PGE2-induced Apoptotic Cell Death in K562 Human Leukaemia Cells

Cytotoxicity, Mitochondrial Functionality, and Redox Status of Human Conjunctival Cells after Short and Chronic Exposure to Preservative-Free Bimatoprost 0.03% and 0.01%: An In Vitro Comparative Study

Prostaglandin analogues (PGAs), including bimatoprost (BIM), are generally the first-line therapy for glaucoma due to their greater efficacy, safety, and convenience of use. Commercial solutions of preservative-free BIM (BIM 0.03% and 0.01%) are already available, although their topical application may result in ocular discomfort. This study aimed to evaluate the in vitro effects of preservative-free BIM 0.03% vs. 0.01% in the human conjunctival epithelial (HCE) cell line. Our results showed that long-term exposure to BIM 0.03% ensues a significant decrease in cell proliferation and viability. Furthermore, these events were associated with cell cycle arrest, apoptosis, and alterations of ΔΨ_m. BIM 0.01% does not exhibit cytotoxicity, and no negative influence on conjunctival cell growth and viability or mitochondrial activity has been observed. Short-time exposure also demonstrates the ability of BIM 0.03% to trigger reactive oxygen species (ROS) production and mitochondrial hyperpolarisation. An in silico drug network interaction was also performed to explore known and predicted interactions of BIM with proteins potentially involved in mitochondrial membrane potential dissipation. Our findings overall strongly reveal better cellular tolerability of BIM 0.01% vs. BIM 0.03% in HCE cells.

Types of necroinflammation, the effect of cell death modalities on sterile inflammation

Distinct types of immune responses are activated by infections, which cause the development of type I, II, or III inflammation, regulated by Th1, Th2, Th17 helper T cells and ILC1, ILC2 and ILC3 cells, respectively. While the classification of immune responses to different groups of pathogens is widely accepted, subtypes of the immune response elicited by sterile inflammation have not yet been detailed. Necroinflammation is associated with the release of damage-associated molecular patterns (DAMP) from dying cells. In this review, we present that the distinct molecular mechanisms activated during apoptosis, necroptosis, pyroptosis, and ferroptosis lead to the release of different patterns of DAMPs and their suppressors, SAMPs. We summarize the currently available data on how regulated cell death pathways and released DAMPs and SAMPs direct the differentiation of T helper and ILC cells. Understanding the subtypes of necroinflammation can be crucial in developing strategies for the treatment of sterile inflammatory diseases caused by cell death processes.

Prostaglandin E2 and F2α Alter Expression of Select Cholesteryl Esters and Triacylglycerols Produced by Human Meibomian Gland Epithelial Cells

PURPOSE

PGF2α analogs are commonly used to treat glaucoma and are associated with higher rates of meibomian gland dysfunction (MGD). The purpose of this study was to evaluate the physiological effects of PGF2α and PGE2 on immortalized human meibomian gland epithelial cells (HMGECs).

METHODS

HMGECs were immunostained for the 4 PGE2 receptors (EP1, EP2, EP3, and EP4) and 1 PGF2α receptor (FP) and imaged. Rosiglitazone-differentiated HMGECs were exposed to PGF2α and PGE2 (10-9 to 10-6 M) for 3 hours. Cell viability was assessed by an adenosine triphosphate-based luminescent assay, and lipid extracts were analyzed for cholesteryl esters (CEs), wax esters (WEs), and triacylglycerols (TAGs) by ESI-MSMSALL in positive ion mode by a Triple TOF 5600 Mass Spectrometer using SCIEX LipidView 1.3.

RESULTS

HMGECs expressed 3 PGE2 receptors (EP1, EP2, and EP4) and the 1 PGF2α receptor (FP). Neither PGE2 nor PGF2α showed signs of cytotoxicity at any of the concentrations tested. WEs were not detected from any of the samples, but both CEs and TAGs exhibited a diverse and dynamic profile. PGE2 suppressed select CEs (CE 22:1, CE 26:0, CE 28:1, and CE 30:1). PGF2α dose dependently increased several CEs (CE 20:2, CE 20:1, CE 22:1, and CE 24:0) yet decreased others. Both prostaglandins led to nonspecific TAG remodeling.

CONCLUSIONS

PGE2 and PGF2α showed minimal effect on HMGEC viability. PGF2α influences lipid expression greater than PGE2 and may do so by interfering with meibocyte differentiation. This work may provide insight into the mechanism of MGD development in patients with glaucoma treated with PGF2α analogs.

ASIC1a Inhibitor mambalgin-2 Suppresses the Growth of Leukemia Cells by Cell Cycle Arrest

Although tyrosine kinase inhibitors have brought significant success in the treatment of chronic myelogenous leukemia, the search for novel molecular targets for the treatment of this disease remains relevant. Earlier, expression of acid-sensing ion channels, ASIC1a, was demonstrated in the chronic myelogenous leukemia K562 cells. Three-finger toxins from the black mamba (Dendroaspis polylepis) venom, mambalgins, have been shown to efficiently inhibit homo- and heteromeric channels containing the ASIC1a subunit; however, their use as possible antitumor agents had not been examined. In this work, using the patch-clamp technique, we detected, for the first time, an activation of ASIC1a channels in the leukemia K562 cells in response to an extracellular pH decrease. Recombinant mambalgin-2 was shown to inhibit ASIC1a activity and suppress the proliferation of the K562 cells with a half-maximal effective concentration (EC₅₀) ~ 0.2 μM. Maximum mambalgin-2 inhibitory effect is achieved after 72 h of incubation with cells and when the pH of the cell medium reaches ~ 6.6. In the K562 cells, mambalgin-2 caused arrest of the cell cycle in the G1 phase and reduced the phosphorylation of G1 cell cycle phase regulators: cyclin D1 and cyclin-dependent kinase CDK4, without affecting the activity of CDK6 kinase. Thus, recombinant mambalgin-2 can be considered a prototype of a new type of drugs for the treatment of chronic myelogenous leukemia.

