Lithium increases cAMP responsive element (CRE)/CRE binding protein directed gene transcription in vitro by enhancing transactivation of transcription

Lithium salts (Li) are clinically relevant treatments for bipolar disorder. Their mood stabilizing action occurs after a lag period of up to several months and is not yet understood. CREB (cAMP responsive element binding protein) is a transcription factor which has been implicated with important brain functions and action of psychogenic agents. CREB is phosphorylated after activation of cAMP-and Ca2+-depending signal cascades and binds as dimer to a cAMP responsive element (CRE). Our goal was to investigate the influence of Li on CRE/CREB-directed transcription in vitro. HIT-T15 cells were transfected with CRE reporter gene constructs and transcription was stimulated with 8Br-cAMP. Li treatment resulted in massive increase in CRE/CREB-directed reporter gene expression. 20 mM LiCl caused an enhancement up to 10-fold. The half maximal effect was reached at 4,1 mM. Li are known glycogen synthase 3ß (GSK3) inhibitors. However, when two GSK3-inhibitors were tested no enhancement of transcription occured. Li also block inositol monophosphate phosphatase (IMP) and subsequent cause inositol depletion. Addition of inositol did not abolish the Li effect. Thus there seems to be no correlation between both GSK3 and IMP inhibition and enhancement of transcription by Li. Li did not elevate phospho-CREB levels as determined by western blot analysis. They also not increased DNA binding of CREB to the CRE as shown by gel shift assays. However, Li enhanced transactivation of transcription when a fusion construct containing Gal4 DNA binding domain and full length CREB was used. When coexpressed with a Gal4 reporter gene Li increased Gal4-driven reporter gene expression by 2,5 fold. Thus, Li enhanced at least partially the CRE/CREB directed transcription by intensifying the interaction of DNA-bound CREB with the transcription machinery. This findings show that Li increase CRE/CREB-directed gene expression. Thus, we support the hypothesis that Li treatment gradually leads to neuronal adaptation and suggest CREB as a possible candidate for mediating this action.