Tumor suppression by the Fbw7 ubiquitin ligase: mechanisms and opportunities

Abstract

Tumor suppressors with widespread impact on carcinogenesis control broad spectra of oncogenic pathways. Protein degradation is an emerging mechanism by which tumor suppressors regulate a diversity of pathways and is exemplified by the SCF(Fbw7) ubiquitin ligase. Rapidly accumulating data indicate that SCF(Fbw7) regulates a network of crucial oncoproteins. Importantly, the FBXW7 gene, which encodes Fbw7, is one of the most frequently mutated genes in human cancers. These studies are yielding important new insights into tumorigenesis and may soon enable therapies targeting the Fbw7 pathway. Here, we focus on the mechanisms and consequences of Fbw7 deregulation in cancers and discuss possible therapeutic approaches.

Figure 1. Dimeric SCF^Fbw7 regulates a broad network of substrates

(A) Fbw7 binds to both phosphorylated substrates and the rest of the SCF complex (comprised of Skp1, Cullin-1, Rbx1, and an E2 enzyme), resulting in substrate poly-ubiquitylation and degradation by the proteasome. The network of Fbw7 substrates contains proteins with clear roles in carcinogenesis (shown in green) and others with emerging roles in Fbw7-associated tumors (shown in blue) (see text). Optimal (high-affinity) substrates have recognition signals termed CPDs that contain two phosphorylated residues (orange “P”); other, lower-affinity, CPDs contain a negatively charged amino acid (yellow “E”) in place of the second phosphate. (B) Fbw7 exists as three protein isoforms (α, β, and γ) that differ only by their N-terminal exons. All isoforms share three functional domains that are critical to their function as ubiquitin ligases: (1) the dimerization domain (“DD”) mediates Fbw7 dimerization, (2) the F-box binds to the rest of the SCF complex via Skp1 (Fig. 1A), and (3) the WD40 domain binds to phosphorylated substrates, and includes the three arginine residues that are mutational hotspots in cancers.

Figure 2. Possible mechanisms of Fbw7^ARG missense mutations in cancers

(A) Normal interactions of Fbw7 dimers with monomeric (left) or dimeric (right) substrates. Each protomer within an Fbw7 dimer can interact with a separate substrate CPD, leading to greatly increased binding affinity between substrates and Fbw7. The two substrate CPDs can be present within a monomeric substrate (e.g. cyclin E, left), or separated onto two interacting proteins (e.g. SREBP, right). The reduced number of contacts made by suboptimal degrons is indicated by a dashed line. (B) Model of heterozygous Fbw7^ARG dominant-negative activity in cancers resulting from the formation of impaired Fbw7^WT-Fbw7^ARG heterodimers. We speculate that Fbw7^WT-Fbw7^ARG heterodimers differentially affect substrates, depending on CPD affinity. The degradation of high-affinity substrates may still be driven by the normal protomer of an Fbw7^WT-Fbw7^ARG heterodimer (left), whereas suboptimal substrates (depicted by glutamate instead of a second CPD phosphate) rely on the concerted biding of two CPDs to an Fbw7 dimer, and will not be ubiquitylated by Fbw7^WT-Fbw7^ARG heterodimers (right). (C) Fbw7 truncation mutants may also generate heterodimers with Fbw7^WT, but are not nearly as frequent in tumors as Fbw7^ARG. We therefore speculate that a full length Fbw7 protein is critical for the dominant-negative effect of Fbw7^ARG, perhaps by retaining sufficient residual binding affinity (depicted by dashed line) for substrates with intermediate affinity.

Figure 3. Potential therapeutic opportunities targeting Fbw7 pathway mutations in cancer

(A) Tumors with heterozygous Fbw7^ARG mutations could be treated with a small-molecule agonist (shown in purple) that restores the binding affinity between the mutated Fbw7^ARG β–propeller and a phosphorylated substrate, leading to oncoprotein ubiquitylation and proteasomal destruction. (B) In cases where deregulation of particular Fbw7 substrates is necessary for tumor cell survival, inhibiting the functions of the relevant substrates and/or pathways, rather than targeting Fbw7 itself, may lead to anti-tumor activities. This approach would be applicable to tumors containing any type of Fbw7 mutation. (C) Synthetic lethality between decreased Fbw7 function and secondary pathways (“Pathway X”) could lead to selective killing of tumor cells, while sparing other cells that have normal Fbw7 function. Similar to (B), synthetic lethality could be applied to tumors containing any type of Fbw7 mutation. (D) Because leukemia-initiating cells are exquisitely sensitive to Myc abundance (Reavie et al., 2013; Takeishi et al., 2013), Fbw7 inhibition by small molecules (or other approaches) may sufficiently increase Myc abundance to cause LIC exhaustion and inhibit disease progression.