Studies on the regulation of the concentration of androgens and androgen receptors in nuclei of prostatic cells

Abstract

Experiments were performed to assess the effect of intracellular androgen metabolism and the availability of cytoplasmic receptors on the concentration of androgens and androgen receptors in nuclei of prostatic cells. It was found that androgens are incorporated into the nucleus by a regulated, selective process which appears to limit the type and amount of androgen transported across the nuclear membrane. The metabolic conversion of testosterone to dihydrotestosterone which takes place in cytoplasm does not reduce transport and, very likely, affects only the ratio of testosterone and dihydrotestosterone transferred into the nucleus. In vivo, when the intranuclear concentration of androgens approaches 250 nM (8 pmol per mg DNA), an apparent concentration ceiling is reached even in the presence of a downward concentration gradient that would be expected to promote further transport across the nuclear membrane. This finding strongly suggests that in vivo the nuclear membrane acts as a barrier to the passage of androgens and, therefore, mitigates against the possibility that passive diffusion is an important mechanism of afferent transport of androgens into the nucleus. The ability of the nucleus to concentrate testosterone and dihydrotestosterone was clearly demonstrated in vivo when cytoplasmic concentrations of androgens of approximately 20 nM were accompanied by intranuclear concentrations in the vicinity of 250 nM. Since the measured concentration of testosterone and dihydrotestosterone in prostate of several species fall within the 5-20 nM range, it is evident that androgen concentrations in the nucleus as high as 250 nM may be typical of the physiological steady state. At the latter concentration the nucleus contains 60 000 androgen molecules: in approximate terms one third of this total is bound to a large molecular weight component of the nucleus, one third is bound to a 3.3 S receptor and one third is free or loosely bound. Since 60 000 androgen molecules and 20 000 receptor molecules appear in the nucleus before transport stops, it seems that the quantity of 4.4 S cytoplasmic receptor estimated at 174 plus or minus 24 pmol per mg protein (equivalent to about 8000 molecules per cell) is insufficient to account for the total influx of androgens and androgen receptors into the nucleus. Thus, although these results support the view that cytoplasmic receptors and the capacity to transport androgens are closely linked phenotypic markers of intracellular steroid hormone action, they suggest that the control of androgen concentration in the nucleus is achieved in a more intricate fashion than simply through a dependence on the presumed translocation of 4.4 S androgen-receptor complex into the nucleus.