Maternal Embryonic Leucine Zipper Kinase Inhibitor Research Articles

High expression of maternal embryonic leucine-zipper kinase (MELK) impacts clinical outcomes in patients with ovarian cancer and its inhibition suppresses ovarian cancer cells growth ex vivo.

ObjectiveMaternal embryonic leucine zipper kinase (MELK) is receiving an attention as a therapeutic target in various types of cancers. In this study, we aimed to evaluate the prognostic significance of MELK expression in ovarian cancer using clinical samples, and assessed the efficacy of a small molecule MELK inhibitor, OTS167, using patient-derived ovarian cancer cells as well as cell lines.MethodsExpression levels of MELK in 11 ovarian cancer cell lines were confirmed by western blotting. Inhibitory concentration of OTS167 was determined by colorimetric assay. MELK messenger RNA (mRNA) expression was evaluated in 228 ovarian cancer patients by quantitative polymerase chain reaction. Growth inhibition of OTS167 was also evaluated using freshly-isolated primary ovarian cancer cells including spheroid formation condition.ResultsMELK mRNA expression was significantly higher in ovarian cancer than in normal ovaries (p<0.001), and high MELK mRNA expression was observed in patients with advanced stage, positive ascites cytology and residual tumor size. Patients with high MELK mRNA expression showed shorter progression-free survival (p=0.001). Expression of MELK was also confirmed in 10 of 11 ovarian cancer cell lines tested, and the half maximal inhibitory concentration of MELK inhibitor, OTS167, ranged from 9.3 to 60 nM. Additionally, OTS167 showed significant growth inhibitory effect against patient-derived ovarian cancer cells, regardless of their tumor locations, histologic subtypes and stages.ConclusionsWe demonstrated MELK as both a prognostic marker and a therapeutic target for ovarian cancer using clinical ovarian cancer samples. MELK inhibition by OTS167 may be an effective approach to treat ovarian cancer patients.

Inhibition of MELK Protooncogene as an Innovative Treatment for Intrahepatic Cholangiocarcinoma.

Background and Objectives: Intrahepatic cholangiocarcinoma (iCCA) is a pernicious tumor characterized by a dismal outcome and scarce therapeutic options. To substantially improve the prognosis of iCCA patients, a better understanding of the molecular mechanisms responsible for development and progression of this disease is imperative. In the present study, we aimed at elucidating the role of the maternal embryonic leucine zipper kinase (MELK) protooncogene in iCCA. Materials and Methods: We analyzed the expression of MELK and two putative targets, Forkhead Box M1 (FOXM1) and Enhancer of Zeste Homolog 2 (EZH2), in a collection of human iCCA by real-time RT-PCR and immunohistochemistry (IHC). The effects on iCCA growth of both the multi-kinase inhibitor OTSSP167 and specific small-interfering RNA (siRNA) against MELK were investigated in iCCA cell lines. Results: Expression of MELK was significantly higher in tumors than in corresponding non-neoplastic liver counterparts, with highest levels of MELK being associated with patients’ shorter survival length. In vitro, OTSSP167 suppressed the growth of iCCA cell lines in a dose-dependent manner by reducing proliferation and inducing apoptosis. These effects were amplified when OTSSP167 administration was coupled to the DNA-damaging agent doxorubicin. Similar results, but less remarkable, were obtained when MELK was silenced by specific siRNA in the same cells. At the molecular level, siRNA against MELK triggered downregulation of MELK and its targets. Finally, we found that MELK is a downstream target of the E2F1 transcription factor. Conclusion: Our results indicate that MELK is ubiquitously overexpressed in iCCA, where it may represent a prognostic indicator and a therapeutic target. In particular, the combination of OTSSP167 (or other, more specific MELK inhibitors) with DNA-damaging agents might be a potentially effective therapy for human iCCA.

MEK/MELK inhibition and blood-brain barrier deficiencies in atypical teratoid/rhabdoid tumors.

