Characterization of Ca2+ influx induced by dimethylphytosphingosine and lysophosphatidylcholine in U937 monocytes

Elevated 18:0 lysophosphatidylcholine contributes to the development of pain in tissue injury

ABSTRACT

Tissue injuries, including burns, are major causes of death and morbidity worldwide. These injuries result in the release of intracellular molecules and subsequent inflammatory reactions, changing the tissues' chemical milieu and leading to the development of persistent pain through activating pain-sensing primary sensory neurons. However, the majority of pain-inducing agents in injured tissues are unknown. Here, we report that, amongst other important metabolite changes, lysophosphatidylcholines (LPCs) including 18:0 LPC exhibit significant and consistent local burn injury-induced changes in concentration. 18:0 LPC induces immediate pain and the development of hypersensitivities to mechanical and heat stimuli through molecules including the transient receptor potential ion channel, vanilloid subfamily, member 1, and member 2 at least partly via increasing lateral pressure in the membrane. As levels of LPCs including 18:0 LPC increase in other tissue injuries, our data reveal a novel role for these lipids in injury-associated pain. These findings have high potential to improve patient care.

The Channel-Kinase TRPM7 as Novel Regulator of Immune System Homeostasis

The enzyme-coupled transient receptor potential channel subfamily M member 7, TRPM7, has been associated with immunity and immune cell signalling. Here, we review the role of this remarkable signalling protein in lymphocyte proliferation, differentiation, activation and survival. We also discuss its role in mast cell, neutrophil and macrophage function and highlight the potential of TRPM7 to regulate immune system homeostasis. Further, we shed light on how the cellular signalling cascades involving TRPM7 channel and/or kinase activity culminate in pathologies as diverse as allergic hypersensitivity, arterial thrombosis and graft versus host disease (G_VHD), stressing the need for TRPM7 specific pharmacological modulators.

TRPM channels as potential therapeutic targets against pro-inflammatory diseases

The immune system protects our body against foreign pathogens. However, if it overshoots or turns against itself, pro-inflammatory diseases, such as rheumatoid arthritis, inflammatory bowel disease, or diabetes develop. Ions, the most basic signaling molecules, shape intracellular signaling cascades resulting in immune cell activation and subsequent immune responses. Mutations in ion channels required for calcium signaling result in human immunodeficiencies and highlight those ion channels as valued targets for therapies against pro-inflammatory diseases. Signaling pathways regulated by melastatin-like transient receptor potential (TRPM) cation channels also play crucial roles in calcium signaling and leukocyte physiology, affecting phagocytosis, degranulation, chemokine and cytokine expression, chemotaxis and invasion, as well as lymphocyte development and proliferation. Therefore, this review discusses their regulation, possible interactions and whether they can be exploited as targets for therapeutic approaches to pro-inflammatory diseases.

Tipifarnib-induced apoptosis in acute myeloid leukemia and multiple myeloma cells depends on Ca2+ influx through plasma membrane Ca2+ channels

A major contributing factor to the high mortality rate associated with acute myeloid leukemia and multiple myeloma is the development of resistance to chemotherapy. We have shown that the combination of tipifarnib, a nonpeptidomimetic farnesyltransferase inhibitor (FTI), with bortezomib, a proteosome inhibitor, promotes synergistic death and overcomes de novo drug resistance in acute myeloid leukemia cell lines. Experiments were undertaken to identify the molecular mechanisms by which tipifarnib produces cell death in acute myeloid leukemia and multiple myeloma cell lines (U937 and 8226, respectively). Tipifarnib, but not other FTIs tested [N-[4-[2(R)-amino-3-mercaptopropyl]amino-2-phenylbenzoyl]methionine methyl ester trifluoroacetate salt (FTI-277) and 2'-methyl-5-((((1-trityl-1H-imidazol-4-yl)methyl)amino)methyl)-[1,1'-biphenyl]-2-carboxylic acid (FTI-2153), promotes elevations in intracellular free-calcium concentrations ([Ca(2+)](i)) in both cell lines. These elevations in [Ca(2+)](i) were accompanied by highly dynamic plasmalemmal blebbing and frequently resulted in membrane lysis. The tipifarnib-induced elevations in [Ca(2+)](i) were not blocked by thapsigargin or ruthenium red, but were inhibited by application of Ca(2+)-free extracellular solution and by the Ca(2+) channel blockers Gd(3+) and La(3+). Conversely, 2-aminoethoxydiphenyl borate (2-APB) potentiated the tipifarnib-evoked [Ca(2+)](i) overload. Preventing Ca(2+) influx diminished tipifarnib-evoked cell death, whereas 2-APB potentiated this effect, demonstrating a link between tipifarnib-induced Ca(2+) influx and apoptosis. These data suggest that tipifarnib exerts its effects by acting on a membrane channel with pharmacological properties consistent with store-operated channels containing the Orai3 subunit. It is noteworthy that Orai3 transcripts were found to be expressed at lower levels in tipifarnib-resistant 8226/R5 cells. Our results indicate tipifarnib causes cell death via a novel mechanism involving activation of a plasma membrane Ca(2+) channel and intracellular Ca(2+) overload.

