Chloride channels and hepatocellular function: prospects for molecular identification

Chloride ions in health and disease

Chloride is a key anion involved in cellular physiology by regulating its homeostasis and rheostatic processes. Changes in cellular Cl- concentration result in differential regulation of cellular functions such as transcription and translation, post-translation modifications, cell cycle and proliferation, cell volume, and pH levels. In intracellular compartments, Cl- modulates the function of lysosomes, mitochondria, endosomes, phagosomes, the nucleus, and the endoplasmic reticulum. In extracellular fluid (ECF), Cl- is present in blood/plasma and interstitial fluid compartments. A reduction in Cl- levels in ECF can result in cell volume contraction. Cl- is the key physiological anion and is a principal compensatory ion for the movement of the major cations such as Na+, K+, and Ca2+. Over the past 25 years, we have increased our understanding of cellular signaling mediated by Cl-, which has helped in understanding the molecular and metabolic changes observed in pathologies with altered Cl- levels. Here, we review the concentration of Cl- in various organs and cellular compartments, ion channels responsible for its transportation, and recent information on its physiological roles.

NAD(H) Regulates the Permeability Transition Pore in Mitochondria through an External Site

The opening of the permeability transition pore (mPTP) in mitochondria initiates cell death in numerous diseases. The regulation of mPTP by NAD(H) in the mitochondrial matrix is well established; however, the role of extramitochondrial (cytosolic) NAD(H) is still unclear. We studied the effect of added NADH and NAD⁺ on: (1) the Ca²⁺-retention capacity (CRC) of isolated rat liver, heart, and brain mitochondria; (2) the Ca²⁺-dependent mitochondrial swelling in media whose particles can (KCl) or cannot (sucrose) be extruded from the matrix by mitochondrial carriers; (3) the Ca²⁺-dependent mitochondrial depolarization and the release of entrapped calcein from mitochondria of permeabilized hepatocytes; and (4) the Ca²⁺-dependent mitochondrial depolarization and subsequent repolarization. NADH and NAD⁺ increased the CRC of liver, heart, and brain mitochondria 1.5-2.5 times, insignificantly affecting the rate of Ca²⁺-uptake and the free Ca²⁺ concentration in the medium. NAD(H) suppressed the Ca²⁺-dependent mitochondrial swelling both in KCl- and sucrose-based media but did not induce the contraction and repolarization of swollen mitochondria. By contrast, EGTA caused mitochondrial repolarization in both media and the contraction in KCl-based medium only. NAD(H) delayed the Ca²⁺-dependent depolarization and the release of calcein from individual mitochondria in hepatocytes. These data unambiguously demonstrate the existence of an external NAD(H)-dependent site of mPTP regulation.

Chloride intracellular channels as novel biomarkers for digestive system tumors (Review)

Digestive system malignant tumors are common tumors, and the traditional treatment methods for these tumors include surgical resection, radiotherapy, chemotherapy, and molecularly targeted drugs. However, diagnosis remains challenging, and the early detection of postoperative recurrence is complicated. Therefore, it is necessary to explore novel biomarkers to facilitate clinical diagnosis and treatment. Accumulating evidence supports the crucial role of chloride channels in the development of multiple types of cancers. Given that chloride channels are widely expressed and involved in cell proliferation, apoptosis and cell cycle, among other processes, they may serve as a promising diagnostic and therapeutic target. Chloride intracellular channels (CLICs) are a class of chloride channels that are upregulated or downregulated in certain types of cancer. Furthermore, in certain cases, during cell cycle progression, the localization and function of the cytosolic form of the transmembrane proteins of CLICs are also altered, which may provide a key target for cancer therapy. The aim of the present review was to focus on CLICs as biomarkers for digestive system tumors.

A Funnel-Shaped Chloride Nanochannel Inspired By ClC Protein

Chloride transport participates in a great variety of physiological activities, such as regulating electrical excitability and maintaining acid-base equilibrium. However, the high flux is the prerequisite to ensure the realization of the above functions. Actually, the high flux of ion transport is significant, not only for living things but also for practical applications. Herein, inspired by chloride channel (ClC) protein, a novel NH₂-pillar[5]arene functionalized funnel-shaped nanochannel was designed and constructed. The introduction of functional molecules changed surface charge property and endowed the nanochannel with Cl^- selectivity, which facilitated Cl^- transport. Moreover, by adjusting the asymmetric degree of the nanochannel, the Cl^- transport flux can be improved greatly. The successful construction of an artificial ion channel with high flux will be much useful for practical applications like microfluidic devices, sensors, and ion separation.

Chloride channel 3 (CIC-3) predicts the tumor size in hepatocarcinoma

Chloride channel 3 (CIC-3) has been suggested to be implicated in the carcinogenesis though; it still remains ill understood in hepatocarcinoma, especially in terms of clinicopathological meaning of its expression. Given this, herein, to understand the clinicopathological significance of CIC-3 expression in hepatocarcinoma, Immunohistochemistry was performed to examine the level of CIC-3, followed by statistical analysis of the correlation between expression versus clinicopathological variables, including gender, age, TNM classifications, tumor size, lymph node metastasis and overall prognosis. It was shown that positive staining of CIC-3 can be present in both hepatocarcinoma and its paired normal controls; and that CIC-3 was significantly over-expressed in hepatcarcioma on the whole relative to paired normal controls. Moreover, up-regulation of CIC-3 markedly correlated with tumor size and overall prognosis, suggesting that CIC-3 expression could predict both tumor size and overall prognosis in hepatocarcinoma.

A Purified Resin Glycoside Fraction from Pharbitidis Semen Induces Paraptosis by Activating Chloride Intracellular Channel-1 in Human Colon Cancer Cells

Pharbitidis Semen has worldwide recognition in traditional medicine for the treatment of several illnesses apart from its purgative properties, and it is also reported to show anticancer effect. However, limited pharmacological studies are available on the extract or resin glycosides fraction of Pharbitidis Semen. The purpose of this study was to determine the mechanism of the colon cancer cell cytotoxic effect of a purified resin glycoside fraction from Pharbitidis Semen (RFP). Our results showed that the RFP-induced cell death was mediated by the caspase-independent and autophagy-protective paraptosis, a type of cell death that is characterized by the accumulation of cytoplasmic vacuoles and mitochondria swelling. RFP significantly stimulated endoplasmic reticulum stress, inhibited proteasome-dependent degradation, and activated the MAPK signaling pathway in human colon cancer cell lines. Furthermore, we found that RFP activated chloride intracellular channel-1 (CLIC1) and increased the intracellular Cl⁻ concentration. Blockage of CLIC1 by DIDS (disodium 4,4′-diisothiocyanato-2,2′-stilbenedisulfonate hydrate) attenuated cell death, cytoplasmic vacuolization, and endoplasmic reticulum stress, suggesting that CLIC1 acts as a critical early signal in RFP-induced paraptosis. In conclusion, results obtained indicated that the cytotoxic effect of RFP in colon cancer cells was the outcome of paraptosis mediated by activation of CLIC1.

