Inhibition of the activity of T lymphocyte Kv1.3 channels by extracellular zinc

Co-Application of Statin and Flavonoids as an Effective Strategy to Reduce the Activity of Voltage-Gated Potassium Channels Kv1.3 and Induce Apoptosis in Human Leukemic T Cell Line Jurkat

Voltage-gated potassium channels of the Kv1.3 type are considered a potential new molecular target in several pathologies, including some cancer disorders and COVID-19. Lipophilic non-toxic organic inhibitors of Kv1.3 channels, such as statins and flavonoids, may have clinical applications in supporting the therapy of some cancer diseases, such as breast, pancreas, and lung cancer; melanoma; or chronic lymphocytic leukemia. This study focuses on the influence of the co-application of statins-simvastatin (SIM) or mevastatin (MEV)-with flavonoids 8-prenylnaringenin (8-PN), 6-prenylnarigenin (6-PN), xanthohumol (XANT), acacetin (ACAC), or chrysin on the activity of Kv1.3 channels, viability, and the apoptosis of cancer cells in the human T cell line Jurkat. We showed that the inhibitory effect of co-application of the statins with flavonoids was significantly more potent than the effects exerted by each compound applied alone. Combinations of simvastatin with chrysin, as well as mevastatin with 8-prenylnaringenin, seem to be the most promising. We also found that these results correlate with an increased ability of the statin-flavonoid combination to reduce viability and induce apoptosis in cancer cells compared to single compounds. Our findings suggest that the co-application of statins and flavonoids at low concentrations may increase the effectiveness and safety of cancer therapy. Thus, the simultaneous application of statins and flavonoids may be a new and promising anticancer strategy.

Statins as inhibitors of voltage-gated potassium channels Kv1.3 in cancer cells

Voltage-gated potassium channels are integral membrane proteins selectively permeable for potassium ions and activated upon change of membrane potential. Voltage-gated potassium channels of the Kv1.3 type were discovered both in plasma membrane and in inner mitochondrial membrane (mito Kv1.3 channels). For some time Kv1.3 channels located both in plasma membrane and in mitochondria are considered as a potentially new molecular target in several pathologies including some cancer disorders. Lipophilic nontoxic organic inhibitors of Kv1.3 channels may potentially find a clinical application to support therapy of some cancer diseases such as breast, pancreas and lung cancer, melanoma or chronic lymphocytic leukaemia (B-CLL). Inhibition of T lymphocyte Kv1.3 channels may be also important in treatment of chronic and acute respiratory diseases including severe pulmonary complication in corona virus disease Covid 19, however further studies are needed to confirm this supposition. Statins are small-molecule organic compounds, which are lipophilic and are widely used in treatment of hypercholesterolemia and atherosclerosis. Electrophysiological studies performed in our laboratory showed that statins: pravastatin, mevastatin and simvastatin are effective inhibitors of Kv1.3 channels in cancer cells of human T cell line Jurkat. We showed that application of the statins in the concentration range from 1.5 μM to 50 μM inhibited the channels in a concentration-dependent manner. The inhibitory effect was the most potent in case of simvastatin and the least potent in case of pravastatin. The inhibition was partially irreversible in case of simvastatin and fully reversible in case of pravastatin and mevastatin. It was accompanied by a significant acceleration of the current inactivation rate without any significant change of the activation rate. Mechanism of the inhibition is probably complex, including a direct interaction with the channel protein and perturbation of lipid bilayer structure, leading to stabilization of the inactivated state of the channels.

