Cystic fibrosis transmembrane conductance regulator—emerging regulator of cancer

Abstract

Mutations of cystic fibrosis transmembrane conductance regulator (CFTR) cause cystic fibrosis, the most common life-limiting recessive genetic disease among Caucasians. CFTR mutations have also been linked to increased risk of various cancers but remained controversial for a long time. Recent studies have begun to reveal that CFTR is not merely an ion channel but also an important regulator of cancer development and progression with multiple signaling pathways identified. In this review, we will first present clinical findings showing the correlation of genetic mutations or aberrant expression of CFTR with cancer incidence in multiple cancers. We will then focus on the roles of CFTR in fundamental cellular processes including transformation, survival, proliferation, migration, invasion and epithelial–mesenchymal transition in cancer cells, highlighting the signaling pathways involved. Finally, the association of CFTR expression levels with patient prognosis, and the potential of CFTR as a cancer prognosis indicator in human malignancies will be discussed.

Introduction

Cystic fibrosis transmembrane conductance regulator (CFTR) is known as a major anion channel for Cl⁻ and HCO₃⁻, which belongs to the adenosine triphosphate (ATP)-binding cassette (ABC) transporter family [1, 2]. While CFTR was initially found to be expressed in a wide variety of epithelial tissues, subsequent studies indicated that CFTR was expressed in other tissues/cell types as well including the heart [3], smooth muscles [4], neurons [5], endothelial cells [6], sperm [7] and oocytes [8]. Epithelial cells with defective CFTR produce excessive viscous mucus, which consequently accumulates and obstructs the lumen of pancreatic ducts, airway, biliary and reproductive tracts [9], and may result in various clinical manifestations [10]. Thus, mutations of CFTR cause the most common lethal genetic disease in Caucasians—cystic fibrosis (CF) [11, 12]. Although CF is still a disease with relentless morbidity and mortality, more than half of the CF patients can live till age 18 or above in recent years [13], due to an improvement of the treatment strategies. As CF patients now reach adulthood, there is an increasing interest in the association of cancer incidence with the genetic variations of the CFTR gene. In North American and European, large cohort studies have reported increased risk of overall cancers, especially in digestive system, in CF patients [14–18]. Specifically, the mean age at diagnosis of cancer was 40.4 years in CF patients, while 64.2 years in male and 54.8 years in females with one mutated copy of CFTR gene [17]. The risk was even elevated in patients with organ transplantation [18, 19]. Moreover, hyper-methylation of CFTR gene has been reported in several cancer cell lines and primary tumors [20–23]. These studies suggest a potential link between CFTR and the pathogenesis of cancers.

In this review, we will summarize the correlation between the aberrant expression of CFTR, including its expression pattern, genetic mutations/polymorphisms and epigenetic regulation, and cancer incidence in multiple cancers. This review will also discuss various CFTR-regulated signaling pathways involved in the process of cancer development, and the potential implication of CFTR as a regulator of cancer progression and an indicator of cancer prognosis.

Mutations and aberrant expression of CFTR in cancers

CFTR is a 1480 amino acid membrane bound glycoprotein with a molecular mass of 170,000. CFTR protein possesses two membrane-spanning domains (MSDs), each containing six helices and conjoined to a cytoplasmic nucleotide-binding domain (NBD1 and NBD2), where ATP binds to regulate channel gating [24–27]. Over 2000 mutations have been identified in the CFTR gene to date [28], which are situated throughout the entire coding region and the promoter region of the gene. The ΔF508 mutation accounts for approximately 70% of mutations in CF patients. The mutated CFTR anion channel is not fully glycosylated and shows minimal activity in epithelial cells of CF patients. Low temperature or inhibition of histone deacetylases partly rescues ∆F508 CFTR cellular processing defects and function. The majority of the other CFTR mutations are rare, i.e., the four other mutations (G551D, G542X, N1303 K and W1282X) have overall frequencies of above 1%, which are unique to a particular individual or family across the world [29]. In addition, genomic rearrangements, such as large deletions or insertions within the CFTR gene, e.g., Δexons 4–10, Δ95.7 kb starting in intron 1, have also been identified in CF patients, albeit the incidence is low [28].

Since the first report showing increased incidence of gastrointestinal malignancies in CF patients [14], there have been accumulating evidence indicating the association of cancer incidence with the genetic variations of the CFTR gene. Large cohort study in North American and European patients with CF found that there was a significant increase in the risk of cancers affecting the gastrointestinal tract, pancreas and hepatobiliary system [14, 30]. Of note, in a recent study conducted at the University of Minnesota, early colonoscopic screening of adult CF patients revealed a high incidence of colon tumors, especially in males [31]. Conversely, CF gene mutations have also been reported to be associated with a lower risk in several cancers, such as melanoma [32], breast cancer [33], colon cancer [34], prostate cancer [35] and lung cancer [36]. On the other hand, aberrant expression levels of CFTR have been reported by various studies, based on hospital-based case–control studies or laboratory observations. Downregulation of CFTR has been observed in multiple cancers including nasopharyngeal carcinoma (NPC) [37], hepatocellular carcinoma (HCC) [23, 38], intestinal cancer [39, 40], prostate cancer [41], lung cancer [42, 43] and bladder cancer [22, 44]. In contrast, upregulation of CFTR has been found in other types of cancer such as gastric cancer [45], ovarian cancer [46, 47], cervical cancer [48, 49] and endometrial cancer [50, 51].

In our opinion, while numerous genetic variants and aberrant expression levels of CFTR have been frequently identified in different types of cancer, there are considerable contradictory findings regarding the association of CFTR mutations/expression with cancer incidence. Although it is quite clear that different tissue contexts may contribute to the different findings, these discrepancies might also be due to the differences in sample size, study design and various CFTR mutations included in the individual studies. On the other hand, these studies emphasize the importance of understanding the exact physiological role of CFTR in cancer development. Since the association of CFTR mutations or aberrant expression with cancers has been reviewed previously [52, 53], this article will not focus on this aspect and the related information is summarized in Table 1.

Table 1.

