RA induces phosphorylation of rpS6 in NB-4 cells, but not in RA-resistant NB-4.007/6 (NB-4R) cells. (A) NB-4 or NB-4.007/6 cells were treated with RA (1 μM) for the indicated times. Equal amounts of total cell lysates were analyzed by SDS-PAGE and immunoblotted with an antibody against the phosphorylated form of the rpS6 on Ser235/236. (B) The blot shown in panel A was stripped and reprobed with an anti-rpS6 antibody. (C) The same blot, shown in panels A and B, was stripped and reprobed with an anti-tubulin antibody. (D) NB-4 or NB-4.007/6 cells were treated with RA (1 μM) for the indicated times. Equal amounts of total cell lysates were analyzed by SDS-PAGE and immunoblotted with an antibody against the phosphorylated form of the rpS6 on Ser240/244. (E) The blot shown in panel D was stripped and reprobed with an anti-rpS6 antibody. (F) The same blot, shown in panels D and E, was stripped and reprobed with an anti-tubulin antibody.