DDC expression is not regulated by NFAT5 (TonEBP) in dopaminergic neural cell lines

The nuclear factor of activated T-cells 5 (NFAT5), also known as tonicity-responsive enhancer-binding protein (TonEBP), is a transcription factor that regulates osmoadaptive response in multiple tissues and is highly expressed in the developing central nervous system. A former study reported that NFAT5 activation through hypertonic stress increases the expression of the dopa decarboxylase enzyme (DDC), also known as aromatic-l-amino-acid decarboxylase (AADC), in human renal proximal tubule cells, leading to an increase of dopamine synthesis. In a previous study, we identified NFAT5 as a candidate gene for cocaine dependence, a complex psychiatric disorder in which dopaminergic neurotransmission plays an important role. Therefore, to test the hypothesis that NFAT5 may also affect dopamine levels in the nervous system through the regulation of DDC expression, we examined this regulation using two neural dopaminergic cell lines, SH-SY5Y and PC12. The effect of NFAT5 on the expression of the neuronal isoform of DDC was evaluated by qRT-PCR. Upon hypertonic stress, NFAT5 was activated and accumulated into the nuclei and, subsequently, the expression of NFAT5 and its known targets sodium/myo-inositol cotransporter 1 (SMIT) and sodium chloride/taurine cotransporter (TAUT) increased, as expected. However, the expression of DDC decreased. When silencing the expression of NFAT5 with a specific shRNA we observed that the downregulation of DDC is independent from NFAT5 in both cell lines and is due to hypertonic stress. In conclusion, NFAT5 does not regulate the expression of the neuronal isoform of DDC in neural dopaminergic cell lines and, consequently, it does not modulate dopamine synthesis through DDC.