The Journal of Clinical Endocrinology & Metabolism Vol. 84, No. 9 3344-3350
Copyright © 1999 by The Endocrine Society
Expression of Prolactin and Its Receptor in the Baboon Uterus during the Menstrual Cycle and Pregnancy1
Jonna Frasor,
Christina A. Gaspar2,
Kathleen M. Donnelly,
Geula Gibori and
Asgerally T. Fazleabas
Departments of Physiology and Biophysics (J.F., G.G., A.T.F.) and
Obstetrics and Gynecology (C.A.G., K.M.D., A.T.F.), University of
Illinois, Chicago, Illinois 60612
Address all correspondence and requests for reprints to: Asgi Fazleabas, Ph.D., Department of Obstetrics and Gynecology, University of Illinois, 820 S. Wood Street (M/C 808), Chicago, Illinois 60612.
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Abstract
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PRL is known to be a major secretory product of the human decidua.
However, the physiological role of decidual PRL during the menstrual
cycle and pregnancy has not been fully defined, primarily due to the
lack of an appropriate nonhuman primate model for in
vivo studies. Therefore, this initial study examined the
expression of PRL and its receptor in the baboon uterus during the
cycle and pregnancy. PRL and PRL receptor messenger ribonucleic acid
expression were detected by semiquantitative RT-PCR, and protein was
localized by immunocytochemistry. PRL was shown to be expressed in
myometrial smooth muscle during the follicular phase by both RT-PCR and
immunocytochemistry. Expression of PRL messenger ribonucleic acid and
protein was first observed in the epithelial cells of the deep basal
glands during the late luteal phase. With the onset of pregnancy, PRL
expression increased steadily and was evident primarily in the decidual
tissue. In contrast to PRL, its receptor was expressed at constant
levels in both the myometrium and endometrium during the cycle. An
increase in receptor expression was evident in both the decidua and
placenta throughout pregnancy. In summary, these results demonstrate
that the baboon uterus is a site of both PRL production and action
during the cycle and pregnancy. These studies establish the baboon as a
nonhuman primate model to investigate the potential roles of PRL in
implantation and maintenance of pregnancy.
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Introduction
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DECIDUALIZATION of the endometrium involves
a series of morphological and molecular changes in which stromal
fibroblasts differentiate into secretory decidual cells. In the human,
stromal cells undergo a predecidual response during the last week of
the menstrual cycle, whereas in nonhuman primates, decidualization
requires the presence of a conceptus (1). One of the characteristic
markers of decidualization in both the human and the baboon is the
production and secretion of insulin-like growth factor-binding
protein-1 (IGFBP-1). In the human, IGFBP-1 begins to be secreted by the
stromal cells surrounding the spiral arteries during the late luteal
phase, whereas in the baboon, IGFBP-1 is secreted from the glands in
response to progesterone (2, 3). Stromal cell production of IGFBP-1 is
up-regulated in the baboon in response to pregnancy, and in both the
human and the baboon is correlated with the process of
decidualiza-tion (4).
A second characteristic secretory product of the human decidua is PRL.
The production of decidual PRL begins by day 22 of the cycle in the
human and continues to increase throughout gestation (5). High levels
of PRL are reached in the amniotic fluid, which then decrease with the
onset of labor (6). Although high levels of PRL are measurable in
baboon amniotic fluid (7), whether PRL is produced within the baboon
uterus, as it is in the human, is as yet unknown. Therefore, this study
was undertaken to characterize PRL production within the baboon uterus
during the menstrual cycle and pregnancy. In addition, expression of
the PRL receptor (PRL-R) was characterized to identify possible targets
for PRL action within the uterus.
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Materials and Methods
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Tissue collection
Uterine tissue was obtained by hysterectomy or endometriectomy
from adult female baboons (Papio anubis) at various points
during the menstrual cycle or pregnancy (n = 2/day examined). The
animal care committee of the University of Illinois approved all
experiments and procedures. The stage of the cycle was determined by
the following criteria: menstrual cycle history, sex skin tumescence,
and serum levels of estradiol and progesterone (8, 9). The stage of
pregnancy was confirmed by ultrasound and serum levels of CG,
estradiol, and progesterone (10, 11). The myometrial tissue was
separated from the endometrium for tissues obtained from cycling
baboons. Tissue from pregnant baboons was divided into placenta and
decidua. All tissue was either frozen in liquid nitrogen for
ribonucleic acid (RNA) extraction or fixed in Bouins solution for
paraffin embedding and immunocytochemistry (ICC).
