Cell-Cycle-Dependent Regulation of Ca2+-Activated K+ Channel in Jurkat T-Lymphocyte

K+ and Ca2+ Channels Regulate Ca2+ Signaling in Chondrocytes: An Illustrated Review

An improved understanding of fundamental physiological principles and progressive pathophysiological processes in human articular joints (e.g., shoulders, knees, elbows) requires detailed investigations of two principal cell types: synovial fibroblasts and chondrocytes. Our studies, done in the past 8–10 years, have used electrophysiological, Ca²⁺ imaging, single molecule monitoring, immunocytochemical, and molecular methods to investigate regulation of the resting membrane potential (E_R) and intracellular Ca²⁺ levels in human chondrocytes maintained in 2-D culture. Insights from these published papers are as follows: (1) Chondrocyte preparations express a number of different ion channels that can regulate their E_R. (2) Understanding the basis for E_R requires knowledge of (a) the presence or absence of ligand (ATP/histamine) stimulation and (b) the extraordinary ionic composition and ionic strength of synovial fluid. (3) In our chondrocyte preparations, at least two types of Ca²⁺-activated K⁺ channels are expressed and can significantly hyperpolarize E_R. (4) Accounting for changes in E_R can provide insights into the functional roles of the ligand-dependent Ca²⁺ influx through store-operated Ca²⁺ channels. Some of the findings are illustrated in this review. Our summary diagram suggests that, in chondrocytes, the K⁺ and Ca²⁺ channels are linked in a positive feedback loop that can augment Ca²⁺ influx and therefore regulate lubricant and cytokine secretion and gene transcription.

Inhibition of TRPC6 reduces non-small cell lung cancer cell proliferation and invasion

Recent studies indicate that the transient receptor potential canonical 6 (TRPC6) channel is highly expressed in several types of cancer cells. However, it remains unclear whether TRPC6 contributes to the malignancy of human non-small cell lung cancer (NSCLC). We used a human NSCLC A549 cell line as a model and found that pharmacological blockade or molecular knockdown of TRPC6 channel inhibited A549 cell proliferation by arresting cell cycle at the S-G2M phase and caused a significant portion of cells detached and rounded-up, but did not induce any types of cell death. Western blot and cell cycle analysis show that the detached round cells at the S-G2M phase expressed more TRPC6 than the still attached polygon cells at the G1 phase. Patch-clamp data also show that TRPC whole-cell currents in the detached cells were significantly higher than in the still attached cells. Inhibition of Ca²⁺-permeable TRPC6 channels significantly reduced intracellular Ca²⁺ in A549 cells. Interestingly, either blockade or knockdown of TRPC6 strongly reduced the invasion of this NSCLC cell line and decreased the expression of an adherent protein, fibronectin, and a tight junction protein, zonula occluden protein-1 (ZO-1). These data suggest that TRPC6-mediated elevation of intracellular Ca²⁺ stimulates NSCLC cell proliferation by promoting cell cycle progression and that inhibition of TRPC6 attenuates cell proliferation and invasion. Therefore, further in vivo studies may lead to a consideration of using a specific TRPC6 blocker as a complement to treat NSCLC.

Molecular identification of the dominant-negative, splicing isoform of the two-pore domain K(+) channel K(2P)5.1 in lymphoid cells and enhancement of its expression by splicing inhibition

The two-pore domain background K(+) channel K2P5.1 is expected as a possible therapeutic target for autoimmune and inflammatory disorders and cancers because it plays an important role in maintaining the resting membrane potential and regulation of Ca(2+) signaling in T lymphocytes and cancer cells. However, the lack of selective K2P5.1 blockers has led to difficulties conducting experimental studies on this K(+) channel. We identified a novel splicing isoform of K2P5.1, K2P5.1B from the mammalian spleen, which lacked the N-terminus of full-length K2P5.1A. A co-immunoprecipitation assay using mice spleen lysates revealed an interaction between K2P5.1A and K2P5.1B in the cytoplasmic C-terminal domain. In a heterologous HEK293 expression system, K2P5.1B inhibited the trafficking of K2P5.1A to the plasma membrane. The alkaline pHe-induced hyperpolarizing response was significantly suppressed in K2P5.1B-transfected human leukemia K562 cells. Enhancement in cell proliferation by the overexpression of K2P5.1A in K562 was significantly prevented by the transfection of K2P5.1B. The spliceosome inhibitor pladienolide B significantly enhanced the relative expression of K2P5.1B in K562, resulting in decreases in the activity of K2P5.1A. K2P5.1B suppresses the function of the K2P5.1 K(+) channel in a dominant-negative manner, suggesting that the mRNA splicing mechanisms underlying the transcriptional regulation of K2P5.1B may be a new therapeutic strategy for autoimmune and inflammatory disorders and cancers.

