Kamin Johnson, Ph.D.
|Robert W. Mason, Ph.D.
N. Carolyn Schanen, M.D., Ph.D.
Reproductive tract malformations such as cryptorchidism and hypospadias are the most common male birth defects in developed countries. An environmental cause is suggested by epidemiological studies and exposure of rats to endocrine disrupting agents that recapitulate human malformations. However, there is little data linking human endocrine disruptor exposure and malformations. Participants at a recent NIH-sponsored translational workshop concluded that human studies are constrained by a lack of mechanistically relevant biomarkers of exposure and adverse effect. This is the important knowledge gap we propose to close.
Phthalates are endocrine disruptors with ubiquitous human exposure that, in rats, lead to male reproductive maldevelopment via inhibition of fetal Leydig cell testosterone production. This phenotype is due to decreased steroidogenic and lipid metabolism gene expression, but upstream causative factors are unknown. In rat Leydig cells, steroidogenesis requires arachidonic acid mobilization via action of fatty acyl coA synthetase (Acsl4 and Acsbg1) and acyl-CoA thioesterase (Acot), and gonadotropin releasing hormone receptor (Gnrhr) signaling within Leydig cells inhibits steroidogenesis. Intriguingly, rat phthalate exposure alters testicular mRNA levels of these genes. Arachidonic acid metabolism is a component of Leydig cell Gnrhr signaling. Plasma Gnrh levels are relatively high in fetal and neonatal rats but nearly undetectable in postpubertal rat testis or plasma, which coincides with susceptibility to phthalate endocrine disruption. Unlike rats, mice are resistant to phthalate endocrine disruption, and mouse phthalate exposure produces no change in Acot, Acsl4, Ascbg1, or Gnrhr expression. In fact, the refractory mouse testis does not express Gnrhr. This constellation of data suggests that Leydig cell Gnrhr signaling is required for endocrine disruption by phthalates.
We aim to identify mechanistically relevant biomarkers of phthalate exposure that can be applied to human translational studies. We will use placenta as a surrogate tissue because it is a readily available, steroidogenically active tissue in both rodents and humans. In a preliminary microarray study, we identified several hundred rat placental genes with altered expression following phthalate exposure, and (like the testis) lipid metabolism gene expression was significantly affected. Based upon these observations, we hypothesize that placental biomarkers of phthalate exposure predict fetal testis endocrine disruption that requires testicular Gnrhr signaling.