LTBP1

LTBP1 is a multi-domain extracellular matrix glycoprotein that functions as the principal scaffold for sequestering, localizing, and regulating the activation of latent TGF-β1 in the ECM. Its third 8-Cys/TB3 domain covalently binds the latency-associated peptide (LAP) of TGF-β1 through disulfide exchange at an exposed Cys2–Cys6 bond, while three independent ECM-binding domains and a C-terminal fibrillin-1-interacting region anchor the complex to microfibrils (PMID:8617200, PMID:14607119, PMID:11112702, PMID:17293099). Activation of sequestered TGF-β1 proceeds through integrin αvβ6- and fibronectin-dependent mechanical mechanisms requiring a 24-amino-acid hinge sequence, and through MT1-MMP-mediated proteolytic cleavage of the hinge region (PMID:16260650, PMID:18602101). Beyond TGF-β regulation, LTBP1 promotes TGF-β-independent incorporation of fibrillin-1 and fibrillin-2 into the ECM, undergoes regulatory post-translational modifications including S-nitrosylation (targeting it for proteasomal degradation via a PTPS/iNOS complex), O-glucosylation by POGLUT2/3, and KAT8-dependent lactylation at K752 that promotes collagen synthesis, and its transcription is repressed by AhR-recruited HDAC2 (PMID:35452817, PMID:31628042, PMID:34411563, PMID:39102921, PMID:18508077).

1. 1996 High

   Established how LTBP1 physically links latent TGF-β1 to the ECM: the TB3 domain forms a disulfide bond with Cys33 of β1-LAP, while the N-terminal ~400 residues anchor independently and covalently to the matrix.

   Evidence Co-expression of TGF-β1 and LTBP1 domain fragments in mammalian cells with site-directed mutagenesis of LAP cysteines

   PMID:8617200

   Open questions at the time

   • Identity of the LTBP1 cysteine participating in exchange was not pinpointed
   • ECM-anchoring receptor/ligand on the N-terminal side not identified
2. 2001 High

   Resolved a long-standing question of how many ECM-attachment sites exist: three independent 8-Cys domains each bind covalently to fibroblast ECM and compete with native LTBP1 for incorporation, explaining multivalent matrix anchoring.

   Evidence Recombinant fragment binding and competition assays against fibroblast ECM with deoxycholate resistance controls

   PMID:11112702

   Open questions at the time

   • Specific ECM receptors for each binding domain not identified
   • Relative contribution of each domain in vivo unknown
3. 2003 High

   Identified the structural basis for LAP recognition: NMR of TB3 showed a two-residue insertion exposes the Cys2–Cys6 disulfide for exchange, surrounded by a negatively charged surface facilitating complex formation.

   Evidence Solution NMR structure of TB3 domain combined with mutagenesis and NMR perturbation studies

   PMID:14607119

   Open questions at the time

   • No co-structure of TB3 bound to LAP obtained
   • Role of the charged ring in binding kinetics not quantified
4. 2005 High

   Defined the integrin-dependent TGF-β activation mechanism: a 24-amino-acid hinge sequence in LTBP1 is required for αvβ6-mediated activation, which additionally depends on fibronectin/α5β1 for proper matrix incorporation.

   Evidence Binding assays with LTBP1 polypeptides plus fibronectin-null and α5β1-null cell lines and latent TGF-β activation assays

   PMID:16260650

   Open questions at the time

   • Whether mechanical force through integrin directly opens the complex or requires cofactors was not resolved
   • Hinge–integrin direct interaction not demonstrated
5. 2007 Medium

   Showed that LTBP1 connects the latent TGF-β complex to fibrillin-1 microfibrils through its C-terminal region, competing with LTBP2 for the same N-terminal fibrillin-1 binding site.

   Evidence Solid-phase and competitive binding assays with recombinant C-terminal fragments of LTBP1 and LTBP2

   PMID:17293099

   Open questions at the time

   • In vivo validation of competitive binding with LTBP2 lacking
   • Stoichiometry and affinity constants not precisely determined
6. 2008 High

   Identified two parallel regulatory axes: MT1-MMP proteolytically releases latent TGF-β1 from ECM-bound LTBP1 via PKC/ERK signaling in endothelial cells, while AhR represses Ltbp1 transcription by recruiting HDAC2 to maintain promoter hypoacetylation.

