ANDROLOGY
ISSN: 2047-2919
REVIEW ARTICLE
Correspondence: AliReza Alizadeh, Department of Embryology at Reproductive Biomedicine Research Center, Royan Institute for Reproductive Biomedicine, ACECR, Tehran, Iran. E-mail: [email protected]
Keywords: dietary fatty acids, male fertility, sperm fatty acid profiles
Dietary fatty acids affect semen quality: a review 1
V. Esmaeili, 1A. H. Shahverdi, 2M. H. Moghadasian and 1,3A. R. Alizadeh
1 Department of Embryology at Reproductive Biomedicine Research Center, Royan Institute for Reproductive Biomedicine, ACECR, Tehran, Iran, 2Department of Human Nutritional Sciences, St Boniface Hospital Research Centre, University of Manitoba and Canadian Centre for Agri-food Research in Medicine, Winnipeg, MB, Canada, and 3Department of Animal Science, Saveh Branch, Islamic Azad University, Saveh, Iran
Received: 19-Nov-2014 Revised: 12-Jan-2015 Accepted: 11-Feb-2015 doi: 10.1111/andr.12024
SUMMARY Mammalian spermatozoa are characterized by a high proportion of polyunsaturated fatty acids (PUFA) which play a crucial role in fertilization. This review focuses on analysis of sperm fatty acid profiles and the effects of omega-3, saturated and trans dietary and sperm fatty acids on sperm parameters. Two major points have been pivotal points of investigation in the field of sperm fatty acid profiles: first, the comparison between fatty acid profiles of fertile and infertile men and second, the effect of dietary fatty acids on sperm fatty acid profiles as well as sperm quality and quantity. Docosahexaenoic acid (DHA, C22:6n-3), and palmitic acid (C16:0) are the predominant PUFA and saturated fatty acids, respectively, in human sperm cells. Higher levels of DHA are concentrated on the sperm’s head or tail varying among different species. However, the human sperm head contains a higher concentration of DHA. Dietary fatty acids influence on sperm fatty acid profiles and it seems that sperm fatty acid profiles are most sensitive to dietary omega-3 PUFA. Although improvements in sperm parameters are a response to omega-3 sources after more than 4 weeks of supplementation in the male diet, time-dependent and dose-dependent responses may explain the failure in some experiments. In human spermatozoa, elevated saturated or trans fatty acid concentration and a low DHA level is a concern. The regulations of the sperm fatty acid mean melting point as well as expression regulation of peroxisome proliferator-activated receptor gamma (PPARG) alongside with spermatozoon assembly, anti-apoptosis effects, eicosanoid formation, and hormone activity are the putative key factors that induce a response by inclusion of omega-3 PUFA.
INTRODUCTION Almost 50% of infertility cases are attributed to male factors and numerous genetic and non-genetic ambient conditions which contribute to male infertility (Oliva et al., 2001; Shah et al., 2003). For example, translocation of autosomal chromosomes, numeric and structural abnormalities of the sex chromosomes, and mutations of the cystic fibrosis genes are among common genetic factors (Oliva et al., 2001). Non-genetic factors may include dietary behavior, professional circumstances, and environmental toxins (Mendiola et al., 2008). Andrological diseases such as varicocoele (Nasr-Esfahani et al., 2009), male accessory gland infection (Krause, 2008), and immunological causes (Hinting et al., 1996) negatively affect male fertility in humans as well. Other factors may include the excessive use of tobacco (Said et al., 2005), alcohol (Dunphy et al., 1991), narcotic drugs (Fronczak et al., 2012), regular exposure to high © 2015 American Society of Andrology and European Academy of Andrology
temperatures as in a sauna (Jung & Schuppe, 2007), psychological stress (Bhongade et al., 2014), and particularly nutritional habits. These are the factors that research studies over the previous decades suggested as important contributors to fertility rates in men (Rato et al., 2014). Although the relation between body mass index and male fertility is controversial issue (Teerds et al., 2011; Hammiche et al., 2012), infertility in overweight/obese males may be explained by leptin insensitivity. Among nutrients, supplemented carbohydrates and proteins do not have a remarkable effect on improved male fertility (Eslamian et al., 2012). More recently, it was suggested that the overconsumption of high-energy diets alter the functioning of the male reproductive axis and consequently affects the testicular physiology, disrupting its metabolism and bioenergetics capacity (Rato et al., 2014). On the other hand, some dietary fats (Safarinejad, 2011; Esmaeili et al., 2014), micro-minerals Andrology, 1–12
