Normal human plasma contains procoagulant albumin (PC-Al), an anionic form of albumin that induces tissue factor (TF) activity in human umbilical vein endothelial cells (HUVEC) and monocytes. In this study, we investigated both the interactions between HUVEC and PC-Al and the mechanism by which PC-Al induces TF activity. Binding of PC-Al to HUVEC was specific and reversible. Further studies indicated that membrane- bound PC-Al was not internalized by HUVEC. A potential receptor on HUVEC was suggested by studies in which the capacity of a variety of reagents to inhibit the activity of PC-Al was quantitated. Induction of TF activity by PC-Al was antagonized by dextran sulfate, heparin, fucoidan, and concanavalin A but not by ovalbumin, polyglutamic acid, or polyvinyl sulfate. This competition profile bears similarities to those reported for scavenger receptors that have been identified on both HUVEC and monocytes. Involvement of protein kinase C (PKC) in the PC-Al-induced enhancement of TF activity was suggested by experiments in which staurosporine, an inhibitor of PKC, suppressed the activity of PC-Al. The induction of TF activity by PC-Al was further characterized by using a quantitative polymerase chain reaction assay. Increased TF mRNA was first seen after 1 hour of incubation with PC-Al. Maximal observed expression occurred at 2 hours, but at 5 hours, expression had significantly decreased. Monocytes could also be induced to express TF mRNA after a 2-hour incubation with PC-Al. These results suggest that the functionally relevant binding of PC-Al to HUVEC may be mediated through interactions with a membrane constituent that has some of the properties of a scavenger receptor and that this interaction augments TF activity by enhancing transcription of TF mRNA, at least in part, by a mechanism that is dependent on activation of PKC.

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