Lack of evidence supporting a role of TMC4-rs641738 missense variant-MBOAT7- intergenic downstream variant-in the Susceptibility to Nonalcoholic Fatty Liver Disease

Current knowledge on the genetic basis of nonalcoholic fatty liver disease (NAFLD) suggests that variants contributing not only to the disease predisposition but histological severity as well are located in genes that regulate lipid metabolism. We explored the role of rs641738 C/T located in TMC4 (transmembrane channel-like 4) exon 1 (p.Gly17Glu) and 500 bases- downstream of MBOAT7 gene (TMC4/MBOAT7), in the genetic risk for developing NAFLD in a case-control study. Our sample included 634 individuals (372 patients with NAFLD diagnosed by liver biopsy and 262 control subjects); genotyping was performed by a Taqman assay. Genotype frequencies in controls (CC: 84, CT: 137, TT: 41) and patients (CC: 134, CT: 178, TT: 60) were in Hardy-Weinberg equilibrium; minor allele frequency 40.8%. Our sample had 84–99% power if an additive genetic model is assumed for estimated odds ratios of 1.3–1.5, respectively. We found no evidence of association between rs641738 and either NAFLD (Cochran-Armitage test for trend, p = 0.529) or the disease severity (p = 0.61). Low levels of MBOAT7 protein expression were found in the liver of patients with NAFLD, which were unrelated to the rs641738 genotypes. In conclusion, the role of rs641738 in the pathogenesis of NAFLD is inconclusive.

The cellular chloride channels CLIC1 and CLIC4 contribute to virus-mediated cell motility

Ion channels regulate many aspects of cell physiology, including cell proliferation, motility, and migration, and aberrant expression and activity of ion channels is associated with various stages of tumor development, with K⁺ and Cl⁻ channels now being considered the most active during tumorigenesis. Accordingly, emerging in vitro and preclinical studies have revealed that pharmacological manipulation of ion channel activity offers protection against several cancers. Merkel cell polyomavirus (MCPyV) is a major cause of Merkel cell carcinoma (MCC), primarily because of the expression of two early regulatory proteins termed small and large tumor antigens (ST and LT, respectively). Several molecular mechanisms have been attributed to MCPyV-mediated cancer formation but, thus far, no studies have investigated any potential link to cellular ion channels. Here we demonstrate that Cl⁻ channel modulation can reduce MCPyV ST-induced cell motility and invasiveness. Proteomic analysis revealed that MCPyV ST up-regulates two Cl⁻ channels, CLIC1 and CLIC4, which when silenced, inhibit MCPyV ST-induced motility and invasiveness, implicating their function as critical to MCPyV-induced metastatic processes. Consistent with these data, we confirmed that CLIC1 and CLIC4 are up-regulated in primary MCPyV-positive MCC patient samples. We therefore, for the first time, implicate cellular ion channels as a key host cell factor contributing to virus-mediated cellular transformation. Given the intense interest in ion channel modulating drugs for human disease. This highlights CLIC1 and CLIC4 activity as potential targets for MCPyV-induced MCC.

Ion Channels and Oxidative Stress as a Potential Link for the Diagnosis or Treatment of Liver Diseases

Oxidative stress results from a disturbed balance between oxidation and antioxidant systems. Reactive oxygen species (ROS) and reactive nitrogen species (RNS) may be either harmful or beneficial to the cells. Ion channels are transmembrane proteins that participate in a large variety of cellular functions and have been implicated in the development of a variety of diseases. A significant amount of the available drugs in the market targets ion channels. These proteins have sulfhydryl groups of cysteine and methionine residues in their structure that can be targeted by ROS and RNS altering channel function including gating and conducting properties, as well as the corresponding signaling pathways associated. The regulation of ion channels by ROS has been suggested to be associated with some pathological conditions including liver diseases. This review focuses on understanding the role and the potential association of ion channels and oxidative stress in liver diseases including fibrosis, alcoholic liver disease, and cancer. The potential association between ion channels and oxidative stress conditions could be used to develop new treatments for major liver diseases.

Transmembrane voltage potential of somatic cells controls oncogene-mediated tumorigenesis at long-range

The microenvironment is increasingly recognized as a crucial aspect of cancer. In contrast and complement to the field's focus on biochemical factors and extracellular matrix, we characterize a novel aspect of host:tumor interaction - endogenous bioelectric signals among non-excitable somatic cells. Extending prior work focused on the bioelectric state of cancer cells themselves, we show for the first time that the resting potentials of distant cells are critical for oncogene-dependent tumorigenesis. In the Xenopus laevis tadpole model, we used human oncogenes such as mutant KRAS to drive formation of tumor-like structures that exhibited overproliferation, increased nuclear size, hypoxia, acidity, and leukocyte attraction. Remarkably, misexpression of hyperpolarizing ion channels at distant sites within the tadpole significantly reduced the incidence of these tumors. The suppression of tumorigenesis could also be achieved by hyperpolarization using native CLIC1 chloride channels, suggesting a treatment modality not requiring gene therapy. Using a dominant negative approach, we implicate HDAC1 as the mechanism by which resting potential changes affect downstream cell behaviors. Based on published data on the voltage-mediated changes of butyrate flux through the SLC5A8 transporter, we present a model linking resting potentials of host cells to the ability of oncogenes to initiate tumorigenesis. Antibiotic data suggest that the relevant butyrate is generated by a native bacterial species, identifying a novel link between the microbiome and cancer that is mediated by alterations in bioelectric signaling.

Requirement for chloride channel function during the hepatitis C virus life cycle

Hepatocytes express an array of plasma membrane and intracellular ion channels, yet their role during the hepatitis C virus (HCV) life cycle remains largely undefined. Here, we show that HCV increases intracellular hepatic chloride (Cl(-)) influx that can be inhibited by selective Cl(-) channel blockers. Through pharmacological and small interfering RNA (siRNA)-mediated silencing, we demonstrate that Cl(-) channel inhibition is detrimental to HCV replication. This represents the first observation of the involvement of Cl(-) channels during the HCV life cycle.

