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Inhibin A

Activin A
Phorbol ester

Figure 1.7 Factors regulating the release of activin and inhibin from human placental cells in culture

and activin A assay may be of value in diagnosis and short-term follow-up of
molar pregnancy, as levels are highest in molar pregnancies (Florio et al., 2002e);
also of early pregnancy viability (Luisi et al., 2003), fetal demise (Petraglia et al.,
1999b), pregnancy-induced hypertension (Muttukrishna et al., 2000) and PE
(Petraglia et al., 1999b; Florio et al., 2002a; 2003a) (Table 1.2). In particular, the
measurement of inhibin A and activin A may offer important prognostic informa-
tion in predicting early the onset of PE several months before the onset of symptoms
(Florio et al., 2003a; Lambert-Messerlian et al., 2000; Silver et al., 2002). Although
placental mRNA expression of the and A-subunits is increased in PE (Florio
et al., 2003a; Silver et al., 2002), the increased levels of activin A appear to be more
speci¬cally a re¬‚ection of increased placental production than do the increased
levels of inhibin A (Silver et al., 2002).

Placental control of hCG secretion

Placental GnRH (Box 1.9) stimulates secretion of hCG in vitro (Khodr and Siler-
Khodr, 1980), in agreement with the neuroendocrine action of the hypothalamic
counterpart (Figure 1.8). In addition, since several factors regulating the release of
GnRH from hypothalamus also regulate the secretion of placental GnRH, they
may also have a local role in modulating indirectly the placental hCG release.
Indeed, activin A stimulates, whilst inhibin A inhibits, the placental secretion of
hCG, 17 -estradiol and progesterone (Petraglia et al., 1989d; Keelan et al., 1994).
In details, the effects of activin A and inhibin A on placental hCG release is com-
parable to pituitary/FSH regulation. In addition, the cellular colocalization of
activin A, inhibin A and GnRH suggests the occurrence of autocrine events in the
regulation of hCG release (Petraglia et al., 1992b). The addition of an inhibin anti-
serum increases the placental hCG release (Petraglia et al., 1987b; 1989d), whilst
54 F. Petraglia et al.

Box 1.9 Gonadotropin-releasing hormone

The GnRH is a decapeptidic hormone produced by hypothalamic neurons that
controls reproduction in vertebrates through the hypothalamic“pituitary“
gonadal axis by stimulating LH and FSH secretion (Stojilkovic et al., 1994). The
GnRH is also produced by human placental tissue and by cultured placental
cells (Khodr and Siler-Khodr, 1978; 1980) and is immunologically and chemi-
cally identical to hypothalamic GnRH (Khodr and Siler-Khodr, 1978; 1980; Tan
and Rousseau, 1982; Gohar et al., 1996).

Expression and localization
Placental GnRH message has been found in human placenta, from the ¬rst
trimester to term, by in situ reverse transcription-polymerase chain reaction
and immunocytochemistry, with abundant signals both in the cyto- and syncy-
tiotrophoblast (Wolfahrt et al., 1998). The GnRH staining is reported to
be intense in cytotrophoblast and in the villous stroma from early placentae
(8 weeks of pregnancy) (Miyake et al., 1982), but GnRH immunoreacivity has
been also demonstrated in the syncytiotrophoblast of the normal human pla-
centa from the ¬rst half of pregnancy, in syncytiotrophoblast cells of hydatidi-
form mole and choriocarcinoma (Seppala et al., 1980). The total placental
concentration of immunoreactive GnRH, as measured by RIA, progressively
increases during the ¬rst 24 weeks of gestation and remains relatively constant
in the third trimester (Siler-Khodr and Khodr, 1978) whilst, on the contrary, the
mRNA expression remains constant throughout gestation (Kelly et al., 1991).

The human placenta contains speci¬c binding sites for GnRH that interact with
GnRH agonists and antagonists (Leung and Peng, 1996). By in situ hybridization,
GnRH receptor (GnRHR) mRNAs were detected in the human placenta and
localized to the cytotrophoblast and syncytiotrophoblast cell layers (Lin et al.,
1995). Using primers speci¬c to the human GnRHR, the predicted PCR product
was obtained from human placenta cells (Boyle et al., 1998; Wolfahrt et al., 1998)
and choriocarcinoma cell line (JAR and JEG-3) (Lamharzi et al., 1998; Yin et al.,
1998) and the receptor expressed in the placenta is identical to the counterpart of
pituitary (Cheng et al., 2000). The placental GnRHR is coupled to the protein
kinase C (PKC) and cAMP/protein kinase A (PKA) pathways (Cheng et al., 2000).
Moreover, there is evidence that GnRH induces activation of the mitogen-
activated protein kinase (MAPK) signaling pathway in normal and carcinoma
cells of the human ovary and placenta (Kang et al., 2000).
55 Placental expression of neurohormones and other neuroactive molecules

