be attributed to LPS. One can hypothesize that cerebral endothelium in vivo efficiently contributes, by MHC Ag expression and production of ILs, to the activation and/or recruitment of leukocytes to the brain and thus plays an active role in the pathogenesis of cowdriosis and in the immune response to this pathogen.

Infection of bovine brain microvessel endothelial cells with Cowdria ruminantium elicits IL-1 beta, -6, and -8 mRNA production and expression of an unusual MHC class II DQ alpha transcript

Cowdria ruminantium is a bacterial parasite that infects ruminants, causing an acute and often fatal disease. These obligate intracellular Gram-negative bacteria preferentially infect neutrophils and vascular endothelial cells, especially in the brain. The present study was performed with bovine brain microvessel endothelial cells in culture, infected by C. ruminantium in the presence or absence of IFN-gamma. Infection induced the production of IL-1 beta, -6, and -8 mRNAs, and this effect was potentiated by IFN-gamma. A semi-quantitative PCR analysis indicated that similar amounts of IL-1 beta and IL-6 mRNAs were produced in response to C. ruminantium infection and to treatment with 30 to 40 ng/ml LPS. In addition, although IFN-gamma induced the synthesis of an MHC class II DQ alpha transcript (1.3 kb), an unusual transcript (1.5 kb) was induced by infection and not after LPS treatment. Infection did not affect MHC class I, class II DQ beta, and invariant chain mRNA levels. The present results suggest that C. ruminantium infection raises the immune activity of brain endothelial cells in vitro and that only part of this response can be attributed to LPS. One can hypothesize that cerebral endothelium in vivo efficiently contributes, by MHC Ag expression and production of ILs, to the activation and/or recruitment of leukocytes to the brain and thus plays an active role in the pathogenesis of cowdriosis and in the immune response to this pathogen.

Predictive quantitative structure-activity relationships (QSAR) analysis of beta 3-adrenergic ligands

A novel quantitative structure-activity relationships strategy was used to analyze seventeen beta-adrenergic ligands for which we had previously evaluated pharmacological properties in Chinese hamster ovary cells transfected with the human beta 1-, beta 2- or beta 3-adrenergic gene (Blin et al., 1993, Mol. Pharmacol., 44: 1094-1104). These ligands were classified into pharmacological activity categories in order to determine the extent to which molecular structural features may be involved in the selectivity of the interaction with the beta 3-AR, or to define molecular features and properties characteristic of a beta 3-AR high affinity ligand or of a potent beta 3-adrenergic agonist. Topological and physico-chemical molecular descriptors were obtained using a novel software combining calculations with multivariate statistical methods, such as principal component analysis and discriminant analysis. This study showed that beta 1/beta 2-antagonists beta 3-agonists could be differentiate from beta 1/beta 2/beta 3-agonists on the basis of their topological molecular descriptors weighted by partial atomic charge and lipophilicity logP values. Bulky lipophilic groups at the end of the alkylamine chain and an ethoxy function, extending the flexible portion of the molecule and modifying the electron density distribution, were requirements for selective agonism at the beta 3-site. Charge and logP weighted 2D-autocorrelation vectors were properties able to discriminate between classes of agonists in terms of their affinity, potency or intrinsic activity, thus emphasizing the part these molecular descriptors play in determining beta 3-adrenergic ligands. These results, in association with the powerful activity-prediction model evaluated in the test, provide a framework to rationalize the synthesis of new beta 3-AR specific compounds.

Cultured astrocytes release a factor that decreases endothelin-1 secretion by brain microvessel endothelial cells

Endothelin-1 (ET-1), originally characterized as a potent vasoconstrictor peptide secreted by vascular endothelial cells, has now been described to possess a wide range of biological activities within the cardiovascular system and in other organs. Brain microvessel endothelial cells, which, together with perivascular astrocytes, constitute the blood-brain barrier, have been shown to secrete ET-1, whereas specific ET-1 receptors are expressed on astrocytes. It is reported here that conditioned medium from primary cultures of mouse embryo astrocytes could significantly, and reversibly, attenuate the accumulation of both ET-1 and its precursor big ET-1 in the supernatant of rat brain microvessel endothelial cells by up to 59 and 76%, respectively, as assessed by immunometric assay. This inhibitor of ET-1 production was purified by gel-exclusion and ion-exchange chromatography as a 280-Da iron-containing molecule, able to release nitrites upon degradation. These results suggest that astrocytes, via release of an iron-nitrogen oxide complex, may be involved in a regulatory loop of ET-1 production at the level of the blood-brain barrier.

