Effects of Tamoxifen on the Sex Determination Gene and the Activation of Sex Reversal in the Developing Gonad of Mice

Tamoxifen, as well as most endocrine-disrupting chemicals, affects the reproductive system and sexual development, but little is known about its disruption of the molecular pathways regulating mammalian sex determination. In fetal mice, the expression levels and pattern of key genes involved in controlling sexually dimorphic balance were analyzed both in vivo and in vitro by using whole-mount in situ hybridization and quantitative-PCR. Developmental tamoxifen exposure induced abnormal up-regulation of the testis differentiation marker Pdfgra in Leydig cells and of Sox9 and Fgf9 in Sertoli cells in XX gonad. Immunohistochemistry analysis confirmed the over-expression of SOX9 protein. Accordingly, the ovary development marker Foxl2 was depressed at both the mRNA and protein levels. The increase in testosterone and the reduction in 17β-estradiol and progesterone were observed by using the in vitro assay with organotypic cultures. Taken together, results indicated that tamoxifen induced the ectopic expression of well-established sex-specific genes during the critical developmental period, thus resulting in abnormal testicular development in the XX gonad of mammals. This study facilitates a better understanding of the molecular mechanisms of antiestrogens and possibly of compounds that interrupt estrogen signaling by other modes of action, and the association with the pathogenesis of human sexual developmental disorders.