Estrogen Receptor Null Mice - Endocrine Reviews

June, 1999 ESTROGEN RECEPTOR NULL MICE 399
also be true in the �ERKO male, which exhibit slightly
shorter femur lengths in the context of only a 2-fold increase
in serum androgens. The possibility that a loss of ER� during
bone development results in abnormal genomic imprinting
and increased ER� and/or AR levels as a compensatory
mechanism must also be considered.
B. Cardiovascular system
Since the early part of this century, a remarkable genderrelated
contrast in the risk of cardiovascular disease has been
known. Although women generally possess a greater incidence
of the multiple risk factors associated with cardiovascular
disease when compared with men, e.g., obesity, diabetes,
elevated blood pressure, and plasma cholesterol,
epidemiological studies continue to indicate their relative
risk of developing this disease is significantly lower (453,
454). It is now believed that the protective factor against
cardiovascular disease in females is their inherently increased
exposure to estrogens (reviewed in Refs. 433 and
454). The protective effects of estrogens have been documented
in a number of epidemiological studies documenting
the reduced rate of cardiovascular disease in postmenopausal
women receiving estrogen replacement therapy (433,
454). This correlation between the administration of estradiol
and a reduced risk of vascular disease has been reproduced
in laboratory animal studies involving several different species
(454).
The possible mechanisms by which estrogens reduce vascular
disease remain unclear but are likely to include positive
modifications of a number of physiological parameters that
are believed to impinge upon the development of cardiovascular
pathlogy (reviewed in Ref. 454). Most notable of
these is the ability of estrogen replacement therapy to significantly
lower total cholesterol, with a profound effect on
levels of the more damaging low-density lipoproteins (LDLs)
(454). Hypercholesterolemia is strongly associated with the
development of cardiovascular disease. The ability of estrogen
therapy to reduce total cholesterol levels is believed to
account for a large portion of the cardiovascular protective
effects of the hormone (433, 454). Lundeen et al. (455) reported
that the significant decreases in serum cholesterol are
specific to estrogen action in the ovariectomized rat, whereas
other gonadal steroids tested had little effect. Furthermore,
cotreatment with the antagonist, ICI-182,780, was shown to
inhibit the cholesterol-lowering effects of estrogens, strongly
indicating that this is a receptor-mediated effect (455). One
possible site at which the actions of estradiol may converge
with the pathways of cholesterol metabolism is via the upregulation
of the apo E protein and the apolipoprotein (apo)
B/E LDL receptor within and on the cell surface of hepatocytes,
respectively (454). During hydrolysis of triglycerides
in the circulation, the apo E protein is integrated into the apo
B-LDL complexes and thereby functions as a ligand for the
hepatocyte apo B/E LDL receptor (454). When the apo Econtaining
LDL complex is bound by the apo B/E LDL receptor
on the cell surface of hepatocytes, the complex is
internalized, thereby providing for a means of clearing LDL
from the blood (454). The importance of the apo E protein in
maintaining serum cholesterol levels and providing protec-
tion against vascular disease is evident from studies in transgenic
mice that either overexpress the protein or possess a
targeted disruption of the apo E gene. Transgenic mice in
which an apo E gene is overexpressed are significantly protected
from atherosclerosis (456), whereas the apo E knockout
is highly susceptible to the vascular disease (457).
Therefore, much of the cholesterol-lowering ability of estradiol
is thought to be due to increased clearance of LDL
from the circulation via the mechanism described above
(454). Regulation of the apo E gene by estradiol has been
demonstrated in the rat (458, 459). Furthermore, Srivastava
et al. (460) recently reported that hepatic levels of apo E
mRNA are similar in wild-type and �ERKO male littermates,
although a slight decrease in serum levels of the protein was
observed in the �ERKO. However, 6 days of estradiol exposure
(via implantation of a 3 �g E 2/g body weight/day
pellet) elicited an almost 2-fold increase in serum apo E levels
in the wild-type compared with a 1.2-fold increase in the
�ERKO (460). Therefore, these data provide evidence that
ER�, acting at the level of translation, is required to mediate
the estradiol up-regulation of apo E expression in mice (460).
In addition to the favorable effects of estrogens on the lipid
profile, it is now believed the steroid may also play a beneficial
function directly at the level of the vasculature. The
proposed mechanisms of estrogen action on the vasculature
include modulations of vascular adhesion molecules, chemoattractants,
vasodilators (e.g., nitric oxide), vasoconstrictors
(e.g., endothelin-1), as well as possibly acting as an antioxidant
(reviewed in Refs. 454 and 461). Several of the
effects of estrogens, as well as other steroid hormones, on
blood vessel physiology have been proposed to be nongenomic
and independent of the classical nuclear activational
pathway of steroid receptors (reviewed in Ref. 355).
However, ER has been detected in both the endothelial and
smooth muscle cells of the vasculature in several species,
suggesting a role for receptor-mediated actions as well (reviewed
in Refs. 431, 454, and 462). Furthermore, a recent
study reported that the level of ER in atherosclerotic plaques
from human coronary arteries was reduced compared with
levels found in nonlesioned sections (463). Studies have indicated
the presence of ER� in the vasculature as well. Analysis
by ribonuclease protection assay for ER� and ER�
mRNA in the mouse indicate only detectable levels of ER�
transcripts in the aorta (93). However, Iafrati et al. (464) report
the detection of ER� transcripts by RT-PCR in mouse aorta
and blood vessels, including those of the �ERKO. The apparent
contrast in ER� detection between these two reports
in the �ERKO vasculature is most likely due to differences
in the sensitivity of the techniques used, since ER� mRNA
was detected only by the more sensitive RT-PCR method.
This is likely a reflection of the low levels of this receptor
compared with ER�. In the rat aorta, Petersen et al. (70)
described the detection of full-length and variant ER�
mRNA by RT-PCR. Recent reports also describe the detection
of ER� transcripts by RT-PCR in aortic smooth muscle cells
and coronary artery (465), aorta, and cardiac muscle (96)
from monkey. An intriguing report of the tissue distribution
for ER� and ER� mRNA in human vasculature was that of
Tschugguel et al., which described the presence of ER�
mRNA only in cultures from larger blood vessels, i.e., aorta