Desensitization of the beta-adrenergic receptor-coupled adenylate cyclase in cultured mammalian cells. Receptor sequestration versus receptor function

Complexity of agonist- and cyclic AMP-mediated downregulation of the human beta 1-adrenergic receptor: role of internalization, degradation, and mRNA destabilization

Prolonged agonist exposure often induces downregulation of G protein-coupled receptors (GPCRs). Although downregulation of the prototypical beta(2)-adrenergic receptor (beta(2)AR) has been extensively studied, the underlying mechanisms have yet to be resolved. As even less is known about the beta(1)-subtype, we investigated the downregulation of human beta(1)AR stably expressed in Chinese hamster fibroblasts in response to the agonist isoproterenol or the cell-permeable, chlorophenylthio-cAMP (CPT-cAMP). While either effector mediated decreases in both beta(1)AR binding activity and steady-state beta(1)AR mRNA levels, there were significant differences in their actions. Whereas agonist-mediated downregulation of beta(1)AR followed first-order kinetics, that induced by CPT-cAMP was delayed for several hours and approximately 50% of the former. Furthermore, agonist but not CPT-cAMP induced beta(1)AR internalization, and inhibiting internalization also suppressed agonist-mediated downregulation. The latter, however, was more sensitive than the former to agonist concentration (EC(50) of 0.3 vs 48 nM). Thus, at < or =1 nM agonist, downregulation occurred without internalization and with a pattern similar to that mediated by CPT-cAMP. The amounts of beta(1)AR downregulated or internalized were proportional to initial receptor levels but reached saturation at approximately 2 and 3 pmol/mg of protein, respectively. The fate of beta(1)AR protein during downregulation was determined by immunoblotting with anti-C-terminal antibodies. In agonist-treated cells, beta(1)AR protein disappeared with time and without any immunoreactive degradation products. Agonist-mediated downregulation of the human beta(1)AR appears to be a complex process that consists of both agonist- and cAMP-specific components. The former involves both receptor internalization and degradation whereas the latter involves a reduction in receptor mRNA.

Carboxyl-terminal splicing of the rat mu opioid receptor modulates agonist-mediated internalization and receptor resensitization

The rat mu opioid receptor is alternatively spliced into two isoforms (MOR1 and MOR1B) which differ in length and amino acid composition at the carboxyl terminus. When stably expressed in HEK 293 cells, both splice variants bind the mu receptor agonist [D-Ala2,N-Me-Phe4,-Gly-ol5]enkephalin (DAMGO) with similar affinity and exhibit functional coupling to adenylyl cyclase with similar efficiency. However, the shorter isoform, MOR1B, desensitized at a slower rate during prolonged DAMGO exposure (4 h) but resensitized at a faster rate than MOR1 during agonist withdrawal (20 min). Immunocytochemical analysis revealed that DAMGO-induced internalization of MOR1B proceeded much faster than that of MOR1 followed by rapid recycling of the receptor to the cell surface. In addition, the greater resistance of MOR1B to homologous desensitization compared with MOR1 as well as MOR1B resensitization was abolished when receptor reactivation/recycling was blocked with monensin, an inhibitor of endosomal acidification. It is concluded that the sequence at the cytoplasmic tail of MOR1B facilitates clathrin-coated vesicle-mediated endocytosis which, in turn, promotes accelerated receptor reactivation. Taken together, our findings suggest that carboxyl-terminal splicing of the rat mu opioid receptor modulates agonist-induced internalization and receptor resensitization.

