Introduction
Polycystic ovary syndrome (PCOS) is the most common reproductive endocrine disease
among women of reproductive age today, with an incidence of about 10–15% [1 ]. According to the consensus of ESHRE/ASRM,
the diagnosis of PCOS should meet at least two of the following three
characteristics: 1) ovulation dysfunction (rare ovulation and/or anovulation); 2)
hyperandrogenism biochemical or clinical characteristics of excessive androgen; and
3) the appearance of polycystic ovaries in ultrasound examination. In addition to
its effects on reproduction and body shape, PCOS is also closely associated with a
high risk of metabolic disorders, including insulin resistance (IR) and compensatory
hyperinsulinemia, obesity, lipid metabolism disorders, and often accompanied by a
low-grade inflammatory state, which interact with each other and collectively
contribute to diabetes, cardiovascular diseases, and gynecologic tumor in the long
term [1 ]
[2 ]
[3 ]
[4 ].
The etiology and pathogenesis of PCOS is still confusing, which seriously hinders the
advancement of clinical therapies. Nowadays, the impact of autophagy on the
pathological mechanism of PCOS has deeply fascinated scholars. Autophagy is an
evolution-conserved self-digestion pathway of cells, which realizes energy cycling
and tissue remodeling by degrading non-functional cytoplasmic proteins and
organelles [5 ]
[6 ], and this process can be initiated by a variety of stressors and is a
mechanism that kills stress cells and maintains environmental balance in the body
[7 ]. The onset of apoptosis is related to
changes in the mitochondrial membrane caused by cellular stimulation, allowing
pro-apoptotic proteins to enter the cytoplasm through mitochondria [8 ]. Both apoptosis and autophagy are important
ways to ensure the orderly renewal of cells and maintain the stability of the
internal environment of the organism. Studies in recent years have found that some
pathological changes in PCOS seem to be related to the disruption of this balance
[9 ]. In the context of PCOS, studies have
revealed that abnormal autophagy can lead to abnormal ovulation, disorder of glucose
and lipid metabolism, and obesity, which are closely related to the pathological
manifestations of PCOS [10 ]
[11 ]. Therefore, we review in this manuscript
relevant literatures in PubMed, MEDLINE, Web of Science and Scopus, with the search
terms of "autophagy", "PCOS", "polycystic ovary
syndrome", "ovulation", "hyperandrogenemia", "insulin
resistance", "inflammatory state", "circadian rhythm" and
"treatment". And then, we summarize the association between autophagy and
abnormal ovulation, hyperandrogenemia and IR in PCOS. In addition, we also sort out
the current therapeutic methods that can improve the pathological mechanism of PCOS
by regulating autophagy, thus providing a new direction for the treatment of
PCOS.
Autophagy is involved in the pathological mechanism of PCOS
Autophagy is a highly conserved process of self-renewal in eukaryotic cells,
characterized by the cytoplasmic material being swallowed into double membrane
vesicles (autophagosomes) and then degraded in lysosomes [12 ]. In general, autophagy is necessary for the
clearance of dysfunctional proteins and organelles [13 ], of which the components can be recycled to produce new cellular
structures, or can be further processed and used as an energy source. Studies have
corroborated that autophagy is involved in the physiological and pathological
processes of reproduction by regulating the growth, atresia and differentiation of
follicles [5 ]
[13 ]. For example, active autophagy in ovaries induced by accumulated
reactive oxygen is closely related to ovarian granulocyte death in obese women [14 ]
[15 ],
leading to an increased incidence of anovulation-induced infertility. In addition,
autophagy has been verified to participate in PCOS-related metabolic disorders, such
as lipid metabolism and insulin sensitivity [15 ]. All of these findings suggest that autophagy may play an influential
role in the reproductive and metabolic problems of PCOS. Therefore, understanding
the role of autophagy in the pathological mechanism of PCOS is expected to provide
novel clues for the discovery of precise targeted treatment strategies for PCOS.
