Manuela Polimeni

Classical Inhibitors of NOX NAD(P)H Oxidases Are Not Specific

NAD(P)H oxidases (NOXs) are a family of enzymes catalyzing the univalent reduction of oxygen to produce the superoxide anion radical, which in turn can be converted in other reactive oxygen species (ROS) and may participate to the formation of reactive nitrogen derivatives, such as peroxynitrite. By virtue of their activity, NOXs may represent a double-edged sword for the organisms homeostasis. On one hand ROS participate in host defence by killing invading microbes and may regulate several important physiological functions, such as cell signalling, regulation of cell growth and differentiation, oxygen sensing, angiogenesis, fertilization and control of vascular tone. On the other hand ROS may play an important role in pathological processes such as hypertension, atherosclerosis, diabetes, cancer, ischemia/reperfusion injury, neurodegenerative diseases. Many roles suggested for NOXs in various tissues and physiopathological situations have been inferred by the in vitro and in vivo effects of several NOX inhibitors. In particular, most studies are based on the use of two compounds, diphenyleneiodonium and apocynin. Aim of this review is to describe the main features of these two compounds, to show that they cannot be used as specific NOX inhibitors and to solicit researchers to find other tools for investigating the role of NOXs.

Phagocytosis of Hemozoin Enhances Matrix Metalloproteinase-9 Activity and TNF-α Production in Human Monocytes: Role of Matrix Metalloproteinases in the Pathogenesis of Falciparum Malaria

Matrix metalloproteinase-9 (MMP-9), secreted by activated monocytes, degrades matrix proteins, disrupts basal lamina, and activates TNF-α from its precursors. In turn, TNF-α enhances synthesis of MMP-9 in monocytes. We show here that trophozoite-parasitized RBCs/hemozoin-fed adherent human monocytes displayed increased MMP-9 activity and protein/mRNA expression, produced TNF-α time-dependently, and showed higher matrix invasion ability. MMP-9 activation was specific for trophozoite/hemozoin-fed monocytes, was dependent on TNF-α production, and abrogated by anti-TNF-α Ab and by a specific inhibitor of MMP-9/MMP-13 activity. Hemozoin-induced enhancement of MMP-9 and TNF-α production would have a 2-fold effect: to start and feed a cyclic reinforcement loop in which hemozoin enhances production of TNF-α, which in turn induces both activation of MMP-9 and shedding of TNF-α into the extracellular compartment; and, second, to disrupt the basal lamina of endothelia. Excess production of TNF-α and disruption of the basal lamina with extravasation of blood cells into perivascular tissues are hallmarks of severe malaria. Pharmacological inhibition of MMP-9 may offer a new chance to control pathogenic mechanisms in malaria.

The NADPH oxidase inhibitor apocynin induces nitric oxide synthesis via oxidative stress.

We have recently shown that apocynin elicits an oxidative stress in N11 mouse glial cells and other cell types. Here we report that apocynin increased the accumulation of nitrite, the stable derivative of nitric oxide (NO), in the extracellular medium of N11 cell cultures, and the NO synthase (NOS) activity in cell lysates. The increased synthesis of NO was associated with increased expression of inducible NOS (iNOS) mRNA, increased nuclear translocation of the redox-sensitive transcription factor NF-kappa B and decreased intracellular level of its inhibitor IkB alpha. These effects, accompanied by increased production of H2O2, were very similar to those observed after incubation with bacterial lipopolysaccharide (LPS) and were inhibited by catalase. These results suggest that apocynin, similarly to LPS, induces increased NO synthesis by eliciting a generation of reactive oxygen species (ROS), which in turn causes NF-kappa B activation and increased expression of iNOS. Therefore, the increased bioavailability of NO reported in the literature after in vivo or in vitro treatments with apocynin might depend, at least partly, on the drug-elicited induction of iNOS, and not only on the inhibition of NADPH oxidase and the subsequent decreased scavenging of NO by oxidase-derived ROS, as it is often supposed.

Host matrix metalloproteinases in cerebral malaria: new kids on the block against blood–brain barrier integrity?

Cerebral malaria (CM) is a life-threatening complication of falciparum malaria, associated with high mortality rates, as well as neurological impairment in surviving patients. Despite disease severity, the etiology of CM remains elusive. Interestingly, although the Plasmodium parasite is sequestered in cerebral microvessels, it does not enter the brain parenchyma: so how does Plasmodium induce neuronal dysfunction? Several independent research groups have suggested a mechanism in which increased blood–brain barrier (BBB) permeability might allow toxic molecules from the parasite or the host to enter the brain. However, the reported severity of BBB damage in CM is variable depending on the model system, ranging from mild impairment to full BBB breakdown. Moreover, the factors responsible for increased BBB permeability are still unknown. Here we review the prevailing theories on CM pathophysiology and discuss new evidence from animal and human CM models implicating BBB damage. Finally, we will review the newly-described role of matrix metalloproteinases (MMPs) and BBB integrity. MMPs comprise a family of proteolytic enzymes involved in modulating inflammatory response, disrupting tight junctions, and degrading sub-endothelial basal lamina. As such, MMPs represent potential innovative drug targets for CM.

