Implication of Voltage-Gated Potassium Channels in Neoplastic Cell Proliferation

Voltage-gated potassium channels (Kv) are the largest group of ion channels. Kv are involved in controlling the resting potential and action potential duration in the heart and brain. Additionally, these proteins participate in cell cycle progression as well as in several other important features in mammalian cell physiology, such as activation, differentiation, apoptosis, and cell volume control. Therefore, Kv remarkably participate in the cell function by balancing responses. The implication of Kv in physiological and pathophysiological cell growth is the subject of study, as Kv are proposed as therapeutic targets for tumor regression. Though it is widely accepted that Kv channels control proliferation by allowing cell cycle progression, their role is controversial. Kv expression is altered in many cancers, and their participation, as well as their use as tumor markers, is worthy of effort. There is an ever-growing list of Kv that remodel during tumorigenesis. This review focuses on the actual knowledge of Kv channel expression and their relationship with neoplastic proliferation. In this work, we provide an update of what is currently known about these proteins, thereby paving the way for a more precise understanding of the participation of Kv during cancer development.


Potassium Channels: Classification and Function
Ion channels are transmembrane proteins that form aqueous pores and drive the selective flow of ions, participating in the electrochemical gradient across the cell membrane. They are fundamental for excitable cells but are also involved in cell functions, such as proliferation, migration, cell volume, and specific processes such as insulin release or muscular contractibility [1]. Their participation in such highly diverse phenomena highlights a crucial biological relevance. Thus, mutations and alterations of the normal function of these proteins trigger alterations, called channelopathies, in cardiovascular and nervous systems as well as autoimmune and metabolic diseases. [2,3].
The British Pharmacological Society (BPS) and the International Union of Basic and Clinical Pharmacology (IUPHAR) (http://www.guidetopharmacology.org/) classify ion channels as (i) voltage-gated ion channels, (ii) ligand-gated ion channels, or (iii) channels using other gating mechanisms, including aquaporins, chloride channels, and store-operated calcium channels. Following these criteria, 141 members are included in the voltage-gated ion channel superfamily, making it one of the largest groups of signal transduction proteins [4,5].

Kv Channels and Cancer
Kv exhibit specific physiological and pharmacological properties, and cells could express a variable repertoire of channels. According to their functional properties, Kv are grouped into four families [1,5]. In this review, we will structure the information considering this functional classification.

Delayed Rectifier Channels (IDR)
Delayed rectifier channels exhibit a delay before activation ( Figure 2). They generate an outward current of K + following membrane depolarization triggered by an influx of Na + ions inside the cell. To counteract this cation influx, IDR channels allow the exit of K + ions from the cell. Therefore, the membrane repolarizes, shortening the duration of the nerve impulse. This is crucial in excitable cells such as neurons or muscle cells, but their presence is ubiquitous in the human body. This group includes members of the Shaker-related family (Kv1.1-Kv1.3, Kv1.5-Kv1.8), the Shab-related family (Kv2), some Shaw-related members (Kv3.1, Kv3.2), the Kv7 group and Kv10.1, from the ether-à-gogo (EAG) family.
Some IDR participate in neoplastic phenomena. Shaker-related members, such as Kv1.3 and Kv1.5, play an important role in cell proliferation (e.g., in macrophage; astrocytes; and muscular, vascular, and skeletal cells). These channels remodel their expression during both physiological and neoplastic cell growth. In fact, evidence has demonstrated altered expression in several types of tumors and cancer cell lines [23]. Kv1.3 and Kv1.5 are the major Kv channels in leucocytes, and, because Kv control crucial functions such as cell proliferation, activation, migration, or apoptosis, it is not surprising that blood cancers remodel these channels. However, their pattern is not always similar. For instance, Kv1.5 is differentially expressed in various tumors. Furthermore, Kv1.5 is inversely correlated with tumor aggressiveness in non-Hodgkin's lymphomas [24], whereas Kv1.3 is Figure 1. Remodeling of voltage-gated K + channels (Kv) channel expression in human cancers. Schematic representation of the human body highlights the Kv distribution in tumors. Many studies document changes in Kv channel expression (see text for details). Colors represent differential levels of expression: Red, down-regulation; green, up-regulation; orange, altered expression (evidence claim opposite effects in the Kv channel abundance).

