No abstract

An age-associated increase in chronic, lowgrade sterile inflammation termed "inflammaging" is a characteristic feature of mammalian aging that shows a strong association with occurrence of various ageassociated diseases. However, the mechanism(s) responsible for inflammaging and its causal role in aging and age-related diseases are not well understood. Ageassociated accumulation of damage-associated molecular patterns (DAMPs) is an important trigger in inflammation and has been proposed as a potential driver of inflammaging. DAMPs can initiate an inflammatory response by binding to the cell surface receptors on innate immune cells. Programmed necrosis, termed necroptosis, is one of the pathways that can release DAMPs, and cell death due to necroptosis is known to induce inflammation. Necroptosis-mediated inflammation plays an important role in a variety of age-related diseases such as Alzheimer's disease, Parkinson's disease, and atherosclerosis. Recently, it was reported that markers of necroptosis increase with age in mice and that dietary restriction, which retards aging and increases lifespan, reduces necroptosis and inflammation. Genetic manipulations that increase lifespan (Ames Dwarf mice) and reduce lifespan (Sod1 −/− mice) are associated with reduced and increased necroptosis and inflammation, respectively. While necroptosis evolved to protect cells/tissues from invading pathogens, e.g., viruses, we propose that the age-related increase in oxidative stress, mTOR signaling, and cell senescence results in cells/tissues in old animals being more prone to undergo necroptosis thereby releasing DAMPs, which contribute to the chronic inflammation observed with age. Approach to decrease DAMPs release by reducing/blocking necroptosis is a potentially new approach to reduce inflammaging, retard aging, and improve healthspan.

Mice deficient in the antioxidant enzyme Cu/ Zn-superoxide dismutase (Sod1KO mice) have a significant reduction in lifespan, exhibit many phenotypes of accelerated aging, and have high levels of oxidative stress in various tissues. Age-associated cognitive decline is a hallmark of aging and the increase in oxidative stress/damage with age is one of the mechanisms proposed for cognitive decline with age. Therefore, the goal of this study was to determine if Sod1KO mice exhibit an accelerated loss in cognitive function similar to that observed in aged animals. Cognition was assessed in Sod1KO and wild type (WT) mice using an automated home-cage testing apparatus (Noldus PhenoTyper) that included an initial discrimination and reversal task. Comparison of the total distance moved by the mice during light and dark phases of the study demonstrated that the Sod1KO mice do not show a deficit in movement. Assessment of cognitive function showed no significant difference between Sod1KO and WT mice during the initial discrimination phase of learning. However, during the reversal task, Sod1KO mice showed a significantly greater number of incorrect entries compared to WT mice indicating a decline in cognition similar to that observed in aged animals. Markers of oxidative stress (4-Hydroxynonenal, 4-HNE) and neuroinflammation [proinflammatory cytokines (IL6 and IL-1β) and neuroinflammatory markers (CD68, TLR4, and MCP1)] were significantly elevated in the hippocampus of male and female Sod1KO compared to WT mice. This study provides important evidence that increases in oxidative stress alone are sufficient to induce neuroinflammation and cognitive dysfunction that parallels the memory deficits seen in advanced aging and neurodegenerative diseases.

Mice deficient in the antioxidant enzyme Cu/Zn-superoxide dismutase (Sod1-/- or Sod1KO mice) develop spontaneous hepatocellular carcinoma (HCC) with age. Similar to humans, HCC development in Sod1KO mice progresses from fatty liver disease to non-alcoholic steatohepatitis (NASH) with fibrosis, which eventually progresses to HCC. Because liver inflammation is the main mechanism that drives the disease progression in chronic liver disease (CLD) and because necroptosis is a major source of inflammation, we tested the hypothesis that increased necroptosis in the liver plays a role in increased inflammation and progression to fibrosis and HCC in Sod1KO mice. Phosphorylation of MLKL (P-MLKL), a well-accepted marker of necroptosis, and expression of MLKL protein were significantly increased in the livers of Sod1KO mice compared to WT mice indicating increased necroptosis. Similarly, phosphorylation of RIPK3 and RIPK3 protein levels were also significantly increased. Markers of pro-inflammatory M1 macrophages, NLRP3 inflammasome, and transcript levels of pro-inflammatory cytokines and chemokines, e.g., TNFα, IL-6, IL-1β, and Ccl2 that are associated with human NASH and HCC, were significantly increased. Markers of fibrosis and oncogenic transcription factor STAT3 were also upregulated in the livers of Sod1KO mice. Short term treatment of Sod1KO mice with necrostatin-1s (Nec-1s), a necroptosis inhibitor, significantly reduced necroptosis, pro-inflammatory cytokines, fibrosis markers and STAT3 activation. Our data show for the first time that necroptosis-mediated inflammation contributes to fibrosis and HCC progression in Sod1KO mice, a mouse model of accelerated aging and progressive HCC development. These findings suggest that necroptosis might be a target for treating NASH and HCC.