Functional effects of long-term activation on human beta 2- and beta 3-adrenoceptor signalling

The functional effects of long-term activation of beta-adrenoceptors were investigated by measuring adenylyl cyclase activity, cyclic AMP accumulation and cyclic AMP-dependent protein kinase activity in CHW and L cells expressing either human beta 2- or beta 3-adrenoceptors. Pre-incubation of CHW and L cells expressing beta 2-adrenoceptors with 10 microM isoprenaline for 24 h produced a marked reduction in the total receptor number and dramatically reduced the capacity of the receptor to stimulate adenylyl cyclase maximally. In contrast, the ability of beta 3-adrenoceptors number was observed in L but not in CHW cells. Maximal levels of intracellular cyclic AMP concentrations were reached during the first hour of receptor activation with isoprenaline in all four cell lines. In the absence of phosphodiesterase inhibitors, cyclic AMP decreased to basal levels within 24 h of continuous stimulation. This phenomenon occurred more rapidly in cells expressing the beta 2- than the beta 3-adrenoceptors. These results confirm that, at the level of adenylyl cyclase stimulation and cyclic AMP accumulation, the beta 3-adrenoceptor is more resistant than the beta 2-adrenoceptor to long-term desensitization. However, when cyclic AMP-dependent protein kinase activity was considered, a 24 h stimulation of beta 2- and when cyclic AMP-dependent protein kinase activity was considered, a 24 h stimulation of beta 2- and beta 3-adrenoceptor expressing cells led to the desensitization of the kinase in L but not in CHW cells. In conclusion, long-term desensitization may have distinct functional effects on cell signalling depending on the receptor subtype and the cell type considered. These findings might have practical implications for future strategies involving long-term therapies with receptor agonists.

Angiotensin II AT2 receptors are functionally coupled to protein tyrosine dephosphorylation in N1E-115 neuroblastoma cells

Murine N1E-115 neuroblastoma cells are shown to express a single class of angiotensin II (Ang II) receptors that display all the pharmacological properties defining the Ang II receptor subtype 2 (AT2): high affinity for 125I-labelled AT2-selective agonist CGP 42112 (Kd 91 +/- 19 pM); expected rank order of potency (CGP 42112 = (Sar1,Ile8)Ang II > or = Ang II > PD 123319 >> DUP 753) for several Ang II analogues; increased binding in the presence of the reducing reagent dithiothreitol (DTT); and insensitivity to analogues of GTP. Molecular cloning of cDNA encoding AT2 receptors from N1E-115 cells reveals nucleotide sequence identity with the AT2 subtype expressed in fetal tissue. Murine AT2 receptors transiently expressed in COS cells display the same pharmacological profile as endogenous Ang II receptors of N1E-115 cells. Taken together, these data reveal the exclusive presence of the AT2 receptor subtype in N1E-115 cells. Incubation of N1E-115 cells with Ang II leads to a marked decrease in the level of tyrosine phosphorylation of several proteins with apparent molecular masses of 80, 97, 120, 150 and 180 kDa respectively. Tyrosine dephosphorylation of the same set of proteins is observed after treatment with the AT2-specific agonist CGP 42112. The response to both effectors is rapid and transient, showing a maximum between 5 and 10 min, and returning to basal levels after 20-30 min. In both cases, tyrosine dephosphorylation can be prevented by co-incubation with an excess of the antagonist Sarile. These data thus establish that AT2 receptor activation leads to protein tyrosine dephosphorylation in N1E-115 cells, and support a possible role for AT2 receptors in the negative regulation of cell proliferation.

Quantitative analysis of transforming growth factor beta 1 messenger RNA in the liver of patients with chronic hepatitis C: absence of correlation between high levels and severity of disease

