Current implications of obesity in male fertility

1. Introduction

En Mexico obesity is a complex and harmful metabolic condition afflicting children and adults, inducing a variety of pathologies among which is male infertility (Palou and Bonet, 2013). Adverse effects of obesity are also reported in female patients, and the mechanisms that cause them have been clearly explained, unlike male infertility, for which the mechanisms are unclear, due to the contradictory results that several authors have reported (Hall et al., 2010). The World Health Organization (WHO) has provided statistics and predicted that approximately 2.3 billion adults would be in the overweight classification, and 700 million in obesity, during 2015. One cause for obesity is the sedentary lifestyle, which becomes an important risk factor for reproductive disorders.

2. Etiology of obesity

The clearest and most commonly used definition for obesity is the presence of a fat excess in the adipose tissue, caused by an imbalance between the calories consumed and expended by the individual (Cachofeiro, 2006). Like a state of low-grade chronic inflammation (Blancas et al., 2010). However, it is known that its causes may be related to multiple endogenous and exogenous factors, such as genotype or lifestyle, which until now are scarcely known and understood, that, if not further studied, could perpetuate the condition phenotype.

2.1 Metabolic processes involved in obesity

Energy reserves are always necessary for the organism, however, these are well used when different interconnected processes interact, such as the consumed energy control, the nutrients distribution between tissues, anabolic and catabolic processes, adipogenesis and thermogenesis.
In humans, obesity can be characterized either by an increase of adipocytes (hyperplasia), or by an increase in the size of these cells (hypertrophy). Of course, this entails other metabolic complications, as happens in the adipogenic function (Couillard et al., 2000), since the adipose tissue, as an endocrine organ, can secrete some peptides called adipokines. Obesity is particularly related to the presence of molecular markers, such as reactive protein C (CRP), interleukin 6 (IL-6) and tumor necrosis factor (TNF-α), associated with low-grade chronic inflammation and resistance to insulin, which characterizes individuals with obesity. However, it has been reported that only TNF-α causes insulin resistance in the case of rodents, but not in humans (Hotamisligil et al., 1993). Low levels of adiponectin are also an indicator of a weakened metabolism (Fruhbeck, 2004).

2.2 Genetics and obesity

Obesity is genetically regulated, so it is a known fact that there is a variety of obesity inheriting genes, which have a high probability of resulting in the obesity phenotype (Herrera et al., 2011). These genes have been proposed based on certain criteria, both biological and pharmacological, using murine models of obesity, knockout and transgenic (Sandholt et al., 2012). From these studies, at least 127 obesity genes have been suggested, related to the control of energy balance, adipogenesis, adaptive thermogenesis and insulin signaling. All the more, some studies have identified up to 40 places in the gene related to human obesity, between body fat and the gene associated with obesity (OTF), because it is the most replicated, according to statistical analysis (Rankinen et al., 2006). However, despite the advances in the genetics of obesity study, and the identified places in the gene, we know only 2-4% of the total different obesity forms that can be inherited (±40-70%). Therefore, the final expression of genotypes to determine the obese phenotype is related to external factors, such as the environment (Sandholt et al., 2012).

2.3 Food and lifestyle

Another cause of obesity, perhaps the most important one, is the accessibility to, and palatability of, abundant meals with a high fat and/or carbohydrates content. Nowadays in Mexico, it is more and more frequent that some usual lifestyles prevent adequate nourishment, for example: office or domestic work, demanding schedules that involve little physical exercise and prevent any activity that requires a high expenditure of energy (Palou and Bonet, 2013).

There are environmental and psychological factors that can influence an obesity phenotype. The brain plays an important role in the diet: it regulates the consumption, as well as the energy balance –the control over the quantity and quality of the ingested calories, through the satiety signaling pathways, related both to the amount of food and to the energy deposits, through adiposity signals, such as insulin and leptin (Wood et al., 2009).

Still more, regardless of food availability, the social factor is important as well –memories of past experiences, hedonic factors, among others, are also controlled by the brain (Petrovich et al., 2012). That is why one fundamental question in the study of obesity is the deciphering of the cerebral mechanisms dependent on the environmental factor that motivate eating, that is, not physiological mechanisms, but those linked to the regulation of food intake.

