title: Histidase expression in human epidermal keratinocytes: regulation by differentiation status and all-trans retinoic acid.
abstract: BACKGROUND
  Histidase (histidine ammonia lyase) converts histidine into urocanic acid, the main ultraviolet (UV) light absorption factor of the stratum corneum. It is unknown if and how histidase is regulated in the epidermis.
  
  
  OBJECTIVE
  We have investigated the transcriptional regulation of histidase expression in epidermal keratinocytes.
  
  
  METHODS
  Human epidermal keratinocytes were cultured in vitro and exposed to UV irradiation, a number of cytokines and all-trans retinoic acid (ATRA) (1 microM). Keratinocyte differentiation was triggered by maintaining confluent cells in monolayer culture and by establishing three-dimensional skin equivalents. The mRNA expression level of histidase in keratinoytes as well as in the epidermis and other tissues was determined by quantitative real-time PCR. Protein expression was determined by Western blot analysis.
  
  
  RESULTS
  Human epidermis contained higher levels of histidase transcripts than all other tissues investigated. Expression of histidase strongly increased at the mRNA and protein levels during differentiation of primary keratinocytes in vitro. Treatment of keratinocytes with UVA and UVB did not significantly change the expression level of histidase. By contrast, ATRA suppressed histidase expression almost completely.
  
  
  CONCLUSIONS
  Our results show that histidase is upregulated during keratinocyte differentiation and that ATRA but not UV irradiation modulates the expression level of histidase. Suppression of histidase-mediated production of urocanic acid may contribute to the increase in UV sensitivity that is caused by treatment with retinoids.
authors.affiliation: Department of Dermatology, Medical University of Vienna, Waehringer Guertel 18-20, A-1090 Vienna, Austria.
authors.forename: Leopold
authors.initials: L
authors.lastname: Eckhart
authors.forename: Martina
authors.initials: M
authors.lastname: Schmidt
authors.forename: Michael
authors.initials: M
authors.lastname: Mildner
authors.forename: Veronika
authors.initials: V
authors.lastname: Mlitz
authors.forename: Arby
authors.initials: A
authors.lastname: Abtin
authors.forename: Claudia
authors.initials: C
authors.lastname: Ballaun
authors.forename: Heinz
authors.initials: H
authors.lastname: Fischer
authors.forename: Paul
authors.initials: P
authors.lastname: Mrass
authors.forename: Erwin
authors.initials: E
authors.lastname: Tschachler
mesh_terms: D002454:Cell Differentiation / Q000187:drug effects / Q000502:physiology; D002478:Cells, Cultured; D000078404:Epidermal Cells; D004817:Epidermis / Q000201:enzymology; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects / Q000502:physiology / Q000528:radiation effects; D006638:Histidine Ammonia-Lyase / Q000235:genetics* / Q000378:metabolism; D006801:Humans; D015603:Keratinocytes / Q000166:cytology* / Q000187:drug effects / Q000201:enzymology*; D007641:Keratolytic Agents / Q000494:pharmacology*; D014158:Transcription, Genetic / Q000187:drug effects / Q000502:physiology / Q000528:radiation effects; D014212:Tretinoin / Q000494:pharmacology*; D014466:Ultraviolet Rays; D015854:Up-Regulation / Q000187:drug effects / Q000502:physiology / Q000528:radiation effects
chemical_list: D007641:Keratolytic Agents; D014212:Tretinoin; D006638:Histidine Ammonia-Lyase