Note: Descriptions are shown in the official language in which they were submitted.
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ORPHAN RECEPTOR
This invention relates to cellular nuclear receptors and their uses..
A large family of nuclear receptors which confer cells with responsiveness to
molecules
such as retinoid acid, vitamin D, steroid hormones and thyroid hormones has
been
identified. Extensive studies have shown that the members of this superfamily
of nuclear
receptors activate and/or repress gene transcription through direct binding to
discrete
cis-acting elements termed "hormone response elements" (HRE). It has been
shown that
these HRE's comprise repeats of consensus palindromic hexanucleotide DNA
motifs. The
specificity of the HRE's is determined by the orientation of, and spacing
between, halfsites
(i.e. half a palindromic sequence)(Umenesono K., et al, 1991 Cell 65, 1255-
1266).
Specific DNA binding is mediated by a strongly-conserved DNA binding domain,
containing two zinc fingers, which is conserved among all thus discovered
nuclear
receptors. Three amino acids at the C-terminal base of the first zinc finger
(known as the
"P-box") are important for the recognition of the half site nucleotide
sequence. Members
of the nuclear receptor superfamily have been classified into different groups
on the basis
of the amino acid sequence within the P box.
All members of the nuclear receptor superfamily also contain a hypervariable N-
terminal
domain and a ligand-binding domain containing some "patches" of conserved
sequence.
One of these is called the "Ti-domain".
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Molecules which are thought to be nuclear receptors, as they are structurally
related to
characterised receptors, but for which no ligand has been found, are termed
"orphan
receptors". Many such orphan receptors have been identified (see for example
Evans R.M.
(1988) Science 240, 889-895 and O'Malley, B. (1990) Mol. Endocrinol. 4 363-
369)
We have now unexpectedly identified, initially in rat a new orphan receptor,
which is
related to the known estrogen receptor ERa, and which we have designated
"ER(3"
(specifically "rER(3" in rat). In this specification "Er(3" will be used to
refer to the
receptors hER(3 or rER~i or related receptors. The nucleotide and amino acid
sequences of
rER(3 have now been determined and are shown in Fig. 1. We have also
identified a
human Er(3 - "hER(3", the amino acid DNA and sequences of which are shown in
Fig. 13A
and 13B respectively.
According to one aspect of the invention there is provided a novel estrogen
receptor-related nuclear receptor, hereinafter termed "ER(3" having the amino
acid
sequence of Figs. 1, Fig. 13A or 16A or substantially the same amino acid
sequence as the
amino acid sequence shown in Figs. 1, 13A or 16A or an amino acid sequence
functionally
similar to those sequence. The isolated receptor may be particularly useful in
the search for
molecules for use in treatment of diseases or conditions such as
cardiovascular diseases,
central nervous system diseases or conditions or osteoporosis, prostate cancer
or benign
prostatic hyperplasia.
The receptor of the invention may also be used in the testing of environmental
chemicals
for estrogenic activity. There has been increasing concern over the effect of
various
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3
chemicals released into the environment on the reproduction of humans and
animals.
Threats to the reproductive capabilities of birds, fish, reptiles, and some
mammals have
become evident and similar effects in humans have been proposed. Substantial
evidence is
now emerging which shows that exposure to certain chemicals during critical
periods of
foetal life may distort the development of the reproductive organs and the
immune and
nervous systems. On the basis of possible parallels between actual wildlife
effects, seen
for example in birds and seals living in highly polluted areas, and proposed
effects in
humans, in combination with documented human reproductive effects caused by
prenatal
exposure to the pharmaceutical estrogen, diethyl stilbestrol (DES),
"estrogenic" chemicals
have been proposed to threaten the reproductive capability of both animals and
humans.
Among the chemicals known or suspected to act as estrogen mimics on the human
body,
or in other ways disturb the human endocrine system, there are several which
have already
been identified as environmental hazards. Among the chemicals that have been
mentioned
as potential causes of disruption of reproductive function in animals and
humans are
chlorinated organic compounds such dieldrin, endosulfans, chlordanes, endrins,
aldrin,
DDT and some PCBs, plastics such as Bisphenol A, phthalates and nonylphenol,
and
aromatic hydrocarbons. Some of the proposed effects on humans have been
suggested to
be due to an increasing exposure to environmental estrogens - in fact,
exposure to
chemical compounds to which higher organisms during the foetal period react in
a way
that is similar to when they are exposed to high dosages of estrogens. The
effects are
manifested by for example perturbations of the sex characteristics and
impaired
reproductive potential. In humans, elevated risks of breast cancer and other
hormone-related disease has also been discussed as possible effects. In
addition, to the
documented "estrogenic" effects, it has recently been demonstrated that
environmental
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pollutants may also act on hormonal pathways other than the estrogenic pathway
- it has
been shown that p,p' - DDE the main metabolite of DDT (also in humans) is a
fairly anti-
androgenic agent (Kelce W.R. et al Nature 1995 375:581-585). Epidemiological
studies
on these issues are, however, presently difficult to interpret. Nevertheless,
there is a
growing opinion against these potentially hormone disrupting chemicals, and
very palpable
public and environmental demand for the governmental agencies and industry to
act. In
view of the similarities between the receptor of the present invention, Er(3
and the classical
estrogen receptor, Er(3 may be used in the testing of chemicals for estrogenic
effect.
