Combined Targeting of Distinct c-Myc and JunB Transcriptional Programs Inducing Synergistic Anti-Myeloma Activity

INTRODUCTION

Transcription factors (TFs) are convergence points of signaling cascades that coordinate cell differentiation, proliferation and survival and are commonly deregulated in cancer, including multiple myeloma (MM). They contribute to the initiation of MM and promote tumor cell growth and drug resistance. Both cMyc, a merging point of the PI3K-, and JunB, a merging point of the MEK/MAPK-signaling pathway, play pivotal roles in MM. Exciting novel approaches to inhibit TFs like proteolysis-targeting-chimera (PROTAC) promise to lead to selective tumor cell death with little/no consequence for normal cells. However, redundancy phenomena of transcriptional programs are likely to challenge their efficacy. Here, we report our final results on combined targeting of distinct c-Myc & JunB transcriptional programs for MM therapy.

METHODS

MM cell lines and patient MM cells were analyzed. Following CRISPR-loss-of-function screens for cMyc & JunB across MM cell lines and correlation analyses in MM patient datasets, the functional relevance of BRD4/c-Myc- and MEK/JunB-induced TF programs was delineated using genomic and chemical approaches in 2D and 3D models of the bone marrow (BM) microenvironment. Specifically, effects of single or combined targeting of cMyc- and JunB-induced TF-programs were analyzed by flow cytometry, western blot, RNAseq, qPCR and luciferase assays. In vitro and ex vivo results were finally verified in a MM xenograft mouse model.

RESULTS

While CRISPR loss-of-function screens across various MM cell lines confirmed their growth dependency on cMyc and JunB, we did not observe correlative expression levels among these TFs, neither in the publicly available GSE6477 nor in the CoMMpass dataset. In contrast, a significant positive correlation was observed between Brd4 and cMyc, and MEK and JunB expression levels, respectively. The existence of two distinct Brd4/cMyc and MEK/JunB transcriptional programs in MM cells was subsequently supported by a lack of changes in cMyc mRNA/protein levels and resultant transcriptional activity upon JunB knockdown, and vice versa. Likewise, MZ-1, a novel PROTAC which targets Brd4, resulted in the inhibition of BMSC/IL-6- induced cMyc- but not JunB- upregulation. Conversely, neither the MEK inhibitor trametinib nor doxycycline-induced knockdown of BMSC/IL-6- induced JunB upregulation in TetshJunB/MM.1S cells reduced Brd4/c-Myc mRNA/protein levels. Importantly, the activity of MZ-1 and trametinib was predicted by Brd4 and JunB expression levels using mathematical models, respectively. Further, combination of MZ-1 with trametinib or JunB knockdown synergistically inhibited tumor cell proliferation, and induced cell death in a 2D and a dynamic 3D model of the MM-BM milieu. Finally, our in vitro and ex vivo results were confirmed in vivo, utilizing BMSC:TetshJunB/MM.1S vs. BMSC:TetshSCR/MM.1S-carrying NSG mice treated with MZ-1 with/without doxycycline or trametinib.

CONCLUSION

In summary, our data demonstrate for the first time the existence of non-overlapping cMyc and JunB-regulated TF programs providing a rationale for combined cMyc:JunB targeting treatment strategies in MM.