Hepatic ischemia/reperfusion (I/R) injury is a primary cause of liver dysfunction in both pediatric and adult patients. During the previous funding period, using an established model of murine hepatic I/R, we found that old mice (12-13 months of age) have significantly greater liver injury after I/R than do young mice (4-6 weeks of age). These findings are consistent with clinical studies of trauma patients which suggest that pediatric patients have a far lower incidence of multiple organ dysfunction syndrome than do adult patients. We discovered that the increased injury was associated with decreased activation of the transcription factor, NF- kB, in hepatocytes of old mice. Our previous work suggests that NF-kB has divergent functions that are dependent upon the cell type involved. For example, activation of NF-kB in Kupffer cells appears to drive the inflammatory response, while NF-kB activation in hepatocytes appears to function in a cell-protective mechanism. Our more recent studies have begun to explore the mechanism by which NF-kB activation is different in young and old mice. We have found a major defect in the processing of the cytoplasmic inhibitor of NF-kB, IkBa, in old mice. First, we have shown that gene expression of specific subunits of the 19S regulatory particle of the 26S proteasome are significantly reduced in old mice. These protein subunits are involved in recruitment of ubiquitinylated substrates to the proteasome for degradation. Our studies suggest that decreased expression of these subunits reduce the degradation of IkBa and therefore decrease NF-kB activation in the livers of old mice. Secondly, we have preliminary data suggesting that a mode of recruitment of ubiquitinlylated IkBa to the proteasome is altered in old mice. The cytoplasmic chaperone, valosin- containing protein (VCP), is known to bind to ubiquitinylated proteins, including IkBa, and facilitate their recruitment to the proteasome. Phosphorylation of VCP by Akt regulates VCP's association with ubiquitinylated proteins. Our preliminary data demonstrate a defect in Akt activation in the livers of old mice. As such, the global hypothesis of this project is that age-dependent alterations to components of the proteasome and to substrate recruitment mechanisms in hepatocytes of old mice results in decreased activation of NF-kB which renders them more susceptible to ischemic stress than hepatocytes from young mice, resulting in increased oxidative tissue injury and cell death. We will test this hypothesis with three specific aims.
Aim 1 will determine the mechanism by which age alters the chaperone function of VCP.
Aim 2 will determine the specific roles of individual subunits of the 19S regulatory particle on substrate recruitment and degradation by the 26S proteasome. Finally, Aim 3 will determine whether in vivo protein transfection of 19S subunits and/or constitutively active Akt restores normal activation of NF-kB and reduces liver injury induced by I/R in old mice. These studies may identify therapeutic targets that could help lead to new treatments for acute liver injury resulting from a number of different clinical events including trauma, surgery, transplantation, vascular disease, etc.
This proposal will identify biochemical changes in liver cells that occur with age. A better understanding of these changes may lead to the development of therapeutic treatments that could be applied to patients undergoing liver resection and transplantation that would enhance organ and graft function and reduce morbidity and mortality.
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