1
ANDROLOGY
V. Esmaeili et al.
(Safaralizadeh et al., 2005; Prasad, 2013), and vitamins (Vujkovic et al., 2009) may have a role in maintaining and improving semen quality in humans and animals. The bulk of studies have shed light on the pivotal role of fatty acids especially polyunsaturated fatty acids (PUFA) in sperm biology. However, effects, both beneficial and detrimental, of dietary fatty acids are the current focus of research in the field of nutrition and reproduction in males (Wathes et al., 2007). The aim of the present review is to focus on analysis of fatty acid profiles in fertile and infertile men and the effects of dietary omega-3 fatty acids as well as saturated and trans sperm fatty acids on male fertility.
DIETARY LIPIDS AND FATTY ACIDS Lipids comprise a wide-range class of molecules that play a crucial role in the structure and function of cells in mammals. They serve as storage compounds, cellular metabolism, signaling molecules, and various membrane-related functions such as trafficking, regulation of proteins, and creating membrane subcompartments. The main lipids found in animal and plant cells are phospholipids, sterols, and triglycerides (Shevchenko & Simons, 2010; Dunning et al., 2014). Fatty acids are chains of carbons with a methyl group (CH3) at one end and a carboxyl group (COOH) at the other one. The carbon chain may be saturated or it may contain one or more double bonds as in mono- and PUFA, respectively. PUFA can serve as the precursors of eicosanoids. Eicosanoids play a role in male fertility as they contribute to sperm structure (Gill & Valivety, 1997). In PUFA the number, position, and cis/trans state of the double bond have dramatic effects on cell function as well as membrane fluidity. The first carbon of the methyl group is called omega in the omega system; based on the distance of other carbons from omega, they are called omega-3 (n-3), omega-6 (n-6), and omega-9 (n-9) (Fig. 1). Although dietary fatty acids are found in triglyceride form, fatty acids are phospholipids in the cell membrane (Gill & Valivety, 1997; Mazza et al., 2007). Researchers have shown that PUFAs maintain the integrity of the cellular membrane’s lipid bilayer. Plants and animals may provide sources of several fatty acids; the highest concentration of omega-9 fatty acids such as oleic acid (C18:1 n-9) has been measured in olive and canola oils. Omega-6 fatty acids (linoleic acid: C18:2 n-6) is the major fatty acid in safflower seed, sunflower seed, cottonseed as well as soybean oils (Dubois et al., 2007). There are three major dietary n-3
Figure 1 Structure of a number of fatty acids found in dietary lipids (Rustan & Drevon, 2001).
2
Andrology, 1–12
fatty acids: a-linolenic acid (C18:3 n-3) from linseed oil, eicosapentaenoic acid (EPA; C20:5 n-3), and docosahexaenoic acid (DHA; C22:6 n-3) from fish oil. Linoleic acid (C18:2 n-6) and linolenic acid (C18:3 n-3) as PUFAs, are essential fatty acids as they are not synthesized in humans and livestock.
PUFAs METABOLISM IN TESTES Human and animals cannot synthesize some n-6 or n-3 fatty acids de novo due to a lack of appropriate fatty acid desaturase enzymes. Therefore, linoleic acid (C18:2 n-6) and linolenic acid (C18:3 n-3) need to be provided in the diet as these PUFAs are essential for numerous processes including general growth and development of the brain, vision, and reproductive system (Kochhar, 2002). In mammalian cells, dietary linoleic acid and linolenic acid are converted into important fatty acids C20:4n-6, C20:5n-3, C22:5n-6, and C22:6n-3 by alternating steps of elongation and desaturation (Fig. 2) (Rustan & Drevon, 2001). Fatty acids accumulate in testicular cells through two distinct processes: passive diffusion through the lipid bilayer and/or protein-facilitated transport mediated by CD36 glycoprotein, which is widely expressed in Sertoli cells. Lipids are pivotal and function as ‘fuel’ for Sertoli cells, and are also used in membrane remodeling of developing germ cells (Rato et al., 2014). Neurons, photoreceptor cells, and spermatozoa are three cell types that display high DHA content. DHA content in these cells is associated with the supporting role of strocytes, retinal pigment epithelial cells, and Sertoli cells, respectively (Saether et al., 2007). Similar to the liver, the testis is an extraordinary organ in terms of PUFA metabolism. In contrast to other PUFA-rich tissues such as the brain and retina, the testis is continuously drained of these fatty acids, as the spermatozoa are transported to the epididymis. Testicular cells and spermatozoa contain high amounts of 20 and 22 carbon n-3 and n-6 PUFAs. There is a noticeable difference between species in composition of testicular unsaturated fatty acids (Saether et al., 2007). The capacities for desaturation and elongation of unsaturated fatty acids in the testis are high. It appears that human and rat testicular cells are more active in the conversion of 18 and 20 carbon n-3 PUFA into 22 carbon n-3 PUFA than in the conversion of the corresponding 18 and 20 carbon n-6 into 22 carbon n-6 PUFA (Retterstøl et al., 2001). Findings of the previous
Figure 2 Synthesis of n-3 and n-6 polyunsaturated fatty acids (Rustan & Drevon, 2001).
