Chapter 23 / Animal Models, Fetal Programming of PCOS
259
23
Animal Models and Fetal Programming
of the Polycystic Ovary Syndrome
David H. Abbott, Daniel A. Dumesic, Jon E. Levine,
Andrea Dunaif, and Vasantha Padmanabhan
SUMMARY
At least 28 animal models provide insight into the etiological and pathophysiological basis of
polycystic ovary syndrome (PCOS). About 50% of them, however, either do not show sufficient
traits meriting designation of a PCOS phenotype or exhibit alternate features mimicking other disorders, such as hyperprolactinemia. In contrast, animal models of fetal programming through androgen
excess show remarkable resilience and reliability in replicating PCOS, including metabolic defects in
males, and therefore strongly implicate a fetal etiology in the developmental origins of PCOS. This
chapter reviews the relevance of animal models for PCOS and their potential value for providing
insight into the etiology and pathophysiology of this disorder.
Key Words: Polycystic ovary syndrome; rhesus monkeys; androgens; prenatal; fetal programming; animal
model; anovulation; hyperandrogenism.
1. INTRODUCTION
The etiology and pathophysiology of polycystic ovary syndrome (PCOS) in women are poorly
understood. PCOS is multifaceted and includes reproductive, metabolic, and general health disorders
(Table 1). The syndrome is strongly familial in origin, with 67–93% of daughters born to women
with PCOS developing the PCOS syndrome as adults (1–3). Clinical or biochemical manifestation of
androgen excess is the most reliably transmitted PCOS trait (4). PCOS is the most common endocrinopathy of women in their reproductive years (5,6), with a prevalence of 6–7% (7), and one of its most
troubling general health disorders is early-onset type 2 diabetes (8,9).
Nevertheless, PCOS has a heterogeneous and unpredictable clinical presentation (5,6), and most
putative gene candidates studied to date have been unable to adequately explain its phenotype (10),
suggesting that PCOS has multiple (albeit undiscovered) genetic origins modified by environmental
factors and perhaps fetal programming (11). Therefore, the development and application of animal
models for PCOS provide timely insight into the origins and pathophysiological mechanisms that are
difficult to resolve from human studies. This chapter reviews the many animal models proposed as
relevant to PCOS, with particular emphasis on models that reliably and reproducibly emulate the
PCOS syndrome through androgen excess fetal programming.
From: Contemporary Endocrinology: Androgen Excess Disorders in Women:
Polycystic Ovary Syndrome and Other Disorders, Second Edition
Edited by: R. Azziz et al. © Humana Press Inc., Totowa, NJ
259
260
Abbott et al.
Table 1
Common Signs and Symptoms of PCOS
A. Consensus diagnostic criteriaa
Two out of three of the following:
1. Clinical or biochemical hyperandrogenism, as determined by elevated circulating levels of total or
unbound testosterone or hirsutism
2. Intermittent or absent menstrual cycles
3. Polycystic ovaries (as visualized by ultrasound)
The following conditions must also be excluded: classical and nonclassical congenital adrenal hyperplasia,
Cushing’s syndrome, thyroid dysfunciton, hyperprolactinemia, androgen-secreting tumors, and drug-induced
androgen excess.
B. PCOS signs and symptoms outside those required for diagnosis (some, all, or none of these may be present
in an individual)
Reproductive and endocrine
Luteinizing hormone (LH) hypersecretion
Reduced steroid-negative feedback on LH release
Increased recruitment and persistence of ovarian follicles
Ovarian hyperresponsiveness to gonadotropic therapy for in vitro fertiliztion (IVF)
High rates of miscarriage
Endometrial hyperplasia and cancer
Adrenal hyperandrogenism
Gestational diabetes
Metabolic
Insulin resistance and compensatory hyperinsulinemia
Imparied glucose tolerance
Type 2 diabetes
Obesity (including abdominal adiposity)
Pancreatic impairments in insulin responses to glucose
Hyperlipidemia
General health disorders
Cardiovascular disease
Sleep apnea
Acne
Chronic inflammation
Intra-uterine growth retardation
aThese criteria (13) are extented from the previous 1990 NIH Consensus diagnosis (12) which specified the first two
criteria, alone, as a basis for PCOS diagnosis, following exclusion of conditions that mimic PCOS (listed above). Revisions of these criteria by Azziz (14) suggest requiring criterion 1 together with either criteria 2 or 3, whereas revisions by
Chang (15) relegate criterion 3 to nondiagnostic status. PCOS, polycystic ovary syndrome. (Modified from ref. 11.)
