Abstract
Acyl coenzyme A:diacylglycerol acyltransferase 1 (DGAT1) is one of the four
intestinal membrane bound acyltransferases implicated in dietary fat
absorption. Recently, it was found that, in addition to acylating
diacylglycerol (DAG), DGAT1 also possesses robust enzymatic activity for
acylating monoacylglycerol (MAG) (Yen, C. L., Monetti, M., Burri, B. J., and
Farese, R. V., Jr. (2005)
J. Lipid Res.
46, 1502–1511). In the
current paper, we have conducted a detailed characterization of this reaction
in test tube, intact cell culture, and animal models. Enzymatically, we found
that triacylglycerol (TAG) synthesis from MAG by DGAT1 does not behave
according to classic Michaelis-Menten kinetics. At low concentrations of 2-MAG
(<50 μ
m
), the major acylation product by DGAT1 was TAG;
however, increased concentrations of 2-MAG (50–200 μ
m
)
resulted in decreased TAG formation. This unique product/substrate
relationship is similar to MGAT3 but distinct from DGAT2 and MGAT2. We have
also found that XP620 is an inhibitor that selectively inhibits the acylation
of MAG by DGAT1 (IC
50
of human DGAT1: 16.6 ± 4.0
n
m
(MAG as substrate) and 1499 ± 318 n
m
(DAG as
substrate); IC
50
values of human DGAT2, MGAT2, and MGAT3 are
>30,000 n
m
). Using this pharmacological tool, we have shown that
∼76 and ∼89% of the
in vitro
TAG synthesis initiated from MAG
is mediated by DGAT1 in Caco-2 cell and rat intestinal mucosal membranes,
respectively. When applied to intact cultured cells, XP620 substantially
decreased but did not abolish apoB secretion in differentiated Caco-2 cells.
It also decreased TAG and DAG syntheses in primary enterocytes. Last, when
delivered orally to rats, XP620 decreased absorption of orally administered
lipids by ∼50%. Based on these data, we conclude that the acylation of
acylglycerols by DGAT1 is important for dietary fat absorption in the
intestine.