MYRISTOYLATED ALANINE-RICH C-KINASE SUBSTRATE (MARCKS) - A MOLECULAR TARGET FOR THE THERAPEUTIC ACTION OF MOOD STABILIZERS IN THE BRAIN

Background: Lithium remains a first-line treatment for the acute and p rophylactic management of bipolar illness. Previous studies in our lab oratory have demonstrated that chronic, but not acute, exposure to the rapeutic concentrations of lithium significantly reduces the expressio n of the protein kinase C (PKC) substrate MARCKS (myristoylated alanin e-rich C kinase substrate) in the rat hippocampus and an immortalized hippocampal cell Line (HN33). The anticonvulsant drugs valproate and c arbamazepine are emerging as efficacious alternative and adjunctive tr eatments for bipolar disorder. In the present study, we sought to dete rmine the effects of valproate and carbamazepine on MARCKS protein lev els by using our hippocampal cell model. Method: HN33 immortalized hip pocampal cells were exposed acutely or chronically to sodium valproate 1 mM, carbamazepine 100 mu M, lithium chloride 5 mM, or lithium chlor ide 5 mM + sodium valproate 1 mM. Additionally, cells were exposed to lithium chloride 5 mM in the absence or presence of inositol 5 mu M, o r sodium valproate 1 mM in the absence or presence of inositol 40 mu M . After drug exposure, cells were collected, separated into soluble an d membrane fractions, and MARCKS protein assayed by Western blot analy sis using polyclonal rabbit antibody. Immunoreactive bands were quanti tated by densitometric analysis. Results: We report that chronic expos ure of HN33 cells to either Lithium or valproate produced a time-depen dent down-regulation of MARCKS protein. Maximal reduction in MARCKS le vels were observed after 3 days of exposure to valproate and after 7 d ays of exposure to lithium. The reduction of MARCKS produced by lithiu m and valproate alone were additive when the two drugs were combined. The reduction in MARCKS produced by lithium was reversed by the additi on of inositol to the media, whereas the reduction produced by valproa te was unaffected by the addition of inositol. Carbamazepine failed to affect MARCKS protein levels at each dose and time tested. Conclusion : These data provide evidence that, like lithium, chronic exposure to valproate produces a significant time-dependent down-regulation of the PKC substrate MARCKS, whereas carbamazepine is without effect. The MA RCKS reduction produced by valproate appears to occur independently of inositol concentrations yet is additive with the reduction produced b y lithium, which is inositol-reversible. Valproate- and lithium-induce d regulation of MARCKS expression appears to be mediated by different mechanisms that may utilize PKC, and may be associated with the clinic al profile of these mood stabilizers. Regulation of MARCKS expression may be associated with the prophylactic efficacy of lithium in the lon g-term stabilization of the recurrent affective episodes in bipolar di sorder, and valproate may share this property.