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

Abstract

SummaryThe role of maternal and embryonic leucine zipper kinase (MELK) in cancer cell proliferation has been contentious, with recent studies arriving at disparate conclusions. We investigated the in vitro dependency of cancer cells on MELK under a range of assay conditions. Abrogation of MELK expression has little effect under common culture conditions, in which cells are seeded at high densities and reach confluence in 3–5 days. However, MELK dependency becomes clearly apparent in clonogenic growth assays using either RNAi or CRISPR technologies to modulate MELK expression. This dependency is in sharp contrast to that of essential genes, such as those encoding classic mitotic kinases, but is similar to that of other oncogenes including MYC and KRAS. Our study provides an example demonstrating some of the challenges encountered in cancer target validation, and reveals how subtle, but important, technical variations can ultimately lead to divergent outcomes and conclusions.

Enigmatic MELK: The controversy surrounding its complex role in cancer

“…CRISPR-mediated depletion of AURKB and PLK1 slowed proliferation of cells seeded at high or low densities, indicating that the effects of knockout of truly essential genes occur independently of assay conditions. Depletion of KRAS and MYC caused moderate decreases in proliferation (50%) when cells were seeded at high densities and more markedly impaired proliferation (Ͼ90%) when seeded at lower densities (19). Collectively, these results suggest that MELK is likely not universally essential in cancer cells, but under specific growth conditions, MELK expression seems to be important.…”

“…demonstrating that MELK is not required for TNBC cell proliferation, based upon genomic MELK knockout and other experimental techniques (16). To further complicate the situation, members of this same group published another study in 2018, asserting that TNBC cells actually have a conditional dependence on MELK for proliferation (19). No additional studies have been completed to specifically address this controversy.…”

“…One possible explanation is that MELK is dispensable under some conditions, but required, or at least highly important, under others. In this vein, members of the group that initially labeled MELK essential for BLBC cell proliferation (17), and later showed it was not necessary for proliferation of these cells (16), recently published a study indicating that TNBC cells have a conditional dependence on MELK for growth (19). In this study, TNBC cells were transduced with a gRNA (plus Cas9) that caused efficient reduction of MELK expression and infected cells were selected with antibiotic.…”

“…Future efforts to better understand the functions of MELK in cancer will likely shed light on the mechanism underlying discordant results observed with CRISPR and RNAi, which could be broadly applicable to functional studies of other proteins using these techniques. Some studies have begun to investigate the specific conditions under which MELK expression seems to be required, but the conditions tested thus far are certainly not comprehensive (19,45). Efforts to elucidate the specific conditions under which MELK is important for cancer cell proliferation will be a crucial step toward a more complete functional understanding of MELK.…”

PIG-1 MELK-dependent phosphorylation of nonmuscle myosin II promotes apoptosis through CES-1 Snail partitioning

“…The MELK gene is overexpressed in different types of cancers, including 'triple negative' breast cancer (TNBC), the most aggressive form of this type (Pitner et al, 2017). Importantly, it was recently demonstrated that MELK function is required for clonogenic growth of TNBC-derived cells, and this has made MELK a target for the development of novel cancer therapeutics (Lin et al, 2017;Toure et al, 2016;Wang et al, 2014;Wang et al, 2018). However, the physiological function(s) of MELK and the mechanism(s) through which its overexpression contributes to tumorigenesis remain largely unknown.…”