An amino acid sequence functionally-similar to the sequence shown in Fig. 1,
13A or 14A
may be from a different mammalian species.
An amino acid sequence which is more than about 89%, identical with the
sequence
shown in Fig. 1, 13A or 14A is substantially the same amino acid sequence for
the
purposes of the present application. Preferably, the amino acid sequences is
more than
about 95% identical with the sequence shown in Fig. 1, 13A or 14A.
According to another aspect of the invention there is provided a DNA sequence
encoding a
nuclear receptor according to the first aspect of the invention. Preferably,
the DNA
sequence is that given in Fig. 1, 13A or 14A or is a DNA sequence encoding a
protein or
polypeptide having the functionality of ER(3.
ER(3 is unique in that it is extremely homologous to the rat estrogen
receptor, in particular
in its DNA binding domain. It appears that ER(3 has a very limited tissue
distribution. In
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female rats, it appears to be present only in the ovaries, and in male rats in
the prostate and
testes. As these tissues are classic targets for estrogen action, it can be
deduced that ER(3
may mediate some of the effects of estrogen.
The different ligand specificity of ERa and ER(3 may be exploited to design
pharmaceutical agents which are selective for either receptor. In particular,
the differences
in ligand specificity may be used to develop drugs that specifically target
cardiovascular
disease in postmenopausal women or osteoporosis.
The nuclear receptor of the invention, ER~3, a method of producing it, and
tests on its
functionality will now be described, by way of example only, with reference to
the
accompanying drawings, Figs. 1 to 15 in which:
Fig. 1 shows the amino acid sequence of ER~3 and the nucleotide sequence of
the gene
encoding it;
Fig. 2A is a phylogenetic tree showing the evolution of ER~3 and other
receptors;
Fig. 2B shows the homology between the different domains in ER(3 and certain
other
receptors;
Fig. 2C is an alignment of the amino acid sequence in the ligand binding
domains of rER(3,
rERa, mERa and hERa;
Fig. 2D is an alignment of the amino acid sequence in the DNA binding domains
of rER(3,
rERa, mERa and hERa;
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Fig. 3A is a film autoradiograph of prostate gland showing strong expression
of a clone of
the receptor of the invention, clone 29;
Fig. 3B is a darkfield image showing prominent signal for clone 29 in
epithelium (e) of
prostatic alveoli. The stroma(s) exhibits) weaker signal;
Fig. 3C is a bipolarization image of cresyl violet counterstained section
showing silver
grains over epithelium (e), whereas the stroma(s) contains) less grains;
The bar represents 0.7 mm for Fig. 3A, 200 pm for Fig. 3B and 30 pm for Fig.
3C;
Fig. 4A shows a film autoradiograph of ovary showing strong expression of
clone 29 in
follicles at different developmental stages (some are indicated by arrows).
The interstitial
tissue (arrowheads) shows low signal;
Fig. 4B shows a darkfield image showing high expression of clone 29 in
granular cells of
primary (1), secondary (2), tertiary (3) and mature (4) follicles. Low signal
is present in
interstitial tissue (it);
Fig. 4C is a bipolarization image of ovary a showing strong signal in granular
cells (gc),
whereas the oocyte (oc) and the cainterna (ti) are devoid of clear signal;
The bar represents 0.9 mm for Fig. 4A, 140 pm for Fig. 4B and 50 pm for Fig.
4C;
Fig. SA illustrates the results of saturation ligand binding analysis of
cloned ER~3;
Fig. 5B illustrates the specificity of ligand binding by cloned ER~3;
Fig. SC illustrates E2 binding by ER(3;
Fig. 6 illustrates the activation of transcription by cloned ER(3;
Fig. 7 and 7A illustrates stimulation by various ligands by cloned ER(3;
Fig. 8 illustrates the results of RT-PCR experiments on the expression of rat
estrogen
receptors;
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Fig. 9 illustrates the results of RT-PCR experiments on the expression of
human Er~i
(hER(3);
Fig. l0A is a Hill plot comparing binding of '25 I-E2 by hERa and rER(3;
Fig. lOB is a Scatchard plot comparing binding of'z5I-E2 by hERa and rER(3;
Fig. 11 A illustrates the relative binding affinity of hERa and rER(3 for
various ligands;
Fig. 11B is a detail of Fig. 12A;
Fig. 12 is an alignment of various estrogen receptors;
Fig. 13A shows the amino acid sequence of human ER~3;
Fig. 13B shows the DNA sequence of human Er(3;
Fig. 14A shows the amino acid sequence of mER~3;
Fig. 14B shows the DNA sequence of mouse Er~3; and
Fig. 15 illustrates ligand binding affinities for various phytoestrogens by
ER's of the
invention.