Involvement of p38 MAPK- and JNK-modulated expression of Bcl-2 and Bax in Naja nigricollis CMS-9-induced apoptosis of human leukemia K562 cells

CMS-9, a phospholipase A(2) (PLA(2)) isolated from Naja nigricollis venom, induced apoptosis of human leukemia K562 cells, characterized by mitochondrial depolarization, modulation of Bcl-2 family members, cytochrome c release and activation of caspases 9 and 3. Moreover, an increase in intracellular Ca2+ concentration and the production of reactive oxygen species (ROS) was noted. Pretreatment with BAPTA-AM (Ca2+ chelator) and N-acetylcysteine (NAC, ROS scavenger) proved that Ca2+ was an upstream event in inducing ROS generation. Upon exposure to CMS-9, activation of p38 MAPK and JNK was observed in K562 cells. BAPTA-AM or NAC abrogated CMS-9-elicited p38 MAPK and JNK activation, and rescued viability of CMS-9-treated K562 cells. SB202190 (p38 MAPK inhibitor) and SP600125 (JNK inhibitor) suppressed CMS-9-induced dissipation of mitochondrial membrane potential, Bcl-2 down-regulation, Bax up-regulation and increased mitochondrial translocation of Bax. Inactivation of PLA(2) activity reduced drastically the cytotoxicity of CMS-9, and a combination of lysophosphatidylcholine and stearic acid mimicked the cytotoxic effects of CMS-9. Taken together, our data suggest that CMS-9-induced apoptosis of K562 cells is catalytic activity-dependent and is mediated through mitochondria-mediated death pathway triggered by Ca2+/ROS-evoked p38 MAPK and JNK activation.

Trypanosoma cruzi infection is enhanced by vector saliva through immunosuppressant mechanisms mediated by lysophosphatidylcholine

Trypanosoma cruzi, the etiological agent of Chagas disease, is transmitted by bug feces deposited on human skin during a blood meal. However, parasite infection occurs through the wound produced by insect mouthparts. Saliva of the Triatominae bug Rhodnius prolixus is a source of lysophosphatidylcholine (LPC). Here, we tested the role of both triatomine saliva and LPC on parasite transmission. We show that vector saliva is a powerful inducer of cell chemotaxis. A massive number of inflammatory cells were found at the sites where LPC or saliva was inoculated into the skin of mice. LPC is a known chemoattractant for monocytes, but neutrophil recruitment induced by saliva is LPC independent. The preincubation of peritoneal macrophages with saliva or LPC increased fivefold the association of T. cruzi with these cells. Moreover, saliva and LPC block nitric oxide production by T. cruzi-exposed macrophages. The injection of saliva or LPC into mouse skin in the presence of the parasite induces an up-to-sixfold increase in blood parasitemia. Together, our data suggest that saliva of the Triatominae enhances T. cruzi transmission and that some of its biological effects are attributed to LPC. This is a demonstration that a vector-derived lysophospholipid may act as an enhancing factor of Chagas disease.

N,N-Dimethyl-D-erythro-sphingosine inhibits store-operated Ca2+ entry in U937 monocytes

Calcium is a ubiquitous second messenger that controls a broad range of cellular functions, and store-operated calcium entry (SOCE) is the primary mechanism of regulated Ca(2+) entry in non-excitable immunocytes. In this study, we found that N,N-dimethyl-D-erythro-sphingosine (DMS) inhibited SOCE. In U937 cells, treatment with DMS for 2 h inhibited thapsigargin-induced SOCE by about 70%. DMS inhibited SOCE in a concentration-dependent manner when it was added to the cells after SOCE reached a plateau. DMS-induced SOCE inhibition was also confirmed by the Mn(2+)-quenching method, which monitors only Ca(2+) influx. Because sphingosine kinase inhibitors or protein kinase C (PKC) inhibitors could not mimic the SOCE inhibition, sphingosine kinase and PKC could be excluded as targets of DMS-induced inhibition of SOCE. Furthermore, disruption of lipid rafts with methyl-beta-cyclodextrin and bacterial sphingomyelinase did not influence DMS-induced inhibition of SOCE. DMS-induced inhibition of SOCE in U937 human monocytes is a unique observation and could serve as a basis to study modulation of intracellular Ca(2+) concentration by sphingolipids, although the precise mechanism should be elucidated in the future.

