These studies were performed to identify biomarkers that could be linked to cognitive impairments in HIV-1-infected women. Previous data on other HIV NeuroAIDS cohorts have been predominately from male participants. A longitudinal NeuroAIDS cohort with antiviral therapy was established in 2002 as part of a Specialized Neuroscience Research Program, at the University of Puerto Rico. This cohort is well characterized [17
]. 2D-DIGE analysis of monocyte samples from this cohort provided six possible protein targets that appeared to be differentially regulated between normal cognition and HAD. All six of these proteins were decreased in HAD. Although differences in age were found between those with normal cognition and those with HAD, they did not correlate with the expression of antioxidants (data not shown).
We chose to validate thioredoxin, peroxiredoxin, and myeloperoxidase because of their role in the control of oxidative stress. Thioredoxin and peroxiredoxin were validated by flow cytometry, but myeloperoxidase did not differ between patients with and those without HAD. The downregulation of both thioredoxin and peroxiredoxin in peripheral monocytes implies decreased antioxidant capabilities of peripheral monocytes in patients with HAD. Because we previously demonstrated a downregulation in SOD-1 in monocytes from women with HAD by ELISA, we wanted to validate this finding by flow cytometry as well, as viable cells from the longitudinal cohort and this method do not require physical separation of cells which is best for analysis of different cell populations in a small sample size. We obtained comparable results, further confirming that peripheral monocytes have decreased antioxidant proteins in individuals with HAD.
Thioredoxin is an antioxidant protein that facilitates the reduction of proteins by cysteine thiol-disulfide exchange through two redox-active cysteine residues in its active center. Thioredoxin operates with NADPH and thioredoxin reductase to reduce exposed disulfides [20
]. Thioredoxin cooperates with peroxiredoxins for an antiapoptotic effect through scavenging of intracellular hydrogen peroxide [21
]. Serum thioredoxin levels are known to be elevated during HIV-1 infection as compared with levels in uninfected controls [22
]. Elevated thioredoxin levels during HIV-1 infection demonstrate a mechanism by which cells work to control oxidative stress levels and prolong cell survival. With a decrease in thioredoxin expression during HAD, there is presumably an increase in oxidative stress, resulting in increased apoptosis and inflammatory signals.
Peroxiredoxins, thioredoxin-dependent peroxidases, protect against apoptosis by scavenging hydrogen peroxide. Peroxiredoxin 3 is expressed within the mitochondria [23
]. Natural killer enhancing factors (NKEF)-A and NKEF-B, members of the peroxiredoxin family, are upregulated during HIV-1 infection in CD8+ T-cells and are thought to contribute to the antiviral activity of CD8+ T-cells [24
]. Some peroxiredoxins interfere with HIV transcription by inactivation of HIV-1 long terminal repeat, thereby suppressing levels of p24 [25
]. Decreases in peroxiredoxin levels of individuals with HAD may contribute to increased HIV replication and intracellular oxidative stress, later translating to neurotoxicity.
Many neurodegenerative disorders are characterized by increased oxidative stress, such as Parkinson’s disease, Alzheimer’s disease, and amyotrophic lateral sclerosis to name a few. Recently some oxidative stress-related proteins related to HAD have been elucidated. Hypoxia-inducible factor 1(HIF-1) is activated upon HIV-1 infection and has been found to be elevated in brains of AIDS patients [26
]. The HIV-1 accessory protein, Vpr, has also been implicated in mediating HIF-1 induction [26
]. In other studies, CSF and monocytes from HAD patients had decreased expression and activity of Cu/Zn superoxide dismutase, indicating that oxidative stress may exacerbate HAD [19
Traditionally, HIV-1 levels are known to be highest in the basal ganglia and hippocampus [27
]. Argawal et al. investigated the loss of dopaminergic neurons in the substantia niagra due to reactive oxygen species (ROS) and GP120 administration, which was attenuated by administration of SOD or glutathione peroxidase [28
]. Degeneration of the substantia nigra is a hallmark of Parkinson’s disease, and therefore has been investigated in HAND due to some similarities seen between these two neurodegenerative diseases. SOD delivery in rodent models of GP120 neurotoxicity results in protection of dopaminergic neurons [28
]. Recently, we identified SOD downregulation in monocytes of HAND-affected individuals [19
]. This downregulation was confirmed in our current study in a larger number of individuals with HAND. We have also identified a down-regulation in SOD in in-vitro
assays comparing viral isolates from individuals with HAD to those without neurocognitive impairment [30
]. These studies prompted further investigation into changes that occur in monocyte protein expression during HAD. Downregulation of SOD combined with the loss of more specific antioxidant proteins results in unprotected ROS damage to cells.
Decreased antioxidant levels can have detrimental effects on both the affected cell and surrounding cells. Losing the ability to scavenge reactive oxygen species results in detrimental effects not just on MP, but also on astrocytes and neurons. Astrocytes are known to compensate for toxic MP secretions in in vitro
models of HIV-1 infection [31
], but as disease progresses this careful control of homeostasis erodes and neurotoxicity increases uncontrolled by these cells [32
]. They may become overwhelmed by ROS production and therefore cannot sufficiently protect neurons in advanced stages of disease.
This study has uncovered antioxidant proteins that are downregulated in HAD. These proteins will be further investigated to understand the cause for downregulation and how to prevent or treat this deficiency. While we understand the effects of a decrease in antioxidant capabilities, what would cause such a decrease in some individuals with HIV-1 infection and not in others still remains a mystery.