Volume-sensitive outwardly rectifying chloride channel in white adipocytes from normal and diabetic mice

The volume-sensitive outwardly rectifying (VSOR) chloride channel is ubiquitously expressed and involved in cell volume regulation after osmotic swelling, called regulatory volume decrease (RVD), in various cell types. In adipocytes, the expression of the VSOR channel has not been explored to date. Here, by employing the whole-cell patch-clamp technique, we examined whether or not the VSOR channel is expressed in white adipocytes freshly isolated from epididymal fat pads of normal (C57BL/6 or KK) and diabetic (KKA(y)) mice. Whole cell voltage-clamp recordings revealed that Cl(-) currents were gradually activated upon cell swelling induced by application of a hypotonic solution, both in normal and diabetic adipocytes. Although both the mean cell size (or cell capacitance) and the current magnitude in KKA(y) adipocytes were larger than those in C57BL/6 cells, the current density was significantly lower in KKA(y) adipocytes (23.32 +/- 1.94 pA in C57BL/6 adipocytes vs. 13.04 +/- 2.41 pA in KKA(y) adipocytes at +100 mV). Similarly, the current density in diabetic KKA(y) adipocytes was lower than that in adipocytes from KK mice (a parental strain of KKA(y) mice), which do not present diabetes until an older age. The current was inhibited by Cl(-) channel blockers, 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and glibenclamide, or hypertonic solution, and showed outward rectification and inactivation kinetics at large positive potentials. These electrophysiological and pharmacological properties are consistent with those of the VSOR channel in other cell types. Moreover, adipocytes showed RVD, which was inhibited by NPPB. In KKA(y) adipocytes, RVD was significantly slower (tau; 8.42 min in C57BL/6 adipocytes vs. 11.97 min in KKA(y) adipocytes) and incomplete during the recording period (25 min). It is concluded that the VSOR channel is functionally expressed and involved in volume regulation in white adipocytes. RVD is largely impaired in adipocytes from diabetic mice, presumably as a consequence of the lower density of the functional VSOR channel in the plasma membrane.

Plasmodium berghei-infection induces volume-regulated anion channel-like activity in human hepatoma cells

Parasite infection can lead to alterations in the permeability of host plasma membranes. Presented here is the first demonstration that this phenomenon occurs in Plasmodium-infected liver cells. Using the whole-cell patch-clamp technique, volume-regulated anion channel (VRAC) activity was characterized in Huh-7 cells (a human hepatoma cell line) before and after infection with Plasmodium berghei. Consistent with the presence of VRACs, hypotonic bath solution induced large ion currents in Huh-7 cells that rectified outwardly, reversed close to the equilibrium potential for Cl^- and were inhibited by tamoxifen, clomiphene, mefloquine and 5-nitro-2, 3-(phenylpropylamino)-benzoic acid (NPPB), with IC₅₀ values of 4 ± 1, 4 ± 2, 2 ± 1 and 52 ± 12 μM respectively. In isotonic conditions, initial current recordings measured in uninfected and immature (24 h post invasion) parasite-infected Huh-7 cells were similar (with conductances of 14 ± 3 versus 19 ± 5 pS/pF). However, in mature (48–72 h post invasion) parasite-infected Huh-7 cells there was a sevenfold increase in currents (with a conductance of 98 ± 16 pS/pF). The elevated currents observed in the latter are consistent with VRAC-like activity and the possible reasons for their activation are discussed.

Ionic determinants of pH of acidic compartments under hypertonic conditions in trout hepatocytes

Exposure of trout hepatocytes to hypertonicity induced a decrease in acridine orange (AO) fluorescence, indicating a corresponding decrease in pH inside the lumen of acidic compartments (pH(L)). Pre-exposure of cells to the specific V-ATPase inhibitor bafilomycin A1 (0.3 micromol l(-1)) increased AO fluorescence - unmasking H(+) leaks under steady-state conditions - and partially removed the hypertonicity-induced pH(L) decrease. The sustainability of the luminal acidification, but not the acidification itself, appeared to depend on a low K(+) and a high Cl(-) conductance under hypertonic conditions. Increasing K(+) conductance using the specific ionophore valinomycin (10 micromol l(-1)) or removal of extracellular Cl(-) after an instant drop in AO fluorescence resulted in a reversal of luminal acidity. The alkalinization measured under hypertonic conditions in the absence of Cl(-) was largely attenuated when cells were bathed in HCO(3)(-)-free medium, signifying the possible presence of Cl(-)/HCO(3)(-) exchange. Under steady-state conditions, while a slight and brief pH(L) increase was measured upon exposure of cells to valinomycin, Cl(-) removal, unexpectedly, induced a decrease in pH(L), indicating a role for extracellular Cl(-) in limiting luminal acidification. This was confirmed by the substantial pH(L) decrease measured upon exposure of cells to the anion exchanger inhibitor SITS (0.5 mmol l(-1)). Furthermore, hypertonicity-induced acidification was still noticeable in the presence of SITS. On the other hand, the hypertonicity-induced acidification was significantly reduced in the absence of extracellular Na(+) or Ca(2+). However, BAPTA-AM induced an increase in steady-state pH(L) that was independent of V-ATPase inhibition. Moreover, the BAPTA-induced alkalinization was still apparent after depletion of intracellular Ca(2+) using the Ca(2+) ionophore A23187 in Ca(2+)-free medium. We conclude that pH(L) of trout hepatocytes is sensitive to hypertonicity and ionic determinants of hypertonicity. Thus, changes in pH(L) should be considered when studying pH adaptations to hypertonic stress.