The Influence of 6-Prenylnaringenin and Selected Non-prenylated Flavonoids on the Activity of Kv1.3 Channels in Human Jurkat T Cells

The influence of a prenylated flavonoid-6-prenylnaringenin (6-PR) and selected non-prenylated flavonoids: acacetin, chrysin, baicalein, wogonin, and luteolin on the activity of voltage-gated potassium channels Kv1.3 was investigated in human leukemic Jurkat T cells. Electrophysiological measurements were accompanied by studies on the cytotoxic effect of the examined compounds on Jurkat T cells. Electrophysiological studies were performed using the whole-cell patch-clamp technique. Cell viability was determined using the MTT assay. 6-PR inhibited Kv1.3 channels in Jurkat T cells in a concentration-dependent manner. The estimated value of the half-blocking concentration (EC₅₀) was about 5.76 µM. Among non-prenylated flavonoids, acacetin and chrysin inhibited Kv1.3 channels in Jurkat T cells when applied at the concentration of 30 µM, whereas baicalein, wogonin, and luteolin were ineffective at this concentration. The inhibitory effects of acacetin and chrysin on Kv1.3 channels were significantly less potent than the inhibition caused by 6-PR. All tested compounds inhibited growth of Jurkat T cells in a concentration-dependent manner. Wogonin and chrysin were the most cytotoxic flavonoids tested, whereas baicalein and 6-PR were the least cytotoxic compounds. In accordance to our hypothesis the prenylated flavonoid (6-PR) was much more effective inhibitor of Kv1.3 channels than non-prenylated compounds selected for this study. The inhibition of Kv1.3 channels by 6-PR, acacetin, and chrysin was not related to cytotoxicity of these compounds. The channels' inhibition might be involved in anti-proliferative and pro-apoptotic effects of 6-PR, acacetin and chrysin observed in cancer cell lines expressing these channels.

Direct Binding of the pH-Regulated Protein 1 (Pra1) from Candida albicans Inhibits Cytokine Secretion by Mouse CD4+ T Cells

Opportunistic infections with the saprophytic yeast Candida albicans are a major cause of morbidity in immunocompromised patients. While the interaction of cells and molecules of innate immunity with C. albicans has been studied to great depth, comparatively little is known about the modulation of adaptive immunity by C. albicans. In particular, direct interaction of proteins secreted by C. albicans with CD4⁺ T cells has not been studied in detail. In a first screening approach, we identified the pH-regulated antigen 1 (Pra1) as a molecule capable of directly binding to mouse CD4⁺ T cells in vitro. Binding of Pra1 to the T cell surface was enhanced by extracellular Zn²⁺ ions which Pra1 is known to scavenge from the host in order to supply the fungus with Zn²⁺. In vitro stimulation assays using highly purified mouse CD4⁺ T cells showed that Pra1 increased proliferation of CD4⁺ T cells in the presence of plate-bound anti-CD3 monoclonal antibody. In contrast, secretion of effector cytokines such as IFNγ and TNF by CD4⁺ T cells upon anti-CD3/ anti-CD28 mAb as well as cognate antigen stimulation was reduced in the presence of Pra1. By secreting Pra1 C. albicans, thus, directly modulates and partially controls CD4⁺ T cell responses as shown in our in vitro assays.

Molecular mechanism of Zn2+ inhibition of a voltage-gated proton channel

Voltage-gated proton (Hv1) channels are involved in many physiological processes, such as pH homeostasis and the innate immune response. Zn²⁺ is an important physiological inhibitor of Hv1. Sperm cells are quiescent in the male reproductive system due to Zn²⁺ inhibition of Hv1 channels, but become active once introduced into the low-Zn²⁺-concentration environment of the female reproductive tract. How Zn²⁺ inhibits Hv1 is not completely understood. In this study, we use the voltage clamp fluorometry technique to identify the molecular mechanism of Zn²⁺ inhibition of Hv1. We find that Zn²⁺ binds to both the activated closed and resting closed states of the Hv1 channel, thereby inhibiting both voltage sensor motion and gate opening. Mutations of some Hv1 residues affect only Zn²⁺ inhibition of the voltage sensor motion, whereas mutations of other residues also affect Zn²⁺ inhibition of gate opening. These effects are similar in monomeric and dimeric Hv1 channels, suggesting that the Zn²⁺-binding sites are localized within each subunit of the dimeric Hv1. We propose that Zn²⁺ binding has two major effects on Hv1: (i) at low concentrations, Zn²⁺ binds to one site and prevents the opening conformational change of the pore of Hv1, thereby inhibiting proton conduction; and (ii) at high concentrations, Zn²⁺, in addition, binds to a second site and inhibits the outward movement of the voltage sensor of Hv1. Elucidating the molecular mechanism of how Zn²⁺ inhibits Hv1 will further our understanding of Hv1 function and might provide valuable information for future drug development for Hv1 channels.

