Our recent study demonstrated miR-15a/16-1 downregulation in mantle cell lymphoma (MCL). Here, we investigated mechanisms of miR-15a/16-1 transcriptional repression and its epigenetic regulation by c-Myc and histone deacetylase (HDAC) in MCL. c-Myc expression was detected in MCL cell lines and in the primary MCL samples, and pri-miR-15a/16-1 mRNAs were significantly upregulated in Mino and Jeko-1 cells with c-Myc knockdown by small interfering RNAs (siRNAs). Our co-immunoprecipitation analysis showed that c-Myc interacted with HDAC3. Moreover, using chromatin immunoprecipitation, we demonstrated that both c-Myc and HDAC3 co-localized to the two promoters of the miR-15a/16-1 cluster gene, DLEU2, and inhibition of HDAC3 increased histone acetylation of the DLEU2 promoters. Luciferase reporter assay confirmed the dependence of Myc-mediated DLEU2 transcriptional repression on HDAC3. Treatment with the pan-HDAC inhibitor, suberoylanilide hydroxamic acid and HDAC3 siRNA resulted in increased miR-15a/16-1 expression. The regulatory mechanism of miR-15a/16-1 was further demonstrated in Burkitt lymphoma and Myc overexpressing cell lines. These findings highlight the role of HDAC3 in Myc-induced miR-15a/16-1 changes and reveal novel mechanisms for c-Myc-driven microRNA suppression and malignant transformation in aggressive B-cell malignancies.

A major obstacle for clinicians in the treatment of advanced prostate cancer is the inevitable progression to chemoresistance, especially to docetaxel. It is essential to understand the molecular events that lead to docetaxel resistance in order to identify means to prevent or interfere with chemoresistance. In initial attempts to detect these events, we analysed genomic differences between non-resistant and docetaxel-resistant prostate tumor cells and, of the genes modulated by docetaxel treatment, we observed Stat1 and clusterin gene expression heightened in the resistant phenotype. In this study, we provide biochemical and biological evidence that these two gene products are related. Stat1 and clusterin protein expression was induced upon docetaxel treatment of DU145 cells and highly overexpressed in the docetaxelresistant DU145 cells (DU145-DR). The increase in total Stat1 corresponded to an increase in phosphorylated Stat1. Interestingly, there was no detectable difference between DU145 and DU145-DR cells expression of total Stat3 and phosphorylated Stat3. Treatment of DU145-DR cells with small interfering RNA targeted for Stat1 not only resulted in the knockdown of Stat1 expression, but it also caused the inhibition of clusterin expression. Thus, Stat1 appears to play a key role in the regulation of clusterin. Remarkably, inhibition of Stat1 or clusterin expression resulted in the re-sensitization of DU145-DR cells to docetaxel. These results offer the first evidence that Stat1, and its subsequent regulation of clusterin, are essential for docetaxel resistance in prostate cancer. Targeting this pathway could be a potential therapeutic means for intervention of docetaxel resistance.