Increased androgen receptor expression correlates with development of age-dependent, lobe-specific spontaneous hyperplasia of the Brown Norway rat prostate

Androgens are essential for development and differentiated function, as well as proliferation and survival of cells within the prostate gland. Age-related changes in the hormonal milieu, marked by a decrease in the serum androgen to estrogen ratio may contribute to the evolution of pathological changes, such as benign prostatic hyperplasia and carcinoma of the prostate gland, in older men. A similar phenomenon occurs in Brown Norway rats, in which the serum testosterone to estradiol ratio declines with age, and despite the lower serum testosterone level, age-dependent prostatic hyperplasia develops in the dorsal and lateral lobes, but not in the ventral lobe. To evaluate a role for changes in androgen action in the evolution of prostatic hyperplasia, we compared the immunostaining intensity of androgen receptor in the different prostate lobes from young (4 months of age) and old (24 months of age) Brown Norway rats. Androgen receptor immunostaining was present in the nuclei of all epithelial cells and some stromal cells throughout the prostatic ducts of each lobe from both young and old rats. Whereas androgen receptor immunostaining intensity decreased in luminal epithelial cells of the ventral prostate from old rats, it increased in luminal epithelial cells of the dorsal and lateral lobes from old rats, when compared with young rats. To validate immunocytochemical studies, Western blot analyses were performed. The total tissue level of androgen receptor decreased by 30% in the ventral lobe of old rats, whereas tissue levels of androgen receptor increased 2.7-fold and 1.3-fold in the dorsal and lateral lobes, respectively, of old rats. Similarly, the percentage of epithelial cells staining positive for the proliferation marker, proliferating cell nuclear antigen, was increased approximately 2-fold in the dorsal and lateral lobes as a function of older age. The presence of higher levels of androgen receptor and increased number of proliferating cell nuclear antigen-positive cells in the dorsal and lateral lobes of old rats suggest that changes in androgen receptor levels may be related to the lobe-specific proliferation of cells that occurs with increasing age. Additional evidence for lobe-specific regulation of androgen receptor expression was obtained from Western blots and by immunocytochemistry following castration. Androgen receptor levels in the ventral and dorsal lobes, but not the lateral lobe, of young and old rats were down-regulated in the absence of testicular androgen. However, nuclear immunostaining of androgen receptor returned by 7-10 d after castration in the ventral and dorsal lobes in the continued absence of androgen. Moreover, up-regulation of the androgen receptor level occurred more rapidly in the ventral and dorsal lobes of old, compared with young, castrated rats. Taken together, these results suggest that lobe-specific and age-dependent differences in the regulation of androgen receptor expression might lead to changes in tissue androgen responsiveness and the consequent development of lobe-specific hyperplasia in the Brown Norway rat prostate gland.