Inhibition of the αvβ6 integrin leads to limited alteration of TGF-α-induced pulmonary fibrosis

A number of growth factors and signaling pathways regulate matrix deposition and fibroblast proliferation in the lung. The epidermal growth factor receptor (EGFR) family of receptors and the transforming growth factor-beta (TGF-beta) family are active in diverse biological processes and are central mediators in the initiation and maintenance of fibrosis in many diseases. Transforming growth factor-alpha (TGF-alpha) is a ligand for the EGFR, and doxycycline (Dox)-inducible transgenic mice conditionally expressing TGF-alpha specifically in the lung epithelium develop progressive fibrosis accompanied with cachexia, changes in lung mechanics, and marked pleural thickening. Although recent studies demonstrate that EGFR activation modulates the fibroproliferative effects involved in the pathogenesis of TGF-beta induced pulmonary fibrosis, in converse, the direct role of EGFR induction of the TGF-beta pathway in the lung is unknown. The alpha(v)beta(6) integrin is an important in vivo activator of TGF-beta activation in the lung. Immunohistochemical analysis of alpha(v)beta(6) protein expression and bronchoalveolar analysis of TGF-beta pathway signaling indicates activation of the alpha(v)beta(6)/TGF-beta pathway only at later time points after lung fibrosis was already established in the TGF-alpha model. To determine the contribution of the alpha(v)beta(6)/TGF-beta pathway on the progression of established fibrotic disease, TGF-alpha transgenic mice were administered Dox for 4 wk, which leads to extensive fibrosis; these mice were then treated with a function-blocking anti-alpha(v)beta(6) antibody with continued administration of Dox for an additional 4 wk. Compared with TGF-alpha transgenic mice treated with control antibody, alpha(v)beta(6) inhibition significantly attenuated pleural thickening and altered the decline in lung mechanics. To test the effects of genetic loss of the beta(6) integrin, TGF-beta transgenic mice were mated with beta(6)-null mice and the degree of fibrosis was compared in adult mice following 8 wk of Dox administration. Genetic ablation of the beta(6) integrin attenuated histological and physiological changes in the lungs of TGF-alpha transgenic mice although a significant degree of fibrosis still developed. In summary, inhibition of the beta(6) integrin led to a modest, albeit significant, effect on pleural thickening and lung function decline observed with TGF-alpha-induced pulmonary fibrosis. These data support activation of the alpha(v)beta(6)/TGF-beta pathway as a secondary effect contributing to TGF-alpha-induced pleural fibrosis and suggest a complex contribution of multiple mediators to the maintenance of progressive fibrosis in the lung.