Resistance to BET protein inhibitors is orchestrated by Protein Kinase CK2 in Mantle Cell Lymphoma

Mantle Cell Lymphoma (MCL) is a subtype of B-cell non-Hodgkin lymphoma associated with poor prognosis and high relapse rate. Despite the introduction of novel therapeutic agents, relapse/resistance may occur; therefore, it is crucial to define new therapeutic targets. Bromodomain-Containing Protein 4 (BRD4) is a member of the BET (Bromodomain and Extra- terminal domain) protein family that acts as a transcriptional co-activator and epigenetic regulator. BRD4 plays a crucial role in driving the transcription of genes involved in cell cycle progression and growth, including c-Myc and pro-survival proteins associated with B cell receptor (BCR) signaling. Protein kinase CK2, a serine/threonine kinase, supports MCL cell viability by maintaining chronic activation of BCR-dependent survival pathways, such as NF-κB and PI3K/AKT. Phosphorylation of BRD4 by CK2 enhances the transcription of critical genes involved in cell cycle regulation (e.g., c-Myc) and BCR-mediated signaling. Thus, CK2 may serve as a molecular bridge between BCR signaling and chromatin remodeling. There are several CK2 chemical inhibitors available, including CX-4945 (the most studied) and the novel compound SGC-CK2-1. BET protein inhibitors (BETi) are a class of epigenetic drugs that selectively target BET family proteins, by binding to their bromodomains and preventing their interaction with acetylated histones. This inhibition disrupts the transcription of oncogenes and survival-related genes, thereby suppressing tumor growth. Examples of (BETi) include JQ-1 and INCB054329, both of which have shown anti-proliferative effects in cancer models. BETi are promising therapeutic opportunities for several cancer types, for which clinical trials are ongoing. It has been demonstrated that JQ-1 or INCB054329 could overcome ibrutinib resistance in MCL. However, resistance to BETi may arise. Therefore, in this study, we developed a BET inhibitor (JQ-1) resistant MCL cell line (JQ-1R) to investigate underling mechanisms of resistance and identify potential therapeutic targets. We performed the molecular characterization of JQ-1 R cells, focusing on the analysis of key intracellular survival signaling pathways, some of which are known to be modulated by CK2, including the PI3K/AKT pathway, the expression of anti-apoptotic Bcl 2 family members, the oncogene c-Myc, and β-catenin, which are critically involved in lymphoma cell survival, proliferation, and drug resistance. By comparing signaling profiles between parental and JQ-1 R MCL cells, we aimed to determine whether CK2-regulated pathways could contribute to the resistant phenotype; moreover, we explored whether CK2 inhibition with CX-4945 or SGC-CK2-1 could overcome or reverse resistance to BETi. We found that the JQ-1 R cells exhibited elevated expression of CK2 and increased phosphorylation/activation of some downstream targets such as PI3K/AKT, NF-κB, β-catenin and Bcl 2. CK2 inhibition of JQ-1 R MCL cells with CX-4945 or SGC-CK2-1, by disrupting selected survival signaling pathways, reduced JQ-1 R cells viability and increased apoptosis, overcoming JQ-1 resistance and restoring drug sensitivity. Hence, CK2 may serve as a promising therapeutic target to counteract resistance to BETi, offering a potential novel treatment strategy for MCL patients.