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Cellular response to DNA damage from a potent carcinogen involves stabilization of p53 without induction of
Q A Khan1, K H Vousden, A Dipple
1Chemistry of Carcinogenesis, ABL-Basic Research Program, NCI-Frederick Cancer Research and Development Center, MD 21702, USA.
Abstract:
The effect of a potent mammary carcinogen, anti benzo[g]chrysene 11,12-dihydrodiol 13,14-epoxide, on the progress of human mammary carcinoma MCF-7 cells through the cell cycle was investigated. While these cells, which express wild-type p53, were arrested in G1 after treatment with actinomycin D (a positive control), treatment with the mammary carcinogen did not cause G1 arrest but instead delayed the cells in the DNA synthesis phase. In concert with the absence of a G1 arrest, it was found that though both chemical treatments led to increased levels of p53, only the p53 induced by actinomycin D was transcriptionally active and increased the levels of the cyclin dependent kinase inhibitor, p21(waf1/cip1). Since treatment of the cells with the mammary carcinogen did not abrogate the G1 arrest induced by actinomycin D, the lack of p21(waf1/cip1) and of G1 arrest, resulting from treatment with the mammary carcinogen alone, was not due to some general inhibition of transcription or translation. An analogous difference between these two chemicals was demonstrated also in other human cell systems. The stealth-like property of the mammary carcinogen that allows it to damage DNA without turning on the cells' 'guardian of the genome' defense mechanism presumably increases the likelihood of malignant change because DNA replication continues on a damaged template. It is suggested that this stealth characteristic may be a major contributor to the high carcinogenic potency of this mammary carcinogen and possibly to that of other highly potent carcinogens.
Insights
A potent mammary carcinogen delays cancer cells in the DNA synthesis phase, bypassing the G1 arrest. This occurs because the carcinogen damages DNA without activating the p53
Area of Science:
- Molecular Biology
- Cell Biology
- Carcinogenesis
Background:
- The cell cycle is tightly regulated by checkpoints to prevent DNA replication errors.
- The p53 protein acts as a 'guardian of the genome', halting the cell cycle upon DNA damage.
- MCF-7 cells, a human mammary carcinoma cell line, express wild-type p53.
Purpose of the Study:
- To investigate the effect of a potent mammary carcinogen, anti benzo[g]chrysene 11,12-dihydrodiol 13,14-epoxide, on the cell cycle progression of MCF-7 cells.
- To compare the cellular response to this carcinogen with that of actinomycin D, a known G1 arrest agent.
Main Methods:
- MCF-7 cells were treated with the mammary carcinogen or actinomycin D.
- Cell cycle progression was monitored.
- Levels of p53 and p21(waf1/cip1) were assessed.
- Transcriptional activity of p53 was evaluated.
Main Results:
- The mammary carcinogen, unlike actinomycin D, did not induce G1 arrest but delayed cells in the DNA synthesis (S) phase.
- Both treatments increased p53 levels, but only actinomycin D-induced p53 was transcriptionally active, leading to increased p21(waf1/cip1).
- The carcinogen's effect was not due to general inhibition of transcription or translation.
Conclusions:
- The mammary carcinogen possesses a 'stealth-like' property, damaging DNA without activating the p53-mediated G1 arrest.
- This evasion of cellular defense mechanisms allows DNA replication to proceed on a damaged template, potentially increasing malignant transformation.
- This characteristic may significantly contribute to the high carcinogenic potency of this and other potent carcinogens.