Targeting Oncoprotein Translation with Rocaglates in MYC-Driven Lymphoma

Background: c-MYC (MYC) is commonly dysregulated in aggressive B cell lymphomas. MYC associated lymphoma, especially Double Hit lymphoma (DHL) and Double-Expression Lymphoma (DEL) which are characterized by MYC and BCL2 dual overexpression usually present with the inferior outcome as rapid disease progression and poor response to standard chemotherapy regimen. Nevertheless, MYC is considered as an "undruggable" target and targeting strategies such as suppressing MYC transcription by bromodomain (BRD)-4 inhibitors have been widely investigated in both preclinical models and clinical trials. However, increasing evidence has shown that lymphoma cells displayed a wide range of resistance to BRD-4 inhibition, due to transcription adaptation or kinome reprogramming. Hence, alternative approaches for suppressing MYC or its function are urgently needed. Strategies directed against oncoprotein translation may efficiently repress key oncoproteins regardless of the abundant MYC mRNA due to genetic aberrations and secondary transcription up-regulated by MYC. Rocaglate is a class of natural products derived from plants of the Aglaia genus that have been demonstrated to potently inhibit protein translation initiation via eIF4A. The use of rocaglates for anti-cancer treatment was limited due to the scarcity and instability of these natural products e.g. Silvestrol. Recent chemical modification and screening studies have unveiled a few synthetic rocaglates that are more potent than Silvestrol such as SDS-1-021-(−), which unlocked the potential use of rocaglates for clinical applications.

Methods and Results: To probe effective reagents for MYC-driven lymphoma, a screening of 50 drugs targeting common oncogenic pathways and tumorigenic machinery was performed in two isogenic B-lymphoma lines. Several protein translation inhibitors, such as mTOR kinase inhibitors (TORKi), and eIF4A inhibitor Silvestrol were identified as the most potent drugs in all of the tested cells. Further, we found that Silvestrol but not TORKi efficiently repressed MYC protein translation in MYC-driven B lymphoma cells, whereas neither of them inhibited BCL2 expression. Moreover, we demonstrated that eIF4E knockdown or eIF4E/eIF4G disruptor Briciclib did not significantly affect MYC expression, whereas eIF4A inhibitor hippuristanol and rocaglates derivate SDS-1-021-(−) diminished MYC expression similar to that observed in Silvestrol treated cells. By using dual luciferase assay, we demonstrated that rocaglates had stronger cap-dependent and IRES translation inhibition than TORKi in lymphoma cells. Next, by native RNA immunoprecipitation and siRNA knocking down, we found that rocaglates repressed MYC translation not only via eIF4A1 but also via eIF4A2, however, with different underlying mechanisms. Furthermore, to explore the molecular targets of rocaglates treatment in B cell lymphoma, we performed TMT-Mass Spectrometry which identified multiple oncoproteins including NEK2, MYC, MCL1, TCF3, BCL6, PLK1, AURKA, and WEE1 were significantly down-regulated by SDS-1-021-(−) treatment. Finally, we demonstrated that SDS-1-021-(−) is highly potent as a single agent and synergized with ABT199 at a low dose (0.2mg/kg) in PDX models with DHL/DEL.

Brief summary: Our pre-clinical study provided strong evidence that rocaglates but not TORKi efficiently suppress MYC protein translation because 1). Rocaglates exhibit strong inhibition on both Cap- and IRES-dependent translation, 2). Rocaglates decrease PLK1 and AURKA/B thus destabilizing MYC protein. The synthetic rocaglate SDS-1-021-(−) is a potent agent that exhibits significant synergistic killing effect with ABT199 on DHL/DEL cells in the pre-clinical animal study.

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