HIV-1 Tat-Mediated Calcium Dysregulation and Neuronal Dysfunction in Vulnerable Brain Regions

Despite the success of combined antiretroviral therapy, more than half of HIV-1-infected patients in the USA show HIV-associated neurological and neuropsychiatric deficits. This is accompanied by anatomical and functional alterations in vulnerable brain regions of the mesocorticolimbic and nigrostriatal systems that regulate cognition, mood and motivation-driven behaviors, and could occur at early stages of infection. Neurons are not infected by HIV, but HIV-1 proteins (including but not limited to the HIV-1 trans-activator of transcription, Tat) induce Ca(2+) dysregulation, indicated by abnormal and excessive Ca(2+) influx and increased intracellular Ca(2+) release that consequentially elevate cytosolic free Ca(2+) levels ([Ca(2+)]in). Such alterations in intracellular Ca(2+) homeostasis significantly disturb normal functioning of neurons, and induce dysregulation, injury, and death of neurons or non-neuronal cells, and associated tissue loss in HIV-vulnerable brain regions. This review discusses certain unique mechanisms, particularly the over-activation and/or upregulation of the ligand-gated ionotropic glutamatergic NMDA receptor (NMDAR), the voltage-gated L-type Ca(2+) channel (L-channel) and the transient receptor potential canonical (TRPC) channel (a non-selective cation channel that is also permeable for Ca(2+)), which may underlie the deleterious effects of Tat on intracellular Ca(2+) homeostasis and neuronal hyper-excitation that could ultimately result in excitotoxicity. This review also seeks to provide summarized information for future studies focusing on comprehensive elucidation of molecular mechanisms underlying the pathophysiological effects of Tat (as well as some other HIV-1 proteins and immunoinflammatory molecules) on neuronal function, particularly in HIV-vulnerable brain regions.

Upregulation of KCNQ1/KCNE1 K+ channels by Klotho

Klotho is a transmembrane protein expressed primarily in kidney, parathyroid gland, and choroid plexus. The extracellular domain could be cleaved off and released into the systemic circulation. Klotho is in part effective as β-glucuronidase regulating protein stability in the cell membrane. Klotho is a major determinant of aging and life span.Overexpression of Klotho increases and Klotho deficiency decreases life span. Klotho deficiency may further result in hearing loss and cardiac arrhythmia. The present study explored whether Klotho modifies activity and protein abundance of KCNQ1/KCNE1, a K(+) channel required for proper hearing and cardiac repolarization. To this end, cRNA encoding KCNQ1/KCNE1 was injected in Xenopus oocytes with or without additional injection of cRNA encoding Klotho. KCNQ1/KCNE1 expressing oocytes were treated with human recombinant Klotho protein (30 ng/mL) for 24 h. Moreover, oocytes which express both KCNQ1/KCNE1 and Klotho were treated with 10 μM DSA L (D-saccharic acid-1,4-lactone), a β-glucuronidase inhibitor. The KCNQ1/KCNE1 depolarization-induced current (I(Ks)) was determined utilizing dual electrode voltage clamp, while KCNQ1/KCNE1 protein abundance in the cell membrane was visualized utilizing specific antibody binding and quantified by chemiluminescence. KCNQ1/KCNE1 channel activity and KCNQ1/KCNE1 protein abundance were upregulated by coexpression of Klotho. The effect was mimicked by treatment with human recombinant Klotho protein (30 ng/mL) and inhibited by DSA L (10 μM). In conclusion, Klotho upregulates KCNQ1/KCNE1 channel activity by “mainly” enhancing channel protein abundance in the plasma cell membrane, an effect at least partially mediated through the β-glucuronidase activity of Klotho protein.

Upregulation of the creatine transporter Slc6A8 by Klotho

BACKGROUND/AIMS

The transmembrane Klotho protein contributes to inhibition of 1,25(OH)2D3 formation. The extracellular domain of Klotho protein could function as an enzyme with e.g. β-glucuronidase activity, be cleaved off and be released into blood and cerebrospinal fluid. Klotho regulates several cellular transporters. Klotho protein deficiency accelerates the appearance of age related disorders including neurodegeneration and muscle wasting and eventually leads to premature death. The main site of Klotho protein expression is the kidney. Klotho protein is also appreciably expressed in other tissues including chorioid plexus. The present study explored the effect of Klotho protein on the creatine transporter CreaT (Slc6A8), which participates in the maintenance of neuronal function and survival.

METHODS

To this end cRNA encoding Slc6A8 was injected into Xenopus oocytes with and without additional injection of cRNA encoding Klotho protein. Creatine transporter CreaT (Slc6A8) activity was estimated from creatine induced current determined by two-electrode voltage-clamp.

RESULTS

Coexpression of Klotho protein significantly increased creatine-induced current in Slc6A8 expressing Xenopus oocytes. Coexpression of Klotho protein delayed the decline of creatine induced current following inhibition of carrier insertion into the cell membrane by brefeldin A (5 µM). The increase of creatine induced current by coexpression of Klotho protein in Slc6A8 expressing Xenopus oocytes was reversed by β-glucuronidase inhibitor (DSAL). Similarly, treatment of Slc6A8 expressing Xenopus oocytes with recombinant human alpha Klotho protein significantly increased creatine induced current.

CONCLUSION

Klotho protein up-regulates the activity of creatine transporter CreaT (Slc6A8) by stabilizing the carrier protein in the cell membrane, an effect requiring β-glucuronidase activity of Klotho protein.