Few studies report the relation between the subjects’ obesity and the alterations in the neurological and behavioral mechanisms, for example: the food reward in some situations is similar to what happens with drugs and food addiction (Palou and Bonet, 2013). By favoring neurophysiological responses through these mechanisms, abundant food overconsumption is obtained (other factors can also have an influence, e.g. impact of publicity) (Priego et al., 2010).

3. Mechanisms proposed in obesity and male infertility

3.1 Negative effects on testicles

Spermatogenesis, the formation of male gametes called spermatozoa, is a process carried out in the testicles. It has been defined as a complex, long and very precise process of cell division and differentiation, under the regulation of endocrine signals, with the participation of gonadotrophin-releasing hormone (GnRH), luteinizing hormone (LH), inhibin and the follicle stimulating hormone (FSH) and, through paracrine signals, derived from the interrelation between different types of cells of the seminiferous tubules (Sertoli cells) and the interstitium (Leydig cells) and autocrine signals (Hurtado, 2011) . Throughout this process, testosterone (T) plays a fundamental role.

T is the most important male hormone of steroid origin and plays a critical role in testicular development, spermatogenesis and regular maintenance of masculinization (Zhao, 2014). Inside the seminiferous tubules this hormone reaches concentrations a hundred times higher than circulating T in the blood (Ruwanpura et al., 2010), and is secreted by Leydig cells under the stimulation of LH (McLachlan et al., 2002). It is known that primary or secondary hypogonadism is caused by the low production of T, and that obesity is a direct cause of the changes in hormonal concentrations (De Maddalena et al., 2012), since it has been reported that obesity is associated with low plasma T levels (Tsai et al., 2004).

In order to confirm the relation between obesity and male fertility, seminal parameters have been examined: motility, concentration, viability and sperm morphology; and it has been reported that low levels of T in plasma are related to the reduction of sperm motility (Saboor Aftab et al., 2013), damage to Leydig cells with increased apoptosis (Vigueras et al., 2011) and decrease of the gonads weight (testis-epididymis) in obese mice fed a high-fat diet (Hurtado, 2011).

Other studies suggest that there is a significant negative correlation between the body mass index (BMI) and seminal parameters such as sperm count; it has been reported that individuals with normal range BMI (BMI 20-25 kg/m2) present a sperm count of 102.5 x 106, against a 66 x 106 of individuals with obesity (BMI ≥30 kg/m2); a reduction in the percentage of sperm motility has also been observed, identifying in individuals with BMI in the normal range a percentage of 40.9 ± 1.01% and in obese individuals 38.1 ± 2.06% (Chavarro et al., 2010); regarding morphology, a 6.48 x 106 count of normal spermatozoa is reported in individuals with normal range BMI, and a value of 3.30 x 106 in obese individuals (Martini et al., 2010). These changes may be directly related to the spermatogenesis alteration, precisely due to the modification of the sex hormone levels in obese individuals, since there is a reduction in T levels compared to those of subjects with normal BMI (16.5 ± 0.26 nmol • L-1 to 15.7 ± 0.44 nmol • L-1), as well as a decrease in plasma LH levels in obese subjects (from 4.7 ± 0.24 mU • mL-1 in subjects with normal BMI at 4.4 ± 0.33 mU • mL-1, in individuals with obesity) (Paasch et al., 2010).

A high percentage of seminal alteration has also been reported, showing a high number of spermatozoa with abnormal forms in obese mice (Adham et al., 2001), once again reaffirming that obesity can alter spermatogenesis (Fan et al., 2015), since in addition to the hormonal alteration, there is also a deterioration of the hematotesticular barrier (HTB), which consists of anatomical/physical, physiological and immunological components, necessary for this structure to properly function inside the testicles. The main purpose of this barrier is to create the microenvironment for the proper functioning of spermatogenesis and the development of germ cells (mitosis, meiosis and differentiation) (Mital et al., 2011).

It has been observed at an ultrastructural level that obesity, regarding the tight junctions between Sertoli cells (which forms the HTB), favors a discontinuity between them, indicating that the integrity of the HTB is severely compromised (Fan et al., 2015). The analysis of testicular morphology has shown that the epithelium of seminiferous tubules in obese mice is severely disorganized and atrophic, as well as a disruption of cell adhesions between Sertoli cells and spermatogenic cells (Fan et al., 2015). This is due, quite possibly, to the decrease in plasma testosterone levels that negatively affected the maintenance of HTB, as well as its function and participation in spermatogenesis (Mital et al., 2011).