Transforming growth factor beta 1 (TGF beta 1) is a cytokine involved in liver fibrogenesis. Previous semiquantitative studies of patients with chronic viral hepatitis showed that liver TGF beta 1 messenger RNA (mRNA) was increased, compared with normal controls and with patients with chronic hepatitis C virus (HCV) infection who responded favorably to interferon alfa (IFN alpha) treatment. To evaluate its potential prognostic significance, we measured liver TGF beta 1 mRNA, using a new competitive reverse gene amplification assay, in a total of 35 patients with chronic HCV. This technique was reproducible and sensitive; we could measure as few as 5,000 molecules of TGF beta 1 mRNA per microgram of total liver RNA. In patients with chronic HCV, the mean level of TGF beta 1 mRNA was 200-fold higher than in controls. However, no correlation could be found between TGF beta 1 mRNA and either the biological (serum amino-terminal peptide of type III procollagen) and histological (Knodell scores) indices of liver fibrosis or a favorable response to IFN alpha therapy. In 9 patients, second liver specimens were obtained after treatment; in most cases, TGF beta 1 mRNA levels and hepatic histological findings varied in parallel. These data are consistent with the hypothesis that TGF beta 1 plays a role in stimulating liver fibrogenesis during chronic HCV, despite the lack of prognostic value of TGF beta 1 mRNA levels measured before treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Species difference in the G protein selectivity of the human and bovine A1-adenosine receptor

The purified bovine brain A1-adenosine receptor has previously been shown to discriminate among closely related G protein alpha-subunits. To obtain analogous information for the human receptor, the cDNA coding for the human A1-adenosine receptor was inserted into a plasmid placing the synthesis of the receptor protein under the control of the MalE promoter. Following induction by maltose, active receptor accumulated in Escherichia coli membranes. Binding of the antagonist 8-[3H]cyclopentyl-1,3-dipropylxanthine to E. coli membranes (KD approximately 2 nM, Bmax approximately 0.2-0.4 pmol/mg) showed the appropriate pharmacological profile. Incubation of E. coli membranes with purified Go,i-reconstituted guanine nucleotide-sensitive high affinity binding of the agonist (-)[125I] N6-3-(iodo-4-hydroxyphenylisopropyl)adenosine to the receptor (KD approximately 1 nM). In the presence of purified beta gamma-subunit, the recombinant receptor interacted equally well with the recombinant G protein alpha-subunits Gi alpha-1, Gi alpha-2, Gi alpha-3; G(o) alpha displayed a lower affinity for the receptor while Gs alpha was inactive. Parallel experiments were carried out in bovine and human brain membranes pretreated with N-ethylmaleimide to inactivate the endogenous G(o)/Gi proteins; Gi alpha-3 was most potent in reconstituting 125I-HPIA binding to bovine membranes, while Gi alpha-1, Gi alpha-2, and G(o) alpha displayed similar affinities. However, in human membranes, Gi alpha-1, Gi alpha-2, and Gi alpha-3, were equipotent and high concentrations of G(o) alpha were required to promote 125I-HPIA binding. These observations show (i) that functional human A1-adenosine receptors were synthesized in E. coli; (ii) that the pattern of G protein coupling is identical for the recombinant human A1-receptor and its counterpart in the native membrane; (iii) and that species differences between bovine and human receptor exist not only in their pharmacological profile but also in their G protein specificity suggesting that species homologues of receptors may use different signaling mechanisms.

Endothelin induces tyrosine phosphorylation and GRB2 association of Shc in astrocytes

While the mitogen-activated protein kinase (MAPK) pathway coupled to receptor tyrosine kinases has been largely clarified, little is known about MAPK activation mediated by heterotrimeric G protein-coupled receptors. In a previous study, it has been shown that endothelin-1 (ET-1) signaling through heterotrimeric G protein-coupled receptors stimulates MAPK activity in primary cultures of astrocytes (Cazaubon, S., Parker, P. J., Strosberg, A.D., and Couraud, P.O. (1993) Biochem. J. 293, 381-386). To clarify the molecular mechanism responsible for this response, involvement of the adapter proteins, Shc and Grb2, has now been investigated. It is shown here that in these cells, ET-1 stimulates tyrosine phosphorylation of Shc, resulting in its stable association with Grb2 but not with Grb3-3, a Grb2 isoform with partially deleted SH2 domain. These results demonstrate that tyrosine-phosphorylated Shc specifically interacts with the SH2 domain of Grb2. This response was rapid and transient, showing a maximum at 10 min and declining at 60 min. Interestingly, direct activation of G proteins by fluoroaluminate mimics the ET-1 effect. In addition, a shift to a higher apparent molecular mass of Raf-1 kinase, likely reflecting its hyperphosphorylation, was also detected in ET-1-treated cells. These data strongly suggest that ET-1-induced MAPK activation is a G protein-coupled pathway that involves Shc, Grb2, and probably Raf-1. In conclusion, the Shc-Grb2 complex may be involved in the activation of the MAPK pathway, not only by several receptor tyrosine kinases but also by heterotrimeric G protein-coupled receptors, such as ET-1 receptors.

