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-β 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 for survival 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 regulation, and identifying CK2 phosphorylation of BRD4 as a potential target to overcome resistance in this cancer.

靶向表观基因组以调节驱动癌症发展的基因表达程序已成为治疗干预的令人兴奋的途径。在这方面，已证明对染色质衔接蛋白的溴结构域和末端外（BET）家族的药理抑制作用是有效的，主要通过下调c-MYC癌基因及其下游转录程序来抑制多种癌症类型的生长。虽然最初有效，但对BET抑制剂（BETi）的耐药性通常是通过重新激活MYC表达的机制发生的。我们以前已经表明，JQ1通过抑制FOSL1抑制了肺腺癌（LAC），这表明来自不同起源和分化状态的肿瘤细胞的表观遗传环境影响BETi反应。这里，我们通过建立JQ1敏感和耐药LAC细胞系的同基因对，评估了这些差异如何影响BETi耐药机制。我们发现在LAC中对JQ1的抗性独立于FOSL1而发生，而MYC水平在抗性细胞及其经JQ1处理的亲代对等体之间保持不变。此外，尽管在抗药性上观察到上皮-间质转化（EMT），但TGF-β诱导的EMT并未在JQ1敏感的LAC系中赋予抗药性，表明这是结果，而不是模型系统中BETi抗性的驱动因素。重要的是，siRNA敲低表明JQ1抗性细胞系的存活仍然依赖于BRD4表达，我们发现BRD4的磷酸化在抗性LAC中升高，确定酪蛋白激酶2（CK2）作为介导此作用的候选蛋白。CK2的抑制以及磷酸化BRD4的下游转录靶标（包括AXL和PI3K途径的激活剂）与JQ1协同作用，以抑制BETi抗性LAC。总体而言，这表明对BETi的耐药机制因癌症类型而异，LAC细胞独立于MYC调控而发展JQ1耐药性，并将BRD4的CK2磷酸化确定为克服该癌症耐药性的潜在靶标。