Elsevier

Medical Hypotheses

Volume 100, March 2017, Pages 54-58
Medical Hypotheses

Should metformin be included in fertility treatment of PCOS patients?

https://doi.org/10.1016/j.mehy.2017.01.012Get rights and content

Abstract

Metformin, a drug developed for the treatment of patients with type II diabetes, has become commonly prescribed medication for PCOS patients. Initially, metformin was prescribed for patients with impaired glucose tolerance at the pre conception period, however more recently its use was expanded to many of the PCOS patients and for the whole duration of pregnancy. Several studies examining the effects of Metformin during pregnancy reported a lower pregnancy loss, reduced gestational diabetes and no increased risk for birth defects, however, several more recent studies also raised concerns about its safe use.

The therapeutic effect of metformin stems from its ability to inhibit the action of the first complex of the electron transport resulting in reduced ATP production. At the initial stages of embryo development, the only source of ATP is the mitochondrial electron transport chain. Lowering ATP production at the critical stage of early embryo development may impair oocyte maturation and embryo development as well as reprogram the metabolic characteristics of the offspring.

Introduction

Polycystic ovarian syndrome (PCOS) is the most common endocrinopathy, diagnosed in 5–18% of women in the fertile age. It is the most common cause of anovulatory infertility and is also associated with hyperandrogenism, obesity and insulin resistance. Insulin resistance is more common in overweight and obese women with PCOS and severity may range from impaired glucose tolerance to overt type II Diabetes [1], [2]. The ratio between lean to obese PCOS changes from one country to another, however countries in which the population is transitioning from a traditional diet to the western diet experience a surge in the incidence of obesity related PCOS [3].

In patients with PCOS there is an increased risk for adverse obstetric and neonatal outcomes, and the risk varies widely according to the different phenotypes and features of PCOS [4].

There are a few molecular changes that were found to be unique to lean or obese PCOS [5], but these unique molecular signatures did not however assist in elucidating the pathogenesis of this condition as of yet.

Metformin is the oldest and one of the most effective oral treatments for type 2 diabetes. Metformin, is a biguanide, chemically it is 1,1 dimethyl-biguanide hydrochloride.

It reduces hepatic gluconeogenesis and insulin secretion, weight gain and does not increase the risk for hypoglycemia. For PCOS patients and patients with type II diabetes, Metformin is frequently given as a single agent to induce ovulation or as an adjunct to fertility treatment before conception as well as during the entire course pregnancy. For patients with gestational diabetes mellitus metformin was shown to be as efficient and as safe as insulin during the pregnancy [6]. Several studies looking at the effects of Metformin during pregnancy reported a lower pregnancy loss, reduced gestational diabetes and no increased risk for birth defects [7], [8], [9], [10], but other studies showed that Metformin treatment from first trimester to delivery did not reduce pregnancy complications in PCOS patients [11].

The Cochrane 2014 didn’t find any conclusive evidence that metformin treatment before or during ART cycles improved live birth rates in women with PCOS [12].

However, several recent studies also raised concerns about its safe use.

Metformin has been in clinical use over half a century and as such was not exposed to the same rigorous testing that a modern drug would go through, especially if meant to treat pregnant women at all stages of pregnancy. Despite the long duration of clinical use of Metformin, only recently several publications had begun shedding light on its mechanism of action [13], [14], [15].

Recently, it was shown that the action of Metformin in reducing androgen production is mediated by its ability to inhibit the first complex of the electron transport chain (ETC) [16], [17], [18], [19]. This leads to a reduced production of NAD+ that serves as a substrate for key enzymes involved in androgen production. The inhibition of complex I, a key component of the electron transport chain, results in reduced ATP production (Fig. 1).

Oocytes as well as the early embryos are lacking the enzymes that permit ATP production through glycolysis and are therefore totally depended on their mitochondrial citric acid cycle and the oxidative phosphorylation via the ETC for energy production. As a result, metformin may impair the metabolic capacity needed for the completion of oocyte maturation and embryo development.

