E3 Ubiquitin Ligases: Key Regulators of TGFβ Signaling in Cancer Progression

Abstract

Transforming growth factor β (TGFβ) is a secreted growth and differentiation factor that influences vital cellular processes like proliferation, adhesion, motility, and apoptosis. Regulation of the TGFβ signaling pathway is of key importance to maintain tissue homeostasis. Perturbation of this signaling pathway has been implicated in a plethora of diseases, including cancer. The effect of TGFβ is dependent on cellular context, and TGFβ can perform both anti- and pro-oncogenic roles. TGFβ acts by binding to specific cell surface TGFβ type I and type II transmembrane receptors that are endowed with serine/threonine kinase activity. Upon ligand-induced receptor phosphorylation, SMAD proteins and other intracellular effectors become activated and mediate biological responses. The levels, localization, and function of TGFβ signaling mediators, regulators, and effectors are highly dynamic and regulated by a myriad of post-translational modifications. One such crucial modification is ubiquitination. The ubiquitin modification is also a mechanism by which crosstalk with other signaling pathways is achieved. Crucial effector components of the ubiquitination cascade include the very diverse family of E3 ubiquitin ligases. This review summarizes the diverse roles of E3 ligases that act on TGFβ receptor and intracellular signaling components. E3 ligases regulate TGFβ signaling both positively and negatively by regulating degradation of receptors and various signaling intermediates. We also highlight the function of E3 ligases in connection with TGFβ's dual role during tumorigenesis. We conclude with a perspective on the emerging possibility of defining E3 ligases as drug targets and how they may be used to selectively target TGFβ-induced pro-oncogenic responses.

Keywords: E3 Ligase; PROTAC; SMAD; SMURF; TGFβ; cancer; signaling; tumor; ubiquitin.

Figure 1

Overview of the transforming growth factor β (TGFβ) signaling pathway. Canonical and non-canonical signaling are shown. TF corresponds to transcription factors. Signaling initiates with binding of TGFβ to the receptors and ends with regulating transcription of target genes. I and II represent TβRI and TβRII, phosphorylation is depicted by P.

Figure 2

Tumor suppressor and promoting effects of the TGFβ pathway. TGFβ exerts its anti-tumor and tumor promoting effects by regulating the expression of various target genes. These functions are performed by various means shown with arrows. The genes whose expression are enhanced by TGFβ are shown in green, whereas the genes whose expression are suppressed by TGFβ are marked in red.

Figure 3

Schematic representation of the ubiquitin-proteasomal degradation pathway. (A) Sequential action of E1 (ubiquitin activating enzyme), E2 (ubiquitin conjugating enzyme), and E3 (ubiquitin ligase) enzymes in substrate ubiquitination. Ubiquitin is denoted as Ub. After multiple ubiquitin molecules are attached to the substrate forming lysine (K)-48, it can get degraded by the 26S proteasome. (B) Different kinds of polyubiquitin chains formed inside the cell and their cellular functions. Seven different ubiquitin lysine residues can be used for sequential chain formation and, based on the position of chain formation, functions vary.

Figure 4

Schematic representation of negative regulation of TGFβ signaling with major E3 ubiquitin ligases. (A) Direct inhibition of TβRI by different E3 ligases. SMURF2, WWP1 interact with SMAD7 to inhibit TβRI but SMURF1 can combine with either I-SMADs (SMAD6 or SMAD7) to inhibit TβRI. These E3 ligases induce K-48 mediated polyubiquitination of the receptor, which ultimately induces degradation. (B) Inhibition of R-SMAD phosphorylation and stability by diverse E3 ligases. SMURF1 is specific for BMP SMADs (SMAD1/5). PRAJA and NEDD4-2 induce K-48 linked polyubiquitination of SMAD3 and SMAD2, respectively. SMURF2 can ubiquitinate (K-48 linked) R-SMADs (SMAD 1/2 and SMAD3). (C) Inhibition and degradation of co-SMAD by different E3 ligases and the role of I-SMADs in this process. SKP2 specifically targets mutant co-SMAD for ubiquitination.

Figure 5

Schematic representation of positive regulation of TGFβ signaling with major E3 ubiquitin ligases and regulation of non-SMAD signaling by TRAF4 or TRAF6 ubiquitin ligases. TβRI and TβRII are indicated as I and II. (A) Positive regulation of TGFβ signaling by different E3 ligases. TRAF6 causes ubiquitination and release of the intracellular domain of TβRI, which translocates to the nucleus, associates with p300, and affects transcription of genes. RNF111/ARKADIA inhibits the negative regulator of TGFβ induced transcription SNoN. (B) Regulation of JNK/p38 signaling by TRAF4 or TRAF6 E3 ligases.