Serotonin metabolism in rat mesangial cells: Involvement of a serotonin transporter and monoamine oxidase A

Background. Serotonin is one of the factors regulating mesangial cell proli feration, and convergent evidence supports its involvement in the developme nt of glomerulonephritis. In this study, we identified a serotonin transpor ter and the amine-degrading enzyme monoamine oxidases (MAOs) in mesangial c ells, and we studied their involvement in serotonin degradation. Methods. MAOs were characterized in membrane preparations and intact mesang ial cells by enzyme assay using [C-14]5-hydroxytryptamine and [C-14]beta-ph enylethylamine as specific substrates for MAO-A and MAO-B, respectively, an d by Western blot analysis. The expression of a serotonin transporter was d etermined by [C-14]5-hydroxytryptamine uptake experiments and Western blot. Mesangial cell proliferation was measured by BrdU incorporation. Results. Quantitation of the MAO isoforms by enzyme assay and Western blot analysis showed that MAO-A was largely predominant in mesangial cells, acco unting for approximately 90% of the total enzyme population. The MAO substr ate [C-14]serotonin was transported into mesangial cells by a saturable upt ake system (V-max 310 +/- 36 pmol/30 min/mg protein; K-m 5.9 +/- 1.4 mu M) displaying the pharmacological properties of a serotonin transporter. The e xpression of a serotonin transporter was confirmed by Western blot analysis . MAO activity measured in intact cells showed that after accumulation into mesangial cells, [C-14]serotonin was metabolized by MAO-A. Finally, seroto nin-mediated mesangial cell proliferation was significantly increased after irreversible MAO inhibition. Conclusions. Our results suggest that serotonin concentration and function in glomeruli may be regulated in part by its transport into mesangial cells and degradation by MAO-A.