Transcriptional regulation of myelin associated glycoprotein gene expression by cyclic AMP

The treatment of rat glioma C6 cells with 10μ isoproterenol (Ipt) for 4 days upregulated the expression of the myelin‐associated glycoprotein (MAG) gene by approximately 55‐fold over the control value. The constant presence of Ipt in the medium was required for the maximal upregulation, as time‐restricted exposures to the drug produced only partial, or no upregulation of the gene. No difference in the MAG mRNA stability could be detected in Ipt‐treated vs untreated cells indicating that the drug upregulates the MAG gene at transcriptional level. Serum (FCS) strongly attenuated the response of the MAG gene to Ipt. The stimulatory effect of Ipt was profoundly reduced by supermine and H‐89, indicating that protein kinase A‐dependent protein phosphorylation is involved in the MAG gene activation. Within 30 min after Ipt administration, the c‐fos gene was upregulated by 10‐fold, and thereafter, its message level decreased and stabilized at approximately 3‐fold over control. In contrast, the c‐jun gene was downregulated to approximately 20% of control within 30 min after Ipt administration. Subsequently, its message level rose and fell once again within 12 h to approximately half of control, and returned to control level within 72 h. Transfection of C6 cells with progressively deleted pCAT constructs containing up to 861 bp of the MAG gene upstream region demonstrated that active core promoter of the gene is located within 138 bp upstream from the initiation start site. Additional sequences contain both activating and repressive cis‐elements. Practically no upregulation of the CAT activity could be elicited by challenging the transfected cells with Ipt, indicating 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 transcriptional unit. Wiley‐Liss, Inc.