Reduction in ATP Levels Triggers Immunoproteasome Activation by the 11S (PA28) Regulator during Early Antiviral Response Mediated by IFNβ in Mouse Pancreatic β-Cells (Publication with Expression of Concern. See vol. 15, 2020)

Autoimmune destruction of insulin producing pancreatic beta-cells is the hallmark of type I diabetes. One of the key molecules implicated in the disease onset is the immunoproteasome, a protease with multiple proteolytic sites that collaborates with the constitutive 19S and the inducible 11S (PA28) activators to produce immunogenic peptides for presentation by MHC class I molecules. Despite its importance, little is known about the function and regulation of the immunoproteasome in pancreatic beta-cells. Of special interest to immunoproteasome activation in beta-cells are the effects of IFN beta, a type I IFN secreted by virus-infected cells and implicated in type I diabetes onset, compared to IFN gamma, the classic immunoproteasome inducer secreted by cells of the immune system. By qPCR analysis, we show that mouse insulinoma MIN6 cells and mouse islets accumulate the immune proteolytic beta 1(i), beta 2(i) and beta 5(i), and 11S mRNAs upon exposure to IFN beta or IFN gamma. Higher concentrations of IFN beta than IFN gamma are needed for similar expression, but in each case the expression is transient, with maximal mRNA accumulation in 12 hours, and depends primarily on Interferon Regulatory Factor 1. IFNs do not alter expression of regular proteasome genes, and in the time frame of IFN beta-mediated response, the immune and regular proteolytic subunits co-exist in the 20S particles. In cell extracts with ATP, these particles have normal peptidase activities and degrade polyubiquitinated proteins with rates typical of the regular proteasome, implicating normal regulation by the 19S activator. However, ATP depletion rapidly stimulates the catalytic rates in a manner consistent with levels of the 11S activator. These findings suggest that stochastic combination of regular and immune proteolytic subunits may increase the probability with which unique immunogenic peptides are produced in pancreatic beta-cells exposed to IFN beta, but primarily in cells with reduced ATP levels that stimulate the 11S participation in immunoproteasome function.