A. CLONING OF RAT ER13
1. PCR-amplification and complementary DNA cloning.
A set of degenerate primers (DBD 1,2,3 and WAK/FAK) were designed previously
according to the most highly conserved sequences of the DNA-binding domain (P-
box) and ligand binding domain (Ti-stretch) of members of the nuclear receptor
family (Enmark, E., Kainu, T., Pelto-Huikko, M., & Gustafsson, J-t~ (1994)
Biochem. Biophys. Res. Commun. 204, 49-56). Single strand complementary DNA
reverse transcribed from rat prostate total RNA was employed with the primers
in
PCR reactions as described in Enmark, E., Kainu, T., Pelto-Huikko, M., &
Gustafsson, J-~ (1994) Biochem. Biophys. Res. Commun. 204, 49-56. The
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amplification products were separated on a 2% low melting agarose gel and DNA
products between 400 and 700 by were isolated from the gel and ligated to TA
cloning vector (Invitrogen). As alternatives, we also used the RP-I/RP-2 and
DBD66-100/DBD210-238 primer sets in the DNA-binding domain of nuclear
receptors exactly as described by Hirose T., Fijimoto, W., Yamaai, T., Kim,
K.H.,
Matsuura, H., & Jetten, A.M (1994) Mol. Endocrinol. 8, 1667-1677 and Chang,
C.,
Lopes Da Silva, S., Ideta, R., Lee, Y., Yeh, S., & Burbach, J.P.H (1994) Proc.
Natl.
Acad. Sci. 91, 6040-6044 respectively. Clone number 29 (obtained with the
DBD-WAK/FAK set) with a length of 462 by showed high homology (65%) with
the rat estrogen receptor cDNA (65%), which was previously cloned from rat
uterus (Koike, S., Sakai, M., & Muramatsu, M. (1987) Nucleic Acids Res 15,
2499-2513). The amino acid residues predicted by clone 29 DNA sequences
suggested that this DNA fragment encoded part of the DNA-binding domain, hinge
region and the beginning of the ligand binding domain of a novel member of the
nuclear receptor family. Two PCR primers (Figure 1) were used to generate a
probe of 204 by consisting of the hinge region of the novel receptor, which
was
used to screen a rat prostate cDNA library (Clontech gtl0) under stringent
conditions resulting in four strongly positive clones with a size of 0.9 kb,
l.8kb,
2.Skb and 5-6kb respectively. The clone of 2.Skb was sequenced and Figure 1
shows the nucleotide sequence determined in the core facility (CyberGene AB)
by
cycle sequencing using fluorescent terminators (Applied Biosystems) on both
strands, with a series of internal primers and deduced amino acid sequence of
clone
29. Two in frame ATG codons are located at nucleotide 424 and nucleotide 448,
preceding by an in-frame stop codon at nucleotide 319, which suggests that
they
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are possible start codons. The open reading frame encodes a protein of 485
amino
acid residues (counted from the first methionine) with a calculated molecular
weight of 54.2 kDa. Analysis of the proteins by synthesized by in-vitro
translation
from the clone 29 cRNA in rabbit reticulocyte lysate revealed a doublet
protein
band migrating at approximately 57 kDa on SDS-PAGE gels (data not shown),
confirming the open reading frame. The doublet protein band is probably caused
by the use of both ATG codons for initiation of protein synthesis. The amino
acid
sequence of clone 29 protein shows the characteristic zinc module DNA-binding
domain, hinge region and a putative ligand binding domain, which are the
characteristic features of members of the nuclear receptor family (Tsai, M-J.,
&
O'Malley, B.W (1994) Ann. Rev. Biochem. 63, 451-486; Hard, T., & Gustafsson,
J-~. (1993) Acc. Chem. Res. 26, 644-650; Laudet, V., Hanni, C., Coli, J.,
Catzeflis,
F., & Stehelin, D ( 1992) EMBL J. 11, 1003- 1012).
Protein sequence comparison with several representative members of the nuclear
receptor family (Figure 2) showed the clone 29 protein is most related to the
rat
estrogen receptor (ERa), cloned from uterus (Koike, S., Sakai, M., &
Muramatsu,
M. (1987) Nucleic. Acids Res. 15, 2499-2513), with 95% identity in the
DNA-binding domain (amino acid residues 103-167) (Griffiths, K., Davies, P.,
Eaton, C.L, Harper, M.E., Turkes, A., & Peeling, W.B. (1991) in Endocrine
Dependent Tumours, eds. Voigt, K-D. & Knabbe, C. (Raven Press), pp. 83-125).
A number of functional characteristics have been identified within the
DNA-binding domain of nuclear receptors (Hard, T., & Gustafsson, J-~. (1993)
Acc. Chem. Res 26, 644-650 and Zilliacus, J., Carlstedt-Duke, J., Gustafsson,
J-fir.,
& Wright, A.P.H. (1994) Proc. Nutl. Acad. Sci. USA 91, 4175-4179). The
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so-called P-box specifies nucleotide sequence recognition of the core half
site
within the response element, while the D- box mediates dimerization between
receptor monomers. The clone 29 protein P-box and D-box sequences of EGCKA
and PATNQ, respectively, are identical to the corresponding boxes in ERa
(Hard,
T., & Gustafsson, J-t~. (1993) Acc. Chem. Res 26, 644-650 and Koike, S.,
Sakai,
M., & Muramatsu, M. (1987) Nucleic Acids Res. 15, 2499-2513), thus predicting
that clone 29 protein binds to ERE sequences.
