Receptor-specific desensitization of the S49 lymphoma cell adenylyl cyclase. Unaltered behavior of the regulatory component

The same mutation in Gsalpha and transducin alpha reveals behavioral differences between these highly homologous G protein alpha-subunits

Mutating Arg-238 to Glu (R238E) in the switch 3 region of a transducin alpha (*Talpha) in which 27 aa of the GTPase domain have been replaced with those of the alpha-subunit of the inhibitory G protein 1 (Gi1alpha), was reported to create an alpha-subunit that is resistant to activation by GTPgammaS, is devoid of resident nucleotide, and has dominant negative (DN) properties. In an attempt to create a DN stimultory G protein alpha (Gsalpha) with a single mutation we created Gsalpha-R265E, equivalent to *Talpha-R238E. Gsalpha-R265E has facilitated activation by GTPgammaS, a slightly facilitated activation by GTP but much reduced receptor plus GTP stimulated activation, and an apparently unaltered ability to interact with receptor as seen in ligand binding studies. Further, the activity profile of Gsalpha-R265E is that of an alpha-subunit with unaltered or increased GTPase activity. The only change in Gsalpha that is similar to that in *Talpha is that the apparent affinity for guanine nucleotides is decreased in both proteins. The molecular basis of the changed properties are discussed based on the known crystal structure of Gsalpha and the changes introduced by the same mutation in a *Talpha (Gtalpha*) with only 23 aa from Gi1alpha. Gtalpha*-R238E, with four fewer mutations in switch 3, was reported to show no evidence of DN properties, is activated by GTPgammaS, and has reduced GTPase activity. The data highlight a critical role for the switch 3 region in setting overall properties of signal-transducing GTPases.

Characterization of mammalian Gs-alpha proteins expressed in yeast

The guanine nucleotide regulatory protein, GS, mediates transmembrane signaling by coupling membrane receptors to the stimulation of adenylyl cyclase activity. The full length coding sequences for the M(r) = 42-45,000, short form (S), and M(r) = 46-52,000, long form (L), of the alpha-subunits of rat GS were placed in yeast expression vectors under the regulatory control of the copper-inducible CUP1 promoter and transformed into Saccharomyces cerevisiae. In the presence of 100 microM CuSO4, the transformed yeast expressed GS-alpha mRNAs and proteins. In reconstitution experiments, rat GS-alpha(S and L), solubilized from yeast membranes with 1% cholate, conferred NaF-, (-)isoproterenol-, and guanine nucleotide-dependent sensitivity to adenylyl cyclase catalytic units in S49 lymphoma cyc- cell membranes, which are devoid of endogenous GS-alpha. GS-alpha (S) demonstrated twice the activity of GS-alpha(L) in reconstitution assays of fluoride-stimulated adenylyl cyclase activity. Comparison of GS-alpha (S) expressed in yeast with GS purified from rabbit liver or human erythrocytes showed that the crude recombinant protein was fully competent in reconstituting NaF-stimulated adenylyl cyclase activity, but was only 2-5% as potent as purified GS. Addition of bovine brain beta gamma subunits during reconstitution enhanced all parameters of adenylyl cyclase activity for GS-alpha(S and L) obtained from yeast. In contrast, transducin beta gamma only enhanced agonist-stimulated adenylyl cyclase activity for GS-alpha (S and L) following reconstitution. These results demonstrate that the expression of functional mammalian GS-alpha subunits in yeast may be useful for their biochemical characterization.

Hypothalamic hormones modulate G protein levels and second messenger responsiveness in GH3 rat pituitary tumour cells

Thyroliberin (TRH), vasoactive intestinal peptide (VIP) and somatostatin (SRIF) act through receptors that are coupled to guanine nucleotide-binding regulatory proteins (G proteins). Regulation of hormone action may occur at the level of G protein coupling to the receptor or effector systems. In this study we demonstrate that prolonged exposure (for up to 48 hr) of cultured rat pituitary adenoma GH3 cells to these hormones caused homologous and to some extent heterologous attenuation of the adenylyl cyclase (AC) (EC 4.6.1.1) responsiveness. In addition, TRH and SRIF diminished both TRH- and guanosine 5'-[beta gamma-imido]-triphosphate-enhanced phospholipase C (PLC) (EC 3.1.4.3) activity within the same time-course. Measurements of cells membrane levels of Gs protein alpha-subunit (Gs alpha), G(i)-1 alpha/G(i)-2 alpha, G(i)-3 alpha, G(o) alpha and G beta by immunoblotting were performed. TRH and VIP upregulated levels of all G proteins except G(o) alpha and G beta. In contrast, SRIF caused a marked reduction of G beta levels. Thus, TRH and VIP, both acting through Gs, both modulated the alpha-subunit levels of this signal transducer, whereas SRIF, which possibly acts through G(i)-2, did not change the steady state level of G(i)-2 alpha. The actions of TRH, VIP and SRIF are multifaceted at the G protein level, where modulations of subtypes not directly involved in their actions may occur. These findings emphasize the complexity expected to be found in the in vivo situation.

