Báo cáo khoa học: Phosphorylation of Hrs downstream of the epidermal growth factor receptor docx

Phosphorylation of Hrs downstream of the epidermal growth factor receptor Kristi G. Bache 1 , Camilla Raiborg 1 , Anja Mehlum 1 , Inger Helene Madshus 2 and Harald Stenmark 1 1 Department of Biochemistry, Institute for Cancer Research, the Norwegian Radium Hospital, Montebello, Oslo, Norway; 2 Institute of Pathology, the National Hospital, Oslo, Norway The hepatocyte growth factor-regulated tyrosine kinase substrate Hrs is an early endosomal protein that is thought to play a regulatory role in the trafficking of growth factor/ receptor complexes through early endosomes. Stimulation of cells with epidermal growth factor (EGF) rapidly leads to phosphorylation of Hrs, raising the question whether the receptor tyrosine kinase phosphorylates Hrs directly. Here, we present evidence that a downstream kinase, rather than the active receptor kinase is responsible. We show that the nonreceptor tyrosine kinase Src is able to phosphorylate Hrs in vitro and in vivo, but that Hrs is nevertheless phosphory- lated in Src-, Yes- and Fyn-negative cells. Moreover, we show that only 10–20% of Hrs is phosphorylated following EGF stimulation, and that phosphorylation occurs at mul- tiple tyrosines located in different parts of Hrs. These results suggest that Hrs is a substrate for several kinases down- stream of the EGF receptor. Keywords: endosome; membrane trafficking; Src; tyrosine kinase. The growth factor or cytokine stimulated phosphorylation of macromolecules regulates cellular functions such as adhesion, cytoskeletal function, membrane trafficking and gene transcription (reviewed in [1]). The hepatocyte growth factor regulated tyrosine kinase substrate Hrs is a protein that becomes rapidly phosphorylated upon growth factor and cytokine stimulation [2,3]. It was first shown to be phosphorylated in cells treated with HGF as well as platelet- derivedgrowthfactor(PDGF)andepidermalgrowthfactor (EGF) [2]. Later, Hrs was also identified as a tyrosine phosphorylated protein in hematopoietic cells treated with the cytokines interleukin-2 (IL-2) and granulocyte-macro- phage colony-stimulating factor (GM-CSF) [3]. Even though Hrs was identified as a phosphorylated protein, neither the responsible kinase(s), the exact phosphorylation site(s) nor the functional significance of the phosphorylation have been identified so far. Recent results suggest that in order for Hrs phosphorylation to take place, endosomal localization of Hrs is required [4]. Hrs is localized to early endosomes via a phosphatidylinositol 3-phosphate (PtdIns3P)-binding FYVE domain [5] and a coiled-coil domain [6]. It has been shown to bind the signal transducing adaptor molecules STAM [3] and STAM 2/Hbp [7,8], the SNARE protein SNAP-25 [9], and the signal transmitter downstream of the transforming growth factor-b/activin receptors, Smad 2 [10]. In addition, a functional clathrin box motif has recently been found in the C-terminal part of Hrs, and direct binding between Hrs and clathrin was discovered, suggesting that clathrin is recruited to early endosomes via Hrs [11]. Putative homologues of Hrs have been identified in various eukaryotes, including the class E vacuolar protein sorting molecule Vps27p in Saccharomyces cerevisiae [12]. More than 40 vps mutants have been identified in Saccharomyces cerevisiae [13–15], and a subset of 13 of these are designated as the class E vps mutants. These mutants accumulate vacuolar, endocytic and trans-Golgi markers in an aberrant multilamellar structure, the class E compartment [16–18]. Mice that lack Hrs die during early embryogenesis, demonstrating that Hrs is an essential protein. The embryos show severe defects in ventral folding morphogenesis and contain enlarged transferrin receptor- positive structures [19]. Drosophila larvae that lack Hrs likewise contain enlarged endosomes, and electron micro- scopy has revealed that the formation of endosomal invaginations is inhibited in the absence of Hrs [20]. Because Hrs is an essential protein with functions in membrane trafficking and signal