Latent Transforming Growth Factor-β (TGF-β) Binding Proteins: Orchestrators of TGF-β Availability

The latent transforming growth factor-β (TGF-β) 1The abbreviations used are: TGF-β, transforming growth factor-β; LTBP, latent TGF-β binding protein; SLC, small latent complex; LLC, large latent complex; LAP, latency associated protein; EGF, epidermal growth factor; ECM, extracellular matrix; IHH, Indian hedgehog; PTHrP, parathyroid hormone-related protein; BMP, bone morphogenic protein.1The abbreviations used are: TGF-β, transforming growth factor-β; LTBP, latent TGF-β binding protein; SLC, small latent complex; LLC, large latent complex; LAP, latency associated protein; EGF, epidermal growth factor; ECM, extracellular matrix; IHH, Indian hedgehog; PTHrP, parathyroid hormone-related protein; BMP, bone morphogenic protein. binding proteins (LTBP-1– 4) are secreted multidomain glycoproteins, three of which are central to TGF-β regulation (1Koli K. Saharinen J. Hyytiainen M. Penttinen C. Keski-Oja J. Microsc. Res. Tech. 2001; 52: 354-362Crossref PubMed Scopus (222) Google Scholar, 2Mangasser-Stephan K. Gressner A.M. Cell Tissue Res. 1999; 297: 363-370Crossref PubMed Scopus (41) Google Scholar). LTBP-1 is a component of the latent TGF-β complex that consists of TGF-β, the TGF-β propeptide, and LTBP (Fig. 1) (3Miyazono K. Hellman U. Wernstedt C. Heldin C.H. J. Biol. Chem. 1988; 263: 6407-6415Abstract Full Text PDF PubMed Google Scholar). TGF-β is synthesized as a homodimeric proprotein, and the dimeric propeptide is cleaved intracellularly from the growth factor (4Taipale J. Saharinen J. Keski-Oja J. Adv. Cancer Res. 1998; 75: 87-134Crossref PubMed Google Scholar). Unlike most propeptides that have little affinity for the mature protein, the TGF-β propeptide strongly binds to TGF-β, and the proteins are secreted in complex (5Annes J.P. Munger J.S. Rifkin D.B. J. Cell Sci. 2003; 116: 217-224Crossref PubMed Scopus (1286) Google Scholar). In this small latent complex (SLC), TGF-β cannot bind to its surface receptors. Therefore, the propeptide is called the latency-associated protein (LAP). The dissociation or activation 2The cleavage of the TGF-β propeptide from the TGF-β precursor molecule is referred to as processing. The dissociation of TGF-β from LAP in a manner that permits binding of the cytokine to its receptor is called activation.2The cleavage of the TGF-β propeptide from the TGF-β precursor molecule is referred to as processing. The dissociation of TGF-β from LAP in a manner that permits binding of the cytokine to its receptor is called activation. of TGF-β from LAP is a critical regulatory event as all TGF-β is secreted in a latent form. The LAP dimer is usually disulfide-bound to a second gene product, LTBP, and the trimolecular aggregate is called the large latent complex (LLC). LTBP-1 cDNA has a predicted translation product of 1394 amino acids with a calculated Mr of 151,000 (6Kanzaki T. Olofsson A. Moren A. Wernstedt C. Hellman U. Miyazono K. Claesson-Welsh L. Heldin C.H. Cell. 1990; 61: 1051-1061Abstract Full Text PDF PubMed Scopus (367) Google Scholar). The protein contains 17 epidermal growth factor (EGF)-like domains, 14 of which contain consensus Ca2+ binding sequences. In addition, there are four modules, each of ∼70 amino acids with eight cysteines. These eight cysteine (8-Cys) domains are unique to the LTBPs and the fibrillins (5Annes J.P. Munger J.S. Rifkin D.B. J. Cell Sci. 2003; 116: 217-224Crossref PubMed Scopus (1286) Google Scholar). The most N-terminal 8-Cys domain, sometimes called a hybrid domain, has a more divergent sequence and may have derived from a former splicing event between an 8-Cys domain and an EGF-like domain. Between the 8-Cys2 and the central core of repeating EGF-like motifs is a basic amino acid and proline-rich sequence called the hinge domain because of its predicted flexibility (see below). Northern blotting revealed LTBP-1 transcripts of 5.2 and 7.0 kb (7Noguera I. Obata H. Gualandris A. Cowin P. Rifkin D.B. Gene (Amst.). 