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Apoptosis and necroptosis are the primary modes of eukaryotic cell death, with apoptosis being non-inflammatory while necroptosis is highly inflammatory. We previously demonstrated that, once activated, necroptosis is enhanced by hyperglycemia in several cell types. Here, we determine if hyperglycemia affects apoptosis similarly. We show that hyperglycemia does not enhance extrinsic apoptosis but potentiates a shift to RIP1-dependent necroptosis. This is due to increased levels and activity of RIP1, RIP3, and MLKL, as well as decreased levels and activity of executioner caspases under hyperglycemic conditions following stimulation of apoptosis. Cell death under hyperglycemic conditions was classified as necroptosis via measurement of markers and involvement of RIP1, RIP3, and MLKL. The shift to necroptosis was driven by RIP1, as mutation of this gene using CRISPR–Cas9 caused cell death to revert to apoptosis under hyperglycemic conditions. The shift of apoptosis to necroptosis depended on glycolysis and production of mitochondrial ROS. Importantly, the shift in PCD was observed in primary human T cells. Levels of RIP1 and MLKL increased, while executioner caspases and PARP1 cleavage decreased, in cerebral tissue from hyperglycemic neonatal mice that underwent hypoxia-ischemia (HI) brain injury, suggesting that this cell death shift occurs in vivo. This is significant as it demonstrates a shift from non-inflammatory to inflammatory cell death which may explain the exacerbation of neonatal HI-brain injury during hyperglycemia. These results are distinct from our previous findings where hyperglycemia enhanced necroptosis under conditions where apoptosis was inhibited artificially. Here we demonstrate a shift from apoptosis to necroptosis under hyperglycemic conditions while both pathways are fully active. Therefore, while our previous work documented that intensity of necroptosis is responsive to glucose, this work sheds light on the molecular balance between apoptosis and necroptosis and identifies hyperglycemia as a condition that pushes cells to undergo necroptosis despite the initial activation of apoptosis.

Necroptosis is a RIP1-dependent programmed cell death (PCD) pathway distinct from apoptosis. Downstream effector pathways of necroptosis include formation of advanced glycation end products (AGEs) and reactive oxygen species (ROS), both of which are influenced by glycolysis, suggesting that increased cellular glucose may prime necroptosis. Here, we establish that exposure to hyperglycemic levels of glucose enhances necroptosis in U937 monocytes. Pharmacologic or siRNA inhibition of RIP1 prevented the enhanced death, confirming it as RIP1-dependent necroptosis. Hyperglycemia induces a shift from apoptotic death to necroptosis in response to the extrinsic apoptotic stimulus TNFα. Increased membrane permeability and decreased phosphatidylserine exposure were seen in the hyperglycemic condition as compared to euglycemia. Additionally, cell death was shown to be caspase dependent under euglycemic conditions and shifted to RIP1 dependency in hyperglycemic conditions. Total levels of RIP1, RIP3, and MLKL increased following treatment with high levels of glucose suggesting a potential positive feedback mechanism. Hyperglycemic enhancement of necroptosis depends upon glycolysis with AGEs and ROS playing a role. Furthermore, the hyperglycemic state resulted in dysregulated calcium flux in a ROS dependent manner. Hyperglycemia resulted in increased infarct size in a mouse model of brain hypoxia-ischemia injury. The increased infarct size was prevented by inhibition of RIP1 with nec-1s, strongly suggesting that increased necroptosis accounts for exacerbation of this injury in conditions of hyperglycemia. This work reveals that hyperglycemia represents a condition in which cells are extraordinarily susceptible to necroptosis, that local glucose levels alter the balance of PCD pathways, and that clinically relevant outcomes may depend on glucose-mediated effects on PCD.