Abstract
Retinoic acid (RA) receptors (RARs) are nuclear receptors whose specific natural ligands are all-trans RA (atRA) and 9-cis RA. RARs control aspects of vision, cell differentiation, immune response, and embryonic development in vertebrates. However excess RAR signaling can cause various teratogenic effects on developing vertebrates. In our previous studies, we have revealed that municipal wastewater generally contains RAR agonists, and identified that the major causative compounds are RAs [atRA and 13-cis RA (13cRA) ] and 4-oxo-RAs (4-oxo-atRA and 4-oxo-13cRA), which are oxidative metabolites of RAs. In this study, to elucidate the fates of RAs and 4-oxo-RAs during wastewater treatment, we investigated the variations in their concentrations and the overall RAR agonistic activity in municipal wastewater treatment plants in Osaka, Japan. Results of this study suggested that RAs and 4-oxo-RAs in wastewater can be readily removed by activated sludge treatment, regardless of season and treatment type, and that unidentified RAR agonists are produced during the treatment and persist in the final effluent under certain conditions. However, the current RAR agonist level in the final effluent is unlikely to cause deleterious biological effects.
