Measuring faculty effort and contributions in medical education

A national panel on medical education was appointed as a component of the AAMC's Mission-based Management Program and charged with developing a metrics system for measuring medical school faculty effort and contributions to a school's education mission. The panel first defined important variables to be considered in creating such a system: the education programs in which medical school faculty participate; the categories of education work that may be performed in each program (teaching, development of education products, administration and service, and scholarship in education); and the array of specific education activities that faculty could perform in each of these work areas. The panel based the system on a relative value scale, since this approach does not equate faculty performance solely to the time expended by a faculty member in pursuit of a specific activity. Also, a four-step process to create relative value units (RVUs) for education activities was developed. This process incorporates quantitative and qualitative measures of faculty activity and also can measure and value the distribution of faculty effort relative to a school's education mission. When adapted to the education mission and culture of an individual school, the proposed metrics system can provide critical information that will assist the school's leadership in evaluating and rewarding faculty performance in education and will support a mission-based management strategy in the school.

Carboxyl-terminal domains in the avian beta 1-adrenergic receptor that regulate agonist-promoted endocytosis

Most G protein-coupled receptors, including the mammalian beta 2-adrenergic receptor, are endocytosed to an intracellular, vesicular compartment upon continued exposure to agonist. The long form of the avian beta 1-adrenergic receptor, which contains a carboxyl-terminal 59-amino acid extension, does not undergo agonist-promoted endocytosis. We constructed and expressed turkey beta 1-adrenergic receptor cDNAs with regularly spaced carboxyl-terminal truncations and studied their agonist-promoted endocytosis. Removal of 34-86 amino acids from the carboxyl terminus of the turkey receptor allowed its efficient endocytosis, with optimal endocytosis observed upon removal of 59 residues. Removal of only 18 residues allowed some endocytosis. A receptor that lacks the entire carboxyl-terminal region (124 residues) was not endocytosed. We also constructed a chimeric hamster beta 2-adrenergic receptor with the added 59-residue carboxyl-terminal domain of the turkey receptor. The chimera was not significantly endocytosed. These data indicate that residues 450-465 in the carboxyl-terminal region of the beta 1-adrenergic receptor can act independently to block agonist-promoted endocytosis and that other carboxyl-terminal structures nearer to the seventh membrane span are required for endocytosis.

Differential effects of antimycin A on endocytosis and exocytosis of transferrin also are observed for internalization and externalization of beta-adrenergic receptors

In many cells catecholamines induce a translocation of beta-adrenergic receptors from the cell surface to intracellular vesicular sites. We have postulated that the translocation event is the result of ligand-induced endocytosis of the receptor, probably via clathrin-coated pits. Previously, we demonstrated that reduction of cellular ATP content with antimycin A completely blocked endocytosis of epidermal growth factor and translocation of beta-adrenergic receptors in 1321N1 astrocytoma cells. However, the effect of reduction in ATP content on endocytosis remains controversial. In the present report, we demonstrate that reduction of ATP content to a level < 5% of that in control cells is sufficient to prevent endocytosis of [125I]iodotransferrin and translocation of beta-adrenergic receptors. The further demonstration that reactions leading to the return of internalized transferrin or beta-adrenergic receptors to the cell surface are blocked after relatively modest reductions in ATP content provides further evidence of the similarity in the processes subserving diacytosis of beta-adrenergic receptors and transferrin. The differential requirement for ATP of the two arms of diacytosis provides the basis for an explanation of the controversy regarding a requirement for ATP in endocytosis via clathrin-coated pits.

Intensive dose ifosfamide, carboplatin, and etoposide followed by autologous stem cell rescue: results of a phase I/II study in breast cancer patients

We have recently treated 66 women with breast cancer with escalating doses of ifosfamide, carboplatin, and etoposide (ICE) followed by autologous stem cell rescue (ASCR). Patients received ifosfamide (6000-24,000 mg m-2), carboplatin (1200-2100 mg m-2), and etoposide (1800-3000 mg m-2) divided over 6 days with ASCR 48 h after completion of chemotherapy. Our patient population consisted of seven patients with stage II disease with eight or more positive nodes being treated in the adjuvant setting, 16 patients with a history of stage III or inflammatory breast cancer, and 43 patients with stage IV disease. Six patients were not evaluable for response due to early death from infection (three patients) and incomplete restaging (three patients). The overall response rate in patients with measurable metastatic disease was 50%. Of those patients with stage II disease, 85% remain alive and progression-free with a median follow-up of greater than one year. The two most frequent toxicities encountered were reversible elevations of liver function tests and mucositis/enteritis. The dose-limiting toxicities were central nervous system toxicity and nephrotoxicity.

Isoproterenol-initiated beta-adrenergic receptor diacytosis in cultured cells

The kinetics of the return of internalized beta-adrenergic receptors to the plasma membrane were measured in human astrocytoma cells. The movement of [125I]iodopindolol-labeled receptors back to the plasma membrane was measured directly and was shown to occur with a t1+2 of 3-4 min. Unlabeled receptors appeared to exhibit the same kinetics of externalization. The process was not inhibited by low concentrations (1-10 microM) of propranolol or even high concentrations of isoproterenol (0.1-1.0 mM). Higher concentrations of propranolol (0.1-1.0 mM) and other lipophilic amines inhibited externalization. The results are consistent with the proposal that catecholamine-induced beta-adrenergic receptor internalization and externalization (diacytosis) occur via the clathrin-coated pit/endosome pathway.