The mutations, expression and clinical implication of CFTR in different cancers

Cancer type	Expression, Methylation and Polymorphisms	Clinical implication	Citations
Reproduction system
Ovarian cancer	Up-regulation	High CFTR expression is correlated with advanced stage, poor histological grades and high CA-125	Xu et al. [47] and Zhu et al. [46]
Endometrial carcinoma	Up-regulation	Unknown	Xia et al. [50]
Cervical cancer	Up-regulation	The combination of CFTR and NF-κB is positively associated with stage, histological grade, lymph node metastasis, and invasive interstitial depth	Peng et al. [49], Wu et al. [143] and Royse et al. [48]
Breast cancer	DeltaF508 carrier related to higher grade III (1) Down-regulation (2)	Low mRNA expression is correlated with poor prognosis	(1) Southey et al. [183] (2) Zhang et al. [76]
Prostate cancer	IVS8-5T allele protects against prostate cancer (1) Hyper-methylation (2) Down-regulation (3)	CFTR methylation indicates the population at higher risk of therapeutic failure	(1) Qiao et al. [35] (2) Ashour et al. [21] (3) Xie et al. [41]
Respiratory system
Nasopharyngeal carcinoma	Down-regulation	CFTR protein expression is positively correlated with survival	Tu et al. [37]
Lung cancer	DeltaF508 deletion is found in 1/9 lung cancer patients(1) CFTR deletion carrier protects against lung cancer (2) Down-regulation(3) Hyper-methylated(4)	CFTR methylation is correlated with worse survival in young patients Low mRNA expression is correlated with advanced staging, metastasis and poor prognosis	(1) Jung et al. [189] (2) Li et al. [36] (3) Son et al. [20], Li et al. [43] and Tian et al. [42] (4) Son et al. [20]
Urinary system
Bladder cancer	Hyper-methylation	Unknown	Yu et al. [22] and Zhao et al. [44]
Endocrine system
Papillary thyroid cancer	Promoter SNP (Rs4148682, rs213950)	Unknown	Oh et al. [190]
Digestive system
Esophageal adenocarcinoma	A risk loci is within or near CFTR	Unknown	Gharahkhani et al. [191]
Hepatocellular carcinoma	Hyper-methylation	Unknown	Ding et al. [23] and Moribe et al. [38]
Pancreatic adenocarcinoma	CFTR mutations is not significantly different in patients (1) CFTR mutation is a risk factor for smokers and young patients (2)	Unknown	(1) Malats et al. [192], Matsubayashi et al. [193], Piepoli et al. [194] and Schubert et al. [195] (2) McWilliams et al. [30], McWilliams et al. [196] and Hamoir et al. [197]
Gastric cancer	Up-regulated in advanced stage	Serum level of CFTR, along with CD199 and CEA, can be diagnosis factors	Suh et al. [45] and Liu et al. [184]
Colorectal cancer	Down-regulation	CFTR mRNA expression is positively correlated with survival Independent prognostic determinant	Sun et al. [39] and Than et al. [40]

CFTR regulates multiple malignant phenotypes of cancer cells

Increasing evidence suggest that CFTR plays important roles in modulating multiple cellular processes involved in cancer development, such as transformation, proliferation, survival, migration and invasion (Table 2).

Table 2.

CFTR-related signaling involved in cancer regulation

Cancer type	Signaling factors	Cancer cell behavior	Citations
Reproduction system
Endometrial carcinoma	NF-kB/uPAR and mir-125b	Cell viability and invasiveness	Huang et al. [51] and Xia et al. [50]
Ovarian cancer	Phosphor-Src	Invasiveness and motility	Zhu et al. [46] and Xu et al. [47]
Cervical cancer	Regulated by NF-kB	Unknown	Wu et al. [143]
Breast cancer	NF-kB/uPAR	Migration, invasion, EMT and metastasis	Zhang et al. [76]
Prostate cancer	miR-193b/uPA	Cell viability, clone formation, migration and invasion	Xie et al. [41]
Respiratory system
Nasopharyngeal carcinoma	Unknown	Migration, invasion and EMT	Tu et al. [37]
Lung cancer	uPA/uPAR	Migration, invasion, EMT and metastasis	Li et al. [43]
Digestive system
Esophageal adenocarcinoma	Unknown	Unknown
Hepatocellular carcinoma	IP(3)/G-proteins/PLC/Ca2+	Apoptosis	Kim et al. [134]
Pancreatic adenocarcinoma	MUC4	Unknown	Singh et al. [142]
Gastric cancer	Unknown	Unknown
Intestinal cancer	Wnt/β-catenin pathway; AF-6/MAPK	Tumor formation; cell viability, adhesion, migration, invasion and metastasis	Than et al. [40] and Sun et al. [39]
Urinary system
Bladder cancer	Unknown	Unknown
Endocrine system
Papillary thyroid cancer	Unknown	Unknown

Malignant transformation

While the correlation of CFTR mutations with high incidence of gastrointestinal cancers has been observed for a long time [14], the direct link between CFTR dysfunction and GI cancer has not been established until recently. To investigate the effects of CFTR dysregulation on GI cancer, Than et al. created Apc^Min mice that carried an intestinal-specific knockout of Cftr and showed that Apc^Min mice with either Cftr heterozygous or homozygous mutants developed more tumors than Apc^Min Cftr wild-type mice in the small intestine (SI) and the colon [40]. Strikingly, in Apc^+/+ mice aged to one year, Cftr deficiency alone is sufficient to cause the development of intestinal tumors in > 60% of mice. Since both CF patients and Cftr KO mice develop a wide range of dysfunctions in the GI tract, including deficient fluid transport, altered cellular pH, impaired clearance of mucus, abnormal bacterial colonization, microbial dysbiosis and abnormal innate immune responses that lead to chronic inflammation, it is not clear whether all these factors contribute to the development of colon cancer [53]. Importantly, this study firstly revealed that colon organoid formation was significantly increased in Cftr mutant mice compared with wild-type controls, suggesting a role of Cftr in regulating intestinal stem cells (ISC) [40]. Additionally, a number of canonical ISC genes were dysregulated in Cftr KO mouse intestine. In particular, Wnt target genes were aberrantly expressed in Cftr KO tumors which show a significant overlapping with Apc^Min Cftr^+/+ tumors. These data suggest that Cftr functions as a tumor suppressor gene in the intestinal tract, loss of which may initiate tumor transformation process originated from ISCs. While the question of how Cftr may alter ISC dynamics is unknown, it is well established that the microenvironment is a key regulator of ISCs, including their capacity for malignant transformation. Thus, it is plausible that the inflammatory landscape in the GI tract characterized by chronic inflammation and activation of β-catenin contributes to the malignant transformation of ISCs in CF.

