Topical retinoic acid for photoaged skin: Therapeutic effects and mechanisms

Topical all-trans-retinoic acid actively repairs photoaged skin. Roughness is the first parameter to show improvement, and is related to epidermal hyperproliferation, compaction of the stratum corneum and increase in epidermal glycosaminoglycans. Lightening of hyperpigmented lesions is correlated with a reduction in epidermal melanin content. Reduction in wrinkling occurs later, and is probably related to deposition of new collagen in the upper dermis. Epidermal atypia and dysplasia are also improved. Side-effects of dryness, peeling, erythema and irritation are dose-dependent, and diminish with continued treatment. The extent of the role of non-specific irritation, if any, is yet to be determined. Specific nuclear retinoid receptors exist, and are present in skin. Within the nucleus the vitamin - receptor complex binds to specific DNA sequences - response elements - situated in specific retinoid target genes, thereby regulating transcription of these target genes. The precise genes targeted by the retinoid receptors, and the exact mechanism of action of retinoic acid in the process of repairing photoageing, remain to be established.