The contemporary presence of GnRH and GnRHR in identical cells strongly
suggests an autocrine/paracrine regulation by GnRH in human placenta. On
this regard, two classes of placental GnRH-binding sites have been described
to date: high af¬nity (Kd 10 8 mol/l) and low af¬nity (Kd 10 5 mol/l)
(Cheng et al., 2000; Kang et al., 2000). From ¬rst trimester to term, the human
placenta contains low-af¬nity GnRH-binding sites that interact with GnRH
agonist or antagonist (Bramley et al., 1992; 1994; Cheng et al., 2000; Kang et al.,
2000). The GnRHR levels decrease observed between 10 and 20 weeks of gesta-
tion is probably due to a decreased expression/synthesis (or increased catabo-
lism) of placental GnRHR, or increased occupancy (or downregulation) of
placental GnRHR by an endogenous GnRH-like ligand (Bramley et al., 1992;
1994). The mRNA of the high-af¬nity GnRH-binding site is expressed
in human cytotrophoblast and syncytiotrophoblast cell layers (Lin et al.,
1995). The GnRH administration to pregnant women increases serum levels
of hCG in the ¬rst trimester, but not in the third trimester (Iwashita et al., 1993),
probably due to the decreased number of GnRHRs in the term placenta (Lin
et al., 1995).

Levels in biological ¬‚uids
The GnRH is measurable in the maternal circulation, and levels are signi¬cantly
higher during pregnancy than in non-pregnant cycling women, in particular in
the ¬rst half of pregnancy (Siler-Khodr et al., 1984). Pulsatile changes of mater-
nal GnRH values have been shown, with highest amplitude in the ¬rst trimester
and lowest at term (Petraglia et al., 1994a) and, maternal levels at 25“35 weeks
of gestation are higher in women who later had post-term pregnancies (Gohar
et al., 1996).

the addition of recombinant inhibin inhibits it secretion (Petraglia et al., 1987b;
1989d). These effects are mediated at least in part by GnRH, as preincubation with
a GnRH antagonist partially reduced the increase of hCG after immunoneutrali-
zation of inhibin (Petraglia et al., 1987b). Activin A increases hCG and GnRH-
induced hCG release from cultured human placental cells collected at ¬rst
trimester and at term (Petraglia et al., 1987b; 1994c). The effect of activin A on
GnRH release is potentiated by estradiol or estriol, reduced by progesterone and
antagonized by tamoxifen or RU486. In addition, progesterone reverses the effect
of estriol, thus suggesting that estrogens and progesterone have opposite effects on
placental hCG release. Finally, follistatin inhibits activin A-induced hCG release, as
it binds and inactivates activin A (Petraglia et al., 1994c) (Figure 1.8).
56 F. Petraglia et al.


Activin A Inhibin A




Figure 1.8 Paracrine control of hCG release by the placental syncytiotrophoblasts. The stimulatory
effects of GnRH and activin, and the inhibitory effects of inhibin resemble the control
of pituitary FSH release

Opioid peptides ( -END and dynorphins (DYN)) play a role in regulating secre-
tion of hPL (Newnham et al., 1983; Petraglia et al., 1990b; 1996a; Petraglia, 1991;
Ahmed et al., 1992; Reis et al., 2001) and hCG release from trophoblast tissue
(Cemerikic et al., 1992). Indeed, DYN has a signi¬cant stimulatory effect upon pul-
satile hCG secretion in the ¬rst trimester placenta cell cultures (Barnea et al., 1991a),
whilst -END in vitro inhibits hCG secretion (Barnea et al., 1991b) (Figure 1.8).


The physiological maternal and fetal adaptations during human gestation are reg-
ulated by human placenta through the secretion of several neurohormones. Thus,
¬‚uid balance, blood pressure, digestion, respiration, fuel and mineral metabolism,
immune response, and several behavioral functions are reprogrammed during
pregnancy and occur under the modulation of hormonal changes, from very early
gestation till after the fetal delivery. The excessive/reduced release of some pla-
cental neurohormones in association with gestational diseases may be part of an
adaptive response of placenta and fetal membranes to adverse environmental
conditions, such as hypertension, hypoxia and infection, or to malformations of
the fetus and placenta. In a scenery of maternal and/or fetal stress elicited by a
number of pathological conditions, the neurohormones produced and secreted
by the human placenta appear to play a role in coordinating the adaptive changes
in uterine perfusion, maternal metabolism, ¬‚uid balance and possibly uterine
57 Placental expression of neurohormones and other neuroactive molecules


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