2003; 308: 31-41Crossref PubMed Scopus (18) Google Scholar). The shorter transcript corresponds to the initial cDNA isolate, LTBP-1S, whereas the larger transcript corresponds to a form of LTBP-1, LTBP-1L, that has an additional N-terminal 346 amino acids. This sequence encodes one EGF-like repeat and a four-cysteine module and is quite basic. LTBP-1S and LTBP-1L derive from the use of separate promoters and alternative splicing between codons 145 and 146 of LTBP-1S (8Koski C. Saharinen J. Keski-Oja J. J. Biol. Chem. 1999; 274: 32619-32630Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar). Subsequent cloning and expression experiments defined LTBP-2, -3, and -4 (9Bashir M.M. Han M.-D. Abrams W.R. Tucker T. Ma R.-I. Gibson M. Ritty T. Mecham R. Rosenbloom J. Int. J. Biochem. Cell Biol. 1996; 28: 531-542Crossref PubMed Scopus (20) Google Scholar, 10Moren A. Olofsson A. Stenman G. Sahlin P. Kanzaki T. Claesson-Welsh L. ten Dijke P. Miyazono K. Heldin C.H. J. Biol. Chem. 1994; 269: 32469-32478Abstract Full Text PDF PubMed Google Scholar, 11Saharinen J. Taipale J. Monni O. Keski-Oja J. J. Biol. Chem. 1998; 273: 18459-18469Abstract Full Text Full Text PDF PubMed Scopus (106) Google Scholar, 12Yin W. Smiley E. Germiller J. Mecham R.P. Florer J.B. Wenstrup R.J. Bonadio J. J. Biol. Chem. 1995; 270: 10147-10160Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar). The overall structures of these proteins are similar with four 8-Cys domains and multiple EGF-like modules (Fig. 2). All LTBPs are expressed as two major transcripts, and the other LTBPs probably also have long and short forms. LTBP-1S and -1L show different distribution patterns; LTBP-1L is highly expressed in heart, kidney, lung, testes, and placenta, whereas LTBP-1S is broadly distributed (7Noguera I. Obata H. Gualandris A. Cowin P. Rifkin D.B. Gene (Amst.). 2003; 308: 31-41Crossref PubMed Scopus (18) Google Scholar). LTBP-2 mRNA is present at high levels in lung, placenta, and heart in addition to liver and muscle (10Moren A. Olofsson A. Stenman G. Sahlin P. Kanzaki T. Claesson-Welsh L. ten Dijke P. Miyazono K. Heldin C.H. J. Biol. Chem. 1994; 269: 32469-32478Abstract Full Text PDF PubMed Google Scholar). LTBP-3 is abundant in heart, lung, and bone (12Yin W. Smiley E. Germiller J. Mecham R.P. Florer J.B. Wenstrup R.J. Bonadio J. J. Biol. Chem. 1995; 270: 10147-10160Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar), whereas LTBP-4 is prominent in heart, aorta, uterus, and small intestine (11Saharinen J. Taipale J. Monni O. Keski-Oja J. J. Biol. Chem. 1998; 273: 18459-18469Abstract Full Text Full Text PDF PubMed Scopus (106) Google Scholar). Although LTBP distribution patterns correspond, in part, to the distribution of tissue phenotypes in mice with mutations in Ltbp genes, cultured cells often produce multiple LTBP isoforms (13Penttinen C. Saharinen J. Weikkolainen K. Hyytiainen M. Keski-Oja J. J. Cell Sci. 2002; 115: 3457-3468PubMed Google Scholar, 14Weiskirchen R. Moser M. Gunther K. Weiskirchen S. Gressner A.M. Gene (Amst.). 2003; 308: 43-52Crossref PubMed Scopus (11) Google Scholar). Among the four LTBPs, the hinge domain has the highest degree of sequence diversity. The hinge sequences of LTBP-1 and -2 are sensitive to proteolytic cleavage (15Taipale J. Lohi J. Saarinen J. Kovanen P.T. Keski-Oja J. J. Biol. Chem. 1995; 270: 4689-4696Abstract Full Text Full Text PDF PubMed Scopus (341) Google Scholar, 16Hyytiainen M. Taipale J. Heldin C.H. Keski-Oja J. J. Biol. Chem. 1998; 273: 20669-20676Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar), but LTBP-3 and -4 are not cleaved in this region (13Penttinen C. Saharinen J. Weikkolainen K. Hyytiainen M. Keski-Oja J. J. Cell Sci. 2002; 115: 