2. BACKGROUND
2.1. Overview of PCOS
Because PCOS is a diagnosis of exclusion, animal models need to replicate traits consistent with
PCOS without having features that might mimic other clinical diseases. Such a goal is difficult and
compounded by recent changes in the manner in which PCOS is clinically diagnosed. For example,
the 1990 National Institutes of Health (NIH) consensus diagnosis for PCOS specifies
hyperandrogenism accompanied by oligo- or amenorrhea, excluding conditions that mimic PCOS,
such as classic and nonclassical congenital adrenal hyperplasia, thyroid dysfunction,
hyperprolactinemia, androgen-producing tumors, and drug-induced androgen excess (12).
Chapter 23 / Animal Models, Fetal Programming of PCOS
261
The Revised 2003 Rotterdam consensus diagnosis adds polycystic ovaries, as diagnosed by
tranvaginal ultrasound (TVUS), to the above 1990 NIH consensus diagnosis for PCOS, specifying
that two of the three criteria are required to diagnose PCOS (13). A recent reappraisal of the 1990
NIH and Revised 2003 Rotterdam consensus diagnoses (14) combines criteria from both consenses
and specifies PCOS as androgen excess accompanied by oligo- or amenorrhea or by TVUS-confirmed polycystic ovaries while still excluding clinical conditions mimicking PCOS. An additional
suggestion (15) is to continue using the 1990 NIH consensus for the diagnosis of PCOS, while relegating sonographic imaging of polycystic ovaries to confirmatory, rather than diagnostic, status.
Equally complex, individual women with PCOS can exhibit different combinations of diagnostic
criteria with varying degrees of severity, can show abnormalities other that those used for diagnosing
PCOS (5,6), and can experience onset of symptoms at puberty (16,17), which can resolve during
middle age (18).
Thus, for an animal model to truly approximate the complexities of the PCOS phenotype, it must
not only exhibit ovarian hyperandrogenism, oligo- or amenorrhea, and/or an increased number of
medium-sized ovarian follicles in the absence of features mimicking other clinical diseases, but also
show an extraordinary array of relevant traits, permitting a heterogeneous phenotype.
2.2. Animal Models for PCOS
A variety of mammalian species have been employed as animal models of PCOS, ranging from
rodents to nonhuman primates. Each species has differences in reproductive function compared to
humans, and such reproductive differences need to be considered when translating experimental findings into clinical applications (19). For example, rats and mice undergo spontaneous ovulation
approximately every 4–5 days (not every 26–34 days), and they complete follicle luteinization and
form corpora lutea only if mating occurs, with the luteotropic support of prolactin (not luteinizing
hormone [LH]).
2.2.1. Animal Models for PCOS That Produce Large Ovarian Follicular Cysts
Much has been made of the experimental induction of ovarian cysts in animal models for PCOS.
Unlike PCOS ovarian morphology, follicular cysts in animal models are frequently large (preovulatory sized or larger) and are thus not a diagnostic trait for the PCOS syndrome. These animal models,
while providing insight into the development of cystic ovarian morphology, may not be relevant in
resolving the etiology and pathophysiology of PCOS. Models inducing large ovarian follicular cysts
are found in a variety of experimental treatment paradigms (summarized in Table 1), with and without additional induction of PCOS and non-PCOS diagnostic traits. In this context it is important to
note that rodents are generally multiovular and thus normally exhibit multifollicular ovarian morphology.
2.2.2. Animal Models for PCOS That Fail to Demonstrate PCOS Diagnostic Traits
When animal models lack elevated androgen levels or exhibit regular ovulatory cycles, they fail to
demonstrate the basic tenets of PCOS. Those that fall into this category (Table 1) include treatment of
adult females with constant exposure to light (probably the oldest animal model proposed for PCOS),
acute estrogen, and valproic acid, as well as treatment of newborn females with testosterone. Estradiol
valerate treatment of female rats, while inhibiting ovulatory cycles, also induces traits that are uncharacteristic of PCOS: growth hormone excess and hypothalamic degeneration. Such animal models hold
little promise for understanding etiological and pathophysiological mechanisms of PCOS.
