Myeong H. Jung

Regulation of Androgen Action

Publisher Summary This chapter describes the different aspects of regulation of androgen action. Androgens belong to a class of C-19 steroids secreted primarily by the testis and adrenal cortex. Hormonally active androgens promote reproductive and anabolic functions. Both reproductive and anabolic effects of androgens are mediated by their interaction with the androgen receptor (AR), a member of the steroid-thyroid hormone-retinoid-vitamin D superfamily of nuclear receptors (NRs) that function as ligand-activated transcription factors. Almost all of the androgen functions, except its conversion to estrogen by the enzyme aromatase in certain target cells are known to be mediated by the androgen receptor. The androgen receptor are coded by a single copy gene, which is located on the X chromosome. The functional relevance of the segmented domain structure of the NR superfamily is supported by the results of deletion mutagenesis and domain swapping among various receptor proteins. The interaction between the amino terminal and the steroid-binding carboxy-terminal end in the AR transactivation function was initially indicated by the finding that a segment within the hormone-binding domain exerts an inhibitory influence in the transcription regulatory activity of the AR, and the deletion of this region results in ligand-independent activation of the receptor.

Tissue-specific and Androgen-repressible Regulation of the Rat Dehydroepiandrosterone Sulfotransferase Gene Promoter

Dehydroepiandrosterone sulfotransferase (Std) catalyzes sulfonation of androgenic steroids and certain aromatic procarcinogens. In rats, this enzyme is selectively expressed in the liver, and its expression is strongly repressed by androgens. DNase I footprinting and electrophoretic mobility shift analyses revealed two hepatocyte nuclear factor-1 (HNF1), three CCAAT/enhancer-binding protein (C/EBP), and one consensus palindromic thyroid hormone response elements within the first 215 base pairs (bp) of the promoter sequence of rat Std. This promoter is normally inactive in fibroblast-derived NIH 3T3 cells. However, overexpression of HNF1 and C/EBP resulted in synergistic activation of the Std promoter in this cell type, indicating essential roles of these two trans-regulators in liver-selective expression of the rat Std gene. On the other hand, point mutations at any one of five cis elements proximal to the −215 bp region markedly reduced reporter gene expression, suggesting that all of these sites are important for overall promoter function. Androgenic repression of the Std gene in rat liver can be recapitulated in androgen receptor (AR)-negative HepG2 hepatoma cells after cotransfection with an AR expression plasmid. Functional assay of a nested set of 5′-deleted promoters mapped the negative androgen response region between positions −235 and −310. Antibody supershift and oligonucleotide competition identified three OCT-1 and two C/EBP elements between bp −231 and −292. An additional OCT-1 site was found to overlap with a C/EBP element at the −262/−252 position. Mutational inactivation of any one of five cis elements within the −231/−292 region abolished negative androgen response. However, none of these cis elements showed DNase I protection by recombinant AR in footprinting assay, suggesting the absence of a direct AR-DNA interaction. Thus, these studies on rat Std promoter function indicate that (i) HNF1 and C/EBP are responsible for liver specificity of the ratStd gene; (ii) androgenic repression of the gene requires the presence of all of the OCT-1 and C/EBP elements between positions −231 and −292; and (iii) AR may exert its negative regulatory effect indirectly through transcriptional interference of OCT-1 and C/EBP rather than through a direct DNA-AR interaction.

THE EVOLUTIONARY TANGLE OF AGING, SEX, AND REPRODUCTION AND AN EXPERIMENTAL APPROACH TO ITS MOLECULAR DISSECTION

Exchange of genetic materials by two individual members of the same species is considered to be the origin of primitive sex. During evolution, this primitive form of molecular sex has been transformed into a complex biological function involving specialized sexual structures and multiple hormonal interactions. Development and maintenance of these reproductive structures are also dependent on hormones and hormone receptors. Furthermore, reproductive specialization in higher forms of life has led to customized species-specific rates of aging and life-span potentials that are commensurate with the reproductive needs of the particular type of organism. Because of this reproductive imposition on aging of the organism, temporal regulation of the hormone response is a significant component of the genetics of aging. We have observed a marked age-dependent alteration in the hepatic expression of the rat androgen receptor (rAR) gene. Among the large number of transcription factors that control the rAR gene, at least three appear to participate in its age-dependent regulation. Two of these are positively acting and yet/to be characterized transcription factors, while the third is a negative regulator the nuclear factor kappa B (NF-kappa B). NF-kappa B is the major trans-regulator for genes involved in the immune response, inflammation, and oxidative stress. Involvement of NF-kB in the modulation of both oxidative stress and sex function provides the first example of a common molecular link between sex and aging.