The Immunoproteasome in oxidative stress, aging, and disease

Abstract

The Immunoproteasome has traditionally been viewed primarily for its role in peptide production for antigen presentation by the major histocompatibility complex, which is critical for immunity. However, recent research has shown that the Immunoproteasome is also very important for the clearance of oxidatively damaged proteins in homeostasis, and especially during stress and disease. The importance of the Immunoproteasome in protein degradation has become more evident as diseases characterized by protein aggregates have also been linked to deficiencies of the Immunoproteasome. Additionally, there are now diseases defined by mutations or polymorphisms within Immunoproteasome-specific subunit genes, further suggesting its crucial role in cytokine signaling and protein homeostasis (or "proteostasis"). The purpose of this review is to highlight our growing understanding of the importance of the Immunoproteasome in the management of protein quality control, and the detrimental impact of its dysregulation during disease and aging.

Keywords: 20S core; Adaptation; IFN-γ; IκBα; Nrf2; proteostasis.

Figure 1

Panel A Side view of the 20S Proteasome which is comprised of two identical α and β rings, each consisting of seven α and β subunits, respectively. The outer α rings recognize, bind, and feed the protein substrate into the catalytically active inner β rings. The beta rings contain three unique proteolytic subunits: β1, β2, β5, which are sequestered into the internal region of the Proteasome for its catalytic activity. Panel B. Side view of the Immunoproteasome. Oxidative stress and inflammation triggers the transcriptional upregulation and formation of the Immunoproteasome. Similar to the 20S core, the Immunoproteasome consists of two alpha and two beta rings, but with the three catalytically active beta subunits substituted with the Immunoproteasome-specific beta-subunits: β1i, β2i, β5i. Panel C. Top-views of the 20S proteasome (left) and the Immunoproteasome (right). Similar to the 20S beta ring, which has trypsin-like activity (β2), chymotrypsin-like activity (β5), and caspase-like activity (β1, which is also known as peptidyl glutamyl-peptide hydrolyzing activity), the Immunoproteasome catalytic subunits consist of two subunits with chymotrypsin-like activity (β2i & β5i), and one with trypsin-like activity (β1i).

Figure 2

The primary role of the Ubiquitin-Proteasome-System (UPS) is to ensure proper functioning of the cellular proteome. The UPS relies upon the ATP-dependent 26S proteasome to conduct the normal turnover of poly-ubiquitin tagged proteins, such as Nrf2. In contrast, the 20S Proteasome preferentially degrades oxidatively damaged protein substrates, without need for ATP or ubiquitin. During unstressful conditions (left-half of the figure), Nrf2 is polyubiquitinated, disassociates from Keap1, and is targeted for degradation by the 26S proteasome. During oxidative stress, however (bottom right-side of the figure), the Nrf2-Keap1 complex disassociates, and phosphorylated-Nrf2 translocates into the nucleus, where it binds to the Electrophile Responsive Elements (EpRE, which is also called the Antioxidant Response Element or ARE) contained in various stress response genes, including the subunits of the 20S proteasome. This leads to de novo synthesis of the 20S Proteasome subunits, and new intact 20S Proteasomes. Although Proteasome transcription and translation are effective in increasing cellular capacity to degrade oxidized proteins and prevent their aggregation and accumulation, this adaptive process takes several hours. Fortunately (as shown in the top right-side of the figure), cells also have an immediate response mechanism to increase the capacity to degrade oxidized proteins, which is disassembly of 26S Proteasomes by ECM 29 and HSP70. Upon exposure to oxidative stress, the 19S regulator is removed (from 26S proteasomes) by ECM29, producing additional free 20S Proteasomes. The 19S regulators are sequestered and protected from degradation by HSP70. Remarkably, following 3–5 hour recovery period, the 26S Proteasome is reassembled just as newly synthesized 20S Proteasomes (signaled by Nrf2) begin to appear.

Figure 3

Unlike the 20S proteasome, the subunits of the Immunoproteasome lack functional EpRE binding domains, and so cannot be regulated by Nrf2. Instead, the Interferon gamma (IFNγ) pathway may be activated. When IFNγ is added to cells it triggers the activation of Jak1 and Jak2, resulting in the dimerization and phosphorylation of Stat1. Activation of Stat1 is also possible through oxidative stress, (without IFNγ addition) as hydrogen peroxide has been found to cause dimerization of Stat1. In turn, Stat1 translocates into the nucleus, where it binds to interferon-1 (IRF-1). Following translation, IRF-1 moves back into the nucleus to increase expression of the immunoproteasome subunits. A potential alternative route for Immunoproteasome regulation is through the NF-κB pathway. Upon an oxidative insult, Protein Kinase D (PKD) becomes phosphorylated, which acts to disassociate IκBα from NF-κB (the DNA binding domain protein dimers p50 and p65). In turn, IκBα is degraded by the proteasome and NF-κB can migrate into the nucleus where it may potentially interact with, and regulate the expression of, the Immunoproteasome subunits.