transduction, it will be important to determine the functional significance of its phosphorylation. In this paper, we have investigated whether Hrs is phosphorylated by the EGF receptor or a downstream kinase. We find that Hrs is phosphorylated on multiple tyrosines by kinases downstream of the receptor. While the nonreceptor tyrosine kinase Src is able to phosphorylate Hrs both in vitro and in vivo, other tyrosine kinases also phosphorylate Hrs. MATERIALS AND METHODS Antibodies and plasmid constructs Anti-myc Ig was from the 9E10 hybridoma [21]. The horse- radish peroxidase (HRP)-conjugated anti-phosphotyrosine Correspondence to H. Stenmark, Department of Biochemistry, The Norwegian Radium Hospital, Montebello N-0310 Oslo, Norway. Fax: + 47 22508692, Tel.: + 47 22934951, E-mail: stenmark@ulrik.uio.no Abbreviations: Hrs, hepatocyte growth factor-regulated tyrosine kinase substrate; EGF, epidermal growth factor; GM-CSF, granulo- cyte-macrophage colony-stimulating factor; HGF, hepatocyte growth factor; PDGF, platelet-derived growth factor; PtdIns3P, phosphati- dylinositol 3-phosphate; STAM, signal transducing adaptor molecule; Vps, vacuolar protein sorting; MBP, maltose binding protein; TGFa, transforming growth factor-a. (Received 15 March 2002, revised 3 May 2002, accepted 17 June 2002) Eur. J. Biochem. 269, 3881–3887 (2002) Ó FEBS 2002 doi:10.1046/j.1432-1033.2002.03046.x Ig (RC20H) was from Transduction Laboratories. Anti- serum against Hrs was raised against the recombinant protein [11]. Human anti-EEA1 serum was a gift from Ban- Hock Toh Monash University, Melbourne, Australia. Mouse anti-Src Ig and recombinant active Src (p60 c–Src ) were from Upstate (Waltham, MA, USA). [c- 33 P]ATP was from Amersham. HRP-labelled secondary antibodies were from Jackson Immunoresearch (West Grove, PE, USA). pGEM-myc-Hrs1–289 and pGEM-myc-Hrs287–775 were made by PCR with mouse Hrs [2] as the template. pSGT- Src Y527F used for expression of a constitutive active Src construct was a gift from S. Courtneidge (Van Andel Research Institute, Grand Rapids, MI, USA). Non-fusion pEGFP-EGFR used for expression of EGF-receptor in SYF-2 cells was a gift from A. Sorkin (University of Colorado Health Science Center, Denver, CO, USA). Transient expression in HEp-2 and HeLa cells The pSGT-Src Y527F construct was expressed in HEp-2 cells using the FugeneÒ (Roche) system according to the manufacturer’s instructions, and the cells were analyzed 24 h after transfection. pGEM constructs were expressed in cells using modified Ankara T7 RNA polymerase recom- binant vaccinia virus and lipofection as described previously [22]. Confocal immunofluorescence microscopy For studying localization of Hrs in starved and EGF stimulated cells, Hep-2 cells were grown on coverslips, starved for 4 h in serum-free medium, and stimulated (or not) for 8 min with EGF (100 ngÆmL )1 ). The cells were then permeabilized with 0.05% saponin and fixed with 3% paraformaldehyde before staining with the antibodies indicated, as described [23]. Coverslips were examined using a Leica TCS NT confocal microscope equipped with a Kr/Ar laser and a PL Fluotar 100·/1.30 oil immersion objective. EGF/TGFa stimulation and immunoprecipitation of Hrs Hep-2 cells were starved for 4 h in serum-free medium and then stimulated (or not) for 8 min with EGF or TGFa (both 100 ngÆmL )1 ) for 10 or 20 min at 4 °C with EGF. The cells were washed three times with ice-cold NaCl/P i and lysed for 15mininlysisbuffer(125m M KAc, 25 m M Hepes, 25 m M MgAc, 5 m M EGTA, 0.5% NP40, 1 m M dithiothreitol, pH 7.2) at 37 °C or supplemented with mammalian phos- phatase inhibitor cocktail II and protease inhibitor cocktail (Sigma). After preclearance of the lysate by centrifugation, it was incubated with 25 lL protein A–Sepharose (Pharma- cia) preincubated with 20 lL anti-Hrs Ig. Immunoprecip- itates were washed three times with washing buffer (125 m M KAc, 25 m M Hepes, 2.5 m M MgAc, 5 m M EGTA, 1 m M dithiothreitol, pH 7.2) supplemented with phosphatase inhibitor cocktail II, and Hrs was eluted in sample buffer for sodium dodecyl sulfate polyacrylamide gel electrophor- esis (SDS/PAGE) or two dimensional PAGE (8 M urea, 65 m M dithiothreitol, 2% (w/v) Pharmalyte (Pharmacia AB) and 0.5% (v/v) Triton X-100) (10% gels). For the 2D PAGE the material was subjected to isoelectric focusing on linear Immobiline Dry Strips, pH 3–10 (Amersham Phar- macia Biotech, Uppsala, Sweden), with the Pharmacia Multipor II apparatus and procedure. After equilibration in 2% (w/v) SDS, the strips were placed on top of a vertical SDS/PAGE gel and subjected to electrophoresis with a Bio- Rad Protean II apparatus. Following SDS/PAGE, proteins were transferred to 0.45 lm pore size poly(vinylidene fluoride) membranes (Millipore) and incubated with prima- ry and secondary antibodies before detection with the SuperSignal chemiluminescence kit from Pierce. In all experiments, the membranes were stripped and reblotted with anti-Hrs Ig to verify equal loadings of the samples. Immunoprecipitation of Hrs from HeLa cells transfected with a constitutively active Src mutant, or SYF-2 cells transfected with EGF-receptor HeLa cells transfected or not with the plasmid expressing active Src were starved for 4 h before lysis and immuno- precipitation of Hrs as described above. Hrs was eluted in sample buffer for SDS/PAGE. peGFP-EGFR was expressed in SYF-2 cells and and starved for 4 h before stimulating or not with EGF (100 ngÆmL )1 ) for 8 min. The cells were lysed and Hrs immunoprecipitated as described, and eluted in sample buffer for SDS/PAGE. Following SDS/PAGE, proteins were transferred to 0.45-lm pore size poly(vinylidene difluoride) membranes (Millipore) and incubated with primary and secondary antibodies before detection with SuperSignal chemiluminescence from Pierce. In vitro phosphorylation of Hrs Mops buffer (10 m M MgCl 2 ,50m M Mops pH 7.0, 150 m M NaCl) supplemented with 25 lCiÆmL )1 [c- 33 P]ATP was mixedwith1lg maltose binding protein (MBP), 1 lg MBP–Hrs or buffer only, and 0.5 U recombinant active Src was added after heating of the samples to 30 °C. After 20 min at 30 °C, the reactions were stopped by adding 3·SDS-sample buffer. Following SDS/PAGE, the gel was fixed and dried and left overnight in a phosphorimager (Molecular Dynamics). RESULTS Phosphorylation of Hrs at 37 °C and 4 °C Inhibition of endocytosis by dominant-negative dynamin expression and hypertonic media prevents the EGF-induced phosphorylation of Hrs. This suggests that the phosphory- lation of Hrs takes place on endosomes [4]. As the activation of the EGF receptor occurs efficiently at 4 °C [24,25], whereas endocytosis is blocked, we studied the EGF- induced phosphorylation of Hrs at this temperature in order to further investigate the relationship between endocytosis and Hrs phosphorylation. As shown in Fig. 1, essentially no Hrs phosphorylation of Hrs was detected in unstimulated cells (lane 1), whereas EGF stimulation for 8 min at 37 °C caused a strong tyrosine phosphorylation of Hrs (lane 2). When the cells were stimulated with EGF for 10 or 20 min at 4 °C (lanes 3–4), the phosphorylation was much weaker than that detected at 37 °C, supporting the idea that endocytosis of the receptor/ligand complex is important for the phosphorylation of Hrs. Nevertheless, some phosphory- lation occurred even at 4 °C, indicating that endocytosis is 3882 K. G. Bache et al. (Eur. J. Biochem. 269) Ó FEBS 2002 not an absolute requirement for Hrs phosphorylation and that a small fraction of Hrs may be phosphorylated at the plasma membrane. Hrs phosphorylation does not require an active EGF receptor kinase It has been proposed previously that the EGF receptor tyrosine kinase may be responsible for the phosphorylation of Hrs [4,26], and our finding that Hrs is phosphorylated to some extent at 4 °C does not exclude this possibility. To investigate if the activated EGF receptor kinase phosphory- lates Hrs directly, we measured Hrs phosphorylation upon the stimulation of EGF receptors with two ligands that have different properties. While EGF remains bound to the receptor at the acidic endosomal pH, thus activating the receptor kinase, transforming growth factor a (TGFa) rapidly dissociates and leaves the EGF receptor