Roscovitine-induced P-Ser-46 wt p53 activates p53AIP1 protein resulting in apoptosis in human breast cancer MCF-7 cells

The efficacy of distinct anti-cancer drugs used in the chemotherapy of human malignancies varies between tumor tissues and depends primarily on the ability of the therapeutic agents to simultaneously inhibit cell proliferation and to eliminate malignant cells by apoptosis. Human breast cancer MCF-7 cells are relatively resistant to conventional chemotherapy due to the lack of caspase-3 activity. Exposure of MCF-7 cells to roscovitine (ROSC), a potent CDK inhibitor, resulted in a strong inhibition of cell proliferation. Detailed analysis revealed that ROSC arrested MCF-7 cells in G2 phase of the cell cycle and concomitantly induced apoptosis. Post-incubation of G2 arrested cells for 24h in a drug--free medium did not diminish the number of the G2 cell population indicating that ROSC-mediated cell cycle arrest was prolonged. ROSC activated wt p53 tumor suppressor protein in MCF-7 cells in a time and dose-dependent manner. ROSC-mediated stabilization of p53 protein was attributable to its site-specific phosphorylation. At 4h after ROSC administration a strong phosphorylation of serine at position 46 was observed. The onset of Ser-46-p53 phosphorylation preceded the induction of apoptosis. P-Ser-46-p53 upregulated p53AIP1 protein, a component of mitochondria. Upregulated p53AIP1 protein accumulated in the cytosol and then was tranclocated into mitochondria and altered the potential of the mitochondrial membrane resulting in a release of mitochondrial proteins such as AIF and cytochrome C as well as activation of the apoptotic protease Htr2/Omi. Reconstitution of MCF-7 cells with human caspase-3 did not enhance the ROSC mediated apoptosis.