Transcriptional down-regulation of MARCKS gene expression in immortalized hippocampal cells by lithium

The gene (Macs) for the mouse myristoylated alanine-rich C kinase substrate (MARCKS) encodes a prominent substrate for protein kinase C that has been implicated in processes requiring signal dependent changes in actin-membran e plasticity and cytoskeletal restructuring. We have previously demonstrate d that MARCKS protein is significantly downregulated in rat hippocampus and in an immortalized hippocampal cell line (HN33.dw) following long-term exp osure to lithium at clinically relevant concentrations (1 mm). Our current studies have examined transcriptional and posttranscriptional events that m ay underlie the lithium-induced down-regulation of MARCKS protein in the cu ltured hippocampal cell model system. MARCKS mRNA and protein expression we re found to be concomitantly down-regulated following exposure of the HN33. dw cells to chronic lithium. Whereas the stability of MARCKS mRNA remained unchanged in the presence of lithium, nuclear run-off assay indicated that the transcription of nascent MARCKS mRNA was significantly reduced (similar to 50%) in the cells that had been treated with lithium for 7 days. Transi ent transfection of HN33.dw cells with a mouse cloned Macs promoter (993-bp ) showed that the Macs promoter activity was attenuated to the same extent after chronic (7-10 days), but not subacute (24 h), lithium exposure. The i nhibition of the Macs promoter was found to be dependent upon the presence of a 280-bp promoter region between -993-bp and -713-bp relative to the tra nslation start site, suggesting that this region is a potential lithium-res ponsive region of Macs promoter (LRR). Mutant promoter lacking the LRR not only did not respond to chronic lithium exposure but also had significantly reduced promoter activity, suggesting that chronic lithium exposure repres ses the transcriptional activity of activator(s) bound to the promoter. Tak en together, our data indicate that transcriptional inhibition of the Macs gene underlies the lithium-induced down-regulation of MARCKS expression in the immortalized hippocampal cells.