2.2.3. Animal Models for PCOS That Exhibit Diagnostic Traits
But Show Other Traits Excluding a PCOS Diagnosis
Four animal models, while exhibiting PCOS diagnostic traits, show other traits that are inconsistent with PCOS in women,for example, thyroid dysfunction in hypothyroid rats treated with human
chorionic gonadotropin (hCG) (Table 2). The remaining three models, transgenic overexpression of
Species
Androgen
excess
Rat
Estradiol valerate
262
–
+
+
–
+
–
–
+
+
–
?
?
–
–
?
–
–
–
–
+
–
+
–
–
RU486 treatment (antiprogestagenic and antiglucocorticoid)
DHEA treatment
Hypothyroid treatment
with hCG
Transgenic overexpression
of LH
+
+
+
+
+b
+b
+
Mouse
Immature rat
Adult rat
Rat
+
+
Rat
?
–
–
–
–
+
Models with PCOS diagnostic traits, but also diagnostic traits that exclude a PCOS diagnosis
Chronic estrogen
Rat
Acute estrogen
Guinea pig
Valproic acid treatment
Rat
(anticonvulsant and
fatty acid analog)
Valproic acid treatment
Rhesus monkey
PMSG treatment (pregnant
Rat
mare serum gonadotropin)
Neonatal testosterone
Newborn rat
treatment
Rat
Constant light
Models with insufficient traits for a PCOS diagnosis and no known exclusion traits
Model
N/A
N/A
N/A
N/A
N/A
N/A
–
N/A
N/A
?
N/A
N/A
N/A
?
?
?
?
+
?
?
?
?
?
?
?
?
+
+
+
+?
+
?
?
?
?
?
?
+
+
Hyperprolactinemia
Criteria that exclude a PCOS diagnosis
Multiple,
Intermittent or medium-sized Adrenal
absent ovulatory ovarian antral 17OHP
Thyroid
cycles
follicles
excess dysfunction
Criteria for PCOS diagnosis
Ovarian cancer, renal pathology,
large, blood-filled ovarian
follicular cysts
Large ovarian follicular cysts
Large ovarian follicular cysts
Large ovarian follicular cysts
with precocious luteinization
Hypothyroidism
Large ovarian follicular cysts,
small ovaries
Large ovarian follicular cysts
Large ovarian follicular cysts
Many large, cystic ovarian
follicles, loss of endogenous
endocrine rhythms
Elevated growth hormone,
large ovarian follicular cysts,
hypothalamic degeneration
Model characteristics
not found in PCOS
Table 2
Characteristics of Animal Models of PCOS Relevant to PCOS Diagnostic Criteria and Criteria for Diagnostic Exclusion from PCOS
23
21,22
20
67
73
71
72
22
69
70
68
35
Ref.
262
Abbott et al.
263
+
?
?
+
–
–
?
–
–
–
+
–
+
+
–
–
–
?
?
?
N/A
?
N/A
N/A
?
?
N/A
N/A
?
N/A
?
N/A
–
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
–
?
?
?
?
?
?
?
?
?
?
?
?
?
?
–
Virilized genitalia
Virilized genitalia
Virilized genitalia only in females
exposed during early gestation
Virilized genitalia only in females
exposed during early gestation
Large ovarian follicular cysts
Large ovarian follicular cysts
Large ovarian follicular cysts
Large ovarian follicular cysts
Ovaries and uterus immature
Glucocorticoid excess
Excessive sexual behavior
Large ovarian follicular cysts
bExogenous
serum levels of free unbound testosterone.
androgen excess.
cFunctional ovarian hyperandrogenism (V. Padmanabhan, unpublished results).
N/A, not applicable—rodent adrenal glands do not synthesize corticosterone from 17OHP.
