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

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.
(Copyright © 2017 Japanese Society for Investigative Dermatology. Published by Elsevier B.V. All rights reserved.)