Usp28 Counteracts Fbw7 in Intestinal Homeostasis and Cancer

Abstract

The stability of several oncoproteins, including c-Myc, is regulated by ubiquitin-dependent degradation mediated by the SCF (Fbw7) ubiquitin ligase. This activity is antagonized by the deubiquitinase Usp28, which is highly expressed in murine and human intestinal cancers. Usp28 was previously shown to interact with its substrates via a "piggyback" interaction with Fbw7, which suggested that Fbw7 is required for Usp28 activity. Unexpectedly, we found that genetic deletion of Usp28 rescued the lethality of Fbw7-deficient primary fibroblasts. Moreover, Usp28 inactivation in the intestine (Usp28 DIEC ) ameliorated the hyperproliferation and the impaired goblet and Paneth cell differentiation observed in Fbw7 DIEC mice. The aggressive intestinal tumor formation of APC Min/þ ; Fbw7 DIEC mice was restrained when Usp28 was inactivated concomitantly. In both fibroblasts and intestinal cells, Usp28 deficiency corrected the accumulation of SCF(Fbw7) substrate proteins, including NICD1, c-Jun, and c-Myc. These findings suggested that Usp28 function does not depend on the presence of Fbw7, but instead independently recognizes and deubiquitylates the same substrates as SCF(Fbw7). Fbw7 binds to a phosphorylated motif termed the phosphodegron and we found that Usp28 also interacted with this same motif, but only when it is unphosphorylated, offering a mechanistic explanation for identical substrate selection by Fbw7 and Usp28. Our results indicate an unusually direct antagonism between an E3 ligase and a deubiquitinase, Fbw7 and Usp28, in modulating intestinal homeostasis and cancer. Cancer Res; 75(7);

Deubiquitinating c-Myc: USP36 steps up in the nucleolus

“…Using peptide binding assays, it was shown that USP28 binds to the same Myc phosphodegron recognized by Fbw7 in MBI; yet USP28 binds to the non-phosphorylated form whereas Fbw7 binds to the phosphorylated form of this Myc degron. 58 These studies demonstrate a complex regulation of Myc by the USP28-Fbw7 interplay, suggesting an important role in the dynamic and finely tuned regulation of Myc activity necessary for normal cellular function.…”

“…57 Further complexity was revealed in an intestine study, where it was shown that deletion of USP28 rescued the cytotoxic effects of Fbw7-deficiency in primary fibroblasts and meliorated the hyperproliferation and the impaired goblet and Paneth cell differentiation in mice with intestine-specific deletion of Fbw7 (Fbw7 DIEC ). 58 This correlates with correcting the aberrant accumulation of Fbw7 substrates such as Myc, c-Jun and NICD1. These results suggest that USP28 can function independent of a direct interaction with Fbw7.…”

“…Conversely, it is interesting to test whether USP36 could suppress Fbw7g autoubiquitination and stabilize it, similar to the role of USP28 on Fbw7. 57,58 It is also important to differentiate the role of USP36 as a DUB to directly deubiquitinate Myc or as an Fbw7g inhibitor to suppress Fbw7g-mediated Myc ubiquitination, or a combined effect of both.…”

Fbw7 and its counteracting forces in stem cells and cancer: Oncoproteins in the balance

“…Pulldown experiments using c-Myc peptides showed that USP28 and Fbw7 bind to the phosphorylated CPD motif; however, USP28 only bound to non-phosphorylated peptide in the absence of Fbw7 [91]. This suggests that the phosphorylated CPD motif is specifically recognised by Fbw7, which can then recruit USP28, but in the absence of Fbw7, USP28 interacts with the unmodified substrate.…”

“…Moreover, inactivation of USP28 in established APC min/+ tumours reduced c-Myc protein levels, decreased tumour cell proliferation, and increased tumour cell differentiation and death, resulting in increased life expectancy. A follow up study investigated the potential benefit of inactivating USP28 in Fbw7-deleted colorectal cancers [91], as this situation is commonly found in patients. Protein levels of the Fbw7 substrates c-Myc, c-Jun, NICD1 and cyclin E1, which were all elevated in Fbw7-deleted mice, were downregulated in double knock out animals.…”