Abstract
Maternal embryonic leucine zipper kinase (MELK) belongs to the subfamily of AMP-activated Ser/Thr protein kinases. The expression of MELK is very high in glioblastoma-type brain tumors, but it is not clear how this contributes to tumor growth. Here we show that the siRNA-mediated loss of MELK in U87 MG glioblastoma cells causes a G1/S phase cell cycle arrest accompanied by cell death or a senescence-like phenotype that can be rescued by the expression of siRNA-resistant MELK. This cell cycle arrest is mediated by an increased expression of p21(WAF1/CIP1), an inhibitor of cyclin-dependent kinases, and is associated with the hypophosphorylation of the retinoblastoma protein and the down-regulation of E2F target genes. The increased expression of p21 can be explained by the consecutive activation of ATM (ataxia telangiectasia mutated), Chk2, and p53. Intriguingly, the activation of p53 in MELK-deficient cells is not due to an increased stability of p53 but stems from the loss of MDMX (mouse double minute-X), an inhibitor of p53 transactivation. The activation of the ATM-Chk2 pathway in MELK-deficient cells is associated with the accumulation of DNA double-strand breaks during replication, as demonstrated by the appearance of γH2AX foci. Replication stress in these cells is also illustrated by an increased number of stalled replication forks and a reduced fork progression speed. Our data indicate that glioblastoma cells have elevated MELK protein levels to better cope with replication stress during unperturbed S phase. Hence, MELK inhibitors hold great potential for the treatment of glioblastomas as such or in combination with DNA-damaging therapies.
Highlights
Protein kinase MELK is expressed at very high levels in glioblastomas, but it is not understood how this benefits tumor growth
Our results disclose a key role for MELK in progression through unperturbed S phase and show that MELK is needed to cope with replication stress, in particular DNA double-strand breaks
A loss of MELK results in a G1/S delay that, during subsequent cell cycles, gradually culminates in a cell cycle arrest that results in either cell death or a senescence-like phenotype
Summary
Protein kinase MELK is expressed at very high levels in glioblastomas, but it is not understood how this benefits tumor growth. The activation of the ATM-Chk pathway in MELK-deficient MELK substrates are unknown, and a unifying hypothesis on cells is associated with the accumulation of DNA double-strand the biological function of MELK is missing. It can be argued breaks during replication, as demonstrated by the appearance of that at least some of the cellular effects of the loss or overex-. Our results disclose a key role for MELK in progression through unperturbed S phase and show that MELK is needed to cope with replication stress, in particular DNA double-strand breaks. MELK has potential as a therapeutic target for the treatment of glioblastomas
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