Our understanding of the role of protein O-GlcNAcylation in the regulation of the cardiovascular system has increased rapidly in recent years. Studies have linked increased O-GlcNAc levels to glucose toxicity and diabetic complications; conversely, acute activation of O-GlcNAcylation has been shown to be cardioprotective. However, it is also increasingly evident that O-GlcNAc turnover plays a central role in the delicate regulation of the cardiovascular system. Therefore, the goals of this minireview are to summarize our current understanding of how changes in O-GlcNAcylation influence cardiovascular pathophysiology and to highlight the evidence that O-GlcNAc cycling is critical for normal function of the cardiovascular system.The O-linked attachment of -N-acetylglucosamine (O-GlcNAc) to serine/threonine residues of proteins is a dynamic, transient, and reversible process that is an essential, metabolically regulated, signal transduction mediator in all cells (1). O-GlcNAc transferase (OGT) 3 catalyzes O-GlcNAc formation, utilizing UDP-GlcNAc as the substrate. UDPGlcNAc is the end product of the hexosamine biosynthesis pathway (HBP), which is regulated primarily by L-glutamine:Dfructose-6-phosphate amidotransferase (GFAT). GFAT catalyzes the formation of glucosamine 6-phosphate from fructose 6-phosphate. It has been estimated that 2-5% of glucose entering glycolysis is diverted through GFAT, thereby contributing to UDP-GlcNAc and O-GlcNAc synthesis (2); however, quantitative analysis of glucose flux via the HBP in the heart has yet to be performed. Although glucose availability is an important factor in O-GlcNAc synthesis, glutamine is critical as the amine donor for glucosamine 6-phosphate, whereas fatty acid metabolism is likely the primary source for the acetyl moiety. Thus, multiple nutrients contribute to both UDP-GlcNAc and protein O-GlcNAc synthesis. In addition to its synthesis, the levels of protein O-GlcNAc are also regulated by the activity of -Nacetylhexosaminidase (O-GlcNAcase (OGA)), which catalyzes removal of this post-translational modification.A little over a decade following the identification of O-GlcNAc protein modification by Torres and Hart (3), the small heat shock protein ␣B-crystallin was shown to be an O-GlcNAc target in rat heart (4). Using vascular smooth muscle cells, Han and Kudlow (5) demonstrated in 1997 that O-GlcNAcylation of the transcription factor Sp1 modulated its susceptibility to proteasomal degradation, concluding that this may provide a link between nutritional status and transcriptional regulation. In the same year, reported that OGT activity was significantly higher in rat heart compared with liver, fat, and other types of striated muscle; they also hypothesized that O-GlcNAcylation could be involved in mediating glucose toxicity in insulin-responsive tissues. To our knowledge, these were the first reports of O-GlcNAcylated proteins in cardiac or vascular tissues, as well as the first to suggest that protein O-GlcNAcylation may contribute to the adverse effects of i...