Enhanced proteolysis of I kappa B alpha and I kappa B beta proteins in astrocytes by Moloney murine leukemia virus (MoMuLV)-ts1 infection: A potential mechanism of NF-kappa B activation

Moloney murine leukemia virus (MoMuLV)-ts1-mediated neuronal degeneration i n mice is likely due to loss of glial support and release of inflammatory c ytokines and neurotoxins from surrounding tsi-infected glial cells includin g astrocytes. NF-kappaB is a transcription factor that participates in the transcriptional activation of a variety of immune and inflammatory genes. W e investigated whether ts1 activates NF-kappaB in astrocytes and examined t he mechanism(s) responsible for the activation of NF-kappaB by tsi infectio n in vitro. Here we present evidence that tsi infection of astrocytes in vi tro activates NF-kappaB by enhanced proteolysis of the NF-kappaB inhibitors , I kappaB alpha and I kappaB beta. In in vitro studies using protease inhi bitors, I kappaB alpha proteolysis in ts1-infected astrocytes was significa ntly blocked by a specific calpain inhibitor calpeptin but not by MG-132, a specific proteasome inhibitor, whereas rapid I kappaB beta proteolysis was blocked by MG-132. Furthermore, treatment with MG-132 increased levels of multi-ubiquitinated I kappaB beta protein in ts1-infected astrocytes. These results indicate that the calpain proteolysis is a major mechanism of I ka ppaB alpha proteolysis in ts1-infected astrocytes. Additionally, tsi infect ion of astrocytes in vitro increased expression of inducible nitric oxide s ynthase (iNOS), a NF-kappaB-dependent gene product. Our results suggest tha t NF-kappaB activation in tsi-infected astrocytes is mediated by enhanced p roteolysis of I kappaB alpha and I kappaB beta through two different proteo lytic pathways, the calpain and ubiquitin-proteasome pathways, resulting in increased expression of iNOS, a NF-kappaB-dependent gene.