Steroidogenic enzymes in skin

ARTICLE

The major androgen testosterone is principally formed in the testes of males and ovaries of females. Its synthesis in the Leydig cells of the testis involves four enzymes, namely, cytochrome P-450 side-chain cleavage enzyme, cytochrome P-450 17alpha-hydroxylase/lyase, 3beta-hydroxysteroid dehydrogenase (HSD) type 2 and 17beta-HSD type 3 [1]. In men, approximately 95% of the circulating testosterone is formed by the testes, and the remaining 5% is formed in peripheral tissues from adrenal derived dehydroepiandrosterone (DHEA) and androstendione by the 3beta-HSD type 1 isoenzyme and 17beta-HSD(s). A significant part of the plasma-borne testosterone is converted in androgen target tissues to the more potent androgen dihydrotestosterone (DHT) by the enzyme steroid 5alpha-reductase. An important observation is that men have only a 3 times higher plasma concentration of DHT than that of women [2]. The fact that plasma testosterone, the substrate of 5alpha-reductase, is 10-fold higher in men than in women, suggests that the peripheral synthesis of DHT is very efficient at low substrate concentrations.

Steroid 5alpha-reductases

Androgens play a pivotal role in the growth of pubic and axillary hair. Paradoxically, androgens stimulate growth of beard hair, whereas in the scalp, androgens inhibit hair growth. Clinical studies by Dr. James B. Hamilton in the early 1940's suggested that the balding process is promoted by androgens in genetically predisposed men, referred to as androgenetic alopecia [3]. The androgen acting in the scalp to promote the balding process was not known at the time. It was not until the 1960's when it was discovered that testosterone is converted to the more bioactive androgen dihydrotestosterone by the microsomal, NADPH-dependent enzyme steroid 5alpha-reductase in peripheral androgen-responsive tissues such as the prostate and skin [4]. Two genes encoding two isoenzymes of steroid 5alpha-reductase exist, designated type 1 and type 2 [5, 6]. The type 1 isoenzyme is the major 5alpha-reductase in skin, has a neutral pH optimum and a low affinity for testosterone. Steroid 5alpha-reductase type 2, on the other hand, is the major isoenzyme in the prostate, has an acidic pH optimum and a high affinity for testosterone. Mutations in the 5alpha-reductase 2 gene leading to failure to synthesize DHT in the anlage of the male external genitalia during fetal development manifests in a rare form of male pseudohermaphroditism, termed 5alpha-reductase 2 deficiency, in which the male external genitalia develop as female structures [7]. The notion that DHT, and not testosterone, is the androgen responsible for benign prostate hyperplasia and androgenetic alopecia stems from the observation that men with steroid 5alpha-reductase 2 deficiency present with a hypoplastic prostate and do not develop male pattern baldness [8]. Consequently, the 5alpha-reductase type 2 isoenzyme-specific inhibitor finasteride has proven efficacious in promoting hair growth as a consequence of lowering scalp and plasma DHT levels, and in the treatment of benign prostate hyperplasia by lowering prostatic DHT [9-12]. By immunohistochemical analysis of human scalp, it has been shown that the type 2 isoenzyme is predominantly expressed in the root sheath of the hair follicle, in contrast to 5alpha-reductase type 1, which is expressed to high levels in the sebaceous glands [13-15]. Hence, it has been proposed that the function of the type 1 isoenzyme is to locally convert circulating testosterone to dihydrotestosterone for sebum production.

Hydroxysteroid dehydrogenases

DHT is metabolized in peripheral tissues by reductive 3alpha-HSD, 3beta-HSD, and oxidative 17beta-HSD isoenzymes [16-20]. An important finding is that the biologically inactive 5alpha-androstan-3alpha,17beta-diol (3alpha-Adiol) and its 17-keto derivative androsterone can be back-converted to DHT by oxidative 3alpha-HSDs and reductive 17beta-HSDs. The reductive 17beta-HSD isoenzymes that are expressed in peripheral tissues include the types 1, 3, and 5, however, their expression in scalp skin have not to date been reported. The most recent discovery regarding isoenzymes involved in androgen metabolism stem from the cloning and expression of the oxidative 3alpha-HSDs [21, 22]. These isozymes belong to the retinol dehydrogenase (RoDH) gene family and catalyze the conversion of retinol to retinal as well as efficient oxidative 3alpha-HSD activity with 3alpha-Adiol and androsterone as substrates. To date, three genes encoding oxidative 3alpha-HSD/RoDH isoenzymes have been identified in the human genome (Table 1). These isozymes share approximately 60% amino acid sequence identity and, like the 17beta-HSDs, belong to the short-chain alcohol dehydrogenase superfamily by virtue of distinct structural motifs.

The first human RoDH to be cloned was 11-cis-RoDH, nowadays referred to as RoDH 5 [23]. The enzyme is highly expressed in the retinal pigment epithelium and is involved in formation of the 11-cis-retinal chromophore covalently bound to rhodopsin. The physiological role of the isoenzyme in vision is underscored by the observation that deleterious substitution mutations in the RDH5 gene is a cause of fundus albipunctatus, a rare form of stationary night blindness [24]. Interestingly, the enzyme is also expressed to low levels in a wide variety of tissues including liver and mammary gland, in which the enzyme may be involved in the conversion of 9-cis-retinol to 9-cis-retinal [25]. The expression of RoDH 5 in scalp skin is not known.

During the cloning of prostatic oxidative 3alpha-HSD by Russell and coworkers it was discovered that RoDH 5 possesses 3alpha-HSD activity with 3alpha-Adiol and androsterone as substrates [25]. The human prostatic oxidative 3alpha-HSD was isolated by expression and cross-hybridization cloning using radioactive 3alpha-Adiol as substrate, hence, the isozyme is referred to oxidative 3alpha-HSD/RoDH [26]. RNA blotting analysis has revealed that the gene is highly expressed in human liver and to lesser degree in prostate, testis and spleen, but expression of this isoenzyme in human scalp has not been reported.

A third human RoDH, designated RoDH 4 or RoDH-E, was recently cloned by RT/PCR from liver RNA as well as human epidermal keratinocyte RNA [27, 28]. The enzyme is an efficient oxidative 3alpha-HSD and it also possesses RoDH activity showing a preference for all-trans-retinol as substrate. The gene is highly expressed in the epidermis, hence its role in retinoic acid synthesis in the skin has consequently been proposed. It is conceivable that this isoenzyme may also participate in the formation of DHT in scalp skin.

The potential role of the oxidative 3alpha-HSD/RoDH isoenzymes in concert with 5alpha-reductases in the local synthesis of DHT in the scalp may be important factors in the progression of androgenetic alopecia.

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