BackgroundAtypical teratoid/rhabdoid tumors (AT/RT) are rare, but highly aggressive. These entities are of embryonal origin occurring in the central nervous system (CNS) of young children. Molecularly these tumors are driven by a single hallmark mutation, resulting in inactivation of SMARCB1 or SMARCA4. Additionally, activation of the MAPK signaling axis and preclinical antitumor efficacy of its inhibition have been described in AT/RT.MethodsWe established and validated a patient-derived neurosphere culture and xenograft model of sonic hedgehog (SHH) subtype AT/RT, at diagnosis and relapse from the same patient. We set out to study the vascular phenotype of these tumors to evaluate the integrity of the blood–brain barrier (BBB) in AT/RT. We also used the model to study combined mitogen-activated protein kinase kinase (MEK) and maternal embryonic leucine zipper kinase (MELK) inhibition as a therapeutic strategy for AT/RT.ResultsWe found MELK to be highly overexpressed in both patient samples of AT/RT and our primary cultures and xenografts. We identified a potent antitumor efficacy of the MELK inhibitor OTSSP167, as well as strong synergy with the MEK inhibitor trametinib, against primary AT/RT neurospheres. Additionally, vascular phenotyping of AT/RT patient material and xenografts revealed significant BBB aberrancies in these tumors. Finally, we show in vivo efficacy of the non-BBB penetrable drugs OTSSP167 and trametinib in AT/RT xenografts, demonstrating the therapeutic implications of the observed BBB deficiencies and validating MEK/MELK inhibition as a potential treatment.ConclusionAltogether, we developed a combination treatment strategy for AT/RT based on MEK/MELK inhibition and identify therapeutically exploitable BBB deficiencies in these tumors.

MELK Promotes Endometrial Carcinoma Progression via Activating mTOR Signaling Pathway

Maternal embryonic leucine zipper kinase (MELK) is upregulated in a variety of human tumors, where it contributes to a malignant phenotype and correlates with a poor prognosis. Nonetheless, the biological function of MELK in endometrial carcinoma (EC) progression remains largely unknown. In this study, we investigated the mRNA and protein expression of MELK in EC by in silico and immunohistochemical (IHC) methods. We found that MELK expression was significantly higher in patients with EC, and that high expression of MELK was associated with serous EC, high histological grade, an advanced clinical stage, and shorter overall survival and disease-free survival. A MELK knockdown decreased the capacity for cell proliferation and migration in vitro and subcutaneous tumorigenesis in vivo. Chromatin immunoprecipitation and a luciferase reporter assay were performed to demonstrate that high expression of MELK may be caused by transcription factor E2F1. Moreover, we found that MELK directly interacts with MLST8 and then activates the mTORC1 and mTORC2 signaling pathway to promote EC progression. OTSSP167, an effective MELK inhibitor, suppressed the cell proliferation driven by MELK. Thus, we identified the crucial role of the E2F1-MELK-mTORC1/2 axis in the progression of EC, which may contain potential therapeutic targets of EC. Funding Statement: This work was supported by National Natural Science Foundation of China (Nos.81672565). Declaration of Interests: The authors declare no conflict of interest. Ethics Approval Statement: All of the patients provided written informed consent, and all the experiments were approved by the Research Ethics Committee of Shanghai Jiao Tong University Affiliated Sixth People’s Hospital and complied with the Declaration of Helsinki.

MELK inhibition targets cancer stem cells through downregulation of SOX2 expression in head and neck cancer cells.

Maternal embryonic leucine zipper kinase (MELK) has been reported to serve critical roles in the maintenance of stemness of cancer cells, although its mechanism remains unclear. Since SRY‑box 2 (SOX2) was demonstrated to be involved in self‑renewal and tumorigenicity of head and neck squamous cell carcinoma (HNSCC) and is aberrantly expressed in HNSCC tumors, the association between MELK and SOX2 was examined. Firstly, MELK inhibition was performed by small interfering RNA or MELK inhibitor OTS167, and it was determined that MELK inhibition by these approaches could decrease the SOX2 expression in HNSCC cells and OTS167 could suppress the SOX2 expression in a dose‑dependent manner. The present results indicated that MELK inhibition may target cancer stem cells (CSCs) through downregulation of the SOX2 gene. To further confirm the transcriptional regulation of SOX2, the transcription factors (TFs) were screened for SOX2 using a promoter‑binding TF assay followed by reverse transcription‑quantitative polymerase chain reaction and a decrease of the majority of the SOX2 TFs following MELK knockdown was observed. The present results provide evidence that MELK serves a key role in CSCs through the regulation of SOX2 and further indicates that MELK inhibition may also be promising for clinical applications in the treatment of HNSCC.

