Effects of endocrine disruptors on adreno-cortical and Leydig cell steroidogenesis

Abstract

A common sequential pathway for steroidogenesis, designed to produce steroid hormones, is expressed in the adrenal cortex, testes and ovaries. In this process, cholesterol is converted to all known steroids by specific enzymes in a tissue-specific manner. In addition to the maintenance of reproduction, fertility and normal sexual development by androgens and estrogens, mineralocorticoids and glucocorticoids, other steroids produced by the adrenal cortex, play important roles in the regulation of salt balance and responses to stress, respectively. Steroidogenesis is controlled by interactions between the hypothalamus and pituitary in the form of a negative feedback loop, and, disruption of any step in this loop exerts an impact on the entire axis.

In recent years, numerous anthropogenic or naturally occurring compounds in the environment have been reported to influence the homeostasis of endocrine systems. These xenobiotics, referred to as endocrine disrupting chemicals (EDCs), may disrupt the development and/or functioning of various physiological systems, including reproduction, fertility, sexual differentiation and stress responses, both in wildlife and humans. Such disruption may be involved in the increased incidences of testicular cancer, hypospadias and cryptorchidism, as well as the decline in sperm count observed in humans during the past decades. Accordingly, the aim of the present thesis was to explore the effects of several xenobiotics (i.e., the anti-androgen procymidone, the phytoestrogens genistein and resveratrol, and sesquiterpene lactone helenalin and phthalates) on the function of the pituitary-gonadal and pituitary-adrenal axes and the biosynthesis of sex steroids and glucocorticoids, employing in vitro and ex vivo cultures of rat Leydig and adrenocortical cells as a model system.

Dietary administration of procymidone to male rats resulted in elevated serum levels of luteinizing hormone (LH) and testosterone. Furthermore, Leydig cells isolated from these animals displayed an enhanced capacity for the production of testosterone in response to stimulation by human chorionic gonadotropin (hCG) or (Bu)2cAMP, as well as elevated expression of steroidogenic acute regulatory (StAR) protein, cytochrome P450scc (P450scc) and cytochrome P450c17 (P450c17). In contrast, diets containing genistein inhibited the ex vivo steroidogenic response of Leydig cells to hCG or (Bu)2cAMP by down-regulating their expression of P450scc. In a similar manner, phytoestrogen resveratrol suppressed corticosterone production by primary cultures of adrenocortical cells both in vitro and ex vivo in association with a decrease in the expression of cytochrome P450 c21-hydroxylase. Helenalin, a sesquiterpene lactone produced by several species of the Asteracea family of plants, inhibited both adrenocorticotropic hormone (ACTH) - and hCG-activated steroidogenesis in primary cultures of rat adrenocortical and Leydig cells as a result of attenuated expression of the StAR protein, which mediates cholesterol transport into mitochondria.

Further experiments demonstrated that administration of di-2-ethylhexyl phthalate (DEHP) to male rats causes age-dependent alterations in their pituitary-adrenocortical axis in vivo and adrenocortical steroidogenesis ex vivo. Thus oral exposure to this phthalate ester elevated serum levels of ACTH and corticosterone in rats 20 and 40 days of age, but not in adult, 60-day-old animals. In addition, primary cultures of adrenocortical cells isolated from the two younger groups of rats treated with DEHP exhibited enhanced production of corticosterone in response to stimulators, as well as elevated ACTH-stimulated transport of endogenous cholesterol into mitochondria.

Together, these findings reveal that the EDCs examined can influence the function and regulation of the pituitary-gonadal and pituitary-adrenal axes in rats, as well as impair their adrenocortical and Leydig cell steroidogenesis by suppressing the expression of several proteins involved in steroidogenesis (e.g., StAR, P450c21 and P450scc). Such effects might disrupt reproductive potential, fertility and homeostasis in both humans and wildlife.