Ammonia induces upregulation of aquaporin-4 in neocortical astrocytes of rats through the p38 mitogen-activated protein kinase pathway.
BACKGROUND
Astrocyte swelling is an important consequence of hepatic encephalopathy, and aquaporin-4 has been reported to play a vital role in this swelling. Ammonia causes astrocyte swelling and is also known to modulate aquaporin-4 expression in the astrocyte foot processes. The purpose of this study was to explore the mechanism of ammonia-induced aquaporin-4 expression, which has been suggested to involve the p38 mitogen-activated protein kinase pathway.


METHODS
We exposed cultured astrocytes to ammonium chloride, an in vitro model of hepatic encephalopathy. The purity of cultured astrocytes was evaluated by fluorescent glial fibrillary acidic protein labeling; cell morphology was assessed by light microscopy; the expression of aquaporin-4, phospho-p38, and p38 were detected by Western blotting analysis. Statistical analysis was performed by one-way factorial analysis of variance, and the relationship between variables was calculated by linear regression using SPSS version 13.0 program for Windows (SPSS, Chicago, IL, USA).


RESULTS
The purity of cultured astrocytes was (96.6 +/- 1.4)%. Astrocytes swelled significantly when exposed to 5 mmol/L ammonium chloride for 24 hours as compared to non-exposed astrocytes. Co-treatment with 10 micromol/L SB203580 (an inhibitor of p38) attenuated the degree of ammonium chloride induced astrocyte swelling. Western blotting analysis revealed that the expression levels of phospho-p38 and aquaporin-4 in ammonium chloride treated cells were significantly increased relative to the control group (P < 0.001); SB203580 co-treatment inhibited the increased expression of phospho-p38 and aquaporin-4 relative to the ammonium chloride treated group (P = 0.002 and P = 0.015 respectively). The phosphorylation of p38 and upregulation of aquaporin-4 were highly correlated (r = 0.909). There were no significant differences in total p38 expression among the groups (P = 0.341).


CONCLUSIONS
Ammonium chloride induced upregulation of aquaporin-4 in astrocytes is regulated by the p38 mitogen-activated protein kinase pathway. Inhibiting p38 activation prevented ammonium chloride induced aquaporin-4 protein upregulation.
Pan
Cai-Fei
CF
Department of Anesthesiology, First Affiliated Hospital, School of Medicine, Zhejiang University, Hangzhou, Zhejiang 310003, China.
Zhu
Sheng-Mei
SM
Zheng
Yue-Ying
YY
D000643:Ammonium Chloride / Q000494:pharmacology*; D000818:Animals; D051401:Aquaporin 4 / Q000235:genetics / Q000378:metabolism*; D001253:Astrocytes / Q000187:drug effects* / Q000378:metabolism*; D015153:Blotting, Western; D002478:Cells, Cultured; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007093:Imidazoles / Q000494:pharmacology; D010766:Phosphorylation / Q000187:drug effects; D011725:Pyridines / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D015398:Signal Transduction / Q000187:drug effects; D048051:p38 Mitogen-Activated Protein Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism*
D051401:Aquaporin 4; D004791:Enzyme Inhibitors; D007093:Imidazoles; D011725:Pyridines; D000643:Ammonium Chloride; D048051:p38 Mitogen-Activated Protein Kinases; C093642:SB 203580