(A) Dose dependence of tRA-induced NB-4 aggregation. NB-4 cells were exposed to increasing concentrations (0 to 1,000 nmol/L) of tRA for 72 hours. (B) Time dependence of tRA-stimulated NB-4 aggregation. NB-4 cells were exposed to two concentrations of tRA (100 nmol/L and 1,000 nmol/L) for 24, 48, 72, and 168 hours. Homotypic aggregation was measured as described in Materials and Methods. The graph (A) and histogram (B) depict the percent aggregation ± SD. Data represent results from three separate experiments in duplicate.