Endogenous beta 3- but not beta 1-adrenergic receptors are resistant to agonist-mediated regulation in human SK-N-MC neurotumor cells

Although there is considerable interest in the regulation of the different beta-adrenergic receptor (AR) subtypes, most previous studies have utilized stably transfected cells expressing recombinant receptors under the control of viral promoters. Human SK-N-MC neurotumor cells appear to be novel, since they express both endogenous beta 1AR and beta 3AR based on radioligand binding and on functional response. Saturation binding of either the hydrophilic ligand (-)-[3H]CGP-12177 or the more hydrophobic (-)-[125I]iodocyanopindolol indicated the presence of two populations of binding sites with high and low affinities. With either ligand, the beta 1AR antagonist CGP-20712A preferentially inhibited binding to the high-affinity sites. This is consistent with the latter representing beta 1AR whereas the low-affinity sites represent beta 3AR. Both subtypes appeared to be functional on the basis of isoproterenol stimulation of cyclic adenosine monophosphate (cAMP) in intact cells and adenylyl cyclase activity in cell membranes in the absence and presence of CGP-20712A. SK-N-MC-IXC cells, derived by twice subcloning the parental cells, also expressed both beta AR subtypes, indicating that they co-exist in the same cell. SK-N-MC cells exposed to isoproterenol exhibited a rapid sequestration and a slower downregulation of beta 1AR. The latter subtype also underwent desensitization, as indicated by a rightward shift to less sensitivity in the EC50 for isoproterenol stimulation of adenylyl cyclase activity. In contrast, the beta 3AR subtype was resistant to agonist-mediated sequestration, downregulation, and desensitization. Thus, when endogenously expressed in the same cell line, human beta 1AR and beta 3AR display differences in their ability to be regulated by agonist.

Anomalous behavior of CGP 12177A on beta 1-adrenergic receptors

CGP 12177A originally was developed as a hydrophilic antagonist to detect cell surface beta 1- and beta 2-adrenergic receptors, and subsequently was found to be a partial agonist for the atypical or beta 3-adrenergic receptor. Using hamster cells stably expressing either the human beta 1-, human beta 2- or rat beta 1-adrenergic receptor, we found that CGP 12177A behaved as an agonist of beta 1-adrenergic receptors. Whereas at low concentrations, CGP 12177a behaved as an antagonist and inhibited isoproterenol stimulation of adenylyl cyclase activity, at higher concentrations, it stimulated a response even in the absence of isoproterenol. The agonistic properties of CGP 12177A were positively correlated with the level of beta 1-adrenergic receptor expression. Thus, at low receptor of densities, CGP 12177A behaved as a weak, partial agonist whereas as high receptor densities, the drug was a full agonist. At similar high densities of the beta 2-adrenergic receptor, CGP 12177A acted only as a partial agonist. Competition binding studies to membranes from cells expressing beta 1-adrenergic receptors indicated that approximately 90% of the receptors were in a high affinity, guanine nucleotide-insensitive state for CGP 12177A whereas approximately 10% of the receptors were in a lower affinity, guanine nucleotide-sensitive state for CGP 12177A. We propose that the latter receptors are precoupled to stimulatory G proteins and recognize CGP 12177A as an agonist whereas the high affinity, uncoupled receptors recognize CGP 12177A as an antagonist.

Enhancement of beta-adrenergic-induced cAMP accumulation in activated T-cells

Agonist stimulation of the beta-adrenergic receptor on T-cells results in the production of cAMP, which has been correlated with modulation of T-cell function. In previous studies, we have demonstrated that the mitogen PHA can synergistically enhance the accumulation of cAMP in T-cells in response to the agonist isoproterenol. In this report we have investigated the mechanisms by which dual stimulation of T-cells acts to synergistically enhance cAMP accumulation. The results demonstrate that increasing the levels of intracellular calcium with ionomycin or thapsigargin enhanced isoproterenol-induced cAMP accumulation in T-cells. In contrast, PHA enhanced isoproterenol-induced cAMP by a calcium-independent mechanism as evidenced by stimulation with isoproterenol plus PHA in calcium-free medium. Further studies revealed that PHA prevented both sequestration of the beta-adrenergic receptor and its dissociation from Gs protein in response to isoproterenol stimulation. In contrast, PHA did not prevent the functional uncoupling of the beta-adrenergic receptor from adenylyl cyclase, suggesting that additional mechanisms are likely involved. In summary, these studies demonstrate that dual receptor signalling of T-cells increases cAMP accumulation and offers a potential mechanism for catecholamine modulation of T-cell function.

