Estrogen Receptor Null Mice - Endocrine Reviews

394 COUSE AND KORACH Vol. 20, No. 3
males are in stark contrast to the significantly elevated levels
found in the female �ERKO mice (282). This contrast between
the sexes may reflect differences in the inhibin/activin levels
or may represent a definitive sexual differentiation in the
transcriptional regulation of the FSH� gene in the mouse,
indicating that androgens are the primary acting steroids in
the male. However, although not as extreme as those found
in the �ERKO female, pituitary LH� mRNA and serum LH
levels are increased 2-fold in the adult �ERKO male (Table
2) (317).
In a series of experiments, Lindzey et al. (317) demonstrated
that castration results in the expected elevated levels
of serum LH in the wild-type, and a further increase in the
already elevated LH levels in �ERKO males. The rise in
serum LH that occurs upon castration even in the �ERKO
male suggests that either estradiol-ER� or androgen-mediated
mechanisms are maintaining the lower LH levels in the
intact animal. Once again however, the mouse pituitary (including
the �ERKO) appears to possess very little if any ER�
mRNA (93), although ER� is expressed normally in the hypothalamic
regions in the �ERKO (93, 352). Estradiol treatment
of castrated animals over a period of 3 weeks reduced
the serum LH levels to normal in the wild-type males,
whereas no effect was observed in the �ERKO, indicating a
requirement for ER� in this process (317). Although treatments
of similar castrated males with testosterone was completely
effective in producing an inhibitory effect on LH
release in the wild-types, it was only partially effective in the
�ERKO (317). The authors therefore concluded that the ability
of testosterone to fully restore normal levels of LH in the
sera of castrate wild-type males but only partially in the
�ERKO males suggests that local aromatization of testosterone
to estradiol and subsequent activation of ER�-mediated
pathways act to enhance the negative feedback effects of
androgens in the male hypothalamic-pituitary axis (317).
However, the inability of testosterone to completely suppress
the serum LH in the �ERKO male may be related to the
dosage used in these studies. Strong evidence of AR-dependent
regulation of LH secretion in the male �ERKO is found
in preliminary experiments in which treatment with an antiandrogen
(flutamide) increased serum LH by 3- to 10-fold
in wild-type and �ERKO males, respectively. This suggests
that �ERKO males have come to rely entirely on AR-mediated
actions to regulate LH secretion, whereas the ER� continues
to play a role in the wild-type.
In these same studies, Lindzey et al. (317) illustrated that
prolonged treatment with DHT, the more potent and nonaromatizable
androgen, resulted in no reduction in the castrate
levels of serum LH in wild-type but was partially effective
in the �ERKO male. However, the DHT was effective
in restoring hypothalamic GnRH content levels to normal in
castrate males of both genotypes (317). Therefore, the enhanced
effect of DHT in negatively affecting the hypothalamic-pituitary
regulation of serum LH, including the inhibition
of hypothalamic GnRH release, remains a puzzling
phenomenom unique to the �ERKO male. It is possible that
a lack of ER� action during development resulted in a “reorganization”
of the hypothalamic-pituitary axis in the
�ERKO male, and thereby somehow allowed for an increased
sensitivity to androgens (317). Further studies in the
�ERKO as well as the �ERKO males may help elucidate these
unexpected results.
4. PRL regulation. PRL possesses more biological actions than
all of the other anterior pituitary hormones combined. A
recent review by Bole-Feysot et al. (279) thoroughly covered
the current knowledge of the diverse actions of PRL, including
its functions as a hormone, growth factor, neurotransmitter,
and immunoregulator. A reflection of the multiple
functions of PRL is the equally broad distribution of PRLbinding
sites throughout the many physiological systems in
vertebrates (279). The well known effects of PRL in reproduction
include a critical role in the differentiation and function
of the lactating mammary gland, as a luteotrophic hormone
in the function of the corpus luteum and thereby as a
promotor of blastocyst implantation, and an overall enhancement
of the physiological functions in the tissues of the male
reproductive tract (279). Nonreproductive roles of PRL include
an involvement in osmoregulation; promotion of
growth, development, and differentiation in several tissues;
enhancement of metabolic activities in the brain, liver, pancreas,
and adrenals; and various actions in immunoregulation
(279).
It has long been known that estradiol is a critical hormone
in the regulation of PRL synthesis and secretion from the
lactotrophs in the anterior pituitary. Estradiol has also been
shown to stimulate lactotroph cell growth (reviewed in Ref.
281) and has been implicated as a possible factor in the
promotion of PRL-secreting tumors in humans (reviewed in
Ref. 339). Furthermore, the lactotrophs of several species
have been shown to possess significant levels of ER�,
strongly suggesting that the actions of estradiol are receptormediated
(reviewed in Ref. 339). As discussed above, recent
descriptions have indicated the presence of ER� in the lactotrophs
of the rat anterior pituitary, although contrasting
reports exist, possibly due to strain variations. Whereas Wilson
et al. (98) describe the presence of only ER� in lactotrophs,
Mitchner et al. (99) report variable levels of ER�
mRNA throughout the various cell types of the rat anterior
pituitary. Shupnik et al. (100) have also reported the detection
of ER� transcripts in human PRL-secreting tumors. Once
again, we find only low to undetectable levels of ER� mRNA
in the pituitary of the adult mouse, including the �ERKO
(93).
The upstream regulatory sequences of the rat PRL gene
have been found to possess an estrogen-responsive element
that binds ER� and functions synergistically with the pituitary-specific
factor, Pit-1, to promote expression (281, 408,
409). The required function of the ER� in the positive regulation
of the PRL gene is nicely illustrated in the �ERKO
mouse. The �ERKO females exhibit a 20-fold decrease in PRL
mRNA levels in the anterior pituitary, whereas the �ERKO
males exhibit a 10-fold decrease when each is compared with
sex-matched wild-type controls (282). Although not as drastic,
this reduction in the expression of the PRL gene is mirrored
in the serum levels of the hormone in the �ERKO
female, which possess an approximate 5-fold reduction in
serum PRL (Table 2). Therefore, given the plethora of roles
in which PRL is involved, it is likely that several of the
phenotypes observed in the �ERKO mice may be due to a