A truncation mutation in the avian beta-adrenergic receptor causes agonist-induced internalization and GTP-sensitive agonist binding characteristic of mammalian receptors

Recombinant turkey erythrocyte beta-adrenergic receptors expressed in murine L cells exhibited characteristic avian subtype selectivity for agonists and antagonists. In 10 of the 11 clones studied, no agonist-induced internalization of receptor was observed, although agonist-induced uncoupling of receptor and adenylyl cyclase occurred rapidly. GTP caused little or no decrease in affinity for beta-adrenergic agonists. Such behavior is commonly observed in avian erythrocytes. In contrast, one clone was susceptible to agonist-induced receptor internalization and down-regulation even though it exhibited characteristic avian beta-adrenergic ligand-binding properties. The affinity of this variant receptor for agonists was also notably reduced by GTP. Electrophoresis of affinity-labeled receptor from this clone indicated an apparent size of about 33 kDa, about 12 kDa less than that of the native or recombinant turkey beta-adrenergic receptor. Genomic DNA from this cell line that encodes the receptor was cloned and partially sequenced. The coding region of the original receptor cDNA was interrupted after codon 412 (out of 483) and was followed by 36 base pairs of novel sequence prior to the first in-frame stop codon. These results suggest that the lack of both hormone-induced internalization and GTP-sensitive, high affinity binding of agonists that is characteristic of the beta-adrenergic receptor in avian erythrocytes is due to intrinsic properties of the receptor. The restoration of these phenomena in a C-terminally truncated mutant receptor suggests the importance of the C-terminal domain in determining these processes.

Agonists and phorbol esters desensitize beta-adrenergic receptors by different mechanisms

Exposure of 1321N1 human astrocytoma cells to the protein kinase C (PKC) activator phorbol 12-myristate, 13-acetate (PMA) led to a rapid and concentration-dependent decrease in isoproterenol (ISO)-stimulated adenylate cyclase (AC) activity in cell lysates. This desensitization of beta-adrenergic receptor (BAR) function was mimicked by mezerein, which also activates PKC, but not by 4-O-methyl-PMA, which is a very weak activator of PKC. Pretreatment with PMA led to desensitization of AC activity stimulated by ISO and by prostaglandin E1, in contrast to the beta-receptor-specific desensitization induced by ISO. Stimulation of AC activity by forskolin and by fluoride remained unaltered. The extent of desensitization observed with PMA plus ISO was greater than with either agent alone. Desensitization with PMA did not result in internalization of BAR, as assessed by sucrose density gradient centrifugation assays and by assays of competition by the hydrophilic ligand ISO for radioligand binding to intact cell receptors. PMA pretreatment did not alter the apparent affinity of the agonist ISO for intact cell BAR, nor was the potency of ISO for stimulation of AC activity altered. The protein kinase inhibitor H7 [1-(5-isoquinolinesulfonyl)-2-methylpiperazine] inhibited the desensitization induced by PMA but not that induced by ISO. These results indicate that activation of PKC can lead to desensitization of receptor-stimulated AC activity but that agonist-induced desensitization of BAR-stimulated AC activity occurs by a different mechanism.

Sequential appearance of epidermal growth factor in plasma membrane-associated and intracellular vesicles during endocytosis

Receptor-mediated internalization of epidermal growth factor (EGF) occurs by a process involving initially clathrin-coated pits on the cell surface and the subsequent formation of ligand-containing endosomes. Using a modified acid wash technique, cell surface-bound EGF was removed. Utilizing sucrose density centrifugation, the residual cell-associated EGF was separated into plasma membrane-associated and intracellular vesicle-associated forms. Using these procedures we have identified a transient form of cell-associated EGF that is still attached to the plasma membrane but not accessible to the extracellular fluid. This form of EGF appears to be the precursor for endosomic EGF. We suggest that this intermediate form represents the receptor-ligand complex shown by electronmicroscopy to be located in narrow-necked plasma membrane invaginations (Willingham, M. C., and Pastan, I. (1980) Cell 21, 67-77).

The involvement of cellular ATP in receptor-mediated internalization of epidermal growth factor and hormone-induced internalization of beta-adrenergic receptors

Beta-Adrenergic receptors and epidermal growth factor receptors are both expressed on the cell surface of human astrocytoma cells. Incubation with a catecholamine or epidermal growth factor results in rapid internalization of the respective receptor. The internalized receptors co-migrate in light fractions on sucrose gradients. Astrocytoma cells maintain a constant ATP concentration by either glycolytic or mitochondrial ATP production. When cells are incubated in a medium depleted of substrates for glycolysis and gluconeogenesis, addition of inhibitors of mitochondrial ATP synthesis causes a rapid reduction in cellular ATP content. An immediate return to control ATP levels occurs upon addition of an appropriate nutrient, such as glucose. Decreasing the cellular ATP content to less than 10% of control markedly inhibits internalization of beta-adrenergic receptors and epidermal growth factor. The inhibition of endocytosis is reversed as soon as the intracellular ATP content is restored. Previous work by others (Clarke, B.L., and Weigel, P.H. (1985) J. Biol. Chem. 260, 128-133) suggested that ATP is not required for internalization (per se) of asialoglycoprotein in hepatocytes but was required for recycling of the asialoglycoprotein receptor. In contrast, our results indicate that in astrocytoma cells the process of internalization of epidermal growth factor and beta-adrenergic receptors, per se, is highly ATP dependent.

