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Main article: Polycystic Ovary Syndrome Overview
Updated: 5-April-2024
Causes of Polycystic Ovary Syndrome
The exact cause of PCOS is unknown, however researchers are slowly piecing together the puzzle. So far, researchers believe PCOS is most likely caused by elevated androgen and or AMH levels in utero, high sucrose diets consumed prepuberty or gene mutations.
Although theca cells in follicles, which produce androgen, is the main suspect. Exactly how or why remains to be discovered.
Early studies of theca cells from patients with PCOS, undergoing IVF, revealed elevated androgen production and higher levels of dehydroepiandrosterone (DHEA) and 17-OH progesterone secretion, compared to non-PCOS patients in response to LH. This may be caused by pro‐inflammatory cytokine Interleukin 18 which, in a bovine study, increased basal and LH‐induced cell proliferation alongside increased secretion of androstenedione and 17‐hydroxyprogesterone.
Using mouse models of PCOS, the addition of dihydrotestosterone (DHT) blocks the growth of preovulatory follicles and corpus luteum formation. A further study showed the effects of DHT was significantly lessened in mice with depleted androgen receptors (AR) within their theca cells. This suggests AR present in theca cells play a significant role, in the arrest of follicles, in an androgen excess environment.
Another mouse study observed that depletion of phosphatase and tensin homolog (PTEN) protein in theca cells, leads to an increased expression of LHCGR and CYP17A1, increased response to LH, elevated androgen levels, higher antral follicle count and enlarged ovaries, similar to human PCOS. Targeting this pathway (phosphatidylinositol 3-kinase) may potentially influence androgen production.
Source: Lan Z J, et al. (2017)
However FSH also plays a significant role in the metabolism of glucose within preovulatory follicles, specifically granulosa cells.
FSH stimulates the expression and activity of IRS-2 (Insulin receptor substrate 2) protein involved in the regulation of insulin, IGF-1, interleukin, interferon, growth hormone and integrins. FSH stimulated glucose uptake and synthesis within granulosa cells of women with PCOS, irrespective of insulin resistance, was found to be impaired, suggestive of a defect (or interference) in the FSH receptors. On the other hand, hCG which targets LH receptors, inhibits FSH stimulation of IRS-2 expression and thus glucose uptake and synthesis, demonstrating the alternating effects of FSH and LH, via the IRS-2 pathway, on glycogen levels and follicular development.
Nevertheless, a recent study revealed that ferroptosis in ovarian granulosa cells is induced by hyperandrogenism, which is produced by defective theca cells, and may produce a new treatment strategy pending further studies.
Source: Li X, et al. (2024)
A different group of researchers identified autoantibodies (immune proteins mistakenly targeting own tissues or organs) in a sub-group of women with PCOS, which significantly increased GnRH receptor activity. Using a rat model and a synthetic peptide to induce the production of GnRH receptor antibodies, the authors showed a resulting increase in LH and testosterone levels along with inflammation. A more recent study however revealed the prevalence of autoantibodies, in women with PCOS (n=1051), to be only 0.31% challenging the hypothesis that these autoantibodies are the main cause of PCOS.
Interestingly androgen excess also results in endoplasmic reticulum stress and apoptosis, through sustaining insulin overexpression in β cells, which consequently impairs proinsulin maturation and secretion.
Activation of endoplasmic reticulum stress within the granulosa cells of women with PCOS significantly disrupts cellular function, causing the induction of pro-fibrotic growth factors (TGF-β1), apoptosis and accumulation of advanced glycation end products, via the aryl hydrocarbon receptor (AHR). Not surprisingly, apoptosis of granulosa cells correlates negatively with the number of day 5 or 6 blastocysts formed. Administration of an AHR blocker in a PCOS mice model decreased the number of atretic antral follicles and restored estrous cycling, however human studies are now required to test these findings further.
Interestingly, gene expression of α1AMPK in granulosa cells is also significantly lower among (lean) women with PCOS. Deletion of α1AMPK in mice suggest this as a possible cause of PCOS with increased levels of androgen, AMH and pre-antral follicles leading to a decrease in ovulation rates.
Source: Froment P, et al. (2022)
Overall the growing follicle is exposed to an abnormal environment with increased luteinizing hormone (LH), insulin, androgen, anti-Mullerian hormone (AMH) and insufficient follicle-stimulating hormone (FSH).
Source: Franks S and Hardy K, (2018)
This hyper elevated androgen environment during pregnancy potentially alters neuronal wiring and programmes genes associated with ovarian steroidogenesis (androgen, progesterone), gonadotrophin secretion, insulin metabolism and ovarian follicle development leading to the development of PCOS in their children.
