Chloride intracellular channel 1 (CLIC1; also known as NCC27) belongs to the highly conserved CLIC family of chloride ion channels
[39][15]. It can reside in the cytoplasm and temporarily in plasma and internal cell membranes
[40][16]. CLIC1 participates in various cellular functions, including the maintenance of pH homeostasis, cell survival, cell cycle regulation, cell volume regulation, membrane potential modulation, and organelle acidification
[40,41,42,43,44,45,46,47,48][16][17][18][19][20][21][22][23][24]. This channel is upregulated in various cancer type such as prostate
[46][22], gallbladder (GBC)
[48][24], colon cancer
[47][23], gastric
[49][25], clear cell renal cell carcinoma
[50][26], and glioblastoma stem cells
[51,52][27][28]. Overexpressed CLIC1 in patients with HCC
[53][29] positively correlates with HCC proliferation and metastasis
[54][30]. CLIC1 participates in hypoxia-induced colonic carcinoma metastasis via the MAPK/ERK pathway
[45][21]. Moreover, CLIC1 is recruited to the plasma membrane in response to chemotaxis, such as directional treatment with epidermal growth factor (EGF) and mechanotaxis and its ectopic expression of CLIC1 enhances migratory apparatus such as lamellipodia and invadopodia
[54][30]. Hypoxia-induced tumor cells possess irregular microvascular networks and blood flow
[55][31] and can be transformed to promote cancer metastasis
[56][32]. Mechanistically, limited blood perfusion or altered flow due to hypoxic conditions might contribute to the migration and invasion of cancer cells
[57][33]. Upregulated CLIC1 expression correlates with lymph node metastasis and lymphatic invasion
[49][25] as well as lung cancer migration and invasion
[40][16]. Cell growth is promoted by CLIC1 via the MAPK/ERK pathway in prostate cancer
[46][22] and CLIC1 is expressed in pancreatic ductal adenocarcinoma (PDAC)
[58][34] where it plays an important role in promoting cancer cell survival, proliferation, and invasion
[46,59][22][35]. In various regulatory processes involving CLIC1, small interfering (si)RNAs of CLIC1 induce the downregulation of cell proliferation, growth, and invasiveness of pancreatic cancer cell lines such as PANC-1 and MIAPaca-2 compared with control cells
[58,60][34][36]. Furthermore, CLIC1 is associated with proteasome activator 28 β (PA28 β), and its specific siRNA downregulates CLIC1 in gastric cancer
[61][37]. A regulatory volume decrease (RVD) is a critical process in cancer cell motility, such as migration and invasion
[62][38].
2.5. Voltage-Gated Chloride Channels
Voltage-gated chloride channel 3 (CLC-3; also known as CLCN3) is expressed in cell membranes and intracellular vesicles where it exchanges chloride for hydrogen. CLC-3 protein is expressed in prostate carcinoma
[70][39], nasopharyngeal
[62][38], neuroendocrine
[71][40], and brain cells
[72][41] and is significantly overexpressed in HCC, compared with normal control tissues
[73][42]. Moreover, upregulated CLC-3 is associated with HCC tumor size and prognosis
[73][42]. Overexpressed CLC-3 protein participates in cell proliferation and migration. The regulation of cell volume by CLC-3 is involved in the development and metastasis of NPC and prostate cancer
[62,70,74][38][39][43]. Signaling by Wnt/β-catenin contributes to metastasis and adhesion by regulating the EMT process in tumorigenesis
[75,76][44][45]. The expression of CLC-3 is more abundant in tissues at the late stage of CRC and in the CRC LOVO and SW620 compared with that in normal cells. SiRNA-CLC-3 (siCLC-3) inhibits CRC cell viability, proliferation, and metastasis by inhibiting Wnt/β-catenin signaling, whereas the Wnt/β-catenin activator lithium chloride rescues the effect of siCLC-3
[77][46]. CLC-3 could be a prognostic marker for HCC, CRC, NPC, and prostate cancer. Patients with breast cancer are treated with tamoxifen, a non-steroidal anticancer agent
[78][47] that inhibits the migration, chloride current, and volume regulatory mechanisms in HCC MHCC97H cells in vitro
[79][48]. The activator of protein kinase C (PKC) phorbol-12-myristate-13 acetate (PMA) inhibits PKC expression in the presence of tamoxifen and reduces the migration of cells with CLC-3 knockdown, suggesting that CLC-3 is involved in the mechanism of anticancer drug and cellular volume regulation
[80][49]. CLC-4 is expressed on the cell surface and intracellular endosomal membranes in the CRC cell lines RKO and LS174
[81][50]. The migration and invasion of CRC cells is reduced by CLC-4 siRNA or shRNA
[82][51].
2.6. Cystic Fibrosis Transmembrane Conductance Regulator
Cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-activated chloride channel that regulates the balance of electrolytes in the respiratory and endocrine systems, exocrine glands, and other tissues. Malfunctioning and/or abnormal expression of CFTR have been found in various types of cancer. The upregulated expression of CFTR is associated with an invasive phenotype in cervical and ovarian carcinomas
[85,86][52][53]. Conversely, the mRNA and protein expression of CFTR are reduced in NPC 5-8F, 6-10B, and HNE-1, compared with normal cells, whereas CFTR knockdown increases NPC cell migration and invasion
[87][54]. Enhanced CFTR protein expression in NPC 5-8F cells increases epithelial markers such as occludin and E-cadherin, and attenuates the mesenchymal marker smooth muscle actin
[87][54]. In addition, protein expression of CFTR is decreased in CRC, compared with normal tissues
[88][55]. However, CFTR mRNA overexpression decreases cell proliferation, migration, and invasion in the CRC cell lines HCT116 and CaCo-2
[88][55]. The results of studies on the roles of CFTR in cancer have been contradictory. Enhanced CFTR expression inhibits various cancerous processes such as EMT in breast carcinoma
[89][56], lung cancer
[90][57], NPC
[87][54], endometrial carcinoma cells
[91][58], prostate cancer
[92][59], and intestinal carcinoma
[93][60].
