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Metformin, the aspirin of the 21st century: its role in gestational diabetes mellitus, prevention of preeclampsia and cancer, and the promotion of longevity

American Journal of Obstetrics and Gynecology, September 2017, Volume 217, Issue 3, Pages 282-302

Metformin: from the pharaohs to the present

Metformin (dimethylbiguanide hydrochloride) is a constituent of many herbal remedies, and the Ebers Papyrus, written in 1500 BCE, 22 records its use in Egypt since the time of the Pharaohs ( Figure 2 ). In Europe, herbal remedies derived from the plant Galega officinalis ( Figure 3 ) containing metformin have been prescribed to treat polyuria and other symptoms of diabetes mellitus since the Middle Ages, 22 23 24 but it was not until the early 1900s that guanidine was identified as the organic base responsible for the hypoglycemic effects of G officinalis extracts. 25 26

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Figure 2
Ebers papyrus
This document, 20 m long, contains a collection of medical texts considered to be the most comprehensive account of practice in Egyptian medicine. Its encyclopedic content addresses multiple illnesses (eg, treatment for diabetes mellitus, crocodile bites, mental illness, and treatment for death [half an onion and froth of a beer…]). The papyrus was purchased by the Chief of Egyptology (Georg Ebers) and donated to the University of Leipzig in Germany, where it currently resides. The story goes that the papyrus was discovered between the legs of a mummy.
Romero. Metformin, the aspirin of the 21st century. Am J Obstet Gynecol 2017 .
Courtesy Rami Aapasuo/Alamy. Used with permission.

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Figure 3
Galega officinalis
This plant (also known as goat’s rue, French lilac, or Italian fitch) was used for many years to treat the symptoms of diabetes mellitus. In 1920, the antidiabetic class of drugs called biguanides, originating from this plant, was introduced for the treatment of diabetes mellitus.
Romero. Metformin, the aspirin of the 21st century. Am J Obstet Gynecol 2017 .
Courtesy Science History Images/Alamy. Used with permission.

 

Guanidine was too toxic for clinical use, and isoamylene guanidine (galegine) was used as an antidiabetic agent in the 1920s until the development of metformin and phenformin. 27 28 Phenformin was withdrawn from clinical use because it caused lactic acidosis, 29 and, although metformin did not have this side-effect, its use, as well as the use of other biguanide derivatives to treat diabetes mellitus, was displaced by insulin, which was purified and synthesized in 1921 and used clinically to treat diabetes mellitus in humans the next year. 30 31

Nevertheless, research with biguanides continued because they were effective in the treatment of malaria; the hypoglycemic effects of the antimalarial agent chloroguanidine hydrochloride eventually paved the way for the development of metformin to treat diabetes mellitus by Professor Jean Sterne at the L’ Hôpital Laennec in Paris, who coined the name “glucophage” (“glucose eater”) for metformin ( Figure 4 ). 32 Two unexpected side-effects of some biguanides (lactic acidosis and increased cardiac death) then caused their withdrawal from clinical use in the United States; 23 33 34 35 36 37 38 however, metformin was relatively safe and, after 20 years of clinical use in Europe, was approved by the Food and Drug Administration in 1995 for the treatment of diabetes mellitus in the United States. A joint consensus statement by the American Diabetes Association and the European Association for the Study of Diabetes now recommends metformin as the initial oral therapy for patients with type 2 diabetes mellitus. 39 40 Recently, the Professional Practice Committee of the American Diabetes Association recommended the use of metformin for patients with prediabetes mellitus (fasting glucose 100–125 mg/dL, 2-hour post-load glucose 140–199 mg/dL, or A1C 5.7–6.4%), especially in those who are <60 years old, have a body mass index >35 kg/m 2 , or have a history of gestational diabetes mellitus. 41 42 43

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Figure 4
Professor Jean Sterne at L'Hôpital Laennec, Paris, France
Introduction of metformin (“glucophage”) into clinical medicine.
Romero. Metformin, the aspirin of the 21st century. Am J Obstet Gynecol 2017 .
(Reproduced with permission from Bailey CJ, Day C. Metformin: its botanical background. Practical Diabetes International 2004;21:115-7.)

 

Several mechanisms of action considered responsible for this effect include (1) a decrease in hepatic glucose production by the suppression of gluconeogenesis, 44 45 (2) an increased insulin suppression of endogenous glucose production by the liver, 44 45 and (3) a reduction of glucose absorption by the gastrointestinal tract. 44 45 By far the most important mechanism is the reduction in hepatic glucose production considered to be mediated by the activation of the global energy sensor in cells, adenosine monophosphate-activated protein kinase ( Figure 1 ). 46

Metformin was mostly used in nonpregnant diabetic patients until Coetzee and Jackson 47 48 49 50 reported its use in the late 1970s with pregnant diabetic women from South Africa, after which metformin became the treatment of choice for gestational diabetes mellitus 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 and, in the 1990s, for type 2 diabetes mellitus 69 70 because of its ease of administration and high compliance rate.

