Inhibition of receptor-interacting protein 3 upregulation and nuclear translocation involved in Necrostatin-1 protection against hippocampal neuronal programmed necrosis induced by ischemia/reperfusion injury

RIP3 induces ischemic neuronal DNA degradation and programmed necrosis in rat via AIF

We have reported that nuclear translocation of Receptor-interacting protein 3 (RIP3) involves in neuronal programmed necrosis after 20-min global cerebral ischemia/reperfusion (I/R) injury. Herein, the underlying mechanisms and the nuclear role of RIP3 were investigated further. The necroptosis inhibitor necrostatin-1 (Nec-1), the autophagy inhibitor 3-methyladenine (3-MA), and the caspase-3 inhibitor acetyl-L-aspartyl-L-methionyl-L-glutaminyl-L-aspart-1-al (Ac-DMQD-CHO) were administered intracerebroventricularly 1 h before ischemia. Protein expression, location and interaction was determined by western blot, immunofluorescence or immunoprecipitation. Most CA1 neuronal death induced by 20-min global cerebral I/R injury was TUNEL-positive. Neuronal death and rat mortality rates were greatly inhibited by Nec-1 and 3-MA pre-treatment, but not by Ac-DMQD-CHO. And no activation of caspase-3 was detected after I/R injury. Caspase-8 was expressed richly in GFAP-positive astrocytes and Iba-1-positive microglia, but was not detected in Neun-positive neurons. The nuclear translocation and co-localization of RIP3 and AIF, and their interaction were detected after I/R injury. These processes were inhibited by Nec-1 and 3-MA pre-treatment, but not by Ac-DMQD-CHO. The formation of an RIP3-AIF complex and its nuclear translocation are critical to ischemic neuronal DNA degradation and programmed necrosis. Neurons are more likely to enter the programmed necrosis signal pathway for the loss of caspase-8 suppression.

Necroptosis-independent signaling by the RIP kinases in inflammation

Recent advances have identified a signaling cascade involving receptor interacting protein kinase 1 (RIPK1), RIPK3 and the pseudokinase mixed lineage kinase domain-like (MLKL) that is crucial for induction of necroptosis, a non-apoptotic form of cell death. RIPK1-RIPK3-MLKL-mediated necroptosis has been attributed to cause many inflammatory diseases through the release of cellular damage-associated molecular patterns (DAMPs). In addition to necroptosis, emerging evidence suggests that these necroptosis signal adaptors can also facilitate inflammation independent of cell death. In particular, the RIP kinases can drive NF-κB and inflammasome activation independent of cell death. In this review, we will discuss recent discoveries that led to this realization and present arguments why cell death-independent signaling by the RIP kinases may have a more important role in inflammation than necroptosis.

Therapeutic hypothermia attenuates tissue damage and cytokine expression after traumatic brain injury by inhibiting necroptosis in the rat

Necroptosis has been shown as an alternative form of cell death in many diseases, but the detailed mechanisms of the neuron loss after traumatic brain injury (TBI) in rodents remain unclear. To investigate whether necroptosis is induced after TBI and gets involved in the neuroprotecton of therapeutic hypothermia on the TBI, we observed the pathological and biochemical change of the necroptosis in the fluid percussion brain injury (FPI) model of the rats. We found that receptor-interacting protein (RIP) 1 and 3, and mixed lineage kinase domain-like protein (MLKL), the critical downstream mediators of necroptosis recently identified in vivo, as well as HMGB1 and the pro-inflammation cytokines TNF-α, IL-6 and IL-18, were increased at an early phase (6 h) in cortex after TBI. Posttraumatic hypothermia (33 °C) led to the decreases in the necroptosis regulators, inflammatory factors and brain tissue damage in rats compared with normothermia-treated TBI animals. Immunohistochemistry studies showed that posttraumatic hypothermia also decreased the necroptosis-associated proteins staining in injured cortex and hippocampal CA1. Therefore, we conclude that the RIP1/RIP3-MLKL-mediated necroptosis occurs after experimental TBI and therapeutic hypothermia may protect the injured central nervous system from tissue damage and the inflammatory responses by targeting the necroptosis signaling after TBI.

