The effect of perfluorinated compounds on reproduction

1. Introduction

Throughout history, humanity has faced a series of circumstances that have put all individuals (plants and animals) in danger. These circumstances have been of a varied nature such as natural phenomena, wars, epidemics or contamination with various compounds found in the environment. Most of these compounds were synthesized by man to improve the quality of life, however, these substances are capable of affecting the exposed individuals, even without being the target organisms for which they were created for, pesticides and herbicides for example (Sacchettini et al., 2015). Today, there is great concern about the effects of xenobiotic agents on wild animals and on the quality of life of humans, since this can be altered when organisms are exposed to pollutants of various kinds, whether physical, chemical or biological. In the last decade, research has focused on evaluating the effect of exposure to substances such as polychlorinated biphenyls (PCBs), organochlorine and organophosphorus pesticides, phthalates, perfluorinated compounds (PFCs), etc., on human health. In this review we will focus on analyzing the effects of PFCs on reproduction.

1.1 Reproductive toxicology

Reproductive toxicology is the branch of toxicology that studies the deleterious effects produced by xenobiotics on reproduction; including disorders on fertility, embryonic development and alterations in offspring (Repetto, 1997). Xenobiotics can be natural chemical agents or synthetic, biological and physical (Bonilla et al., 2001). Disorders in fertility include effects on libido, sexual behavior, spermatogenesis and oogenesis, hormonal activity, the fertilization process and the development of the zygote until the implantation phase (Caméan and Repetto, 2006).

1.2 The perfluorinated compounds

PFCs are a group of chemicals that are not naturally found in the environment, their use has been widespread since the 1950s. The structure of these products is constituted by a chain of carbons surrounded by fluorine atoms and an acid or amide group located at the end of the carbon chain. They are very stable, hydrophobic and oleophobic compounds (ATSDR, 2009). These unique properties have led to a widespread use of PFCs, for example, in protective carton formulas, carpets and leather products, and in the textile industry as a repellent to water and grease. PFCs have also been used in firefighting foams and in the production of non-stick coatings in kitchen utensils and some clothing (DRAFT, 2011).

The PFCs with the highest volume of production are perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA). Followed by these, with a lower volume of production are perfluorohexane sulfonate (PFHxS) and perfluorononanoic acid (PFNA). In the US alone, the reported production of PFOS and PFOA in 2002 was 6,804 kg and 226,796 kg respectively (DRAFT, 2011). The precursors of the PFCs are fluorotelomeric alcohols (FTOHs) which production volumes have been estimated at 11,000-14,000 tons after 2002, resulting in a global production of 44,000-80,000 tons for perfluorinated carboxylic acids (PFCA) (between 1951-2004) and of 96,000 tons of perfluorooctanesulfonyl fluoride (PFOSF) (between 1970-2002), respectively (Zhao et al., 2012). Table 1 shows the most known and abundant PFCs found in food products for human consumption of the population of Catalonia, Spain (Castells et al., 2012).

Table 1. Eighteen most abundant PFCs in various foods (Castells et al., 2012).

PFCs are highly persistent in the environment and are able to withstand extreme conditions. Because of these characteristics they become suitable for application in kitchen utensils that resist high temperatures and for the preparation of materials that have contact with acids or strong bases. The carbon-fluorine bonds are very strong, which causes the persistence of these substances in the environment (ATSDR, 2009; Weiss et al., 2009). They decompose very slowly in the air and the soil, passing directly from soil to the underground water, travelling great distances through the oceanic currents to the most remote places (Giesy et al., 2001). The precursors of the PFCs, the FTOHs, are very volatile substances capable of being transported for great distances and being degraded to PFCs through abiotic and biotic mechanisms. Because of this, considerable levels of PFCs have been detected in places far away from production and consumption sites such as the Arctic and the North Atlantic, where reported concentrations range from 130-650 pg/L and the Greenland Sea, where concentrations range between 45 and 280 pg/L (Gabrielsen, 2012; Zhao et al., 2012).

