Pre-receptor regulation of the androgen receptor

Abstract

The human androgen receptor (AR) is a ligand-activated nuclear transcription factor and mediates the induction of genes involved in the development of the male phenotype and male secondary sex characteristics, as well as the normal and abnormal growth of the prostate. We have identified the pair of hydroxysteroid dehydrogenases (HSDs) that regulate ligand access to the AR in human prostate. We find that type 3 3alpha-HSD (aldo-keto reductase (AKR)1C2) catalyzes the NADPH dependent reduction of the potent androgen 5alpha-dihydrotestosterone (5alpha-DHT) to yield the inactive androgen 3alpha-androstanediol (3alpha-diol). We also find that RoDH like 3alpha-HSD (RL-HSD) catalyzes the NAD(+) dependent oxidation of 3alpha-diol to yield 5alpha-DHT. Together these enzymes are involved in the pre-receptor regulation of androgen action. Inhibition of AKR1C2 would be desirable in cases of androgen insufficiency and inhibition of RL-HSD might be desirable in benign prostatic hyperplasia.

Figure 1

Pre-receptor regulation of hormone action mediated by HSDs. The example of 11β-HSDs. Regulation of the mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) by 11β-hydroxysteroid dehydrogenase type 2 and type 1, respectively (A); The example of 3α-HSDs. Regulation of the androgen receptor (AR) by 3α-hydroxysteroid dehydrogenase isoforms (AKR1C2 and RL-HSD), (B)

Figure 2

Sources of testosterone and 3α-androstanediol in adult male. Adult male (A) and andropause (B) The solid box shows steroidogenesis in Leydig cells, and the dotted-box shows steroidogenesis in the adrenal gland. The pathway in gray shows a backdoor pathway to 5α-DHT via 3α-diol which does not involve the intermediates Δ⁴-androstene-3,17-dione or testosterone. Enzymes involved in the individual steps are italicized.

Figure 3

Molecular modeling explains why AKR1C1 and AKR1C2 preferentially reduce 5α-DHT to yield 3β-diol and 3α-diol, respectively. DHT is docked into the active site of AKR1C1 (light gray) and AKR1C2 (dark gray). Adapted from Jin and Penning 2006a.

Figure 4

Trans-activation of the androgen receptor by 3α-androstanediol in mammalian cells. Trans-activation of the AR by 3α-diol in the presence of oxidative 3α-HSDs. (A), Activation of the (ARE)₂-tk-CAT reporter gene by the AR in the presence of co-transfected HSDs versus the concentration of 3α-diol (10⁻¹² to 10⁻⁶ M); (B) the calculated EC₅₀ values to reach a 100% trans-activation; where 100% trans-activation is the maximal response seen with 5α-DHT. The fold increase in chloramphenicol acetyl transferase (CAT) activity seen at maximal response was 30-fold; and (C) The cellular basis of the assay. Abbreviations, T = testosterone. Adapted from Bauman et al., 2006b.

Figure 5

Expression of AKR1C2, RL-3α-HSD and AR in prostate stromal and epithelial cells. Representative scatter box plots of transcripts of enzymes (AKR1C2 and RL-HSD) that regulate ligand access to the androgen receptor (AR) in epithelial and stromal cells from normal and diseased patients (CaP and BPH) using real-time RT-PCR panels A–D. Expression of the AR in the same cell types panels E–F. One μg of total RNA was reverse-transcribed to cDNA from the epithelial and stromal cells and 50 ng of cDNA was added to each real-time PCR experiment that was performed in triplicate with the mean shown for each sample. Data is normalized to the housekeeping gene PBGD and is expressed as fg of each transcript per ng of total cDNA. Normal (n=14), CaP (n=14) and BPH (n=6) epithelial cells and normal (n=15), CaP (n=16) and BPH (n=21) stromal cells were used for the study, adapted from Bauman et al., 2006a.