Vijaya Karoor and Craig C. Malbon

Transcription

1 THE JOURNAL OF BIOLOGICAL CHEMISTRY Vol. 271, No. 46, Issue of November 15, pp , by The American Society for Biochemistry and Molecular Biology, Inc. Printed in U.S.A. Insulin-like Growth Factor Receptor-1 Stimulates Phosphorylation of the 2 -Adrenergic Receptor in Vivo on Sites Distinct from Those Phosphorylated in Response to Insulin* (Received for publication, August 7, 1996) Vijaya Karoor and Craig C. Malbon From the Department of Molecular Pharmacology, Diabetes and Metabolic Diseases Research Center, School of Medicine, State University of New York, Stony Brook, New York G-protein-linked receptors have been shown to be substrates for growth factor receptors with intrinsic tyrosine kinase activity typified by the ability of insulin to both phosphorylate tyrosyl residues in the C terminus of and to counter-regulate the action of the 2 -adrenergic receptor (Karoor, V., Baltensperger, K., Paul, H., Czech, M. P., and Malbon, C. C. (1995) J. Biol. Chem. 270, ). Insulin-like growth factor-1 (IGF-1), another member of the growth factor family operating via receptors with intrinsic tyrosine kinase, is shown in the present work to stimulate in vivo the phosphorylation of the 2 -adrenergic receptor. Analysis of tryptic digests prepared from phosphorylated 2 -adrenergic receptors of IGF-1-treated, metabolically labeled smooth muscle cells was performed using reversed-phase high performance liquid chromatography, two-dimensional peptide mapping, and matrixassisted laser desorption/ionization time-of-flight mass spectrometry. The results of these separate analyses reveal that IGF-1 stimulates phosphorylation predominantly on tyrosyl residues Y132/141 of the second intracellular loop of the 2 -adrenergic receptor rather than the C-terminal region targeted by the activated insulin receptor (Y350/354, Y364), although both growth factors block -adrenergic agonist action. These data demonstrate selective phosphorylation of a G-protein-linked receptor by receptor tyrosine kinases for insulin and IGF-1 mapping to spatially distinct regions of this heptihelical membrane receptor. G-protein-linked receptors (GPLR) 1 and growth factor receptors with intrinsic tyrosine kinase activity transduce signals fundamental to cell growth, differentiation, and metabolism (1 4). Cross-talk between these two major cell signaling pathways occurs at several levels, including receptor to receptor (5 7), tyrosine kinase to G-protein (8), and at downstream levels in the mitogen-activated protein kinase regulatory network (9 12). A paradigm in which the two most proximal elements of the GPLR and tyrosine kinase receptor pathways * This work was supported by United States Public Health Service Grant DK25410 from the NIDDK, National Institutes of Health. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. Current address: Weiss Research Center, Geisinger Clinic, Danville, PA To whom all correspondence should be addressed. Tel.: ; Fax: The abbreviations used are: GPLR, G-protein-linked receptor; IGF-1, insulin-like growth factor-1; MALDI TOF, matrix-assisted laser desorption/ionization time-of-flight; HPLC, high performance liquid chromatography. This paper is available on line at operating as substrate (former) and protein kinase (latter) provides a new dimension to our understanding of the integrations of intracellular signaling (13). The counter-regulatory effects of IGF-1 and insulin on catecholamine action are well known, and we sought to explore whether the actions of IGF-1 included phosphorylation of a prominent member of the GPLR, the 2 -adrenergic receptor. Earlier we reported that purified IGF-1 receptor and insulin receptor both were able to phosphorylate the 2 -adrenergic receptor directly in an in vitro reconstitution assay using recombinant 2 -adrenergic receptor (7). In this work we explore the ability of IGF-1 to stimulate phosphorylation of