Inhibition of Kv1.3 Channels in Human Jurkat T Cells by Xanthohumol and Isoxanthohumol

Using whole-cell patch-clamp technique, we investigated influence of selected compounds from groups of prenylated chalcones and flavonoids: xanthohumol and isoxanthohumol on the activity of Kv1.3 channels in human leukemic Jurkat T cells. Obtained results provide evidence that both examined compounds were inhibitors of Kv1.3 channels in these cells. The inhibitory effects occurred in a concentration-dependent manner. The estimated value of the half-blocking concentration (EC50) was about 3 μM for xanthohumol and about 7.8 μM for isoxanthohumol. The inhibition of Kv1.3 channels by examined compounds was not complete. Upon an application of the compounds at the maximal concentrations equal to 30 μM, the activity of Kv1.3 channels was inhibited to about 0.13 of the control value. The inhibitory effect was reversible. The application of xanthohumol and isoxanthohumol did not change the currents' activation and inactivation rate. These results may confirm our earlier hypothesis that the presence of a prenyl group in a molecule is a factor that facilitates the inhibition of Kv1.3 channels by compounds from the groups of flavonoids and chalcones. The inhibition of Kv1.3 channels might be involved in antiproliferative and proapoptotic effects of the compounds observed in cancer cell lines expressing these channels.

Extracellular zinc ion regulates transient receptor potential melastatin 5 (TRPM5) channel activation through its interaction with a pore loop domain

The transient receptor potential melastatin 5 (TRPM5) channel is a monovalent cation channel activated by intracellular Ca(2+). Expression of this channel is restricted to taste cells, the pancreas and brainstem, and is thought to be involved in controlling membrane potentials. Its endogenous ligands are not well characterized. Here, we show that extracellular application of Zn(2+) inhibits TRPM5 activity. In whole-cell patch-clamp recordings, extracellular application of ZnCl2 inhibited step-pulse-induced TRPM5 currents with 500 nM free intracellular Ca(2+) in a dose-dependent manner (IC50 = 4.3 μM at -80 mV). ZnSO4 also inhibited TRPM5 activity. Extracellular application of ZnCl2 inhibited TRPM5 activation at several temperatures. Furthermore, inhibition by 30 μM ZnCl2 was impaired in TRPM5 mutants in which His at 896, and Glu at 926 and/or Glu at 939 in the outer pore loop were replaced with Gln. From these results, we conclude that extracellular Zn(2+) inhibits TRPM5 channels, and the residues in the outer pore loop of TRPM5 are critically involved in the inhibition.

The influence of 8-prenylnaringenin on the activity of voltage-gated Kv1.3 potassium channels in human Jurkat T cells

Using the whole-cell patch-clamp technique, we investigated the influence of 8-prenylnaringenin on the activity of the voltage-gated Kv1.3 potassium channels in the human leukemic T lymphocyte cell line Jurkat. 8-prenylnaringenin is a potent plant-derived phytoestrogen that has been found to inhibit cancer cell proliferation. The results show that it inhibited the Kv1.3 channels in a concentration-dependent manner. Complete inhibition occurred at concentrations higher than 10 μM. The inhibitory effect of 8-prenylnaringenin was reversible. It was accompanied by a significant acceleration of channel inactivation without any pronounced change in the activation rate. Of the naringenin derivatives tested to date, 8-prenylnaringenin is the most potent inhibitor of the Kv1.3 channels. The potency of the inhibition may be due to the presence of a prenyl group in the molecule of this flavonoid. The inhibition of the Kv1.3 channels might be involved in the antiproliferative and pro-apoptotic effects of 8-prenylnaringenin that have been observed in cancer cell lines expressing these channels.