Important results from the metaanalyses: metformin reduces the frequency of gestational hypertension in gestational diabetes mellitus

In 2013, the efficacy and safety of metformin in the management of gestational diabetes mellitus had been compared to that of insulin in 5 randomized clinical trials. 56 71 72 73 74 Gui et al 75 published a systematic review and metaanalysis in which metformin was shown to be superior to insulin in the reduction of maternal weight gain during pregnancy and in the frequency of gestational hypertension ( Figure 5 , A); however, metformin did not change the frequency of large-for-gestational-age (LGA) or small-for-gestational-age (SGA) fetuses or of hypoglycemia and preeclampsia. 75 Two subsequent metaanalyses confirmed metformin’s effect on maternal weight gain during pregnancy and on gestational hypertension; 76 77 furthermore, Butalia et al 77 reported that, compared to insulin, metformin significantly decreased the frequency of neonatal hypoglycemia, LGA neonates, and admissions to a neonatal intensive care unit ( Figure 5 , B). 56 72 73 74 77 78 79 80 81 82 83 Gui et al 75 suggested that metformin reduced the rate of gestational hypertension because of its effects on endothelial function and its decrease in the production of reactive oxygen species, the 2 mechanisms implicated in the pathophysiologic condition of preeclampsia. 84 85 86 87 88 89 90 91 92 93 94

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Figure 5
Results of the metaanalyses that compared the efficacy of treatment with metformin vs insulin in women with gestational diabetes mellitus
A, The panels present the beneficial effects of metformin vs insulin in women with gestational diabetes mellitus that indicate a reduction in (1) maternal weight gain during pregnancy and (2) gestational hypertension. (Reproduced with permission from Gui J, Liu Q, Feng L. Metformin vs insulin in the management of gestational diabetes mellitus: a meta-analysis. PLoS One 2013;8:e64585.) B, The panels present the beneficial effects of metformin vs insulin in women with gestational diabetes mellitus that indicate a reduction in (1) neonatal hypoglycemia, (2) large-for-gestational-age neonates, and (3) admissions to the neonatal intensive care unit.
CI , confidence interval; IV , inverse variance; M-H , Mantel-Haenszel.
Romero. Metformin, the aspirin of the 21st century. Am J Obstet Gynecol 2017 .
(Reproduced with permission from Butalia S, Gutierrez L, Lodha A, Aitken E, Zakariasen A, Donovan L. Short- and long-term outcomes of metformin compared with insulin alone in pregnancy: a systematic review and meta-analysis. Diabet Med 2017;34:27-36.)

 

Two trials from the United Kingdom report conflicting results for the treatment of obese, nondiabetic, pregnant women with metformin

In a randomized controlled clinical trial, “Efficacy of Metformin in Pregnant Obese Women” (EMPOWaR), the effects of metformin and placebo were compared in nondiabetic, obese (defined here as a body mass index >30 kg/m 2 ), pregnant women. 95 Women randomly assigned to receive metformin began treatment at 12–16 weeks of gestation. The starting dose of 500 mg/day was increased, as necessary, to a maximum tolerable dose not exceeding 2500 mg. In this trial, metformin had no significant effect on birthweight, maternal weight gain during gestation, preeclampsia, or combined adverse pregnancy outcomes, which included miscarriage, termination of pregnancy, or fetal or neonatal death. 95

The second randomized clinical trial compared the treatment of obese (defined here as a body mass index >35 kg/m 2 ), nondiabetic, pregnant women with metformin to placebo. 96 In this trial conducted by the Fetal Medicine Foundation, women randomly assigned to receive metformin started treatment at 12–18 weeks of gestation at a dose of 1 g/day, increased by 0.5 g/week to a maximum dose of 3 g/day. The goal was to reduce the rate of LGA infants: 96 the primary outcome was a reduction of median neonatal birthweight by 0.3 standard deviations, representing a 50% reduction in the incidence of LGA neonates. Although metformin did not reduce the frequency of LGA neonates, it significantly reduced the frequency of preeclampsia and maternal weight gain, although not the rate of gestational diabetes mellitus. 96 The finding that metformin decreased the frequency of preeclampsia was consistent with the results of a previous metaanalysis reported by Feng and Yang. 76

Trial design and execution may explain the contradictory results of the 2 United Kingdom trials

Differences in trial design, execution, and compliance are the most likely explanations for the contradictory results obtained in these 2 randomized clinical trials from the United Kingdom. In the Fetal Medicine Foundation trial, women had a higher body mass index (>35 vs >30 kg/m 2 ), were treated with higher starting and maximum doses of metformin, and were more compliant than those in the EMPOWaR trial. 95 96 In the Fetal Medicine Foundation trial, almost 80% of women took at least 50% of the total number of tablets prescribed, and 91% of those who were prescribed ≥2.5 g/day did so. 96 By contrast, in the EMPOWaR trial, compliance was defined as ingestion of at least 1 tablet for at least 29% of the days between randomization and delivery; only 66% of women fulfilled this criterion. 95 Suboptimal compliance in randomized trials is well known to cause negative results. 97 98 99 100 101 It is also noteworthy that only 13% of the eligible patients (434/3329) consented to participate in the EMPOWaR study, whereas 47% of eligible women (400/844) were recruited to the Fetal Medicine Foundation study, which made the latter group more representative of women who meet the study’s eligibility criteria and to whom the study’s results apply.