PCOS, polycystic ovary syndrome; hCG, human chorionic gonadotropin; LH, luteinizing hormone; DHEA, dehydroepiandrosterone; IGF, insulin-like growth factor.
aLow
+
Fetal testosterone treatment Rhesus monkey
+
+
+
+
(+)
Seasonal
anovulation only
+
+
+
+b
+
+
+c
+
+b
+
+
+b
+
+
+
+
+
+
+
+
+
+
Chronic estrone treatment Rhesus monkey
Chronic hCG treatment
Rat
Chronic insulin and hCG
Rat
treatment
Chronic IGF-1 and hCG
Rat
treatment
Fetal testosterone treatment
Mouse
Fetal testosterone treatment
Rat
Fetal testosterone treatment
Sheep
+
+
+
+a
Models with PCOS diagnostic traits and no known exclusion traits
Nymphomania
Cow
Immunization against
Rat
testosterone
Letrozole treatment
Rat
(aromatase inhibitor)
Aromatase knockout (ArKO)
Mouse
Dexamethasone treatment
Pig
(synthetic glucocorticoid)
Chronic testosterone
Immature rat
treatment
Chronic testosterone
Rhesus monkey
treatment
Chronic androstenedione
Rhesus monkey
treatment
11,40
48
49
42
28
35
26
27
34
33
25
31
29
30
22
24
Chapter 23 / Animal Models, Fetal Programming of PCOS
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264
Abbott et al.
LH (20), and treatment with either dehydroepiandrosterone (21,22) or RU486 (an antiprogestagen
and antiglucocorticoid) (23), all induce hyperprolactinemia, a well-known mimic of PCOS in women
(12,13). All but the hypothyroid rats also exhibit large ovarian follicular cysts (Table 2). These animal models might thus provide useful information in determining mechanisms underlying the pathophysiology of conditions that resemble PCOS, but are not PCOS itself.
2.2.4. Animal Models for PCOS That Exhibit Diagnostic Traits
and Lack Traits Excluding a PCOS Diagnosis
The remaining 16 animal models all demonstrate traits consistent with those for PCOS diagnosis,
and none exhibit additional traits for exclusion (Table 2). The nymphomaniac cow is the only naturally occurring animal model for PCOS (22), but its unpredictable occurrence and unknown mechanism have yet to make it useful. Although manipulation of adult female rats by immunization against
testosterone (24), chronic treatment with testosterone (25), hCG (26), insulin and hCG (27), and
insulin-like growth factor-1 and hCG (28) all induce hyperandrogenic females that have intermittent
or absent ovulatory cycles, the treatments induce large ovarian follicular cysts, unlike the smallersized cysts found in women with PCOS. Treatment of adult female pigs with dexamethasone, a synthetic glucocorticoid, induces PCOS diagnostic criteria provided that adrenergic innervation to the
ovaries remains intact (29), yet glucocorticoid excess is not a common symptom accompanying
PCOS. Use of an aromatase inhibitor, letrozole, on adult female rats produces a phenotype remarkably similar to that of PCOS, including LH hypersecretion (30). Not surprisingly, however, serum
levels of estradiol are greatly diminished, a steroidogenic abnormality not found in PCOS. Although
complete aromatase knockout female mice are hyperandrogenic and anovulatory, they do not represent a PCOS phenotype because their ovaries and uteri fail to mature (31). In this context it is interesting
to note that fetal female monkeys exposed to a highly specific aromatase inhibitor have greatly diminished ovarian follicular development that is prevented by simultaneous treatment with estradiol (32).
Chronic exposure of adult female monkeys to testosterone (33), androstenedione (34), or estrone
(35) induce females with hyperandrogenism and intermittent or absent menstrual cycles (Table 1).
None of these adult models, however, exhibit traits commonly associated with PCOS beyond the
diagnosis, such as PCOS-like ovarian morphology, LH hypersecretion, or metabolic dysfunction (36).
Such chronic manipulation of the adult steroid hormone environment, including that induced by
aromatase inhibition, has generated four animal models that more closely approximate the
symptomology of PCOS than the models previously discussed. Neither these nor the previous models, however, match the replication of PCOS phenotype generated by animal models of fetal androgen excess that reprogram differentiation and development of multiple organ systems. Acute exposure
of normal adult female monkeys to testosterone, accelerates the early stages of ovarian follicular
development (37) and may mimic accelerated the early follicular development found in women with
PCOS associated with diminished intraovarian expression of anti-Müllerian hormone (38).
