Abstract
Studies have indicated that deregulated oncogene expression can result in either programmed cell death or proliferation, depending on the cellular microenvironment. However, little is known about whether oncogenic signals in themselves are able to activate a cellular apoptotic program. We have tested the hypothesis that oncogenic signals in the absence of gene expression are sufficient to induce cell death, which would indicate that constitutive expression of antiapoptotic genes is necessary for maintenance of the transformed state. Using two highly specific RNA polymerase (RNAP) II inhibitors, 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) and alpha-amanitin, which inhibit RNAP II function by two distinct mechanisms, we found that inhibition of gene expression substantially increased apoptosis in a time- and dose-dependent manner in p53+/+- and p53(-/-)-transformed mouse embryonic fibroblasts and in HeLa cells, demonstrating that this type of apoptosis does not require wild-type p53. Engineered expression of an alpha-amanitin resistance RNAP II gene rendered cells resistant to induction of apoptosis by alpha-amanitin without affecting their sensitivity to DRB, indicating that alpha-amanitin induces apoptosis solely by inhibiting RNAP II function and not by a nonspecific mechanism. DRB-induced apoptosis was independent of the cell cycle or ongoing DNA replication, since DRB induced similar levels of apoptosis in asynchronous cells and cells synchronized by collection at mitosis. Inhibition of RNAP II in untransformed cells like Rat-1 or human AG1522 fibroblasts resulted not in apoptosis but in growth arrest. In contrast, deregulated expression of c-Myc in Rat-1 cells dramatically increased their sensitivity to DRB, directly demonstrating that apoptosis following inhibition of RNAP II function is greatly enhanced by oncogenic expression. The requirement for RNAP II function to prevent oncogene-induced apoptosis implies the need for the constitutive expression of an antiapoptotic gene(s) to maintain the transformed state. The differential sensitivities of untransformed and transformed cells to induction of apoptosis by transcriptional inhibition, coupled with the finding that this type of apoptosis is independent of p53 status, suggest that inhibition of RNAP II may be exploited therapeutically for the design of successful antitumor agents.