Abstract
Moloney murine leukemia virus (MoMuLV)- ts 1-mediated neuronal degeneration in mice is likely due to loss of glial support and release of inflammatory cytokines and neurotoxins from surrounding ts 1-infected glial cells including astrocytes. NF- s B is a transcription factor that participates in the transcriptional activation of a variety of immune and inflammatory genes. We investigated whether ts 1 activates NF- s B in astrocytes and examined the mechanism(s) responsible for the activation of NF- s B by ts 1 infection in vitro . Here we present evidence that ts 1 infection of astrocytes in vitro activates NF- s B by enhanced proteolysis of the NF- s B inhibitors, I s B f and I s B g . In in vitro studies using protease inhibitors, I s B f proteolysis in ts 1-infected astrocytes was significantly blocked by a specific calpain inhibitor calpeptin but not by MG-132, a specific proteasome inhibitor, whereas rapid I s B g proteolysis was blocked by MG-132. Furthermore, treatment with MG-132 increased levels of multiubiquitinated I s B g protein in ts 1-infected astrocytes. These results indicate that the calpain proteolysis is a major mechanism of I s B f proteolysis in ts 1-infected astrocytes. Additionally, ts 1 infection of astrocytes in vitro increased expression of inducible nitric oxide synthase (iNOS), a NF- s B-dependent gene product. Our results suggest that NF- s B activation in ts 1-infected astrocytes is mediated by enhanced proteolysis of I s B f and I s B g through two different proteolytic pathways, the calpain and ubiquitin-proteasome pathways, resulting in increased expression of iNOS, a NF- s B-dependent gene.