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Institute for Molecular Bioscience and School of Biomedical Sciences, University of Queensland, St Lucia, Australia 4072
(Requests for offprints should be addressed to M J Waters; Email: m.waters{at}imb.uq.edu.au)
| Abstract |
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| Growth hormone (GH) receptor (GHR) activation |
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This plausible model rests upon hormone-dependent receptor dimerization, but is this actually the case? The mAb agonist studies of Fuh et al.(1992) used a chimeric receptor stably expressed in FDCP-1 cells, where the GHR ECD was fused to the ECD, transmembrane domain (TMD) and cytoplasmic domain of the granulocyte colony-stimulating factor (G-CSF) receptor. When we repeated these experiments with a panel of mAbs to the GHR ECD and cell lines expressing full-length GHR, we found only one of 14 mAbs to have significant agonist activity even though eight were able to dimerize and activate the chimeric receptor (Rowlinson et al. 1998). We have subsequently undertaken high-resolution epitope mapping of this mAb (mAb 263), and shown that although it does not bind within the hormone-binding surface, it binds in a way which would promote relative receptor subunit twisting. Torsional force is evidently required, since the monovalent antibody fragment does not act as an agonist (Wan et al. 2003). The alternate view that receptor dimerization is particularly stringent for receptor subunit alignment is not supported by other studies which use particular mAbs to the ECD to document conformational changes which occur upon hormone-dependent activation of the receptor (Jiang et al. 2004). Hormone-dependent conformational change is also supported by protease K digestion studies, which show differences in susceptibility to proteolysis dependent on whether the hGH agonist or the G120R antagonist is bound (van Kerkhof et al. 2002). While it could be argued that hormone-dependent dimerization could account for the latter differences, Harding et al.(1996) showed that the G120R antagonist can form a trimeric complex at the cell surface, contrary to in vitro studies with the isolated ECDs. One final piece of evidence is not consistent with the hormone-induced dimerization model: we have recently reported that the potency of porcine GH can be increased up to 5-fold by substituting hGH residues into porcine GH. Based on the diffusion-dependent dimerization model of Pearce et al.(1999), it should not be possible to increase the biopotency of GH given the kinetic parameters evident for binding of these porcine GH analogues (Wan et al. 2004).
These disagreements with the hormone-dependent dimerization paradigm encouraged us to re-evaluate the model (Brown et al. 2005). In agreement with recent studies from Strous group (Gent et al. 2002), we found that, in co-immunoprecipitation studies, a portion of the full-length GHR exists as constitutive dimers in vivo. This was evident using co-immunoprecipitation of hemagglutinin and FLAG N-terminally tagged receptors. However, the proportion of dimer was found to increase in the presence of hormone. This could be a result of hormone-dependent dimerization of a partially monomeric population or because hormone-binding stabilizes the trimeric complex during detergent solubilization and immunoprecipitation. This is a result of the binding energy imparted from the two hormonereceptor interactions as well as the receptorreceptor interaction in the membrane-proximal ECD (Chen et al. 1997, Bernat et al. 2003). In order to resolve this issue, we have used fluorescence and bioluminescence resonance energy transfer (FRET and BRET), techniques which can be applied to full-length receptors in the cell membrane. Using transient co-expression of receptors labelled with a fluorescence donor or an acceptor, we find high ratios consistent with constitutive dimer formation (i.e. receptor fluorophores within 100 Å). Further, by serial truncating of the receptor ECD and cytoplasmic domains, we conclude that this association is mediated by the transmembrane/juxtamembrane domain. Importantly, addition of hormone even at high concentration was without effect on the BRET and FRET ratios in plasma membrane fractions, indicating that at the cell membrane GH does not induce receptor dimerization, at least for transiently expressed GHR. Appropriate controls for non-specific receptor aggregation showed that homodimerization of the GHR was specific.
Constitutive dimerization of the receptor implies that the activation process involves a conformational change. In order to determine the nature of this, we determined the crystal structure of the unliganded hGHR ECD, and compared it with the receptor ECDs in the 2:1 complex at 2.7 Å (Brown et al. 2005). This revealed only minor conformational change on hormone binding, notably some relative rotation of the upper and lower domains, and movement in the Trp-104 residue, the key hormone-binding residue for both receptors. Importantly, the unliganded receptor ECD does not exist as a dimer in the crystal structure, contrary to the homologous erythropoietin receptor (EPOR). This does not support involvement of a scissor movement in the activation process, currently believed to be the basis for EPOR activation (Kubatsky et al. 2001).
Lack of major conformational change on hormone binding restricts signaling options to a rotation or a plunger movement, and both are possible given that receptor 1 is rotated with respect to receptor 2 in the 2:1 crystallographic complex, as well as sitting higher than receptor 2 relative to the cell surface. To test this we have induced relative subunit rotations of 100 ° by introducing alanines into the TMD and just below. These show that four Ala insertions are able to induce constitutive activation of JAK2 and STAT5 and proliferation seen in cell populations expressing equivalent low levels of receptor. Four Ala residues would induce a 40 ° rotation past the initial position in a helix. These results are not consistent with a plunger movement, since both receptors would be at equivalent height to the Ala insertions. However, they are consistent with activation by relative subunit rotation, a mechanism in agreement with a study showing constitutive activation of the EPOR when receptor subunits are rotated by replacement of the ECD with a coiled coil (Seubert et al. 2003).
