Abstract
Autoimmune destruction of insulin producing pancreatic β-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 β-cells. Of special interest to immunoproteasome activation in β-cells are the effects of IFNβ, a type I IFN secreted by virus-infected cells and implicated in type I diabetes onset, compared to IFNγ, 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 β1i, β2i and β5i, and 11S mRNAs upon exposure to IFNβ or IFNγ. Higher concentrations of IFNβ than IFNγ 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 IFNβ-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 β-cells exposed to IFNβ, but primarily in cells with reduced ATP levels that stimulate the 11S participation in immunoproteasome function.
Highlights
T cell-mediated destruction of insulin producing pancreatic bcells is the hallmark of type I diabetes, an autoimmune disease associated with various genetic and environmental factors
IFNb induces accumulation of immunoproteasome and 11S mRNAs in a manner preceeded by activation of Interferon Regulatory Factor 1 (IRF1) gene expression and prevented by IRF1 gene knockout
Little is known about the function and regulation of the immunoproteasome in pancreatic b-cells, especially in response to IFNb, a type I IFN secreted by virus-infected cells and implicated in type I diabetes onset
Summary
T cell-mediated destruction of insulin producing pancreatic bcells is the hallmark of type I diabetes, an autoimmune disease associated with various genetic and environmental factors. Viral infections have been recognized as one potential environmental trigger of onset, but the molecular basis of the link between antiviral defenses and autoimmunity is still unclear [1,2]. Type I IFNs are produced shortly after viral infection and regulate the early steps of antiviral defense. This class includes IFNc that is synthesized by infected cells in the absence of other cell types, and several types of IFNa that are synthesized primarily by leukocytes recruited to the infection site. IFNc, the only type II interferon, is synthesized by cells of the immune system upon their recruitment to infected cells, which takes place days or even weeks after infection
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