1.3 Routes of exposure to perfluorinated compounds

In recent years, studies in animals and in humans exposed to PFCs have increased, since there has been evidence of the accumulation of these compounds in organisms (Cassone et al., 2012). Some reports indicate that, in human blood plasma, higher concentrations of PFCs are found compared to other compounds. Concentrations of PFCs can be up to 50 times higher than PCBs and 450 more than hexachlorobenzene (Kärrman et al., 2006).

PFCs can enter the body through the lungs when breathing air containing these compounds, such as dust particles. There is currently little information on the amount, speed and type of PFCs that enter in greater quantity in this fashion (Fraser et al., 2013). These substances also enter through the digestive tract through the consumption of contaminated food or water, or through the skin when in contact with dusts, aerosols or liquids (Domingo, 2012). In Figure 1 the possible routes of absorption of some toxic compounds, among which are the PFCs, are depicted.

Figure 1. Possible routes of penetration of some toxic compounds and their distribution in an organism (modified from Bello et al., 2001).

Different concentrations of PFCs have been reported in products as varied as personal products, food containers, disposable containers, hermetic containers, cooking utensils and even some foods such as honey (Eom et al., 2014; Surma et al., 2016). However, sea products such as fish and seafood are the main source of exposure to PFCs (Haug et al., 2010). The blood plasma analysis of populations located in northern Norway and the Faroe Islands (Kingdom of Denmark) confirmed the relationship between the presence of these compounds and their daily diet (Weihe et al., 2008; Rylander et al., 2009).

Currently, there has been concern about the possible exposure to these compounds through contaminated water, so that both the European Food Safety Authority (EFSA), in Europe, and the EPA and the Agency for Toxic Substances and the Registry of Diseases (ATSDR), in the USA, have issued recommendations on the permissible limits of these compounds in water for human consumption. For example, in the state of Minnesota (USA), the permissible limits of PFOS and PFOA are 300 ng/L (Grandjean and Budtz-Jørgensen, 2013).

It is possible for small amounts of PFCs to enter the body through the skin. Once inside the organism, they bind to the albumin of the blood serum through which they are transported by the organism and bioaccumulate in organs like the liver or the kidney, to mention a few (Zhang et al., 2009; Governini et al., 2011; Kato et al., 2011; Buck Louis et al., 2015). Figure 2 shows the percentages of intake of PFCs according to the products consumed daily in a population of Catalonia.

Figure 2. Percentage of PFCs in a daily diet.

In humans, PFOA and PFOS tend to remain unchanged for long periods. It has been reported that the average life of these contaminants is 3.5 to 7.5 years in blood plasma (Shankar et al., 2011; Buck Louis et al., 2015).

1.4 Pathways for the elimination of perfluorinated compounds

Unlike other compounds such as PCBs, PFCs cannot be eliminated by body transpiration (Genuis et al., 2013). The main route of elimination of PFCs is urine. It has also been observed that breast milk may be another route of elimination and that it may contribute to the exposure of infants to these agents, since significant concentrations of these compounds have been detected in breast milk (Goosey et al., 2012; Mondal et al., 2014). In a study conducted in the female Japanese population, evidence was found about the elimination of PFCs through menstrual bleeding, since the levels of these compounds in blood plasma decreased more rapidly in women than in men from the same population (Harada and Koizumi, 2009).

It has been reported that in women during pregnancy or lactation periods, the concentration of PFCs decreased in blood plasma due to the transfer of these contaminants to the fetus through the umbilical cord and to the newborn through breast milk (in which up to nine PFCs have been detected) (von Ehrensteina et al., 2009; Whitworth et al., 2012). These data indicate a high risk of exposure of individuals to different toxins from the beginning of their life.

2. Perfluorinated compounds in reproduction

The high concentrations of PFCs found in the plasma of animals and humans have been related to conditions that affect exposed individuals. In rats, for example, the appearance of tumors in the liver has been observed. In vivo studies in rodents have also shown various effects on neuroendocrinology, reproduction and embryonic development (EPA, 2012).