Physiological Function and Characterization of TRPCs in Neurons

Ca²⁺ entry is essential for regulating vital physiological functions in all neuronal cells. Although neurons are engaged in multiple modes of Ca²⁺ entry that regulates variety of neuronal functions, we will only discuss a subset of specialized Ca²⁺-permeable non-selective Transient Receptor Potential Canonical (TRPC) channels and summarize their physiological and pathological role in these excitable cells. Depletion of endoplasmic reticulum (ER) Ca²⁺ stores, due to G-protein coupled receptor activation, has been shown to activate TRPC channels in both excitable and non-excitable cells. While all seven members of TRPC channels are predominately expressed in neuronal cells, the ion channel properties, mode of activation, and their physiological responses are quite distinct. Moreover, many of these TRPC channels have also been suggested to be associated with neuronal development, proliferation and differentiation. In addition, TRPCs also regulate neurosecretion, long-term potentiation and synaptic plasticity. Similarly, perturbations in Ca²⁺ entry via the TRPC channels have been also suggested in a spectrum of neuropathological conditions. Hence, understanding the precise involvement of TRPCs in neuronal function and in neurodegenerative conditions would presumably unveil avenues for plausible therapeutic interventions for these devastating neuronal diseases.

Prostaglandin E2 induces apoptosis in cultured rat microglia

Prostaglandin E2 (PGE2) plays a critical role in the modulation of microglial function including migration and phagocytosis through EP2, which increases intracellular cyclic adenosine monophosphate (AMP) concentration. In the present study, we found that PGE2 reduces cell viability in microglia. PGE2 decreased 3-(4,5-dimethylthiazol-2-thiazolyl)-2,5-diphenyltetrazolium bromide (MTT) reduction and increased lactate dehydrogenase release, deoxyribonucleic acid fragmentation, and poly(ADP-ribose) polymerase cleavage after 24h incubation, suggesting that PGE2 induces apoptosis in these cells. An EP2 agonist, butaprost, and an EP4 agonist, PGE1 alcohol, also induced apoptosis, while an EP1 agonist, 17-phenyl trinor PGE2, or an EP3 agonist, sulprostone, at 10(-6)M did not. On the other hand, EP1-EP4 antagonists, SC-51322, AH6809, L-798106, or GW627368X, up to 10(-5)M did not affect the decrease in MTT reduction by PGE2. Intracellular cyclic AMP accumulation was induced by butaprost, but not 17-phenyl trinor PGE2, sulprostone, or PGE1 alcohol at 10(-6)M. Additionally, we previously reported that PGE2-induced intracellular cyclic AMP accumulation was reversed by AH6809. Besides EP receptors, one of other targets was thought to be prostaglandin transporter, but its inhibitors, bromocresol green or U-46619 up to 10(-5)M did not affect the decrease in MTT reduction by PGE2. These results suggest that PGE2 induces apoptosis in microglia independent of intracellular cyclic AMP concentration, and there are different mechanisms between PGE2-induced apoptosis and the modulation of microglial function.

Inhibitors of second messenger pathways and Ca(2+)-induced exposure of phosphatidylserine in red blood cells of patients with sickle cell disease

The present work investigates the contribution of various second messenger systems to Ca²⁺-induced phosphatidylserine (PS) exposure in red blood cells (RBCs) from sickle cell disease (SCD) patients. The Ca²⁺ dependence of PS exposure was confirmed using the Ca²⁺ ionophore bromo-A23187 to clamp intracellular Ca²⁺ over 4 orders of magnitude in high or low potassium-containing (HK or LK) saline. The percentage of RBCs showing PS exposure was significantly increased in LK over HK saline. This effect was reduced by the Gardos channel inhibitors, clotrimazole and charybdotoxin. Nevertheless, although Ca²⁺ loading in the presence of an outwardly directed electrochemical gradient for K⁺ stimulated PS exposure, substantial exposure still occurred in HK saline. Under the conditions used inhibitors of other second messenger systems (ABT491, quinacrine, acetylsalicylic acid, 3,4-dichloroisocoumarin, GW4869 and zVAD-fmk) did not inhibit the relationship between [Ca²⁺] and PS exposure. Inhibitors of phospholipase A2, cyclooxygenase, platelet-activating factor, sphingomyelinase and caspases, therefore, were without effect on Ca²⁺-induced PS exposure in RBCs, incubated in either HK or LK saline.

Downregulation of the renal outer medullary K(+) channel ROMK by the AMP-activated protein kinase

The 5'-adenosine monophosphate-activated serine/threonine protein kinase (AMPK) is stimulated by energy depletion, increase in cytosolic Ca(2+) activity, oxidative stress, and nitric oxide. AMPK participates in the regulation of the epithelial Na(+) channel ENaC and the voltage-gated K(+) channel KCNE1/KCNQ1. It is partially effective by decreasing PIP(2) formation through the PI3K pathway. The present study explored whether AMPK regulates the renal outer medullary K(+) channel ROMK. To this end, cRNA encoding ROMK was injected into Xenopus oocytes with and without additional injection of constitutively active AMPK(γR70Q) (AMPK(α1)-HA+AMPK(β1)-Flag+AMPKγ1(R70Q)), or of inactive AMPK(αK45R) (AMPK(α1K45R)+AMPK(β1)-Flag+AMPK(γ1)-HA), and the current determined utilizing two-electrode voltage-clamp and single channel patch clamp. ROMK protein abundance was measured utilizing chemiluminescence in Xenopus oocytes and western blot in whole kidney tissue. Moreover, renal Na(+) and K(+) excretion were determined in AMPK(α1)-deficient mice (ampk ( -/- )) and wild-type mice (ampk ( +/+ )) prior to and following an acute K(+) load (111 mM KCl, 30 mM NaHCO(3), 4.7 mM NaCl, and 2.25 g/dl BSA) at a rate of 500 μl/h. As a result, coexpression of AMPK(γR70Q) but not of AMPK(αK45R) significantly decreased the current in ROMK1-expressing Xenopus oocytes. Injection of phosphatidylinositol PI((4,5))P(2) significantly increased the current in ROMK1-expressing Xenopus oocytes, an effect reversed in the presence of AMPK(γR70Q). Under control conditions, no significant differences between ampk ( -/- ) and ampk ( +/+ ) mice were observed in glomerular filtration rate (GFR), urinary flow rate, serum aldosterone, plasma Na(+), and K(+) concentrations as well as absolute and fractional Na(+) and K(+) excretion. Following an acute K(+) load, GFR, urinary flow rate, serum aldosterone, plasma Na(+), and K(+) concentration were again similar in both genotypes, but renal absolute and fractional Na(+) and K(+) excretion were higher in ampk ( -/- ) than in ampk ( +/+ ) mice. According to micropuncture following a K(+) load, delivery of Na(+) to the early distal tubule but not delivery of K(+) to late proximal and early distal tubules was increased in ampk (-/-) mice. The upregulation of renal ROMK1 protein expression by acute K(+) load was more pronounced in ampk (-/-) than in ampk ( +/+ ) mice. In conclusion, AMPK downregulates ROMK, an effect compromising the ability of the kidney to excrete K(+) following an acute K(+) load.

