Ubiquitination in the regulation of inflammatory cell death and cancer

Abstract

The ubiquitin system is complex, multifaceted, and is crucial for the modulation of a vast number of cellular processes. Ubiquitination is tightly regulated at different levels by a range of enzymes including E1s, E2s, and E3s, and an array of DUBs. The UPS directs protein degradation through the proteasome, and regulates a wide array of cellular processes including transcription and epigenetic factors as well as key oncoproteins. Ubiquitination is key to the dynamic regulation of programmed cell death. Notably, the TNF signaling pathway is controlled by competing ubiquitin conjugation and deubiquitination, which governs both proteasomal degradation and signaling complex formation. In the inflammatory response, ubiquitination is capable of both activating and dampening inflammasome activation through the control of either protein stability, complex formation, or, in some cases, directly affecting receptor activity. In this review, we discuss the enzymes and targets in the ubiquitin system that regulate fundamental cellular processes regulating cell death, and inflammation, as well as disease consequences resulting from their dysregulation. Finally, we highlight several pre-clinical and clinical compounds that regulate ubiquitin system enzymes, with the aim of restoring homeostasis and ameliorating diseases.

Fig. 1. Ubiquitin proteasome system.

Ubiquitination is a multistep process that involves ubiquitin activation by E1 enzymes, ubiquitin conjugation to E2 enzymes, and ubiquitin ligation to the substrate protein via E3 enzymes. Ubiquitination can result in proteasomal degradation of the substrate or in recruitment of the substrate to multiprotein complexes, depending on the topology of the polyubiquitin chain linkages. X and Y indicate ubiquitin chain-binding proteins.

Fig. 2. Ubiquitination in inflammatory signaling.

Signaling mediated by TNFR1, Il-1R, TLR3/4, or NOD2 relies on complex ubiquitination involving multiple ubiquitin chains to activate inflammatory gene expression. Green indicates ubiquitin ligases and yellow deubiquitinase. Ubiquitin linkage types are indicated in the figure.

Fig. 3. Ubiquitination in the regulation of cell death and inflammasome.

Ubiquitination of the key components of TNF-stimulated cell death and NLRP3 inflammasome–mediated signaling regulate cell survival and inflammation. Green indicates ubiquitin ligases and yellow deubiquitinase. Ubiquitin linkage types are indicated in the figure.