The Androgen Cascade in Ageing Men: Blessing or Curse?

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1 European Urology Supplements European Urology Supplements 2 (23) 8 12 The Androgen Cascade in Ageing Men: Blessing or Curse? J.A. Schalken * Department of Urology, University Hospital Nijmegen, Geert Groote Plein 16, P.O. Box 911, 65 HB Nijmegen, The Netherlands Abstract Dihydrotestosterone (DHT) is a dominant force in the development of the adult prostate. The action of DHT is critical in modulating the interaction of epithelial cells in the urogenital sinus mesenchyme during the organogenesis of the prostate. During adolescence, DHT is required to complete the normal growth phase of the prostate. In older men, if the response to DHT is altered, the resulting population of cells may be dominated by the transient proliferation of intermediate cells that are very sensitive to DHT for their amplifying potential. In benign prostatic hyperplasia, these mechanisms are disturbed and the usual control of final stromal and epithelial cell mass is ineffective. The importance of the two isozymes of 5a reductase in regulating the continuing stromal:epithelial cell balance might provide a good scientific basis for moderating both pathways in an attempt to alter the clinical consequences of abnormal prostate growth. # 23 Elsevier B.V. All rights reserved. Keywords: Prostate; BPH; Dutasteride; Cancer; Androgen; DHT 1. Introduction It has been known since the late 19th century that testicular factors play a role in the growth of the prostate [1]. More recently, the specific role of individual androgens in embryological development and in later life has been identified. Proliferation within the prostate is rapid in the first half of gestation, after which the growth rate is slow until adolescence [2]. The epithelial stem cells expand through transiently proliferating/amplifying cells into the highly specialized exocrine and neuroendocrine population of cells [3]. 2. Development of the prostate The function of the adult prostate in health and disease can be said to be dominated by two important characteristics: the inherent potential of stem cells features and the surrounding cellular milieu. During development stem cells undergo asymmetrical division in what is the first stage in a programmed developmental hierarchy. * Tel. þ ; Fax: þ address: j.schalken@uro.umcn.nl (J.A. Schalken). This division gives rise to a copy of the original cell and a more committed cell the amplifying cell (Fig. 1). With state of the art multi-parameter staining techniques it is possible to identify the population of cells that lie between the basal and luminal cells of the prostate ducts in some detail (Fig. 2). These more committed intermediate stem cells can give rise to the differentiation lineages that lead to the exocrine and neuro-endocrine cells. During development, the relative population of stem cells and intermediate cells changes. In the postembryonic prostate, there is a preponderance of the stem cell phenotype in the glandular bud. This compares with the adult gland, where the cells are primarily of a terminally differentiated cell type. In the diseased gland, there are more of the intermediate stem cells and due to their amplifying potential, the epithelial cell mass expands dramatically. The development, organization and maintenance of the epithelium and the total number of epithelial cells are critically dependent on an interaction with the stroma. The stroma controls the epithelium in a paracrine fashion, and vice versa, through both positive and negative signalling. The determinant of exocrine cell differentiation is hormonal, with intracellular dihydrotestosterone (DHT) being the dominating influence /$ see front matter # 23 Elsevier B.V. All rights reserved. doi:1.116/j.eursup

2 J.A. Schalken / European Urology Supplements 2 (23) Fig. 1. Microscopic view of the normal prostatic epithelium demonstrating the undifferentiated basal layer and a terminally differentiated luminal layer primarily secreting prostate-specific antigen. 3. The role of DHT The most potent androgen responsible for the growth and differentiation of both the prostate and male external genitalia is DHT, which is converted from testosterone in the foetal testes [4 7]. DHT is formed from the catalysis of testosterone by 5a reductase, which exists in two isoforms, types 1 and 2 [8], each Fig. 2. A two-parameter confocal scanning laser micrograph of a transverse section of a prostate duct, with markers to identify the basal cells and the luminal cells (keratin 5 in red, keratin 14 in purple and keratin 18 in green).