Vasoactive intestinal peptide: autocrine growth factor in neuroblastoma

Neuroblastoma is the most common solid tumor of children less than 5 years of age; yet the biology of this tumor is poorly understood. Neuroblastoma tumors are derived from neural crest precursors; they synthesize both adrenergic and peptidergic neurotransmitters. This study determined VIP receptor expression in primary neuroblastoma tumors prior to chemotherapy. The VIP receptor was expressed in 12 of 15 neuroblastoma tumors as determined by direct binding studies (KD = 1.3-12.4 nM) and VIP-mediated stimulation of adenylate cyclase. The VIP stimulation index for adenylate cyclase in the primary tumor was inversely correlated with the VIP content of the tumor, suggesting that VIP regulates its own receptor expression. Similar observations were made in vitro by comparison of two human neuroblastoma cell lines, IMR32 and SKNSH. Both cell lines were demonstrated to express specific, high affinity VIP receptors (KD = 4 nM and 2.5 nM for IMR32 and SKNSH, respectively). IMR32 cells contained very low levels of VIP (0.6 pg VIP/10(6) cells). Exogenous VIP stimulated adenylate cyclase 22-fold over basal activity and VIP inhibited proliferation of IMR32 cells by 49% in 6-day cultures. On the other hand, SKNSH cells synthesized high levels of VIP (6.3 pg/10(6) cells), metabolized VIP rapidly and demonstrated a low level of VIP-mediated stimulation of adenylate cyclase; their proliferation rate was minimally inhibited by exogenous VIP. These observations help validate the hypothesis that VIP serves as an autocrine growth factor in neuroblastoma.

Clinical and biochemical aspects of depressive disorders: II. Transmitter/receptor theories

The present document is the second of three parts in a review that focuses on recent data from clinical and animal research concerning the biochemical bases of depressive disorders, diagnosis, and treatment. Various receptor/transmitter theories of depressive disorders are discussed in this section. Specifically, data supporting noradrenergic, serotonergic, cholinergic, dopaminergic, GABAergic, and peptidergic theories, as well as interactions between noradrenergic and serotonergic, or cholinergic and catecholaminergic systems are presented. Problems with the data and future directions for research are also discussed. A previous publication, Part I of this review, dealt with the classification of depressive disorders and research techniques for studying the biochemical mechanisms of these disorders. A future publication, Part III of this review, discusses treatments for depression and some of the controversies in this field.

Insulin sensitizes a cultured rat osteogenic sarcoma cell line to hormones which activate adenylate cyclase

Pretreatment of the osteogenic sarcoma cell line UMR-106-01 with insulin results in sensitization to both parathyroid hormone (PTH) and isoproterenol. In insulin-pretreated cells, the two hormones cause a significantly greater cyclic AMP (cAMP) accumulation than in noninsulin-treated cells. In the presence of cholera toxin, which enhances cAMP production by these cells in both the basal and PTH-stimulated state, the effect of insulin is maintained. In the presence of pertussis toxin, which has no effect on basal cAMP accumulation but enhances both PTH and isoproterenol stimulation, insulin sensitization for both hormones is abolished. These data suggest that insulin sensitizes these cells to subsequent hormone stimulation by lessening the action of an inhibitory guanine nucleotide regulatory protein, possibly Gi.