2.3. Animal Models of Androgen Excess Fetal Programming of PCOS
Compared with other animal models for PCOS, models of fetal programming have one clear advantage: simultaneous exposure of multiple organ systems to a specific developmental insult during differentiation and maturation (Fig. 1). Altered structure and function is commonly permanent, such as
fetal androgen excess virilization of the female urogenital tract, resulting in expression of fetal programming in adulthood (11). The best known example of fetal programming is described by Barker
and colleagues (39), in which human fetal undernutrition and low birthweight are associated with
adult cardiovascular disease, hypertension, insulin resistance, and type 2 diabetes, some key hallmarks of PCOS outside of those required for its diagnosis.
In the previous edition of this book, Abbott and colleagues (40) provided the first description of a
fetal programming model for PCOS: the prenatally androgenized female rhesus monkey. As illustrated in Fig. 1 and in Tables 2 and 3, early gestation exposure of the female monkey fetus to fetal
Chapter 23 / Animal Models, Fetal Programming of PCOS
265
Fig. 1. Gestational progression of aspects of differentiation and maturation of hypothalamic–pituitary–
ovarian function and pancreas and E-cell function in rhesus monkeys. The timing of exposure of females to
androgen excess (early or late in gestation) is indicated in relation to fetal developmental progress. GnRH,
gonadotropin-releasing hormone; LH, luteinizing hormone; FSH, follicle-stimulating hormone. (Modified from
ref. 11.)
male levels of testosterone results in adult females with PCOS traits. Early gestation in the rhesus
monkey provides a stage in development when multiple organ systems regulating reproductive and
metabolic function are undergoing differentiation. A single insult during such a sensitive stage of
development can permanently alter disparate organ systems, producing a phenotypic mimic of PCOS.
2.3.1. Fetal Androgen Programming of Reproductive Defects
Prenatally androgenized female monkeys exhibit heterogeneity in their presentation of PCOS
traits, an inherent complexity in the human syndrome. Approximately 70% of prenatally androgenized
female monkeys exposed to androgen excess during early gestation have serum testosterone levels in
excess of the mean value in normal adult female monkeys of the same age, weight, and body mass
index; approximately 40% are anovulatory (~10 times the normal rate, while the remainder have
mostly intermittent menstrual cycles); and approximately 40% have increased numbers of mediumsized ovarian antral follicles (approximately twice the normal incidence) (41). Differing degrees of
virilization of both internal and external genitalia, however, are found in all female monkeys exposed
to androgen excess during early gestation. Because virilized genitalia is not a feature of PCOS in
women, exposure of female monkeys to androgen excess during late gestation, when the urogenital
tract is no longer responsive to androgen reprogramming, produces a closer PCOS phenotype that
retains heterogeneity of trait expression, but without genital virilization (Fig. 1) (11,41). Such results
from early and late gestationally exposed prenatally androgenized female monkeys, confirmed in
prenatally androgenized ewes (42), suggest that exposure of human female fetuses to androgen excess
during the latter part of gestation (second to third trimesters) from genetic and/or environmental factors
might induce organ system reprogramming, causing PCOS. Thus, the key element in the etiology and
initial pathophysiology of PCOS may be appropriately timed fetal hyperandrogenism derived from
hyperandrogenic fetal ovaries (43), fetal adrenal cortex (44), or from hyperandrogenemia of PCOS
mothers (45) reflected in the fetal circulation (46).
266
Abbott et al.
Table 3
Common Signs and Symptoms Associated With PCOS and Shown
by Animal Models of Androgen Excess Fetal Programming of PCOS
Monkey
(early/late gestation)d
Mousea
Ratb
Sheepc
+
+
???
+
???
???
+
+
???
+
???
???
+
+
+
+
+
???
+
+
+
+
+
(???) Poor embryo
development
???
N/A
???
???
N/A
???
???
N/A
???
(+) Hyperplasia
+
???
???
???
???
???
???
???
+
+
???
???
+
???
+
???
???
???
???
???
+
+
+
+
+
+
???
???
???
???
???
???
???
+
+
???
???
+
???
???
???
–
???
???