Mechanisms by which metformin may prevent preeclampsia

The role of angiogenic and antiangiogenic factors in the genesis of preeclampsia

For more than 100 years, preeclampsia was thought to be caused by the release of “toxic factors” from an ischemic placenta, hence, the name “toxemia.” 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 The most widely known “toxins” at this time are soluble fms-like tyrosine kinase-1 (sFlt-1 or soluble vascular endothelial growth factor receptor 1 [sVEGFR-1]) and soluble endoglin. 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142

It has been hypothesized that sVEGFR-1 is produced in preeclampsia because the placenta is ischemic or hypoxic, and sVEGFR-1 antagonizes angiogenic molecules, such as vascular endothelial growth factor (VEGF) and placental growth factor (PlGF). 119 143 144 145 146 147 148 149 150 Soluble endoglin transforming growth factor is a cell surface co-receptor for transforming growth factor-β1, which blocks TGF-β1-mediated activation of endothelial nitric oxide synthase and promotes vasorelaxation. 124 There is excessive production of sVEGFR-1 and soluble endoglin in the uterus of patients with preeclampsia. 121 This is proportional to the severity of the disease, 151 152 153 and maternal plasma concentrations of sVEGFR-1 and soluble endoglin increase before preeclampsia is diagnosed, making them potential biomarkers for the disease. 111 120 128 129 132 133 137 138 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 The increase of sVEGFR-1 causes a parallel decrease in the maternal plasma concentration of PlGF in preeclampsia. 119 143 144 145 146 147 148 149 150

The administration of sFlt-1 (or sVEGFR-1) and/or soluble endoglin to animals produces changes characteristic of preeclampsia. For example, if sVEGFR-1 is administered to rats using an adenovirus vector, the animals develop hypertension, proteinuria, and glomerular endotheliosis. 180 Additionally, if sVEGFR-1 and soluble endoglin are given to pregnant rats, the animals not only develop preeclampsia but also liver dysfunction, thrombocytopenia, and intrauterine growth restriction; 124 similar results have been obtained in mice. 141 This condition is indistinguishable from the hemolysis, elevated liver enzymes, and low platelet count (HELLP) syndrome observed in humans. 181 182 183 184

Although the emphasis on the toxic factors produced by an ischemic placenta has been focused on the balance between angiogenic and antiangiogenic factors, evidence now suggests that cytokines (such as tumor necrosis factor-α and interleukin-10) are altered in early- and late-onset preeclampsia and that the changes correlate with the type of histopathologic changes in the placenta. 185 The importance of ischemic placental disease, not limited to preeclampsia, has been the subject of several recent studies (ie, preterm labor, preterm premature rupture of the membranes, fetal growth restriction, fetal death, and other complications of pregnancy). 186 187 188 189

Brownfoot et al, 20 from the Translational Obstetrics Group of the Department of Obstetrics and Gynecology, Mercy Hospital for Women, at the University of Melbourne, Heidelberg, Victoria, Australia, reported that metformin, when administered in a dose-dependent manner, reduced the production of sFlt-1 and soluble endoglin by the endothelial cells, villous trophoblast, and villous explants ( Figure 6 ); the report also suggested that metformin regulates these antiangiogenic factors at the level of the mitochondria. Metformin also decreased the expression of vascular cell adhesion molecule 1, expressed by endothelial cells that either are dysfunctional or have been stimulated by incubation with tumor necrosis factor-α, a cytokine increased in the circulation of patients with preeclampsia. 190 191 192 193 194 195

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Figure 6
Effect of metformin on soluble fms-like tyrosine kinase-1/soluble vascular endothelial growth factor receptor-1 secretion, soluble endoglin, and isoforms e15a and i13 expression in endothelial cells and placental tissue
Metformin reduced, in a dose-dependent manner, soluble fms-like tyrosine kinase-1 from A, endothelial cells, B, villous cytotrophoblast cells, and C, preterm preeclamptic placental villous explants. Metformin also reduced endothelial cell expression of D, the soluble fms-like tyrosine kinase-1 i13 isoform, E, villous cytotrophoblast cells, and F, preterm preeclamptic placental villous explant messenger RNA expression of soluble fms-like tyrosine kinase-1 e15a . Metformin reduced soluble endoglin secretion from G, endothelial cells and H, villous cytotrophoblast cells, but it did not change soluble endoglin secretion from I, preterm preeclamptic placental villous explants. The single asterisk indicates P <.05; the double asterisk indicates P <.01; the triple asterisk indicates P <.0001; and the quadruple asterisk indicates P <.00001.
mM , millimolar; sENG , soluble endoglin; sFlt-1 , soluble fms-like tyrosine kinase-1.
Romero. Metformin, the aspirin of the 21st century. Am J Obstet Gynecol 2017 .
(Modified with permission from Brownfoot FC, Hastie R, Hannan NJ, et al. Metformin as a prevention and treatment for preeclampsia: effects on soluble fms-like tyrosine kinase 1 and soluble endoglin secretion and endothelial dysfunction. Am J Obstet Gynecol 2016;214:356.e1-15.)