5-Keto-3-cyano-2,4-diaminothiophenes as selective maternal embryonic leucine zipper kinase inhibitors

Maternal embryonic leucine zipper kinase (MELK) is involved in several key cellular processes and displays increased levels of expression in numerous cancer classes (colon, breast, brain, ovary, prostate and lung). Although no selective MELK inhibitors have yet been approved, increasing evidence suggest that inhibition of MELK would constitute a promising approach for cancer therapy. A weak high-throughput screening hit (17, IC50 ≈ 5 μM) with lead-like properties was optimized for MELK inhibition. The early identification of a plausible binding mode by molecular modeling offered guidance in the choice of modifications towards compound 52 which displayed a 98 nM IC50. A good selectivity profile was achieved for a representative member of the series (29) in a 486 protein kinase panel. Future elaboration of 52 has the potential to deliver compounds for further development with chemotherapeutic aims.

MELK Inhibition in Diffuse Intrinsic Pontine Glioma.

Purpose: Diffuse intrinsic pontine glioma (DIPG) is a highly aggressive pediatric brain tumor, for which no effective therapeutic options currently exist. We here determined the potential of inhibition of the maternal embryonic leucine zipper kinase (MELK) for the treatment of DIPG.Experimental Design: We evaluated the antitumor efficacy of the small-molecule MELK inhibitor OTSSP167 in vitro in patient-derived DIPG cultures, and identified the mechanism of action of MELK inhibition in DIPG by RNA sequencing of treated cells. In addition, we determined the blood-brain barrier (BBB) penetration of OTSSP167 and evaluated its translational potential by treating mice bearing patient-derived DIPG xenografts.Results: This study shows that MELK is highly expressed in DIPG cells, both in patient samples and in relevant in vitro and in vivo models, and that treatment with OTSSP167 strongly decreases proliferation of patient-derived DIPG cultures. Inhibition of MELK in DIPG cells functions through reducing inhibitory phosphorylation of PPARγ, resulting in an increase in nuclear translocation and consequent transcriptional activity. Brain pharmacokinetic analyses show that OTSSP167 is a strong substrate for both MDR1 and BCRP, limiting its BBB penetration. Nonetheless, treatment of Mdr1a/b;Bcrp1 knockout mice carrying patient-derived DIPG xenografts with OTSSP167 decreased tumor growth, induced remissions, and resulted in improved survival.Conclusions: We show a strong preclinical effect of the kinase inhibitor OTSSP167 in the treatment of DIPG and identify the MELK-PPARγ signaling axis as a putative therapeutic target in this disease. Clin Cancer Res; 24(22); 5645-57. ©2018 AACR.

Abstract 837A: Combining CRISPR/Cas9 mutagenesis and a small-molecule inhibitor to probe the function of MELK in cancer

Abstract The Maternal Embryonic Leucine Zipper Kinase (MELK) has been identified as a promising therapeutic target in multiple cancer types. MELK over-expression is associated with aggressive disease, and MELK has been implicated in numerous cancer-related processes, including chemotherapy resistance, stem cell renewal, and tumor growth. Based on these preclinical results, the MELK inhibitor OTS167 is currently being tested as a novel chemotherapy agent in several clinical trials. Here, we report that cancer cell lines harboring CRISPR/Cas9-induced null mutations in MELK exhibit wild-type growth in vitro, under environmental stress, in the presence of cytotoxic chemotherapies, and in vivo. Furthermore, MELK-knockout lines remain sensitive to OTS167, suggesting that this drug blocks cell division through an off-target mechanism. By combining our MELK-knockout clones with a new highly-specific MELK inhibitor, we further demonstrate that the acute inhibition of MELK results in no specific anti-proliferative phenotype. Analysis of gene expression data from cancer patients identifies MELK expression as a correlate of mitotic activity, explaining its association with poor prognosis. In total, our results demonstrate the power of CRISPR/Cas9-based genetic approaches to investigate cancer drug targets, and call into question the rationale for treating patients with anti-MELK monotherapies. Citation Format: Christopher J. Giuliano, Ann Lin, Joan C. Smith, Ann C. Palladino, Jason M. Sheltzer. Combining CRISPR/Cas9 mutagenesis and a small-molecule inhibitor to probe the function of MELK in cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 837A.

Abstract 3218: Maternal embryonic leucine zipper kinase (MELK) confers radioresistance in triple-negative breast cancers (TNBC) through a nonhomologous end joining (NHEJ)-mediated pathway

Abstract Background: Increased rates of locoregional recurrence have been observed in TNBC despite the use of radiation (RT); therefore, approaches that result in radiosensitizaton in TNBC may improve clinical outcomes. Despite some controversy as to the role of MELK kinase as an oncogene, we previously established its clear role in conferring radioresistance in TNBC. Here we extend those studies to understand the mechanism of conferred radioresistance. Methods: Clonogenic survival assays were used to quantify the degree of radiosensitivity after MELK inhibition. Mass spectrometry was utilized to identify proteins associated with MELK. NHEJ assays were performed using linearized pEYFP vector and quantitated by FACS. qRT-PCR was used to determine MELK RNA expression. Rad51 and γH2AX foci counting were performed to evaluate homologous recombination (HR). Neutral comet assay was used to quantitate dsDNA break repair. Laser microirradiation studies were performed to determine the location and kinetics of protein shuttle to and from sites of dsDNA breaks, including MELK, Ku70 and Ku80 proteins. Fluorescent microscopy was performed for localization of endogenous MELK protein post RT. To study the effect of MELK kinase activity on Ku70-Ku80 complex formation and recruitment at the DNA damage site, cells were lysed and immunoprecipitation was performed using MELK, Ku70 or Ku80-specific antibodies. Results: We previously demonstrated that genomic or pharmacologic inhibition of MELK confers significant radiosensitization. Mechanistic studies were undertaken to understand the pathways critical to MELK-mediated radioresistance. When MELK is genomically or pharmacologically inhibited, NHEJ and HR reporter assays demonstrate that this radiosensitization is driven through impaired NHEJ at 4, 16, and 24 hrs after RT and not mediated by HR in multiple TNBC cell lines. Tandem mass spectrometry studies of tagged MELK protein identify Ku70 and Ku80 as direct interactors with MELK protein, which is confirmed by Co-IP. Laser microirradiation studies confirm that MELK, Ku70 and Ku80 co-localize to sites of dsDNA breaks, and that MELK kinase function is required to stabilize the Ku70/Ku80 complex at these sites. Comet assay confirms that rapid dissolution of the Ku70/Ku80 complex when MELK is inhibited or functionally dead is not a result of accelerated dsDNA break repair. Conclusion: MELK kinase function is critical in conferring radioresistance in TNBC and inhibition of function confers radiosensitivity through an NHEJ-mediated pathway. MELK also stabilizes Ku70/Ku80 proteins at sites of dsDNA breaks and allows for more efficient repair of breaks induced by ionizing RT. These results further support the rationale for developing clinical strategies to inhibit MELK in combination with RT treatment as a novel radiosensitizing strategy in TNBC. Citation Format: Shyam Nyati, Ben Chandler, Eric Olsen, Leah Moubadder, Meilan Liu, Meleah Cameron, Kari Wilder-Romans, Theodore S. Lawrence, Powel H. Brown, Fellix Y. Fang, Lori J. Pierce, Corey Speers. Maternal embryonic leucine zipper kinase (MELK) confers radioresistance in triple-negative breast cancers (TNBC) through a nonhomologous end joining (NHEJ)-mediated pathway [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3218.

Inhibition of maternal embryonic leucine zipper kinase with OTSSP167 displays potent anti-leukemic effects in chronic lymphocytic leukemia.

