Cucurbitacin B inhibits the translational expression of hypoxia-inducible factor-1α

Cucurbitacin B is a triterpenoid compound isolated from Trichosanthes kirilowii Maximowicz, which has been used in oriental medicine for its antitumor activities. However, the mechanisms by which cucurbitacin B inhibits tumor growth are not fully understood. We here demonstrated the effect of cucurbitacin B on hypoxia-inducible factor-1 (HIF-1) activation. Cucurbitacin B showed the potent inhibitory activity against HIF-1 activation induced by hypoxia in various human cancer cell lines. This compound markedly decreased the hypoxia-induced accumulation of HIF-1α protein dose-dependently, whereas it did not affect the expressions of HIF-1β. Further analysis revealed that cucurbitacin B inhibited HIF-1α protein synthesis, without affecting the expression level of HIF-1α mRNA or degradation of HIF-1α protein. Rather, we found that suppression of HIF-1α accumulation by cucurbitacin B correlated with strong dephosphorylation of mammalian target of rapamycin (mTOR) and its effectors ribosomal protein S6 kinase (p70S6K) and eukaryotic initiation factor 4E-binding protein-1 (4E-BP1) and extracellular signal-regulated kinase-1/2 (ERK1/2), a pathway known to regulate HIF-1α expression at the translational level. Cucurbitacin B also activated Akt, a mechanistic feature exhibited by established mTOR inhibitors in many tumor cells. Furthermore, cucurbitacin B prevented hypoxia-induced expression of HIF-1 target genes and suppresses the invasiveness of tumor cells. In vivo studies further confirmed the inhibitory effect of cucurbitacin B on the expression of HIF-1α proteins, leading to a decrease growth of HeLa cells in a xenograft tumor model. These results show that cucurbitacin B is an effective inhibitor of HIF-1 and provide new perspectives into the mechanism of its anticancer activity.