 

However, the most persuasive evidence that metformin has a vascular effect is the finding that it reverses the impairment of vascular relaxation induced by incubating the maternal blood vessels obtained from the omentum at the time of cesarean delivery with placental-conditioned media of patients diagnosed with preeclampsia. 20 Metformin also abrogated the reduction of angiogenic sprouting induced in human omental vessel explants by incubation with sVEGFR-1 ( Figure 7 ). 20 Overall, the findings of Brownfoot et al 20 suggest that metformin may have a role in the prevention of preeclampsia through its effect on cell metabolism, on the antiangiogenic state, and, most likely, on other processes associated with this obstetric syndrome. 20

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Figure 7
Effect of soluble fms-like tyrosine kinase-1/soluble vascular endothelial growth factor receptor-1 and metformin on angiogenesis
Omental vessel rings cultured with soluble fms-like tyrosine/soluble vascular endothelial growth factor receptor-1 reduced the vessel outgrowth. The white arrows point out vessel outgrowth. This effect was resolved when metformin (1 mmol/L) was added to the culture media (right panel).
sFlt-1 , soluble fms-like tyrosine kinase-1.
Romero. Metformin, the aspirin of the 21st century. Am J Obstet Gynecol 2017 .
(Reproduced with permission from Brownfoot FC, Hastie R, Hannan NJ, et al. Metformin as a prevention and treatment for preeclampsia: effects on soluble fms-like tyrosine kinase 1 and soluble endoglin secretion and endothelial dysfunction. Am J Obstet Gynecol 2016;214:356.e1-15.)

 

Preeclampsia as a mitochondrial disorder and the effect of metformin on mitochondrial function

Torbergsen et al 196 first suggested that mitochondria might be involved in the pathogenesis of preeclampsia based on the high frequency of preeclampsia in a family that experienced mitochondrial dysfunction. Six of 10 women with a mitochondrial disease had at least 1 pregnancy complicated by preeclampsia or eclampsia, whereas none had been hypertensive in the nonpregnant state. Torbergsen et al suggested that these women experienced preeclampsia because their dysfunctional mitochondria could not meet the increased energy demands of pregnancy and because the failure of adequate cellular energy production led to an accumulation of adenosine pyrophosphate, which is able to mediate most of the changes seen in preeclampsia. 89 197 Adenosine pyrophosphate, a vasoconstrictor, causes platelet aggregation and breaks down to uric acid.

In 1990, Shanklin and Sibai 198 reported that mitochondria from small vessels, myometrial smooth muscle, myometrial interstitial cells, circulating leukocytes, epidermal and dermal cells, and hepatocytes in women with preeclampsia all had morphologic changes quite different from those seen in normal pregnancies. 198 The tissues of women with preeclampsia showed central disruption in mitochondrial morphology and changes in the Golgi apparatus, endoplasmic reticulum, and small, unidentified microvesicles; these mitochondrial changes were not limited to the uterine tissues, thus implying that mitochondrial dysfunction is a systemic disorder. That same year, Berkowitz et al 199 reported a case of abnormal mitochondrial morphology observed in an endomyocardial biopsy obtained from a 20-year-old nulliparous woman, known to have mitochondrial myopathy, who had severe preeclampsia.

Since that time, the following findings have further implicated mitochondria in the pathogenesis of preeclampsia: (1) a comparative proteomics analysis of placental mitochondria in patients with a normal pregnancy and those with preeclampsia has shown up-regulation of 4 proteins and down-regulation of 22 proteins. Using bioinformatic tools, differentially expressed proteins in this study were identified as participating in many critical processes of preeclampsia, such as reactive oxygen species generation, apoptosis, fatty acid oxidation, respiratory chain function, and the tricarboxylic acid cycle; 200 (2) the median maternal whole-blood mitochondrial DNA copy number was higher in women with preeclampsia than in those who experienced a normal pregnancy ( P < .001 201 ), which suggests that an influx of mitochondrial DNA into the maternal circulation may act as a danger signal, or alarmin, responsible for the sterile (absence of infection) intravascular inflammatory processes of this condition; 202 203 (3) the placentas of patients with preeclampsia, 204 as well as those from pregnancies with preeclampsia and SGA, 205 overexpress microRNA (miR)-210, which has, as a potential target, the regulation of transcription of the innate immune response. Additionally, it was demonstrated that miR-210 was induced by hypoxia, and RNA interference knockdown resulted in autophagosomal and siderosomal iron accumulation, which implicated siderosis or interstitial trophoblasts as mechanisms in preeclampsia and SGA; 206 and (4) aside from its effects on sterile inflammation and iron metabolism, miR-210 also modulates mitochondrial function. The laboratory of Professor Leslie Myatt provided evidence that placental mitochondrial dysfunction is mediated, at least in part, by miR-210, by demonstrating that mitochondrial complexes I, III, and IV are decreased in preeclampsia, along with a decrease in the iron-sulfur cluster scaffold homologue. 207 Importantly, transfection of cells with miR-210 resulted in a significant reduction in oxygen consumption by mitochondria, a mitochondrial respiratory deficiency, and the production of reactive oxygen species. 207