the 2 -adrenergic receptor in DDT 1 MF-2 smooth muscle cells in culture following metabolic labeling. In addition, we demonstrate the power of matrix-assisted laser desorption/ ionization time-of-flight (MALDI TOF) mass spectrometry to analyze phosphopeptides of 2 -adrenergic receptor, revealing the precise sites of phosphorylation to tyrosyl residues (Y132/ 141) localized to the second intracellular loop of this heptihelical GPLR spatially distinct from those tyrosyl residues (Y350/ 354, Y364) targeted by the insulin receptor (6, 7). EXPERIMENTAL PROCEDURES Phosphorylation of 2 -Adrenergic Receptor in Vivo DDT 1 MF-2 hamster vas deferens smooth muscle cells were cultured in Dulbecco s modified Eagle s medium, metabolically labeled in phosphate-free Dulbecco s modified Eagle s medium containing 0.5% fetal bovine serum and 1 mci/ml [ 32 P]orthophosphate for 4hat37 C(5,14), treated with either hormone (IGF-1 or insulin, 100 nm) or saline for 2 40 min at 37 C. The cultures were then lysed (1% Triton X-100, 0.1% sodium dodecyl sulfate, 6.0 M dithiothreitol, 5 g/ml aprotinin, 5 g/ml leupeptin, 100 g/ml bacitracin, 100 g/ml benzamidine, 1 mm sodium orthovanadate, 150 mm NaCl, 5 mm EDTA, 50 mm NaF, 40 mm sodium pyrophosphate, 50 mm KPO 4,10mMsodium molybdate, 20 mm Tris- HCl, ph 7.4) and the 2 -adrenergic receptor immunoprecipitated with anti-receptor antibody CM-4 as described (4, 15), but the samples were not boiled. Proteins were denatured for 5 min at 95 C and then separated by SDS-polyacrylamide gel electrophoresis (10% w/v acrylamide). Phosphorylated proteins were visualized by exposing the dried gel to X-Omat AR film (Eastman Kodak Co.). Stoichiometry of Phosphorylation The cells were labeled with [ 32 P]orthophosphate as described above. After 4 h, the medium was aspirated and the proteins were precipitated with 0.5 M perchloric acid. The extract was spun down and the supernatant neutralized with KOH. The specific activity of the [ 32 P]ATP in the supernatant was determined essentially as described by England and Walsh (15). The specific activity of the receptor was determined by immunoprecipitation from a duplicate labeled plate. The amount of receptor was quantified by ICYP labeling (4). The phosphate content is reported in units of mol phosphate/mol receptor. Reversed-phase HPLC and Separation of Tryptic Phosphopeptides 32 P-Labeled 2 -adrenergic receptor was immunoprecipitated from metabolically labeled DDT 1 MF-2 cells on SDS-polyacrylamide gel electrophoresis as described above. Synthetic peptides containing tyrosine residues 350, 354, and 364 were labeled in vitro with [ - 32 P]ATP and separated on Tricine gels as described (16). The bands corresponding to 2 -adrenergic receptor or the synthetic peptides

2 29348 Phosphorylation of 2 -Adrenergic Receptor in Response to IGF-1 versus Insulin TABLE I IGF-1 abolishes 2 -adrenergic agonist-stimulated cyclic AMP accumulation Cyclic AMP accumulation in DDT 1 MF-2 smooth muscle cells was measured in response to vehicle (none), isoproterenol (10 M), IGF-1 (100 nm), or both in combination for 15 min at 37 C. The values are means S.E.M. from three different experiments, each performed in triplicate. Treatment Cyclic AMP accumulation pmol/10 5 cells None Isoproterenol, 10 M IGF-1, 100 nm Isoproterenol, 10 M IGF-1, 100 nm FIG. 1.IGF-1 stimulates phosphorylation of the 2 -adrenergic receptor in vivo. Phosphorylation of 2 -adrenergic receptor by IGF-1 in vivo: dose dependence (A) and time course (B). Metabolically labeled DDT 1 MF-2 cells were treated with increasing concentrations of IGF-1 for 2 min. The cells were lysed and the labeled 2 -adrenergic receptor immunoprecipitated as described under Experimental Procedures. The immunoprecipitates were subjected to SDS-polyacrylamide gel electrophoresis and the phosphorylated 2 -adrenergic receptor made visible by autoradiography of the dried gel. The data are representative of three