Critical Role of Nitric Oxide During the Apoptosis of Peripheral Blood Leukocytes from Patients with AIDS

Oxidative Stress during HIV Infection: Mechanisms and Consequences

It is generally acknowledged that reactive oxygen species (ROS) play crucial roles in a variety of natural processes in cells. If increased to levels which cannot be neutralized by the defense mechanisms, they damage biological molecules, alter their functions, and also act as signaling molecules thus generating a spectrum of pathologies. In this review, we summarize current data on oxidative stress markers associated with human immunodeficiency virus type-1 (HIV-1) infection, analyze mechanisms by which this virus triggers massive ROS production, and describe the status of various defense mechanisms of the infected host cell. In addition, we have scrutinized scarce data on the effect of ROS on HIV-1 replication. Finally, we present current state of knowledge on the redox alterations as crucial factors of HIV-1 pathogenicity, such as neurotoxicity and dementia, exhaustion of CD4+/CD8+ T-cells, predisposition to lung infections, and certain side effects of the antiretroviral therapy, and compare them to the pathologies associated with the nitrosative stress.

Nitric oxide levels in HIV-infected, untreated patients and HIV-infected patients receiving antiretroviral therapy

The role of nitric oxide (NO) in HIV infection is ambiguous; controversy exists around whether the levels of serum NO are increased or decreased in HIV-infected patients. Thus, it is necessary to reassess NO levels in HIV-infected patients. The aim of this study was to investigate the nitrite/nitrate metabolite (NOx) levels in HIV-infected untreated patients and in HIV-infected patients receiving highly active antiretroviral therapy (HAART), compared with HIV-uninfected individuals (control group). The HIV-infected patients enrolled in this study had been receiving HAART for at least 6 months (HIV-treated) or had not received HAART for at least 6 months (HIV-untreated group). New recommendations encourage initiating treatment in HIV-infected adults at a CD4 cell count of 500 cells/mm(3) or less. We also investigated whether levels of NOx were associated with immunophenotypic characteristic of HIV-infected patients. Our results showed a statistically significant increase in NOx levels in the HIV-untreated group (164.0 ± 166.6 μmol/L), compared with both the control (98.9 ± 59.4 μmol/L) and HIV-treated group (71.7 ± 53.3 μmol/L). Multiple regression analysis showed that the differences in NOx level were independent of gender, liver enzyme level, lipid measurement, and hematological parameters. In addition, a lower CD4/CD8 ratio was associated with higher NOx levels in HIV-infected patients. The results further revealed that NOx levels were increased in HIV infection, and that derangement of immune system function was associated with increased NO levels. The levels of NOx were found to decline with the use of HAART, which may contribute to cardiovascular disease in HIV-infected patients.

The oxidative stress-induced niacin sink (OSINS) model for HIV pathogenesis

Although several specific micronutrient deficiencies are associated with disease progression and increased mortality risk in HIV/AIDS, and even a simple multivitamin/mineral supplement can prolong survival, this is typically viewed merely as nutritional support of the immune system, and only necessary if there are deficiencies to be rectified. However, the reality is more complex. Several striking nutrient-related metabolic abnormalities have been consistently documented in HIV infection. One is chronic oxidative stress, including a drastic depletion of cysteine from the glutathione pool, and a progressive decline of serum selenium that is correlated with disease progression and mortality. Another is decreased blood levels of tryptophan, with an associated intracellular niacin deficiency. Tryptophan depletion or "deletion" by induction of indoleamine-2,3-dioxygenase (IDO), the first step in oxidative tryptophan metabolism, is a known mechanism for immune suppression that is of critical importance in cancer and pregnancy, and, potentially, in HIV/AIDS. Existing evidence supports the hypothesis that these nutrient-related metabolic abnormalities in HIV infection regarding antioxidants, selenium, sulfur, tryptophan and niacin are interrelated, because HIV-associated oxidative stress can induce niacin/NAD+ depletion via activation of poly(ADP-ribose) polymerase (PARP), which could lead to tryptophan oxidation for compensatory de novo niacin synthesis, thereby contributing to immune tolerance and T-cell loss via tryptophan deletion and PARP-induced cell death. This "oxidative stress-induced niacin sink" (OSINS) model provides a mechanism whereby the oxidative stress associated with HIV infection can contribute to immunosuppression via tryptophan deletion. This model is directly supported by evidence that antioxidants can counteract indoleamine-2,3-dioxygenase (IDO), providing the critical link between oxidative stress and tryptophan metabolism proposed here. The OSINS model can be used to guide the design of nutraceutical regimens that can effectively complement antiretroviral therapy for HIV/AIDS.

