Nuclear factor-kB(NF-kB)and hypoxia-inducible factor-1(HIF-1)are regulators in the tumorigenesis and therefore become important molecular targets in the development of anticancer drugs.In the search for NF-kB and HIF-1 inhibitors from natural origins and chemical library,we found that cucurbitacin B showed the potent inhibitory activity against HIF-1 activation induced by hypoxia in various human cancer cell lines.This compound markedlydecreased the hypoxia-induced accumulation of HIF-1α protein dose-dependently,whereas it did not affect the expressions of HIF-1βand topoisomerase-I.Further analysis revealed that cucurbitacin Binhibited HIF-1α protein synthesis,without affecting the expression level of HIF-1α mRNA ordegradation 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.Furthermore,cucurbitacin B prevented hypoxia-induced expression of HIF-1 target genes and suppresses the invasiveness of tumor cells.