The relation between adipose tissue and reproduction is due to the participation of androgens (Figure 1). So if the concentration of testosterone is altered, this can cause an alteration of the hypothalamus pituitary gonad axis, oxidative stress and negative effects on testicular steroidogenesis.

Figure 1. WAT participation in the HPG axis regulation. The kisspeptins promote the GnRH release, this hormone will stimulate the release of FSH and LH from the adenohypophysis to exert its effect on the gonad (testis), where it will favor the T secretion that later will be converted to estradiol by the P450 aromatase. Furthermore, FSH is regulated by Inhibin B, which is synthesized by Sertoli cells. In WAT, estradiol and leptin are secreted –both participate in regulating the HPG axis and the secretion of T. Kisspeptins also stimulate the neuropeptide Y release, which participates in the energy balance.

3.2 Negative effect on the epididymis

Currently, the possible negative effects of obesity on the epididymis –results of a histological analysis– have been reported, showing that the lumen and the epithelium of the rat epididymis with obesity have more apoptotic bodies. The epididymis is an important organ, in which the spermatozoa acquire the fertilizing potential, and when affected, compromises the optimal functionality of the male gamete (Cooper, 2007).

Some studies have indicated that in the case of scrotal lipomatosis, increased fat deposits at the subcutaneous level and around the scrotum, a rise in local temperature may favor and generate an environment of oxidative stress in spermatozoa, i.e. an imbalance between free radicals and antioxidant enzymes of the spermatozoid is generated (Kasturi et al., 2008).

An example of this was provided by Vigueras et al. (2011), determining lipoperoxidation through the increase of malondialdehyde (MDA, product of lipid peroxidation), in obese rats, 4.74 ± 0.63 vs 2.38 ± 0.22 MDA, in control subjects. It is important to remember that lipoperoxidation is a result of the oxidative stress present in the epididymis –in addition to observing apoptosis.

At the end of the study, it was found that in an obese male rat the morphology of the head and tail of the epididymis is apparently unaffected. The immunopositivity to androgen receptors in the nucleus of most cells and in some basal cells was observed both in obese rats and in control subjects, which means that obesity does not affect the androgen receptors nor the sperm located in the tubular lumen. However, apoptotic bodies were observed in the lumen of the head of obese rats epididymis –in several sections of the epithelium of the head of the epididymis and in the tubular lumen of obese rats (Vigueras et al., 2011).

However, until now no studies have been reported to elaborate on the consequences of obesity in the process in which spermatozoa acquire the ability to fertilize the oocyte as they move through the epididymis, epididymal sperm maturation. This is why part of these investigations has begun in our research group, determining the post-translational modifications of the spermatozoa, which occur as part of the maturation process, among which are: the glycoconjugates in the spermatozoa membrane, and the protein phosphorylation; as well as techniques to determine if the increase in scrotal temperature favors a process of oxidative stress in the epididymal spermatozoa (Vigueras et al., 2011).

Many mechanisms are involved in epididymal sperm maturation, and many of them (lipid composition changes, acrosome restructuring, total negative charge increase, modification of surface proteins, reorganization in membrane proteins, acquisition of motility and fertilizing potential) may be affected by obesity, so this is only a small part of what remains to be elucidated.

4. Obesity and inflammation

It has been reported that one of the causes of low-grade chronic inflammation, such as obesity, is the excessive reserve of adipose tissue (Gomez et al., 2005). It is important to remember that although this type of inflammation does not respond to the characteristics of a classic inflammatory response, some markers are present in obesity (Catalan et al., 2007). One of them is the concentration of adipokines, which can be altered, triggering pathologies like type 2 diabetes (Calle and Kaaks, 2004).

The proinflammatories are markers too (interleukins 1α, 2β, 6, 8 and tumor necrosis factor [TNF α,β], and  interferon γ), that will recruit the immune system to try to counteract the inflammation (Yang and Ming, 2011). However, the infiltration of immune system cells, such as macrophages, forming specialized structures that surround dead adipocytes, could affect the optimal functioning of the adipose tissue, as an endocrine organ that regulates metabolism and reproduction (Yang and Ming, 2011).

