Retinoids (natural and synthetic derivatives of vitamin A) are known to affect the growth and differentiation of any types of cells. Retinoids regulate gene transcription by binding to and activating retinoic acid receptors (RAR-a,-b,and-c) and 9-cis-retinoic acid(9cRA) receptors (RXR-a,-b, and -c). All-trans-retinoicacid, atRA, is generally thought to exert most biological actions of retinoids. Retinol is metabolized to atRA in a tightly controlled two-step oxidative process, starting with the conversion of all-trans-retinol to all-trans-retinaldehyde, followed by the conversion of all-trans-retinaldehyde to atRA (for a review, see [1] or [2] and references therein). Oxidoreductases known as retinol dehydrogenases (RDHs) catalyze the first reaction, which appears to be rate limiting [2] while cytosolic retinaldehyde dehydrogenases are thought to catalyze the second reaction [3,4].
Several isoenzymes belonging to the short chain de-hydrogenase/reductase (SDR) superfamily with various co-factor and substrate preferences have been cloned from mammals. Some of these only recognize all-trans-retinol as substrate [5–8], whereas others may also metabolizecis-retinols (9-, 11-, or 13-cis-retinol) [9–16]. Many of these SDRs also oxidize androgens, primarily the 3a-hydroxysteroids 3a-adiol (5a-androstan-3a,17b-diol) and androsterone (5a-androstan-3a-ol-17-one) into DHT (dihydrotestosterone) and androstanedione (5a-androstan-3,17-dione), respectively [2]. Recently, the first human counterpart to a RDH-like SDR, called RoDH-4, was cloned [17]. RoDH-4, a NADþ-preferring oxidoreductase expressed in the liver, is able to oxidize all-trans-retinol and 13-cis-retinol as well as 3a-adiol and androsterone, thus being the first human micro-somal enzyme capable of metabolizing all-trans-retino to its corresponding aldehyde. nterestingly, the oxidative 3a-HSD activity of RoDH-4 is competitively inhibited by both all-trans-retinol and 13-cis-retinol
More recently, RoDH-4 has also been reported in human epidermis under the name hRDH-E [18]. This is of particular interest since vitamins in the form of all-trans- and 3,4-didehydroretinol are metabolized to reti-noic acids in the skin [19,20], and androgens, such as DHT, are potent stimulators of sebocyte growth and differentiation [21,22]. For years, skin disorders such as acne and psoriasis have been treated with natural (atRA and 13-cis-retinoic acid, 13cRA) or synthetic (e.g., acitretin and adapalene) retinoids either topically or orally