Recent research suggests that compounds we believed were estrogenic may really be disrupting endocrine signaling via the progesterone receptor . Recently some progesterone disruptors have been identified in personal care products . In utero exposure to natural or synthetic progesterones can induce hypospadias in mice in which the urethral opening occurs more proximally than normal; the synthetic progesterone medroxyprogesterone acetate (MPA) feminizes male and virilizes female genital tubercles (EW et al. in preparation). In the current work, the relatively low dose of vinclozolin used caused hypospadias in the mice, but it also virilized the females, reflecting our findings with MPA. We also have observed a more proximal urethral opening in females treated with natural progesterone, but have not quantified these data (unpublished data).
Our morphological findings with vinclozolin, in the context of our findings with MPA and progesterone, led us to investigate the effects of vinclozolin on progesterone receptor expression, and on the expression of other steroid hormone receptors. In a study with rats, an in vitro assessment of the ability of two vinclozolin metabolites to bind progesterone receptor indicated that it could outcompete the natural ligand; however, the concordant in vivo studies from these investigations using adult male rats showed no in vivo effect . The discrepancy between these data and our current results could be attributable to species differences or differential responses based on timing of exposure. Our mice experienced vinclozolin exposure in utero, rather than as adults, and a central assumption of the endocrine-disruption model is that in utero exposures exert different and more powerful effects than do exposures in adults . In addition, our results are consistent between the morphological and molecular levels in males.
Vinclozolin-exposed males exhibited up-regulated estrogen receptor α and up-regulated progesterone receptor, effects we have also seen with exposure to the synthetic estrogen, ethinyl estradiol (unpublished data). Ethinyl estradiol up-regulates estrogen receptor α in females and upregulates progesterone receptor in both sexes. MPA also up-regulates estrogen receptor α and progesterone receptor in males, which it feminizes.
Vinclozolin can cause an increase in aromatase activity , which would presumably result in an increase in estradiol, especially in a system where testosterone is being made in abundance, as is the case with the male differentiation pathway. In fact, in the fathead minnow, a species that serves as a model of vertebrate endocrine disruption, vinclozolin exposure resulted in a slight increase in estradiol concentrations in adult male fish . Thus, vinclozolin could act on aromatase, which in turn could act on testosterone, both producing estradiol inappropriately and depriving 5-alpha-reductase of the substrate it needs to produce dihydrotesterone, the androgen necessary for appropriate male development of secondary sex characteristics. One possible explanation for the up-regulation of progesterone receptor mRNA in males could be the boosted estrogen receptor expression; estrogen receptor binds the progesterone receptor promoter. But another potential explanation, given the fact that vinclozolin binds progesterone receptor in vitro, is that it is affecting the receptor directly.
At the molecular level, vinclozolin down-regulated expression of estrogen receptors α and β in females and up-regulated progesterone receptor expression. Intuitively, the results with estrogen receptor suggest a lessening effect of estrogen, which would be consistent with our findings of virilization in the females. Androgen receptor expression remained unchanged in females. We speculate that the virilization that occurred in the female genital tubercle may have been the result of indirect estrogen antagonism; vinclozolin does not appear to be an estrogen receptor agonist or antagonist . It may enhance aromatase activity, and the up-regulation in aromatase activity may exhaust available substrate with the ultimate effect of lessening available estrogen. In addition, if signaling through the progesterone receptor is involved in the development of endpoints that are thought to be androgen-mediated, the increase in progesterone receptor in females may provide more opportunity for the development of these ostensibly androgen-associated phenotypes.