HIV‐1 Tat inhibits NGF‐Induced Egr‐1 transcriptional activity and consequent p35 expression in neural cells

Abstract

Infection with HIV-1 causes degeneration of neurons leading to motor and cognitive dysfunction in AIDS patients. One of the key viral regulatory proteins, Tat, which is released by infected cells, can be taken up by various uninfected cells including neurons and by dysregulating several biological events induces cell injury and death. In earlier studies, we demonstrated that treatment of neuronal cells with Tat affects the nerve growth factor (NGF) signaling pathway involving MAPK/ERK. Here we demonstrate that a decrease in the level of Egr-1, one of the targets for MAPK, by Tat has a negative impact on the level of p35 expression in NGF-treated neural cells. Further, we demonstrate a reduced level of Egr-1 association with the p35 promoter sequence in NGF-treated cells expressing Tat. As p35, by associating with Cdk5, phosphorylates several neuronal proteins including neurofilaments and plays a role in neuronal differentiation and survival, we examined kinase activity of p35 complexes obtained from cells expressing Tat. Results from H1 kinase assays showed reduced activity of the p35 complex from Tat-expressing cells in comparison to that from control cells. Accordingly, the level of phosphorylated neurofilaments was diminished in Tat-expressing cells. Similarly, treatment of PC12 cells with Tat protein or supernatant from HIV-1 infected cells decreased kinase activity of p35 in these cells. These observations ascribe a role for Tat in altering p35 expression and its activity that affects phosphorylation of proteins involved in neuronal cell survival.HIV-1 proteins may induce neuronal apoptosis in AIDS patients with neurological disorders by dysregulating the regulatory events responsible for cell growth, survival, and programmed cell death (Adle-Biassette et al., 1995;New et al., 1997;Macho et al., 1999;Kaul et al., 2001Kaul et al., , 2005Xu et al., 2004;Kaul and Lipton, 2006;King et al., 2006). Upon entry into the CNS, HIV-1 productively infects brain macrophages and microglia leading to production of large amounts of viral proteins including Tat, which, in turn, can induce neuronal damage. Tat is a viral transcriptional activator that plays a critical role in the replication of HIV-1 and has the ability to interact with several important cellular regulatory proteins and alter their function (Dingwall et al., 1989;Jeang et al., 1993;Cujec et al., 1997;Greenberg et al., 1997;Hottiger and Nabel, 1998;Wei et al., 1998;Sawaya et al., 2000;Liu et al., 2002). It is thought that Tat possesses neurotoxic activity through its unusual ability to be released by infected cells and be taken up in a biologically active form by the neighboring uninfected cells including neurons (Frankel and Pabo, 1988;Ensoli et al., 1993;Philippon et al., 1994;Li et al., 1995;Ma and Nath, 1997 understood, earlier studies revealed that activation of glycogen synthase kinase 3β (GSK3β) by Tat via PI3 kinase and Akt may contribute to neurotoxicity of this protein (Maggirwar et al., 1999; Sui et al., 2006a,b). It has also been found tha...

Cell Membrane-bound Kaposi's Sarcoma-associated Herpesvirus-encoded Glycoprotein B Promotes Virus Latency by Regulating Expression of Cellular Egr-1

“…However, the specific function for the HIV Tat protein to stimulate lytic KSHV infection is still being debated (70,71). Moreover, recent evidence suggests that the Tat protein may inhibit Egr-1 expression in target cells (72). Thus, it is possible that the Tat protein may be able to inhibit lytic KSHV infection and promote tumor progression in a manner similar to KSHV gB.…”

“…The gB-and gL-positive cells were sorted using a flow cytometer based on the expression of GFP. The time at which the mixture of cells was seeded onto 12-well plates was considered as 0 h. At different time points (1,4,8,12,24,48,72,96, and 120 h) post-seeding, the cells were lysed, RNA was extracted, cDNA was prepared, and the expression of ORF50 was monitored by qRT-PCR using specific primers. The relative copy numbers of the ORF50 transcripts were calculated from the standard graph plotted using the Ct values for different dilutions of in vitro transcribed ORF50 (using ORF50/pGEM-T plasmid).…”

Neurotrophins modulate the expression of chemokine receptors in the brain

“…Likewise, alteration of the NGF pro-survival pathway by another HIV protein, Tat, has been suggested to play a role in neuronal apoptosis evoked by HIV (Darbinian et al 2008). Moreover, neurodegeneration in HIV subjects is also associated with a decrease in the expression of other growth factors such as acidic fibroblast growth factor (Everall et al 2001).…”

Role of Neurotrophic Factor Alterations in the Neurodegenerative Process in HIV Associated Neurocognitive Disorders

“…On the other hand, gp120 has been shown to affect the expression of both NGF and nitric oxide synthase in rodent models of HIV disease (Bagetta et al, 1996; Corasaniti et al, 1998). Also, the tat has been shown to cooperate with p35 signaling in neurons to dysregulate the NGF pathway, thereby disrupting neuronal survival and differentiation (Peruzzi et al, 2002; Darbinian et al, 2008). …”