The Role of Myc and the miR-17~92 Cluster in Histone Deacetylase Inhibitor Induced Apoptosis of Solid Tumors

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In recent years histone deacetylase inhibitors (HDACi’s) have emerged as promising therapeutics for cancer. While favorable responses to HDACi’s as single agents have been shown in several hematological malignancies, very little efficacy has been demonstrated in solid tumors. c-Myc (Myc), an oncoprotein commonly over-expressed in cancer, has been shown by several studies to play a critical role in HDACi-mediated cellular death. To expand upon these findings and determine the role that Myc plays in this process in solid tumors, we compared the effect of two HDAC inhibitors, SAHA and LAQ824, on the proliferation of solid tumor cell lines expressing high versus low levels of Myc. We found that cells expressing high levels of Myc were more sensitive to HDACi. In addition, there were significant differences in the type of response to HDACi treatment between the two cell types with prominent apoptosis in cells expressing higher levels of Myc while cell cycle arrest was more commonly observed in cells expressing lower levels of Myc. Interestingly, HDACi reduced the expression of Myc and one of its well-known oncogenic miRNA targets, miR-17~92 cluster, resulting in an increase in the expression of the master pro-apoptotic protein Bim. We propose that this novel mechanism may play a role in the potent anti-proliferative effects mediated by HDACi. Furthermore, these studies suggest that Myc expression could be used as a predictive biomarker to select patients with solid tumors who may be more responsive to HDACi treatment.

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