Mechanism of transforming growth factor-β1 signaling: Role of the mitogen-activated protein kinase
2000; Elsevier BV; Volume: 58; Linguagem: Inglês
10.1046/j.1523-1755.2000.07709.x
ISSN1523-1755
Autores Tópico(s)Parathyroid Disorders and Treatments
ResumoMechanism of transforming growth factor-β1 signaling: Role of the mitogen-activated protein kinase.Transforming growth factor-β1 (TGF-β1) regulates diverse biologic activities including cell growth, cell death or apoptosis, cell differentiation, and extracellular matrix (ECM) synthesis. TGF-β1 is believed to be a key mediator of tissue fibrosis as a consequence of ECM accumulation in pathologic states such as progressive renal diseases including diabetic nephropathy. TGF-β1 actions are mediated by the heteromeric interactions of types I and II serine/threonine kinase receptors. Initiation of signaling requires binding of TGF-β1 to TGF-β type II receptor (TβR-II), a constitutively active serine/threonine kinase, which subsequently transphosphorylates TGF-β type I receptor (TβR-I). However, the signaling pathway following the initial receptor interaction with ligand remains poorly understood. Much of current investigation, including in our laboratory, is now focused on the elucidation of the intracellular signaling components that mediate TGF-β1 signals downstream of the cell-surface receptors. An emerging body of evidence implicates the mitogen-activated protein kinase (MAPK) as an important TGF-β1 signaling pathway. Mechanism of transforming growth factor-β1 signaling: Role of the mitogen-activated protein kinase. Transforming growth factor-β1 (TGF-β1) regulates diverse biologic activities including cell growth, cell death or apoptosis, cell differentiation, and extracellular matrix (ECM) synthesis. TGF-β1 is believed to be a key mediator of tissue fibrosis as a consequence of ECM accumulation in pathologic states such as progressive renal diseases including diabetic nephropathy. TGF-β1 actions are mediated by the heteromeric interactions of types I and II serine/threonine kinase receptors. Initiation of signaling requires binding of TGF-β1 to TGF-β type II receptor (TβR-II), a constitutively active serine/threonine kinase, which subsequently transphosphorylates TGF-β type I receptor (TβR-I). However, the signaling pathway following the initial receptor interaction with ligand remains poorly understood. Much of current investigation, including in our laboratory, is now focused on the elucidation of the intracellular signaling components that mediate TGF-β1 signals downstream of the cell-surface receptors. An emerging body of evidence implicates the mitogen-activated protein kinase (MAPK) as an important TGF-β1 signaling pathway. Transforming growth factor-β1 (TGF-β1) is a 25 kD homodimeric polypeptide, belonging to a superfamily of multifunctional cytokines, which participates in a broad array of biologic activities such as normal development and wound repair, as well as pathologic processes1.Roelen B.A. Lin H.Y. Knezevic V. Freund E. Mummery C.L. Expression of TGF-βs and their receptors during implantation and organogenesis of the mouse embryo.Dev Biol. 1994; 166: 716-728Crossref PubMed Scopus (127) Google Scholar, 2.Choi M.E. Liu A. Ballermann B.J. Differential expression of transforming growth factor-β receptors in rat kidney development.Am J Physiol. 1997; 273: F386-F395PubMed Google Scholar, 3.Attisano L. Wrana J.L. Signal transduction by members of the transforming growth factor-β superfamily.Cytokine Growth Factor Rev. 1996; 7: 327-339Abstract Full Text PDF PubMed Scopus (140) Google Scholar, 4.Shull M.M. Ormsby I. Kier A.B. Pawlowski S. Diebold R.J. Yin M. Allen R. Sidman C. Proetzel F. Calvin D. Annunziata N. Doetschman T. Targeted disruption of the mouse transforming growth factor-β1 gene results in multifocal inflammatory disease.Nature. 