inactive after internalization into early endosomes [27,28]. There- fore, if Hrs is phosphorylated by the receptor kinase, we would expect that only EGF, and not TGFa,causesits phosphorylation. HEp-2 cells, which are well characterized with respect to EGF and TGFa signalling [27], were serum-starved andstimulatedwithEGForTGFa before the cells were washed and Hrs was immunoprecipitated from the lysate. We chose to stimulate the cells with the growth factors for 8 min, as the growth factor-stimulated phosphoryla- tion of Hrs is maximal at this time point [2–4]. The tyrosine phosphorylation was analyzed by immunoblot- ting using an antibody reacting specifically to phospho- tyrosine (Fig. 2). The membranes were stripped and reblotted with anti-Hrs Ig to verify equal loadings of the samples. We found that both EGF and TGFa had the same ability to stimulate Hrs phosphorylation. This indicates that Hrs is not phosphorylated by the EGF receptor kinase and implies the involvement of a downstream kinase. Hrs is a substrate for Src phosphorylation in vitro and in vivo When considering tyrosine kinases that are activated downstream of the EGF receptor, we regarded Src family kinases as possible candidates, as these kinases are activated downstream of all known receptors that mediate Hrs phosphorylation [29–34]. To study whether Src can phos- phorylate Hrs in vitro, active Src was mixed with recom- binant Hrs fused to maltose-binding protein (MBP). As a negative control we used MBP only. The phosphoryla- tion was measured by the incorporation of radioactive phosphate, and we found that MBP-Hrs, but not MBP alone, was phosphorylated by Src (Fig. 3A). In addition we observed a strong phosphorylation product of about 60 kDa in all lanes, corresponding to autophosphorylated Fig. 3. Src phosphorylates Hrs in vitro and in vivo. (A) Radioactive ATP was mixed with Hrs fused to maltose binding protein (MBP) (lane 3) or MBP alone (lane 2). An additional control containing buffer only was also included (lane 1). Recombinant active Src was added, and the samples were incubated at 30 °C for 20 min before the reac- tions were stopped by adding 3· sample buffer for SDS/PAGE. The gel was fixed and dried and exposed in a phosphoimager over night. (B) HeLa cells were transfected or not with an active Src construct and serum-starved for 4 h before Hrs was immunoprecipitated and phos- phorylation analyzed by immunoblotting with anti-phosphotyrosine Ig (upper panel) or anti-Hrs Ig (lower panel). Fig. 1. Phosphorylation of Hrs is reduced at 4 °C. HEp-2 cells were serum-starved for 4 h before stimulation with EGF for 8 min at 37 °C (lane 2, upper panel) or 10 and 20 min at 4 °C(lanes3and4,upper panel). Hrs was immunoprecipitated and phosphorylation analyzed by immunoblotting with anti-phosphotyrosine Ig (upper panel) or anti- HrsIg(lowerpanel). Fig. 2. Hrs phosphorylation is caused by both EGF and TGFa. Hep-2 cells were serum-starved for 4 h and stimulated with EGF or TGFa as indicated. Hrs was immunoprecipitated and phosphorylation analyzed by immunoblotting with anti-phosphotyrosine Ig (upper panel) or anti-Hrs Ig (lower panel). Ó FEBS 2002 Tyrosine phosphorylation of Hrs (Eur. J. Biochem. 269) 3883 Src. These results show that Hrs is a substrate for Src phosphorylation in vitro. In order to study if Hrs is a substrate for Src phosphorylation in vivo, we investigated whether a consti- tutively active Src construct was able to phosphorylate Hrs in serum-starved cells. HeLa cells were transfected with the active Src construct and serum-starved for 4 h before lysing the cells and immunoprecipitating Hrs. Phosphorylation was examined by immunoblotting. As expected, no phos- phorylated Hrs was detected in the serum-starved cells without the active Src construct. However, in the trans- fected cells we could observe a strong phosphorylation of Hrs (Fig. 3B). These results indicate that Src is able to phosphorylate Hrs in vivo. After transfection of HeLa cells with the active Src construct and analysis by confocal fluorescence microsco- py, we observed a number of large vesicular structures that were positively