Recently, Professor Gennady Sukhikh’s Research Institute in Moscow has reported the following findings indicating that mitochondrial functional changes in the placenta occur in both early- and late-onset preeclampsia when compared to normal pregnancy: 208 women with early-onset preeclampsia had (1) a 2-fold increase in the mRNA expression of OPA1 (optic atrophy, type 1) and a 3-fold increase in OPA-1 protein expression (cleaved and uncleaved forms). This gene is involved in mitochondrial fusion and in the cristae structure of the inner mitochondrial membrane, a fine-tuned process crucial for mitochondrial quality control; (2) a 5-fold decrease in the mitochondrial transcription factor A; (3) a 1.5-fold increase of the relative placental mitochondrial DNA copy number; and (4) increased mitochondrial respiration in the presence of complex I substrates. 208

An increase in the phosphate/oxygen ratio (a measure for how much adenosine triphosphate is synthesized per 2 electrons transferred to oxygen) was observed in early- and late-onset preeclampsia. Thus, early-onset preeclampsia is associated with mitochondrial activation, up-regulation of OPA-1, active DNA replication (resulting from a high respiration rate), and mitochondrial transcription factor down-regulation, although both early- and late-onset preeclampsia are associated with an elevated phosphate/oxygen ratio. 208

Collectively, this evidence, coupled with the conduction of a workshop recently held by the International Federation of Placenta Associations on the role of mitochondria in placental function, 209 suggests that mitochondrial dysfunction plays a major role in the pathogenesis of preeclampsia.

Brownfoot et al 20 found that metformin can improve endothelial dysfunction, reduce the expression of vascular adhesion molecule 1 mRNA induced by tumor necrosis factor-α, and improve whole-blood vessel angiogenesis impaired by sFlt-1 or sVEGFR-1. Given the evidence that metformin acts through the mitochondrial electron transport chain by inhibiting complex I, 46 210 211 212 Brownfoot et al 20 investigated whether the effects on sFlt-1 and soluble endoglin production are regulated through the mitochondrial electron transport chain ( Figure 8 ).

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Figure 8
Effect of metformin on the mitochondrial respiratory transport chain complex 1
Metformin crosses the plasma membrane of the cell by passive diffusion; the mitochondria is its main intracellular target. Metformin inhibits the mitochondrial respiratory transport chain complex 1 and induces a decrease in reduced nicotinamide adenosine dinucleotide (NADH) oxidation, proton pumping across the inner mitochondrial membrane, and the oxygen consumption rate, leading to a reduction of adenosine triphosphate (ATP) synthesis from adenosine diphosphate (ADP) and inorganic phosphate (Pi).
Cyt c , cytochrome complex; e - , electrons; FAD , flavin adenine dinucleotide; H + , hydrogen ion; H 2 O , water; NAD , oxidized nicotinamide adenine dinucleotide; OCT , organic cation transporter; Q , coenzyme Q, or ubiquinone.
Romero. Metformin, the aspirin of the 21st century. Am J Obstet Gynecol 2017 .
(Reproduced with permission from Viollet B, Guigas B, Sanz Garcia N, Leclerc J, Foretz M, Andreelli F. Cellular and molecular mechanisms of metformin: an overview. Clin Sci (Lond) 2012;122:253-70.)

 

The reduction in the secretion of antiangiogenic factors by metformin was reversed when succinate (whose metabolism via complex II bypasses complex I) was used as an electron source. 20 However, succinate is also an inhibitor of hypoxia-inducible factor (HIF)-1α, 213 which has been implicated in the exaggerated production of sFlt-1 and in the pathophysiologic condition of preeclampsia; 214 215 therefore, succinate could also be acting by inhibiting HIF-1α rather than the mitochondrial electron transport chain. However, Brownfoot et al 20 demonstrated that other electron transport chain inhibitors (antimycin and rotenone) blocked secretion of sFlt-1 and soluble endoglin, which suggests that the electron transport chain is the major effector of the benefits of metformin. Indeed, succinate in the absence of metformin appears to have no effect. 20 The role that reactive oxygen species and the electron transport chain play in the genesis of preeclampsia could be parsed both pharmacologically and genetically. 216 217 218 219 The changes in the mitochondrial function in preeclampsia suggest that there are perturbations in the cellular energy balance of patients with this syndrome that affect cell growth and division (especially in the placenta and fetus); all are modified by metformin.