Downregulation of Kv1.5 K channels by the AMP-activated protein kinase

BACKGROUND

The voltage gated K(+) channel Kv1.5 participates in the repolarization of a wide variety of cell types. Kv1.5 is downregulated during hypoxia, which is known to stimulate the energy-sensing AMP-activated serine/threonine protein kinase (AMPK). AMPK is a powerful regulator of nutrient transport and metabolism. Moreover, AMPK is known to downregulate several ion channels, an effect at least in part due to stimulation of the ubiquitin ligase Nedd4- 2. The present study explored whether AMPK regulates Kv1.5.

METHODS

cRNA encoding Kv1.5 was injected into Xenopus oocytes with and without additional injection of wild-type AMPK (α1 β 1γ1), of constitutively active (γR70Q)AMPK (α1 β 1γ1(R70Q)), of inactive mutant (αK45R)AMPK (α1(K45R)β1γ1), or of Nedd4-2. Kv1.5 activity was determined by two-electrode voltage-clamp. Moreover, Kv1.5 protein abundance in the cell membrane was determined by chemiluminescence and immunostaining with subsequent confocal microscopy.

RESULTS

Coexpression of wild-type AMPK(WT) and constitutively active AMPK(γR70Q), but not of inactive AMPK(αK45R) significantly reduced Kv1.5-mediated currents. Coexpression of constitutively active AMPKγR70Q further reduced Kv1.5 K(+) channel protein abundance in the cell membrane. Co-expression of Nedd4-2 similarly downregulated Kv1.5-mediated currents.

CONCLUSION

AMPK is a potent regulator of Kv1.5. AMPK inhibits Kv1.5 presumably in part by activation of Nedd4- 2 with subsequent clearance of channel protein from the cell membrane.

AMP-activated protein kinase in BK-channel regulation and protection against hearing loss following acoustic overstimulation

The energy-sensing AMP-activated serine/threonine protein kinase (AMPK) confers cell survival in part by stimulation of cellular energy production and limitation of cellular energy utilization. AMPK-sensitive functions further include activities of epithelial Na+ channel ENaC and voltage-gated K+ channel KCNE1/KCNQ1. AMPK is activated by an increased cytosolic Ca2+ concentration. The present study explored whether AMPK regulates the Ca2+-sensitive large conductance and voltage-gated potassium (BK) channel. cRNA encoding BK channel was injected into Xenopus oocytes with and without additional injection of wild-type AMPK (AMPKα1+AMPKβ1+AMPKγ1), constitutively active AMPKγR70Q, or inactive AMPKαK45R. BK-channel activity was determined utilizing the 2-electrode voltage-clamp. Moreover, BK-channel protein abundance in the cell membrane was determined by confocal immunomicroscopy. As BK channels are expressed in outer hair cells (OHC) of the inner ear and lack of BK channels increases noise vulnerability, OHC BK-channel expression was examined by immunohistochemistry and hearing function analyzed by auditory brain stem response measurements in AMPKα1-deficient mice (ampk-/-) and in wild-type mice (ampk+/+). As a result, coexpression of AMPK or AMPKγR70Q but not of AMPKαK45R significantly enhanced BK-channel-mediated currents and BK-channel protein abundance in the oocyte cell membrane. BK-channel expression in the inner ear was lower in ampk-/- mice than in ampk+/+ mice. The hearing thresholds prior to and immediately after an acoustic overexposure were similar in ampk-/- and ampk+/+ mice. However, the recovery from the acoustic trauma was significantly impaired in ampk-/- mice compared to ampk+/+ mice. In summary, AMPK is a potent regulator of BK channels. It may thus participate in the signaling cascades that protect the inner ear from damage following acoustic overstimulation.

Integration of transient receptor potential canonical channels with lipids

Transient receptor potential canonical (TRPC) channels are the canonical (C) subset of the TRP proteins, which are widely expressed in mammalian cells. They are thought to be primarily involved in determining calcium and sodium entry and have wide-ranging functions that include regulation of cell proliferation, motility and contraction. The channels are modulated by a multiplicity of factors, putatively existing as integrators in the plasma membrane. This review considers the sensitivities of TRPC channels to lipids that include diacylglycerols, phosphatidylinositol bisphosphate, lysophospholipids, oxidized phospholipids, arachidonic acid and its metabolites, sphingosine-1-phosphate, cholesterol and some steroidal derivatives and other lipid factors such as gangliosides. Promiscuous and selective lipid sensing have been detected. There appear to be close working relationships with lipids of the phospholipase C and A₂ enzyme systems, which may enable integration with receptor signalling and membrane stretch. There are differences in the properties of each TRPC channel that are further complicated by TRPC heteromultimerization. The lipids modulate activity of the channels or insertion in the plasma membrane. Lipid microenvironments and intermediate sensing proteins have been described that include caveolae, G protein signalling, SEC14-like and spectrin-type domains 1 (SESTD1) and podocin. The data suggest that lipid sensing is an important aspect of TRPC channel biology enabling integration with other signalling systems.