The mitochondria have been described as the ‘powerhouse’ of the cell, owing to their prominent role in energy production. The ETC is located within the inner membrane of the mitochondria. It consists of a sequential group of compounds which accepts electrons from reducing equivalents. These electrons are transferred along the chain ultimately combining with oxygen to produce water. The ETC consists of 5 complexes (Fig. 1) [20]. Highly charged electrons produced by the Krebs cycle are transferred into complex I and II of the ETC. Ubiquinol accepts electrons carried by Complex I and II and relays them to complex III, which passes them to cytochrome C and ultimately to complex IV. This process results in the transfer of proton ions (H+) into the transmembrane space and increases the transmembrane electrochemical potential gradient (chemical and electrical charge gradient). Dissipation of the gradient through a channel (complex V) is coupled with phosphorylation of ADP and generation of ATP [20].

Mitochondria are also the site in which most of the steroidogenesis takes place and mitochondrial function was shown to be associated with the efficacy of this process [21]. Lastly, mitochondria are also a source of metabolic byproducts such as fumarate, and acetate, which together with ATP are used for posttranslational modification of hundreds of proteins (e.g. phosphorylation, acetylation and succination).

Small numbers of mitochondria are present in primordial germ cells, oogonia and at the early stages of oocyte development. As the oocyte matures, extensive mitochondrial replication leads to expansion of the existing pool to ∼200,000 mitochondria/oocyte, a larger number of mitochondria than any other cell [22]. Mitochondria replication halts when the oocyte matures and only resumes at the blastocyst stage when the embryo has well over a hundred cells. The number of mitochondria is therefore diluted with each embryo cleavage so that each daughter cell retains only half the amount of mitochondria. As a result, towards the blastocyst stage, the amount of mitochondria in each cell is hardly enough to support the energetic demands of the embryo [23], [24], [25]. The oocytes are surrounded by several layers of supporting cells called granulosa cells. The subset of granulosa cells that is intimately associated with the oocyte are the cumulus cells and together with it form the cumulus oocyte complex. Paracrine hormone secretion and gap junctions allow bidirectional communication between the oocytes and cumulus cells, which allows for effective nutritional feedback between these two cell types. Normal signaling and adequate energy supplies are essential for oocyte maturation, development and viability [26]. The oocyte itself is unable to utilize glucose directly. It relies on the cumulus cells, which are capable of metabolizing glucose by glycolysis into metabolic substrates that the oocyte can utilize; the most important substrates being pyruvate and lactate [26], [27], [28]. Oocytes also depend on cumulus cells for supply of amino acids and fatty acids for its metabolism. Other metabolic pathways are less active but remain essential for oocyte maturation. These include the pentose phosphate pathway required for nuclear maturation, the hexosamine biosynthetic pathway required for cumulus expansion and the polyol pathway [26]. Some of the ATP is likely produced by cumulus cells that supply the ATP to the oocytes. Cumulus cells, similarly to oocytes exhibit similar mitochondrial defects as germ cells, when maternal metabolism is compromised [29] and thus can be used as a surrogate cell type for analysis of metabolic outcomes.

In mammals, embryos utilize nutrients from their surroundings (fallopian tubes or culture media). Mitochondrial ATP is absolutely required for egg and embryo development and maturation [30]. All the mitochondria in embryo are maternally inherited and their number remains constant during preimplantation development. Severe mitochondrial dysfunction in the embryo may induce cell death, however, if less severe may also result in reprogramming of metabolic pathways leading to aberrant metabolism in the adult. Preimplantation early embryo development prior to the 8 cell stage is entirely dependent on the Krebs cycle and oxidative phosphorylation for ATP production [31]. Energy production and utilization at this stage is relatively low, however, ATP is indispensable and both glutamine and pyruvate are utilized for its production whereas glycolysis is inhibited and the presence of glucose in culture media may be detrimental [30] In mammals, glycolysis and oxidative phosphorylation rates increase steeply starting at the compaction stage and the embryo becomes increasingly metabolically active [30].