Currently, a decrease in fertility in humans has been reported. Nowadays, it is estimated that 15% of couples of the reproductive age worldwide suffer some problem related to fertility. This figure could reach 70 million couples approximately. This has been related to exposure to different xenobiotic agents, among which are the PFCs (Teg-Nefaah et al., 2013; Agarwal et al., 2015).

2.1 Alterations related to male reproduction

Epidemiological studies relate male human fertility to PFCs during the early years, unlike studies that have been conducted in experimental animals that suggest that these compounds alter fertility. In rodents exposed to these contaminants, a decrease in serum testosterone and a reduction in the weight of accessory sex glands have been reported (Bookstaff et al., 1990). In humans, despite several epidemiological reports, there is still controversy between the relationship between PFCs and reproductive diseases. Male infertility is a widely researched problem. It has been estimated that out of the total reported cases of couple infertility, the man is the cause from 20% to 30% of recorded cases (Agarwal et al., 2015).

Currently, there are few reports on an association between exposure to PFCs and various alterations in male health. Nordström et al., (2009) and Toft et al. (2012) reported problems related to semen quality as defects in sperm morphology and damage to DNA integrity. Similar effects have been observed by other authors (Vested et al., 2013), who indicate a possible association between exposure to PFOS and PFOA during intrauterine life and its impact on semen quality, testicular volume and the levels of reproductive hormones in male adults. Exposure to PFOA during intrauterine life indicated a trend in increased levels of LH and FSH in the adult, in addition to a low sperm concentration. In a Danish population of 19-year olds, an association was found between high concentrations of PFOS, PFOA and PFHxS in blood plasma and a decrease in normal sperm concentration from 15.5 to 6.2 million sperm (Nordström et al., 2009). There is still controversy between the relationship of PFOS and PFOA with male fertility, since in a subsequent study carried out in men of the same nationality, and submitted to studies in infertility clinics, no association was found between the concentration of PFOS and PFOA with plasma and semen volume, concentration and sperm motility (Raymer et al., 2012).

In a later work, six PFCs were analyzed: 2-(N-Methyl-perfluorooctane sulfonamido) acetic acid (Me-PFOSA-AcOH), perfluorodecanoic acid (PFDA), perfluorononanoic acid (PFNA), perfluorooctanesulfonamide (PFOSA), perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) and their association with the sperm quality in 17 semen samples of American men. PFOSA was associated with smaller heads, a lower percentage of DNA staining capacity and a higher percentage of two-headed and immature spermatozoa. The PFDA, PFNA, PFOA and PFOS had a relation with a lower percentage of sperm with coiled flagella. All this suggested that some of these compounds may have a deleterious effect, but it indicates a need for more work to corroborate these facts (Buck Louis et al., 2015).

2.2 Alterations related to female reproduction

Although there are studies in animals and humans that indicate that there is a relation between exposure to PFCs and alterations in reproductive functions, there are few studies on the effect of these compounds on female fertility. Domínguez et al. (2016) reported an adverse effect of PFOS on the in vitro viability and maturation of oocytes of prepubescent sows exposed during maturation, being 32 and 22 μM the lethal concentration 50 and the inhibition of maturation 50, respectively.

The PFCs, as already mentioned, can be found in blood plasma. In women, its presence in the follicular fluid has been reported. These compounds have been related to various pathologies such as polycystic ovarian syndrome (Vagi et al., 2014), endometriosis (Buck Louis et al., 2012), alterations in the menstrual cycle (Lyngsø et al., 2014) and the appearance of precocious menopause (Konkel, 2014). In addition, they have been related to a decrease in fertilization and pregnancy rates (Governini et al., 2011, Jacquet et al., 2012). Following, is the relation that exists between the PFCs, some pathologies and the studies carried out.

