TRANSCRIPTIONAL REGULATION OF MYELIN-ASSOCIATED GLYCOPROTEIN GENE-EXPRESSION BY CYCLIC-AMP

The treatment of rat glioma C6 cells with 10 mu M isoproterenol (Ipt) for 4 days upregulated the expression of the myelin-associated glycopr otein (MAG) gene by approximately 55-fold over the control value. The constant presence of Ipt in the medium was required for the maximal up regulation, as time-restricted exposures to the drug produced only par tial, or no upregulation of the gene. No difference in the MAG mRNA st ability could be detected in Ipt-treated vs untreated cells indicating that the drug upregulates the MAG gene at transcriptional level. Seru m (FCS) strongly attenuated the response of the MAG gene to Ipt.The st imulatory effect of Ipt was profoundly reduced by spermine and H-89, i ndicating that protein kinase A-dependent protein phosphorylation is i nvolved in the MAG gene activation. Within 30 min after Ipt administra tion, the c-fos gene was upregulated by 10-fold, and thereafter, its m essage level decreased and stabilized at approximately 3-fold over con trol. In contrast, the c-jun gene was downregulated to approximately 2 0% of control within 30 min after Ipt administration. Subsequently, it s message level rose and fell once again within 12 h to approximately half of control, and returned to control level within 72 h. Transfecti on of C6 cells with progressively deleted pCAT constructs containing u p to 861 bp of the MAG gene upstream region demonstrated that active c ore promoter of the gene is located within 138 bp upstream from the in itiation start site. Additional sequences contain both activating and repressive cis-elements. Practically no upregulation of the CAT activi ty could be elicited by challenging the transfected cells with Ipt, in dicating that AP-1 sites (14 within the 861 bp region) are not solely responsible for the Ipt-induced upregulation of the MAG gene, and that the responsible cis-elements map further upstream from the transcript ional unit. (C) 1994 Wiley-Liss, Inc.