The putative ligand binding domain (LBD) of clone 29 protein (amino acid
residues 259-457) shows closest homology to the LBD of the rat ERa (Figure 2),
while the homology with the human ERR1 and ERR2 proteins (Giguere, V., Yang,
N., Segui, P., & Evans R.M. (1988) Nature 331, 91-94) is considerably less.
With
the human, mouse and xenopus estrogen receptors the homology in the LBD is
also
around 55%, while the homology with the LBD of other steroid receptors is not
significant (Figure 2). Cysteine residue 530 in human ERa has been identified
as
the covalent attachment site of an estrogenic affinity label (Harlow, K.W.,
Smith
D.N., Katzenellenbogen, J.A., Greene, G.L., & Katzenellenbogen, B.S. (1989) J.
Biol. Chem. 264, 17476- 17485). Interestingly, clone 29 protein (Cys-436) as
well
as the mouse, rat and xenopus ERas have a cysteine residue at the
corresponding
position. Also, two other amino acid residues described to be close to or part
of the
ligand-binding pocket of the human ERa-LBD (Asp 426 and Gly 521) are
conserved in the LBD of clone 29 protein (Asp 333 and Gly 427) and in the LBD
of ERas from various species (20,21 ). The ligand-dependent transactivation
function TAF-2 identified in ERa (Danielian, P.S., White, R., Lees, J.A., &
Parker,
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M.G. (1992) EMB~ J. 11, 1025-1033), which is believed to be involved in
contacting other transcription factors and thereby influencing activation of
transcription of tarteg genes, is almost completely conserved in clone 29
protein
(amino acid residues 441-457). Steroid hormone receptors are phosphoproteins
(Kuiper, G., & Brinkmann, A.O. (1994) Mol. Cell. Endocrinol. 100, 103-107),
and
several phosphorylation sites identified in the N-terminal domain and LBD of
ERa (Arnold, S.F., Obourn, J.D., Jaffe, H., & Notides. A.C. (1995) Mol.
Endocrinol. 9, 24-33 and Le Goff, P., Montano, M.M., Schodin, D.J., &
Katzenellenbogen, B.S (1994),1. Biol. Chem. 269, 4458-4466) are conserved in
clone 29 protein (Ser 30 and 42, Tyr 443). Clone 29 protein consists of 485
amino
acid residues while ERas from human, mouse and rat consist of 590-600 amino
acid residues. The main difference is a much shorter N-terminal domain in
clone
29 protein i.e 103 amino acid residues as compared to 185-190 amino acid
residues
in the other receptor proteins. Also the non-conserved so-called F-domain at
the
C-terminal end of ERas is 15 amino acid residues shorter in clone 29 protein.
The
cDNA insert of a positive clone of 2.6 kb was subcloned into the EcoRl site of
pBluescript (trademark) (Stratagene). The complete DNA sequence of clone 29
was determined (CyberGene AB) by cycle sequencing using fluorescent
terminators (Applied Biosystems) on both strands, with a series of internal
primers.
Figs 2C and 2D respectively compare the ligand and DNA binding domain of Er~3
compared to rat, mouse and human Era's.
2. Saturation ligand binding analysis and ligand competition studies:
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Clone 29 cDNA was subcloned in pBluescript*downstream of the T7 promoter to
give p29-T7. Clone 29 protein was synthesized in vitro using the TnT-coupled
reticulocyte lysate system (Promega). Translation reaction mixtures were
diluted
five times with TEDGMo buffer (40 mm Tris/HC1, pH 7.4, 1mM EDTA, 10%
(v/v) glycerol, 10 mM NazMoOa, 10 mM DTT) and 0.1 ml aliquots were incubated
for 16 h at 8° C with 0.3- 6.2 nM [2,4,6,7 3H]-17~i-estradiol (NEN-
Dupont; specific
radioactivity 85 Ci/mmol) in the presence or absence of a 200-fold excess of
unlabelled E2.
Fig. SA illustrates the results of a saturation ligand analysis of clone 29
protein.
Reticulocyte lysate containing clone 29 protein was incubated with 6
concentrations of [3H]E2 between 0.3 and 6.0 nM. Parallel tubes contained an
additional 200 fold of non-radioactive E2. Bound and free ligand were
separated
with a dextran-coated charcoal assay. The Kd (0.6 nM) was calculated from the
slope of the line in the Scatchard plot shown (r = 0.93), and the number of
binding
sites was extrapolated from the intercept on the abscissa (Bmax = 1400 fmol/ml
undiluted translation mixture).
