ENDOGENOUS FIBRONECTIN OF BLOOD POLYMORPHONUCLEAR LEUKOCYTES - STIMULUS-INDUCED SECRETION AND PROTEOLYSIS BY CELL SURFACE-BOUND ELASTASE

In an accompanying study, we described the presence of intact fibronec tin, a large adhesive molecule, in the specific granules of blood PMNs . Secretion of fibronectin by blood PMNs is poorly understood, and the fate of this fibronectin is practically unknown. In the present study we demonstrate that nanomolar concentrations of phorbol ester or the chemoattractants fMLP, PAF, and LTB4 induce fibronectin secretion from blood PMNs. Phorbol ester induced secretion of approximately 85% of t he total fibronectin content, as well as expression of small amounts o n the cell surface of the activated PMNs. Secreted fibronectin was pro teolytically cleaved and, after 20 min, four major fragments of 150, 1 20, 90, and 80 kDa containing a midchain epitope were identified by We stern blot analysis. Kinetic studies indicated that fibronectin was ra pidly secreted as an intact molecule and that proteolysis started with in minutes and proceeded for at least I h. If cells were removed after 5 min TPA treatment, no further proteolysis of the secreted fibronect in was observed, indicating participation of cell-bound proteinases. F rom a cocktail of proteinase inhibitors, PMSF was the most active in s uppressing fibronectin proteolysis. Studies with specific peptidyl inh ibitors of human leukocyte elastase and cathepsin G, major serine prot einases of PMNs, demonstrated some inhibition with the cathepsin G inh ibitor, while the human leukocyte elastase inhibitor almost completely abolished fibronectin proteolysis. A monoclonal antibody to the elast ase had a similar effect. The results indicate that intact fibronectin is a secretory product of blood PMNs and that this endogenous adhesiv e molecule is within minutes extracellularly processed by cell surface -bound elastase. (C) 1997 Academic Press.