The Placental Hormones (1)

Chapter 6. The Placental Hormones ( I )

Jan. 17, 1997 2nd year resident J. W. Lee. M.D.

I. Overview

Halban (1905), Placenta ; "endocrine organ
Steroid hormone : Table 6-1
Peptide & protein hormone ; hPL, hCG, ACTH, other products of POMC, chorionic
thyrotropin, GH variant, PTH-rP, calcitonin, relaxin, TRH, GnRH, CRH, somatostatin,
GHRH, inhibin, activins, ANP
Figure 6-1

II. Human Chorionic Gonadotropin

"Pregnancy hormone"
Glycoprotein hormone with biologic activity like LH, both of which act via plasma
membrane LH/hCG receptor.
Produced almost exclusively in the placenta (but, β-subunit is synthesized in fetal
kidney,& a number of fetal tissues produce the intact hCG molecules)

A. Chemical characteristics

Glycoprotein with the highest carbohydrate content
Carbohydrate component (esp, terminal sialic acid) ; protects the molecule from catabolism
Plasma half-life ; 24 hrs (cf. LH; 2 hrs)
Comprised of α &β subunit

-> noncovalently linked ; no intrinsic LH-like biologic activity of either separated subunit
Structurally, similar to hCG : LH, FSH, TSH ; α-subunit identical

B. Biosynthesis

Synthesis of the α-& β- chain ; regulated separately

* α-subunit ; single gene ( chromosome 6 at q12-q21)

* 8 separate genes ( chromosome 19 ) of β- hCG/β-LH family
7 genes -> code for β-hCG
1 gene -> code for β-LH

--> only 3 of the β-hCG genes are expressed.
7β-hCG genes likely evolved from an ancestral gene for LH.

* As with hCG, a number of genes that are expressed in human placenta seem to
have evolved from genes normally expressed in adult tissues.

ex) hPL <-- prolactin gene

CYP19 (aromatase, the enzyme required for estrogen synthesis)

: Promoters used to initiate gene transcription in trophoblast are different from the
promoters used to initiate transcription of the same gene in nontrophoblastic cells

(ex. granulosa & adipose tissue stromal cells)
α-&β-subunits of hCG : synthesized as larger molecular wt. precursors

--> Signal sequence are cleaved by microsomal endopeptidase.

--> Once intact hCG is assembled, the molecule is rapidly released from the cell.
Rate limiting step for hCG synthesis : synthesis β-subunits
Trophoblasts of normal placenta & tissue of H-mole & choriocarcinoma tissues secretes
free α-&β-subunits as well as intact hCG.

--> Excess of hCG α-subunits (+) in placenta & plasma of pregnant women but,
β-subunits : present in plasma in only small quantities.

C. Cellular Sites of Origin

Complete hCG molecule is synthesized primarily in the syncytiotrophoblast.
Immunoreactive hCG is present in cytotrophoblasts very early in pregnancy, before 6wks.
Cellular shift in the formation of hCG & hPL --> occurs at about 6wk

1. Regulation of HCG Subunit Biosynthesis

The amounts of mRNA for both the α-&β-subunits of hCG in syncytiotrophoblast

: 1st trimester >> at term. --> important consideration in the measurement of hCG in
plasma as a screening procedure to identify abnormal fetuses.
The finding of mRNA for the α-&β-subunits of hCG in cytotrophoblasts or in
intermediate trophoblasts --> hCG genes are expressed before full differentiation of
trophoblast. : treatment of cytotrophoblasts in culture with cAMP --> promote hCG
secretion without the formation of syncytium.