+
+
Reproductive and endocrine
Ovarian hyperandrogenism
Intermittent or absent ovulatory cycles
Multiple medium-sized ovarian follicles
LH hypersecretion
Reduced steroid negative feedback on LH
Ovarian endocrine hyper-responsiveness to
gonadotropic hyperstimulation for IVF
High rates of miscarriage
Endometrial hyperplasia and cancer
Adrenal hyperandrogenism
Gestational diabetes
Metabolic
Insulin resistance and compensatory hyperinsulinemia
Impaired glucose tolerance
Type 2 diabetes
Obesity (including abdominal adiposity)
Pancreatic impairments in insulin responses to glucose
Hyperlipidemia
General health disorders
Cardiovascular disease
Sleep apnea
Chronic inflammation
Low birthweight
Heterogeneity of PCOS trait expression
N/A, not applicable because adrenal glands from nonprimate species do not normally synthesize androgens.
aFrom ref. 48.
bFrom ref. 49 and J. E. Levine, unpublished results.
cFrom refs. 51, 56, and 62, and V. Padmanabhan, unpublished results.
dFrom refs. 11, 41, 50, and 66 and D. H. Abbott, unpublished results.
LH, luteinizing hormone; IVF, in vitro fertilization.
Recent findings from prenatally androgenized female sheep (47), mice (48), and rats (49) confirm
and extend those obtained from prenatally androgenized female monkeys (Tables 2 and 3). All
express PCOS diagnostic traits, as well as traits commonly associated with the syndrome (e.g., LH
hypersecretion). Anovulation in prenatally androgenized female mice (48) and rats (49) is induced by
fetal exposure to the nonaromatizable androgen dihydrotestosterone (DHT), suggesting an androgen
receptor-mediated neuroendocrine defect. In prenatally androgenized monkeys, although prenatal
DHT exposure can induce similar behavioral outcomes to those achieved by testosterone (11), prenatal DHT-exposed animals are not available for PCOS studies. Certainly in prenatally androgenized
female mice, treatment of such adults with the antiandrogen flutamide restores ovulatory cycles and
implicates adult excess androgen, acting via the androgen receptor, in the neuroendocrine mechanism of anovulation (48).
Chapter 23 / Animal Models, Fetal Programming of PCOS
267
2.3.2. Fetal Androgen Programming of Oocyte Quality
Beyond ovulatory dysfunction, diminished oocyte quality in both prenatally androgenized female
monkeys and women with PCOS provides an additional barrier to fertility (50). Following controlled
ovarian hyperstimulation for in vitro fertilization (IVF), retrieved oocytes from either early or late
gestation exposed, prenatally androgenized female monkeys exhibit reduced competence as defined
by the ability of the resulting diploid zygotes to reach the blastocyst stage. In women with PCOS,
diminished quality of retrieved oocytes contributes to implantation failure and pregnancy loss. Both
prenatally androgenized female monkeys and PCOS women exhibit abnormal intrafollicular steroidogenic responses to controlled ovarian hyperstimulation, which in the former is associated with
an inability to normally suppress circulating insulin levels between the first day of the recombinant
human follicle-stimulating hormone (rhFSH) treatment and the day of oocyte retrieval (50). Women
with PCOS, however, are hyperresponsive to rhFSH and exhibit intrafollicular hyperandrogenism at
oocyte retrieval following recombinant human chorionic gonadotropin (rhCG) injection. Prenatally
androgenized female monkeys, however, are hyporesponsive to rhFSH relative to normal females,
and early gestation-exposed female monkeys exhibit diminished intrafollicular androgen and estrogen levels along with an exaggerated shift in intrafollicular steroidogenesis from androgen and estrogen to progesterone at oocyte retrieval following rhCG injection. The early gestation-exposed female
monkey response to controlled ovarian hyperstimulation, therefore, is reminiscent of that shown by
normoandrogenic ovulatory women with reduced ovarian responsiveness to rhFSH (50). Collectively,
in addition to ovulatory defects, the timing of fetal androgen excess exposure impairs oocyte quality
through ovarian and/or metabolic dysfunction, with such oocyte defects possibly having
transgenerational consequences for female offspring of prenatally androgenized monkeys and for
daughters of women with PCOS.
