INHIBITION OF PROTEIN KINASE CK2 AND BCL-2: A NOVEL THERAPEUTIC STRATEGY TO ANTAGONIZE MANTLE CELL LYMPHOMA CELL GROWTH

ABSTRACT

Background: Mantle cell lymphoma (MCL) is a B-cell tumor which often relapses. BCR inhibitors (Ibrutinib, Acalabrutinib) and antiapoptotic BCL2-family members blockers BH3-mimetics (Venetoclax, ABT-199) are effective drugs to fight MCL. However, the disease remains incurable, due to therapy resistance, even to the promising Venetoclax and Ibrutinib combination. Therefore, there is a profound need to explore novel useful therapeutic targets. CK2 is a S/T kinase overexpressed in several solid and blood tumors. We demonstrated that CK2, operating through a 'non-oncogene addiction' mechanism promotes tumor cell survival, and counteracts apoptosis, by activating pro-survival signaling cascades, such as NF-κ B, STAT3 and AKT. CK2 could regulate also BCL2 family members. The CK2 chemical inhibitor CX-4945 (Silmitasertib, Sil) is already under scrutiny in clinical trials in relapsed multiple myeloma, solid tumors and COVID-19. Aims: In this work, we tested the effect of CK2 chemical inhibition or knock down on Venetoclax (Ven)-induced cytotoxicity in MCL pre-clinical models to effectively reduce MCL cell growth and clonal expansion. Methods: CK2 expression and BCR/BCL2 related signaling components were analyzed in MCL cells and control cells by Western blotting. CK2 and BCL2 inhibition was obtained with Sil and Ven, respectively and with CK2 gene silencing through the generation of anti-CK2 shRNA IPTG-inducible MCL cell clones. Survival, apoptosis, mitochondrial membrane depolarization and proliferation were investigated by FACS analysis of AnnexinV/PI and JC-10 staining. The synergic action of Ven and Sil was analyzed by the Chou-Talalay combination index (CI) method. CK2 knock down in vivo was obtained in xenograft NOD-SCID mouse models Results: CK2 inactivation (with Sil or CK2 silencing) determined a reduction in the activating phosphorylation of S529 p65/RelA and S473 and S129 AKT, important survival cascades for MCL. Sil or CK2 silencing caused BCL2 and related MCL1 protein reduction, causing cell death. Importantly, we confirmed these results also in an in vivo xenograft mouse model of CK2 knockdown in MCL. Sil +Ven combination increased MCL cell apoptosis, as judged by the augmented frequency of Annexin V positive cells and expression of cleaved PARP protein, and JC-10 mitochondrial membrane depolarization, with respect to the single treatments. Captivatingly, Sil or CK2 gene silencing led to a substantial reduction of the Ven-induced increase of MCL-1, potentially counteracting a deleterious Ven-induced drawback. Analysis of cell cycle distribution confirmed an increased frequency of apoptotic cells in the sub G1 phase in CK2-silenced cells and a modulation of the other phases of the cell cycle. Remarkably, the calculated CI less than 1 suggested a strong synergic cell-killing effect between Sil and Ven, on all the cell lines tested, including those less sensitive or resistant to Ven Summary/Conclusion: We demonstrated that the simultaneous inhibition/knock down of CK2 and BCL2 synergistically cooperates in inducing apoptosis and cell cycle arrest of MCL malignant B-lymphocytes and has the potential of reducing MCL clonal growth, also counterbalancing mechanism of resistance that may arise with Ven. Therefore, CK2 is a rational therapeutic target for the treatment of MCL to be tested in combination with Ibrutinib or Ven.