Therefore, it is very important to address the inflammatory response associated with obesity and some therapeutic targets, as the thermogenesis process. It is necessary to remember that every organism tries to maintain a dynamic equilibrium, known as homeostasis. When there is a stressor, in this case an excessive energy gain that causes inflammation, the body will seek to recover its homeostasis, and one of the known ways is the immune response to counteract this inflammatory process. On the other hand, thermogenesis, as the capacity of an organism to produce heat, is an important component for energy disbursement. This process is modulated by three main factors: environmental temperature, quantity and quality of nutrients, and systemic inflammation.

4.1 Thermogenesis

The brown adipose tissue (BAT) has the capacity to carry out the process of adaptive thermogenesis, which is the production of regulated heat, catabolically mediated by energy substrates, and the release of chemical energy, from the breakdown of adenosine triphosphate (ATP). This energy breakdown is possible because of an uncoupling protein 1 (UCP-1) present in the inner mitochondrial membrane of brown adipocytes. This protein, as its name implies, is responsible for decoupling the production of adenosine triphosphate from the catabolic pathways of lipids and carbohydrates (Long et al., 2014). This protein is only expressed in BAT, so an increase in BAT may favor energy dissipation in obese patients (Montanari et al., 2017).

The process of thermogenesis is controlled by the sympathetic nervous system (SNS), depending on adrenergic receptors, which regulate the effects of norepinephrine and catecholamines. The three essential environmental factors on which this process relies are the environmental temperature (such as cold, that can induce thermogenesis), the quantity and the quality of nutrients (diet can induce thermogenesis, and systemic inflammation in response to tissue damage and infection, such as fever) (Virtaken et al., 2009).

An increase in norepinephrine in the heart, and white adipose tissue (WAT) and BAT, has already been reported in response to thermogenic stimuli caused by diet and cold. Also, the binding of catecholamines to  adrenergic receptors of brown adipose tissue that activate the thermogenic program, including the induction of lipolysis and the activity of (UCP-1) and gene expression (Montanari et al., 2017).

The signaling of adrenergic receptors in WAT plays an important role supporting the process of thermogenesis and fatty acids supplying, as a substrate for lipolysis, being transported to BAT by an increase in blood flow (Cannon and Nedergaard, 2004). The energy derived is released by BAT in the form of heat dissipated by the body thanks to its vascularization, in conjunction with some endogenous factors (Montanari et al., 2017).

This is why, in individuals with obesity, the conversion of energy from WAT to BAT, through the endogenous factors participating in BAT can be a therapeutic target to solve this condition generated by the imbalance of energy (Figure 2), and thus, to avoid obesity becoming a factor in pathologies such as type 2 diabetes, metabolic syndrome and male and female infertility.

Figure 2. Obesity effect in the white and brown adipose tissue. An increase in visceral adiposity, caused by the number and size of WAT increase, will alter thermogenesis, decreasing that which takes place in BAT due to the loss of activity leaded by the SNS, and favoured by the leptin increases and the adiponectin reduction, causing a decrease in energy gain.

5. Associated metabolic syndrome to male infertility

Another health problem caused by the imbalance of energy is the metabolic syndrome (MS), which encompasses a set of disorders, including obesity, dyslipidemia, hypertension, diabetes and vascular diseases. In addition, it causes men hypogonadism, erectile dysfunction and psychological imbalance (Michalakis et al., 2013).

As part of the relationship between obesity, MS and infertility, it has been described that hyperinsulinemia and hyperglycemia are shared traits in obese individuals and in rodents used in studies on the subject (Kasturi et al., 2008). These two conditions have been shown to have an inhibitory effect on the quantity and quality of sperm, noticed in the low fertility seen in obese men (Lotti et al., 2013).

Adverse effects on male fertility would occur through three mechanisms:

a) the peripheral conversion of T to estrogen in adipose tissue, which can cause secondary hypogonadism through the hypothalamic-pituitary-gonadal (HPG) axis; b) oxidative stress at the level of the testicular microenvironment, which can result in a decrease in spermatogenesis and sperm damage; c) suprapubic accumulation of adipose tissue and internal fat of the thigh can lead to an increase in scrotal temperature (35 ° C) in men with severe obesity (Filippi et al., 2009).

