296 IN VITRO SYNTHESIS OF SURFACTANT ASSOCIATED APOLIPOPROTEIN A: DEVELOPMENTAL ASPECTS

Abstract

Apolipoprotein A is a major surfactant associated protein localized to Type II cells in adult lung. We now demonstrate in vitro synthesis of apo A in lung slices, adult Type II epithelial cells and fetal epithelial cells in organotypic culture. Synthesis and secretion of apo A was assessed in cells and media after 35S-methionine labelling; apo A was immunoprecipitated with antisera and identified by 2D-IEF-SDS-PAGE. Fully sialated apo A (Mr=26-36,000) was identified in media from adult rat lung slices and purified adult Type II cells. Apo A synthesis by adult Type II cells was optimal in freshly isolated cells: labelling was undetectable after 24 hrs. in culture. Intracellular forms of apo A were less acidic, Mr=30-34,000, representing partially glycosylated peptides. Apo A synthesis was undetectable in slices of fetal rat lung (day 18 of gestation). However, organotypic cultures of fetal epithelial cells isolated on day 19 of gestation, synthesized fully glycosylated forms of apo A after culture for 3-4 days. These studies demonstrate that apo A synthesis and secretion is developmentally regulated in Type II epithelial cells. Apo A synthesis is rapidly lost in primary culture of adult Type II cells but maintained in organotypic cultures of fetal cells. Major intracellular forms of apo A lack complete glycosylation and only a small fraction of cellular apo A exists as the mature secreted form.