Extracellular protease trypsin activates amiloride-insensitive sodium channels in human leukemia cells

Sodium influx is tightly regulated in the cells of blood origin. Amiloride-insensitive sodium channels were identified as one of the main sodium-transporting pathways in leukemia cells. To date, all known regulatory pathways of these channels are coupled with intracellular actin cytoskeleton dynamics. Here, to search for physiological mechanisms controlling epithelial Na⁺ channel (ENaC)-like channels, we utilized leukemia K562 cells as a unique model to examine single channel behavior in a whole-cell patch-clamp experiments. We have shown for the first time that extracellular serine protease trypsin directly activates sodium channels in plasma membrane of K562 cells. The whole-cell single current recordings clearly demonstrate no inhibition of trypsin-activated channels by amiloride or benzamil. Involvement of proteolytic cleavage in channel opening was confirmed in experiments with soybean trypsin inhibitor. More importantly, stabilization of F-actin with intracellular phalloidin did not prevent trypsin-induced channel activation indicating no implication of cytoskeleton rearrangements in stimulatory effect of extracellular protease. Our data reveals a novel mechanism modulating amiloride-insensitive ENaC-like channel activity and integral sodium permeability in leukemia cells.

Senescence-messaging secretome factors trigger premature senescence in human endometrium-derived stem cells

Accumulating evidence suggests that the senescence-messaging secretome (SMS) factors released by senescent cells play a key role in cellular senescence and physiological aging. Phenomenon of the senescence induction in human endometrium-derived mesenchymal stem cells (MESCs) in response to SMS factors has not yet been described. In present study, we examine a hypothesis whether the conditioned medium from senescent cells (CM-old) may promote premature senescence of young MESCs. In this case, we assume that SMS factors, containing in CM-old are capable to trigger senescence mechanism in a paracrine manner. A long-term cultivation MESCs in the presence of CM-old caused deceleration of cell proliferation along with emerging senescence phenotype, including increase in both the cell size and SA-β-Gal activity. The phosphorylation of p53 and MAPKAPK-2, a direct target of p38MAPK, as well as the expression of p21Cip1 and p16Ink4a were increased in CM-old treated cells with senescence developing whereas the Rb phosphorylation was diminished. The senescence progression was accompanied by both enhanced ROS generation and persistent activation of DNA damage response, comprising protein kinase ATM, histone H2A.X, and adapter protein 53BP1. Thus, we suggest that a senescence inducing signal is transmitted through p16/MAPKAPK-2/Rb and DDR-mediated p53/p21/Rb signaling pathways. This study is the first to demonstrate that the SMS factors secreted in conditioned medium of senescent MESCs trigger a paracrine mechanism of premature senescence in young cells.

Time-Dependent Regulation of IL-2R α-Chain (CD25) Expression by TCR Signal Strength and IL-2-Induced STAT5 Signaling in Activated Human Blood T Lymphocytes

The expression of the IL-2R α-chain (IL-2Rα) is regulated at the transcriptional level via TCR- and IL-2R-signaling. The question is how to precede in time the activation signals to induce the IL-2Rα expression in native primary T cells. By comparing the effects of selective drugs on the dynamics of CD25 expression during the mitogen stimulation of human peripheral blood lymphocytes, we identified distinct Src- and JAK-dependent stages of IL-2Rα upregulation. PP2, a selective inhibitor of TCR-associated Src kinase, prevents CD25 expression at initial stages of T cell activation, prior to the cell growth. This early IL-2Rα upregulation underlies the T cell competence and the IL-2 responsiveness. We found that the activated with “weak” mitogen, the population of blood lymphocytes has some pool of competent CD25+ cells bearing a high affinity IL-2R. A distinct pattern of IL-2R signaling in resting and competent T lymphocytes has been shown. Based on the inhibitory effect of WHI-P131, a selective drug of JAK3 kinase activity, we concluded that in quiescent primary T lymphocytes, the constitutive STAT3 and the IL-2-induced prolonged STAT5 activity (assayed by tyrosine phosphorylation) is mostly JAK3-independent. In competent T cells, in the presence of IL-2 JAK3/STAT5 pathway is switched to maintain the higher and sustained IL-2Rα expression as well as cell growth and proliferation. We believe that understanding the temporal coordination of antigen- and cytokine-evoked signals in primary T cells may be useful for improving immunotherapeutic strategies.

Endogenous expression of TRPV5 and TRPV6 calcium channels in human leukemia K562 cells

In blood cells, changes in intracellular Ca2+ concentration ([Ca2+]i) are associated with multiple cellular events, including activation of cellular kinases and phosphatases, degranulation, regulation of cytoskeleton binding proteins, transcriptional control, and modulation of surface receptors. Although there is no doubt as to the significance of Ca2+ signaling in blood cells, there is sparse knowledge about the molecular identities of the plasmalemmal Ca2+ permeable channels that control Ca2+ fluxes across the plasma membrane and mediate changes in [Ca2+]i in blood cells. Using RNA expression analysis, we have shown that human leukemia K562 cells endogenously coexpress transient receptor potential vanilloid channels type 5 (TRPV5) and type 6 (TRPV6) mRNAs. Moreover, we demonstrated that TRPV5 and TRPV6 channel proteins are present in both the total lysates and the crude membrane preparations from leukemia cells. Immunoprecipitation revealed that a physical interaction between TRPV5 and TRPV6 may take place. Single-channel patch-clamp experiments demonstrated the presence of inwardly rectifying monovalent currents that displayed kinetic characteristics of unitary TRPV5 and/or TRPV6 currents and were blocked by extracellular Ca2+ and ruthenium red. Taken together, our data strongly indicate that human myeloid leukemia cells coexpress functional TRPV5 and TRPV6 calcium channels that may interact with each other and contribute into intracellular Ca2+ signaling.