3 1 J.A. Schalken / European Urology Supplements 2 (23) 8 12 of which has different tissue distribution profiles. Much of our understanding concerning the role of 5a reductase and therefore the importance of DHT, has been gained through the study of men with type 2 5a-reductase deficiency. These men have feminised external genitalia and a small or absent prostate [9]. Although DHT is generally essential for prostate development, it has been shown that at least one key stage of prostate development, which occurs following development of the prostatic buds, does not rely entirely on it [1]. DHT has two modes of effect: it acts directly on the stroma to induce stromal cell proliferation and it also drives terminal differentiation in the epithelial compartment. During the development of the prostate, from the onset of DHT synthesis glandular buds occur and subsequently these develop into a complex branching ductal system with a differentiated epithelium. Importantly, this entire process of development of the prostate from stem cells to an adult gland is critically dependent on DHT. It would appear that the differences in potency and specific effects of testosterone and DHT, which both bind and act via the androgen receptor (AR), can be explained in part by their receptor kinetics. In addition, recent evidence suggests that the testosterone-ar and DHT-AR complexes may have differential effects on gene expression [11]. The AR is a phosphoprotein that mediates the actions of androgens by acting as a transcription factor. The AR is thought to be in an inactive state prior to binding to its ligand and that when the AR has bound to DHT there is a conformational change in the AR. It then interacts with coactivators and binds to the androgen response element in the promoter of androgen-responsive genes, causing initiation of their transcription. The number of ARs reflects the degree of androgen-dependent activity during normal development of male sexual characteristics and these are down-regulated in the penis after puberty, but not in the prostate. In the development of BPH, the balance between the indirect effect and the direct effect of DHT is disturbed. DHT remains the primary androgenic stimulator of prostate growth beyond puberty, not only stimulating prostate cell proliferation [12], but also inhibiting apoptosis [13] leading to a disturbed stromal:epithelial ratio [14]. The result is an imbalance between an increasing number of proliferating cells and a reduction in number of cells dying, resulting in net growth of the prostate. Consequently, prostate growth in BPH can be explained by an imbalance in the way the stem cells differentiate into the terminally differentiated epithelial cells. This is evidenced clinically in the subgroup Index.2.1 After castration Apoptosis Proliferation After testosterone re-suppletion Time (days) 25 3 of men with symptomatic BPH who have an enlarged prostate. The role of DHT in driving proliferation and differentiation in the prostate creates two potential points at which the DHT axis in the prostate can be attacked. The power of DHT modulation can be seen in the changes brought about in cell kinetics with changes within the androgen milieu. Under total androgen ablation, the number of epithelial cells will decrease by nearly 9% [15]. When testosterone is replaced, there is a massive effect on the epithelial cell content reverting to a near normal epithelial cell population. These changes can be explained by an increase in apoptosis when androgens are removed [16], and a strong increase in proliferation, activation of amplifying cells when testosterone is restored to normal levels (Fig. 3) [15]. Under normal conditions there is no overgrowth of the epithelial cell population. When testosterone concentration is increased fivefold, the prostate nevertheless grows to its original size, indicating that there is a poorly characterized negative feedback loop in normal glandular tissue. Under androgen withdrawal the intermediate stem cells become far more abundant in the prostate (Fig. 4) [17]. When adult dogs are castrated, the removal of androgen drive results in a large decrease in luminal epithelial cells, but basal cells persist [18]. Importantly, when androgens are reintroduced to castrated animals there is a marked increase in transiently proliferating/amplifying cells preceding the development of BPH, underscoring the importance of this cell type and the androgen drive, in the pathogenesis of BPH. Although much remains to be understood, it has been suggested that changes in hormonal homeostasis in later life may trigger the development of cells intermediate between basal and luminal cells and that an accumulation of specific cell types may occur [19]. This may contribute to the development of BPH and possibly prostate cancer Fig. 3. The relation between apoptosis and androgen milieu in the prostate [15]. Index