Supported by the fact that cytotrophoblasts begin to disappear from the placenta at
the end of the 1st trimester : but, in some abnormal pregnancy in which there is a
reappearance of cytotrophoblasts. -- hCG ↑

(ex. D-antigen isoimmunization * G-DM)
GnRH (produced in cytotrophoblast) --> may act on the syncytium to stimulate
hCG formation

D. Molecular Forms of HCG in Plasma & Urine

Multiple forms of hCG in maternal plasma & urine (+)

1. Enzymatic degradation
2. Modification during the normal cellular sequence of synthesis/processing of the
hCG molecule

1. β-Subunits

Plasma levels of the free α-&β-subunits in pregnant women : substantially differed
from those of the intact molecule. (result of rate-limiting synthesis of β-subunits)
Figure 2-4

2. Free α-Subunits

Free α &α-subunits of intact hCG : identical. but 2 forms of the α-subunits differ
in oligosaccharide structure.
↑ size of the oligosaccharide of free α: prevent dimerization with β-hCG.
Plasma levels of free α increase gradually, but steadily, until about 36wk, when a
plateau is attained that is maintained for the remainder of pregnancy.

(--> this pattern is similar to that of hPL)
α-hCG secretion : roughly correspondent to placental mass.

c.f. Rate of secretion of complete hCG molecule : maximal at 8~10wk
Always < 10% than that of intact hCG.

3. hCG Glycosylation.

4. Nicks in the HCG Molecule.

Nicks : missing peptide linkages
Site : between β-subunit amino acid 44 ~ 45 & 47 ~ 48
Extent of nicking in standard preparation from pooled urine.

: 10 ~ 20% but, in individual samples, (0 ~ 100%)
Origin : believed to be through enzymatic action on the molecule that occurs near the
cellular site of synthesis of the β-subunit (ex. reaction catalyzed by leukocyte elastase)
Biological importance of nicked hCG.

: unknown, but. bioactivity ↓ (20%) & immunoreactivity to monoclonal antibody

(--> severely attenuated but variable among different antibody)

--> Issue of some concern when monitoring changes in the levels of hCG as a
function of time, treatment or both, as in the clinical management of persons with
neoplastic trophoblastic Dz.)

5. The " β-core fragment" of HCG.

Composition : aminoacids 6-40 that are disulfide bonded to aminoacids 55-92
Not biologically active and not combine with the α-subunit to produce intact hCG.
More easily identified in urine because in plasma this moiety is bound to protein in a
manner that precludes ready identification/quantification by immunoassay.

E. Concentration of HCG in Serum & Urine

Intact hCG molecule : detectable in plasma about 7.5 ~ 9.5 days after the midcycle surge
of LH. --> enters maternal blood at the time of blastcyst implantation.
Max. level : about 8 ~ 10wks.
During day, rhythmicity of hCG secretion (-), but fluctuation (+).
Concentration in maternal urine --> closely parallel to that in plasma.
Figure 2-4
Pattern of appearance of hCG in fetal blood --> similar to that in the mother (but,
levels in fetal plasma --> only 3% in maternal plasma.)
Concentration in amniotic fluid early in pregnancy --> similar to that in maternal
plasma. but, at near term, only 1/5.

1. Elevated or Depressed HCG levels in Maternal Plasma. / Urine

Significantly higher plasma levels

Multiple fetus

Single erythroblastotic fetus resulting from maternal D-Ag isoimmunization

H-mole & choriocarcinoma.
Relatively higher levels

: in midtrimester, Down syndrome (reason : ?, placenta - less mature)
Relatively lower levels

Ectopic pregnancy

Impending spontaneous abortion.

F. Regulation of HCG Synthesis

Butyrated derivatives of cAMP
Hypothalamic - like hormone ( GnRH, CRH )
IL-1 & IL-6
TNF-α
GF ( TGF-β & fibroblast GF )
Colony-stimulating factors
Thyroid hormone.

1. Metabolic Clearance of HCG

MCR (Metabolic Clearance Rate) : 3ml/min, (4L/day)

: Renal clearance as native molecule --> 30% of total MCR,
remainder --> liver & kidney

c.f MCR : β-subunit (*10), α-subunits (* 30)

G. Biological Factor of HCG

1. Rescue of the Corpus Luteum

Continued progesterone production
Progesterone synthesis by corpus luteum ↓ at 6wk despite continued & increasing
hCG production

2. HCG Stimulation of Fetal Testis.

Testosterone secretion (max, at max. levels of hCG)
At a critical time in sexual differentiation of the male fetus, entering fetal plasma
from syncytiotrophoblast acts as an LH surrogate, stimulating the replication of fetal
testicular Leydig cells & testosterone synthesis to promote male sexual differentiation.
Before about 110 days, no vascularization of the fetal ant. pituitary from hypothalamus.

