Abstract
O-GlcNAc transferase (OGT) is an enzyme that catalyzes the O-GlcNAc modification of nucleocytoplasmic proteins and is highly expressed in many types of cancer. However, the mechanism regulating its expression in cancer cells is not well understood. This study shows that OGT is a substrate of the E3 ubiquitin ligase X-linked inhibitor of apoptosis (XIAP) which plays an important role in cancer pathogenesis. Although LSD2 histone demethylase has already been reported as an E3 ubiquitin ligase in lung cancer cells, we identified XIAP as the main E3 ubiquitin ligase in colon cancer cells. Interestingly, OGT catalyzes the O-GlcNAc modification of XIAP at serine 406 and this modification is required for the E3 ubiquitin ligase activity of XIAP toward specifically OGT. Moreover, O-GlcNAcylation of XIAP suppresses colon cancer cell growth and invasion by promoting the proteasomal degradation of OGT. Therefore, our findings regarding the reciprocal regulation of OGT and XIAP provide a novel molecular mechanism for controlling cancer growth and invasion regulated by OGT and O-GlcNAc modification.
Highlights
Introduction TheX-linked inhibitor of apoptosis protein (XIAP), known as BIRC4, is a member of the IAP family and a potent inhibitor of the caspase-mediated apoptosis pathway[1]
Glutathione S-transferase (GST) conjugated O-GlcNAc transferase (OGT) were used for a pull-down assay and a subsequent mass spectrometry (MS) analysis to identify the proteins that interact with OGT in HCT116 cells (Supplementary Table S1)
Co-immunoprecipitation was performed between XIAP and OGT to investigate the effect that substituting serine 406 with alanine on protein–protein interaction. Both XIAP serine 406 to alanine (S406A) and XIAP lacking the really interesting new gene (RING) domain interacted with OGT as much as XIAP wild-type HCT116 (WT), indicating that the interaction between XIAP and OGT was still functional (Supplementary Fig. S3). These results show that XIAP is modified by O-GlcNAc at serine 406 and that this modification directly influences the degradation of OGT
Summary
X-linked inhibitor of apoptosis protein (XIAP), known as BIRC4, is a member of the IAP family and a potent inhibitor of the caspase-mediated apoptosis pathway[1]. XIAP possesses three N-terminal baculovirus IAP repeat domains, which mediate direct binding to caspases 3, 7, and 9 along with flanking residues. These residues are part of a C-terminal really interesting new gene (RING) domain that mediates the E3 ubiquitin ligase activity of XIAP and its involvement in the ubiquitin-proteasome pathway[2,3]. The ubiquitin-associated (UBA) domain and dimerization through the RING domain are important parts of XIAP binding to K63- and, in some cases, K48-linked polyubiquitin chains[4]. XIAP has many other roles besides inhibiting cancer cell death, so its involvement in cancer cell biology should be explored further[7,8,9,10,11,12,13]
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