As illustrated in Fig. 1
we propose that identical receptor subunits are held in an equivalent and inactive orientation by TMD/juxtamembrane interactions, as recently described for the EPOR (Seubert et al. 2003). In this conformation, locking of the dimerization domains of the ECDs is prevented by steric and charge repulsions involving Tyr-200, Pro-198, His-150 and Asp-152. Hormone binding to receptor 1 initiates activation, with the limited flexibility of the individual membrane connector sequences facilitating alignment of the second receptor through thermal movement, allowing the second site on the hormone to bind its receptor. As the second receptor-binding site is closer to the membrane, and not symmetrically placed, site 2 binding would drive the second receptor down and rotate it relative to the first receptor, allowing the dimerization domains to lock together in the signaling-productive mode. The combined energy of the three binding surfaces (Bernat et al. 2003) would then be sufficient to rotate the TMD helices to the active orientation, and, given rigidity in the submembrane linker, align the box 1 sequences so that the JAK2s can transphosphorylate each other, initiating the signaling cascade. Rotation of the JAK2-binding sites (box 1) in the EPOR is sufficient for such activation (Seubert et al. 2003).
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< 6Hz,
=65 ± 35 °). No explicit H-bond restraints were included in calculations. The final 15 lowest-energy structures contained no dihedral angle (> 5 °) or distance (> 0.3 Å) violations and are displayed in Fig 2b
-helical turns (from V1 to M12), with eight i
i+4 hydrogen bonds between NH protons of F4, K6, Q8, R9, K11, M12, L13 and I14 and CO oxygens of the N-terminal acetyl group, F2, F4, S5, Q7, Q8, R9 and I10; and a looser turn from residues (L13, I14, L15) that are neighboring the proline-rich stretch. The average backbone pairwise root mean squared deviation was 1.22 Å for the helical segment. The C-terminal nine residues (PPVPVPKIK-NH2) displayed strong d
(i,i+1) nuclear Overhauser effects (NOEs) for L15P16, P16P17, V18-P19 and V20P21 consistent with trans-amide geometry, and strong d
N(i +1; i+2) NOEs throughout indicate ß-turn conformations for the box 1 motif.
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| The downstream growth signal |
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We elected to address this issue by creating mice with targeted mutations in their cytoplasmic domains which would eliminate key signaling pathways, then to observe the effect of this on growth (Rowland et al. 2005). Currently, three such mice have been created involving (1) truncation at residue 569 with the two distal tyrosines converted to phenylalanine, to remove the majority of STAT5 signaling and the SHP2-binding site, originating from the distal 80 residues of the intact receptor, (2) truncation at 391, removing all STAT5 generation and presumably other signals in the central segment of the cytoplasmic domain and (3) mutation of the box 1 sequence, to abrogate JAK2 activation. This should leave the Src family kinase intact, and indicate the extent of GH action through JAK2.
The phenotypic consequences of these mutations can be seen in Fig. 3
. It is clear that GHR signaling is essential for postnatal allometric growth, and that loss of STAT5 signaling correlates progressively with loss of postnatal growth enhancement. The latter is in agreement with the recent study of Woelfe et al. (2003a), which has identified a functional STAT5 element in the second intron of the insulin-like growth factor (IGF)-I promoter, and a study by this group showing blockade of GH-dependent hepatic IGF-I generation in vivo with dominant negative STAT5 (Woelfe et al. 2003b). Further, specific STAT5b gene mutation results in major retardation of postnatal growth in humans (Kofoed et al. 2004). In our mice harboring GHR truncated at 569, loss of around 70% of STAT5 signaling results in around 50% loss of postnatal growth enhancement by GH, and around 75% loss of circulating IGF-I. Interestingly, although the loss of hepatic IGF-I transcript is only around 30%, because this is accompanied by major falls in IGF-binding protein 3 and acid labile unit, the clearance of circulating IGF-I increases, preferentially decreasing serum IGF-I. Truncation at residue 391 removes all STAT5a/b signaling, and results in a growth phenotype similar to, but significantly larger than, the complete GHR knockout (Zhou et al. 1997). The residual 15% of GH-dependent growth evidently represents the STAT5a/b-independent component (Rowland et al. 2005).
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| Conclusion |
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| Acknowledgements |
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| References |
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Brown RJ, Adams JJ, Pelekanos RA, Wan Y, McKinstry WJ, Palethorpe K, Seeber RM, Monks TA, Eidne KA, Parker MW & Waters MJ 2005 A model for GH receptor activation based on subunit rotation within a receptor dimer. Nature Structural & Molecular Biology 12 814821.
