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The apoptotic network and expression of BH3-containing proteins predict phenotypic response to BET bromodomain inhibitors

ABSTRACT: Small molecule inhibitors of the bromodomain and extraterminal (BET) family of proteins are in clinical trials for a variety of cancers, but patient selection strategies are limited. This is due in part to the heterogeneity of response following BET inhibition (BETi), which includes differentiation, senescence, and cell death in subsets of cancer cell lines. To elucidate the dominant features defining response to BETi, we carried out phenotypic and gene expression analysis of both treatment naïve cell lines and engineered tolerant lines. We found that both de novo and acquired tolerance to BET inhibition are driven by the robustness of the apoptotic response and that genetic or pharmacological manipulation of the apoptotic signaling network can modify the phenotypic response to BETi. We further identify that ordered expression of the apoptotic genes BCL2, BCL2L1, and BAD significantly predicts response to BETi. Our findings highlight the role of the apoptotic network in response to BETi, providing a molecular basis for patient stratification and combination therapies.

ORGANISM(S): Homo sapiens

PROVIDER: GSE69383 | GEO | 2016/01/07

SECONDARY ACCESSION(S): PRJNA285337

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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The apoptotic network and expression of BH3-containing proteins predict phenotypic response to BET bromodomain inhibitors

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