Skin is the outermost organ and acts as a barrier between the organism and environment. Skin protects the organism from environmental insults, such as chemicals, pathogens, and UV light. Much of the protective function of skin is dependent on the epidermis, a multi-layered epithelium that is composed of various cell types such as keratinocytes and melanocytes. Keratinocytes produce protective components through a sophisticated differentiation process. Disturbance of keratinocyte differentiation is related to several skin diseases such as psoriasis and atopic dermatitis. In this study, we prepared extract of combined medicinal plants (ECMP) consisting of Taraxacum platycarpum H. Dahlstedt, Heartleaf Houttuynia, Glycyrrhiza uralensis Fischer, and root bark of Ulmus davidiana. We demonstrated that ECMP enhanced keratinocyte differentiation and barrier functionality using an in vitro cell culture system and in vivo animal test. Treatment of cultured keratinocytes with ECMP resulted in induction of keratinocyte differentiation, as evidenced by increased differentiation markers such as involucrin, loricrin, and filaggrin. In line with these results, ECMP decreased proliferation of keratinocytes cultured in vitro. ECMP applied topically to tape-stripped mouse skins accelerated reduction of transepidermal water loss (TEWL), indicating fast recovery of barrier function. Immunohistochemistry showed that ECMP increased the filaggrin level in tape-stripped mouse skins. These results suggest that ECMP may be applicable for keratinocyte differentiation-related skin diseases.

Introduction
 Materials and Methods
  Cell culture
  Extract of combined medicinal plants (ECMP)
  MTT assay
  Western blot analysis
  Luciferase reporter assay
  Cell growth analysis
  Transepidermal water loss (TEWL)
  Immunohistochemistry
  Statistical analysis
 Results
  Cytotoxicity of ECMP
  Effect of ECMP on keratinocyte differentiation
  Effect of ECMP on keratinocyte proliferation
  Effect of ECMP on skin barrier functionality
 Discussion
 Acknowledgements
 ORCID
 References

• Xue Mei Li(Department of Dermatology, First Affiliated Hospital of Dalian Medical University, Dalian, China)
• Dae-Kyoung Choi(Department of Dermatology, School of Medicine, Chungnam National University, Daejeon 301-747, Korea)
• Seul Ki Lim(Department of Dermatology, School of Medicine, Chungnam National University, Daejeon 301-747, Korea)
• In Suk Ahn(Department of Obstetrics & Gynecology, College of Korean Medicine, Dongguk University, Seoul 100-715, Korea)
• Dong-Il Kim(Department of Obstetrics & Gynecology, College of Korean Medicine, Dongguk University, Seoul 100-715, Korea)
• Yong Jun Piao(Department of Dermatology, First Affiliated Hospital of Dalian Medical University, Dalian, China)
• Chang Deok Kim(Department of Dermatology, School of Medicine, Chungnam National University, Daejeon 301-747, Korea)
• Jeung-Hoon Lee(Department of Dermatology, School of Medicine, Chungnam National University, Daejeon 301-747, Korea) Corresponding author