Chen C-M, Brinkworth R & Waters MJ 1997 The role of receptor dimerization domain residues in GH signalling. Journal of Biological Chemistry 272 51335140.
deVos AM, Ultsch M & Kossiakoff AA 1992 Human growth hormone and extracellular domain of its receptor: crystal structure of the complex. Science 255 306312.
Fuh G, Cunningham BC, Fukunaga S, Nagata S, Goeddel DV & Wells JA 1992 Rational design of potent antagonists to the human growth hormone receptor. Science 256 16771680.
Gent J, van Kerkhof P, Roza M, Bu G & Strous GJ 2002 Ligand-independent growth hormone receptor dimerization occurs in the endoplasmic reticulum and is required for ubiquitin system-dependent endocytosis. PNAS 99 98589863.
Harding PA, Wang X, Okada S, Chen WY, Wan W & Kopchick JJ 1996 Growth hormone (GH) and a GH antagonist promote GH receptor dimerization and internalization. Journal of Biological Chemistry 271 67086712.
Jiang J, Wang X, He K, Li X, Chen C-M, Sayeski PP, Waters MJ & Frank SJ 2004 A conformationally sensitive GHR antibody; impact on GH signalling and proteolysis. Molecular Endocrinology 18 29812996.
Kofoed EM, Hwa V, Little B, Woods KA, Buckway CK, Tsubaki J, Pratt KL, Bezrodnik L, Jasper H, Tepper A et al. 2003 Growth hormone insensitivity associated with a STAT5b mutation. New England Journal of Medicine 349 11391147.
Kubatzky KF, Ruan W, Gurezka R, Cohen J, Ketteler R, Watowich SS, Neumann D, Langosch D & Klingmuller U 2001 Self assembly of the transmembrane domain promotes signal transduction through the erythropoietin receptor. Current Biology 11 110115.[CrossRef][ISI][Medline]
Pearce KH Jr, Cunningham BC, Fuh G, Teeri T & Wells JA 1999 GH binding affinity for its receptor surpasses the requirements for cellular activity. Biochemistry 38 8189[CrossRef][Medline]
Rowland JE, Lichanska AM, Kerr LM, White M, DAniello EM, Maher SL, Brown RJ, Teasdale R, Noakes PG & Waters MJ 2005 In vivo analysis of growth hormone receptor signalling domains and their associated transcripts. Molecular Cell Biology 25 6677.
Rowlinson SW, Behncken SN, Rowland JE, Clarkson RW, Strasburger CJ, Wu Z, Baumbach W & Waters MJ 1998 Activation of chimeric and full-length growth hormone receptors by growth hormone receptor monoclonal antibodies. A specific conformational change may be required for full-length receptor signaling. Journal of Biological Chemistry 273 53075314.
Schneider H, Chaovapong W, Matthews DJ, Karkaria C, Cass RT, Zhan H, Boyle M, Lorenzini T, Elliott SG & Giebel LB 1997 Homodimerization of erythropoietin receptor by a bivalent monoclonal antibody triggers cell proliferation and differentiation of erythroid precursors. Blood 89 473482.
Seubert N, Royer Y, Staerk J, Kubatzky KF, Moucadel V, Krishnakumar S, Smith SO & Constantinescu SN 2003 Active and inactive orientations of the transmembrane and cytosolic domains of the erythropoietin receptor dimer. Molecular Cell 12 12391250.[CrossRef][ISI][Medline]
van Kerkhof P, Smeets M & Strous GJ 2002 The ubiquitin-proteasome pathway regulates the availability of the GH receptor. Endocrinology 143 12431252.
Wan Y, Zheng YZ, Harris JM, Brown R & Waters MJ 2003 Epitope map for a GH receptor agonist monoclonal antibody, MAb 263. Molecular Endocrinology 17 22402250.
Wan Y, McDevitt A, Shen B, Smythe ML & Waters MJ 2004 Increased Site 1 affinity improves biopotency of porcine growth hormone: evidence against diffusion dependent receptor dimerization. Journal of Biological Chemistry 279 4477544784.
Woelfe J, Chia DJ & Rotwein P 2003a Mechanisms of growth hormone (GH) action. Identification of conserved Stat5 binding sites that mediate GH-induced insulin-like growth factor-I gene activation. Journal of Biological Chemistry 278 5126151266.
Woelfe J, Billiard J & Rotwein P 2003b Acute control of IGF-I gene transcription by growth hormone through Stat5b. Journal of Biological Chemistry 278 2269622702.
Zhou Y, Xu BC, Maheshwari HG, He L, Reed M, Lozykowski M, Okada S, Cataldo L, Coschigamo K, Wagner TE et al. 1997 A mammalian model for Laron syndrome produced by targeted disruption of the mouse growth hormone receptor/binding protein gene (the Laron mouse). PNAS 94 1321513220.
Received 11 August 2005
Accepted 23 November 2005
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