CLIC4, skin homeostasis and cutaneous cancer: surprising connections

Chloride intracellular channel 4 (CLIC4) is a putative chloride channel for intracellular organelles. CLIC4 has biological activities in addition to or because of its channel activity. In keratinocytes, CLIC4 resides in the mitochondria and cytoplasm, and CLIC4 gene expression is regulated by p53, TNF-alpha, and c-Myc. Cytoplasmic CLIC4 translocates to the nucleus in response to cellular stress conditions including DNA damage, metabolic inhibition, senescence, and exposure to certain trophic factors such as TNF-alpha and LPS. Nuclear translocation is associated with growth arrest or apoptosis, depending on the level of expression. In the nucleus CLIC4 interacts with several nuclear proteins as demonstrated by yeast two-hybrid screening and co-immunoprecipitation. Nuclear CLIC4 appears to act on the TGF-beta pathway, and TGF-beta also causes CLIC4 nuclear translocation. In human and mouse cancer cell lines, CLIC4 levels are reduced, and CLIC4 is excluded from the nucleus. CLIC4 soluble or membrane-inserted status is dependent on redox state, and redox alterations in cancer cells could underly the defect in nuclear translocation. CLIC4 is reduced and excluded from the nucleus of many human epithelial neoplasms. Paradoxically, CLIC4 is reciprocally upregulated in tumor stroma in conjunction with the expression of alpha-smooth muscle actin in the fibroblast to myofibroblast transition. Overexpression of CLIC4 in cancer cells inhibits tumor growth in vivo. Conversely, overexpression of CLIC4 in tumor stromal cells stimulates tumor growth in vivo. Thus, CLIC4 participates in normal and pathological processes and may serve as a useful target for therapies in disturbances of homeostasis and neoplastic transformation.

CLIC4 mediates and is required for Ca2+-induced keratinocyte differentiation

Keratinocyte differentiation requires integrating signaling among intracellular ionic changes, kinase cascades, sequential gene expression, cell cycle arrest, and programmed cell death. We now show that Cl(-) intracellular channel 4 (CLIC4) expression is increased in both mouse and human keratinocytes undergoing differentiation induced by Ca(2+), serum and the protein kinase C (PKC)-activator, 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Elevation of CLIC4 is associated with signaling by PKCdelta, and knockdown of CLIC4 protein by antisense or shRNA prevents Ca(2+)-induced keratin 1, keratin 10 and filaggrin expression and cell cycle arrest in differentiating keratinocytes. CLIC4 is cytoplasmic in actively proliferating keratinocytes in vitro, but the cytoplasmic CLIC4 translocates to the nucleus in keratinocytes undergoing growth arrest by differentiation, senescence or transforming growth factor beta (TGFbeta) treatment. Targeting CLIC4 to the nucleus of keratinocytes via adenoviral transduction increases nuclear Cl(-) content and enhances expression of differentiation markers in the absence of elevated Ca(2+). In vivo, CLIC4 is localized to the epidermis in mouse and human skin, where it is predominantly nuclear in quiescent cells. These results suggest that CLIC4 participates in epidermal homeostasis through both alterations in the level of expression and subcellular localization. Nuclear CLIC4, possibly by altering the Cl(-) and pH of the nucleus, contributes to cell cycle arrest and the specific gene expression program associated with keratinocyte terminal differentiation.

Overexpression of CLIC1 in human gastric carcinoma and its clinicopathological significance

Gastric cancer is the second most common cancer worldwide and the fifth leading cause of cancer-related death in Taiwan. Identification of biomarkers is essential to improve patient survival. Fifty aberrantly expressed proteins were identified using 2-DE combined with MALDI TOF MS and were grouped based on their function. The overexpression of proteins was confirmed using real-time quantitative RT-PCR, Western blot, and immunohistochemical analysis. The clinicopathological correlations and prognostic significance of these aberrantly expressed proteins were evaluated to determine the novel gastric cancer biomarkers. In this study, expression of chloride intracellular channel 1 (CLIC1) is significantly up-regulated in 67.9% of gastric patients and was selected for further study. The CLIC1 expression in tumor tissues was increased by 1.95-fold (range, 0.01-6.19-fold) compared with that expressed by adjacent noncancerous mucosa. Elevated CLIC1 expression was strongly correlated with lymph node metastasis, lymphatic invasion, perineural invasion, and pathological staging. Additionally, the 5-year survival rate for the low CLIC1 expression group (n = 28; <1.72-fold) was higher than that for the high CLIC1 expression group (n = 28; >or=1.72-fold) (log rank, p = 0.0300). Experimental results indicate that overexpression of CLIC1 is a potential prognostic marker for gastric cancer.

Hydrophobic bile salts trigger ceramide formation through endosomal acidification

Hydrophobic bile salts activate NADPH oxidase through a ceramide- and PKCζ-dependent pathway as an important upstream event of bile salt-induced hepatocyte apoptosis. The mechanisms underlying bile salt-induced ceramide formation have remained unclear to date and thus were studied in rat hepatocytes. Proapoptotic bile salts, such as taurolithocholylsulfate (TLCS), lowered the apparent pHveswithin seconds from 6.0 to 5.6 in an FITC-dextran-accessible endosomal compartment that also contains acidic sphingomyelinase. Simultaneously, a rapid decrease inN-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide (MQAE) fluorescence was observed, suggestive of an increase in cytosolic [Cl-], which is known to activate vacuolar-type H+-ATPase. No vesicular acidification or increase in cytosolic [Cl-] was found in response to the non-apoptotic bile salt taurocholate or the anti-apoptotic bile salt tauroursodesoxycholate. Inhibition of TLCS-induced endosomal acidification by bafilomycin or 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid largely abolished the TLCS-induced ceramide-formation and downstream ceramide-dependent processes, such as p47phox-serine phosphorylation, NADPH oxidase activation, CD95 activation and apoptosis. These responses were also abolished after knockdown of acidic sphingomyelinase in rat hepatocytes. In conclusion, hydrophobic, proapoptotic bile salts stimulate ceramide formation through chloride-dependent acidification of endosomes, with subsequent activation of acidic sphingomyelinase. Our data suggest that changes in ion homeostasis underlie the stimulation of ceramide formation in response to hydrophobic bile acids as an important upstream event of bile salt-induced apoptosis.

CD95 activation in the liver: ion fluxes and oxidative signaling

Apoptosis is characterized by typical features as cell shrinkage, nuclear condensation, DNA fragmentation, and apoptotic body formation. Whereas some signs of apoptosis are cell type-and death signal-dependent, apoptotic cell volume decrease is an early and ubiquitous event and little is known about the signalling events, which are localized upstream of the plasma membrane transport steps leading to apoptotic cell volume decrease and the proapoptotic events, which are induced by osmolyte loss and cell shrinkage. Ion fluxes and oxidative signaling were recently shown to play an important role in signal transduction with respect to apoptotic cell death within the liver, as a ceramide-dependent activation of the NADPH oxidase was identified as the source of reactive oxygen species generation in rat hepatocytes upon treatment with CD95 ligand, hydrophobic bile salts or hyperosmolarity. The NADPH oxidase-derived ROS signal then allows via Yes, JNK, and EGFR activation for CD95 tyrosine phosphorylation as a prerequisite for CD95 targeting to the plasma membrane and formation of the death inducing signalling complex. Other covalent modifications such as CD95-tyrosine-nitration or CD95-serine/threonine-phosphorylation can interfere with the CD95 activation process. The findings not only provide a mechanistic explanation for the high susceptibility of dehydrated cells for apoptosis, but also give insight into the role of ion fluxes and oxidative signaling with respect to apoptotic cell death within the liver.

