Volume-Sensitive Chloride Channels Do Not Mediate Activation-Induced Chloride Efflux in Human Neutrophils

The function of TRP channels in neutrophil granulocytes

Neutrophil granulocytes are exposed to widely varying microenvironmental conditions when pursuing their physiological or pathophysiological functions such as fighting invading bacteria or infiltrating cancer tissue. Examples for harsh environmental challenges include among others mechanical shear stress during the recruitment from the vasculature or the hypoxic and acidotic conditions within the tumor microenvironment. Chemokine gradients, reactive oxygen species, pressure, matrix elasticity, and temperature can be added to the list of potential challenges. Transient receptor potential (TRP) channels serve as cellular sensors since they respond to many of the abovementioned environmental stimuli. The present review investigates the role of TRP channels in neutrophil granulocytes and their role in regulating and adapting neutrophil function to microenvironmental cues. Following a brief description of neutrophil functions, we provide an overview of the electrophysiological characterization of neutrophilic ion channels. We then summarize the function of individual TRP channels in neutrophil granulocytes with a focus on TRPC6 and TRPM2 channels. We close the review by discussing the impact of the tumor microenvironment of pancreatic ductal adenocarcinoma (PDAC) on neutrophil granulocytes. Since neutrophil infiltration into PDAC tissue contributes to disease progression, we propose neutrophilic TRP channel blockade as a potential therapeutic option.

Control of volume-sensitive chloride channel inactivation by the coupled action of intracellular chloride and extracellular protons

The volume-sensitive chloride current (I(ClVol)) exhibit a time-dependent decay presumably due to channel inactivation. In this work, we studied the effects of chloride ions (Cl(-)) and H(+) ions on I(ClVol) decay recorded in HEK-293 and HL-60 cells using the whole-cell patch clamp technique. Under control conditions ([Cl(-)](e) = [Cl(-)](i) = 140 mM and pH(i) = pH(e) = 7.3), I(ClVol) in HEK cells shows a large decay at positive voltages but in HL-60 cells I(ClVol) remained constant independently of time. In HEK-293 cells, simultaneously raising the [Cl(-)](e) and [Cl(-)](i) from 25 to 140 mM (with pH(e) = pH(i) = 7.3) increased the fraction of inactivated channels (FIC). This effect was reproduced by elevating [Cl(-)](i) while keeping the [Cl(-)](e) constant. Furthermore, a decrease in pH(e) from 7.3 to 5.5 accelerated current decay and increased FIC when [Cl(-)] was 140 mM but not 25 mM. In HL-60 cells, a slight I(ClVol) decay was seen when the pH(e) was reduced from 7.3 to 5.5. Our data show that inactivation of I(ClVol) can be controlled by changing either the Cl(-) or H(+) concentration or both. Based on our results and previously published data, we have built a model that explains VRAC inactivation. In the model the H(+) binding site is located outside the electrical field near the extracellular entry whilst the Cl(-) binding site is intracellular. The model depicts inactivation as a pore constriction that happens by simultaneous binding of H(+) and Cl(-) ions to the channel followed by a voltage-dependent conformational change that ultimately causes inactivation.

Chloride movements in human neutrophils during phagocytosis: characterization and relationship to granule release

Chloride ion efflux is an early event occurring after exposure of human neutrophils to several soluble agonists. Under these circumstances, a rapid and reversible fall in the high basal intracellular chloride (Cl-i) levels is observed. This event is thought to play a crucial role in the modulation of several critical neutrophil responses including activation and up-regulation of adhesion molecules, cell attachment and spreading, cytoplasmic alkalinization, and activation of the respiratory burst. At present, however, no data are available on chloride ion movements during neutrophil phagocytosis. In this study, we provide evidence that phagocytosis of Candida albicans opsonized with either whole serum, complement-derived opsonins, or purified human IgG elicits an early and long-lasting Cl- efflux accompanied by a marked, irreversible loss of Cl-i. Simultaneous assessment of Cl- efflux and phagocytosis in cytochalasin D-treated neutrophils indicated that Cl- efflux occurs without particle ingestion. These results suggest that engagement of immune receptors is sufficient to promote chloride ion movements. Several structurally unrelated chloride channel blockers inhibited phagocytosis-induced Cl- efflux as well as the release of azurophilic-but not specific-granules. It implicates that different neutrophil secretory compartments display distinct sensitivity to Cl-i modifications. Intriguingly, inhibitors of Cl- exchange inhibited cytosolic Ca2+ elevation, whereas Cl- efflux was not impaired in Ca2+-depleted neutrophils. We also show that FcgammaR(s)- and CR3/CR1-mediated Cl- efflux appears to be dependent on protein tyrosine phosphorylation but independent of PI3K and phospholipase C activation.