Kv Channels and Cancer
Kv exhibit specific physiological and pharmacological properties, and cells could express a variable repertoire of channels. According to their functional properties, Kv are grouped into four families [1,5]. In this review, we will structure the information considering this functional classification.

Delayed Rectifier Channels (I DR )
Delayed rectifier channels exhibit a delay before activation ( Figure 2). They generate an outward current of K + following membrane depolarization triggered by an influx of Na + ions inside the cell. To counteract this cation influx, I DR channels allow the exit of K + ions from the cell. Therefore, the membrane repolarizes, shortening the duration of the nerve impulse. This is crucial in excitable cells such as neurons or muscle cells, but their presence is ubiquitous in the human body. This group includes members of the Shaker-related family (Kv1.1-Kv1.3, Kv1.5-Kv1.8), the Shab-related family (Kv2), some Shaw-related members (Kv3.1, Kv3.2), the Kv7 group and Kv10.1, from the ether-à-go-go (EAG) family.
Some I DR participate in neoplastic phenomena. Shaker-related members, such as Kv1.3 and Kv1.5, play an important role in cell proliferation (e.g., in macrophage; astrocytes; and muscular, vascular, and skeletal cells). These channels remodel their expression during both physiological and neoplastic cell growth. In fact, evidence has demonstrated altered expression in several types of tumors and cancer cell lines [23]. Kv1.3 and Kv1.5 are the major Kv channels in leucocytes, and, because Kv control crucial functions such as cell proliferation, activation, migration, or apoptosis, it is not surprising that blood cancers remodel these channels. However, their pattern is not always similar. For instance, Kv1.5 is differentially expressed in various tumors. Furthermore, Kv1.5 is inversely correlated with tumor aggressiveness in non-Hodgkin's lymphomas [24], whereas Kv1.3 is decreased in lymphoma and leukemia samples but is not always related to tumor malignancy [25,26]. In fact, Kv1.3 could function as a tumor suppressor in blood cancers by a mechanism that implies apoptosis [27,28].
Kv1.3 expression is well documented in solid tumors. Both pro-and anti-proliferative properties have been assigned to this channel, depending on the tissue and the stage and degree of malignancy of the tumor. Kv1.3 is differentially remodeled in breast, colon, lung, glioma, muscle, brain, or prostate cancers [23,26]. Its role in tumor progression is not clear, and different implications are described depending on the cancer. Sometimes Kv1.3 expression is aberrant and related to proliferation and apoptosis [19,[29][30][31], whereas only cell migration and adhesion are altered in others [20].
Similarly, some examples inversely correlate Kv1.5 and malignancy, linking the channel with apoptosis and impairing cancer progression [23,32]. However, Kv1.5 is overexpressed in some malignant and aggressive neoplasia, such as gastric, bone or colon cancers, where it participates in tumor proliferation and calcium homeostasis [25,33,34]. Furthermore, Kv1.5 is overexpressed in muscle sarcoma and is related to tumor malignancy [35,36]. By contrast, and similarly to lymphomas, a Kv1.5 abundance is inversely correlated with the degree of malignancy in gliomas [37]. Moreover, the methylation of Kv1.3 [38] and Kv1.5 [39,40] promoters silences channel expression in some neoplastic phenotypes, which supports their roles as tumor suppressors. Both Kv1.3 and Kv1.5 are upregulated during the initial phases of the cell cycle, thus promoting cell cycle progression. Therefore, both channels undergo cell cycle-dependent regulation; however, the molecular mechanisms remain poorly understood [41].
Mitochondria, playing a pivotal role in cell metabolism, participate in apoptosis. Mitochondria contribute to reactive oxygen species (ROS) production and the onset of signaling pathways [42]. Bcl-2 family members, such as Bax, inhibit mitochondrial channels, such as mitoKv1.3 and K Ca 3.1, downstream of pro-apoptotic signals to promote cell survival [27]. Other mitochondrial K + channels, such as KCa2.x or TASK-3, have been related to cell death, suggesting a potential link between K + channel modulation and intrinsic apoptosis [42,43]. Cancer therapies targeting mito-channels, such as mitoKv1.3, selectively reduce tumor cells and control cancer development and progression in mouse models of pancreatic ductal adenocarcinomas (PDAC) and melanoma [43][44][45]. Considering the above, ion channels, such as Kv1.3 and Kv1.5, should be considered multifunctional proteins; therefore, assuming a single role is a misinterpretation.
Kv1.1 has been documented in breast cancer and, similar to Kv1.3, plays different roles. Kv1.1 functions as a tumor suppressor when it changes its cell location, affecting cellular senescence and transformation [46]. On the other hand, breast cancer cell lines show Kv1.1 overexpression and, similar to other Kv channels, implicates it in cell migration and tumor development [20]. Kv2.1 is also altered in several cancers, such as gastric [33], medulloblastoma [47], or endometrial cancer [48]. As we will explain later, Kv2.1 exhibits a cell cycle-dependent subcellular distribution, concentrating in raft-like lipid domains during M phase [49]. Thus, anti-neoplastic treatments targeting lipid rafts affect Kv2.1 function [50].
Some members of the KCNQ (Kv7) family are also related to cell proliferation and cancer. Kv7.1 remodels in some tumors, and channel inhibition reduces cell proliferation. Kv7.1 is increased in colon cancer as well as in seminoma and germinal cell line tumors [51,52]. Additionally, Kv7.5, which contributes to the vascular smooth muscle tone, participates at the G1/S phase transition during cell cycle progression in myoblasts [53].
Ether-a-go-go potassium channels (hEAG/Kv10.1) are characterized by the correlation between the speed of activation and membrane potential before the stimulus. These channels undergo cell cycle regulation. In fact, Kv10.1 was the first voltage-gated channel related to oncogenesis. Many cell and tumor models document the relationship between Kv10.1 expression and tumor growth [54,55]. Kv10.1 expression is mostly restricted to the central nervous system under healthy conditions. However, noticeable Kv10.1 levels are detected in clinical tumors from several different origins, including neuroblastomas [56], glioblastomas and derived brain metastasis [21], breast cancer [57,58], colon and gastric cancers [59,60], or osteosarcomas [61,62]. This evidence supports Kv10.1 as a potential marker for several cancers, such as cervical and colon cancer. Aberrant expression of Kv10.1 correlates with a malignant phenotype and a poor survival rate, probably because the channel provides a good environment for tumor development. Kv10.1 stimulates vascularization and certain resistance to hypoxia, both of which are an advantage for the survival of tumor cells against an immune attack. Kv10.1 is also related to cytoskeletal regulation, which may be associated with proliferation, cell adhesion, and metastasis [58,63,64].
correlates with a malignant phenotype and a poor survival rate, probably because the channel provides a good environment for tumor development. Kv10.1 stimulates vascularization and certain resistance to hypoxia, both of which are an advantage for the survival of tumor cells against an immune attack. Kv10.1 is also related to cytoskeletal regulation, which may be associated with proliferation, cell adhesion, and metastasis [58,63,64].