© 2015 American Society of Andrology and European Academy of Andrology
DIETARY FATTY ACIDS AND SPERM FATTY ACIDS
studies suggest that high capacities for the metabolism of unsaturated fatty acids and the specialization of testicular enzymes for incorporation of PUFA into spermatozoa may regulate the patterns of fatty acid composition in the sperm phospholipids (Retterstol et al., 2001; Saether et al., 2007). Research studies have also suggested that compared with germ cells, Sertoli cells be more active in the metabolism of 18–22 carbon PUFAs. However, higher levels of 22 carbon PUFAs have been found in germ cells in comparison to Sertoli cells (Retterstøl et al., 2001). Studies on laboratory animal testes revaluated high levels desaturase mRNA and elongase enzyme (Cho et al., 1999; Leonard et al., 2000). Several hormones such as luteinizing hormone (LH) and adrenocorticotropin hormone (ACTH) possibly change unsaturated fatty acid composition in the testis by changing activities of the enzymes (Hurtado de Catalfo & de Gomez Dumm, 2002). In reaction to LH-stimulation, together with an increased testosterone secretion, the stored lipid is quickly vanished. By prompting the expression of steroidogenesis involved genes, both production and testosterone are bolstered by LH (Aoki & Massa, 1975). The D5-desaturase activity in the testicular cells was prone to change by administering ACTH to the mature normal rats. The total fatty acid composition of the Sertoli cells isolated from ACTH-treated rats showed a significant increase in the relative percentage of 18:2n-6 and a decrease in 20- and 22-carbon PUFA biosynthesis. The data suggested that ACTH exert an inhibitory effect on D5- and D6desaturase (Hurtado de Catalfo et al., 1992). The testes can metabolize some fatty acids and this process may be a part of the scenario of keeping the proper fluidity and producing functional sperm.
SPERM FATTY ACID PROFILES IN SEVERAL SPECIES As early as 1897 it was shown that spermatozoa contained a considerable amount of intracellular lipids which acted as energy sources (Ahluwalia & Holman, 1969; Safarinejad et al., 2010). In 1951, gas chromatography (GC) was developed as a reliable method for analysis of fatty acid profiles. Introduction of this method expedited the research studies on structure, chemistry, and biochemistry of lipids and led to rapid advancements in knowledge in these fields (Hammond, 2002). Therefore, accurate sperm fatty acid profile analysis prior to the 1950s was not possible. Improvements in GC and subsequent techniques such as GC mass, high-performance liquid chromatography (HPLC), and thin layer chromatography caused further development of the capability for fatty acid analyses in biological systems. Although some studies emphasized the pivotal roles of PUFAs concentration in sperm samples, the sperm fatty acid profiles have not yet been analyzed and monitored comprehensively by measuring the entire set of saturated and unsaturated fatty acids (Chavarro et al., 2014; Esmaeili et al., 2014). The structural integrity of the spermatozoa cell membrane plays a pivotal role in successful fertilization. This is because both the acrosome reaction and sperm–oocyte fusion are associated with the membrane’s fatty acid profile (De Vriese & Christophe, 2003). Early attempts to analyze sperm lipids date back to the pioneering studies of mammalian and non-mammalian sperm biochemistry and have shown the presence of neutral fatty acids, cholesterol, phospholipids (particularly lecithin, cephalin, and sphingomyelin), and glycolipids. The flurry of © 2015 American Society of Andrology and European Academy of Andrology
