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Accumulating evidence suggests that Bcl-xL, an anti-apoptotic member of the Bcl-2 family, also functions in cell cycle progression and cell cycle checkpoints. Analysis of a series of phosphorylation site mutants reveals that cells expressing Bcl-xL(Ser62Ala) mutant are less stable at the G 2 checkpoint and enter mitosis more rapidly than cells expressing wild-type Bcl-xL or Bcl-xL phosphorylation site mutants, including Thr41Ala, Ser43Ala, Thr47Ala, Ser56Ala and Thr115Ala. Analysis of the dynamic phosphorylation and location of phospho-Bcl-xL(Ser62) in unperturbed, synchronized cells and during DNA damage-induced G 2 arrest discloses that a pool of phospho-Bcl-xL(Ser62) accumulates into nucleolar structures in etoposide-exposed cells during G 2 arrest. In a series of in vitro kinase assays, pharmacological inhibitors and specific siRNAs experiments, we found that Polo kinase 1 and MAPK9/JNK2 are major protein kinases involved in Bcl-xL(Ser62) phosphorylation and accumulation into nucleolar structures during the G 2 checkpoint. In nucleoli, phospho-Bcl-xL(Ser62) binds to and co-localizes with Cdk1(cdc2), the key cyclin-dependent kinase required for entry into mitosis. These data indicate that during G 2 checkpoint, phospho-Bcl-xL(Ser62) stabilizes G 2 arrest by timely trapping of Cdk1(cdc2) in nucleolar structures to slow mitotic entry. It also highlights that DNA damage affects the dynamic composition of the nucleolus, which now emerges as a piece of the DNA damage response.

Tissue homeostasis requires a carefully-orchestrated balance between cell proliferation, cellular senescence and cell death. Cells proliferate through a cell cycle that is tightly regulated by cyclin-dependent kinase activities. Cellular senescence is a safeguard program limiting the proliferative competence of cells in living organisms. Apoptosis eliminates unwanted cells by the coordinated activity of gene products that regulate and effect cell death. The intimate link between the cell cycle, cellular senescence, apoptosis regulation, cancer development and tumor responses to cancer treatment has become eminently apparent. Extensive research on tumor suppressor genes, oncogenes, the cell cycle and apoptosis regulatory genes has revealed how the DNA damage-sensing and -signaling pathways, referred to as the DNA-damage response network, are tied to cell proliferation, cell-cycle arrest, cellular senescence and apoptosis. DNA-damage responses are complex, involving "sensor" proteins that sense the damage, and transmit signals to "transducer" proteins, which, in turn, convey the signals to numerous "effector" proteins implicated in specific cellular pathways, including DNA repair mechanisms, cell-cycle checkpoints, cellular senescence and apoptosis. The Bcl-2 family of proteins stands among the most crucial regulators of apoptosis and performs vital functions in deciding whether a cell will live or die after cancer chemotherapy and irradiation. In addition, several studies have now revealed that members of the Bcl-2 family also interface with the cell cycle, DNA repair/recombination and cellular senescence, effects that are generally distinct from their function in apoptosis. In this review, we report progress in understanding the molecular networks that regulate cell-cycle checkpoints, cellular senescence and apoptosis after DNA damage, and discuss the influence of some Bcl-2 family members on cell-cycle checkpoint regulation.

A lysosomal pathway, characterized by partial rupture of lysosomal membranes and cathepsin B activation, is activated during camptothecin (CPT)-induced apoptosis in U937 and Namalwa cancer cells. These lysosomal events occur simultaneously with mitochondrial permeabilization and caspase activation. In U937 cells, blocking mitochondrial permeability transition pore with cyclosporin A and bongkrekic acid reduces mitochondrial and lysosomal rupture, suggesting that lysosomal rupture may be dependent, in part, on mitochondrial disruption. Overexpressing bcl-xL, an antiapoptotic protein known to preserve mitochondrial functions, also impedes lysosomal and mitochondrial disruption in both cell lines, indicating signaling between the two organelles. In addition, no evidence was obtained of bcl-2-like proteins targeting lysosomes. Caspase activities, including caspase-2L, are required for lysosomal and mitochondrial disruption, and lysosomal cathepsin B slightly participates in apoptosis propagation after CPT, although not essential for apoptosis activation. Our study provides evidence for the participation of a lysosomal pathway during DNA damage-induced cell death. Our data suggest that caspase activation and mitochondrial disruption represent cell-context-specific mechanisms by which DNA damage leads to lysosomal rupture, and that lysosomal cathepsins could slightly participate in apoptosis propagation after CPT.

In response to cancer chemotherapeutic drugs, cells rapidly trigger the apoptotic program or undergo growth arrest and senescence at specific phases of the cell cycle. Mitochondrial bcl-xL plays a central role in preventing alteration of mitochondrial dysfunction, cytochrome c release, caspase activation, DNA fragmentation and apoptosis. However, its pleitropic function depends on its subcellular localization. Here, we show that in addition to its mitochondrial effect that delays apoptosis, bcl-xL colocalizes and binds to cdk1(cdc2) during G2/M cell-cycle checkpoint and its overexpression stabilizes a G2/M-arrest senescence program in surviving cells after DNA damage. Bcl-xL potently inhibits cdk1(cdc2) kinase activity, which is reversible by a synthetic peptide between the 41st amino acid and 60th amino acid surrounding of the Thr47 and Ser62 phosphorylation sites, and Asn52 deamidation site, within the flexible loop domain of bcl-xL. A mutant deleted of this region does not alter the antiapoptotic function of bcl-xL, but impedes its effect on cdk1(cdc2) activity and on the G2/M-arrest senescence program after DNA damage. The nuclear interaction of bcl-xL and cdk1(cdc2) suggests that bcl-xL is coupled to the stabilization of a cell-cycle checkpoint induced by DNA damage, and this effect is genetically distinct from its function on apoptosis.Oncogene (2007) 26, 5851–5865; doi:10.1038/sj.onc.1210396; published online 19 March 2007 [ABSTRACT FROM AUTHOR]