Phorbol diesters promote β-adrenergic receptor phosphorylation and adenylate cyclase desensitization in duck erythrocytes

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  • Vol. 121, No. 3, 1984 BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS June 29, 1984 Pages 973-979


    David R. Sibley, Ponnal Nambi, Jack R. Peters and Robert J. Lefkowitz

    Howard Hughes Medical Institute Research Laboratories Departments of Medicine (Cardiology) and Biochemistry

    Duke University Medical Center Durham, North Carolina 27710

    Received May 21, 1984

    Preincubation of duck erythrocytes with tumor promoting phorbol diesters or catecholamines leads to attenuation of adenylate cyclase activity. 12-O-Tetradecanoyl phorbol-13-acetate (TPA) and phorbol 12,13-dibutyrate treatment induced a 38% and 30% desensitization of isoproterenol-stimulated adenylate cyclase activity, respectively. In contrast, the inactive phorbol diester, 4a-phorbol 12,13-didecanoate, was without effect in promoting adenylate cyclase desensitization. 51% desensitization. Incubation of

    35' catecholamine isoproterenol induced a Pi labeled erythrocytes with TPA

    promoted a 3- to O-fold increase in phosphorylation of the %-adrenergic receptor as did incubation with isoproterenol. Treatment of the cells with both TPA and isoproterenol together resulted in desensitization and receptor phosphorylation which were no greater than those observed with either agent alone. These data suggest a potential role for protein kinase C in regulating %-adrenergic receptor function.

    The tumor-promoting phorbol diesters have been shown to directly affect a

    variety of cellular functions when administered in vivo or in vitro. These

    actions of phorbol diesters appear to be mediated by a Ca 2+

    and phospholipid

    dependent protein kinase identified by Nishizuka and coworkers (1,2) and

    termed protein kinase C. In the presence of concentrations of Ca 2+ comparable

    to those found in most cells , protein kinase C also requires diacylglycerol

    which is produced from phosphatidylinositol turnover in a receptor-dependent

    fashion (2,3). Phorbol diesters can substitute for diacylglycerol by directly

    binding to and activating protein kinase C (4-6).

    Recently, treatment with the potent phorbol diester, 12-O-tetradecanoyl

    phorbol-13-acetate (TPA)' has been shown to decrease %-adrenergic

    responsiveness in mouse epidermis (7,8) and in cultured epidermal (9) and

    astrocytoma (10) cells. In the mouse epidermis system, this TPA-induced

    desensitization was shown to be due to uncoupling of the %-adrenergic

    1 The abbreviations used are: TPA, 12-O-tetradecanoyl phorbol-13-acetate; PDB, phorbol 12,13-dibutyrate; 4a-PDD, 4a-phorbol 12,13-didecanoate; CYP, cyanopindolol; pABC, para-azidobenzylcarazolol; SDS, sodium dodecyl sulfate; ISO, isoproterenol; ALP, alprenolol; BSA, bovine serum albumin.

    0006-291X/84 $1.50

    973 Copyright 0 1984 by Acudemic Press, Inc.

    All ri,qhts of reproducfion in any form rexenaed.


    receptors from adenylate cyclase with no change in receptor number (7,8).

    This form of desensitization is similar to that seen in avian erythrocytes

    where incubation with catecholamines promotes desensitization of adenylate

    cyclase and uncoupling of the B-adrenergic receptors (11,12). Recently, we

    have shown that, in turkey erythrocytes, the S-adrenergic receptor is a

    phosphoprotein and that the degree of receptor phosphorylation is highly

    correlated with the extent of catecholamine-induced desensitization of

    adenylate cyclase (13,14). We now report that in duck erythrocytes phorbol

    diesters can also promote desensitization of adenylate cyclase activity in

    conjunction with phosphorylation of the 6-adrenergic receptor.


    32 P Incorporation into Duck Erythrocyte S-Adrenergic Receptors: Washed

    duck erythrocytes were suspended to 20% hematocrit in 157.5 mM NaCl, 2.5 mM KCl, 11.1 mM glucose, 10 mM HEPES, pH 7.4 (HEPES buffer) with 300 U/ml penicillin and 1 mgfml streptomycin ag2described by Sibley et al. (14). The 1&1s were incubated with 0.5 mCi/ml Pi at 41C for 20 hr. to allow maximal

    P incorporation into the intracellular ATP. After the 20 hr. preincubation, the cells were further incubated with various phorbol diesters and/or isoproterenol for 3 hr. The cells were then washed three times with HEPES buffer and lysed by hypotonic shock in 5 mM MgCl , 5 mM Tris-HCl, pH 7.5. The membranes were isolated by centrifugation at 30,600 x g and washed three times by resuspension and centrifugation. Adenylate cyclase assays were performed using these membranes as previously described (14). The membranes were then solubilized with 2% digitonin, 100 mM NaCl, 10 mM Tris-HCl, pH 7.2 and the solubilized $-adrenergic receptors purified by alprenolol-Sepharose affinity chromatography as described (14). The affinFty purified receptor fractions were concentrated by ultrafiltration, desalted on Sephadex w 0 columns and assayed for maximal receptor binding activity with 300 pM [ IICYP (2,200 Ci/mmol, New England Nuclear Corp.) as previously described (14). Each sample was next lyophilized to dryness and resuspended in electrophoresis sample buffer containing 25 mM Tris-HCl, pH 6.8, 10% SDS, 10% glycerol and 5% B-mercaptoethanol.

