Dyslipidaemia is on the rise in young people in both developed and developing countries. It is believed that with increasing prevalence of sedentary life styles and dietary changes, hyperlipidaemia is emerging as an important cause of adverse health outcomes including cardiovascular complications, obesity, metabolic disorders, infertility and so on [1].

According to an estimate global infertility ranges between 10- 15% of couple, affecting 50-80 million people all over the world. Male infertility has a substantial share of the total infertility burden [2,3]. Over the years, a number of population based studies have highlighted a trend towards deterioration of semen quality [4,5]. Due to environment contamination and change of life style, the infertility rate is going to increase in future.

Lipids have an important role in the functional activity of sperm cells, sperm viability, maturity, capacitation and fertilization [6,7]. Excessive intake of high cholesterol or high fat diet may induce hypercholesterolemia/hyperlipidemia and disturb cholesterol homeostasis in the body which may adversely affect normal male reproductive functions. Several animal and clinical studies have been conducted to focus on the association of hyperlipidaemia/ hypercholesterolemia with male infertility. However, these studies show some variation in the results and also the exact molecular mechanism(s) of action are still poorly known. Therefore, the purpose of this review is to critically explore the links between hypercholesterolemia/ hyperlipidaemia and male infertility, to address how it disrupts the male reproductive function and fertility.

Lipids play multiple roles that either individually or collectively influence many cell processes. Cholesterol is one of the most important bio-molecule in animals and has significant role in cellular function and integrity. It is essential for membrane composition, permeability, fluidity, endocytosis and intracellular signalling. It is also a precursor of all sexual hormones [8,9]. Cholesterol has crucial functions in the area of male and female reproductive physiology, from sex differentiation to gamete formation.

The sperm membrane is composed from heterogeneous mixture of phospholipids, glycolipids and sterol [10] and plays an important role in sperm capacitation and fertilization. It is known that the acrosome reaction and sperm-oocyte fusion both are membrane associated events [11]. Besides this, the lipids of the spermatozoa have been suggested to be important for viability, maturation and function of spermatozoa [12]. Cholesterol’s ability to order saturated phospholipids contributes to the formation of rafts that have distinctive protein composition and are supposed to play an important role in signal transduction pathway [13].

Spermatozoa leaving testes are neither mobile nor fertile. As spermatozoa transverse through the epididymis and female genital tract, they undergo multiple biochemical and physiological modifications, such as the removal of seminal plasma proteins/glycoproteins absorbed to the surface of ejaculated spermatozoa, as well as modification and reorganization of sperm plasma membrane molecules [14]. As spermatozoa travel through the epididymis, modification in the content of cholesterol and different phospholipids takes place to promote membrane fluidity [15-19]. The modification of the sperm cholesterol during epididymal maturation has been investigated in several mammalian species. A significant (50%) decrease in the level of sperm cholesterol has been reported in hamster [20], mouse [21], and rat [22] and ram [23] whereas no significant change in sperm cholesterol content was observed in boar [24] and a significant increase was observed in goat [25]. During epididymis transit, the relative proportion of Polyunsaturated Fatty Acids (PUFAs) in sperms has been reported to increase [26]. These biochemical modifications of sterols and fatty acids occurring in the epididymis have a direct influence on the sperm plasma membrane architecture and dynamics [27]. The loss of cholesterol results in the decrease of cholesterol/phospholipid ratio and consequently increases of the fluidity of sperm membrane. Cholesterol depletion is important in the remodeling of lipid rafts on the sperm surface [28]. Cholesterol diffusion was higher in the sperm head region than in the tail and shows heterogeneous distribution when detected with filipin [29]. Cholesterol efflux from the sperm plasma membrane triggers signal transduction pathway, facilitates Ca⁺² influx into sperm and is associated with changes in sperm membrane dynamics and structure, triggering the acrosomal reaction and spermoocyte fusion [17,30-32].

In recent years much attention has been focused on the association of serum lipid profile with seminal plasma or sperm lipid content, semen quality and male infertility. A number of human studies have indicated an association between hypercholesterolemia and subsequently male infertility [33-36]. Contrary to these findings, others have found no correlation between serum cholesterol and the amount of cholesterol in the semen and in sperm or seminal plasma suggesting that sperm cholesterol content is regulated locally within the male reproductive tract [7,18,37-39]. It has been suggested that in male reproductive tract, lipid homeostasis is performed by testicular and post-testicular mechanism [7,40]. Khalili et al., [41] also showed that concentration of serum lipid was not generally related with the quality of sperm parameters.