Modulation of doxorubicin resistance by the glucose-6-phosphate dehydrogenase activity.

How anti-neoplastic agents induce MDR (multidrug resistance) in cancer cells and the role of GSH (glutathione) in the activation of pumps such as the MRPs (MDR-associated proteins) are still open questions. In the present paper we illustrate that a doxorubicin-resistant human colon cancer cell line (HT29-DX), exhibiting decreased doxorubicin accumulation, increased intracellular GSH content, and increased MRP1 and MRP2 expression in comparison with doxorubicin-sensitive HT29 cells, shows increased activity of the PPP (pentose phosphate pathway) and of G6PD (glucose-6-phosphate dehydrogenase). We observed the onset of MDR in HT29 cells overexpressing G6PD which was accompanied by an increase in GSH. The G6PD inhibitors DHEA (dehydroepiandrosterone) and 6-AN (6-aminonicotinamide) reversed the increase of G6PD and GSH and inhibited MDR both in HT29-DX cells and in HT29 cells overexpressing G6PD. In our opinion, these results suggest that the activation of the PPP and an increased activity of G6PD are necessary to some MDR cells to keep the GSH content high, which is in turn necessary to extrude anticancer drugs out of the cell. We think that our data provide a new further mechanism for GSH increase and its effects on MDR acquisition.

Physicochemical determinants in the cellular responses to nanostructured amorphous silicas.

Amorphous silicas, opposite to crystalline polymorphs, have been regarded so far as nonpathogenic, but few studies have addressed the toxicity of the wide array of amorphous silica forms. With the advent of nanotoxicology, there has been a rising concern about the safety of silica nanoparticles to be used in nanomedicine. Here, we report a study on the toxicity of amorphous nanostructured silicas obtained with two different preparation procedures (pyrolysis vs. precipitation), the pyrogenic in two very different particle sizes, in order to assess the role of size and origin on surface properties and on the cell damage, oxidative stress, and inflammatory response elicited in murine alveolar macrophages. A quartz dust was employed as positive control and monodispersed silica spheres as negative control. Pyrogenic silicas were remarkably more active than the precipitated one as to cytotoxicity, reactive oxygen species production, lipid peroxidation, nitric oxide synthesis, and production of tumor necrosis factor-α, when compared both per mass and per unit surface. Between the two pyrogenic silicas, the larger one was the more active. Silanols density is the major difference in surface composition among the three silicas, being much larger than the precipitated one as indicated by joint calorimetric and infrared spectroscopy analysis. We assume here that full hydroxylation of a silica surface, with consequent stable coverage by water molecules, reduces/inhibits toxic behavior. The preparation route appears thus determinant in yielding potentially toxic materials, although the smallest size does not always correspond to an increased toxicity.

Haemozoin Induces Early Cytokine-Mediated Lysozyme Release from Human Monocytes through p38 MAPK- and NF-kappaB- Dependent Mechanisms

Malarial pigment (natural haemozoin, HZ) is a ferriprotoporphyrin IX crystal produced by Plasmodium parasites after haemoglobin catabolism. HZ-fed human monocytes are functionally compromised, releasing increased amounts of pro-inflammatory molecules, including cytokines, chemokines and cytokine-related proteolytic enzyme Matrix Metalloproteinase-9 (MMP-9), whose role in complicated malaria has been recently suggested. In a previous work HZ was shown to induce through TNFalpha production the release of monocytic lysozyme, an enzyme stored in gelatinase granules with MMP-9. Here, the underlying mechanisms were investigated. Results showed that HZ lipid moiety promoted early but not late lysozyme release. HZ-dependent lysozyme induction was abrogated by anti-TNFalpha/IL-1beta/MIP-1alpha blocking antibodies and mimicked by recombinant cytokines. Moreover, HZ early activated either p38 MAPK or NF-kappaB pathways by inducing: p38 MAPK phosphorylation; cytosolic I-kappaBalpha phosphorylation and degradation; NF-kappaB nuclear translocation and DNA-binding. Inhibition of both routes through selected molecules (SB203580, quercetin, artemisinin, parthenolide) prevented HZ-dependent lysozyme release. These data suggest that HZ-triggered overproduction of TNFalpha, IL-1beta and MIP-1alpha mediates induction of lysozyme release from human monocytes through activation of p38 MAPK and NF-kappaB pathways, providing new evidence on mechanisms underlying the HZ-enhanced monocyte degranulation in falciparum malaria and the potential role for lysozyme as a new affordable marker in severe malaria.