A-Type Channels (IA)
IA channels generate a transient-outward K + current with little delay after depolarization ( Figure 2). Characterized by rapid inactivation, these channels open when depolarization occurs after hyperpolarization, and they increase the interval between action potentials. Thus, IA help neuronal repetitive firing. This group includes members of the Shaker (Kv1.4), Shaw-related (Kv3.3, Kv3.4), and Shal-related (Kv4) families.
Kv1.4 expression is impaired in gastric cancer because of hypermethylation of the promoter, resulting in a loss of channel expression [65]. Kv3.4 is present in oral carcinomas, head and neck cancers, and leukoplakia samples, together with altered ROS production patterns, hypoxia-related tumor processes, and cell cycle arrest-mediated control of proliferation [66][67][68][69][70]. Kv1.4, as well as Kv3.4, are also present in bone cancer and show changes in expression and in function when related to pain. However, the role of the channels in proliferation or tumorigenesis is not clear [71]. Cell migration and invasion are altered in aggressive lung adenocarcinoma cell lines overexpressing Kv3.4 [69]. On the other hand, the aberrant expression of Kv4.1 is documented in gastric and mammary cancers. In this vein, Kv4.1 inhibition halts cancer proliferation by arresting cells at the G1/S transition of the cell cycle [72,73].