Zinc inactivates melastatin transient receptor potential 2 channels via the outer pore

Zinc ion (Zn(2+)) is an endogenous allosteric modulator that regulates the activity of a wide variety of ion channels in a reversible and concentration-dependent fashion. Here we used patch clamp recording to study the effects of Zn(2+) on the melastatin transient receptor potential 2 (TRPM2) channel. Zn(2+) inhibited the human (h) TRPM2 channel currents, and the steady-state inhibition was largely not reversed upon washout and concentration-independent in the range of 30-1000 μM, suggesting that Zn(2+) induces channel inactivation. Zn(2+) inactivated the channels fully when they conducted inward currents, but only by half when they passed outward currents, indicating profound influence of the permeant ion on Zn(2+) inactivation. Alanine substitution scanning mutagenesis of 20 Zn(2+)-interacting candidate residues in the outer pore region of the hTRPM2 channel showed that mutation of Lys(952) in the extracellular end of the fifth transmembrane segment and Asp(1002) in the large turret strongly attenuated or abolished Zn(2+) inactivation, and mutation of several other residues dramatically changed the inactivation kinetics. The mouse (m) TRPM2 channels were also inactivated by Zn(2+), but the kinetics were remarkably slower. Reciprocal mutation of His(995) in the hTRPM2 channel and the equivalent Gln(992) in the mTRPM2 channel completely swapped the kinetics, but no such opposing effects resulted from exchanging another pair of species-specific residues, Arg(961)/Ser(958). We conclude from these results that Zn(2+) inactivates the TRPM2 channels and that residues in the outer pore are critical determinants of the inactivation.

Influence of specific immunotherapy on the activity of human T lymphocyte Kv1.3 voltage-gated potassium channels in insect venom allergic patients

Kv1.3 channels play an important role in T lymphocytes function. CD4(+) and CD4(+)CD25(+) T cells are two broad categories of T cells that are critically involved in the immunoresponse to allergens and that are also a major target for allergen immunotherapy. The aim of the study was to evaluate the effects of venom immunotherapy (VIT) on the activity of Kv1.3. channels on noncultured subsets: CD4(+) and CD4(+)CD25(+) T cells of insect venom allergic patients. Eleven patients with allergic reactions to bee or wasp venoms participated in the study. The patients were provided VIT according to the ultrarush protocol. CD4(+) and CD4(+)CD25(+) T cells were isolated from peripheral blood mononuclear cells of VIT-treated patients by an immunomagnetic method. We used the whole-cell patch clamp technique to investigate the whole potassium chord conductance (gK) of Kv1.3. channels in CD4(+) and CD4(+)CD25(+) T cells of venom-sensitive patients before and during the course of VIT. The conductance of Kv1.3. channels on CD4(+)CD25(+) T cells decreased during the course of VIT. On day 0 it was 0.054 ± 0.07 [nS], and on day 70 it was 0.008 ± 0.09 [nS] (P = 0.03). The observed decrease of the gK of the Kv1.3 channels in the subpopulation of activated T cells may contribute to T cell tolerance and functional unresponsiveness of these cells to allergen in the early stages of VIT.

The effect of zinc on glycinergic inhibitory postsynaptic currents in rat spinal dorsal horn neurons

