Modulation of hypoxia-inducible factor-1α expression by mitochondrial NADP+-dependent isocitrate dehydrogenase.

The transcription factor hypoxia-inducible factor-1 (HIF-1) is an important regulator of the tumor response to hypoxia, including increased angiogenesis, glycolytic metabolism, and resistance to apoptosis. In the current study, small interfering RNA (siRNA)-mediated knockdown of mitochondrial NADP(+)-dependent isocitrate dehydrogenase (IDPm) suppressed hypoxia-induced stimulation of HIF-1α protein expression in PC3 human prostate cancer cells. Treatment with the 26S proteasome inhibitor MG132 failed to abrogate the suppression of HIF-1α accumulation induced by IDPm knockdown, whereas HIF-1α levels were reduced by cycloheximide treatment in both control and IDPm siRNA-transfected cells. These results suggested that the suppression of HIF-1α accumulation by IDPm knockdown in PC3 cells was due to an inhibition of HIF-1α transcription. Inactivation of the phosphoinsotide-3 kinase (PI3K)/Akt pathway decreased HIF-1α expression through inactivation of Sp1. Thus, IDPm siRNA functioned as a potentially useful agent for targeting chemo- and radio-resistant hypoxic cells within solid tumors through inhibition of HIF-1α expression.