Abstract LB-101: The expression of death receptor 4 is positively regulated by MEK/ERK signaling and suppressed upon MEK inhibition

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Abstract Death receptor 4 (DR4) is a cell surface receptor for tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and triggers apoptosis upon ligation with TRAIL or aggregation. MEK/ERK signaling is a well-known and the best-studied effector pathway downstream of Ras and Raf. This study focuses on determining the impact of pharmacological MEK inhibition on DR4 expression and elucidating the underlying mechanism.We found that several MEK inhibitors, including MEK162, AZD6244 and PD0325901, effectively decreased the levels of DR4 protein including cell surface DR4 in different cancer cell lines while suppressing ERK1/2 phosphorylation. Accordingly, pre-treatment of TRAIL-sensitive cancer cell lines with a MEK inhibitor attenuated TRAIL-induced apoptosis. These results indicate that MEK inhibition negatively regulates DR4 expression and cell response to TRAIL-induced apoptosis. MEK inhibitors did not alter DR4 protein stability, rather decreased its mRNA levels, suggesting a transcriptional regulation of DR4 expression. In contrast, enforced activation of MEK/ERK signaling by expressing ectopic B-Raf (V600E) or constitutively activated MEK1 (MEK1-CA) or MEK2 (MEK2-CA) activated ERK1/2 and increased DR4 expression; these effects were inhibited when a MEK inhibitor was present. Moreover, enforced expression of MEK1-CA enhanced DR4promoter activity; this effect could be abolished by the presence of a MEK inhibitor. Hence it is clear that activation of the MEK/ERK kinase cascade positively regulates DR4 transcription, resulting in increased DR4 expression. Detailed promoter analysis through deletion and mutation identified the AP-1 binding site as an essential response element for MEK enhancement of DR4 transactivation. In addition, inhibition of AP-1 by knocking down c-Jun abrogated the ability of MEK1-CA to increase DR4 promoter activity and DR4 expression. These results suggest an essential role of AP-1 in mediating MEK/ERK activation-induced DR4 expression. Our findings together highlight a previously undiscovered mechanism that positively regulates DR4 expression through activation of the MEK/ERK/AP-1 signaling pathway. (WY and YTO contribute equally to this work. This study was supported by Emory Winship Cancer Institute Robbins Scholar awards to YTO and to JD and Halpern Research Scholar award to SYS. SYS is a Georgia Research Alliance Distinguished Cancer Scientist and Halpern Research Scholar) Citation Format: You-Take Oh, Weilong Yao, Jiusheng Deng, Ping Yue, Shi-Yong Sun. The expression of death receptor 4 is positively regulated by MEK/ERK signaling and suppressed upon MEK inhibition. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-101.