Microenvironment and Immunology MiR-10b Downregulates the Stress-Induced Cell Surface MoleculeMICB, a Critical Ligand for Cancer Cell Recognition by Natural Killer Cells

Natural killer cells (NK) are a component of innate immunity well known for their potent ability to kill virusinfected or neoplastically transformed cells following stimulation of the NK cell receptor NKG2D. One of the various ligands of NKG2D isMICB, a stress-induced ligand that has been found to be upregulated on the surface of tumor cells. However, there is little knowledge about how this upregulation may occur or how it may be selected against in tumors as a mechanism of immune escape. Here, we report that the metastasis-associated microRNA (metastamir) miR-10b directly binds to the 30 untranslated region of MICB and downregulates its expression. Notably, antagonizing miR-10b action enhanced NKG2D-mediated killing of tumor cells in vitro and enhanced clearanceof tumors in vivo. Conversely, overexpressionofmiR-10bdownregulatedMICBand impairedelimination of tumor cells. Together, our results define MICB as a novel immune target of miR-10b, implying a direct link between metastasis capability and immune escape from NK cells. Cancer Res; 72(21); 5463–72. 2012 AACR. Introduction MiRNAs are short noncoding RNA molecules that usually repress gene expression, by binding to their target mRNAs [mainly in the 30 untranslated region (UTR)] and either repress translation, or cause mRNA degradation (1, 2). The miRNAs' effect ismoderate; nevertheless, thesemolecules are important gene regulators (3–5) and it is estimated that the activity of more than 50% of all cellular genes is controlled bymiRNAs (5). MiRNAs can be either beneficial or detrimental to the developing tumors as they could serve either as tumor suppressors or as tumor initiators (6–10). MiRNAs are also involved in the metastatic process and the capacity of several miRNAs to initiate and to promote metastasis formation set the term "metastamir" (11–13). The most prominent metastamirs are: miR-10b, miR-21, miR-210, miR-373, and miR-520d (11–14). Among these miRNAs, miR-10b is probably the most famous one as it promotes both invasion and metastasis in various types of cancers by targeting multiple genes (15–18). Surprisingly, miR-10b is also expressed in the majority of normal tissues (19). Furthermore, it is significantly downregulated in the initial process of breast cancer initiation and is later upregulated in tumor metastases (20, 11). These observations suggest that miR-10b also functions under normal conditions; however, its role under these conditions is poorly understood. Developing tumors are sensed by both innate and adaptive immune cells (21). Natural killer (NK) cells, which are part of the innate immune system, are known for their capacity to kill various tumor cells (22, 23). NK cell activity is controlled by a balance of signals derived from inhibitory and activating receptors (24, 25). One of the most powerful activating receptors expressed by NK cells (and also by subsets of T cells) is NKG2D (26, 27). This receptor recognizes stress-induced ligands that appear on the cell surface following various stresses, such as viral infection and cell transformation (28). The human stress-induced ligands include the major histocompatibility complex class I polypeptide–related sequences A and B (MICA and MICB, respectively) and the UL16-binding proteins (ULBP) 1–6 (28–30). Numerous mechanisms were developed by tumors and by viruses to escape NKG2D-mediated recognition, emphasizing the importance of this receptor (29, 31–35). For example, several distinct cellular and viral miRNAs were shown to target MICB, MICA, and ULBP3 to evade the NKG2D-mediated elimination (31–35). The identification of miRNA targets is a difficult task. This difficulty arises because miRNAs are not fully matched with their target mRNAs and because the exact mechanisms controlling miRNA-mRNA interactions are not fully understood. Most algorithms that were so far developed to predict miRNA targets are very inaccurate with a false positive rate of about 65% (36). Because several of the cellular MICB/A-targeting miRNAs are overexpressed in tumors (35) and as NKG2D plays an important role in tumor cell recognition, we wondered whether tumors might use metastamirs not only to promote the Authors' Affiliation: Lautenberg Center for General and Tumor Immunology, The HebrewUniversity, The BioMedical Research Institute, Hadassah Medical School, Jerusalem, Israel Note: Supplementary data for this article are available at Cancer Research Online (http://cancerres.aacrjournals.org/). Corresponding Author: Ofer Mandelboim, The Hebrew University, Ein Karem, Jerusalem, MA 91120, Israel. Phone: 972-2-6757515; Fax: 972-26424653; E-mail: [email protected]. doi: 10.1158/0008-5472.CAN-11-2671 2012 American Association for Cancer Research. Cancer Research www.aacrjournals.org 5463 on April 14, 2017. © 2012 American Association for Cancer Research. cancerres.aacrjournals.org Downloaded from Published OnlineFirst August 21, 2012; DOI: 10.1158/0008-5472.CAN-11-2671