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K+ channels are membrane-integral proteins composed of differing combinations of pore-forming α-subunits and peripheral β-subunits. Thus far, more than 80 K+ channel genes have been cloned. Five functional classes of K+ channels have been defined on the basis of their topological arrangements and gating mechanisms: voltage-gated K+ channels (Kv channels), Ca2+ activated K+ channels (KCa channels), ATP-sensitive K+ channels (KATP channels), inwardly rectifying K+ channels and two-pore domain K+ channels1, 2. K+ channels are ubiquitous in a variety of cells: neurons, cardiac myocytes, skeletal muscles, smooth muscles, pancreatic β-cells, lymphocytes and tumor cells2, 3, 4, 5. Traditionally, K+ channels play an important role in regulating cell excitability. For example, in excitable cells, they contribute to maintaining the resting membrane potential, to determining the shape and duration of action potentials and to modulating neurotransmitter release1. In non-excitable cells, however, K+ channels regulate cell volume, cell differentiation and proliferation, apoptosis, and several other physiological functions5. The alteration of K+ channel activity and expression can cause profound pathophysiological events in many different cell types. Thus, pharmacological agents that interact with K+ channels can lead to beneficial therapeutic or untoward side effects.

KATP, KCa, and Kv2.1 channels in insulin secretion

There are at least three types of K+ channels that are involved in glucose-stimulated insulin secretion in pancreatic β-cells6. First, there are the KATP channels, which have been implicated in the regulation of glucose homeostasis. KATP channels are composed of four pore-forming Kir6 subunits and four regulatory subunits of sulfonylurea receptor7. These channels are known to be critical for setting the resting membrane potential of pancreatic β-cells and are regulated by the intracellular concentrations of ATP and ADP which have opposing effects on KATP channels8. After the characterization of KATP channels in pancreatic β-cells, it is reported that KATP channel blockers modulate the release of insulin. Sulphonylurea drugs are currently being used in the treatment of type 2 diabetes9. The primary pharmacological action of sulphonylurea is the block of the KATP channels in the β-cells plasma membrane. This drug then depolarizes the membrane potential of β-cells by reducing the outward K+ currents, which induces insulin secretion.

A second type of K+ channels in pancreatic β-cells includes the KCa channels, which activate in response to a rise in intracellular Ca2+ concentrations10. Although KCa channels regulate the relationships between membrane potential and intracellular Ca2+ influx, which is the major determinant in exocytosis, the role of KCa channels in glucose-stimulated insulin release is not well understood. Low concentrations of TEA and charybdotoxin, which inhibit KCa channels, have no effect either on the membrane potential of β-cells or on insulin release from isolated islets11. These results suggest that KCa channels are not the target on which glucose acts to regulate electrical activity and insulin release in pancreatic β-cells, although the exact role of KCa channels in insulin secretion remains controversial. The third type of Kv channels can be divided into four conserved gene families, comprising the Kv1-4 (Shaker, Shab, Shaw, Shal-type subunits). In pancreatic β-cells, a number of different Kv channel subunits, Kv1.4, Kv1.6, Kv2.1, and Kv3.2, are expressed4. In addition, Kv1.5 is highly expressed in human insulinoma12. The Kv2.1, Shab-type Kv channel, is the primary Kv channel and is highly expressed in pancreatic β-cells. This channel mediates the majority of repolarizing delayed rectifier K+ currents in pancreatic β-cells. The dominant negative knockout of Kv2.1decreased delayed rectifier K+ currents by about 60% and caused a much larger enhancement of glucose-stimulated insulin secretion in isolated rat islets4. Hanatoxin, a specific Kv2.1 inhibitor, increased the membrane potential and the intracellular Ca2+ oscillation response of insulin secreting cells to glucose, and enhanced insulin secretion from β-cells in a glucose-dependent manner13. These results suggest that Kv2.1 may represent a novel target for the treatment of type 2 diabetes.

