Role of potassium and chlorine channels in the regulation of thymocyte volume in rats

Inorganic polyphosphate regulates functions of thymocytes via activation of P2X purinoreceptors

Inorganic polyphosphate (polyP) is an ancient polymer, which was proven to be a signalling molecule in the mammalian brain, mediating the communication between astrocytes via activation of P2Y1 purinoreceptors and modulating the activity of neurons. There is very limited information regarding the ability of polyP to transmit the information as an agonist of purinoreceptors in other cells and tissues. Here, we show that application of polyP to the suspension of primary thymocytes increases the concentration of intracellular calcium. PolyP evoked calcium signal was dependent on the presence of P2X inhibitors but not P2Y1 inhibitor. PolyP dependent increase in intracellular calcium concentration caused mild mitochondrial depolarization, which was dependent on inhibitors of purinoreceptors, extracellular calcium and inhibitor of mitochondrial calcium uniporter but wasn't dependent on cyclosporin A. Application of polyP modulated cell volume regulation machinery of thymocytes in calcium dependent manner. Molecular docking experiments revealed that polyP can potentially bind to several types of P2X receptors with binding energy similar to ATP - natural agonist of P2X purinoreceptors. Further molecular dynamics simulations with P2X4 showed that binding of one molecule of polyP dramatically increases permeability of this receptor-channel for water molecules. Thus, in this research we for the first time showed that polyP can interact with P2X receptors in thymocytes and modulate physiological processes.

Effect of Glycyrrhetic Acid Derivatives on Regulation of Thymocyte Volume

We studied the effects of glycyrrhetinic acid (bioactive aglycone of glycyrrhizin) and its ester derivatives at positions C-3 and C-30 on the cell volume regulation in rat thymocytes under conditions of hypoosmotic stress. Native glycyrrhetinic acid completely suppressed this process with half-maximal concentration of 12.7±1.4 μM and Hill coefficient of 3.1±0.6. Formation of esters at C-3 (esters with the acetic, cinnamic and methoxi-cinnamic acid) and at C-30 (methyl ester) drastically decreased the inhibitory activity of the molecule, suggesting that intact hydroxyl group at C-3 and carboxyl group at C-30 are structurally important determinants of biological activity of glycyrrhetinic acid towards volume regulation of thymic lymphocytes.

Lytic and sublytic effects of gossypol on red blood cells and thymocytes

Gossypol is a natural polyphenol presently considered as a promising biological phytochemical with a range of activities including anticancer. We examined volume regulation-dependent effects of gossypol using erythrocytes and thymic lymphocytes. Gossypol effectively lysed human red blood cells (RBC) with a half-maximal concentration of 67.4 ± 1.6 μmol/L and in a non-colloid osmotic manner. Sublytic gossypol doses of 1-10 μmol/L significantly protected RBC from osmotic hemolysis, but potentiated their sensitivity to the colloid-osmotic lysis induced by a pore-former nystatin. When added to the thymocytes suspension, gossypol caused a strong depression of the ability of cells to restore their volume under hypoosmotic stress with a half-maximal activity at 2.1 ± 0.3 μmol/L. Gossypol suppressed regulatory volume decrease under experimental conditions, when cationic permeability was controlled by gramicidin D, and volume recovery depended mainly on anionic conductance, suggesting that the polyphenol inhibits the swelling-induced anion permeability. In direct patch-clamp experiments, gossypol inhibited the volume-sensitive outwardly rectifying (VSOR) chloride channel in thymocytes and in human HCT116 and HeLa cells, possibly by a mechanism when gossypol molecule with a radius close to the size of channel pore plugs into the narrowest portion of the native VSOR chloride channel. Micromolar gossypol suppressed proliferation of thymocytes, HCT116 and HeLa cells. VSOR blockage may represent new mechanism of anticancer activity of gossypol in addition to its action as a BH3-mimetic.

Effect of plant flavonoids on the volume regulation of rat thymocytes under hypoosmotic stress

BACKGROUND

Cell volume regulation and volume-regulated anion channels are critical for cell survival in non-isosmotic conditions, and dysregulation of this system is detrimental. Although genes and proteins underlying this basic cellular machinery were recently identified, the pharmacology remains poorly explored.

METHODS

We examined effects of 16 flavonoids on the regulatory volume decrease (RVD) of thymocytes under hypoosmotic stress assessed by light transmittance and on the activity of volume-sensitive chloride channel by patch-clamp technique.

RESULTS

Comparison of effects of flavonoids on RVD revealed a group of four active substances with lehmannin being the strongest inhibitor (IC50 = 8.8 μM). Structure-functional comparison suggested that hydrophobicity brought about by methoxy, prenyl or lavandulyl groups as well as by the absence of glucosyl fragment together with localization of the phenyl ring B at the position C2 (which is at C3 in totally inactive isoflavones) are important structural determinants for the flavonoids activity as volume regulation inhibitors. All active flavonoids suppressed RVD under Gramicidin D-NMDG hypotonic stress conditions when cationic permeability was increased by an ionophore, gramicidin D, with all extracellular monovalent cations replaced with bulky NMDG+ suggesting that they target volume-sensitive anionic permeability. While effects of hispidulin and pulicarin were only partial, lehmannin and pinocembrin completely abolished RVD under Gramicidin D-NMDG conditions. In direct patch-clamp experiments, lehmannin and pinocembrin produced a strong inhibiting effect on the swelling-induced whole-cell chloride conductance in a voltage-independent manner.

