Turnover of fibronectin and laminin by alveolar epithelial cells. Dunsmore, Sarah E., Cara Martinez-Williams, Robert A. Goodman, and D. Eugene Rannels. Departments of Cellular & Molecular Physiology and Anesthesia, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033
APStracts 2:0129L, 1995.
Type II pulmonary epithelial cells in primary culture synthesize and assemble a multi-component extracellular matrix which exhibits biological activity in vitro. Simultaneously the pneumocytes degrade components of the underlying matrix, such that matrix composition may be determined by the balance of synthesis and turnover. The present work defines turnover of the specific matrix glycoproteins, fibronectin and laminin, both in the type II cell and in its extracellular matrix. Pulse-chase experiments demonstrate that both fibronectin and laminin, identified by immunoprecipitation, turn over rapidly in the cell and extracellular matrix compartments, with half -lives under 10 hours. In the cell compartment, initial rates of laminin turnover are more rapid than those of fibronectin on culture day 2, but these rates are similar on day 6. Matrix fibronectin also turns over rapidly, with similar rates on day 2 and day 6. During the chase interval, small but increasing amounts of immunoprecipitable fibronectin are detected in the medium, suggesting that a portion of the glycoprotein may be released to the extracellular compartment, rather than degraded. Alternatively, release of immunoreactive glycoprotein may involve ongoing processing and secretion of residual radiolabeled fibronectin by the cells. The results suggest that matrix composition may be determined by turnover, as well as synthesis, of its components. 

Received 10 January 1995; accepted in final form 19 July 1995.
APS Manuscript Number L8-5.
Article publication pending Am. J. Physiol. (Lung Cell. Mol.
Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 14 August 1995.