Placing ion channels into a signaling network of T cells: from maturing thymocytes to healthy T lymphocytes or leukemic T lymphoblasts

T leukemogenesis is a multistep process, where the genetic errors during T cell maturation cause the healthy progenitor to convert into the leukemic precursor that lost its ability to differentiate but possesses high potential for proliferation, self-renewal, and migration. A new misdirecting "leukemogenic" signaling network appears, composed by three types of participants which are encoded by (1) genes implicated in determined stages of T cell development but deregulated by translocations or mutations, (2) genes which normally do not participate in T cell development but are upregulated, and (3) nondifferentially expressed genes which become highly interconnected with genes expressed differentially. It appears that each of three groups may contain genes coding ion channels. In T cells, ion channels are implicated in regulation of cell cycle progression, differentiation, activation, migration, and cell death. In the present review we are going to reveal a relationship between different genetic defects, which drive the T cell neoplasias, with calcium signaling and ion channels. We suggest that changes in regulation of various ion channels in different types of the T leukemias may provide the intracellular ion microenvironment favorable to maintain self-renewal capacity, arrest differentiation, induce proliferation, and enhance motility.

Intermediate-conductance Ca2+-activated K+ channel, KCa3.1, as a novel therapeutic target for benign prostatic hyperplasia

Recently, a new experimental stromal hyperplasia animal model corresponding to clinical benign prostatic hyperplasia (BPH) was established. The main objective of this study was to elucidate the roles of the intermediate-conductance Ca(2+)-activated K(+) channel (K(Ca)3.1) in the implanted urogenital sinus (UGS) of stromal hyperplasia BPH model rats. Using DNA microarray, real-time polymerase chain reaction, Western blot, and/or immunohistochemical analyses, we identified the expression of K(Ca)3.1 and its transcriptional regulators in implanted UGS of BPH model rats and prostate needle-biopsy samples and surgical prostate specimens of BPH patients. We also examined the in vivo effects of a K(Ca)3.1 blocker, 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34), on the proliferation index of implanted UGS by measurement of UGS weights and proliferating cell nuclear antigen immunostaining. K(Ca)3.1 genes and proteins were highly expressed in implanted UGS rather than in the normal host prostate. In the implanted UGS, the gene expressions of two transcriptional regulators of K(Ca)3.1, repressor element 1-silencing transcription factor and c-Jun, were significantly down- and up-regulated, and the regulations were correlated negatively or positively with K(Ca)3.1 expression, respectively. Positive signals of K(Ca)3.1 proteins were detected exclusively in stromal cells, whereas they were scarcely immunolocalized to basal cells of the epithelium in implanted UGS. In vivo treatment with TRAM-34 significantly suppressed the increase in implanted UGS weights compared with the decrease in stromal cell components. Moreover, significant levels of K(Ca)3.1 expression were observed in human BPH samples. K(Ca)3.1 blockers may be a novel treatment option for patients suffering from BPH.

K(bg) and Kv1.3 channels mediate potassium efflux in the early phase of apoptosis in Jurkat T lymphocytes

Microelectrode ion flux estimation (MIFE) and patch-clamp techniques were combined for noninvasive K(+) flux measurements and recording of activities of the dominant K(+) channels in the early phases of apoptosis in Jurkat cells. Staurosporine (STS, 1 microM) evoked rapid (peaking around 15 min) transient K(+) efflux, which then gradually decreased. This transient K(+) efflux occurred concurrently with the transient increase of the K(+) background (K(bg)) TWIK-related spinal cord K(+) channel-like current density, followed by a drastic decrease and concomitant membrane depolarization. The Kv1.3 current density remained almost constant. Kv1.3 activation was not altered by STS, whereas the inactivation was shifted to more positive potentials. Contribution of K(bg) and Kv1.3 channels to the transient and posttransient STS-induced K(+) efflux components, respectively, was confirmed by the effects of bupivacaine, predominantly blocking K(bg) current, and the Kv1.3-specific blocker margatoxin. Channel-mediated K(+) efflux provoked a substantial cellular shrinkage and affected the activation of caspases.

