Involvement of PTEN in TPA-mediated p53-activation in mouse skin epidermal JB6 cells.

Our recent studies suggest a unique role of p53 during the early stage of cancer development. However, how p53 activation is regulated during TPA treatment remains elusive. We used murine skin epidermal JB6 promotion-sensitive (P+) and promotion resistant (P-) cells to observe differential expression of PTEN during TPA-induced p53 activation. Total PTEN expression was decreased in only P+ cells. Nuclear expression of PTEN increased and complex formation between PTEN and p53 occurred in P+ cells treated with TPA. Knocking down PTEN expression via siRNA inhibited TPA-induced Bax expression. Similar effects were seen with the p53 inhibitor, pifithrin-alpha. Cells that were transfected with siRNA to PTEN exhibited enhanced tumorigenicity. Our findings suggest PTEN mediates TPA-induced p53 activation.

Mutant p53 exhibits trivial effects on mitochondrial functions which can be reactivated by ellipticine in lymphoma cells.

Increasing evidence has shown that a fraction of the wild-type (wt) form of the tumor suppressor p53, can translocate to mitochondria due to genotoxic stress. The mitochondrial targets of wt p53 have also been studied. However, whether mutant p53, which exists in 50% of human cancers, translocates to mitochondria and affects mitochondrial functions is unclear. In this study, we used doxorubicin, a chemotherapeutic drug, to treat five human lymphoma cell lines with wt, mutant or deficient in p53, to induce p53 activation and mitochondrial translocation. Our results demonstrated that mutant p53, like wt p53, was induced upon doxorubicin treatment. Similarly, a fraction of mutant p53 also translocated to mitochondria. However, Complex I and II activities in the mitochondria were compromised only in wt p53-bearing cells after doxorubicin treatment, but not in mutant p53-bearing cells. Similarly, doxorubicin treatment caused greater cell death only in wt p53-bearing cells, but not in mutant p53-bearing cells. When p53 deficient Ramos cells were transfected with mutant p53 (249S), the cells showed resistance to doxorubicin-induced cell death and decreases in complex activities. To reactivate mutant p53 and reverse chemoresistance, ellipticine (5,11-dimethyl-6H-pyrido[4,3-b]carbazole) was used to treat mutant p53 cells. Ellipticine enhanced p53 mitochondrial translocation, decreased Complex I activity, and sensitized p53 mutant cells to doxorubicin-induced apoptosis. In summary, our studies suggest that mutations in p53 may not hinder p53's mitochondrial translocation, but impair its effects on mitochondrial functions. Therefore, restoring mutant p53 by ellipticine may sensitize these cells to chemotherapy.