Cell growth and survival

CFTR has been implicated in the regulation of cell growth in various types of cancer [33, 41, 47]. The growth inhibitory effect of CFTR on cancer was first reported in breast cancer cell lines. Abraham et al. [33] showed that CFTR suppressed breast cancer cell growth due to high extracellular ATP concentration. In addition, mouse breast cancer cell line transfected with human CFTR exhibited a slower growth rate in vivo [33]. In our recent study, we evaluated the role of CFTR in prostate cancer development using three prostate cancer cell lines (PC-3, DU-145, LNcap). Our results showed that overexpression of CFTR significantly decreased the growth rate of cancer cells, whereas knockdown of CFTR promoted cancer cell growth. Interestingly, suppression of CFTR function by channel inhibitors exhibited the same effect (data not shown), indicating alternation of the channel function itself affects cancer cell growth. Given that CFTR is an anion channel providing a major pathway for Cl⁻ and HCO₃⁻ efflux, which have been demonstrated to be important in keeping characteristics of cancer cells, such as anchorage‐dependent growth and apoptosis [54–56], it is not surprising that CFTR plays essential role in the regulation of cancer cell growth and/or apoptosis. However, it should be noted that knockdown of CFTR in normal prostate epithelial cells does not influence their own growth but rather induces stromal cell proliferation via production of PGE₂ [57]. In addition, in contrast to breast cancer and prostate cancer, CFTR has been shown to promote cell growth in ovarian cancer. It was reported that suppression of CFTR inhibited cell proliferation and colony formation in ovarian cancer cell lines, SKOV3 and A2780 [47]. These results suggest that CFTR might play divergent role in regulating normal and malignant cell growth depending on the cellular context and/or their microenvironment. For instance, in cancer cells, pH_i is increased compared to normal cells (∼ 7.3–7.6 vs. ∼ 7.2), while extracellular pH (pH_e) is decreased (∼ 6.8–7.0 vs. ∼ 7.4). The increased pH_i is maintained in cancer cells through the increased expression or activity of plasma membrane ion transporters and pH_i regulators [58–60]. This reversed pH gradient in cancer cells is an early event in cancer development and increases during cancer progression [61]. Dysregulated pH in cancer cells enables cellular processes that are sensitive to small changes in pH_i, including cell proliferation, migration and metabolism. Interestingly, it was shown recently that crypt epithelial cells in Cftr KO mice maintained an alkaline (pHi) and increased intracellular Cl⁻ concentration [62]. Thus, while there is no direct evidence to date showing the differences in cancer cell growth are due to the channel function of CFTR, it is likely that CFTR channel function related to ion and pHi dynamics contributes to the regulatory role of CFTR in cancer cell growth.

Besides cell proliferation and apoptosis, it has been reported recently that CFTR regulates cancer cell autophagy as well [63]. In CFTR-overexpressing HeLa and A549 cells, the levels of endogenous LC3B-II were enhanced compared with the vector control, whereas CFTR^F508Δ overexpression had no obvious effect on cell autophagy. In contrast, rapamycin decreased the expression of CFTR, which could be recovered by E64d plus pepstatin A or MG132, indicating that CFTR degradation involves both proteasome and autophagy pathways [63]. These results suggest that CFTR promotes autophagosome formation both in normal and stress states in HeLa and A549 cells. More interestingly, using protein interaction profiling and global bioinformatics analysis, recent study revealed that ∆F508 was associated with mTOR signaling components, which are closely correlated with autophagy regulation [64]. Inhibition of the PI3K/Akt/mTOR pathway with 6 different inhibitors demonstrated an increase in CFTR stability and expression. In addition, the study identified that CFTR promoted autophagy via Bcl-2-associated athanogene 3 (BAG3), a regulator of autophagy and aggresome clearance [64].

Cell migration and invasion

In carcinomas, acquisition of a mesenchymal-like phenotype that is reminiscent of an oncogenic EMT (epithelium–mesenchymal transition) is associated with pro-metastatic properties, including increased motility, invasion, anoikis resistance, and cancer stem cell characteristics [65, 66]. Several lines of evidence suggest that core polarity proteins are important for the formation and maintenance of the structural organization of epithelial cells, suggesting that their loss could induce or at least contribute to EMT [67, 68]. This notion is further supported by the findings that polarity proteins are cellular targets of an increasing list of oncogenes and tumor suppressors [69–72]. Given the critical role of CFTR in maintaining epithelial cell polarity [73] and cell–cell contact [74, 75], it is foreseeable that dysfunction of CFTR might play a role in the EMT process and cancer progression. Indeed, accumulating evidence has demonstrated that suppression of CFTR expression or activity promotes malignant phenotype of cancer cells, including adhesion, migration and invasion [37, 39, 41, 43, 76]. More importantly, EMT, the key step of cancer progression and metastasis, could be induced by downregulation of CFTR in breast [76], non-small cell lung cancer (NSCLC) [43] and nasopharyngeal carcinoma (NPC) [37] cells. The inhibitory role of CFTR in cancer metastasis has also been demonstrated in mouse model, as loss of CFTR enhances focal invasion of breast cancer cells and NSCLC cells [43, 76]. On the contrary, CFTR appears to promote cell migration, invasion and adhesion in ovarian cancer cells since knockdown of CFTR suppresses the malignant phenotypes [77]. However, whether this effect is related to EMT and whether CFTR exerts the malignancy-promoting effect on ovarian cancer in vivo are not clear.

Taken together, it appears that except for ovarian cancer, CFTR suppresses cancer development via regulating a wide range of cellular processes, supporting its identity as a tumor suppressor [40]. Elevated expression of CFTR has been reported in ovarian cancer tissues and cell lines by two independent studies [46, 47]. In this regard, the promotion of ovarian cancer by CFTR appears to be consistent with the clinical observations. However, it should be noted that while the origin and pathogenesis of ovarian cancer are poorly understood, it is well recognized that ovarian cancer is a heterogeneous disease composed of different types of tumors with widely differing clinicopathologic features and behavior. Thus, a larger sample size and more intensive biological studies are necessary to fully understand the regulatory role of CFTR in ovarian cancer.

Regulators of CFTR

Numerous reports have shown that both genetic and epigenetic abnormalities lead to aberrant expression levels of CFTR in cancer cells. Gene mutations or polymorphisms are the most common genetic changes that cause non-function or low expression allele of CFTR. On the other hand, promoter hyper-methylation of tumor suppressor genes is one of the most well-recognized epigenetic mechanism associated with cancer. In the case of CFTR, hyper-methylation has been observed in prostate cancer [21], NSCLC [20], HCC [23, 38] and bladder cancer [22, 44], supporting CFTR as a tumor suppressor (Table 1). Besides that, the expression levels of CFTR can be regulated by environmental cues, which is of paramount importance for cancer development (Fig. 1).

Fig. 1.

CFTR-mediated signaling pathways in cancer cells. The expression levels of CFTR can be regulated by environmental cues, including hypoxia, TGF-β and cigarette smoke in cancer cells (red arrows), with NF-κB, Wnt/β-catenin, PDZ domain-mediated protein–protein interaction and microRNAs identified as downstream signaling pathways (black arrows)

Hypoxia

Hypoxia, which is closely related to tumor progression, EMT and metastasis [78–80], has been linked to CFTR expression level and its channel-related physiological function such as phosphorylation, ATP hydrolysis and apical membrane recycling [81–84]. Both negative and positive correlation between hypoxia and CFTR have been demonstrated in different tissues, which suggests that the regulation could be tissues specific [81, 83, 84]. For instance, in mice subjected to low oxygen in vivo, CFTR mRNA expression in airways, gastrointestinal tissues, and liver was repressed. In addition, CFTR mRNA expression was diminished in pulmonary human tissues taken from hypoxemic subjects at the time of lung transplantation [81]. The subsequent study revealed that HIF (hypoxia induced factor)-1α was bound to the promoter of CFTR gene, which suppressed both expression and function of CFTR protein in intestinal epithelium and human colon carcinoma cell line—Caco-2 [82] (Fig. 1). This study proposes novel insights into the presence of an HIF-1α-orchestrated mechanism involving CFTR that controls epithelial ion and water transport, supporting the notion that reduced oxygenation of cells caused by chronic hyperventilation plays the crucial role in triggering CFTR abnormalities and the development and pathogenesis of CF. However, the effect of hypoxia on channel function is not specific for CFTR, other studies unrelated to CF showed that low oxygen levels decreased active transport of sodium, chloride and water across primary epithelial cells in a dose-dependent manner [85–88]. In addition, whether CFTR mediates hypoxia-induced downstream pathways and cellular behavior is still an open question.

