Destabilization of glial fibrillary acidic protein mRNA in astrocytes by ammonia and protection by extracellular ATP.
The expression of the astrocyte-specific intermediate filament protein, glial fibrillary acidic protein (GFAP), is decreased in hepatic encephalopathy and increased in numerous neurological conditions including brain injury. However, little is known about the molecular mechanisms that regulate GFAP expression. Here it is reported that treatment of cultured astrocytes with ammonium chloride reduces GFAP mRNA by up to 85% without inhibiting total RNA synthesis. The effect of NH4Cl was time and dose dependent. The reduction in GFAP mRNA was detected 3 h after initiation of ammonia treatment with a maximum effect observed at 24 h. Significant decreases in GFAP mRNA were observed at 2, 5, and 10 mM NH4Cl. Concurrent treatment with extracellular ATP prevented the loss of GFAP mRNA, possibly by activation of purinergic receptors. In addition, removal of ammonium chloride restored GFAP mRNA to normal levels. Nuclear runoff experiments indicated that NH4Cl did not inhibit GFAP mRNA transcription. Studies using alpha-amanitin, an inhibitor of RNA polymerase II, showed that NH4Cl decreased the stability of GFAP mRNA by approximately 50%. This destabilization of GFAP mRNA may be an important factor in the pathogenesis of hepatic encephalopathy. Because increased GFAP is an important component of reactive gliosis, understanding the mechanisms that destabilize GFAP mRNA may facilitate strategies to minimize the gliosis associated with brain injury.
Laboratory of Neuropathology, Research Service, VA Medical Center, Miami, Florida.
J T
JT
Neary
S R
SR
Whittemore
Q
Q
Zhu
M D
MD
Norenberg
D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000643:Ammonium Chloride / Q000494:pharmacology*; D000818:Animals; D001253:Astrocytes / Q000378:metabolism*; D015152:Blotting, Northern; D002478:Cells, Cultured; D004355:Drug Stability; D005786:Gene Expression Regulation / Q000187:drug effects*; D005904:Glial Fibrillary Acidic Protein / Q000235:genetics*; D006501:Hepatic Encephalopathy / Q000378:metabolism; D007700:Kinetics; D012333:RNA, Messenger / Q000378:metabolism*; D051381:Rats; D014529:Uridine / Q000378:metabolism; D014746:Vimentin / Q000235:genetics
D005904:Glial Fibrillary Acidic Protein; D012333:RNA, Messenger; D014746:Vimentin; D000643:Ammonium Chloride; D000255:Adenosine Triphosphate; D014529:Uridine