3457-3468PubMed Google Scholar). The LTBP-1 hinge contains a putative heparin binding sequence. The central core of EGF-like domains is reminiscent of the tandem EGF-like repeats in the fibrillins (17Downing A.K. Knott V. Werner J.M. Cardy C.M. Campbell I.D. Handford P.A. Cell. 1996; 85: 597-605Abstract Full Text Full Text PDF PubMed Scopus (368) Google Scholar). The repeating EGF-like modules may form a rigid sequence that extends the growth factor-binding C-terminal region away from the matrix-binding N-terminal domain (18Handford P.A. Downing A.K. Reinhardt D.P. Sakai L.Y. Matrix Biol. 2000; 19: 457-470Crossref PubMed Scopus (114) Google Scholar) facilitating the interaction of latent TGF-β with its activators. However, removal of the EGF-like core from LTBP-1S does not impede latent TGF-β activation by the integrin αvβ6 (19Annes J.P. Chen Y. Munger J.S. Rifkin D.B. J. Cell Biol. 2004; 165: 723-734Crossref PubMed Scopus (365) Google Scholar). The EGF-like domains adjacent to 8-Cys domains may restrain 8-Cys (19Annes J.P. Chen Y. Munger J.S. Rifkin D.B. J. Cell Biol. 2004; 165: 723-734Crossref PubMed Scopus (365) Google Scholar, 20Saharinen J. Taipale J. Keski-Oja J. EMBO J. 1996; 15: 245-253Crossref PubMed Scopus (184) Google Scholar) domain motion and enhance interactions with matrix or fibular proteins as described for fibrillin (18Handford P.A. Downing A.K. Reinhardt D.P. Sakai L.Y. Matrix Biol. 2000; 19: 457-470Crossref PubMed Scopus (114) Google Scholar, 21Lee S.S. Knott V. Jovanovic J. Harlos K. Grimes J.M. Choulier L. Mardon H.J. Stuart D.I. Handford P.A. Structure (Camb.). 2004; 12: 717-729Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar). The 8-Cys3 domain of LTBP-1, -2, and -4 binds the SLC (20Saharinen J. Taipale J. Keski-Oja J. EMBO J. 1996; 15: 245-253Crossref PubMed Scopus (184) Google Scholar, 22Gleizes P.-E. Beavis R.C. Mazzieri R. Shen B. Rifkin D.B. J. Biol. Chem. 1996; 271: 29891-29896Abstract Full Text Full Text PDF PubMed Scopus (110) Google Scholar). LAP binds via its unpaired most N-terminal cysteines. The remaining 8-Cys motifs in LTBP-1, -3, and -4, as well as all of the 8-Cys domains in LTBP-2 and the fibrillins, do not bind SLC. The basis for the differential binding of LAP to 8-Cys3 domains was suggested by Saharinen and Keski-Oja (23Saharinen J. Keski-Oja J. Mol. Biol. Cell. 2000; 11: 2691-2704Crossref PubMed Scopus (212) Google Scholar), who pointed out that the three LAP-binding 8-Cys domains have a dipeptide insertion between cysteines 6 and 7 compared with the sequences of the nonLAP-binding 8-Cys. The structural basis for LLC complex formation was clarified with the elucidation of the solution structure of the LTBP-1 8-Cys3 domain (24Lack J. O'Leary J.M. Knott V. Yuan X. Rifkin D.B. Handford P.A. Downing A.K. J. Mol. Biol. 2003; 334: 281-291Crossref PubMed Scopus (39) Google Scholar) and comparison of this structure to the structure of the fibrillin-1 non-LAP-binding 8-Cys6. The 8-Cys fold is globular with six β-strands and two α-helices. The pairing of the eight cysteines is 1–3, 2– 6, 4 –7, and 5– 8, creating a fairly rigid structure. Whereas the general structures of the TGF-β-binding and non-binding 8-Cys domains are similar, the Phe-Pro insertion between cysteines 6 and 7 projects a β-strand and displaces the 2– 6 cysteine pair into the solution (Fig. 3). The increased surface accessibility probably enhances reactivity of the 2– 6 disulfide bond permitting covalent attachment to the pair of cysteine 33 residues in TGF-β 1 LAP. The NMR structure also revealed a “ring” of five negatively charged amino acids surrounding the 2– 6 disulfide, suggesting that electrostatic interactions between LAP and the LTBP-1 8-Cys3 domain initiate complex formation. LAP and 8-Cys3 electrostatic interactions, also demonstrated by fluorescence resonance energy transfer experiments, imply that non-covalent protein-protein associations are possible with 8-Cys domains. Covalent bonding may be only one end of a spectrum of 8-Cys domain binding interactions that might include propeptides of other members of the TGF-β superfamily (25Altmann C.R. Chang C. Munoz-Sanjuan I. Bell E. Heke M. Rifkin D.B. Brivanlou A.H. Dev. Biol. 2002; 248: 118-127Crossref PubMed Scopus (24) Google Scholar, 26Charbonneau N.L. Ono R.N. Corson G.M. Keene D.R. Sakai L.Y. Birth Defects Res. Part C Embryo Today. 