The sweet nature of cardioprotection.

GLUCOSE, GLUTAMINE, AND GLUCOSAMINE are metabolites that can alter cardioprotection (1, 3, 8, 9, 13–15). Numerous studies suggest that one molecular mechanism by which these metabolites protect cells is though their conversion to uridine diphosphate-N-acetylglucosamine (UDP-GlcNAc) via the hexosamine biosynthetic pathway (Fig. 1) and the subsequent modification of nuclear and cytoplasmic proteins with O-linked -N-acetylglucosamine (O-GlcNAc, a novel posttranslation modification) (1, 3, 7, 9, 10, 13–15). Inhibiting the conversion of glucose, glutamine, and glucosamine to UDP-GlcNAc or adding O-GlcNAc to the protein backbone blocks the cardioprotective effects of these metabolites (1, 3, 7, 9, 10). Moreover, elevating the O-GlcNAc protein modification is protective in numerous models of cellular injury (1, 3, 6, 7, 9–11, 16, 17, 19, 20). These and other studies have introduced the idea of O-GlcNAc-mediated cellular protection (Fig. 2, left). In this issue of the American Journal of Physiology-Heart and Circulatory Physiology, Liu et al. (8) show that specifically raising O-GlcNAc protein modification levels during reperfusion is sufficient for cardioprotection, thus identifying glucosamine and O-(2-acetamido-2-deoxy-D-glucopyranosylidene)aminoN-phenylcarbamate (PUGNAc, an inhibitor of O-GlcNAc removal) as potentially usefully cardioprotective agents. Over 600 proteins in the nucleus and cytoplasm of metazoans are modified by O-GlcNAc (4). Deletion of the UDP-GlcNAc polypeptide O-N-acetylglucosaminyltransferase (OGT), the enzyme that adds O-GlcNAc, is lethal, highlighting the importance of this simple posttranslational modification in cellular regulation (12). O-GlcNAc is thought to act as a modulator of protein function, analogous to protein phosphorylation; the addition of O-GlcNAc to the protein backbone is dynamic, responding to morphogens, the cell cycle, changes in glucose metabolism, and cellular stress. O-GlcNAc occurs at sites on the protein backbone similar to those modified by protein kinases and is reciprocal with phosphorylation on some wellstudied proteins, such as RNA Pol II, estrogen receptor, SV-40 large T-antigen, endothelial nitric oxide synthase, and the c-Myc protooncogene product. These data suggest that one mechanism by which O-GlcNAc modulates cellular function is by competing with phosphorylation (Fig. 1, bottom). Numerous cellular pathways within the cell are modulated by OGlcNAc, including transcription, translation, signal transduction, and protein degradation (4). Both in vitro and in vivo data support a model where increased UDP-GlcNAc levels, due to hyperglycemia, result in increased O-GlcNAc levels, leading to insulin resistance, a hallmark of type II diabetes. It has been suggested that elevated O-GlcNAc protein modification may be associated with many of the complications associated with type II diabetes (4). In response to multiple forms of cellular stress, levels of the O-GlcNAc protein modification are elevated rapidly and dynamically on myriad nucleocytoplasmic proteins (19). Several studies demonstrate that elevation of O-GlcNAc preceding heat stress (6, 16, 19), trauma hemorrhage (11, 17, 20), and ischemia-reperfusion injury (1, 3, 7–10, 13–15) is protective, suggesting that increased O-GlcNAc in response to stress is a survival response of cells. The exact molecular mechanism(s) by which O-GlcNAc regulates protein function leading to cellular protection have not been identified. However, OGlcNAc has been shown to regulate the following pathways in a manner consistent with stress tolerance: 1) heat shock protein levels (19), 2) protein solubility (6, 16), 3) p53 stability (18), 4) cytosolic Ca influx (9, 10, 13–15), 5) calcineurin activation and NFAT (nuclear factor of activated T cells) translocation (1), 6) p38 MAP kinase phosphorylation (3), and 7) circulating IL-6 and TNFlevels