Modifier/Silencer Subunits
Several groups have similar sequences and structures to those of some Kv families but are not functional in homotetrameric compositions. Instead, they mostly heterotetramerize with members of the Kv2 family, modulating their activity. This group includes the Kv5, Kv6, Kv8, and Kv9 families. These channels present a restricted tissue expression, indicating a tissue-specific function for the heterotetrameric channels. Scarce information is related to these channels regarding cancer. However, their expression is impaired in some cancer cell lines, and evidence suggests that they are involved in cell proliferation, both acting as nonconduction proteins or associated with Kv2.1 [74]. For example, Kv9.3 and Kv2.1 could be major components of Kv channels in cervical adenocarcinoma cells, linking with cell cycle regulation [48]. By contrast, in colon and lung adenocarcinomas, Kv9.3

A-Type Channels (I A )
I A channels generate a transient-outward K + current with little delay after depolarization ( Figure 2). Characterized by rapid inactivation, these channels open when depolarization occurs after hyperpolarization, and they increase the interval between action potentials. Thus, I A help neuronal repetitive firing. This group includes members of the Shaker (Kv1.4), Shaw-related (Kv3.3, Kv3.4), and Shal-related (Kv4) families.
Kv1.4 expression is impaired in gastric cancer because of hypermethylation of the promoter, resulting in a loss of channel expression [65]. Kv3.4 is present in oral carcinomas, head and neck cancers, and leukoplakia samples, together with altered ROS production patterns, hypoxia-related tumor processes, and cell cycle arrest-mediated control of proliferation [66][67][68][69][70]. Kv1.4, as well as Kv3.4, are also present in bone cancer and show changes in expression and in function when related to pain. However, the role of the channels in proliferation or tumorigenesis is not clear [71]. Cell migration and invasion are altered in aggressive lung adenocarcinoma cell lines overexpressing Kv3.4 [69]. On the other hand, the aberrant expression of Kv4.1 is documented in gastric and mammary cancers. In this vein, Kv4.1 inhibition halts cancer proliferation by arresting cells at the G1/S transition of the cell cycle [72,73].

Modifier/Silencer Subunits
Several groups have similar sequences and structures to those of some Kv families but are not functional in homotetrameric compositions. Instead, they mostly heterotetramerize with members of the Kv2 family, modulating their activity. This group includes the Kv5, Kv6, Kv8, and Kv9 families. These channels present a restricted tissue expression, indicating a tissue-specific function for the heterotetrameric channels. Scarce information is related to these channels regarding cancer. However, their expression is impaired in some cancer cell lines, and evidence suggests that they are involved in cell proliferation, both acting as nonconduction proteins or associated with Kv2.1 [74]. For example, Kv9.3 and Kv2.1 could be major components of Kv channels in cervical adenocarcinoma cells, linking with cell cycle regulation [48]. By contrast, in colon and lung adenocarcinomas, Kv9.3 overexpression by itself could also be related to tumor progression. Silencing Kv9.3 but not Kv2.1 in these cancer cell lines inhibits cell proliferation, causing G0/G1 cell cycle arrest [75].

Others
Some channels cannot be grouped into any of the abovementioned categories according to their properties. For example, Kv10.2 is sometimes defined as a noninactivating outward-rectifying potassium channel. In addition, Kv11.1, a member the hERG family, is a voltage-gated potassium channel with inwardly rectifying properties. Finally, K Ca 3.1 channels are activated in response to voltage and Ca 2+ changes.
Kv11.1 is mainly expressed in the heart but shows certain ubiquitous expression in the remaining healthy human tissues. This channel is present in several tumors from multiple origins, such as gastric, colorectal, pancreatic, neuroblastoma, leukemia, or endometrial cancers [22,[76][77][78][79][80]. In these neoplasias, Kv11.1 causes resting-potential variations along different stages of the cell cycle in tumor cells. Evidence correlates Kv11.1 with malignancy and prognosis of the cancer [79,81,82]. During cancer progression, Kv11.1 participates with the stimulation of angiogenesis and the recruitment of cytokines or growth factors. Thus, this channel functions during differentiation, cell migration, invasiveness, and proliferation, which can be an advantage for cancer cells [83][84][85].