Agonist-induced transfer of the alpha subunits of the guanine-nucleotide-binding regulatory proteins Gq and G11 and of muscarinic m1 acetylcholine receptors from plasma membranes to a light-vesicular membrane fraction

A clone of a Chinese hamster ovary (CHO) cell line expressing high levels of the human muscarinic M1 acetylcholine (Hm1) receptor undergoes a substantial agonist-specific down-regulation of both Hm1 receptors and the alpha subunits of the guanine-nucleotide-binding (G)-proteins Gq and G11 which is accompanied by a desensitization of inositol-phospholipid-specific-phospholipase-C response [Mullaney, I., Dodd, M. W., Buckley, N. J. & Milligan, G. (1993) Biochem. J. 289, 125-131]. To examine early events in this process, the effect of agonist on subcellular distribution of Gq alpha and G11 alpha and of Hm1 receptors was assessed after short-term and long-term treatment with carbachol. Short-term (30 min) incubation with carbachol (1 mM) induced a simultaneous transfer of a proportion of both Gq alpha and G11 alpha and Hm1 receptors from plasma membranes to distinct light vesicular membranes. The total number of receptors and of Gq alpha and G11 alpha in each cell remained unchanged under these conditions. A similar transfer was noted for the G-protein Gs alpha but not for intrinsic plasma membrane markers. The plasma membrane, as well as light vesicular membrane, pool of Gs alpha subunit was unaffected by further sustained incubation with carbachol (16 h), whereas Hm1 receptors and both Gq alpha and G11 alpha proteins were down-regulated to 25% and 40%, respectively, when compared with untreated cells. Such observations support the idea that down-regulation of both the Hm1 receptor and its associated inositol-phospholipid-specific-phospholipase-C-linked G-proteins is produced by two sequential steps. The first is a transfer of signal-transducing polypeptides from the plasma membrane to a non-plasma membrane light vesicle fraction. The second step is represented by an agonist-specific down-regulation pathway. Both the Hm1 receptor and Gq alpha/G11 alpha appear to follow similar sequestration and down-regulation patterns.

Independent and coordinate regulation of beta 1- and beta 2-adrenergic receptors in rat C6 glioma cells

Rat C6 glioma cells have both beta 1- and beta 2-adrenergic receptors in approximately 7:3 ratio. When the cells were exposed to the beta-adrenergic agonist isoproterenol, there was a rapid sequestration of up to 50% of the surface receptor population over a 30-min period as measured by the loss of binding of the hydrophilic ligand [3H] CGP-12177 to intact cells. Using the beta 1-selective antagonist CGP 20712A to quantify the proportion of the two subtypes, it was found that although both beta 1 and beta 2 receptors were sequestered, the latter were sequestered initially twice as fast as the former. More prolonged agonist exposure led to a down-regulation of approximately 90% of the total receptor population by 6 h as measured by the loss of binding of the more hydrophobic ligand [125I]iodocyanopindolol to cell lysates. The two subtypes, however, underwent down-regulation with similar kinetics. Treatment of the cells with agents that raise cyclic AMP levels such as cholera toxin and forskolin resulted in a slower, but still coordinated down-regulation of both subtypes. Thus, there appears to be both independent and coordinate regulation of endogenous beta 1-and beta 2-adrenergic receptors in the same cell line.

Multiple mechanisms of serotonin 5-HT2 receptor desensitization

Desensitization of serotonin 5-HT2 receptor-mediated enhancement of the N-methyl-D-aspartate (NMDA) depolarization was studied in rat cortical neurons. Serotonin and (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) induced long term desensitization. Staurosporine, a nonspecific protein kinase C inhibitor, potentiated the serotonin and DOI facilitation, suggesting acute desensitization was operative. In the case of DOI, long term desensitization was prevented by staurosporine. Activators of protein kinase C abolished the serotonin facilitation, an action prevented by staurosporine. Concanavalin A potentiated the facilitation at 100 microM, but not 30 microM serotonin, suggesting these receptors undergo dose dependent internalization. Calmodulin antagonists prevent long term desensitization induced by serotonin. The depolarization induced by NMDA alone was not altered by staurosporine, protein kinase C activators, concanavalin A or calmodulin antagonists. Serotonin at 100 microM, but not 30 microM, induced heterologous desensitization of phenylephrine and carbachol induced facilitation of the NMDA depolarization. We conclude that serotonin 5-HT2 receptors both induce and undergo several forms of desensitization.