    Photoaffinity Labeling of Duck Erythrocyte Membranes: Duck erythrocyte membranes were diluted in 75 mM Tris-HCl, pH 7.5, 12.5 mM MerCl,, 1.5 mM EDTA (g$,fer A) to a receptor concentration of 30-50 pM and incuba& with 30-50 pM t I]pABC (2,200 Ci/mmol, New England Nuclear Corp.) for 90 min. at 25C in the dark. The incubation mixture was then diluted with Buffer A containing 0.5% fatty acid free bovine serum albumin (BSA) and centrifuged at 40,000 x g for 10 min. The washing with the BSA buffer was repeated twice and one final wash was performed without BSA. The membranes were then suspended in 15 ml of Buffer A and irradiated as described (15). The irradiated samples were pelleted by centrifugation and dissolved in electrophoresis sample buffer.

    SDS Polyacrylamide Gel Electrophoresis: Gel electrophoresis was performed according to the method of Laemmli (16) as previously described (14) using 8% homogeneous slab gels. Upon completion of the electrbphoresis run, gels were dried prior to autoradiography at -70C for 24-36 hr. After visualization of the phosphorylated receptor bands, the gel was rehydrated, permeabilized with protosol gel solubilizer and the quantitated by liquid scintillation spectroscopy.


    When duck erythrocytes are preincubated with the potent phorbol diester,

    TPA, basal adenylate cyclase activity is unchanged (data not shown) whereas




    106M TPA

    Figure 1. Desensitization of isoproterenol-sensitive adenylate cyclase of duck erythrocytes. The cells were preincubated with the indicated concentrations of desensitizing agents followed by preparation of membranes and assessment of basal, fluoride and isoproterenol-stimulated adenylate cyclase activity as described in ref. 14. Typical control enzyme activities were : basal, 4.5 pm01 CAMP/assay; isoproterenol-stimulated, 30 pmol CAMP/assay; fluoride-stimulated, 230 pmol cAi+/assay. The data represent the mean + SEM percent desensitization of the isoproterenol-stimulated activity from between 3-10 experiments.

    isoproterenol-stimulated activity is attenuated by 38 + 2% (Figure 1).

    Exposure of the cells to phorbol 12,13-dibutyrate (PDB) also desensitizes

    isoproterenol-stimulated enzyme activity by 30 + 6% (Figure 1). In contrast,

    preincubation with 4cc-phorbol 12,13-didecanoate, which is devoid of

    tumor-promoting activity and which does not activate protein kinase C, fails

    to promote desensitization in these cells (Figure 1). Incubation with the

    catecholamine, isoproterenol, also promotes a 51 ? 7% desensitization of the

    isoproterenol-stimulated response (Figure 1). Interestingly, when the cells

    are exposed to both TPA and isoproterenol, the desensitization seen (49 + 5%)

    is essentially nonadditive, never being greater than that observed with

    isoproterenol alone (Figure 1). Both TPA and isoproterenol also promote

    desensitization of fluoride-stimulated adenylate cyclase activity to a smaller

    but still significant extent (14.5 + 3%, n = 10 and 12.5 ? 5%, n = 5,

    desensitization, respectively).

    Since the phorbol diesters promote desensitization of isoproterenol-

    stimulated adenylate cyclase, we wished to examine their effects on

    phosphorylation of the B-adrenergic receptor. This is tested by first

    incubating the erythrocytes with 32 Pi in order to prelabel the intracellular

    ATP pool followed by incubation with the desensitizing agent(s). Membranes

    are then prepared and the receptors are solubilized with digitonin and




    67K -

    43K -



    Figure 2. SDS polyacrylamide gel electrophoresis of lz51]pABC labeled B-adrenergic receptor peptidfj?