separate experiments. were excised from the gel and treated with L-1-tosylamido-2-phenylethyl chloromethyl ketone-treated trypsin (40 g/ml) for 18 h (16). The tryptic eluate was then separated on a microbore HPLC (Applied Biosystems) using a 220-mm Aquapore OD-300 column and a gradient of acetonitrile (0 50% in 45 min) in 0.1% trifluoroacetic acid at a flow rate of 200 l/min. Fractions were collected at 1-min intervals and counted for Cerenkov radiation. Two-dimensional Peptide Mapping Tryptic digests of 2 -adrenergic receptor isolated from metabolically labeled cells (see above) and phosphorylated synthetic peptide markers (6, 7) were subjected to reversedphase HPLC. The peaks identified in the HPLC eluates of tryptic digests were subjected to two-dimensional peptide mapping on cellulose thin-layer plates (6, 7). An aliquot (10 l) of the tryptic eluate was spotted onto a TLC plate and subjected to high voltage (1000 V) electrophoresis for 60 min in a buffer composed of formic acid/glacial acetic acid/water (50:156:1794, ph 1.9). Following electrophoresis, the TLC plate was air-dried overnight and then subjected to chromatography (at a right angle to the direction of electrophoresis) in a phosphochromatography buffer composed of 1-butanol/pyridine/acetic acid/water (15:10:3:12). The plates were dried and the peptides identified by autoradiography (14). Mass Spectrometry of Receptor Phosphopetides Tryptic digests of the 2 -adrenergic receptor were prepared as described above. Phosphopeptides then were purified from the tryptic digest by ferric chelation chromatography (17). Briefly, the tryptic eluates were dried and resuspended in buffer A (50 mm 4-morpholineethanesulfonic acid, 1 M NaCl, ph 5.5). The samples were loaded onto a column of IDA beads (Pierce) charged with ferric chloride. Following adsorption of the sample, the column was washed with buffer A and again with buffer B (50 mm 4-morpholineethanesulfonic acid, ph 6.0) and then washed with buffer C (500 mm NH 4 HCO 3, ph 8.0) to release the phosphopeptides. The samples were dried, washed several times with water to rid them of salts, and then subjected to mass analysis using a Bruker Protein MALDI TOF mass spectrometer fitted with a UV nitrogen laser of 337 nm and a pulse length of 3 ns. The samples were mixed with the matrix sinapinic acid and 2- l aliquots spotted for analysis. The molecular mass was determined for each peak of the MALDI TOF mass spectrum. FIG. 2.Analysis by reversed-phase HPLC and two-dimensional peptide mapping of tryptic digests of 2 -adrenergic receptor phosphorylated in vivo in response to IGF-1 identifies Y132/141 as predominant sites for phosphorylation. Tryptic peptides of 2 - adrenergic receptor of control (A) and IGF-1 treated (B) metabolically labeled cells as well as 32 P-phosphorylated synthetic peptide markers for tryptic fragments (6, 7) I135 (C) and Y132 (D) were subjected to reversed-phase HPLC as described under Experimental Procedures. The amount of Cerenkov radioactivity in each fraction of a typical chromatogram is displayed. The data are representative of three separate experiments. Two-dimensional peptide mapping was performed on the tryptic digest of the phosphorylated synthetic peptide Y132 (6, 7) and that of 2 -adrenergic receptor obtained from IGF-1-treated metabolically labeled cells as described above. High voltage electrophoresis (TLE) and thin-layer chromatography (TLC) were performed as described under Experimental Procedures. The data are representative of three separate experiments. Arrows indicate the origin of the twodimensional peptide mapping. Cyclic AMP Determination Cells were seeded at a density of cells/well in 96-well microtiter plates 48 h before each experiment. On the day of the experiment, Dulbecco s modified Eagle s medium was aspirated and the cells washed and resuspended in Krebs-Ringer phos-

