Submitted on November 16, 2006
Revised on July 17, 2007
Integrin
6 Mediates Phospholipid and Collectin Homeostasis by Activation of Latent TGF
1
Laura L Koth1*, Byron T Alex1, Samuel Hawgood2, Michael A Nead3, Dean Sheppard1, David J Erle1, and David G Morris3
1 Department of Medicine, University of California, San Francisco, Lung Biology Center, San Francisco General Hospital, San Francisco, CA, USA,
2 Department of Pediatrics, University of California, San Francisco, San Francisco, CA, USA,
3 Department of Medicine, Yale School of Medicine, Section of Pulmonary and Critical Care Medicine, New Haven, CT, USA
* To whom correspondence should be addressed. E-mail: Laura.Koth{at}ucsf.edu.
Surfactant lines the alveolar surface and prevents alveolar collapse. Derangements of surfactant cause respiratory failure and interstitial lung diseases. The collectins, surfactant proteins A and D, are also important in innate host defense. However, surfactant regulation in the post-natal lung is poorly understood. We found that the epithelial integrin,
v
6, regulates surfactant homeostasis in vivo by activating latent TGF
. Adult mice lacking the
-subunit of
v
6 (Itgb6-/-) developed increased bronchoalveolar lavage phospholipids and surfactant proteins A and D and demonstrated abnormal appearing alveolar macrophages, reminiscent of the human disease pulmonary alveolar proteinosis. Using lung-specific expression of constitutively active TGF
1 in Itgb6-/- mice we found that TGF
1 was sufficient to normalize these abnormalities. Tgf
1-deficient mice also demonstrated increased phospholipids and surfactant proteins A and D but mice lacking the key TGF
signaling molecule, SMAD3, did not. Therefore, integrin-mediated activation of latent TGF
1 regulates surfactant constituents independent of intracellular SMAD3. In vivo increases in surfactant protein A and D were not associated with increases in mRNA for these proteins in alveolar tissue from Itgb6-/- mice. On the other hand, isolated alveolar macrophages from Itgb6-/- mice were defective in processing phospholipids in vitro, suggesting that reduced surfactant clearance contributes to altered surfactant homeostasis in these mice in vivo. These findings show that
v
6 and TGF
regulate homeostasis of phospholipids and collectins in adult mouse lungs and may have implications for anti-fibrotic therapeutics that inhibit active TGF
in the lung.