Xanthine oxidoreductase is required for genotoxic stress-induced NKG2D ligand expression and gemcitabine-mediated antitumor activity.

Xiulong Xu,Geetha Rao,Yi Li

doi:10.18632/oncotarget.11042

Xiulong Xu, Geetha Rao + Show 1 more

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https://doi.org/10.18632/oncotarget.11042

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Abstract

MICA/B (the major histocompatibility antigen-related chain A and B) and Rae I are stress-inducible ligands for the immune-receptor NKG2D. Mechanisms by which genotoxic stress and DNA damage induce the expression of NKG2D ligands remain incompletely understood. Here, we report that inhibition of xanthine oxidoreductase (XOR) activity by allopurinol or inhibition of XOR expression by gene knockdown abrogated genotoxic stress-induced expression of MICA/B and Rae I in three tumor cell lines. XOR knockdown also blocked gemcitabine-mediated antitumor activity in an orthotopic syngeneic mouse model of breast cancer. As a rate-limiting enzyme in the purine catabolic pathway, XOR generates two end-products, uric acid and reactive oxygen species (ROS). ROS scavenging had an insignificant effect on genotoxic drug-induced MICA/B expression but modestly inhibited radiation-induced MICA/B expression. Exogenous uric acid (in the form of monosodium urate) induced MICA/B expression by activating the MAP kinase pathway. Allopurinol blocked genotoxic stress-induced MAP kinase activation. Our study provides mechanistic insights into genotoxic stress-induced activation of the MAP kinase pathway and suggests that XOR is required for genotoxic stress-induced NKG2D ligand expression and gemcitabine-mediated antitumor activity.

Highlights

The ligands for the NKG2D immunoreceptor include one group of retinoic acid early transcript-1 (Rae-1), H60, and Mult1 in rodents, four UL-16 binding proteins and the major histocompatibility complex (MHC) class I-related chain A and B (MICA/B) in humans
Our study provides mechanistic insights into genotoxic stress-induced activation of the mitogen-activated protein (MAP) kinase pathway and suggests that xanthine oxidoreductase (XOR) is required for genotoxic stress-induced NKG2D ligand expression and gemcitabinemediated antitumor activity
Our study provides several lines of evidence showing that uric acid production was responsible for the genotoxic stress-induced NKG2/D ligand expression: 1) Inhibition of XOR activity by allopurinol or XOR expression by XOR miRNA abrogated the genotoxic stress-induced NKG2D ligand expression, MAP kinase activation, and uric acid production; 2) Exogenous uric acid induced MHC class I-related chain A and B (MICA/B) expression; 3) Intracellular uric acid concentrations in monosodium urate (MSU)-treated cells were comparable to that in the cells exposed to genotoxic stress; 4) A375 cells that failed to uptake uric acid did not respond to MSU to induce MICA/B expression and to activate the MAP pathway

Summary

Introduction

The ligands for the NKG2D immunoreceptor include one group of retinoic acid early transcript-1 (Rae-1), H60, and Mult in rodents, four UL-16 binding proteins and the MHC class I-related chain A and B (MICA/B) in humans. These ligands bind the NKG2D receptor expressed on natural killer cells, γδ T cells, and CD8+ T cells, and play an important role in activating innate immunity [1, 2]. Activation of the MAP kinase pathway by oncogene mutations such as RAS or BRAF induces MICA/B expression in thyroid tumor cell lines [12]. Presence of DNA in the cytoplasm and activation of STING in DNA-damaged cells leads to the activation of IRF-3 transcription factor and induction of NKG2D ligand expression [21, 22]

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