Transforming growth factor-β receptors: versatile mechanisms of ligand activation

Abstract

Transforming growth factor-β (TGF-β) signaling is initiated by activation of transmembrane TGF-β receptors (TGFBR), which deploys Smad2/3 transcription factors to control cellular responses. Failure or dysregulation in the TGF-β signaling pathways leads to pathological conditions. TGF-β signaling is regulated at different levels along the pathways and begins with the liberation of TGF-β ligand from its latent form. The mechanisms of TGFBR activation display selectivity to cell types, agonists, and TGF-β isoforms, enabling precise control of TGF-β signals. In addition, the cell surface compartments used to release active TGF-β are surprisingly vibrant, using thrombospondins, integrins, matrix metalloproteinases and reactive oxygen species. The scope of TGFBR activation is further unfolded with the discovery of TGFBR activation initiated by other signaling pathways. The unique combination of mechanisms works in series to trigger TGFBR activation, which can be explored as therapeutic targets. This comprehensive review provides valuable insights into the diverse mechanisms underpinning TGFBR activation, shedding light on potential avenues for therapeutic exploration.

Keywords: TGFBR; integrins; matrix metalloproteinases; receptor Smads; thrombospondins; transactivation.

Fig. 1. The TGF-β complex and TGFBR signaling pathways.

a Transforming growth factor (TGF)-β is synthesized as pro-TGF-β peptide before being cleaved to form latency associated peptide (LAP) and mature TGF-β ligand. The mature TGF-β is non-covalently associated with LAP, forming small latent complex (SLC). SLC is then linked to latent TGF-β binding protein (LTBP) to form large latency complex (LLC) before being secreted to the extracellular domain and deposited in the extracellular matrix. b Active TGF-β will first bind to transforming growth factor-β receptor II (TGFBRII) dimer before forming a complex with transforming growth factor-β receptor I (TGFBRI) dimer. Activated TGFBRI recruits Smad2/3 and phosphorylates the carboxyl-terminal of Smad2/3. Phosphorylated Smad2/3 will associate with Smad4 and other co-factors before forming a transcription factor complex to regulate gene expression.

Fig. 2. Liberation of mature TGF-β by TSP-1 and MMPs.

Left side: Thrombospondin-1 (TSP-1) is a protein found in the extracellular matrix. The AAWSHW domain in TSP-1 can recognize the latency associated peptide (LAP) portion of the transforming growth factor (TGF)-β complex and deposit TSP-1 to LAP, which will then disrupt the structure of LAP with its KRFK motif. The modified LAP loses the ability to maintain the latency of mature TGF-β, therefore releasing the mature TGF-β for transforming growth factor-β receptor (TGFBR) activation. Right side: Matrix metalloproteinases (MMPs) are enzymes that hydrolyze peptides into smaller sections. The LAP is susceptible to MMP proteolysis, thereby disturbing the structural integrity of LAP, enabling the release of mature TGF-β for TGFBR activation.

Fig. 3. The liberation of mature TGF-β by RGD motif-recognizing integrins (αvβ3, αvβ5, αvβ6, αvβ8).

RGD motif-recognizing integrins can identify the RGD domain on latency associated peptide (LAP). Left side: αvβ3, αvβ5 and αvβ6 upon identify the RGD domain on LAP, secure the LAP for tension force exerted by cytoskeleton, which physically disrupts the architecture of LAP, allowing the release of mature transforming growth factor (TGF)-β, which is ready to bind to transforming growth factor-β receptor (TGFBR). Right side: αvβ8 does not require cytoskeleton for the release of mature TGF-β. Upon αvβ8 secures the LAP, membrane type-1 matrix metalloproteinase (MT1-MMP) is used to cleave LAP, enabling mature TGF-β to escape from the LAP for the activation of TGFBR.

Fig. 4. The GPCRs transactivation of TGFBR.

Upon activation, G-protein coupled receptors (GPCRs) recruit Gαq to relay signals to Rho/ROCK, which will then activate cytoskeleton contraction to disrupt the structure of latency associated peptide (LAP) for the release of mature transforming growth factor (TGF)-β with the assistance of RGD motif-recognizing integrin. The active TGF-β will then bind to transforming growth factor-β receptor (TGFBR), fulfilling the command from GPCRs transactivation of TGFBR.

Fig. 5. The TLR4 transactivation of TGFBR.

Lipopolysaccharides (LPS)-bound toll-like receptor 4 (TLR4) activates matrix metalloproteinase-2 (MMP-2) but not matrix metalloproteinase (MMP)-9 to hydrolyze latency associated peptide (LAP). Hydrolyzed LAP loses the ability to maintain the latency of mature transforming growth factor (TGF)-β, allowing the release of mature TGF-β to activate transforming growth factor-β receptor (TGFBR), thereby relaying the signal from the TLR4 signaling pathway to the TGFBR signaling pathway.