Hypoxia-Inducible Factor-1α is upregulated in Natural Killer cells by Interleukin-2 and hypoxia via PI3K/mTOR signaling pathway

Abstract Natural killer (NK) cells have cytotoxic functions that aid in the recognition and killing of tumors. Hypoxia is a major characteristic of the tumor microenvironment (TME) and cellular adaptive responses are mediated by hypoxia inducible factors (HIFs). This study investigates how activated NK cells adapt to hypoxia through the upregulation of HIF-1α. Both NK cell lines, NKL and NK92, upregulated HIF-1α when exposed to hypoxia but only when stimulated with IL-2. NKL cells activated with IL-2 and exposed to 5% O2 did not increase expression of HIF-1α. Treatment with translational inhibitor cycloheximide (CHX) blocked HIF-1α expression indicating that the IL-2 mediated signaling stimulates HIF-1α protein synthesis under hypoxia. The synthesis of HIF-1α through IL-2 induced signaling via the PI3K/mTOR pathway was essential for HIF-1α expression by NK cells under hypoxia. Treatment of PI3K inhibitor LY294002 or mTOR inhibitor Rapamycin blocked the expression of HIF-1α, while MAPK inhibitor PD98059 had no effect on IL-2 stimulated NKL cells. Importantly, NKL cells exposed to hypoxia demonstrated a reduced anti-tumor cytotoxicity against the colorectal cancer cell line DLD-1 in a real time impedance-based cytotoxicity assay (xCelligence RTCA). Similar results were obtained with NK-92 cells and primary NK cells derived from human PBMCs. Increase in HIF-1α expression under hypoxia were observed in NK cells treated with IL-15, another common cytokine receptor gamma chain cytokine. Together, these data illustrate that the activation of the PI3K/mTOR signaling pathway through the common cytokine receptor gamma chain is critical for HIF-1α upregulation in NK cells under hypoxia and provide a molecular basis for reduced NK cell functions.