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B Activity in Human Osteoblastic U2-OS Cells through Estrogen Receptor (ER)
, and Not through ER
1
Hubrecht Laboratory (M.E.Q., C.E.V.D.B., S.W., R.H.M.M.S., P.T.V.D.S., B.V.D.B.), Netherlands Institute for Developmental Biology, Uppsalalaan 8, 3584 CT Utrecht, the Netherlands; and Department of Medical Nutrition (J.-Å.G.), Karolinska Institute, Novum, S-14186 Huddinge, Sweden
Address all correspondence and requests for reprints to: Dr. Bart van der Burg, Hubrecht Laboratory, Netherlands Institute for Developmental Biology, Uppsalalaan 8, 3584 CT Utrecht, The Netherlands. E-mail: bvdb{at}niob.knaw.nl
Estrogens are important mediators of bone homeostasis, and
postmenopausal estrogen replacement therapy is extensively used to
prevent osteoporosis. The biological effects of estrogen are mediated
by receptors belonging to the superfamily of steroid/thyroid nuclear
receptors, estrogen receptor (ER)
and ER
. ER
, not only
trans-activates target genes in a hormone-specific
fashion, but it can also neutralize other transcriptional activators,
such as nuclear factor (NF)-
B, causing repression of their target
genes. A major mechanism by which estrogens prevent osteoporosis seems
to be repression of transcription of NF-
B target genes, such as the
osteoclast-activating cytokines interleukin-6 and interleukin-1. To
study the capacity of both ERs in repression of NF-
B signaling in
bone cells, we first carried out transient transfections with ER
or
ER
of the human osteoblastic U2-OS cell line, in which
endogenous NF-
B was stimulated by tumor necrosis factor
.
Repression by ER
was already observed without 17
-estradiol,
whereas addition of the ligand increased repression to 90%. ER
,
however, was able to repress NF-
B activity only in the presence of
ligand. Because it is known that some antiestrogens can also display
tissue-specific agonistic properties, 4-hydroxytamoxifen was tested for
its capacity in repressing NF-
B activity and was found to be active
(albeit less efficient than 17
-estradiol) and, interestingly, only
with ER
. The pure antagonist ICI 164,384 was incapable of repressing
through any ER subtypes. Deletion analysis and the use of receptor
ER
/ER
-chimeras showed that the A/B domain, containing activation
function-1, is essential for this suppressive action. Next, we
developed stable transfectants of the human osteoblastic U2-OS cell
line containing ER
or ER
in combination with an NF-
B
luciferase reporter construct. In these cell lines, repression of
NF-
B activity was only mediated through ER
and not through ER
.
These findings offer new insights into the specific role of both ER
subtypes in bone homeostasis and could eventually help in developing
more specific medical intervention strategies for osteoporosis.
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