→ little LH secretion from the pituitary

→ hCG acts as LH before this times

→ as hCG levels fall, pituitary LH maintains a lower level of fetal testicular stimulation

3. HCG Stimulation of the Maternal Thyroid.

In neoplastic trophoblastic disease → biochemical & clinical evidence of hyperthyroidsm
(+) (Past, d/t chorionic thyrotropins by neoplastic trophoblast. Later, some forms of hCG
bind to the TSH receptors of thyroid cells)
hCG stimulates thyroid activity via the LH/hCG receptor & by the TSH receptor as well

4. Other Proposed Function.

Promote relaxin secretion by the corpus luteum.
LH/hCG receptor are found in myometrium & in uterine vascular tissue

--> promote uterine vascular dilatation & myometrial smooth muscle relaxation.

III. Human Placental Lactogen

Potent lactogenic & GH-like bioactivity ( and an immunochemical resemblance to human
GH ), --> In past, called chorionic GH, chorionic somatomammotropin
In syncytiotrophoblast
Detected in the trophoblast as early as the 2nd or 3rd weeks after fertilization of the ovum.

A. Chemical Characteristics.

Single nonglycosylated polypeptide chain with a molecular Wt. 22.279d

<-- derived from a precursor of 25.000d than contains a 26aminoacids signal sequence.
191 aminoacid residues (GH: 94%, hPRL: 67% --> aminoacids sequence homology)

<-- ? genes for hPL, hPRL, hGH evolved from a common ancestral gene (probably)
by repeated gene duplication
Production Site :

Trophoblast

Direct radioimmunoassay in sera from men & women with varies malignancy

--> broncogenic carcinoma, hepatoma, lymphoma, pheochromocytoma.

B. HPL Gene Structure & Expression.

5 genes on chromosome 17.
2 of these genes, hCS-A & hCS-B --> both code for hPL.

C. HPL Secretion & Metabolism

2 of these genes, hCS-A & hCS-B --> both code for hPL.
MCR : 175L/day (>> hCG)
Production rate at near term : 1g/day --> the greatest (by far) of any known
hormone in humans.

1. HPL Serum Concentration.

Demonstrable in placenta within 5 ~` 10days after conception & detected in serum
as early as the 5th week after LMP.
In maternal plasma, : rises steadily until about 34th ~ 36th weeks & proportional to
placental mass.
Figure 2-4
Half-life : very short, 10 ~ 30MIN.
In fetal blood / urine of the mother / newborn : very little

c.f. in amniotic fluid, somewhat lower than that in maternal plasma.

(d/t, secreted primarily into the maternal circulation, with only very small amounts in
cord blood)

2. Regulation of HPL Biosynthesis

mRNA levels in syncytiotrophoblast : relatively constant throughout pregnancy.
Prolonged maternal starvation in the 1st half of pregnancy --> ↑ HPL

Synthesis ↑ : insulin, cAMP

Synthesis ↓ : PGE2 & PGF2

3. Metabolic Actions of HPL

Lipolysis & ↑FFA (--> providing a source of energy for maternal metabolism &
fetal nutrition)
Anti-insulin action, (leading to ↑maternal insulin level)
--> favors protein synthesis and provides a mobilizable source of aminoacid for
transports to the fetus.

4. Pregnancy with no Detectable HPL

Not required for a successful pregnancy outcome.
hPL deficiency : ca. 1/12.000 pregnancy
hPL function primarily as a fail-safe mechanism to ensure nutrient supply to the fetus,
possibly, in times of maternal starvation.

