Regulation of phospholipid biosynthesis by Ca2+ -calmodulin-dependent protein kinase inhibitors

Inhibitors of Ca²⁺-calmodulin (CaM)-dependent protein kinases strongly modify phospholipid metabolism. Two compounds, KN62 and KT5926 recognized as blockers of Ca²⁺ -CaM-dependent protein kinase II, induced a specific increase in phosphatidylserine (PtdSer) synthesis without noticeable changes in phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) biosynthesis. The increase of PtdSer synthesis was dependent on the presence of Ca²⁺ in the incubation medium and was impaired in cells whose Ca²⁺ stores were depleted by pretreatment with CD3 mAb, thapsigargin or EGTA. The mechanism of the stimulation of PtdSer synthesis by these two compounds seems to involve an accumulation of Ca²⁺ into the endoplasmic reticulum, possibly due to an increased activity of the endoplasmic reticulum Ca²⁺ -ATPase. By contrast, ML-7 and ML-9, two inhibitors of the myosin light chain kinase (MLCK), another Ca²⁺-CaM-dependent kinase, were both capable of increasing PtdSer synthesis and decreasing PtdCho and PtdEtn synthesis, reproducing the effect previously described with CaM-antagonists. The increase of PtdSer caused by ML-7 and ML-9 was Ca²⁺-dependent while the inhibition of PtdCho and PtdEtn synthesis was not. The use of these four protein kinase inhibitors thus suggests the possible existence of two CaM-dependent pathways that differentially regulates phospholipid metabolism in T cells.