The effect of zinc on glycinergic spontaneous inhibitory postsynaptic currents (IPSCs) was investigated using the whole-cell patch-clamp technique in mechanically dissociated rat spinal dorsal horn neurons. Zinc at a concentration of 10 microM reversibly increased the spontaneous IPSC frequency without changing the current amplitudes, suggesting that zinc increases spontaneous glycine release from presynaptic nerve terminals. At a low concentration of 1 microM, on the other hand, zinc potentiated the amplitude of spontaneous IPSCs but had no effect on the frequency. At a high concentration of 100 microM, zinc increased the spontaneous IPSC frequency while it inhibited the IPSC amplitude. The current evoked by exogenously applied glycine was potentiated and inhibited by low and high concentrations of zinc, respectively. The increase in spontaneous IPSC frequency by 10 microM zinc was inhibited by blocking the voltage-dependent Ca(2+) channels in the presence of both omega-conotoxin-MVIIC and nifedipine. The facilitatory effect of zinc on spontaneous IPSC frequency was also inhibited in the presence of tetrodotoxin. In the slice preparation, 30 microM zinc potentiated the evoked IPSC amplitude and decreased the paired pulse ratio. These results suggest that, in addition to an action on the postsynaptic glycine receptors, zinc may depolarize the presynaptic nerve terminals, leading to an activation of voltage-dependent Na(+) and Ca(2+) channels that in turn increases glycine release. Since dorsal horn neurons receive nociceptive inputs, zinc may play an important role in the regulation of sensory transmission.

Inhibition of the Activity of Human Lymphocyte Kv1.3 Potassium Channels by Resveratrol

The whole-cell patch-clamp technique was applied to study the modulatory effect of resveratrol on voltage-gated potassium channel Kv1.3 expressed in human lymphocytes. Results demonstrate that application of resveratrol in the concentration range 1-200 muM: inhibited the channel activity in a concentration-dependent manner to about 18% of the control value. The half-blocking concentration of resveratrol was 40.9 microM: , whereas the Hill coefficient was 1.05. The inhibition was time-dependent and slowly reversible. The inhibitory effect of resveratrol was correlated in time with a significant slowing of the current activation, whereas the inactivation rate remained unaffected upon application of resveratrol. The inhibition of Kv1.3 channels was voltage-independent. The steady-state activation of the currents remained unchanged upon resveratrol application. The magnitude of the inhibitory effect of resveratrol was not altered when resveratrol was coapplied with genistein. The possible mechanism of the inhibitory effect and its significance for biological activity of resveratrol are discussed.

The influence of protons and zinc ions on the steady-state inactivation of Kv1.3 potassium channels

Using the whole-cell patch-clamp technique, we investigated the influence of extracellular pH and zinc ions (Zn(2+)) on the steady-state inactivation of Kv1.3 channels expressed in human lymphocytes. The obtained data showed that lowering the extracellular pH from 7.35 to 6.8 shifted the inactivation midpoint (V(i)) by 17.4 +/- 1.12 mV (n = 6) towards positive membrane potentials. This shift was statistically significant (p < 0.05). Applying 100 microM Zn(2+) at pH 6.8 further shifted the Vi value by 16.55 +/- 1.80 mV (n = 6) towards positive membrane potentials. This shift was also statistically significant (p < 0.05). The total shift of the Vi by protons and Zn(2+) was 33.95 +/- 1.90 mV (n = 6), which was significantly higher (p < 0.05) than the shift caused by Zn(2+) alone. The Zn(2+)-induced shift of the V(i) at pH 6.8 was almost identical to the shift at pH = 7.35. Thus, the proton-and Zn(2+)-induced shifts of the V(i) value were additive. The steady-state inactivation curves as a function of membrane voltage were compared with the functions of the steady-state activation. The total shift of the steady-state inactivation was almost identical to the total shift of the steady-state activation (32.01 +/- 2.10 mV, n = 10). As a result, the "windows" of membrane potentials in which the channels can be active under physiological conditions were also markedly shifted towards positive membrane potentials. The values of membrane voltage and the normalised chord conductance corresponding to the points of intersection of the curves of steady-state activation and inactivation were also calculated. The possible physiological significance of the observed modulatory effects is discussed herein.

