FIG. 9.
Overexpression of cyclins D1, E, and A in c-myc−/− cells. (A) Analysis of protein expression. HO15.19 (c-myc−/−) cells were infected singly with retrovirus vectors, and clonal cell lines were established. Immunoblotting was performed with exponentially growing cells. Cell lines are designated above each lane. HO15LX1 and HO15LX4 are two clones infected with empty LXSH vector. The exponential-phase doubling times for each clone are indicated below the lanes. The antibodies are indicated to the left of each panel. The cyclin D1 and cyclin A antibodies recognize both the human and rodent proteins; the cyclin E antibody is human specific. (B) Phosphorylation of Rb in cyclin D1-overexpressing cells. Cells were rendered quiescent by serum starvation and subsequently stimulated to reenter the cell cycle by serum addition. The zero time point is the time of serum addition. Samples were taken at the time points indicated above each lane and analyzed by immunoblotting. Cell lines and antibodies are indicated on the left. Equal amounts of protein were loaded in each lane.