1992; 359: 693-699Crossref PubMed Scopus (2527) Google Scholar, 5.Border W.A. Noble N.A. Transforming growth factor β in tissue fibrosis.N Engl J Med. 1994; 331: 1286-1292Crossref PubMed Scopus (2899) Google Scholar. TGF-β1 regulates multiple cellular functions including inhibition and stimulation of cell growth, cell death or apoptosis, and cellular differentiation. TGF-β1 is also a potent inducer of extracellular matrix (ECM) protein synthesis and has been implicated as the key mediator of fibrogenesis in various tissues5.Border W.A. Noble N.A. Transforming growth factor β in tissue fibrosis.N Engl J Med. 1994; 331: 1286-1292Crossref PubMed Scopus (2899) Google Scholar. In the kidney, the critical role of TGF-β1 has been well recognized in several renal diseases, including diabetic nephropathy, characterized by progressive glomerular accumulation of ECM, which leads to the development of glomerulosclerosis6.Yamamoto T. Nakamura T. Noble N.A. Ruoslahti E. Border W.A. Expression of transforming growth factor β is elevated in human and experimental diabetic nephropathy.Proc Natl Acad Sci USA. 1993; 90: 1814-1818Crossref PubMed Scopus (785) Google Scholar, 7.Nakamura T. Fukui M. Ebihara I. Osada S. Nagaoka I. Tomino Y. Koide H. mRNA expression of growth factors in glomeruli from diabetic rats.Diabetes. 1993; 42: 450-456Crossref PubMed Google Scholar, 8.Sharma K. Jin Y. Guo J. Ziyadeh F.N. Neutralization of TGF-β by anti-TGF-β antibody attenuates kidney hypertrophy and the enhanced extracellular matrix gene expression in streptozotocin-induced diabetic mice.Diabetes. 1996; 45: 522-530Crossref PubMed Scopus (0) Google Scholar. Enhanced glomerular expression of TGF-β1 has been well demonstrated in both human and experimental diabetic nephropathy. For instance, Yamamoto et al have demonstrated progressive increase in the expression of TGF-β1 mRNA in glomeruli of rats made diabetic by the administration of streptozotocin6.Yamamoto T. Nakamura T. Noble N.A. Ruoslahti E. Border W.A. Expression of transforming growth factor β is elevated in human and experimental diabetic nephropathy.Proc Natl Acad Sci USA. 1993; 90: 1814-1818Crossref PubMed Scopus (785) Google Scholar. Furthermore, increased glomerular TGF-β1 immunostaining with concomitant increase in ECM proteins induced by TGF-β1 were observed in both diabetic rats and patients with diabetic nephropathy6.Yamamoto T. Nakamura T. Noble N.A. Ruoslahti E. Border W.A. Expression of transforming growth factor β is elevated in human and experimental diabetic nephropathy.Proc Natl Acad Sci USA. 1993; 90: 1814-1818Crossref PubMed Scopus (785) Google Scholar. A separate study by Nakamura et al also confirmed the increased glomerular expression of TGF-β1 mRNA in the streptozotocin-induced diabetic rat7.Nakamura T. Fukui M. Ebihara I. Osada S. Nagaoka I. Tomino Y. Koide H. mRNA expression of growth factors in glomeruli from diabetic rats.Diabetes. 1993; 42: 450-456Crossref PubMed Google Scholar. Treatment of streptozotocin-induced diabetic mice with a neutralizing TGF-β antibody significantly attenuated the increase in TGF-β1 and ECM expression8.Sharma K. Jin Y. Guo J. Ziyadeh F.N. Neutralization of TGF-β by anti-TGF-β antibody attenuates kidney hypertrophy and the enhanced extracellular matrix gene expression in streptozotocin-induced diabetic mice.Diabetes. 1996; 45: 522-530Crossref PubMed Scopus (0) Google Scholar. Moreover, Sharma and Ziyadeh reported augmented TGF-β1 expression in the kidneys of two experimental models of spontaneous diabetic BB rat and the NOD mouse9.Sharma K. Ziyadeh F.N. Renal hypertrophy is associated with upregulation of TGF-β1 gene expression in diabetic BB rat and NOD mouse.Am J Physiol. 1994; 267: F1094-F1101PubMed Google Scholar. In addition, elevated glucose concentrations in vitro significantly increased TGF-β receptor number as well as mRNA and immunoreactive protein expression in glomerular mesangial cells10.Riser B.L. Ladson-Wofford S. Sharba A. Cortes P. Drake K. Guerin C. Yee J. Choi M.E. Segarini P.R. Narins R. TGF-β receptor expression and binding in rat mesangial cells by glucose and cyclic mechanical strain.Kidney Int. 1999; 56: 428-439Abstract Full Text Full Text PDF PubMed Scopus (64) Google Scholar. Collectively, these findings implicate a key role of TGF-β1 in the pathogenesis of both human and experimental diabetic nephropathy. Glomerulosclerosis, a