HIV-1 induced decrease of nitric oxide production and inducible nitric oxide synthase expression during in vivo and in vitro infection

Nitric oxide (*NO) has been implicated in immunopathogenesis of HIV-1 infection. Initial reports using low sensitive techniques showed elevated levels of *NO in sera and tissues from seropositive patients. These results were not further supported using similar experimental approaches. To gain insight on *NO deregulation during HIV-1 infection, we used recently described fluorescent probes with enhanced sensitivity to assess *NO levels combined with iNOS mRNA expression in peripheral blood mononuclear cells (PBMC) from HIV-infected patients or after in vitro HIV-1 infection of normal cells. We demonstrate that PBMC from HIV-infected patients display a significant decrease of *NO production and iNOS mRNA expression. Results from in vitro infection showed that HIV-1 induces a significant decrease in *NO production and iNOS mRNA expression. Since *NO could play a role in some key processes like apoptosis, regulation of immune responses and viral replication, these results could help in elucidating HIV-1 immunopathogenesis.

Protein S-nitrosation: Biochemistry and characterization of protein thiol–NO interactions as cellular signals

The interaction of nitric oxide with thiols is complex and still an active area of research. Herein, we provide an overview of the ways in which nitric oxide can be biologically transformed into species capable of adding an NO moiety to protein sulfhydryls, emphasizing how protein S-nitrosation differs from nitrosation of low molecular weight thiols. Protein S-nitrosation is being revealed as a post-translational means of chemically modifying and functionally altering proteins. Changes in protein function, which persist on a physiologically relevant time scale, effectively transmit biological signals and thus provide a framework for elucidating signaling networks. A description of recently developed methodology facilitating inquiry into this area is provided, along with a sketch of various proteins reported to be targets for nitrosation and the functional consequences therein. Protein denitrosation appears to be an active and perhaps enzymatically catalyzed process. Here, we summarize the evidence that suggests this and proffer a précis of proteins possessing denitrosation activity.

Role of nitric oxide in HIV-1 infection: friend or foe?

Nitric oxide (NO) is an important biologically active molecule that plays a key part in host defence against bacteria, protozoa, and tumour cells. NO has antiviral effects against several DNA viruses, such as murine poxvirus, herpes simplex virus, and Epstein-Barr virus, and some RNA viruses, such as coxsackievirus. In several studies, in vitro and in vivo, overproduction of NO has been noted in the presence of HIV-1 infection. Furthermore, increased NO production may contribute directly to the pathogenesis of HIV-1-associated dementia. The mechanisms of virus infection mediated by NO may be related to: direct antiviral effects of NO; impairment of antiviral defence mediated by T-helper-1 immune response by suppressing T-helper-1 functions; NO-induced cytotoxic effects by oxidative injury with cellular and organ dysfunctions; and NO-induced oxidative stress leading to rapid viral evolution with productions of drug-resistant and immunologically tolerant mutants. By contrast, there is some evidence of NO activity--directly, indirectly, or both--decreasing or blocking HIV-1 replication, through inhibition of viral enzymes, such as reverse transcriptase, protease, or cellular nuclear transcription factor (NF-kappa B) and long-terminal repeat-driven transcription. Therefore, although NO surely plays an important part in HIV-1 infection, that role is sometimes helpful and other times damaging to the host. Future challenges are to learn more about the beneficial and harmful effects of NO in HIV-1 infection, and how to selectively inhibit excessive NO production or to use NO-releasing drugs to decrease viral replication. This review discusses the role of NO in the pathogenesis of HIV-1 infection, inasmuch as its role against HIV-1 is unequivocal in inhibiting or increasing viral replication.

