Caveolin-1 expression and caveolae biogenesis during cell transdifferentiation in lung alveolar epithelial primary cultures

Caveolae are omega-shaped invaginations of the plasmalemma possessing a cyt oplasmic membrane protein coat of caveolin. Caveolae are present in the in vivo alveolar epithelial type I (ATI) lung cell, but absent in its progenit or, the alveolar epithelial type II (ATII) cell. In primary culture ATII ce lls grown on a plastic substratum acquire with time an ATI-"like" phenotype . We demonstrate that freshly isolated rat ATII cells lack caveolae and exp ression of caveolin-1 (a critical caveolae structural protein). As the ATII cells acquire an ATI-like phenotype in primary culture caveolin-1 expressi on increases, with caveolin-1 signal at 192 h postseeding up to 50-fold gre ater than at 60 h; caveolae were morphologically evident only after 132 h. When maintaining the differentiated ATII phenotype with time, i.e., culture upon collagen with an apical interface of air, a temporal increase in cave olin-1 expression was not observed, with only very faint signals evident ev en at 192 h postseeding; at no time did these cultures display caveolae. In late primary ATII cultures caveolin-1 expression and caveolae biogenesis o ccur as a function of in vitro transformation from the ATII to the ATI-like phenotype. The results have broad implications for the in vitro study of t he role of caveolae and caveolin in alveolar epithelial cell biology. (C) 1 999 Academic Press.