Abstract
The induction of senescence has emerged as a potentially important contributor to the effects of chemotherapeutic agents against tumors. We have demonstrated that depletion of CTP induced by cyclopentenyl cytosine (CPEC, NSC 375575), a specific inhibitor of the enzyme CTP synthetase, induces irreversible growth arrest and senescence characterized by altered morphology and expression of senescence-associated U+0392-galactosidase activity (SA-U+0392-gal) in MCF-7 breast cancer cells expressing wild type p53. In contrast, differentiation in the absence of senescence resulted from CPEC treatment in MDA-MB-231 breast cancer cells that express a mutated p53. Both senescence of MCF-7 cells and differentiation of MDA-MB-231 cells were prevented by repletion of CTP through the cytidine salvage pathway. Senescence in MCF-7 cells was associated with a G2 and S phase arrest, whereas differentiation in MDA-MB-231 cells was associated with arrest in G1 phase at 5 days. Mechanistic studies revealed that CTP depletion induced a rapid translocation of nucleolar proteins including nucleostemin and nucleolin into the nucleoplasm. This nucleolar stress response resulted in a sustained elevation of p53 and the p53 target genes, p21 and Mdm2, in cells with wild-type p53. Furthermore, siRNA-induced knockdown of p53 in MCF-7 cells treated with CPEC prevented cellular senescence and increased apoptotic cell death. We conclude that CTP depletion and the resulting nucleolar stress response results in a senescence-like growth arrest through activation of p53, while cells with mutated p53 undergo differentiation or apoptotic cell death.
- Received December 2, 2010.
- Revision received March 31, 2011.
- Accepted April 4, 2011.
- The American Society for Pharmacology and Experimental Therapeutics