CONCLUSION

Lehmannin, pinocembrin, and possibly hispidulin and pulicarin may serve as leads for developing effective low-toxic immunomodulators.

Tannins, novel inhibitors of the volume regulation and the volume-sensitive anion channel

The volume-sensitive outwardly rectifying anion channel (VSOR) is a key component of volume regulation system critical for cell survival in non-isosmotic conditions. The aim of the present study was to test the effects of four tannin extracts with defined compositions on cell volume regulation and VSOR. Preparation I (98% of hydrolysable tannins isolated from leaves of sumac Rhus typhina L.) and Preparation II (100% of hydrolysable tannins isolated from leaves of broadleaf plantain Plantago major L) completely and irreversibly abolished swelling-activated VSOR currents in HCT116 cells. Both preparations profoundly suppressed the volume regulation in thymocytes with half-maximal effects of 40.9 μg/ml and 12.3 μg/ml, respectively. The inhibition was more efficient at lower concentrations but reverted at higher doses due to possible non-specific membrane-permeabilizing activity. Preparations III and IV (54,7% and 54.3% of hydrolysable tannins isolated, respectively, from roots and aboveground parts of Fergana spurge Euphorbia ferganensis B.Fedtch) inhibited VSOR activity in a partially reversible manner and suppressed the volume regulation with substantially higher half-maximal doses of 270 and 278 μg/ml, respectively, with no secondary reversion at higher doses. Hydrolysable tannins represent a novel class of VSOR channel inhibitors with the capacity to suppress the cell volume regulation machinery.

Volume-sensitive anion channels mediate osmosensitive glutathione release from rat thymocytes

Glutathione (GSH) is a negatively charged tripeptide, which is a major determinant of the cellular redox state and defense against oxidative stress. It is assembled inside and degraded outside the cells and is released under various physiological and pathophysiological conditions. The GSH release mechanism is poorly understood at present. In our experiments, freshly isolated rat thymocytes were found to release GSH under normal isotonic conditions at a low rate of 0.82±0.07 attomol/cell/min and that was greatly enhanced under hypoosomotic stimulation to reach a level of 6.1±0.4 attomol/cell/min. The swelling-induced GSH release was proportional to the cell density in the suspension and was temperature-dependent with relatively low activation energy of 5.4±0.6 kcal/mol indicating a predominant diffusion mechanism of GSH translocation. The osmosensitive release of GSH was significantly inhibited by blockers of volume-sensitive outwardly rectifying (VSOR) anion channel, DCPIB and phloretin. In patch-clamp experiments, osmotic swelling activated large anionic conductance with the VSOR channel phenotype. Anion replacement studies suggested that the thymic VSOR anion channel is permeable to GSH(-) with the permeability ratio P(GSH)/P(Cl) of 0.32 for influx and 0.10 for efflux of GSH. The osmosensitive GSH release was trans-stimulated by SLCO/OATP substrates, probenecid, taurocholic acid and estrone sulfate, and inhibited by an SLC22A/OAT blocker, p-aminohippuric acid (PAH). The inhibition by PAH was additive to the effect of DCPIB or phloretin implying that PAH and DCPIB/phloretin affected separate pathways. We suggest that the VSOR anion channel constitutes a major part of the γ-glutamyl cycle in thymocytes and, in cooperation with OATP-like and OAT-like transporters, provides a pathway for the GSH efflux from osmotically swollen cells.

Swelling-activated anion channels are essential for volume regulation of mouse thymocytes

Channel-mediated trans-membrane chloride movement is a key process in the active cell volume regulation under osmotic stress in most cells. However, thymocytes were hypothesized to regulate their volume by activating a coupled K-Cl cotransport mechanism. Under the patch-clamp, we found that osmotic swelling activates two types of macroscopic anion conductance with different voltage-dependence and pharmacology. At the single-channel level, we identified two types of events: one corresponded to the maxi-anion channel, and the other one had characteristics of the volume-sensitive outwardly rectifying (VSOR) chloride channel of intermediate conductance. A VSOR inhibitor, phloretin, significantly suppressed both macroscopic VSOR-type conductance and single-channel activity of intermediate amplitude. The maxi-anion channel activity was largely suppressed by Gd(3+) ions but not by phloretin. Surprisingly, [(dihydroindenyl)oxy] alkanoic acid (DIOA), a known antagonist of K-Cl cotransporter, was found to significantly suppress the activity of the VSOR-type single-channel events with no effect on the maxi-anion channels at 10 μM. The regulatory volume decrease (RVD) phase of cellular response to hypotonicity was mildly suppressed by Gd(3+) ions and was completely abolished by phloretin suggesting a major impact of the VSOR chloride channel and modulatory role of the maxi-anion channel. The inhibitory effect of DIOA was also strong, and, most likely, it occurred via blocking the VSOR Cl(-) channels.