    3'P labeled and

    52 from duck erythrocytes. For P labeling, the cells were preincubated with Pi prior to desensitization with the indicated agents as described in Methods. The percent desensitization of isoproterenol-stimulated adenylate cyclase activity in each sample was: 1 !JM TPA, 35%; 10 nM isoproterenol, 31%; 1 uM TPA + 10 UM isoproterenol, 39%.

    the amount of receptor binding was . The amounts of receptor loaded on the gel

    were: control, 1.42 pmol; 1 PM TPA, 1.57 pmol; 10 UM ISO, 1.6 pmol; 32 1 !JM TPA + 10 UM ISO, 1.34 pmol. Subsequent to electrophoresis, the P incorporation into the receptor peptides ~2 s quantitated as described in Methods. The specific activities of the P labeled receptors in the samples were : control, 85 cpm/pmol; 1 PM TPA, 349l~~m/pmol; 1 UM TPA + 10 UM ISO, 339 cpm/p~s&. For [

    10 PM ISO, 261 cpm/pmol~ I]pABC labeling, duck erythrocyte

    membranes were incubated with [ IlpABC in the absence (control) or presence of 10 PM alprenolol (+ 10 PM ALP) and processed as described in Methods. The experiment shown was performed twice with similar results.

    purified by affinity chromatography prior to characterization on SDS-PAGE.

    Figure 2 shows the results of such an experiment. There is little 32P

    incorporation into the receptor under control conditions, whereas after

    exposure to TPA or isoproterenol two major phosphorylated peptides of

    molecular weight Mr = 48,000 and Mr = 40,000 are clearly detectable. TPA and

    isoproterenol increase the 32 P content of these peptides by 3-4 fold (Figure

    2) * These peptides are identical to those observed when the photoaffinity

    probe [ 125 I]pABC is used to visualize the receptors by covalent incorporation

    in membranes (Figure 2). The labeling of these peptides by [ 125 I]pABC is

    completely blocked by the B-adrenergic antagonist alprenolol indicating the

    B-adrenergic receptor nature of these peptides (Figure 2). These

    photoaffinity labeling results agree well with those previously reported for

    duck erythrocyte B-adrenergic receptors (17).



    In addition to the major Mr = 48,000 and Mr = 40,000 peptides, there is a

    minor phosphopeptide seen at Mr = 34,000 which seems to be selectively

    phosphorylated in response to TPA, and one at Mr = 30,000 whose

    phosphorylation is promoted by both TPA and isoproterenol (Figure 2). Whether

    or not these peptides represent receptor degradation products or are simply

    nonreceptor contaminants is currently not known. Strikingly, when the cells

    are exposed to both TPA and isoproterenol, the phosphorylation of the receptor

    is not additive being no greater than that observed with either agent alone

    (Figure 2). DISCUSSION

    The major findings of the present study are: 1) that phorbol diesters

    promote desensitization of isoproterenol-stimulated adenylate cyclase

    activity; 2) that this desensitization is associated with phosphorylation of

    S-adrenergic receptors and 3) that phorbol diesters and catecholamines are

    nonadditive with respect to promoting adenylate cyclase desensitization and

    receptor phosphorylation.

    Several lines of evidence suggest that the phorbol diester-induced

    desensitization is specific in nature. Firstly, the phorbol diesters display

    the expected order of potency with TPA being more potent than PDB, and 4o-PDD,

    which does not activate protein kinase C, being inactive (Figure 1).

    Secondly, addition of TPA to the membranes during the adenylate cyclase assay

    does not affect enzymatic activity (data not shown). Additionally, the

    phorbol diester effects are seen primarily with the isoproterenol-stimulated

    enzyme activity with either no or little desensitization of the basal and

    fluoride activities, respectively. This is similar to the pattern observed

    with catecholamine-induced desensitization (12). Finally, the desensitization

    of adenylate cyclase by TPA is associated with a specific phosphorylation of

    the S-adrenergic receptors.

    It should be noted that both TPA and isoproterenol promote

    phosphorylation of both B-adrenergic receptor peptides in proportion to their

    relative preponderance as identified by [ 125 I]pABC labeling (Figure 2). This

    suggests that neither peptide is selectively phosphorylated. Using turkey

    erythrocytes, we have recently investigated the relationship between the two

    S-adrenergic receptor peptides which are characteristically seen in avian

    erythrocytes and found that the smaller peptide is derived from the larger

    (18). We have also previously attempted to elicit phorbol diester-induced

    desensitization in intact turkey erythrocytes with little success (14).

    However, using a turkey erythrocyte lysate system (19), TPA does promote

    desensitization similar to that seen in this study (Nambi, P. et al., in -- preparation). This suggests that intact turkey erythrocytes may be relatively

    impermeable to phorbol diesters which must partition into intracellular

    compartments in order to activate protein kinase C.



    The fact that phorbol diesters, which activate protein kinase C, and

    catecholamines, which activate CAMP-dependent protein kinase, promote

    %-adrenergic receptor phosphorylation in a nonadditive fashion suggests a

    common mechanism or pathway of action. One possibility is that both enzymes

    phosphorylate the receptor on the same site(s). Another possibility is that

    both enzymes phosphorylate the receptor at separate sites but that

    phosphorylation of one set of sites inhibits the phosphorylation of the other

    sites. Alternatively, both protein kinase C and CAMP-dependent protein kinase

    might activate a third or additional kinase(s) which in turn phosphorylate the


    Phorbol diesters have now been shown to decrease the affinity and/or

    responsiveness of a number of membrane receptors including those for EGF...


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