Clusterin is a Gene Specific Target of MicroRNA-21 in Head and Neck Squamous Cell Carcinoma. PURPOSE: MicroRNA-21 (miRNA-21) has proto-oncogenic properties, though no miRNA-21 specific targets have been found in head and neck squamous cell carcinoma (HNSCC) . Further study of miRNA-21 and its specific targets is essential to understanding HNSCC biology. EXPERIMENTAL DESIGN: miRNA expression profiles of 10 HNSCC and 10 normal mucosa samples were investigated using a custom miRNA microarray. 13 HNSCC and 5 normal mucosa primary tissue specimens underwent mRNA expression microarray analysis. To identify miRNA-21 downstream targets, oral keratinocyte cells were subjected to microarray analysis after miRNA-21 transient transfection. miRNA and mRNA expression were validated by RT-qPCR in a separate cohort of 16 HNSCC and 15 normal mucosal samples. Microarray and bioinformatics analyses were integrated to identify potential gene targets. In vitro assays looked at the function and interaction of miRNA-21 and its specific gene targets. RESULTS: miRNA-21 was upregulated in HNSCC and stimulated cell growth. Integrated analyses identified Clusterin (CLU) as a potential miRNA-21 gene target. CLU was downregulated after forced expression of miRNA-21 in normal and HNSCC cell lines. The activity of a luciferase construct containing the 3'UTR of CLU was repressed by the ectopic expression of miRNA-21. CLU was also found to be downregulated in primary HNSCC and correlated with miRNA-21 over-expression. A CLU variant 1 (CLU-1) was the predominant splice variant in HNSCC, and showed growth suppression function that was reversed by miRNA-21 over-expression. CONCLUSIONS: CLU is a specific, functional target of oncogenic miRNA-21 in HNSCC. CLU-1 isoform is the predominant growth suppressive variant targeted by miRNA-21.