Transforming growth factor (TGF)-β

Transforming growth factor (TGF)-β, a key inflammatory cytokine, has been found to be negatively correlated with the expression, function and translocation of CFTR in normal epithelial cells [89–91]. TGF-β downregulates CFTR expression and function during chronic inflammation, which may serve as a noxious loop to exaggerate the inflammatory response. During cancer development, TGF-β functions as a tumor suppressor in the early stage of cancer. However, in the later stage, the activation of TGF-β pathway promotes EMT, invasion and metastasis [92, 93]. Interestingly, our recent study found that as a strong EMT activator, TGF-β significantly downregulated the expression of CFTR, and promoted EMT in breast cancer cells [76]. There are no published data on how TGF-β inhibits CFTR mRNA level or whether a putative TGF-β consensus site exists in the CFTR promoter, thus it is unclear whether TGF-β suppresses CFTR expression by transcriptional or post-transctiprional mechanisms [94]. Nevertheless, the complexity and versatility of the TGF-β pathway indicate that several mechanisms, including direct and indirect pathways may play a role in modulating CFTR expression.

Cigarette smoke

Accumulated evidence has demonstrated that cigarette smoke, which is a stimulator of inflammation and oxidative/nitrosative stress that related to a high risk of cancer, induces downregulation and dysfunction of CFTR in epithelial cells [95–98]. Xu et al. observed that the cigarette extract could induce lysosomal degradation of CFTR in bronchial epithelium, which is associated with mitogen-activated protein kinase (MAPK) pathway [99]. Cadmium, a component of cigarette [100], appears to downregulate the expression of the CFTR protein and subsequent chloride transport in human airway epithelial cells [101]. In addition, PDZ domain-mediated interaction between CFTR and MRP2, the tobacco carcinogen NNK transporter, indicates the potential involvement of CFTR in cigarette smoke-mediated transforming of airway epithelial cells [102]. Moreover, it is demonstrated that CFTR, α7 Nicotinic acetylcholine receptor (α7 nAChR) and adenylyl cyclase-1 are physically associated in a macromolecular complex at the apical membrane of surface and glandular airway epithelium [103]. This study establishes the role of α7 nAChR in the pathogenesis of smoking-related lung diseases via interaction with CFTR [103].

Sex hormones

It has been well established that the CFTR expression level in the uterus is dynamically changed during the estrous cycle. Estrogen can stimulate CFTR expression on both mRNA and protein levels in the female reproductive tract [104–106]. As endometrial, ovarian and cervical cancers are known to be estrogen-dependent disorders [107–109], the upregulation of CFTR in these female sex hormone-related cancers is very likely to be related to estrogen accumulation, which possibly explains the distinctive expression pattern of CFTR in cancers originated from the female reproductive tract. In the male reproductive system, we have found that castration results in declined CFTR expression, whereas testosterone treatment upregulates CFTR expression both in cultured primary prostate epithelial cells and in post-castration rat prostate tissue [41]. On the other hand, the mRNA expression level of Cftr is downregulated to about 25% in 23 months aged rat prostate compared to 3 months rat prostate [57], and the protein expression level also downregulated in 30 days rat prostate compared to 3 days rat prostate [41], which is consistent with the age-dependent prevalence of prostate hyperplasia and cancer. Thus, it is likely that in aging prostate, reduced level of testosterone is responsible for the loss of CFTR, which confers cellular susceptibility to malignant transformation.

Altogether, these studies support the notion that CFTR regulation in response to environmental changes contributes to a wide variety of biological processes. In contrast, disturbance of the microenvironment under pathological conditions may lead to dysregulation of CFTR, which is involved in the pathological processes of various diseases, such as cancer. Indeed, it is easy to understand that localized at the apical or basolateral membrane of the cells, ion channels/transporters are in a strategic position to sense and transmit extracellular signals into the intracellular machinery to regulate various cell functions. In this regard, CFTR may function as a key regulator by sensing environmental changes and transducing the micro-environmental signals to intracellular machinery, which subsequently leads to cellular adaptive responses in health and disease.

CFTR regulates cancer cell behavior through both genetic and epigenetic mechanisms

The knowledge base surrounding the structure and function of CFTR that has accumulated in the last 20 years is remarkable. On the other hand, whole-genome and transcriptome sequencing data have shed new light on the molecular mechanisms underlying the function of CFTR. Of note, CFTR has been identified as one of the hub genes that is critical for carcinogenesis [110, 111]. In addition, recent cell functional and animal studies have provided convincing evidence that CFTR is involved in cancer development via regulation of multiple signaling pathways (Fig. 1 and Table 2).

Ion channel-related signaling

The pancreatic duct epithelium secretes HCO₃⁻ at a concentration of around 140 mM by a mechanism that is only partially understood. In the proximal part of pancreatic ducts close to acinar cells, HCO₃⁻ secretion across the apical membrane is largely mediated by SLC26A6 CI⁻/HCO₃⁻ exchanger [112–116]. In distal ducts where the luminal HCO₃⁻ concentration is already high, most of the HCO₃⁻ secretion has been proposed to be mainly mediated by HCO₃⁻ conductance of CFTR [117]. However, CFTR Cl⁻ channel has a limited permeability to HCO₃⁻ in typical physiologic conditions [118], implying an Cl⁻/HCO₃⁻-independent pathway. While subsequent studies revealed that CFTR HCO₃⁻ permeability was dynamically regulated by intracellular Cl⁻ concentration-sensitive WNK1-SPAK/OSR1 kinase pathway [114], this model is still unable to account for the ability of the ducts to secrete HCO₃⁻ at high concentrations in many species, including humans [115]. Despite these confounding factors, the loss of pancreatic bicarbonate secretion in patients with CF [119] indicates that CFTR plays a critical role in bicarbonate secretion. While markedly alteration of HCO₃⁻ and pH-regulatory transport proteins have been observed in cancer cells [120], the great majority of research has focused on proton transporters and pH regulators. However, it is interesting to note that at least under some conditions, including during embryonic development or reproduction, alteration of bicarbonate levels triggers the activation of various signaling pathways including miR-125b and CREB [121, 122]. Interestingly, CFTR has been demonstrated to be expressed in the female reproductive tract, including the ovary [123, 124], and involved in uterine HCO₃⁻ secretion [7] which is required for sperm capacitation [125, 126]. Of interest, we have found recently that CFTR-mediated HCO₃⁻ transport regulates cell proliferation in granulosa cells, which promotes HCO₃⁻/sAC/PKA pathway leading to ERK phosphorylation and cyclin D2 upregulation, defect of which might contribute to the pathogenesis of polycystic ovary syndrome (PCOS) [122, 127]. Of note, PCOS patients show increased risk of ovarian cancer [128]. On the other hand, CFTR is not only a cAMP and PKA-regulated Cl⁻ and HCO₃⁻ channel, but also a regulator of other ion channels as well as other signaling [129–132]. It was reported that Glibenclamide, an antidiabetic drug, induced downregulation of CFTR and increased apoptosis in HepG2 human hepatoblastoma cells [133, 134]. Further mechanistic study revealed that downregulation of CFTR triggered intracellular Ca²⁺ release via regulating formation of inositol 1,4,5-trisphosphate [IP(3)] and PTx-sensitive G-proteins, and induced apoptosis in cancer cells [134].

