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  • Several alterations have been or can be

    2024-04-01

    Several alterations have been or can be associated to the decreased tolerance of steatotic liver to I/R injury. Previous studies demonstrated that upregulation of the mitochondrial uncoupling protein-2 (UP2) is strictly related to the increase of I/R injury in steatotic liver [4]. UPR induction is cause of release of oxidant species (OS) and decreased ATP synthesis [4], [5] and can thus account for the massive hepatocyte necrosis of steatotic liver exposed to I/R described in earlier research [3]. Endoplasmic reticulum (ER) stress is an adaptive process that can recover cell homeostasis upon a damaging insult or lead to cell death if is protracted or excessive [6]. ER stress is promoted in liver by numerous conditions that include transplantation, I/R injury, steatosis, steatohepatitis and oxidative stress [7] and its inhibition reduces damage of fatty and lean liver exposed to I/R [8]. Lipotoxicity is critically involved in steatosis evolution to steatohepatitis and is associated to the activation of a critical inductor of hepatocyte death and liver damage, the c-Jun N-terminal kinase-1/2 (JNK) [10], [11]. It is possible, that in presence of a mild steatosis, “per se” unable to induce directly damage, I/R can sensitize hepatic RK-33 to the damaging effects of lipids and of JNK activation. The researches, by now, performed on I/R injury of fatty liver were mainly conducted by “in vivo” studies and with different models of hepatic steatosis [3], [4], [5], [8]. These approaches might have limited the possibilities to identify critical mediators of the process. The different models of hepatic steatosis, in fact, might dissimilarly respond to I/R injury and some of these models induce effects the do not reproduce those observed in humans [11]. Moreover, I/R exposure might induce, in separate types of steatotic hepatic cell, divergent responses that may be not evident, analyzing the whole liver. To overcome this problem, we here induced hepatic steatosis by feeding mice with high fat diet (HFD), a procedure that more closely reproduce the features of human hepatic steatosis [11]. Additionally, we contextually analyzed “cellular” models of steatotic-I/R injury. This, with the purpose to start dissecting the outcomes of I/R on two main populations of steatotic liver cells: hepatocytes and Kupffer Cells (KC). By using primary hepatocytes, we investigated, on the bases of the “in vivo” evidences, if a molecular interplay among cellular steatosis, ER stress mediators and OS will converge to a common mechanism responsible to increase hepatocyte damage by hypoxia/reoxygenation. The effects of hypoxia/reoxygenation, both in absence or in presence of an increased intracellular lipid content, are largely unexplored in primary KC. Such knowledge, however, can help the understanding of the molecular base of I/R injury in control and fatty liver. KC, in fact, are first sensors of liver insults and contribute to give rise and maintain the inflammatory responses that can further increase or modulate hepatocyte damage [12], [13]. In the present study, we will thus analyzed, in both mice hepatocytes and KC, how lipids influence their molecular responses and their susceptibility to hypoxia/reoxygenation. This, with the purpose to extrapolate a common pathogenic mediator of I/R injury of fatty liver and to investigate the capacity of its inhibition to exert hepatoprotective effects.
    Materials and methods
    Results
    Discussion Apoptosis signal-regulating kinase 1 (ASK1) mediates the cellular responses to numerous stresses such as oxidative stress, hypoxia/ischemia, calcium overload, inflammatory signals or ER stress consequent to sustained unfolded protein response (UPR) [20]. Upon ER stress, ASK1 takes part of the IRE1α pathway, as one of the adaptive ER proximal unfolded protein response (UPR) transmembrane proteins [6], [7], [20]. IRE1α collaborates with adaptor-like tumor necrosis factor receptor (TNFR)-associated factor2 (TRAF2) and recruits ASK1 that can in turn activate the downstream p38 and JNK pathways [6], [7], [20]. We found that in control hepatocytes, cold hypoxia alone was unable to activate the TRAF2-ASK1 axis that was instead stimulated after warm reoxygenation and inhibited by preventing ER stress. This indicates that ER stress and the subsequent stimulation of the TRAF2-ASK1 cytotoxic pathway has a role in the damaging effects of liver reperfusion, independently from the alterations produced during the ischemic phase.