Occludin is involved in adhesion, apoptosis, differentiation and Ca2+-homeostasis of human keratinocytes: implications for tumorigenesis

PLoS One. 2013;8(2):e55116. doi: 10.1371/journal.pone.0055116. Epub 2013 Feb 4.

Abstract

Tight junction (TJ) proteins are involved in a number of cellular functions, including paracellular barrier formation, cell polarization, differentiation, and proliferation. Altered expression of TJ proteins was reported in various epithelial tumors. Here, we used tissue samples of human cutaneous squamous cell carcinoma (SCC), its precursor tumors, as well as sun-exposed and non-sun-exposed skin as a model system to investigate TJ protein alteration at various stages of tumorigenesis. We identified that a broader localization of zonula occludens protein (ZO)-1 and claudin-4 (Cldn-4) as well as downregulation of Cldn-1 in deeper epidermal layers is a frequent event in all the tumor entities as well as in sun-exposed skin, suggesting that these changes result from chronic UV irradiation. In contrast, SCC could be distinguished from the precursor tumors and sun-exposed skin by a frequent complete loss of occludin (Ocln). To elucidate the impact of down-regulation of Ocln, we performed Ocln siRNA experiments in human keratinocytes and uncovered that Ocln downregulation results in decreased epithelial cell-cell adhesion and reduced susceptibility to apoptosis induction by UVB or TNF-related apoptosis-inducing ligand (TRAIL), cellular characteristics for tumorigenesis. Furthermore, an influence on epidermal differentiation was observed, while there was no change of E-cadherin and vimentin, markers for epithelial-mesenchymal transition. Ocln knock-down altered Ca(2+)-homeostasis which may contribute to alterations of cell-cell adhesion and differentiation. As downregulation of Ocln is also seen in SCC derived from other tissues, as well as in other carcinomas, we suggest this as a common principle in tumor pathogenesis, which may be used as a target for therapeutic intervention.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Aged
  • Aged, 80 and over
  • Calcium / metabolism
  • Carcinoma, Squamous Cell / genetics*
  • Carcinoma, Squamous Cell / metabolism
  • Carcinoma, Squamous Cell / pathology
  • Cell Adhesion / radiation effects
  • Cell Differentiation / radiation effects
  • Cell Transformation, Neoplastic / metabolism
  • Cell Transformation, Neoplastic / pathology
  • Cell Transformation, Neoplastic / radiation effects*
  • Claudins / genetics
  • Claudins / metabolism
  • Epithelial-Mesenchymal Transition / radiation effects*
  • Female
  • Gene Expression Regulation, Neoplastic / radiation effects*
  • Homeostasis / radiation effects
  • Humans
  • Keratinocytes / cytology
  • Keratinocytes / metabolism
  • Keratinocytes / radiation effects*
  • Male
  • Middle Aged
  • Neoplasm Grading
  • Occludin / antagonists & inhibitors
  • Occludin / genetics*
  • Occludin / metabolism
  • RNA, Small Interfering / genetics
  • Signal Transduction / radiation effects
  • Skin Neoplasms / genetics*
  • Skin Neoplasms / metabolism
  • Skin Neoplasms / pathology
  • Tight Junctions / metabolism
  • Tight Junctions / pathology
  • Tight Junctions / radiation effects
  • Young Adult
  • Zonula Occludens-1 Protein / genetics
  • Zonula Occludens-1 Protein / metabolism

Substances

  • Claudins
  • OCLN protein, human
  • Occludin
  • RNA, Small Interfering
  • TJP1 protein, human
  • Zonula Occludens-1 Protein
  • Calcium

Grant support

Deutsche Forschungsgemeinschaft, Forschergruppe 721/2, TP9, Johann und Anny Thomas Stiftung, COST Action BM 0903 SkinBAD STSM. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.