eIF4B Is a Novel Target of CAMK2G and Promotes Proliferation of Malignant Cells in Myelofibrosis
Authors: Sasaki Ken , Mizuno Hideaki , Ogawa Tadayuki , Masamoto Yosuke , Kurokawa Mineo 
Published: November 2021

Abstract
Background
Myelofibrosis (MF) is a myeloproliferative neoplasm which is associated with megakaryocytic atypia, fibrosis in the bone marrow (BM), and extramedullary hematopoiesis, followed by progressive hematopoietic failure and leukemic transformation. JAK1/JAK2 inhibitors are currently available and reduce spleen volume, improve symptoms related to MF and prolong the overall survival (OS). Although the benefits associated with JAK1/JAK2 inhibitors are well established, not all patients respond to these inhibitors, and long-term exposure to these inhibitors results in the emergence of resistant clones based on the reactivation of the JAK-STAT pathway. Therefore, new therapeutic strategies targeting other molecules can provide additional benefits to patients with MF. In the previous study, we identified Calcium/Calmodulin Dependent Protein Kinase II Gamma (CAMK2G) as a new therapeutic target for MF by performing compound screening. Furthermore, CAMK2G inhibition can overcome drug-resistance against JAK2 inhibitors. Therefore, it is important to investigate the mechanisms underlying the therapeutic effect of CAMK2G inhibition. In this study, to explore the mechanism underlying the therapeutic effect of CAMK2G inhibition, we performed the immunoprecipitation mass spectrometry to find out the protein that has the direct interaction with CAMK2G.
Methods
Quantitative Proteomic Analysis of the Target Proteins of CAMK2G
The protein complexes with FLAG-tagged CAMK2G (FLAG-CAMK2G) were trapped by anti-FLAG antibody. The eluted assay mixtures were reduced, alkylated and digested into peptides. Each peptide solution and control mixture were labeled with differential stable isotope tags. The samples were quantitatively analyzed using a Q Exactive mass spectrometer (Thermo Fisher Scientific). The spectra were searched against the SWISS-PROT databases using SEQUEST on Proteome Discoverer software 2.2 (Thermo Scientific).
Results
To reveal the , we conducted the immunoprecipitation assays by FLAG-CAMK2G or FLAG-tag alone, overexpressed in MF model cells. Because CAMK2G has kinase activity, we attempted to reveal the kinase-activity-dependent targets using unhydrolyzable ATP analog, AMP-PNP（5'-adenylyl-imidodiphosphate）that can maintain the strong-binding state of kinase with its target. The protein complexes with FLAG-CAMK2G were trapped by anti-FLAG antibody. After samples were digested into peptides and the candidate proteins were identified and quantitatively analyzed by mass spectrometry. As a result, we identified eukaryotic translation initiation factor 4B (eIF4B) as a protein that has a direct interaction with CAMK2G. eIF4B is a part of the complex involved in the initiation of translation. It has been reported that eIF4B plays a role in the translation of factors involved in anti-apoptosis (Bcl2, Bclxl, Mcl1), cell cycle (Cdc25c), and cell proliferation (c-Myc). We then checked whether knockdown of eIF4B decrease proliferation of MF model cell line. shRNA-mediated silencing of eIF4B decreased cell growth of these cells. Furthermore, the phosphorylation of eIF4B was increased by the ectopic expression of MPL W515L, one of the common mutations found in MF. Also, the phosphorylation of eIF4B was increased by the overexpression of CAMK2G. We then explored the proteins regulated by eIF4B in MF and identified that knockdown of eIF4B decreased the amount of Bcl2 protein.
Conclusion
In our study, we performed immunoprecipitation mass spectrometry and identified eIF4B as a partner protein of CAMK2G. Since CAMK2G inhibition was shown to be effective against MF in vitro and in vivo, we focused on eIF4B as a potential effector in MF. We also showed that overexpression of MPL W515L and CAMK2G phosphorylates eIF4B and that knockdown of eIF4B inhibited proliferation of MF cells. Furthermore, Bcl2 can be one of the target proteins regulated by eIF4B. Based on these, eIF4B plays an important role in MF. We further perform ribosome profiling to comprehensively understand the regulation of translation by eIF4B. Our research not only elucidate the pathogenesis of MF but also identify new therapeutic targets for MF.

Disclosures
Masamoto: MSD K.K.: Speakers Bureau; Eisai Co., Ltd.: Speakers Bureau; Otsuka Pharmaceutical Co., Ltd.: Speakers Bureau; ONO PHARMACEUTICAL CO., LTD.: Speakers Bureau; Takeda Pharmaceutical Company Limited.: Speakers Bureau; Chugai Pharmaceutical Company: Speakers Bureau; Kyowa Hakko Kirin Co., Ltd.: Speakers Bureau; Nippon Shinyaku Co., Ltd.: Speakers Bureau; AbbVie GK: Speakers Bureau; Janssen Pharmaceutical K.K.: Speakers Bureau; SymBio Pharmaceuticals: Speakers Bureau; Bristol Myers Squibb: Speakers Bureau. Kurokawa: Otsuka Pharmaceutical Co., Ltd.: Research Funding, Speakers Bureau; ONO PHARMACEUTICAL CO., LTD.: Research Funding, Speakers Bureau; Takeda Pharmaceutical Company Limited.: Research Funding, Speakers Bureau; Chugai Pharmaceutical Company: Research Funding, Speakers Bureau; Sumitomo Dainippon Pharma Co., Ltd.: Research Funding, Speakers Bureau; Kyowa Hakko Kirin Co., Ltd.: Research Funding, Speakers Bureau; Nippon Shinyaku Co., Ltd.: Research Funding, Speakers Bureau; Daiichi Sankyo Company.: Research Funding, Speakers Bureau; AbbVie GK: Research Funding, Speakers Bureau; Teijin Limited: Research Funding, Speakers Bureau; Pfizer Japan Inc.: Research Funding, Speakers Bureau; Eisai Co., Ltd.: Research Funding, Speakers Bureau; MSD K.K.: Research Funding, Speakers Bureau; Astellas Pharma Inc.: Research Funding, Speakers Bureau.