Autophagy and abnormal ovulation
Increasing evidence supports the essential role of autophagy in the atresia,
development and maturation of follicles, and the cellular mechanisms of rare
ovulation or anovulation in PCOS are closely related to abnormal autophagy. Data
have shown that only about 0.1% of the follicles in mammals are expelled [16 ], and the rest are atretic. It has been
suggested that autophagy in granulocyte contributes to digesting abnormal
proteins and damaged organelles in the atretic follicles so as to achieve energy
cycle and support the development and excretion of the dominant follicle [17 ]
[18 ]. Studies verify that the LC3 protein staining is weak in the
early/middle stage of unatresia follicles and in the granulosa layer of large
sinus follicles, while LC3 immunoreactivity is strong in the atretic follicles
[19 ]. LC3 is currently the most widely
used autophagosome marker, which can conjugate to phosphatidylethanolamine to
form LC3-II. And the amount of LC3-II reflects the number of autophagosomes and
autophagy-related structures. Besides, the current study displays that
follicular atresia depends on the degree of the granulosa cell apoptosis, which
can be facilitated by autophagy, thus determining the fate of follicles and
female fertility [20 ]. Cellular autophagy
and apoptosis coexist and are dynamic at different follicular stages. Autophagy
is commonly observed in medium-sized follicles, whereas apoptosis is more common
in large follicles [21 ]. In young healthy
women of normal weight, cells choose to self-repair by inducing autophagy rather
than undergo apoptosis in response to low levels of reactive oxygen species. In
contrast, in obese women granulosa autophagy is activated by excess oxidized
low-density lipoprotein (oxLDL), which in turn leads to apoptosis, resulting in
dysfunction of follicular development and ovulation [22 ]. As granulosa cells express FSH
receptors at cell surface, excessive apoptosis of granulosa cells leads to a
lack of binding receptors for FSH, which in turn affects its follicle-promoting
biological effects [23 ]. Therefore, we
speculate that the abnormal autophagy level is closely related to the occurrence
of clinical ovulation disorders in PCOS. Yi et al. found that autophagy of
granulosa cells in PCOS patients was increased, characterized by a high level of
autophagosomes, Beclin-1 and LC3-II and a decreased level of the autophagy
substrate p62 [24 ], which was consistent
with the findings of Li et al. who uncovered that transcription profiles of 34
autophagy-related genes were upregulated and autophagy was active in ovarian
tissue of PCOS model rats [25 ]. In
addition, disturbances in the balance of autophagy and apoptosis were also
observed in PCOS [26 ]
[27 ]. In contrast, ovulatory function can be
improved in PCOS when over-activated autophagy of granulosa cells was alleviated
[28 ]
[29 ]. From the above research, we project that the abnormal autophagy
level of granulosa cells may be one of the pivotal mechanisms for the
pathogenesis of PCOS. The involvement of autophagy in ovulation dysregulation in
PCOS is shown in [Fig. 1 ]. However, due
to the complex mechanisms regulating cellular autophagy, the molecular cascade
activating autophagy in granulocytes of PCOS patients is currently poorly
understood. Thus, an in-depth exploration of this area is imminent so as to
provide potential targets for precision therapy of PCOS.
Fig. 1 Autophagy is involved in abnormal ovulation of PCOS by
regulating the growth, atresia, and differentiation of follicles.
Autophagy and hyperandrogenemia
Hyperandrogenemia (HA) is a prominent feature of PCOS [30 ]. HA can cause clinical symptoms such as
hirsutism, obesity, acne, and alopecia, which also tends to hinder the normal
growth of follicles, resulting in anovulation or rare ovulation. Through
literature, we found that HA and autophagy of granule cells can affect each
other and jointly contribute to the complex reproductive endocrine disorder in
PCOS [25 ]
[31 ]
[32 ]. As we mentioned above,
active autophagy in granulosa cells of PCOS patients leads to increased cell
apoptosis, which can then bring about disfunction of hormone transformation from
testosterone and androstenedione to estradiol and estrone, thus facilitating HA.
