Dexamethasone increases the incorporation of [H-3] serine into phosphatidylserine and the activity of serine base exchange enzyme in mouse thymocytes: A possible relation between serine base exchange enzyme and apoptosis

The exposure of phosphatidylserine toward the external surface of the membr ane is a well-established event of programmed cell death. The possibility t hat an apoptotic stimulus influences the metabolism of this phospholipid co uld be relevant not only in relation to the previously mentioned event but also in relation to the capability of membrane phosphatidylserine to influe nce PKC activity. The present investigation demonstrates that treatment of mouse thymocytes with the apoptotic stimulus dexamethasone, enhances the in corporation of [H-3]serine into phosphatidylserine. Cell treatment with dex amethasone also enhanced the activity of serine base exchange enzyme, assay ed in thymocyte lysate. Both the effects were observed at periods of treatm ent preceding DNA fragmentation. The addition of unlabelled ethanolamine, t ogether with [3H]serine to the medium containing dexamethasone-treated thym ocytes lowered the radioactivity into phosphatidylserine. Serine base excha nge enzyme activity was influenced by the procedure used to prepare thymocy te lysate and was lowered by the addition of fluoroaluminate, that is widel y used as a G-protein activator. The increase of serine base exchange enzym e activity induced by dexamethasone treatment was observed independently by the procedure used to prepare cell lysate and by the presence or absence o f fluoroaluminate.