central pathologic feature of progressive renal diseases, is thought to represent the final common response to injury, and the role of TGF-β1 in this process is further demonstrated in other models of glomerular diseases. For instance, increased TGF-β1 expression has been observed in the kidneys of patients and experimental animals with glomerulonephritis; suppression of excess ECM deposition and glomerulosclerosis by neutralizing TGF-β1 antibody has been shown in experimental glomerulonephritis11.Okuda S. Languino L.R. Ruoslahti E. Border W.A. Elevated expression of transforming growth factor-β and proteoglycan production in experimental glomerulonephritis.J Clin Invest. 1990; 86: 453-462Crossref PubMed Scopus (517) Google Scholar,12.Border W.A. Okuda S. Languino L.R. Sporn M.B. Ruoslahti E. Suppression of experimental glomerulonephritis by antiserum against transforming growth factor-β1.Nature. 1990; 346: 371-374Crossref PubMed Scopus (917) Google Scholar. Furthermore, in vivo transfection of TGF-β1 gene into the rat kidney has been shown to induce glomerulosclerosis13.Isaka Y. Fujiwara Y. Ueda N. Kaneda Y. Kamada T. Imai E. Glomerulosclerosis induced by in vivo transfection of transforming growth factor-β or platelet-derived growth factor gene into the rat kidney.J Clin Invest. 1993; 92: 2597-2601Crossref PubMed Scopus (489) Google Scholar. Similarly, development of glomerulosclerosis has been described in mice that express the transgene for TGF-β1, under the control of a murine albumin promoter (Alb/TGF-β1), and that elevated circulating levels of TGF-β114.Kopp J.B. Factor V.M. Mozes M. Nagy P. Sanderson N. Böttinger E.P. Klotman P.E. Thorgeirsson S.S. Transgenic mice with increased plasma levels of TGF-β1 develop progressive renal disease.Lab Invest. 1996; 74: 991-1003PubMed Google Scholar. Thus, an ever-growing body of data implicates TGF-β1 as the key mediator in the pathogenesis of glomerulosclerosis, and much of our current investigations are now being focused on the elucidation of TGF-β1 signaling mechanisms. TGF-β1 exerts its multiple biologic actions by the interaction with two transmembrane serine/threonine kinase receptors, types I and II, that are coexpressed by most cells including glomerular mesangial and endothelial cells15.Massagué J. Cheifetz S. Boyd F.T. Andres J.L. TGF-beta receptors and TGF-beta binding proteoglycans: recent progress in identifying their functional properties.Ann NY Acad Sci. 1990; 593: 59-72Crossref PubMed Scopus (203) Google Scholar, 16.Choi M.E. Kim E.G. Huang Q. Ballermann B.J. Rat mesangial cell hypertrophy in response to transforming growth factor-β1.Kidney Int. 1993; 44: 948-958Abstract Full Text PDF PubMed Scopus (80) Google Scholar, 17.Choi M.E. Ballermann B.J. Inhibition of capillary morphogenesis and associated apoptosis by dominant negative mutant transforming growth factor-β receptors.J Biol Chem. 1995; 270: 21144-21150Crossref PubMed Scopus (127) Google Scholar. Initiation of signaling requires the binding of TGF-β1 to TβR-II, a constitutively active serine/threonine kinase, resulting in the recruitment and phosphorylation of TβR-I to produce a heteromeric signaling complex that in turn activates downstream signaling pathways18.Wrana J.L. Attisano L. Wieser R. Ventura F. Massagué J. Mechanism of activation of the TGF-β receptor.Nature. 1994; 370: 341-347Crossref PubMed Scopus (2038) Google Scholar,19.Vivien D. Attisano L. Wrana J.L. Massagué J. Signaling activity of homologous and heterologous transforming growth factor-β receptor kinase complexes.J Biol Chem. 1995; 270: 7134-7141Crossref PubMed Scopus (80) Google Scholar. Molecular cloning has revealed that a distinct type II receptor exists for TGF-β1, and for members of the TGF-β superfamily, including activin and bone morphogenetic protein (BMP), and that the type II receptor is capable of binding its respective ligands directly and interacting with different type I receptors20.Lin H.Y. 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ALK-5 has been identified as the predominant TβR-I in most cell types and has been shown to mediate TGF-β1 signaling23.Franzén P. Dijke P. Ichijo H. Yamashita H. Schulz P. Heldin C.