Resistance to BET inhibitors in lung adenocarcinoma is mediated by casein kinase phosphorylation of BRD4

Abstract

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.

Post-Translational Modifications of BRD4: Therapeutic Targets for Tumor

“…In the JQ1-resistant lung adenocarcinoma (LAC) cell line, the phosphorylation of BRD4 was increased, and CK2 was identified as its kinase (63). This suggests that CK2 phosphorylation of BRD4 may be related to the JQ1-resistant lung adenocarcinoma (LAC) cell line (63).…”

“…In the JQ1-resistant lung adenocarcinoma (LAC) cell line, the phosphorylation of BRD4 was increased, and CK2 was identified as its kinase (63). This suggests that CK2 phosphorylation of BRD4 may be related to the JQ1-resistant lung adenocarcinoma (LAC) cell line (63). Further studies showed that JQ1 combined with CK2 inhibitor (CX-4945) could effectively induce the death of lung adenocarcinoma cells and determined the CK2 phosphorylation of BRD4 as a potential target to overcome this cancer resistance (63,64).…”

BET Proteins as Attractive Targets for Cancer Therapeutics

“…Possible mechanisms of resistance to BETis encompass AMPK-ULK1mediated autophagy in AML [76,77], NF-κB in colorectal cancer [78], PP2A phosphatase and BCL2L1/BCL-X in breast cancer [79], the GLI2-dependent Hedgehog pathway in pancreatic cancer [80] and kinome reprogramming in ovarian cancer [81], among others. Further studies demonstrated multiple mechanisms of resistance to BETis in solid tumours, including triple-negative breast cancer (TNBC), CRPC, and lung cancer [82][83][84]. The multitude of resistance mechanisms in diverse cancer models indicates that sensitivity to BETis might be cancer-cell-type-dependent.…”

Discovery of Novel Dual-Target Inhibitor of Bromodomain-Containing Protein 4/Casein Kinase 2 Inducing Apoptosis and Autophagy-Associated Cell Death for Triple-Negative Breast Cancer Therapy

“…30,31 The latest study in 2021 suggested that the drug resistance mechanism of BET inhibitors varies with the cancer type, and inhibiting the phosphorylation of BRD4 by casein kinase 2 (CK2) is a potential strategy to overcome drug resistance in cancer patients. 32 CK2 is upregulated in various types of cancers, presenting a vital function in essential biological processes. It is considered as a potential therapeutic target for cancers.…”

“…BRD4 inhibitors are capable of inhibiting the transcription of oncogenes in TNBC, serving as promising therapeutic agents. , BRD4 inhibitors have drawn significant attention in recent years. Multiple BET inhibitors have been reported, including JQ1 ( 1 ), I-BET726 ( 2 ), RVX-208 ( 3 ), mivebresib ( 4 ), ZL0516 ( 5 ), ZL0454 ( 6 ), GSK046 ( 7 ), SJ018 ( 8 ), GSK789 ( 9 ), and LT052 ( 10 ) (Figure A), and clinical trials with some BRD4 inhibitors evaluating the therapeutic efficacy against cancers are in progress. ,, A growing number of BRD4 inhibitors have been discovered and applied to the treatment of TNBC, but the emergence of drug resistance was one of the reasons that greatly limited the clinical application. , The latest study in 2021 suggested that the drug resistance mechanism of BET inhibitors varies with the cancer type, and inhibiting the phosphorylation of BRD4 by casein kinase 2 (CK2) is a potential strategy to overcome drug resistance in cancer patients …”