Inhibition of Receptor Interacting Protein Kinases Attenuates Cardiomyocyte Hypertrophy Induced by Palmitic Acid

Palmitic acid (PA) is known to cause cardiomyocyte dysfunction. Cardiac hypertrophy is one of the important pathological features of PA-induced lipotoxicity, but the mechanism by which PA induces cardiomyocyte hypertrophy is still unclear. Therefore, our study was to test whether necroptosis, a receptor interacting protein kinase 1 and 3 (RIPK1 and RIPK3-) dependent programmed necrosis, was involved in the PA-induced cardiomyocyte hypertrophy. We used the PA-treated primary neonatal rat cardiac myocytes (NCMs) or H9c2 cells to study lipotoxicity. Our results demonstrated that cardiomyocyte hypertrophy was induced by PA treatment, determined by upregulation of hypertrophic marker genes and cell surface area enlargement. Upon PA treatment, the expression of RIPK1 and RIPK3 was increased. Pretreatment with the RIPK1 inhibitor necrostatin-1 (Nec-1), the PA-induced cardiomyocyte hypertrophy, was attenuated. Knockdown of RIPK1 or RIPK3 by siRNA suppressed the PA-induced myocardial hypertrophy. Moreover, a crosstalk between necroptosis and endoplasmic reticulum (ER) stress was observed in PA-treated cardiomyocytes. Inhibition of RIPK1 with Nec-1, phosphorylation level of AKT (Ser473), and mTOR (Ser2481) was significantly reduced in PA-treated cardiomyocytes. In conclusion, RIPKs-dependent necroptosis might be crucial in PA-induced myocardial hypertrophy. Activation of mTOR may mediate the effect of necroptosis in cardiomyocyte hypertrophy induced by PA.

Protective mechanisms of CA074-me (other than cathepsin-B inhibition) against programmed necrosis induced by global cerebral ischemia/reperfusion injury in rats

Many studies have demonstrated the key role of lysosomes in ischemic cell death in the brain and have led to the "lysosomocentric" hypothesis. In this hypothesis, the release of cathepsin-B due to a change of lysosomal membrane permeabilization (LMP) or rupture is critical, and this can be prevented by its inhibitors CA074 and CA074-me. However, the role of CA074-me in neuronal death and its effect on the change of lysosomal membrane integrity after global cerebral ischemia/reperfusion (I/R) injury is not clear, so we investigated this here. Rat hippocampal CA1 neuronal death was evaluated after 20-min global cerebral I/R injury. CA074-me (1 μg, 10 μg) were given intracerebroventricularly 1h before ischemia or 1h post reperfusion. The changes of heat shock protein 70 (Hsp70), cathepsin-B, lysosomal-associated membrane protein 1 (LAMP-1), receptor-interacting protein 3 (RIP3), and the change of lysosomal pH were evaluated respectively. Hippocampal CA1 neuronal programmed necrosis induced by global cerebral I/R injury was prevented by CA074-me both pre-treatment and post-treatment. Diffuse cytoplasmic cathepsin-B and LAMP-1 immunostaining synchronized with the pyknotic nuclear changes 2 days post reperfusion, and a rise of lysosomal pH with the leakage of DND-153, a dye of lysosomes, after oxygen-glucose deprivation (OGD) was detected. Both of these changes demonstrated the rupture of lysosomal membrane and the leakage of cathepsin-B, and this was strongly inhibited by CA074-me pre-treatment. The overexpression and nuclear translocation of RIP3 and the reduction of NAD(+) level after I/R injury were also inhibited, while the upregulation of Hsp70 was strengthened by CA074-me pre-treatment. Delayed fulminant leakage of cathepsin-B due to lysosomal rupture is a critical harmful factor in neuronal programmed necrosis induced by 20-min global I/R injury. In addition to being an inhibitor of cathepsin-B, CA074-me may have an indirect neuroprotective effect by maintaining lysosomal membrane integrity and protecting against lysosomal rupture.