CLIC4 mediates and is required for Ca²⁺-induced keratinocyte differentiation

Keratinocyte differentiation requires integrating signaling among intracellular ionic changes, kinase cascades, sequential gene expression, cell cycle arrest, and programmed cell death. We now show that Cl^- intracellular channel 4 (CLIC4) expression is increased in both mouse and human keratinocytes undergoing differentiation induced by Ca²⁺, serum and the protein kinase C (PKC)-activator, 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Elevation of CLIC4 is associated with signaling by PKC, and knockdown of CLIC4 protein by antisense or shRNA prevents Ca²⁺-induced keratin 1, keratin 10 and filaggrin expression and cell cycle arrest in differentiating keratinocytes. CLIC4 is cytoplasmic in actively proliferating keratinocytes in vitro, but the cytoplasmic CLIC4 translocates to the nucleus in keratinocytes undergoing growth arrest by differentiation, senescence or transforming growth factor (TGF) treatment. Targeting CLIC4 to the nucleus of keratinocytes via adenoviral transduction increases nuclear Cl^- content and enhances expression of differentiation markers in the absence of elevated Ca²⁺. In vivo, CLIC4 is localized to the epidermis in mouse and human skin, where it is predominantly nuclear in quiescent cells. These results suggest that CLIC4 participates in epidermal homeostasis through both alterations in the level of expression and subcellular localization. Nuclear CLIC4, possibly by altering the Cl^- and pH of the nucleus, contributes to cell cycle arrest and the specific gene expression program associated with keratinocyte terminal differentiation.