For ligand competition studies diluted reticulocyte lysate was incubated with
5 nM
[2,4,6,7 3H]-173-estradiol in the presence of either 0, 5, S0, 500 or 5,000 nM
of
non- radioactive E2, estrone, estriol, testosterone, progesterone,
corticosterone, Sa-
androstane-3~3,17(3-diol, Sa-androstanc-3a,17(3-diol and diethylstilbestrol
(DCES)
for 16 h at 8°C. Bound and unbound steroids were separated with a
dextran-coated
*trade-mark
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charcoal assay (Ekman, P., Barrack, E.R., Greene, G.L., Jensen, E.V., & Walsh,
P.C (1983) J. Clin. Endocrinol Metab. 57, 166-176).
Fig. 5B illustrates the specificity of ligand binding by clone 29 protein.
Reticulocyte lysate containing clone 29 protein was equilibrated for 16 h with
5
nM [3H]E2 and the indicated fold excess of competitors. Data represent [3H]E2
bound in the presence of unlabelled E2, testosterone (T), progesterone (grog),
corticosterone (cortico), estrone (E1), diethylstilbestrol (DES), 5a-
androstane-3a,
17(3-diol (3a-AD), 5a-androstane- 33,17(3-diol (3~3-AD) and estriol (E3).
[3H]E2
binding in the absence of competitor was set at 100%.
3. In-situ hybridisation:
In-situ hybridisation was carried out as previously described (Dagerlind A.,
Friberg, K., Bean, A.J., & Hokfelt, T (1992) Histochemistry 98, 39-49).
Briefly,
two oligonucleotide probes directed against nucleotides 994-1041 and 1981-2031
were each labelled at the 3'-end with 33P-dATP using terminal
deoxynucleotidyltransferase (Amersham, UK). Adult male and female
Sprague-Dawley rats (age 2 to 3 months n=10) were used for this study. The
rats
were decapitated and the tissues were rapidly excised and frozen on dry ice.
The
tissues were sectioned in a Microm HM500 cryostat at 14 pm and thawed onto
Probe-On*glass slides (Fisher Scientific, PA, USA). The slides were stored at
-20°C until used. The slides were incubated in humidifed boxes at
42°C for 18 h
with 1x106 cpm of the probe in a hybridization solution containing 50%
formamide, 4 x SSC (1 x SSC = 0.15 M NaCI, 0.015 M sodium citrate), 1 x
*trade-mark
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Denhardt (0.02 % BSA, 0.02 % Ficoll, 0.02 % PVP), 1 % sarkosyl, 0.02 M sodium
phosphate (pH 7.), 10% dextransulphate, 500 pg/ml salmon sperm DNA and 200
mM DTT. Slides were subsequently rinsed in 1 x SSC at 55°C for 60 min
with
four changes of SSC and finally in 1 x SSC starting at 55°C and slowly
cooled to
room temperature, transferred through distilled water and briefly dehydrated
in
50% and 95% ethanol for 30 sec each, air-dried, and covered with Amersham
(3-man autoradiography film for 15 to 30 days. Alternatively the slides were
dipped
in Kodak NTB2 nuclear track emulsion (diluted 1:1 with distilled water) and
exposed for 30 to 60 days at 4°C. Finally, the sections were stained
with cresyl
violet.
Clear expression of clone 29 was observed in the reproductive tract of both
male
and female rats, while in all other rat tissues the expression was very low or
below
the level of detection with in-situ hybridisation (not shown). In male
reproductive
organs high expression was seen in the prostate gland (Figure 3), while very
low
expression was observed in testis, epididymis and vesicula seminalis (not
shown).
In dipped sections, expression was clearly visible in prostate epithelial
cells
(secreting alveoli) while the expression in smooth muscle cells and
fibroblasts in
the stroma was low (Figure 3). In female reproductive organs expression was
seen
in the ovary (Figure 4), while uterus and vagina were negative (not shown). In
dipped sections high expression was seen in the granulosa cell layer of
primary,
secondary and mature follicles (Figure 4), whereas primordial follicles,
oocytes and
corpora lutes appeared completely negative. Low expression was seen in the
interstitial cells of the ovary. Both anti-sense oligonucleotide probes used
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produced similar results. Addition of a 100 fold excess of the respective
unlabelled
oligonucleotide probes during the hybridisation reactions abolished all
signals.
4. Transactivation analysis in CHO-cells:
The expression vector pCMV29 was constructed by inserting the 2.6 kb clone 29
fragment in the EcoRI site of the expression vector pCMVS (Andersson, S.,
Davis,
D.L., Dahlback, H., Jornvall, H., & Russell, D.W. (1989) J. Biol. Chem. 264,
8222-
8229). The pERE-ALP reporter construct contains a secreted form of the
plancental alkaline phosphatase gene (Berger, J., Hauber, J., Hauber, R.,
Geiger,
R., & Cullen, B.R. (1988) Gene 66, 1-10) and the MMTV-LTR in which the
glucocorticoid response elements were replaced by the vitellogenin promoter
estrogen response element (ERE).
