Abstract Rationale The tetraspanins CD151 & CD9 are known to play a prominent role in cancer progression by modulating cell adhesion and migration/invasion. CD151 is commonly overexpressed (metastasis enhancer) and tends to predict poor prognosis in many solid malignancies, whereas the metastasis suppressor CD9 is typically down-regulated which correlates with poor patient outcomes. However, progress in developing CD151 and CD9 as prognostic biomarkers & therapeutic targets has been hampered by a lack of understanding of how they are regulated in normal cells and how they are deregulated in cancers. Objective Many micro-RNAs are deregulated in prostate and breast cancers. Bio-informatic analysis has identified 250 miRNAs that potentially regulate CD151 or CD9. Therefore, we investigated whether miRNAs regulate CD151 and CD9 in a range of normal and cancerous breast and prostate cell lines in vitro. Methods & Results A dual luciferase reporter assay using the 3′UTR of CD151 and CD9 provided evidence that breast & prostate cancer cell lines endogenously express miRNA that regulate tetraspanins and that the extent of regulation varies across the cell lines. Analysis of miRNA expression in normal and cancer cell lines using miRNA arrays and qPCR has confirmed that some miRNAs which are predicted to target CD9, are up-regulated in many prostate and/or breast cancer cell lines compared to non-tumourigenic cells. Furthermore, miRNAs were identified with altered expression that were predicted to target CD151. These miRNA have also been shown to directly regulate CD151 or CD9 using the dual luciferase 3′UTR reporter assay with miRNA mimics. Conclusion Altered miRNA expression may contribute to prostate and breast cancers by manipulating tetraspanins to allow cancer progression and metastasis. Therefore, miRNA may be effective prognostic and/or therapeutic targets for prostate and breast cancers in the future. Citation Format: Danielle R. Bond, Crystal Passfield, Murray Cairns, Leonie K. Ashman, Judith Weidenhofer. Posttranscriptional regulation of tetraspanins CD151 & CD9 in breast & prostate cancers. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4364. doi:10.1158/1538-7445.AM2014-4364