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Chloride intracellular channel (CLIC)4 is a p53- and tumor necrosis factor α (TNFα)-regulated chloride channel protein that is localized to the mitochondria and cytoplasm of mouse and human keratinocytes. CLIC4 protein increases in differentiating keratinocytes and in keratinocytes exposed to DNA-damaging agents and metabolic inhibitors. Increasing CLIC4 levels by transduction of recombinant CLIC4 causes apoptosis. CLIC4 translocates to the nucleus under a variety of conditions of cell stress, and nuclear CLIC4 is associated with cell cycle arrest and accelerated apoptosis. Reduction of CLIC4 and several other CLIC family members by expressing a doxycycline-regulated CLIC4 antisense also causes apoptosis in squamous cancer cell lines. Expressing antisense CLIC4 in tumors derived from transplanting these cells into nude mice inhibits tumor growth, increases tumor apoptosis, and reduces tumor cell proliferation. Co-administration of TNFα intraperitoneally enhances the tumor-inhibitory influence of CLIC4 antisense expression. Together, these results suggest that CLIC4 is important for keratinocyte viability and may be a novel target for anti-cancer therapy.
Chloride is a major intracellular anion of all living organisms. Intracellular chloride is essential for regulating electrogenic cation transport across intracellular and plasma membranes and maintaining membrane potential in organelles. As a consequence, both organelle volume and pH are maintained, and these are fundamental to organelle function. The importance of intracellular chloride ion regulation is emphasized by the evolution of at least five independent chloride channel families in mammalian cells (Table I). The number of family members in each class of chloride channels is variable, but mutations in genes from several families are associated with neuromuscular, respiratory, osseous, and renal diseases in humans (
The most recently discovered and still evolving family of chloride channel proteins is the chloride intracellular channel (CLIC) family. Currently, seven highly homologous proteins are recognized that are encoded by independent genes, except for p64 (CLIC5B) and CLIC5A (Table II). The parent member of this family, p64, was isolated from microsomes of bovine kidney and trachea, and have been shown to possess chloride-selective channel function in lipid bilayers (
). All subsequently discovered members of the CLIC family possess the central core domains of p64, including a transmembrane region, but vary in the amino and carboxy termini Figure 1. Consensus phosphorylation sites for PKA, PKC, CKII, and tyrosine kinases on CLIC family members suggest that post-translational modification contributes to the physiology of CLIC proteins. A structural and functional breakthrough in understanding CLIC took place with the crystal structure studies of CLIC1 (
). This analysis confirmed the similarity of CLIC1 to the omega class of glutathione transferase proteins, and provided a structural basis through protein folding for CLIC proteins to partition into soluble or membranous compartments of mammalian cells. Structural studies also indicated that channel activity was likely related to the formation of homodimers or tetramers of CLIC creating a hydrophilic pocket for ion transport.
