The Center for Alternatives to Animal Testing is an academic center affiliated with the Division of Toxicological Sciences in the Department of Environmental Health Sciences of the Johns Hopkins University Bloomberg School of Public Health.

 

Johns Hopkins School of Public Health

Abstract for TestSmart -- Endocrine Disruptors

Androgen receptor structure and transcriptional activity

Elizabeth M. Wilson
Laboratories for Reproductive Biology, Departments of Pediatrics and Biochemistry, University of North Carolina, Chapel Hill NC 27599

The androgen receptor (AR) mediates androgen dependent transcriptional activation required for normal male sexual development. The overall structural arrangement of the AR is similar to other steroid receptors, with a carboxyl-terminal high affinity steroid binding domain, a central DNA binding domain, and a major transactivation domain in the NH2-terminal region. AR functional activity can be disrupted or stimulated by environmental compounds. A number of environmental antiandrogens have been reported and include metabolites of pesticides and fungicides. It remains to be established whether levels of these compounds are sufficiently high in the environment to be significant health risks. More recently environmental androgens that result in masculinization of female fish were identified in river water. Polluted waters downstream of paper mills present a significant risk to wildlife development and propagation as well as a potential threat to human populations. A challenge in this field is to establish in vitro assays that reliably detect and reflect androgen agonist and antagonist activities as they occur in vivo. The development of such assays is confounded by several factors. These include the frequent requirement for in vivo metabolism of environmental compounds to active metabolites, the availability of steroid receptor expression and reporter plasmids for transfection assays to measure agonist-induced or antagonist-inhibited transcriptional activity, and the reliability of reporter gene assays for the detection of activity that reflects what would occur in vivo. A unique aspect of AR structure and function is an androgen specific interaction between the NH2-terminal and carboxyl-terminal domains (N/C interaction). The importance of the N/C interaction in AR functional activity in vivo is supported by single amino acid mutations in the AR gene that disrupt this interaction without altering high affinity androgen binding affinity. These mutations result in partial or complete loss of AR function which is expressed phenotypically as the androgen insensitivity syndrome. Evidence indicates that under normal physiological conditions, true AR agonists induce the N/C interaction and AR antagonists generally inhibit it. However, some steroid derivatives that are not androgen agonists in vivo and accordingly fail to induce the N/C interaction can nevertheless be active in traditional reporter gene transient transfection studies. Thus a number of factors must be considered in the evaluation of environmental chemicals with potential impact on AR functional activity.