Interestingly prenatal AMH levels also alters maternal serum hormones and placental gene expression, to create an elevated androgen environment in utero, leading to reproductive dysfunction in female offspring. This effect was reversed in mouse models with androgen receptors removed from kisspeptin neurons in the arcuate nucleus suggesting better control of AMH (or androgen) levels during pregnancy may prevent prenatal PCOS development in children.
On the other hand, non-hyperandrogenic women with PCOS display impaired follicles that don’t respond to estradiol as normal.
Source: Marie C, et al. (2023)
Ovarian Follicle Development
AMH
At the very beginning of egg development, a number of follicles are activated, from a pool of resting follicles. AMH promotes the growth of these early follicles (preantral) to the antral stage independent of gonadotropin (LH, FSH) levels, in non-human primates. Peak AMH concentration is found in antral stage follicles.
Once estradiol concentrations from FSH stimulated granulosa cells reach the required threshold, AMH expression is suppressed. This is important because elevated AMH can inhibit (antral) follicle maturation and selection of a dominant follicle. Not surprisingly, elevated AMH also interferes with ovulation induction medication (clomiphene citrate, metformin).
Androgen
Androgen receptors, through which androgen works, can be found in the ovaries within theca cells, granulosa cells, oocytes and stromal cells.
Source: Sen A and Hammes S R, (2010)
Theca cells within the ovary are formed from mesenchymal cells and stromal (WT1+) cells. Interestingly only mesenchymal derived theca cells form the androgen producing theca cells. Androgen produced by theca cells is essential for the growth of follicles during early development (preantral and antral stages).
In granulosa cells androgen also amplifies FSH receptor (FSHR) expression during early development (preantral and early antral stages). This is combined with an increase in aromatase expression (CYP19A1), connective tissue growth factor (CTGF) and LH/chorionic gonadotropin receptors (LHCGR).
Source: Weil S, et al. (1999)
As the follicle grows beyond 6μm in size, androgen receptor gene expression levels decline. This coincides with a shift from the growth of follicles to orderly death, as one dominant follicle from the group is selected.
Source: Jeppesen J V, et al. (2012)
Sex Hormone-Binding Globulin
SHBG is a protein in the blood which binds to testosterone and controls the level of free androgens circulating in the body.
Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH)
The theca and granulosa cells also feature LH and FSH receptors. This allows both LH and FSH to stimulate the differentiation of theca and granulosa cells within the growing antral follicles.
Luteinizing hormones activate the LH receptor (LHR/LHCGR) in theca cells, which increases steroidogenesis (via CYP11A1, CYP17A1 genes) and androgen production.
In the same theca cells CYP17A1 is blocked by estradiol binding to estrogen receptor (ESR1). This provides a means within the follicle by which androgen production can be controlled.
Selection of a Dominant Follicle
With increasing estrogen secretion by the growing follicles, FSH secretion from the pituitary gland is reduced. This leads to a reduction in aromatization (conversion of androgen to estrogen) and increase in inhibin-B and AMH. In the normal ovary, this hostile environment ensures only the most dominant follicle with the highest number of receptors survive. In particular LHCGR which increases the follicles response to LH as FSH levels reduce. On the other hand non-dominant follicles undergo degeneration.
Pathophysiology of PCOS
The effect of PCOS on the body is widespread altering various physiological processes such as:
- Kisspeptin
- Insulin
- Humanin
- Prolactin
- VEGF
- Homocysteine
- Myonectin
- ADAMTS1
- DHEAS
- Androstenedione
- NAD+
- Inflammation
- Iron homeostasis
- Sympathetic Nervous System
- Genetics
Kisspeptin
Kisspeptin is the primary neuropeptide which controls the gonadotropin-releasing hormone (GnRH) neurons, and subsequent release of LH and FSH.
Source: Plant T M, (2019)
In a detailed study, 71 women with PCOS had their serum kisspeptin and LH sampled every 10 minutes over a 2 hour period. Women with PCOS and regular periods (eumenorrheic, 21 to 45 days) demonstrated coordinated secretion of kisspeptin and LH, while women with PCOS and irregular periods (oligomenorrheic, greater than 45 days) did not have coordinated secretion of both kisspeptin and LH.
Source: Katulski K, et al. (2018)
Dysregulation of kisspeptin among women with PCOS is believed to be one of the primary causes of abnormal sex hormone levels.
Source: Zarei E, et al. (2021)
In fact, kisspeptin was the missing link between hyperandrogenism and endoplasmic reticulum stress, which itself is linked to insulin resistance and type 2 diabetes. However a recent study, reported kisspeptin is influenced more by obesity than PCOS itself. Instead neuropeptide Y (interlinked with kisspeptin) was identified as significantly elevated in both non-obese and obese adolescents girls with PCOS. Neuropeptide Y and its receptors are also found in the ovaries and involved in follicle growth (folliculogenesis).
