5.3 Cell death
The I/R process of organisms is dynamic, and the mechanism for producing ROS is also complex. The ROS produced by the above pathways may accumulate in cells during the ischemic stage and inhibit antioxidants. However, after tissue restoration of blood supply, if ischemia is severe, ROS induced oxidative stress may further cause cell damage and even cell death[94].
Apoptosis is a process of programmed cell death, mainly caused by calcium overload and ROS activation in I/R. There are two pathways of cell apoptosis that can interact with each other: endogenous apoptosis pathway and exogenous apoptosis pathway. Endogenous pathway, also known as mitochondrial pathway. In the cells injured by ischemia/reperfusion, a large amount of calcium influx and ROS production will cause the opening of mitochondrial mPT pore and the activation of apoptosis promoting Bcl-2 family, increase the permeability of mitochondrial membrane, release cytochrome C into the cytoplasm, and then combine with apoptosis protease activating factor 1 (APAF-1) to activate Caspase-9 and form a complex, and then trigger the apoptosis cascade reaction to promote apoptosis[95].The exogenous pathway, also known as the death receptor pathway, is mainly activated by death factors or receptors. Important death factors mainly include TNF-α,Fas ligands, TRAIL, and TL1A. As mentioned earlier, during I/R, reperfusion cells release TNF-α,Activate the JNK pathway to stimulate the production of ROS. The accompanying oxidative stress will further stabilize the phosphorylation of c-JunN-terminal kinase and accelerate cell death[96]. If TNF-α Persistent increase will induce the TNFR related death domain (TRADD) to combine with it to synthesize TNF α- TRADD. TNF α- TRADD and Fas FADD can induce and activate caspase 8 and 10, then enzymolysis activates downstream caspase 3, 6 and 10, and then starts the process of cell apoptosis[97]. However, in ischemia-reperfusion injury, cell apoptosis is not as common as necrosis mentioned below.
Necrotizing apoptosis is also a type of programmed cell death, but its impact on organisms is completely different from previous cell apoptosis. The main characteristics of necrotic apoptosis are cell swelling, disintegration of organelle and leakage of intracellular components, which often cause severe inflammatory reaction in ischemic tissues[98, 99]. As a regulatory mode of death, the main factors triggering necrotic apoptosis are the interacting serine threonine kinase 3 (RIPK3) and mixed lineage kinase like domain (MLKL)[100]. RIP3 can enable TNF-α driven cell death changes from apoptosis to necrotic apoptosis. When Caspase-8 is depleted or cIAP is deficient, TNFR1 will promote necrotic apoptosis[99]. The assembly of necrotic complex is mainly related to RIPK1/RIPK3 interaction and MLKL activation. RIPK3 induces phosphorylation of MLKL, leading to oligomerization and translocation of MLKL to the lobules within the plasma membrane, ultimately increasing plasma membrane permeability and cell death[100]. ROS induced DNA damage also promotes the formation of necrotic complex by activating poly(ADP-ribose)polymerase (PARP), further accelerating cell death. Due to the close relationship between necrotic apoptosis and the occurrence of inflammation in the human body, understanding the molecular mechanism and pathophysiological significance of necrotic apoptosis remains an important goal of therapeutic I/R research.
The role of autophagy in I/R is bidirectional, which can both protect cells and disrupt them. During ischemia, appropriate mitochondrial autophagy can clear partially damaged mitochondria and reduce subsequent damage[101].During the reperfusion phase, the levels of Ca2+and ROS increase in the cells, while oxidative stress inhibits the activity of rapamycin mTOR, leading to the formation of ULK-1 complexes and PI3K III class, which induce autophagy. However, autophagy cannot clear all damaged mitochondria, and when autophagy clearance capacity is exceeded, it can lead to cell death[102, 103].