Polycystic ovarian syndrome (PCOS): It is a chronic disorder characterized by hyperandrogenemia, increased LH, hyperestrogenemia and hyperprolactinemia, often associated with insulin resistance. It is the most common endocrinopathy in women of reproductive age able to increase the risk of infertility, endometrial pathology and cardiometabolic disease (Builes et al., 2006; Bremer, 2010; Barthelmess and Naz, 2015). In a study conducted by Vagi et al. (2014), a correlation was made between the concentrations of specific environmental pollutants such as organochlorine insecticides, phthalates and PFCs and PCOS. The results of the study with regard to PFCs indicated a higher probability of PCOS in women with high serum concentrations of PFOA and PFOS.

Endometriosis: It is a chronic disease that is characterized by the presence of endometrial tissue outside the uterus and can occur in women of every ethnic or social group. The prevalence reported is around 10% in the general population (Moradi et al., 2014). Currently, the incidence of endometriosis has been related to exposure to persistent environmental pollutants. Buck Louis et al. (2013) observed a correlation between the plasma concentration of PFOS and PFOA and a higher probability of moderate or severe endometriosis. Despite this, more studies are required to clarify this possible association and if so, find out the possible mechanisms of action that trigger this pathology.

Alterations in the menstrual cycle: the presence of PFOA in the blood plasma was associated with a lengthening of menstrual cycles in women from Greenland, Ukraine and Poland. In the Danish population A lag in the time required to achieve pregnancy and low fecundity were also attributed to exposure to PFOS and PFOA (Lyngsø et al., 2014).

Early menopause: menopause is a physiological process in which a woman stops ovulating. This usually occurs between the 40 and 50 years of age. Early menopause occurs before the age of 40 and it has been linked to increased cardiovascular risk and may be an indicator of biological aging. The increased cardiovascular risk at menopause may be a consequence of estrogen deprivation, or as a result of hypertension, diabetes, visceral obesity, dyslipidemia and endothelial dysfunction, which occur with aging (Ebong et al., 2014). In a study conducted in American women, the possible association between plasma concentrations of PFOA, PFOS, PFNA and PFHxS and the onset of early menopause were evaluated (Konkel, 2014, Taylor et al., 2014). The results showed that women with high plasma concentrations of PFOA, PFNA and PFHxS were more likely to have early menopause compared to women with low levels of these compounds. This study also revealed that women with high PFHxS concentrations were more than 3.5 times as likely to have a hysterectomy.

3. Perfluorinated compounds as endocrine disruptors

PFCs are considered endocrine disruptors. These are exogenous substances capable of interfering with the synthesis, secretion, transport, metabolism, binding to the target site or the elimination of natural hormones present in the body that are responsible for homeostasis, reproduction and development (Diamanti-Kandarakis et al., 2009; Knox et al., 2011). In Fields (2008), it was reported that in the rat PC-12 cell line, PFOSA, PFOS, PFOA and PFBS decreased viability and the ability to synthesize DNA and to reproduce. PFOSA showed the most obvious damage, since it almost completely avoided DNA synthesis, cell division and increased the production of oxidative stress, even in low concentrations.

Data collected by the National Health and Nutrition Examination Survey (NHANES) in 2007 indicated that 98% of blood samples obtained from adults in the USA revealed the presence of PFOS and PFOA (Haug et al., 2010). This indicates that even the non-occupationally exposed population presents the risk of accumulating these potentially toxic substances that are found in the environment or consumer products. These compounds are capable of altering the regulation of the hypothalamic-pituitary-ovarian axis and cause alterations in menstrual cycles and delay ovulation. In addition, PFOA and PFOS are capable of altering hormonal function (Jensen and Leffers, 2008; Zhao et al., 2010). PFOA can alter the production of steroid hormones via the ovaries as an endocrine disruptor. In rodents, exposure during pregnancy produced alterations in the short or long term, but it has been suggested that the possibility of damage is not the same in all species (White et al., 2011).