Rapid agonist-induced beta-adrenergic receptor kinase translocation in C6 glioma cells

Exposure of C6 glioma cells to 1 microM isoproterenol leads to fast desensitization of the beta-adrenergic receptor/adenylyl cyclase system and transient receptor sequestration. It also triggers a very rapid and transient translocation to the plasma membrane of beta-adrenergic receptor kinase (beta ARK), a specific cytoplasmic kinase that phosphorylates only the agonist-occupied form of several G protein-coupled receptors. beta ARK-mediated receptor phosphorylation appears to be a suitable mechanism for the rapid regulation of adrenergic receptor function in the nervous tissue.

Pretreatment of cerebellar granule cells with concanavalin A potentiates quisqualate-stimulated phosphoinositide hydrolysis

The hydrolysis of phosphoinositides (PI) elicited in cerebellar granule cell cultures by agonists of metabolotropic glutamate receptors, glutmate and quisqualate, was enhanced when the cells were pretreated with concanavalin A (Con-A). A similar effect was produced by wheat germ agglutinin, but not by several other lectins tested. Con-A produced a dose-dependent effect (EC50 = 3 microM) and increased the efficacy but not the potency of the agonists. In contrast, Con-A failed to enhance PI hydrolysis evoked by N-methyl-D-aspartate, kainate, carbachol, the calcium ionophore A23187, or 50 mM K+. The Con-A stimulatory effect was prevented by simultaneous pretreatment with the agonists of ionotropic quisqualate receptors quisqualate, kainate, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, but not by the antagonist 6-cyano-7-nitroquioxaline-2,3-dione (CNQX). CNQX, which did not inhibit quisqualate-stimulated PI hydrolysis in untreated cells, abolished the component of quisqualate response enhanced by Con-A pretreatment. The pretreatment with Con-A also increased the influx of 45Ca2+ in granule cells stimulated by quisqualate. This increase was inhibited by CNQX. Moreover, the potentiation of PI hydrolysis by Con-A, but not the response to quisqualate alone, was abolished in the absence of Ca2+ and Na+. Pretreatment of granule cells with pertussis toxin inhibited PI hydrolysis stimulated by the metabolotropic quisqualate receptor and the Con-A-potentiated response by the same percentage, but Ca2+ influx induced by quisqualate was not affected.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization and regulation of beta 1-adrenergic receptors in a human neuroepithelioma cell line

Intact human neuroepithelioma SK-N-MC cells bound the beta-adrenergic antagonist (-)-[3H]-CGP 12177 with a KD of 0.13 nM and a Bmax of 17,500 sites/cell. When the cells were exposed to beta-adrenergic agonists, they accumulated cyclic AMP in the following order of potency: isoproterenol much greater than norepinephrine greater than epinephrine, which is indicative of a beta 1-subtype receptor. Membranes prepared from the cells bound (-)-3-[125I]iodocyanopindolol with a KD of 11.5 pM. Inhibition of agonist-stimulated cyclic AMP production and competition binding experiments indicated that the beta 1-selective antagonists CGP 20712A and ICI 89,406 were much more potent than the beta 2-selective antagonist ICI 118,551. Analysis of the displacement curves indicated that the cells contained only beta 1-adrenergic receptors. Northern blot analysis of SK-N-MC mRNA using cDNA probes for the beta 1- and beta 2-adrenergic receptors revealed the presence of a very strong beta 1-adrenergic receptor mRNA signal, while under the same conditions no beta 2-adrenergic receptor mRNA was observed. Thus, SK-N-MC cells appear to express a pure population of beta 1-adrenergic receptors. When the cells were exposed to isoproterenol, there was no observable desensitization during the first hour. After longer exposure, desensitization slowly occurred and the receptors slowly down-regulated to 50% of control levels by 24 h. Other agents that elevate cyclic AMP levels, such as forskolin, cholera toxin, and cyclic AMP analogues, caused no or little substantial receptor loss.

