Current Cancer Drug Targets - Volume 16, Issue 2, 2016

BRCA1, a breast and ovarian tumor suppressor, maintains genome stability through its functions in DNA repair, cell-cycle checkpoints, heterochromatin formation and centrosome amplification. BRCA1 interacts with BARD1 to constitute a RING heterodimer-type E3 ubiquitin ligase. BRCA1-associated protein 1 (BAP1) is a deubiquitinating enzyme that also regulates similar cellular events, including cell-cycle control, transcription, chromatin modification and DNA damage response. Germline mutations in BRCA1 predispose individuals to breast, ovarian, fallopian tube, peritoneal, pancreatic and prostate cancers, whereas BAP1 mutations combined with certain types of DNA damage provoke malignant mesothelioma, uveal and cutaneous melanoma, lung adenocarcinoma and renal cell carcinoma. Although BAP1 was initially discovered as a BRCA1-associated protein, recent mass-spectrometric screens of BAP1 interactors failed to detect BRCA1, raising questions about their presumed endogenous interaction. However, in addition to physical interaction, new evidence indicates a functional correlation between the two proteins. This review summarizes BAP1 function in histone modification and the DNA damage response, focusing on BAP1’s relevance to BRCA1 function. An understanding of the cooperative functions between BRCA1 and BAP1 may uncover opportunities for new drug targets in a variety of related cancers.

Epithelial-mesenchymal transition (EMT) plays an important role in the development of tumor metastases by facilitating cell migration and invasion. One of the hallmarks of EMT is the diminished expression of E-cadherin and gain of mesenchymal traits, which are regulated by core EMT-inducing transcriptional factors (EMT-TFs), such as Snail/Slug, ZEB1/ZEB2, and Twist1. EMT-TFs are known to be extremely labile proteins, and their protein levels are tightly controlled by the ubiquitin-proteasome system (UPS). Several E3 ubiquitin ligases have been shown to play crucial roles in the regulation of EMT, and genetic aberrations and alterations in these ligases have been detected in human cancer. In this review, we focused on EMT-TFs, describing the UPS controlling their activities and functions in cancer. A deeper understanding of the role of UPS in the regulation of EMT will provide valuable information for the development of effective anti-metastatic drugs to modulate the malignant processes mediated by EMT.

The ubiquitin system controls protein stability and function. F-box proteins form SCF (SKP1-Cullin1-F-box protein)-type ubiquitin (E3) ligases to selectively target their substrates for degradation via the ubiquitin–proteasome pathway. Here, we review F-box proteins associated with cancer development. S-phase kinase-associated protein 2 (SKP2) (also known as FBXL1) is often overexpressed in human cancers, and functions as an oncogenic E3 ligase to degrade tumor suppressor gene products. Moreover, F-box/WD repeat-containing protein 7 (FBXW7) (also known as Fbw7) is often mutated in human cancers and functions as a tumor suppressive E3 ligase targeting oncogenic proteins for degradation. SKP2 is a potential drug target for cancer therapy and FBXW7 is useful in determining patient diagnosis, prognosis, and drug sensitivity. In this review, we also discuss other F-box proteins involved in cancer-associated cellular processes such as cell cycle control, epigenetic regulation, epithelial mesenchymal transition, apoptosis/survival, drug resistance, and DNA-damage responses.

The ubiquitin-proteasome pathway is involved in various biological processes. Several oncogenic E3 ligases target tumor suppressor proteins for ubiquitin-mediated degradation. Alternatively, some other E3 ligases play as a tumor suppressor specifically targeting oncogene products. Deregulation of these E3 ligases induces unbalance between oncogenic signal and tumor suppressor pathway and leads to cellular transformation, tumor growth and metastasis in various human malignancies including oral, and head and neck cancers. Facilitated degradation of the cyclin-dependent kinase (CDK) inhibitor p27Kip1 has been observed in oral, and head and neck cancers, and is correlated with their poor prognosis. SCFSkp2, KPC complex, Pirh2 and CRL4DDB2-Artemis have been reported as E3 ligases targeting p27Kip1 for degradation. In oral cancers, it is reported that overexpression of Skp2 and Pirh2 is associated with poor prognosis. Thus, chemical inhibitors against these E3 ligases are applicable for oral cancer therapy. Some potential compounds that inhibit E3 ligase activity of SCFSkp2 have been reported. Moreover, the HECT-type E3 ligase WWP family and Smurf1 are also involved in the development and growth of human oral cancers. Therefore, small molecule inhibitors against HECT-type E3 ligases are discussed as anti-oral cancer drugs.