TP53 pathway defects contributed to therapy resistance and adverse clinical outcome in chronic lymphocytic leukemia (CLL), which represents an unmet clinical need with few therapeutic options. Maternal embryonic leucine zipper kinase (MELK) is a novel oncogene, which plays crucial roles in mitotic progression and stem cell maintenance. OTSSP167, an orally administrated inhibitor targeting MELK, is currently in a phase I/II clinical trial in patients with advanced breast cancer and acute myeloid leukemia. Yet, no investigation has been elucidated to date regarding the oncogenic role of MELK and effects of OTSSP167 in chronic lymphocytic leukemia (CLL). Previous studies confirmed MELK inhibition abrogated cancer cell survival via p53 signaling pathway. Thus, we aimed to determine the biological function of MELK and therapeutic potential of OTSSP167 in CLL. Herein, MELK over-expression was observed in CLL cells, and correlated with higher WBC count, advanced stage, elevated LDH, increased β2-MG level, unmutated IGHV, positive ZAP-70, deletion of 17p13 and inferior prognosis of CLL patients. In accordance with functional enrichment analyses in gene expression profiling, CLL cells with depletion or inhibition of MELK exhibited impaired cell proliferation, enhanced fast-onset apoptosis, induced G2/M arrest, attenuated cell chemotaxis and promoted sensitivity to fludarabine and ibrutinib. However, gain-of-function assay showed increased cell proliferation and cell chemotaxis. In addition, OTSSP167 treatment reduced phosphorylation of AKT and ERK1/2. It decreased FoxM1 phosphorylation, expression of FoxM1, cyclin B1 and CDK1, while up-regulating p53 and p21 expression. Taken together, MELK served as a candidate of therapeutic target in CLL. OTSSP167 exhibits potent anti-tumor activities in CLL cells, highlighting a novel molecule-based strategy for leukemic interventions.

Elucidating the Role of the Maternal Embryonic Leucine Zipper Kinase in Adrenocortical Carcinoma.

Adrenocortical carcinoma (ACC) is an aggressive cancer with a 5-year survival rate <35%. Mortality remains high due to lack of targeted therapies. Using bioinformatic analyses, we identified maternal embryonic leucine zipper kinase (MELK) as 4.1-fold overexpressed in ACC compared with normal adrenal samples. High MELK expression in human tumors correlated with shorter survival and with increased expression of genes involved in cell division and growth. We investigated the functional effects of MELK inhibition using newly developed ACC cell lines with variable MELK expression, CU-ACC1 and CU-ACC2, compared with H295R cells. In vitro treatment with the MELK inhibitor, OTSSP167, resulted in a dose-dependent decrease in rates of cell proliferation, colony formation, and cell survival, with relative sensitivity of each ACC cell line based upon the level of MELK overexpression. To confirm a MELK-specific antitumorigenic effect, MELK was inhibited in H295R cells via multiple short hairpin RNAs. MELK silencing resulted in 1.9-fold decrease in proliferation, and 3- to 10-fold decrease in colony formation in soft agar and clonogenicity assays, respectively. In addition, although MELK silencing had no effect on survival in normoxia, exposure to a hypoxia resulted in a sixfold and eightfold increase in apoptosis as assessed by caspase-3 activation and TUNEL, respectively. Together these data suggest that MELK is a modulator of tumor cell growth and survival in a hypoxic microenvironment in adrenal cancer cells and support future investigation of its role as a therapeutic kinase target in patients with ACC.

MELK expression correlates with tumor mitotic activity but is not required for cancer growth.

The Maternal Embryonic Leucine Zipper Kinase (MELK) has been identified as a promising therapeutic target in multiple cancer types. MELK over-expression is associated with aggressive disease, and MELK has been implicated in numerous cancer-related processes, including chemotherapy resistance, stem cell renewal, and tumor growth. Previously, we established that triple-negative breast cancer cell lines harboring CRISPR/Cas9-induced null mutations in MELK proliferate at wild-type levels in vitro (<xref ref-type="bibr" rid="bib34">Lin et al., 2017</xref>). Here, we generate several additional knockout clones of MELK and demonstrate that across cancer types, cells lacking MELK exhibit wild-type growth in vitro, under environmental stress, in the presence of cytotoxic chemotherapies, and in vivo. By combining our MELK-knockout clones with a recently described, highly specific MELK inhibitor, we further demonstrate that the acute inhibition of MELK results in no specific anti-proliferative phenotype. Analysis of gene expression data from cohorts of cancer patients identifies MELK expression as a correlate of tumor mitotic activity, explaining its association with poor clinical prognosis. In total, our results demonstrate the power of CRISPR/Cas9-based genetic approaches to investigate cancer drug targets, and call into question the rationale for treating patients with anti-MELK monotherapies.