A comparison of catecholamine-induced internalization of beta-adrenergic receptors and receptor-mediated endocytosis of epidermal growth factor in human astrocytoma cells. Inhibition by phenylarsine oxide

The ligand-induced internalization of beta-adrenergic receptors and the receptor-mediated internalization of epidermal growth factor were blocked, under similar conditions, by phenylarsine oxide (PAO) in human astrocytoma cells (1321N1). The inhibition was not prevented or reversed by monofunctional sulfhydryl agents such as 2-mercaptoethanol or glutathione; however, the inhibitory action of PAO was blocked and reversed by bifunctional thiols such as 2,3-dimercaptoethanol or dithiothreitol. The results are consistent with the interaction of PAO with vicinal sulfhydryl groups to form a stabile ring structure. PAO did not prevent isoproterenol-induced uncoupling (desensitization) of beta-adrenergic receptors even though receptor internalization was completely blocked. The effects of PAO on receptor internalization could not be explained by any action of the trivalent arsenical to lower ATP levels. Ligand binding to both receptors was not detectably altered by PAO under conditions selective for inhibition for endocytosis. The results suggest a common mechanism for internalization of beta-adrenergic receptors and epidermal growth factor by a process that involves vicinal sulfhydryl groups.

Effects of tunicamycin on the expression of beta-adrenergic receptors in human astrocytoma cells during growth and recovery from agonist-induced down-regulation

Tunicamycin, which inhibits formation of asparagine-linked glycoproteins, caused a concentration-dependent blockade of beta-adrenergic receptor (beta-AR) accumulation in 1321N1 human astrocytoma cells during growth in culture. A concentration of tunicamycin (0.1 microgram/ml) that inhibited receptor accumulation and [3H]mannose or [3H]glucosamine incorporation into glycoproteins by 90% had only a small effect (10%) on [3H]leucine incorporation into protein, and reduced the rate of cell growth. Incubation in drug-free medium subsequent to treatment of 1321N1 cells with tunicamycin for 48 hr resulted in recovery of beta-AR to control levels within an additional 48 hr. Exposure of cultures to isoproterenol (0.1 microM, 12 hr) caused an 80-90% loss of beta-AR in both pre- and postconfluent cultures; beta-AR recovered to control levels upon removal of isoproterenol. Although both tunicamycin and the protein synthesis inhibitor cycloheximide blocked beta-AR accumulation during growth of 1321N1 cells, neither agent inhibited the appearance of beta-AR during recovery from the down-regulated state in preconfluent cultures. However, cycloheximide, but not tunicamycin, blocked recovery of beta-AR after isoproterenol-induced loss of receptors in postconfluent cultures. In a previous report (Mol. Pharmacol. 26:424-429, 1984), we provided direct evidence that recovery of beta-AR from down-regulation in postconfluent cultures requires de novo synthesis of receptor protein. Thus, the results with tunicamycin are consistent with the idea that recovery of beta-AR in postconfluent cultures requires the synthesis of new beta-AR molecules, but as aglycoproteins that exhibit radioligand-binding characteristics similar to those of native glycoprotein beta-AR.

Cellular redistribution of beta-adrenergic receptors in a human astrocytoma cell line: a comparison with the epidermal growth factor receptor in murine fibroblasts

The redistribution of beta-adrenergic receptors (beta-AR) during agonist-induced desensitization has been compared to the process of receptor-mediated endocytosis of epidermal growth factor (EGF) in human astrocytoma cells (1321N1). [125I]EGF exhibited saturable binding to high affinity (KD = 1-2 nM) receptor sites on intact 1321N1 cells. [125I]EGF was found to internalize rapidly using an acid wash technique to remove surface bound hormone. Sucrose density gradient fractionation following exposure to EGF revealed a redistribution of EGF binding sites from high density (heavy peak) to low density (light peak) regions of the gradient. The light peak binding probably represents EGF in internalized vesicles formed during endocytosis. Low temperature (4 degrees C) or the presence of the lectin concanavalin A (con A) inhibited this ligand-induced movement of EGF receptors. When cells were incubated simultaneously with EGF and the beta-AR agonist isoproterenol, both receptors were found to co-migrate in the low density regions of sucrose gradients. No evidence of heterologous ligand-induced receptor endocytosis was found. These results suggest that the EGF receptors and beta-AR are processed in parallel by 1321N1 cells.

Use of a density shift method to assess beta-adrenergic receptor synthesis during recovery from catecholamine-induced down-regulation in human astrocytoma cells

Exposure of postconfluent 1321N1 human astrocytoma cells to 1.0 microM isoproterenol for 12-24 hr results in a 90% loss of beta-adrenergic receptors. Upon removal of agonist, recovery of beta-receptors to control levels occurs within 72 hr. The recovery of receptors is completely blocked by cycloheximide [R. C. Doss, J. P. Perkins, and T. K. Harden, J. Biol. Chem. 256:12281-12286 (1981)]. In contrast cycloheximide does not block recovery of beta-receptors after down-regulation in preconfluent cultures. To determine unambiguously if beta-receptor synthesis accounts for the recovery of receptors after down-regulation, post confluent cultures were incubated with isoproterenol and then transferred to agonist-free medium containing either normal or "heavy" (2H, 13C, 15N) amino acids. The rate and extent of beta-receptor recovery were similar in both normal and heavy amino acid-containing medium. When beta-receptors that had recovered in the heavy amino acid-containing medium were labeled with 125I-cyanopindolol, solubilized in Lubrol PX, and subjected to centrifugation on a 5-15% sucrose density gradient, they exhibited an increased mass compared to beta-receptors that recovered in the presence of normal amino acids. These results confirm that the density shift method is a useful approach for the study of beta-receptor synthesis and that new receptor synthesis occurs during recovery of beta-receptors from catecholamine-induced down-regulation in postconfluent cultures.