NF-κB-mediated pathway

As a key transcriptional regulator in inflammation, NF-κB has long been identified as a downstream effector of defective CFTR, which plays essential role in CF pathogenesis [135, 136]. Aberrant expression level, membrane localization and channel functions of CFTR have been shown to induce the expression and activity of NF-κB and the downstream inflammatory signaling [137, 138]. The negative correlation of CFTR with NF-κB-mediated signaling has been demonstrated in various cancers. Loss of CFTR led to NF-κB-mediated activation of urokinase-type plasminogen activator (uPA) expression and activity in breast cancer cells, which is known to be involved in EMT process and metastasis [76]. More importantly, the inhibition of NF-κB activity completely abrogated the CFTR knockdown-induced activation of uPA activity and enhanced cell invasion and migration in breast cancer cells, indicating that the upregulation of NF-κB activity targeting uPA is the major mechanism leading to increased malignancies. The link between CFTR and NF-κB/uPA/uPAR system, was also shown in NSCLC and NPC [37, 43]. In addition to uPA system, our previous studies have demonstrated that CFTR functions as a negative regulator of NF-κB/COX-2/PGE₂-mediated pro-inflammatory response in airway and prostate epithelial cells, defective of which results in excessive activation of NF-κB and over production of PGE₂ [57, 139]. Given the importance of COX-PGE₂ pathway in cancer development, these findings point toward a scenario that defective CFTR leads to exaggerated NF-κB-mediated responses that are likely linked to cancer development. Of interest, CFTR downregulation was also shown to disrupt testicular tight junctions through up-regulation of NF-κB/COX-2/PGE2 in rat model of cryptorchidism [140], which is one of the risk factors for testicular cancer [141].

In pancreatic adenocarcinoma, CFTR was shown to regulate the expression of a tumor-linked mucin MUC4 [142]. The authors identified a NF-κB binding site in the promoter region of MUC4, however, whether NF-κB contributes to the transcriptional regulation of MUC4 is not known. While most of the studies have indicated a negative correlation between CFTR and NF-κB, positive association of CFTR with NF-κB was observed in ovarian cancer cell line and primary tissues [143]. In addition, both CFTR and NF-κB expression levels are positively associated with stage, histological grade, lymph node metastasis, and invasive interstitial depth in ovarian cancer patients. Despite the observed link, it is not clear whether and how CFTR regulates NF-κB expression in ovarian cancer.

It should be noted that while the inverse relationship between CFTR and NF-κB activation has been well established in different experimental settings, the mechanistic link between CFTR dysfunction to aberrant NF-κB activation remains mysteriously missing. It has been reported that Src tyrosine kinase-induced activation of NF-κB is involved in CFTR-mediated inflammation and permeability in biliary epithelium [144]. In addition, a recent study showed that CFTR physically interacted with TRADD, a key adaptor molecule in TNFα signaling, and modulated its degradation, therefore, regulating NF-κB signaling [145] in bronchial epithelial cells. In addition, our recent study has shown that CFTR forms a complex with β-catenin and NF-κB, which regulates NF-κB nuclear translocation and thus NF-κB-mediated inflammatory response in intestinal epithelial cells [146]. However, whether these mechanisms are involved in cancer development awaits further investigation.

Wnt/β-catenin pathway

The Wnt/β-catenin signaling cascade is considered to play a key role in cancer development via modulating numerous cellular processes including cell survival, cancer stem cell activity and EMT process [147]. Interestingly, a recent comprehensive study on human lung epithelial cells has found that the CFTR interactome includes several hundred proteins, with β-catenin listed as a potential interacting protein [148]. Moreover, RNA Seq analysis of adenomas derived from the Cftr knockout (Apc^+/+ Cftr^fl/fl-Villin-Cre) mouse small intestine and colon reveals the accumulation of Wnt/β-catenin target genes (Ccnd1, CD44, Axin2, Lgr5, Mmp7, Wnt10A and Ptgs2) and intestinal stem cell genes (Lgr5, CD44, Aldh1a1, Aqp4, Ascl2 and Hopx), supporting the notion that loss of Cftr initiates tumor formation via a dysregulated Wnt/β-catenin signaling pathway [40]. Importantly, colon organoid formation is significantly increased in organoids created from Cftr mutant mice compared with wild-type controls, suggesting Cftr plays critical role in regulating the intestinal stem cell compartment and tumor initiation by regulating Wnt/β-catenin pathway. While it is clear that canonical β-catenin pathway is activated in Cftr null small intestine, this study did not provide a mechanical link between Cftr loss and activation of β-catenin. Noticeably, our recent study showed that CFTR co-localized and physically interacted β-catenin in human colon cancer cell line Caco-2, therefore, stabilizing β-catenin [146]. In addition, Cftr mutation or downregulation promoted the degradation of β-catenin and subsequently reduced the nuclear translocation and activity of β-catenin [146]. Indeed, β-catenin activity in the ΔF508 mouse intestine is significantly reduced compared to WT mouse intestine, which is associated with activation of NF-κB and exaggerated inflammatory phenotype. The seemingly contradictory results observed in these two studies can be explained by the following possibilities: first, given the fact that β-catenin plays a central role in maintaining intestinal homeostasis by regulating the self-renewal and differentiation balance between crypt and villi cells, it is plausible that CFTR exerts distinctive regulatory effect on β-catenin activity in different cellular compartments of the intestine. For instance, CFTR negatively regulates β-catenin activity and suppresses ISC growth in the crypt region, whereas positively regulates β-catenin activity by stabilizing it in the villi area. This kind of divergent regulatory role of CFTR might be attributable to the distinctive expression level of CFTR in the different segment and compartment of intestine. It was shown clearly that in the porcine intestine, the pCFTR protein was expressed in the crypt epithelial cells of all intestinal segments with a distinct increase in signal intensities from small to large intestine and a decrease along the crypt–villus axis [149]. Second, CFTR may exert differential effect on Wnt/β-catenin signaling via interacting with different components of the Wnt/β-catenin pathway. The direct interaction between CFTR and β-catenin itself has not only been observed in Caco-2 cells [146], but also in mouse embryonic stem cells (ESC) [150]. Interestingly, CFTR-β-catenin interaction is critical for ESC differentiation, loss of which leads to dramatic defect in mesendoderm differentiation. On the other hand, CFTR has been found to be physically interact with dishevelled segment polarity protein 2 (Dvl2), a key adaptor of Wnt/β-catenin signaling, thereby suppressing the activation of β-catenin in kidney epithelial cells [151]. Over-activation of Wnt/β-catenin pathway due to dysregulated or mutant inhibitors of the pathway has been extensively observed in stem cells and cancer cells. Therefore, it is possible that CFTR plays divergent role on undifferentiated cells and mature cells via its differential regulatory effect on β-catenin signaling, depending on the interacting partners. Finally, ΔF508 protein may have alternative binding partner compared to CFTR WT protein. Up to 90% of ∆F508 CFTR protein is retained in the endoplasmic reticulum (ER) and subsequently targeted for proteolytic degradation by the ER-associated degradation pathway (ERAD) [152]. Therefore, models have been proposed in which differential protein interactions with ∆F508 CFTR contribute to dysregulated signaling pathway and CF pathogenesis [153]. Indeed, a ∆F508 mutation-specific interactome has been identified using a novel deep proteomic analysis method recently [148]. Taken together, despite unanswered questions, in view of the importance of Wnt/β-catenin pathway in embryonic development and tumorigenesis, the demonstrated role of CFTR in regulating β-catenin signaling provides novel insights into the molecular mechanism underlying various physiological and pathological conditions.