The pulses of the GnRH and the normal operation of the HPG axis depend on the normal energy balance, and in case of an energy imbalance, damage to the reproductive axis can occur. For example, when hyperinsulinemia occurs, it affects the liver and is considered the major determinant for the decrease in the concentrations of the sex hormone-binding globulin (SHBG). When there is a reduction in T concentrations, binding with SHBG is not possible, due to the hyperestrogenemia that lowers the pulses of the LH (Michalakis et al., 2013).

Thus, the relationship between a body mass index >30 kg / m2 and the hormone SHBG has been strengthened by what was observed in men who lost weight: that SHBG increases its binding capacity and plasma levels return to their normal range (Michalakis et al., 2013). In most studies, obese individuals have a decrease in T.

In addition, the metabolic syndrome has been associated with thermo-oxidative changes, due to the accumulation of adipose tissue in the suprapubic area or abdominal region and in the thigh, causing an elevated scrotal temperature and affecting spermatogenesis (Michalakis et al., 2013).

Therefore, it is considered that this accumulation of adipose tissue is related to the increase of oxidative stress and lipoperoxidation. Reactive oxygen species (ROS), by causing lipoperoxidation, become toxic compounds for the human spermatozoon, positioning oxidative stress as a cause of male infertility (Kasturi et al., 2008).

6. Erectile dysfunction as a result of obesity

Erectile dysfunction (ED), a man’s inability to achieve or maintain a firm erection for sexual intercourse, is one of the main causes for the decrease in the seminal quality in an individual’s life, and it affects approximately 30 million men (Traish et al., 2009). One study reports that severe or moderate erectile dysfunction is present in 12% of men under 59 years of age; 22% in men from 60 to 69 years old, and 30% in men older than 69 years. However, men with a BMI ˃29 have this problem, or have a 30% risk of suffering it (Gunduz et al., 2004).

Obesity has been directly related to endothelial dysfunction and to the increase in serum concentrations of markers of vascular inflammation, and both ED and endothelial dysfunction share some metabolic and vascular pathways. Obese men with ED are evidence of an endothelial dysfunction, the indicators of this are elevated serum concentrations of inflammatory markers such as IL-6 and IL-8, reactive protein C (PRC). When performing a study in 110 obese men, a relation between the index that determines ED and the endothelial dysfunction indexes was observed, supporting the presence of common vascular pathways (Somani et al., 2010).

Therefore, it is likely that hypogonadism and increased cardiovascular risk, associated with obesity, may justify the high prevalence of ED in overweight and obese individuals (Mah and Wittert, 2010).

A control measure has been proposed, based on the behaviors shown by both men and animals when receiving pleasure, or when evading that which causes some nonconformity. This measure of control is that if a pleasant sexual life is ensured for obese individuals, it would encourage an attachment to the diet and the promotion of exercise. A sexual dissatisfaction, due to impotence, embodies a motivation to consult with specialists, representing an opportunity to treat obese patients with ED (Bacon et al., 2003).

It is important to point out that obesity is a multifactorial condition and even the psychological factor can play an important role. Weight loss along with changes in lifestyle can reduce ED in obese men. Such is the example of the study carried out in Massachusetts where overweight men who present a high risk of developing ED, when losing weight, reduce the risk of developing ED. Similarly, men who perform some physical activity in the middle of their lives have a 70% chance of not having ED compared to those who do not (Isidori et al., 2014).

Abbreviations

ATP: adenosine triphosphate.
HTB: hematotesticular barrier.
UCP-1: uncoupling protein 1.
RPC: reactive protein C.
ED: erectile dysfunction.
ROS: reactive oxygen species.
FSH: follicle stimulating hormone.
GnRH: gonadotrophin-releasing hormone.
HPG: hypothalamus pituitary gonad.
BMI: body mass index.
IL6: interleukin 6.
IL8: interleukin 8.
LH: luteinizing hormone.
MDA: malondialdehyde.
WHO: World Health Organization.
TNF-α: tumor necrosis factor alpha.
OTF: a gene associated with obesity.
SHBG: sex hormone binding globulin.
MS: metabolic syndrome.
PNS: parasympathetic nervous system.
T: testosterone.
WAT: white adipose tissue.
BAT: brown adipose tissue.