CHO-K1 cells were seeded in 12-well plates at approximately 1.7 x 105 cells
per
well in phenol-red free Ham F12 medium with 5% FCS (dextran-coated charcoal
treated) and 2 mM Lglutamine. After 24 h the cells were transfected with 250
ng
pERE-ALP vector and 50 ng pCMV29 using lipofectamine (Gibco) according to
the manufacturer's instructions. After five hours of incubation the cells were
washed and refed with 0.5 ml phenol-red free Coon's F-12 medium containing 5%
serum substitute (SRC 3000, Tissue Culture Services Ltd., Botolph Claydon,
Buckingham, UK) 2 mM Lglutamine and 50 pg/ml gentamicin plus hormones as
indicated. After 48 h the medium was assayed for alkaline phosphatase (ALP)
activity by a chemiluminescence assay. A 10 pl aliquot of the cell culture
medium
was mixed with 200 ~1 assay buffer (10 mM diethanolamine pH 10.0 1 mM MgCl2
and 0.5 mM CSPD (Tropix Inc. Boston, USA) ) and incubated for 20 min at
37°C
CA 02340475 2001-03-26
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before measurement in a microplate luminometer (Luminoska~; Labsystems,
Finland) with integral measurement for 1 second. The ALP activity of
ERE-reporter alone was set at 1.
5. Ligand binding characteristics and transactivation function of clone 29
protein:
On the basis of the described high homology between clone 29 protein and rat
ERa in the DBD and LBD it was hypothesized that clone 29 protein might encode
a novel ER. Furthermore, biological effects of estrogens on rat prostate and
ovary,
which show high expression of clone 29 RNA, are well known (Griffiths, K.,
Davies, P., Eaton, C. L, Harper, M.E., Turkes, A., & Peeling W. B. (1991) in
Endocrine Dependent Tumours, eds Voigt, K-D. & Knabbe, C. (Raven Press), pp
83- .125; Richards, J.S (1994) Endocrine Rev. 15, 725-745; and Habenicht, U-
F.,
Tunn, U.W., Senge, Th., Schroder, R.H., Schweikert, H.U., Bartsch, G., & El
Etreby, M.F. (1993) J. Steroid Biochem. Molec. Biol. 44, 557-563). In order to
analyze the steroid binding properties of clone 29 protein synthesized in
vitro, the
reticulocyte lysate was incubated at 8°C for 16 h with increasing
concentrations
(0.3-6.0 nM) of [3H]E2 in the presence or absence of a 200 fold molar excess
of
unlabelled E2. Linear transformation of saturation data revealed a single
population of binding sites for E2 with a ICd (dissociation constant) of 0.6
nM
(Figure SA and C). Steroid binding specificity was measured by incubating
reticulocyte lysate with 5 nM [3H]E2 in the presence of 0.5, 50, 500 and 5,000
nM
unlabelled competitors. Competition curves generated are indicative of an
estrogen
receptor in that only estrogens competed efficiently with [3H]E2 for binding
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(Figure SB). Fifty percent inhibition of specific binding occured by 0.6 fold
excess
of unlabelled E2; diethylstilbestrol, estriol, estrone and Sa-androstane-
3(3,17(3-diol
were 5, 15, 50 and 150 times, respectively, less effective as competitors.
Neither
testosterone, progesterone, corticosterone nor Sa- androstane-3x,17(3-diol
were
efficient competitors, even at the highest concentrations used ( 1000 fold
excess).
The dissociation constant and the steroid binding specificities measured are
in
good agreement with data previously reported for ERs in rat and human
prostate,
rat granulosa cells, rat antral follicles and whole rat ovarian tissue (Ekman,
P.,
Barrack, E.R., Greene, G.L., Jensen, E.V., & Walsh. P.C (1983) J. Clin.
Endocrinol. Metab. 57, 166-176; van Beurden-Lamers, W.M.O., Brinkmann, A.O.,
Minder, E., & van der Molen, H. (1974) Biochem. J 140, 495-502; Kudolo, G.B.,
Elder, M.G., & Myatt, L. (1984) J. Endocrinol. 102, 83-91; and Kawashima, M.,
&
Greenwald, G.S. (1993) Biology ofReprod. 48 172-179).
When clone 29 protein was labelled with a saturating dose of [3H]E2 and
analyzed
on sucrose density gradients, a single peak of specifically bound
radioactivity was
observed. The sedimentation coefficient of this complex was about 7S, and it
shifted to 4S in the presence of 0.4 M NaCI (not shown). To investigate the
transcriptional regulatory properties of clone 29 protein, we performed
co-transfection experiments in which CHO cells were transfected with a clone
29
protein expression vector and/or an estrogen-responsive reporter gene
construct.
Cells were incubated in the absence of E2 (clone 29) or in the presence of 100
nM
E2 (Clone 29 + E2) or in the presence of 100 nM E2 and 12 pM Tamoxifen (Clone
29 + E2/Tam). In the absence of exogenously added E2 clone 29 protein showed
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18
considerable transcriptional activity which could be further increased by the
addition of 100 nM E2 (Figure 6). Simultaneous addition of a 10 fold excess of
the
antiestrogen Tamoxife~ partially suppressed the E2 stimulated activity (Figure
6).
