Hypoxia-inducible factor-1 (HIF-1) is one of the most promising pharmacological targets for all types of cancer, including ovarian cancer. Ovarian clear cell carcinoma (OCCC) has poor prognosis because of its insensitivity to chemotherapy. To elucidate the characteristics of this troublesome cancer, we examined HIF-1α expression under normoxia or hypoxia in various ovarian cancer cell lines. HIF-1α was highly expressed under normoxia only in RMG-1, an OCCC cell line. To examine whether HIF-1 is involved in the tumorigenesis of RMG-1 cells, we established HIF-1α-silenced cells, RMG-1HKD. The proliferation rate of RMG-1HKD cells was faster than that of RMG-1 cells. Furthermore, the activity of MEK/ERK in the Ras pathway increased in RMG-1HKD cells, whereas that of mTOR in the PI3K pathway did not change. Activation of the Ras pathway was attributable to the increase in phosphorylated MEK via PP2A inactivation. To confirm the crosstalk between the PI3K and Ras pathways in vivo, RMG-1 or RMG-1HKD cells were transplanted into the skin of nude mice with rapamycin (an inhibitor of mTOR), PD98059 (an inhibitor of MEK), or both. RMG-1HKD cells showed higher sensitivity to PD98059 than that observed in RMD-1 cells, whereas the combination therapy resulted in synergistic inhibition of both cells. These findings suggest that inhibition of HIF-1, a downstream target of mTOR in the PI3K pathway, activates the Ras pathway on account of the increase in MEK phosphorylation via PP2A inactivation, and the crosstalk between the 2 pathways could be applied in the combination therapy for HIF-1-overexpressing cancers such as OCCC.
Keywords: 4E-BP, eukaryotic initiation factor 4E-binding-protein; ERK, extracellular signal-regulated kinase; HIF-1; HIF-1, hypoxia-inducible factor-1; MEK inhibitor; MEK, mitogen-activated kinase/extracellular signal-regulated kinase kinase; OCCC, ovarian clear cell carcinoma; OE, ovarian endometrioid carcinoma; OM, ovarian mucinous carcinoma; OS, ovarian serous carcinoma; PI3K pathway; PI3K, phosphatidylinositol 3-kinase; PP2A; PP2A, protein phosphatase 2A; RCC, renal cell carcinoma; RCCC, renal clear cell carcinoma; Ras pathway; S6K, p70 ribosomal S6 kinase; mTOR; mTOR, mammalian target of rapamycin; ovarian clear cell carcinoma; pVHL, von Hippel–Lindau protein.