Abstract 764: Transient up-regulation of Myc in B-cell lymphomas enhances p53-independent apoptotic responses to chemotherapy.

Abstract

Abstract The c-MYC oncogene deregulation is a common trait in human cancers, with programmed cell death (or apoptosis) among Myc-regulated events. Myc is well-known to induce apoptosis in a p53-dependent manner, via the Myc→ARF ┤MDM2 ┤p53 pathway. We had previously shown that this pathway couldbe exploited to increase sensitivity to anti-cancer drugs. Specifically, small increases in Myc levels obtained by inhibiting Myc-targeting microRNA-34a resulted in enhanced apoptotic response of B-cells to bortezomib (Sotillo et al, Oncogene 2010). Because many Myc-driven tumors are defective for p53 activity, we sought to determine whether transient up-regulation of Myc also increases cell death in response to chemotherapeutic drugs in a p53-deficient background. For this purpose, we used B-lymphoma cells isolated from bone marrows of p53ER™ knock-in mice and subsequently transduced with a Myc-expressing retrovirus (Amaravadi et al, J Clin Inv 2007, Yu et al, Blood 2007). This inducible system allowed us to distinguish between p53-dependent (cells treated with tamoxifen) and independent (no tamoxifen added) cell death. Additionally, instead of modulating Myc levels through miRNA inhibitors, which currently have limited clinical utility, we increased Myc stability with a small molecule inhibitor of GSK3β, which promotes proteosomal degradation of Myc. We observed that the activation of p53 via the addition of tamoxifen is sufficient to induce robust apoptosis, with neither modulation of Myc levels nor additional treatment with doxorubicin being necessary. On the other hand, in the absence of active p53 doxorubicin was needed to kill Myc/p53ER™ cells, but cell killing was rather inefficient. Remarkably, pharmacological stabilization of Myc strongly enhanced p53-independent doxorubicin-induced apoptosis. This enhancement was accompanied by selective activation of pro-apoptotic genes BIM, BAX and BAK, but not PUMA or NOXA, suggesting that the response was indeed p53-independent. To validate this effect in a more relevant setting, we inhibited GSK3β in various Burkitt's lymphoma cell lines and achieved consistent up-regulation of Myc. Most importantly, even in p53mut Ramos cells, Myc stabilization resulted in sharply increased responses to doxorubicin. Our results suggest that transient up-regulation of Myc could be a viable adjuvant therapy even for tumors with p53 loss or MDM2 amplification. Citation Format: Elena Sotillo-Piñeiro, Andrei Thomas-Tikhonenko. Transient up-regulation of Myc in B-cell lymphomas enhances p53-independent apoptotic responses to chemotherapy. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 764. doi:10.1158/1538-7445.AM2013-764