Research Article Isorhamnetin Suppresses Skin Cancer through Direct Inhibition of MEK1 and PI3-K Jong-Eun Kim 1,2 , Dong-Eun Lee 1,2 , Ki Won Lee 1,3 , Joe Eun Son 1 , Sang Kwon Seo 1 , Jixia Li 2 , Sung Keun Jung 1,2 , Yong-Seok Heo 4 , Madhusoodanan Mottamal 2 , Ann M. Bode 2 , Zigang Dong 2 , and Hyong Joo Lee 1 Abstract 3 0 -Methoxy-3,4 0 ,5,7-tetrahydroxyflavone (isorhamnetin) is a plant flavonoid that occurs in fruits and medicinal herbs. Isorhamnetin exerts anticancer effects, but the underlying molecular mechanism for the chemopreventive potential of isorhamnetin remains unknown. Here, we report anti–skin cancer effects of isorhamnetin, which inhibited epidermal growth factor (EGF)-induced neoplastic cell transformation. It also suppressed anchorage-dependent and -independent growth of A431 human epithelial carcinoma cells. Isorhamnetin attenuated EGF-induced COX-2 expression in JB6 and A431 cells. In an in vivo mouse xenograft using A431 cells, isorhamnetin reduced tumor growth and COX-2 expression. The EGF-induced phosphorylation of extracellular signal-regulated kinases, p90 and p70 ribosomal S6 kinases, and Akt was suppressed by isorhamnetin. In vitro and ex vivo kinase assay data showed that isorhamnetin inhibited the kinase activity of MAP (mitogen-activated protein)/ERK (extracellular signal regulated kinase) kinase (MEK) 1 and PI3-K (phosphoinositide 3-kinase) and the inhibition was due to direct binding with isorhamnetin. Notably, isorhamnetin bound directly to MEK1 in an ATP-noncompetitive manner and to PI3-K in an ATP-competitive manner. This report is the first mechanistic study identifying a clear molecular target for the anticancer activity of isorhamnetin. Overall, these results indicate that isorhamnetin has potent anticancer activity and it primarily targets MEK and PI3-K, which might contribute to the chemopreventive potential of certain foods. Cancer Prev Res; 4(4); 582–91. Ó2011 AACR. Introduction Nonmelanoma skin cancer (NMSC) is the most com- monly diagnosed cancer in the United States. About 1.3 million individuals suffer from this disease (1). The most common types of NMSC are basal cell carcinoma (BCC) and squamous cell carcinoma (SCC), which develop from basal cells and keratinocytes, respectively. BCC comprises 75% of all NMSC cases and SCC makes up 20%. SCC, but not BCC, can metastasize to other areas of the body (2). Because NMSC is readily detectable and has limited malig- nancy, the mortality of NMSC is low compared with other cancers (3). However, because of its high incidence, NMSC is the fifth most costly cancer in the United States. Medicare spends $13 billion each year on skin cancer treatment (4). Changing environmental conditions, such as the destruc- tion of the ozone layer and environmental pollution, and alterations in eating habits and increased longevity enhance the incidence of NMSC (5). A suitable chemopre- ventive agent could prevent and cure NMSC, which would help people avoid the pain of skin cancer and reduce Medicare costs (6). UV radiation is the most important cause of skin cancer and chronic illnesses, and carcinogenic chemical exposures are also major factors (1). These causes of skin cancer activate multiple cellular signaling pathways and convert normal cells to cancerous cells. An important mediator of these signaling pathways is the epidermal growth factor (EGF) receptor (EGFR), which belongs to the ErbB family of receptor tyrosine kinases (RTK). This receptor is dimer- ized by binding a ligand, such as EGF (7), which activates the intercellular tyrosine kinase domain. The activated EGFR tyrosine kinase domain activates the mitogen-acti- vated protein (MAP) kinases and phosphoinositide 3- kinase (PI3-K; ref. 8). Previous studies have shown that Authors' Affiliations: 1 WCU Major in Biomodulation, Department of Agricultural Biotechnology, Research Institute for Agriculture and Life Sciences and Center for Agricultural Biomaterials, Seoul National Uni- versity, Seoul, Republic of Korea; 2 The Hormel Institute, University of Minnesota, Minnesota; and Departments of 3 Bioscience and Biotechnol- ogy and 4 Chemistry, Konkuk University, Seoul, Republic of Korea Note: Supplementary data for this article are available at Cancer Preven- tion Research Online (http://cancerprevres.aacrjournals.org/). J-E. Kim, D-E. Lee, and K.W. Lee contributed equally to this work. Corresponding Author: Zigang Dong, The Hormel Institute, University of Minnesota, 801 16th Avenue NE, Austin, MN 55912. Phone: 507-437- 9600; Fax: 507-437-9606; E-mail: zgdong@hi.umn.edu or Hyong Joo Lee, Department of Agricultural Biotechnology, Seoul National University, Seoul 151-742, Republic of Korea. Tel.: +82-2-880-4860; Fax: +82-2- 873-5095; E-mail: leehyjo@snu.ac.kr doi: 10.1158/1940-6207.CAPR-11-0032 Ó2011 American Association for Cancer Research. Cancer Prevention Research Cancer Prev Res; 4(4) April 2011 582 Research. on October 2, 2021. © 2011 American Association for Cancer cancerpreventionresearch.aacrjournals.org Downloaded from Published OnlineFirst February 17, 2011; DOI: 10.1158/1940-6207.CAPR-11-0032