S-phase-coupled apoptosis in tumor suppression

Abstract

DNA replication is essential for accurate transmission of genomic information from parental to daughter cells. DNA replication is licensed once per cell division cycle. This process is highly regulated by both positive and negative regulators. Over-replication, under-replication, as well as DNA damage in a cell all induce the activation of checkpoint control pathways such as ATM/ATR, CHK kinases, and the tumor suppressor protein p53, which provide "damage controls" via either DNA repairs or apoptosis. This review focuses on accumulating evidence, with the emphasis on recently discovered Killin, that S-phase checkpoint control is crucial for a mammalian cell to make a life and death decision in order to safeguard genome integrity.

Fig. 1

RFP-Killin inhibits DNA replication in vivo. The RFP-Killin in-frame fusion protein (upper), RFP control (middle) or truncated RFP-Killin (low) expression vectors were transiently transfected into Cos-E5 cells by using FUGEN-6. Twenty-four hours after transfection, S-phase cells undergoing DNA replication were visualized after 30-min pulse label with BrdU followed by FITC-labeled anti-BrdU antibody staining (in green), under a Zeiss fluorescent microscope (×40). For cells in which the RFP-Killin (red) and BrdU signals (green) colocalized, the bulk of DNA replication foci (origins of replication) were missing in the area where the RFP-Killin foci reside. The overlay of fluorescent signals from BrdU labeling with RFP-Killin (merge) always exhibit a mutually exclusive pattern, in contrast to control cells transfected with RFP alone. DAPI was used to stain DNA (nuclei). Results shown were representative of multiple cells from at least two independent experiments

Fig. 2

A positive-feedback model showing the role of Killin in p53-mediated apoptosis. Genotoxic stress (e.g., radiation or the 5-FU treatment) is used to activate p53 via the ATR/ATM and Chk kinase checkpoint pathways, which function to sense and control DNA damage/repair. Subsequent induction of the p53 target gene, p21, will lead to growth arrest of cells that are in the G1-phase, whereas cells that have already entered the S-phase can only be stopped by Killin. The stalled replication forks caused by Killin further activates ATR/ATM and Chk1/Chk2 kinase checkpoint pathways, leading to persistent activation of p53 and induction of its apoptotic target genes (including known genes, such as PUMA, Bax, Noxa, etc., as well as those that may have yet to be identified). In contrast, this positive feedback loop cannot be activated in p21-arrested G1