Estrogen Receptor Null Mice - Endocrine Reviews
Estrogen Receptor Null Mice - Endocrine Reviews
Estrogen Receptor Null Mice - Endocrine Reviews
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June, 1999 ESTROGEN RECEPTOR NULL MICE 377<br />
cells of antral follicles, were apparently preserved in the<br />
follicles of the �ERKO ovary. Follicular atresia in the ovary<br />
is a hormonally controlled process that is critical to oocyte<br />
selection. Although the factors that trigger atresia are not<br />
well understood, it is characterized by apoptosis of the granulosa<br />
cells of the follicle (reviewed in Refs. 223 and 224).<br />
Estradiol has been shown to be one of the many factors<br />
reported to protect the follicle from becoming atretic (250).<br />
Furthermore, androgens reportedly accelerate the process,<br />
and it may be an altered estrogen/androgen synthesis ratio<br />
in the follicle that leads to atresia (250). However, despite<br />
elevated androgen production, the presence of androgen<br />
receptor (AR) mRNA, and a lack of ER� action in the mature<br />
�ERKO ovary, an inordinate amount of apoptosis is not<br />
observed in the follicles (142). Estradiol has also been shown<br />
to facilitate the FSH induction of LH receptors in the granulosa<br />
cells of the mature ovulatory follicle in both in vivo (225,<br />
231, 252) and in vitro experiments (229, 230). Nonetheless, the<br />
granulosa cells of the growing follicles, in addition to the<br />
enlarged cysts in the �ERKO ovary, possess significant levels<br />
of LH receptor mRNA when assayed by in situ hybridization<br />
(142). The most plausible explanation for these observations<br />
is that these estrogen actions are mediated by ER�, which has<br />
been shown to be expressed in a normal pattern in the granulosa<br />
cells of the �ERKO ovary, and concomitant with the<br />
expression of LH receptor (93, 142).<br />
Therefore, with data suggesting that disruption of the ER�<br />
gene did not result in an ovary completely refractory to estrogen,<br />
other aspects of ovarian physiology must be considered as<br />
possible factors in the etiology of the �ERKO ovarian phenotype.<br />
As previously discussed, ovarian function is tightly controlled<br />
by pituitary gonadotropins (see Section III.D.1). In turn,<br />
gonadotropin synthesis and secretion from the anterior pituitary<br />
are at least partially regulated by gonadal steroids acting<br />
via classical feedback mechanisms in the hypothalamic-pituitary<br />
axis (reviewed in Ref. 251). Indeed, disruption of the ER�<br />
gene has resulted in significant phenotypes in the hypothalamic-pituitary<br />
axis of the �ERKO female (see Section VI.A). Most<br />
notable is the increased and chronic secretion of LH in the<br />
�ERKO female that results in levels that are 4–7 times that<br />
found in wild-type females (Table 2) (252). As discussed above,<br />
synchronized increases in serum FSH and LH levels are critical<br />
to follicular maturation and ultimate ovulation in the ovary.<br />
However, it has been proposed that the follicular requirements<br />
for LH are finite, and the presence of abnormally high levels<br />
may force maturing follicles to either prematurely luteinize or<br />
become atretic (253).<br />
Therefore, the �ERKO ovarian phenotype is likely caused<br />
by the chronic exposure to abnormally high levels of LH.<br />
Support for this hypothesis can be drawn from a number of<br />
studies. Investigations involving prolonged treatment with<br />
antiestrogens over a period of at least 28 days have produced<br />
an ovarian phenotype in both mice (195, 196) and rats (197)<br />
that is similar to that of the �ERKO. Of course, interpretation<br />
of these studies is complicated by the ability of the antiestrogens<br />
to inhibit both ERs as well as estrogen action in both<br />
the ovary and hypothalamic-pituitary axis. However, these<br />
studies reported that the “�ERKO” phenotype of enlarged<br />
cystic follicles was produced only after prolonged treatments<br />
with those antiestrogens that possessed the ability to cross<br />
the blood-brain barrier and concurrently produce serum LH<br />
levels that were several fold higher than controls. Therefore,<br />
whereas the estrogen antagonists ZM-189,154 (197) and EM-<br />
800 (195, 196) produced chronically elevated LH levels and<br />
an ovarian phenotype strikingly similar to that of the<br />
�ERKO, tamoxifen did neither (195–197). More definitively,<br />
Risma et al. (254, 255) report that targeted transgenic overexpression<br />
of the LH�-subunit in the mouse that results in a<br />
15-fold increase in serum LH levels produces females that are<br />
anovulatory and exhibit an ovarian phenotype almost indistinguishable<br />
from that of the adult �ERKO female.<br />
Therefore, the similarity in the ovarian phenotypes described<br />
in the above studies, in which the models presumably<br />
possessed normal levels of ovarian ER�, combined with our<br />
observations in the �ERKO, strongly indicate that the ER� is<br />
not directly involved in the development of ovarian cysts due<br />
to hypergonadotropism. However, there are descriptions of<br />
at least two models in which serum LH is chronically elevated,<br />
yet do not manifest an ovarian phenotype similar to<br />
the �ERKO or those induced by antiestrogens or transgenics<br />
as described above. Female mice that are homozygous for a<br />
targeted disruption of the FSH�-subunit gene exhibit an<br />
approximate 5-fold increase in serum LH but do not show<br />
indications of enlarged cystic follicles in the ovary (241),<br />
indicating a role for FSH in this process as well. Bogovich<br />
(256) has provided supporting evidence by demonstrating<br />
that FSH is required along with prolonged exposure to LH<br />
(in the form of human CG) to induce follicular cysts in the<br />
rat. A likely role for FSH is the induction of LH receptor in<br />
the granulosa cells of the maturing follicles, thereby rendering<br />
the follicle responsive to the increased levels of LH.<br />
Another contrasting knockout model is that of the P450 arom<br />
gene (ArKO), in which the homozygous females possess<br />
significantly elevated serum LH and FSH levels but lack the<br />
capacity to synthesize estradiol (257). Although folliculogenesis<br />
is arrested at an antral stage in the ArKO ovary, no<br />
�ERKO-like cystic structures are reported (257). Therefore,<br />
assuming that a lack of estradiol synthesis would disrupt<br />
ligand-dependent activity of the ER� in the granulosa cells,<br />
the lack of ovarian cysts in the ArKO indicate an intraovarian<br />
role for estradiol in this phenotype. Therefore, although the<br />
�ERKO phenoytpe may be triggered by hyperstimulation of<br />
the follicles by LH, it is likely influenced by both FSH and<br />
ER� actions in the granulosa cells as well. Current studies<br />
utilizing prolonged treatment of �ERKO females with a<br />
GnRH antagonist to reduce serum gonadotropins are being<br />
carried out to further define the etiology of the ovarian<br />
phenotype (J. F. Couse, D. O. Bunch, J. Lindzey, D. W.<br />
Schomberg, and K. S. Korach, manuscript in preparation).<br />
Since the �ERKO ovarian phenotype develops and worsens<br />
only after sexual maturity, we have began studies to<br />
characterize ovarian function in the immature �ERKO female<br />
(J. F. Couse, D. O. Bunch, J. Lindzey, D. W. Schomberg,<br />
and K. S. Korach, manuscript in preparation). Although superovulation<br />
with exogenous gonadotropins was not successful<br />
in eliciting ovulation in the older �ERKO females,<br />
immature (�28 days) females do respond and produce viable<br />
oocytes that can be collected from the oviduct. However, the<br />
average number of oocytes collected from superovulated<br />
�ERKO females is significantly less than that yielded from