RT-PCR
RNA was prepared from baboon endometrium, myometrium, decidua,
and placenta at various days of pregnancy using Trizol reagent
(Life Technologies, Inc., Gaithersburg, MD) according to
the manufacturers instructions. For detection of PRL or PRL-R
messenger RNA (mRNA), 1 or 3 µg total RNA, respectively, were reverse
transcribed at 42 C using random hexamer primers (Pharmacia Biotech, Piscataway, NJ) and Moloney murine leukemia virus
reverse transcriptase (Life Technologies, Inc.). Primers
for PRL were designed to amplify a region of high homology between the
human and rhesus monkey decidual PRL (5'-GCCCCCTTGCCCATCTGTCC-3' and
5'-AGAAGCCGTTTGGTTTGCTCC-3'). To detect expression of the long and
short forms of the PRL-R, sense and antisense strand oligonucleotides
corresponding a common extracellular domain and unique intracellular
domains were used as previously described (12). In addition, two
oligonucleotides (5'-CGTTCACCTTGATGAGAGCCAGT-3' and
5'-TCC-AAGGGTCCGCTGCAGTC-3') specific for ribosomal protein S16 mRNA
were included in each PCR reaction as an internal control.
Amplification was carried out using 2.5 U Taq DNA polymerase
(Perkin Elmer Corp., Foster City, CA) and
[
-32P]deoxy-CTP (Amersham Pharmacia Biotech, Arlington Heights, IL) for 30 cycles with an annealing
temperature of 65 C. All PCRs were carried out in the linear phase of
amplification. PCR products were electrophoresed on an 8%
polyacrylamide gel and analyzed using a PhosphorImager (Molecular Dynamics, Inc., Sunnyvale, CA).
ICC
Six-micron sections of paraffin-embedded tissue were cut and
mounted on poly-L-lysine-coated slides. Immunocytochemical
localization of PRL was performed using a polyclonal antibody to
recombinant human PRL at a dilution of 1:750 (Scripps Laboratory, San
Diego, CA). A monoclonal antibody (T6; 12.5 µg/mL) to the rat PRL-R
(provided by Dr. Paul Kelly) was used for ICC localization of the
receptor. Immunostaining was visualized using avidin-biotin complexed
to horseradish peroxidase (Vectastain ABC kit,
Vector Laboratories, Inc., Burlingame, CA) and the
chromogen diaminobenzidine (Sigma Chemical Co., St. Louis,
MO). Negative controls included the substitution of he primary antibody
with preimmune rabbit serum for PRL and mouse ascites fluid for
PRL-R.
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Results and Discussion
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PRL production in the baboon uterus
RNA obtained from endometrium and myometrium of cycling animals
and from decidua and placenta of pregnant animals were analyzed for PRL
expression by RT-PCR (Fig. 1A
). PRL mRNA
levels were quantitated and expressed relative to S16 mRNA levels in
each sample (Fig. 1B
). PRL mRNA could be detected in late luteal
endometrium as well as follicular phase myometrium. The decidua
expressed higher levels of PRL mRNA, which increased throughout
pregnancy. Very low levels of PRL mRNA could also be detected in
placental tissues.

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Figure 1. Detection of PRL mRNA by RT-PCR. A,
Endometrium and myometrium from the follicular phase (FP) and the
luteal phase (days 5, 10, and 12 postovulation) as well as decidua and
placenta from days 29, 39, and 52 and at term of pregnancy were
analyzed for PRL mRNA expression by RT-PCR. B, Quantitation of PRL mRNA
levels relative to S16 mRNA levels.
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To determine which cell types were producing PRL in the baboon uterus,
PRL was localized by ICC using a polyclonal antibody to recombinant
human PRL. In the endometrium, staining for PRL protein was absent
during the follicular phase, but was evident in glandular epithelium
and individual stromal cells during the late luteal phase (Fig. 2
, A and B). In contrast, the myometrial
smooth muscle stained positively for PRL during the follicular phase,
but was absent in the luteal phase (Fig. 2
, C and D). During pregnancy,
both the decidua and placental syncytiotrophoblasts stained positively
for PRL on days 39 and 59 and at term (Fig. 3
). No staining was evident when
preimmune serum was used in place of the primary antibody (data not
shown).

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Figure 2. ICC localization of PRL during the menstrual
cycle. Endometrium from the follicular (A) and luteal (B) phases as
well as myometrium from the follicular (C) and luteal (D) phases were
stained using a polyclonal antibody to recombinant human PRL.
Magnification, x800.
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Figure 3. ICC localization of PRL during pregnancy.
Decidua and placenta from day 39 (A and B) day 59 (C and D) of
pregnancy and at the time of cesarean section (E and F) were stained
using a polyclonal antibody to recombinant human PRL. Magnification,
x800.
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In the human, estradiol has been shown to stimulate pituitary PRL
production, whereas progesterone is a potent stimulator of decidual PRL
expression (13, 14). In contrast, myometrial PRL is down-regulated by
progesterone (15). Our study suggests that regulation of uterine PRL by
progesterone in the baboon is similar to that in the human. During the
follicular phase, when low levels of progesterone are present, the
myometrium expresses PRL mRNA and protein. However, during the luteal
phase, as progesterone levels increase, myometrial PRL is
down-regulated, and as progesterone levels rise, the glandular
epithelium and stromal cells begin to produce PRL. However, it is the
decidua tissue of pregnancy that produces high levels of PRL, which
increase throughout pregnancy. Preliminary results in our laboratory
have also shown that when baboon stromal cells are induced to
decidualize in vitro, PRL mRNA expression is increased.
