O-GlcNAcylation is a dynamic post-translational modification of cytonuclear proteins for intracellular signaling. Elevated O-GIcNAcylation is a general feature of cancer and contributes to cancer progression, and recent studies indicate the contribution to increasing incidence of various types of cancer in diabetic patients. However, the role of O-G1cNAcylation in tumor progression is not fully elucidated. Forkhead box Ml (FOXM1), a master mitotic transcription factor, has been implicated in all major hallmarks of cancer, and is wildly expressed in solid tumors. Given that FOXM1 expression was reported to be elevated in gastric cancer, we examined the effect of high glucose or an inhibitor of O-GIcNAc hydrolase, Thiamet G (TMG), on FOXM1 protein expression in a human gastric cancer cell line, MKN45 cells, and confirmed that FOXM1 protein level and the cell proliferation were upregulated. To investigate the molecular mechanisms by which FOXM1 protein expression is regulated by O-G1cNAcylation, the effect of high glucose and TMG on FOXM1 ubiquitination was examined in MKN45 cells. As a result, the ubiquitination and degradation of FOXM1 protein were both suppressed by high glucose and TMG treatment. However, the O-GIcNAcylation was not detected on FOXM1 but not on GSK-3 beta. High glucose and TMG treatment increased phospho-serine 9 GSK-3 beta, an inactive form, and the degradation of FOXM1 protein was suppressed by treatment of GSK-3 beta inhibitors in MKN45 cells. Taken together, we suggest that high glucose and elevated O-G1cNAcylation stabilize FOXM1 protein by its reduced degradation via GSK-3 beta inactivation in MKN45 cells, suggesting that the higher risk of gastric cancer in diabetic patients could be partially due to O-GlcNAcylation-mediated FOXM1 stabilization. (C) 2017 Elsevier Inc. All rights reserved.