Platelet activating factor causes a rapid increase in activity of prior expressed tissue factor on monocyte surface membrane

It is known that tissue factor (TF) activity depends on cell membrane phosp holipids. However, the mechanism involved in the regulation of TF activity by the modulation of the phospholipids has not yet been described in detail . To determine whether some mediators regulate TF activity by such a mechan ism, we investigated the effect of platelet activating factor (PAF). Additi on of PAF to TF-expressed monocytes caused a rapid and marked increase in t he activity, but no increase in the antigen. Kinetic analyses were performe d on TF-expressed monocytes with or without the addition of PAF, and on pur ified TF The former revealed that the activity enhancement by PAF was assoc iated with reduced Km, with Vmax remaining unaltered. The latter showed tha t the additional phosphatidylserine produced greater TF activity in purifie d TF, with an alteration pattern of kinetic parameters similar to that obse rved in the addition of PAF From these results, we conclude that PAF regula tes TF activity at the cell surface by alteration of the phospholipid compo sition of the membrane, and not by fresh production of TF apoprotein. The r ole of PAF as described in this paper must be one of the major regulatory s ystems in TF activity. (C) 1999 The Japanese Society of Hematology.