Complementary effect of hydroquinone and retinoic acid on corneocyte desquamation with their combination use

Abstract Background Retinoic acid (RA) enhances skin-lightening capabilities of hydroquinone (HQ), at least in part, by facilitating desquamation which leads to increase penetration of HQ. The desquamation also affects skin irritation levels. The mechanism of RA-induced desquamation, however, has not been completely explored and no such data has been available for HQ uses. Objective To examine the role of HQ, RA, and their combination in the desquamation. Methods Primary cultured normal human keratinocytes, which were treated with HQ and/or RA in presence or absence of serine-specific inhibitor Kazal type5 (SPINK5)/lympho-epithelial Kazal-type-related inhibitor (LEKTI) knockdown or recombinant human SPINK5/LEKTI, and biopsied skin samples applied with HQ or RA were examined. Expression levels of corneodesmosin (CDSN), desmocollin1 (DSC1), kallikrein5 (KLK5), KLK7, and SPINK5/LEKTI, and proteolysis activity against extracted human skin epidermal protein were determined using time-course real-time PCR, Western blotting, ELISA, and immunofluorescence staining. Results HQ increased but RA decreased the synthesis of CDSN and DSC1. HQ reduced corneodesmosome degradation by the upregulation of SPINK5/LEKTI, whereas RA showed opposite results without upregulation of SPINK5/LEKTI. The combination of HQ and RA was close to the sum of the individual components. Conclusions HQ reduced corneocyte desquamation. However, RA enhanced desquamation. The combination induced more desquamation than HQ but less than RA.