Porcine reproductive and respiratory syndrome virus non-structural protein 1 suppresses tumor necrosis factor-alpha promoter activation by inhibiting NF-κB and Sp1

The objective of this study was to identify porcine reproductive and respiratory syndrome virus (PRRSV)-encoded proteins that are responsible for the inhibition of TNF-alpha expression and the mechanism(s) involved in this phenomenon. Using a TNF-alpha promoter reporter system, the non-structural protein 1 (Nsp1) was found to strongly suppress the TNF-alpha promoter activity. Such inhibition takes place especially at the promoter's proximal region. Both Nsp1 alpha and Nsp1 beta, the two proteolytic fragments of Nsp1, were shown to be involved in TNF-alpha promoter suppression. Furthermore, using reporter plasmids specific for transcription factors (TFs) that bind to TNF-alpha promoter, Nsp1 alpha and Nsp1 beta were demonstrated to inhibit the activity of the TFs that bind CRE-kappa B-3 and Sp1 elements respectively. Subsequent analyses showed that Nsp1 alpha moderately inhibits NF-kappa B activation and that Nsp1 beta completely abrogates the Sp1 transactivation. These findings reveal one of the important mechanisms underlying the innate immune evasion by PRRSV during infection. (C) 2010 Elsevier Inc. All rights reserved.