The biologic basis for the effects of metformin on fetal growth

Nutrient sensing: key for the survival of all living forms

“Cell growth and division are the two most fundamental features of life.” 220 All organisms must be able to detect nutrient levels in their environment to coordinate growth and development. This is true of bacteria that must choose whether to grow or to remain stationary, and, in the case of motile bacteria, to determine in which direction to move, depending on the availability of nutrients. Bacteria evolved specific chemoreceptors for this purpose that coordinate information received from the environment with specialized structures, such as flagella, to move toward nutrients. The evolutionary history of nutrient-sensing pathways from bacteria to humans is displayed in Figure 9 . 221

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Figure 9
Nutrient-sensing pathways in the evolution of species from unicellular to multicellular organisms
Pathways specific to unicelullar organisms are denoted , followed by the sensing pathways that are conserved from yeast to man. Blue bars indicate the presence of the nutrient-sensing pathways used by different organisms.
AMPK , 5' adenosine monophosphate-activated protein kinase; GCN2 , general control nonderepressible 2; MEP2 , methylamine permease protein 2, extracellular ammonium sensor; PII , nitrogen regulatory proteins; Snf3/Rgt2 , extracellular glucose sensor; SPS , Ssy1-Ptr3-Ssy5, extracellular amino acid sensor; TOR , target of rapamycin.
Romero. Metformin, the aspirin of the 21st century. Am J Obstet Gynecol 2017 .
(Reproduced with permission from Chantranupong L, Wolfson RL, Sabatini DM. Nutrient-sensing mechanisms across evolution. Cell 2015;161:67-83.)

 

The evolution from prokaryotes (a 2-compartment system) to eukaryotes ( Figure 10 ) created unique opportunities for storage: eukaryotes have a third compartment that allows intracellular storage; here, nutrient sensing can also occur ( Figure 10 ). 221 Nutrient sensing became more complex in metazoans because they were required to maintain homeostasis of different tissues and organs. Indeed, over the course of millions of years, specific pathways have evolved for glucose, amino-acids, and energy, and metazoans eventually evolved the endocrine and paracrine systems to meet their requirements for nutrient sensing. The coordinated actions of hormones (such as insulin, leptin, and ghrelin) regulate the organism’s response to the presence or absence of nutrients, modulate anabolic and catabolic processes, and control feeding behavior by signaling the brain. 222 223 224

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Figure 10
Mechanisms of nutrient-sensing within unicellular and multicellular organisms
A, Prokaryotes can sense amino acids through a variety of sensors present in the cytosol and extracellular compartments. B, Similarly, yeast cells sense extracellular amino acids via plasma membrane transporters and cytosolic sensors. Eukaryote cells have another potential compartment, such as a vacuole, where sensing may occur. C, In mammalian cells, sensing may occur via cell membrane transporters in the cytosol and within the lysosome.
GCN2 , general control nonderepressible 2; mTORC1 , mechanistic target of rapamycin complex 1; PII , nitrogen regulatory proteins; SPS , Ssy1-Ptr3-Ssy5.
Romero. Metformin, the aspirin of the 21st century. Am J Obstet Gynecol 2017 .
(Reproduced with permission from Chantranupong L, Wolfson RL, Sabatini DM. Nutrient-sensing mechanisms across evolution. Cell 2015;161:67-83).

 

Fetal-placental nutrient sensing

Nutrient sensing is a crucial requirement for fetal development. The key structure in the placenta responsible for nutrient sensing is the syncytiotrophoblast, which covers the villous tree and is in direct contact with the maternal blood in the intervillous space. In this strategic location, the nutrient-sensing mechanisms of the syncytiotrophoblast can detect changes in maternal blood composition, which enables the placenta to monitor it constantly, not only regarding the nutritional status of maternal circulation but also for any threat it contains to the fetus (ie, microorganisms). Thus, it is easy to envision that the syncytiotrophoblast and other components of the placenta contain the nutrient-sensing systems that allow the fetus to regulate its own growth by extracting nutrition from the maternal blood.

Disorders in nutrient-sensing pathways may lead to fetal growth disorders, and studies by Powell and Jansson, 225 Roos et al, 226 and Rosario et al 227 have provided unique insights into nutrient sensing by the human placenta. For example, down-regulation of the mechanistic target of rapamycin (mTOR) in the placenta has been reported in SGA/growth-restricted fetuses ( Figure 11 ). 226 Whether this represents a primary defect in the sensing mechanisms or an adaptive response by the fetoplacental unit remains to be determined. Similarly, when the placenta senses an excess of nutrients (as in obesity, diabetes mellitus, or other metabolic disorders), mTOR activation and fetal growth acceleration may be expected to occur.

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Figure 11
Effect of maternal nutrition on the placental nutritional sensing system and fetal growth
The placenta plays a critical role in modulating maternal-fetal resource allocation, thereby affecting fetal growth and the long-term health of the offspring. Maternal under-nutrition decreases circulating levels of insulin growth factor-1, leptin, and insulin and increases maternal serum adiponectin concentrations, which leads to low fetal nutrient availability. Maternal over-nutrition is associated with increased insulin, insulin-like growth factor-1, and leptin concentrations in the maternal circulation and decreased levels of circulating levels of adiponectin, which leads to fetal overgrowth. The placenta integrates maternal and fetal nutritional signals with information from intrinsic nutrient sensors such as mTOR.
IGF-1 , insulin-like growth factor-1; mTOR , mechanistic target of rapamycin.
Romero. Metformin, the aspirin of the 21st century. Am J Obstet Gynecol 2017 .
(Modified with permission from Jansson T, Powell TL. Role of placental nutrient sensing in developmental programming. Clin Obstet Gynecol 2013;56:591-601; and Jansson T, Aye IL, Goberdhan DC. The emerging role of mTORC1 signaling in placental nutrient-sensing. Placenta 2012;33(suppl2):e23-9; and https://clipartfest.com/download/2ccb316331956e87398d72be2d6d14e49c1a9be8.html .)

