Diametrically opposed effects of hypoxia and oxidative stress on two viral transactivators

Abstract

BackgroundMany pathogens exist in multiple physiological niches within the host. Differences between aerobic and anaerobic conditions are known to alter the expression of bacterial virulence factors, typically through the conditional activity of transactivators that modulate their expression. More recently, changes in physiological niches have been shown to affect the expression of viral genes. For many viruses, differences in oxygen tension between hypoxia and normoxia alter gene expression or function. Oxygen tension also affects many mammalian transactivators including AP-1, NFkB, and p53 by affecting the reduced state of critical cysteines in these proteins. We have recently determined that an essential cys-x-x-cys motif in the EBNA1 transactivator of Epstein-Barr virus is redox-regulated, such that transactivation is favoured under reducing conditions. The crucial Tat transactivator of human immunodeficiency virus (HIV) has an essential cysteine-rich region, and is also regulated by redox. Contrary to EBNA1, it is reported that Tat's activity is increased by oxidative stress. Here we have compared the effects of hypoxia, oxidative stress, and cellular redox modulators on EBNA1 and Tat.ResultsOur results indicate that unlike EBNA1, Tat is less active during hypoxia. Agents that generate hydroxyl and superoxide radicals reduce EBNA1's activity but increase transactivation by Tat. The cellular redox modulator, APE1/Ref-1, increases EBNA1's activity, without any effect on Tat. Conversely, thioredoxin reductase 1 (TRR1) reduces Tat's function without any effect on EBNA1.ConclusionsWe conclude that oxygen partial pressure and oxidative stress affects the functions of EBNA1 and Tat in a dramatically opposed fashion. Tat is more active during oxidative stress, whereas EBNA1's activity is compromised under these conditions. The two proteins respond to differing cellular redox modulators, suggesting that the oxidized cysteine adduct is a disulfide bond(s) in Tat, but sulfenic acid in EBNA1. The effect of oxygen partial pressure on transactivator function suggests that changes in redox may underlie differences in virus-infected cells dependent upon the physiological niches they traffic to.

Contributions of the EBNA1 Protein of Epstein-Barr Virus Toward B-Cell Immortalization and Lymphomagenesis

“…Therefore, oxidized cysteine adducts reduced by APE1 are not acted on by b-ME and DTT. We determined that treating cells with b-ME or DTT did not alter EBNA1's ability to transactivate, or ameliorate the effect of oxidative stress on transactivation (Washington, Singh et al 2010). Therefore, it is unlikely that oxidative stress reduces EBNA1's ability to transactivate by creating intra-or intermolecular disulfide bridges.…”

“…Many viral and mammalian transactivators, including AP-1, NFκB, the Tat protein of human immunodeficiency virus (HIV), and the E2 protein of papillomavirus, are regulated by redox (Hutchison, Matic et al 1991;Xanthoudakis and Curran 1992;Xanthoudakis, Miao et al 1992;Huang and Adamson 1993;Mitomo, Nakayama et al 1994;Xanthoudakis and Curran 1996;Jayaraman, Murthy et al 1997;Okamoto, Tanaka et al 1999;Kalantari, Narayan et al 2008;Wan, Ottinger et al 2008;Washington, Singh et al 2010). These proteins contain one or several cysteine residues susceptible to oxidation, whose redox status regulates the capacity to transactivate, due to the effect of oxidative stress on protein structure.…”

“…Proteins containing zinc fingers essential for their function are especially sensitive to oxidative stress because the thiol group, but not oxidized thiol adducts, can coordinate zinc (Kiley and Storz 2004). In light of EBNA1's UR1 domain containing two conserved cysteines that are critical for transactivation, we tested whether transactivation was susceptible to oxidative stress or alterations in environmental oxygen tension Washington, Singh et al 2010). Exposure of EBNA1-expressing cells to low levels of agents that generate intracellular hydroxyl and superoxide radicals, such as menadione and paraquat, dramatically reduced transactivation without substantially affecting cell survival or proliferation.…”

“…Cellular and viral transactivators with redox-sensitive cysteines are often regulated by cellular redox effectors such as thioredoxin, thioredoxin reductase and the bifunctional enzyme AP-endonuclease 1 (APE1) (Hutchison, Matic et al 1991;Xanthoudakis and Curran 1992;Xanthoudakis, Miao et al 1992;Huang and Adamson 1993;Mitomo, Nakayama et al 1994;Xanthoudakis and Curran 1996;Jayaraman, Murthy et al 1997;Okamoto, Tanaka et al 1999;Kalantari, Narayan et al 2008;Wan, Ottinger et al 2008;Washington, Singh et al 2010). Thioredoxin and thioredoxin reductase can reduce disulfide bridges that result from cysteine oxidation, and therefore function similarly to small molecules such as bmercaptoethanol (b-ME) and dithiothreitol (DTT) (Lothrop, Ruggles et al 2009).…”

Hypoxia Induces the Gene Expression and Extracellular Transmission of Persistent Lymphocytic Choriomeningitis Virus

“…There is also growing evidence that hypoxia and HIF affect the host-virus interaction and hence the pathogenesis of the viruses that either posses a DNA genome or pass through a DNA stage, contain hypoxia-responsive elements in their promoters, and need the nucleus for their replication. For example, hypoxia enhances gene expression of human B19 erythrovirus (9,29) and replication of oncolytic herpes simplex virus (1) and induces lytic replication of Epstein-Barr virus (18,41) and Kaposi sarcoma-associated herpesvirus (KSHV) (12,15), but it suppresses replication of the oncolytic parvovirus minute virus of mice (35), adenovirus (36), Moloney murine leukemia virus (M-MuLV) (30), and HIV-1 (11,41).…”