MELK Promotes Melanoma Growth by Stimulating the NF-κB Pathway

Melanoma accounts for more than 80% of skin cancer-related deaths, and current therapies provide only short-term benefit to patients. Here, we show in melanoma cells that maternal embryonic leucine zipper kinase (MELK) is transcriptionally upregulated by the MAPK pathway via transcription factor E2F1. MELK knockdown or pharmacological inhibition blocked melanoma growth and enhanced the effectiveness of BRAFV600E inhibitor against melanoma cells. To identify mediators of MELK function, we performed stable isotope labeling with amino acids in cell culture (SILAC) and identified 469 proteins that had downregulated phosphorylation after MELK inhibition. Of these proteins, 139 were previously reported as substrates of BRAF or MEK, demonstrating that MELK is an important downstream mediator of the MAPK pathway. Furthermore, we show that MELK promotes melanoma growth byactivating NF-κB pathway activity via Sequestosome 1 (SQSTM1/p62). Altogether, these results underpin an important role for MELK in melanoma growth downstream of the MAPK pathway.

Maternal embryonic leucine zipper kinase is a novel target for proliferation-associated high-risk myeloma.

Treatment of high-risk patients is a major challenge in multiple myeloma. This is especially true for patients assigned to the gene expression profiling-defined proliferation subgroup. Although recent efforts have identified some key players of proliferative myeloma, genetic interactions and players that can be targeted with clinically effective drugs have to be identified in order to overcome the poor prognosis of these patients. We therefore examined maternal embryonic leucine zipper kinase (MELK) for its implications in hyper-proliferative myeloma and analyzed the activity of the MELK inhibitor OTSSP167 both in vitro and in vivo. MELK was found to be significantly overexpressed in the proliferative subgroup of myeloma. This finding translated into poor overall survival in patients with high vs. low MELK expression. Enrichment analysis of upregulated genes in myeloma cells of MELKhigh patients confirmed the strong implications in myeloma cell proliferation. Targeting MELK with OTSSP167 impaired the growth and survival of myeloma cells, thereby affecting central survival factors such as MCL-1 and IRF4. This activity was also observed in the 5TGM.1 murine model of myeloma. OTSSP167 reduced bone marrow infiltration and serum paraprotein levels in a dose-dependent manner. In addition, we revealed a strong link between MELK and other proliferation-associated high-risk genes (PLK-1, EZH2, FOXM1, DEPDC1) and MELK inhibition also impaired the expression of those genes. We therefore conclude that MELK is an essential component of a proliferative gene signature and that pharmacological inhibition of MELK represents an attractive novel approach to overcome the poor prognosis of high-risk patients with a proliferative expression pattern.

MELK is not necessary for the proliferation of basal-like breast cancer cells.

Thorough preclinical target validation is essential for the success of drug discovery efforts. In this study, we combined chemical and genetic perturbants, including the development of a novel selective maternal embryonic leucine zipper kinase (MELK) inhibitor HTH-01-091, CRISPR/Cas9-mediated MELK knockout, a novel chemical-induced protein degradation strategy, RNA interference and CRISPR interference to validate MELK as a therapeutic target in basal-like breast cancers (BBC). In common culture conditions, we found that small molecule inhibition, genetic deletion, or acute depletion of MELK did not significantly affect cellular growth. This discrepancy to previous findings illuminated selectivity issues of the widely used MELK inhibitor OTSSP167, and potential off-target effects of MELK-targeting short hairpins. The different genetic and chemical tools developed here allow for the identification and validation of any causal roles MELK may play in cancer biology, which will be required to guide future MELK drug discovery efforts. Furthermore, our study provides a general framework for preclinical target validation.

