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.