The constitutive transcriptional activity of clone 29 protein could be
suppressed by
the anti-estrogen ICI-1624384 (not shown). It has been shown previously that
the
wild-type mouse and human ERs are constitutive activators of transcription,
and
that the transcriptional activity can be stimulated further by the addition of
E2
(Txukerman, M., Xiao-Kun Zhang., Hermann, T., Wills, K. N., Graupner, G., &
Phal, M. (1990) New Biologist 2, 613-620 and Lees, J.A., Fawell, S.E., &
Parker,
M.G. (1989) Nucl. Acids Res. 17, 5477-5488). To obtain more insight into what
concentrations of E2 effect clone 29 protein transcriptional activity,
transient
transfection experiments were carried out in the presence of increasing
concentrations of E2. CHO-cells were transiently transfected with the
ERE-reporter plasmid and the clone 29 protein expression plasmid. Cells were
incubated with increasing concentrations of E2 (0.1 - 1000 nM), estrone (E1,
1000
nM), Sa-androstane-3(3,17(3-diol (3(3-AD, 1000 nM) or no ligand added.
Alkaline
phosphatase activity (ALP) was measured as described and the activity in the
absence of ligand (control) was set at 1. The figure shows relative ALP-
activities
(LSD) from three independent experiments. Clone 29 protein began to respond at
0.1 nM E2 and maximal stimulation was observed between 1 nm and 10 nM E2
(Figure 7). The maximal stimulation factor was 2.6 t 0.5 fold (mean t SD. n =
9)
as compared to incubation in the absence of E2. Apart from E2 also estrone and
Sa-androstane- 3(3,17~i-diol could stimulate transcriptional activity, albeit
at higher
concentrations (Figure 7). Dexamethasone, testosterone, progesterone,
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Sa-androstane-3a,17~3-diol, thyroid hormone and all-traps-retinoic acid could
not
stimulate transcriptional activity of clone 29 protein, even at the highest
concentration (1000 nM) tested (not shown). The results of the co-transfection
experiments are in agreement with the ligand binding and specificity data of
clone
29 protein presented in Figure 5. In control experiments, wild-type human ERa
also showed transcriptional activity in the absence of E2, which could be
increased
by the addition of E2 (not shown).
6. Detection of rat ER expression by RT-PCR
The tissue specificity of expression of rat ER(3 and ERa was determined using
reverse transcriptase polymerase chain reaction (RT-PCR). The results of the
experiment are shown in Fig. 8.
B. Isolation of human Er(3
1. A human version of Er~3 (hER(3) has also been cloned from human ovary. The
tissue specificity of hER(3 expression in a variety of cells was also
determined
using the RT-PCR technique. The results are shown in Fig. 9. It will be
noticed
that there is a very high level of mRNA of hER(3 in human umbilical vein
endothelial cells (HUVEC) but no detection of hERa in the same cells. In
addition, it will be seen that in human osteosarcoma cell line (HOS-D4), hER(3
is
expressed in greater quantities compared to hERa.
I. A human version of ER(3 (hER(3) has also been cloned. The tissue
specificity
of hER(3 expression in a variety of cells was also determined using the
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RT-PCR technique. The results are shown in Fig. 9. It will be noticed that
there is a very high level of mRNA of hER(3 in human umbilical vein
endothelial cells (HUVEC) but no detection of hERa in the same cells. In
addition, it will be seen that in human osteosarcoma cell line (HOS-D4),
hER(3 is expressed in greater quantities compared to hERoc.
The partial DNA sequence of hER~3 is shown in Fig. 10 and a derived amino acid
sequence is shown in Fig. 11.
Cloning of human Er(3 from testis
A commercially available cDNA from human testis (Clontech, article no.
HL1161x) was screened, using a fragment containing the ligand-binding
domain of the rat Er(3 cDNA as probe. Approximately 106 recombinants
were screened, resulting in one positive clone. Upon sequencing of this
clone, it was seen that the insert was 1156 by (Figure 13A and 13B). This
corresponds to most of the translated region of a receptor with an overall
homology of 90.0% to rat Er~3, therefore deduced to represent the human
form of Er~3.
The cloned hER~i, however, lacks approximately 47 amino acids at the
N-terminal end and 61 amino acids at the C- terminal end (as compared to
the rat sequence). Further screening of the same library was unsuccessful.
PCR technology was therefore used to obtain the remaining parts. For
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oligonucleotides were sunthesised; two degenerate oligonucleotides
containing all possible codons for the amino acids adjacent to the initiation
methionine and the stop codon, respectively, of the rat Er(3, and two
specific oligonucleotides containing the sequence of the clone isolated from
the human testis library and situated approximately 100 by from respective
end of this clone. PCR with the N-terminal and C-terminal pair of oligos
yielded specific bands, that were subcloned and sequenced. The parts of
these new clones that overlap the original cDNA clone are identical to this.
It was thus possible to construct peptide and DNA sequences corresponding
to the whole open reading frame (Fig. 13A and 13B).
