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