Latent TGF-β structure and activation

Abstract

Transforming growth factor (TGF)-β is stored in the extracellular matrix as a latent complex with its prodomain. Activation of TGF-β1 requires the binding of αv integrin to an RGD sequence in the prodomain and exertion of force on this domain, which is held in the extracellular matrix by latent TGF-β binding proteins. Crystals of dimeric porcine proTGF-β1 reveal a ring-shaped complex, a novel fold for the prodomain, and show how the prodomain shields the growth factor from recognition by receptors and alters its conformation. Complex formation between αvβ6 integrin and the prodomain is insufficient for TGF-β1 release. Force-dependent activation requires unfastening of a ‘straitjacket’ that encircles each growth-factor monomer at a position that can be locked by a disulphide bond. Sequences of all 33 TGF-β family members indicate a similar prodomain fold. The structure provides insights into the regulation of a family of growth and differentiation factors of fundamental importance in morphogenesis and homeostasis.

Targeting TGF-β Signaling for Therapeutic Gain

“…There is reciprocal regulation between TGF-b by ECM. Latent TGF-b bound to ECM components, such as fibronectin and fibrillin, is inactive until physiological or pathological processes initiate its release (Chaudhry et al 2007;Shi et al 2011). Moreover, interaction between integrins and ECM can generate tension that feeds back to more TGF-b expression and activation, generating a cycle toward cumulative fibrosis.…”

“…Dimeric LAP also binds noncovalently to cell surface integrins, particularly integrins b1, b6, and b8, which can bind latent TGF-b1 and b3 via an RGD site within the corresponding LAP domains (Munger et al 1999;Annes et al 2004;Yang et al 2007;Arnold et al 2014). This interaction results in TGF-b activation, triggered by the molecular tension resulting from physical stretch between LAP-integrin binding at the cell surface and LAP-LTBP anchorage to the ECM (Yang et al 2007;Shi et al 2011;Dong et al 2014;Mi et al 2015). Proteolytic cleavage of the amino-terminal motif of LAP can also activate TGF-b by releasing the entrapped active homodimer from its latent protein cage (Dong et al 2014).…”

“…The prodomain, otherwise termed the latency-associated peptide (LAP), is cleaved from the carboxy-terminal mature peptide by a subtilisin-like proprotein convertase, furin, but remains noncovalently associated with the mature homodimeric ligand (Annes et al 2003). Dimeric LAP encapsulates the mature dimeric peptide within a cage-like structure, keeping it in an inactive state that facilitates spatial and temporal regulation of storage, delivery, and activation (Shi et al 2011). On the outer surface of this "cage," one of the two LAP arms also binds covalently to a LTBP (latent TGF-b-binding protein), which is a large ECM component that anchors latent TGF-b within the ECM rather than permitting free diffusion within the tissue (Munger et al 1997;Ota et al 2002;Annes et al 2003).…”

“…Thrombin, matrix metalloproteinases (MMP)-2 and -9, and thrombospondin-1 (THBS1), which can all be enriched in carcinomas, are each capable of cleaving LAP to activate TGF-b (SchultzCherry et al 1994). Other means of ligand activation include acidic pH, reactive oxygen species (ROS) and ionizing radiation, all of which are relevant to tumor progression and cancer treatment (Barcellos-Hoff 1993;Barcellos-Hoff and Dix 1996;Annes et al 2004;Jobling et al 2006;Shi et al 2011).…”

TGF-β Family Signaling in Connective Tissue and Skeletal Diseases

“…LAP prodomains are cleaved during posttranslational processing and remain associated with the corresponding TGF-b molecule through noncovalent interactions (Munger and Sheppard 2011). Disruption of noncovalent interactions between TGF-b and LAP molecules, through LAP degradation (Lyons et al 1988;Yu and Stamenkovic 2000), chemical modification, such as that caused by reactive oxygen species or low pH (Lyons et al 1988;Barcellos-Hoff and Dix 1996), and/or through binding-induced conformational change (Munger et al 1997;Shi et al 2011), results in conversion of latent TGF-b into active TGF-b. TGF-b -LAP dimers (SLC) covalently bind to one of the latent TGF-b binding proteins (LTBP-1, -3, and -4) through disulfide bonds formed between LAP and LTBPs (Todorovic and Rifkin 2012), resulting in the large latent complex, or LLC.…”