The Placental Hormones (2)

CHAPTER 6. THE PLACENTAL HORMONE (II)


24th, Feb. 1997

1st year resident J. T. Moon. M.D.




I. ESTROGEN




  • Steroid biosynthesis in human syncythium is dependent upon blood-borne steroid precursors
  • Near term hyperestrogenic state





    • Last few weeks of pregnancy each day estrogen production -- > 1000 ovulatory women
    • During one normal pregnancy estrogen amount ; 200 ovulatory women 40weeks
    • Hyperestrogenic state of human pregnancy is one of continually increasing magnitude as pregnancy progress ; terminating abruptly after delivery of the fetus and placenta



  • Pregnancy 7th weeks ; >50% of estrogen - produced in placenta

    ex) 78th day of pregnancy, after BSO the level of urinary estrogen were not reduced



A. Placental Estrogen

Biosynthesis

  • Ovarian estrogen synthesis pathway ; de novo, from acetate or cholesterol
    androstenedione ( ovary theca cell ) -> follicular fluid -> estradiol-17 (granulosa cell)

    cf) progesterone ( corpus luteum ) <- LDL cholesterol



  • In human placenta

    precursor ; acetate or cholesterol is not,
    C19 steroid ( dehydroepiandrosterone, androstenedion, testosterone ) to estrogen



B. Plasma C19-steroids as Precursors for Placental Estrogen Formation



  • Frandsen and Stakemann
    ; Anencephalic fetus mother urine estrogen - 1/10 normal fetus mother
    due to absence of hypothalamic-pituitary function --> fetal adrenal gland atrophied




  • Baulieu and Dray 1963
    ; RI - DS to pregnant women -> convert RI urinary estrogen




1. Placental Aromatase Enzyme





  • Androstenedione to estrogen ; catalazed by enzyme complex referred to aromatase




  • Composed of

    specific cytochrome P-450 monooxygenase

    aromatase cytochrome P-450 (P-450AROM ; P-450XIX, the product of the CYP19 gene)
    flavoprotein

    NADPH-cytochrome P-450 reductase



  • Principal cellular location of P-450 AROM : syncytiotrophoblast, ovary granulosa cell




  • cf) CYP19 expressed in much low level at adipose tissue stromal cells, Sertoli and Leydig
    cells of testis, brain (hypothalamus), fetal liver (not adult), some breast cancer

    cells, stromal cells of endometriosis tissue ( but not normal endometrium )



2. Aromatase Gene (CYP19)




  • Simpson and colleagues characterized the gene encoding P-450.CYP19


    (1) largest P-450 gene identified

    (2) the first exon of CYP19 in placenta is untranslated



  • Alternate promoters are used in a tissue-specific manner for initiation of aromatase gene transcription



  • In trophoblast, the primary aromatase transcript is attributable to a different promoter than that used in granulosa cells or adipose tissue stromal cells



    --> indicative of tissue-specific regulation of cis- and trans-regulatory element for
    controlling aromatase gene expression and thereby aromatase activity



  • The rate of transcription of the CYP19 gene -> regulate primarily aromatase enzyme and
    enzyme activity



3. Secreted Estrogens




  • Dependent on the nature of the substrate available the particular 17 -hydroxysteroid
    dehydrogenase isozyme



  • Ovary and testis : estradiol-17

    adipose tissue : estrone

    human placenta : estradiol-17

    estriol (16 -hydroxyandrostenedione -> 16 -hydroxyestrone -> E3 )




C. Metabolism of Maternal Plasma Dehydroepiandrosterone Sulfate




  • Gant and co-workers(1971) ; 10- to 20- fold increase in matabolic clearance rate of
    plasma dehydroepiandrosterone sulfate (normal term pregnant woman compared to man
    and nonpregnant)



  • MCR of plasma DS by two process



    (1) Removal through conversion to estradiol-17 in syncytium

    (2) Accelerated 16 -hydroxylation of DS (probably in maternal liver)



