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
integrin alpha8beta1 is expressed by alveolar interstitial cells in normal lung and is upregulated during the development of
fibrosis. TGFbeta1 is an important mediator of the inflammatory response in
pulmonary fibrosis. TGFbeta1 is secreted as a latent
protein that is non-covalently associated with latency-associated
peptide (LAP) and requires activation to exert its effects. LAP-TGFbeta1 and LAP-TGFbeta3 contain the tripeptide sequence,
arginine-glycine-aspartic acid (RGD), a known
integrin recognition motif. The
integrin alpha8beta1 binds to several
ligands such as
fibronectin and
vitronectin through the RGD sequence. Recent reports demonstrate that the
integrins alphavbeta1, alphavbeta6 and alphavbeta8 adhere to LAP-TGFbeta1 through the RGD site. Therefore, we asked whether LAP-TGFbeta1 might be a
ligand for alpha8beta1 and whether this may be important in the development of
fibrosis. We found that cell lines transfected with alpha8 subunit were able to spread on and adhere to recombinant LAP-TGFbeta1 significantly better than mock transfected cell lines. alpha8-transfected cells were also able to adhere to LAP-TGFbeta3 significantly better than mock transfected cells. Adhesion to LAP-TGFbeta1 was enhanced by activation of alpha8beta1 by Mn(2+), or 8A2, an
integrin beta1 activating antibody. Furthermore, cell adhesion was abolished when we used a recombinant LAP-TGFbeta1
protein in which the RGD site was mutated to RGE. alpha8beta1 binding to LAP-TGFbeta1 increased cell proliferation and phosphorylation of FAK and ERK, but did not activate of TGFbeta1. These data strongly suggest that LAP-TGFbeta1 is a
ligand of alpha8beta1 and interaction of alpha8beta1 with LAP-TGFbeta1 may influence cell behavior.