Regulation of Cell Cycle Progression by Kv Channels
Several redundant and independent mechanisms finely control proliferation and cell cycle progression in all cell types (Figure 3). Such a set of molecular events operates through a checkpoint system which guarantees the initiation of an event only after the successful completion of the preceding step. This checkpoint system organizes the cell cycle in different phases named G1 (gap 1), S (DNA synthesis), G2 (gap 2), and M (mitosis). After mitosis, the cell either can move on to a new G1 phase or enter into a quiescent state. This latter state is representative of end-differentiated cells, which will last for the rest of their lifetime. Transition between phases is regulated by the cyclic activation/inactivation of cyclin-dependent kinases (CDKs) by cyclins and CDK inhibitors (CKIs), respectively. K + channels can control the upstream biochemical events leading to cell cycle progression by the regulation of biophysical properties such as the membrane potential and cell volume, in addition to mechanisms involving protein-protein interactions-all of which converge in tight regulation of Ca 2+ oscillations.
The observation that the membrane potential is not constant during the cell cycle dates back to the last century. However, at that time, the causal relationship was not clear [86,87]. Since then, evidence has suggested the bioelectric control of the cell cycle [88]. An increase in K + permeability hyperpolarizes the cell at the end of the G1 phase. By contrast, depolarization is found at the G2/M border. These changes in the membrane potential are gradual rather than instantaneous and have been proven to be essential for the proliferation of many cell types. Thus, inhibition of K + channels activity produces cell cycle arrest, typically by hindering G1/S transition [89,90]. Interestingly, although cell cycle-dependent fluctuations in the membrane potential are observed under both physiological and pathophysiological conditions, cells with a high proliferative phenotype tend to be more depolarized at each step than their normal analogs. Therefore, malignant cancerous cells show a depolarized phenotype, and depolarization itself can induce cancerous transformation. Indeed, depolarization has been suggested as a hallmark of cancer [90]. Growing evidence is unravelling a complex scenario where not only the type of current but also the molecular identity of the potassium channel is important to regulate a function that, in most cases, is time and place dependent. throughout G1 progression. Knocking down each of these three genes impairs proliferation [94]. In spinal cord astrocytes, the downregulation of inwardly rectifying K + currents is important for G1/S transition, whereas blockade of delayed outwardly rectifying currents causes G1 arrest. Conversely, a recovery of Kir currents is critical for mitosis. Furthermore, S-phase cell cycle arrest accumulates delayed outwardly rectifying currents [95]. . Participation of potassium channels on the control of the cell cycle progression. Kv channels participate during the cell cycle in a series of events that control the progression. Events indicated from outside to inside circles. Outer grey circle: Physical and biochemical properties of ion channels affecting cell cycle progression: (i) Ion flux-dependent properties due to K + conduction; (ii) Kv conformational changes may associate with other down-stream signalling partners; and (iii) Kv channels can also induce membrane reorganization phenomena and promote the formation of subcellular structures. These connected events, related to no specific phase, contribute to the regulation of Ca 2+ oscillations leading to cell cycle progression. Inner colored circles: Events regulated by Kv channels in specific phases of the cell cycle. Colors correspond to sequential phases of the cycle. Color gradients represent transitions between phases. Kv channels regulate membrane potential, cell volume, mitogen-dependent signal transduction pathways, and other processes involved in cell cycle progression, such as the primary cilium resorption and mitochondrial ROS production. Cell cycle representation: M (yellow) and S (blue) phases of the cell cycle are separated by G1 (green) and G2 (red) gap phases. Several CDK-Cyclin complexes and CDK-inhibitors regulate transitions between phases. In the inner circle, colored complexes are active at specific stages of the cell cycle. Figure 3. Participation of potassium channels on the control of the cell cycle progression. Kv channels participate during the cell cycle in a series of events that control the progression. Events indicated from outside to inside circles. Outer grey circle: Physical and biochemical properties of ion channels affecting cell cycle progression: (i) Ion flux-dependent properties due to K + conduction; (ii) Kv conformational changes may associate with other down-stream signalling partners; and (iii) Kv channels can also induce membrane reorganization phenomena and promote the formation of subcellular structures. These connected events, related to no specific phase, contribute to the regulation of Ca 2+ oscillations leading to cell cycle progression. Inner colored circles: Events regulated by Kv channels in specific phases of the cell cycle. Colors correspond to sequential phases of the cycle. Color gradients represent transitions between phases. Kv channels regulate membrane potential, cell volume, mitogen-dependent signal transduction pathways, and other processes involved in cell cycle progression, such as the primary cilium resorption and mitochondrial ROS production. Cell cycle representation: M (yellow) and S (blue) phases of the cell cycle are separated by G1 (green) and G2 (red) gap phases. Several CDK-Cyclin complexes and CDK-inhibitors regulate transitions between phases. In the inner circle, colored complexes are active at specific stages of the cell cycle.