2.7. Voltage-Dependent Anion Channels
The expression of voltage-dependent anion channels (VDACs) on the mitochondrial membrane of all eukaryotes, including mammals
[96][61], is increased in various tumor tissues, such as carcinoma of the breast
[97][62], colon
[98][63], thyroid gland
[99][64], lung
[100][65], pancreas
[101][66], and liver
[102][67] compared with that in normal tissues. The VDAC1, 2, and 3 isotypes of these channels play different roles; VDAC1 and VDAC2 participate in pore formation within the mitochondrial membrane
[96][61] and VDAC3 participates in the regulation of mitochondrial membrane potential
[103][68]. The expression of VDAC is associated with neurodegenerative disorders and muscular and myocardial diseases including various types of cancers
[104][69]. The progression of tumorigenesis is decreased in HeLa cells with depleted VDAC1
[105][70]. The expression of VDAC1 is more abundant in cancer, A549, and HeLa cells, than in normal WI-38 fibroblasts derived from lung tissue
[106][71], HCC tissues, HepG2 and SMMC7721 cells, as well as lung adenocarcinoma tumors
[107][72]. Small interfering RNA-VDAC1 and miR-7 downregulate cell growth, proliferation, migration, and invasion in HCC tissues
[102][67], lung cancer A549 cells
[100][65], and cervical cancer HeLa cells
[105][70]. Furthermore, miR-490-3p is significantly associated with the carcinogenesis of various cancers
[98][63], and it can regulate the growth and EMT of HCC cells
[108][73] and the invasiveness of triple-negative breast cancer cells, MDA-MB-231, and MDA-MB-436
[109][74]. MiR-490-3p downregulates VDAC1 through the mammalian target of rapamycin (mTOR) pathway in CRC tissues and cell lines
[98][63]. The expression of VDAC2 is upregulated in melanoma cells and HCC cell lines such as HepG2
[110][75] but downregulated in glioma stem cells
[111][76] and it plays an anti-apoptotic role in primary cultured mouse embryonic fibroblasts. Although its different role of VDACs is defined, precise roles of VDAC family in different cancers remain unresolved and await identification in future studies.
2.8. Volume-Regulated Anion Channel
Volume regulation is critical function to maintain cellular fate. Volume-regulated anion channels (VRACs; also called volume-sensitive organic osmolyte anion channel or swelling-induced chloride current ICl
swell) are considered as regulatory channels of cellular volume
[112][77]. VRAC is involved in the RVD and regulates proliferation of nasopharyngeal carcinoma cell
[113,114[78][79][80],
115], OSCC HST-1 cells
[27][81], and gastric cancer
[116][82]. Inhibited VRAC by 4-(2-Butyl-6,7-dichlor-2-cyclopentyl-indan-1-on-5-yl) oxybutyric acid (DCPIB) reduces proliferation, migration, and invasion of glioblastoma U251 and U87 cells
[117][83]. Leucine-rich-repeat-containing 8A (LRRC8A, also called SWELL1) is component protein of VRAC. LRRC8A expresses in HCC tissues and induced cellular proliferation and migration in HCC SMMC-7721, Sk-hep-1, Huh7, and HCCLM3 cells
[118][84]. Moreover, survival of cisplatin-resistant A549 cells or A2780 cells is modulated by the LRRC8A
[119,120][85][86]. Although dual function of VRAC components LRRC8A and LRRC8D on drug resistance has been addressed
[121][87], VRACs may be associated with chemo-resistant mechanism and are needed to verify its precise mechanism in various cancers.
2.9. Chloride-Bicarbonate Exchangers
Chloride-bicarbonate (CB) exchangers consist of solute carrier (SLC) families, including anion exchangers (AEs) and SLC26As. The CB exchangers mediate the electroneutral or electrogenic exchange of bicarbonate for chloride (respective stoichiometry of chloride: bicarbonate, 1:1 or 1:2) and are associated with the regulation of intracellular pH. The anion exchangers AE1, AE2, AE3, and AE4
[122,123,124,125][88][89][90][91] are expressed in various tissues and localize in the plasma membrane. Both AE1 and AE2 are involved in cancer whereas other AEs are unknown. Histologic findings have shown that AE1 is expressed in the cytoplasm of gastric cancer cells
[126,127][92][93]. Knockdown of AE1 induces the release of p16INK4A and inhibits gastric cancer growth
[126,128][92][94]. The expression of AE1 positively correlates with cancer size and metastasis
[127][93]. Although the modulation of AE1 expression is poorly verified, miR-24-mediates AE1 attenuation in gastric cancer cells
[129][95]. The expression of AE1 is associated with tumor progression through crosstalk with MAPK and hedgehog signaling pathways in esophageal carcinoma
[130][96]. Although AE2 is expressed in most tissues, it has been addressed that the AE2 gene is highly expressed in HCC cells, gastric, and colorectal cancers
[131,132,133,134,135][97][98][99][100][101].