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Abstract Background Casein kinase II (CK2) is involved in multiple tumor-relevant signaling pathways affecting proliferation and apoptosis. CK2 is frequently upregulated in acute B-lymphoblastic leukemia (B-ALL) and can be targeted by the ATP-competitive CK2 inhibitor CX-4945. While reduced proliferation of tumor entities including B-ALL after CX-4945 incubation has been shown in vitro and in vivo, the detailed way of action is unknown. Here, we investigated the influence on the PI3K/AKT and apoptosis cascades in vivo and in vitro for further clarification. Methods A B-ALL xenograft model in NSG mice was used to perform in vivo longitudinal bioluminescence imaging during six day CX-4945 treatment. CX-4945 serum levels were determined at various time points. Flow cytometry of bone marrow and spleen cells was performed to analyze CX-4945-induced effects on tumor cell proliferation and distribution in B-ALL engrafted mice. ALL cells were enriched and characterized by targeted RNA sequencing. In vitro, B-ALL cell lines SEM, RS4;11 and NALM-6 were incubated with CX-4945 and gene expression of apoptosis regulators BCL6 and BACH2 was determined. Results In B-ALL-engrafted mice, overall tumor cell proliferation and distribution was not significantly influenced by CK2 inhibition. CX-4945 was detectable in serum during therapy and serum levels declined rapidly after cessation of CX-4945. While overall proliferation was not affected, early bone marrow and spleen blast frequencies seemed reduced after CK2 inhibition. Gene expression analyses revealed reduced expression of anti-apoptotic oncogene BCL6 in bone marrow blasts of CX-4945-treated animals. Further, BCL6 protein expression decreased in B-ALL cell lines exposed to CX-4945 in vitro. Surprisingly, levels of BCL6 opponent and tumor suppressor BACH2 also declined after prolonged incubation. Simultaneously, increased phosphorylation of direct CK2 target and tumor initiator AKT was detected at respective time points, even in initially pAKT-negative cell line NALM-6. Conclusions The CK2 inhibitor CX-4945 has limited clinical effects in an in vivo B-ALL xenograft model when applied as a single drug over a six day period. However, gene expression in B-ALL cells was altered and suggested effects on apoptosis via downregulation of BCL6. Unexpectedly, the BCL6 opponent BACH2 was also reduced. Interactions and regulation loops have to be further evaluated.

Abstract Background The tumor suppressor protein phosphatase and tensin homolog (PTEN) is a key regulator of the PI3K/AKT pathway which is frequently altered in a variety of tumors including a subset of acute B-lymphoblastic leukemias (B-ALL). While PTEN mutations and deletions are rare in B-ALL, promoter hypermethylation and posttranslational modifications are the main pathways of PTEN inactivation. Casein Kinase II (CK2) is often upregulated in B-ALL and phosphorylates both PTEN and DNA methyltransferase 3A, resulting in increased PI3K/AKT signaling and offering a potential mechanism for further regulation of tumor-related pathways. Methods Here, we evaluated the effects of CK2 inhibitor CX-4945 alone and in combination with hypomethylating agent decitabine on B-ALL proliferation and PI3K/AKT pathway activation. We further investigated if CX-4945 intensified decitabine-induced hypomethylation and identified aberrantly methylated biological processes after CK2 inhibition. In vivo tumor cell proliferation in cell line and patient derived xenografts was assessed by longitudinal full body bioluminescence imaging and peripheral blood flow cytometry of NSG mice. Results CX-4945 incubation resulted in CK2 inhibition and PI3K pathway downregulation thereby inducing apoptosis and anti-proliferative effects. CX-4945 further affected methylation patterns of tumor-related transcription factors and regulators of cellular metabolism. No overlap with decitabine-affected genes or processes was detected. Decitabine alone revealed only modest anti-proliferative effects on B-ALL cell lines, however, if combined with CX-4945 a synergistic inhibition was observed. In vivo assessment of CX-4945 in B-ALL cell line xenografts resulted in delayed proliferation of B-ALL cells. Combination with DEC further decelerated B-ALL expansion significantly and decreased infiltration in bone marrow and spleen. Effects in patient-derived xenografts all harboring a t(4;11) translocation were heterogeneous. Conclusions We herein demonstrate the anti-leukemic potential of CX-4945 in synergy with decitabine in vitro as well as in vivo identifying CK2 as a potentially targetable kinase in B-ALL.