Inhibition of Kir2.1 (KCNJ2) by the AMP-activated protein kinase

The inward rectifier K(+) channel Kir2.1 participates in the maintenance of the cell membrane potential in a variety of cells including neurons and cardiac myocytes. Mutations of KCNJ2 encoding Kir2.1 underlie the Andersen-Tawil syndrome, a rare disorder clinically characterized by periodic paralysis, cardiac arrhythmia and skeletal abnormalities. The maintenance of the cardiac cell membrane potential is decreased in ischaemia, which is known to stimulate the AMP-activated serine/threonine protein kinase (AMPK). This energy-sensing kinase stimulates energy production and limits energy utilization. The present study explored whether AMPK regulates Kir2.1. To this end, cRNA encoding Kir2.1 was injected into Xenopus oocytes with and without additional injection of wild type AMPK (AMPKα1+AMPKβ1+AMPKγ1), of the constitutively active (γR70Q)AMPK (α1β1γ1(R70Q)), of the kinase dead mutant (αK45R)AMPK (α1(K45R)β1γ1), or of the ubiquitin ligase Nedd4-2. Kir2.1 activity was determined in two-electrode voltage-clamp experiments. Moreover, Kir2.1 protein abundance in the cell membrane was determined by immunostaining and subsequent confocal imaging. As a result, wild type and constitutively active AMPK significantly reduced Kir2.1-mediated currents and Kir2.1 protein abundance in the cell membrane. Expression of wild type Nedd4-2 or of Nedd4-2(S795A) lacking an AMPK phosphorylation consensus sequence downregulated Kir2.1 currents. The effect of wild type Nedd4-2 but not of Nedd4-2(S795A) was significantly augmented by additional coexpression of AMPK. In conclusion, AMPK is a potent regulator of Kir2.1. AMPK is at least partially effective through phosphorylation of the ubiquitin ligase Nedd4-2.

Inhibition of the heterotetrameric K+ channel KCNQ1/KCNE1 by the AMP-activated protein kinase

The heterotetrameric K(+)-channel KCNQ1/KCNE1 is expressed in heart, skeletal muscle, liver and several epithelia including the renal proximal tubule. In the heart, it contributes to the repolarization of cardiomyocytes. The repolarization is impaired in ischemia. Ischemia stimulates the AMP-activated protein kinase (AMPK), a serine/threonine kinase, sensing energy depletion and stimulating several cellular mechanisms to enhance energy production and to limit energy utilization. AMPK has previously been shown to downregulate the epithelial Na(+) channel ENaC, an effect mediated by the ubiquitin ligase Nedd4-2. The present study explored whether AMPK regulates KCNQ1/KCNE1. To this end, cRNA encoding KCNQ1/KCNE1 was injected into Xenopus oocytes with and without additional injection of wild type AMPK (AMPKα1 + AMPKβ1 + AMPKγ1), of the constitutively active (γR70Q)AMPK (α1β1γ1(R70Q)), of the kinase dead mutant (αK45R)AMPK (α1(K45R)β1γ1), or of the ubiquitin ligase Nedd4-2. KCNQ1/KCNE1 activity was determined in two electrode voltage clamp experiments. Moreover, KCNQ1 abundance in the cell membrane was determined by immunostaining and subsequent confocal imaging. As a result, wild type and constitutively active AMPK significantly reduced KCNQ1/KCNE1-mediated currents and reduced KCNQ1 abundance in the cell membrane. Similarly, Nedd4-2 decreased KCNQ1/KCNE1-mediated currents and KCNQ1 protein abundance in the cell membrane. Activation of AMPK in isolated perfused proximal renal tubules by AICAR (10 mM) was followed by significant depolarization. In conclusion, AMPK is a potent regulator of KCNQ1/KCNE1.

TRPC channels in smooth muscle cells

Transient receptor potential canonical (TRPC) proteins constitute a family of seven (TRPC1-7) nonselective cation channels within the wider TRP superfamily. TRPC1, TRPC3, TRPC4, TRPC5 and TRPC6 channels are expressed in vascular smooth muscle cells from human vessels of all calibers and in smooth muscle from organs such as the uterus and the gastrointestinal tract. TRPC channels have recently emerged as important players in the control of smooth muscle function. This review will focus on the retrospective analysis of studies proposing contributions of TRPC channels to native calcium entry pathways in smooth muscle and to physiological and pathophysiological responses with emphasis on the vascular system.

Silver ion-induced suicidal erythrocyte death

Owing to its antibiotic activity, silver is used for water purification, wound care and a wide variety of implants. Silver metal and silver compounds ionize in solution, and silver ions interfere with the function of a wide variety of proteins. In mammalian cells, silver ions may trigger apoptosis by stimulation of cytochrome c release from mitochondria. The present study explored the effect of AgNO3 on eryptosis, the suicidal death of erythrocytes, cells devoid of mitochondria. Similar to apoptosis of nucleated cells, eryptosis is characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the cell surface. Eryptosis is triggered by energy depletion, cellular depletion of nitric oxide (NO) and activation of protein kinase C (PKC). Phosphatidylserine exposure was determined by annexin V-binding, cell volume by forward scatter, cellular ATP by a luciferin-luciferase assay kit, and hemolysis by photometry. A 48 h exposure to AgNO3 (> or =100 nm) but not to NaNO3 significantly enhanced the percentage of annexin V-binding cells, slightly but significantly decreased forward scatter and significantly decreased cytosolic ATP. Furthermore, inhibition of PKC by staurosporine and donation of NO by sodium nitroprusside significantly blunted silver-induced eryptosis. In conclusion, AgNO3 triggers cell membrane scrambling, an effect attributed to ATP depletion, PKC activation and decrease of cellular NO.