Protein–protein interaction

While Ford et al. uncovered the structural features of CFTR indicating a bottom-closed conformation of CFTR in crystals [154], recent studies on CFTR structural determination by single particle electron microscopy revealed an unexpected asymmetric feature of the two NBDs upon binding ATP [27, 155]. These pioneering works reveal major structural rearrangements and conformational changes related to channel activation. Since CF can be defined as a protein misfolding disease, the interactome of CFTR which may affect the protein stability has been characterized by recent studies. Of note, CFTR has been shown to form multiple-protein macromolecular complexes with ion channels, cell adhesion and other signaling molecules [129, 148, 156]. Pankow et al. identified 638 CFTR-interacting proteins which are related to apoptosis (4 proteins, including p53), adhesion (16 proteins, including E-cadherin, Integrins and β-catenin), cell skeleton (56 proteins, including Vimentin) and cellular signaling (22 proteins, including EGFR) [148]. In addition, the authors discovered a deltaF508 deletion-specific interactome, which could be extensively remodeled upon RNAi-mediated rescue [148], indicating the aberrant interactome of CFTR may underlie the pathogenesis of CF. This notion is further supported by several biochemical studies showing physical interaction of CFTR with various functional proteins. For instance, it was reported that CFTR regulated RANTES expression via its PDZ-interacting motif [157].

A recent study using The Cancer Genome Atlas (TCGA) database identified 482 differentially expressed genes (DEGs) in lung squamous cell carcinoma (SCC) with and without metastasis. Among them, 49 DEGs were used to construct a protein–protein interaction network and the results found that CFTR functioned as the hub protein connecting with 182 genes [42]. Cell–cell adhesion via structures such as adherent junctions (AJ) and tight junctions are responsible for establishing the polarity and maintaining the barrier function and strength of the epithelial cells. In colon cancer cells, Sun et al. reported that CFTR interacted with adherent junction molecule AF-6/afadin via PDZ domain, thus regulating epithelial polarity and affecting cancer metastasis [39]. Loss of CFTR resulted in degradation of AF-6/afadin protein and activation of mitogen-activated protein kinase (MAPK), which reduced epithelial tightness and enhanced EMT and malignancies in colon cancer cells [39]. Interestingly, several kinase inhibitors, including MAPK inhibitor, exhibit strong rescue effect on DeltaF508-CFTR function or expression in epithelial cells [158], suggesting a negative regulatory loop between CFTR and MAPK pathway. CFTR has been demonstrated to interact with another tight junction-associated protein ZO-1 as well [159]. Of note, this effect was observed in the absence of cAMP stimulation, a condition when CFTR channel activity is inactivated, suggesting that the channel function of CFTR is not required for its role in tight junction assembly. Since ZO-1/Zonab pathway is implicated in EMT process in different cancers [160, 161], it is very likely that the interaction between CFTR and ZO-1 contributes to cancer development. In addition, the physical interaction between CFTR and heat shock proteins (Hsp), Hsp70 and Hsp90, has long been identified [162, 163]. The Hsp proteins, especially Hsp90, are essential for cancer progression, inhibition of which has been considered as therapeutic targets in multiple cancers [164, 165]. Therefore, the protein–protein interaction between Hsp and CFTR could be another potential mechanism underlying cancer development.

MicroRNAs

Emerging studies have provided evidence indicating the ability of CFTR in linking environmental cues to miRNA changes [166, 167]. MiRNA profiling in wild type and ΔF508-CFTR lung epithelial cell lines revealed 22 differentially expressed miRNAs [168]. In nasal epithelial tissues of CF patients, the expression of miR-145 was elevated with downregulation of its target SMAD3, a key element of the TGF-β1 pathway [169]. In addition, an analysis of CFTR and hypoxia-sensitive miRNAs has revealed common miRNAs that are implicated in cancer development, such as miR-155, miR-21 and miR-23 [170, 171]. These studies suggest the possibility that CFTR regulates cancer development through alteration of miRNAs. Indeed, our recent study has revealed a direct effect of CFTR on miR-193b, which has been proposed to be a putative tumor suppressor targeting uPA in various types of cancers [41]. Interestingly, overexpression of miR-193b was shown to significantly reverse the CFTR knockdown-enhanced proliferation, migration, or invasion and completely abrogated the CFTR knockdown-elevated uPA activity in prostate cancer, indicating that CFTR regulates prostate cancer development via miR-193b/uPA pathway. In another study on endometrial cancer, CFTRinh172, a specific inhibitor of CFTR channel function, reduced the expression of miR-125b and promoted cell viability and invasion in endometrial cancer cell line ISK. Mimic of miR-125b reduced endometrial cancer cell proliferation and progression [50]. While the study did not identify the molecular mechanism underlying the regulatory effect of CFTR on miR-125b or the target of miR-125b in ISK cells, another study has reported that CFTR-mediated HCO₃⁻ transport could regulate miR-125b expression, which further modulated the expression level of p53 during embryonic development [121]. It would be of interest to see whether similar CFTR-regulated miRNA mechanism is involved in cancer development.