In addition, HA can directly promote the autophagy and apoptosis of granulosa
cells and affect the synthesis of steroid hormones and the development of
follicles. Li et al. found that compared with non-PCOS patients, autophagy of
granulosa cells in PCOS was induced. In addition, the beclin1 mRNA abundance in
human granulosa cells was positively correlated with serum total testosterone
level, and the expression of beclin1 mRNA and LC3II/LC3I in granulosa cells
induced by androgen was dose-dependent [31 ]. Beclin1 has been confirmed to be involved in membrane transport
and recombination process in autophagy and can interact with multiple autophagy
factors to promote autophagy pathway. Including Bcl-2 homology domain 3 (BH3),
central coiled coil domain (CCD), and C-terminal evolutionarily conserved domain
(ECD) [33 ]. Similarly, another study
confirmed that the LC3II and beclin1 expression levels were increased and the
p62 and p-mTOR levels were decreased in vivo in ovarian tissue from the PCOS
mice [34 ]. p62 is a selective autophagy
adaptor protein, which is involved in the degradation of ubiquitin-proteasome
and autophagy lysosome. And elevated p62 implies that autophagic substrates are
not efficiently degraded. p-mTOR can activate Unc-51 like autophagy activating
kinase 1 (ULK1), block the activation of Adenosine 5′-monophosphate-activated
protein kinase (AMPK), and thus inhibit autophagy process. Besides, the in vitro
data were similarly with the in vivo by stimulation of mouse granulosa cells
with dihydrotestosterone. These effects could be diminished by the autophagy
inhibitor (MHY1485) or by androgen receptor antagonists (ARN509) [34 ]. To sum up, the active autophagy and
increased apoptosis of granulosa cells can promote the increase of serum
androgen levels, while hyperandrogenemia in turn can further lead to the
increase of autophagy and apoptosis in granulosa cells, forming a recurrent
pathological ring and promoting the development of PCOS. The involvement of
autophagy in hyperandrogenemia in PCOS is shown in [Fig. 2 ]. Besides, future studies are
needed to further focus on the effects of hyperandrogenemia on autophagy levels
in different tissues and cells of PCOS, so as to fully understand the regulatory
role of autophagy in the pathogenic mechanism of hyperandrogenemia in PCOS.
Fig. 2 Autophagy is involved in hyperandrogenemia, insulin
resistance, and abnormal circadian rhythm of PCOS.
Autophagy and insulin resistance
Insulin resistance (IR) is present in 50–70% of women with PCOS [35 ] and is a high risk factor for metabolic
syndrome. Compensatory hyperinsulinemia in PCOS can lead to increased value of
free testosterone through synergy with luteinizing hormone (LH) and decreased
hepatic synthetic sex hormone binding globulin (SHBG) [36 ]
[37 ]
[38 ], and jointly
participate in the complex reproductive endocrine disorders in PCOS. Recently,
studies have displayed that the significant decreased level of autophagy in fat,
liver, skeletal muscle and other tissues is closely related to the occurrence of
IR. Wang et al. established IR model in vitro, and observed decreased level of
autophagy [39 ]. On the contrary, when
autophagy level was increased, IR was mitigated. Besides, in order to further
study the role of autophagy in regulating insulin sensitivity, the autophagy
related gene (ATG)7 or ATG5 were knocked out, respectively, which significantly
interfered with the establishment of IR model. Based on the above experiments,
we can find that the abnormal level of autophagy is closely related to the
occurrence of IR, and targeting the regulatory factors of autophagy is expected
to become a potential therapeutic direction to improve IR in the future. Whether
autophagy is also involved in the development of IR in PCOS is a question worth
exploring. Scholars have confirmed that the regulation of autophagy contributes
to improving IR in PCOS. Sumarac-Dumanovic et al. observed that mRNA profiling
of autophagy-related molecules are significantly reduced in endometrium of PCOS.
Moreover, metformin treatment (2 g/d, 3 months) can significantly increase the
mRNA level of ATG14, beclin-1, and ATG3 in endometrium of patients with PCOS
[40 ]. In addition, a study by Song et
al. demonstrated that DHEA-exposed mice presented IR in whole-body, along with
autophagy inhibition [41 ]. Besides,
DHEA-induced IR could be alleviated by electroacupuncture, and this effect was
reversed by autophagy inhibitor 3-MA [42 ].
Abuelezz et al. suggested that the mechanism of ameliorating IR by modulating
autophagy may be related to its protection against oxidative stress and the
effects of chronic inflammation on the function and fate of b cells [43 ]. Therefore, it can be concluded that
autophagy in PCOS is closely related to IR, which may be a novel target for the
treatment of IR in PCOS. The involvement of autophagy in IR in PCOS is shown in
[Fig. 2 ].
