The importance of regulatory ubiquitination in cancer and metastasis

Abstract

Ubiquitination serves as a degradation mechanism of proteins, but is involved in additional cellular processes such as activation of NFκB inflammatory response and DNA damage repair. We highlight the E2 ubiquitin conjugating enzymes, E3 ubiquitin ligases and Deubiquitinases that support the metastasis of a plethora of cancers. E3 ubiquitin ligases also modulate pluripotent cancer stem cells attributed to chemotherapy resistance. We further describe mutations in E3 ubiquitin ligases that support tumor proliferation and adaptation to hypoxia. Thus, this review describes how tumors exploit members of the vast ubiquitin signaling pathways to support aberrant oncogenic signaling for survival and metastasis.

Keywords: deubiquitinase; ligase; metastasis; ubiquitination.

Figure 1.

The misregulated expression of E2 ubiquitin conjugating enzymes and E3 ubiquitin ligases in various human cancers. The ubiquitination reaction initiates with the activation of ubiquitin by ATP, in which ubiquitin is then transferred to the active site of E1 ubiquitin conjugating enzyme. The E1 transfers the ubiquitin to a Cys in the catalytic active site of the E2 ubiquitin conjugating enzyme. The HECT domain E3 ligases ubiquitinate the target substrates by 2 mechanisms: first, the ubiquitin is transferred from the active site of the E2 to the Cys in the active site of the E3, which then ubiquitinates the Lys residue in the target substrate. RING- and RING-related domain E3 ligases, in contrast, serve as scaffolds to ubiquitinate target substrates in one step: the E2 transfers the ubiquitin directly to the Lys residue in the target substrate. Various tumors take advantage of the misregulated expression of E2s and E3s for the aberrant activation of oncogenic pathways. E2s and E3s colored in green indicate the importance of their expression or overexpression in cancer, while those in red indicate their downregulated expression in cancer. These genomic events result in cancer cell proliferation, migration, invasion, angiogenesis, hypoxia, EMT and metastasis.

Figure 2.

Misregulated expression of members of the ubiquitin cascade contributes to the aberrant signaling of various pathways in cancer. (LEFT) The UBE2N/UBE2V1 E2 ubiquitin conjugating enzyme complex catalyzes the Lys63-linked ubiquitination of NEMO that recruits the TAK1/TAB1/2 complex to activate the IKK complex, which is composed of IKKβ, IKKα and NEMO. IKKβ phosphorylates IκBα, which is Lys48-linked ubiquitinated and subsequently degraded by the 26S-proteasome. This event releases NFκB to translocate into the nucleus to mediate the transcription of a signature of genes involved in inflammatory response. STUB1 E3 ligase negatively regulates NFκB signaling by catalyzing the degradation-inducing Lys48-linked ubiquitination of p65 subunit of NFκB. (CENTER) In addition, cancer cells take advantage of overexpressed TRAF4 to modulate TGFβ signaling. TGFβ activation culminates in the nuclear translocation of SMAD2/3/4 complex to modulate gene transcription. SMAD7 is a negative regulator of TGFβ signaling by recruiting SMURF2 E3 ubiquitin ligase to ubiquitinate TβRI, leading to the proteasomal degradation of the receptor and mitigation of signaling TGFβ signaling is regulated by TRAF4 E3 ligase mediated Lys48-linked ubiquitination of SMURF2 E3 ligase. The latter E3 ligase catalyzes the degradation signal of TGFβ receptor I (TβRI), in which these events mitigate the activation of the signaling pathway. TRAF4, on the other hand, is conjugated to Lys63-linked ubiquitin polymers to activate TAK1/TAB1/2 complex that induces the signaling of p38 MAPK and NFκB. TRAF4 further interacts with deubiquitinating enzyme USP15 and USP4, which remove the degradation signal from TGFβ receptor I. These events contribute to the stabilization of TGFβ signaling. (RIGHT) Tumor adaptation to hypoxia is highly attributed to HIF signaling. Under normal oxygen level condition (normoxia), VHL E3 ligase binds to hydroxylated Proline residue in HIF catalyzed by PHD proteins. VHL then catalyzes the Lys48-linked ubiquitination of HIF, leading to proteasome degradation. UBE2S enzyme controls VHL protein stability. Under low oxygen conditions (hypoxia), inactivating of VHL (either by mutations or decreased expression) contributes to HIF isoform stabilization, which mediates the transcription of genes involved in tumor adaptation to hypoxia, including angiogenesis. These signaling events governed by members of the ubiquitin cascade all contribute to EMT, cellular proliferation, migration, invasion, chemotherapy resistance and metastasis.

Figure 3.

E3 ubiquitin ligases in pluripotent stem cells. (A) Cancer stem cells are characterized by the upregulation of Sox2, Oct4, and Nanog, which activates self-renewal associated genes and inhibits cellular differentiation. Cell surface markers, CD133 and CD44 are associated with CSC properties. Conventional chemotherapeutic agents target differentiated cells, thus quiescent CSCs are innately chemo-resistant. Moreover, CSCs show increased ABC multi-drug transporters, which pump the cytotoxic drugs out of the cells. CSCs also show high aldehyde dehydrogenase activity (ALDH), which detoxifies the aldehydes generated by the chemotherapeutic agents. As a result, the surviving CSCs can re-populate or metastasize, and these cancer cells that possess self-renewal advantages are very challenging to eradicate by using conventional chemotherapeutic agents. (B) E3 ligases can either promote or suppress CSCs in different cancers.

Figure 4.

Proposed model of the mechanism by which the misregulated expression of E2s, E3s and DUB's may contribute to tumorigenesis and metastasis. The information from the reports described in this review reveals that inhibition of E2s, E3s and DUB's with either small interfering RNA or small molecule inhibitors is sufficient to inhibit the progression and metastasis of tumors. However, some of these reports show that the misregulated expression of the members of the ubiquitination pathway occurs in multiple tumors. It is not clear whether a subpopulation of cells within a tumor, for instance, may rely on the misregulated expression of one member only or on a combination of the misregulated expression of these proteins to support aberrant oncogenic signaling. In addition, it is not clearly demonstrated whether the misregulated expression of E2s and E3s stabilizes oncogenic signaling via the catalysis of non-proteasomal ubiquitin linkages, while DUB's remove ubiquitin linkages that would otherwise lead to proteasomal degradation. Nevertheless, we describe a common conclusion from multiple reports that members of the ubiquitination pathway drive tumor aggressiveness and culminate in metastasis.