For instance, as abovementioned, Kv1.5 but not Kv1.3 activity is important for myoblast proliferation; however, both channels are transcriptionally upregulated during G1 phase of the cell cycle. Kv1.5 controls myoblast proliferation through a mechanism involving the accumulation of the CDKIs p21 cip1 and p27 kip1 [91]. In oligodendrocyte progenitors, a similar increase in Kv1.3 and Kv1.5 expression is found at the G1 phase. However, Kv1.3 activity, rather than Kv1.5 activity, is involved in G1 progression in these cells. Similar to myoblasts, this mechanism involves the accumulation of CDKIs [92,93]. Many other examples of fluctuations in K + channels expression and activity along the cell cycle are reported. For example, Kv1.2 and Kv2.1 mRNA are decreased from early to late G1, while K Ca 3.1 increases in mesenchymal stem cells from the bone marrow. Such remodeling implies a decrease in voltage-gated delayed rectifier K + currents and an increase in Ca-activated K + currents throughout G1 progression. Knocking down each of these three genes impairs proliferation [94]. In spinal cord astrocytes, the downregulation of inwardly rectifying K + currents is important for G1/S transition, whereas blockade of delayed outwardly rectifying currents causes G1 arrest. Conversely, a recovery of Kir currents is critical for mitosis. Furthermore, S-phase cell cycle arrest accumulates delayed outwardly rectifying currents [95].
K + channels can also exhibit cell cycle-dependent localization. Kv2.1 is an intriguing example. This channel, which clusters at ER-PM junctions during mitosis, diffusely distributes during interphase. Such Kv2.1 transient localization is dependent on the phosphorylation state, which increases at M phase [49]. Kv2.1 stabilizes/enhances contacts between the endoplasmic reticulum and the plasma membrane, named as ER-PM junctions [96]. Therefore, the channel indirectly regulates localized Ca 2+ movements and the composition of lipidic microenvironments, suggesting a structural role for Kv2.1 during mitosis. Kv10.1 is another example of cell cycle-dependent localization of K + channels. This protein is located at the centrosome and primary cilium [97]. Disregarded for many years, the primary cilium is assembled at the plasma membrane of nearly all quiescent cells. Increasing evidence has pointed to the primary cilium as an important organelle for the transduction of extracellular information. However, the mechanism-either mechanical, chemical, or both-is still unclear. The primary cilium consists of a microtubule-based protrusion whose basal body derives from the mother centriole. Upon cell cycle entry, the primary cilium resorbs, and the centriole is redistributed to form the microtubule-organizing center (MTOC) [98]. Transient Kv10.1 expression is transcriptionally induced during G2/M transition by the direct binding of E2F1 to the Kv10.1 promoter [99]. The channel is then located at the basal primary cilium membrane where it promotes cilium resorption. The hypothesized mechanism postulates that local membrane hyperpolarization, due to Kv10.1 activity, would lead to increased Ca 2+ influx and PIP2 dispersion from the basal cilium membrane; both events are necessary for primary cilium retraction [98]. Further examples of the importance of K + channels localization for cell cycle regulation include Kv1.3. Inhibition of Kv1.3 activity at the plasma membrane blocks G1/S transition in many cell types. However, a recent study has shown that specific blockade of mitochondrial Kv1.3, with a low concentration of PAP-1 mitochondriotropic inhibitors, slightly favors proliferation, most likely by a mechanism involving mitochondrial ROS production [30].
The regulation of cell volume is intrinsically linked to changes in the membrane potential, which is crucial for cell cycle progression. Hyperpolarization via K + channels activation favors Cl − exits by increasing its electrical driving force. The consequent leakage of KCl implies cell shrinkage by osmotic water loss that, in turn, favors the initiation of Ca 2+ oscillations driving cell proliferation. Moreover, water fluxes can modify the crowding of nutrients and other intracellular solutes, such as enzymes and co-factors involved in cell cycle regulation [100]. For instance, Kv11.1 peaks the expression at the G1 phase and directly connects with the cell volume regulation during the cell cycle. Sustained inhibition of channel activity causes cell bursting, which can be counteracted when decreasing intracellular osmotic pressure [101]. Interestingly, transient swelling, required for cell division, produces normal-sized daughter cells and regulates cell shape and cell-cell contacts [102].
In addition to flux-dependent abilities of K + channels, the cell cycle can also be regulated by nonconducting properties of the channels. As transmembrane proteins, K + channels can contribute to the initiation of many biochemical events by direct protein-protein interactions, leading to the initiation of many intracellular pathways. For instance, depolarization activates Kv1.3 by inducing a conformational change on its voltage sensor domain. This structural switch into the open state of Kv1.3 is sufficient to induce the channel pro-proliferative activity, independently of K + conduction. Thus, a pore-less Kv1.3 promotes proliferation only if the voltage-dependence of gating is conserved [103]. Activation of Kv1.3 exposes a C-terminal docking domain, which contains different phosphosites essential to induce proliferation [104]. Similarly, Kv10.1 induces proliferation through the activation of the mitogen-activated protein kinase (MAPK) cascade in the absence of conducting properties [105]. These observations corroborate the importance of voltage-sensing flux-independent properties of K + channels in the regulation of proliferation, which include conformational changes and the consequent gating currents.