Hyperosmotic Activation of the CD95 System

Cell shrinkage, nuclear condensation, DNA fragmentation, and apoptotic body formation are hallmarks of programmed apoptotic cell death. Herein, apoptotic volume decrease (AVD) is an early and ubiquitous event. Conversely, in hepatocytes, hyperosmotic cell shrinkage leads to an activation of the CD95 death receptor system, which involves CD95 tyrosine phosphorylation, CD95 oligomerization, and subsequent trafficking of the CD95 to the plasma membrane, and sensitizes hepatocytes toward CD95 ligand (CD95L)-induced apoptosis. Early signaling events leading to CD95 activation by hyperosmolarity have been identified. In hepatocytes, hyperosmotic exposure induces an almost instantaneous acidification of an acidic sphingomyelinase (ASM) containing endosomal compartment, which is followed by an increase in the intracellular ceramide concentration. Inhibition of anion channels or the vacuolar-type H(+)-ATPase abolishes not only endosomal acidification and subsequent ceramide generation, but also the otherwise observed hyperosmotically induced generation of reactive oxygen species (ROS) by NADPH oxidase isoforms. Hyperosmolarity-induced ROS formation then leads to a Src-family kinase Yes-mediated activation of the epidermal growth factor receptor (EGFR) and to an activation of the c-Jun-N-terminal kinase (JNK). JNK then provides a signal for CD95/EGFR association and subsequent CD95 tyrosine phosphorylation, which is mediated by the EGFR tyrosine kinase activity. CD95 tyrosine phosphorylation then allows for CD95 receptor oligomerization, translocation of the CD95/EGFR protein complex to the plasma membrane, and formation of the death inducing signaling complex (DISC). Mild hyperosmotic exposure, that is, 405 mosmol/liter, does not lead to a reduction of cell viability, even if DISC formation and subsequent caspase 8 and 3 activation occur, but sensitizes hepatocytes to CD95L-induced apoptosis. However, activation of the CD95 system by a more severe hyperosmotic challenge (>505 mosmol/liter) is followed by execution of the apoptotic cell death. Other covalent modifications of CD95, such as CD95 tyrosine nitration or CD95 serine/threonine phosphorylation, were shown to inhibit the CD95 activation process.

c-Src control of chloride channel support for osteoclast HCl transport and bone resorption

Bone degradation by osteoclasts depends upon active transport of hydrogen ions to solubilize bone mineral. This transport is supported by the parallel actions of a proton ATPase and a chloride channel located in the osteoclast ruffled membrane. We have previously identified a novel chloride channel, p62, which appears to be the avian counterpart to CLIC-5b and is expressed coincident with the appearance of acid secretion as avian osteoclasts differentiate in culture. In this article, we show that suppression of CLIC-5b in differentiating avian osteoclasts results in decreased acidification by vesicles derived from these cells and decreased ability of the cells to resorb bone. Acidification is rescued by the presence of valinomycin, consistent with a selective loss of chloride channel but not proton pump activity. Osteoclast bone resorption is known to be dependent on the expression of the tyrosine kinase, c-Src. We show that CLIC-5b from osteoclasts has affinity for both Src SH2 and SH3 domains. We find that suppression of expression of Src in developing osteoclasts results in decreased vesicular acidification, which is rescued by valinomycin, consistent with the loss of chloride conductance in the proton pump-containing vesicles. Suppression of c-Src causes no change in the steady state level of CLIC-5b expression, but does result in failure of proton pump and CLIC-5b to colocalize in cultured osteoclast precursors. We conclude that suppression of c-Src interferes with osteoclast bone resorption by disrupting functional co-localization of proton pump and CLIC-5b.

Two distinct chloride ion requirements in the constitutive protein secretory pathway

The role of chloride ions in regulated secretion is well described but remains poorly characterised in the constitutive system. In the liver, newly synthesised proalbumin is transported to the trans Golgi network where it is converted to albumin by a furin protease and then immediately secreted. We used this acid-dependent hydrolysis and the measurement of specific protein secretion rates to examine the H+ and Cl- ion dependence of albumin synthesis and secretion, a major constitutive protein secretory event in all mammals. Using permeabilised primary rat hepatocytes we show that ordinarily chloride ions are essential for the processing of proalbumin to albumin. However Cl- is not required for transport which continues but releases solely proalbumin. Prior treatment of the cells with Tris (used as a membrane-permeable weak base to neutralise Golgi luminal pH) both eliminated the formation of albumin and very greatly reduced secretion. After washing out Tris, both authentic secretion and processing could be restarted if Cl-, ATP, GTP, cAMP, Ca2+ and cytosolic proteins were added. Hence a requirement for chloride in transport, in addition to processing, can be uncovered by first neutralising pH gradients. Furthermore, the chloride channel blocker DIDS (4,4-diisothiocyanostilbene 2,2-disulphonic acid) reversibly inhibited the constitutive secretory pathway. However, the total mass of proalbumin detectable in DIDS-treated cells fell to 36% of control while the fraction processed to albumin remained almost constant. This clearly dissociates a large part of the Cl- requirement of the constitutive protein secretory pathway from the function of known liver Golgi Cl- channels.