Although low levels of PRL mRNA were detected in the placenta, this
tissue has not been shown to produce PRL. This may be attributed to
decidual cells contaminating the placental tissue. In contrast, ICC
staining for PRL was extremely high. This may be due to placental
PRL-like proteins cross-reacting with the antibody or to the
sequestration of PRL into the syncytiotrophoblast cells.
Expression of PRL-R in the baboon uterus
RNA isolated from endometrium and myometrium from cycling animals
and from decidua and placenta of pregnant animals were examined for
PRL-R expression by RT-PCR (Fig. 4A
). In
all tissues examined, only the long form of the PRL-R could be
detected. Throughout the menstrual cycle, endometrial and myometrial
PRL-R levels were relatively constant when normalized to S16 (Fig. 4B
).
PRL-R mRNA expression was also observed in both the decidua and
placenta on day 39 and at term pregnancy.

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Figure 4. Detection of PRL-R mRNA by RT-PCR. A,
Endometrium (E) and myometrium (M) from the follicular phase (FP) and
from the luteal phase (days 5, 10, and 12 postovulation) as well as
decidua (D) and placenta (P) from days 39 and term of pregnancy were
analyzed for PRL-R mRNA expression by RT-PCR, B, Quantitation of PRL-R
mRNA levels relative to S16 mRNA levels.
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To determine which cells within these tissues express the PRL-R, ICC
was performed using a monoclonal antibody to rat PRL-R. In tissue from
cycling baboons, glandular epithelium and myometrial smooth muscle
stained positively for PRL-R throughout the cycle (Fig. 5
). Similarly, the decidua and placental
syncytiotrophoblasts stained positively throughout pregnancy (Fig. 6
). In addition, both the chorionic and
amniotic fetal membranes demonstrated high levels of staining for the
PRL-R at the time of delivery (data not shown). Again, no staining was
observed when tissue sections were incubated with preimmune mouse
ascites (data not shown).

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Figure 5. ICC localization of PRL-R during the
menstrual cycle. Endometrium from the follicular (A) and luteal (B)
phases as well as myometrium from the follicular (C) and luteal (D)
phases were stained using a monoclonal antibody to rat PRL-R.
Magnification, x800.
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Figure 6. ICC localization of PRL-R during pregnancy.
Decidua and placenta from days 29 (A and B) and 71 (C and D) of
pregnancy and at the time of cesarean section (E and F) were stained
using a monoclonal antibody to rat PRL-R. Magnification, x800.
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A recent study has shown that the human endometrium expresses the PRL-R
in a pattern similar to that found for PRL: low in the proliferative
and early secretory phases, increased in the mid- to late secretory
phase, and further increased in the decidua of early pregnancy (16).
The PRL-R has also been shown to be up-regulated in the human decidua
and correlates with PRL gene expression (17). Furthermore, the PRL-R
was expressed in human chorion, amnion, decidua, and placenta in late
pregnancy (18). Similar to that in the human, baboon PRL-R expression
in the endometrium was low during the menstrual cycle, but as
decidualization progressed, both PRL and PRL-R expression increased.
However, the PRL-R appears to be constituitively expressed in the
myometrium during the cycle as well as in the placenta and fetal
membranes during pregnancy.
The presence of both PRL and its receptor implies that PRL may play an
autocrine/paracrine role within the uterus. However, what function PRL
serves remains speculative. Its up-regulation at the time of
decidualization suggests that PRL may play a role in preparing the
endometrium for implantation, whereas its high level in amniotic fluid
may indicate a role in the maintenance of pregnancy. Preliminary
studies in the rat suggest that decidual PRL may play a critical role
in the control of apoptosis during early pregnancy (19). Thus, one
potential function of PRL may be to ensure cell survival during
gestation. In further support of a role for PRL in the maintenance of
pregnancy, Bethea et al. demonstrated that intrauterine
infection leads to an increase in proinflammatory cytokines, a decrease
in amniotic PRL, and the onset of labor in rhesus monkeys (20). In
general, PRL has been shown to alter the immune response, with high
levels being stimulatory and low levels being inhibitory (21). In
addition, within the myometrium, PRL may cause an increase in smooth
muscle cell number, as has been shown in vitro (22).
In conclusion, this study demonstrates for the first time that both PRL
and the PRL-R are expressed in the baboon uterus. Furthermore, the
regulation of PRL and PRL-R expression within the uterus is similar to
that described in the human. These results indicate that the baboon may
be an excellent model for in vivo studies related to the
role of PRL in maintaining decidual integrity during gestation in the
primate.
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Footnotes
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1 This work was supported by NIH Grants HD-29964 (to A.T.F.) and
HD-11119 (to G.G.) and a Lusto American Foundation Fellowship (to
C.A.G.). This work was performed as part of the National Cooperative
Program for Markers of Uterine Receptivity for Blastocyst
Implantation. 
2 Current address: Faculty of Pharmacology, University of Porto, 4000
Porto, Portugal. 
Received September 17, 1998.
Revised April 2, 1999.
Accepted May 20, 1999.
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