stained for Src and partly colocalized with Hrs and EEA1 (Fig. 4). These enlarged structures might represent enlarged endosomes resulting from the hyper-phosphorylation of Hrs or other endosomal pro- teins. However, it has been shown that v-Src induces constitutive macropinocytosis in rat fibroblasts resulting in large pinocytic vesicles [35], and the enlarged structures we observed might also be related to this phenomenon. In any case, the colocalization between Src and Hrs is consistent with the idea that Hrs is a substrate for Src phosphorylation. Fig. 4. Hrs colocalizes with constitutively active Src on large endocytic structures. Hep-2 cells were transfected with the pSGT-Src Y527F construct that expresses constitutively active Src, and permeabilized with 0.05% saponin before fixation with 3% paraformaldehyde. Src was stained with anti-Src Ig (rhodamine) (a), Hrs with anti-Hrs Ig (FITC) (b) and early endosomes with anti-EEA1 Ig (Cy5) (c). Merged images are shown to illustrate colo- calization between active Src and Hrs (yellow) (d),HrsandEEA1(turquoise)(e)andactive Src and EEA1 (purple) (f). Examples of enlarged structures are pointed out (arrows). Size bar ¼ 5 lm. 3884 K. G. Bache et al. (Eur. J. Biochem. 269) Ó FEBS 2002 Hrs is phosphorylated in cells that lack Src, Yes and Fyn The previous results led us to suspect that Src plays an important role in the phosphorylation of Hrs. Therefore, we wanted to examine whether EGF-induced phosphorylation of Hrs can take place in cells lacking Src. The mouse fibroblast cell line SYF-2, which is negative for Src and the related kinases Yes and Fyn, was used for this experiment. The EGF receptor is not endogenously expressed in this cell line, and therefore had to be transfected in before serum- starvation and EGF stimulation. As expected, no tyrosine phosphorylation of Hrs was detected in starved SYF-2 cells (Fig. 5, lane 1). However, when the cells were stimulated with EGF (lane 2), we observed a significant phosphoryla- tion of Hrs. This result demonstrates that kinases other than Src, Yes and Fyn are able to phosphorylate Hrs. Hrs is phosphorylated on multiple tyrosines, and only a minor fraction is phosphorylated Even though the ligand-induced phosphorylation of Hrs has been documented in many studies, the fraction of Hrs that is phosphorylated has never been determined. Furthermore, it is not known if Hrs is phosphorylated on single or multiple tyrosine residues. In order to investigate this, we stimulated Hep-2cellsfor8minwithEGFandanalyzedthe phosphorylation of Hrs by 2D PAGE, which should enable us to separate phosphorylated Hrs from nonphosphorylated Hrs based on differences in the pI. We expected phosphory- lated Hrs to have a slightly lower pI than nonphosphory- lated Hrs, and to observe multiple spots if there are multiple phosphorylation sites. In unstimulated cells, Hrs migrated as a strong spot at pI  6, with a minor spot at slightly lower pI (Fig. 6A). The nature of this minor spot is not known. Significantly, in cells that were stimulated with EGF, we observed at least three additional spots towards the acidic side of the gel (Fig. 6B). When reprobing the blots with anti- phosphotyrosine Ig, we confirmed that these three addi- tional spots corresponded to phosphorylated Hrs (Fig. 6D), whereas we observed no phosphorylation in the unstimu- lated cells (Fig. 6C). To study if the phosphorylation occurs in the N- or C-terminal regions of Hrs, we transfected an N-terminal (Hrs1–289) and a C-terminal (Hrs287–775) Hrs construct into HeLa cells and studied their phosphorylation (Fig. 7) in the presence of EGF. Interestingly, both of these constructs were phosphorylated, although Hrs287–775 showed the relatively strongest phosphorylation. We con- clude from these experiments that Hrs is phosphorylated on Fig. 5. Hrs is phosphorylated in cells lacking Src, Yes and Fyn. SYF-2 cells, which are negative for the Src family kinases Src, Yes and Fyn, were transfected with EGF receptor, serum-starved for 4 h and sti- mulated or not for 8 min with EGF. Hrs was immunoprecipitated and phosphorylation analyzed by immunoblotting with anti-phospho- tyrosine Ig (upper panel) or anti-Hrs Ig (lower panel). Fig. 6. 