Understanding the impact of Fbw7 mutations on substrate function and cancer cell biology
Final Report Abstract
Fbw7 ubiquitin ligase is a key tumor suppressor in human cancer, with mutations occurring in T- cell leukemia and a range of solid cancers including lung, colorectal, ovarian and bile duct carcinomas. Such cancer-associated mutations have been shown to abrogate substrate recognition leading to the accumulation of oncogenic substrates. However, published studies provided contrasting data on the impact of Fbw7 mutations on substrate binding and their oncogenic potential, arguing that further analysis is required to understand the role of such mutations in cancer biology. We studied the role of Fbw7 mutations on Myc function using Fbw7-proficient and Fbw7- deficient isogenic cell lines. Our data show that mutant Fbw7 retains the ability to bind Myc and promotes Myc ubiquitination and turnover, albeit less efficiently than the wildtype Fbw7. Analysis of Myc-dependent transcription showed that mutant Fbw7 has a non-linear impact on the expression of Myc target genes. For some genes mutant Fbw7 mimics complete gene deletion, whereas for others it recapitulates the effects of wildtype Fbw7. In agreement with transcriptional effects, mutant Fbw7-expressing cells show mixed phenotypic characteristics. In the absence of Fbw7, Myc inhibits cell-cell and cell-matrix interactions via regulation of genes encoding integrins and claudins and strongly stimulates anchorage-independent growth. Coexpression of wildtype Fbw7 completely reverses these effects, whereas mutant Fbw7 produces a partial phenotype and allows efficient proliferation both in suspension and in monolayer. These data suggest that mutations in Fbw7 promote cellular plasticity in the requirement for adhesion, allowing adaption to different in vivo contexts and facilitate both cell survival and transitions between epithelial and mesenchymal cell states. Furthermore, we show that ubiquitination of Myc is required for Myc-driven transcriptional activation by regulating its interactions with the elongation factor Paf1. Importantly, Fbw7 can specifically modify Myc at key functional ubiquitin acceptor residues and thereby can directly control Myc-dependent transcriptional activation and repression. Fbw7 mutation selectively modulates Myc binding to Paf1 subunits and not only regulates Mycdependent transcription but also alleviates Myc-induced replicative stress and DNA damage and desensitizes Myc-overexpressing cells to blockade of DNA replication and inhibition of DNA damage checkpoint signaling. Collectively, these data suggest that another critical function of Fbw7 mutations is to attenuate Myc-induced replicative stress and DNA damage to facilitate tumor initiation and progression.
Publications
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(2016) Regulating Fbw7 on the road to cancer. Semin. Cancer Biol. 36: 62–70
Xu W, Taranets L & Popov N
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(2016) Ubiquitin-Dependent Turnover of MYC Antagonizes MYC/PAF1C Complex Accumulation to Drive Transcriptional Elongation. Molecular Cell 61: 54–67
Jaenicke LA, von Eyss B, Carstensen A, Wolf E, Xu W, Greifenberg AK, Geyer M, Eilers M & Popov N
