Integrin αVβ6-mediated activation of latent TGF-β requires the latent TGF-β binding protein-1

Abstract

Transforming growth factor-βs (TGF-β) are secreted as inactive complexes containing the TGF-β, the TGF-β propeptide, also called the latency-associated protein (LAP), and the latent TGF-β binding protein (LTBP). Extracellular activation of this complex is a critical but incompletely understood step in TGF-β regulation. We have investigated the role of LTBP in modulating TGF-β generation by the integrin αVβ6. We show that even though αvβ6 recognizes an RGD on LAP, LTBP-1 is required for αVβ6-mediated latent TGF-β activation. The domains of LTBP-1 necessary for activation include the TGF-β propeptide-binding domain and a basic amino acid sequence (hinge domain) with ECM targeting properties. Our results demonstrate an LTBP-1 isoform-specific function in αVβ6-mediated latent TGF-β activation; LTBP-3 is unable to substitute for LTBP-1 in this assay. The results reveal a functional role for LTBP-1 in latent TGF-β activation and suggest that activation of specific latent complexes is regulated by distinct mechanisms that may be determined by the LTBP isoform and its potential interaction with the matrix.

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

“…Moreover, the fact that activation by the integrin ␣v␤6 requires the Arg-Gly-Asp within the LAP sequences of TGF-␤1 or ␤3 (TGF-␤2 LAP does not have an Arg-Gly-Asp) (40) suggested that LTBP is not part of that activation pathway. However, LTBP-1 is required for latent TGF-␤ activation by ␣v␤6 (19). Unexpectedly cells expressing ␣v␤6 did not activate added or endogenously expressed SLC.…”

“…The repeating EGF-like modules may form a rigid sequence that extends the growth factorbinding C-terminal region away from the matrix-binding N-terminal domain (18) facilitating the interaction of latent TGF-␤ with its activators. However, removal of the EGF-like core from LTBP-1S does not impede latent TGF-␤ activation by the integrin ␣v␤6 (19). The EGF-like domains adjacent to 8-Cys domains may restrain 8-Cys (19,20) domain motion and enhance interactions with matrix or fibular proteins as described for fibrillin (18,21).…”

“…However, removal of the EGF-like core from LTBP-1S does not impede latent TGF-␤ activation by the integrin ␣v␤6 (19). The EGF-like domains adjacent to 8-Cys domains may restrain 8-Cys (19,20) domain motion and enhance interactions with matrix or fibular proteins as described for fibrillin (18,21).…”

“…Bonding to the 8-Cys3 domain ensures correct SLC folding and disulfide bonding, thereby accelerating secretion. In this scenario, LTBP acts as chaperone for SLC, a concept reinforced by the observation that replacement of the LTBP-binding cysteines in LAP with serines promotes SLC secretion (19).…”

“…This result suggested that LLC formation is crucial for ␣v␤6-mediated activation. Annes et al (19) demonstrated that the region of LTBP-1 required for TGF-␤ activation is a 46-amino acid sequence within the hinge domain; neither the core of EGF-like repeats nor the N-or-C-terminal 8-Cys domains are necessary. Latent TGF-␤ activation by ␣v␤6 can work in trans, as one cell can utilize the LLC produced by another cell.…”