Effects of mitochondrial inhibitors on cell viability in U937 monocytes under glucose deprivation

We studied cytotoxic mechanism of mitochondrial inhibitors in U937 cells. U937 cells were sensitive to cytotoxicity of mitochondrial inhibitors under glucose deprivation condition, whereas PC12 neuronal cells were not. In glucose deprivation condition, intracellular ATP content is decreased and thereby AMP-activated protein kinase (AMPK) is activated. And also activation of JNK, inactivation of ERK, and enhanced expression of Bcl-2 were observed. Mitochondrial inhibitors such as rotenone, TTFA, antimycin A, sodium azide, oligomycin, and valinomycin were used in this study. Inhibitors did not much influence intracellular ATP contents and activity of AMPK under glucose deprivation condition. Activities of Akt and p38 MAPK, however, were decreased by the inhibitors under glucose deprivation condition except TTFA. Furthermore, intracellular Ca2+ concentration was also greatly increased by the inhibitors. Finally, mitochondrial membrane potential was decreased by the inhibitors but TTFA increase the potential and oligomycin maintains it. In the present study, results suggest that under glucose deprivation condition mitochondrial inhibitors may induce severe cytotoxicity of U937 cells through inhibition of Akt and p38 MAPK, increase of [Ca2+]i, and decrease of MMP, but not through inhibition of ATP production and activation of AMPK.

N, N-dimethyl-D-erythro-sphingosine increases intracellular Ca2+ concentration via Na+-Ca2+-exchanger in HCT116 human colon cancer cells

N,N-dimethyl-D-erythro-sphingosine (DMS), an N-methyl derivative of sphingosine, is an inhibitor of protein kinase C (PKC) and sphingosine kinase (SK). In previous reports, DMS-induced intracellular Ca2+ increase concentration ([Ca2+]i) was studied in T lymphocytes, monocytes, astrocytes and neuronal cells. In the present study, we studied DMS-induced increase of [Ca2+]i in HCT116 human colon cancer cells. We found that the DMS-induced increase of [Ca2+]i in colon cancer cells is composed of Ca2+ release from intracellular Ca2+ stores and subsequent Ca2+ influx. The Ca2+ release is not related to modulation of inositol 1,4,5-trisphosphate (IP3) receptor or ryanodine receptor. On the other hand, the Ca2+ influx is mediated largely through Ca2+ channels sensitive to verapamil, nifedipine, Ga3+, and La3+. Furthermore, we found that the response is inhibited by bepridil and Ni2+, specific inhibitors of Na+-Ca2+-exchanger, suggesting involvement of Na+-Ca2+ exchanger in the DMS-induced [Ca2+]i increase in colon cancer cells. This inhibition was also observed in U937 monocytes, but not in 1321N1 astrocytes.

Sphingolipid metabolites selectively elicit increases in nuclear calcium concentration in cell suspension cultures and in isolated nuclei of tobacco

Sphingolipids are known to interfere with calcium-based signalling pathways. Here we report that these compounds modulate nuclear calcium signalling in tobacco BY-2 cells. Nuclear protein kinase activity phosphorylated endogenous sphingoid long-chain bases (LCBs), suggesting that LCBs are actively metabolized in the nucleus of tobacco BY-2 cells. The Delta4-unsaturated LCB D-erythro-sphingosine and the saturated LCB D-ribo-phytosphingosine elicited increases in free calcium in the nucleus in a dose-dependent and structure-related manner. However, neither sphingosine-1-phosphate nor C2-ceramide was able to stimulate nuclear calcium changes. N-,N-Dimethyl-D-erythro-sphingosine, a structural analogue of D-erythro-sphingosine, was the most efficient LCB so far tested in eliciting nuclear calcium changes both in intact tobacco BY-2 cells and in isolated nuclei. TRP channel inhibitors prevent the effect of DMS, suggesting that LCBs may activate TRP-like channels located on the inner nuclear membrane Collectively, the obtained data show that nuclei respond to LCBs on their own independently of the cytosolic compartment.

TRPM channels, calcium and redox sensors during innate immune responses

Melastatin-related TRPM ion channels have emerged as novel therapeutic targets due to their potential ability to modulate the function and fate of immune cells during inflammation, innate, and adaptive immunity. Four family members, TRPM1, TRPM2, TRPM4 and TRPM7 have a strong presence in the immune system. TRPM channels regulate ion-homeostasis by sensing cellular redox status and cytoplasmic calcium levels. TRPM2 for example, is highly expressed in phagocytes. This channel is activated by intracellular ADP-ribose upon exposure to oxidative stress and induces cell death. Here we will review the functional links between TRPM-mediated ion conductance, chemotaxis, apoptosis, and innate immunity.