Selective counteractive effect of RARα antagonist on upregulation of TM activity on leukemia cells. Changes in TM cofactor activity in protein C activation on NB4 (A) and U937 (B) cell surfaces were measured after exposure to 0.01 μmol/L ATRA (lane 2), Am80 (lane 3), or Ch55 (lane 4) for 24 hours. Cells were also incubated with Ro41-5253 (1 μmol/L), and ATRA (0.01 μmol/L) was added 1 hour later (lane 5). Basal ΔOD_405nm per minute levels were 0.033 ± 0.002/min/5 × 10⁶ NB4 cells (31 ± 2 ng activated protein C/10⁶ cells) and 0.063 ± 0.002/min/5 × 10⁶ U937 cells (61 ± 2 ng activated protein C/10⁶ cells).