Subunit-specific modulation of T-type calcium channels by zinc

Zinc (Zn2+) functions as a signalling molecule in the nervous system and modulates many ionic channels. In this study, we have explored the effects of Zn2+ on recombinant T-type calcium channels (CaV3.1, CaV3.2 and CaV3.3). Using tsA-201 cells, we demonstrate that CaV3.2 current (IC50, 0.8 microm) is significantly more sensitive to Zn2+ than are CaV3.1 and CaV3.3 currents (IC50, 80 microm and approximately 160 microm, respectively). This inhibition of CaV3 currents is associated with a shift to more negative membrane potentials of both steady-state inactivation for CaV3.1, CaV3.2 and CaV3.3 and steady-state activation for CaV3.1 and CaV3.3 currents. We also document changes in kinetics, especially a significant slowing of the inactivation kinetics for CaV3.1 and CaV3.3, but not for CaV3.2 currents. Notably, deactivation kinetics are significantly slowed for CaV3.3 current (approximately 100-fold), but not for CaV3.1 and CaV3.2 currents. Consequently, application of Zn2+ results in a significant increase in CaV3.3 current in action potential clamp experiments, while CaV3.1 and CaV3.2 currents are significantly reduced. In neuroblastoma NG 108-15 cells, the duration of CaV3.3-mediated action potentials is increased upon Zn2+ application, indicating further that Zn2+ behaves as a CaV3.3 channel opener. These results demonstrate that Zn2+ exhibits differential modulatory effects on T-type calcium channels, which may partly explain the complex features of Zn2+ modulation of the neuronal excitability in normal and disease states.

The influence of zinc on the modulatory effect of sphingosylphosphorylcholine on Kv1.3 channels in human T lymphocytes

In the present study, the whole-cell patch-clamp technique was applied to investigate the influence of co-application of zinc ions and sphingosylphosphorylcholine (SPC) on the SPC-induced shift of the activation midpoint and slowing of activation kinetics of Kv1.3 channels in human T lymphocytes. The results obtained provided evidence that the effects exerted by SPC and Zn were not additive. The shift was significantly diminished in a concentration-dependent manner upon co-application of 10 microM SPC and Zn in the concentration range 10-300 microM. However, the shift was not abolished in the presence of 100 and 300 microM of Zn co-applied with SPC. It was shown that the extent of the shift upon SPC and Zn co-application was similar to the shift observed for Zn applied without SPC. The slowing of the activation kinetics was also diminished upon SPC and Zn co-application; however, no clear dependence on concentration was observed. Moreover, the slowing was not abolished in the presence of 100 and 300 microM of Zn. It was shown that the slowing of the activation kinetics upon Zn and SPC co-application was primarily due to the effect exerted by SPC. The steepness of the voltage dependence of steady-state activation of the channels was not changed upon SPC and Zn co-application. Possible mechanisms underlying the observed phenomena and their possible physiological significance are discussed.

The voltage- and time-dependent blocking effect of trifluoperazine on T lymphocyte Kv1.3 channels

Phenothiazines are well-known calmodulin inhibitors that interact with many receptors and channels including a variety of potassium channels. In this study, we report a blocking effect of trifluoperazine (TFP) on voltage-gated Kv1.3 channels expressed in human T lymphocytes. Application of TFP in the concentration range from 1 to 20 microM reduced the current amplitude to about a half of the control value. The currents were blocked to less than 0.05 of the control value at 50 microM TFP concentration. The blocking effect was accompanied by a substantial increase in the current inactivation rate, whereas the activation rate and the steady-state activation and inactivation were not changed significantly. The blocking effect of TFP was voltage dependent being most potent at +60mV and least potent at -20mV. The blocking effect of TFP on the currents and the recovery from block was time dependent. Other calmodulin antagonists: tamoxifen (TMX) and thioridazine also inhibited the channels at micromolar concentrations. The effects exerted by TMX and thioridazine resembled the inhibitory effect of TFP. The blocking effect of thioridazine was time dependent and appeared to be more potent that the inhibition by TFP and TMX. TFP, TMX and thioridazine inhibited the activity of Kv1.3 channels only when applied extracellularly. The inhibitory effect of all the compounds was reversible. The possible physiological significance of the current inhibition is discussed.