Possible involvement of amino acid transporters on S-nitroso-cysteine-induced inhibition of arachidonic acid release in PC12 cells

Measuring tryptophan dynamics using fast scan cyclic voltammetry at carbon fiber microelectrodes with improved sensitivity and selectivity

Despite the fact that tryptophan (Trp) is an essential amino acid that humans typically obtain through diet, there are several interesting tryptophan dynamics at play in the body. Quantifying and understanding these dynamics are crucial in studies of depression, autism spectrum disorder, and other disorders that involve neurotransmitters directly synthesized from tryptophan. Here we detail the optimization of waveform parameters in fast scan cyclic voltammetry at carbon fiber microelectrodes to yield four-fold higher sensitivity and six-fold higher selectivity compared to previously reported methods. We demonstrate the utility of our method in measuring (1) exogenous Trp dynamics from administration of Trp to PC-12 cells with and without overexpression of tryptophan hydroxylase-2 and (2) endogenous Trp dynamics in pinealocyte cultures with and without stimulation via norepinephrine. We observed interesting differences in Trp dynamics in both model systems, which demonstrate that our method is indeed sensitive to Trp dynamics in different applications.

Hyperoxia inhibits nitric oxide treatment effects in alveolar epithelial cells via effects on L-type amino acid transporter-1

AIMS

The aims of this study were to determine hyperoxia effects on S-nitrosothiol (SNO) accumulation and L-type amino acid transporter 1 (LAT1) expression/function in alveolar epithelium and to determine whether hyperoxia impairs exogenous nitric oxide (NO) treatment effects in alveolar epithelium through effects on LAT1 expression and/or function.

RESULTS

SNO accumulation in vitro and in vivo after NO treatment was dependent on the LAT1 system transport. Hyperoxia (60% or 90%) impaired NO effects on SNO accumulation and soluble guanylyl cyclase activation in proportion to the magnitude of hyperoxia and the duration of exposure, up to 12 h, in type I-like (R3/1) and type II-like (L2) rat and human (A549) alveolar epithelial cells. LAT function, determined by sodium-independent (3)H-leucine uptake, was impaired in a parallel manner. Hyperoxia impaired LAT1 expression in alveolar epithelial cells, determined by immunoblots and immunofluorescence, and in newborn rats exposed to 60% O2 for 4 days, determined by immunohistochemistry.

INNOVATION

Despite significant preclinical evidence, inhaled NO has shown disappointing limitations in clinical applications. Our studies suggest an important explanation: oxidative stress, a common feature of diseases in which therapeutic NO would be considered, impairs LAT1 expression and function, blocking a major route for inhaled NO (iNO) action, that is, the uptake of S-nitrosocysteine via LAT1.

CONCLUSIONS

SNO uptake after NO treatment is dependent on LAT1. Hyperoxia impairs SNO uptake and NO effects during NO exposure and impairs LAT system function and LAT1 expression. Effects on SNO formation and transport must be considered for rational optimization of NO-based therapeutics.

The mechanism of transmembrane S-nitrosothiol transport

S-nitrosothiols have been suggested to play an important role in nitric oxide (NO)-mediated biological events. However, the mechanisms by which an S-nitrosothiol (or the S-nitroso functional group) is transferred across cell membrane are still poorly understood. We have demonstrated previously that the degradation of S-nitrosoglutathione (GSNO) by cells absolutely required the presence of cystine in the extracellular medium and proposed a mechanism that involved the reduction of cystine to cysteine, followed by the reaction of cysteine with GSNO to form S-nitrosocysteine (CysNO), mixed disulfides, and nitrosyl anion. In the present study we have assessed the effect of cystine on the transfer of the S-nitroso functional group from the extracellular to the intracellular space. Using RAW 264.7 cells, we found that the presence of L-cystine enhanced GSNO-dependent S-nitrosothiol uptake, increasing the intracellular S-nitrosothiol level from approximately 60 pmol/mg of protein to approximately 3 nmol/mg of protein. The uptake seems to depend on the reduction of L-cystine to L-cysteine, which involves the xc- amino acid transport system, the transnitrosation between GSNO and L-cysteine to form L-CysNO, and uptake of L-CysNO via amino acid transport system L. Compared with GSNO, (Z)-1-[N-(3-ammoniopropyl)-N-[4-(3-aminopropylammonio)butyl]-amino]diazen-1-ium-1,2-diolate, an NO donor, is much less effective at intracellular S-nitrosothiol formation in the presence of L-cystine or L-cysteine, suggesting that the biochemical changes that occur after exposure of cells to S-nitrosothiol, with respect to thiol chemistry, are distinctly different from those observed with NO.

Involvement of the system L amino acid transporter on uptake of S-nitroso-L-cysteine, an endogenous S-nitrosothiol, in PC12 cells

Previously, we proposed that S-nitroso-L-cysteine, an endogenous S-nitrosothiol, was incorporated via the system L-like amino acid transporter(s) in rat brain slices. In this study, we investigated the effect of S-nitroso-L-cysteine on L-[3H]leucine uptake in PC12 cells (a neuronal cell line). L-[3H]Leucine uptake in PC12 cells was Na(+) independent and significantly inhibited by an inhibitor of system L and by L-phenylalanine, L-cysteine, L-methionine and L-leucine at 1 mM. The effects of L-alanine, L-serine and L-threonine were limited. S-Nitroso-L-cysteine, but not other nitric oxide compounds, inhibited L-[3H]leucine uptake, and this inhibitory effect was eliminated by washing with buffer. System L is composed of the 4F2 light chains (LAT1 or LAT2) and the heavy chain, and the transcripts of these components were detected in RNA from PC12 cells. These findings suggest that S-nitroso-L-cysteine is incorporated via the system L amino acid transporter and thus regulates cell responses in PC12 cells.