There is a very little information regarding the direct association of serum Triglycerides (TG) with sperm parameters, independent of hypercholesterolemia. Zmuda et al., [42] reported a decrease in endogenous testosterone associated with an increase in triglycerides level. Alquabaty [43] also reported significant negative correlation between serum total testosterone level and triglycerides and insignificant correlation between serum testosterone and total cholesterol level in infertile men. Vignon et al., [44] also found that an increase in triglycerides have deleterious effect on spermatogenesis in men. Ergun et al., [45] have observed that increased triglycerides may have deleterious effect on spermatogenesis correlated with decrease sperm motility and testosterone in a group of infertile men. Contrary to these findings, Kulka et al., [46] showed no correlation between triglycerides concentration and sperm parameters (sperm concentration, motility and morphology). However, alteration in phospholipid concentration was associated with abnormal semen analysis. Change in fatty acid and phospholipid composition of human sperm membrane has also been reported in human asthenozoospermic samples. The cholesterol/ phospholipid ratio was found to be higher in spermatozoa obtained from patients with idiopathic infertility [47].

In the past few years, a large number of studies have also been carried out on high cholesterol/high fat diet fed animal models (mouse, rat and rabbit) to determine the association between serum lipid profile, seminal plasma or sperm lipid content and infertility (Table 1). The result of these studies have shown detrimental effect of hypercholesterolemia on testicular histology and functions including spermatogenesis and steroidogenesis, epididymal sperm maturation process, sperm quality parameters, sperm fertilitizing capacity and fertility index [48-57].

Table 1: Effect of Diet Induced Hyperlipidaemia/Hypercholesterolemia on Male Reproductive Function in Various Experimental Animals

It has been demonstrated that modifying the dietary intake of fatty acids, modified the fatty acid composition of sperm plasma membrane in rabbit [58] and in boar [59,60]. In normal goats, level of HDL cholesterol is positively correlated with testicular histology, seminal parameters and serum testosterone level while high serum triglycerides level exert adverse effect on testicular and seminal parameters and serum testosterone level [61]. Diaz-fondevila et al., [62] and Diazfondevila and Bustos-Obregan, [63] have reported that rabbits fed high cholesterol diet did not show increased cholesterol content of testis and seminal plasma. They suggested an adverse effect of cholesterol enriched diet on Leydig and Sertoli cells secretory function. Similar to this, Yammamoto et al., [51] also reported a significant reduction in sperm concentration, sperm motility and morphology in hypercholestrolemic rabbit. However, there was no significant difference in cholesterol concentration of seminal plasma or testicular tissue when compared with control. They suggested that hypercholesterolemia has a detrimental effect on Leydig and Sertoli cell secretory function, sperm maturation process and the overall sperm fertilizing capacity. Hypercholesterolemia in animals may cause a significant difference in filipin-sterol complex in plasma membrane of acrosome region of sperm which could modify the sperm membrane fusion capacity and functionality and decrease in kinetics of acrosome reaction [63].

Tanaka et al., [52] have suggested that hypercholesterolemia is an independent risk factor for testicular dysfunction in rats and the reduction in serum and testicular testosterone level is due, at least in part, to a reduction in testicular LH/HCG binding. Martinez-mortos et al., [64] have found that dietary cholesterol modulates bioactive peptides of the rennin-angiotensin system localized in the testis which result in inhibition of steroidogenesis and consequently decrease in testosterone production in Balb C mice.

Ouvrier et al., [56] have reported that an overload of dietary cholesterol in Lxr- deficient male mice (liver X receptor deficient mouse model) causes complete infertility. Spermatozoa of cholesterol fed Lxr deficient male mice were found to be dramatically less viable, less motile and highly susceptible to a premature acrosome reaction, although spermatogenesis was not affected, as shown by normal sperm count, testicular weight and histology. It was suggested that infertility resulted from post-testicular defect. High cholesterol diet alter the caput epididymal epithelium in a segment and cell specific manner, characterized by peritubular accumulation of cholesterol ester lipid droplets in smooth muscle lining of the epididymal duct which impair peristaltic contraction and consequently sperm progression in the lumen of the tubule, perturbing the finny orchestrated process of sperm maturation.

It has also been proposed that hypercholesterolemia induces reproductive and testicular damage by excessive generation of free radicals and increased oxidative stress, which are cytotoxic to spermatozoa [53,65-67]. Administration of antioxidants and lipid lowering agents has been shown to protect the testis and reproductive functions during hypercholesterolemia [53,65,66]. Furthermore, treatment of hypercholesterolemia patients with statins at the dose effective in improving lipid profile has shown modulating effect on semen parameters and gonadal endocrine dysfunction [68,69].

