Pesticides Research No. 111 2007 – Effects of azole fungicides on the function of sex and thyroid hormones

Effects of azole fungicides on the function of sex and thyroid hormones

6 Perspectives

6.1 Research perspectives
6.2 Regulatory perspectives

6.1 Research perspectives

The results from this study demonstrate that the triazole fungicides, tebuconazole, propiconazole, and epoxiconazole, like the imidazole fungicide prochloraz, are able to act as endocrine disruptors during several pathways. In addition, the results strongly indicate that interference with steroid hormone synthesis is the major mechanism behind the observed effects on reproductive development in offspring after prenatal exposure. The results indicate that inhibition of CYP17 (an enzyme involved in the conversion of cholesterol to progesterone and testosterone) may be one of the targets but other steroidogenic enzymes may also be affected. This issue deserves further studies in order to elucidate which enzymes are affected and the relative impact of this for the total response in vivo.

Male fetuses are considered to be more sensitive towards endocrine disruptors than female fetuses because development of the male phenotype requires more hormonal regulation than female development (Sharpe, 2006). However, the two triazoles tebuconazole and epoxiconazole both virilized female offspring while the effects on male offspring were less pronounced. Further studies on virilizing effects on female pups after exposure to endocrine disrupters are wanted to confirm these observations and to elucidate the mechanism behind. The impact of virilization for the female fertility as adults would also be interesting to investigate.

Some phthalates esters (DBP, DEHP, and BBP) disrupt male reproductive development after in utero exposure in rats (Foster, 2006) although these substances do not block the AR (Mylchreest et al., 1999). The effects include decreased AGD, nipple retention and malformations of the external genitalia including hypospadias and cryptorchidism. The critical mechanism for induction of these effects is a marked reduction in fetal testicular testosterone production at the critical window for development of the reproductive tract. Hence, in several ways the effects observed for the azole fungicides resembles those seen for the phthalates. Phthalates also had other effect on Leydig cell function including reduced production of insulin-like factor 3 (insl3) and changes in gene-expression of a number of enzymes and transport proteins involved in normal testosterone biosynthesis and transport. It would be obvious to further investigate if azole fungicides have similar effects on Leydig cell function as the phthalates.

The recent observation, that fetal exposure to vinclozolin or methoxychlor during gonadal development caused decreased spermatogenetic capacity, not only in the exposed males, but also in subsequent generations is a matter of concern. Hence, the ability of other endocrine disruptors, inclusive azole fungicides, to reprogram the germ line and to produce a trans-generational effect on reproductive capacity should be investigated. These studies should include examination of sperm production and apoptosis in spermatogenic cells.

6.2 Regulatory perspectives

The endocrine disrupting properties of azole fungicides demonstrated in this study, as well as in our previous studies on prochloraz, identify azole fungicides as a group of endocrine disruptors capable of inducing effects via several different mechanisms. This makes the risk assessment more complex since many different end-points have to be considered but in general we strongly recommend that all azole fungicides are regarded and regulated as endocrine disruptors until the individual substances have been properly tested.

The results from this study indicate that interference with key-enzymes involved in steroid hormone biosynthesis may be a central mechanism behind the observed effects on reproductive development. This illustrates the importance of applying assays for disruption of steroidogenesis when chemicals are screened for potential endocrine disrupting properties, since chemicals with weak or no affinity for the ER and/or AR may still induce endocrine disruption in vivo during this mechanism.

Since azole fungicides are commonly used in agriculture, the population might be exposed from residues in food products and from drifting from sprayed areas. In addition, occupational exposure takes place in agriculture and horticulture. The azole fungicides seem to share several modes of actions and therefore the effects might be additive if the population are exposed to more azole fungicides simultaneously. This is very likely to occour via the diet. Hence, there is a need for better surveillance of the exposure situation of these fungicides. This could be obtained by including more of the azole compounds in the surveys of food products. Especially those used in largest amounts, such as epoxiconazol, should be included. In addition, development of biomarkers to investigate the exposure level in occupationally exposed groups and in samples from the general population is warranted. Until more is known about the potential risk of these compounds, efforts should be taken to minimize the exposure of especially women in the childbearing age group as well as children to these compounds.

Prochloraz is no longer approved for sale in Denmark after 2006. However, the compound has not been banned for used by the authorities and therefore remains can be used legally and according to experience from other products this process may take several years. A more effective way to reduce the exposure of the Danish population would be a regular ban, sine this would minimize (ideally eliminate) occupational exposures and exposure from Danish produced food stuffs immediately. In addition, a ban would also give a signal to other countries about the potential health risk assessed for this pesticide.