Selective degradation of pathogenic proteins by small molecules in cells is a novel approach for development of therapeutic agents against various diseases, including cancer. We and others have developed a protein knockdown technology with a series of hybrid small compounds, called SNIPERs (Specific and Nongenetic IAP-dependent Protein ERasers); and peptidic chimeric molecules, called PROTACs (proteolysis-targeting chimeric molecules), which induce selective degradation of target proteins via the ubiquitin-proteasome pathway. These compounds include two different ligands connected by a linker; one is a ligand for a ubiquitin ligase and the other is a ligand for the target protein, which are expected to crosslink these proteins in cells. Theoretically, any cytosolic protein can be targeted for degradation by this technology. To date, several SNIPERs and PROTACs against various oncogenic proteins have been developed, which specifically induce polyubiquitylation and proteasomal degradation of the oncogenic proteins, resulting in cell death, growth arrest, or impaired migration of cancer cells. Thus, this protein knockdown technology has a great potential for cancer therapy.

Tribbles-related protein (TRB) family members are the mammalian orthologs of Drosophila tribbles. Tribbles was originally identified as a cell cycle regulator during Drosophila development. Tribbles genes are evolutionary conserved, and three TRB genes (TRB1, TRB2 and TRB3) have been identified in mammals. TRBs are considered pseudokinases because they lack an ATP binding site or one of the conserved catalytic motifs essential for kinase activity. Instead, TRBs play important roles in various cellular processes as scaffolds or adaptors to promote the degradation of target proteins and to regulate several key signaling pathways. Recent research has focused on the role of TRBs in tumorigenesis and neoplastic progression. In this review, we focus on the physiological roles of TRB family members in tumorigenesis through the regulation of the ubiquitin-proteasome system and discuss TRBs as biomarkers or potential therapeutic targets in cancer.

In the ubiquitylation system, E3 ubiquitin ligases play a key role in determining substrate specificity and catalyzing the transfer of ubiquitin from E2 enzymes to the substrate. Growing evidence has shown that E3 ubiquitin ligases are involved in cancer development and progression. The RING-type and HECT-type E3 ligases are the classically categorized groups of E3 ubiquitin ligases, and more of these enzymes are being shown to be potential targets for cancer therapy. The recently classified RBR E3 ligases catalyze the transfer of ubiquitin by a RING/HECT hybrid-like mechanism. Notably, these ligases are also emphasized as important potential candidates for targets of cancer treatment drugs. The present review provides an overview of the RING-, HECT- and RBR-type E3 ligases, and discusses their roles in cancer and cancer therapy.

Gastrointestinal cancer treatment is based more on molecular biology that has provided increasing knowledge about cancer pathogenesis on which targeted therapy is being developed. Precisely, targeted therapy is defined as a “type of treatment that uses drugs, such as monoclonal antibodies or tyrosine kinase inhibitors, to identify and attack specific cancer cells”. Nowadays, the United States Food and Drug Administration has approved many targeted therapies for gastrointestinal cancer treatment, as many are in various phases of development as well. In a previous review we discussed the main monoclonal antibodies used and studied in gastrointestinal cancer. In addition to monoclonal antibodies, tyrosine kinase inhibitors represent another class of targeted therapy and following the approval of imatinib for gastrointestinal stromal tumours, other tyrosine kinase inhibitors have been approved for gastrointestinal cancers treatment such as sunitinib, regoragenib, sorafenib and erlotinib. Moving forward, the purpose of this review is to focus on the efficacy data of main tyrosine kinase inhibitors commonly used in the personalized treatment of each gastrointestinal tumour and to provide a comprehensive overview about experimental targeted therapies ongoing in this setting.

Epidemiological data suggest that Non Steroidal Anti Inflammatory Drugs (NSAIDs) and Cyclooxygenase 2 (COX2) inhibitors (COXibs) can exert chemopreventive and antitumour effects in many human neoplasia. This is particularly true in colon cancer (CC), where the regular assumption of these molecules has been shown to exert chemopreventive and chemotherapeutic effects. Since the late ‘90s, there has been a progressive increase in experimental evidence, indicating that in CC the antiproliferative effects of NSAIDs and COXibs could be both dependent on and independent of COXs inhibition, and that these effects do not necessarily exclude each other. This review will examine some of these COX-independent cellular pathways, with a focus on those involved in the inhibition of CC cells proliferation through transcription factors crosstalk.