Hypoxia downregulates the expression of cell surface MICA without increasing soluble MICA in osteosarcoma cells in a HIF-1α-dependent manner

Abstract

Abstract. Tumor cells express NKG2D ligands on their cell surface, which are the ligands of the activating receptor, NKG2D, that is expressed on the surface of NK cells. The binding of NK cells to tumor cells through the interaction of NKG2D and its ligands induces the cytolysis of the tumor cells. In the present study, we investigated the effects of hypoxia on the expression of NKG2D ligands on the surface of human osteosarcoma cells using three cell lines. To produce hypoxic and normoxic conditions, the osteosarcoma cell lines were cultured under 1 and 20% O 2 conditions, respectively. The osteosarcoma cells expressed NKG2D ligands such as MHC class I-related chain molecules A and B (MICA and MICB) and the UL16-binding proteins 1, 2 and 3 (ULBP 1, 2 and 3). MICA was the most frequently expressed NKG2D ligand in the osteosarcoma cells. Hypoxia decreased the expression of cell surface MICA only without increasing the secretion of soluble MICA, which is produced by proteolytic cleavage of cell surface MICA. Hypoxia consistently decreased the susceptibility of the osteosarcoma cells to the cytotoxicity of the NK cells. Hypoxia induced the expression of hypoxia-inducible factor-1α (HIF-1α), and knockdown of the expression of HIF-1α using small interfering RNA increased the expression of cell surface MICA and concomitantly increased the level of soluble MICA. Hypoxia decreased the production of nitric oxide (NO) metabolites (nitrite and nitrate), thus, indicating a decreasing effect on NO production. However, a NO donor, NOC18, decreased the expression of cell surface MICA without any apparent effects on the expression of HIF-1α under both hypoxic and normoxic conditions. The present results indicate that hypoxia downregulates the expression of cell surface MICA without increasing the level of soluble MICA in a HIF-1α-dependent manner and suggest that the effects of hypoxia are not linked to the hypoxia-induced reduction of NO production.

A hypoxia-induced decrease of either MICA/B or Hsp70 on the membrane of tumor cells mediates immune escape from NK cells

“…Hypoxia has been described to induce tumor immune escape (Barsoum et al 2011;Yamada et al 2012;Barsoum et al 2014;Noman et al 2012). Herein, we show that hypoxia reduces the sensitivity of different human tumor cells to NK cell-mediated lysis by a differential downregulation of the NK cell ligands such as MICA/B and Hsp70.…”

L’hypoxie tumorale

“…[21]. Les TAM sont localisés préférentiel-lement au niveau des zones tumorales hypoxiques où, MICA à la surface cellulaire [18,19]. MICA étant un ligand du récepteur activateur NKG2D (natural killer group 2D) présent à la surface des cellules NK et des lymphocytes T, cette diminution aboutit à l'échappe-ment des cellules tumorales à la lyse par les NK et CTL [18,19].…”

“…Les TAM sont localisés préférentiel-lement au niveau des zones tumorales hypoxiques où, MICA à la surface cellulaire [18,19]. MICA étant un ligand du récepteur activateur NKG2D (natural killer group 2D) présent à la surface des cellules NK et des lymphocytes T, cette diminution aboutit à l'échappe-ment des cellules tumorales à la lyse par les NK et CTL [18,19]. Dans le contexte des mutations VHL observées dans les carcinomes du rein à cellules claires, l'inhibition de HIF-1 dans des cellules tumorales stabilisant HIF-1 et HIF-2 est associée à une résistance aux cellules NK par une diminution de l'expression des molécules du système HLA de classe I [20].…”

Hypoxic microenvironment and hepatocellular carcinoma treatment

“…NKG2D is critical in directing NK cell responses against tumors. Yamada et al [37] show that hypoxia promotes downregulation of the NKG2D ligand MICA by tumor cells via a HIF1α-dependent mechanism. Under hypoxia and in the presence of TGF-β, CD4 + T cells upregulate Foxp3 through direct binding of HIF1 to Foxp3 promoter region, inducing Treg formation and immune tolerance [38] .…”