The biology of beta-adrenergic receptors: analysis in human epidermoid carcinoma A431 cells

1. G-protein-linked transmembrane signaling has emerged as a major pathway for information transduction across the cell membrane. 2. In addition to photopigments that propagate the signal from light, cell-surface receptors for hormones, neurotransmitters, and autacoids propagate signals from ligand binding to membrane-bound effector units via G-proteins. 3. Biochemical and molecular features of one prominent member of these receptors, the beta-adrenergic receptor, will be highlighted in the present article. 4. The role of the human epidermoid carcinoma A431 cells as a model for the study of the structure and biology of beta-adrenergic receptors will be emphasized. 5. A model for receptor regulation, gleaned from recent advances in the biochemistry, cell and molecular biology of beta-adrenergic receptors, is discussed.

Altered adrenoceptor responsiveness during adrenaline infusion but not during mental stress: differences between receptor subtypes and tissues

1. Effects of 3 h infusions of adrenaline (0.4 nmol kg-1 min-1) or placebo and of mental stress evoked by a colour word test (CWT) on adrenergic receptor function were investigated in healthy men. Responses of heart rate, blood pressure, plasma catecholamines, plasma cyclic AMP and plasma free fatty acids (FFA) were evaluated during infusions and CWT. In vitro beta 2-adrenoceptor numbers [( 125I]-HYP binding) and function (isoprenaline induced cyclic AMP accumulation) were studied on lymphocytes in all experiments. alpha 2-adrenoceptor binding [( 3H]-yohimbine and adrenaline) to intact platelets was evaluated in the infusion experiments only. 2. Placebo infusion evoked no major alterations of any parameter. 3. Adrenaline infusion raised venous plasma adrenaline levels to 4-5 nmol l-1, increased heart rate by 14 +/- 3 beats min-1 and plasma cyclic AMP by 17 +/- 3 nmol l-1, and decreased diastolic blood pressure by 15 +/- 5 mm Hg. These responses persisted throughout the infusion. Plasma FFA levels, on the other hand, increased at 30 min of infusion (from 236 +/- 44 to 717 +/- 92 mumol l-1) and returned to basal levels after 3 h of infusion. 4. In vitro, lymphocytes showed increased beta 2-responsiveness after 30 min of adrenaline infusion (delta cyclic AMP increased from 1.86 +/- 0.24 to 3.06 +/- 0.58 pmol/10(6) cells), but a decreased response (0.47 +/- 0.10 pmol/10(6) cells) after 3 h of infusion. [125I]-HYP binding to lymphocyte membranes showed a three-fold increase of Bmax at 30 min of adrenaline infusion followed by a return to basal values after 3 h of infusion. [125I]-HYP binding reflected the functional responsiveness of the lymphocytes in vitro poorly. alpha 2-adrenoceptors on platelets were not altered with regard to Bmax or Kd for [3H]-yohimbine binding or Ki for adrenaline displacement of [3H]-yohimbine binding. 5. CWT evoked marked circulatory changes, a four-fold increase in plasma adrenaline and a 60% increase in beta 2-adrenoceptor binding sites without changes in functional responsiveness of the lymphocytes. 6. We conclude that exposure to high physiological levels of adrenaline in vivo alters lymphocyte beta-adrenoceptor responsiveness in a biphasic manner, with an early increase followed by a later decrease, but that most beta-adrenoceptor mediated responses to adrenaline in vivo remain intact. Lymphocyte alterations may reflect recruitment of cells into the circulation during sympathoadrenal stimulation. Platelet alpha 2-adrenoceptors are apparently not easily subjected to agonist induced dynamic receptor regulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Desensitization of the stimulatory A2 adenosine receptor-adenylate cyclase system in vascular smooth muscle cells from rat aorta