Autophagy and abnormal circadian rhythm
The circadian rhythm is a physiological and behavioral pattern controlled by the
circadian clock located in the suprachiasmatic nucleus (SCN) and mainly affected
by light. Secondary oscillators are present in many brain regions and peripheral
organs, which can affect function of local tissue [44 ]
[45 ]
[46 ]
[47 ]
[48 ]. At the molecular level, circadian rhythms are generated by a
transcription-translation feedback loop regulated by molecular clock
transcription factors, in which brain and muscle arant-like-1 (BMAL1), CLOCK,
cryptochrome (CRY), and period circadian regulator (PER) are the core circadian
rhythm genes [49 ]. In 2007, Nakao et al.
found the role of circadian rhythm genes in Japanese quail ovaries, which broke
the inherent research model around neuroendocrine control of ovarian
steroidogenesis and ovulation [50 ].
Evidence of large-scale cohort epidemiological studies shows that the circadian
rhythm disorder caused by shift work will cause women to face such reproductive
physiological abnormalities as menstrual disorder, infertility or abortion [51 ]. Moreover, the disorder of circadian
rhythm is closely related to the occurrence and development of reproductive
diseases such as PCOS. It has been reported that pinealectomy or long-term light
exposure can induce polycystic ovary and androgen excess in rodents [52 ]
[53 ], while androgen excess can cause abnormal expression of PER2, one
of the main control genes as one of the core elements of molecular oscillation
[54 ]
[55 ], thus leading to disorder of steroid hormone production in
granulosa cells and promoting the development of PCOS [56 ]. In addition, scholars also confirmed
that the expression of BMAL1 in granulosa cells of PCOS decreased significantly,
which was closely related to the expression of FSHR, aromatase, abnormal
follicular development and ovulation disorder [57 ]
[58 ]
[59 ]. Besides, the disorder of circadian
rhythm is closely related to the abnormality metabolism of PCOS. Several studies
have shown a significant increase in the incidence of IR and metabolic syndrome
in women exposed to night-light changes [60 ]
[61 ]. Simon et al. conducted
a cross-sectional study comparing circadian rhythms in women with PCOS (n=559)
and obese girls without PCOS (n=533). The result displayed that women with PCOS
showed a later secretion of melatonin during puberty than controls, with a
delayed circadian rhythm, which showed a significant correlation with serum free
testosterone and insulin sensitivity [62 ].
In addition, Chu et al. also corroborated that circadian rhythm could promote
the occurrence and development of PCOS through gut microbiota. In their studies,
female Sprague Dawley (SD) rats exposed to continuous light showed changes in
gut microbiota, estrous cycle, and ovarian morphology, which was similar to that
of PCOS [63 ]. Based on the above
experimental results, it can be found that abnormalities in the circadian system
can affect many aspects in PCOS such as endocrine, reproduction and metabolism
systems.
Circadian rhythm is also influenced by autophagy. Studies by Toledo et al. have
revealed that autophagy controls the liver’s circadian clock by degrading a
circadian protein, cryptochrome 1 (Cry1), which can reduce glucose production in
the liver. While the degradation of CRY1 is aggravated by autophagy in obese
individuals. Olsvik et al. confirmed that the circadian rhythm protein BMAL1,
CLOCK, REV- ERB a and CRY1 were the targets of lysosome, and autophagy can
selectively degrade CRY1 to influence circadian rhythm. In addition, they also
documented CRY1 contained light chain 3 (LC3)-interacting region (LIR) motifs
[64 ] that promoted the interaction of
cargo proteins with LC3-labeled autophagosome, thereby regulating the autophagic
degradation of CRY1 to achieve circadian glycemic control [65 ]. In recent years, active autophagy has
been observed in the ovarian tissues of PCOS women and rat models, especially in
ovarian granulosa cells [25 ]. Therefore,
excessive autophagy of PCOS may be closely related to the degradation of
circadian proteins, thus affecting glucose and lipid metabolism, steroid hormone
synthesis and follicular development. In the future, the correlation between
autophagy and changes of circadian genes in the pathological mechanism of PCOS
should be further explored in vivo or in vitro, which is conducive for us to
understanding the disease and discovering new direction for the treatment. The
involvement of autophagy in abnormal circadian rhythm in PCOS is shown in [Fig. 2 ].
