Sepsis is a systemic response to infection characterized by hemodynamic and metabolic derangements that result in shock, multiple organ system failure and death. The incidence and mortality is disproportionately increased in elderly patients, whose treatment represents a major clinical challenge. Thus, the investigation of the molecular mechanisms underlying the age-dependent susceptibility to sepsis is of utmost importance. Scientific evidence suggests that profound changes in mitochondrial function and bioenergetics play a role in the disease process. Accumulation of dysfunctional mitochondria may further increase oxidative stress and cell death. During the past funding cycle, we have demonstrated that the age-dependent susceptibility to sepsis is associated with a marked dysfunction of the nuclear hormone receptors, peroxisome proliferator-activated receptor-? (PPAR?), PPAR?, and liver X receptor-? (LXR?), well-known regulators of metabolic and anti- inflammatory cellular responses. In new preliminary studies, we have observed that mitochondrial complex I function is impaired in liver of old mice (11-12 months of age) when compared to young animals (2-3 months of age) during sepsis. This event in old mice is associated with reduced expression of LC3 II, a marker of autophagy, an important process that enables the cells to dispose defective mitochondria. We also have found that liver of old mice exhibits reduced nuclear expression of the PPAR? coactivator 1-? (PGC-1?), the master regulator of mitochondrial biogenesis, and reduced activation of AMP-activated protein kinase (AMPK), a crucial energy status sensor, which is known to activate PGC-1? and negatively regulate the autophagy controller, the mammalian target of rapamycin complex 1 (mTORC1). Interestingly, treatment with an AMPK activator was able to ameliorate liver function and improve early survival rate in septic old mice. Thus, these preliminary data raise the novel hypothesis that an age-related dysregulation of AMPK may lead to a vicious cycle of impaired autophagy and mitochondrial biogenesis, thus enhancing susceptibility to sepsis and impairing organ recovery.
Three specific aims are proposed to validate this novel concept.
In aim 1 we will investigate the changes of autophagy and mitochondrial biogenesis and their correlation with AMPK activation from infancy through senescence in polymicrobial sepsis in mice. With pharmacological gain-of-function and genetic loss-of-function studies, in aim 2 we will establish the precise role of AMPK in modulating autophagy and mitochondrial biogenesis through the downstream mTORC1 and PGC-1? pathways. Pharmacological studies will also establish whether AMPK activators mitigate sepsis-induced systemic inflammatory response, multiple organ failure and death. With in vitro studies in primary hepatocytes and myocytes from young or old mice, in aim 3 we will test the hypothesis that AMPK also affects the nuclear function of PPAR?, PPAR? and LXR?, which then contribute to the mechanisms of autophagy and mitochondrial biogenesis. These studies may have an impact in developing novel therapies to decrease sepsis morbidity and mortality.
Sepsis is the leading cause of mortality in non-cardiac intensive care units with estimates of over 215,000 deaths in the United States each year. Elderly patients have poor outcomes when compared with young patients. Our project is aimed to understand the molecular mechanisms that regulate the reparative capacity of organ function during aging and sepsis. We will focus on autophagy, a process that allows the cell to dispose dysfunctional organelles, and mitochondria biogenesis, a process that allows the cell to restore functional organelles. The contribution of these reparative processes will be examined in an experimental model of sepsis using genetically altered mice as well as drug interventions directed at a specific protein, the AMP activated kinase, which is thought to be involved in the regulation of these processes.
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