3 Phosphorylation of 2 -Adrenergic Receptor in Response to IGF-1 versus Insulin TABLE II Tryptic fragments of the intracellular domains of the 2 -adrenergic receptor which harbor potential sites for phosphorylation The tryptic fragments derived from intracellular domains (intracellular loops 1, 2, 3 and the C-terminal tail) of the 2 -adrenergic receptor which harbor potential sites for protein phosphorylation are displayed in increasing mass. Residues Sequence a Mass SSK SSSK ESER SGHLR YQSLLTK NCSTNDSPL YIAITSPFK AYGNGYSSNSNGK FHSPNLGQVEQDGR TDYMGEASGCQLGQEK LCEDPPGTESFVNCQGTVPSLSLDSQGR 2975 a Phosphorylation sites in boldface. phate buffer containing 10 M Ro (cyclic AMP phosphodiesterase inhibitor). Cells were challenged with isoproterenol (10 M) inthe absence and presence of IGF-1 (100 nm) or with IGF-1 alone for 15 min at 37 C. The reaction was terminated by the addition of HCl (0.1 M final). Cyclic AMP accumulation was measured using a competition binding assay (18). FIG. 3.Analysis by MALDI TOF mass spectrometry of tryptic phosphopeptides of the 2 -adrenergic receptor identifies Y141 as the site phosphorylated in response to IGF-1 in vivo. Tryptic phosphopeptides of the 2 -adrenergic receptor were prepared from metabolically labeled DDT 1 MF-2 cells as described in the legends to Figs. 1 and 2. The phosphopeptides were purified by ferric chelation and subjected to analysis on a Bruker MALDI TOF mass spectrometer using sinapinic acid as the matrix. Mass spectra were obtained from phosphopeptides isolated from cells treated with 100 nm IGF-1 for 2 min (a c). The analysis of the peaks and the deduced identity of the peptides are summarized in Table III. RESULTS AND DISCUSSION Having demonstrated the ability of activated purified IGF-1 receptor to catalyze phosphorylation of recombinant 2 -adrenergic receptor in a reconstituted in vitro system (7), we explored whether the 2 -adrenergic receptor was indeed phosphorylated in DDT 1 MF-2 hamster vas deferens smooth muscle cells stimulated with IGF-1. First we investigated the effect of IGF-1 treatment on the ability of the -catecholamine isoproterenol to stimulate cyclic AMP accumulation in these cells. Isoproterenol (10 M) stimulated a 6-fold increase in cyclic AMP accumulation (Table I). Although failing to alter cyclic AMP levels by itself, IGF-1 (100 nm) effectively abolished the rise in intracellular cyclic AMP stimulated by isoproterenol, demonstrating the cross-regulatory effects of this insulin-like growth factor on catecholamine action. For study of phosphorylation, cells first were labeled metabolically with [ 32 P]P i, washed, and then stimulated with IGF-1 (Fig. 1). Immunoprecipitation of the 2 -adrenergic receptor from metabolically labeled cells subjected to SDS-polyacrylamide gel electrophoresis reveal basal phosphorylation of 2 -adrenergic receptor in the absence of IGF-1 or insulin as previously noted (3, 5 7). IGF-1 stimulated a dose-dependent increase in the phosphorylation of the 2 -adrenergic receptor in these smooth muscle cells in culture. The phosphorylation of receptor peaked at 10 nm IGF-1, resulting in a doubling of the content of phosphate. The time course for IGF-1 stimulation of 2 -adrenergic receptor phosphorylation in DDT 1 MF-2 cells was explored. Phosphorylation of the 2 -adrenergic receptor nearly doubles within 30 s of stimulation of the cells with 100 nm IGF-1 (Fig. 1B). The phosphorylation reaches a peak value within 2 min and declines from 5 min thereafter. By 10 min, the phosphorylation declines to near base-line values, revealing the transient nature of the IGF-1-induced phosphorylation. The stoichiometry of the phosphorylation was determined. In units of mol P i /mol 2 -adrenergic receptor, basal unstimulated phosphorylation is , whereas that stimulated by 10 nm IGF-1 is (n 4, p 0.05 for the difference). Phosphoamino acid analysis (5 7) revealed that the IGF-1-stimulated phosphorylation was confined exclusively to phosphotyrosine (not shown). Metabolically labeled 2 -adrenergic receptor from control and IGF-1-stimulated DDT 1 MF-2 cells were excised from the SDS-polyacrylamide gel electrophoresis gels, subjected to digestion with trypsin, and the tryptic digests resolved by reversed-phase HPLC as developed earlier (6, 7). In the absence of IGF-1 stimulation, the HPLC profile displayed several peaks (Fig. 2A) previously shown to be composed largely of phosphoserine and phosphothreonine (6, 7). For tryptic digests of 2 - adrenergic receptor isolated from metabolically labeled IGF-1-