Autophagy and state of chronic inflammation
Recently, PCOS has been identified as a chronic inflammatory disease [66 ], manifested as significantly increased
levels of C-reactive protein (CRP), tumor necrosis factor-a (TNF-a), and
interleukin-6 (IL-6) in the peripheral circulation. In addition to disrupting
ovarian function, chronic low-grade inflammation in PCOS patients also affects
glucose and lipid metabolism, exerting a vital part in PCOS [67 ]
[68 ]. Studies documented that low-grade inflammatory response and
autophagy were involved in the crosstalk of signaling pathways to promote the
progression of pathological mechanisms [69 ]. Toll-like receptor (TLR) and NOD-like receptor (NLR) signaling
pathways mainly contribute to chronic inflammation state, which can interact
with autophagy and jointly promote the process of disease. Gu et al. confirmed
that the expression of TLR-related proteins, TLR4 and TLR9, were upregulated in
the cumulus cells of PCOS, and were associated with decreased oocyte quality and
low embryo rate [70 ]. Besides, TLR is the
main pathogen associated molecular pattern (PAMP) cell sensors, which can
activate autophagy. Xu et al. has shown that lipopolysaccharide (LPS), acted as
PAMP molecular, is able to induce autophagy flux in macrophages by stimulating
TLR4 so as to promote the clearance to pathogen [71 ]. Later, Shi et al. further demonstrated that not only TLR4, but
also other TLR family members could interact with beclin-1 (a key factor of
autophagosome formation) through the cohesion protein MyD88 or TRIF, to reduce
the binding of beclin-1 and bcl-2, thus removing the inhibition of beclin-1 by
bcl-2 and promote the occurrence of autophagy. Furthermore, TLR4 and TLR9 on the
surface of granulosa cells are controlled by LH and androgen. So, we suspect
that the TLR signaling pathway of oocyte granulosa cells in PCOS patients is
activated by high levels of LH and androgen, which lead to autophagic cell death
in granulosa cells and ovulation disorders. Of course, this hypothesis needs to
be further validated in vivo and in vitro. The involvement of autophagy in
chronic inflammation state in PCOS is shown in [Fig. 3 ].
Fig. 3 Autophagy is involved in the state of chronic inflammation
of PCOS.
In addition, autophagy can in turn regulate the TLR signaling pathway and affect
inflammatory response. Lee et al. found that autophagosomes could capture the
RNA of Sendai virus and deliver it to the endosomes. Then, TLR on the endosome
membrane is activated, promoting the induction of interferon (IFN) [72 ]. Other studies have shown that human
follicular granulosa cells exposed to resveratrol (RES) can induce protective
autophagy to reduce the expression of TLR4, CD36, and LX1, and thus playing a
protective role on granulosa cells [73 ].
In addition, NLRP3, as one of the important members of the NLR family, has also
been proved to participate in the inflammation response of PCOS by promoting the
release of IL-1 and IL-18 [74 ], while
autophagy can negatively regulate the activation of NLRP3 and thereby inhibits
the inflammation state of PCOS [75 ].
Besides, low-grade inflammatory response and autophagy are linked by intestinal
microorganisms. Studies have shown that patients with PCOS show imbalance in gut
microbiota, with an increased number of Gram-negative bacteria. Thus, level of
LPS is increased and then enter the blood circulation, which can bind to
lipopolysaccharide binding protein (LBP) synthesized by the liver and acts on
TLR4 on the surface of immune cells to activate the downstream MyD88 signaling
pathway, then promote the expression of TNF-α, IL-6, etc. Moreover, TNF-α can
also upregulate the expression of autophagy genes (LC3) and beclin1 [76 ]
[77 ]. In turn, hyperactivated autophagy may promote inflammation
through gut microbiota. Mice lacking autophagy-related gene Atg7 showed abnormal
fecal microbiota and significantly increased Gram-negative bacteria such as
Bacteroides fragilis
[78 ]. In
addition, autophagy can also influence the intestinal barrier function by
inducing lysosomal degradation of tight junction protein CLDN2, increasing its
permeability, accelerating the entry of LPS into the circulation process, and
causing systemic inflammation response [79 ]. Therefore, we can conclude that that autophagy can promote the
contribution of gut microbiota to chronic inflammation in PCOS by influencing
the number of bacteria and the permeability of host intestinal wall.
To sum up, autophagy and inflammatory signaling pathways can influence each other
and jointly affect the pathological mechanism development in PCOS. However, the
mechanisms are intricate and complicated, which remain to be further
explored.
Drugs targeting autophagy may have potential to treat PCOS
Over the past decades, the treatment strategies of PCOS have remained controversial.
