Related Experiment Videos
Genetic determinants that influence hypoxia-induced apoptosis
R M Alarcon1, N C Denko, A J Giaccia
1Stanford University School of Medicine, Department of Radiation Oncology, Division of Radiation & Cancer Biology, CA 94305-5152, USA.
Abstract:
The p53 tumour suppressor gene is capable of activating both death receptor and mitochondrial-signalled forms of apoptotic cell death in response to diverse stimuli. Studies have suggested that impairment of the mitochondrial-signalled Apaf/caspase 9 pathway and not the death receptor Fas pathway results in almost complete resistance to apoptotic cell death induced by a low oxygen environment. However, it is unclear how p53 signals the activation of this pathway and whether it is through already identified p53 effector genes such as the pro-apoptotic gene bax, or through novel effectors such as BNIP-3/BNIP-3L. Comparison of cell lines genetically matched at the bax, cytochrome c, apaf, caspase 9 and caspase 3 loci indicated that except for bax, all of these genes were essential for hypoxia induced apoptosis both in cell culture and in transplanted tumours. These data imply that cytochrome c plays a pivotal role in signalling cell death by apoptosis under hypoxic conditions, and that the release of cytochrome c is independent of both Bax and p53. In contrast to cytochrome c, p53 modulates the magnitude of apoptosis under hypoxic conditions, but in itself is not required for the activation of the caspase cascade.
Insights
The p53 tumor suppressor gene influences apoptosis magnitude under low oxygen, but cytochrome c release is key for initiating cell death, independent of p53 or Bax. This clarifies the mitochondrial apoptosis pathway in hypoxic conditions.
Area of Science:
- Cell Biology
- Molecular Biology
- Cancer Research
Background:
- The p53 tumor suppressor gene activates apoptosis via death receptor and mitochondrial pathways.
- Hypoxia-induced apoptosis resistance is linked to the mitochondrial Apaf/caspase 9 pathway, not the Fas pathway.
- The precise signaling mechanism of p53 in activating the mitochondrial pathway under hypoxia remains unclear, with potential roles for effector genes like BAX or novel factors like BNIP-3/BNIP-3L.
Purpose of the Study:
- To elucidate the role of p53 and its effector genes in signaling mitochondrial apoptosis under hypoxic conditions.
- To determine the involvement of specific apoptotic pathway components (Bax, cytochrome c, Apaf, caspase 9, caspase 3) in hypoxia-induced cell death.
- To investigate whether p53 is required for the activation of the caspase cascade during hypoxia.
Main Methods:
- Utilized genetically matched cell lines differing at key apoptotic loci (bax, cytochrome c, Apaf, caspase 9, caspase 3).
- Assessed the necessity of these genes for hypoxia-induced apoptosis in both cell culture and transplanted tumor models.
- Compared the signaling roles of p53, Bax, and cytochrome c in the activation of the apoptotic cascade under low oxygen.
Main Results:
- Except for Bax, all examined genes (cytochrome c, Apaf, caspase 9, caspase 3) were essential for hypoxia-induced apoptosis.
- Cytochrome c release plays a pivotal role in signaling apoptosis under hypoxic conditions.
- Cytochrome c release was found to be independent of both Bax and p53.
- p53 modulates the magnitude of apoptosis under hypoxia but is not required for caspase cascade activation.
Conclusions:
- Cytochrome c release is a critical, p53/Bax-independent event in initiating apoptosis during hypoxia.
- While p53 influences the extent of cell death, it is not essential for the core caspase cascade activation in response to low oxygen.
- The findings highlight the central role of the mitochondrial pathway and cytochrome c in cellular response to hypoxic stress.