The Myc proto-oncogene is known to regulate p53-dependent apoptosis through the Myc -> ARF -| MDM2 -| p53 pathway. We had previously shown that this pathway could be exploited to increase sensitivity to anti-cancer drugs. Specifically, small increases in Myc levels obtained by inhibiting the Myc-targeting microRNA-34a resulted in enhanced apoptotic response of B-cells to bortezomib (Sotillo et al, Oncogene 2010). However, many Myc-driven tumors are deficient in p53 activity. Thus, we sought to determine whether transient up-regulation of Myc also increases cell death in response to chemotherapeutic drugs in p53-deficient backgrounds. We chose to study this in human Burkitt's lymphoma cell lines and murine B-lymphoid 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 latter system allowed us to distinguish between p53-dependent (cells treated with tamoxifen) and independent (no tamoxifen added) cell death. We also considered that Myc expression is regulated not only transcriptionally, most notably by the bromodomain containing protein 4 (BRD4), but also post-transcriptionally by the glycogen synthase kinase 3β (GSK3-β). GSK3-β phosphorylates Myc on its Thr58 residue, targeting it for proteasomal degradation, and inhibition of GSK3-β activity results in stabilization of Myc protein (Chung et al, J Clin Inv 2012). Using the GSK3-β inhibitor CHIR 99021, we transiently increased Myc protein levels prior to treating B-lymphoma cells with chemotherapeutic drugs. Remarkably, pharmacological stabilization of Myc strongly enhanced p53-independent doxorubicin-induced apoptosis in the p53ER™/Myc system as well as in Burkitt's lymphoma cell lines with inactive p53. This enhancement was accompanied by cleavage of caspase 8 with concomitant de-regulation of the death receptor pathway genes, indicating engagement of the extrinsic apoptotic pathway. In contrast, we did not observe any significant differences in the intrinsic pro-apoptotic genes (BAX, BAK, BIM). To determine whether chemosensitization was mediated by Myc or other GSK3-β targets, we used a BRD4 inhibitor (iBET-151) at concentrations that were sufficient to prevent the CHIR 99021-mediated increase in Myc levels but didn't grossly affect cell viability. Inhibition of Myc with iBET-151 abrogated the increase in apoptosis brought about by CHIR 99021 and doxorubicin, suggesting that Myc is key to chemosensitization by CHIR 99021. Our results suggest that Myc stabilization could be a viable adjuvant therapy, even for tumors with p53 loss or MDM2 amplification.

Citation Format: Elena Sotillo-Piñeiro, Colleen T. Harrington, Daniel Soto De Jesus, Andrei Thomas-Tikhonenko. Transient upregulation of Myc with GSK3-β inhibitors in B-cell lymphomas enhances p53-independent apoptotic responses to chemotherapy. [abstract]. In: Proceedings of the AACR Special Conference on Myc: From Biology to Therapy; Jan 7-10, 2015; La Jolla, CA. Philadelphia (PA): AACR; Mol Cancer Res 2015;13(10 Suppl):Abstract nr B33.