Objective Gastrointestinal stromal tumor(GIST)is the most common mesenchymal tumors of the gastrointestinal tract and originates from interstitial cells of Cajal(ICCs).Activating mutations of the receptor tyrosine kinase KIT(75-80%)or platelet-derived growth factor receptor alpha(PDGFRA)(5-8%)are often the initial driver for most GIST progression.Imatinib,a specific tyrosine kinase inhibitor(TKI),was approved as a first-line treatment for unresectable or metastatic GISTs,and a number of clinical studies demonstrated that imatinib significantly increased overall survival and progression free survival in patients with GIST.Despite the clinical success,50%of patients develop resistance to imatinib within 2 years of treatment.Increasing reports have demonstrated that cell cycle checkpoint play critical roles in GIST development.WEE1 kinase acts as a tyrosine kinase that is a crucial component of the G2-M cell cycle checkpoint and plays an oncogenic role in various tumors.MK1775,a selective small molecule WEE1 inhibitor,showed potent antitumor effect in single or combined treatment with conventional chemotherapy or radiotherapy.However,the potential role of WEE1 in GIST remains largely unknown.Methods The Oncomine public database,western blotting,and immunohistochemistry were used to determine the expression of WEE1 in GISTs.Using MTT assays,colony formation analysis,and flow cytometry,we examined the role of WEE1 and the antitumor activity of inhibitor MK1775 alone,or combination with imatinib,in GIST-T1 and GIST-882 cells.Immunofluorescence staining for γ H2AX was used to analyze the level of DNA damage.A cycloheximide chase assay was performed to explore the antitumor mechanism of MK1775 in GISTs.Results From the Oncomine database,we found that WEE1 levels were upregulated in 6 GISTs compared to that in 19 normal tissues.To further assesse WEE1 expression in our GIST specimens,western blotting analysis showed that WEE1 expression was also higher in 41 GIST tissues than 16 normal tissues.For immunohistochemistry,43.9%(18/41)of samples showed negative staining and 56.1%(23/41)exhibited positive staining in GIST tissues and positive expression of WEE1 was correlated with tumor size,mitotic count,and risk grade.Inhibition of WEE1 with small interfering RNAs or MK1775 significantly inhibited GIST cell proliferation and induced apoptosis and cell cycle arrest.MK1775 significantly reduced the viability of GIST cells in a dose-dependent manner and the IC50 value was 0.650 ± 1.539 μ M and 1.532 ± 1.111 μ M for GIST-T1 and GIST-882,respectively.Moreover,MK1775 treatment increased γ H2AX expression,indicating the occurrence of DNA double-strand breaks.A combination imatinib and MK1775 enhanced the antitumor activity for GIST cells.As determined using CompuSyn software,the combination index(CI)of imatinib and MK1775 was 0.6238 in GIST-T1 cells and 0.5687 in GIST-882 cells,which indicated synergistic effects.Moreover,a greater induction of apoptosis and DNA damage were observed in the combination treatment group of imatinib and MK1775.In addition,a cycloheximide chase assay demonstrated MK1775 promoted KIT protein degradation,which may contribute to strength the antitumor activity of combination imatinib and MK1775.Conclusions Our data suggest that WEE1 plays a pivotal role in GIST proliferation and is a potential prognostic biomarker.Targeting of WEE1 might be a beneficial strategy for treatment of GISTs.