The role of protein kinase C and cyclic AMP in the ammonia-induced shift of the taurine uptake/efflux balance towards efflux in C6 cells

Disturbance of hepatocyte growth and metabolism in a hyperammonemia microenvironment

BACKGROUND

We found through previous research that hyperammonemia can cause secondary liver damage. However, whether hepatocytes are target cells of ammonia toxicity and whether hyperammonemia affects hepatocyte metabolism remain unknown.

AIMS

The purpose of the current study is to examine whether the hepatocyte is a specific target cell of ammonia toxicity and whether hyperammonemia can interfere with hepatocyte metabolism.

METHODS

Cell viability and apoptosis were analyzed in primary hepatocytes and other cells that had been exposed to ammonium chloride. Western blotting was adopted to examine the expression of proteins related to ammonia transport. We also established a metabolomics method based on gas chromatography-mass spectrometry to understand the characteristics of the hepatocyte metabolic spectrum in a hyperammonemia microenvironment, to screen and identify differential metabolites, and to determine the differential metabolic pathway. Different technologies were used to verify the differential metabolic pathways.

RESULTS

Hepatocytes are target cells of ammonia toxicity. The mechanism is related to the ammonia transporter. Hyperammonemia interferes with hepatocyte metabolism, which leads to TCA cycle, urea cycle, and RNA synthesis disorder.

CONCLUSIONS

This study demonstrates that hepatocyte growth and metabolism are disturbed in a hyperammonemia microenvironment, which further deteriorates hepatocyte function.

Resveratrol prevents ammonia toxicity in astroglial cells

Ammonia is implicated as a neurotoxin in brain metabolic disorders associated with hyperammonemia. Acute ammonia toxicity can be mediated by an excitotoxic mechanism, oxidative stress and nitric oxide (NO) production. Astrocytes interact with neurons, providing metabolic support and protecting against oxidative stress and excitotoxicity. Astrocytes also convert excess ammonia and glutamate into glutamine via glutamine synthetase (GS). Resveratrol, a polyphenol found in grapes and red wines, exhibits antioxidant and anti-inflammatory properties and modulates glial functions, such as glutamate metabolism. We investigated the effect of resveratrol on the production of reactive oxygen species (ROS), GS activity, S100B secretion, TNF-α, IL-1β and IL-6 levels in astroglial cells exposed to ammonia. Ammonia induced oxidative stress, decreased GS activity and increased cytokines release, probably by a mechanism dependent on protein kinase A (PKA) and extracellular signal-regulated kinase (ERK) pathways. Resveratrol prevented ammonia toxicity by modulating oxidative stress, glial and inflammatory responses. The ERK and nuclear factor-κB (NF-κB) are involved in the protective effect of resveratrol on cytokines proinflammatory release. In contrast, other antioxidants (e.g., ascorbic acid and trolox) were not effective against hyperammonemia. Thus, resveratrol could be used to protect against ammonia-induced neurotoxicity.

Synergistic action of hypoosmolarity and glutamine in inducing acute swelling of retinal glial (Müller) cells

High blood ammonia, elevated glutamine, and hyponatremia are pathogenic factors contributing to astrocytic swelling and brain edema in liver failure. We investigated the effects of hypoosmolarity, ammonia, and glutamine on the induction of glial cell swelling in freshly isolated slices of the rat retina. Glutamine, but not ammonia or hypoosmolarity per se, evoked a rapid (within one minute) swelling of retinal glial (Müller) cell bodies under hypoosmotic conditions. Under isoosmotic conditions, glutamine evoked a delayed swelling after 10 min of exposure. The effect of glutamine was concentration-dependent, with half-maximal and maximal effects at ∼ 0.1 and 0.5 mM. Glutamine in hypoosmotic solution induced a dissipation of the mitochondrial membrane potential. The effects on the mitochondrial membrane potential and the glial soma size were reduced by (i) agents which inhibit the transfer of glutamine into mitochondria and its hydrolysis there, (ii) inhibition of the mitochondrial permeability transition, (iii) inhibitors of oxidative-nitrosative stress, and (iv) inhibitors of phospholipase A(2) and cyclooxygenase. Glutamine-induced glial swelling was also prevented by ATP and adenosine, acting at adenosine A(1) receptors. The data suggest that hypoosmolarity accelerates the swelling-inducing effect of glutamine on retinal glial cells, and that swelling induction by glutamine is mediated by inducing oxidative-nitrosative stress, inflammatory lipid mediators, and mitochondrial dysfunction.

cAMP mediates ammonia-induced programmed cell death in the microglial cell line BV-2

