Hypoxia-inducing factor-1 alpha (HIF-1alpha), is a major survival factor for
tumor cells growing in a low
oxygen environment. The anti-
cancer agent
imexon binds
thiols and causes accumulation of
reactive oxygen species (ROS) in
pancreatic cancer cells. Unlike many
cytotoxic agents,
imexon is equi-cytotoxic in human MiaPaCa-2 and Panc-1 cells grown in normoxic (21% O(2)) and hypoxic (1% O(2)) conditions. Western blot analyses of
imexon-treated cells demonstrated that
imexon reduces HIF-1alpha
protein levels in both normoxic and hypoxic conditions in a time- and concentration-dependant fashion.
Gemcitabine did not similarly affect HIF-1alpha levels.
Imexon did not reduce transcription of new HIF-1alpha
mRNA, but did reduce the synthesis of new
proteins, including HIF-1alpha, measured by (35)S
methionine/
cysteine (Met/Cys) incorporation. Concurrently, the half-life of existing HIF-1alpha
protein was increased by
imexon, in association with a marked inhibition of chymotryptic activity in the
20S proteasome. The inhibition of HIF-1alpha translation was not specific, rather it was part of a general decrease in protein translation caused by
imexon. This inhibitory effect on translation did not involve phosphorylation of eukaryotic initiation factor-2alpha (eIF-2alpha) and was not closely correlated to cell growth inhibition by
imexon, suggesting that mechanisms other than
protein synthesis inhibition contribute to the drug's cytotoxic effects. In summary,
imexon blocks the translation of new
proteins, including HIF-1alpha, and this effect overcomes an increase in the stability of preformed HIF-1alpha due to
proteasome inhibition by
imexon. Because net HIF-1alpha levels are reduced by
imexon, combination studies with other drugs affected by HIF-1alpha survival signaling are warranted.