Irrespective of the actual molecule, dysregulation of the pulse generator within the brain directly affects the ovaries and oocyte development.
Source: Guo F, et al. (2023); Sun P, et al. (2023)
Preliminary trials of 2 drugs that target these pathways (Peripherally-Restricted Kappa Receptor Agonist, Fezolinetant) in animals and humans, show a consistent reduction in LH and testosterone levels, with the potential to restore menstrual cycles. Further studies over a longer duration are now required to validate the efficacy and safety of these options.
Insulin
Insulin growth factor receptors (IGFR) are found in theca cells. Insulin-like growth factor 1 (IGF1), Insulin-Like Peptide 3 (INSL3) and Insulin-Like Peptide 5 (INSL5) combined with LH enhances androgen production. Studies show insulin levels correlate positively with AMH levels. However AMH remains a better diagnostic marker of PCOS.
DOES INSULIN CAUSE PCOS
According to a study involving normal and at-risk female rhesus monkeys, development of insulin resistance alone during adulthood does not induce (or cause) PCOS like features.
Source: Zhou R, et al. (2022)
Humanin
Humanin is a mitochondria-derived peptide, associated with insulin resistance (IR), found in both serum and follicular fluid. In women with PCOS + IR, humanin concentration in follicular fluid is significantly lower (186.56 vs 250.20 pg/mL) that women with PCOS – IR, and also significantly lower than non-PCOS ± IR.
Humanin supplementation in PCOS rat models improved fasting plasma glucose and fasting insulin levels via IRS1, PI3K, Akt and GLUT4 insulin related proteins, similar to metformin and quercetin mechanism of action, with knock down experiments further implicating humanin in the regulation of glucose metabolism, however primate studies are now required before future trials in humans.
Source: Wang Y, et al. (2021)
Prolactin
Lower levels of prolactin (<187 mIU/L) within normal range is associated with insulin resistance and functional deficiency of beta-cells in women with PCOS. Excess dopamine secretion, caused by the emotional burden of PCOS and reduced quality-of-life, is suggested to decrease prolactin levels and potentially explains the inverse association between prolactin and LH in women with PCOS.
Source: Yang H, et al. (2021)
Conversely, women with PCOS and hyperprolactinemia have decreased LH levels and LH/FSH ratios, and increased estradiol levels compared to those without hyperprolactinemia. This suggests hyperprolactinemia potentially masks the severity of PCOS in these specific women.
Source: Davoudi Z, et al. (2021)
VEGF (Vascular Endothelial Growth Factor)
Angiogenesis and its factors (VEGF) play an important role in the changes within the ovaries during a normal ovarian cycle. However in obese women with PCOS, levels of VEGF are significantly higher (up to 4x greater) indicating dysregulation of angiogenesis and a possible contributing factor to subfertility, ovulatory dysfunction and ovarian hyperstimulation syndrome (OHSS).
In vitamin D deficient women with PCOS, vitamin D supplementation decreases VEGF levels (-10.5%).
Source: Irani M, et al. (2017)
Homocysteine
Homocysteine is an amino acid which is metabolised (along with vitamin B6, B12 and folate) to form methione and cysteine. Elevated levels of homocysteine is linked to PCOS women with insulin resistance and hyperinsulinemia. This suggests vitamin deficiency, hyperinsulinemia or drugs which alter homocysteine levels may positively influence / regulate one another.
Homocysteine levels greater than or equal to 15μmol/L negatively impacts IUI clinical pregnancy rates.
Source: Cao J and Wang H, (2022)
Myonectin
A recently discovered muscle factor, myonectin plays an important role in regulating glucose and lipid metabolism. Unsurprisingly, myonectin levels were found significantly lower in women with PCOS, while correlation analysis showed that the level of myonectin was negatively associated with BMI, LH, fasting blood glucose, HOMA-IR and testosterone, whilst positively associated with SHBG and risk of PCOS. Further studies are now required to confirm if this a potential cause or effect of PCOS in women. However this finding goes some way to explaining why exercise has such a noticeable effect on women with PCOS.
Endurance based exercise increases myonectin levels and reduces inflammatory response to lipopolysaccharide in mice.
Source: Otaka N, et al. (2018)
ADAMTS1
ADAMTS1 (A disintegrin and metalloproteinase with thrombospondin motifs 1) is an extracellular matrix metalloproteinase involved in embryo development and ovulation. ADAMTS1 is significantly increased in the granulosa cells of women with PCOS, compared to controls. However silencing ADAMTS1 inhibits (granulosa) cell proliferation and E2 secretion required for oocyte maturation. Retrospective analysis of clinical data found that expression levels of ADAMTS1 correlated positively with oocyte maturation rate and the rate of good quality embryos (Day 3 and 5). Further studies are required to understand the underlying cause of elevated ADAMTS1 levels and the best way to reduce this back to normal levels.
