S. Lamon-fava et al., Estrogen increases apolipoprotein (Apo) A-I secretion in Hep G2 cells by modulating transcription of the apo A-I gene promoter, ART THROM V, 19(12), 1999, pp. 2960-2965
Estrogen administration to postmenopausal women has been shown to increase
plasma levels of apolipoprotein (apo) A-I. A human hepatoma cell line, Hep
G2, was used to test the hypothesis that estrogen increases the hepatic pro
duction of apo A-I by modulating gene expression. When Hep G2 cells were tr
eated fur 24 hours with E-2, the apo A-I content in the medium increased 4.
3+/-1.0-fold at 10 mu mol/L E-2 and 1.8+/-0.4-fold at 1 mu mol/L E-2 compar
ed with untreated cells. A time-course experiment indicated that there was
no E-2-dependent (10 mu mol/L) increase in apo A-I medium content at 1 hour
and 2 hours and that apo A-I was 165% of controls at 6 hours and 440% at 2
4 hours. Hep G2 cells were transfected, by the cationic lipid method, with
constructs containing serial deletions of the 5' region of the apo A-I gene
(-41/+397, -256/+397, and -2500/+397) cloned in front of the luciferase ge
ne and with or without a 7-kb region spanning the apo C-III/A-IV intergenic
region, which has been shown to contain regulatory elements for the expres
sion of the apo A-I gene. With the exception of the construct containing on
ly the basal promoter (-41/+397), the expression of all constructs was 2- t
o 3-fold greater in the presence of E-2. The smallest construct that mainta
ined E-2 responsiveness,the -256/+397 construct, does not contain a typical
estrogen-responsive element. In the same transfection experiments, the 4-f
old increase in apo A-I in the culture medium was preserved. However, when
the same set of transfections was performed by the calcium phosphate precip
itation method, the E-2 effect on the apo A-I content in the culture medium
and on transcription activation was nearly abolished. This effect was prob
ably mediated by Ca2+ because incubation of cells with 20 mmol/L CaCl2 abol
ished the E,response. In conclusion, E-2 increases apo A-I production in he
patic cells by increasing the transcription of the apo A-I gene.