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SF2523: Dual PI3K/BRD4 inhibitor blocks tumor immunosuppression and promotes adaptive immune responses in cancer

ABSTRACT: Macrophages (MQs) are key immune infiltrates in solid tumors and serve as major drivers behind tumor growth, immune suppression, and inhibition of adaptive immune responses in the tumor microenvironment (TME). Bromodomain and extraterminal (BET) protein, BRD4, which binds to acetylated lysine on histone tails has recently been reported to promote gene transcription of pro-inflammatory cytokines, but has rarely been explored for its role in IL4-driven macrophage transcriptional programming and macrophage-mediated immunosuppression in the TME. Herein, we report that BET bromodomain inhibitor JQ1, blocks association of BRD4 with promoters of arginase and other IL4-driven macrophage genes, which promote immunosuppression in TME. Pharmacological inhibition of BRD4 using JQ1 and/or PI3K using dual PI3K/BRD4 inhibitor SF2523 (previously reported by our group as a potent inhibitor to block tumor growth and metastasis in various cancer models) suppresses tumor growth in syngeneic and spontaneous mice models; reduces infiltration of MDSCs; blocks polarization of immunosuppressive MQs; restores CD8+ T-cell activity and stimulates anti-tumor immune responses. Finally, our results suggest that BRD4 controls the immunosuppressive myeloid tumor microenvironment and provide opportunity to test BET inhibitors and dual PI3K/BRD4 inhibitors to treat cancers driven by the MQ-dependent immunosuppressive TME.

ORGANISM(S): Mus musculus

PROVIDER: GSE123154 | GEO | 2020/04/14

REPOSITORIES: GEO

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Project description:Targeting the epigenome to modulate gene expression programs driving cancer development has emerged as an exciting avenue for therapeutic intervention. Pharmacological inhibition of the bromodomain and extraterminal (BET) family of chromatin adapter proteins has proven effective in this regard, suppressing growth of diverse cancer types mainly through downregulation of the c-MYC oncogene and its downstream transcriptional program. While initially effective, resistance to BET inhibitors (BETi) typically occurs through mechanisms that reactivate MYC expression. We have previously shown that lung adenocarcinoma (LAC) is inhibited by JQ1 through suppression of FOSL1, suggesting that the epigenetic landscape of tumor cells from different origins and differentiation states influences BETi response. Here, we assessed how these differences affect mechanisms of BETi resistance through the establishment of isogenic pairs of JQ1 sensitive and resistant LAC cell lines. We found that resistance to JQ1 in LAC occurs independent of FOSL1 while MYC levels remain unchanged between resistant cells and their JQ1 treated parental counterparts. Furthermore, while epithelial-mesenchymal transition (EMT) is observed upon resistance, TGF- induced EMT did not confer resistance in JQ1 sensitive LAC lines, suggesting this is a consequence, rather than a driver of BETi resistance in our model systems. Importantly, siRNA knockdown demonstrated that JQ1 resistant cell lines are still dependent on BRD4 expression and we found that phosphorylation of BRD4 is elevated in resistant LACs, identifying casein kinase 2 (CK2) as a candidate protein mediating this effect. Inhibition of CK2, as well as downstream transcriptional targets of phosphorylated BRD4 - including AXL and activators of the PI3K pathway - synergize with JQ1 to inhibit BETi resistant LAC. Overall, this demonstrates that the mechanism of resistance to BETi varies depending on cancer type, with LAC cells developing JQ1 resistance independent of MYC re-activation, and identifying CK2 phosphorylation of BRD4 as a potential target to overcome resistance in this cancer.

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SF2523: Dual PI3K/BRD4 inhibitor blocks tumor immunosuppression and promotes adaptive immune responses in cancer

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