PFCs as endocrine disruptors have been analyzed in different in vitro systems. In a study on the effect of these substances in human adrenocortical carcinoma cells (H295R), the results showed that PFOS altered steroidogenesis and increased the secretion of estradiol, progesterone and testosterone in a concentration-response manner, these last two hormones in lesser amounts. PFOA and PFNA decreased viability; the latter, in high concentrations, induced apoptosis in these cells (Kraugerud et al., 2011). In the breast cancer MFC7 BOS cell line, both PFOS and PFOA had estrogenic effects in concentrations of 0.03-30 μg/ml and 30 μg/ml respectively, while a co-exposure with PFCs and 17 estradiol (E2) presented an anti-estrogenic effect, which is why these compounds can be a source of xenoestrogens in animals and in humans (Henry and Fair, 2013).

4. Effect of perfluorinated compounds in fertilization

Fertilization is the process in which the male and the female gamete unite forming a zygote; its development originates from a new organism. Fertilization involves several processes such as the adequate maturation of the oocyte and the sperm and the recognition and union of the gametes so that the subsequent embryonic development is carried out (Betancourt et al., 2003). The fertilization process may be vulnerable, and recent studies indicate that PFCs can affect human fertility.

There are reports that indicate that PFCs alter fertility in women. In patients undergoing in vitro fertilization and embryo transfer programs, a relation was observed between high concentrations of PFCs in the follicular fluid and a decrease in the quality of the oocytes, lower percentages of fertilization and number of embryos produced, compared with the group that did not present these compounds in said fluid (Governini et al., 2011). However, there is controversy in these facts, since in one study it was reported that high concentrations of PFOS and PFOA increased 2.1% the probability of subfertility in Norwegian multiparous women, whereas in nulliparous women there was no association between these xenobiotics and subfertility (Whitworth et al., 2012). In a more recent study, Buck et al. (2013) observed that the presence of PFOSA in blood plasma coincided with the decrease in female fertility.

5. Alterations during pregnancy and embryonic development

In recent years, studies on the effects of PFCs during pregnancy and human embryonic development have increased, because of the evidence of adverse effects of these compounds in animals. Fei et al. (2007) reported that PFOS and PFOA can alter the homeostasis of sex hormones and that they have been associated with the incidence of fetal resorptions and the loss of gestation in laboratory animals. Studies carried out with hamster embryos showed that the PFCs affected their morphology when the females were exposed to them during pregnancy, observing a decrease in the body mass at birth and a decrease in the growth of the organs and skeleton of these rodents (Fei et al., 2008; Jacquet et al., 2012; Koustas et al., 2014). Another in vivo study, also in rodents, determined that prenatal exposure to PFOA was able to increase liver size and decrease breast tissue in individuals (Macon et al. 2011).

Woodruff et al. (2011) showed that about 100% of pregnant American women had different environmental contaminants in the blood plasma, among these, PFCs, the results showed the presence of at least four of them. Currently it has been reported that, during pregnancy, the mother, in addition to providing nutrients to the fetus, can also expose it to certain contaminants such as PFCs. This has been reported in several studies, since PFOA has been found in the umbilical cord and amniotic fluid (Mondal et al., 2012, Stein et al. 2012).

PFCs have also been linked to alterations in women’s health, such as preeclampsia during pregnancy. Starling et al., (2014) evaluated a possible association between PFCs and preeclampsia in Norwegian women. In this study, 466 women, all of them with different levels of PFCs in their blood plasma, participated. However, the results indicated that there was no correlation between the concentrations of PFCs and an increased risk of suffering preeclampsia in nulliparous women.

6. Perfluorinated compounds and neurotoxic effect

It has been reported that PFCs are potentially neurotoxic substances in highly exposed animals, and that they can also affect protein levels of functional importance during neuronal growth and synaptogenesis (Chun-Yang et al., 2008; Johansson et al. 2009). White et al. (2011) postulated PFOA as a developmental toxicant, since said compound was responsible for various health problems in rodents. They also reported that the magnitude of the endocrine alterations depended on the degree of exposure and the response in each species.

Some studies have focused on analyzing the effect of these compounds on the development of individuals. Fei and Olsen (2011) conducted a study in a cohort of births in Denmark, in which the possible effect on the behavior or coordination problems in 7-year old infants exposed in the prenatal stage to PFCs was analyzed. Although every infant that participated in the study was exposed to PFOA and PFOS, the results showed that, in the prenatal stage, regardless of the levels of PFCs of the mothers, there was no correlation between the levels of these xenobiotics and behavior issues or motor coordination in childhood.