Endosomal Acidification and Activation of NADPH Oxidase Isoforms Are Upstream Events in Hyperosmolarity-induced Hepatocyte Apoptosis

Hyperosmotic exposure of rat hepatocytes induced a rapid oxidative-stress(ROS) response as an upstream signal for proapoptotic CD95 activation. This study shows that hyperosmotic ROS formation involves a rapid ceramide- and protein kinase Czeta (PKCzeta)-dependent serine phosphorylation of p47phox and subsequent activation of NADPH oxidase isoforms. Hyperosmotic p47phox phosphorylation and ROS formation were sensitive to inhibition of sphingomyelinases and were strongly blunted after knockdown of acidic sphingomyelinase (ASM) or of p47phox protein. Hyperosmolarity induced a rapid bafilomycin- and 4,4 '-diisothiocyanostilbene-2,2 '-disulfonic acid disodium salt (DIDS)-sensitive acidification of a vesicular compartment, which was accessible to endocytosed fluorescein isothiocyanate-dextran and colocalized with ASM, PKCzeta, and the NADPH oxidase isoform Nox 2 (gp91phox). Bafilomycin and DIDS prevented the hyperosmolarity-induced increase in ceramide formation, p47phox phosphorylation, and ROS formation. As shown recently (Reinehr, R., Becker, S., Höngen, A., and Häussinger, D. (2004) J. Biol. Chem. 279, 23977-23987), hyperosmolarity induced a Yes-dependent activation of JNK and the epidermal growth factor receptor (EGFR), followed by EGFR-CD95 association, EGFR-catalyzed CD95-tyrosine phosphorylation, and translocation of the EGFR-CD95 complex to the plasma membrane, where formation of the deathinducing signaling complex occurs. These proapoptotic responses were not only sensitive to inhibitors of sphingomyelinase, PKCzeta, or NADPH oxidases but also to ASM knockdown, bafilomycin, and DIDS, i.e. maneuvers largely preventing hyperosmolarity-induced endosomal acidification and/or ceramide formation. In hepatocytes from p47phox knock-out mice, hyperosmolarity failed to activate the CD95 system. The data suggest that hyperosmolarity induces endosomal acidification as an important upstream event for CD95 activation through stimulation of ASM-dependent ceramide formation and activation of NADPH oxidase isoforms.

The ClC-3 Cl− channel in cell volume regulation, proliferation and apoptosis in vascular smooth muscle cells

The volume-regulated Cl(-) current (I(Cl.vol)) is responsible for the transmembrane Cl(-) transport that is involved in cell volume regulatory mechanisms. Although the regulation of cell volume is a fundamental function of healthy cells for maintaining constant size, the molecular genetic identification of I(Cl.vol) is still being debated. Recent studies in vascular smooth muscle support the idea that ClC-3, a member of the voltage-gated ClC Cl(-) channel family, is the molecular component involved in the activation or regulation of I(Cl.vol). Moreover, gene-targeting studies in vascular smooth muscle cells (VSMCs) and other cell types indicate emerging roles of ClC-3 in cell proliferation and apoptosis. These findings indicate that ClC-3 might be involved in modulating vascular remodeling in hypertension and arteriosclerosis.

Anion channels transport ATP into the Golgi lumen

Anion channels provide a pathway for Cl(-) influx into the lumen of the Golgi cisternae. This influx permits luminal acidification by the organelle's H(+)-ATPase. Three different experimental approaches, electrophysiological, biochemical, and proteomic, demonstrated that two Golgi anion channels, GOLAC-1 and GOLAC-2, also mediate ATP anion transport into the Golgi lumen. First, GOLAC-1 and -2 were incorporated into planar lipid bilayers, and single-channel recordings were obtained. Low ionic activities of K(2)ATP added to the cis-chamber directly inhibited the Cl(-) subconductance levels of both channels, with K(m) values ranging from 16 to 115 microM. Substitution of either K(2)ATP or MgATP for Cl(-) on the cis, trans, or both sides indicated that ATP is conducted by the channels with a relative permeability sequence of Cl(-) > ATP(4-) > MgATP(2-). Single-channel currents were observed at physiological concentrations of Cl(-) and ATP, providing evidence for their importance in vivo. Second, transport of [alpha-(32)P]ATP into sealed Golgi vesicles that maintain in situ orientation was consistent with movement through the GOLACs because it exhibited little temperature dependence and was saturated with an apparent K(m) = 25 microM. Finally, after transport of [gamma-(32)P]ATP, a protease-protection assay demonstrated that proteins are phosphorylated within the Golgi lumen, and after SDS-PAGE, the proteins in the phosphorylated bands were identified by mass spectrometry. GOLAC conductances, [alpha-(32)P]ATP transport, and protein phosphorylation have identical pharmacological profiles. We conclude that the GOLACs play dual roles in the Golgi complex, providing pathways for Cl(-) and ATP influx into the Golgi lumen.

CLIC4, an intracellular chloride channel protein, is a novel molecular target for cancer therapy

Chloride intracellular channel (CLIC)4 is a p53- and tumor necrosis factor alpha (TNFalpha)-regulated chloride channel protein that is localized to the mitochondria and cytoplasm of mouse and human keratinocytes. CLIC4 protein increases in differentiating keratinocytes and in keratinocytes exposed to DNA-damaging agents and metabolic inhibitors. Increasing CLIC4 levels by transduction of recombinant CLIC4 causes apoptosis. CLIC4 translocates to the nucleus under a variety of conditions of cell stress, and nuclear CLIC4 is associated with cell cycle arrest and accelerated apoptosis. Reduction of CLIC4 and several other CLIC family members by expressing a doxycycline-regulated CLIC4 antisense also causes apoptosis in squamous cancer cell lines. Expressing antisense CLIC4 in tumors derived from transplanting these cells into nude mice inhibits tumor growth, increases tumor apoptosis, and reduces tumor cell proliferation. Co-administration of TNFalpha intraperitoneally enhances the tumor-inhibitory influence of CLIC4 antisense expression. Together, these results suggest that CLIC4 is important for keratinocyte viability and may be a novel target for anti-cancer therapy.

Inhibition of cellular responses to insulin in a rat liver cell line. A role for PKC in insulin resistance

The initial response of liver cells to insulin is mediated through exocytosis of Cl- channel-containing vesicles and a subsequent opening of plasma membrane Cl- channels. Intracellular accumulation of fatty acids leads to profound defects in metabolism, and is closely associated with insulin resistance. It is not known whether the activity of Cl- channels is altered in insulin resistance and by which mechanisms. We studied the effects of fatty acid accumulation on Cl- channel opening in a model liver cell line. Overnight treatment with amiodarone increased the fat content by approximately 2-fold, and the rates of gluconeogenesis by approximately 5-fold. The ability of insulin to suppress gluconeogenesis was markedly reduced indicating that amiodarone treatment induces insulin resistance. Western blot analysis showed that these cells express the same number of insulin receptors as control cells. However, insulin failed to activate exocytosis and Cl- channel opening. These inhibitory effects were mimicked in control cells by exposures to arachidonic acid (15 microm). Further studies demonstrated that fatty acids stimulate the PKC activity, and inhibition of PKC partially restored exocytosis and Cl- channel opening in insulin-resistant cells. Accordingly, activation of PKC with PMA in control cells potently inhibited the insulin responses. These results suggest that stimulation of PKC activity in insulin resistance contributes to the inhibition of cellular responses to insulin in liver cells.