Kv1.3 in insulin sensitivity

The Kv1.3 channel is a Shaker-type, delayed-rectifier K+ channel and was originally identified in T lymphocytes as an n-type K+ channel. Kv1.3 is involved in T-lymphocyte activation and proliferation by regulating membrane potential and Ca2+ signaling pathways3. This channel is expressed in many tissues, including lymphocytes, CNS, kidney, osteoclasts, testis and insulin-sensitive tissues such as adipose tissue, liver and skeletal muscle14, 15. The Kv1.3 channel has been implicated in the regulation of a wide range of physiological functions, including cell volume regulation, cell proliferation and apoptosis in non-excitable cells, and neurotransmitter release and neuronal excitability in excitable cells. Several lines of evidence indicate that Kv1.3 may participate in the pathways that regulate energy homeostasis and body weight. For instance, Kv1.3 channels are expressed in postganglionic sympathetic neurons and influence the function of sympathetic activity that regulates body weight and energy homeostasis16. The gene-targeted deletion of Kv1.3 in a genetic model of obese mice reduced adiposity and total body weight by increasing locomotor activity and thereby total energy expenditure without changing blood glucose and insulin levels17. In addition, mice bearing a disrupted Kv1.3 gene had a lower body weight and an increased basal metabolic rate which made them resistant to diet-induced obesity14. Although the exact mechanism by which Kv1.3 regulates energy homeostasis is not understood yet, these results highlight the Kv1.3 channels as an important component of the pathways that regulate body weight and energy homeostasis and as a potential target for the development of the drug in the management of obesity.

Insulin signaling systems have been shown to be involved in the regulation of food intake and body weight18. The potential phosphorylation site for tyrosine kinase has been identified in Kv1.3 channels and tyrosine phosphorylation has been linked to the regulation of the activity of Kv1.3 in the olfactory bulb neurons19. These findings suggest that the functional linkage between Kv1.3 and insulin receptor has been implicated in insulin signaling. Indeed, a polymorphism in the human Kv1.3 gene is associated with decreased insulin sensitivity and impaired glucose tolerance but not with insulin secretion, suggesting a candidate gene for the pathogenesis of insulin resistance15. Another mechanistic study showed that Kv1.3 gene deletion and channel inhibition enhanced peripheral insulin sensitivity by increasing the amount of glucose transporter 4 (GLUT4) at the plasma membrane and the uptake of glucose in skeletal muscle and adipose tissue via intracellular Ca2+ signaling20, 21. Therefore, these results indicated that Kv1.3 may play a critical role in insulin action by controlling peripheral insulin sensitivity, although this channel is not associated with insulin secretion in pancreatic β-cells. Furthermore, it is reported that long-term treatment of fluoxetine, a selective serotonin reuptake inhibitor, is associated with a reduction in food intake and body weight and seems to improve insulin sensitivity, possibly by an increase in glycogen synthase activity in skeletal muscle tissue22. Although the mechanism mediating this effect is unknown, the fact that fluoxetine potently blocks Kv1.3 may lead to the hypothesis that increased insulin sensitivity may be attributed to the inhibitory effect of this drug on Kv1.323. The role of Kv1.3 in insulin sensitivity is summarized in Figure 1. Although Kv channel blockers are currently used for several diseases, recent studies have spotlighted Kv1.3 as a potential therapeutic target for treatment of autoimmune disease, type 2 diabetes, asthma, aplastic anemia and periodontal disease24, 25, 26, 27, 28.

Figure 1
figure 1

A cellular mechanism for Kv1.3-involved glucose uptake in insulin-sensitive cells. IR, insulin receptor; IRS-1, insulin receptor substrate-1; GLUT4, glucose transporter 4; TNFα, tumor necrosis factor α; JNK, c-JUN terminal kinase; IL-6, interleukin-6; up and down arrows, increased and decreased activities, respectively.

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Kv1.3 as a potential drug target in diabetic therapy

Type 2 diabetes is a metabolic disease, mainly caused by pancreatic β-cell dysfunction and insulin resistance in the peripheral target tissues. Thiazolidinedione derivatives are widely used in the management of type 2 diabetes as an insulin sensitizer. Although the mechanism of action for thiazolidinedione derivatives is not completely understood, it is well recognized that they do not directly stimulate insulin secretion from pancreatic β-cells but the binding of these drugs to peroxisome proliferator-activated receptor-γ (PPARγ) enhances the expression of the glucose transporter and increases insulin sensitivity in target tissues such as, adipose tissue and skeletal muscle29. Our recent study showed that rosiglitazone and troglitazone, thiazolidinedione derivatives, potently blocked Kv1.3 in a concentration-dependent manner (Figure 2)30. Because the pharmacological regulation of Kv1.3 may be an important mechanism mediating insulin sensitivity as described previously, these results suggest that besides activation of PPARγ, part of the increased peripheral insulin sensitivity promoted by these drugs may be mediated through the inhibition of Kv1.3 channel activity. From these results, thiazolidinedione derivatives can be used in the treatment of diabetes as a new paradigm of a combination effect involving conventional pharmacological actions and K+ channel-blocking properties. K+ channels are the most diverse family of ion channels and contribute to many important physiological functions in various tissues. Thus, K+ channel blocker, as an ideal and safe therapeutic drug, should act selectively and potently on the specific target K+ channels without affecting other ion channels. However, most drugs currently used in clinics display variable effects on several ion channels, which can cause undesirable cardiovascular side effects, especially when blocking cardiac ion channels. Since several K+ channels are involved in insulin release and actions as described above, a drug that acts on more than a single ion channel could be more effective in the treatment of type 2 diabetes. For instance, thiazolidinedione derivatives not only inhibit Kv1.3 but also KATP channel activities30, 31, which provide evidence for a dual mechanism underlying hypoglycemic action of this drug: 1) increased peripheral insulin sensitivity of target tissues, and 2) increased insulin secretion in pancreatic β-cells. Thus, a potential combined therapeutic effect might be expected from the blocking properties of drugs on Kv1.3, Kv2.1, and KATP channels.

