Hypoxia occurs in association with cancer development, the result being a more aggressive and metastatic cancer phenotype. Hypoxia, which activates hypoxia-inducible factor-1 alpha (HIF-1α), is associated with a number of cellular changes including increased apoptotic resistance. The authors hypothesized that HIF-1α is central to the cell's ability to resist apoptosis induced during the hypoxia selection process.
PWR-1E, LNCaP, LNCaP-HOF, PC-3, and DU-145 cells were cultured in normoxic and hypoxic conditions. Apoptosis was assessed by propidium iodide DNA staining. Cleavage of specific substrates was used to assess caspase activity and Western blotting was used to assess mitochondrial release of cytochrome c and second mitochondria-derived activator caspase (SMAC)/Diablo. A dominant negative HIF-1α construct was transfected into the PC-3 and LNCaP cells to block HIF-1α activity.
PC-3 and DU-145 were resistant to apoptosis induced by exposure to hypoxia, but the PWR-1E and LNCaP cells were susceptible. This induction of apoptosis in the LNCaP cells was caspase dependent but independent of cytochrome c release. Blocking the activity of HIF-1α had no effect on increased apoptotic susceptibility in the PC-3 cells. LNCaP-HOF cells, which were resistant to hypoxia-induced apoptosis, showed no increase in HIF-1α expression or activity.
Apoptotic resistance is already established in cells that survive a hypoxic insult and whereas increased HIF-1α activity may be essential for the development of a more aggressive cancer phenotype, it may not be responsible for the initial selection of an apoptotic resistance phenotype. Cancer 2005. © 2005 American Cancer Society.