Functional analysis of choline transporters in rheumatoid arthritis synovial fibroblasts

Objectives: In this study, we examined the functional characteristics of choline uptake and sought to identify the transporters in rheumatoid arthritis synovial fibroblasts (RASFs).Methods: The expression of choline transporters was evaluated by quantitative real-time PCR, western blotting, and immunocytochemistry. Time course, Na+-dependency, and kinetics of [H-3]choline uptake were investigated. Effects of cationic drugs on the uptake of [H-3]choline, cell viability, and caspase-3/7 activity were also examined. Finally, we investigated the influence of choline uptake inhibitor, hemicholinium-3 (HC-3), and choline deficiency on cell viability and caspase-3/7 activity.Results: Choline transporter-like protein 1 (CTL1) and CTL2 mRNA and protein were highly expressed in RASFs and were localized to the plasma membrane. [H-3]Choline uptake occurred via a Na+-independent and pH-dependent transport system. The cells have two different [H-3]choline transport systems, high- and low-affinity. Various organic cations, HC-3 and choline deficiency inhibited both [H-3]choline uptake and cell viability, and enhanced the caspase-3/7 activity. The functional inhibition of choline transporters could promote apoptotic cell death. In RASFs, [H-3]choline uptake was significantly increased compared with that in OASFs without a change in gene expression.Conclusions: These results suggest that CTL1 (high-affinity) and CTL2 (low-affinity) are highly expressed in RASFs and choline may be transported by a choline/H(+)antiport system. Identification of this CTL1- and CTL2-mediated choline transport system should provide a potential new target for RA therapy.