Figure 2
figure 2

Concentration-dependent inhibition of Kv1.3 currents expressed in CHO cells by rosiglitazone and troglitazone. Superimposed currents were elicited by applying a 200-ms depolarizing pulse from a holding potential of −80 mV to +40 mV every 30 s in the absence and presence of different concentrations of drugs. Concentration-response curves for the inhibition of Kv1.3 currents. The drug-induced inhibition were measured at the end of a 200-ms depolarizing pulse of +40 mV and were normalized to the current under control conditions. The normalized currents were fitted to the Hill equation (n=7−8). Data are expressed as means±SEM30.

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Additionally, the inflammatory cytokines such as interleukin-6 (IL-6) and tumor necrosis factor-α (TNFα) have been known to play an important role in insulin resistance32, 33. IL-6 exerted long-term inhibitory effects on the insulin receptor substrate-1 (IRS-1), GLUT4 and PPARγ, which impaired insulin signaling and action. Kv1.3 is also involved in the regulation of synthesis and secretion of inflammatory cytokines, thereby affecting the specific immune responsiveness. Selective inhibition of Kv1.3 activity led to a reduction in the production of IL-6 and TNFα and in Ca2+ influx, improving the inflammatory reaction that is an important pathogenesis of insulin resistance in type 2 diabetes20, 34.

It is well known that vascular diseases are the most common complications of diabetes. Vascular smooth muscle cell (VSMC) dysfunction, hypercoagubility and inflammation are the key factors in the etiology of vascular complications in diabetes35, 36. Recent studies have found that the functional expression of Kv1.3 is increased in proliferating VSMC and the selective blockade of Kv1.3 currents by margatoxin decreased the migration and proliferation of VSMC37. These results indicated that Kv1.3 is involved in the proliferative response of VMSC. Considering the fact that the major features of abnormal VSMC functions in diabetes include abnormal growth, proliferation and hypertrophy of VSMC, therefore, targeting the Kv1.3 channel represents a promising approach to treating vascular disorders in diabetes. In addition, the dysfunction of platelets is regarded as an important factor in the pathophysiology of vascular disease in diabetes36. Recent study has shown that Kv1.3 is the exclusive Kv channel in platelets, which sets the resting membrane potential, regulates Ca2+ signaling and influences the circulating platelet number38. Kv1.3 may play a potential role in the development of vascular complications in diabetes, and the therapeutic significance of Kv1.3 could be expanded to the treatment and prevention of diabetic vascular complications. Therefore, a drug-targeted Kv1.3 channel may have a therapeutic advantage over conventional therapies in that the Kv1.3 channel blocker stabilizes cellular function by regulating the membrane potential and the resultant intracelluar Ca2+ influx that influences multiple signal transduction pathways and cellular events, regardless of the etiology of cellular dysfunction.

Future study

Recently, a number of drugs have been shown to regulate the functional ion channel expression on cell membranes by regulating the channel trafficking pathways in addition to affecting the ion permeability of channels upon chronic administration39. Also, the densities of several cardiac Kv channel genes are reportedly significantly altered in some pathologic conditions such as atrial fibrillation and heart failure. For example, in cases of chronic atrial fibrillation, there is a reduction in Kv1.5 protein levels and a decrease in current density, although it is not clear that the reduction in Kv1.5 current density is directly caused by atrial fibrillation events40. Consistent with this view, mutations in the genes encoding sulfonylurea receptor can contribute to defective trafficking and function of the pancreatic KATP channel and be linked to an impaired pancreatic β-cell response41. This finding has important pharmacological implications in that drugs could exhibit different sensitivities to normal or pathological situations42. To the best of our knowledge, no data are available concerning Kv1.3 channel expression in diabetes. In future studies, it will be interesting to examine the regulation of Kv1.3 channel expression and the effect of several drugs on the activity and expression of Kv1.3 channels on the cell membrane of insulin-sensitive tissues in diabetic patients.