2004; 72: 37-50Crossref PubMed Scopus (85) Google Scholar), thereby expanding the binding repertoire of the LTBPs. An interesting question is whether the LTBPs have distinct binding specificities for TGF-β isoforms. Saharinen and Keski-Oja (23Saharinen J. Keski-Oja J. Mol. Biol. Cell. 2000; 11: 2691-2704Crossref PubMed Scopus (212) Google Scholar) reported that in coexpression experiments the LTBP-1 and -3 8-Cys3 domains bound all three TGF-β isoforms well, but the LTBP-4 protein weakly bound only TGF-β1. However, because the Ltbp-4 3Uppercase letters (LTBP) refer to the human proteins or the protein in general, whereas lowercase letters (Ltbp) refer to the mouse protein. hypomorphic mouse displays TGF-β-related phenotypes (see below), Ltbp-4 LLC formation in vivo must be sufficient to affect TGF-β action. LTBP-1S interacts with the matrix through both N- and C-terminal motifs (27Nunes I. Gleizes P.E. Metz C.N. Rifkin D.B. J. Cell Biol. 1997; 136: 1151-1163Crossref PubMed Scopus (345) Google Scholar, 28Unsold C. Hyytiainen M. Bruckner-Tuderman L. Keski-Oja J. J. Cell Sci. 2001; 114: 187-197Crossref PubMed Google Scholar). Peptides containing either the first, second, or fourth 8-Cys domains of LTBP-1S associate with the ECM, with the 8-Cys2 region showing the most rapid and extensive incorporation (27Nunes I. Gleizes P.E. Metz C.N. Rifkin D.B. J. Cell Biol. 1997; 136: 1151-1163Crossref PubMed Scopus (345) Google Scholar). Matrix binding was enhanced by cell-conditioned medium. The association of the N-terminal sequence with the ECM, the requirement for a cell-derived factor, and the demonstration of a transglutaminase site in 8-Cys2 (27Nunes I. Gleizes P.E. Metz C.N. Rifkin D.B. J. Cell Biol. 1997; 136: 1151-1163Crossref PubMed Scopus (345) Google Scholar) suggest that the factor is a transglutaminase. The C-terminal domains of LTBP-1 and -4 interact with fibrillin-1 (29Isogai Z. Ono R.N. Ushiro S. Keene D.R. Chen Y. Mazzieri R. Char-bonneau N.L. Reinhardt D.P. Rifkin D.B. Sakai L.Y. J. Biol. Chem. 2003; 278: 2750-2757Abstract Full Text Full Text PDF PubMed Scopus (439) Google Scholar). The LTBP-1L N-terminal extension contains sequences that facilitate matrix incorporation as LTBP-1L associates more readily with ECM than does LTBP-1S (30Olofsson A. Ichijo H. Moren A. ten Dijke P. Miyazono K. Heldin C.H. J. Biol. Chem. 1995; 270: 31294-31297Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar). It is interesting that whereas LTBP-1 and -4 are effectively incorporated into the ECM, LTBP-3 is not (13Penttinen C. Saharinen J. Weikkolainen K. Hyytiainen M. Keski-Oja J. J. Cell Sci. 2002; 115: 3457-3468PubMed Google Scholar). Forms of LTBP-1 that contain only the LAP-binding domains (8-Cys3) fail to target TGF-β to the ECM and yield animals with V. H. J. A. E. P. E. and B. for LTBP-1 matrix binding may initiate sequences at either end of the protein, but ECM incorporation may binding by the N-terminal domains by transglutaminase LTBPs interact with a of matrix and J. Saharinen J. K. Keski-Oja J. J. 1996; PubMed Scopus Google Scholar). (29Isogai Z. Ono R.N. Ushiro S. Keene D.R. Chen Y. Mazzieri R. Char-bonneau N.L. Reinhardt D.P. Rifkin D.B. Sakai L.Y. J. Biol. Chem. 2003; 278: 2750-2757Abstract Full Text Full Text PDF PubMed Scopus (439) Google Scholar) demonstrated binding interactions between LTBP-1 and fibrillin-1 and