Chemical and Radioisotope Pollution: Hazards to Male Fertility, 1995
The British Andrology Society organized this conference in Manchester, UK on 14 & 15 September 1995. The meeting was supported by The International Society of Andrology, The Wellcome Trust and Serono Laboratories.
The controversial practice of extrapolating animal reproductive toxicology data to human male fertility was addressed in the opening lecture by Dr. Purchase (UK). Dr. Purchase showed how public pressure could drive experimental research to address the question of the risk of pollution to male fertility. Epidemiological criteria to test the cause and effect relationships, strength, consistency, specificity, temporality, biological gradient, plausibility, coherence, experiment and analogy must all be considered if the link between falling male fertility and pollution is to be proven. Clearly the criteria have not been satisfied so that animal studies must be used to assess the risk to human health and caution must be the keyword before assigning blame.
Dr. Skakkebaek (Denmark) reviewed the human data which suggest that there are environmental effects upon human male fertility. He pointed out that there were clearly national and regional trends in the increase of the incidence of testicular cancer, hypospadias and undescended testes as well as the well publicized decline in semen quality. Dr. Skakkebaek offered the hypothesis that one of the most plausible reasons for the detrimental changes in the reproductive systems of men is the presence of pollutants in the environment especially those with estrogenic activity. However it was conceded that proof of this hypothesis will be difficult to obtain.
Industrial toxicology is the area which has provided the most convincing evidence for the impairment of male fertility by chemicals. Dr. Foster (UK) described a rationale which illustrated how a systematic approach could determine the mode of action and target for specific chemicals. Two examples were chosen. The phthalate esters are targeted to an FSH modulating mechanism within the Sertoli cell especially in young animals. Nitroaromatic compounds such as dinitrobenzene act via a totally different mechanism but also target the Sertoli cell. The Sertoli cell will metabolize the DNB to an active compound which establishes a futile redox cycle and therefore reduces cellular protectants. By establishing the dose dependency in animals and the concentration in the blood of manufacturing workers, it was possible to suggest that the human risk was negligible.
Dr. Moore (UK) identified mechanisms in the fetus which could potentially be affected by pollutants. Although the SRY (TDF) gene, the genetic basis for sexual determination, is well defined, the molecular mechanisms still remain unclear as their regulation has not been determined. Estrogens have a profound effect upon testis formation and sexual development but the effects of the maternal environment are unknown and physically make the fetus difficult to study. The marsupial opposum avoids these complications and Dr. Moore showed how low doses of estradiol administered to the neonate lead to failure of testis formation and sexual differentiation. Future studies will involve the Xenoestrogens.
One of the main proponents of the hypothesis that environmental estrogens impair human male fertility, Dr. Sharpe (UK), described some of his recent experiments to investigate whether fetal exposure to the Xenoestrogens was the likely cause. The Xenoestrogens, transferrin. The Sertoli cell in the seminiferous epithelium manufactures its own transferrin-iron transport system allowing access of potentially noxious isotopes (iron,indium and plutonium) to the germ cell. Indium-114, as a model isotope, is retained within the testis and will kill germ cells and induce dominant lethal mutations. Transferrin dependent binding of the isotopes can be demonstrated in the Sertoli cell, seminiferous tubule and spermatozoa. Taken together these data suggested a mechanism by which germ cells could selectively acquire genetic damage from pollutant isotopes.
Fish unavoidably are exposed to significant levels of aquatic pollutants so it was not surprising that some of the most compelling evidence for the bioactivity of these agents was discovered by studies of hermaphrodite fish living in polluted water courses. Dr. Sumpter (UK) showed that effluent from sewage treatment works contains chemicals which are estrogenic to fish. Several candidates were identified including ethinyl estradiol (from oral contraceptive tablets), natural estrogens, alkylphenolic surfactants and plasticizers. These substances induce estrogen dependent vitellogenin synthesis in male fish and decrease testicular growth. The determination of the potency of the compounds in wild and domestic species will aid in the assessment of risk to men.
Dr. Safe (USA) reviewed the conflicting epidemiological evidence for a fall in male fertility. Dr. Safe also challenged the estrogen hypothesis by estimating the dietary human exposure to food and pollutant estrogens and other substances which exhibit antiestrogenic activity. Based on mass/balance and mass/potency estimations he concluded that the highest estrogenicity intake arose from food sources. As human populations which consume high levels of dietary estrogens do not show any evidence of increased incidence of diseases associated with estrogen exposure he concluded that the hypothesis cannot be supported.