Relationship between an altered membrane form and a low affinity form of the beta-adrenergic receptor occurring during catecholamine-induced desensitization. Evidence for receptor internalization

We have investigated the relationship between the catecholamine-induced occurrence in 1321N1 human astrocytoma cells of beta-adrenergic receptors that exhibit low apparent affinity for hydrophilic ligands in short-time assays with intact cells and a population of beta-adrenergic receptors that migrate in a light vesicle fraction on sucrose density gradients. Pretreatment of cells with concanavalin A prevents the generation of both of these forms of the receptor during incubation with agonists but does not prevent the agonist-induced decrease in isoproterenol-stimulated cyclic AMP production that also occurs during desensitization. Selective labeling of the low affinity beta-receptors with 125I-pindolol followed by centrifugation on sucrose density gradients revealed that all of the receptors in the light vesicle fraction from desensitized cells were of the low affinity type, but that a portion of the low affinity receptors also migrated in a heavier sucrose fraction together with the plasma membrane. In contrast, in control cells, no low affinity receptors were present in the heavy sucrose fractions. The agonist-induced occurrence of these various forms of the beta-adrenergic receptor can be explained on the basis of current models of desensitization involving agonist-induced internalization of beta-adrenergic receptors.

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.

Agonist-induced changes in beta-adrenergic receptors on intact cells

Competition by beta-adrenergic agonists and antagonists for 125I-pindolol binding sites on intact cells (1321N1 human astrocytoma and C62B rat glioma) was measured using short time binding assays as previously described (Toews, M. L., Harden, T. K., and Perkins, J. P. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 3553-3557). Preincubation of cells with agonists converted about half of the cellular beta-adrenergic receptors from a form exhibiting high affinity for the agonists isoproterenol and epinephrine and the antagonist sotalol to a form exhibiting much lower apparent affinity for these ligands in short time assays. Exposure to agonists did not alter the affinity of receptors for the antagonist metoprolol. This change in the ligand binding properties of the receptor was rapid (t1/2 = 1-2 min following a lag of about 0.5 min), reversible (t1/2 = 6-8 min), and dependent on the agonist concentration present during the preincubation (K0.5 = 15 nM for isoproterenol). Both isoproterenol and sotalol attained equilibrium with the high affinity receptors very rapidly but equilibrated only slowly with those receptors exhibiting low apparent affinity in short time assays. These results are interpreted in terms of a model which postulates that both the low apparent affinity in short time assays and the subsequent slow equilibration of hydrophilic ligands with these receptors result from agonist-induced internalization of a fraction of cell surface beta-adrenergic receptors. The relationship of this change in receptor binding properties to other aspects of agonist-induced desensitization of the beta-adrenergic receptor-coupled adenylate cyclase system is discussed.

Characterization of an altered membrane form of the beta-adrenergic receptor produced during agonist-induced desensitization

Incubation of 1321N1 human astrocytoma cells with 1 microM isoproterenol rapidly results in the conversion of a portion of the beta-adrenergic receptors to a membrane form that can be separated from markers for the plasma membrane by sucrose density gradient or differential centrifugation. This "light peak" form of the receptor reaches a maximal level within 10 min of incubation of cells with catecholamine. Two types of experiments suggest that the early phase of catecholamine-induced desensitization of the beta-adrenergic receptor-linked adenylate cyclase can be separated into at least two reactions. First, the agonist-induced loss of catecholamine-stimulated adenylate cyclase activity precedes the appearance of beta-adrenergic receptors in the light peak fraction by 1-2 min. Second, pretreatment of cells with concanavalin A prior to induction of desensitization blocks the formation of the light peak form of beta-adrenergic receptors without blocking the "uncoupling" reaction as measured by catecholamine-stimulated adenylate cyclase activity. Specificity for the reaction that converts beta-adrenergic receptors to the light peak form is indicated by the lack of a catecholamine-induced alteration in the sucrose density gradient distribution of muscarinic cholinergic receptors, adenylate cyclase or the guanine nucleotide-binding proteins, Ns and Ni. The light peak of beta-adrenergic receptors migrates at a density similar to that of at least a portion of the activity of galactosyltransferase, a marker for Golgi. Enzyme marker activities for lysosomes and endoplasmic reticulum are not associated with this population of beta-adrenergic receptors. Taken together, these and other data suggest that incubation of 1321N1 cells with isoproterenol results in a rapid uncoupling of beta-adrenergic receptors from adenylate cyclase which is followed by a change in the membrane form of the receptor. This latter step most likely represents internalization of receptors into a vesicular form which may then serve as the precursor state from which receptors are eventually lost from the cell.