Cellular signaling by an agonist-activated receptor/Gs alpha fusion protein

The consequences of agonist-dependent activation of guanine nucleotide-binding protein (G protein)-coupled receptors vary from cell to cell, depending on a complex network of regulations between components of the signaling cascade. Specific interactions between receptors, G proteins, and effectors are difficult to analyze in intact cells. Engineering of receptor-transducer fusion proteins might be an effective strategy to target cellular effectors more efficiently and specifically. As a model, we evaluated the ability of a fusion protein of beta 2-adrenergic receptor bound to the alpha subunit of adenylyl cyclase-stimulatory G protein (Gs alpha) to restore the defective activation of adenylyl cyclase in S49 cyc- cells that lack endogenous Gs alpha. The coupling between the two partners of the fusion protein was functional, and the agonist-dependent activation of the effector was more potent and more productive in transfected than in wild-type S49 cells. The covalent link between receptor and Gs alpha could thus convey an advantage over freely interacting components. Such receptor-G alpha fusion proteins may help to elucidate the complex interactions between members of signaling pathways and may also constitute a useful tool for studying the effects of single effector activation.

Antibodies for the immunochemistry of the human beta 3-adrenergic receptor

Based on the amino acid sequence deduced from the recently cloned human beta 3-adrenergic receptor (hu beta 3AR) gene, polyclonal antibodies were prepared against synthetic peptides, corresponding to regions of hu beta 3AR presumed to be exposed at the outer or the inner side of the membrane on the basis of the putative three-dimensional structure of the previously characterized beta 1 and beta 2 adrenergic receptors. Affinity-purified antibodies directed against N-terminal, extracellular or intracellular loops and C-terminal peptides reacted specifically with the hu beta 3AR and not with either the human beta 1 or beta 2 adrenergic receptor. Using these antibodies, it was demonstrated that the receptor is present at the surface of Chinese Hamster Ovary (CHO) cells transfected with the hu beta 3AR gene; in addition, the presence of the receptor protein was established in a human tissue (gall bladder). Immuno-affinity chromatography of solubilized CHO hu beta 3AR-containing cell membranes allowed the isolation of hu beta 3AR protein with an overall yield of 30%. The degree of purity of the receptor was more than 80%, as assessed by N-terminal sequencing of the protein eluted from the column. Sequence analysis demonstrated the absence of a methionine residue at the N-terminal position, and suggested that the side chain of the asparagine residue at position 7 is glycosylated.

Mediation of most atypical effects by species homologues of the beta 3-adrenoceptor

1. A wide panel of compounds acting on beta-adrenoceptors active either in mammalian heart or in rodent digestive tract and adipose tissues, were investigated for their effects on Chinese hamster ovary cells transfected with the human or murine beta 3-adrenoceptor gene. 2. The beta 3-agonists, bucindolol, CGP 12177A and pindolol exhibited the highest binding affinities; BRL 37344, LY 79771, ICI 201651 and SR 58611A presented high potencies in stimulating adenylyl cyclase; bupranolol appeared as the most efficient beta 3-antagonist. 3. This pharmacological analysis further established that the beta 3-adrenoceptor is the prototype of the adipose tissue atypical beta-adrenoceptor, since these receptors share a number of pharmacological properties which differ strikingly from those of beta 1- and beta 2-adrenoceptors: low affinities for conventional beta-adrenoceptor agonists and antagonists, high potencies for novel compounds active in adipose tissues, partial agonistic activities for several beta 1/beta 2-antagonists. 4. Although the pharmacological profiles of the human and murine beta 3-receptor were very similar, some quantitative or even qualitative differences were observed for particular compounds such as propranolol, which exhibited weak and partial agonistic effects at the human beta 3-receptors and antagonistic effects at the murine beta 3-receptors. These differences may result from key amino-acid substitutions between the human and the murine beta 3-receptor sequences, which may alter the binding site or signal processing. 5. Compounds active on atypical beta-sites of other tissues such as heart and digestive tract were also potent on the beta 3-adrenoceptor expressed in Chinese hamster ovary cells, suggesting that this receptor mediates most of the atypical properties described in various tissues, and that differences in ligand effects may result from tissue-related specificities.

