Original StudyDifferent Immunohistochemical Localization of Fatty Acid Binding Protein 5 in Actinic Keratosis Compared with That in Bowen's Disease: A Retrospective StudyAhn, Bokyung MD*; Kim, Chul Hwan MD, PhD†; Chae, Yang-Seok MD, PhD†; Lee, Jeong Hyeon MD, PhD†; Lee, Youngseok MD, PhD†; Ahn, Hyo Hyun MD, PhD‡; Lee, Yoo Jin MD, PhD†Author Information *Department of Pathology, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea; and Departments of †Pathology, and ‡Dermatology, Korea University Anam Hospital, Korea University College of Medicine, Seoul, Korea. Correspondence: Yoo Jin Lee, MD, PhD, Department of Pathology, Korea University Anam Hospital, Sungbuk-Gu, Inchon-Ro 73, Seoul 136-705, Korea (e-mail: [email protected]). Supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2020R1I1A1A01068917). The authors declare no conflicts of interest. The American Journal of Dermatopathology: May 2021 - Volume 43 - Issue 5 - p 356-361 doi: 10.1097/DAD.0000000000001823 Buy Metrics Abstract Actinic keratosis (AK) and Bowen's disease (BD) are common premalignant lesions of invasive squamous cell carcinoma that have different pathogenesis and clinical significance. Fatty acid–binding protein 5 (FABP5) is responsible for keratinocyte homeostasis and differentiation; however, no study has revealed its expression in AK and BD. Our study aimed to investigate the differential expression and significance of FABP5 in these lesions. Patients with pathologically confirmed cases of AK (n = 37) and BD (n = 12) were included in this study. FABP5 immunostaining pattern was assessed in the normal skin, AK and BD lesions, with a focus on the staining patterns of basal cells, atypical keratinocytes, and uninvolved epidermal keratinocytes. All patients with AK showed negative FABP5 expression in the atypical cells in the basal layer, whereas the uninvolved upper layers showed diffuse, strong FABP5 expression, regardless of the grade of AK. All patients with BD showed heterogeneous and diffuse FABP5 expression in atypical cells of all layers of the epidermis. This study is the first to investigate the role of FABP5 in premalignant skin lesions. The unique immunohistochemical localization of the FABP5 can be a helpful diagnostic marker, and altered fatty acid metabolism may be the key in understanding the different pathophysiology of AK and BD. Copyright © 2020 Wolters Kluwer Health, Inc. All rights reserved.