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A number of investigations during the last two decades describe adverse trends in male reproductive health, which have been proposed to be caused by environmental factors with endocrine disrupting properties. In contrast to many other toxicants, endocrine disruptors often do not show linear dose-response relationships typical of those found in traditional toxicological studies. For many compounds, low-dose exposure causes effects opposite to the ones seen after high-dose exposure. In addition, the timing of exposure has been found to be critical. Hence, to correctly assess the impact of endocrine disruptors on reproductive health requires in-depth knowledge of their mechanisms of action.

This thesis aimed at identifying the mechanisms underlying the effects of cadmium (Cd), phthalates and phytoestrogens on testicular steroidogenesis. For this purpose, in vitro as well as in vivo models were used. Cd was found to inhibit testosterone synthesis in vivo by down-regulating LH receptor gene expression and reducing the testicular levels of cAMP and StAR protein. In addition, Cd caused a pronounced increase in testicular prostaglandin F_2ɑ (PGF_2ɑ), suggesting that Cd exerts its suppressive effect on steroidogenesis also by inducing the inhibitory PKC pathway. Pre-treatment with zinc (Zn) protected completely against Cd-induced effects on testosterone and PGF_2ɑ. Furthermore, we observed that Cd exposure increased glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA expression in the testis. GAPDH is a potent coactivator of androgen receptor-mediated transcription and the up-regulation found in our study is probably a compensatory response to reduced testosterone concentrations. This finding is interesting since GAPDH has been proposed to have an important role in the regulation of apoptosis as well as sperm motility. We discovered that mono-(2-ethylhexyl) phthalate (MEHP), the active metabolite of the frequently used phthalate di-(2-ethylhexyl) phthalate (DEHP), stimulates Leydig cell steroidogenesis in vitro, by a cAMP- and StAR-independent mechanism. MEHP exposure caused a similar effect in granulosa cells. Gene expression analysis revealed that MEHP is likely to stimulate steroidogenesis by increasing the amount of cholesterol available for steroid synthesis. In the last investigation, we examined the effects of low-dose phytoestrogen exposure on testosterone synthesis during puberty in male goats. Isoflavones present in clover increased plasma concentrations of testosterone and free as well as total triiodothyronine (T₃). T₃ has previously been shown to induce testosterone synthesis and it is possible that an elevated T₃ secretion underlies the increased plasma testosterone levels.

Reduced fertility and reproductive tract malformations affect both the individual and the society. Hence, a sound knowledge of reproductive toxicants is of crucial importance. The findings presented in this thesis provide new insights into the reproductive toxicology of endocrine disruptors and may be valuable for risk assessment purposes.