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Ubiquitination and Cancer
Post-translational modification of proteins is a generally used mechanism in eukaryotic cell signaling. Ubiquitination, or ubiquitin conjugation to proteins, is a crucial process for cell cycle progression and cell proliferation and development. Although ubiquitination usually serves as a signal for protein degradation through the 26S proteasome, it could also serve for other fundamental cellular processes, e.g. in endocytosis , enzymatic activation , and DNA repair. Moreover, since ubiquitination functions to tightly regulate the cellular level of cyclins, its misregulation is highly expected to have severe impacts. First evidence of the importance of the ubiquitin/proteasome pathway in oncogenic processes was observed due to the high antitumor activity of proteasome inhibitors. Various studies have shown that defects or alterations in ubiquitination processes are commonly associated with or presence in human carcinoma. Malignancies could be developed through loss of function mutation directly at the tumor suppressor gene, increased activity of ubiquitination, and/or indirect attenuation of ubiquitination due to mutation in related proteins.

Renal Cell Carcinoma
The VHL (Von Hippel–Lindau) gene encodes a component of an E3 Ubiquitin Ligase. VHL complex targets member of the hypoxia-inducible transcription factor family (HIF) for degradation by interacting with the oxygen-dependent destruction domain under normoxic condition. HIF activates downstream targets such as the vascular endothelial growth factor (VEGF), promoting angiogenesis. Mutations in VHL prevent degradation of HIF and thus lead to the formation of hypervascular lesions and renal tumors.

Breast Cancer
The BRCA1 gene is another tumor suppressor gene in human which encodes the BRCA1 protein that is involved in response to DNA damage. The protein contains a RING motif with E3 Ubiquitin Ligase activity. BRCA1 could form dimer with other molecules, such as BARD1 and BAP1, for its ubiquitination activity. Mutations that affect the ligase function are often found and associated with various cancers.

Cyclin E
As processes in cell cycle progression is the most fundamental processes for cellular growth and differentiation, and are the most common to be altered in human carcinomas, it is expected for cell cycle-regulatory proteins to be under tight regulation. The level of cyclins, as the name suggests, are high only at certain time point during cell cycle. This is achieved by continuous controlled of cyclins/CDKs level through ubiquitination and degradation. When cyclin E is partnered with CDK2 and get phosphorylated, an SCF-associated F-box protein Fbw7 recognizes the complex and thus target it for degradation. Mutations in Fbw7 have been found in more than 30% of human tumors, characterizing is as a tumor suppressor protein.

Cervical Cancer
Oncogenic types of the human papillomavirus (HPV) are known to hijack cellular ubiquitin-proteasome pathway for viral infection and replication. The E6 proteins of HPV will bind to the N-terminus of the cellular E6-AP E3 ubiquitin ligase, redirecting the complex to bind p53, a well-known tumor suppressor gene that inactivation is found in many types of cancer. Thus, p53 undergoes ubiquitination and proteasome-mediated degradation. Meanwhile E7, another one of the early-expressed HPV genes, will bind to Rb, also a tumor suppressor gene, mediating its degradation. The loss of p53 and Rb in cells allows limitless cell proliferation to occur.

p53 Regulation
Gene amplification often occur in various tumor cases, including of MDM2, a gene encodes for a RING E3 Ubiquitin ligase responsible for downregulation of p53 activity. MDM2 targets p53 for ubiquitination and proteasomal degradation thus keeping its level appropriate for normal cell condition. Overexpression of MDM2 causes loss of p53 activity and therefore allowing cells to have a limitless replicative potential.

p27
Another gene that is a target of gene amplification is SKP2. SKP2 is an F-box protein that roles in substrate recognition for ubiquitination and degradation. SKP2 targets p27Kip-1, an inhibitor of cyclin-dependent kinases (CDKs). CDKs2/4 partner with the cyclinsE/D, respectively, family of cell cycle regulator to control cell cycle progression through the G1 phase. Low level of p27Kip-1 protein is often found in various cancers and is due to overactivation of ubiquitin-mediated proteolysis through overexpression of SKP2.

Efp
Efp, or estrogen-inducible RING-finger protein, is an E3 ubiquitin ligase that overexpression has been shown to be the major cause of estrogen-independent breast cancer. Efp's substrate is 14-3-3 protein which negatively regulates cell cycle.

Colorectal Cancer
The gene associated with colorectal cancer is the adenomatous polyposis coli (APC), which is a classic tumor suppressor gene. APC gene product targets beta-catenin for degradation via ubiquitination at the N-terminus, thus regulating its cellular level. Most colorectal cancer cases are found with mutations in the APC gene. However, in cases where APC gene is not mutated, mutations are found in the N-terminus of beta-catenin which renders it ubiquitination-free and thus increased activity.

Glioblastoma
As the most aggressive cancer originated in the brain, mutations found in patients with glioblastoma are related to the deletion of a part of the extracellular domain of the epidermal growth factor receptor (EGFR). This deletion causes CBL E3 ligase unable to bind the receptor for its recycling and degradation via a ubiquitin-lysosomal pathway. Thus, EGFR is constitutively active in the cell membrane and activates its downstream effectors that are involved in cell proliferation and migration.

Phosphorylation-dependent Ubiquitination
The interplay between ubiquitination and phosphorylation has been an ongoing research interest since phosphorylation often serves as a marker where ubiquitination leads to degradation. Moreover, ubiquitination can also act to turn on/off the kinase activity of a protein. The critical role of phosphorylation is largely underscored in the activation and removal of autoinhibition in Cbl protein. Cbl is an E3 ubiquitin ligase with a RING finger domain that interacts with its tyrosine kinase binding (TKB) domain, preventing interaction of the RING domain with an E2 ubiquitin-conjugating enzyme. This intramolecular interaction is an autoinhibition regulation that prevents its role as a negative regulator of various growth factors and tyrosine kinase signaling and T-cell activation. Phosphorylation of Y363 relieves the autoinhibition and enhances binding to E2. Mutations that renders the Cbl protein dysfunctional due to the loss of its ligase/tumor suppressor function and maintenance of its positive signaling/oncogenic function have been shown to cause development of cancer.

Screening for Ubiquitin Ligase Substrates
Identification of E3 ligase substrates is critical to understand its implication in human diseases since deregulation of E3-substrate interactions are often served as major cause in many. To overcome the limitation of mechanism used to identify the substrates of the E3 Ubiquitin Ligase, a system called the 'Global Protein Stability (GPS) Profiling' was developed in 2008. This high-throughput system made use of reporter proteins fused with thousands of potential substrates independently. By inhibition of the ligase activity (through the making of Cul1 dominant negative thus renders ubiquitination not to occur), increased reporter activity shows that the identified substrates are being accumulated. This approach added a large number of new substrates to the list of E3 ligase substrates.

==== Possible Therapeutic Intervention ==== Blocking of specific substrate recognition by the E3 ligases, e.g. Bortezomib

Challenge
Finding a specific molecule that selectively inhibits the activity of a certain E3 ligase and/or the protein-protein interactions implicated in the disease remains as one of the important and expanding research area. Moreover, as ubiquitination is a multi-step process with various players and intermediate forms, consideration of the much complex interactions between components needs to be taken heavily into account while designing the small molecule inhibitors.