Human dihydrofolate reductase is a substrate of protein kinase CK2α.

Dihydrofolate reductase (DHFR) is a prominent molecular target in antitumor, antibacterial, antiprotozoan, and immunosuppressive chemotherapies, and CK2 protein kinase is an ubiquitous enzyme involved in many processes, such as tRNA and rRNA synthesis, apoptosis, cell cycle or oncogenic transformation. We show for the first time that CK2α subunit strongly interacted with and phosphorylated DHFR in vitro. Using quartz crystal microbalance with dissipation monitoring (QCM-D) we determined DHFR-CK2α binding kinetic parameters (Kd below 0.5 µM, kon = 10.31 × 104 M-1s-1 and koff = 1.40 × 10-3s-1) and calculated Gibbs free energy (-36.4 kJ/mol). In order to identify phosphorylation site(s) we used site-directed mutagenesis to obtain several DHFR mutants with predicted CK2-phosphorylable serine or threonine residues substituted with alanines. All enzyme forms were subjected to CK2α subunit catalytic activity and the results pointed to serine 168 as a phosphorylation site. Mass spectrometry analyses confirmed the presence of phosphoserine 168 and revealed additionally the presence of phosphoserine 145, although the latter phosphorylation was on a very low level.

Synthesis, in vitro antiproliferative activity and kinase profile of new benzimidazole and benzotriazole derivatives.

Protein kinase 2 (CK2), a member of the serine/threonine kinase family, has been established as a promising target in anticancer therapy. New derivatives of known CK2 inhibitors 4,5,6,7-tetrabromo-1H-benzimidazole (TBBi) and 4,5,6,7-tetrabromo-1H-benzotriazole (TBBt) bearing azide or substituted triazole groups were synthesized. Their influence on the activity of human recombinant CK2α and cytotoxicity against normal and cancer cell lines were evaluated. TBBi derivatives with triazole substituted with carboxyl substituent (7 and 10) exhibited the most potent inhibitory activity against CK2 with Ki value in the range of 1.96-0.91µM, respectively. New TBBi derivatives 2, 3, 5 and 9 have demonstrated the EC50, in the range of 12-25µM and 13-29µM respectively towards CCRF-CEM and MCF-7 cells. Derivatives TBBi decreased viability of cancer cells more efficiently than BALB cells and the biggest differences were observed for the azide substituted compounds 3 and 5. The effect of the most active compounds on the activity of eight off-target kinases was evaluated. Inhibitory efficiency of CK2-mediated p65 phosphorylation was demonstrated for the TBBi and compound 12.

Inhibition of Protein Kinase CK2 Affects Thymidylate Synthesis Cycle Enzyme Level and Distribution in Human Cancer Cells.

Thymidylate synthase (TS), dihydrofolate reductase (DHFR), and serine hydroxymethyltransferase (SHMT) constitute the thymidylate synthesis cycle providing thymidylate for DNA synthesis and repair. Our previous studies indicated that TS and DHFR are the substrates of protein kinase CK2. This work has been aimed at the elucidation of the effect of CK2 activity on cell cycle progression, thymidylate synthesis enzyme expression and localization, and the role of CK2-mediated TS phosphorylation in in vitro di- and trimolecular complex formation. The results were obtained by means of western blot, confocal microscopy, flow cytometry, quantitative polymerase chain reaction (QPCR), quartz crystal microbalance with dissipation monitoring (QCM-D), and microthermophoresis (MST). Our research indicates that CK2 inhibition does not change the levels of the transcripts; however, it affects the protein levels of DHFR and TS in both tested cell lines, i.e., A549 and CCRF-CEM, and the level of SHMT1 in CCRF-CEM cells. Moreover, we show that CK2-mediated phosphorylation of TS enables the protein (pTS) interaction with SHMT1 and leads to the stability of the tri-complex containing SHMT1, DHFR, and pTS. Our results suggest an important regulatory role of CK2-mediated phosphorylation for inter- and intracellular protein level of enzymes involved in the thymidylate biosynthesis cycle.

Simultaneous Inhibition of Protein Kinase CK2 and Dihydrofolate Reductase Results in Synergistic Effect on Acute Lymphoblastic Leukemia Cells.

BACKGROUND/AIM: Recently, we demonstrated the ability of inhibitors of protein kinase 2 (casein kinase II; CK2) to enhance the efficacy of 5-fluorouracil, a thymidylate synthase (TYMS)-directed drug for anticancer treatment. The present study aimed to investigate the antileukemic effect of simultaneous inhibition of dihydrofolate reductase (DHFR), another enzyme involved in the thymidylate biosynthesis cycle, and CK2 in CCRF-CEM acute lymphoblastic leukemia cells. MATERIALS AND METHODS: The influence of combined treatment on apoptosis and cell-cycle progression, as well as the endocellular level of DHFR protein and inhibition of CK2 were determined using flow cytometry and western blot analysis, respectively. Real-time quantitative polymerase chain reaction was used to examine the influence of silmitasertib (CX-4945), a selective inhibitor of CK2 on the expression of DHFR and TYMS genes. RESULTS: The synergistic effect was correlated with the increase of annexin V-binding cell fraction, caspase 3/7 activation and a significant reduce in the activity of CK2. An increase of DHFR protein level was observed in CCRF-CEM cells after CX-4945 treatment, with the mRNA level remaining relatively constant. CONCLUSION: The obtained results demonstrate a possibility to improve methotrexate-based anti-leukemia therapy by simultaneous inhibition of CK2. The effect of CK2 inhibition on DHFR expression suggests the important regulatory role of CK2-mediated phosphorylation of DHFR inside cells.

Effect of Simultaneous Inhibition of Protein Kinase CK2 and Thymidylate Synthase in Leukemia and Breast Cancer Cells.

BACKGROUND/AIM: Protein kinase CK2 was recently identified as a promising therapeutic target for combination therapy. Our study aims to investigate the anticancer effect of a simultaneous inhibition of thymidylate synthase (TS) and CK2 in MCF-7 breast cancer and CCRF-CEM leukemia cells. MATERIALS AND METHODS: The type of interaction between CK2 inhibitors: CX-4945, 4,5,6,7-tetrabromo-1H-benzimidazole (TBBi), or recently obtained 4,5,6,7-tetrabromo-2-methyl-1H-benzimidazol-1-yl)acetonitrile (2b) and TS-directed anticancer drug, 5-fluorouracil (5-FU) was determined using the MTT assay and a combination index method. The influence of the combined treatment on apoptosis in leukemia cells, as well as on cell-cycle progression and the levels of TS, CK2α and P-Ser529-p65 were determined in both cell lines, using flow cytometry and western blot analysis, respectively. RESULTS: The best synergistic effect was observed in CCRF-CEM cell line with the combination of 5-FU and 2b which correlated with a decrease in the endocellular CK2 activity and enhancement of the pro-apoptotic effect. CONCLUSION: The obtained results demonstrate the ability of CK2 inhibitors to enhance the efficacy of 5-FU in anticancer treatment, indicating a different molecular mechanism of the studied CK2 inhibitors interaction with 5-FU.