We have previously shown that adenylate cyclase present in rat aorta vascular smooth muscle cells can be stimulated by adenosine, its analogs and other agonists. In the present studies, we have examined the effect of preexposure of aorta vascular smooth muscle cells to N-ethylcarboxamide adenosine (NECA) on adenylate cyclase activity stimulated by NECA and other agonists. The vascular smooth muscle cells, when exposed to NECA, resulted in a concentration- and time-dependent loss of NECA-stimulated adenylate cyclase activity. NECA stimulated adenylate cyclase activity by about 120% in control cells, which was decreased to 20% in cells pretreated with 50 microM NECA for 30 min at 37 degrees C. However, GTP-, isoproterenol-, and forskolin-sensitive adenylate cyclase activities were not affected by such treatment, suggesting that NECA treatment of the cells resulted in homologous desensitization. Similarly, the exposure of the cells to isoproterenol resulted in the desensitization of isoproterenol-stimulated adenylate cyclase activity without affecting the NECA-stimulated adenylate cyclase activity. Furthermore, when NECA-treated cells were washed free of agonist, the desensitized state was reversed and the cells regained about 75% responsiveness to NECA stimulation of adenylate cyclase.

Human lymphocyte and myocardial beta-adrenoceptors: up and down regulation

Radioreceptor assays have extended the knowledge of beta-adrenoceptor regulation. To evaluate the usefulness of this method in cardiovascular pharmacology, this study was designed to determine: (1) the correlation between beta-adrenoceptor density in human lymphocytes and myocardial cell membranes. This was achieved by using simultaneously harvested left auricles and whole blood samples in 10 patients scheduled to undergo cardiothoracic surgery. The correlation was found to be linear (r = 0.65; P less than 0.05; N = 10); (2) the changes in lymphocyte receptor density and affinity in heart failure. Lymphocytes were harvested from 6 healthy volunteers, 8 patients with moderate heart failure (NYHA class I or II) and 8 patients with severe heart failure (NYHA class III or IV). Mean densities observed were 75.6 +/- 11, 46.3 +/- 18 and 26.4 +/- 5.9 fmol/mg protein, respectively, and dissociation constants were 62.8 +/- 16; 67.8 +/- 14, and 46 +/- 26 pM. The number of receptors fell significantly from one heart failure class to that immediately above it (P less than 0.01; N = 22); (3) the beta-adrenoceptor regulatory properties of a class I antiarrhythmic drug, propafenone, the chemical structure of which is similar to that of the beta-blocking drug propranolol. Five patients needing antiarrhythmic treatment were given 10 d of oral propafenone treatment (450-900 mg/day). Mean adrenoceptor densities (Bmax) were 22.7 +/- 9 and 43.7 +/- 9 fmol/mg protein, and dissociation constants (KD) values were 24.7 +/- 20 and 27 +/- 18 pM respectively, before and after 10 d of chronic treatment. The number of receptors increased significantly after 10 d of treatment (P less than 0.01; N = 5).(ABSTRACT TRUNCATED AT 250 WORDS)

Differential sensitivity of the insulin receptor to proteolysis after beta-adrenergic stimulation

The cellular mechanism by which the specific binding of [125I]insulin to intact rat adipocytes is inhibited by isoproterenol has been studied. By exposing control and isoproterenol-treated cells to trypsin (0-150 micrograms/ml for 20 min at 4 degrees C) and measuring the intact insulin receptor pool following detergent solubilization, a differential sensitivity to proteolysis of the cell membrane receptor was observed. At low trypsin concentration (less than 30 micrograms/ml), approximately 40% of the specific insulin binding in isoproterenol-treated cells was insensitive to proteolysis as compared to control cells. At higher levels of trypsin (50-150 micrograms/ml) both groups displayed similar levels of trypsin-insensitive receptors which, at the highest trypsin concentration, accounted for 10% of the total receptors in intact cells. Detergent-solubilized receptors from isoproterenol-treated cells, on the other hand, exhibited the same sensitivity to trypsin proteolysis as solubilized receptors from control cells. The time course of the onset and reversal of the isoproterenol-induced binding alteration in intact adipocytes has been analyzed by mild trypsinization (20 micrograms/ml). Results indicated that insulin receptors resistant to trypsin under these conditions mediated the decreased surface binding and were re-expressed on the cell surface upon removal of isoproterenol. Experiments in which adipocytes were fractionated into plasma membrane and Golgi-enriched fractions indicated that the loss of surface insulin binding was not accompanied by a decrease in the proportion of receptors in the adipocyte plasma membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenergic receptor properties of hepatocytes from male and female rats

