Immunoproteasome Deficiency Modifies the Alternative Pathway of NFκB Signaling

Immunoproteasome is a protease abundant in immune cells and also present, albeit at lower concentrations, in cells outside the immune system. Recent evidence supports a novel role for the immunoproteasome in the cellular stress response potentially through regulation of NF kappa B signaling, which is the primary response to multiple stressors. The current study tests whether the Classical or Alternative Pathways are regulated by immunoproteasome following chronic TNF alpha exposure in cultured retinal pigment epithelial cells isolated from wild-type mice and mice deficient in one (LMP2, L2) or two (LMP7 and MECL-1, L7M1) immunoproteasome subunits. Assays were performed to assess the expression of NF kappa B responsive genes, the content and activity of NF kappa B transcription factors (p65, p50, p52, cRel, RelB), and expression and content of regulatory proteins (I kappa B alpha, A20, RPS3). Major findings include distinct differences in expression of NF kappa B responsive genes in both KO cells. The mechanism responsible for the altered gene expression could not be established for L7M1 since no major differences in NF kappa B transcription factor content or activation were observed. However, L2 cells exhibited substantially higher content and diminished activation of NF kappa B transcription factors associated with the Alternative Pathway and delayed termination of the Classical Pathway. These results provide strong experimental evidence supporting a role for immunoproteasome in modulating NF kappa B signaling.