-H. Miyazono K. Cloning of a TGFβ type I receptor that forms a heteromeric complex with the TGFβ type II receptor.Cell. 1993; 75: 681-692Abstract Full Text PDF PubMed Scopus (702) Google Scholar. ALK-1 and ALK-2 are thought to be activin type I receptors, but both have also been demonstrated to bind TGF-β24.Attisano L. Cárcamo J. Ventura F. Weis F.M.B. Massagué J. Wrana J.L. Identification of human activin and TGF-β type I receptors that form heteromeric kinase complexes with type II receptors.Cell. 1993; 75: 671-680Abstract Full Text PDF PubMed Scopus (589) Google Scholar,25.Ebner R. Chen R.-H. Shum L. Lawler S. Zioncheck T.F. Lee A. Lopez A.R. Derynck R. 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Furthermore, besides the characteristic cytoplasmic serine/threonine kinase domain, type I receptors have a region between the transmembrane and the kinase domains, containing a conserved TSGSGSG motif, denoted the GS domain. Mutational analyses have revealed that phosphorylation of serine and threonine residues in the GS domain of the type I receptor by the type II receptor is essential for TGF-β1 signaling28.Saitoh M. Nishitoh H. Amagasa T. Miyazono K. Takagi M. Ichijo H. Identification of important regions in the cytoplasmic juxtamembrane domain of type I receptor that separate signaling pathways of transforming growth factor-β.J Biol Chem. 1996; 271: 2769-2775Crossref PubMed Scopus (74) Google Scholar,29.Weis-Garcia F. Massagué J. Complementation between kinase-defective and activation-defective TGF-β receptors reveals a novel form of receptor cooperativity essential for signaling.EMBO J. 1996; 15: 276-289Crossref PubMed Scopus (133) Google Scholar. TβR-I is thought to determine the specificity of the cellular response to TGF-β1, whereas TβR-II determines the ligand specificity. TβR-I alone is unable to bind TGF-β1, based on 125I-labeled TGF-β1 cross-linking studies, and TβR-II is unable to signal without TβR-I18.Wrana J.L. Attisano L. Wieser R. Ventura F. Massagué J. Mechanism of activation of the TGF-β receptor.Nature. 1994; 370: 341-347Crossref PubMed Scopus (2038) Google Scholar. Thus, interaction of TGF-β type II receptor with different type I receptors to form the heteromeric signaling complex may be a mechanism used to mediate the multiple TGF-β1 actions and thus confer multifunctionality of TGF-β1. Although ALK-1, ALK-2, and ALK-5 have all been demonstrated to complex with ligand-bound TβR-II, ALK-5 is the predominant TGF-β signaling type I receptor in most cell types23.Franzén P. Dijke P. Ichijo H. Yamashita H. Schulz P. Heldin C.-H. Miyazono K. Cloning of a TGFβ type I receptor that forms a heteromeric complex with the TGFβ type II receptor.Cell. 1993; 75: 681-692Abstract Full Text PDF PubMed Scopus (702) Google Scholar. Much progress has been made in identifying signaling pathways for TGF-β by molecular cloning of the various receptors. We have previously reported the cloning of a full-length rat TGF-β type II receptor cDNA and two distinct type I receptor cDNAs, namely ALK-5 and ALK-2, also known as Tsk7L2.Choi M.E. Liu A. Ballermann B.J. Differential expression of transforming growth factor-β receptors in rat kidney development.Am J Physiol. 1997; 273: F386-F395PubMed Google Scholar,16.Choi M.E. Kim E.G. Huang Q. Ballermann B.J. Rat mesangial cell hypertrophy in response to transforming growth factor-β1.Kidney Int. 1993; 44: 948-958Abstract Full Text PDF PubMed Scopus (80) Google Scholar. Our recent studies have identified that there are variant forms of the TGF-β type I receptor ALK-530.Choi M.E. Cloning and characterization of a naturally-occurring soluble form of TGF-β type I receptor.Am J Physiol. 1999; 276: F88-F95PubMed Google Scholar. From a neonatal rat kidney cDNA library, we have isolated three unique cDNA clones that encode multiple receptor forms of ALK-5. Two of the rat cDNA clones encode membrane-spanning receptors that differ in the C-terminal region of the extracellular domain. The third lacked the entire transmembrane and the cytoplasmic serine/threonine domains and encodes a previously undescribed soluble form of TGF-β type I receptor (sTβR-I)30.Choi M.E. Cloning and characterization of a naturally-occurring soluble form of TGF-β type I receptor.Am J Physiol. 