CLIC4 is a tumor suppressor for cutaneous squamous cell cancer

Carcinogenesis. 2012 May;33(5):986-95. doi: 10.1093/carcin/bgs115. Epub 2012 Mar 1.


Chloride intracellular channel (CLIC) 4 is a member of a redox-regulated, metamorphic multifunctional protein family, first characterized as intracellular chloride channels. Current knowledge indicates that CLICs participate in signaling, cytoskeleton integrity and differentiation functions of multiple tissues. In metabolically stressed skin keratinocytes, cytoplasmic CLIC4 is S-nitrosylated and translocates to the nucleus where it enhances transforming growth factor-β (TGF-β) signaling by protecting phospho-Smad 2 and 3 from dephosphorylation. CLIC4 expression is diminished in multiple human epithelial cancers, and the protein is excluded from the nucleus. We now show that CLIC4 expression is reduced in chemically induced mouse skin papillomas, mouse and human squamous carcinomas and squamous cancer cell lines, and the protein is excluded from the nucleus. The extent of reduction in CLIC4 coincides with progression of squamous tumors from benign to malignant. Inhibiting antioxidant defense in tumor cells increases S-nitrosylation and nuclear translocation of CLIC4. Adenoviral-mediated reconstitution of nuclear CLIC4 in squamous cancer cells enhances TGF-β-dependent transcriptional activity and inhibits growth. Adenoviral targeting of CLIC4 to the nucleus of tumor cells in orthografts inhibits tumor growth, whereas elevation of CLIC4 in transgenic epidermis reduces de novo chemically induced skin tumor formation. In parallel, overexpression of exogenous CLIC4 in squamous tumor orthografts suppresses tumor growth and enhances TGF-β signaling. These results indicate that CLIC4 suppresses the growth of squamous cancers, that reduced CLIC4 expression and nuclear residence detected in cancer cells is associated with the altered redox state of tumor cells and the absence of detectable nuclear CLIC4 in cancers contributes to TGF-β resistance and enhances tumor development.

Publication types

  • Research Support, N.I.H., Intramural

MeSH terms

  • Animals
  • Cell Line, Tumor
  • Cell Nucleus / genetics
  • Cell Nucleus / metabolism
  • Cell Transformation, Neoplastic / genetics
  • Cell Transformation, Neoplastic / metabolism
  • Chloride Channels / biosynthesis*
  • Chloride Channels / genetics
  • Chloride Channels / metabolism
  • Fibroblasts / metabolism
  • Humans
  • Keratinocytes / metabolism
  • Male
  • Mice
  • Mice, Inbred BALB C
  • Mice, Inbred SENCAR
  • Mitochondrial Proteins / biosynthesis*
  • Mitochondrial Proteins / genetics
  • Mitochondrial Proteins / metabolism
  • Neoplasms, Squamous Cell / genetics
  • Neoplasms, Squamous Cell / metabolism*
  • Oxidation-Reduction
  • Papilloma / genetics
  • Papilloma / metabolism
  • Protein Transport
  • Signal Transduction
  • Skin Neoplasms / genetics
  • Skin Neoplasms / metabolism*
  • Transforming Growth Factor beta / genetics
  • Transforming Growth Factor beta / metabolism
  • Tumor Suppressor Proteins / biosynthesis*
  • Tumor Suppressor Proteins / genetics
  • Tumor Suppressor Proteins / metabolism


  • CLIC protein, mouse
  • CLIC4 protein, human
  • Chloride Channels
  • Mitochondrial Proteins
  • Transforming Growth Factor beta
  • Tumor Suppressor Proteins