ANDROLOGY studies that followed revealed that phospholipids were the most representative lipid fraction of the sperm cell membranes and phosphatidylcholine and phosphatidylethanolamine which were the major contributors (Mann & Lutwak-Mann, 1982). Some omega-3 and omega-6 fatty acids such as DHA (C22:6n-3) for humans (Safarinejad, 2011) and ruminants (Esmaeili et al., 2014; Fair et al., 2014), docosapantaenoic acid (DPA; C22:5n-6) for boars, rodents, and rabbits and docosatetraenoic acid (C22:4n-6) for domestic birds were recognized as major elements in spermatozoa phospholipids (Alizadeh et al., 2014). The establishment of a negative correlation between increased saturated fatty acids (De Vriese & Christophe, 2003) or trans fatty acids (Chavarro et al., 2014) and normal sperm parameters revealed the significance of comprehensively reporting fatty acid profiles in human. The negative correlation between decreased spermatozoa total lipid, increased saturated fatty acid content, and sperm parameters was previously reported in infertile boars (Am-in et al., 2011). Therefore, fatty acids which are involved in the profiles could influence sperm parameter, and some indices such as the desaturase index (DI) could be used for comparison. Fatty acid profiles in head and tail of spermatozoa Whereas the tail is mostly associated with sperm movement, the head is related with acrosome reaction and membrane fusion (Argov-Argaman et al., 2013a,b). In monkeys, Connor et al. have shown that approximately 99% of total sperm DHA belongs to the tail. Their findings suggested that DHA density in sperm tail be directly related to fluidity and flexibility (Connor et al., 1998). By contrast, some studies in humans and bulls measured higher concentrations of PUFA, total n-3 fatty acid (especially DHA) in the head compared with the tail (Zalata et al., 1998; Argov-Argaman et al., 2013a,b). This characteristics is critical for acrosome biogenesis. Roqueta-Rivera et al (2011) reported that when mice consumed an omega-3 fatty acid-deficient diet there was no acrosomal reaction. Therefore, DHA deficiency might stop the second stage of spermatogenesis, even in species whose spermatozoa contain high levels of n-6 fatty acids. In addition, the percentage of DHA in sperm membrane phospholipids was higher than that of DHA in other cells. Hence, PUFA metabolism was more active in the testes during spermatogenesis and epididymal sperm maturation than these of PUFA metabolism of other cells (Moore, 1998). Surprisingly, Zalata et al. have suggested that DHA in human spermatozoa may have specific functions unrelated to fluidity, which is similar to the functions of DHA in the brain and retina. More recently, it has been suggested that lipid concentrations may affect semen parameters, and this effect is more pronounced in sperm head morphology (Schisterman et al., 2014). Altogether, dietary omega-3 fatty acids may improve sperm function by manipulation of head and tail fatty acid profiles.
SPERM FATTY ACID PROFILES OF FERTILE AND INFERTILE MEN The lipid composition of the sperm membrane has a significant effect upon the functional characteristics of spermatozoa. Chavarro et al. (2011) reported that higher trans-fatty acid levels in spermatozoa were associated with lower sperm concentration. Similarly, there were lower levels of sperm DHA reported in oligoasthenozoospermic compared with normospermic men. Andrology, 1–12
