Estrogen Receptor Null Mice - Endocrine Reviews

June, 1999 ESTROGEN RECEPTOR NULL MICE 401
reproduced with prolonged antiestrogen treatments in intact
females (197, 476, 477). Interestingly, a similar increased
body weight is observed in �ERKO females of the C57/BL
background. Gross observations upon necropsy of adult
�ERKO females indicate an obvious increase in the amount
of white adipose fat in the pads of the mammary gland and
those lining the lateral-ventral portions of the body cavity. As
early as 4 months of age, �ERKO females exhibit an increased
body weight compared with female wild-type littermates, at
28.7 g (�0.91) vs. 23.3 g (�0.43), respectively. This difference
in body weight between wild-type and �ERKO females increases
at 8 months of age, at which time the average weight
of �ERKO females exceeds that of age-matched wild-types
by almost 35% (t test; P � 0.01). However, by 12 months of
age, increases in the body weight of wild-type females appear
to decrease the gap between the two genotypes.
The fact that the increased fat stores in the �ERKO female
are similar to those observed in the classical ovariectomized
model indicate that a loss of ER�-mediated estrogen action
may alter metabolism and adipocyte physiology. Fisher et al.
(257) reported a similar phenotype in the ArKO female mice,
in which the weight of the gonadal and mammary fat pads
were increased by 50–80%. In both �ERKO and ArKO females,
serum testosterone levels are substantially elevated
but do not appear to have a lessening effect on fat pad weight
(257). Furthermore, there are no reports of increased body
weight in the androgen-resistant Tfm mice (478), indicating
that androgen action may play a lesser role than estrogen in
fat storage. It is unknown at this time whether a phenotype
similar to the �ERKO and ArKO mice may also exist in the
�ERKO mice. However, sexually mature �ERKO mice of
both sexes exhibit no significant differences in body weight
and appear to possess a relatively normal distribution of
white adipose tissue in the peritoneum at the ages examined.
Although the above evidence strongly supports a role for
ER�-mediated estrogen action in adipocyte physiology, the
mechanisms involved remain unclear. Adipose tissue has been
shown to possess ER� (479, 480, 486–488) and the enzymes
necessary for estradiol synthesis (292, 480). Recent studies have
also indicated that the gonadal sex steroids can alter the activity
of lipoprotein lipase, a critical enzyme in adipocyte growth and
fat storage (474). Interestingly, Lubahn et al. (481) reported that
�ERKO female mice fed a diet enriched with the phytoestrogen,
genistein, exhibited a reduced body weight compared with
�ERKO females fed the control diet. This may indicate a non-
ER�-mediated yet estrogenic action of genistein, a naturally
occurring isoflavone shown to possess hormone-like actions in
mammalian cells that are devoid of ER (482). Therefore, the
ER�-independent actions of genistein, such as the inhibition of
protein tyrosine kinases and influence on growth factor action,
complicate the interpretation of the above study in terms of
defining a role for ER� in fat stores.
VIII. Comparison with Human Disease and Models of
Deficient Estrogen Action
A. Ovarian carcinogenesis
Ovarian cancer is the leading cause of death from gynecological
cancers and accounts for 5% of all cancer deaths in
Western countries (483). However, the etiology of ovarian
carcinoma is complicated by paradoxes similar to those concerning
breast cancer, i.e., although risk is significantly reduced
with each pregnancy, the use of oral steroid contraceptives
also appears to reduce the risk (483). Approximately
80–90% of human ovarian cancers are derived from the ovarian
surface epithelium (484), a portion of the ovary known to
be rich in ER� expression. Although the causal factors remain
unclear, evidence indicates that incessant ovulation, resulting
in constant rupture and estrogen-mediated repair of the
surface epithelium, increases the probability of spontaneous
genetic abnormalities that may lead to tumorigenesis (483,
484). The exact role that estrogen and the ER may play in the
induction and promotion of ovarian carcinoma remains unclear
(reviewed in Refs. 483–485). A number of immortalized
cell lines have been generated and characterized from human
ovarian tumors and exhibit varied levels and responses to
estrogen agonists and antagonists (reviewed in Ref. 484).
Brandenberger et al. (94) recently reported the detection of
ER� and ER� mRNA in the human ovary, ovarian tumors,
and ovarian tumor cell lines. Their findings include the description
of ER� mRNA predominantly in the granulosa cells
of normal ovaries and a marked reduction in the levels of
ER� transcripts in ovarian carcinomas (94). In contrast, a cell
line derived from a human ovarian surface epithelium and
several human ovarian carcinomas were reported to express
high levels of ER� mRNA (94). As mentioned previously,
ovarian tumors are most often derived from the outer surface
epithelium, whereas granulosa cell-derived carcinoma in humans
is rare. Interestingly, we observe an approximately 40%
incidence of granulosa/thecal cell tumors of the ovary in
�ERKO females between the ages of 15–20 months. No such
ovarian tumors have been observed in the wild-type or heterozygous
littermates of the �ERKO. A similar incidence and
type of spontaneous tumor is reported in transgenic mice
possessing significantly elevated levels of LH caused by
overexpression of the LH� subunit gene (254, 255). Therefore,
hypergonadotropin stimulation appears to play a significant
role in the etiology of this type of ovarian tumor in
the mouse. The similarities in the incidence and type of
tumor observed between the �ERKO and bLH�-CTP mouse
indicate that ER� probably plays a minor role. Preliminary
analysis in tumor samples for the �ERKO females indicates
normal to elevated levels of ER� expression. Therefore, elevated
gonadotropins and estradiol may result in chronic
induction of the granulosa cells to proliferate and is the likely
stimulus for the ovarian tumors observed in both transgenic
models. Future investigations to determine the incidence of
tumors in the �ERKO ovary will prove useful in further
elucidating the etiology of this neoplasia.
B. Chronic anovulation
Targeted gene disruption of the ER genes has resulted in
partial and complete anovulation in the �ERKO and �ERKO
females, respectively. In the clinical setting, chronic anovulation
is often categorized by the presence or lack of estrogen
synthesis (204). Chronic anovulation in the absence of estrogen
is diagnosed in women who experience little to no
menstrual bleeding after progesterone withdrawal and is