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OBJECTIVES/SPECIFIC AIMS: Unlike the high cure rates (90%) of children with acute lymphoblastic leukemia (ALL), that of adults is still lagging behind and better therapies are needed. Maternal embryonic leucine-zipper kinase (MELK) is aberrantly upregulated in cancer, and implicated in cancer stem cell survival. A recent study has identified FOXM1, a MELK substrate, as a therapeutic target in B cell ALL (B-ALL). Thus, we hypothesized that MELK may act as a therapeutic target in ALL via targeting FOXM1 activity. METHODS/STUDY POPULATION: Western blot and qPCR were used to assess MELK expression in 14 ALL cell lines. Knock-down and kinase inhibition approaches targeting MELK expression and function, followed by CCK-8 and Annexin V (flow cytometry) assays to measure cell viability and apoptosis, respectively. RESULTS/ANTICIPATED RESULTS: MELK was significantly upregulated in patients with ALL (oncomine data analysis). MELK was also significantly higher in B-ALL and T-ALL cell lines compared with that in blood cells of healthy donors. MELK knock-down significantly decreased cell viability (40%–70%, p&lt;0.05, Fig. 1) in ALL cells, and induced apoptosis (~40%). OTS167, a potent MELK inhibitor exhibited cytotoxic activities in both B and T-ALL cells. The IC50 of OTS167 ranged from 20 to 60 nM; we also found a significant increase in apoptosis (p&lt;0.05). Mechanistically, MELK inhibition resulted in decrease of FOXM1 protein levels 3 hours post-treatment. DISCUSSION/SIGNIFICANCE OF IMPACT: MELK is highly expressed in ALL and represents a novel therapeutic target likely via modulating FOXM1 activity. Functional and mechanistic studies will complement and ensure the success of the undergoing Phase I/II clinical trial of OTS167 in patients with refractory or relapsed AML, ALL, and other advanced hematologic malignancies.

Phosphorylation of the HIV-1 capsid by MELK triggers uncoating to promote viral cDNA synthesis.

Regulation of capsid disassembly is crucial for efficient HIV-1 cDNA synthesis after entry, yet host factors involved in this process remain largely unknown. Here, we employ genetic screening of human T-cells to identify maternal embryonic leucine zipper kinase (MELK) as a host factor required for optimal uncoating of the HIV-1 core to promote viral cDNA synthesis. Depletion of MELK inhibited HIV-1 cDNA synthesis with a concomitant delay of capsid disassembly. MELK phosphorylated Ser-149 of the capsid in the multimerized HIV-1 core, and a mutant virus carrying a phosphorylation-mimetic amino-acid substitution of Ser-149 underwent premature capsid disassembly and earlier HIV-1 cDNA synthesis, and eventually failed to enter the nucleus. Moreover, a small-molecule MELK inhibitor reduced the efficiency of HIV-1 replication in peripheral blood mononuclear cells in a dose-dependent manner. These results reveal a previously unrecognized mechanism of HIV-1 capsid disassembly and implicate MELK as a potential target for anti-HIV therapy.

Abstract 3526: The expression of MELK, a growth regulator of triple-negative breast cancer, is regulated by p53

