Aj. Chu, MECHANISM BY WHICH ETHANOL INHIBITS PHOSPHATIDYLCHOLINE BIOSYNTHESIS IN HUMAN LEUKEMIC MONOCYTE-LIKE U937 CELLS, Cell biochemistry and function, 12(1), 1994, pp. 45-55
A previous study showing that ethanol (ETOH) blocked [H-3]choline inco
rporation into phosphatidylcholine (PC) suggested an inhibition of PC
biosynthesis in human leukemic monocyte-like U937 cells. The mechanism
of the inhibitory action of ETOH was investigated. Cells were pulsed
with [H-3]choline for 30 min and chased in the presence or absence of
ETCH for up to 6 h. PC biosynthesis was inhibited drastically within 1
h after exposure to ETOH which increased intracellular cAMP appreciab
ly. After a 3-h treatment, ETOH significantly inhibited both choline k
inase (CK) and the cytosolic CTP: cholinephosphate cytidylyltransferas
e (CT). The inactivated CT was no longer stimulated by exogenous phosp
hatidylglycerol (PG). There was no evidence for redistribution of CT a
ctivity between cytosol and microsomes. When cells were exposed to 8-B
romo-cAMP ranging from 100 to 300 mu M, PC biosynthesis remained unaff
ected despite the drastically elevated cAMP. These results seem to sug
gest that the raised cAMP is not a prerequisite for the inhibition of
PC biosynthesis in U937 cells. Following pretreatment with protein kin
ase inhibitors (H-89 and K-252a), PC biosynthesis was decreased signif
icantly and the inhibitory effect of ETOH was potentiated. Taken toget
her, our results suggest that the inhibition of PC biosynthesis and th
e inhibitory effect of ETOH are independent of the activation of cAMP-
dependent protein kinase. Unlike protein kinase inhibitors, pretreatme
nt with tyrosine kinase inhibitors (erbstatin, genistein and tyrphosti
n 25) resulted in differential effects on PC biosynthesis and on the i
nhibitory action of ETOH. Genistein stimulated PC biosynthesis by 30 p
er cent as well as partially reventing /reversing the ETOH action, whi
le tyrphostin 25 produced a synergistic inhibition. The relevance of t
yrosine phosphorylation/dephosphorylation to the regulation of PC bios
ynthesis and ETOH action remains to be established.