Different effects of ribosome biogenesis inhibition on cell proliferation in retinoblastoma protein- and p53-deficient and proficient human osteosarcoma cell lines

Objectives: To evaluate the effects of rRNA synthesis inhibition on cell cycle progression and cell population growth according to the RB and p53 status. Material and methods: RB and p53-proficlent U20S cells and the RB- and p53-deficient SAOS-2 cells were used, rRNA transcription hindered by actinomycin D, and cell cycle analysed by flow cytometry. Results: One hour of actinomycin D treatment induced in U20S cells a block at the cell cycle checkpoints G(1)-S and G(2)-M, which was removed only after rRNA synthesis was resumed. rRNA synthesis inhibition did not influence cell cycle progression in SAOS-2 cells. No effect on cell cycle progression after actinomycin D-induced rRNA inhibition was also found in U20S cells silenced for RB and p53 expression. A mild perturbation of cell cycle progression was observed in U20S cells silenced for the expression of either RB or p53 alone. We also treated U20S and SAOS-2 cells with actinomycin D for 1 h/day for 5 days. This treatment lightly reduced growth rate of the U20S cell population, whereas cell population growth of SAOS-2 cells was completely inhibited. A marked reduction of ribosome content occurred in SAOS-2 cells after the long-term actinomycin D treatment, whereas no modification was observed in U20S cells. Conclusions: These results demonstrate that inhibition of ribosome biogenesis does not hinder cell cycle progression in RB- and p53-deficient cells. A daily-repeated transitory inhibition of ribosome biogenesis leads to a progressive reduction of ribosome content with the consequent extinction of cancer cell population lacking RB and p53.