High-affinity binding of agonists to beta-adrenergic receptors on intact cells

The interactions of agonists and antagonists with beta-adrenergic receptors on intact 1321N1 human astrocytoma and C62B rat glioma cells were studied by using the radioligand (-)-[125I]iodopindolol. Competition binding assays were performed at 37 degrees C under equilibrium conditions and in short-time nonequilibrium assays that approximated initial velocity conditions for binding of the radioligand. The theoretical basis and experimental validation of the initial velocity approach for determining binding affinities of rapidly equilibrating ligands are presented. For the agonists isoproterenol and epinephrine, high binding affinities that approximated their apparent affinities for binding in membranes and for increase of cyclic AMP concentrations in intact cells could be demonstrated only in short-time assays; in contrast, much lower affinities were observed with equilibrium (60-min) assays as reported previously for various cell lines. High-affinity binding of isoproterenol to 1321N1 cells also was observed in equilibrium (6-hr) binding assays carried out on ice. These results indicate that in the native state the intact cell beta-adrenergic receptor has a high binding affinity for agonists and suggest that incubation at 37 degrees C in the presence of an agonist converts the receptors to a form with low affinity for agonists.

Recovery of beta-adrenergic receptors following long term exposure of astrocytoma cells to catecholamine. Role of protein synthesis

As a part of the process of agonist-induced desensitization, 1321N1 human astrocytoma cells lose up to 95% of their beta-adrenergic receptors, as detected by 125I-hydroxybenzylpindolol (125IHYP) binding, after 12-24 h of exposure to isoproterenol. In preconfluent cultures the loss of beta-receptors is completely reversible upon removal of isoproterenol, with receptor levels reaching 100% of control levels within 48-72 h. Addition of cycloheximide (5 micrograms/ml) upon removal of agonist does not prevent the recovery of receptors. After an initial 4-h lag, receptors accumulate in the presence of cycloheximide until the same receptor level is reached that was present at the onset of desensitization. Confluent cultures, which have a reduced number of receptors per cell, recover beta-receptors to only 60 to 70% of control levels following removal of isoproterenol. In addition, cycloheximide blocks the recovery of receptors in these cultures. The effects of cycloheximide on the accumulation of receptors during cell growth suggest that receptors are stable in preconfluent cultures and that turnover only occurs later when cultures are confluent. The data also indicate that long term exposure of cells to catecholamine results in a form of the beta-adrenergic receptor that is undetectable by 125IHYP binding but, nonetheless, retains its primary amino acid structure. The undetectable receptors appear to be retained until agonist is removed, whereupon they become detectable by 125IHYP binding with a t1/2 of about 36 h in the presence of cycloheximide.

Cystic fibrosis fibroblasts respond normally to isoproterenol

Thirteen cystic fibrosis and 12 normal strains of skin fibroblasts obtained from the Institute for Medical Research were compared for their degree of production of cyclic adenosine 3':5'-monophosphate in response to isoproterenol and prostaglandin E1. There were no significant differences in their quantitative responses in content of cyclic AMP at two different times whether these cells were growing exponentially or were already confluent. All strains responded similarly to the presence of two types of phosphodiesterase inhibitor. The averaged initial rates of the response to isoproterenol in exponentially growing cells were also similar for the two sets of strains. Although response differed greatly between strains, the response of each strain was relatively reproducible.

Catecholamine-induced alteration in sedimentation behavior of membrane bound beta-adrenergic receptors

Incubation of astrocytoma cells with catecholamines results in a decrease in catecholamine-stimulated adenylate cyclase activity and a concomitant alteration in the sedimentation properties of particulate beta-adrenergic receptors. The altered receptors exhibit agonist binding properties similar to those of receptors that are "uncoupled" from adenylate cyclase.

Killing of fibroblasts by dexamethasone or dibutyryl adenosine 3',5'-monophosphate is not a valid test for cystic fibrosis

Assays based on the counting of total cells and of colony-forming cells were used to demonstrate that neither dexamethasone nor dibutyryl adenosine 3',5'-monophosphate (cyclic AMP) kills human fibroblasts under a variety of conditions. These results contradict those of previous studies showing that dexamethasone and dibutyryl cyclic AMP kill a higher percentage of fibroblasts from normal humans than from individuals with cystic fibrosis.

Isolation of adenylate cyclase-enriched membranes from mammalian cells using concanavalin A

Plasma membrane vesicles containing adenylate cyclase and beta-adrenergic receptors were prepared from 1321N1 human astrocytoma cells by a procedure involving the use of concanavalin A to stabilize the plasma membrane to fragmentation and vesiculation upon cell lysis. Treatment of cells with concanavalin A causes these plasma membrane markers to sediment to a higher density of sucrose and in a narrower band than observed with untreated cells. Upon treatment of the heavy membrane fragments with alpha-methylmannoside to remove bound concanavalin A, the enzyme markers again sediment a lower densities of sucrose. This reversible change in sedimentation behavior has been used to obtain preparations of plasma membranes enriched 14- to 21-fold (recovery 25%) in adenylate cyclase activity and about 12-fold (recovery 16%) in beta-adrenergic receptor density, as compared to lysates. The adenylate cyclase of purified membranes responded normally to isoproterenol and prostaglandin E1. Experiments with S49 and YAC mouse lymphoma cells and human skin fibroblasts indicate that this procedure may be adaptable to the isolation of plasma membranes from a variety of cultured cell lines.

Adaptive changes in the responsiveness of adenylate cyclase to catecholamines

Exposure of the intact astrocytoma cell to isoproterenol not only causes the activation of adenylate cyclase and the accumulation of cyclic AMP but sets in motion a complicated series of events designed to down-regulate the system if exposure to the agonist is extended in time. We have identified at least three of these processes: (1) a rapid uncoupling of the beta-receptor--adenylate cyclase system with subsequent loss of beta-receptors; (2) a slower, nonspecific desensitization of adenylate cyclase to the effects of all classes of receptor agonists by a process that may be mediated by cyclic AMP; and (3) a slow induction of phosphodiesterase activity that is probably mediated by cyclic AMP.

