Up-regulation of protein l-isoaspartyl methyltransferase expression by lithium is mediated by glycogen synthase kinase-3 inactivation and β-catenin stabilization

During cell aging, proteins accumulate damages, which affect their structure and activity. The protein l-isoaspartyl methyltransferase (PIMT) is involved in the repair of proteins containing abnormal l-isoaspartyl residues. Although its mechanism of action is well defined, little is known about the pathways involved in the regulation of PIMT expression. In this study, we demonstrated that glycogen synthase kinase-3 (GSK-3) and β-catenin are involved in the regulation of PIMT expression. Treatment of astrocytoma cells (U-87) with direct pharmacological GSK-3 inhibitors such as lithium, SB-216763 and SB-415286 stimulated PIMT expression (∼twofold). As expected, GSK-3 inhibition led to an increase of phosphorylated GSK-3β (Ser9) and to β-catenin accumulation. PIMT induction by lithium was dependent on increased protein synthesis. In addition, RT-PCR analysis showed higher level of PIMT mRNA following GSK-3 inhibition, which was abolished by the transcriptional inhibitor actinomycin D. These results demonstrated regulation of PIMT expression by lithium at both the transcriptional and the translational levels. Additionally, inhibition by siRNA of GSK-3 and β-catenin modulated the expression of the PIMT in accordance with GSK-3 pharmacological inhibition. Valproic acid, an antiepileptic drug with mood-stabilizing properties, up-regulated phospho-GSK-3β (Ser9), β-catenin and PIMT levels similarly to lithium. This study reports that PIMT expression is up-regulated by GSK-3 inhibition and β-catenin stabilization upon treatments with lithium and valproic acid. These findings suggest a possible therapeutic role for PIMT in certain brain diseases including epilepsy.