Abnormal Ubiquitination of Ubiquitin-Proteasome System in Lung Squamous Cell Carcinomas

Ubiquitination is an important post-translational modification. Abnormal ubiquitination is extensively associated with cancers. Lung squamous cell carcinoma (LUSC) is the most common pathological type of lung cancer, with unclear molecular mechanism and the poor overall prognosis of LUSC patient. To uncover the existence and potential roles of ubiquitination in LUSC, label-free quantitative ubiquitomics was performed in human LUSC vs. control tissues. In total, 627 ubiquitinated proteins (UPs) with 1209 ubiquitination sites were identified, including 1133 (93.7%) sites with quantitative information and 76 (6.3%) sites with qualitative information. KEGG pathway enrichment analysis found that UPs were significantly enriched in ubiquitin-mediated proteolysis pathway (hsa04120) and proteasome complex (hsa03050). Further analysis of 400 differentially ubiquitinated proteins (DUPs) revealed that 11 subunits of the proteasome complex were differentially ubiquitinated. These findings clearly demonstrated that ubiquitination was widely present in the ubiquitin-proteasome pathway in LUSCs. At the same time, abnormal ubiquitination might affect the function of the proteasome to promote tumorigenesis and development. This book chapter discussed the status of protein ubiquitination in the ubiquitin-proteasome system (UPS) in human LUSC tissues, which offered the scientific data to elucidate the specific molecular mechanisms of abnormal ubiquitination during canceration and the development of anti-tumor drugs targeting UPS.

Lys63-Linked Polyubiquitination of Transforming Growth Factor β Type I Receptor (TβRI) Specifies Oncogenic Signaling

Transforming growth factor β (TGFβ) is a multifunctional cytokine with potent regulatory effects on cell fate during embryogenesis, in the normal adult organism, and in cancer cells. In normal cells, the signal from the TGFβ ligand is transduced from the extracellular space to the cell nucleus by transmembrane serine–threonine kinase receptors in a highly specific manner. The dimeric ligand binding to the TGFβ Type II receptor (TβRII) initiates the signal and then recruits the TGFβ Type I receptor (TβRI) into the complex, which activates TβRI. This causes phosphorylation of receptor-activated Smad proteins Smad2 and Smad3 and promotes their nuclear translocation and transcriptional activity in complex with context-dependent transcription factors. In several of our most common forms of cancer, this pathway is instead regulated by polyubiquitination of TβRI by the E3 ubiquitin ligase TRAF6, which is associated with TβRI. The activation of TRAF6 promotes the proteolytic cleavage of TβRI, liberating its intracellular domain (TβRI-ICD). TβRI-ICD enters the cancer cell nucleus in a manner dependent on the endosomal adaptor proteins APPL1/APPL2. Nuclear TβRI-ICD promotes invasion by cancer cells and is recognized as acting distinctly and differently from the canonical TGFβ-Smad signaling pathway occurring in normal cells.

New Discoveries on the Roles of “Other” HECT E3 Ubiquitin Ligases in Disease Development

HECT E3 ubiquitin ligases selectively recognize, bind, and ubiquitylate their substrate proteins to target them for 26S proteasomal degradation. There is increasing evidence that HECT E3 ubiquitin ligase dysfunction due to misfolding and/or the gene encoding the protein being mutated is responsible for the development of different diseases. Apart from the more prominent and well-characterized E6AP and members of the NEDD4 family, new studies have begun to reveal how other members of the HECT E3 ubiquitin ligase family function as well as their links to disease and developmental disorders. This chapter provides a comprehensive discussion on the more mysterious members of the HECT E3 ubiquitin ligase family and how they control intracellular processes. Specifically, AREL1, HACE1, HECTD1, HECTD4, G2E3, and TRIP12 will be examined as these enzymes have recently been identified as contributors to disease development.

Branching and Mixing: New Signals of the Ubiquitin Signaling System

Posttranslational modifications allow cells and organisms to adapt to their environment without the need to synthesize new proteins. The ubiquitin system is one of the most versatile modification systems as it does not only allow a simple on–off modification but, by forming a chain of ubiquitin molecules, allows conveying multiple signals. The structure of the chains is dependent on the linkage to the previous ubiquitin molecule as every lysine can serve as an acceptor point for this modification. Different chain types code for specific signals ranging from protein degradation to protein targeting different cellular compartments. Recently the code of ubiquitin signals has been further expanded as branching and mixing of different chain types has been detected. As an additional layer of complexity, modifications of the ubiquitin chain by ubiquitin-like modifiers, like NEDD8, SUMO, or ISG15, have been found. Here we will discuss the different chain types and the technical challenges which are associated with analyzing ubiquitin topology-based signaling.

Ubiquitin-Independent Proteasomal Degradation Mediated by Antizyme

Most of the proteins in eukaryotic cells are degraded by the proteasome in an ubiquitin-dependent manner. However, ubiquitin-independent protein degradation pathway by the 26S proteasome exists in the cells. Ornithine decarboxylase (ODC) is a well-known protein that is degraded by the 26S proteasome without ubiquitination. Degradation of ODC requires the protein, “antizyme (AZ),” that is induced by polyamine and binds to the ODC monomer to inhibit ODC activity and target it to the 26S proteasome for proteolytic degradation. Namely, AZ contributes the feedback regulation of intracellular polyamine level. ODC has been considered to be the only protein that AZ binds and accelerates its degradation. However, recently AZ-mediated proteasomal protein degradation will gradually increase. Most recently, we found that one of the antizyme families, AZ2, accelerates c-Myc degradation by the proteasome without ubiquitination. In this chapter, we introduce latest several ubiquitin-independent proteasomal degradation mediated by antizyme.

Ubiquitination and Deubiquitination in Melanoma Research and Clinically Relevant Outcomes

Malignant melanoma is one of the most invasive tumors with increasing mortality, low overall survival rates and limited effective therapeutic strategies. Ubiquitination is a post-translational protein modification, which is regulated by a series of ubiquitination-associated enzymes. Ubiquitination plays a critical role in diverse pathophysiological activities of cellular and participates in the pathogenesis of various cancers, including melanoma. This study aims to provide a conclusive of ubiquitination and deubiquitination, and their potential clinical application value in melanoma in the following aspects: melanoma pathogenesis-related components and processes in the ubuiquitin-proteasome system (UPS), ubiquitination in melanoma immunological microenvironment modulation, ubiquitination of key transcription factors in melanoma and melanoma therapeutic strategy via targeting the UPS.