3
ANDROLOGY
V. Esmaeili et al.
Aksoy et al. (2006) reported higher omega-6:omega-3 ratios in spermatozoa of infertile vs. fertile men, respectively. For many years, it has been known that PUFAs are the main substrate of lipid peroxidation, changes in sperm lipid composition cause alterations in sperm functional characteristics (Gulaya et al., 2001). It is still a controversial issue whether male infertility is a result of the damage induced by reactive oxygen species (ROS) generated by spermatozoa endogenously, or alternatively a lower concentration of some PUFAs. However, inadequate DHA concentration is the main cause of low-quality spermatozoa (Aksoy et al., 2006; Safarinejad et al., 2010). Mean melting point (MMP) is proposed to be an index of fluidity. MMP is determined on the basis of upon the whole set of fatty acids (Holman et al., 1991). All saturated and unsaturated fatty acids not only have the same contribution in MMP, but it also appears that minute highly influential concentrations of some fatty acids play effective roles on regulation of MMP. In some biological compounds such as milk fat the crucial role of these minute fatty acids such as
ISSN: 2047-2919
REVIEW ARTICLE
Correspondence: AliReza Alizadeh, Department of Embryology at Reproductive Biomedicine Research Center, Royan Institute for Reproductive Biomedicine, ACECR, Tehran, Iran. E-mail: [email protected]
Keywords: dietary fatty acids, male fertility, sperm fatty acid profiles
Dietary fatty acids affect semen quality: a review 1
V. Esmaeili, 1A. H. Shahverdi, 2M. H. Moghadasian and 1,3A. R. Alizadeh
1 Department of Embryology at Reproductive Biomedicine Research Center, Royan Institute for Reproductive Biomedicine, ACECR, Tehran, Iran, 2Department of Human Nutritional Sciences, St Boniface Hospital Research Centre, University of Manitoba and Canadian Centre for Agri-food Research in Medicine, Winnipeg, MB, Canada, and 3Department of Animal Science, Saveh Branch, Islamic Azad University, Saveh, Iran
Received: 19-Nov-2014 Revised: 12-Jan-2015 Accepted: 11-Feb-2015 doi: 10.1111/andr.12024
SUMMARY Mammalian spermatozoa are characterized by a high proportion of polyunsaturated fatty acids (PUFA) which play a crucial role in fertilization. This review focuses on analysis of sperm fatty acid profiles and the effects of omega-3, saturated and trans dietary and sperm fatty acids on sperm parameters. Two major points have been pivotal points of investigation in the field of sperm fatty acid profiles: first, the comparison between fatty acid profiles of fertile and infertile men and second, the effect of dietary fatty acids on sperm fatty acid profiles as well as sperm quality and quantity. Docosahexaenoic acid (DHA, C22:6n-3), and palmitic acid (C16:0) are the predominant PUFA and saturated fatty acids, respectively, in human sperm cells. Higher levels of DHA are concentrated on the sperm’s head or tail varying among different species. However, the human sperm head contains a higher concentration of DHA. Dietary fatty acids influence on sperm fatty acid profiles and it seems that sperm fatty acid profiles are most sensitive to dietary omega-3 PUFA. Although improvements in sperm parameters are a response to omega-3 sources after more than 4 weeks of supplementation in the male diet, time-dependent and dose-dependent responses may explain the failure in some experiments. In human spermatozoa, elevated saturated or trans fatty acid concentration and a low DHA level is a concern. The regulations of the sperm fatty acid mean melting point as well as expression regulation of peroxisome proliferator-activated receptor gamma (PPARG) alongside with spermatozoon assembly, anti-apoptosis effects, eicosanoid formation, and hormone activity are the putative key factors that induce a response by inclusion of omega-3 PUFA.
INTRODUCTION Almost 50% of infertility cases are attributed to male factors and numerous genetic and non-genetic ambient conditions which contribute to male infertility (Oliva et al., 2001; Shah et al., 2003). For example, translocation of autosomal chromosomes, numeric and structural abnormalities of the sex chromosomes, and mutations of the cystic fibrosis genes are among common genetic factors (Oliva et al., 2001). Non-genetic factors may include dietary behavior, professional circumstances, and environmental toxins (Mendiola et al., 2008). Andrological diseases such as varicocoele (Nasr-Esfahani et al., 2009), male accessory gland infection (Krause, 2008), and immunological causes (Hinting et al., 1996) negatively affect male fertility in humans as well. Other factors may include the excessive use of tobacco (Said et al., 2005), alcohol (Dunphy et al., 1991), narcotic drugs (Fronczak et al., 2012), regular exposure to high © 2015 American Society of Andrology and European Academy of Andrology
temperatures as in a sauna (Jung & Schuppe, 2007), psychological stress (Bhongade et al., 2014), and particularly nutritional habits. These are the factors that research studies over the previous decades suggested as important contributors to fertility rates in men (Rato et al., 2014). Although the relation between body mass index and male fertility is controversial issue (Teerds et al., 2011; Hammiche et al., 2012), infertility in overweight/obese males may be explained by leptin insensitivity. Among nutrients, supplemented carbohydrates and proteins do not have a remarkable effect on improved male fertility (Eslamian et al., 2012). More recently, it was suggested that the overconsumption of high-energy diets alter the functioning of the male reproductive axis and consequently affects the testicular physiology, disrupting its metabolism and bioenergetics capacity (Rato et al., 2014). On the other hand, some dietary fats (Safarinejad, 2011; Esmaeili et al., 2014), micro-minerals Andrology, 1–12
