Essential role of human leukocyte antigen-encoded proteasome subunits in NF-κB activation and prevention of tumor necrosis factor-α-induced apoptosis

The multisubunit proteasome complex is the principal mediator of nonlysosomal protein degradation. The proteasome subunit varies minimally between cells with the exception of LMP2, LMP7, and LMP10 subunits in rodent and human cells. LMP2 and LMP7 subunits are encoded by the human lymphocyte antigen region, and they optimize proteolytic mediated antigen presentation. The proteasome is also important for the function of transcription factor nuclear factor-kappa B (NF-kappa B). It is required for NF-kappa B subunits p50 and p52 generation and catalyzes degradation of phosphorylated I kappa B alpha. These proteasome-mediated reactions have now been shown to be defective in T2 cells, a human lymphocyte cell line that lacks both LMP2 and LMP7. Although T2 cells contain normal expression of p100 and p105, the abundance of p50 and p52 was greatly reduced. Tumor necrosis factor-alpha (TNF-alpha) induced normal phosphorylation of I kappa B alpha but failed to induce degradation of phosphorylated I kappa B alpha. Both DNA binding assays and luciferase assays revealed that TNF-alpha-induced NF-kappa B activation is defective in T2 cells. Unlike parental cells, T2 cells were susceptible to TNF-alpha-induced apoptosis. These data indicate human leukocyte antigen-linked proteasome subunits are essential for NF-kappa B activation and protection of cells from TNF-alpha-induced apoptosis.