Although ammonia is a well-known neuropathogenic factor, the cellular mechanisms of ammonia toxicity are less characterized. Up to now, the main focus of ammonia toxicity has been on astrocytes and neurons. Despite the significance of microglia in neurodegenerative diseases, little is known about their responsiveness to ammonia. In the present study, we found that ammonia triggered mitosis at concentrations between 30 microm and 3.0 mm but apoptosis at concentrations >or= 1.0 mm in the murine microglial cell line BV-2. Most apoptotic cells showed an accumulation of condensed chromatin at the nuclear envelope, blebbing of the plasma membrane, formation of apoptotic bodies and an increase in caspase 3/7 activity. Blockade of caspase 3/7 activity by Ac-DEVD-CHO suppressed ammonia-induced apoptosis. Surprisingly, some BV-2 cells exposed to ammonia displayed clear signs of mitotic catastrophe, a type of cell death occurring during mitosis. In a further series of experiments, we found that cyclic adenosine 3',5'-monophosphate (cAMP) mediated the apoptogenic effects of ammonia, because (i) ammonia dose-dependently elevated the intracellular cAMP level, (ii) blockade of the adenylyl cyclase by SQ-22536 suppressed ammonia-induced apoptosis, (iii) inhibition of phosphodiesterases (PDEs) by the nonselective PDE inhibitor IBMX, or by the PDE4-selective inhibitor rolipram, increased the relative number of apoptotic cells, and (iv) the cAMP analogues 8-bromoadenosine cAMP and Sp-cAMP mimicked the effect of ammonia and triggered apoptosis. Taken together, our results indicate that distinct concentrations of ammonia trigger opposite signalling pathways in microglial cells.

Ammonia-induced alteration in S100B secretion in astrocytes is not reverted by creatine addition

Hyperammonemia is a major element in the pathogenesis of hepatic encephalopathy (HE) and ammonia neurotoxicity involves an effect on the glutamatergic neurotransmitter system. Astrocytes are intimately related to glutamatergic neurotransmission and, in fact, many specific glial alterations have been reported as a result of ammonia exposure. S100B protein, particularly extracellular S100B, is used as a parameter of glial activation or commitment in several situations of brain injury. However, there is little information about this protein in ammonia toxicity and none about its secretion in astrocytes under ammonia exposure. In this study, we investigated S100B secretion in rat cortical astrocytes acutely exposed to ammonia, as well astrocyte morphology, glial fibrillary acidic protein (GFAP) content and glutamine synthetase (GS) activity. Moreover, we studied a possible effect of creatine on these glial parameters, since this compound has a putative role against ammonia toxicity in cell cultures. We found an increase in S100B secretion by astrocytes exposed to ammonia for 24h, accompanied by a decrease in GFAP content and GS activity. Since elevated and persistent extracellular S100B plays a toxic effect on neural cells, altered extracellular content of S100B induced by ammonia could contribute to the brain impairment observed in HE. Creatine addition did not prevent this increment in S100B secretion, but was able to prevent the decrease in GFAP content and GS activity induced by ammonia exposure.

Astrocytes protect neurons from ammonia toxicity

Ammonia is a neurotoxin that is implicated in the CNS dysfunction associated with hepatic encephalopathy, urea cycle disorders, Reye's syndrome and other neurological conditions. While in vivo studies suggest that astrocytes are the principal target of ammonia toxicity, recent in vitro investigations suggest that neurons may also be directly affected by ammonia. To further examine the issue of neural cell sensitivity to ammonia, pure rat cortical neuronal cultures, as well as co-cultures of neurons and astrocytes, were exposed to 5 mM NH4Cl for 48 h. Cultures were examined for morphological changes by light microscopy, measures of cell death, free radical production and changes in the mitochondrial inner membrane potential. Ammonia caused extensive degenerative changes in pure cultured neurons, while such neuronal changes were minor in the co-cultures. Similarly, processes of pure cultured neurons displayed a significant loss of the mitochondrial inner membrane potential, as compared to neurons in co-cultures. Cell death (LDH release) in ammonia-treated neuronal cultures was twice as great as untreated controls, while in co-cultures ammonia did not significantly increase cell death. Free radical production at 3 min was increased (69%, P<0.05) in pure neuronal cultures but not in co-cultures. The neuroprotective effects observed in co-cultures may have been mediated by the astrocyte's ability to scavenge free radicals, by their detoxification of ammonia and/or by their neurotrophic actions. The neuroprotective action of astrocytes may explain the failure to detect significant pathological changes in neurons in ammonia toxicity in vivo.