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

Abstract

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 S