High fat diet induces nutritional obesity, hyperlipemia, adverse effect on testicular development and hormonal profile in pubertal rat [70,71] and rabbit [72]. There was a significant decline in the testosterone and testosterone to estradiol ratio (T/Es). High lipid diet can induce obesity and increases the apoptosis of testicular spermatogenic cell population [71].

Accumulative evidences also suggest that hyperlipidaemia is associated with erectile dysfunction. A high level of total cholesterol and low level of High Density Lipoprotein (HDL) cholesterol are important risk factors for erectile dysfunction [73-75]. Nikoobakht et al., [76] reported that plasma cholesterol and LDL level in individual suffering from erectile dysfunction were significantly higher than control. However, no difference in the mean plasma TG and HDL level was seen. Impairment of endothelium dependent relaxation of numerous vascular beds in men with hypercholesterolemia has been established by many workers [77-81]. These impairments have also been found to be reversible using lipid lowering therapies [82]. Oxidized low density lipoprotein is the major causative cholesterol responsible for impaired relaxation response [83].

Studies based on animal models of hypercholesterolemia also found a correlation between high concentration of LDL cholesterol and ejaculatory dysfunctions [84-86]. These hypercholestrolemic animals show both deficient endothelium and neurogenic dependant cavernously relaxation [84,85]. Manning et al., [87] also observed a correlation between high LDL and organic erectile dysfunction and a clear positive correlation between LDL and caverno-venus insufficiency. In contrast to above findings, other workers did not find any relationship between high blood cholesterol and TG levels and ejaculatory dysfunction [88-91]. There are also reports where high HDL-cholesterol [88] and low LDL-cholesterol [92] was inversely correlated with erectile dysfunction.

Consumption of high fat diet for long term may cause blood hyperlipidaemia and may induce obesity. In diet induced obese male mice, decrease in sperm motility [93,94], fertilization rate [92], as well as increase in sperm DNA damage and Reactive Oxygen Species (ROS) without showing any significant change in sperm concentration in cauda epididymis have been reported [94]. It has also been reported that in obese mice, proteins regulating acetylation and DNA damage repair systems are altered [95]. In contrast to these reports, no impairment of fertility in male DBA/2J mice fed with high fat diet was observed by Tortoriello et al., [96]. Fernandez et al., [97] observed a significant increase in obesity index and serum leptin levels without any significant effect on serum testosterone level and sperm counts in testes and epididymis of rats fed high fat diet. However, there was an alteration in sperm motility parameters, reduction in fertility and an increase in estradiol level in these rats.

Vigueras-villasenor et al., [98] also reported a significant increase in circulating leptin and estradiol levels; however, the testosterone level was reduced in rats fed high fat diet. Although, they observed no structural alteration in seminiferous tubules but there was significant reduction in sperm viability, motility and concentration with an increase in lipid peroxidation.

Wang et al., [70] and Yang et al., [71] reported that in pubertal rats, the high fat diet induced an increase in total cholesterol and triglycerides levels, and poor development of testicles. There was a significant decrease in blood testosterone level, testosterone to estradiol ratio and increase in estradiol level. Chen et al., [99] reported significant increase in apoptotic index of spermatogenic cells in testes of pre-pubertal rats fed high fat diet.

The available evidences on the role of obesity and Body Mass Index (BMI) on male infertility are controversial [100-104]. Some studies related to man have shown association of BMI with reproductive parameters like poor semen quality [105], decreased sperm concentration [106], decreased number of normal motile sperm cells [107,108] increased Reactive Oxygen Species (ROS) and increased DNA fragmentation index [109,110]. Contrary to this, some workers showed little or no relation between obesity and sperm concentration [109,111,112], sperm motility or morphology [108,113] even when the serum concentration of sex hormones were altered [111,113]. Shukla et al., [114] concluded that obesity negatively affects male reproductive potential not only by reducing sperm quality but in particular, it alters physical and molecular structure of germ cells in the testes and ultimately affects the maturity and functions of sperm cells.

Hajshafiha et al., [115] reported that BMI was not found to be associated with mean values of semen parameters including sperm count, sperm morphology and sperm motility. However, a significant correlation was found between BMI and estradiol, Sex Hormone Binding Globulin (SHBG) levels and also the testosterone to estradiol ratio. Similarly, Strain et al., [116] and Pauli et al., [117] found no significant difference in the spermatic parameters between obese and normal humans.

Several studies have documented a negative effect of obesity on semen quality both in normal fertile [118,119] and sub fertile-infertile males [100,120]. Obese and overweight men exhibit a high incidence of infertility in association with metabolic disturbances and altered hormonal profiles (decreased serum testosterone, FSH, inhibin B levels and increased levels of estrogen) [100,120-122]. Evidences suggest that increased estrogen as a result of aromatization in the adipose tissue may be an important mechanism for hypoandrogenemia and altered sperm parameters [102].

