Mild Chronic Hypoxia-Induced HIF-2α Interacts with c-MYC Through Competition with HIF-1α in Hepatocellular Carcinoma Proliferation

Abstract

Background: Hepatocellular carcinoma (HCC) has emerged as a major cause of cancer deaths globally, in which hypoxia and activated hypoxia-inducible factors (HIFs) play an important role. The sibling rivalry between HIF-1α and HIF-2α in hypoxic tumor growth and progression is still debated, including in HCC. This may be associated with the regulation of unique target genes, like c-MYC and mTOR. Methods: In the current study, twenty-six corresponding tumor- and non-tumor tissues, taken from patients with HCC, who underwent liver resection, were analyzed. In vitro, co-immunoprecipitation (Co-IP), Western blot, MTT assay, colony formation assay and Annexin V-FITC/PI staining apoptosis Assay were used to elucidate the relationship between HIF-1α and HIF-2α in hypoxic HCC cell proliferation and involved mechanism.Results: HIF-2α, but not HIF-1α, has a positive correlation with the expression of c-MYC in tumor tissues. In vitro, rapid HCC cell proliferation and increased interaction of HIF-2α/c-MYC were observed in mild chronic hypoxia. It was confirmed in situ proximity ligation assay and co-immunoprecipitation assay that although mild hypoxia up-regulated all HIF-1α, HIF-2α, and c-MYC, mTORC2-regulated HIF-2α competed with HIF-1α to bind c-MYC. HIF-2α knockdown, but not HIF-1α knockdown, decreased the expression of downstream c-MYC and mTORC1, suppressed hypoxic cell proliferation, and induced more cell apoptosis. Moreover, the inhibition of upstream protein PI3K by inhibitor Apitolisib counteracted this mechanism of adaptation to mild hypoxia hereby inducing cancer cells to apoptosis in HCC. Conclusions: In summary, this study highlights the role of HIF-2α but not HIF-1α in activating and binding c-MYC in HCC cell proliferation during mild chronic hypoxia response. PI3K/mTORC2/ HIF-2α/c-MYC axis plays a key role in this process. PI3K inhibitor Apitolisib is suggested as a potential treatment option for HCC, especially for rapidly growing HCC in mild chronic hypoxia.