Cyclin B1 is both necessary and sufficient for γ-radiation–induced apoptosis.

(A-C) Cyclin B1-antisense oligonucleotides inhibit γ-radiation–induced apoptosis. Ramos cells were treated with 5 μmol/L sense or antisense oligonucleotides and subjected to γ-irradiation 4 hours later. Four hours following γ-irradiation, (A) cyclin B1 protein levels were measured by Western blot, indicating that antisense oligonucleotides, but not the sense control, prevent cyclin B1 protein accumulation following γ-irradiation. Untreated cells were irradiated but not exposed to sense or antisense oligonucleotides. (B) Apoptosis, as measured by Annexin staining (quadrants 2 and 4), is decreased by antisense treatment relative to the sense control. Results are representative of three experiments. (D-E) Ectopic cyclin B1 expression is sufficient to induce apoptosis. Ramos cells were transfected with 5 μg of either a control pCDNA3 plasmid or cyclin B1. (D) Transfection of cyclin B1 (white profile), relative to the pCDNA3 control (black profile), resulted in an increase of apoptosis, as measured by an increase in the number of Annexin staining cells. (E) Twenty-four hours after transfection with cyclin B1 or pCDNA3, cells were exposed to 200 rad of γ-radiation. Transfection of cyclin B1 increased the number of Annexin staining cells following irradiation (white profile) relative to the control pCDNA3 transfected cells (black profile).