Cell Stress Gives a Red Light to the Mitochondrial Cell Death Pathway

Although the ultimate outcome of prolonged exposure of cells to stress is often death, the early response appears to be the activation of survival pathways that are likely to give the cell an opportunity to repair low-level damage. How these stress-initiated survival pathways influence B cell lymphoma/leukemia 2(Bcl-2) proteins, the core cell death machinery, has remained unclear; however, two papers now provide insight into stress-mediated survival mechanisms. The liver is unusually resistant to p53-mediated apoptosis. It appears that p53-mediated induction of the gene that encodes insulin-like growth factor-binding protein-1 (IGFBP1) attenuates the cell death response in hepatocytes by preventing the formation of a complex between p53 and the proapoptotic protein BAK. This is especially interesting as IGFBP1 is not a member of the Bcl-2 family, yet it inhibited BAK. In three unrelated cell lines, another regulatory interaction that influences cell survival occurs at the mitochondria. In this case, protein phosphatase 1 gamma (PP1 gamma) regulated the phosphorylation status of the Bcl-2/Bcl-X-L-associated death promoter (BAD). The prefold in family member URI is normally phosphorylated by S6 kinase 1, which liberates PP1 gamma from a URI-PP1 gamma complex. However, the withdrawal of growth factors or nutrients stabilizes this complex, which renders PP1 gamma inactive. The net response of this stress stimulus is an increased abundance of phosphorylated BAD, which raises the threshold required to trigger cell death. These two studies have identified new players and mechanisms that integrate stressr esponses and cell death.