2D PAGE reveals that only a minor fraction of Hrs is phos- phorylated upon EGF stimulation. Hep-2 cells were serum-starved for 4 h and then stimulated (b and d) or not (a and c) for 8 min with EGF. Hrs was then immunoprecipitated and analyzed by 2D gel electro- phoresis and immunoblotting with anti-Hrs Ig (a and b) or anti- phosphotyrosine Ig (c and d). Arrowheads indicate additional spots appearing after stimulation, corresponding to phosphorylated Hrs. Fig. 7. Both N- and C-terminal parts of Hrs are phosphorylated. Myc- tagged wild-type Hrs or the deletion constructs Hrs1–289 and Hrs287– 775 were transfected into HeLa cells, which were stimulated for 8 min with EGF. The expressed constructs were immunoprecipitated from cell lysates using an antibody against the myc epitope, and analyzed by immunoblotting with anti-phosphotyrosine Ig (upper panel) or anti- myc Ig (lower panel). Arrows indicate the position of the Ig heavy chain. Ó FEBS 2002 Tyrosine phosphorylation of Hrs (Eur. J. Biochem. 269) 3885 multiple tyrosine residues. When comparing the intensity of the spots corresponding to phosphorylated forms with that corresponding to nonphosphorylated Hrs, it is striking that the former is much weaker than the latter one (10–20% as measured by quantitation of the gel spots). This indicates that only a fraction of Hrs becomes phosphorylated upon EGF stimulation of cells. DISCUSSION In the present work, we show evidence that the phos- phorylation of Hrs occurs downstream of the EGF receptor kinase. Our findings that Src colocalizes with Hrs and can phosphorylate Hrs in vitro and in vivo suggest that this kinase may be involved in the phosphorylation of Hrs. On the other hand, as EGF-dependent phosphory- lation of Hrs occurs in cells that lack Src, Yes and Fyn, Hrs must also be a substrate for other kinases. Our 2D-gel analysis, which indicates that Hrs is phosphoryl- ated on multiple tyrosine residues upon EGF stimulation, is consistent with this view. There are 29 tyrosines in Hrs, and four of these are conserved in Hrs from nematode, fly, mouse, rat and human. Recent work has shown that tyrosine 334 (and possibly tyrosine 329) of Hrs is phosphorylated [36]. We found that the N-terminal part (residues 1–289) and the C-terminal part (residues 287–775) were both phosphory- lated, with the latter showing the relatively highest phos- phorylation. Together with the fact that multiple spots representing phosphorylated Hrs could be observed by 2D PAGE, these findings lead us to conclude that multiple tyrosines in Hrs are phosphorylated upon EGF stimulation. By transfecting cells with a constitutively active mutant of Src, we found that active Src has a strong ability to phosphorylate Hrs in the absence of growth factor/cytokine stimuli. In addition we observed phosphorylation of Hrs in vitro when incubating MBP fusion of Hrs with recom- binant active Src. Src is activated in several signalling pathways, including those known to lead to Hrs phos- phorylation, and it localizes to the plasma membrane and endosomes [37–41]. The facts that Hrs has functions in membrane trafficking and the signalling pathway of growth factors and cytokines [2,19], that it is localized to the limiting membrane of early endosomes [26], and that it partially colocalizes with Src (this study), are consistent with our finding that Hrs is a substrate for Src phosphorylation. It is evident, however, that kinases other than Src, Yes and Fyn may also have Hrs as a substrate, because Hrs is still phosphorylated in Src/Yes/Fyn negative cells upon EGF stimulation. It is possible that different kinases phosphory- late different tyrosine residues in Hrs, or that alternative kinases can phosphorylate Hrs in the absence of Src. The functional relevance of the Src-induced phosphorylation of Hrs thus needs to be investigated further. 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As the activation of the EGF receptor. Phosphorylation of Hrs downstream of the epidermal growth factor receptor Kristi G. Bache 1 , Camilla Raiborg 1 ,