2.3.3. Fetal Androgen Programming of Metabolic Defects
Insulin resistance and diminished pancreatic insulin response to glucose are integral defects in the
development of type 2 diabetes in women with PCOS and are exacerbated by obesity (5,6). Prenatally androgenized female monkeys (11), sheep (51), and rats (J. E. Levine, unpublished results) all
exhibit such insulin dysfunction, while early gestation-exposed androgenized female monkeys also
exhibit abdominal obesity, hyperlipidemia, and an increased incidence of type 2 diabetes (Table 3).
Insulin sensitivity in early, but not late, gestationally exposed, prenatally androgenized female monkeys is reduced to that found in normal male monkeys and in normal females during the luteal phase
of the menstrual cycle, and a similar degree of insulin resistance is found in prenatally androgenized
sheep (51). Such parallels in metabolic dysfunction between androgen excess fetal programming
models of PCOS and women with PCOS provide strong evidence for a fetal origin of metabolic
defects in both cases, possibly through fetal programming of preferential accumulation of abdominal
fat (Fig. 2).
2.3.4. Fetal Androgen Programming of General Health Disorders
Poor intrauterine growth and low birthweight are associated with the development of precocious
puberty and PCOS in northern Spanish women (52) and with PCOS pregnancies in Chilean women
(53), but not in larger studies of Finnish (54) and Dutch (55) women. Prenatally androgenized female
sheep (56) and rats (57) exhibit clear evidence of intrauterine growth restriction and low birthweight,
whereas prenatally androgenized female monkeys do not (41). Prenatally androgenized sheep and
rats may thus provide more suitable animal models for women with PCOS who have placental insufficiency. Perhaps not surprisingly, in this context, prenatally androgenized sheep have enlarged left
ventricles of the heart, kidneys, and adrenal glands suggestive of developing cardiovascular disease
(42), and prenatally androgenized female rats have increased mortality (58).
268
Abbott et al.
Fig. 2. Diagrammatic representation of our hypothesis for early gestation, fetal androgen excess programming of adult polycystic ovary syndrome traits. Genetic or environmental mechanisms induce fetal
hyperandrogenism (see text) that result in permanent changes in both reproductive and metabolic function.
Reproductive consequences include (1) altered hypothalamic–pituitary function leading to luteinizing hormone
(LH) hypersecretion, (2) ovarian hyperandrogenism that may or may not be the result of LH hypersecretion, (3)
reduced steroid hormone negative-feedback regulation of LH, which may be a component of the initial permanent alteration in hypothalamic–pituitary function, and (4) increased anovulation. Metabolic consequences include (1) increased abdominal adiposity, which may be responsible for increased circulating total free fatty acid
levels, (2) impaired pancreatic insulin secretory response to glucose, (3) impaired insulin action and compensatory hyperinsulinemia, (4) hyperglycemia, and (5) increased incidence of type 2 diabetes. Insulin resistance and
compensatory hyperinsulinemia may be functionally implicated in the anovulatory mechanism. (Modified from
ref. 41.)
2.3.5. Comparison of Androgen Excess Fetal Programming Models for PCOS
Table 3 illustrates the comparability of androgen excess fetal programming models for PCOS.
Although most PCOS traits to date have been identified in prenatally androgenized female monkeys
(11), increasing numbers of reports are describing PCOS traits in prenatally androgenized females in
nonprimate species (42,49,48). The only clear difference in symptomology between primate and
nonprimate models involves low birthweight in the latter and not the former, indicating an otherwise
remarkable degree of concurrence among models. Because low birthweight is found in some (52,53),
but not all (54,55) populations of women with PCOS, this heterogeneity in animal models may prove
extremely useful in identifying mechanisms underlying the different fetal phenotypes found in PCOS.
Rodent models, nevertheless, may not be ideal for ovarian phenotype determination as they normally
develop multiple large follicles prior to ovulation, unlike sheep or monkeys. On the other hand,
Chapter 23 / Animal Models, Fetal Programming of PCOS
269
transgenic rodent models hold much promise for determining fetal programming consequences of
specific gene changes, such as those involving neuroendocrine regulation of ovarian function (48).
Sheep models are particularly useful for understanding abnormal ovarian follicular recruitment and
persistence through sequential ultrasonography, repeated blood sampling, and ovarian manipulation.
Nonhuman primate models, with more than 90% genetic similarity to humans and the closest
symptomology to PCOS, provide the most straightforward translation of experimental findings into
improved clinical application.
