Rainer Voegeli Department of Research and Development, Pentapharm Ltd., Basel, Switzerland Christos C. Zouboulis Departments of Dermatology and Immunology, Dessau Medical Center, Dessau, Germany Peter Elsner Department of Dermatology and Allergology, Friedrich-Schiller-University Jena, Jena, Germany
Department of Research and Development, Pentapharm Ltd., Basel, Switzerland
5a-dihydrotestosterone (DHT) exerts a 2 to 10 times stronger androgen activity than testosterone. Thus, this steroid hormone was supposed to play a more important role than testosterone in many androgen-dependent diseases such as acne, hirsut- ism, androgenetic alopecia, benign prostatic hyperplasia, and prostate carcinoma. DHT is converted from testosterone by the enzyme 5a-reductase. There are two iso- enzymes of 5a-reductase (types 1 and 2) that differ in their localization within the body and even in the skin. Activity of the type 1 isoenzyme predominates in sebac- eous glands, where it may be involved in regulation of sebum production. There- fore, specific inhibition of type 1 5a-reductase may represent a therapeutic approach to acne. This review refers to the features both of 5a-reductases and of their inhibitors.
ANDROGEN METABOLISM IN THE SKIN
The androgenic provoked stimulation of sebum production plays a crucial role in the pathogenesis of acne. Although numerous factors contribute to the develop- ment of acne, the requirement for androgens is absolute (1). Thiboutot et al. (2) and Fritsch et al. (3) demonstrated that human skin is indeed a steroidogenic tissue. The skin functions as an independent peripheral endocrine organ (4) expres- sing all the enzymes necessary for androgen synthesis and catabolism. It can syn- thesize cholesterol from acetate and can further metabolize steroids such as dehydroepiandrosterone sulfate into potent androgens such as testosterone and DHT. DHT itself further induces enzymes such as 3a-hydroxysteroid dehydrogen- ase and 17b-hydroxysteroid dehydrogenase (5).
The skin compartment responsible for both the initiation of steroidogenesis and the local production of potent androgens seems to be the sebaceous gland. Therefore, selective local interruption of this pathway can be of therapeutic benefit in androgen-mediated diseases of the skin and allows for effective treatments (3).
Further details on the androgen metabolism in the skin are explicitly described in Chapter 7.
5a-REDUCTASE Historical Background
It was not until 1954 that a series of 5a-reduced steroids, produced from the incu- bation of androst-4-ene-3,17-dione with rabbit liver homogenate, was recognized by paper chromatography. Two years later, DHT was first identified as the major metabolite of the incubation of testosterone with rat liver homogenate. Both inde- pendent experiments indicated the presence of 5a-reductase in the liver tissues of rabbits and rats. Since then, this membrane-bound enzyme has been demonstrated to be present in many other organs and androgen-sensitive tissues of mammalian species (6). Subsequent investigations revealed that the enzyme catalyzes the reduction of a variety of steroids, while nicotinamide adenine dinucleotide phosphate-oxidase (NADPH) is needed as an essential cofactor (Fig. 1) (7).
The realization that 5a-reductase plays an important role in androgen metab- olism began in the early 1960s, when in bioassays, DHT was demonstrated to be a more potent androgen than testosterone in the prostate. Further studies revealed that administration of radiolabeled testosterone led to an accumulation of DHT in the nuclei of ventral prostate cells. Moreover, DHT was shown to bind preferen- tially to the specific nuclear androgen receptor in target organs. The androgen receptor is a member of the steroid-thyroid-retinoid family of transcription regulat- ory factors. The DHT-receptor complex interacts with specific portions of the genome to regulate gene activities. Developmental studies revealed that 5a- reductase activity in mammalian embryos was high in the primordia of the prostate and external genitalia prior to their virilization but very low in Wolffian duct struc- tures, suggesting that the enzyme was crucial for the development of the normal male phenotype during embryogenesis. Taken together, these data implied that conversion of testosterone to DHT by 5a-reductase was a critical step in male sexual differentiation and focused attention on the role of 5a-reductase in androgen physiology and pathophysiology (7).