 

Nutrient sensing occurs through several pathways, 225 the most important of which are (1) the adenosine monophosphate-activated protein kinase pathway, a global energy sensor in cells; (2) glycogen synthase 3, which acts as a glucose sensor; (3) the hexose-amino signaling pathway, which depends on the availability of nutrients such as glucose, glutamine, and acetyl-CoA; (4) the amino-acid response signal transduction pathway, which is activated under conditions of essential amino-acid deficiency or imbalance; and (5) the mTOR complex 1 (mTORC1) pathway, which integrates nutrient and growth factor signaling. Each of these nutrient-sensing pathways is present in the human placenta. 225 226 227 There are also changes in these pathways in cases of fetal growth restriction caused by maternal starvation or impaired placentation where there is down-regulation of placental nutrient transport. 225 226

The mTOR complex translates maternal nutritional status via the placenta to the fetus

Rapamycin, an antifungal agent produced by Streptomyces higroscopicus , 228 inhibits the growth of Candida albicans and was first discovered in soil on Easter Island (Rapa Nui). The story of its discovery is similar to that of penicillin: its isolation from Aspergillus penicillinum and its capacity to inhibit bacterial growth. However, rapamycin's popularity is due to its powerful immunosuppressive, rather than its antimicrobial, properties, which made it a drug of choice for patients with renal transplants. 229

Rapamycin can also prolong the life cycle of many species as diverse as worms and mice, and its signaling system has been implicated as the master regulator of cellular growth and metabolism. A family of molecules, the mTOR complex has been attributed a major role in the biology of cell metabolism, growth, longevity, and even preterm birth. 230 231 232 233 234 235 236

Rapamycin acts by interacting with a family of TOR molecules highly conserved, functionally and evolutionarily, from yeast to humans, and it contains 2 multiprotein complexes functionally and structurally different: the first, mTORC1, is rapamycin-sensitive; the second, mTORC2, is rapamycin-resistant. 237 Both complexes control cell growth and metabolism in response to the availability of nutrients, energy, and growth factors. On activation, the serine/threonine kinases activate a cascade of intracellular processes that involve the mitochondria and the nucleus of the cell to promote cellular growth and, to a certain extent, aging. Rapamycin prolongs lifespan by inhibiting the effects of mTOR. 238 239

mTOR is a major human nutrient-sensing receptor in the placenta. 225 This signaling system is highly expressed in the syncytiotrophoblast, and its activity is regulated by glucose and amino-acid concentrations. 226 227 240

mTOR has key properties in the placenta 225 226 that include its activation by insulin, insulin-like growth factor-1, and leptin, and its inhibition by cortisol. mTOR regulates 2 key amino-acid transport systems (A and L). The activation of mTOR is correlated positively with the first-trimester maternal body mass index, which links maternal over-nutrition and nutrient-sensing by the placenta, although its expression is down-regulated when fetal growth is restricted (in both animal models and humans). The Akt-mTOR-HIF-1α signaling pathway also affects placental angiogenesis by its ability to increase VEGF and endoglin expression in response to hypoxia in a trophoblast cell line. 241

Safety of metformin during pregnancy

Metformin has a molecular weight of 129 Daltons and crosses the placenta by direct diffusion without affecting the facilitated transfer of glucose. 242 Indeed, an ex vivo dually perfused human placental lobule demonstrated the rapid transfer of metformin from maternal to fetal circulation with a lag time of 1.7±0.28 minutes, similarly observed in women with normal pregnancies and those diagnosed with gestational diabetes mellitus. 242 In vivo studies reported the detection of metformin in the umbilical cord blood of neonates from women with polycystic ovarian syndrome who were treated with metformin throughout gestation. 243 244 Similar concentrations of metformin in the umbilical artery and umbilical vein suggest negligible metformin metabolism by the fetus. 244 Several metaanalyses that studied the teratogenic effect of metformin on embryonic development found that this drug carries no increased risk for congenital malformations 245 246 and is currently classified as category B in the United States and as category C in Australia. 247 248 Similarly, no excess of fetal or neonatal complications could be demonstrated when the administration of metformin was compared to glyburide and insulin. 249 In addition, a study that investigated the neurodevelopmental effect at 2 years of age could not identify a significant difference between children exposed in utero to metformin and those exposed to insulin. 250 Moreover, there were no differences in fat measurements, total fat mass, and percentage of body fat. 251 Those who were exposed to metformin during fetal life had a larger upper arm circumference and bigger subscapular skin folds and biceps, which suggests a better fat distribution than children who are exposed to insulin. 251

Maternal side-effects that have been reported with the use of metformin are mainly gastrointestinal (ie, nausea and diarrhea). 248 The rate of hypoglycemia is lower than that reported with insulin. 79 In addition, rare side-effects such as mild erythema and decreased vitamin B 12 absorption have been associated with long-term administration. 252

