Selective DNA binding by the androgen receptor as a mechanism for hormone-specific gene regulation

Steroid hormones control many physiological processes by activating specifi c receptors that act as transcription factors. In vivo, each of these recep tors has a specific set of target genes, but in vitro the glucocorticoid, p rogesterone, mineralocorticoid and androgen receptors (class I receptors) a ll recognise response elements which are organised as inverted repeats of 5 '-TGTTCT-3'-like sequences with a three nucleotide spacer. This poses the q uestion how the in vivo specificity of the different steroid responses is m ediated. To unravel the mechanisms involved, we have compared the structura l features of the androgen-selective enhancers of the probasin, the secreto ry component and the sex-limited protein genes with those of non-selective enhancers in the mouse mammary tumour viral promoter and the C3(1) gene. Th e probasin promoter contains an androgen response element which is recognis ed with high affinity by the androgen receptor, but not by the other class I receptors. Swapping experiments between the DNA-binding domains of the an drogen and glucocorticoid receptor revealed that it is not the first zinc f inger, but rather the second zinc finger and part of the hinge region which contribute to this specificity. Three AR-specific aminoacids are involved in the probasin ARE recognition, but not in the C3(1) ARE binding by the AR . The location of these residues strongly suggests that an alternative dime risation interface is involved in the probasin ARE binding. We could subseq uently demonstrate that the AR binds direct repeats of 5'-TGTTCT-3'-like se quences in gel retardation assays as well as in transfection experiments. M oreover, the androgen-specific enhancers all contain direct repeats, and po int mutations that change the nature of these elements into inverted repeat s result in a change of specificity. It seems, therefore, that direct repea t elements can be the determinants of the AR-specificity, It will be exciti ng to learn how such DNA elements will affect the properties of the recepto r dimer with respect to ligand binding, interactions between the aminotermi nal domain and the ligand-binding domain, the recruitment of co-activators and cooperativity with other transcription factors. (C) 2001 Elsevier Scien ce Ltd. All rights reserved.