Ion channels as promising biomarkers in cancer

The emerging role of voltage-gated ion channels in cancer has gained increasing interest in recent years, revealing how they contribute to different aspects and stages of the cancer process, including metastasis.

In this blog post, we share bits of evidence about the involvement of voltage-gated ion channels in human cancers, highlighting their promising use as biomarkers.

Voltage-gated ion channels (VGICs) are one of the three main types of ion channels, along with the extracellular ligand-gated and the intracellular ligand-gated ion channels (and other small groups of various ion channels).

VGICs control rapid bioelectrical signalings such as action potential and contraction. The VGIC superfamily includes calcium channels, chloride channels, potassium channels, zinc-activated ion channels, and sodium channels. The opening of ion channels, due to a change in the membrane potential, allows the diffusion of the correspondent ions Ca2+, Cl−, K+, Zn2+, and Na+.

Other, smaller VGICs categories are the vanilloid transient receptor potential channels (TRPV, mainly located on the plasma membrane), the ATP-gated channels, the cyclic nucleotide-gated channels (CNG, activated by the binding of cGMP or cAMP), and the aquaporins (water channels, that play critical roles in controlling the water contents of cells).

Voltage-gated ion channels, cancer development, and metastasis

VGICs, in general, significantly contribute to a variety of mechanisms involved in cell survival and are crucial for maintaining normal tissue homeostases, such as cell proliferation (Rao et al., 2015), cell migration (Schwab and Stock 2014), gene expression (Mycielska et al., 2005), vesicular patterning (Krasowska et al., 2004), apoptosis (Bortner and Cidlowski 2014), and more.

All these mechanisms are critically important in maintaining and promoting cell activities but are also part of cancer cell proliferation. Increasing evidence supports ion channels in cancer cells in vitro and in vivo, revealing how they contribute to different aspects and stages of the cancer process.

According to their expression levels, several VGICs have been found to play essential roles during the cell cycle. Thus, aberrant ion channels' expression or malfunction can impair these processes, driving the transformation of normal cells into malignant ones that exhibit uncontrolled multiplication and spreading.

There is plenty of data about the involvement of VGICs in cancer mechanisms. Since 2020, in line with PubMed (the database of life sciences and biomedical references), the number of publications discussing the role of functionally expressed VGICs in different human cancers and their use for diagnostic, prognostic, or predictive purposes in cancer is quadruplicated.

The following are a couple of examples:

Potassium channels: 

With 77 genes coding and many splice variants, the potassium channels are the largest, most diverse group of ion channels in the human genome. Voltage-gated potassium (Kv) channels play a pivotal role in the progression of various cancer types, including blood cancers such as leukemia and lymphoma. Several studies have demonstrated an altered expression of the potassium channel subunits in cancer compared to normal tissues. However, the changes depend on the type and the stage of the disease. 

In breast cancer, a significantly up-regulated expression of Kv1.3 channel mRNA is already observed in the first stage of the disease (Jang S et al.,2009). Similarly, the human voltage-gated potassium channel ether à go-go 1 (EAG1, Kv 10.1) is overexpressed in most types of tumors, including leukemia (Hemmerlein B. et al., 2006). 

In the case of prostate cancer, instead, there is a significant inverse correlation between the expression of the Kv1.3 channels in the epithelium of human prostate tissue and the grade of the tumor (Abdul M. et al., 2006).

Sodium channels: 

in a study by Fraser et al. (2003), two rat prostate cancer cell lines with different metastatic abilities such as MAT-LyLu (strongly metastatic cell line expressing functional sodium channels) and AT-2 (weakly metastatic cell line with no functional sodium channels), were used in a comparative approach. The results plainly show that only the MAT-LyLu cells with functional VGSC expression have enhanced prostate cancer cells' cellular motility (hence metastatic process). 

Another example is the expression of the sodium Nav1.5 ion channel in breast cancer. A study by Nelson et al. (2015) shows that the Nav1.5 α subunit regulates breast tumor growth and potentiates migration and invasion, supporting the notion that compounds targeting Nav1.5 may help reduce metastasis.

A similar example is a study by Fraser et al. (2005) showing that Nav1.5 expression is significantly up-regulated in metastatic human breast cancer cells and tissues compared with matched normal breast tissue and that Nav1.5 activity potentiates cellular directional motility, endocytosis, and invasion.

