Myc Represses miR15a/Mir-16-1 Expression through Recruitment of HDAC3 In Mantle Cell Lymphoma

Abstract 3121

MicroRNAs (miR)-15a and miR-16-1 are known to act as tumor suppressors. Expression of these miRNAs inhibits cell proliferation, promotes apoptosis of cancer cells, and suppresses tumorigenicity by targeting multiple oncogenes. Loss or down-regulation of these miRNAs has been reported in a variety of hematopoietic and solid tumors. Our recent study demonstrated that miR-15a/16-1 are down-regulated in the majority of patients with mantle cell lymphoma (MCL, Zhao et al., Blood. 2010 1;115(13):2630-9). However, the mechanism responsible for miR-15a/16 suppression was unknown. Here, we have investigated mechanism of miR-15a/16-1 repression and its transcriptional and epigenetic regulation by Myc and histone deacetylase (HDAC) in MCL. Over-expression of Myc protein was detected in all MCL cell lines, and miR-15a/16 and DLEU-2 mRNA were significantly up-regulated in Mino and Jeko-1 cells when Myc was depleted by c-Myc siRNA. Treatment with pan HDAC inhibitor, SAHA and specific HDAC-3 shRNA resulted in increase of miR15a/16 expression. Co-immunoprecipitation study showed that c-Myc interacted with HDAC3. Moreover, chromatin immunoprecipitation (ChIP)demonstrated that both Myc and HDAC3 co-localized to the two promoters of miR-15a/16 cluster gene, DLEU-2. Luciferase reporter assay confirmed that both c-Myc and HDAC3 inhibited transcriptional activity of two E-box (Myc-binding)-specific DLEU-2 promoters. Computational analysis and mutagenesis study further supported direct association of Myc with miR-15a/16 promoter resulting in HDAC3-mediated transcriptional repression. These findings highlight an important role for HDAC-3 in Myc-driven lymphoma cell proliferation and reveal novel transcriptional and posttranscriptional mechanisms in Myc-mediated malignant transformation in MCL.