Topical phenolic agents, including hydroquinone (HQ) and non-phenolic agents, including retinoids are used for hyperpig- mentation disorders. Topical HQ is the most popular treatment modality and is frequently used in combination therapies, such as a triple combination with topical all-trans retinoic acid and corticosteroids [1–4]. Although HQ is used in over-the-counter formulations for lightening skin pigmentation, it can cause skin irritation [5]. The most common adverse event associated with topical retinoic acid (RA), is also an irritant reaction of variable intensity, which presents with dryness, scaling, erythema, burning, and/or stinging [6–8]. Because skin irritation could lead to post-inflammatory hyperpigmentation, efforts have focused on the reduction of the irritant potential without loss of efficacy. Skin irritation can be induced by different mechanisms, such as skin barrier disruption, induction of a cytokine cascade, and involve- ment of the oxidative stress network [9,10]. Different classes of substances could potentially exert skin irritation through differ- ent mechanisms. A dose–response relationship is seen for adverse events, as well as effectiveness [11–13]. Irritant reaction from the same chemical shows variable intensities in vivo, if their concentrations are different. Gene expression may also differ at different intensities of irritation. Although concentrations that inhibit cell viability by 50% have been used as the threshold concentration of irritation on human skin [14], the exact proportion of cell survival or cytotoxicity that is indicative of irritation in vivo is not defined. Therefore, validation using sequentially increasing concentrations [1]
including concentration of 50% cell survival may be helpful in the identification of molecular
mechanism of irritation. Skin irritation is one of the most common adverse reactions to chemicals. Every substance is an irritant to some degree. To reduce irritant reactions, the molecular mechanisms require to be identified. This study was performed to elucidate the molecular mechanisms associated with skin irritation by RA and HQ. Toward this aim, a genome-wide transcriptional profiling analysis and a Manhattan plot were performed after treatment of cultured normal human keratinocytes, a main constituent of skin, with subcytotoxic doses of HQ or RA. The data were validated by in vitro mRNA studies using increasing concentrations of each chemical (subcytotoxic, 50%, and 20–30% cell survival). The expression of corresponding proteins was examined in vivo by using increasing concentrations which induced mild erythema, definite erythema, and severe erythema with focal erosion, to corroborate the in vitro result.