Up to now, more than 160 recommendations and practical guidelines have been
published around the world [80 ]
[81 ], which generally include lifestyle
intervention drug therapy and surgical procedures. Treatment targets include
improving abnormal ovulation, hyperandrogen, and metabolic abnormalities. In
addition to symptom management, treatment objectives also contain the prevention of
long-term complications associated with PCOS, such as diabetes and cardiovascular
disease [82 ]. But in general, current
treatments are symptomatic rather than parenchymal, due to a superficial
understanding of disease mechanisms. For example, oral contraceptives are the
first-line treatment for menstrual androgen disorders, clomiphene and letrozole are
used for ovulation induction, and metformin is recommended for metabolic therapy,
etc. [83 ]. Current studies have confirmed that
current therapeutic drugs can also play a therapeutic role in PCOS by targeting
autophagy. Xu et al. demonstrated that metformin ameliorated PCOS in a rat model by
downregulating autophagy in granulosa cells, and metformin could decrease the levels
of oxidative stress and autophagy in H2 O2 -induced granulosa
cells and affected the PI3K/AKT/mTOR signaling pathway [19 ]. Taken together, these results indicate
that metformin ameliorates PCOS by decreasing excessive autophagy in granulosa
cells. In addition, a study by Liu revealed that Guizhi Fuling pill, a traditional
Chinese medicine used for PCOS, could inhibit granulosa cell autophagy and promote
follicular development to attenuate ovulation disorder in PCOS-IR rats [84 ]. In order to broaden the decision-making
range of PCOS and make up for the deficiencies of existing treatment plans, this
study proposes several potential methods that can improve the pathological mechanism
of PCOS by regulating autophagy ([Fig.
4 ]).
Fig. 4 The treatment strategies of PCOS.
Lifestyle intervention
Various lifestyle interventions for PCOS are suggested, including diet. So far,
nearly 10 studies have been published involving dietary interventions, including
low-calorie diets, vegan or soy diets. However, there are no guidelines or
studies confirming the benefits of restrictive feeding (RF) for PCOS. Currently
intermittent fasting (for example, limiting energy intake by 60% two days a week
or every other day), periodic fasting (for example, a 5-day diet providing
750–1100 kcal), and time restrictive feeding (TRF, limiting daily food intake to
8 hours or less) in normal and overweight subjects have been shown to be
effective in weight loss and has improved various health indicators and reduced
risk factors for cardiovascular disease [85 ]. So far, nine trials of TRF on human have been shown to reduce
weight, improve glucose metabolism, inflammation cytokines and extend life span,
even when food intake was comparable to that of the control group [86 ]
[87 ]. Later, scholars further explored the molecular mechanism of the
metabolic improvement of RF, which revealed that the benefits of intermittent
fasting are partly due to regulation on autophagy [88 ]. Jamshed et al. conducted a randomized,
cross-sectional study in overweight adults who were required to take food
between 8 AM and 2 PM (early TRF, eTRF) or between 8 AM and 8 PM (control
schedule). The results showed that fasting glucose and insulin in the morning,
fasting insulin in the evening, and 24-hour glucose fluctuations were reduced in
TRF group. And the expression of LC3 was increased by 22% in eTRF group, which
encoded an important structural component of the autophagosome membrane [89 ]. This study reveals that the
improvement of glucose metabolism by RF may be related to the influence of
autophagy. Martinez et al. confirmed the effect of trial-a-day (ITAD) feeding on
autophagy and metabolism through animal experiments. They established an
isocaloric twice-a-day (ITAD) bean model wherein ITAD-fed mice consume the same
food amount as controls but were limited to take food at two short windows: 8–10
AM (feeding window 1) and 5–7 PM (feeding window 2). The results displayed that
ITAD fasting activated autophagy in the liver, fat and muscle tissues, promotes
diverse metabolic benefits in multiple systems, and prevents the age-and
obesity-associated metabolic defects [90 ].
However, the efficacy of intermittent fasting in the treatment of PCOS is still
lacking. According to the above experiments, we speculate that RF tends to
improve the glucose and lipid metabolism disorders in PCOS, and whether it can
improve the secretion of steroid hormones and restore ovulation remains to be
confirmed by experiments.
