Ammonia-induced activation of p53 in cultured astrocytes: role in cell swelling and glutamate uptake

Abstract

Cytotoxic brain edema, due principally to astrocyte swelling, is a major neurological complication of the acute form of hepatic encephalopathy (HE) (acute liver failure, ALF), a condition likely caused by elevated levels of brain ammonia. Potential mediators of ammonia-induced astrocyte swelling include oxidative/nitrosative stress (ONS), the mitochondrial permeability transition (mPT), mitogen-activated protein kinases (MAPKs) and nuclear factor-kappaB (NF-kappaB), since blockade of these factors reduces the extent of astrocyte swelling. As p53, a tumor suppressor protein and transcription factor, is a downstream target of ONS and MAPKs, we examined its potential role in the mechanism of ammonia-induced astrocyte swelling. Astrocytes exposed to NH(4)Cl (5mM) showed increased phosphorylation (activation) of p53((Ser392)) at 1h and such phosphorylation was significantly reduced by inhibitors of MAPKs (ERK1/2, JNK and p38-MAPK), antioxidants (vitamin E, catalase, PBN, desferoxamine, MnTBAP), as well as by L-NAME, an inhibitor of nitric oxide synthase, indicating a key role of oxidative/nitrosative stress and MAPKs in the ammonia-induced activation of p53. Since p53 is known to induce the mPT and to activate NF-kappaB (factors leading to ONS and implicated in ammonia-induced astrocyte swelling), we examined whether inhibition of p53 activation blocked mPT induction, NF-kappaB activation, as well as cell swelling. Pifithrin-alpha (PFT), an inhibitor of p53, blocked these processes. Impairment of astrocytic glutamate uptake is another important feature of HE and hyperammonemia. We therefore examined the potential role of p53 in the ammonia-induced inhibition of glutamate uptake and found that PFT also reversed the ammonia-induced inhibition of glutamate uptake. Our results indicate that a potentially important downstream target of ammonia neurotoxicity is p53, whose activation contributes to astrocyte swelling and glutamate uptake inhibition, processes likely a consequence of ONS derived from the mPT and activation of NF-kappaB.