Increased cell proliferation and enlargement of ventral prostate in rats kept on the diet rich in cholesterol or saturated fat has been reported by many workers [123-129]. Inclusion of saturated fat in the diet changed the expression of androgen receptor and peroxisome proliferation activated receptor Y (PPARY) [130]. Rahman et al., [126] reported increased expression of alpha-adrenergic receptors in the hyper-lipidemic rats. Increased oxidative stress and incidence of prostate adreno-carcinoma and hyperplasia were also observed in the rats kept on high cholesterol diet for long period [131]. Increased expression of NADPH oxidase subunits, activation of NF-KB signalling and decreased expression of glutathione peroxidase clearly indicated the increased oxidative stress and activation of inflammatory response in ventral prostate of rats fed high fat diet [132,133].

Presence of dyslipidaemia is a frequently observed condition in Benign Prostate Hyperplasia (BPH) patients. High levels of total cholesterol, LDL-cholesterol, triglycerides, low level of HDL-cholesterol increases the risk of BPH and cholesterol lowering treatment may reduce the risk [134,135]. Physical activity, which is known to decrease serum lipid level, is also associated with decreased risk for BPH [136]. Hyperlipidemia is closely associated with the obesity, higher Body Mass Index (BMI) and these parameters show a positive correlation with BPH [137-142].

Excessive fat intake is associated with adiposity, development of insulin-resistance and obesity, and these conditions are known to increase the expression of Autotoxin gene (ATX) and therefore, Lysophosphatic Acid (LPA) levels [143]. Kulkarni and Getzenberg, [144] proposed ATX-LPA axis as a possible link between excessive dietary fat intake and prostatic hyperplasia.

Epidemiological and preclinical studies suggest that high level of serum cholesterol plays a role in incidence and progression of prostate cancer, even hypercholesterolemia does not raise circulating androgen levels [145-154]. Circulating cholesterol level is directly correlated in tumoral expression of the key steroidogenic enzyme CYP17A, testosterone levels and tumor size with castration sensitive LNCaP xenograft mouse model, suggesting that hypercholesterolemia increases intra-tumoral de novo steroidogenesis resulting in acceleration of the growth of prostate tumor [155]. Some in vitro studies have suggested that elevated levels of cholesterol in prostate tumor cells could be due to dysregulation of the key regulators of cholesterol homeostasis [99,156] which could have significant role in progression of prostate cancer into androgen independent state [157,158] while reduction in cholesterol level retards prostate cancer growth, possibly by inhibition of tumor angiogenesis [159,160]. A number of studies have also shown that use of cholesterol lowering drugs may reduce the risk of prostate cancer when used prior to cancer development [161-169]. However, some studies do not support these findings [170-172]. Godwin [173] suggested that serum cholesterol is not associated with the overall incidence of prostate cancer.

Hypercholesterolemic diet stimulates growth of LNCaP human PCa xenograft [160,174]. In Hypercholesterolemic environment, tumour accumulated more cholesterol in their membranes, exhibited enhanced activation of Akt (a kinase linked to aggressive Pca) and lower level of apoptosis [175-177].

Lipids, especially cholesterol play significant role in the structural and functional activity of spermatozoa. Excessive intake of high cholesterol/high fat diet may induce hypercholesterolemia and disturbs cholesterol homeostasis in the body. Experimental studies carried out in various Hypercholesterolemic animal models during the last few decades clearly demonstrate a negative impact of hypercholesterolemia on the structure and functions of testes and accessory sex organs, sperm maturation, sperm quality parameters and ejaculatory functions leading to male infertility. Numerous mechanisms have been proposed to explain such effects, like deregulation of hypothalamic- pituitarygonadal axis, impairment of steroid genesis and secretory function of Sertoli and Leydig cells, enhanced oxidative stress and excessive generation of free radicals and altered expression of some testicular genes. A large number of human studies have also indicated an association between hypercholesterolemia and poor semen quality and subsequent male infertility. However, most of these studies have been conducted on normal infertile or obese men. Whether the hypercholesterolemia has any impact on sperm/seminal plasma cholesterol/lipid content is still poorly known. Thus, on the basis of above reports it can be proposed that dyslipidaemia is an important factor contributing to male infertility. Therefore, regulation of serum lipid profile may be useful to some extent, for proper male reproductive functions and management of male infertility.

The authors are highly thankful to the Head of the Department and the CAS coordinator for providing necessary facilities. The first author acknowledges Indian Council of Medical Research, New Delhi for awarding JRF