  • Maternal adrenal gland do not produce sufficient amount of DS during pregnancy



  • Fetal adrenal glands are the quantitatively important source of placental E precursor





II. THE FETAL ADRENAL GLANDS





A. Fetal Adrenal Contribution to Estrogen Formation in Placenta



  • Fetal adrenal cortex ; principal source of placental E precursor




    • Frandsen and Stakemann (1961) ; anencephalic fetus pregnant ; low urine E excretion
    • High level of DS in cord blood of normal newborn
    • Bolte' and co-workers ; RI - labeled DS to umbilical A. -> placenta in situ ; convert
      into E2


  • Near term E2 produced in placenta ; maternal plasma DS- 50 %

    fetal plasma DS- 50%




B. Fetal Adrenal Participation in Placental E3 Synthesis




  • In nonpregnant women, urinary E3/E1+E3 = 1
    in near term > 10 <= E3 formation increase



  • Brown (1956) ; E2 matabolism in pregnant women not differ from nonpregnant
    E1, E2 not convert to E3



    Ryan (1959); 16 -hydroxylated C19-steroids--16 -hydroxydehydroepiandrosterone,

    16 -hydroxy- -androstenedione and 16a-hydroxytestosterone were convert to

    E3 in placental tissue



  • Direct placental synthesis of E3 from plasma borne 16a-DS

    16a-DS ; produced in fetal adrenal gland

    16a- hydroxylation of plasma DS in fetal liver
    near term the fetus is the source of 90 % of placenta E3 precursor
    mother ; maternal plasma DS -> 16a-DS in liver -> E3 in placenta




C. Fetal Adrenal Function : An overview



  • Adrenal cortex-the largest organ of the fetus compared with adult organ

    at term adrenal gland weight - similar to adult adrenal gland

    > 85% of the gland : composed of peculiar fetal zone



  • Figure 6 - 3




  • Daily production of steroids by fetal adrenal gland : 100 - 200 ng/day

    cf) in adult : 30 - 40 mg/day



  • Adrenal cortex begin a process of involution immediately after birth weight of adrenal
    gland - first few weeks of life ↓



D. Fetal Adrenal Development




1. Embryonic Adrenal Cortex



; composed of fetal zone

-- hypothalamic-anterior pituitary is not operative early in gestation

-> fetal cortisol act at the level of the ant. pituitary gland -> inhibit ACTH release

vascularization (hypothalamus to ant. pituitary) completed in 110 days of gestation



ex) CAH ; caused by gene mutation - steroid 21-hydroxylase,

steroid 17a - hydroxylase,steroid 11b-hydroxylase

reduced enzymatic activity protein gene

--> cortisol formation defect --> ACTH excessive increase


cf) ordinarily cortisol inhibit ACTH secretion <-- cortisol inhibit CRH release




2. Fetal Adrenal Growth




- ACTH promote hypertrophy, not hyperplasia
determined by growth factors



3. Fetal Adrenal Steroid Precursor



# What is the Precursor used for steroid biosynthesis in the fetal adrenal gland ?



  • progesterone -> cortisol
  • pregnenolone - > DS ==> small fraction
  • cholesterol - fetal adrenal steriodogenesis precursor
  • fetal cholesterol pool turnover : adult 6 배



4. LDL Cholesterol and Fetal Adrenal Steroidogenesis



  • several investigator ; fetal adrenal gland in serum free medium-> steroid hormone 생산 가능

    즉 acetate 에서 cholesterol 생산 가능

    small part of steroidogenesis



  • Simpson,Carr and their co-worker

    ; LDL medium 에서 steroidogenesis markedly stimulated

    HDL less effective

    VLDL devoid of stimulatory activity



    ==> fetal adrenal steroidogenesis - highly dependent on circulating LDL



  • Figure 6 - 4

    DS <- fetal zone

    cortisol <- neocortex



5. Regulation of Cholesterol level in the Fetus



== Fetal Plama cholesterol source ?



