DESTABILIZATION OF GLIAL FIBRILLARY ACIDIC PROTEIN MESSENGER-RNA 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 ence phalopathy 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 ti me and dose dependent. The reduction in GFAP mRNA was detected 3 h aft er initiation of ammonia treatment with a maximum effect observed at 2 4 h. Significant decreases in GFAP mRNA were observed at 2, 5, and 10 mM NH4Cl. Concurrent treatment with extracellular ATP prevented the lo ss of GFAP mRNA, possibly by activation of purinergic receptors. In ad dition, removal of ammonium chloride restored GFAP mRNA to normal leve ls. Nuclear runoff experiments indicated that NH4Cl did not inhibit GF AP mRNA transcription. Studies using alpha-amanitin, an inhibitor of R NA polymerase II, showed that NH4Cl decreased the stability of GFAP mR NA by similar to 50%. This destabilization of GFAP mRNA may be an impo rtant factor in the pathogenesis of hepatic encephalopathy. Because in creased GFAP is an important component of reactive gliosis, understand ing the mechanisms that destabilize GFAP mRNA may facilitate strategie s to minimize the gliosis associated with brain injury.