DHEAS (Dehydroepiandrosterone Sulphate)
DHEAS (Dehydroepiandrosterone sulphate) is the most abundant circulating and localised steroid hormone in humans. In women, theca and cumulus-granulosa cells within the ovaries synthesize DHEAS.
A recent study hypothesized that DHEAS regulates Ca2+ transport across membranes to maintain calcium homeostasis and induce oocyte activation. It found that women with PCOS and normal levels of DHEAS (in follicular fluid) yielded good-quality oocytes, with optimal rates of fertilisation, embryo development and live births, compared to women with PCOS and high DHEAS levels.
Optimising DHEAS levels among women with PCOS may significantly improve treatment outcomes.
Androstenedione
Androstenedione is a steroid produced by the ovaries, adrenal glands and peripheral tissue (to a small degree) which is converted into testosterone. Elevated androstenedione and testosterone levels is associated with increased metabolic risk. Meanwhile approximately 1 in 10 women with PCOS have elevated androstenedione but normal total testosterone levels leading to misclassification of PCOS phenotype.
Source: O’Reilly M W, et al. (2014); Knochenhauer E S, et al. (1998)
NAD+ (Nicotinamide Adenine Dinucleotide)
NAD+ (Nicotinamide Adenine Dinucleotide) is an essential enzyme cofactor, participating in various energy metabolism processes in cells, which declines with aging and metabolic diseases. In women with PCOS, granulosa cell levels of NAD+ is significantly lower resulting in mitochondrial dysfunction. In vitro experiments suggest that inflammation is the cause of this decrease in NAD+ levels.
Source: Wang Y, et al. (2021)
Inflammation
Not surprisingly inflammatory proteins (IL-6, IL-8, NLRP3 and CCL20) in granulosa cells of ovulatory follicles are significantly elevated in women with PCOS, particularly those who are obese. Similarly, an alternate study reported significantly altered inflammatory related genes in the ovarian stroma of women with PCOS.
Even within the endometrium of women with PCOS the immune cell population is significantly different featuring elevated levels of macrophages, dendritic cells and T cells. Interestingly immune cells contain androgen receptors, however whether theca cell produced androgen directly alters the immune cells within the ovary is not yet known.
Finally in women with PCOS and endometrial hyperplasia, recent in vivo experiments show that excess androgen activates TLR4/IRF-7/NFkB signalling pathways altering endometrial cytokine synthesis, leading to the onset and progression of endometrial inflammation.
Source: Hu M, et al. (2020)
Iron homeostasis
Women with PCOS display poor regulation of iron levels via hepcidin. Experts suggest elevated testosterone levels and or insulin resistance is the primary cause, although estrogen and progesterone levels can also influence iron homeostasis throughout the menstrual cycle.
Sympathetic Nervous System
The sympathetic nervous system and hypothalamus–pituitary–adrenal (HPA) axis are both hyperactive in women with PCOS. Interestingly, common features of PCOS, like central obesity, hyperinsulinaemia and obstructive sleep apnoea (OSA), is also linked to chronic sympathetic hyperactivity. This occurs due to noradrenaline levels which is predominantly influenced by the alpha-2 adrenoceptor in women with PCOS. These specific women may benefit from alpha-2 adrenergic blockers (agonists) which can normalise this dysfunction somewhat.
Genetics
PCOS is a heritable condition, firstly illustrated in a study of 115 sisters of women with PCOS, which found 22% of sisters also had PCOS and another 24% had hyperandrogenemia with regular periods. A later study, involving over 3200 twins, calculated the heritability of PCOS to be 71% among genetically identical twins, compared to only 38% in non identical twins, substantiating the effect of genes in PCOS.
To date genome wide association studies (GWAS) have identified less than 10% of PCOS heritability meaning a significant amount of research is still required to identify all the genes involved.
In model mice studies, mutations of both LHCGR and ALMS1 genes identified in women with PCOS, significantly increases testosterone and estradiol levels, while decreasing the numbers of late secondary and preovulatory follicles, suggesting these 2 genes combined play a considerable role in the development of PCOS. However a clinically validated GWAS study, also identified various inflammatory regulators (IL-2, IL-4, IL-17, SDF1a, SCGFb, VEGF) specific to women with PCOS women. This only confirms the complex genetic make up of PCOS.
Source: Chen H, et al. (2021); Yu L, et al. (2021)
