Endocrine Abstracts

Androgens influence transcription of their target genes through the activation of the androgen receptor (AR). Binding of the AR to DNA motifs in these genes is a critical step in this signal transduction process. The DNA motifs, called androgen response elements (AREs), can be classified in two classes: the classical AREs, which are in vitro also recognized by the other steroid hormone receptors, and the AR-selective AREs, which display selectivity for the AR. For in vitro interaction with these selective AREs, the AR DNA-binding domain is dependent on specific residues in the D-box in its second zinc-finger, and residues in a carboxyterminal extension of the classical DBD. To evaluate the physiological relevance of selective AREs, we generated a transgenic mouse model, termed SPARKI (Specificity affecting AR knock-in), in which the second zinc-finger of the AR was replaced by that of the glucocorticoid receptor (GR), resulting in a chimeric protein that retains its ability to bind classical AREs, but is unable to bind selective AREs. The reproductive organs of SPARKI males are smaller compared to wild type animals and they are subfertile.

The number of Sertoli cells is reduced to ∼60% and this probably explains why the testis size is reduced similarly. Spermatogenesis is impaired only marginally at the level of the second meiotic division. The epididymides are also affected, and a high percentage of sperm cells have deformed tails.

The expression of several androgen-responsive genes is differentially affected in the SPARKI testes. This is correlated with the presence of different types of response elements. The most affected gene, pem or Rhox5 gene has a selective ARE near its promoter, while the promoter of a less affected gene (eppin) has a classical ARE.

Surprisingly, SPARKI males do not display any anabolic phenotype: the growth curve of male and female SPARKI is super-imposable with that of wild type littermates. Therefore, the selective elements or the second zinc finger of the AR do not seem to be involved in the regulations of bone and muscle homeostasis.

In this report we present in vivo evidence for the existence of two functionally different types of AREs and the role of the second zinc finger in gene specificity. We also demonstrate that AR-regulated gene expression can be targeted based on this distinction.