Regulation of phosphatidylserine synthesis in Jurkat T cell clones: Caffeine bypasses CD3/TCR-induced protein tyrosine kinases and calcium signals

Phosphatidylserine synthesis as measured by the incorporation of [H-3]serin e into phosphatidylserine (PtdSer) through the serine-base exchange enzyme system (serine-BEES) is markedly inhibited in Jurkat cells treated with caf feine. The caffeine-induced inhibition was compared to that observed in cel ls treated with either CD3 mAb or thapsigargin. While CD3- and thapsigargin -induced inhibition was related to the release of Ca2+ from the endoplasmic reticulum (ER), a process that deprives the serine-BEES of its major cofac tor, caffeine modified PtdSer synthesis in the absence of decreased Ca2+ co ntent of ER. Using Jurkat clones differing by the expression of cell surfac e markers or protein tyrosine kinases implicated in the CD3/TCR signal tran smission pathway, we have shown that CD3 mAb-induced inhibition of PtdSer s ynthesis necessitates the expression of both the CD3/TCR and the protein ty rosine phosphatase CD45 at the cell surface as well as the presence of p56( lck) and ZAP-70 protein tyrosine kinases. By contrast, thapsigargin, a bloc ker of the Ca2+-ATPase of the ER, known for its Ca2+ releasing properties, inhibited PtdSer synthesis in all the Jurkat clones tested, indicating that this compound bypasses the CD3/TCR-induced signals. Despite its lack of ef fect on Ca2+ release from ER and on protein tyrosine phosphorylations, caff eine inhibited PtdSer synthesis in all the Jurkat clones. The use of severa l cAMP-inducing drugs and of others xanthine derivatives indicated that caf feine modify PtdSer synthesis either by a direct action on the serine-BEES or by a modification of the structure of the phospholipids used as substrat e by the enzyme. (C) 1999 Academic Press.