The immunoproteasome is induced by cytokines and regulates apoptosis in human islets

In addition to degrading misfolded and damaged proteins, the proteasome regulates the fate of cells in response to stress. The role of the proteasome in pro-inflammatory cytokine-induced human beta-cell apoptosis is unknown. INS-1, INS-1E and human islets were exposed to combinations of IFNγ, IL-1β and TNFα with or without addition of small molecules. Gene expression was assessed by microarray or quantitative PCR. Proteasome activities were analyzed using luminescent assays. Protein oxidation, Western blotting and electron microscopy were used to examine mechanisms underlying cytokine-induced apoptosis. We found that cytokines induce the expression and activity of the immuno-proteasome in INS-1E cells and human islets. Cytokine-induced immuno-proteasome expression but not activity depended upon histone deacetylase 3 activation. Inhibition of JAK1/STAT1 signaling did not affect proteasomal activity. Inhibition of the immuno-proteasome subunit Psmb8 aggravated cytokine-induced human beta-cell apoptosis while reducing intracellular levels of oxidized proteins in INS-1 cells. While cytokines increased phospho-JNK and total cellular NFκB subunit p50 and p52 levels and reduced the cytosolic NFκB subunit p65 and IκB levels, these effects were unaffected by Psmb8 inhibition. We conclude that beta cells upregulate immuno-proteasome expression and activity in response to IFNγ, likely as a protective response to confine inflammatory signaling.