Fig. 3. Regulation of Rb phosphorylation and cdk2 and cdk4 expression by PMA. Wild-type or PMA-resistant K562 clones were treated for 24 h with or without PMA in the presence or absence of the MEK inhibitor PD098059, and the level of phosphorylated and total Rb protein (A) was analyzed by immunoblot as described in "Materials and Methods." The analysis was also carried out in synchronized cell cultures of both wild-type K562 and C1 cells treated with or without PMA. Both phosphorylated Rb (B) and the expression level of cdk2 and cdk4 (C) were analyzed.