4 29350 Phosphorylation of 2 -Adrenergic Receptor in Response to IGF-1 versus Insulin FIG. 4. Analysis by MALDI TOF mass spectrometry of tryptic phosphopeptides of the 2 -adrenergic receptor identifies Y350/354 and Y364 as phosphorylated in response to insulin in vivo. Tryptic phosphopeptides of the 2 -adrenergic receptor were prepared from metabolically labeled DDT 1 MF-2 cells as described in the legends to Figs. 1 and 2. The phosphopeptides were purified by ferric chelation and subjected to analysis on a Bruker MALDI TOF mass spectrometer using sinapinic acid as the matrix. Mass spectra were obtained from phosphopeptides isolated from cells treated with 100 nm insulin (a c) for 2 min. The analysis of the peaks and the deduced identity of the peptides are summarized in Table III.

5 Phosphorylation of 2 -Adrenergic Receptor in Response to IGF-1 versus Insulin TABLE III Analysis by MALDI TOF mass spectrometry of tryptic phosphopeptides of the 2 -adrenergic receptor identifies Y141 as the site phosphorylated in response to IGF-1 in vivo as compared to Y350/354 and Y364 phosphorylated in response to insulin The mass values and identity of the tryrosyl residue-containing peptides of the 2 -adrenergic receptor that compose the peaks identified in the MALDI TOF mass spectrum are shown. Values in parentheses denote the expected molecular weights of the fragments. Major residues IGF-1 Insulin Y (914) Y141 2Na (960) Y (1380) Y350/354 matrix 1Na (1704) Y364 matrix 2Na (2181) treated cells, the HPLC profile (Fig. 2B) was quite distinct from the basal state (Fig. 2A) and that observed for digests from insulin-treated cells (6, 7). Prominent were two peaks eluting in fractions and (Fig. 2B). Based upon the previous elution profiles of tryptic phosphopeptides of 2 -adrenergic receptor used as standards (6, 7), the prominent IGF-1-stimulated phosphopeptides were identified as those containing tyrosyl residues Y132 and Y141 (Fig. 2, C and D). Two-dimensional peptide mapping of the peaks resolved by HPLC was used to verify the nature of the labeled phosphopeptides. Highvoltage electrophoresis followed by thin-layer chromatography (Fig. 2, E and F) confirmed the identity of the phosphopeptides derived from tryptic digests of 2 -adrenergic receptor isolated from IGF-1-treated cells (Fig. 2F), i.e. phosphorylation predominantly of Y132/141 (Fig. 2E). An alternative strategy, i.e. matrix-assisted laser desorption/ ionization time-of-flight (MALDI TOF) mass spectrometry was used to extend the analysis of phosphopeptides as a means to avoid the need to metabolically label cells with [ 32 P]P i. For this approach the phosphopeptides were isolated from the mixture of tryptic peptides by ferric chelation column chromatography (17). The purification of phosphopeptides by ferric chelation simplifies the interpretation of the spectra (Table II and Fig. 3). The full topological analysis of the tryptic peptides derived from intracellular domains of the 2 -adrenergic receptor is displayed elsewhere (6). Analysis of the mass spectrometry data identified those peptides of the 2 -adrenergic receptor that are phosphorylated in response to IGF-1 (Table III). Since the ionization potential of peptides in a mixture can be quite variable (19, 20), the mass size rather than the apparent abundance is most informative. The Y141 tryptic phosphopeptide identified both by