Integrin α8β1 mediates adhesion to LAP-TGFβ1

The development of fibrosis is a common response to a variety of injuries and results in the net accumulation of matrix proteins and impairment of normal organ function. We previously reported that the integrinα8β1 is expressed by alveolar interstitial cells in normal lung and is upregulated during the development of fibrosis. TGFβ1 is an important mediator of the inflammatory response in pulmonary fibrosis. TGFβ1 is secreted as a latent protein that is non-covalently associated with latency-associated peptide (LAP) and requires activation to exert its effects. LAP-TGFβ1 and LAP-TGFβ3 contain the tripeptide sequence,arginine-glycine-aspartic acid (RGD), a known integrin recognition motif. The integrin α8β1 binds to several ligands such as fibronectin and vitronectin through the RGD sequence. Recent reports demonstrate that the integrins αvβ1, αvβ6 and αvβ8 adhere to LAP-TGFβ1 through the RGD site. Therefore, we asked whether LAP-TGFβ1 might be a ligand for α8β1 and whether this may be important in the development of fibrosis. We found that cell lines transfected with α8 subunit were able to spread on and adhere to recombinant LAP-TGFβ1 significantly better than mock transfected cell lines.α8-transfected cells were also able to adhere to LAP-TGFβ3 significantly better than mock transfected cells. Adhesion to LAP-TGFβ1 was enhanced by activation of α8β1 by Mn²⁺, or 8A2, an integrin β1 activating antibody. Furthermore, cell adhesion was abolished when we used a recombinant LAP-TGFβ1 protein in which the RGD site was mutated to RGE. α8β1 binding to LAP-TGFβ1 increased cell proliferation and phosphorylation of FAK and ERK, but did not activate of TGFβ1. These data strongly suggest that LAP-TGFβ1 is a ligand ofα8β1 and interaction of α8β1 with LAP-TGFβ1 may influence cell behavior.