The deubiquitinase USP28 controls intestinal homeostasis and promotes colorectal cancer

Stem cell and progenitor fate in the mammalian intestine: Notch and lateral inhibition in homeostasis and disease

The control of cell fate decisions is vital to build functional organs and maintain normal tissue homeostasis, and many pathways and processes cooperate to direct cells to an appropriate final identity. Because of its continuously renewing state and its carefully organised hierarchy, the mammalian intestine has become a powerful model to dissect these pathways in health and disease. One of the signalling pathways that is key to maintaining the balance between proliferation and differentiation in the intestinal epithelium is the Notch pathway, most famous for specifying distinct cell fates in adjacent cells via the evolutionarily conserved process of lateral inhibition. Here, we will review recent discoveries that advance our understanding of how cell fate in the mammalian intestine is decided by Notch and lateral inhibition, focusing on the molecular determinants that regulate protein turnover, transcriptional control and epigenetic regulation.

The E3 ubiquitin ligase Trim7 mediates c-Jun/AP-1 activation by Ras signalling

The c-Jun/AP-1 transcription factor controls key cellular behaviours, including proliferation and apoptosis, in response to JNK and Ras/MAPK signalling. While the JNK pathway has been well characterized, the mechanism of activation by Ras was elusive. Here we identify the uncharacterized ubiquitin ligase Trim7 as a critical component of AP-1 activation via Ras. We found that MSK1 directly phosphorylates Trim7 in response to direct activation by the Ras-Raf-MEK-ERK pathway, and this modification stimulates Trim7 E3 ubiquitin ligase activity. Trim7 mediates Lys63-linked ubiquitination of the AP-1 co-activator RACO-1, leading to RACO-1 protein stabilization. Consequently, Trim7 depletion reduces RACO-1 levels and AP-1-dependent gene expression. Moreover, transgenic overexpression of Trim7 increases lung tumour burden in a Ras-driven cancer model, and knockdown of Trim7 in established xenografts reduces tumour growth. Thus, phosphorylation-ubiquitination crosstalk between MSK1, Trim7 and RACO-1 completes the long sought-after mechanism linking growth factor signalling and AP-1 activation.

Usp28 counteracts Fbw7 in intestinal homeostasis and cancer

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(ΔIEC)) ameliorated the hyperproliferation and the impaired goblet and Paneth cell differentiation observed in Fbw7(ΔIEC) mice. The aggressive intestinal tumor formation of APC(Min/+); Fbw7(ΔIEC) 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.

Trial Watch: Proteasomal inhibitors for anticancer therapy

The so-called "ubiquitin-proteasome system" (UPS) is a multicomponent molecular apparatus that catalyzes the covalent attachment of several copies of the small protein ubiquitin to other proteins that are generally (but not always) destined to proteasomal degradation. This enzymatic cascade is crucial for the maintenance of intracellular protein homeostasis (both in physiological conditions and in the course of adaptive stress responses), and regulates a wide array of signaling pathways. In line with this notion, defects in the UPS have been associated with aging as well as with several pathological conditions including cardiac, neurodegenerative, and neoplastic disorders. As transformed cells often experience a constant state of stress (as a result of the hyperactivation of oncogenic signaling pathways and/or adverse microenvironmental conditions), their survival and proliferation are highly dependent on the integrity of the UPS. This rationale has driven an intense wave of preclinical and clinical investigation culminating in 2003 with the approval of the proteasomal inhibitor bortezomib by the US Food and Drug Administration for use in multiple myeloma patients. Another proteasomal inhibitor, carfilzomib, is now licensed by international regulatory agencies for use in multiple myeloma patients, and the approved indications for bortezomib have been extended to mantle cell lymphoma. This said, the clinical activity of bortezomib and carfilzomib is often limited by off-target effects, innate/acquired resistance, and the absence of validated predictive biomarkers. Moreover, the antineoplastic activity of proteasome inhibitors against solid tumors is poor. In this Trial Watch we discuss the contribution of the UPS to oncogenesis and tumor progression and summarize the design and/or results of recent clinical studies evaluating the therapeutic profile of proteasome inhibitors in cancer patients.