The following are recently published findings about the mechanism of action of Metformin that raise concern over a potential detrimental effect on embryogenesis:

  • Metformin was shown to exert most of its therapeutic effects by inhibition of the first complex of the mitochondrial electron transport chain as well as the action of the mitochondrial ATP synthase (Complex V). Together, these actions of Metformin reduce the ability of the cell to produce ATP via OXPHOS (oxidative phosphorylation) [32], [13]. Metformin also, independently, stimulate reactive oxygen species production by the complex I Flavin [32]. Early preimplantation embryonic development is entirely dependent on ATP produced from oxidative phosphorylation through the ETC within the mitochondria. Likewise, oocyte maturation is dependent on mitochondrial activity to produce adequate ATP levels necessary for an error free execution of meiosis [30]. Also, the reduced OXPHOS derived ATP production leads to anaerobic respiration and increased lactate formation in human granulosa cells [33], [13]. As a result, metformin may impair or re-program the metabolic capacity needed to complete oocyte maturation and embryo development.

  • The inhibition of complex 1 of the electron transport chain in the mitochondria by Metformin is achieved by the inhibition of the reduction of Ubiquinol (Co Enzyme Q10) thereby creating a shortage in the capacity of carrying electros through the complexes [32].

  • Metformin activates the energy sensing enzyme AMPK directly and as result of the higher AMP/ATP ratio [34], [35]. Due to the ATP deficiency induced by Metformin and the activation of AMPK, energy consuming process such as protein and fatty acid synthesis as well as steroidogenesis are dampened, resulting in lower concentrations of estrogen, progesterone and especially androgens [16], [36]. Activation of AMPK during in vitro maturation of porcine oocytes resulted in inhibition of cumulus expansion and resumption of meiosis [37]. In the brain, as opposed to the rest of the body, metformin inhibits AMPK, similarly to a state of anorexia. It suppresses appetite and reduces the GnRH stimulated FSH and LH secretion [38].

  • Metformin causes an activation of AMPK which is known to be involved in chromatin organization and meiotic spindle formation. It directly associates with the chromatin to regulate transcriptional programing via phosphorylation of histone H2B or by regulating histone deacetylases [39], [40], [41]. These actions potentially allow metformin to induce a multitude of epigenetic reprogramming changes, which could set a landscape for metabolic defects later in life.

  • Metformin, despite being highly polarized has a very good tissue penetration due to the action of the organic cation transporter 1–3 or multidrug and toxic compound extrusion type transporter (MATE1, MARE2) allowing high concentration in multiple tissues including gonads. Metformin also readily crosses the placenta and its level in cord blood is similar to maternal venous blood [42]. Therefore, similar effects seen on the maternal side can be expected on the fetal side as well. The effects of metformin on androgen production in the embryo may affect male genitalia development, reduced fetal gluconeogenesis may affect the availability of glucose that is essential for proper brain development and function, etc.

Initial studies on the effects of Metformin administration in pregnancy reported an improved live birth rate in PCOS with no increased risk for birth defects. However, the emerging understanding of Metformin’s mechanism of action, calls for a more careful inspection of the short term effects on the oocytes and embryos, and the long term effects of exposure to the drug during pregnancy.

Several studies published recently raise some concerns regarding the use of Metformin in peri-conception period and during pregnancy.

Culturing zygotes with metformin reduced the rate of cleavage and the number of zygotes that developed to embryos [43]. Inhibitors of complex I and II were demonstrated to induce cell death by reducing ATP availability and increasing ROS (reactive oxygen species) leading to excessive autophagy [44]. Blastocyst development rate was significantly reduced in murine embryos incubated in culture media containing metformin [45].

According to the Barker hypothesis, maternal condition at the time of pregnancy may have long lasting effects on the health of offspring due to genetic reprogramming. Embryos that develop in suboptimal conditions with reduced nutrient and energy availability tend to develop an adult phenotype suggestive of the metabolic syndrome. The metabolic syndrome is thought to represent a genetic reprogramming to a state of reduced glucose consumption leading to a greater availability of glucose for brain function by means of relative insulin resistance [46]. Metformin, being an inhibitor of the oxidative phosphorylation, when given in pregnancy, may lead to a condition of low energy availability to the developing embryo. The offspring of mice exposed to therapeutically levels of metformin in utero had shown greater body fat content, altered body fat distribution and lower amounts of lean mass as adults. They also had higher total cholesterol levels and larger livers due to fat deposition. Male mice had a higher fasting glucose and both genders had shown a poor glucose tolerance. Microarray analysis showed an altered gene expression of metabolic regulators (insig-1) and glucose transporters was detected [47].

