Thrombin generation by apoptotic vascular smooth muscle cells

Thrombin activation requires assembly of a prothrombinase complex of activated coagulation factors on an anionic phospholipid surface, classically provided by activated platelets, We have previously shown that anionic phosphatidylserine is exposed by rat vascular smooth muscle cells (VSMCs) undergoing apoptosis after serum withdrawal. In this study, using a chromogenic assay, we have shown thrombin generation by apoptotic VSMCs expressing c-myc (VSMC-myc) with an area under the thrombin-generation curve (AUG) of 305 +/- 17 nmol.min/L and a peak thrombin (PT) of 154 +/- 9 nmol/L. The thrombin-generating potential of the apoptotic VSMC-myc cells was greater than that of unactivated platelets (P = .003 for AUG; P = .0002 for PT) and similar to calcium-ionophore activated platelets (AUG of 332 +/- 15 nmol min/L, P = .3; PT of 172 +/- 8 nmol/L, P = .2). Thrombin activation was also seen with apoptotic human VSMCs (AUG of 211 +/- 8 nmol.min/L; PT of 103 +/- 4 nmol/L) and was inhibited by annexin V (P < .0001 for AUC and PT). VSMC-myc cells maintained in serum generated less thrombin than after serum withdrawal (P = .0002 for AUC and PT), VSMCs derived from human coronary atherosclerotic plaques that apoptose even in serum also generated thrombin (AUG of 260 +/- 2 nmol.min/L; PT of 128 +/- 4 nmol/L). We conclude that apoptotic VSMCs possess a significant thrombin-generating capacity secondary to phosphatidylserine exposure, Apoptotic cells within atherosclerotic plaques may allow local thrombin activation, thereby contributing to disease progression. (C) 1997 by The American Society of Hematology.