Sperm Oxidative Stress and DNA Damage

From what is currently known about sperm cell damage and dysfunction, there is certainly scope for RF-EMR to contribute to this affliction. The three main types of cellular damage which can account for the adverse observations made are membrane, protein and DNA damage. The factors responsible for contributing to this damage in the male germ line can be grouped into several categories. Quality of spermatogenesis defines the susceptibility of cells to damage [23]. Biological factors including diet and stress as well as chemical and physical factors may all have direct and/or indirect effects on the "health" of spermatozoa. These categories are not necessarily mutually exclusive and combination between them is most probable. The complex, multifactorial nature of male infertility makes it a challenging area of research; however, cellular damage originating from environmental factors including the impacts of EMR on the male germ line must be understood if this issue is to be managed. The continued effort to determine the clinical significance of environmental-born EMR exposure on reproduction (as well as human health) and to gain an understanding of the mechanisms involved is urgently required as the exposure of RF-EMR to humans will only escalate.

There is growing evidence that environmental factors may be a key factor in male infertility [24]. Sperm concentration or microscopic analysis of sperm quality has dominated focus in the past, when studying xenobiotic or other environmental exposures; however, recently, more attention has been centred on the effects of sperm DNA integrity. There is a wealth of reports that link environmental exposures to sperm DNA damage and reduced fertility [25-27], with RF-EMR recently included. To put the risks in some perspective, evidence suggests the ability of DNA damaged cells to initiate fertilisation is somewhat compromised; however, it is not necessarily precluded from fertilising an oocyte. Indeed, DNA damage in the male germ line has been linked with a range of adverse clinical outcomes, including poor fertility rates in vitro, but also subsequent disruptions of embryonic development, increased rates of miscarriage and poor childhood health [27-29] including cancer. Therefore, it is crucial that we understand the clinical implications and mechanisms of RF-EMR if indeed it plays any part in elevating DNA damage in the male germ line.

Regardless of the recent attention sperm DNA damage has enjoyed, the aetiology of this damage remains unknown. While the cellular mechanisms underpinning these effects have not been completely resolved, it has been suggested that oxidative stress derived from numerous possible pathways could be a key factor [17, 30] . There are several reports that also strongly link this mode of action to the adverse effects observed after EMR exposure [31-34]. strengthening the potential role of this environmental factor in this affliction. Oxidative stress has also been implicated in a range of other infertility pathologies, including loss of sperm motility and vitality, which is also a common observation after RF-EMR exposure. Failure of sperm-oocyte fusion is also another result of oxidative stress [ 35] . We now know that human spermatozoa are capable of generating significant amounts of ROS [36, 37], both spontaneously and when exposed to xenobiotic or physical environmental factors [38]. Furthermore, these highly specialised cells are intrinsically sensitive to ROS and may enter a state of oxidative stress [39] with little hindrance. Therefore, the induction of ROS by environmental factors may account for the majority of cellular damage and dysfunction observed in human spermatozoa. Mobile phone radiation has the potential to elevate ROS leading to a state of oxidative stress which in turn impacts sperm motility vitality and DNA integrity; however, the fundamental mechanism by which ROS is generated is unknown. Two main ideas that lead to a state of oxidative stress in spermatozoa by RF-EMR are the disruption of the sperm mitochondria [40] and the activation of plasma membrane NADH oxidases [41].

Many studies have reported the presence of reactive oxygen species within human spermatozoa and its consequences for the gamete [42-44] . From our understanding of the cell biology of human spermatozoa, it is perhaps no surprise that these cells are then susceptible to oxidative stress and DNA damage. The minute volume of cytoplasm in these highly specialised cells limits the antioxidant capacity usually afforded to other cell types. Once oxidative stress is initiated, fertilisation is compromised through the loss of motility and ability to fuse to the oocyte. These outcomes arise due to lipid peroxidation of the abundant redox-sensitive polyunsaturated fatty acids in the plasma membrane. These peroxides also have the capacity to further propagate oxidative stress by a lipid peroxidation cascade [45]. Oxidative stress also leads to a range of protein damage, including alkylation (by lipid peroxides products) and oxidation [46]. The sperm chromatin does not escape the negative effects of this stress where DNA damage, ranging from oxidation, adduct formation and strand breaks result [26, 39, 47]. Further strengthening the central role of oxida-tive stress in sperm damage, several studies have shown that supplementary antioxi-dants have had some protective role against damage induced by EMR [48-50] . The area of antioxidant treatment for male infertility is a rapidly growing one; nevertheless, it is largely driven by empirical data. The work in this area confirms the major importance of ROS and oxidative stress in spermatozoa exposed to RF-EMR;

however, in a broader sense more understanding of oxidant-antioxidant interactions are needed here before rational clinical applications can be established. Similarly, care must be taken when aligning the adverse observations made after RF-EMR or mobile phone exposure to clinical relevance. The biophysics of EMR is exceedingly complex, evident by some examples of poor experimental design and lack of insight offered to date. There have been few experiments done on spermatozoa and the evidence for low-level genotoxic effects is weak [51] at this current time. More appropriately designed studies need to be conducted if any headway is to be made in this field.

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