1999; 276: F88-F95PubMed Google Scholar. We have been able to clearly demonstrate in vivo expression of a mRNA transcript encoding the sTβR-I, and interestingly, the sTβR-I mRNA was expressed in greater abundance in the rat neonatal kidney compared with the adult kidney, as was the TβR-I mRNA abundance as previously reported, thus indicating a role for sTβR-I in renal development2.Choi M.E. Liu A. Ballermann B.J. Differential expression of transforming growth factor-β receptors in rat kidney development.Am J Physiol. 1997; 273: F386-F395PubMed Google Scholar,30.Choi M.E. Cloning and characterization of a naturally-occurring soluble form of TGF-β type I receptor.Am J Physiol. 1999; 276: F88-F95PubMed Google Scholar. Furthermore, sTβR-I is a functional protein capable of binding TGF-β1 ligand in the presence of TGF-β type II receptor on the cell surface, as determined by affinity cross-linking with 125I-labeled TGF-β1. Moreover, studies using a TGF-β-inducible luciferase reporter construct p3TP-Lux reveal that this novel protein augmented TGF-β1 signaling30.Choi M.E. Cloning and characterization of a naturally-occurring soluble form of TGF-β type I receptor.Am J Physiol. 1999; 276: F88-F95PubMed Google Scholar. Physiologic functions of these variant receptor forms are not yet known, but their variable in vivo expression may represent a mechanism conferring multiple cellular responses to TGF-β1. Moreover, the biologic importance of a soluble TβR-I is yet to be determined, but the findings here suggest that it may serve as a natural potentiator of TGF-β1 signaling. The molecular cloning and identification of a naturally occurring sTβR-I adds a new level of complexity to our present knowledge of the TGF-β receptor system and will facilitate further investigations to expand our understanding of the mechanism of TGF-β1 actions. To understand the mechanisms involved in TGF-β1 signaling from the cell membrane to the nucleus, it is crucial to unravel the intracellular events that provide a link between the activated cell surface TGF-β receptors and the downstream effects of TGF-β1, such as cell growth inhibition and matrix induction. With the identification of the Smad family of signal-transducing proteins involved in mediating TGF-β1 signals downstream of the transmembrane serine/threonine kinase receptors, the focus of many investigators has centered on studies of the Smad proteins. In particular, it is now known that Smad2 and Smad3 both act as signaling proteins and transcription factors and are serine phosphorylated in a TGF-β1-dependent fashion and associate with Smad4 to translocate to the nucleus where they bind target sites on specific gene promoters31.Massagué J. TGFβ signaling receptors, transducers, and Mad proteins.Cell. 1996; 85: 947-950Abstract Full Text Full Text PDF PubMed Scopus (804) Google Scholar. However, a growing body of information is emerging regarding additional important intracellular signaling pathways for TGF-β1, that may either signal independent of the Smads or converge and cross talk with the Smad signaling pathways. We and others have demonstrated the critical involvement of the mitogen-activated protein kinase (MAPK) pathways in TGF-β1 signaling32.Chin B.Y. Petrache I. Choi A.M.K. Choi M.E. Transforming growth factor-β1 rescues serum deprivation-induced apoptosis via the mitogen-activated protein kinase (MAPK) pathway in macrophages.J Biol Chem. 1999; 274: 11362-11368Crossref PubMed Scopus (92) Google Scholar, 33.Hartsough M.T. Frey R.S. Zipfel P.A. Buard A. Cook S.J. McCormick F. Mulder K.M. 