DNA and ribonucleotide binding characteristics of two forms of the androgen receptor from rat prostates

Maintaining and reprogramming genomic androgen receptor activity in prostate cancer

Prostate cancer treatment is dominated by strategies to control androgen receptor (AR) activity. AR has an impact on prostate cancer development through the regulation of not only transcription networks but also genomic stability and DNA repair, as manifest in the emergence of gene fusions. Whole-genome maps of AR binding sites and transcript profiling have shown changes in the recruitment and regulatory effect of AR on transcription as prostate cancer progresses. Defining other factors that are involved in this reprogramming of AR function gives various opportunities for cancer detection and therapeutic intervention.

Mechanism of androgen action: recent observations on the domain structure of androgen receptors and the induction of EGF-receptors by androgens in prostate tumor cells

In this paper two different aspects of androgen action are reviewed. Polyacrylamide gel electrophoresis of androgen receptors, photoaffinity labeled with R1881 showed that receptors isolated from both human prostate cells and calf uterine cytosol cells are proteins with a molecular mass of approx 110 kD. Purification to homogeneity of this form of the receptor from calf uterus also yielded a 110 kD protein. A molecular model for the DNA-binding form of the receptor is presented in which one polypeptide comprises three active domains: one for ligand binding, one for interaction with nuclear acceptor sites, and a third domain which modulates nuclear interaction. Mild digestion with chymotrypsin or a protease from rat prostates removes the modulating domain and leaves the ligand binding and nuclear interaction domain intact. Trypsin treatment yields a fragment of lower molecular mass containing the ligand binding domain with some affinity for RNA, but not DNA. In vitro studies with a human prostate tumor cell line (LNCaP), suggest that androgens not only directly effect cell growth, but also act indirectly. Both epidermal growth factor (EGF) and androgens stimulate cell growth. In addition androgens stimulate synthesis of receptors for EGF. Thus androgens effect tumor cell growth by autocrine or paracrine mechanisms by making the cells more sensitive for growth factor mediated stimuli.

Purification of the intact monomeric 110 kDa form of the androgen receptor from calf uterus to near homogeneity

In the present study, calf uterine tissue has been used for isolation of androgen receptors. This tissue appeared to be a favourable source for large-scale purification of androgen receptors, because of the relatively high level of androgen receptors and the low concentration of proteolytic enzymes. The purification involved differential phosphocellulose and DNA affinity chromatography as first steps. The non-transformed receptor was passed through these matrices in order to remove contaminating DNA-binding proteins. After a transformation step to the DNA-binding state, the receptor was bound to DNA cellulose and subsequently eluted with MgCl2. A 0.5% pure androgen receptor preparation was obtained. Photoaffinity labelling with [3H]R1881 (methyltrienolone) was used to determine the size of the receptor at this stage of purification and during the following steps. Subsequently, isoelectric focussing of the partially purified androgen receptor preparation in an aqueous glycerol gradient was performed. In this step, the progesterone receptor, which is copurified with the androgen receptor protein during the first part of the purification procedure, focussed at pH 5.5 while the androgen receptor could be isolated at pH 5.8. The isoelectric focussing procedure could be applied in a preparative way for further purification of androgen receptors. After this step an approx. 8% pure preparation was obtained. Polyacrylamide gel electrophoresis of S-carboxymethylated androgen receptor was used as the final purification step. The [3H]methyltrienolone labelled androgen receptor from calf uterus was purified to homogeneity and consisted of one polypeptide with a molecular mass of 110 kDa.

Analysis of steroid- and DNA-binding domains of the calf uterine androgen receptor by limited proteolysis

The DNA-binding form of the calf uterine androgen receptor (AR) was subjected to limited protease digestion using chymotrypsin, trypsin and a rat prostate cytosol protease. The properties of the generated polypeptide fragments were identified and compared with those of the intact AR. Physicochemical characterization was achieved through sedimentation analysis, gel filtration chromatography and DEAE anion exchange chromatography. Intactness of functional binding domains was evaluated by measuring the retention of steroid- and DNA-binding capacity. Under non-denaturing conditions the intact AR is a highly asymmetrical molecule with a Stokes radius (RS) of 45A, a sedimentation coefficient of 4.3S and a relative molecular mass of 80,000 daltons. This form of AR has an intrinsic binding affinity for DNA and was eluted from DNA-cellulose with 9 mM MgCl2. Chymotrypsin produced a more globular polypeptide (RS: 31A; 3.1S; 41,000 daltons) with a decreased net negative charge. This fragment also displayed DNA-binding affinity but required a higher concentration of MgCl2 (14 mM) for DNA-cellulose elution, indicating an increased affinity for DNA. The observed reduction in molecular size upon chymotrypsin treatment was confirmed when analysed by SDS-polyacrylamide gel electrophoresis after covalently labelling of the AR with [3H]R1881. Rat prostate cytosol contains a protease which is very active in generating an AR polypeptide with an increased affinity for DNA, without changing the AR net negative charge (RS: 33A; 3.7S; 51,000 daltons). The specificity of this protease remained unknown since none of a large number of inhibitors was able to inactivate this enzyme. The fragment generated is different from that obtained with chymotrypsin since significant differences in size as well as in charge were measured. Trypsin treatment generated a much smaller polypeptide (RS: 25A; 2.9S; 30,000 daltons) which had lost its DNA-binding capacity, but not its steroid binding site. This form probably represents the so-called meroreceptor. When intact AR was treated sequentially with prostate cytosol and trypsin, a polypeptide fragment with identical properties was obtained, indicating the spatial separation of two of the proteolytic cleavage sites. These studies provide evidence for the distinct nature of the molecular domains for androgen and DNA interaction on the calf uterine AR.

