Folic acid inhibits tau phosphorylation through regulation of PP2A methylation in SH-SY5Y cells

Neurofibrillary tangles (NFTs), which are composed of intracellular filamentous aggregates of hyperphosphorylated tau protein, are one of the pathological hallmarks of Alzheimer's disease (AD). Because tau phosphorylation is regulated by phosphatases, abnormal metabolism of protein phosphatase 2A (PP2A) has been proposed to be a contributing factor to the disease process. To determine the function of folic acid on tau phosphorylation, an in vitro model of human neuroblastoma cells (SH-SY5Y) were exposed to folic acid (0-40 mu mol/L) for 96 h, in the presence or absence of the phosphoesterase inhibitor okadaic acid (OA) (10 nmol/L) for 9 h. The data of western blot showed tau phosphorylation at the Ser396 site in OA-incubated SH-SY5Y cells was inhibited by folic acid in a concentration-dependent manner, with the folic acid concentration of 40 mu mol/L providing maximal inhibition. Folic acid can downregulate tau protein phosphorylation by inhibiting the demethylation reactions of PP2A. High folic acid concentrations (20 and 40 mu mol/L) increased SAM:SAH ratios and cell viability. Therefore, we can speculate that folate deficiency may be a cause of PP2A deregulation, which can in turn lead to expression of the abnormal hyperphosphorylated form of tau.