Moreover, it has been shown that CFTR regulates multiple signaling pathways, such as forkhead box O (FOXO) family via miR-146a, miR-155, miR-370, and miR-708 [172]. In addition, dysfunction of CFTR activates PI3K/AKT-mediated regulation on microRNA-199a-5p, which is implicated in hyper inflammatory response in CF [173]. MiR-155, a target of CFTR, is also been found to be regulated by TGF-β and involved in the regulation of TGF-β/Smads signaling, which may respond to the crosstalk between CFTR and TGF-β/Smads signaling [174, 175]. Given that miRNAs are known to play critical roles in the development of various cancers and a large number of miRNAs are shown to be differentially expressed in CFTR defective cells, we propose that the tumor-regulatory effects of CFTR could be mediated through its action on miRNAs, potentially as a link between environmental cues and epigenetic regulation of cancer progression.

Collectively, emerging evidence has shown that CFTR regulates multiple genetic and epigenetic pathways that are involved in cancer development. However, other important potential contributors that are associated with the channel function itself cannot be underestimated [129, 176]. For instance, while Cl⁻ is essential for maintaining electrolyte fluid balance, HCO₃⁻ is critical for mucus formation, epithelial barrier function and luminal pH homeostasis [177, 178]. Thus, CF patients develop a wide range of epithelial dysfunction including deficient fluid transport, altered cellular pH, impaired clearance of mucus, abnormal bacterial colonization, microbial dysbiosis and abnormal innate immune responses that lead to chronic inflammation [10, 52, 53, 179]. Accordingly, a model for tumor development in both humans and mice deficient for CFTR would include long-term chronic inflammation associated with these pathological changes. In particular, chronic upregulation of pro-inflammatory cytokines such as interleukin-6 (IL-6), interleukin-1 (IL-1), tumor necrosis factor-α (TNF-α), hypoxia induced factor (HIF)-1α and chemokines in CF has been well established for tumor promotion [180, 181]. In addition, loss of CFTR may cooperate with defects in other ion channels, ultimately leading to activation of β-catenin [182]. Despite these hints, whether CF-related chronic inflammation contributes to the development of cancer is not clear.

Correlation of CFTR expression levels with cancer stage and patient prognosis

A successful therapy for cancer depends heavily on the biomarkers for monitoring the progression of the disease, and predicting the prognosis and survival after clinical intervention. Given the observed alteration in CFTR expression levels in various types of cancer, it is promising that CFTR can be used as a novel cancer biomarker (Table 1).

Colon cancer

The potential of using CFTR as a novel cancer progression indicator has been explored in recent studies. In colon cancer, it has been shown that the TNM (tumor, node and metastasis) classification is correlated with the mRNA expression level of CFTR. CFTR expression is significantly lower in TNM stage 4 patients than TNM stage 2/3 patients. In addition, CFTR expression in patients with metastasis is significantly lower than patients without metastasis. Furthermore, with a median of 120-month follow-up, the authors reveal that patients with low expression of CFTR and its interacting AJ gene AF-6 have significantly shorter overall survival than patients with high CFTR and AF-6 expression [39]. These data indicate that reduced expressions of CFTR and AF-6 are significantly associated with cancer progression and poor prognosis of human colon cancer patients. Besides, Than et al. analyzed disease free survival in a set of stage II colon cancer samples [40]. The Kaplan–Meier analysis and log-rank test revealed that the 3-year relapse free survival was 85% in the CFTR high-expressing group compared with 56% in the CFTR low-expressing group. In addition, the multivariate analysis showed that CFTR expression is an independent prognostic determinant. The identification of CFTR as a prognostic predictor of cancer progression and survival in colon cancer patients is fully consistent with epidemiological and clinical data, indicating that CF patients are at an increased risk for colon cancer [14].

Lung cancer

In a study to investigate the methylation status of CFTR gene in NSCLC, it was found that CFTR methylation was not significantly associated with the prognosis of the patients. However, when patients were categorized by age, CFTR methylation was associated with significantly worse survival among younger patients (≤ 62 years), but not older patients (> 62 years). In a more recent study, CFTR expression in 296 NSCLC tumor samples and 22 normal tissues was evaluated by real-time PCR and correlated with clinical characteristics, staging, and prognosis [43]. The results showed that low CFTR expression was correlated with advanced staging and lymph node metastasis. In addition, low CFTR expression was significantly associated with poor prognosis (overall survival: 45 vs. 36 months; progression-free survival: 41 vs. 30 months). In addition, the data derived from TCGA database show that CFTR has a tendency of down-regulation in lung SCC with metastasis compared to lung SCC without metastasis [42].

Breast cancer

In our recent study, the expression of CFTR was examined in accordance with patients’ Nottingham Prognostic Index (NPI) in breast cancer [76]. The results showed that patients with poor prognosis (NPI3) had significantly lower levels of CFTR transcripts when compared to patients with good prognosis (NPI1). In addition, patients with poor prognosis, including those with metastasis, local recurrence, and death due to breast cancer, had significantly lower levels of CFTR transcripts compared to disease-free patients. Moreover, patients with higher expression levels of CFTR had longer disease-free survival than patients with a lower CFTR transcript level. Thus, reduced expression of CFTR is significantly associated with disease progression and poor prognosis in breast cancer. This is consistent with a report showing that breast cancer in deltaF508 CFTR carriers were poorly differentiated tumors [183].

Prostate cancer

In prostate cancer study, genome-wide methylation analysis was performed on 83 tumors and 10 normal prostate samples [21]. The study identified that CFTR was more frequently methylated in tumor samples along with clinico-pathological indicators of poor prognosis. Particularly, the authors found that simultaneous hypermethylation of CFTR and HTR1B was frequent in patients with a high Gleason score and high Ki-67 levels, suggesting that simultaneous HTR1B and CFTR hypermethylation could help to discriminate advanced prostate cancers [21].

Nasopharyngeal cancer

CFTR is also expressed in nasopharyngeal mucosa. In a recent study, we found that CFTR expression was downregulated in nasopharyngeal cancer [37]. We used a cohort of 225 paraffin-embedded NPC specimens to examine the prognostic value of using CFTR as a novel biomarker of NPC. Our results showed that CFTR expression was significantly correlated with clinical stage and distant metastasis. In addition, patients with high expression levels of CFTR had longer overall survival than patients with lower CFTR expression levels. Besides, we noticed that patients with high CFTR levels had higher 10-year survival rate (41.7%), compared to those with lower CFTR levels (22.6%). Multivariate analysis showed that CFTR expression level was an independent prognostic factor for NPC. Thus, lower expression of CFTR is significantly associated with disease progression and poor prognosis in NPC.

