A Central Bioactive Region of LTBP-2 Stimulates the Expression of TGF-β1 in Fibroblasts via Akt and p38 Signalling Pathways

Abstract

Latent transforming growth factor-β-1 binding protein-2 (LTBP-2) belongs to the LTBP-fibrillin superfamily of extracellular proteins. Unlike other LTBPs, LTBP-2 does not covalently bind transforming growth factor-β1 (TGF-β1) but appears to be implicated in the regulation of TGF-β1 bioactivity, although the mechanisms are largely unknown. In experiments originally designed to study the displacement of latent TGF-β1 complexes from matrix storage, we found that the addition of exogenous LTBP-2 to cultured human MSU-1.1 fibroblasts caused an increase in TGF-β1 levels in the medium. However, the TGF-β1 increase was due to an upregulation of TGF-β1 expression and secretion rather than a displacement of matrix-stored TGF-β1. The secreted TGF-β1 was mainly in an inactive form, and its concentration peaked around 15 h after addition of LTBP-2. Using a series of recombinant LTBP-2 fragments, the bioactivity was identified to a small region of LTBP-2 consisting of an 8-Cys motif flanked by four epidermal growth factor (EGF)-like repeats. The LTBP-2 stimulation of TGF-β expression involved the phosphorylation of both Akt and p38 mitogen-activated protein kinase (MAPK) signalling proteins, and specific inactivation of each protein individually blocked TGF-β1 increase. The search for the cell surface receptor mediating this LTBP-2 activity proved inconclusive. Inhibitory antibodies to integrins β1 and αVβ5 showed no reduction of LTBP-2 stimulation of TGF-β1. However, TGF-β1 upregulation was partially inhibited by anti-αVβ3 integrin antibodies, suggestive of a direct or indirect role for this integrin. Overall, the study indicates that LTBP-2 can directly upregulate cellular TGF-β1 expression and secretion by interaction with cells via a short central bioactive region. This may be significant in connective tissue disorders involving aberrant TGF-β1 signalling.

Diverse Signaling by TGFβ Isoforms in Response to Focal Injury is Associated with Either Retinal Regeneration or Reactive Gliosis

“…We also detected upregulation of Ltbp isoforms, Ltbp1 , Ltbp2 and Ltbp3 . Their modulation can be directly mediated by ROS production (Jobling et al 2006 ) and activate the p38 MAPK pathway (Sideek et al 2017 ) linked to gliosis (Kaminska et al 2009 ). Thus, we analyzed p38 MAPK signaling and found evidence of activation of the non-canonical p38 MAPK pathway—likely mediated by TGFβ1 and TGFβ2—during gliotic response in mouse.…”

LTBP2 is secreted from lung myofibroblasts and is a potential biomarker for idiopathic pulmonary fibrosis

“…An in vitro study has revealed that LTBP2 competes with LTBP1 by binding to fibrillin-1, suggesting an indirect role of LTBP2 in the storage and activation of TGF-β, a major profibrotic growth factor [34]. In addition, Sideek et al [35] recently reported that a bioactive region of LTBP2 can directly up-regulate TGF-β1 expression. On the other hand, previous studies suggested the antifibrotic role of LTBP2 via inhibition of fibroblast growth factor-2-induced fibroblast proliferation and fibroblast adhesion to fibronectin [36,37].…”

Uncovering the secretome of mesenchymal stromal cells exposed to healthy, traumatic, and degenerative intervertebral discs: a proteomic analysis

“…MSCs secreted several proteins solely following traumatic stimulus, potentially participating in damage repair. Latent transforming growth factor-β1 binding protein 2 (LTBP-2, Table 1) has been reported to stimulate the expression of TGF-β1 in fibroblasts [48]. In the IVD, TGF-β1 has been associated with alleviation of the inflammatory response, inhibition of ECM degradation, Table 1 Proteins identified in secretomes of mesenchymal stromal cells stimulated with healthy, traumatic, or degenerative intervertebral disc conditioned medium or IL1-β, with a log2 fold-change > 1.5 (relative to the baseline control stimulation) (Continued) inhibition of cell death, increased ECM synthesis, and promotion of cell proliferation [49].…”