HPLC and by two-dimensional mapping of 2 -adrenergic receptor peptides from IGF-1-treated metabolically labeled cells (Fig. 2) was observed at m/z and ( 2 Na ) 964, agreeing within 0.4% of the calculated mass (Fig. 3, a c). Other phosphopeptides present correspond to phosphoserine-containing sequences, e.g. sequence (SSSK) with m/z 472, sequence (SGHLR) with m/z 696, and sequence (NCSTNDSPL) corresponding to phosphoserine/threonine containing peptide with m/z and ( 1 Na ) The signature of the matrix itself, sinapinic acid, was prominent at m/z 205 and 225. Analysis by MALDI TOF mass spectrometry was applied to 2 -adrenergic receptor isolated from cells challenged with insulin (Fig. 4, a c) as compared to IGF-1 (Fig. 3, a c). The differences in the spectra clearly demonstrate the unique features of the sites phosphorylated in response to IGF-1. Four peaks from the spectrum of receptor phosphopeptides of unstimulated control cells with signals at m/z 471, 515, 761, and 1046 (not shown) were often observed in the spectra from hormone-stimulated cells and were not investigated further. The analysis, confined to phosphotyrosyl-containing sequences (Table III), reveals signal at m/z corresponding to Y350/354 (peptide coupled matrix 1Na ) and 2186 corresponding to Y364 (peptide coupled matrix 2Na ). These data agree well with analysis of labeled tryptic peptides of 2 -adrenergic receptor from metabolically labeled cells subjected to HPLC and two-dimensional peptide mapping (6, 7), demonstrating the Y350/354 and Y364 are prominent sites of insulin-stimulated phosphorylation both in vivo (5, 6) and in vitro (7). The current work illuminates a broadened understanding of G-protein-linked receptors as substrates for growth factor receptors with tyrosine kinase activity. At the outset, it seemed likely that IGF-1 and insulin might promote phosphorylation of a common site(s) of the 2 -adrenergic receptor through which coupling to G s might be counter-regulated. Although sharing many features in counter-regulation of catecholamine action, IGF-1 and insulin are shown to catalyze the phosphorylation of distinct regions in the 2 -adrenergic receptor. Insulin stimulates phosphorylation of tyrosyl residues localized to the C- terminal tail of the receptor, predominantly Y350/354 and Y364. The sequence about Y364 has features of recognition domain for a growth factor tyrosine kinase (21). Phosphorylation of Y350 creates a Src homology 2 domain (22), which may interact with the adapter molecule GRB2. Stimulation by IGF-1, in contrast, leads to phosphorylation of a sequence embedded in the second intracellular loop of this GPLR (7), largely confined to Y141 and Y132. Phosphorylation of either Y132 (YXXI) or Y141 (YXXL) creates a recognition domain for binding of Shc, an element central to insulin signaling and for binding of members of the Src family of non-receptor tyrosine kinases (23). Previously we demonstrated the ability of IGF-1 receptor and the insulin receptor to phosphorylate the recombinant 2 -adrenergic receptor in vitro in a reconstituted system (7). The data obtained in vivo agree well with the prior data obtained in vitro, creating the possibility for more detailed analysis of the interaction of tyrosine kinase receptors with GPLR substrates in the reconstituted system (7). Most significant is the observation that IGF-1 and insulin counter-regulate signaling via 2 -adrenergic receptors, although promoting phosphorylation at spatially distinct sites of the molecule. In addition, phosphorylation of either the C terminus (in response to insulin) or the second, intracellular loop (in response to IGF-1) of the 2 -adrenergic receptor abolishes signaling to a common end point, the stimulatory G-protein of adenylyl cyclase G s. REFERENCES 1. Gilman, A. G. (1987) Annu. Rev. Biochem. 56, Hausdorff, W. P., Caron, M. G., and Lefkowitz, R. J. 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