The intestinal epithelial cell cycle: uncovering its 'cryptic' nature

PURPOSE OF REVIEW

To discuss the recent landmark findings that have increased our understanding not only of the role of the epithelial cell cycle in the homeostasis of the small intestine, but also its relevance to inflammation and cancer.

RECENT FINDINGS

Recent data have unveiled novel information on protein interactions directly involved in the cell cycle as well as in the pathways that transduce external environmental signals to the cell cycle. A growing body of the recent evidence confirms the importance of food as well as hormonal regulation in the gut on cell cycle. Information on the contribution of the epithelial microenvironment, including the microbiota, has grown substantially in the recent years as well as on the gene-environment interactions and the multiple epigenetic mechanisms involved in regulating cell-cycle proteins and signalling. Finally, further studies investigating the dysregulation of the cell cycle during inflammation and proliferation have increased our understanding of the pathophysiology of chronic inflammatory diseases and cancer.

SUMMARY

This review highlights some of the most recent advances that further emphasize the importance of the cell cycle in the small intestine during homeostasis as well as in inflammation and cancer.

Fbw7 and Usp28 - enemies and allies

The Usp28 deubiquitinase antagonizes Fbw7-mediated turnover of multiple oncoproteins, including Myc, Jun, and Notch, and promotes tumorigenesis in the intestine. Our recent study reveals that Usp28 also counteracts autocatalytic ubiquitination of Fbw7, suggesting a complex role for Usp28 in the regulation of Fbw7 activity and tumor development.

Dual regulation of Fbw7 function and oncogenic transformation by Usp28

Fbw7, the substrate recognition subunit of SCF(Fbw7) ubiquitin ligase, mediates the turnover of multiple proto-oncoproteins and promotes its own degradation. Fbw7-dependent substrate ubiquitination is antagonized by the Usp28 deubiquitinase. Here, we show that Usp28 preferentially antagonizes autocatalytic ubiquitination and stabilizes Fbw7, resulting in dose-dependent effects in Usp28 knockout mice. Monoallelic deletion of Usp28 maintains stable Fbw7 but drives Fbw7 substrate degradation. In contrast, complete knockout triggers Fbw7 degradation and leads to the accumulation of Fbw7 substrates in several tissues and embryonic fibroblasts. On the other hand, overexpression of Usp28 stabilizes both Fbw7 and its substrates. Consequently, both complete loss and ectopic expression of Usp28 promote Ras-driven oncogenic transformation. We propose that dual regulation of Fbw7 activity by Usp28 is a safeguard mechanism for maintaining physiological levels of proto-oncogenic Fbw7 substrates, which is equivalently disrupted by loss or overexpression of Usp28.

Regulation of USP28 deubiquitinating activity by SUMO conjugation

USP28 (ubiquitin-specific protease 28) is a deubiquitinating enzyme that has been implicated in the DNA damage response, the regulation of Myc signaling, and cancer progression. The half-life stability of major regulators of critical cellular pathways depends on the activities of specific ubiquitin E3 ligases that target them for proteosomal degradation and deubiquitinating enzymes that promote their stabilization. One function of the post-translational small ubiquitin modifier (SUMO) is the regulation of enzymatic activity of protein targets. In this work, we demonstrate that the SUMO modification of the N-terminal domain of USP28 negatively regulates its deubiquitinating activity, revealing a role for the N-terminal region as a regulatory module in the control of USP28 activity. Despite the presence of ubiquitin-binding domains in the N-terminal domain, its truncation does not impair deubiquitinating activity on diubiquitin or polyubiquitin chain substrates. In contrast to other characterized USP deubiquitinases, our results indicate that USP28 has a chain preference activity for Lys(11), Lys(48), and Lys(63) diubiquitin linkages.