SHORT COMMUNICATION Screening of Kit Inhibitors: Suppression of Kit Signaling and Melanogenesis by Emodin Seong Jin Lee, 1 Daeyoung Jeong, 1 Woo-Kyu Park, 1 Jae Yang Kong, 1 Gildon Choi, 1 Hojeong Kim, 2 Sangjin Kang 2 and Heeyeong Cho 1 * 1 Pharmacology Research Center, Bio-organic Science Division, Korea Research Institute of Chemical Technology, PO Box 107, Sinseongno19, Yuseong, Daejeon 305-600, Korea 2 Bioactive Material Team, LG Household and Health Care, Yuseong, Daejeon 305-343, Korea In search of novel antipigmentation agents, a set of 3,840 compounds with natural-like synthetic or natural origin were screened against Kit (stem cell factor receptor). Emodin from the seed of Cassia tora and baicalin from Scutellariae radix showed potent inhibitory effects (IC 50 = 4.9 and 9.0 mM, respectively) on the phosphory- lation of Kit. Emodin also blocked other receptor tyrosine kinase activities, such as epithelial growth factor receptor (EGFR), vascular endothelial growth factor receptor 2 (VEGFR-2), fibroblast growth factor receptor 1 (FGFR-1), platelet-derived growth factor receptor b (PDGFR-b). In contrast to emodin, aloe-emodin did not inhibit Kit activity at all. Emodin also blocked the cellular kinase activities of Kit and its down-stream p44/42 mitogen activated protein kinase (MAPK) in MO7e cells and human primary melanocytes. Emodin strongly suppressed the melanin synthesis triggered by stem cell factor (SCF) treatment. Also, emodin showed almost no toxicity up to 10 mM on cultured melanocytes as reported previously by other researchers. The results indicate that emodin is a good candidate for the development of antipigmentation agents since it can radically block the differentiation and proliferation of pigment cells by reducing Kit signaling. Copyright © 2009 John Wiley & Sons, Ltd. Keywords: kit; emodin; RTK inhibitor; melanogenesis; pigment. INTRODUCTION Kit (stem cell factor receptor, SCF-R, c-Kit), a type III transmembrane receptor tyrosine kinase (RTK), is involved in the process of proliferation, differentiation and survival of melanoblasts/melanocytes (Halaban, 2000; Mizoguchi, 2004; Wehrle-Haller, 2003). Long- term exposure to UV increases the number of melano- cytes by promoting melanogenesis and accelerating pigment synthesis in the dermis mainly through the Kit- mediated signaling cascade. Activated Kit is known to stimulate mitogen activated protein (MAP) kinase and in turn mitf (microphthalmia-associated transcription factor), a helix-loop-helix leucine zipper protein. The phosphory- lated mitf can bind to the promoter sequence of tyrosinase/Tyrp-2 (tyrosinase-related protein 2) and turn on their transcription (Kawaguchi et al., 2001). Even though inhibitors of tyrosinase such as hydro- quinone, ascorbic acid, kojic acid, glutathione are cur- rently used as whitening agents, however, most of them have undesired effects or problems in stability, smell or skin penetration (Briganti et al., 2003). In order to dis- cover novel antipigmentation agents, Kit rather than tyrosinase related enzymes could be a superior target since it can diminish the supply and the population of melanin-generating cells in dermis instead of temporal enzymatic inhibition of pigment synthesis. Many compounds derived from herbs and plants are highly bioactive and provide invaluable sources in the development of novel medicine and cosme- ceutical agents (Pieroni et al., 2004). This study screened about 3,840 natural-like synthetic or naturally occurring compounds in order to seek negative modula- tors of melanogenesis. Emodin (1,3,8-trihydroxy-6- methylanthroquinone, C 15 H 10 O 5 ) from Cassia tora, Cassia obtusifolia and Polygonum multiflorum, inhib- ited the phosphorylation of Kit and other RTKs besides previously reported antibacterial, antioxidant and anti- tumor effects (Koyama et al., 2002). MATERIALS AND METHODS Materials. All the chemicals not specified were pur- chased from either CalBiochem (CA, USA) or Sigma Co. Ltd (St Louis, MO, USA). Natural-like synthetic compounds were purchased either from Comgenex * Correspondence to: H. Cho, Pharmacology Research Center, Bio- organic Science Division, Korea Research Institute of Chemical Technol- ogy, PO Box 107, Sinseongno19, Yuseong, Daejeon 305-600, Korea. E-mail: hycho@krict.re.kr Received 07 Augest 2008 Revised 06 May 2009 Copyright © 2009 John Wiley & Sons, Ltd. Accepted 08 May 2009 PHYTOTHERAPY RESEARCH Phytother. Res. 24: 308–312 (2010) Published online 7 July 2009 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/ptr.2928