The effect of PPP on CDK1 activity and the effect of CDK1 inhibition on growth. (A) Serum-starved RPMI 8226 cells were treated with PPP at 1 μM. At indicated time points the cells were treated for 5 minutes with 6.7 nM IGF-1. Cell lysates were subjected to CDK1 immunoprecipitation and subsequent in vitro kinase assay using Histone H1 as a substrate. The amount of labeled H1 was quantified and expressed as fold reduction as compared with relevant IGF-1–treated time controls. Each of the 13 MM cell lines used previously were treated with CGP74512A at indicated concentrations for 48 hours followed by analysis using the resazurin assay. As in Figure 1 the cell lines were divided into 3 categories: (B) IL-6–independent, (C) IL-6–dependent, and (D) the RPMI 8226 cell line and its subclones. Four experiments were performed in triplicate, and 1 representative is shown whereby data are expressed as mean percentage of control ± SD. Error bars not visible are included within the symbols.