ClC-3 Chloride Channels Facilitate Endosomal Acidification and Chloride Accumulation

We investigated the involvement of ClC-3 chloride channels in endosomal acidification by measurement of endosomal pH and chloride concentration [Cl-] in control versus ClC-3-deficient hepatocytes and in control versus ClC-3-transfected Chinese hamster ovary cells. Endosomes were labeled with pH or [Cl-]-sensing fluorescent transferrin (Tf), which targets to early/recycling endosomes, or alpha2-macroglobulin (alpha2M), which targets to late endosomes. In pulse label-chase experiments, [Cl-] was 19 mM just after internalization in alpha2M-labeled endosomes in primary cultures of hepatocytes from wild-type mice, increasing to 58 mM over 45 min, whereas pH decreased from 7.1 to 5.4. Endosomal acidification and [Cl-] accumulation were significantly impaired in hepatocytes from ClC-3 knock-out mice, with [Cl-] increasing from 16 to 43 mM and pH decreasing from 7.1 to 6.0. Acidification and Cl- accumulation were blocked by bafilomycin. In Tf-labeled endosomes, [Cl-] was 46 mM in wild-type versus 35 mM in ClC-3-deficient hepatocytes at 15 min after internalization, with corresponding pH of 6.1 versus 6.5. Approximately 4-fold increased Cl- conductance was found in alpha2M-labeled endosomes isolated from hepatocytes of wild-type versus ClC-3 null mice. In contrast, Golgi acidification was not impaired in ClC-3-deficient hepatocytes. In transfected Chinese hamster ovary cells expressing ClC-3A, endosomal acidification and [Cl-] accumulation were enhanced. [Cl-] in alpha2M-labeled endosomes was 42 mM (control) versus 53 mM (ClC-3A) at 45 min, with corresponding pH 5.8 versus 5.2; [Cl-] in Tf-labeled endosomes at 15 min was 37 mM (control) versus 49 mM (ClC-3A) with pH 6.3 versus 5.9. Our results provide direct evidence for involvement of ClC-3 in endosomal acidification by Cl- shunting of the interior-positive membrane potential created by the vacuolar H+ pump.

The pH of the secretory pathway: measurement, determinants, and regulation

The luminal pH of the secretory pathway plays a critical role in the posttranslational modification and sorting of proteins and lipids. The pH of each one of the organelles that constitute the pathway is unique, becoming more acidic as the biosynthetic cargo approaches its destination. The methods used for measurement of pH in the secretory pathway, its determinants, and its regulation are the subjects of this review.

Halometabolites and cellular dehalogenase systems: an evolutionary perspective

We review the role of iodothyronine deiodinases (IDs) in the evolution of vertebrate thyroidal systems within the larger context of biological metabolism of halogens. Since the beginning of life, the ubiquity of organohalogens in the biosphere has provided a major selective pressure for the evolution and conservation of cellular mechanisms specialized in halogen metabolism. Among naturally available halogens, iodine emerged as a critical component of unique developmental and metabolic messengers. Metabolism of iodinated compounds occurs in the three major domains of life, and invertebrate deuterostomes possess several biochemical traits and molecular homologs of vertebrate thyroidal systems, including ancestral homologs of IDs identified in urochordates. The finely tuned cellular regulation of iodometabolite uptake and disposal is a remarkable event in evolution and might have been decisive for the explosive diversification of ontogenetic strategies in vertebrates.

Cell volume regulation and swelling-activated chloride channels

Maintenance of a constant volume is essential for normal cell function. Following cell swelling, as a consequence of reduction of extracellular osmolarity or increase of intracellular content of osmolytes, animal cells are able to restore their original volume by activation of potassium and chloride conductances. The loss of these ions, followed passively by water, is responsible for the homeostatic response called regulatory volume decrease (RVD). Activation of a chloride conductance upon cell swelling is a key step in RVD. Several proteins have been proposed as candidates for this chloride conductance. The status of the field is reviewed, with particular emphasis on ClC-3, a member of the ClC family which has been recently proposed as the chloride channel involved in cell volume regulation.

Altered properties of volume-sensitive osmolyte and anion channels (VSOACs) and membrane protein expression in cardiac and smooth muscle myocytes from Clcn3-/- mice

ClC-3, a member of the large superfamily of ClC voltage-dependent Cl(-) channels, has been proposed as a molecular candidate responsible for volume-sensitive osmolyte and anion channels (VSOACs) in some cells, including heart and vascular smooth muscle. However, the reported presence of native VSOACs in at least two cell types from transgenic ClC-3 disrupted (Clcn3(-/-)) mice casts considerable doubt on this proposed role for ClC-3. We compared several properties of native VSOACs and examined mRNA transcripts and membrane protein expression profiles in cardiac and pulmonary arterial smooth muscle cells from Clcn3(+/+) and Clcn3(-/-) mice to: (1) test the hypothesis that native VSOACs are unaltered in cells from Clcn3(-/-) mice, and (2) test the possibility that targeted inactivation of the Clcn3 gene using a conventional murine global knock-out approach may result in compensatory changes in expression of other membrane proteins. Our experiments demonstrate that VSOAC currents in myocytes from Clcn3(+/+) and Clcn3(-/-) mice are remarkably similar in terms of activation and inactivation kinetics, steady-state current densities, rectification, anion selectivity (I(-) > Cl(-)>> Asp(-)) and sensitivity to block by glibenclamide, niflumic acid, DIDS and extracellular ATP. However, additional experiments revealed several significant differences in other fundamental properties of native VSOACs recorded from atrial and smooth muscle cells from Clcn3(-/-) mice, including: differences in regulation by endogenous protein kinase C, differential sensitivity to block by anti-ClC-3 antibodies, and differential sensitivities to [ATP](i) and free [Mg(2+)](i). These results suggest that in response to Clcn3 gene deletion, there may be compensatory changes in expression of other proteins that alter VSOAC channel subunit composition or associated regulatory subunits that give rise to VSOACs with different properties. Consistent with this hypothesis, in atria from Clcn3(-/-) mice compared to Clcn3(+/+) mice, quantitative analysis of ClC mRNA expression levels revealed significant increases in transcripts for ClC-1, ClC-2, and ClC-3, and protein expression profiles obtained using two-dimensional polyacrylamide gel electrophoresis revealed complex changes in at least 35 different unidentified membrane proteins in cells from Clcn3(-/-) mice. These findings emphasize that caution needs to be exercised in simple attempts to interpret the phenotypic consequences of conventional global Clcn3 gene inactivation.