In humans, Glueck et al. [48], Niromanesh et al. [49] and Gandhi et al. [50], had shown a reduced birth weight for offspring of mothers treated with Metformin during pregnancy. Conversely, at 12 months of age, Carlsen et al. [51], observed that in utero metformin exposed children were heavier in comparison to placebo in utero treated children. In a prospective study by RØ et al., children between the ages 7 and 9 years, born to mothers treated with Metformin during pregnancy, had a higher fasting blood glucose and increased systolic blood pressure compared to offspring of women treated with placebo [52]. These studies raise a concern regarding the safe use of Metformin during pregnancy and long lasting effects on offspring.

Legro et al. performed a prospective randomized controlled study including infertile women with the polycystic ovary syndrome to receive clomiphene citrate plus placebo, extended-release metformin plus placebo, or a combination of metformin and clomiphene for up to 6 months. They showed that clomiphene was superior to metformin in achieving life birth, and that the combination of metformin plus clomiphene compared to clomiphene alone, didn’t increase the pregnancy rate or life birth [53].

The endocrine society clinical practice guideline suggested against the use of metformin in PCOS patients for prevention of pregnancy complications or for the treatment of obesity. They recommended clomiphene citrate or letrozole as the first line treatment for anovulatory infertility in women with PCOS. They suggested the optional use of metformin as an adjuvant therapy for infertility to prevent OHSS (ovarian hyperstimulation syndrome) in women with PCOS undergoing IVF (in vitro fertilization). They also suggested that metformin may have some use in women with PCOS and obesity as an adjuvant agent together with clomiphene in clomiphene resistant women [54].

The Cochrane in 2010 conclude that there is no evidence that metformin improves live birth rates whether it is used alone or in combination with clomiphene, or when compared with clomiphene. Therefore, the use of metformin in improving reproductive outcomes in women with PCOS appears to be limited [55].

Metformin is widely used for the treatment of PCOS patients as a single agent to induce ovulation or as an adjunct to fertility treatment with questionable efficacy. More recently the treatment of many PCOS patients with metformin was extended for the whole duration of pregnancy in order to reduce the risk or treat gestational diabetes [56].

When prescribed to PCOS patients with co-existing obesity or insulin resistance, the beneficial effects on reducing the embryo-toxic effects of hyperglycemia likely negate the less dramatic effects on metabolism and metabolic reprogramming. This can explain the net positive effect on pregnancy outcome [57]. However, the relative safety of the drug expended it use to lean PCOS patients with normal insulin sensitivity as well. A large survey published recently showed that over 10% of all IVF cycles included metformin and that the main indications for its addition were recurrent pregnancy loss and poor embryo quality rather than PCOS patients with obesity or insulin resistance [58].

Section snippets

Conclusions

Recent studies show no clear advantage to the use of metformin in fertility treatment for PCOS patients. Prescribing metformin in the immediate pre conception period to a patient with PCOS with no proven insulin resistance is therefore not indicated. It may also be associated with disturbing side effects as well as with long term adverse effects on the health of the offspring. Thus, one should carefully consider the potential metabolic impairment at a crucial and sensitive stage in embryo

Abbreviations

PCOS, polycystic ovaries syndrome; IVF, in vitro fertilization; LH, luteinizing hormone; FSH, follicle stimulating hormone; OHSS, ovarian hyperstimulation syndrome; ETC, electron transport chain.

Ethics approval

Not needed – this manuscript is a hypothesis.

Consent for publication

Not applicable.

Availability of data

The data supporting the conclusions of this article is included within the article.

Funding

No funding.

Competing interest

None of the authors have a competing interest.

Authors contribution

J.H and Y.B wrote the manuscript together, read and approved the final manuscript.

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