Beta-3 adrenoceptor selectivity of the dioxolane dicarboxylate phenethanolamines

The beta-1, beta-2 and beta-3 adrenergic properties of several benzodioxole-containing phenethanolamines were determined in vitro in both functional and binding assays. In addition, two of the compounds were evaluated for their effects on radioligand binding and cyclic AMP (cAMP) production in stably transfected Chinese Hamster Ovary (CHO) cells expressing the cloned rat or human beta-3 adrenoceptor or the human beta-2 or beta-1 adrenoceptor. The (+/-)-R*,R*-racemate, CL 314,514, and the pure (-)-R,R enantiomer, CL 316,243, stimulated rat adipocyte lipolysis (beta-3 effect) with EC50 values in the low nanomolar range, while having no effect on the rate of contraction of guinea pig atria (beta-1 effect) and little or no ability to prevent the insulin-stimulated incorporation of [14C]glucose into rat soleus muscle glycogen (beta-2 effect) with concentrations as great as 100 microM. The lack of beta-1 and beta-2 adrenergic activity was confirmed by the low affinity of the compounds for beta-1 or beta-2 adrenoceptors in plasma membranes from rat heart or rat soleus muscle, respectively. In CHO cells expressing each human beta adrenoceptor subtype, CL 314,514 bound to beta-3-CHO cells with a Ki of 2 microM and stimulated cAMP production with an activation constant (Kact) of 1 microM, whereas it did not bind to either beta-1- or beta-2-CHO cells at 100 microM. CL 316,243 bound to membranes from rat beta-3-CHO cells with a Ki of 1 microM and stimulated cAMP production in beta-3-CHO cells with a Kact of 0.7 nM.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell-specific down-regulation of the beta 3-adrenergic receptor

Previous studies have shown that beta 3-adrenergic receptors, in contrast to the beta 1 and beta 2 subtypes, do not undergo desensitization following short term activation (minutes) with agonists. Longer activation (hours) has been shown to induce desensitization of the beta 3-adrenergic receptor in some, but not all, cases, suggesting that cell- or species-specific mechanisms may be involved. We investigated the contribution of the cell type to the pattern of beta 3-adrenergic receptor long term desensitization by studying, in parallel, two cell lines (Chinese hamster fibroblasts and murine Ltk- cells) expressing the human beta 3-adrenergic receptor. Sustained agonist-promoted down-regulation of the beta 3-adrenergic receptor could be induced in Ltk- cells, whereas only a transient and weak reduction of receptor number was observed in Chinese hamster fibroblasts. The half-life of the beta 3-adrenergic receptor was not affected by the agonist activation in either cell line, indicating that in contrast to the beta 2-adrenergic receptor, degradation of preexisting receptor protein does not contribute to down-regulation. Sustained reduction of receptor RNA levels, monitored by reverse transcriptase polymerase chain reaction, was exclusively shown in Ltk- cells and probably accounted for most of the observed down-regulation. Differences in the ability of the receptor to stimulate adenylyl cyclase activity in the two cell lines may be responsible for the distinct patterns of beta 3-adrenergic receptor down-regulation.

Virucidal effect of myeloperoxidase on human immunodeficiency virus type 1-infected T cells

Myeloperoxidase is virucidal to human immunodeficiency virus type 1 (HIV-1) in the persistently infected CEM human T-cell line or in acutely infected human peripheral blood mononuclear cells, as judged by viral infectivity and P24 radioimmunoassay. HIV-1 was specifically inactivated by low doses of the human myeloperoxidase (1.4 to 14.3 mU/ml) and the cells were spared. A higher enzyme concentration (143 mU/m) was cytotoxic, but uninfected CEM cells and normal lymphocytes were resistant to > or = 143 mU of myeloperoxidase per ml. The enzyme was virucidal with the Cl- present in medium and did not require exogenous H2O2. Catalase, an antioxidant enzyme, partially inhibited the virucidal effect of myeloperoxidase. Hence, the H2O2 probably came from the HIV-infected cells themselves. These in vitro findings indicate that the myeloperoxidase system is capable of inactivating HIV-1 of infected cells.