  • Pitkin and co - workers (1972)

    ; 20% 이상 -> maternal transfer



  • Carr and Simpson (1984)
    ; fetal plasma LDL 의 대부분 - fetal liver de novo synthesis

    low level LDL cholesterol - not consequence of impaired fetal LDL synthesis

    but rapid use of LDL by fetal adrenal gland

    == cord plama level of LDL and DS : inverse correlation




E. Fetal induced Modification of Estrogen production




1. Fetal Death



-> striking reduction in urinary E. level

즉 placental E. biosynthesis 의 important source of precursor - fetus



2. Fetal Anencephaly



; adrenal cortex 의 fetal zone absence

placental E esp. E3 severly limited due to limited availability of C19- steroid precursor

therefore maternal plasma DS only - E precursor

placental E production ↓



3. Fetal Adrenal Hypoplasia.



; C19-precursor ↓ => E formation ↓




4. Placental Sulfatase Deficiency




; E formation in placenta - generally regulated by availability of C 19- steroid prohormone



  • France and Liggins (1969)

    ; placental sulfatase deficiency ->

    cause of very low E level in otherwise normal pregnancy



  • X - linked disorder

  • sulfatase ; hydrolysis of C19-steroid sulfate


    first enzymatic step in placental prohormone for E formation



5. Placental Aromatase Deficiency




; fetal adrenal DS -> androstenedione -\-> 17b E2

metabolite of dehydroepiandrosterone (androstenedione and testosterone) secret into maternal & fetal circulation

-> mother and female fetus virilization



  • aromatase gene defect
    ; caused by and insert of 87 base pair in frame

    ->additional 29 amino acid containing abnormal protein

    -> this protein contain aromatase activity ( 6% of normal )



  • outcome - episode of uterine contraction before term

    fetus born alive

    early in life grow normal


6. Aromatase Deficiency in Male



; epiphyseal closure not occur -> become very tall ( 204 Cm Ht )
bone mineralization deficiency





7. Down syndrome




; unconjugated E3 level decrease

the reason for E level decrease are not established

best possibility is inadequate formation of C19-steroid in adrenal gland





8. Deficiency in Fetal LDL cholesterol Biosynthesis




; diminished E formation




9. Decreased fetal Adrenal use of LDL



; most common cause of decreased placental E formation

DS formation reduction -> E precursor ↓

Ex) pregnancy complicated by HT . severe DM




F. Maternal Condition that affect Placental Estrogen Formation




  1. Glucocorticoid therapy

    ; cause a placental E formation

    glucocorticoid - maternal and fetal pituitary gland ACTH secretion inhibit

    -> DS ↓

    -> E formation ↓



  2. Maternal Adrenal Dysfunction


    ; Addison disease -> maternal urinary estrogen level 감소

    principally Estrone and 17b-E2




  3. Maternal Ovarian Androgen- producing Tumor


    ; C19 - steroid aromatization




    1. Edmann and associates

      placental clearance of maternal plasma androgen to E2

      -> similar to placental blood flow

      ; means all of androstenedione -> intervillous space -> syncythium ( E2 )



    2. androgen secreting tumor

      virilized female fetus - rare

      due to placenta efficiently convert testosterone -> E2

      virilized female fetus



      1. nonaromatizable C19-steroid androgen producing tumor

        (eg. 5a- dihydrotestosterone)


      2. if produced testosterone exceed the capacity of placental aromatase



    3. Neoplastic trophoblastic disease

      ; no fetal adrenal source of C19 - steroid precursor

      E formation source -> maternal plasma



III. Estriol Measurement to Assess Fetal Status



Fetus - important role in E synthesis

fetal well-being monitored by urinary and plasma E level

24hr urine E2 level

F 6-5



A. Plasma Estriol




; E3 ( free or nonconjugated )