defined the of the two the N-terminal region of fibrillin-1 binds to the C-terminal of The C-terminal of LTBP-4 also interacts with the N-terminal sequence of The interaction of LTBPs with fibrillin-1 may affect tissue as P.A. L. B. Sakai L.Y. 2003; PubMed Scopus Google Scholar) reported increased activation of latent TGF-β the of fibrillin-1 hypomorphic The of fibrillin-1 in the mice may in LLC to interact with fibrillin-1 LLC interacts with latent TGF-β from This of the mouse that in structural as growth factor action. the of tissue may not from of the proteins but from an to In to LTBP have an association between LTBP and TGF-β LTBP enhances SLC in cultured cells K. Olofsson A. P. Heldin C.H. EMBO J. PubMed Scopus Google Scholar). of LTBP-1 and SLC also with the disulfide binding In the of LTBP-1, disulfide are These proteins may be by and thereby the and degree of to the 8-Cys3 domain SLC and disulfide thereby In this LTBP as for SLC, a by the that of the cysteines in LAP with SLC (19Annes J.P. Chen Y. Munger J.S. Rifkin D.B. J. Cell Biol. 2004; 165: 723-734Crossref PubMed Scopus (365) Google Scholar). of LTBP-1 have by in with R. M. Y. Miyazono K. J. Heldin C.H. Rifkin D.B. J. Cell Biol. PubMed Scopus Google Scholar) reported that a to LTBP-1 latent TGF-β activation in and muscle This a requirement for of LLC to the ECM for TGF-β and was with the that the of transglutaminase of LLC to the ECM latent cytokine activation (27Nunes I. Gleizes P.E. Metz C.N. Rifkin D.B. J. Cell Biol. 1997; 136: 1151-1163Crossref PubMed Scopus (345) Google Scholar). Y. Miyazono K. M. M. T. H. J. Cell Biol. 1997; 136: PubMed Scopus Google Scholar) reported an LTBP-1 requirement for mouse heart a to TGF-β Mol. Dev. PubMed Scopus Google Scholar). to LTBP-1 of cells with and this was by the suggest that most TGF-β is to the matrix by Gualandris A. J.P. M. I. V. Rifkin D.B. Mol. Biol. Cell. 2000; 11: PubMed Scopus Google Scholar) that LTBP-1 is of the regulatory for TGF-β cells in the of either an to LTBP-1 or LAP, which with and TGF-β, the and of cells TGF-β increased and These that with LTBP-1 the of cells in a all three an association of LTBP-1 and TGF-β all of the by cells with an to LTBP have as TGF-β action. Miyazono K. J. Cell Biol. 1995; PubMed Scopus Google Scholar) that to LTBP-1 of by The was not by addition of TGF-β but was by with an LTBP-1 The of an LTBP-2 P. Hyytiainen M. Keski-Oja J. J. Biol. Chem. 2003; 278: Full Text Full Text PDF PubMed Scopus Google Scholar) for cells that LTBPs have However, of the of Miyazono K. J. Cell Biol. 1995; PubMed Scopus Google Scholar) as a of LTBP-1 be of that TGF-β the target in LTBP-1, -3, and -4 from in latent TGF-β activation. of have described for latent TGF-β activation high and but only the integrin αvβ6 and 1 have in latent TGF-β activation by both in and in vivo experiments (5Annes J.P. Munger J.S. Rifkin D.B. J. Cell Sci. 2003; 116: 217-224Crossref PubMed Scopus (1286) Google Scholar). Although a for LTBP-1 in latent TGF-β activation was suggested R. M. Y. Miyazono K. J. Heldin C.H. Rifkin D.B. J. Cell Biol. PubMed Scopus Google Scholar), the of the used in these experiments may have with the activation the that activation by the αvβ6 the the LAP sequences of or LAP does not have an J.S. X. H. J. C. Rifkin D.B. Cell. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar) suggested that LTBP is not of that activation However, LTBP-1 is for latent TGF-β activation by αvβ6 (19Annes J.P. Chen Y. Munger J.S. Rifkin D.B. J. Cell Biol. 2004; 165: 723-734Crossref PubMed Scopus (365) Google Scholar). cells αvβ6 not or expressed SLC. This suggested that LLC formation is for activation. (19Annes J.P. Chen Y. Munger J.S. Rifkin D.B. J. Cell Biol. 2004; 165: 723-734Crossref PubMed Scopus (365) Google Scholar) demonstrated that the region of LTBP-1 for TGF-β activation is a acid sequence the hinge the core of EGF-like repeats the N- 8-Cys domains are TGF-β activation by αvβ6 in as one the LLC by the LTBP-3 hinge sequence cannot for LTBP-1 the LTBP-3 activation. However, the LTBP-1 hinge is in the LTBP activation. not all LTBPs all LTBP hinge in latent TGF-β activation. The suggest a in which activation the of and only be the latent complex is to the matrix or An question is is the hinge binding or hypomorphic mutations for three of the four LTBPs have mice in and J.M. Mecham R.P. E. Bonadio J. Rosenbloom J. C. Mol. Cell. Biol. 2000; PubMed Scopus Google Scholar). The for is but there may be a in The described M. Keski-Oja J. J. Cell Biol. 2003; PubMed Scopus Google Scholar) or P. Hyytiainen M. Keski-Oja J. J. Biol. Chem. 2003; 278: Full Text Full Text PDF PubMed Scopus Google Scholar) of LTBP-2 may be this of may bind molecule for The mouse has a of but is and for B. Chen Y. C. Obata H. L. Rifkin D.B. J. Cell Biol. 2002; PubMed Scopus Google Scholar). animals have and Y. H. V. B. and B. for of the and and a of cells Y. B. C. M. S. Rifkin D.B. J. Cell. 2003; PubMed Scopus (18) Google Scholar). The are multiple B. Chen Y. C. Obata H. L. Rifkin D.B. J. Cell Biol. 2002; PubMed Scopus Google the mice have the long are and as the animals the long and and and phenotypes in mice with TGF-β E. A. J. J. M. R. (Camb.). 1999; Google Scholar, X. Chen L. X. C. C. J. Cell Biol. 2001; PubMed Scopus Google Scholar). the phenotypes are with extracellular levels of TGF-β, but to in TGF-β The from the of the in the of the The increased of of the from a in a regulatory Indian and parathyroid hormone-related protein that The expression of by the the of by the and E. J. 2001; PubMed Scopus Google Scholar). interacts with its expressed by and the of In expression is suggesting that TGF-β The of in levels of TGF-β in to IHH, TGF-β in expression of PTHrP, and This be by expression of TGF-β or by in the the Ltbp-4 hypomorphic mice as of a gene A. K. J. T. K. T. S. Keski-Oja J. H. Dev. 2002; PubMed Scopus Google Scholar). animals have levels of Ltbp-4 protein and than of the of Ltbp-4 The hypomorphic mice at the which is than the in the The are by of larger and The mice of the small intestine with both the high expression of LTBP-4 in the small intestine (11Saharinen J. Taipale J. Monni O. Keski-Oja J. J. Biol. Chem. 1998; 273: 18459-18469Abstract Full Text Full Text PDF PubMed Scopus (106) Google Scholar) and the of TGF-β Y. J.M. Cell. 1998; Full Text Full Text PDF PubMed Scopus Google Scholar). of and tissue from animals revealed a of extracellular TGF-β with of the growth the major phenotypes to extracellular levels of The the Ltbp-4 phenotypes has by K. A. A. M. H. Keski-Oja J. J. Cell Biol. 2004; PubMed Scopus Google Scholar), who demonstrated that cells derived from LTBP-4 mice have a matrix containing of The cells have levels of extracellular TGF-β, increased expression of and expression of the The matrix is by either or a receptor for and expression also in These that the Ltbp-4 TGF-β of in the cells to the The of LTBP in TGF-β is from the phenotypes of Ltbp phenotypes be in these as well as in with to LTBP action. are there Although the LTBPs may have all of the to LTBPs as of TGF-β action. It is also possible that LTBPs in the of other TGF-β superfamily are the of LTBPs the of a tissue different LTBPs may be The phenotypes of the -3, and -4 mice suggest unique or tissue a in which expression for the of the of K. A. A. M. H. Keski-Oja J. J. Cell Biol. 2004; PubMed Scopus Google Scholar) that LTBP-1 to the of Ltbp-4 cells a of the that not all are by the integrin αvβ6 the that LTBPs have unique However, of must as LTBP of the three TGF-β a of in LTBP or may be These may to interactions with surface or matrix of these latent the of in a with the LLC and This permits both and as one activation may be but a second may with or