Drug therapy
Melatonin
Melatonin (N -acetyl-5-methoxytryptamine), an indoleamine hormone
secreted by the pineal gland, exerts a variety of functions in regulating
circadian rhythm, immune response, inflammation, etc. [91 ]. Melatonin is widely distributed in
the body, including follicular fluid, and its concentration is significantly
higher than that in the blood (36.5, 4.8 pg/ml vs. 10.0, 1.4 pg/ml) [92 ]. Accordingly, the expression of
melatonin receptors can be detected throughout the ovary [93 ]
[94 ], especially in the granulosa of sinus follicles. Thus, we
speculate that melatonin is essential for the reproductive and endocrine
functions of the ovaries. Recent studies have confirmed the therapeutic
effect of melatonin on PCOS. For instance, Pai et al. found that the use of
melatonin (1–2 mg/kg, 35 days) in PCOS rats could improve its reproductive
and metabolic disorders [95 ]. Prata
Lima et al. discovered that the number of ovarian cysts and the ovarian
weight were decreased after 4 months of melatonin treatment, suggesting that
melatonin may improve chronic anovulatory symptoms [59 ]. Afterwards, some scholars further
explored the molecular mechanism of melatonin in the treatment of PCOS.
In addition, melatonin is able to alleviate PCOS by targeting autophagy.
Studies have shown that the inhibition of autophagy mediated by melatonin
improves the resistance of cells to harmful stimuli [96 ]
[97 ]. They found that melatonin protects granulosa cells from
oxidative damage by weakening the autophagy signal activated by oxidative
stress. Cell experiments showed that the loss of activity in ovarian
granulosa cells induced by oxidative stress was significantly ameliorated by
melatonin treatment. In addition, PI3K-AKT is the key downstream effector of
melatonin. It not only improves the resistance of granulosa cells to
oxidative stress, but also inhibits autophagic responses, from gene
expression to the formation of autophagy vacuoles [98 ]. Besides, melatonin has been proven
to protect against mitochondrial injury in granulosa cells of PCOS by
enhancing SIRT1 expression and inhibiting excessive PINK1/Parkin-mediated
mitophagy [99 ]. These findings suggest
a novel mechanism that melatonin protects against oxidative damage to
granulosa cells by regulating autophagy, which could be a potential
therapeutic target for anovulatory disorders.
Vitamin D
It is well known that vitamin D helps promote calcium and bone
mineralization. Accumulated evidence shows that deficiency of vitamin D is
also a risk factor for IR, cardiovascular, autoimmune diseases, and low
fertility [100 ]
[101 ]. Interestingly, numerous studies
authenticate that vitamin D is associated with the pathogenesis and symptoms
of PCOS [102 ]
[103 ]. Women with PCOS show a higher
incidence of vitamin D deficiency (defined as a concentration of
25[OH]D<75 nmol/l) when compared with healthy women matched by age and
body mass index (BMI). Current researches confirm that vitamin D can
effectively improve the metabolism, endocrine and fertility of PCOS [104 ]. Wehr et al. [102 ] conducted a cross-sectional study
and they found that 25(OH)D concentration is an independent predictor of
homeostasis model assessment of insulin resistance (HOMA-IR) and BMI. And
serum 25(OH)D concentration was negatively correlated with BMI and other
obesity markers. Trummer summarized current cross-sectional studies and
randomized controlled trials (RCTs) and concluded that vitamin D showed a
regulatory role in PCOS-related symptoms such as ovulation disorders, IR,
and HA [105 ]. Azadi et al. conducted a
meta-analysis to evaluate the effect of vitamin D supplementation on PCOS
patients with HA. This meta-analysis included 6 clinical trials involving
183 participants. The results showed that vitamin D supplementation
significantly reduced total testosterone levels [106 ]. From the above studies, we can
definitely conclude that vitamin D supplementation is of great benefit to
the treatment of PCOS. However, the molecular mechanism remains to be
explored.
Studies have confirmed that vitamin D can enhance autophagy so as to improve
metabolic problems [107 ]
[108 ]. The autophagy process can be
influenced by the levels of Ca2+
[109 ]. And vitamin D is able to regulate Ca2+ levels by
enhancing the effects of Ca2 + pumps and Ca2+ buffers.
