Regulation of apolipoprotein E secretion in rat primary hippocampal astrocyte cultures

Apolipoprotein E isoforms may have differential effects on a number of path ological processes underlying Alzheimer's disease. Recent studies suggest t hat the amount, rather than the type, of apolipoprotein E may also be an im portant determinant for Alzheimer's disease. Therefore, understanding the r egulated synthesis of apolipoprotein E is important for determining its rol e in Alzheimer's disease. We show here that in rat primary hippocampal astrocyte cultures, dibutyryl- cAMP increased apolipoprotein E secretion with time in a dose-dependent man ner (to 177% at 48 h) and that retinoic acid potentiated this effect (to 29 8% at 48 h). Dibutyryl-CAMP also gave a rapid, albeit transient, increase o f apolipoprotein E mRNA expression (to 200% at 1 h). In contrast, the prote in kinase C activator phorbol 12-myristate 13-acetate decreased both apolip oprotein E secretion (to 59% at 48 h) and mRNA expression (to 22% at 1 h). Phorbol 12-myristate 13-acetate also reversed the effects of dibutyryl-CAMP . Apolipoprotein E secretion was also modulated by receptor agonists for th e adenylyl cyclase/CAMP pathway. Isoproterenol (50 nM, a beta -adrenoceptor agonist) enhanced, while clonidine (250 nM, an alpha2-adrenoceptor agonist ) decreased, secreted apolipoprotein E. We also analysed the effects of ago nists for the phospholipase C/protein kinase C pathway. Arterenol (1 muM, a n alpha1-adrenoceptor agonist) and serotonin (2.5 muM) enhanced, whereas ca rbachol (10 muM, an acetylcholine muscarinic receptor agonist) decreased se creted apolipoprotein E. The effects of these nonselective receptor agonist s were modest, probably due to effects on different signalling pathways. Ar terenol also potentiated the isoproterenol-mediated increase. We also show that phorbol 12-myristate 13-acetate and dibutyryl-cAMP have opposite effec ts on nerve growth factor, as compared to apolipoprotein E, secretion, sugg esting that the results obtained were unlikely to be due to a general effec t on protein synthesis. We conclude that astrocyte apolipoprotein E production can be regulated by factors that affect CAMP intracellular concentration or activate protein ki nase C. Alterations in these signalling pathways in Alzheimer's disease bra in may have consequences for apolipoprotein E secretion in this disorder. ( C) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved.