The endocrine disrupting activities of major industrial chemicals - the phthalate esters and 4-nonylphenol

Abstract

A number of widely used industrial chemicals have been shown to possess endocrine-disrupting properties. In this thesis, a series of in vitro tests, and an in vivo reproductive performance test with fathead minnows, were used to clarify the extent of estrogenic activity exhibited by the phthalate esters - a class of compound hitherto referred to as 'estrogenic'. Using a recombinant yeast estrogen screen, I demonstrated that a small number of commercially available phthalates showed extremely weak estrogenic activity. The most potently estrogenic phthalate of those tested was BBP, which was approximately one million-fold less potent than 17B-estradiol. The phthalates which were estrogenic in the yeast screen were also mitogenic on estrogen-responsive human breast cancer cells (MCF-7 and ZR-75 cell lines). The most prolifically used phthalate, DEHP, was not estrogenic in any of these assays. The small number of metabolites of phthalate metabolites tested (including MBuP, MBzP, MEHP and MnOP) were also not estrogenic in the recombinant yeast assay. The ability of BBP (as the most potently estrogenic phthalate in vitro) to induce a vitellogenic response (an indicator of estrogen exposure) in fathead minnows (Pimephales promelas) exposed via the water was assessed. No induction of vitellogenin was observed, indicating that 100 Jlg BBP/L (a concentration higher than would normally be found in the environment) is not estrogenic to this species of fish under the conditions employed for this experiment. In the same study, fecundity of breeding pairs of fathead minnows was assessed; exposure to BBP was not found to affect reproductive performance in these fish. A possible alternative mechanism of action of the way in which the phthalates induce frequently reported reproductive disorders was observed. Some of the phthalates, and, notably, some of their metabolites, were demonstrated to act as anti-androgens in a recombinant yeast androgen assay. 4-Nonylphenol is another industrial chemical which is used in large volumes, and due to the nature of its use (mainly in detergent formulations), is discharged into water systems via sewage effluents. This chemical has been shown to be estrogenic to fish at the concentrations at which it has been detected in the environment. 4-Nonylphenol was tested for its ability to affect plasma and pituitary gonadotropin levels in female recrudescing rainbow trout (Oncorhynchus mykiss). Plasma and pituitary levels of FSH were suppressed in fish exposed to 10 and 100 Jlg 4-NP/L. In addition, FSH gene expression was reduced in these fish, and also in the fish exposed to 1 Jlg 4-NP/L. Pituitary LH content and gene expression of this hormone were suppressed in the fish exposed to 100-, and 10- and 100 Jlg 4-NP/L respectively. Gonadal development in vertebrates is regulated by FSH. Ovarian development ceased in the fish exposed to 100 JAg 4-NP/L, possibly as a result of the suppression of FSH synthesis and/or release in these fish.