Protein kinases induce isoproterenol desensitization of beta-adrenoceptor-coupled adenylate cyclase system: significance of receptor occupancy

Treatment of rat reticulocytes with isoproterenol resulted in dose- and time-dependent desensitization of adenylate cyclase to beta-adrenoceptor agonist stimulation. Cyclic AMP-dependent protein kinase was possibly involved in this desensitization. Treatment of rat reticulocytes with a phorbol ester, tetradecanoyl phorbol acetate (TPA), also induced desensitization to beta-adrenoceptor agonists, indicating the possible involvement of protein kinase C. The addition of a beta-adrenoceptor agonist enhanced the desensitizing effect of dibutyryl cyclic AMP or TPA. T1/2 (time for induction of half maximal desensitization) was decreased from 30 to 2.5 min in accordance with an increase in the concentration of the agonist. The extent of the desensitization was also increased by addition of the agonist. The potency with which the increase was induced was compatible with the potency for binding of the agonist to beta-adrenoceptors (KD values). These results suggest that cyclic AMP- or phorbol ester-induced desensitization is enhanced by receptor occupation by beta-adrenoceptor agonists.

Beta-adrenergic receptor-coupled adenylate cyclase. Biochemical mechanisms of regulation

Beta-adrenergic receptor-coupled adenylate cyclase is regulated by both amplification and desensitization processes. Desensitization of adenylate cyclase is divided into two major categories. Homologous desensitization is initiated by phosphorylation of the receptors by a beta-adrenergic receptor kinase. This reaction serves to functionally uncouple the receptors and trigger their sequestration away from the cell surface. These sequestered receptors can rapidly recycle to the cell surface or, with time, become down regulated, being destroyed within the cell. Dephosphorylation of the receptors is accomplished in the sequestered compartment of the cell, which may functionally regenerate the receptors and allow their return to the cell surface. In heterologous desensitization, receptor function is also regulated by phosphorylation, but in the absence of receptor sequestration or down regulation. In this case, phosphorylation serves only to functionally uncouple the receptors, that is, to impair their interactions with the guanine nucleotide regulatory protein Ns. Several protein kinases are capable of promoting this phosphorylation, including the cAMP-dependent kinase and protein kinase C. In addition to the receptor phosphorylation, heterologous desensitization is associated with modifications at the level of the nucleotide regulatory protein Ns and perhaps Ni. Adenylate cyclase systems are also subject to amplification that involves a protein kinase C-mediated phosphorylation of the catalytic unit of the enzyme. Phosphorylation of the catalytic unit enhances its catalytic activity and results in amplified stimulation by the regulatory protein Ns. Other receptor/effector systems exhibit qualitatively similar regulatory phenomena, suggesting that covalent modification (phosphorylation) may represent a general mechanism for regulating receptor function.

Arachidonic acid metabolites induce beta-adrenoceptor desensitization in rat lung in vitro

The possible involvement of arachidonic acid (AA) or its metabolites in beta-adrenoceptor desensitization has been studied in rat lung parenchyma both from a functional and a biochemical point of view. In vitro perfusion of rat lungs with AA (3 X 10(-5)M for 20 min) reduced the relaxant effect of isoproterenol (ISO) on lung parenchymal strips, shown by a shift to the right of ISO dose-response curve, similar to that obtained using desensitizing concentration of specific beta-agonist. Moreover, AA treatment reduced the capacity of ISO to stimulate adenylate-cyclase activity, whereas the number of beta-receptor binding sites was not significantly modified. Inhibition of cyclo-oxygenase pathway by indomethacin (INDO) (1.5 X 10(-5)M) prevented both the loss of ISO-relaxing capacity and the decrease of adenylate-cyclase activity induced by AA treatment. In order to support the role of eicosanoids in beta-adrenoceptor desensitization, changes of endogenous free AA levels have also been studied in lung homogenates. Perfusion of rat lung with ISO (10(-6)M for 20 min) decreased by about 50% the levels of free AA and the pretreatment with BW755C (9 X 10(-5)M), a lipo- and cyclo-oxygenase inhibitor, prevented this phenomenon. On the basis of these results, we suggest that the activation of AA cascade is actually involved in beta-adrenoceptor desensitization in lung tissues with a possible interference at the site beyond the drug-receptor interaction.

Molecular mechanisms of receptor desensitization using the beta-adrenergic receptor-coupled adenylate cyclase system as a model

Desensitization, the tendency of biological responses to wane over time despite the continuous presence of a stimulus of constant intensity, is observed in organisms as diverse as bacteria and mammals. Recently, new insights into the molecular mechanisms underlying these phenomena have emerged from the study of the receptors coupled to the ubiquitous second messenger-generating system adenylate cyclase. These mechanisms involve sequestration or down-regulation of the receptors from the cell surface as well as functionally significant covalent modifications of the receptors and/or guanine nucleotide regulatory proteins.

