K. Tran et al., VITAMIN-E SUPPRESSES DIACYLGLYCEROL (DAG) LEVEL IN THROMBIN-STIMULATED ENDOTHELIAL-CELLS THROUGH AN INCREASE OF DAG KINASE-ACTIVITY, Biochimica et biophysica acta, L. Lipids and lipid metabolism, 1212(2), 1994, pp. 193-202
The present study has examined the role of vitamin E, a natural lipid
antioxidant, in the production of diacylglycerol (DAG) and phosphatidi
c acid (PA) in thrombin-stimulated human endothelial cells. Cells were
labelled with [H-3]myristate and the incorporation and distribution o
f [H-3]myristate into cellular lipids was not affected by vitamin E. H
owever, in response to thrombin stimulation, considerably more PA and
less DAG were formed in cells enriched with vitamin E. The time-course
of thrombin stimulation indicated that vitamin E attenuated the accum
ulation of sustained DAG levels with a concomitant increase in PA. Dir
ect determination of DAG mass further confirmed that vitamin E suppres
ses the accumulation of DAG induced by thrombin. In the presence of et
hanol, the formation of [H-3]phosphatidylethanol (PEt) in [H-3]myrista
te-labelled cells stimulated by thrombin was unaffected by vitamin E e
nrichment. DL-Propranolol, a PA phosphohydrolase inhibitor, caused an
accumulation of PA, without affecting DAG formation in either vitamin
E-treated and untreated cells. This indicated that the increase in PA
and decrease in DAG in vitamin E-treated cells was not due to a stimul
ation of phospholipase D or an inhibition of PA phosphohydrolase. Dete
rmination of inositol phosphates formation in response to thrombin sho
wed that the change of DAG levels elicited by vitamin E was independen
t of phospholipase C-induced hydrolysis of inositol phospholipids. In
contrast, analysis of DAG kinase activity revealed that vitamin E enri
chment enhanced the activity of the enzyme in both basal and thrombin-
stimulated cells. Taken together, these data indicated that vitamin E
caused an increased conversion of DAG to PA by activating DAG kinase a
ctivity without causing any change in the activities of phospholipase
D, PA phosphohydrolase or phospholipase C.