FIBROBLASTS RESTRICT TISSUE FACTOR FROM VESICLES WHICH FORM IN RESPONSE TO LOW CONCENTRATIONS OF DETERGENT

Studies of tissue factor activity on fibroblasts have found that manif estation of the otherwise cryptic activity is evoked by Triton X-100 o r octyl glucoside at concentrations that lyse the cells. Even though s ublytic concentrations of the detergents extract membrane lipids into the soluble phase, they were without effect on tissue factor activity. Those experiments led us to conclude that either the fibroblasts main tain plasma membrane lipid asymmetry even as lipids are extracted by t he detergents, up to the onset of lysis, or additional mechanisms for regulation of tissue factor specific activity were operative. Using ph ase contrast and immunofluorescent microscopy, we now show that at lea st one additional regulatory mechanism is indeed operative. In respons e to sublytic concentrations of octyl glucoside or Triton X-100, the c ells release vesicles from which tissue factor antigen is excluded. Ly tic concentrations of the detergents preclude this segregation, leavin g only low amounts of tissue factor antigen associated with the adhere nt cytoskeletons. Two-color staining reveals marked tissue factor-acti n filament co-localization, which implies the potential for cytoskelet al participation in the observed tissue factor segregation. We propose that tissue factor activity is indeed regulated by the phospholipids with which it is associated and the degree to which phosphatidylserine is available on the membrane surface, but the cells possess additiona l mechanisms by which the association of tissue factor with potentiall y procoagulant membrane domains is controlled.