Receptor-specific mechanisms of desensitization of beta-adrenergic receptor function

beta-Adrenergic receptor (beta AR)-specific, agonist-induced desensitization of adenylate cyclase can be shown in most mammalian cells examined to involve at least three reactions. An initial 'uncoupling' reaction leads to a 40-60% loss of catecholamine-stimulated adenylate cyclase activity at a time when no detectable loss of beta AR has occurred. This process precedes by 45-90 sec the appearance of beta AR in cytoplasmic vesicles. Such beta AR exhibit ligand binding properties consistent with their existence on the inside of membrane vesicles; thus, they appear to be formed by a process of agonist-induced beta AR internalization (endocytosis). A third process results in the loss of beta AR, at least in some cases due to receptor degradation. In general, agonist-induced desensitization or down-regulation reactions do not require protein synthesis. Recovery from the desensitized states does not require protein synthesis, whereas recovery from beta AR down-regulation (degraded receptors) requires new receptor synthesis. Agonist-induced beta AR desensitization and down-regulation reactions appear to have much in common with the process of polypeptide hormone-induced receptor down-regulation. The availability of a large number of ligands (agonists, partial agonists and antagonists) for the beta AR should allow the use of this receptor system to gain unique insights into the general processes of ligand-induced, cell surface receptor endocytosis.

Regulation of hormone-responsive Sertoli cell adenylyl cyclase in a cell-free system

Homologous desensitization of either FSH- or isoproterenol-responsive adenylyl cyclases in Sertoli cell membranes can be achieved in a cell-free system. Incubation of membrane particles from cultured immature Sertoli cells with either FSH or isoproterenol resulted in a time- and concentration-dependent loss of subsequent adenylyl cyclase response to the homologous hormone. Half-maximal refractoriness was achieved within 20-30 min of incubation. The concentration of FSH required to obtain half-maximal loss of adenylyl cyclase response (400 ng/ml) was in the same order of magnitude as the apparent Km for FSH-stimulated adenylyl cyclase activity (300 ng/ml). Hormone-induced homologous desensitization was dependent on the presence of both ATP and Mg2+. Increasing concentrations of ATP in the presence of FSH caused a concentration-dependent loss of subsequent response to the homologous hormone. Half-maximal desensitization was achieved at an ATP concentration of 0.2 mM. Increasing concentrations of Mg2+ in the presence of either FSH or isoproterenol caused desensitization for the homologous hormone. The concentration of Mg2+ giving half-maximal effect was approximately 5 mM in excess of ATP and EDTA. However, higher concentrations of free Mg2+, in the absence of hormone, caused desensitization of both FSH- and isoproterenol-sensitive adenylyl cyclase with half-maximal effect at approximately 30 mM. Hormone-specific desensitization was obtained in the presence of GTP. However, when GTP was substituted with the non-hydrolysable analogue GMPP(NH)P the hormonal activation remained constant throughout 90 min of incubation.(ABSTRACT TRUNCATED AT 250 WORDS)

The molecular basis for adrenergic desensitization in hamster brown adipose tissue: uncoupling of adenylate cyclase activation

A crude membrane fraction was prepared from hamster brown adipose tissue. Extensive washing of the crude membranes was crucial for the appearance of specific beta-adrenergic receptor binding as assessed by (-)-[3H]dihydroalprenolol. Adrenergic agents competed for the specific binding sites with beta 1-specificity. Binding characteristics were very similar to those earlier found in intact cells, supporting our previous finding that a single (non-tumour) mammalian cell may contain as many as 60,000 beta-adrenergic receptors. Desensitization in situ (i.e. chronic norepinephrine stimulation due to cold acclimation) only marginally affected the number of beta 1-receptors and their affinity (Ki) for norepinephrine. Total (fluoride-stimulated) adenylate cyclase increased somewhat, but the Kact for norepinephrine slightly decreased. Thus the ratio Ki/Kact was rather unaffected by cold acclimation. However, the fraction of adenylate cyclase which could be stimulated by norepinephrine decreased drastically. GTP introduced a low-affinity form (for agonist) of the receptor. The form observed in isolated cells must primarily be the high-affinity form. The basis for desensitization must reside in a diminished ability to transfer the signal from the receptor to the cyclase. This change may be molecularly located in the N-protein or in its interaction with the receptor.

