American Society for Biochemistry and Molecular Biology
Journal of Biological Chemistry vol:288 issue:33 pages:24200-12
MELK, for maternal embryonic leucine zipper kinase, 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, which can be rescued by the expression of siRNA-resistant MELK. This cell-cycle arrest is mediated by an increased expression of p21WAF1/CIP1, an inhibitor of cyclin-dependent kinases, and is associated with the hypophosphorylation of the retinoblastoma protein and the downregulation of E2F target genes. The increased expression of p21 can be explained by the consecutive activation of ATM, 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, an inhibitor of p53 transactivation The activation of the ATMChk2 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 cope better 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.