Abstract Background: Triple negative breast cancer (TNBC) is the most aggressive form of breast cancer with poor prognosis. Due to frequent distant metastasis and lack of successful targeted therapies, the overall survival rate in TNBC patients is significantly lower than estrogen receptor positive (ER) and HER2 positive breast cancers. To identify potential druggable targets, previously, we performed a gene expression analysis and identified protein kinases that are highly expressed in TNBC. Through these studies, we discovered that the expression of maternal embryonic leucine zipper kinase (MELK) is highly elevated in TNBCs and correlated with p53-mutation status and metastasis-free survival. Inhibition of MELK reduced the growth and invasiveness of p53 mutant breast cancer cells. Methods: In this study, we analyzed the association of MELK expression with p53 status using publicly available datasets, and examined the role of wild-type and mutant P53 in regulating MELK expression. MELK expression at mRNA and protein levels were determined through Q-RT-PCR and western blotting analysis respectively. The association of p53 and MELK expression was determined using Oncomine analysis. We also investigated the effect of p53 knockdown and overexpression on MELK expressing using Q-RT-PCR, western blotting and promoter luciferase assays. Results: Analysis of publicly available datasets revealed that MELK expression is highly elevated p53-mutant breast cancer independent of estrogen receptor status. Consistent with observation, high MELK expression was found in p53-mutant cells compared to p53 wild-type cells. Through luciferase, western blotting and Q-RT-PCR assays, we discovered that the increased expression of MELK in TNBC cells is due to inactivation of p53 wild-type function. Consistently, over expression of p53 wild-type in p53-Null cells, using doxycycline inducible system reduced MELK promoter activity and expression. Similarly, inhibition of p53 wild-type by over expression of p53-mutants in p53-wild-type cells also increased MELK expression through dominant-negative effect. In contrast, knockdown of p53-mutants or over expression of p53-mutants in p53-null cells did not alter MELK expression. Finally, through promoter deletion studies coupled with luciferase studies, we identified a potential p53-response region in MELK promoter. Conclusion: Our results suggest that the expression of MELK, a critical regulator of growth and invasion of TNBCs, is regulated by the tumor suppressor gene, p53 wild-type. Inactivation of p53 wild-type either gene deletion or mutations induces the expression of MELK. These results suggest that MELK is a promising target for the treatment of women with p53-mutant TNBC. This work was supported by Susan G Komen Promise Grant (PB, SH, GM), SAB Komen grant (PB) and Young Foundation grant (PB). Citation Format: Lakshmi Reddy Bollu, Powel H. Brown, Abhijit Mazumdar, Dekuang Zhao, Yanxia Ma. The expression of MELK, a growth regulator of triple-negative breast cancer, is regulated by p53 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3526. doi:10.1158/1538-7445.AM2017-3526

CRISPR/Cas9 mutagenesis invalidates a putative cancer dependency targeted in on-going clinical trials.

The Maternal Embryonic Leucine Zipper Kinase (MELK) has been reported to be a genetic dependency in several cancer types. MELK RNAi and small-molecule inhibitors of MELK block the proliferation of various cancer cell lines, and MELK knockdown has been described as particularly effective against the highly-aggressive basal/triple-negative subtype of breast cancer. Based on these preclinical results, the MELK inhibitor OTS167 is currently being tested as a novel chemotherapy agent in several clinical trials. Here, we report that mutagenizing MELK with CRISPR/Cas9 has no effect on the fitness of basal breast cancer cell lines or cell lines from six other cancer types. Cells that harbor null mutations in MELK exhibit wild-type doubling times, cytokinesis, and anchorage-independent growth. Furthermore, MELK-knockout lines remain sensitive to OTS167, suggesting that this drug blocks cell division through an off-target mechanism. In total, our results undermine the rationale for a series of current clinical trials and provide an experimental approach for the use of CRISPR/Cas9 in preclinical target validation that can be broadly applied.

Genome-wide effects of MELK-inhibitor in triple-negative breast cancer cells indicate context-dependent response with p53 as a key determinant

Triple-negative breast cancer (TNBC) is an aggressive, highly recurrent breast cancer subtype, affecting approximately one-fifth of all breast cancer patients. Subpopulations of treatment-resistant cancer stem cells within the tumors are considered to contribute to disease recurrence. A potential druggable target for such cells is the maternal embryonic leucine-zipper kinase (MELK). MELK expression is upregulated in mammary stem cells and in undifferentiated cancers, where it correlates with poor prognosis and potentially mediates treatment resistance. Several MELK inhibitors have been developed, of which one, OTSSP167, is currently in clinical trials. In order to better understand how MELK and its inhibition influence TNBC, we verified its anti-proliferative and apoptotic effects in claudin-low TNBC cell lines MDA-MB-231 and SUM-159 using MTS assays and/or trypan blue viability assays together with analysis of PARP cleavage. Then, using microarrays, we explored which genes were affected by OTSSP167. We demonstrate that different sets of genes are regulated in MDA-MB-231 and SUM-159, but in both cell lines genes involved in cell cycle, mitosis and protein metabolism and folding were regulated. We identified p53 (TP53) as a potential upstream regulator of the regulated genes. Using western blot we found that OTSSP167 downregulates mutant p53 in all tested TNBC cell lines (MDA-MB-231, SUM-159, and BT-549), but upregulates wild-type p53 in the luminal A subtype MCF-7 cell line. We propose that OTSSP167 might have context-dependent or off-target effects, but that one consistent mechanism of action could involve the destabilization of mutant p53.