alpha 1- and beta-adrenergic receptor properties of intact hepatocytes from adult male and female rats were evaluated in ligand binding studies using [3H]prazosin and [3H]CGP-12177 (4-(t-butylamino-2-hydroxypropoxy)-[5,7-3H]benzimidazole-2-one-HCl), a hydrophilic beta antagonist. Prior work had suggested that the response of hepatocytes from males to alpha 1-adrenergic stimulation was greater than that of cells from females. However, little sexual difference in prazosin affinity, number of binding sites or kinetics of association/dissociation with the cells was found. Epinephrine, [3H]prazosin competition for binding sites on intact cells was performed at 2 degrees C and 80-90% of agonist sites remained in a high affinity state with an epinephrine Kd comparable to that previously found in glucose release and phosphorylase alpha activation studies. Agonist Kd inferred from these competition experiments also showed no sexual dimorphism. These data suggest that the greater rise in the concentration of cytosolic free calcium and release of 45Ca from cells of males in response to epinephrine stimulation is not due to male/female alpha 1-receptor differences but, rather, may be a function of the previously observed sexual difference in cell calcium metabolism. [3H]CGP binding to hepatocytes from females was stereospecific, saturable and identified a single, high affinity site. Comparable sites were not found on cells from males, however, [3H]CGP binding to crude membrane preparations from both sexes was identical. This suggests that the loss of hepatic beta-receptor function in the adult male is due to an inaccessibility of beta-receptors at the external surface of the plasma membrane of the intact cell. Further studies with other beta-receptor ligands are being carried out to confirm these initial findings.

Concanavalin A prevents phorbol-mediated redistribution of protein kinase C and beta-adrenergic receptors in rat glioma C6 cells

Exposure of rat glioma C6 cells to the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) caused an activation of protein kinase C wherein the enzyme rapidly became membrane-bound (T 1/2 of 15 min). This translocation of protein kinase C from cytosol to membrane was followed by a sequestration of cell surface beta-adrenergic receptors and a loss of isoproterenol-stimulated adenylate cyclase activity. We had reported previously that prior exposure of rat glioma cells to concanavalin A prevents the TPA-mediated sequestration of receptors and desensitization of adenylate cyclase (Kassis et al., 1985). We now show that the concanavalin A treatment also prevents the translocation and activation of protein kinase C. These results are further evidence that in the TPA-treated cells, sequestration of beta-adrenergic receptors is mediated by membrane-bound protein kinase C.

Down-regulation of protein kinase C in rat glioma C6 cells: effects on the beta-adrenergic receptor-coupled adenylate cyclase

Continuous exposure of rat glioma C6 cells to 12-O-tetradecanoylphorbol-13-acetate (TPA) resulted in a time and dose dependent loss of [3H]phorbol dibutyrate binding sites and protein kinase C activity. Thus, by 24 h, the cells were essentially depleted of protein kinase C activity. In agreement with previous studies, TPA treatment caused a reduction in isoproterenol-stimulated adenylate cyclase activity and a sequestration of beta-adrenergic receptors. Cells were treated with TPA for 24-48 h to completely down-regulate protein kinase C and then exposed to isoproterenol. Agonist-mediated desensitization of adenylate cyclase and sequestration of beta-adrenergic receptors occurred at similar rates in control and TPA-treated cells. In addition, agonist-mediated down-regulation of beta-adrenergic receptors was not impaired by the absence of protein kinase C activity. Although both agonists and phorbol esters cause desensitization of the beta-adrenergic receptor-coupled adenylate cyclase, agonist-mediated events can occur independently of protein kinase C.

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.