BET-bromodomain proteins ‘read’ acetylated lysine residues on histone tails to modulate transcription. Bromodoman inhibitors (BRDi) are an emerging class of anti-cancer agents with selective toxicity against MYC-driven malignancies. Although BRDi downregulate oncogenic MYC transcription in certain cellular contexts, precise mechanism of apoptosis induction and the role of non-MYC targets in tumour responses remain undefined.
The Eμ-Myc model of aggressive B-cell lymphoma has been extensively used for preclinical investigations characterizing MYC-mediated lymphomagenesis and responses to novel therapeutics. We utilised Eμ-Myc lymphomas on informative genetic backgrounds to investigate mechanisms of apoptosis induction by the prototypical BRDi, JQ1.
JQ1 demonstrated potent cytostatic and pro-apoptotic activity at ‘on target’ concentrations (IC50 250-500nM at 24 hrs) that was p53-independent, but abrogated in the presence of constitutively active RAS or through blockade of the intrinsic apoptotic pathway. In contrast to prior reports in human IG-cMYC-translocated cell lines, JQ1 did not suppress Myc transcription. Moreover, ectopic MYC-expression did not rescue the apoptotic phenotype, indicating a Myc-independent mechanism of action. However, JQ1 directly engaged the intrinsic apoptotic pathway through induction of pro-apoptotic BH3-only Bim and concomitant transcriptional downregulation of anti-apoptotic BCL-2 family members, Bcl-2 and Bcl-XL. As RAS mutations facilitate anti-apoptotic signalling via PI3K and MAPK, we next evaluated the potential synergy of combining JQ1 with kinase inhibitors targeting these pathways. PI3K or AKT inhibition (with BEZ235 and MK2206 respectively), but not MEK inhibition (with MEK162), synergized with JQ1 to restore sensitivity to apoptosis in RAS-mutant tumours. Finally, JQ1 treatment of syngeneic mice bearing transplanted Eμ-Myc lymphomas resulted in robust therapeutic responses that were associated with a decreased rate of disease progression (as indicated by peripheral blood leukocytosis and bioluminescence imaging), resulting in improved overall survival.
These data highlight important Myc-independent epigenetic modulation of apoptotic and pro-survival signalling in a Myc-driven context. Further experiments evaluating the mechanisms by which PI3K pathway inhibitors and JQ1 synergise are on-going.