Abstract

Objectives: Senile lentigines is one of the earliest signs of photoaging where chronic UV exposure on sun-exposed skin forms hyperpigmented lesions. While previous studies have characterized morphological changes and gene expression profiles with their functions in SLs, how SLs form, aggravate and fixate with increasing their number, size, and getting darker, has remained largely unknown. To reveal the molecular mechanism of the formation and aggravation of SLs, we aim to identify key transcriptional and epigenetic profiles of SLs by integrating RNA-seq and whole-genome bisulfite sequencing (WGBS). Introduction: Dysfunctional keratinocyte differentiation, enhancement of inflammatory immune response, and melanocyte stimulation occurs in SLs. While transcriptome analyses provide an important aspect of drastic change in gene expression profile of SLs, combinational analyses of genomics and transcriptome are required to understand how genomic landscape including epigenetics links to characteristic gene expression profile in SLs. However, it remains challenge to distinguish resultant change from a causal initial step of gene expression change linking to the formation and aggravation of SLs. Materials / method: This study was approved by the ethics committees at both Kobe University and Shiseido, and all subjects provided written informed consent. The extracted DNAs and RNAs from epidermis of biopsies were used for WGBS and RNA-seq analyses, respectively. Non-invasive measurement of mitochondria metabolism was carried out by fluorescent lifetime imaging microscopy. Immunohistochemical and cell biological analyses with in vitro and ex vivo systems were performed to demonstrate the function of mTOR signaling pathway and the effect of excessive melanin deposition in keratinocytes. Results: The integrated WGBS and RNA-seq analyses showed hypomethylation and upregulated expression of mTOR-related genes. Activation of mTOR signaling promoted keratinocyte proliferation and melanin intake. Furthermore, we also identified downregulation of mitochondria metabolism due to excessive melanin deposition, promoting senescence of keratinocytes. Based on these findings, we further screened ingredients that could prevent mTOR activation and enhance mitochondria metabolism to improve the formation and the aggravation of SL. Conclusion: Our studies revealed that epigenetic change in SLs underlies the life cycle of SLs. Among them, hypomethylation of mTOR signaling-related genes activate downstream factors, which promoted proliferation of keratinocytes along with increasing melanin intake. Additionally, we also revealed that excessive melanin deposition caused metabolic downregulation as well as keratinocyte senescence, “melanoaging”, thereby leading to the aggravation of SLs. To improve SLs based on these findings, we successfully screened ingredients that will approach the formation and aggravation of SLs.