Estrogen Receptor Null Mice - Endocrine Reviews

364 COUSE AND KORACH Vol. 20, No. 3
acting in the form of heterodimers (50, 62, 65, 86). These
studies generally report a tendency of ER� to form homodimers
whereas ER� prefers to heterodimerize with ER�.
However, Giguere et al. report (87) that the heterodimer is the
preferred state when both mouse ERs are present. The transactivational
activity of the heterodimer when assayed in in
vitro mammalian cell transfection assays appears to lie between
that of the more active ER� homodimer and the less
active ER� homodimer (50, 62, 86). A major consideration
when evaluating the possible physiological functions of an
ER�/ER� heterodimer is evidence of coexpression of the two
receptors in the same cell, which has not yet been definitively
reported (88). To this end, studies and reagents are only now
becoming available to directly assess this question.
Several functional characteristics of the two ERs are similar.
The residues critical to function of the AF-2 domain
appear to be identical in the mouse ER� and ER� (87). Tremblay
et al. (50, 89) demonstrated that a tyrosine residue critical
to the function of the AF-2 domain was conserved in both the
ER� and ER� and that mutation of this amino acid resulted
in similar constitutive, ligand-independent transactivational
activity in both receptors. In contrast, the N�-terminal AF-1
domain shows no significant regions of similarity between
the two ERs (87). However, a potential activation site of the
mitogen-activated protein (MAP)-kinase pathway previously
shown for ER� is present and active in the ER� (50, 89).
Additionally, when acting on a basal promoter linked to a
consensus estrogen response element, both ER� and ER�
were able to recruit the coactivator SRC-1 and were equally
susceptible to inhibition by the antiestrogens raloxifene, ICI
164,384, and EM-800 (50).
However, as studies continue, distinct differences at the
molecular level and in the transactivational capacities between
ER� and ER� have been described. Two separate
studies have demonstrated the specificity of the agonist activity
of 4-hydroxytamoxifen to be unique to ER�, although
this appears to be highly dependent on the cell and promoter
context as well as experimental design (50, 90). Furthermore,
Paech et al. (91) reported that when interacting with DNAbound
AP-1 transcription factors, the in vitro transactivational
activity of estrogen agonists and antagonists was quite
different depending on which form of ER was present.
Whereas antagonists, such as raloxifene, tamoxifen, and ICI
164,384, were able to block the stimulatory activity of the
ER�/AP-1 complex, these same compounds acted as potent
agonists when bound to an ER�/AP-1 complex (91). Further
experimental support for the existence of distinct structural
and functional differences between ER� and ER� was recently
provided by Sun et al., who showed that certain nonsteroidal
ligands were receptor selective in their binding and
agonist/antagonist activities (92).
Perhaps the most significant disparity lies in the tissue
distribution of the two receptors. Studies employing the techniques
of RT-PCR and/or ribonuclease protection assay
(RPA) have indicated that ER� mRNA is predominant in the
uterus, mammary gland, testis, pituitary, liver, kidney, heart,
and skeletal muscle, whereas ER� transcripts are significantly
expressed in the ovary and prostate (Fig. 2) (63, 80, 93,
94). These same studies have indicated relatively equal levels
of mRNA for the two receptors in the epididymis, thyroid,
adrenals, bone, and various regions of the brain (80, 93,
95–97). However, as more studies are reported, several discrepancies
in the expression patterns of ER� and ER� among
different species are becoming apparent (80, 93, 96). For
example, whereas ER� mRNA is easily detectable in the
pituitary of the rat (70, 98, 99), human (100), and rhesus
monkey (96), levels in the pituitary of the mouse appear low
to undetectable (93). A similar difference in expression is
apparent in the mammary gland, in which normal and neoplastic
human tissue and cell lines express detectable ER�
mRNA (64, 68, 73, 101, 102), although the mammary gland
of the mouse appears to predominantly express ER� (93).
Furthermore, even in those tissues expressing both ERs, there
is often a distinct expression pattern within the heterogeneous
cell types composing the tissue. In the ovary, ER� is
apparently localized to the granulosa cells of maturing follicles,
whereas ER� is detectable in the surrounding thecal
cells (69, 103, 104). In the prostate of the rat, expression of ER�
and ER� is detectable in the stroma and epithelium, respectively,
but does not appear to be colocalized in any portion
of the tissue (49). However, through the combined use of
immunohistochemistry and in situ hybridization, Shughrue
FIG. 2. RT-PCR for ER� and ER� mRNA in various tissues of the wild-type mouse. RT-PCR was carried out on 0.5 �g of total RNA pooled from
adult wild-type mice using primers specific for the mouse ER� and ER� transcripts (see Refs. 93 and 123). Equal amounts of the individual
RT-PCR reactions were then fractionated on an agarose gel. Note the broad tissue distribution of ER� mRNA, whereas ER� transcripts are
primarily expressed in the ovary, hypothalamus, lung, and male reproductive tract. RT-PCR for �-actin was carried out as a positive control.
(�RT) indicates a negative control, i.e., PCR on total ovarian RNA minus reverse transcriptase, indicating the specificity of the ER primers for cDNAs
generated by the reverse transcriptase enzyme.