POLARIZED SECRETION OF FIBRINOGEN BY LUNG EPITHELIAL-CELLS

The lung epithelium has recently been identified as a novel site of fi brinogen (FBG) biosynthesis. A coordinated upregulation of A alpha, B beta, and gamma chain FBG gene transcription occurs upon stimulation o f A549 lung epithelial cells with dexamethasone (DEX) and the proinfla mmatory mediator interleukin-6 (IL-6). Subsequently, the cells synthes ize and secrete fully assembled FBC. This study addresses the polarity of such FBG secretion by A549 cells cultured on polycarbonate membran e filters. After induction with IL-6 and DEX, cells were metabolically labeled, and FBG was immunopurified from the apical and basolateral c hambers. Analysis by gel electrophoresis revealed that A549 cells secr eted newly synthesized FBG in a polarized manner, with the majority (8 0%) of FBG secreted basolaterally. Consistent with this observation, i mmunoelectron microscopy using Protein A-gold labeling showed FBG with in secretory vesicles in close proximity to the basolateral aspect of the A549 cell membrane. Polarized secretion was microtubule-dependent since depolymerization using colchicine significantly reduced the baso lateral component of secretion, causing intracellular retention of FBG . These data provide evidence that FBG is secreted by lung alveolar ep ithelial cells vectorially toward the basement membrane, which may ref lect in vivo processes associated with local injury, inflammation, and repair mechanisms.