A role for metformin in cancer and aging

Originally introduced for the treatment of diabetes mellitus, metformin is now gaining attention as a potential anticancer agent ( Figure 1 ). 7 8 9 10 The first observation that metformin might reduce the risk of cancer was made in a population-based case-control study of patients with type 2 diabetes who were treated with metformin. 253 A cohort study of type 2 diabetic patients, newly treated with metformin and later followed, reported that the frequency of cancer was significantly lower in patients who received metformin compared to the control subjects who had never received metformin, after adjustments for body mass index, hemoglobin A1C, smoking, and the use of other drugs; 254 this finding has subsequently been confirmed in several other studies. 7 8 9 10 In a metaanalysis, metformin-treated patients with diabetes mellitus had a 31% reduction in the incidence of cancer and a 34% reduction in cancer death after adjustment for body mass index. 255 This epidemiologic evidence has coalesced with experimental work in animals. 256 257 258 259 260 261 262 263 Animal experiments have elucidated the mechanisms underlying these epidemiologic findings (ie, suppression of cancer stem cells; 264 inhibition of epithelial-to-mesenchymal transition 265 implicated in metastasis; and interference with glucose, 266 protein, 267 268 and lipid 269 metabolism of the neoplastic cells. There is further evidence that metformin may also have an adjunctive effect in patients who receive chemotherapy; 270 there are now more than 100 ongoing trials registered in clinicaltrials.gov studying the role of metformin in cancer treatment. 45 271 272

Finally, an unbiased search for genes that extend lifespan has identified a disproportionate number of genes that function in mitochondrial metabolism; 273 similar observations were also found by targeting mitochondrial genes. 274 Not surprisingly, drugs that inhibit mitochondrial function were examined in this context, and metformin was shown to extend longevity in worms 2 and mice 3 but not in Drosophila. 275 Metformin has now been found to target several age-related pathways; 3 276 277 however, the mechanisms by which metformin extends lifespan are far from clear. A randomized clinical trial, TAME (Targeting Aging with Metformin), 6 has been planned to test the effect of metformin on the time to the new occurrence of a composite outcome that includes cardiovascular events, cancer, dementia, and death as an endpoint in 3000 subjects who are 65–79 years old. 6

Metformin exerts many of its growth inhibitory and antineoplastic effects through the nuclear pore complex and the acyl-CoA dehydrogenase family member-10. 278 Nucleocytoplasmic shuttling regulates mTORC1 activity, an effect that can be reversed by RNA silencing. Biguanides inhibit growth by suppressing mitochondrial respiration, which limits the transit of the complex RagA-RagC GTPase heterodimer through the nuclear pore complex. 278 By preventing access to the nucleus, Ras-related GTP binding C becomes incapable of stimulating mTORC1 and therefore stimulates cellular growth ( Figure 12 ). 278 Wu et al 278 proposed that the nuclear pore complex and acyl-CoA dehydrogenase family member-10 are involved in insulin action and the regulation of blood glucose concentration. Whether these mechanisms are also responsible for the antidiabetic actions of metformin and its ability to improve insulin sensitivity is unknown at this time.

gr12
 

Figure 12
Metformin suppresses cell growth and promotes longevity
Metformin slows Caenorhabditis elegans (roundworm) growth by inhibiting the mitochondrial electron transport chain, which limits the transit of the Ras-related GTP binding C protein through the nuclear pore complex that results in a reduced activity of mechanistic target of rapamycin complex 1. The metformin-induced inhibition of mechanistic target of rapamycin complex 1 leads to the up-regulation of the transcription factor protein Skinhead-1/Nuclear factor-erythroid-related factor-2 (a regulator of antioxidant genes) and the expression of acyl-CoA dehydrogenase family member-10 (ACAD10) gene.
ACAD10 , acyl-CoA dehydrogenase family member-10; mTORC1 , mechanistic target of rapamycin complex 1; NPC , nuclear pore complex; RagC , Ras-related GTP binding C; RNAi , RNA interference; Skn-1/Nrf-2 , protein skinhead-1/nuclear-factor-erythroid-related factor-2.
Romero. Metformin, the aspirin of the 21st century. Am J Obstet Gynecol 2017 .
(Reproduced with permission from Wu L, Zhou B, Oshiro-Rapley N, et al. An ancient, unified mechanism for metformin growth inhibition in C elegans and cancer. Cell 2016;167:1705-18.e13.)

 

Conclusion

Metformin, long known to be an herbal medicine, has evolved from its use as a popular treatment for diabetes mellitus into a drug with a significantly wider array of beneficial effects that range from cancer treatment to extending longevity and, in our field, gestational hypertension and preeclampsia in obese women. Current evidence suggests that metformin’s wide-ranging beneficial effects are mediated by at least 2 primary mechanisms: suppression of intracellular metabolic activity of mitochondria and the cellular nutrient-sensing system mediated by mTOR.

This work was supported, in part, by the Perinatology Research Branch, Program for Perinatal Research and Obstetrics, Division of Intramural Research, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, U.S. Department of Health and Human Services (NICHD/NIH/DHHS); and, in part, with Federal funds from NICHD/NIH/DHHS under Contract No. HHSN275201300006C.

The authors report no conflict of interest.

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