IV. Other Protein Hormones of the Placenta

1. Chorionic Adrenocorticotropin

Pro-opiomelanocortin(POMC) --> ACTH, lipotropin, β-endorphin
Dexamethasone Tx -> not alter the levels of ACTH in placental tissue
Physiologic role ; unclear
Plasma level in pregnancy (before labor); lower than those in men and nonpregnant

; increase as pregnancy advances
Not cross the placenta

2. Chorionic Thyrotropin

Significant biologic role
Thyroid-stimulating activity in GTN; by hCG , not thyrotropin

3. Relaxin

Corpus luteum, decidua, placenta
Structually similar to insulin, nerve growth factor
Two relaxin genes; H1, H2

i) corpus luteum; H2

ii) decidua, placenta, fetal membrane; H1, H2
Act on myometrial smooth muscle to stimulate adenyl cyclase -> uterine relaxation

4. Parathyroid Hormone-related protein (PTH-rP)

Synthesis in uterus, corpus luteum, lactating mammary tissues, fetal parathyroid, placenta,
kidney.
Not produced in the normal parathyroid gland of adults.
may serve as the parathromone of fetus
Synthesis/ secretion by cytotrophoblast
Responsive to the extracellular calcium concentration

5. Growth Hormone-variant ( hGH-v)

Placental growth hormone
hGH-v is in maternal plasma by 21-26 wks --> increasing concentration to 36 wks
Correlation with IGF-1 level.
Biologic action is similar to hPL

V. Hypothalamic-like Releasing Hormones

A. Gonadotropin-releasing Hormone (GnRH)

Present in cytotrophoblast
Stimulate the secretion of hCG from placenta
1st trimester trophoblasts; more responsive to GnRH than term placenta
Inhibin, activin ; regulate GnRH synthesis

B. Corticotropin-releasing Hormone (CRH)

CRH gene is also expressed in trophoblast, amnion, chorion laeve, & decidua.
Plasma levels of CRH

i) nonpregnant ; 15 pg/ml

ii) early 3rd trimester ; 250 pg/ml

iii) term ; 1-2 ng/ml

iv) after delivery ; undetectable (half life; 1 hr)
Conditions of ↑ CRH : PIH, preterm labor, fetal asphyxia, fetal growth retardation,
twin pregnancy, during the course of labor

1. CRH-binding protein (BP) in human plasma

Inactivate the CRH
Inhibit CRH-induced ACTH release by pituitary cells
CRH-BP levels decrease during last few weeks of pregnancy at a time when CRH
increase
Deficiency --> Cushing syndrome

2. Placental CRH

Biologic function is not so unclear.
Receptors for CRH ; placenta, adrenal, sympathetic ganglia, lymphocyte, GI tract,
pancreas, gonads, myometrium
Local action in paracrine or neuroendocrine fashion
Many suggestions of its role in pregnancy

i) act to modify pituitary- adrenal function

ii) induction of smooth muscle relaxation and immunosuppression

iii) induction of myometrial contraction (i.e, initiation of parturition)

iv) PG formation

3. Regulation of placental CRH synthesis

(+) feedback loop in placenta

(--> glucocorticosteroid stimulation -> CRH gene expression)

C. Thyrotropin-releasing Hormone

Little known of the biologic role

D. Growth Hormone-releasing Hormone

VI. Other Peptide Hormones Synthesized in Placenta

1. Neuropeptide-Y

Small peptide that distributed in brain, sympathetic neuron
Isolated from placenta, and localized in cytotrophoblast.
Potassium in high concentration NPY↑
Treatment of placental cells with NPY --> release of CRH

2. Inhibin and Activin

Produced by testis, granulosa-cells of ovary (including Corpus luteum)
Structure

1) Inhibin ; composed of α- and β- subunit (βA of βB)

2) Activin; combination of two β-subunits
Inhibin ; inhibit FSH secretion --> preclude ovulation during pregnancy ( greatest levels in term)

* synthesis ; cytotrophoblast

* storage ; syncytiotrophoblast

* hCG and cyclic AMP analogue --> stimulate secretion

* inhibin may act via GnRH to regulate hCG synthesis/ secretion in placenta

3. Atrial Natriuretic Peptide

Natriuresis, diuresis, vasorelaxation
Produced in atrial myocytes, placenta
ANP receptors are found in placenta, myometrium-> vasorelaxation by increasing
cellular levels of cGMP
Promoting uterine relaxation during first 95% of pregnancy