Downregulation of Taurine Transport by~Calcium Blockers in Osteoblast Cells

Abstract

Taurine is found in a high concentration in bone cells and is thought to help enhance bone tissue formation and inhibit bone loss. It is mainly transported by a sodium and chloride ion dependent taurine transporter (TauT), which is expressed in a variety of tissues, such as brain, retina, and placenta, but in bone the transporter has not yet been identified. The purpose of this study is to clarify the uptake mechanism of taurine in osteoblasts using mouse osteoblast cell lines. Mouse stromal ST2 cells and mouse osteoblast-like MC3T3-E1 cells were used as osteoblast cell lines. Detection of TauT mRNA expression in these cells was performed by RT-PCR. The activity of the taurine transporter was assessed by measuring the uptake of [³H]taurine in cell lines in the presence and absence of inhibitors. TauT mRNA was detected in ST2 and MC3T3-E1 cells. [³H]Taurine uptake by these cells exhibited a time dependent increase that was linear for at least 10 min. [³H]Taurine uptake was dependent on the presence of extracellular sodium and chloride ions, and was inhibited by unlabeled taurine, β-alanine and ? -amino-n-butyric acid. Moreover, uptake of [³H]taurine by these cells was dependent on the presence of extracellular calcium. The uptake of [³H]taurine in ST2 cells treated with 4 mM calcium was increased 1.7-fold. The initial rate of [³H]taurine uptake was significantly inhibited by 100 μM nifedipine and 100 μM verapamil. These results suggest that in mouse osteoblast cell lines taurine transport is controlled by extracellular calcium.