Differential expression of collagen-binding receptors in fetal rat lung cells

Interactions of cells with molecules of the extracellular matrix (ECM) are mediated via specific cell surface receptors. Because collagen is an important ECM component in the developing lung, we investigated the expression of collagen receptors by distal fetal lung epithelial cells and fibroblasts. Cell attachment experiments revealed that fibroblasts but not epithelial cells adhered to various types of collagen. With the use of subunit-specific antibodies, we demonstrated the presence of the collagen integrins alpha 1 beta 1, alpha 2 beta 1, and alpha 3 beta 1 at the fibroblast surface but only the integrin alpha 3 beta 1 on epithelial cells. Affinity chromatography on collagen-Sepharose identified only alpha 1 beta 1 and alpha 2 beta 1 as collagen-binding integrins in extracts of 125I surface-labeled fibroblasts. No collagen-binding receptors were detected in extracts of surface-iodinated epithelial cells. Message for alpha 1- and alpha 2-integrin was readily demonstrated for fibroblasts, but both mRNAs were hardly detectable in epithelial cells. In contrast, epithelial cells expressed significantly greater alpha 3 mRNA levels than fibroblasts. These data demonstrate that alpha 1 beta 1- and alpha 2 beta 1-integrins function as collagen-binding receptors in fetal lung fibroblasts. Distal fetal lung epithelial cells do not express the alpha 1 beta 1- and alpha 2 beta 1-integrins and do not adhere to collagen. The alpha 3 beta 1-integrin, which is expressed by both cell types, does not function as a collagen receptor.