Figure 2.

Activation of S-phase checkpoint by β-lapachone (A and B) is accompanied by indirect inhibition of cyclin A/CDK2 (C and D). DU145 prostate cancer cells were treated with vehicle control (A) or β-lapachone at 4 μM (B) for 4 h and incubated in drug-free medium for an additional 4 h. Cells were harvested for flow cytometry analysis (8). (C) Cyclin A/CDK2 was immunoprecipated from untreated cells, and the direct effect of β-lapachone on the kinase activity was determined with histone H1 as the substrate (8, 24). Lane 1, control; lane 2, 2 μM; lane 3, 4 μM. (D) DU145 prostate cancer cells were treated with or without β-lapachone for 4 h and incubated in drug-free medium for an additional 4 h. Nuclear extracts were prepared. Cylin A CDK2 were immunoprecipated, and the kinase activity was determined as described. Lane 1, control; lane 2, 0.5 μM; lane 3, 2 μM; lane 4, 4 μM.