1
ANDROLOGY
V. Esmaeili et al.
(Safaralizadeh et al., 2005; Prasad, 2013), and vitamins (Vujkovic et al., 2009) may have a role in maintaining and improving semen quality in humans and animals. The bulk of studies have shed light on the pivotal role of fatty acids especially polyunsaturated fatty acids (PUFA) in sperm biology. However, effects, both beneficial and detrimental, of dietary fatty acids are the current focus of research in the field of nutrition and reproduction in males (Wathes et al., 2007). The aim of the present review is to focus on analysis of fatty acid profiles in fertile and infertile men and the effects of dietary omega-3 fatty acids as well as saturated and trans sperm fatty acids on male fertility.
DIETARY LIPIDS AND FATTY ACIDS Lipids comprise a wide-range class of molecules that play a crucial role in the structure and function of cells in mammals. They serve as storage compounds, cellular metabolism, signaling molecules, and various membrane-related functions such as trafficking, regulation of proteins, and creating membrane subcompartments. The main lipids found in animal and plant cells are phospholipids, sterols, and triglycerides (Shevchenko & Simons, 2010; Dunning et al., 2014). Fatty acids are chains of carbons with a methyl group (CH3) at one end and a carboxyl group (COOH) at the other one. The carbon chain may be saturated or it may contain one or more double bonds as in mono- and PUFA, respectively. PUFA can serve as the precursors of eicosanoids. Eicosanoids play a role in male fertility as they contribute to sperm structure (Gill & Valivety, 1997). In PUFA the number, position, and cis/trans state of the double bond have dramatic effects on cell function as well as membrane fluidity. The first carbon of the methyl group is called omega in the omega system; based on the distance of other carbons from omega, they are called omega-3 (n-3), omega-6 (n-6), and omega-9 (n-9) (Fig. 1). Although dietary fatty acids are found in triglyceride form, fatty acids are phospholipids in the cell membrane (Gill & Valivety, 1997; Mazza et al., 2007). Researchers have shown that PUFAs maintain the integrity of the cellular membrane’s lipid bilayer. Plants and animals may provide sources of several fatty acids; the highest concentration of omega-9 fatty acids such as oleic acid (C18:1 n-9) has been measured in olive and canola oils. Omega-6 fatty acids (linoleic acid: C18:2 n-6) is the major fatty acid in safflower seed, sunflower seed, cottonseed as well as soybean oils (Dubois et al., 2007). There are three major dietary n-3
Figure 1 Structure of a number of fatty acids found in dietary lipids (Rustan & Drevon, 2001).
2
Andrology, 1–12
fatty acids: a-linolenic acid (C18:3 n-3) from linseed oil, eicosapentaenoic acid (EPA; C20:5 n-3), and docosahexaenoic acid (DHA; C22:6 n-3) from fish oil. Linoleic acid (C18:2 n-6) and linolenic acid (C18:3 n-3) as PUFAs, are essential fatty acids as they are not synthesized in humans and livestock.
PUFAs METABOLISM IN TESTES Human and animals cannot synthesize some n-6 or n-3 fatty acids de novo due to a lack of appropriate fatty acid desaturase enzymes. Therefore, linoleic acid (C18:2 n-6) and linolenic acid (C18:3 n-3) need to be provided in the diet as these PUFAs are essential for numerous processes including general growth and development of the brain, vision, and reproductive system (Kochhar, 2002). In mammalian cells, dietary linoleic acid and linolenic acid are converted into important fatty acids C20:4n-6, C20:5n-3, C22:5n-6, and C22:6n-3 by alternating steps of elongation and desaturation (Fig. 2) (Rustan & Drevon, 2001). Fatty acids accumulate in testicular cells through two distinct processes: passive diffusion through the lipid bilayer and/or protein-facilitated transport mediated by CD36 glycoprotein, which is widely expressed in Sertoli cells. Lipids are pivotal and function as ‘fuel’ for Sertoli cells, and are also used in membrane remodeling of developing germ cells (Rato et al., 2014). Neurons, photoreceptor cells, and spermatozoa are three cell types that display high DHA content. DHA content in these cells is associated with the supporting role of strocytes, retinal pigment epithelial cells, and Sertoli cells, respectively (Saether et al., 2007). Similar to the liver, the testis is an extraordinary organ in terms of PUFA metabolism. In contrast to other PUFA-rich tissues such as the brain and retina, the testis is continuously drained of these fatty acids, as the spermatozoa are transported to the epididymis. Testicular cells and spermatozoa contain high amounts of 20 and 22 carbon n-3 and n-6 PUFAs. There is a noticeable difference between species in composition of testicular unsaturated fatty acids (Saether et al., 2007). The capacities for desaturation and elongation of unsaturated fatty acids in the testis are high. It appears that human and rat testicular cells are more active in the conversion of 18 and 20 carbon n-3 PUFA into 22 carbon n-3 PUFA than in the conversion of the corresponding 18 and 20 carbon n-6 into 22 carbon n-6 PUFA (Retterstøl et al., 2001). Findings of the previous
Figure 2 Synthesis of n-3 and n-6 polyunsaturated fatty acids (Rustan & Drevon, 2001).