Studies in the 1970s and 1980s revealed that the skin, in addition to the pros- tate, is rich in 5a-reductase. In this respect, it appeared very likely that 5a-reductase functions as an autocrine mediator in both the skin growth and skin differentiation (8). In 1976, the biochemical properties of 5a-reductase were investigated in cell- free extracts of fibroblasts cultured from genital and nongenital skin of healthy sub- jects and patients with male hermaphroditism. In these experiments, two types of
5a-reductase activities were distinguished: one detectable mainly in genital skin with an optimal pH of about 5.5 and another with an optimal pH between six
and nine in all fibroblasts, cultured from normal and mutant skin taken from both genital and nongenital areas. Patients with congenital 5a-reductase deficiency seemed to lack the former enzyme only (9). In contrast to type-2 isoenzyme, no human pathology with a deficiency or mutation of the type 1 isoenzyme has been identified so far (10).
In the following, attention has been focused on molecular biological approaches to identifying and isolating 5a-reductase. In 1989, a cDNA encoding a rat liver 5a-reductase was isolated and used to identify a single mRNA species in both liver and prostate of the rat. When this rat 5a-reductase cDNA was used to screen cDNA libraries constructed from human prostate mRNA, a corresponding cDNA was isolated in 1990. However, when the human cDNA was expressed in COS cells, the 5a-reductase activity produced was highest at the physiological pH values, and not at 5.5 as deduced from earlier biochemical studies. Moreover, the enzyme showed different inhibitor specificity than expected. As the gene encod- ing this cDNA was also normal in 5a-reductase-deficient individuals, it was again suggested in 1991 that there must be at least 2 isoforms of human 5a-reductase (11), as already proposed by Moore and Wilson (12) by biochemical means in 1976. Later on, a second 5a-reductase gene was also isolated from a human prostate genomic library. This gene encoded a protein with 50% homology to the former one. Accord- ingly, these enzymes were designated as type 1 and type 2 5a-reductase, respect- ively, in the order in which they were cloned (11).
5a-reductase is the key enzyme in androgen metabolism. Altered enzyme function and/or regulation are responsible for numerous human pathologies. In humans, the normal activity of 5a-reductases routinely maintains testosterone-mediated biological functions, such as anabolic actions (muscle-mass increase, penis enlarge- ment, scrotum enlargement, and vocal cords enlargement), spermatogenesis (male sex drive and performance), and DHT-mediated effects, such as increased facial and body hair, scalp hair recession, acne, and prostate enlargement (6).
The deficiency of 5a-reductase in males results in incomplete differentiation of the external genitalia at birth. At puberty, patients have normal-to-elevated levels of testosterone in plasma, and virilization occurs, but the prostate remains small and there is no acne. Male pseudohermaphroditism is accompanied by low levels of DHT. However, females with 5a-reductase deficiency appear to have no clinical signs, however, the enzyme also metabolizes other 4-ene-3-oxosteroids, such as progesterone and corticosterone. Abnormally, high 5a-reductase activity in humans results in excessively high DHT levels in peripheral tissues, which is implicated in the pathogenesis of prostate carcinoma, benign prostatic hyperplasia, acne, hirsutism, and androgenetic alopecia (6).
The exact physiological role of type 2 5a-reductase in tissues other than pros- tate and accessories of the external genitalia remains unknown, since these organs do not appear to be affected by the genetic absence of this isoenzyme or by its pharmacological inhibition and the women concerned have normal sexual develop- ment and are fertile. The physiological reasons and the factors that mediate this temporal pattern of expression are not known. However, given the role of andro- gens in 5a-reductase regulation, it is intriguing to speculate that the expression of one gene may influence the timing or expression level of the other: type 2 isoen- zyme had a priming effect on tissues, such as the prostate, skin and scalp, on which type 1 isoenzyme will act later in life (13).
The androgen control of sebum production and the role of type 1 5a-reductase in the pathogenesis of acne are confirmed by the following data and clinical observations:
1. Male castrates produce less sebum than normal males and they do not have acne unless testosterone is given.
2. Androgen administration also results in a significant increase in sebum pro- duction in women.
3. Human sebaceous glands are rich in type 1 5a-reductase activity.
4. Sebum production is not detectable in subjects with complete androgen insen- sitivity (13).
5. Adult males with type 2 5a-reductase deficiency have sebum production scores identical to normal age-matched males.
6. Males with benign prostatic hyperplasia treated with Finasteride (5 mg/day for one year) did not show decreased sebum production, although the serum
5a-DHT level was lowered (14).