MiR-26a Inhibits Cell Growth and Tumorigenesis of Nasopharyngeal Carcinoma through Repression of EZH2

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Abstract
Several microRNAs (miRNA) have been implicated in nasopharyngeal carcinoma (NPC), a highly invasive and metastatic cancer that is widely prevalent in southern China. In this study, we report that microRNA miR-26a iscommonly downregulated in NPC specimens and NPC cell lines with important functional consequences. Ectopic expression of miR-26a dramatically suppressed cell proliferation and colony formation by inducing G1-phase cell-cycle arrest. We found that miR-26a strongly reduced the expression of EZH2 oncogene in NPC cells. Similar to the restoring miR-26 expression, EZH2 downregulation inhibited cell growth and cell-cycle progression, whereas EZH2 overexpression rescued the suppressive effect of miR-26a. Mechanistic investigations revealed that miR-26a suppressed the expression of c-myc, the cyclin D3 and E2, and the cyclin-dependent kinase CDK4 and CDK6 while enhancing the expression of CDK inhibitors p14ARF and p21CIP1 in an EZH2-dependent manner. Interestingly, cyclin D2 was regulated by miR-26a but not by EZH2, revealing cyclin D2 asanother direct yet mechanistically distinct target of miR-26a. In clinical specimens, EZH2 was widely overexpressed and its mRNA levels were inversely correlated with miR-26a expression. Taken together, our results indicate that miR-26a functions as a growth-suppressive miRNA in NPC, and that its suppressive effects are mediated chiefly by repressing EZH2 expression. Cancer Res; 71(1); 225–33. ©2010 AACR.