Concluding Remarks
In recent years, countless examples of aberrant expression of Kv channels in several types of cancer have been described (Table 1). Their implication in tumor progression is variable. Thus, neoplastic transformation, proliferation, migration, adhesion, cell volume, or apoptosis-among other properties-can be altered when these proteins are remodeled.
Cell cycle regulation is related to the polarization state of the cells. As far as we know, changes in K + channels expression or function may be a cause and/or consequence of changes in the membrane potential. Interestingly, highly proliferative cells show a depolarized phenotype. Inhibition of Kv has been related to cell cycle arrest and impairment of proliferation. On the other hand, Kv channels exhibit a cell cycle-dependent localization, which can be altered with different tumorigenic scenarios. Though the role of Kv channels in proliferation is highly demonstrated, their relationship to tumor progression is not entirely understood. The wide variety of tissues, cells, tumor stages, degrees of malignancy, and channels with related auxiliary proteins involved that can be affected further complicate our knowledge. Kv channels are, thus, seducing and exciting targets for anti-tumoral treatments. Toxins and blockers, acting selectively against Kv channels, impair tumor progression in vitro, although their use in vivo still deserves further work before use in human anticancer therapies. ↓ Methylation of the gene promoter. [111] ↓ Decreased expression in ductal adenocarcinoma grade II. [25] Bones ↑ Osteosarcoma samples and derived cell lines. [29] Skeletal muscle ↑ Increased expression in skeletal muscle carcinogenesis but no clear relationship with malignancy. [35] Parathyroid ↑ DNA and protein overexpression of Kv1.3. Potential marker to distinguish carcinoma or adenoma. [112] Kv1.5 Blood ↓ Inversely correlates with aggressiveness in non-Hodgkin's lymphomas. [24] Skeletal muscle ↑ Increased expression in skeletal muscle carcinogenesis. Correlation with the degree of malignancy. [35] Breast ↓ Absent or low expression in mammary duct carcinoma samples. [25] Brain ↓ Kv1.5 inversely correlates with glioma malignancy. High in astrocytoma, moderate in oligodendroglioma, and low in glioblastoma. [37] Skin ↑ High expression in squamous skin cell carcinoma. [25] Colon ↑ Overexpression in colon adenocarcinoma. [25] Stomach ↑ Kv1.5 may be involved in tumor cell proliferation by controlling calcium entry. [33] Bone ↓ Promoters of ion channels are highly methylated in Ewing Sarcoma. Inhibiting CpG islands, cancer cells are sensitive to death. Kv1.5 would act as a tumor suppressor. [39,40] ↑ Osteosarcoma samples and cell lines. Silencing Kv1.5 impairs osteosarcoma cell proliferation and induces cell cycle arrest (G0/G1) and apoptosis. [34]