Thus, vitamin D is capable of maintaining a moderate autophagic flux [110 ]. Besides, the effects of vitamin D
in regulating autophagy in PCOS has been confirmed by Lajtai et al. [111 ]. They divided female Wistar rats
into four groups. Two groups were PCOS model groups, one of which received a
low vitamin D diet (D–T+), and the other received a normal vitamin D3 diet
(D+T+). The other two groups without testosterone treatment served as
controls, one of which received vitamin D3 (D+T–) orally, and the other of
which were lack of vitamin D (D–T–). The results displayed that D–T+animals
showed a decrease in LC3 II levels in the liver and increased insulin
levels. This reflected that decreased level of autophagy in the liver of
PCOS patients was closely related to IR. In addition, in (D–T+) group, the
level of LC3 II was reduced and phosphorylated Akt was increased in ovary,
and excessive activation of PI3K/Akt may lead to impaired follicular
development and the appearance of a large number of immature follicles,
which contributes to the formation of polycystic ovarian status. While in
(D+T+) group, IR was alleviated and the level of autophagy was increased,
with decreased level of phosphorylated Akt. Therefore, we can infer that
vitamin D supplementation play a therapeutic role in the recovery of PCOS,
which is closely related to its regulation on autophagy. In the future, it
should be further observed whether vitamin D exerts an effect on PCOS via
mediating autophagy in human, and further exploration should be conducted to
explore the pathway by which vitamin D interferes with autophagy.
Probiotics
Probiotics can improve the balance of intestinal microbiota by regulating
microbial composition and metabolites, enabling the host to obtain health
benefits from living microorganisms [112 ]. Common probiotic strains include lactobacillus,
bifidobacterium, and yeast, etc. Currently, probiotic supplementation is
also used in the treatment of PCOS, as it is suggested that the gut
microbiota of PCOS is abnormal [113 ]
[114 ]. The addition of
probiotics can restore the diversity of gut microbiota and improve the
reproductive and metabolic capacity of PCOS rats [115 ]. Ahmadi et al. conducted a
randomized double-blind placebo-controlled clinical trial on 60 patients
with PCOS who were randomly divided into probiotic capsule group (n=30) and
placebo group (n=30) for 12 weeks of treatment [116 ]. The results showed that probiotic
supplementation could significantly reduce the BMI, fasting blood glucose,
serum insulin concentration, and lipid levels in patients with PCOS. And
another randomized double-blind controlled trial [117 ] found that treatment of probiotics
combined with vitamin D for 12 weeks could significantly reduce total level
of testosterone in patients with PCOS, improve the hairy symptoms, and
alleviate chronic inflammation. In addition, the addition of probiotics can
help to improve the long-term complications of PCOS. Tabrizi R et al.
performed a meta-analysis involving 11 RCTs, which indicated that probiotics
could not only improve HA and IR, but also reduce the level of total
cholesterol and triglyceride, thereby reducing the risk of cardiovascular
disease in patients with PCOS [118 ].
Giorgia et al. studied adult female zebrafish to explore the relationship
between probiotics and autophagy [119 ]. The control group (n=10) was given a commercial diet and the
treatment group (n=10) was fed with diets containing the lyophilized
probiotic Lactobacillus rhamnosus IMC 501. After 10 days of
treatment, the zebrafish were sacrificed, and their ovaries were removed.
The results showed that autophagosomes in the preovulation follicles were
increased in the probiotics treatment group, which were more obvious in
stage IV follicles. The expression of autophagy related genes (i. e.,
Ambra1, beclin1, LC3 and Uvrag) was increased, and the expression profiling
of apoptosis-related genes (i. e., P53, Bax, Apaf and Cas3) was decreased,
leading to a reduced apoptosis rate and an increased survival rate of
follicles. This study confirms the supportive role of probiotics in
follicular development by regulating follicular autophagy and apoptosis.
Moreover, probiotics can regulate inflammation through autophagy. Studies
have shown that probiotics can activate autophagy in bone marrow dendritic
cells in which autophagy were reduced or Atg16L1 was knocked down, thus
reducing the release of inflammatory cytokines [120 ]. In addition, probiotics can also
amplify the effect of beneficial signaling pathways by modulating autophagy
and enhance the therapeutic effect of PCOS. For instance, probiotics has
been confirmed to be able to exert anti-inflammation activity and induce
autophagy by activating the Vitamin D receptor-autophagy signaling pathways
[121 ]
[122 ], which is closely related to the
improvement of PCOS-related pathological mechanisms. However, the effect of
probiotics on autophagy, especially the effects of different probiotics on
the autophagy level of different tissue cells in patients with PCOS needs to
be further clarified.