Intercellular adhesion molecule 1 activation induces tyrosine phosphorylation of the cytoskeleton-associated protein cortactin in brain microvessel endothelial cells

Inflammatory diseases of the central nervous system, such as multiple sclerosis or experimental autoimmune encephalomyelitis, are characterized by adhesion of lymphocytes on cerebral microvascular endothelium, followed by transendothelial migration into the brain parenchyma. T lymphocyte adhesion to vascular endothelial cells is mediated by several types of adhesion molecules, including the integrin leukocyte function-associated molecule 1 and its endothelial counter receptor intercellular adhesion molecule 1 (ICAM-1), of the immunoglobulin superfamily. In order to understand the molecular mechanisms that support lymphocyte extravasation, we intended to investigate a putative role of ICAM-1 in signal transduction in brain microvessel endothelial cells. Here we describe, using a well differentiated rat brain endothelial cell line (RBE4 cells), that ICAM-1 activation by a specific monoclonal antibody, or by syngeneic encephalitogenic T cells, induces tyrosine phosphorylation of several proteins together with stimulation of the tyrosine kinase p60src activity. One of the major tyrosine-phosphorylated proteins, of 85 kDa, has been identified by immunoprecipitation and immunoblotting, as the recently described actin-binding protein, p60src substrate, cortactin. These findings demonstrate that ICAM-1 activation transduces signals in brain endothelial cells, which may lead to cytoskeleton changes and transendothelial migration of lymphocytes into the brain.

Regulation of gamma-glutamyl transpeptidase and alkaline phosphatase activities in immortalized rat brain microvessel endothelial cells

Rat brain microvessel endothelial cells were immortalized by transfection with a plasmid containing the E1A adenovirus gene. One clone, called RBE4, was further characterized. These cells display a nontransformed phenotype and express typical endothelial markers, Factor VIII-related antigen and Bandeiraea simplicifolia binding sites. When RBE4 cells were grown in the presence of bFGF and on collagen-coated dishes, confluent cultures developed sprouts that extend above the monolayer and organized into three-dimensional structures. The activity of the blood-brain barrier-associated enzyme, gamma-glutamyl transpeptidase (gamma GTP), was expressed in these structures, not in the surrounding monolayer. Similar results were obtained with the microvessel-related enzyme alkaline phosphatase (ALP). Addition of agents that elevate intracellular cAMP reduced the formation of three-dimensional structures, but every cell inside the aggregates still expressed gamma GTP and ALP activities. Such structures, associated with high levels of gamma GTP and ALP activities, were also induced by astroglial factors, including (1) plasma membranes from newborn rat primary astrocytes or rat glioma C6 cells, (2) C6 conditioned media, or (3) diffusible factors produced by primary astrocytes grown in the presence of, but not in contact with RBE4 cells. RBE4 cells thus remain sensitive to angiogenic and astroglial factors for the expression of the blood-brain barrier-related gamma GTP activity, as well as for ALP activity, and could constitute the basis of a valuable in vitro model of the blood-brain barrier.

Characterization of a membrane glycoprotein having pharmacological and biochemical properties of an AT2 angiotensin II receptor from human myometrium

The angiotensin II receptors of human myometrial tissue were characterized using ligand binding, cross-linking with radioactive label, detergent solubilization and partial purification by lectin-affinity chromatography. Human myometrial membrane preparations contained variable amount (5-650 fmol/mg protein) of high affinity (Kd = 44-65 pM) binding sites for 125I-CGP42112, a ligand specific for the AT2 subtype of angiotensin II receptors. Competition studies with AT1-specific and AT2-specific compounds indicated that angiotensin II receptors on these membranes were exclusively of the AT2 subtype. The binding sites for 125I-CGP42112 were efficiently solubilized by the detergent Chaps, albeit with a marked decrease in affinity (Kd = 1.2 nM). The proteins in the myometrial membrane preparation were cross-linked to 125I-[Sar1, Ile8]angiotensin II (Sarile) with disuccinimidyl suberate. When low concentrations of cross-linker were used, a single radiolabelled band of about 66-70 kDa was revealed on SDS/PAGE. At higher concentrations additional bands of about 105-120 kDa and 200 kDa were labelled. The 66-70-kDa and 105-120-kDa bands were separated by electroelution of SDS/PAGE gel slices and submitted to trypsin cleavage. The tryptic-peptide maps were identical for both products, suggesting that the additional bands are homodimers and trimers of the labelled polypeptide. The Chaps-solubilized receptor was retained on wheat-germ-agglutinin-Sepharose and specifically eluted by the competing sugar triacetylchitotriose, leading to a fivefold purification factor. Treatment of the 125I-Sarile-labelled protein with N-glycanase caused a shift in its apparent molecular mass on SDS/PAGE from 66-70 kDa to 33 kDa.