Anion channels, including ClC-3, are required for normal neutrophil oxidative function, phagocytosis, and transendothelial migration

NADPH oxidase activity, phagocytosis, and cell migration are essential functions of polymorphonuclear leukocytes (PMNs) in host defense. The cytoskeletal reorganization necessary to perform these functions has been extensively studied, but the role of cell volume regulation, which is likely dependent upon anion channels, has not been defined. Mice lacking the anion channel ClC-3 (Clcn3(-/-)) died from presumed sepsis following intravascular catheter placement, whereas Clcn3(+/+) littermates survived. We hypothesized that ClC-3 has a critical role in host defense and reasoned that PMN function would be compromised in these mice. Clcn3(-/-) PMNs displayed markedly reduced NADPH oxidase activity in response to opsonized zymosan and modestly reduced activity after phorbol 12-myristate 13-acetate. Human PMNs treated with the anion channel inhibitors niflumic acid or 5-nitro-2-(3-phenylpropylamino)benzoic acid had a very similar defect. ClC-3 protein was detected in the secretory vesicles and secondary granules of resting PMNs and was up-regulated to the phagosomal membrane. Clcn3(-/-) PMNs and human PMNs lacking normal anion channel function both exhibited reduced uptake of opsonized zymosan at 1, 5, and 10 min in a synchronized phagocytosis assay. Niflumic acid-treated PMNs also had impaired transendothelial migration in vitro, whereas migration in vivo was not altered in Clcn3(-/-) PMNs. Selective inhibition of the swelling-activated chloride channel with tamoxifen profoundly reduced PMN migration but had no effect on NADPH oxidase activity. In summary, PMNs lacking normal anion channel function exhibited reduced NADPH oxidase activity, diminished phagocytosis, and impaired migration. ClC-3 was specifically involved in the respiratory burst and phagocytosis.

Statin Treatment and Diabetes Affect Myeloperoxidase Activity in Maintenance Hemodialysis Patients

Myeloperoxidase (MPO), which is secreted during activation of neutrophils, may serve as one mechanistic link among persistent inflammation, oxidative stress, and cardiovascular disease. This study related MPO activity to inflammatory and oxidative stress biomarkers, comorbidity, and ongoing medication in prevalent hemodialysis (HD) patients. In a cross-sectional evaluation of 115 prevalent (vintage 25 mo) HD patients (62 men; 63 +/- 1 yr), data on comorbidity (Davies score), diabetes, medication (statins and antihypertensive drugs), nutritional status (subjective global assessment), blood lipids (cholesterol, HDL cholesterol, and triglycerides), inflammatory biomarkers (serum albumin, C-reactive protein, TNF-alpha, and IL-6), oxidative stress biomarkers (pentosidine, 8-hydroxydeoxyguanosine, and MPO activity) were recorded. Patients with MPO activity greater than the median had significantly (P < 0.05) lower serum albumin levels (33.2 +/- 0.7 versus 35.0 +/- 0.5 g/L), higher 8-hydroxydeoxyguanosine levels (1.26 +/- 0.08 versus 1.05 +/- 0.06 ng/ml), and a lower prevalence of statin treatment (18 versus 36%). Therefore, the median MPO activity was significantly (P < 0.05) lower (17.7 versus 26.6 deltaOD630/min per mg protein) in the subgroup of 31 HD patients with ongoing statin treatment. In a multiple regression model, correction for the impact of age, gender, vintage, serum cholesterol, serum albumin, comorbidity, diabetes, and statin use, only diabetes (P < 0.01) and statin use (P < 0.01) were significantly associated to MPO activity. Fourteen patients who had diabetes and were receiving statin treatment had markedly (P = 0.001) lower median (19.9 versus 41.2 deltaOD630/min per mg protein) MPO activity compared with 18 who had diabetes and were not taking statins. This cross-sectional study suggests that both diabetes and statin treatment affect MPO activity in prevalent HD patients.

Enhanced RAGE-mediated NFkappaB stimulation in inflamed hemodialysis patients

Advanced glycation end-products (AGEs), a group of carbohydrate-derived compounds formed by non-enzymatic glycation and oxidation, are markedly elevated in end-stage renal disease (ESRD) and may be related to both inflammation and oxidative stress. The cellular effects of AGE are largely mediated by their interaction with specific surface receptors, such as RAGE. Measurements of biomarkers of inflammation and oxidative stress were conducted in 7 hemodialysis (HD) patients (5 males) with persistent high-grade inflammation (C-reactive protein [CRP]>10 mg/L) and 11 HD-patients (6 males) with low-grade inflammation (CRP<10 mg/L) for at least 6 months. Measured biomarkers for inflammation included hs-CRP, interleukin (IL)-6, white blood cells, neutrophils, S-albumin, peroxisome proliferator-activated receptors (PPAR alpha, beta, gamma) and nuclear factor kappaB (NFkappaB) activity. Markers for oxidative stress were advanced oxidation products (AOPP), myeloperoxidase (MPO)-activity, pentosidine and carboxymethyl lysine (CML). In addition, the effect of increasing doses of CML-modified human serum albumin on NFkappaB activity was tested in mononuclear cells isolated from each patient. As expected, HD-patients with high-grade inflammation had significantly elevated levels of IL-6 (median 9.2 pg/mL versus 2.5 pg/mL; p<0.01), MPO-activity (134.5+/-14.6 DeltaOD(630)/(min mg protein) versus 80.5+/-12.9 DeltaOD(630)/(min mg protein); p<0.05), PPAR-gamma (0.65+/-0.01 OD(655) versus 0.56+/-0.01 OD(655); p<0.01), and AOPP (269+/-54 microM versus 163+/-15 microM; p<0.05) compared with low-grade inflamed patients. Significant associations were demonstrated between hs-CRP and NFkappaB (rho=0.58; p<0.05), AOPP (rho=0.49; p<0.05) and PPAR-gamma (rho=0.62; p<0.05), respectively. In the patient group with high-grade inflammation, stimulation of mononuclear cells with CML-modified human serum albumin caused a rapid dose-dependent rise (p<0.0001) in NFkappaB activity that could be completely blocked by an anti-RAGE antibody. Inflammation and oxidative stress biomarkers are interrelated in ESRD. Inflammatory cell signal pathways, such as NFkappaB, are activated by CML-modification of proteins via RAGE.