Table IIChloride intracellular channels
Chloride channel activity
Activation and features
p64/CLIC5B (49 kDa)
Indanyloxy acetic acid (IAA)-sensitive Cl transporter, voltage-dependent Cl channel activity in microsomal compartment, anti-p64-mediated immunodepletion of Cl-transporting activity
Among the various CLIC family members, the biological activity of CLIC4 has been studied most extensively (Table III). CLIC4 (also known as mtCLIC, p64H1, RS43) is ubiquitously expressed with particularly high expression in the skin, kidney, liver, and brain. The subcellular localization of CLIC4 varies with cell type and has been localized to golgi, endoplasmic reticulum, large dense core vesicles in the brain, and the inner mitochondrial membrane and cytoplasm in mouse and human keratinocytes. In keratinocytes and other cell types, the expression of CLIC4 transcripts is regulated by p53 and tumor necrosis factor α (TNFα), and CLIC4 is a direct response gene for both c-myc and p53, with functional consensus binding sites identified for both of these transcription factors in the CLIC4 promoter (
). Increased or reduced expression of CLIC4 induces apoptosis in several cell types including keratinocytes, and CLIC4 expression increases in keratinocytes undergoing an apoptotic response to an external stimulus. Furthermore, CLIC4 upregulation is a requirement for p53-induced apoptosis (
). The apoptotic response to overexpression of CLIC4 is directed through a mitochondrial pathway, resulting in decreased mitochondrial membrane potential, release of cytochrome C, and activation of caspases ((
). Cytoplasmic CLIC4 translocates to the nucleus in keratinocytes undergoing apoptosis or growth arrest in response to stress including exposure to metabolic inhibitors, DNA-damaging agents, inhibitors of cell cycle progression, and TNFα. Nuclear translocation involves interaction with the Ran/Importin nuclear transport complex (
). Nuclear targeting of CLIC4 through transduction of a modified CLIC4 construct accelerates apoptosis and induces apoptosis in cells that are defective in the mitochondrial apoptotic pathway through genetic deletion of Apaf1 (
). Since this discovery, modifications in CLIC4 have been associated with 3T3-L1 fibroblasts differentiating into adipocytes, and mammary fibroblasts differentiating into myofibroblasts under the influence of transforming growth factor β (
) supports a wider involvement of CLIC proteins in cellular functions beyond or requiring chloride transport. The importance of these functions is emphasized by the discovery that CLIC4 is among a small number of genes whose expression is upregulated in cutaneous stem cells (
The involvement of CLIC4 in the differentiation of several cell types, its importance in maintaining cell viability, its relationship with TNFα, p53, and c-myc, and the essentiality of chloride homeostasis in cell signaling prompted us to test whether CLIC4 could be a target for tumor therapy. To address this question, we created several keratinocyte-derived tumor cell lines constructed to express an antisense CLIC4 under the regulation of a tetracycline-sensitive promoter (antisense paper). Upon withdrawal of doxycycline from culture medium, the antisense construct is induced and CLIC4 levels decrease Figure 2. Subsequent studies have indicated that expression of CLIC4 antisense also reduces the level of CLIC1 and CLIC5 in these cell lines (antisense paper). When these cells are grafted to a dermal graft bed on the dorsum of athymic nude mice, they grow as aggressive squamous cell carcinomas while the mice are fed a diet containing doxycycline Figure 3. If doxycycline is withdrawn from the diet to induce CLIC4 antisense either at the time of grafting or after tumor establishment, however, tumor growth is inhibited. Histological and immunohistochemical analyses of tumors indicate that the expression of the antisense CLIC4 is associated with a 3-fold increase in apoptotic cells (TUNEL assay) and a significant reduction in proliferating cells (BrdU staining). Further reduction in tumor growth is obtained by injecting TNFα intraperitoneally twice weakly, together with antisense expression Figure 3. Together, these results suggest that CLIC4 reduction could be a novel target for therapy of cutaneous tumors.
The forgoing results indicate a role for CLIC4 in both keratinocyte differentiation and carcinogenesis. Previous studies have suggested that chloride transport participates in epidermal homeostasis (
), but the discovery of CLIC proteins adds a new dimension to the process. A number of questions arise as a consequence of this discovery. What is the function of CLIC4 induction in differentiating keratinocytes? Does CLIC4, or other CLIC, proteins participate in the growth arrest and loss of viability associated with epidermal maturation? Does the increase in p53-mediated transcription in differentiating keratinocytes (
) regulate CLIC4 expression? Does the increase in PKC activation in maturing epidermal cells modify CLIC4 function? How does the subcellular localization of CLIC4 contribute to differentiation or carcinogenesis? Can we develop small molecule inhibitors for the CLIC proteins that would have a therapeutic benefit in the treatment of cutaneous or other malignancies? Will combined treatments with inhibitors of CLIC proteins and standard chemotherapeutic drugs enhance cancer regression? All of these questions are readily approachable in experimental models. The real challenge will be to translate results from these basic studies into clinical advances for the prevention and treatment of cutaneous diseases.
Challenging accepted ion channel biology: p64 and the CLIC family of putative intracellular anion channel proteins (review).