Augmented expression of cytosolic phospholipase A2 during phenotypic transformation of cultured type II pneumocytes

Abstract

Over time in culture, rat type II alveolar epithelial cells (AEC) demonstrate increased levels of unesterified arachidonic acid (AA) and increased prostanoid synthesis, while assuming certain morphological and biochemical characteristics of the type I cell phenotype. The objective of this study was to elucidate the enzymatic mechanism(s) responsible for increased AA accumulation in this model. Cells were examined both early in culture (2 days), when they retained type II cell features, and later in culture (7 days), when they are known to express a number of type I cell characteristics. An increase in AA levels at day 7 persisted despite inhibition of AA reacylation, suggesting that differences in deacylation were responsible for differences in free fatty acid levels. These differences in deacylation were not explained by differing susceptibilities to hydrolysis of radiolabeled endogenous lipids from day 2 and day 7 cells. The phospholipase A2 (PLA2) activities at both days in culture were qualitatively similar and typical of the recently described high-molecular-mass cytosolic PLA2 (cPLA2), but activity in day 7 cytosol was threefold greater than that present in day 2 cytosol. A neutralizing anti-cPLA2 antibody reduced the PLA2 activity in day 7 cytosol to the level found in day 2 cytosol. Immunoblot analysis failed to detect expression of low-molecular-mass PLA2 proteins but confirmed that expression of the 97-kDa cPLA2 was greater in day 7 cytosol than in day 2 cytosol. These results indicate that increased levels of unesterified AA in AEC with phenotype altered during culture are due to augmented steady-state expression of cPLA2 and suggest for the first time that expression of cPLA2 is differentiation dependent.