Isoproterenol-induced desensitization of adenylate cyclase in human astrocytoma cells. Relation of loss of hormonal responsiveness and decrement in beta-adrenergic receptors

Incubation of human astrocytoma cells (1321N1) with low concentrations of isoproterenol results in a specific loss of responsiveness to catecholamines as evidenced by a decreased accumulation of cAMP in intact cells, a reduction in isoproterenol-stimulated adenylate cyclase activity, and a decrease in beta-adrenergic receptor density, as measured by the specific binding of 125I-hydroxybenzylpindolol. The kinetics of desensitization suggest the involvement of two different reactions. The initial reaction involves a rapid loss of adenylate cyclase activity with little loss of beta-adrenergic receptors. Subsequently, a slower reaction results in the loss of measurable beta-adrenergic receptors. The degree of loss of both parameters was similar after 24 h of desensitization. It is concluded that the loss of beta-adrenergic receptors is an event that occurs as a result of the initial uncoupling of the beta-receptor-linked adenylate cyclase.

Expression of the regulation of cAMP metabolism in somatic cell hybrids

Normal rat liver cells (BRL-1) that respond to isoproterenol (beta+2), prostaglandin E1 (PGE+1) and adenosine (Ado+) with a rise in adenosine 3':5'-monophosphate (cAMP) content have been hybridized with rat hepatoma cells (H35) which do not respond to any of these agonists (beta-2, PGE-1 and Ado-). Both the initial hybrid line (BF5) and a subclone (BF5-1-1) expressed a beta+2, PGE+1, Ado- phenotype. However, full expression of the responsive phenotype in the BF5 line was apparent only if phosphodiesterase activity was blocked, for example, by methylisobutylxanthine (MIX). Direct measurements showed the rate of degradation of cAMP to be 7 times greater in intact BF5 cells than in the BRL-1 parent. In contrast to BF5 cells, the BF5-1-1 cells did not express maximal responsiveness to any of the agonists even in the presence of MIX. The differential accumulation of intracellular cAMP observed with BRL-1, BF5 and BF5-1-1 cells in response to isoproterenol was shown not to be as a result of differential rates of excretion of cAMP. Furthermore, no differences in the apparent affinities of the beta 2-catecholamine receptors for isoproterenol were observed. It is suggested that the increased degradative capacity of BF5 cells accounts for the difference in cAMP accumulation in these cells compared with the BRL-1 parent. The reduced responsiveness of BF5-1-1 cells, however, does not appear to be solely due to increased phosphodiesterase activity. It appears that the beta 2- phenotype may not always be dominant in hybrid crosses of this type as has been reported previously.

Regulation of adenosine 3':5'-monophosphate content of rous sarcoma virus-transformed human astrocytoma cells. Effects of cholera toxin on the responsiveness to catecholamines and prostaglandins

Human astrocytoma cells (EH118MG) respond to catecholamines and prostaglandins with a marked increase in the rate of formation of cyclic AMP. Treatment of EH118MG cells with cholera toxin (10 to 100 ng/ml) for 45 to 60 min caused an increase in cellular cyclic AMP content (5- to 10-fold over basal). Cholera toxin also decreased the K0.5 for isoproterenol 10- to 50-fold and decreased the K0.5 for prostaglandin E1 (PGE1)30- to 100-fold, while increasing the maximal response to PGE1 by 1.5- to 3-fold. Treatment with cholera toxin did not change the K1 values for beta-adrenergic receptor antagonists such as propranolol, alprenolol, and sotalol. Direct binding studies using [125I]iodohydroxybenzylpindolol indicated no significant changes in the number of beta-receptors or in the kinetics of the interaction of the radioligand with receptors after treatment of cells with the toxin. Competition binding studies with propranolol and sotalol revealed no toxin-induced change in Kd values for these antagonists. Treatment with cholera toxin caused only small decreases (2- to 3-fold) in the Kd values for binding of isoproterenol and norepinephrine. It is concluded that cholera toxin has little direct effect on the binding of agonists or antagonists to beta-receptors, but instead increases the efficiency of coupling of receptor and catalytic moieties of adenylate cyclase.

Glucocorticoids increase the responsiveness of cells in culture to prostaglandin E1

The influence of steroid hormones on the response of human astrocytoma cells (1321N1) to prostaglandin E(1) (PGE(1)) has been investigated. Responsiveness to PGE(1) was determined by measuring the conversion of [(3)H]ATP to cyclic [(3)H]AMP in cells prelabeled with [(3)H]adenine. After incubation of the cells with dexamethasone, a marked increase in both the maximal effect (2- to 3-fold) and the potency (5-fold) of PGE(1) was observed. The effect was specific for the action of PGE(1) in that no change in the response of the cells to isoproterenol was observed. The EC(50) for dexamethasone was 0.001 muM and the effect was dependent on the presence of serum. The effect of dexamethasone was first observed after a 30- to 60-min lag and was maximal by 6-8 hr. Preconfluent cultures (3 days after seeding) exhibited optimal responsiveness to glucocorticoids. Both hydrocortisone and corticosterone mimicked the effect of dexamethasone but both were less potent. The action of dexamethasone was blocked by progesterone, testosterone, and 17alpha-methyltestosterone. Cycloheximide, at a concentration (1.0 mug/ml) that blocked protein synthesis (>90%) in 1321N1 cells, totally prevented the effect of dexamethasone on the response of the cells to PGE(1). Upon removal of dexamethasone from cells treated for 16 hr, responsiveness to PGE(1) returned to control levels with a half-time of 4 hr. Dexamethasone also was found to increase the response to PGE(1) of a Rous sarcoma virus-transformed human astrocytoma cell line and the WI-38 human fibroblast line. The most obvious interpretation of our findings is that glucocorticoids induce the synthesis of a protein that selectively modifies the sensitivity of adenylate cyclase to PGE(1).

