Prevention of Oxidative Stress Injury to Sperm

Abstract

The greatest paradox of aerobic respiration is that oxygen, which is essential for energy production, may also be detrimental because it leads to the production of reactive oxygen species (ROS) (Saleh and Agarwal, 2002). When levels of reactive oxygen species (ROS) overwhelm the body's antioxidant defense system, oxidative stress (OS) occurs. OS is a condition in which the elevated levels of ROS damage cells, tissues, or organs (Moller et al, 1996; Sharma and Agarwal, 1996; Saleh et al, 2003).

ROS are free radicals that play a significant role in many of the sperm physiological processes such as capacitation, hyperactivation, and sperm-oocyte fusion (Aitken et al, 2004; Allamaneni et al, 2004; de Lamirande et al, 1998). However, they also trigger many pathological processes in the male reproductive system, and these processes have been implicated in cancers of the bladder and prostate, as well as in male infertility (Bankson et al, 1993; Hietanen et al, 1994; Agarwal and Saleh, 2002).

Spermatozoa are sensitive to OS because they lack cytoplasmic defenses (Donnelly et al, 1999; Saleh and Agarwal, 2002). Moreover, the sperm plasma membrane contains lipids in the form of polyunsaturated fatty acids, which are vulnerable to attack by ROS. ROS, in the presence of polyunsaturated fatty acids, triggers a chain of chemical reactions called lipid peroxidation (Agarwal et al, 1994; Kobayashi et al, 2001; Zalata et al, 2004). ROS can also damage DNA by causing deletions, mutations, and other lethal genetic effects (Moustafa et al, 2004; Tominaga et al, 2004). It is difficult to block the OS-induced injury to cells or tissues because ROS are continuously produced by cellular aerobic metabolism (Davies, 2000). Several clinical trials are currently attempting to minimize the toxic effects of OS on human spermatozoa (Agarwal et al, 2004). In this review, we highlight the various protective measures available to minimize OS-induced injury to spermatozoa.

There are two main sources of ROS in semen: leukocytes and immature spermatozoa (Garrido et al, 2004). Of these, leukocytes are considered to be the primary source (Aitken et al, 1992). Leukocytes, particularly neutrophils and macrophages, have been associated with excessive ROS production that ultimately leads to sperm dysfunction (Aitken and Baker, 1995; Aitken et al, 1997; Hendin et al, 1999; Ochsendorf, 1999; Pasqualotto et al, 2000; Saleh et al, 2002; Shalika et al, 1996; Sharma et al, 2001).

Spermatozoa produce ROS mainly when a defect occurs during spermiogenesis that results in retention of cytoplasmic droplets (Gomez et al, 1996; Zini et al, 1993). A strong positive correlation exists between immature spermatozoa and ROS production, which in turn negatively affects the sperm quality (Gil-Guzman et al, 2001; Said et al, 2004).

The two main sites of ROS production are the mitochondrion and the sperm plasma membrane. The mitochondrion is the powerhouse of respiration. Hence, it is the major site of ROS generation, which is produced through the nicotinamide adenine dinucleotide-dependent oxido-reductase pathway (Gavella and Lipovac, 1992). In contrast, the sperm plasma membrane produces ROS through the nicotinamide adenine dinucleotide phosphate-dependent oxidase system (Aitken et al, 1992; Agarwal et al, 2003). Xanthine oxidase—a key enzyme in purine catabolism—is also involved in the production of ROS in spermatozoa (Aitken et al, 1993; Sanocka et al, 1996).

Sperm damage can be caused either by the invading pathogens or by the defense mechanisms that are employed against them (Ochsendorf, 1999). For example, when microorganisms invade the human body, it produces polymorphonuclear leukocytes and macrophages, which are the major sources of ROS production (Ochsendorf, 1999; Saran et al, 1999; Zalata et al, 2004). Prostatitis and accessory gland infection increase OS, which severely damages spermatozoa (Potts and Pasqualotto, 2003). In addition, a past infection by the sexually transmitted Neisseria gonorrhea is associated with leukocytospermia (Trum et al, 1998). Although there is no direct evidence that Neisseria gonorrhea directly increases ROS production, the associated leukocytospermia is well known to produce ROS (Trum et al, 1998). According to a study by Depuydt et al, leukocytospermia and male accessory glands infection reduce a man's fertilizing potential by affecting sperm parameters both in vitro and in vivo (Depuydt et al, 1998).

Prolonged in vitro incubation of semen samples that contain high levels of immature spermatozoa before sperm processing increases the risk of OS damage to mature spermatozoa (Gil-Guzman et al, 2001). In a study by Aitken and Clarkson, it was reported that repeated centrifugation mechanically injures spermatozoa and increases ROS production (Aitken and Clarkson, 1988). OS may also damage sperm during cryopreservation. A study by Bilodeau et al revealed that ROS generated during freeze-thaw cycles are detrimental to sperm function and that levels of antioxidants were diminished during each cycle (Bilodeau et al, 2000).

Spermatozoa are under a continuous influence of OS because of excessive generation of ROS. Although spermatozoa are affected in different ways by OS, there are sufficient antioxidant protections that can decrease the progression of the damage. However, when an imbalance exists between levels of ROS and the natural antioxidant defenses, various measures can be used to protect spermatozoa against the OS-induced injury (Figure). Diet forms an important component of the antioxidant protection system; it supplies the major antioxidants such vitamin C, vitamin E, and carotenoids. Therefore, food rich in these elements should form a part of the daily diet. For those patients who are suspected to have high levels of ROS, antioxidant supplements can be considered. Nevertheless, further studies are required to validate their use in this group of patients. In certain cases, it is also essential to modify certain lifestyle behaviors because many habits and environmental factors increase the production of ROS and affect fertility.

Another important method for decreasing OS is the use of antioxidants during various sperm processing techniques. Antioxidants decrease the oxidative damage to spermatozoa induced during these techniques. There are many controversies regarding the doses, types, and combinations that could be used in different sperm-handling techniques. Future research should address these issues to develop standard and reliable protocols.