Radiation as well as chemicals is well known to have adverse effects upon reproduction and it is clear that many man-made isotopes reach the environment. Therefore there may also be a risk to male fertility from another of man's activities, the use of isotopes for defense and energy generation. Dr. Evans (UK) described chromosomal and gene mutations in human infants. Man is clearly at risk from genetic abnormalities as he described data which shows that over 60% of early abortions have some form of genetic abnormality. Mutation rates in single genes are higher in males than females which can lead to cancer in surviving offspring (retinoblastoma, Wilms tumour). In spite of this he suggested that there is little evidence that radiation contributes towards the high incidence of human genetic abnormalities. This high background may confound the power of epidemiology studies.
The epidemiological evidence for the adverse effects of paternal exposure to ionizing radiation was described by Dr. Roman (UK). Much work has arisen from the conclusions of the Gardener report which suggested that paternal exposure to radiation (external and possibly internal via the ingestion of isotopes) leads to leukemia in children. The possible effects that a potentially hazardous exposure could have depends upon a number of factors including dose and such precise information is not available for human epidemiological studies. It was shown that many of the studies which have reported a relationship between preconceptional exposure to radiation and adverse reproductive outcome have been difficult to interpret because of the rarity of the cancers involved. The lack of statistical significance should not necessarily be taken as proof that paternal exposure to radiation has no adverse effects upon the man's children.
In addition to the genetic damage produced by radiation it can also kill spermatogenic cells as described by Dr. de Rooij (Holland). The radiosensitivity of the spermatogonial cells varies greatly during the cycle of the spermatogenic epithelium. Quiescent stem cells are sensitive whilst proliferating cells are resistant. Spermatocytes and spermatids are killed only after very high doses of radiation. Although the cellular associations in the seminiferous epithelium of primates is different from non-primate animals the radiosensitivity of the stem cells appears similar. There is a large pool of radiosensitive quiescent stem cells in primates which contrast with the rodent where the pool is small.
X-rays are the major external source of radiation for experimental studies but Dr. Morris explained how the hazard is increased if the isotopes are ingested and their physico-chemical properties allow access to concentrating mechanisms. Certain isotopes bind to transferrin. The Sertoli cell in the seminiferous epithelium manufactures its own transferrin-iron transport system allowing access of potentially noxious isotopes (iron,indium and plutonium) to the germ cell. Indium-114, as a model isotope, is retained within the testis and will kill germ cells and induce dominant lethal mutations. Transferrin dependent binding of the isotopes can be demonstrated in the Sertoli cell, seminiferous tubule and spermatozoa. Taken together these data suggested a mechanism by which germ cells could selectively acquire genetic damage from pollutant isotopes.
Whilst there is considerable controversy about the reproductive effects of environmental radiation, testicular function is clearly impaired after radiation or chemical treatment for cancer. Dr. Shalet (UK) described the extent of the damage and showed that the potential for recovery is dependent upon the nature of the treatment and the dose employed. Infertility in men is not necessarily permanent as 5% of men who were azoospermic at the nd of treatment recover a normal sperm count 5 years later. Leydig cells may also be damaged which can be detected as raised LH secretion in the presence of normal serum testosterone. What impact this has upon spermatogenesis remains to be determined.
Dr. Jégou (France) posed the question: How can we protect fertility from the effects of cancer treatment? Sperm storage, the development of less toxic therapies and shielding of the testes from external radiation sources were discussed. Dr. Jégou favoured another approach borrowing ideas from the cancer field which tries to protect normal tissue from the cytotoxic therapy. Some animal treatments such as antioxidant agents or hypoxia are not suitable for man. Dr. Jégou has shown that hypothermia will protect the rodent testis from a sterilizing dose of X-rays and suggested that this method would be acceptable in men. Other pharmacological approaches were also discussed. Whether these methods are only applicable to therapeutic radiation or could be used in the event of a nuclear incident remains to be seen.
The conference was closed by Dr. Kavlock (USA) who reviewed the research needs and policy implications related to environmental causes of declining male reproductive function. In the USA a Presidential advisory committee has called for a national research strategy on endocrine disrupting chemicals to assist the federal government in making informed public health decisions. A central need was to determine the risk relative to other environmental and public health concerns. Workshops were convened and scientific strategies developed. Results from this program will influence regulatory decisions which will have to take into consideration legislative mandates, cause and effect relationships, exposure levels, remediation strategies and the international use and global transport of pollutants. Although Dr. Kavlock's presentation addressed chemical pollution there were clear similarities to the problems arising from radioisotope pollution,and without doubt the measures which will have to be taken will transcend international boundaries.
Ian D. Morris, United Kingdom