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Krebs E.G. The MAPK signaling cascade.FASEB J. 1995; 9: 726-798Crossref PubMed Scopus (3099) Google Scholar. Three major subgroups of the MAPK superfamily members have been identified to date: the extracellular signal-regulated kinases 1 and 2 (ERK1 and ERK2), also known as p44/p42 MAPKs, respectively; the c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK); and the p38 MAPK36.Vojtek A.B. Cooper J.A. Rho family members: activators of MAP kinase cascades.Cell. 1995; 82: 527-529Abstract Full Text PDF PubMed Scopus (250) Google Scholar. The signal transduction cascades involved in the activation of MAPKs require a well-coordinated cascade of three protein kinase reactions that transduce signals by sequential phosphorylation and activation of the next kinase in their respective pathways. The MAPKs require dual phosphorylation at the threonine and tyrosine sites by MAPK kinases (the MEKs and MKKs that are specific for ERK, JNK, and p38 MAPK), which are in turn activated by MAPK kinase kinases (MKKKs) through serine/threonine phosphorylation (36). The MAPK cascades display evolutionary conservation and are considered to play essential roles in the signal transduction of many biologic events, such as the regulation of cell growth, differentiation, and apoptosis, and cellular responses to environmental stresses. TGF-β1 has been demonstrated in various cell types to be capable of activating each of the three major MAPK members. The best characterized of the MAPKs is the ERK1/ERK2 cascade, which is the prototypical MAPK pathway activated by mitogenic growth factor stimulation of receptor tyrosine kinases, whereas JNK/SAPK and p38 MAPK are activated predominantly by stress stimuli, such as ultraviolet light, inflammatory cytokines, and osmotic shock agents38.Nishida E. 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Given the rapid kinetics observed for TGF-β1-induced activation of Ras and ERK (typically within 1–5 min), the effect of TGF-β1 on the ERK pathway is likely direct and not secondary to activation of tyrosine kinase receptors by secretion of growth stimulatory factors in response to TGF-β141.Hartsough M.T. Mulder K.M. Transforming growth factor β activation of p44 MAPK in proliferating cultures of epithelial cells.J Biol Chem. 1995; 270: 7117-7124Crossref PubMed Scopus (270) Google Scholar, 42.Reimann T. Hempel U. Krautwald S. Axmann A. Scheibe R. Seidel D. Wenzel K.-W. Transforming growth factor-β1 induces activation of Ras, Raf-1, MEK and MAPK in rat hepatic stellate cells.FEBS Lett. 1997; 403: 57-60Abstract Full Text Full Text PDF PubMed Scopus (97) Google Scholar, 43.Axmann A. Seidel D. Reimann T. Hempel U. Wenzel K.-W. Transforming growth factor-β1-induced activation of the Raf-MEK-MAPK signaling pathway in rat lung fibroblasts via a PKC-dependent mechanism.Biochem Biophys Res Comm. 1998; 249: 456-460Crossref PubMed Scopus (62) Google Scholar. Further, a dominant negative of Ras (RasN17) completely blocked TGF-β-mediated activation of ERK1 and upregulation of cyclin-dependent kinase (Cdk) inhibitors and resulted in reversal of the ability of TGF-β to decrease Cdk2 activity and cyclin A protein expression in intestinal epithelial cells33.Hartsough M.T. Frey R.S. Zipfel P.A. Buard A. Cook S.J. McCormick F. Mulder K.M. Altered transforming growth factor β signaling in epithelial cells when ras activation is blocked.J Biol Chem. 1996; 271: 22368-22375Crossref PubMed Scopus (115) Google Scholar,45.Yue J. Buard A. Mulder K.M. Blockade of TGFβ up-regulation of p27Kip1 and p21 Cip1 by expression of RasN17 in epithelial cells.Oncogene. 1998; 17: 47-55Crossref PubMed Scopus (35) Google Scholar. Collectively, these studies indicate that Ras activation is required for TGFβ-mediated activation of ERK1 and that the Ras/ERK pathway is involved in TGF-β1-mediated growth inhibition. TGF-β1 also exerts growth inhibitory effects on glomerular mesangial and endothelial cells; we have observed rapid activation of ERK1/ERK2 by TGF-β1 in these cells as well16.Choi M.E. Kim E.G. Huang Q. Ballermann B.J. Rat mesangial cell hypertrophy in response to transforming growth factor-β1.Kidney Int. 1993; 44: 948-958Abstract Full Text PDF PubMed Scopus (80) Google Scholar, 17.Choi M.E. Ballermann B.J. Inhibition of capillary morphogenesis and associated apoptosis by dominant negative mutant transforming growth factor-β receptors.J Bio
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