Gastric cancer

In gastric cancer, serum level of CFTR has been reported to be strongly correlated with carbohydrate antigen 199 (CA199), the classical cancer tumor biomarkers in some cases [184]. In addition, the concentrations of CFTR is associated with gastric cancer stage [184]. Combinations of CFTR, CA199, and carcinoembryonic antigen (CEA) yielded the best receiver operating characteristics (ROC) curve, which is used for diagnosis of gastric cancer [184].

Cancers of the female reproductive tract

It has been shown that the expression of CFTR in ovarian cancer is significantly higher than that in benign and normal ovaries. Moreover, CFTR protein level was well-related to advanced clinical stages, poor histological grade and a higher serum Ca-125 level [47]. Similarly, in cervical cancer, the expression of CFTR was reported to be significantly increased in cancer tissues compared to normal cervical tissue. The expression of CFTR and NF-κB activation was positively associated with stage, histological grade, lymph node metastasis, and invasive interstitial depth. Multivariate analysis showed that coexpression of CFTR and NF-κB was an independent prognostic factor for survival [143].

Collectively, these studies indicate that CFTR negatively or positively correlated with cancer stage or patient prognosis, depending on the types of cancer. Furthermore, upregulation of CFTR is correlated with enhanced sensitive to Paclitaxel, a microtubule-stabilizing drug that has been commonly used to treat cancer, in multiple cancer cell lines [185]. In addition, the single nucleotide polymorphisms (SNPs) in CFTR was reported to be associated with the toxicity profile caused by anticancer agent docetaxel [186]. Altogether, these findings suggest the potential role of CFTR as a clinical indicator of diagnosis and/or prognosis in cancer (Table 1).

Concluding remark

Accumulating studies have found a close association of CFTR mutation, hypermethylation or aberrant expression with a wide array of cancers. Both clinical investigation and biological studies have lent support to an important role of CFTR in different stages of cancer development, especially in cancer progression. Compellingly, emerging evidence suggest that CFTR is not merely an ion channel, but also plays a fundamental role in regulating multiple signaling pathways that are involved in a variety of biological processes, pinpointing the potential molecular mechanisms underlying the tumor-regulatory role of CFTR. However, the question of whether channel function is necessary for the regulatory role of CFTR is still unclear. On the one hand, numerous studies have shown that suppression of channel function alters intracellular signaling, suggesting that inactivation of ion channel function itself is sufficient to alter cancer cell behavior [50, 121, 127]. On the other hand, studies from both our group and others have demonstrated that protein–protein interaction via PDZ domain, but not channel function is critical for the tumor-suppressive role of CFTR, indicating that the signal transduction function of CFTR is unlikely to be mediated by its channel function. For instance, various studies have shown that the effect of CFTR knockdown can be mimicked by PDZ binding domain deletion in colon cancer [39], kidney [151], trachea and epididymis [159]. In addition, our recent work reveals that the effect of CFTR knockdown cannot be mimicked by CFTR G551D, a channel function defective mutation with an intact PDZ binding domain (data not shown). Moreover, it should be noted that CFTRinh172 is known to act on the cytoplasmic side of the plasma membrane [187]. Thus, one cannot exclude the possibility that the inhibitor itself disrupts the normal configuration of CFTR and its interaction with other proteins, in addition to the channel blocker function. Despite all these hints, more intensive biochemical studies designed to exclude the gated channel function of CFTR is essential to provide direct evidence for channel-independent role of CFTR. Nevertheless, given the importance of many of the identified CFTR-mediated pathways, such as NF-κB/Cox-2/PGE2 and Wnt/β-catenin pathways, in different cellular processes during cancer development, it is not surprising that CFTR might regulate cancer development in a more complex way than we previously thought.

More importantly, the correlation of CFTR with cancer stages and prognosis may lead to its clinical application as a diagnosis or prognostic predictor in different cancers (Table 1). However, several key issues remain unresolved in this field. For instance, aberrant CFTR expression, either downregulation or upregulation, has been found in different types of cancer, which is considered conflicting results for some time. While it is conceivable that CFTR may be differentially expressed in different types of cancer, the differential microenvironment may affect CFTR expression as well. It should be noted that the expression level and activity of CFTR can be readily modulated by pH, ATP level, hypoxia, sex hormones and cytokines [24–27, 57, 81–84, 104–106, 118]. In addition, though more than 2000 CFTR mutations have been documented so far [188], previous clinical studies have merely focused on several frequent mutations, which may mask the results. This kind of discrepancy also goes to the completely opposite regulatory effects of CFTR on malignant phenotypes, i.e., promoting cell migration and invasion in female reproductive tract cancers but suppressing these malignant phenotypes in other cancers, such as prostate cancer, colon cancer and breast cancer. What determines the functionality of CFTR in different cellular contexts? It is plausible that CFTR reacts differently in response to differential environmental cues. One possibility is that CFTR might have different interacting proteins and that these different interactions might trigger diverse signaling pathways, leading to distinctive effects in different cancer cell types. On the other hand, given the well-established role of CFTR in inflammation, the pathophysiology of interactions between chronic inflammation and malignancy in CF requires focused research. Altogether, future investigation resolving these issues are necessary for the clinical application of CFTR as a diagnostic and prognostic indicator. Given the fact that CFTR is highly accessible cell surface molecule, it may represent an ideal drug target if its exact role in different cancers is identified. Thus, investigation into the role of CFTR in cancer development and the underlying signaling pathways may open up new revenue of prognostic indicators and therapeutic targets for the treatment of cancer.

Abbreviations

ABC

Adenosine triphosphate (ATP)-binding cassette

CF

Cystic fibrosis

CFTR

Cystic fibrosis transmembrane conductance regulator

EMT

Epithelium–mesenchymal transition

Hsp

Heat shock protein

HIF-1α

Hypoxia induced factor-1α

MDR

Multidrug resistance

NF-κB

Nuclear factor kappa-light-chain-enhancer of activated B cells

NPC

Nasopharyngeal carcinoma

NSCLC

Non-small cell lung cancer

PTC

Papillary thyroid cancer

SCC

Squamous cell carcinoma

SNPs

Single nucleotide polymorphisms

TGF-β

Transforming growth factor-β

uPA

Urokinase-type plasminogen activator

Author contributions

JZ and YW have contributed to data collection and manuscript written. Prof. XJ and Prof. HCC have contributed to write and finalize the paper.

Funding

The work was supported in parts by National 973 Project (2013CB967404, 2013CB967401, 2013CB967403), Research Grant Council of Hong Kong (CUHK 466413, CUHK14119516), National Natural Science Foundation of China (81571390), the Focused Investment Scheme of the Chinese University of Hong Kong and L.K.S Foundation.

Compliance with ethical standards

Conflict of interest

None of the authors have any conflict of interests to declare.