Central role for interleukin-4 in regulating nitric oxide-mediated inhibition of T-cell proliferation and gamma interferon production in schistosomiasis

Schistosoma mansoni-infected wild-type (WT) mice develop a Th2 response and chronic disease. In contrast, infected interleukin-4 double-deficient (IL-4(-/-)) mice develop a Th1-like response and an acute, lethal syndrome. Disease severity in these animals correlates with excessive and prolonged production of nitric oxide (NO) associated with enhanced antigen-driven gamma interferon (IFN-gamma) production in the absence of IL-4. Strikingly, splenic lymphocytes from infected IL-4(-/-) mice failed to proliferate as well as those from infected WT mice following stimulation in vitro with antigen or anti-CD3 antibody. Contrary to antigen-driven IFN-gamma responses, anti-CD3 antibody stimulation of splenocytes resulted in significantly less IFN-gamma being produced by CD8 cells from infected IL-4(-/-) mice than by those from infected WT mice or normal mice. NO is largely responsible for the impaired T-cell functions in infected IL-4(-/-) mice, as inhibition of iNOS significantly enhanced proliferation and IFN-gamma production.

The regulatory role of nitric oxide in apoptosis

Nitric oxide (NO) is a multi-faceted molecule with dichotomous regulatory roles in many areas of biology. The complexity of its biological effects is a consequence of its numerous potential interactions with other molecules such as reactive oxygen species (ROS), metal ions, and proteins. The effects of NO are modulated by both direct and indirect interactions that can be dose-dependent and cell-type specific. For example, in some cell types NO can promote apoptosis, whereas in other cells NO inhibits apoptosis. In hepatocytes, NO can inhibit the main mediators of cell death-caspase proteases. Moreover, low physiological concentrations of NO can inhibit apoptosis, but higher concentrations of NO may be toxic. High NO concentrations lead to the formation of toxic reaction products like dinitrogen trioxide or peroxynitrite that induce cell death, if not by apoptosis, then by necrosis. Long-term exposure to nitric oxide in certain conditions like chronic inflammatory states may predispose cells to tumorigenesis through DNA damage, inhibition of DNA repair, alteration in programmed cell death, or activation of proliferative signaling pathways. Understanding the regulatory mechanisms of NO in apoptosis and carcinogenesis will provide important clues to the diagnosis and treatment of tissue damage and cancer. In this article we have reviewed recent discoveries in the regulatory role of NO in specific cell types, mechanisms of pro-apoptotic and anti-apoptotic induction by NO, and insights into the effects of NO on tumor biology.

The adenine nucleotide translocator: a target of nitric oxide, peroxynitrite, and 4-hydroxynonenal

Nitric oxide (NO), peroxynitrite, and 4-hydroxynonenal (HNE) may be involved in the pathological demise of cells via apoptosis. Apoptosis induced by these agents is inhibited by Bcl-2, suggesting the involvement of mitochondria in the death pathway. In vitro, NO, peroxynitrite and HNE can cause direct permeabilization of mitochondrial membranes, and this effect is inhibited by cyclosporin A, indicating involvement of the permeability transition pore complex (PTPC) in the permeabilization event. NO, peroxynitrite and HNE also permeabilize proteoliposomes containing the adenine nucleotide translocator (ANT), one of the key components of the PTPC, yet have no or little effects on protein-free control liposomes. ANT-dependent, NO-, peroxynitrite- or HNE-induced permeabilization is at least partially inhibited by recombinant Bcl-2 protein, as well as the antioxidants trolox and butylated hydroxytoluene. In vitro, none of the tested agents (NO, peroxynitrite, HNE, and tert-butylhydroperoxide) causes preferential carbonylation HNE adduction, or nitrotyrosylation of ANT. However, all these agents induced ANT to undergo thiol oxidation/derivatization. Peroxynitrite and HNE also caused significant lipid peroxidation, which was antagonized by butylated hydroxytoluene but not by recombinant Bcl-2. Transfection-enforced expression of vMIA, a viral apoptosis inhibitor specifically targeted to ANT, largely reduces the mitochondrial and nuclear signs of apoptosis induced by NO, peroxynitrite and HNE in intact cells. Taken together these data suggest that NO, peroxynitrite, and HNE may directly act on ANT to induce mitochondrial membrane permeabilization and apoptosis.