Characterization of androgen receptors after photoaffinity labelling with [3H]methyltrienolone (R1881)

The synthetic androgen 17 beta-hydroxy-17 alpha-[3H]methyl-4,9,11-estratrien-3-one (R1881) has been used as photoaffinity label to characterize androgen receptors in rat prostate, in a human transplantable prostatic adenocarcinoma (PC-82) and in calf uterus. Androgen receptors preparations were partially purified either via differential chromatography on 2',5'-ADP-Sepharose (rat prostate), via anion exchange fast protein liquid chromatography (rat prostate and PC-82) or via DNA-cellulose chromatography (calf uterus). Purification factors obtained with the three different methods were: 245, 75 and 40 respectively. Photolabelling of receptor preparations was performed via irradiation with a high pressure mercury lamp either before or after partial purification. Polyacrylamide gel electrophoresis under denaturing conditions showed that the DNA-binding form of the androgen receptor in calf uterus cytosol is a protein with a molecular mass of approx 95 kD. The covalent attachment of [3H]R1881 to the 95 kD protein could be completely suppressed by a 200-fold molar excess of dihydrotestosterone. In rat prostate cytosol an androgen receptor with a molecular mass of approx 50 kD could be photoaffinity labelled with R1881. A similar size was found for the androgen receptor in the human prostatic adenocarcinoma. Our results show that photoaffinity labelling of androgen receptors with [3H]R1881 as ligand can be applied for characterization of partial purified androgen receptor preparations.

Photoaffinity labelling of androgen receptors with 17 beta-hydroxy-17 alpha-[3H]methyl-4,9,11-estratrien-3-one

The synthetic androgen 17 beta-hydroxy-17 alpha-[3H]methyl-4,9,11-estratrien-3-one (R1881) has been used as photoaffinity label to characterize androgen receptors in calf uterus and rat prostate. Polyacrylamide gel electrophoresis under denaturing conditions showed that the DNA-binding form of the androgen receptor in calf uterus cytosol is a protein with a molecular mass of 98 kD. In rat prostate cytosol an androgen receptor with a molecular mass of 46 kD could be photoaffinity labelled with R1881. The photoaffinity labelling procedure described here provides a method for studying the hormone binding domain of androgen receptors in partial purified preparations.

Characterization of steroid hormone receptors with ion-exchange fast protein liquid chromatography

Androgen, estrogen and progesterone receptors have been characterized with anion exchange Fast Protein Liquid Chromatography (FPLC) on a Mono Q column (Pharmacia). In the presence of sodium molybdate androgen receptors in cytosols from rat prostate, rat epididymis and calf uterus eluted as a single sharp peak at 0.32 M NaCl with recoveries of approx 90%. The molybdate-stabilized form of the androgen receptor from rat prostate was purified about 75-fold. The receptor containing FPLC-peak fractions sedimented in high salt (0.4 M KCl) linear sucrose gradients at 3.6 S (prostate) and at 4.6 S (epididymis and calf uterus) respectively. Multiple forms of the androgen receptor were present in cytosols from rat prostate prepared in the absence of sodium molybdate, probably due to proteolytic breakdown of the native form. Calf uterine estradiol and progesterone receptors prepared in the presence of sodium molybdate (20 mM) eluted from the Mono Q column at 0.32 M NaCl. The molybdate-stabilized forms of the oestradiol and progesterone receptors were purified approx 70-fold and 30-fold respectively. In the absence of molybdate the estradiol receptor dissociated into two major forms eluting at 0.23 M NaCl and 0.37 M NaCl. After heat induced transformation (30 min at 25 degrees C) of the estradiol receptor one major peak was eluted at 0.42 M NaCl, indicating a change in the surface charge of the estradiol receptor as a result of the 4 S to 5 S transformation. It is concluded that the FPLC anion exchange system is a powerful, fast tool for characterization and partial purification of steroid receptors. In addition this technique could be applied as a rapid procedure for the quantitative estimation of steroid receptors in small biological samples.

Interaction of rat prostate androgen receptors with polynucleotides, RNA, DNA and cloned DNA fragments

Androgen receptors were partially purified from prostates of mature (non-castrated) rats by chromatography on 2',5'-ADP-Sepharose and labelled by exchange with 5 alpha-[3H]dihydrotestosterone. The partially purified receptor preparation was free of DNAase activity and sedimented at approx. 3 S. The specificity of the interaction of this androgen receptor with nucleotides was investigated in a competitive binding assay using inhibition of binding of the steroid receptor complex to ADP-Sepharose. Certain polyribonucleotides were strongly bound (e.g., poly(UG), poly(AU), poly(G) and poly(U] and competed more effectively for the receptor binding sites than prostate RNA. Restriction fragments of genomic clones from the genes which code for prostatic binding protein showed only moderate affinity for the 3 S receptor form. These data suggest that the 3 S form of the androgen receptor lacks the specific domain or conformation necessary for specific interaction with DNA, but retains a high affinity for certain forms of RNA. Some potent inhibitors of proteolysis (diisopropylfluorophosphate, leupeptin) did not have any effect on the form of the receptor isolated from mature intact animals. A possible function of the 3 S form in post-transcriptional processing is discussed.