Beta-adrenergic regulation of Sertoli cell adenylyl cyclase: desensitization by homologous hormone

Incubation of Sertoli cell-enriched cultures with D,L-isoproterenol caused a time- and concentration-dependent, homologous desensitization of isoproterenol-responsive adenyl cyclase, whereas the response to FSH was unaffected. Half-maximal desensitization was achieved within 1 h of preincubation, after which a more gradual loss of response was observed. Preincubation of Sertoli cells for 24 h with increasing concentrations of D,L-isoproterenol demonstrated that the concentration required to obtain half-maximal densensitization was approximately 10-fold lower than the Km for activation of adenylyl cyclase. The function of the guanine nucleotide regulatory component (N-component) of the adenylyl cyclase complex in hormonally desensitized Sertoli cells, as evaluated by activation of adenylyl cyclase by GTP, GMPP(NH)P, fluoride and Mg2+, was not affected by the hormone pretreatment. Preincubation of Sertoli cells with a high concentration of dbcAMP (10(-3) M) for 24 h was associated with a 45% reduction in adenylyl cyclase activation by both FSH and isoproterenol. Also in this case fluoride- and GTP-stimulated adenylyl cyclase activities were normal. However, the effects of dibutyryl cyclic AMP occurred much more slowly than agonist-induced desensitization, indicating that cAMP may not be the primary mediator of homologous desensitization of Sertoli cell adenylyl cyclase by isoproterenol.

Forskolin activates adenylate cyclase activity and inhibits mitosis in in vitro in pig epidermis

The novel adenylate cyclase activator forskolin caused rapid and high intracellular accumulation of cyclic AMP in a floating skin (epidermal) slice system. Increased cAMP levels were also detected in the media. Addition of a phosphodiesterase inhibitor to forskolin-containing medium caused only a slight increase in the intracellular cAMP level and forskolin itself did not inhibit phosphodiesterase activity. Ka of forskolin for epidermal adenylate cyclase was about 2-3 X 10(-5) M. This forskolin activation was rapidly reversed after washing. The forskolin stimulation (Ka 5 X 10(-5) M) was also found when tested with an epidermal membrane preparation which contained the catalytic unit of adenylate cyclase but lacked either the GTP or receptor stimulation. With the epidermal slice system, the combination of forskolin and epinephrine (or histamine) stimulated adenylate cyclase synergistically. The data suggest that forskolin activates not only the catalytic unit but also the nucleotide regulatory protein or the receptor-regulatory protein complex of the adenylate cyclase system. The cAMP accumulation caused by forskolin produced a dose-dependent mitotic inhibition of epidermal cells in an in vitro outgrowth system. This inhibitory effect was reversible 48 h after washing out the forskolin.

Functional modification of the guanine nucleotide regulatory protein after desensitization of turkey erythrocytes by catecholamines

Densensitization of turkey erythrocytes by exposure to the beta-adrenergic agonist (-)isoproterenol leads to decreased activation of adenylate cyclase by agonist, NaF, and guanyl-5'-yl imido diphosphate, with no reduction in the number of beta-adrenergic receptors. Interactions between the receptor and the guanine nucleotide regulatory protein (N protein) also seem to be impaired. These observations suggest that a component distal to the beta-adrenergic receptor may be a locus of modification. Accordingly we examined the N protein to determine whether it was altered by desensitization. The rate at which (-)isoproterenol stimulated the release of [3H]GDP from the N protein was substantially lower in membranes prepared from desensitized cells, providing further evidence for uncoupling of the receptor and the N protein. The amount of N protein in membranes from control and desensitized cells was compared by labeling the 42,000 Mr component of the N protein with [32P]NAD+ and cholera toxin; no significant difference was found. However, significantly more N protein (p less than .001) was solubilized by cholate extraction of desensitized membranes, suggesting an altered association of the N protein with the membrane after desensitization. The functional activity of the N protein was measured by reconstitution of cholate extracts of turkey erythrocyte membranes into S49 lymphoma cyc- membranes. Reconstitution of (-)isoproterenol stimulation of adenylate cyclase activity was reduced significantly (p less than .05) after desensitization. These observations suggest that desensitization of the turkey erythrocyte by (-)isoproterenol results in functional modifications of the guanine nucleotide regulatory protein, leading to impaired interactions with the beta-adrenergic receptor and reduced activation of adenylate cyclase.