E3 conjugates - E3 sulfate

E3 sulfate T1/2 > nonconjugate E3 T1/2

--> for ease of measurement and interpretation, nonconjugate E3 check





B. E3 Measurement to Monitor Fetal Well-being



# The Fetus at Risk

== The greatest problem in Obstetrical Management

; proper timing of delivery when pregnancy threaten the fetal life

prematurity vs.high risk for fetus

(if intrauterine existence on a deteriorating environment is continue)




- obstetrical management 에서 urinary or plasma E3 level 의 의미 ?

perinatal mortality or morbidity 감소에 little or no clinical utility







IV. PROGESTERONE




A. Origin of Progesterone in Human Pregnancy



  • pregnancy 6-7 weeks 이후

    -> very little progesterone is produced in ovary


    7 - 10 weeks 이후 corpus luteum removal or BSO

    -> not cause urinary progesterone

    metabolite levels decrease




B. Progesterone Production Rates




- Pearlmann in 1957

; normal singleton pregnancy 250 mg/day

multiple fetus > 600 mg/d



C. Source of Cholesterol for Placental Progesterone Biosynthesis



= cholesterol -> progesterone : two step enzymatic reaction


  1. cholesterol -> pregnenolone in mitochondria

    catalized by cytochrome P450 cholesterol side chain cleavage enzyme



  2. pregnenolone -> progesterone in microsome

    3 - hydroxysteroid dehydrogenase. 5-4 isomerase



  • Hellig and associates (1970)

    ; maternal plasma cholesterol - principal precursor

    90% of progesterone biosynthesis

    즉 trophoblast 의 cholesterol de novo synthesis 는 minimal




1. Placental Use of Maternal Plasma LDL Cholesterol





  • the rate of Progesterone biosynthesis

    ; dependent upon the number of LDL receptor on trophoblast

    independent on uteroplacental blood flow

    this obtain for several reasons :




    1. cholesterol side - chain cleavage activity in placental mitochondria

      --> continually in a highly activated state



    2. De novo synthesis of cholesterol by the placenta

      - is limited but not absent

      - is increased in the absence of maternal plasma LDL


      Reduced placental progesterone formation is observed in woman

      with abetalipoproteinemia,a homozygous deficiency in LDL formation



    3. the fetus contribute few or no precursor for placental progesterone biosynthesis



    4. Maternal levels of LDL cholesterol are not rate limiting in the placental assimilation
      of cholesterol from the maternal circulation


  • Simpson and Burkhart (1980)

    ; progesterone inhibit the activation of cholesterol estrification enzyme



2. Progesterone Synthesis and Fetal Well-being



  • fetal death 후에 no concomitant decrease in plasma P. level



Progesterone metabolism during Pregnancy



  • 5a - dihydroprogesterone level 의 증가

    progesterone also convert to deoxycorticosterone



D. Difectional Secretion of Steroid from Shcytiotrophoblast



; > 90 % E2,E3 enter maternal plasma

> 85 % Progesterone ''

maternal plasma P. cross the placenta

-> fetus ; very little E1 preferentially entry into fetal plasma





E. Transfer of Steroid from Syncytiotrophoblast into Maternal and Fetal Blood



  • directly enter maternal blood

    cf) do not enter fetal blood directly

    syncytiotrophoblast -> cytotrophoblast 경유

    -> intervillous space -> fetal capillary wall 경유

    -> fetal blood (hemochorioendothelial barrier 경유)



    cf) decreased uteroplacental blood flow

    ex) PIH,chronic HT. severe DM pregnant

    - umbilical cord E, P level > normal pregnant

    due to uteroplacental blood flow↓ -> relative stasis of maternal blood

    in intervillous space -> formed steoid redistribution in favor fetal compartment

    ==>> uteroplacental blood flow 장애 있는 fetus 에서

    Accelerated lung maturation 을 설명하는 important concept