Selective counteractive effect of RARα antagonist on TM upregulation by ATRA in NB4 (A) and U937 (B) cells. NB4 and U937 cells were incubated with ATRA (0.01 μmol/L) for 24 hours (lane 2). These cells were also preincubated with 1 μmol/L Ro41-5253 (lane 3) or LE540 (lane 4) for 1 hour. Lane 1 shows control without treatment.