A dominant negative form of p63 inhibits apoptosis in a p53-independent manner

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Abstract

Stem cells are a source of differentiated cells in multiple tissues. If genetic alterations occur in stem cells, the problem persists and malignant cancers may arise. ΔNp63α—a homologue of the tumor suppressor p53—is exclusively expressed in proliferating undifferentiated epithelial cells and cancer cells of epidermal origin. Here, we show that ΔNp63α antagonizes DNA damage-induced apoptosis in a p53-independent manner. We found that upon cellular injury, ΔNp63α must be downregulated before apoptotic program can be activated. The 5637 cell line has abundant levels of ΔNp63α and mutant p53, and it is resistant to DNA damage-induced apoptosis. The knockdown of ΔNp63α by RNA interference sensitized these cells to apoptosis upon genotoxic insult. This suggests that ΔNp63α plays an anti-apoptotic role regardless of the p53 status. Considering the frequent mutations of p53 in tumor cells, our results provide important implications for the treatment of cancers in which p63 is amplified.

Section snippets

Materials and methods

Cell lines and cell culture. 5637 (bladder cancer) and A549 (lung cancer) cells were grown in RPMI-1640 supplemented with 10% v/v fetal bovine serum, penicillin, and streptomycin. Human foreskin keratinocytes (HFK) were isolated and cultured as described previously [11]. Human telomerase reverse transcriptase (hTERT)-introduced human mammary epithelial (HME) cells were kindly provided by Dr. William Hahn (Dana Farber Cancer Center, Boston).

Antibodies, plasmids, and chemical reagents. Antibodies

ΔNp63α is downregulated following various cellular damages that induce apoptosis

In order to test our hypothesis that ΔNp63α may antagonize p53 in tumorigenesis as observed previously during embryogenesis [2], we checked the p63 status of 15 cancer cell lines of epithelial origin by using Western blot analysis and RT-PCR. This screening revealed that A549—a non-small cell lung carcinoma line—was positive for ΔNp63α expression while the 5637 bladder carcinoma cell line was a ΔNp63α-overexpressing cell line. As compared to human foreskin keratinocytes HFK, the level of ΔNp63α

Discussion

Here, we have presented several lines of evidence suggesting that ΔNp63α plays an antiapoptotic role in the cellular response to DNA damage. Our results demonstrate that the overexpression of ΔNp63α, which has been observed in a number of epithelial cancer cell lines, makes the cells resistant to apoptotic signals on DNA damage. Defective apoptotic responses make the cells highly susceptible to acquiring new mutations that can then lead to malignancy.

The most well-known function of ΔNp63α is

Acknowledgments

We are grateful to Dr. David Kimelman (University of Washington, Seattle) for critically reading the manuscript. We also thank Dr. William Hahn (Dana Farber Institute, Boston) for kindly providing us with human mammary epithelial cells. This work was supported by a fund from the Korea Research Foundation (R01-2003-000-10396-0) and National Cancer Center (3344-20050066).

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