Stimulation of adenosine 3':5'-monophosphate formation by prostaglandins in human astrocytoma cells. Inhibition by nonsteroidal anti-inflammatory agents

Prostaglandins (PG) of the E series and catecholamines stimulate adenosine 3':5'-monophosphate (cAMP) formation in human astrocytoma cells (1321N1). These two classes of effectors activated adenylate cyclase upon interaction with different receptor systems. No evidence for a mediatory role for PG in the action of catecholamines was found. PG interacted with 1321N1 cells with an order of potency of PGE1 = PGE2 greater than PGA1 greater than PGF2 alpha. The effect of combinations of the various PG indicated that all efficacious PG interacted with a common receptor. 7-Oxa-13-prostynoic acid and indomethacin were shown to be competitive inhibitors of the effect of PGE1 with Ki values of 4 and 150 micron, respectively. These two compounds did not inhibit the effect of isoproterenol. Polyphloretin phosphate caused a complex pattern of inhibition of the effects of PGE1 and at higher concentrations also inhibited the effects of isoproterenol. The mefenamate class of nonsteroidal anti-inflammatory agents was found to inhibit the effects of PGE1 with a potency order of meclofenamic acid greater than flufenamic acid = mefenamic acid. The inhibitory action of meclofenamic acid was complex involving specific, but partial, insurmountable antagonism of PGE1 as well as competitive inhibition of PGE1 effects. At higher concentrations of meclofenamic acid a nonspecific inhibition of the effects of both PGE1 and isoproterenol was observed. These studies suggest that the inhibition by nonsteroidal anti-inflammatory agents of the physiological effects of PGE1 in animals may occur, at least in part, at the level of adenylate cyclase. The possibility that multiple classes of adenylate cyclase-linked PGE receptors might exist in nature is discussed.

Ontogeny of beta-adrenergic receptors in rat cerebral cortex

The ontogeny of beta-adrenergic receptors in rat cerebral cortex has been studied using [125I]iodohydroxybenzylpindolol as a ligand in an in vitro binding assay. The concentration of beta-adrenergic receptors was very low during the first week after birth. Between days 7 and 14 there was a rapid increase in the density of receptors. Adult levels were reached by the end of the second week. The affinities of 1-isoproterenol and iodohydroxybenzylpindolol for beta-adrenergic receptors did not vary with the age of the animal. Fluoride stimulated adenylate cyclase activity in the cerebral cortex was 40% of the adult level at birth and gradually increased to maximal levels over the next two weeks. On the other hand, catecholamine stimulated cyclic-3',5'-adenosine monophosphate accumulation was barely detectable during the first week after birth, but it increased rapidly to adult levels between days 7 and 14. The results suggest that it is the development of beta-adrenergic receptors that permits the expression of catecholamine sensitive adenylate cyclase activity. Norepinephrine stores in the cerebral cortex developed slowly reaching adult levels approximately two months after birth. There is therefore little correlation between the ontogeny of presynaptic adrenergic nerve terminals and the postsynaptic development of beta-adrenergic receptors.

Regulation of adenosine 3':5'-monophosphate content of human astrocytoma cells: mechanism of agonist-specific desensitization

The mechanism underlying agonist-induced loss of responsiveness to catecholamines and prostaglandins has been investigated in human astrocytoma cells. Pulse-labeling of the cells with [3H] adenine during the time course of exposure to either norepinephrine or prostaglandin E1 (PGE1) demonstrated a reduction of the rate of incorporation of label into cyclic AMP within 5 min after exposure of the cells to either agonist. The loss of responsiveness observed by this technique was essentially agonist-specific during the first 30 min of exposure of the cells to either norepinephrine or PGE1. The rate constant for degradation of cyclic AMP throughout a 60 min exposure to either norepinephrine or PGE1 did not change suggesting that loss of responsiveness is not related to increased phosphodiesterase activity. The results are discussed in terms of a standard theoretical model for the regulation of the steady state level of an intermediate in a reaction sequence in which the rate of synthesis of the intermediate follows zero order kinetics and the rate of degradation follows first order kinetics. The hypothesis is put forth that agonist-induced desensitization is caused by an agonist-specific reduction in the rate of synthesis of cyclic AMP that follows rapidly after the initial stimulation of adenylate cyclase activity.

