Regulation of Phosphatidylserine Synthesis in Jurkat T Cell Clones: Caffeine Bypasses CD3/TCR-Induced Protein Tyrosine Kinases and Calcium Signals

Abstract

Phosphatidylserine synthesis as measured by the incorporation of [3H]serine into phosphatidylserine (PtdSer) through the serine-base exchange enzyme system (serine-BEES) is markedly inhibited in Jurkat cells treated with caffeine. The caffeine-induced inhibition was compared to that observed in cells 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 cofactor, caffeine modified PtdSer synthesis in the absence of decreased Ca2+ content of ER. Using Jurkat clones differing by the expression of cell surface markers or protein tyrosine kinases implicated in the CD3/TCR signal transmission pathway, we have shown that CD3 mAb-induced inhibition of PtdSer synthesis necessitates the expression of both the CD3/TCR and the protein tyrosine phosphatase CD45 at the cell surface as well as the presence of p56lck and ZAP-70 protein tyrosine kinases. By contrast, thapsigargin, a blocker 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 effect on Ca2+ release from ER and on protein tyrosine phosphorylations, caffeine inhibited PtdSer synthesis in all the Jurkat clones. The use of several cAMP-inducing drugs and of others xanthine derivatives indicated that caffeine modify PtdSer synthesis either by a direct action on the serine-BEES or by a modification of the structure of the phospholipids used as substrate by the enzyme.