© 2015 American Society of Andrology and European Academy of Andrology
DIETARY FATTY ACIDS AND SPERM FATTY ACIDS
studies suggest that high capacities for the metabolism of unsaturated fatty acids and the specialization of testicular enzymes for incorporation of PUFA into spermatozoa may regulate the patterns of fatty acid composition in the sperm phospholipids (Retterstol et al., 2001; Saether et al., 2007). Research studies have also suggested that compared with germ cells, Sertoli cells be more active in the metabolism of 18–22 carbon PUFAs. However, higher levels of 22 carbon PUFAs have been found in germ cells in comparison to Sertoli cells (Retterstøl et al., 2001). Studies on laboratory animal testes revaluated high levels desaturase mRNA and elongase enzyme (Cho et al., 1999; Leonard et al., 2000). Several hormones such as luteinizing hormone (LH) and adrenocorticotropin hormone (ACTH) possibly change unsaturated fatty acid composition in the testis by changing activities of the enzymes (Hurtado de Catalfo & de Gomez Dumm, 2002). In reaction to LH-stimulation, together with an increased testosterone secretion, the stored lipid is quickly vanished. By prompting the expression of steroidogenesis involved genes, both production and testosterone are bolstered by LH (Aoki & Massa, 1975). The D5-desaturase activity in the testicular cells was prone to change by administering ACTH to the mature normal rats. The total fatty acid composition of the Sertoli cells isolated from ACTH-treated rats showed a significant increase in the relative percentage of 18:2n-6 and a decrease in 20- and 22-carbon PUFA biosynthesis. The data suggested that ACTH exert an inhibitory effect on D5- and D6desaturase (Hurtado de Catalfo et al., 1992). The testes can metabolize some fatty acids and this process may be a part of the scenario of keeping the proper fluidity and producing functional sperm.
SPERM FATTY ACID PROFILES IN SEVERAL SPECIES As early as 1897 it was shown that spermatozoa contained a considerable amount of intracellular lipids which acted as energy sources (Ahluwalia & Holman, 1969; Safarinejad et al., 2010). In 1951, gas chromatography (GC) was developed as a reliable method for analysis of fatty acid profiles. Introduction of this method expedited the research studies on structure, chemistry, and biochemistry of lipids and led to rapid advancements in knowledge in these fields (Hammond, 2002). Therefore, accurate sperm fatty acid profile analysis prior to the 1950s was not possible. Improvements in GC and subsequent techniques such as GC mass, high-performance liquid chromatography (HPLC), and thin layer chromatography caused further development of the capability for fatty acid analyses in biological systems. Although some studies emphasized the pivotal roles of PUFAs concentration in sperm samples, the sperm fatty acid profiles have not yet been analyzed and monitored comprehensively by measuring the entire set of saturated and unsaturated fatty acids (Chavarro et al., 2014; Esmaeili et al., 2014). The structural integrity of the spermatozoa cell membrane plays a pivotal role in successful fertilization. This is because both the acrosome reaction and sperm–oocyte fusion are associated with the membrane’s fatty acid profile (De Vriese & Christophe, 2003). Early attempts to analyze sperm lipids date back to the pioneering studies of mammalian and non-mammalian sperm biochemistry and have shown the presence of neutral fatty acids, cholesterol, phospholipids (particularly lecithin, cephalin, and sphingomyelin), and glycolipids. The flurry of © 2015 American Society of Andrology and European Academy of Andrology
