E3 ubiquitin ligases as cancer targets and biomarkers

Abstract

E3 ubiquitin ligases are a large family of proteins that are engaged in the regulation of the turnover and activity of many target proteins. Together with ubiquitin-activating enzyme E1 and ubiquitin-conjugating enzyme E2, E3 ubiquitin ligases catalyze the ubiquitination of a variety of biologically significant protein substrates for targeted degradation through the 26S proteasome, as well as for nonproteolytic regulation of their functions or subcellular localizations. E3 ubiquitin ligases, therefore, play an essential role in the regulation of many biologic processes. Increasing amounts of evidence strongly suggest that the abnormal regulation of some E3 ligases is involved in cancer development. Furthermore, some E3 ubiquitin ligases are frequently overexpressed in human cancers, which correlates well with increased chemoresistance and poor clinic prognosis. In this review, E3 ubiquitin ligases (such as murine double minute 2, inhibitor of apoptosis protein, and Skp1-Cullin-F-box protein) will be evaluated as potential cancer drug targets and prognostic biomarkers. Extensive study in this field would lead to a better understanding of the molecular mechanism by which E3 ligases regulate cellular processes and of how their deregulations contribute to carcinogenesis. This would eventually lead to the development of a novel class of anticancer drugs targeting specific E3 ubiquitin ligases, as well as the development of sensitive biomarkers for cancer treatment, diagnosis, and prognosis.

Figure 1

Ubiquitin pathways in the regulation of protein degradation and function. Ubiquitin is first attached to E1 ubiquitin-activating enzyme in the presence of ATP. The activated ubiquitin is then transferred to E2 ubiquitin-conjugating enzyme. E3 ubiquitin ligase recognizes a protein substrate, recruits a E2-ubiquitin complex, and catalyzes ubiquitin transfer from E2 to substrate. A single run of the reaction causes monoubiquitination of a target protein that could change its function, whereas multiple runs of the reaction lead to polyubiquitination of the substrate. Depending on ubiquitin-ubiquitin linkage, polyubiquitinated proteins can either be activated (through K63 linkage), or recognized and degraded by the 26S proteasome (through K48 linkage).

Figure 2

Targeting Mdm2 E3 for p53 accumulation and apoptosis induction. Mdm2 binds to p53 through its N-terminal p53-binding domain and promotes p53 ubiquitination and degradation through its C-terminal RING domain. Two classes of small-molecule compounds were discovered. The first class comprises the inhibitor Mdm2 E3 ligase HLI98C, and the second class comprises inhibitors that either disrupt Mdm2-p53 binding (Nutlin-1 or MI-17) or bind to p53 to inhibit its binding to Mdm2 (RITA). The net effects of these small molecules are p53 accumulation and apoptosis induction.

Figure 3

Targeting IAPs for caspase activation and apoptosis induction. IAP binds to caspases through its BIR2 or BIR3 domain and promotes the ubiquitination and degradation of caspases (Casps) through its C-terminal RING domain. Small-molecule inhibitors that disrupt IAP-caspase binding (AVPI-like) or yet-to-be discovered IAP E3 ligase inhibitor (E3I) would either release caspases or induce accumulation of caspases, leading to apoptosis induction.

Figure 4

Targeting SCF-Skp2 for p27 accumulation and growth inhibition. SCFE3 ubiquitin ligase consists of four components: scaffold protein Cullins to link Skp1 and Rbx/SAG; adaptor protein Skp1 to link Cullins and F-box protein; RING protein Rbx/ROC/SAG to recruit E2; and F-box protein to recognize substrate. In the SCF-Skp2 E3 ligase that promotes the ubiquitination and degradation of p27, a small protein Cks1 is also involved. Small molecules that can either inhibit core E3 ligase activity (E3I) or disrupt CKs1/Skp2/p27 binding would induce p27 accumulation and cause growth arrest.