CLIC4 is a tumor suppressor for cutaneous squamous cell cancer

Abstract

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.

Head and neck squamous cancer progression is marked by CLIC4 attenuation in tumor epithelium and reciprocal stromal upregulation of miR-142-3p, a novel post-transcriptional regulator of CLIC4

“…The reduction of CLIC4 in tumor cells in progressing cancers of certain organs, particularly squamous cancers, suggests a tumor suppressor role for the protein. Indeed, in a cutaneous cancer model, overexpression of CLIC4 in epidermis by transgene targeting or administration of exogenous CLIC4 via viral transduction are effective in reducing tumor growth [24]. We now show three tumor types (cervix, esophagus, and head and neck) with low CLIC4 expression in parenchyma relative to stroma.…”

“…CLIC4 has been implicated in angiogenesis [3][4][5], pulmonary arterial hypertension [6,7], epithelial differentiation [8], myofibroblast differentiation [9][10][11], response to oxidative stress [12][13][14][15], cellular adhesion and integrin trafficking [16][17][18], immunity [19][20][21][22], and cancer [23][24][25][26][27][28][29][30][31]. Despite the elucidation of many CLIC4 functions, little is known regarding the regulation of CLIC4 expression.…”

Three Decades of Chloride Intracellular Channel Proteins: From Organelle to Organ Physiology

“…CLIC4 also influences angiogenesis by acting upstream of hypoxia‐inducible factor alpha (HIF‐1α) and vascular endothelial growth factor (VEGF) signaling. Its role in angiogenesis explains its increased expression in many cancers, suggesting its targeting in the development of new chemotherapeutics (Chiang, Chou, Chien, Tsai, & Chen, ; Peretti et al., ; Suh et al., ; Suh et al., ; Suh et al., ). CLIC4 plays a role in the acidification of intracellular vacuoles that are required for tubulogenesis (Ulmasov et al., ).…”

Intracellular ion channels and cancer

“…Marked changes in expression and subcellular localization of CLIC4 occur early in tumorigenesis. In particular, reduced CLIC4 expression and nuclear localization in cancer cells is associated with the altered redox state and CLIC4 acts as an important suppressor of squamous tumor development and progression (Suh et al, 2012). …”