Regulation of adenosine 3':5'-monophosphate content of human astrocytoma cells: desensitization to catecholamines and prostaglandins

Human astrocytoma cells (1321N1) exhibit adenylate cyclase activities coupled to independent receptors for catecholamines and prostaglandins of the E-series. Exposure of the cells to either norepinephrine or prostaglandin E1 (PGE1) results in an initial rapid accumulation of cyclic AMP but also results in a progressive loss of responsiveness of the cells to agonists. Initially, the desensitization is in large part agonist-specific. However, with continued exposure to high concentrations of norepinephrine, partial loss of responsiveness to PGE1 occurs, and vice versa. The mechanism underlying this phenomenon does not appear to involve inactivation of the effectors, formation of an inhibitory substance in the culture medium or an increase in the rate of excretion of cyclic AMP from the cell. Blockade of protein synthesis (85%) by 5 mug/ml cycloheximide did not change the rate or extent of desensitization. When desensitized cells were incubated in the presence of the effectors, responsiveness was essentially completely recovered with a t1/2 of 5-7 hr. Cycloheximide recovery reduced. Norepinephrine-induced desensitization to either norepinephrine or PGE1 was blocked by sotalol, a beta receptor blocking agent. Incubation of the cells with dibutyryl cyclic AMP caused desensitization to both norepinephrine and PGE). The results suggest that catecholamine-induced desensitization occurs as a result of interaction of the agonist with the same receptor that is linked to activation of adenylate cyclase. Cyclic AMP appears to mediate at least the non-specific aspect of agonist-induced desensitization.

Factors influencing the effect of hormones on the accumulation of cyclic AMP in cultured human astrocytoma cells

The characteristics of the effects of catecholamines, prostaglandins, and adenosine on the adenosine 3',5'-monophosphate (cAMP) content of human astrocytoma cells are described. Catecholamines interact with a typical beta-adrenergic receptor, i.e., the order of potency of catecholamines is isoproterenol larger than or equal to epinephrine greater than norepinephrine greater than dopamine, and propranolol is an inhibitor but phentolamine is not. The prostaglandins interact with a receptor that recognized PGE-1, PGE-2, and PGA-1 but not PGF-2-alpha. The effects of PGE-1 are blocked by 7-oxa-13-prostynoic acid, indomethacin, and meclofenamic acid in a rapid, reversible manner. The cells contain another adenylate cyclase-linked receptor that recognizes adenosine and the adenine nucleotides but not guanosine, deoxyadenosine, or adenine. Theophylline and other methylxanthines are competitive inhibitors of the effect of adenosine. Each class of effector appears to stimulate adenylate cyclase by interacting with a structure-specific receptor. This follows from the observation that the effect of each class of agonists can be blocked selectively by the various inhibitors and is consistant with the observation that co-addition of different agonists results in additive effects on accumulation of cAMP. The magnitude of the effect of any of the classes of agonists can be influenced by a variety of factors, some of which may be related to the peculiarities of growth in culture: (1) The cells secrete cAMP into the medium, and the magnitude of this secretion for a given rise in intracellular cAMP is different for different agonists. (2) The exposure of the cells to catecholamines or prostaglandins leads to a loss of responsiveness to a subsequent challenge by the same agonist. The magnitude of the agonist-induced loss of responsiveness is dependent on the concentration of the agonist and the time of exposure. The process is at least partially agonist specific in that exposure of cells to isoproterenol can lead to greater than 90% loss in catecholamine responsiveness with less than 20% loss in responsiveness to prostaglandins. (3) The responsiveness of the cells also changes as a function of the age of the culture and as a function of cell density. (4) Finally, it can be demonstrated that cells maintained in culture for prolonged periods (months to years) may lose responsiveness to specific agonists while responsiveness to other agonists remains unchanges or actually increases. The advantages and disadvantages of the use of cells in culture for studies of the regulation of cAMP metabolism are discussed.

The responses in culture of human tumour astrocytes and neuroblasts to N 6 , O 2' -dibutyryl adenosine 3',5'-monophosphoric acid

N 6, O2'-dibutyryl cyclic AMP (DBcAMP) (3 x 10-4 M) causes reproducible changes in the morphology, growth rate and terminal cell density of 3 different types of human tumour cells grown in culture.

Human tumour astrocyte lines 1181N1 and EH-118MG show generalized ablation of membrane systems when grown in the presence of this cyclic AMP analogue. This is not consistent with differentiation; it may be a toxic effect but possibly represents the response of these cell types to DBcAMP. These cells also grow more slowly and to lower terminal cell densities as a function of the concentration of DBcAMP. Although the growth kinetics demonstrate a return of these malignant cells to a state of density-dependent growth, micrographic analyses of inter-cellular relationships reveal that the cells still grow in overlapping patterns. Thus as judged by the commonly accepted criteria, DBcAMP does not induce contact inhibition of growth. All observed changes induced by the presence of DBcAMP are rapidly and completely reversed upon its removal from the growth medium. Human neuroblasts (NB1) grown in the presence of DBcAMP show irreversible morphological changes and the rate and extent of cell division is reduced. There is marked hyperplasia of intracytoplasmic microfilaments, which is considered consistent with differentiation.

Regulation of adenosine 3':5'-cyclic monophosphate concentration in cultured human astrocytoma cells by catecholamines and histamine

Norepinephrine, epinephrine, and histamine cause a rapid increase in the concentration of adenosine 3':5'-cyclic monophosphate (cAMP) in a tumor astrocyte cell line derived from a primary culture of a human glioblastoma multiforme. The catecholamine-induced increase in cAMP is dependent on the cell density, being far greater in cells in the log phase of growth than in cells near terminal density. The response to norepinephrine is inhibited 50% by 0.01 muM propranolol, a blocking agent of beta-adrenergic receptors. In contrast, the effect of histamine on cAMP concentration varies only slightly from log-phase growth to terminal density, and is not inhibited by 10 muM propranolol. The results suggest that astrocytoma cells have independent receptors for catecholamines and histamine. Further, if the astrocytoma cell is an adequate model of the normal glial cell, these results suggest that astrocytes in human cerebral cortex may be sensitive to norepinephrine and histamine.