   Evidence shRNA silencing of MT1-MMP with TIMP controls in endothelial cells; ChIP, siRNA, and reporter assays for AhR/HDAC2 at Ltbp1 promoter in MEFs

   PMID:18508077 PMID:18602101

   Open questions at the time

   • MT1-MMP cleavage site on LTBP1 not mapped
   • Whether AhR/HDAC2 regulation is conserved in human tissues not shown
7. 2019 High

   Uncovered a post-translational degradation pathway: under hypoxia, AMPK-phosphorylated PTPS recruits iNOS to S-nitrosylate LTBP1, directing it to proteasomal degradation and thereby limiting TGF-β secretion in tumor cells.

   Evidence Co-immunoprecipitation, mass spectrometry of S-nitrosylation site, proteasome and iNOS inhibitor experiments in tumor cells

   PMID:31628042

   Open questions at the time

   • Exact S-nitrosylation site(s) on LTBP1 and whether modification is reversible not fully mapped
   • In vivo tumor relevance demonstrated in limited models
8. 2021 High

   Showed that EGF repeats on LTBP1 are O-glucosylated by POGLUT2/3, modifications that promote fibrillin-1 secretion, connecting LTBP1 post-translational processing to microfibril assembly.

   Evidence Mass spectrometry site identification; POGLUT2/3 knockout HEK293T cells with fibrillin-1 secretion assays

   PMID:34411563

   Open questions at the time

   • Effect of O-glucosylation on LTBP1 folding versus secretion not separated
   • Whether LTBP1 itself requires O-glucosylation for its own secretion not tested
9. 2022 Medium

   Established a TGF-β-independent function: LTBP1 promotes fibrillin-1 and fibrillin-2 ECM incorporation with isoform-specific differences between LTBP1S and LTBP1L.

   Evidence In vitro ECM incorporation assays comparing LTBP1 isoforms with loss-of-function experiments

   PMID:35452817

   Open questions at the time

   • Molecular basis for isoform-specific activity not determined
   • In vivo microfibril phenotype of LTBP1 isoform-selective loss not characterized
10. 2024 Medium

    Identified lactylation as a new regulatory modification: lactate-driven, KAT8-dependent lactylation of LTBP1 at K752 enhances collagen I/III expression in fibroblasts, linking metabolic signaling to ECM remodeling through LTBP1.

    Evidence KAT8 inhibition/knockdown, MCT1 transport inhibition, western blotting of lactylation and collagen levels in fibroblasts

    PMID:39102921

    Open questions at the time

    • Whether lactylation alters TGF-β binding or activation is unknown
    • Mechanism by which lactylated LTBP1 upregulates collagen not defined
11. 2025 Medium

    Revealed an mRNA-level regulatory circuit: LTA4H induces HNRNPA1 phosphorylation, disrupting HNRNPA1-mediated Ltbp1 mRNA maturation and reducing LTBP1 protein and downstream TGF-β secretion in hepatocellular carcinoma.

    Evidence Co-immunoprecipitation, phosphorylation analysis, nuclear fractionation, and mRNA processing assays in HCC cells

    PMID:40056904

    Open questions at the time

    • Specific mRNA processing step affected (splicing, export, or stability) not fully delineated
    • Whether this pathway operates outside HCC is unknown

• Key open questions include: the precise structural mechanism by which integrin-mediated mechanical force on the LTBP1 hinge releases TGF-β from LAP; the in vivo consequences of individual post-translational modifications (S-nitrosylation, O-glucosylation, lactylation) on ECM homeostasis; and whether LTBP1's TGF-β-independent role in microfibril assembly is essential for connective tissue integrity.
• No co-structure of LTBP1–LAP complex exists
• In vivo genetic dissection of individual LTBP1 ECM-binding domains not performed
• Relative physiological importance of proteolytic versus integrin-mediated TGF-β activation through LTBP1 is unresolved