Extracellular human immunodeficiency virus type type 1 (HIV-1) viral protein R (Vpr) is a pleiotropic protein accomplishing several functions within the viral life cycle. While Vpr has been described extensively as an intracellular protein, very little is known about its role as an extracellular protein. In fact, HIV-1 Vpr has been detected in the blood, serum, and cerebrospinal fluid of HIV-1-infected patients, with concentrations increasingly higher in late-stage disease. To determine the role exogenous Vpr plays in HIV-associated central nervous system dysfunction, primary human fetal astrocytes were exposed to recombinant Vpr and a time- and dose-dependent decrease was demonstrated in two fundamental intracellular metabolites (ATP and glutathione (GSH)). Additionally, exposure to exogenous Vpr led to increased caspase activity and secretion of proinflammatory cytokines IL-6 and IL-8 and chemoattractants, monocyte chemotactic protein-1 and migration inhibition factor. Extracellular Vpr also dampened the glycolytic pathway through impairment of GAPDH activity, causing a decline in the levels of ATP. The reduction in intracellular ATP increased reactive oxygen species buildup, decreasing GSH concentrations, which affected several genes in the oxidative stress pathway. In addition, exposure of the SK-N-SH neuroblastoma cell line to conditioned medium from exogenous Vpr-treated astrocytes decreased synthesis of GSH, leading to their apoptosis. These observations point to a role that Vpr plays in altering astrocytic metabolism and indirectly affecting neuronal survival. We propose a model that may explain some of the neurological damage and therefore neurocognitive impairment observed during the course of HIV-1 disease.

Patients infected with human immunodeficiency virus type 1 (HIV-1) often display neurological complications in late stage disease and increased viral loads directly correlated with higher concentrations of extracellular HIV-1 viral protein r (Vpr) in the blood serum and cerebrospinal fluid. Additionally, HIV-1-infected patients with a low CD4+ T-lymphocyte count displayed lower concentrations of reduced glutathione (GSH), the main intracellular antioxidant molecule, and lower level of survival. To establish a correlation between increased concentrations of extracellular Vpr and an oxidative stress-induced phenotype, the U-87 MG astroglioma cell line has been used to determine the downstream effects induced by Vpr. Conditioned media obtained from the human endothelial kidney (HEK) 293T cell line transfected either in the absence or presence of HIV-1 Vpr contained free Vpr. Exposure of U-87 MG to this conditioned media decreased intracellular levels of both adenosine triphosphate (ATP) and GSH. These observations were recapitulated using purified recombinant HIV-1 Vpr both in U-87 MG and primary human fetal astrocytes in a dose- and time-dependent manner. Vpr-induced oxidative stress could be partly restored by co-treatment with the antioxidant molecule N-acetyl-cysteine (NAC). In addition, free Vpr augmented production of reactive oxygen species due to an increase in the level of oxidized glutathione (GSSG). This event was almost entirely suppressed by treatment with an anti-Vpr antibody or co-treatment with NAC. These studies confirm a role of extracellular Vpr in impairing astrocytic levels of intracellular ATP and GSH. Studies are underway to better understand the intricate correlation between reductions in ATP and GSH metabolites and how they affect neuronal survival in end-stage disease.

Numerous studies published in the past two decades have identified the viral protein R (Vpr) as one of the most versatile proteins in the life cycle of human immunodeficiency virus type 1 (HIV-1). In this regard, more than a thousand Vpr molecules are present in extracellular viral particles. Subsequent to viral entry, Vpr participates in early replicative events by assisting in viral genome nuclear import and, during the viral life cycle, by shuttling between the nucleus and the cytoplasm to accomplish its functions within the context of other replicative functions. Additionally, several studies have implicated Vpr as a proapoptotic protein because it promotes formation of permeability transition pores in mitochondria, which in turn affects transmembrane potential and adenosine triphosphate synthesis. Recent studies have identified Vpr as a virion-free protein in the serum and cerebrospinal fluid of patients infected with HIV-1 whose plasma viremia directly correlates with the extracellular concentration of Vpr. These observations pointed to a new role for Vpr as an additional weapon in the HIV-1 arsenal, involving the use of an extracellular protein to target and possibly inhibit HIV-1-uninfected bystander cells to enable them to escape immune surveillance. In addition, extracellular Vpr decreases aden-osine triphosphate levels and affects the intracellular redox balance in neurons, ultimately causing their apoptosis. Herein, we review the role of Vpr as an extracellular protein and its downstream effects on cellular metabolism, functionality, and survival, with particular emphasis on how extracellular Vpr-induced oxidative stress might aggravate HIV-1-induced symptoms, thus affecting pathogenesis and disease progression.