Analysis of the monovalent ion fluxes in U937 cells under the balanced ion distribution: recognition of ion transporters responsible for changes in cell ion and water balance during apoptosis

Unidirectional (22)Na, Li(+) and Rb(+) fluxes and net fluxes of Na(+) and K(+) were measured in U937 human leukemic cells before and after induction of apoptosis by staurosporine (1 microM, 4 h) to answer the question which ion transporter(s) are responsible for changes in cell ion and water balance at apoptosis. The original version of the mathematical model of cell ion and water balance was used for analysis of the unidirectional ion fluxes under the balanced distribution of major monovalent ions across the cell membrane. The values of all major components of the Na(+) and K(+) efflux and influx, i.e. fluxes via the Na(+),K(+)-ATPase pump, Na(+) channels, K(+) channels, Na/Na exchanger and Na-Cl symport were determined. It is concluded that apoptotic cell shrinkage and changes in Na(+) and K(+) fluxes typical of apoptosis in U937 cells induced by staurosporine are caused by a complex decrease in the pump activity, Na-Cl symport and integral Na(+) channel permeability.

Thymocyte K+, Na+ and Water Balance During Dexamethasone- and Etoposide-Induced Apoptosis

The mechanism of apoptotic cell volume decrease was studied in rat thymocytes treated with dexamethasone (Dex) or etoposide (Eto). Cell shrinkage, i.e. dehydration, was quantified by using buoyant density of the thymocytes in a continuous Percoll gradient. The K+ and Na+ content of cells from different density fractions were assayed by flame emission analysis. Apoptosis was tested by microscopy and flow cytometry of acridine orange stained cells as well as by flow DNA cytometry. Treatment of the thymocytes with 1 microM Dex for 4-5.5 h or 50 microM Eto for 5 h resulted in the appearance of a new distinct high-density cell subpopulation. The cells from this heavy subpopulation but not those with normal buoyant density had typical features of apoptosis. Apoptotic increase of cell density was accompanied by a decrease in cellular K+ content, which exceeded the simultaneous increase in cellular Na+ content. Cellular loss of K+ contributed to most of the estimated loss of cellular osmolytes, but owing to the parallel loss of cell water, the decrease in cytosolic K+ concentration was less than one third. Due to gain of Na+ and loss of cell water the cytosolic Na+ concentration in thymocytes rose following treatment with Dex (5.5 h) or Eto (5 h) by a factor of about 3.6 and 3.1, respectively.

Properties of Mg2+-dependent cation channels in human leukemia K562 cells

The endogenous Mg(2+)-inhibited cation (MIC) current was recently described in different cells of hematopoietic lineage and was implicated in the regulation of Mg2+ homeostasis. Here we present a single channel study of endogenously expressed Mg(2+)-dependent cation channels in the human myeloid leukemia K562 cells. Inwardly directed unitary currents were activated in cell-attached experiments in the absence of Ca2+ and Mg2+ in the pipette solution. The current-voltage (I-V) relationships displayed strong inward rectification and yielded a single channel slope conductance of approximately 30 pS at negative potentials. The I-V relationships were not altered by patch excision into divalent-free solution. Channel open probability (P(o)) and mean closed time constant (tau(C)) were strongly voltage-dependent, indicating that gating mechanisms may underlie current inward rectification. Millimolar concentrations of Ca2+ or Mg2+ applied to the cytoplasmic side of the membrane produced slow irreversible inhibition of channel activity. The Mg(2+)-dependent cation channels described in this study differ from the MIC channels described in human T-cells, Jurkat, and rat basophilic leukemia (RBL) cells in their I-V relationships, kinetic parameters and dependence on intracellular divalent cations. Our results suggested that endogenously expressed Mg(2+)-dependent cation channels in K562 cells and the MIC channels in other hematopoietic cells might be formed by different channel proteins.

Differential transcription of ion transporters, NHE1, ATP1B1, NKCC1 in human peripheral blood lymphocytes activated to proliferation

This work, using RT PCR, studied expression of mRNAs encoding ion transporters, the Na/H antiporter (NHE1), the beta subunit of the Na,K-ATPase pump (ATP1B1), the NaK2Cl symporter (NKCC1), and some proteins unrelated to ion transport: the serum and glucocorticoid dependent kinase (hSGK), beta-actin, a glycolytic enzyme (GAPDH), and regulators of proliferation and apoptosis (p53, Bcl-2) during activation of human lymphocytes with phytohemagglutinin for 4-24 h. Within 24 hours the mRNA levels of NHE1, beta-actin, Bcl-2, and p53 increased by more than 100%, the mRNA levels of ATP1B1, GAPDH, and hSGK, by about 50%, while the mRNA levels of NKCC1 decreased transiently. These results indicate a differential transcriptional control of NHE1, ATP1B1, and NKCC1 following a proliferative stimulus of human lymphocytes.