Diverse roles of the E2/E3 hybrid enzyme UBE2O in the regulation of protein ubiquitination, cellular functions, and disease onset

Abstract

The ubiquitin-proteasome system is an important regulatory machinery involved in proteostasis and cellular signaling. Proteins are ubiquitinated via the concerted action of E1 ubiquitin-activating enzymes, E2 ubiquitin-conjugating enzymes, and E3 ubiquitin ligases. Although most of the studies to date focus on the significance of E3 ubiquitin ligases in disease development and therapeutic treatment, recent discoveries suggest that E2 ubiquitin-conjugating enzymes might also be potential drug targets. The ubiquitin-conjugating enzyme E2 O (UBE2O), an E3-independent E2 (i.e. an E2/E3 hybrid enzyme), can directly mediate the ubiquitination of many substrates. These include 5′-AMP-activated protein kinase catalytic subunit α2 (AMPKα2), tumor suppressor ubiquitin carboxyl-terminal hydrolase BAP1, mixed-lineage leukemia (MLL) protein, SMAD family member 6 (SMAD6), transcription factor c-Maf and aryl hydrocarbon receptor nuclear translocator-like protein 1 (ARNTL or BMAL1), and free ribosomal proteins, which are ubiquitinated in distinct ways, thereby associating UBE2O with a variety of biological functions. Furthermore, UBE2O is frequently amplified or mutated in multiple cancers, and its high expression is associated with low survival rate of gastric, lung, breast, and prostate cancer patients. However, the molecular mechanisms by which UBE2O contributes to tumor initiation and progression are not fully elucidated. This review focuses on emerging insights from genetics, biochemistry, and cell biology to explore the biological functions of UBE2O and its therapeutic potential.