Disruption of Choline Methyl Group Donation for
Phosphatidylethanolamine Methylation in Hepatocarcinoma Cells*
Cynthia J.
DeLong
§,
Amy M.
Hicks§, and
Zheng
Cui
**
From the Departments of Biochemistry and
Cancer Biology, Wake Forest University School of Medicine,
Winston-Salem, North Carolina 27157
Despite being widely hypothesized, the actual
contribution of choline as a methyl source for phosphatidylethanolamine
(PE) methylation has never been demonstrated, mainly due to the
inability of conventional methods to distinguish the products from that of the CDP-choline pathway. Using a novel combination of stable-isotope labeling and tandem mass spectrometry, we demonstrated for the first
time that choline contributed to phosphatidylcholine (PC) synthesis
both as an intact choline moiety via the CDP-choline pathway and as a
methyl donor via PE methylation pathway. When hepatocytes were labeled
with d9-choline containing three deuterium atoms on each of the three methyl groups, d3-PC
and d6-PC were detected, indicating that newly
synthesized PC contained one or more individually mobilized methyl
groups from d9-choline. The synthesis of
d3-PC and d6-PC was
sensitive to the general methylation inhibitor 3-deazaadenosine and
were specific products of PE methylation using choline as a one-carbon
donor. While the contribution to the CDP-choline pathway remained
intact in hepatocarcinoma cells, contribution of choline to PE
methylation was completely disrupted. In addition to a previously
identified lack of PE methyltransferase, hepatocarcinoma cells were
found to lack the abilities to oxidize choline to betaine and to donate
the methyl group from betaine to homocysteine, whereas the usage of
exogenous methionine as a methyl group donor was normal. The failure to
use choline as a methyl source in hepatocarcinoma cells may contribute
to methionine dependence, a widely observed aberration of one-carbon
metabolism in malignancy.
*
This work was supported in part by National Institutes of
Health Grant R01CA7960 (to Z. C.).The costs of publication of this article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
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