4 J.A. Schalken / European Urology Supplements 2 (23) c-met expression (strongly positive cells/5) 125 P = No therapy Androgen deprivation Fig. 4. The changes in stem cell population with androgen deprivation [17]. 4. DHTas a therapeutic target in prostate tissue In light of the classical BPH aetiology review by Isaacs and Coffey [3], modern understanding of the development of BPH is based on a combination of embryonic re-awakening and stem cell theories. The DHT hypothesis, which is more than 2 years old, suggested that in the ageing male, intraprostatic DHT increases producing an increased mass of the prostate [3]. This is clearly not true as it is now documented that DHT levels diminish with age, oestrogens increase, and the ratio of DHT:oestradiol changes quite significantly. A complicating phenomenon is that within the prostate gland at the subcellular level DHT concentrations are likely to vary significantly. The role of DHT in prostate growth and differentiation before and after puberty is reliant on its conversion from testosterone by 5a reductase. The understanding that 5a reductase exists as two isoenzymes has lead to intensive study of their relative distribution within both healthy and diseased tissue. A number of studies suggest that in the normal prostate, both isoforms are expressed but with type 2 5a reductase predominating [2 22]. At a cellular level, there are data demonstrating that type 1 5a reductase is localized to prostatic epithelial cells, while evidence points to a primary localization of type 2 in stromal tissue [23,24], although it has also been located in epithelial cells. The embryonic reawakening theory suggests that the resumption of prostatic growth in adulthood is due to a resurgence of the powerful inductive potential first demonstrated during embryonic development. The signal to induce prostate development is thought to come from the urogenital sinus mesenchyme [25]. This can be simply demonstrated by putting urothelial cells in the urogenital sinus mesenchyme at which point a prostate morphology will develop under the influence of DHT [26]. Multiple pathways are involved in this process, and keratinocyte growth factor, fibroblast growth factor, haptocyte growth factor and transforming growth factor-b are some of the mediators. This experimental model also points to the significant influence of stroma on growth and development in the prostate. A paracrine interaction modulated by DHT exists between the stroma and epithelium, while at the same time DHT also controls the epithelium directly. The explanation for the development of BPH is that the response to DHT is altered so that the balance between proliferation and differentiation is corrupted, resulting in a net increase of epithelial cell numbers in the early stages of development of BPH. The original concept was that 5a reductase was exclusively expressed in the stroma and that manipulation of the T-DHT axis was therefore exclusively a stromal event. With the identification of type 1 5a reductase in the epithelium, it has been proposed that the amount of DHT in both stroma and epithelium can be reduced by using a compound that will interfere with both type 1 and type 2 5a-reductase activity. Interestingly, 5a-reductase inhibitors have been shown to induce apoptosis within the prostate, although they do not appear to have direct effects on cellular proliferation [27]. The relative expression of the two isoenyzmes of 5a reductase appears to differ between normal and pathological prostate tissue. In men with BPH, the expression of both type 1 and type 2 is increased relative to normal tissue. Prostate tumour cells, which are derived from epithelial precursors, have a 3 4 fold increased expression of type 1 5a reductase compared with benign tissue [28]. However, it remains uncertain whether this enhanced expression is found in some or all prostate cancer cells. Conversely, a number of studies have failed to identify the type 2 isoenzyme in prostate cancer cells [29,3]. These data suggest that the type 1 isoform of 5a reductase may have a significant role in both BPH and prostate cancer. Dutasteride is an inhibitor of both type 1 and type 2 5a reductase, in comparison with the mono-inhibition of type 2 5a reductase by finasteride, seen at therapeutic dosing levels. This difference in isoenzyme inhibition profile results in more complete suppression of prostatic DHT for dutasteride in comparison with finasteride. 5. Conclusion There is a high level of organization in the prostate and the problems of prostate growth in older men are

5 12 J.A. Schalken / European Urology Supplements 2 (23) 8 12 primarily associated with a disturbed balance between the stromal and epthelial compartment, which, in turn, is best explained by an imbalance in the stromaepithelial interaction. In this process, the intermediate cell type plays a pivotal role. The amplifying potential of intermediate stem cells explains why a small effect on these cells translates into a profound effect on total epithelial cell number. The intermediate cell type also plays pivotal role in the development of prostate cancer. DHT modulates stromal and epithelial interactions and the intermediate stem cells. Therefore, modulation of DHT becomes an appropriate target to control prostate growth and potentially return the balance of cellular relationships in the prostate gland. References [1] Cabot A. The question of castration for enlarged prostate. Ann Surg 1986;24: [2] Xue Y, Sonke G, Schoots C, Schalken J, Verhofstad A, de la Rosette J, et al. 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