Regulation of ryanodine receptor-mediated Ca(2+) release in vas deferens smooth muscle cells

Ca(2+) release from intracellular store sites via the ryanodine receptor (RyR) and hormonal regulation by flutamide, an androgen-receptor (AR) antagonist, on it were examined in vas deferens (VD) smooth muscle cells (SMCs). VD and VDSMCs were obtained from two groups of male rats that were treated p.o. with 100 mg/kg flutamide (Flu) or vehicle (Vehicle). Both spontaneous and caffeine-induced Ca(2+) releases were markedly smaller in single VDSMCs from Flu than in those from Vehicle. Interestingly, [Ca(2+)](i) rise by 100 muM norepinephrine in VDSMCs from Flu was larger than that in those from Vehicle. The contractions induced by direct electrical stimulation in tissue preparations from Flu showed lower susceptibility to 30 muM ryanodine than those from Vehicle. Real-time PCR analyses revealed that the transcripts of ryanodine receptor (RyR) type 2 and type 3 (RyR2 and RyR3) were expressed in VD and markedly reduced in Flu. The protein expression of total RyR was significantly reduced by flutamide treatment, but that of inositol 1,4,5-trisphosphate receptor (IP3R) was not affected. It can be strongly suggested that long term block of AR by flutamide reduced the expression of RyR and its contribution to the contraction, but not those of IP3R in VDSMCs.

Inhibition of Kv1.3 potassium current by phosphoinositides and stromal-derived factor-1α in Jurkat T cells

The activation of Kv1.3 potassium channel has obligatory roles in immune responses of T lymphocytes. Stromal cell-derived factor-1α (SDF-1α) binds to C-X-C chemokine receptor type 4, activates phosphoinositide 3-kinase, and plays essential roles in cell migration of T lymphocytes. In this study, the effects of phosphoinositides and SDF-1α on Kv1.3 current activity were examined in the Jurkat T cell line using whole cell patch-clamp techniques. The internal application of 10 μM phosphatidylinositol 4,5-bisphosphate (PIP2) or 10 μM phosphatidylinositol-3,4,5-trisphosphate (PIP3) significantly reduced Kv1.3 current, but that of 10 μM phosphatidylinositol-4-monophosphate (PIP) did not. The coapplication of 10 μg/ml anti-PIP3 antibody with PIP2 from the pipette did not change the reduction of Kv1.3 current by PIP2, but the coapplication of the antibody with PIP3 eliminated the reduction. The heat-inactivated anti-PIP3 antibody had no effect on PIP3-induced inhibition. These results suggest that PIP2 per se can reduce Kv1.3 current as well as PIP3. External application of 1 μM Akt-kinase inhibitor VIII did not reverse the effect of intracellular PIP3. External application of 10 and 30 ng/ml SDF-1α significantly reduced Kv1.3 current. Internal application of anti-PIP3 antibody reversed the SDF-1α-induced reduction. These results suggest that, in Jurkat T cells, PIP2, PIP3, and SDF-1α reduce Kv1.3 channel activity and that the reduction by SDF-1α may be mediated by the enhancement of PIP3 production. These novel inhibitory effects of phosphoinositides and SDF-1α on Kv1.3 current may have a significant function as a downregulation mechanism of Kv1.3 activity for the maintenance of T lymphocyte activation in immune responses.

Effects of in vitro brevetoxin exposure on apoptosis and cellular metabolism in a leukemic T cell line (Jurkat)

Harmful algal blooms (HABs) of the toxic dinoflagellate, Karenia brevis, produce red tide toxins, or brevetoxins. Significant health effects associated with red tide toxin exposure have been reported in sea life and in humans, with brevetoxins documented within immune cells from many species. The objective of this research was to investigate potential immunotoxic effects of brevetoxins using a leukemic T cell line (Jurkat) as an in vitro model system. Viability, cell proliferation, and apoptosis assays were conducted using brevetoxin congeners PbTx-2, PbTx-3, and PbTx-6. The effects of in vitro brevetoxin exposure on cell viability and cellular metabolism or proliferation were determined using trypan blue and MTT (1-(4,5-dimethylthiazol-2-yl)-3,5-diphenylformazan), respectively. Using MTT, cellular metabolic activity was decreased in Jurkat cells exposed to 5 - 10 microg/ml PbTx-2 or PbTx-6. After 3 h, no significant effects on cell viability were observed with any toxin congener in concentrations up to 10 microg/ml. Viability decreased dramatically after 24 h in cells treated with PbTx-2 or -6. Apoptosis, as measured by caspase-3 activity, was significantly increased in cells exposed to PbTx-2 or PbTx-6. In summary, brevetoxin congeners varied in effects on Jurkat cells, with PbTx-2 and PbTx-6 eliciting greater cellular effects compared to PbTx-3.