Brain edema and associated intracranial hypertension are major complications of acute liver failure (ALF), which has a high mortality and for which therapeutic options are largely limited to an emergency liver transplantation. It is generally believed that ammonia is responsible for the edema in ALF, and that the edema is principally "cytotoxic", due to astrocyte swelling. Mounting evidence suggests, however, that in addition to ammonia, cytokines and related sepsis and inflammation also contribute to the brain edema in ALF. During the course of investigating mechanisms of ALF-related astrocyte swelling/brain edema, we identified the transcription factor NF-kB as a key mediator of astrocyte swelling/brain edema. It is notable that cytokines are well known activators of NF-kB. Thus, cytokines together with ammonia, appear to be involved in the activation of NF-kB in ALF. We have generated preliminary data indicating that NF-kB is indeed activated by ammonia and cytokines in cultured astrocytes, and that BAY 11-7082, an inhibitor of NF-kB, markedly diminished astrocyte swelling by ammonia and cytokines. We also found that in a rat model of ALF (thioacetamidemediated hepatotoxicity) NF-kB is activated in neural cells, including astrocytes, and that brain edema in ALF could be diminished by the systemic administration of BAY 11-7082. Our hypothesis is that activation of NF-kB represents a central factor in the pathway by which ammonia results in astrocyte swelling/ALF-related brain edema, and that cytokines synergistically interact with ammonia to bring about astrocyte swelling.
Aim #1 will establish synergistic interactions between ammonia and cytokines in astrocyte swelling and NF-kB activation.
Aim #2 will examine the potential contribution of microglia, the major cell in brain mediating inflammation, to the mechanism of astrocytic NF-kB activation and cell swelling.
Aim #3 will examine the mechanisms by which NF-kB brings about cell swelling. We have generated preliminary data that Na-K-Cl cotransporter + + - (NKCC), and the water channel protein aquaporin-4 (AQP4) are both activated in ammonia-treated cultured astrocytes;that such activation appears to be mediated by NF-kB, and that the activation of NKCC and AQP4 may represent the penultimate events leading to astrocyte swelling. The precise involvement of NKCC and AQP4 in the mechanism of cell swelling, along with their interaction with inflammatory enzymes, including inducible nitric oxide synthase (iNOS), phospholipase A2 (PLA2), and cyclooxygenase-2 (COX-2) that are regulated by NF-kB, will be investigated.
Aim #4 will (a) establish whether activation of NF-kB occurs in vivo using the TAA rat model of ALF, and whether inhibiting NF-kB activation diminishes the severity of brain edema, and (b) investigate the status of brain edema by TAA-mediated ALF in transgenic mice that exhibit an astrocytic functional inactivation of NF-kB. We believe our investigations will lead to a better understanding of mechanisms involved in the brain edema associated with ALF, with an aim at developing novel therapeutic strategies for this clinically life-threatening condition.
Severe liver disease can cause extensive brain swelling (brain edema) that may lead to irreversible brain damage, a condition referred to as acute liver failure (ALF). This is a life-threatening condition for which there is currently no treatment available other than an emergency liver transplantation. We propose to examine the role of the transcription factor NF-:B as a critical intermediary responsible for causing brain swelling in ALF.
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