Enhanced insulin sensitivity of gene-targeted mice lacking functional KCNQ1

The pore-forming K+-channel alpha-subunit KCNQ1 is expressed in a wide variety of tissues including heart, skeletal muscle, liver, and epithelia. Most recent evidence revealed an association of the KCNQ1 gene with the susceptibility to type 2 diabetes. KCNQ1 participates in the regulation of cell volume, which is, in turn, critically important for the regulation of metabolism by insulin. The present study explored the influence of KCNQ1 on insulin-induced cellular K+ uptake and glucose metabolism. Insulin (100 nM)-induced K+ uptake was determined in isolated perfused livers from KCNQ1-deficient mice (kcnq1(-/-)) and their wild-type littermates (kcnq1(+/+)). Moreover, plasma glucose and insulin levels, intraperitoneal glucose (3 g/kg) tolerance, insulin (0.15 U/kg)-induced hypoglycemia, and peripheral uptake of radiolabeled 3H-deoxy-glucose were determined in both genotypes. Insulin-stimulated hepatocellular K+ uptake was significantly more sustained in isolated perfused livers from kcnq1(-/-) mice than from kcnq1(+/+)mice. The decline of plasma glucose concentration following an intraperitoneal injection of insulin was again significantly more sustained in kcnq1(-/-) than in kcnq1(+/+) mice. Both fasted and nonfasted plasma glucose and insulin concentrations were significantly lower in kcnq1(-/-) than in kcnq1(+/+)mice. Following an intraperitoneal glucose injection, the peak plasma glucose concentration was significantly lower in kcnq1(-/-) than in kcnq1(+/+)mice. Uptake of 3H-deoxy-glucose into skeletal muscle, liver, kidney and lung tissue was significantly higher in kcnq1(-/-) than in kcnq1(+/+)mice. In conclusion, KCNQ1 counteracts the stimulation of cellular K+ uptake by insulin and thereby influences K+-dependent insulin signaling on glucose metabolism. The observations indicate that KCNQ1 is a novel molecule affecting insulin sensitivity of glucose metabolism.

TRPC channel lipid specificity and mechanisms of lipid regulation

TRPC channels are a subset of the transient receptor potential (TRP) proteins widely expressed in mammalian cells. They are thought to be primarily involved in determining calcium or sodium entry and have broad-ranging functions that include regulation of cell proliferation, motility and contraction. The channels do not respond to a single stimulator but rather are activated or modulated by a multiplicity of factors, potentially existing as integrators at the plasma membrane. This review considers the sensitivity of TRPCs to lipid factors, with focus on sensitivities to diacylglycerols, lysophospholipids, arachidonic acid and its metabolites, sphingosine-1-phosphate (S1P), cholesterol and derivatives, and other lipid factors such as gangliosides. Promiscuous and selective lipid-sensing are apparent. In many cases the lipids stimulate channel function or increase insertion of channels in the membrane. Both direct and indirect (receptor-dependent) lipid effects are evident. Although information is limited, the lipid profiles are consistent with TRPCs having close working relationships with phospholipase C and A2 enzymes. We need much more information about lipid-sensing by TRPCs if we are to fully appreciate its significance, but the available data suggest that lipid-sensing is a key, but not exclusive, aspect of TRPC biology.

Physiology and pathophysiology of canonical transient receptor potential channels

The existence of a mammalian family of TRPC ion channels, direct homologues of TRP, the visual transduction channel of flies, was discovered during 1995-1996 as a consequence of research into the mechanism by which the stimulation of the receptor-Gq-phospholipase Cbeta signaling pathway leads to sustained increases in intracellular calcium. Mammalian TRPs, TRPCs, turned out to be nonselective, calcium-permeable cation channels, which cause both a collapse of the cell's membrane potential and entry of calcium. The family comprises 7 members and is widely expressed. Many cells and tissues express between 3 and 4 of the 7 TRPCs. Despite their recent discovery, a wealth of information has accumulated, showing that TRPCs have widespread roles in almost all cells studied, including cells from excitable and nonexcitable tissues, such as the nervous and cardiovascular systems, the kidney and the liver, and cells from endothelia, epithelia, and the bone marrow compartment. Disruption of TRPC function is at the root of some familial diseases. More often, TRPCs are contributing risk factors in complex diseases. The present article reviews what has been uncovered about physiological roles of mammalian TRPC channels since the time of their discovery. This analysis reveals TRPCs as major and unsuspected gates of Ca(2+) entry that contribute, depending on context, to activation of transcription factors, apoptosis, vascular contractility, platelet activation, and cardiac hypertrophy, as well as to normal and abnormal cell proliferation. TRPCs emerge as targets for a thus far nonexistent field of pharmacological intervention that may ameliorate complex diseases.