Activation and inhibition of cellular calcium and tyrosine kinase signaling pathways identify targets of the HBx protein involved in hepatitis B virus replication

Human hepatitis B virus (HBV) HBx protein is a multifunctional protein that activates cellular signaling pathways and is thought to be essential for viral infection. Woodchuck HBV mutants that lack HBx are unable to replicate in vivo or are severely impaired. HBV replication in HepG2 cells, a human hepatoblastoma cell line, is stimulated 5- to 10-fold by HBx protein. We have utilized the HepG2, HBx-dependent HBV replication system to study the effects of activators and inhibitors of cytosolic calcium and tyrosine kinase signaling pathways on viral replication. By transfecting either a wild-type HBV genome or an HBV genome that does not express HBx and then treating transfected cells with activators or inhibitors of signaling pathways, we identified compounds that either impair wild-type HBV replication or rescue HBx-deficient HBV replication. Geldanamycin or herbimycin A, tyrosine kinase inhibitors, blocked HBV replication. Derivatives of cyclosporine, i.e., cyclosporine A, cyclosporine H, and SDZ NIM811, which block cytosolic calcium signaling and specifically the mitochondrial permeability transition pore (SDZ NIM811), also impaired HBV replication. Treatment of cells with compounds that increase cytosolic calcium levels by a variety of mechanisms rescued replication of an HBx-deficient HBV mutant. Transcription of viral RNA and production of viral capsids were only minimally affected by these treatments. These results define a functional signaling circuit for HBV replication that includes calcium signaling and activation of cytosolic signaling pathways involving Src kinases, and they suggest that these pathways are stimulated by HBx acting on the mitochondrial transition pore.

Amazing chloride channels: an overview

AIM

This review describes molecular and functional properties of the following Cl- channels: the ClC family of voltage-dependent Cl- channels, the cAMP-activated transmembrane conductance regulator (CFTR), Ca2+ activated Cl- channels (CaCC) and volume-regulated anion channels (VRAC). If structural data are available, their relationship with the function of Cl- channels will be discussed. We also describe shortly some recently discovered channels, including high conductance Cl- channels and the family of bestrophins. We illustrate the growing physiological importance of these channels in the plasma membrane and in intracellular membranes, including their involvement in transepithelial transport, pH regulation of intracellular organelles, regulation of excitability and volume regulation. Finally, we discuss the role of Cl- channels in various diseases and describe the pathological phenotypes observed in knockout mice models.

Chloride/anion channels in glial cell membranes

At least seven different chloride/anion currents have now been identified in astrocytes, oligodendrocytes/Schwann cells, and microglia. Only for two of these currents is the corresponding gene known. One of these genes is not encoding for a chloride channel, but for a class of mitochondria-like pores also found in cell membranes. Astrocytes and oligodendrocytes differ in their resting properties: astrocytes accumulate chloride but do not have a significant permeability. Oligodendrocytes have a close to passive distribution and a significant permeability. Under certain circumstances, astrocytes can express a resting chloride conductance. Reactive and neoplastic astrocytes as well as astrocytes with an altered shape exhibit a resting conductance. The function of these channels certainly involves volume regulation. Other possible functions are potassium homeostasis, migration, proliferation (in microglia), and involvement in spreading depression waves. Of greatest interest are two phenomena discovered in situ: The ClC-2 channel is only found in astrocytic endfeet near blood capillaries adjacent to neuronal GABA(A) receptors. In the supraoptic nucleus of the hypothalamus, there is an osmosensitive astrocytic taurine release. This released taurine interacts with glycine receptors in neighboring neurons, causing inhibition. It is assumed that with the future availability of more in situ, rather than in vitro, studies, an increased number of such complex interactions between glial cells, neurons, and blood vessels will be discovered.

A large-conductance anion channel of the Golgi complex

An acidic lumenal pH is vital for the proper posttranslational modifications and sorting of proteins and lipids from the Golgi complex. We characterized ion channels present in Golgi fractions that have been cleared of transiting proteins. A large conductance anion channel was observed in approximately 30% of successful channel incorporations into the planar lipid bilayer. The channel, GOLAC-2, has six levels (one closed and five open). The open states are each approximately 20% increments of the maximal, 325 pS conductance. The channel was approximately 6 times more selective for Cl(-) over K(+). Binomial analysis of percent occupancy for each conducting level supports the hypothesis of five independent conducting pathways. The conducting levels can coordinately gate because full openings and closings were often observed. Addition of 3 to 5 mM reduced glutathione to the cis chamber caused dose-dependent increases in single channel conductance, indicating that the channel may be regulated by the oxidation-reduction state of the cell. We propose that GOLAC-2 is a co-channel complex consisting of five identical pores that have a coordinated gating mechanism. GOALC-2 may function as a source of counter anions for the H(+)-ATPase and may be involved in regulating charge balance and membrane potential of the Golgi complex.

Ultraviolet A1 (340-400 nm) irradiation and systemic lupus erythematosus

Short wavelengths of ultraviolet (UV) light are clearly harmful in systemic lupus erythematosus (SLE), but the action of long UV wavelengths in SLE is more enigmatic. In a series of animal and human studies, long-wavelength UV radiation, i.e., radiation in the ultraviolet-A1 (UVA1) range (340-400 nm), has proven effective in the treatment of SLE. Disease amelioration and a marked decrease in mortality followed ultraviolet-A (UVA) radiation (320-400 nm) of the New Zealand White/New Zealand Black mouse model of lupus. A follow-up study in the same animal suggested that the longer wavelengths (UVA1, 340-400 nm) in the UVA wave band were primarily responsible. There followed four human studies. The first three of these provided data indicating that low-dose UVA1 radiation significantly reduced constitutional symptoms, joint pain, rashes, and the systemic lupus activity measures, a validated gauge of disease activity in SLE. The fourth human study showed that the therapeutic action of low-dose UVA1 action persisted or progressed long term, a period averaging 3.4 y. UVA1 effects on DNA repair, cell-mediated immunosuppression, tumor necrosis factor alpha release, and apoptosis contrast markedly with those of ultraviolet B (UVB, 280-320 nm) radiation and afford a possible basis for the salutary action of this modality of treatment. The unique features of UVA1 wavelengths may be suited to further therapeutic use, not only in SLE but also in other immunologic disorders.