7. Effect of perfluorinated compounds at birth

Children and especially newborns are more sensitive to environmental pollutants compared to adults. In the first months of life, the metabolic pathways are immature and the ability of the newborn to metabolize and eliminate many toxins is different from that of adults. Although the exposures have occurred during the fetal or neonatal period, their effects can sometimes be observed in later years, which is why more studies are needed to clarify the effects of these substances on human health (Ünüvar and Büyükgebiz, 2012).

Betts et al. (2007) suggested that PFOS and PFOA are able to reduce the weight of children at birth by an average of 69 and 104 g, respectively. These data were supported by other researchers. Fei et al. (2008) conducted a study in Danish women who had different concentrations of PFOA and PFOS in their blood during early pregnancy. The results showed a decrease in the length, weight and size of the head of newborns. This suggests that fetal exposure to PFOA can affect the growth of organs and the skeleton; the presence of PFOS was not associated with any of the parameters analyzed. In British girls, the presence of both PFOS and PFOA decreased weight at birth by an average of 140 and 108 g respectively compared to girls that were exposed to lower concentrations in the prenatal stage (Maisonet et al, 2012 ).

In other works, a follow-up on the development and quality of life during childhood of people exposed to PFCs during pregnancy has been carried out. Ode et al. (2014) evaluated a possible association between fetal exposure to PFOS, PFOA and PFNA and attention deficit hyperactivity disorder (ADHD) in a group of Swedish children. The results indicated that there was no relationship between ADHD and the exposure to PFCs during the fetal stage.

In the Canadian population it was shown that the daughters of women with PFOA and PFHxS in their plasma, even in low concentrations (1.61 and 4.46 ng/ml), can present alterations in fertility (Vélez et al., 2015).

8. Perfluorinated compounds and miscarriage

PFCs toxicity has been demonstrated in laboratory animals. There are currently reports indicating that high concentrations of PFOA and PFOS are capable of increasing neonatal mortality, decreasing fetal growth and reducing litter size (Lau et al., 2006; Lau et al., 2007). However, despite these reports, there are few data on these compounds and their relation to this condition in human health.

A study conducted by Kold et al., (2015) evaluated the relationship between plasma concentrations of PFCs and miscarriage in Denmark. The results showed an association between miscarriage and serum concentrations of PFDA, PFHxS and especially PFNA. These findings are of vital importance for public health, but more studies are still needed to acknowledge if this pattern is repeated in every country.

9. Conclusion

PFCs are ubiquitous and able to bioaccumulate in plants, animals and humans. The negative effect of PFCs has been more studied in animals than in humans. In human reproductive health, several authors report the adverse effect of PFCs in different populations; however, these results are not yet completely clear since each organism responds differently to these xenobiotics, depending on the degree of exposure. In addition, not all patterns are repeated in different populations. This could be due to the degree of sensitivity, race or diet of each of them. Despite the studies carried out, more complete analyzes are still required to know the mechanisms of action of the PFCs in the biological systems since these are still in their infancy.

Abbreviations

ATSDR: Agency for Toxic Substances and Disease Registry.
EFSA: European Food Safety Authority.
FTOHs: Fluorotelomer alcohols.
Me-PFOSA-AcOH: 2-(N-Methyl-perfluorooctane sulfonamido) acetic acid.
PCBs: Polychlorinated biphenyls.
PFCs: Perfluorinated compounds.
PFCA: perfluorinated carboxylic acids.
PFDA: Perfluorodecanoic acid.
PFNA: Perfluorononanoic acid.
PFHxS: Perfluorohexane sulfonate.
PFOA: Perfluorooctanoic acid.
PFOS: Perfluorooctane sulfonate.
PFOSA: Perfluorooctanesulfonamide.
PFOSF: Perfluorooctanesulfonyl fluoride.
PCOS: Polycystic ovarian syndrome.