SGK1 dependence of insulin induced hypokalemia

Insulin stimulates cellular K+ uptake leading to hypokalemia. Cellular K+ uptake is accomplished by parallel stimulation of Na+/H+ exchange, Na+,K+,2Cl- co-transport, and Na+/K+ ATPase and leads to cell swelling, a prerequisite for several metabolic effects of the hormone. Little is known about underlying signaling. Insulin is known to activate the serum and glucocorticoid-inducible kinase SGK1, which in turn enhances the activity of all three transport proteins. The present study thus explored the contribution of SGK1 to insulin-induced hypokalemia. To this end, gene-targeted mice lacking SGK1 (sgk1-/-) and their wild-type littermates (sgk1+/+) have been infused with insulin (2 mU kg(-1) min(-1)) and glucose at rates leaving the plasma glucose concentration constant. Moreover, isolated liver perfusion experiments have been performed to determine stimulation of cellular K+ uptake by insulin (100 nM). As a result, combined glucose and insulin infusion significantly decreased plasma K+ concentration despite a significant decrease of urinary K+ excretion in sgk1+/+ but not in sgk1-/- mice. Accordingly, the plasma K+ concentration was within 60 min significantly lower in sgk1+/+ than in sgk1-/- mice. In isolated liver perfusion experiments, cellular K+ uptake was stimulated by insulin (100 nM), an effect blunted by 72% in sgk1-/- mice as compared to sgk1+/+ mice. Accordingly, insulin-induced cell hydration was 63% lower in sgk1-/- mice than in sgk1+/+ mice. Moreover, volume regulatory K+ release was 31% smaller in sgk1-/- mice than in sgk1+/+ mice. In conclusion, the serum and glucocorticoid-inducible kinase SGK1 participates in the signaling mediating the hypokalemic effect of insulin.

Differential Effect of HOE642 on Two Separate Monovalent Cation Transporters in the Human Red Cell Membrane

Residual K(+) fluxes in red blood cells can be stimulated in conditions of low ionic strength. Previous studies have identified both the non-selective, voltage-dependent cation (NSVDC) channel and the K(+)(Na(+))/H(+) exchanger as candidate pathways mediating this effect, although it is possible that these pathways represent different modes of operation of a single system. In the present study the effects of HOE642, recently characterised as an inhibitor of the K(+)(Na(+))/H(+) exchanger, on NSVDC has been determined to clarify this question. Radioisotope flux measurements and conductance determinations showed that HOE642 exerted differential effects on the NSVDC channel and the K(+)(Na(+))/H(+) exchanger, confirming that the salt loss observed in low ionic strength solutions represents contributions from at least two independent ion transport pathways. The findings are discussed in the context of red blood cell apoptosis (eryptosis) and disease.

Synergistic actions of diacylglycerol and inositol 1,4,5 trisphosphate for Ca2+-dependent inactivation of TRPC7 channel

AIM

The aim of the present study was to explore the mechanism for the Ca2+- dependent inactivation of the canonical transient receptor potential (TRPC) 7 channel expressed in human embryonic kidney 293 cells.

METHOD

The whole-cell patch-clamp technique was used in the study.

RESULTS

With Ca2+-free external solution, the perfusion of 100 micromol/L carbachol to, or dialysis of the cell with 100 micromol/L guanosine 5'-3-O-(thio)triphosphate (GTPgammaS), induced large inward currents, respectively. These currents were rapidly inhibited by the addition of 1 mmol/L Ca2+ into the bath, and recovery from this inhibition was only partial after the Ca2+ removal, unless vigorous intracellular Ca2+ buffering with 10 mmol/L 1,2 bis(2- aminophenoxy)ethane-N,N,No,No-tetraacetic acid (BAPTA) (plus 4 mmol/L Ca2+) was employed. In contrast, the current induced by a membrane-permeable analog of diacylglycerol (DAG), 1-oleoyl-2-acetyl-sn-glycerol (OAG; 100 micromol/L) did not undergo the inhibition persisting after Ca2+ removal. Interestingly, the inclusion of inositol 1,4,5 trisphosphate (IP3; 100 micromol/L) in the patch pipette rendered the OAG-induced current susceptible to the persistent Ca2+-mediated inhibition independent of the IP3 receptor in the majority of the tested cells, as evidenced by the inability of heparin and thapsigargin in reversing the effect of IP3.

CONCLUSION

The present results suggest that Ca2+ entry via the activated TRPC7 channel plays a critical role in inactivating the channel where the cooperative actions of DAG and IP3 are essentially involved.

Mechanisms and significance of cell volume regulation

Survival of human and animal cells requires avoidance of excessive alterations of cell volume. The osmolarity amassed by cellular accumulation of organic substances must be compensated by lowering cytosolic ion concentrations. The Na+/K+ ATPase extrudes Na+ in exchange for K+, which can permeate the cell membrane through K+ channels. K+ exit generates a cell-negative potential difference across the cell membrane, driving the exit of anions such as Cl-. The low cytosolic Cl- concentrations counterbalance the excess cellular osmolarity by organic substances. Cell volume regulation following cell swelling involves releasing ions through activation of K+ channels and/or anion channels, KCl-cotransport, or parallel activation of K+/H+ exchange and Cl-/HCO3- exchange. Cell volume regulation following cell shrinkage involves accumulation of ions through activation of Na+,K+,2Cl- cotransport, Na+/H+ exchange in parallel to Cl-/HCO3- exchange, or Na+ channels. The Na+ taken up is extruded by the Na+/K+ ATPase in exchange for K+. Shrunken cells further accumulate organic osmolytes such as sorbitol and glycerophosphorylcholine, and monomeric amino acids by altered metabolism and myoinositol (inositol), betaine, taurine, and amino acids by Na+ coupled transport. They release osmolytes during cell swelling. Challenges of cell volume homeostasis include transport, hormones, transmitters, and drugs. Moreover, alterations of cell volume participate in the machinery regulating cell proliferation and apoptotic cell death. Deranged cell volume regulation significantly contributes to the pathophysiology of several disorders such as liver insufficiency, diabetic ketoacidosis, hypercatabolism, fibrosing disease, sickle cell anemia, and infection.

Inflammation and apoptosis in Clostridium difficile enteritis is mediated by PGE2 up-regulation of Fas ligand

BACKGROUND & AIMS

Clostridium difficile toxin A causes acute inflammation and fluid secretion in experimental animals and patients with C difficile infection. We previously reported that toxin A increased cyclooxygenase-2/prostaglandin E(2) (PGE(2)) expression and apoptosis in human colonocytes. Here, we assessed the role of secreted PGE(2) in inflammation and enterocyte apoptosis in toxin A enteritis.