Other isoforms of Nav sodium channels such as Nav1.6 and Nav1.7 are involved in cervical cancer, breast, prostate, and non-small cell lung cancers.

Calcium channels: 

Calcium ion channels also have confirmed roles in cellular functions, including mitogenesis, proliferation, differentiation, apoptosis, and metastasis. For example, the expression of several calcium channels of the TRP superfamily is elevated in different common carcinomas, such as the TRPC1 in breast cancer, TRPC3 in some breast and ovarian cancer, TRPC6 in breast, liver, stomach, and glial cancers, and TRPM7 in breast, pancreas, ovarian and gastric cancers, to mention some.  

The high voltage-activated Cav1.2 channel is overexpressed in most cancer types, including colorectal, gastric, leukemia, brain, uterus, breast, pancreatic, sarcoma, skin, and prostate. Similarly, Cav1.3 is highly expressed in most types of cancer, including breast and prostate cancer, brain cancer, colorectal, gastric, bladder, lung, esophageal, and uterine tumors (Chen, R. et al., 2014, Wang, CY. et al. 2015a). Also, in breast cancer, a total of 5 VGCC family members (CACNA1A, CACNA1B, CACNA1E, CACNA1G, and CACNA1I) show a reduced expression.

In this recent comprehensive review by Tajada S. et al., (2020), you can read more about the involvement of the calcium channels in human cancers.

Ion channels as promising biomarkers in cancer

The identification of early biomarkers and novel therapies is essential to improve cancer diagnosis and patient survival. Given the enhanced expression and activity of various VGICs in cancer and their role in key events contributing to the initiation and progression of the disease, several ion channels have been proposed as potential tumor markers and therapeutic targets for several cancers. Here are two examples:

1. Eag1 potassium channels in hepatocarcinoma

The potassium channels are involved in cell proliferation and apoptosis in both solid and hematological cancers. Therefore, the Kv potassium channels are considered new targets for designing anti-cancer therapy. Today there are several specific Kv1.3 channel inhibitors such as toxins, small molecules, and specific antibodies able to simultaneously inhibit cancer cell proliferation and induce apoptotic death of the cancer cells (Wulff et al., 2019; Díaz-García and Varela, 2020).

One of the most studied ion channels in cancer is the Eag1 potassium channel  (also known as KCNH1), expressed in most human tumors in contrast to its restricted distribution in healthy tissues. Eag1 channels are particularly overexpressed in human hepatocellular carcinoma (HCC) and detected in cirrhotic and dysplastic rat livers. Two studies by  Chávez-López et al. (2015 and 2016) show that inhibition of either the expression or the activity of Eag1 potassium channels reduces cancer cells’ proliferation, making Eag1 a potential and a promising early-stage marker in the diagnosis of HCC as well as a target for anticancer therapy, such as Astemizole.  

2. Nav1.7 sodium channels α-subunits in prostate cancer

The expression of functional sodium channels α-subunits (VGSCαs), specifically Nav1.7, is associated with strong metastatic potential in prostate cancer in vitro. To investigate VGSCα expression in prostate cancer in vivo, this study uses immunohistochemistry and real-time PCR on human prostate cancer biopsies. The results show that Nav1.7 is ∼20-fold upregulated in human prostate cancer samples, concluding that Nav1.7 can be used successfully as a marker to grade tumor severity and metastatic potential in vivo on human samples (Diss JKJ et al., 2005).

3. CACNA1 and CLCA2 calcium channels in breast cancer

Mutated voltage-gated calcium channels and pumps encourage tumor cell proliferation, catalyze chemoresistance, and delay apoptosis. A recent bioinformatics approach (Nhut Phan N et al.,2017) revealed that, since CACNA1S, CACNA1C, CACNA1D, and CACNA1A were abundantly expressed in normal tissue but not in cancer tissue, they could serve as tumor suppressor gene markers for specific subtypes of cancer.

This study found the calcium-activated chloride channel CLCA2 expression is associated with survival among women with triple-negative breast cancer, an aggressive breast cancer subtype associated with poor prognosis (Purrington KS. et al., 2020)

Primary antibodies targeting VGICs

Below we have selected some of our antibodies targeting voltage-gated ion channels for research purposes in immunohistochemistry, western blot, and immunofluorescence. Each antibody is paired with the respective PrEST antigen control.