The only major difference in trait defects between prenatally androgenized female rhesus monkeys and women with PCOS involves their responses to ovarian hyperstimulation for IVF. Women
with PCOS are hyperandrogenic and hyperresponsive to rhFSH, whereas prenatally androgenized
female monkeys are not (Table 3). It is tempting to speculate that this differential response to gonadotropin stimulation may reflect different regulatory mechanisms governing hyperandrogenism in
ovarian theca cells of women with PCOS vs prenatally androgenized female monkeys, perhaps reflecting an intrinsic, genetically determined hyperandrogenism in the former (59), but not the latter.
2.4. Animal Models for a Male Phenotype of PCOS
Close male relatives of women with PCOS present with metabolic dysfunction similar to that of
their female kin (60–62). Consistent with a fetal origins hypothesis for PCOS, male monkeys exposed to androgen excess during gestation develop insulin resistance and diminished insulin response
to glucose as adults (63). Because fetal monkey androgen treatments induce circulating testosterone
levels in male and female fetuses only within the normal range for fetal males (64), fetal programming of male (and female?) metabolic dysfunction may be induced by mechanisms beyond androgen-mediated action in the fetus, perhaps involving an estrogenic metabolite of testosterone, or the
action of androgen on the placenta or the mother. In support of potential estrogenic involvement in
PCOS fetal programming, exposure of fetal ewes to bisphenol-A, a plasticizer and estrogen mimic,
leads to intrauterine growth restriction and LH surge defects similar to those produced by fetal exposure to testosterone (V. Padmanabhan et al., unpublished).
3. CONCLUSIONS
A variety of animal models have been proposed for PCOS, but only models of androgen excess
fetal programming have reliably produced the spectrum and heterogeneity of traits that closely reflect PCOS in women (Tables 2 and 3). Animal models of fetal programming may thus provide the
elusive etiology and initial pathophysiology for PCOS in women and for metabolic dysfunction in
their close male kin. Such fetal models suggest that permanent alteration of gene expression in multiple organ systems may provide a closer approximation to PCOS phenotypes than differing genotypes alone.
4. FUTURE AVENUES OF INVESTIGATION
Until the etiology of PCOS is determined and the cellular and molecular mechanisms of its pathophysiology are understood, animal models for the syndrome will be essential. Animal models will
continue to lead the way in identifying a probable fetal origin for PCOS until sampling from human
fetuses becomes a low-risk routine procedure and pregnancies at risk for PCOS are identified prospectively. Such PCOS animal models will also continue to be essential in ascertaining the otherwise
ethically unattainable goal of normalized oocyte and embryo quality in women with PCOS. Fetal
programming models thus hold promise for determining the gestational timing of crucial changes in
specific organ and system functions that ultimately result in specific PCOS defects and for determining transcriptional, translational, and posttranscriptional levels of dysfunction that can be targeted for
therapeutic amelioration during pre- and postconception.
270
Abbott et al.
KEY POINTS
• Animal models provide insight into the etiology and pathophysiology of PCOS unattainable in human
studies.
• Many animal models either fail to exhibit PCOS diagnostic traits or exhibit traits that resemble other
clinical disorders, including thyroid dysfunction and hyperprolactinemia (Table 2).
• Androgen excess fetal programming of PCOS in female mice, rats, sheep, and monkeys (Table 3) provides compelling evidence for fetal origins of the syndrome in humans.
• Androgen excess fetal programming of a male PCOS metabolic phenotype suggests fetal origins for male
PCOS beyond the direct effects of androgens on the fetus.
• The intrauterine environment conducive to PCOS may be induced by a variety of genetic or environmental
factors, or a combination of both, resulting in fetal androgen excess.
ACKNOWLEDGMENTS
We thank the many staff members of our respective laboratories and institutions for their multiple
contributions to the work reported here and A.D.M. Abbott for his contribution towards compilation
of Table 2. This work was supported by NIH grants P50 HD044405, U01 HD044650, R01 RR013635,
R21 RR014093, T32 AG000268, P51 RR000167, R01 HD041098, and P01 HD044232 and was partly
conducted at a facility constructed with support from Research Facilities Improvement Program grant
numbers RR15459 and RR020141.
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