The metabolic fate of retinoids is controlled by two classes of intracellular binding proteins: the cellular retinol binding protein type I (CRBP I) and the cellular retinoic acid binding protein type II (CRABP II) (17,18). These sequester the retinoids, so that it is only available for reaction with specific enzymes. The holo-CRBP retinol complex can serve as a substrate for lecithin retinol acyl transferase (LRAT) (19) when the retinoid status is high, conversely when low, it serves as a substrate for retinol dehydrogenase synthesizing retinal, which ultimately gets converted to all-trans-RA (Fig. 4). The transport into the nucleus and further metabolism of all- trans-RA is controlled by CRBP I and CRABP II. It is suggested that CRBP I trans- port all-trans-RA into the nucleus, whereas CRABP II sequesters excess all-trans-RA in the cytoplasm facilitating its degradation. Metabolic inactivation of all-trans-RA to 4-hydroxyretinoic acid occurs via a cytochrome P450 enzyme (CYP26; Fig. 4) (20,21). This hydroxylase activity is actually induced in vivo by RA in human epi- dermis but can be inhibited by azoles. In the presence of low-dose RA or ROH, azoles amplify the human skin responses to retinoids in a manner characteristic
FIGURE 4 Schematic representation of retinol metabolism. Abbreviations: CRABP, cellular retinoic acid binding protein; LRAT, lecithin retinol acyl transferase; RA, retinoic acid; RAL, retinal; RE, retinyl ester; ROL, retinol.
of the retinoids at a higher dose (22). These agents have been called retinoic acid metabolism breakdown agents (RAMBAs) or retinomimetics. Azoles also inhibit the RA-induced expression of cytochrome 24-hydroxylase, which inactivates
In human keratinocytes, all-trans-RA regulates its own biosynthesis from atROH through the regulation of retinol esterification and, as such, RAMBA-type agents are not optimal (23). Treatment with all-trans-RA induces retinol-esterifying activity in proliferating keratinocytes. LRAT was induced by all-trans-RA and reduces the conversion of ROH to RA, resulting in sequestration of ROH in REs. Acyl retinol acyl transferase activity is also present. Several other enzymatic steps have been identified, which if manipulated can deliver improved retinoid responses (24).
EFFECTS OF RETINOIDS
Hypovitaminosis has been related to the expression of acne (25), and so its addition to skin should alleviate the condition. All-trans-RA was the first retinoid used, but it was ineffectual on acne when used systemically. 13-cis-RA (isotretinoin), however, is an extremely effective oral anti-acne drug by causing marked sebosuppression (26). 13-cis-RA has been shown to inhibit significantly sebocyte proliferation, differ- entiation, and lipid synthesis in vivo. Despite its potent biological effects, 13-cis-RA exhibits only low-binding affinity for CRABP and nuclear receptors (27). This unique antisebotrophic activity results from a selective isomerization to all-trans- RA intracellularly in sebocytes, reduces all-trans-RA inactivation compared with all-trans-RA, and mediates inhibition of sebocyte proliferation (28). The synthesis of CRABP II, which facilitates the degradation of intracellular all-trans-RA, was delayed and induced lesser by 13-cis-RA when compared with all-trans-RA, in vitro. Although all-trans-RA in trace amounts promotes sebocyte growth and differ- entiation, larger doses induce atrophy of the sebaceous gland and decrease
sebogenesis. In vitro all-trans-RA and selective RAR agonists increase lipid-forming colonies except at higher doses (1026 M), in which only a small number of colonies grew but they were differentiated more greatly. On the other hand, RXR agonists increased the cell growth slightly and lipid-forming colonies dramatically (29 – 31).
It is important to remember that RA effects in vitro may be completely differ- ent to in vivo. Retinoic acid induces sebocyte differentiation in preputial sebocytes, yet in in vivo the opposite occurs. Equally, the effects of RA on the keratinocyte differentiation process are contradictory in vitro and in vivo. Nevertheless, the effects of retinoids on epidermal differentiation will help to improve the acnegen- esis state. Increased levels of epidermal transglutaminase and involucrin are observed together with increases in K6, as would be expected for a proliferating epidermis, but the differentiation in keratin markers (K1 and K10) are not inhibited in vivo. A unique keratin, K13 is also induced in vivo. Filaggrin and loricrin are increased in the longer term (four months) application of RA. As it has been recently reported that reduced Langerhans cell (LC) activity decreases in acne, the increases in LC activity by RA will also be beneficial (32).
The mechanism of action of isotretinoin has however remained elusive, as it does not bind itself to the retinoid receptors. Multiple actions have been proposed from inhibition of sebaceous gland activity, inhibition of the growth of P. acnes, inhi- bition of inflammation, and improvements in follicular epithelial differentiation. More recently, it has been shown that isotretinoin competitively inhibits 3 alpha- hydroxysteroid oxidation by retinol dehydrogenase, resulting in reduced formation of dhihydrotestosterone and thereby reduced sebogenesis (33). Retinoids are reported to increase the expression of transforming growth factor (TGF beta 1 – 3), and these inhibit keratinocyte proliferation. However, TGF beta-2 and 3, but not TGF beta-1, reduce cell proliferation and lipogenesis in human sebaceous glands in organ culture studies, suggesting that these growth factors may also contribute to the effects of retinoids on sebaceous glands (34 – 37).
At present, Accutane (oral 13-cis-RA) is available for the oral treatment of acne, whereas topical treatments include Retin-A (micronized all-trans-RA)— Differin (adapalene or CD271). Retinoic acid cannot be used in cosmetic products but retinol can (Fig. 5) (36). This does have some efficacy, but several new routes are being proposed to enhance its activity by inhibiting esterification and degra- dation enzymes and also manipulation of HDAC activities (37).
FIGURE 5 Typical structures of retinoids. Abbreviation: RA, retinoic acid.