Digoxin and ouabain induce P-glycoprotein by activating calmodulin kinase II and hypoxia-inducible factor-1α in human colon cancer cells

Digoxin and ouabain are cardioactive glycosides, which inhibit the Na+/K+-ATPase pump and in this way they increase the intracellular concentration of cytosolic calcium ([Ca2+](i)). They are also strong inducers of the P-glycoprotein (Pgp), a transmembrane transporter which extrudes several drugs, including anticancer agents like doxorubicin. An increased amount of Pgp limits the absorption of drugs through epithelial cells, thus inducing resistance to chemotherapy. The mechanism by which cardioactive glycosides increase Pgp is not known and in this work we investigated whether digoxin and ouabain elicited the expression of Pgp with a calcium-driven mechanism. In human colon cancer HT29 cells both glycosides increased the [Ca2+](i) and this event was dependent on the calcium influx via the Na+/Ca2+ exchanger. The increased [Ca2+](i) enhanced the activity of the calmodulin kinase II enzyme, which in turn activated the transcription factor hypoxia-inducible factor-1alpha. This one was responsible for the increased expression of Pgp, which actively extruded doxorubicin from the cells and significantly reduced the pro-apoptotic effect of the drug. All the effects of glycosides were prevented by inhibiting the Na+/Ca2+ exchanger or the calmodulin kinase II. This work clarified the molecular mechanisms by which digoxin and oubain induce Pgp and pointed out that the administration of cardioactive glycosides may widely affect the absorption of drugs in colon epithelia. Moreover, our results suggest that the efficacy of chemotherapeutic agent substrates of Pgp may be strongly reduced in patients taking digoxin.

Quartz inhibits glucose 6-phosphate dehydrogenase in murine alveolar macrophages.

Crystalline silica is well-known to induce oxidative stress as a consequence of both surface-derived generation of free radicals and intracellular production of reactive oxygen species upon phagocytosis; the mechanism of the latter is still partially unknown. In this study, we report that in murine alveolar MH-S macrophages, a 24 h incubation with quartz particles (80 microg/cm(2)) inhibits the glucose 6-phosphate dehydrogenase (G6PD) (1) activity by 70% and the pentose phosphate pathway by 30%. Such effects are accompanied by a 50% decrease of intracellular glutathione, a 35% increase of thiobarbituric acid reactive products (index of lipoperoxidation), and a 5-fold increase of leakage of lactate dehydrogenase in the extracellular medium (index of cytotoxicity). Quartz inhibits G6PD but not other oxidoreductases, and such inhibition is fully prevented by glutathione, suggesting that silica exerts on G6PD an oxidative damage. Our data provide a new additional mechanism by which silica may induce oxidative stress, that is, by inhibiting the pentose phosphate pathway, one of the main antioxidant metabolic pathways of the cell.

Role of 15‐hydroxyeicosatetraenoic acid in hemozoin‐induced lysozyme release from human adherent monocytes

Natural hemozoin (nHZ), a lipid-bound ferriprotoporphyrin IX crystal produced by Plasmodium parasites after hemoglobin catabolism, seriously compromises the functions of human monocytes, and 15-hydroxyeicosatetraenoic acid (15-HETE) and 4-hydroxynonenal (4-HNE), two nHZ lipoperoxidation products, have been related to such a functional impairment. nHZ was recently shown to promote inflammation-mediated lysozyme release from human monocytes through p38 mitogen-activated protein kinase- (MAPK)- and nuclear factor (NF)-κB-dependent mechanisms. This study aimed at identifying the molecule of nHZ lipid moiety that was responsible for these effects. Results showed that 15-HETE mimicked nHZ effects on lysozyme release, whereas 4-HNE did not. 15-HETE-enhanced lysozyme release was abrogated by anti-TNF-α and anti-IL-1β-blocking antibodies and mimicked by recombinant cytokines; on the contrary, MIP-1α/CCL3 was not involved as a soluble mediator of 15-HETE effects. Moreover, 15-HETE early activated p38 MAPK and NF-κB pathways by inducing p38 MAPK phosphorylation; cytosolic I-κBα phosphorylation and degradation; NF-κB nuclear translocation and DNA-binding. Inhibition of both routes through chemical inhibitors (SB203580, quercetin, artemisinin, and parthenolide) prevented 15-HETE-dependent lysozyme release. Collectively, these data suggest that 15-HETE plays a major role in nHZ-enhanced monocyte degranulation.