U50,488 protection against HIV-1-related neurotoxicity: involvement of quinolinic acid suppression
Introduction
Human immunodeficiency virus type 1 (HIV-1) encephalopathy, or acquired immune deficiency syndrome (AIDS) dementia, is a complication of central nervous system (CNS) viral infection, resulting in severe cognitive and motor abnormalities (Price et al., 1988). The histopathological findings associated with this neurological disorder include HIV-1 infection of microglia, multinucleated giant cells, astrocyte proliferation, and loss of neurons in discrete regions of the brain (Navia et al., 1986, Ketzler et al., 1990). For unclear reasons, this brain disease is most prevalent in HIV-infected children (Cohen et al., 1991). The molecular mechanisms underlying HIV-1 encephalopathy have not been completely elucidated. Since neurons do not appear to be permissively infected, secretory products of HIV-1-infected monocytes and microglia have been postulated to be the source of neurotoxic factors (Budka, 1989, Giulian et al., 1990, Pulliam et al., 1991, Gendelman et al., 1994, Masliah et al., 1996).
Mounting evidence has demonstrated that viral proteins, such as gp120 (Dreyer et al., 1990), Tat (Tardieu et al., 1992, New et al., 1997), and gp41 (Adamson et al., 1996) are neurotoxic. In addition to viral proteins, release of a variety of neurotoxins from HIV-1-infected microglia or macrophages, such as platelet-activating factor (Gelbard et al., 1994), arachidonic acid (Genis et al., 1992), tumor necrosis factor-α (Gelbard et al., 1993, Wilt et al., 1995), reactive oxygen (Talley et al., 1995) and nitrogen (Bukrinsky et al., 1995) intermediates, a neurotoxic amine (Giulian et al., 1996), and the N-methyl-d-aspartate (NMDA) receptor ligand quinolinate (Nottet et al., 1996, Kerr et al., 1997), has been proposed to be of pathogenic importance. Also, NMDA receptor activation has been suggested as a final pathway by which these diverse factors induce neuronal dysfunction and death (Lipton and Rosenberg, 1994). In addition to the endogenous NMDA receptor ligand quinolinate (Heyes et al., 1989, Achim et al., 1993, Brew et al., 1995, Venkateshan et al., 1996), a neurotoxic amine (Giulian et al., 1996) could also act at NMDA receptors to induce excitotoxicity.
Based upon the findings of a growing number of studies, opioid peptides recently have been postulated to play an immunomodulatory role in the CNS (Sheng et al., 1997). Of the three major classes of endogenous opioids (μ, δ, and κ) (Reisine, 1995), κ opioid receptor (KOR) ligands have been shown both in vitro and in vivo studies to have neuroprotective effects against certain CNS insults. In the present study, we used human brain cell cultures to test the neurotoxic activity of supernatants derived from acutely infected microglial cells. Because we recently had found that the selective KOR selective ligand U50,488 suppressed HIV-1 p24 antigen (Ag) production in acutely infected microglial cell cultures (Chao et al., 1996), we investigated whether this synthetic KOR ligand would protect against HIV-1-induced neurotoxicity by suppressing the production of viral proteins and quinolinate by infected microglia.
Section snippets
Reagents
The selective KOR ligand U50,488 was a gift of Pharmacia Upjohn Company (Kalamazoo, MI). The κ-selective antagonist nor-binaltorphimine (Nor-BNI) was kindly provided by P.S. Portoghese (University of Minnesota, Minneapolis, MN). Other reagents were purchased from the indicated sources: antibodies to microglial cell CD68 antigen and astrocyte glial fibrillary acidic protein (GFAP) (Dako, Carpinteria, CA); anti-neuron specific enolase antibodies (Polysciences, Warrington, PA); NMDA receptor ion
Effect of U50,488 on HIV-1 expression
Treatment of microglial cell cultures with U50,488 for 24 h prior to infection with HIVSF162 resulted in marked suppression of HIV-1 p24 Ag production on day 14 of cell culture (Fig. 1A). Maximal inhibition (approx. 50%) was detected when U50,488 was added to the cultures at concentrations between 1 and 100 pM. Incubation of microglial cell cultures with Nor-BNI (10 pM) blocked U50,488 (1 pM)-induced inhibition of HIV-1 expression by 84% (Fig. 1B), suggesting that U50,488’s effect was mediated
Discussion
κ-Opioids recently have been found to have neuroprotective properties in animal models of ischemic brain injury (Itoh et al., 1993, Widmayer et al., 1994). In the present study, the neuroprotective potential of U50,488 was extended to a viral infection of the CNS, i.e. HIV-1 encephalopathy. We found that the synthetic KOR ligand U50,488 protected against HIV-1-related neurotoxicity in an in vitro model in which supernatants derived from infected microglia are added to highly enriched neuronal
Acknowledgments
This study was supported in part by United Public Health Service grants DA09924, DA04381, and T32-DA07239 from the National Institute on Drug Abuse.
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