Contributions of Phosphatidylserine Exposure on Leukemic Cells to Unregulated Procoagulant Activity in Acute Promyelocytic Leukemia.

Acute promyelocytic leukemia (APL) causes coagulation, which can worsen following initiation of chemotherapy and is improved or corrected by the differentiating agents, all trans retinoic acid (ATRA) and arsenic trioxide (As2O3). The relationship of procoagulant activity to phosphatidylserine (PS) exposure on leukemic cells has not been clarified, although prior studies indicate that APL cells expose tissue factor (TF). Procoagulant activity of leukemic cells was measured in a modified prothrombin time in which leukemic cells replaced thromboplastin. The presence of phosphatidylserine (PS) in neutrophils and mononuclear cells (MNC) from healthy donors and APL cells from patients was investigated by flow-cytometry and confocal microscopy. The assembly of extrinsic tenase, intrinsic tenase and prothrombinase complexes on these cells was studied using enzymatic assays employing plasma or purified proteins. Lactadherin was utilized as a probe to track PS exposure and as an agent to block exposed PS. Isolated APL cells exhibited patches that stained with lactadherin, but neutrophils and MNC did not, indicating more PS exposure on APL cells than the other two cell types. Exclusion of propidium iodide by the leukemic cells indicated that PS exposure occurs in the absence of frank apoptosis. Coagulant activity increased approx 20-fold after cells were exposed to 0.1 μM daunorubicin for 24 hr with 70% of apoptosis and decreased by 85% after 24 hr treatment with 1 μM ATRA or As2O3 with partial differentiation. Inhibition of procoagulant activity by ATRA and As2O3 corresponded to decreased PS and decrease activity for all three enzyme complexes. Lactadherin, which blocks PS binding sites, inhibited Xase and prothrombin conversion by their respective APL-assembled activating complexes. APL cells support all procoagulant reactions leading to robust thrombin formation. This ability results from concomitant PS and TF exposure at the outer leaflet of cell membrane. In contrast, less PS on neutrophils and MNC do not support the coagulation.