ANDROLOGY studies that followed revealed that phospholipids were the most representative lipid fraction of the sperm cell membranes and phosphatidylcholine and phosphatidylethanolamine which were the major contributors (Mann & Lutwak-Mann, 1982). Some omega-3 and omega-6 fatty acids such as DHA (C22:6n-3) for humans (Safarinejad, 2011) and ruminants (Esmaeili et al., 2014; Fair et al., 2014), docosapantaenoic acid (DPA; C22:5n-6) for boars, rodents, and rabbits and docosatetraenoic acid (C22:4n-6) for domestic birds were recognized as major elements in spermatozoa phospholipids (Alizadeh et al., 2014). The establishment of a negative correlation between increased saturated fatty acids (De Vriese & Christophe, 2003) or trans fatty acids (Chavarro et al., 2014) and normal sperm parameters revealed the significance of comprehensively reporting fatty acid profiles in human. The negative correlation between decreased spermatozoa total lipid, increased saturated fatty acid content, and sperm parameters was previously reported in infertile boars (Am-in et al., 2011). Therefore, fatty acids which are involved in the profiles could influence sperm parameter, and some indices such as the desaturase index (DI) could be used for comparison. Fatty acid profiles in head and tail of spermatozoa Whereas the tail is mostly associated with sperm movement, the head is related with acrosome reaction and membrane fusion (Argov-Argaman et al., 2013a,b). In monkeys, Connor et al. have shown that approximately 99% of total sperm DHA belongs to the tail. Their findings suggested that DHA density in sperm tail be directly related to fluidity and flexibility (Connor et al., 1998). By contrast, some studies in humans and bulls measured higher concentrations of PUFA, total n-3 fatty acid (especially DHA) in the head compared with the tail (Zalata et al., 1998; Argov-Argaman et al., 2013a,b). This characteristics is critical for acrosome biogenesis. Roqueta-Rivera et al (2011) reported that when mice consumed an omega-3 fatty acid-deficient diet there was no acrosomal reaction. Therefore, DHA deficiency might stop the second stage of spermatogenesis, even in species whose spermatozoa contain high levels of n-6 fatty acids. In addition, the percentage of DHA in sperm membrane phospholipids was higher than that of DHA in other cells. Hence, PUFA metabolism was more active in the testes during spermatogenesis and epididymal sperm maturation than these of PUFA metabolism of other cells (Moore, 1998). Surprisingly, Zalata et al. have suggested that DHA in human spermatozoa may have specific functions unrelated to fluidity, which is similar to the functions of DHA in the brain and retina. More recently, it has been suggested that lipid concentrations may affect semen parameters, and this effect is more pronounced in sperm head morphology (Schisterman et al., 2014). Altogether, dietary omega-3 fatty acids may improve sperm function by manipulation of head and tail fatty acid profiles.
SPERM FATTY ACID PROFILES OF FERTILE AND INFERTILE MEN The lipid composition of the sperm membrane has a significant effect upon the functional characteristics of spermatozoa. Chavarro et al. (2011) reported that higher trans-fatty acid levels in spermatozoa were associated with lower sperm concentration. Similarly, there were lower levels of sperm DHA reported in oligoasthenozoospermic compared with normospermic men. Andrology, 1–12
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Aksoy et al. (2006) reported higher omega-6:omega-3 ratios in spermatozoa of infertile vs. fertile men, respectively. For many years, it has been known that PUFAs are the main substrate of lipid peroxidation, changes in sperm lipid composition cause alterations in sperm functional characteristics (Gulaya et al., 2001). It is still a controversial issue whether male infertility is a result of the damage induced by reactive oxygen species (ROS) generated by spermatozoa endogenously, or alternatively a lower concentration of some PUFAs. However, inadequate DHA concentration is the main cause of low-quality spermatozoa (Aksoy et al., 2006; Safarinejad et al., 2010). Mean melting point (MMP) is proposed to be an index of fluidity. MMP is determined on the basis of upon the whole set of fatty acids (Holman et al., 1991). All saturated and unsaturated fatty acids not only have the same contribution in MMP, but it also appears that minute highly influential concentrations of some fatty acids play effective roles on regulation of MMP. In some biological compounds such as milk fat the crucial role of these minute fatty acids such as
