Androgen-Binding Proteins in Human Benign Prostatic Hypertrophy

Abstract

Prostatic samples were surgically removed from 7 patients suffering from benign prostatic hypertrophy. High-speed supernatants (cytosol) containing 20-25 mg of protein/ml were prepared. Glycerol gradient ultracentrifugations were performed, using cytosol labeled at 0 C with 2-5 nM 3H-17beta-hydroxy-androstan-3-one (androstanolone or dihydrotestosterone) alone, or in the presence of 50-250-fold excess of androstanolone, estradiol, or androstane-3alpha, 17beta-diol (androstanediol). Two high-affinity saturable binding components were observed. One binding component was the androgen receptor. Its sedimentation coefficient was 8 S in low-salt medium. It had a high affinity for androstanolone. The binding of 3H-androstanolone was strongly completed by androstanolone itself, less by estradiol, and not by androstanediol. In one case, endogenous androstanolone found in the 8 S region of glycerol gradients was measured by radioimmunoassay, and it was calculated that more than 90% of the cytosol receptor binding sites might be occupied by this steroid while the total binding capacity of the 8 S receptor was estimated to approximate 2.6 pmol of androstanolone/g of prostate. No testosterone was found in the receptor fraction. The second binding component was attributable, at least in part, to the sex steroid-binding plasma protein (SBP), as indicated by its sedimentation coefficient (congruent to 4 S in low salt medium), its high affinity for androstanolone and androstanediol and its lower affinity for estradiol, and finally, its migration on polyacrylamide gel electrophoresis. In one instance, the concentration of the SBP-like protein in prostate cytosol was measured by equilibrium dialysis, and it was calculated that the binding capacity of the prostate SBP-like component corresponded to 4 pmol of androstanolone/g of prostate, a small (less than 5%) value with regard to SBP concentration in the plasma of the same patient. The blood contamination of the cytosol, as obtained from the measurement of hemoglobin, did not account for the amount of SBP found in the prostate sample. Since SBP-like protein is probably of plasma origin, to determine whether SBP was located in the extracellular space or inside the prostate cells, BPH slices from another patient were incubated in the presence of 3H-testosterone, the cytosol was prepared, and was fractionated by Sephadex G-150 column chromatography. The androstanolone/testosterone ratio in the receptor-containing peak was high (1.7), whereas in the incubation medium it was very low (0.08). In the peak containing the SBP-like protein, the ratio was 0.74, which may suggest that all or part had been exposed to the predominant androstanolone environment inside the prostatic cell.

Specific binding of human corticosteroid-binding globulin to cell membranes.

“…Taken together, our results on the CBG and TEBG receptors, along with the recently published report of a TEBG receptor in human decidual endometrium membrane (33), raise the possibility that CBG and TEBG may serve to transport steroid hormones into target tissues via specific membrane receptors. Since it is generally accepted that the specificity of receptor-mediated endocytosis resides exclusively in the initial binding step between a ligand and its receptor (34), this hypothesis both supports and is supported by previous reports indicating the intracellular presence of CBG in various cortisol and progresterone target cells (2, 6-15, 18, 20); the presence of CBG-like molecules on membranes from rat pituitary (17) and rat liver cells (16); the intracellular presence of TEBG in various androgen and estrogen target cells (35)(36)(37)(38)(39); and the observation that cortisol bound to CBG is physiologically active (5,40). However, direct evidence for the above hypothesis is not yet at hand, and it is possible that the binding of CBG to its receptor may subserve an as yet unknown function that is unrelated to the transport of steroids into target cells.…”

Autoradiographic and Biochemical Studies of Hormone Receptor Localization,

“…In the baboon we find equal amounts bound in the nucleus and free in the cytosol. This has been a common finding in human prostate tissue (Rosen et al, 1975;Shain et al, 1978); in fact, these authors report 10 times more receptor bound in the nucleus than is free in the cytosol. Also, more bound nuclear receptor was found in the rat prostate than can be accounted for by a loss in free cytoplasmic receptor (Bruchovsky et al, 1975).…”

Immunocytochemical localization of the sex steroid-binding protein of plasma in tissues of the adult monkey Macaca nemestrina.

“…Various reports have appeared suggesting that, in humans, SBP may enter interstitial fluid and tissues and, therefore, may not be exclusively localized in the plasma (10)(11)(12)(13). Others, however, have suggested that this observed presence of SBP in cell extracts is due to plasma contamination when the tissues are homogenized.…”