METHODS

Effects of PGE(2) and PGE(2) blockade on toxin A-induced apoptosis of human colonocytes (NCM460) and of PGE(2) or toxin A on the Fas ligand (FasL) induction were analyzed by flow cytometry and Western blot. Functional activity of elevated FasL on colonocytes was assessed by coculture of colonocytes with Fas bearing Jurkat T cells. The involvement of PGE(2)-dependent Fas/FasL activation in toxin A enteritis was further assessed in either scid or FasL and Fas deficient mice.

RESULTS

Inhibition of cyclooxygenase-2 by NS-398 and of PGE(2) using a blocking antibody markedly attenuated apoptosis in colonocytes exposed to toxin A. Enhanced expression and release of FasL followed PGE(2) or toxin A exposure in vivo and in vitro and also was significantly attenuated by treatment with NS-398 and PGE(2) blocking antibody. PGE(2) acting through an EP1 receptor activated nuclear factor-kappaB, which induced transcription of FasL. Toxin A enteritis was accompanied by increased cellular infiltration, fluid secretion, and mucosal damage in control mice, but this response was markedly reduced in both Fas(-/-) and FasL(-/-) mice.

CONCLUSIONS

Toxin A enteritis involves release of PGE(2), which activates the Fas/FasL system, causing enterocyte apoptosis and inflammation.

Curcumin induced suicidal erythrocyte death

The natural nutrient component Curcumin with anti-inflammatory and antitumor activity has previously been shown to stimulate apoptosis of several nucleated cell types. The present study has been performed to explore whether Curcumin could similarly induce suicidal death of erythrocytes or eryptosis, which is characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the erythrocyte surface. Phosphatidylserine exposing cells are phagocytosed and thus rapidly cleared from circulating blood. Erythrocyte membrane scrambling may be triggered by increase of cytosolic Ca(2+) activity or formation of ceramide. To test for eryptosis, erythrocyte phosphatidylserine exposure has been estimated from annexin V binding, and erythrocyte volume from forward scatter in FACS analysis. Exposure of erythrocytes to Curcumin (= 1 microM) increased annexin V binding and decreased forward scatter, pointing to phosphatidylserine exposure at the cell surface and cell shrinkage. According to Fluo3 fluorescence Curcumin increased cytosolic Ca(2+) activity and according to immunofluorescence Curcumin increased ceramide formation. As shown previously, hypertonic shock (addition of 550mM sucrose), chloride removal and glucose depletion decreased the forward scatter and increased annexin V binding. The effects on annexin binding were enhanced in the presence of Curcumin. Exposure to Curcumin did, however, not significantly enhance the shrinking effect of hypertonic shock or Cl(-) removal and reversed the shrinking effect of glucose withdrawal. The present observations disclose a proeryptotic effect of Curcumin which may affect the life span of circulating erythrocytes.

Plasma membrane ion channels in suicidal cell death

The machinery leading to apoptosis includes altered activity of ion channels. The channels contribute to apoptotic cell shrinkage and modify intracellular ion composition. Cl(-) channels allow the exit of Cl(-), osmolytes and HCO(3)(-) leading to cell shrinkage and cytosolic acidification. K(+) exit through K(+) channels contributes to cell shrinkage and decreases intracellular K(+) concentration, which in turn favours apoptotic cell death. K(+) channel activity further determines the cell membrane potential, a driving force for Ca(2+) entry through Ca(2+) channels. Ca(2+) may enter through unselective cation channels. An increase of cytosolic Ca(2+) may stimulate several enzymes executing apoptosis. Specific ion channel blockers may either promote or counteract suicidal cell death. The present brief review addresses the role of ion channels in the regulation of suicidal cell death with special emphasis on the role of channels in CD95 induced apoptosis of lymphocytes and suicidal death of erythrocytes or eryptosis.

Inhibition of eryptosis and intraerythrocytic growth of Plasmodium falciparum by flufenamic acid

Non-selective (NSC) cation channels participate in the Ca(2+) leak of human erythrocytes. Sustained activity of these channels triggers suicidal erythrocyte death (eryptosis), which is characterized by Ca(2+)-stimulated cell shrinkage and phosphatidylserine (PS) exposure. PS-exposing erythrocytes are rapidly cleared from circulating blood. PGE(2) activates the NSC channels, and erythrocyte PGE(2) formation is stimulated by a decrease in intra- or extracellular Cl(-) concentration. In addition, the intraerythrocytic malaria parasite Plasmodium falciparum activates the NSC channels, most probably to accomplish Na(+) and Ca(2+) entry into the erythrocyte cytosol required for parasite development. By Ca(2+) uptake the parasite maintains a low Ca(2+) concentration in the erythrocyte cytosol and thus delays the suicidal death of the host erythrocyte. Flufenamic acid has previously been shown to inhibit NSC channels. The present study thus explored the effect of flufenamic acid on erythrocyte Ca(2+) entry, on suicidal erythrocyte death and on intraerythrocytic growth of P. falciparum. Within 48 h, replacement of extracellular Cl(-) with gluconate or application of PGE(2) (50 microM) increased Fluo3 fluorescence reflecting cytosolic Ca(2+) activity, decreased forward scatter reflecting cell volume and increased annexin V binding reflecting PS exposure in FACS analysis. All those effects were significantly blunted in the presence of flufenamic acid (10 microM). Flufenamic acid (25 microM) further significantly delayed the intraerythrocytic growth of P. falciparum and the PS exposure of the infected erythrocytes. The present observations disclose a novel effect of flufenamic acid, which may allow the pharmacological manipulation of erythrocyte survival and the course of malaria.