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

Abstract

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.

Therapeutic Targets for Adrenocortical Carcinoma in the Genomics Era

“…Recent translational and preclinical studies have also demonstrated that ACC may be sensitive to newer cell cycle inhibitors targeting kinases with well-established roles in cell cycle progression, such as CDK4/6 [ 85 , 86 ]. PLK1 [ 87 ], Aurora kinases [ 88 ], and MELK [ 89 ]. These therapeutic approaches have had moderate success in preclinical models of other solid tumors, and the recent advances in design of more selective kinase inhibitors have improved toxicity profiles, enabling assessment of therapeutic efficacy in clinical trials [ 90 ].…”

Targeted Assessment of G0S2 Methylation Identifies a Rapidly Recurrent, Routinely Fatal Molecular Subtype of Adrenocortical Carcinoma

“…We demonstrated that CIMP-high carcinomas are chromosomally noisy, frequently bear somatic alterations leading to activation of cell cycle, and exhibit a transcriptional program characterized by increased expression of steroidogenic enzymes, proliferation machinery, and genes coordinating DNA damage-associated processes. Cell cycle and DNA damage-associated genes upregulated in CIMP-high ACC include MELK, AURKB, CDK6, PLK1, and TOP2A which have been successfully targeted in preclinical and translational models of ACC (34)(35)(36)(37)(38), and may even predict clinical responsiveness to combination therapy with etoposide, doxorubicin, cisplatin, and mitotane (39). Although there is currently little data to support a clinical trial evaluating utility of demethylating agents alone in NOTE: Hazard ratios (HR) and 95% confidence intervals (95% CI) were determined by Cox proportional hazards regression using available clinical and molecular data from all tumors in the FMUSPþUM Primary ACC Cohort.…”

A Conditional Dependency on MELK for the Proliferation of Triple-Negative Breast Cancer Cells

“…The functional dependence on MELK has been reported in models representing an array of tumor types (reviewed by Settleman et al., 2018 ), and more recently in a few others including prostate cancer ( Jurmeister et al., 2018 ), adrenocortical carcinoma ( Kiseljak-Vassiliades et al., 2018 ), chronic lymphocytic leukemia ( Zhang et al., 2018 ), and diffuse intrinsic pontine glioma ( Meel et al., 2018 ). These studies, including our earlier report of MELK as a target for TNBC ( Wang et al., 2014 ), all used RNAi as the genetic approach.…”

“…In addition, hormone receptor-positive breast cancer cells and untransformed cells are insensitive to MELK depletion, indicating a selective dependency of TNBC cells on MELK ( Wang et al., 2014 , Touré et al., 2016 ). Moreover, recent studies using RNA interference (RNAi) techniques have found MELK to be a promising target in other cancer types, including melanoma ( Janostiak et al., 2017 ), prostate cancer ( Jurmeister et al., 2018 ), high-risk neuroblastoma ( Guan et al., 2018 ), adrenocortical carcinoma ( Kiseljak-Vassiliades et al., 2018 ), chronic lymphocytic leukemia ( Zhang et al., 2018 ), and diffuse intrinsic pontine glioma ( Meel et al., 2018 ).…”