When comparing the human Er(3 to rat Er~3, this receptor is 79.6% identical
in the N-terminal domain, 98.5% in the DNA-binding domain, 85.6% in the
hinge and 91.6% in the ligand-binding and F-domains. These numbers
match very well those found when comparing the rat and human forms of
Era.
Studies of the expression of human Er(3 using Northern blot show
expression in testis and in ovaries. The expression in prostate, however,
appears lower than found in the rat.
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The human Er(3 gene has been mapped to chromosome 14 using PCR and
to region 14q22-23 using the FISH technique, whereas the human Er(3 gene
has been mapped to chromosome 6q25.
2. Comparison of ligand binding affinity of hERa and rER[i
The ligand affinity of the two estrogen receptors, human Era (ovary)
(hERa) and rat Er(3 (rER~3) was tested in binding saturation experiments
and in binding competition experiments.
cDNA of the receptor subtypes hERa and rER~3 were in vitro translated in
rabbit reticulocyte lysate in presence of non-radioactive amino acids
according to the instructions supplied by the manufacturer (Promega).
The radioactive ligand used in all experiments was 16a-['ZSI]-17~i-estradiol
(['zsI]-E2) (NEX- 144, New England Nuclear). The method for the binding
experiments was previously described in: Salomonsson M, Carlsson B,
Haggblad J. J. Steroid Biochem. Molec. Biol. Vol. 50, No. S/6 pp. 313-18,
1994. In brief, estrogen receptors are incubated with ['ZSI]-E2 to
equilibrium (16-18 h at +4°C). The incubation was stopped by separation
of protein-bound ['ZSI]-E2 from free ['ZSI]-E2 on Sephadex~G25 columns.
The radioactivity of the eluate is measured in a gamma-counter.
In the competition experiments, non-radioactive ligands were diluted in
DMSO, mixed with ['25I]-E2 (approximately 100-200 pM), aliquoted in
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parallel, and finally hERa or rER(3 was added. The final concentration of
DMSO in the binding buffer was 2%.
The buffer used in the experiments was of the following composition:
Hepes (pH=7.5) 20 mM, KC 1 150 mM, EDTA 1 mM, glycerol (8.7%),
monothioglycerol 6 mM, Na3MOa l OmM.
3. Equilibrium binding saturation experiments (Ka-determinations)
A range of concentrations of ['ZSI]-E2 were mixed with the ER:s and incubated
as
described above, free ['z5I]-E2 was determined by substracting bound ['z5I]-E2
from
added ['z5I]-E2. Binding data was analysed by Hill-plots and by Scatchard
plots
(Figure 11). The equilibrium binding results are shown in Table 1. The
apparent
Kd-values for ['zsI]_E2 differed between the two ERa with approximately a
factor
of four; Kd(hERa):Ka(rER~i) = 1:4.
Table 1. Equilibrium dissociation constants for ['ZSI]-E2 to the two
subtypes.
Receptor subtype Ka (Hill-plot) Kd (Scatchard-plot)
hERa 0.06 nM 0.09 nM
rER(3 0.24 nM 0.42 nM
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4. Competition experiments (ICso determinations)
The experiments were performed as described above. ICso values were obtained
by
applying a four parameter logistic analysis; b=((bm~
bm;~)/(1+(I/ICso)S))+b",;~, where
I is the added concentration of binding inhibitor, ICso is the concentration
of
inhibitor at half maximal binding and S is a slope factor. The free
concentration of
['ZSI]-E2 was determined by sampling an aliquot from the wells at the end of
the
incubation and then substract bound radioactivity from sampled total
radioactivity.
Since the equilibrium binding experiments (above) showed that the Kd-values
for
['ZSI]-E2 differed between the two ERa, K;-values (from the Cheng-Prusoff
equation: KICso/(1+L/Ka) where L is free (['ZSI]-E2]) were calculated for the
compounds investigated. Two approaches for calculating RBA (Relative Binding
Affinity) were used. The RBA values were derived using either the ICso values
or
the K; values. In both approaches, the value for the compound
16a-bromo-estradiol was selected as the reference value (100%). Both
approaches
gave similar results. The results are summarized in Figure 12. In these
Figures
"4-OH-Tam" = 4-hydroxy-tamoxifen; "DES" = diethylstilbestrol; "Hexestr" _
hexestrol; "ICI-164384" = ICI plc compound no. 164382; "17(3-E2" _
17(3-estradiol; "16a-B- E2" = 16a-bromo-estradiol; "Ralox" = Raloxifen; and
"17a-E2" = 17a diol.
The results show that Era and Er(3 have significant different ligand binding
affinities - the apparent Ka-values for ['ZSI]-E2 differed between the two
ER's by a
factor of about 4 (Kd(hERa): Kd (rER~3) ~ 1:4). Some compounds investigated
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showed significant differences in the competition for binding of [lzsl]_E2 to
the
ER's. Certain compounds were found to be more potent inhibitors of ['zsI]_E2
binding to hERa as compared to rER~i whereas others were found to be more
potent inhibitors of ['z5I]-E2 binding to rER~3 than to hERa.
