Mitotic MELK-eIF4B signaling controls protein synthesis and tumor cell survival

Abstract

The protein kinase maternal and embryonic leucine zipper kinase (MELK) is critical for mitotic progression of cancer cells; however, its mechanisms of action remain largely unknown. By combined approaches of immunoprecipitation/mass spectrometry and peptide library profiling, we identified the eukaryotic translation initiation factor 4B (eIF4B) as a MELK-interacting protein during mitosis and a bona fide substrate of MELK. MELK phosphorylates eIF4B at Ser406, a modification found to be most robust in the mitotic phase of the cell cycle. We further show that the MELKeIF4B signaling axis regulates protein synthesis during mitosis. Specifically, synthesis of myeloid cell leukemia 1 (MCL1), an antiapoptotic protein known to play a role in cancer cell survival during cell division, depends on the function of MELK-elF4B. Inactivation of MELK or eIF4B results in reduced protein synthesis of MCL1, which, in turn, induces apoptotic cell death of cancer cells. Our study thus defines a MELK-eIF4B signaling axis that regulates protein synthesis during mitosis, and consequently influences cancer cell survival.aternal and embryonic leucine zipper kinase (MELK) is a serine/threonine kinase with potential roles in mitosis. Similar to other established mitotic factors, such as Aurora kinases and cyclin B1, MELK demonstrates increased protein abundance during mitosis and is degraded when cells progress into G1 phase (1, 2). Our recent study proposed an essential role of MELK in the mitotic progression of specific cancer cell types, with MELK knockdown resulting in multiple mitotic defects, including G2/M arrest and mitotic cell death (2). Despite these advances, there is a lack of mechanistic understanding of the role of MELK during cell division. An immediate question is the identity of the MELK substrates that mediate its role in mitosis, such as promoting mitotic cell survival.Myeloid cell leukemia 1 (MCL1) is an important negative regulator of apoptosis. Uniquely among the Bcl-2 family, it is turned over rapidly by ubiquitin-mediated proteolysis and must be continuously resupplied by translation (3, 4). Protein abundance of MCL1 decreases during prolonged mitotic arrest induced by antimicrotubule drugs, rendering arrested cells highly sensitive to inhibitors of other Bcl-2 family members (5). We thus speculate that synthesis of MCL1 is important for cell survival during normal and drug-arrested mitosis and, conversely, that a drug that decreases MCL1 synthesis during mitosis might have anticancer potential.The rate of protein synthesis and other basic biological processes, such as DNA or RNA biogenesis, fluctuates throughout the cell cycle. Overall protein synthesis significantly decreases when cells enter mitosis (6, 7), consistent with the notion that macromolecule synthesis predominantly occurs in interphase before the segregation of cellular mass in mitosis.A recent study based on ribosome profiling identified a set of genes that are translationally repressed in mitosis, and proposed that suppressed protein synthesis migh...

Inhibition of MELK produces potential anti‐tumour effects in bladder cancer by inducing G1/S cell cycle arrest via the ATM/CHK2/p53 pathway

“…4,5 Maternal embryonic leucine zipper kinase (MELK) is a member of the AMP protein kinase (AMPK) family of serine/threonine kinases, and MELK activates multiple cellular pathways that drive oncogenic growth. [8][9][10][11][12][13] It has been shown that MELK is overexpressed in multiple human tumours, including the following: melanoma, 8 diffuse intrinsic pontine glioma (DIPG), 14 breast cancer, 6,15 gastric cancer, 16 high-grade prostate cancer, 17 hepatocellular carcinoma, 18 kidney cancer, 19 small lung cancer, 20 myeloma, 21 acute myeloid leukaemia (AML) 22 and chronic lymphocytic leukaemia (CLL). [8][9][10][11][12][13] It has been shown that MELK is overexpressed in multiple human tumours, including the following: melanoma, 8 diffuse intrinsic pontine glioma (DIPG), 14 breast cancer, 6,15 gastric cancer, 16 high-grade prostate cancer, 17 hepatocellular carcinoma, 18 kidney cancer, 19 small lung cancer, 20 myeloma, 21 acute myeloid leukaemia (AML) 22 and chronic lymphocytic leukaemia (CLL).…”

“…The ATM-CHK2 pathway is activated when DSBs occur. MELK was proven to be associated with mitotic progression and DNA damage [8][9][10][11][12][13]. Kig et al confirmed that MELK was required for the repair of DNA damage (including dou-…”

Cell death response to anti-mitotic drug treatment in cell culture, mouse tumor model and the clinic

“…It is possible that certain oncogenic pathways generate specific, druggable vulnerabilities in mitosis, although none has yet been found. Recent work suggested MELK kinase activity is required for successful mitosis in certain breast cancer lines, but not others [90, 91]. Whether MELK is a useful target remains to be seen, but this work is encouraging in the context that it is at least theoretically possible to perturb mitosis selectively in some cancer genotypes.…”

MELK Promotes Melanoma Growth by Stimulating the NF-κB Pathway

“…MELK is highly overexpressed in several cancer types and its inhibition has been shown to block the tumor growth of some cancers (Inoue et al, 2016; Joshi et al, 2013; Kato et al, 2016; Wang et al, 2016; Wang et al, 2014). Interestingly, MELK knockout mice are viable and do not show any specific phenotypes (Wang et al, 2014).…”