Axel Behrens: Projects

Loss of Fbw7 reprograms adult pancreatic ductal cells into α, δ, and β cells

The adult pancreas is capable of limited regeneration after injury, but has no defined stem cell population. The cell types and molecular signals that govern the production of new pancreatic tissue are not well understood.

Previous studies from our laboratory and others have shown that Fbw7, the substrate recognition component of an SCF-type E3 ubiquitin ligase, controls the stability of several key cell fate determinants and oncoproteins. Loss of Fbw7 alters the balance of stem and progenitor cells in tissues such as the nervous system and intestine.

Surprisingly, we found that inactivation of Fbw7 in the adult pancreas induces a subset of pancreatic ductal cells to reprogram into β cells, and to a lesser extent α and δ cells. Loss of Fbw7 stabilised the transcription factor Ngn3, a key regulator of endocrine cell differentiation. Moreover, expression of a stable form of Ngn3 in pancreatic ductal cells induced a similar frequency of reprogramming to β cells. The induced β cells resemble islet β cells in morphology and histology and express a comprehensive panel of β cell markers.

Figure 2

Figure 2: Loss of Fbw7 reprograms adult pancreatic ductal cells into α, δ, and β cells

A frequent stumbling block in previous models inducing cell reprogramming has been the functionality of the newly formed β cells. In contrast, Fbw7-mutant induced β cells secrete comparable amounts of insulin after glucose challenge as bona fide β cells isolated from pancreatic islets, suggesting that they are functional. Thus, loss of Fbw7 appears to reawaken an endocrine developmental differentiation program in adult pancreatic ductal cells, identifying Fbw7 as a master regulator of cell fate decisions in the pancreas. Our study highlights the plasticity of seemingly differentiated adult cells and reveals adult pancreatic duct cells as a latent multipotent cell type.

Figure 3

Figure 3: Fbw7 maintains pancreatic ductal cell fate. (Click to view larger image)

Tamoxifen-inducible inactivation of Fbw7 in adult pancreatic ductal cells (expressing CK19) results in an increase in the endocrine transcription factor Ngn3 and emergence of insulin-positive β cells within the ducts. Ngn3 is a direct target of ubiquitination by the Fbw7-containing SCFFbw7 E3 ubiquitin ligase complex, which in wild-type ducts marks Ngn3 for degradation. In the Fbw7 knockout (KO) ducts, Ngn3 protein is stabilised, favouring transdifferentiation of selected ductal cells to an endocrine fate.

The deubiquitinase USP28 controls intestinal homeostasis and promotes colorectal cancer

Colorectal cancer is the third most common cancer worldwide. Although the transcription factor c-MYC is misregulated in the majority of colorectal tumours, it is difficult to inhibit directly. Researchers are therefore looking for alternative ways to target c-MYC, for example by altering its stability.

The deubiquitinase USP28 stabilizes c-MYC as well as other oncogenic factors, but its role in tumourigenesis and in the intestine was unknown. Using murine genetic models, we determined that USP28 antagonises the ubiquitin-dependent degradation of c-MYC, as well as two additional oncogenic factors, c-JUN and NOTCH1, in the intestine. Mice lacking USP28 were healthy, but showed reduced intestinal proliferation and increased differentiation of secretory lineage cells.

In a murine model of colorectal cancer, mice harbouring USP28 deletion developed fewer intestinal tumours. More importantly, even in mice with established tumours, deleting USP28 reduced tumour size and dramatically increased lifespan. USP28 deficiency promoted tumour cell differentiation accompanied by decreased proliferation, suggesting that USP28 acts similarly in intestinal homeostasis and colorectal cancer models. Moreover, we identified USP28 as a c-MYC target gene highly expressed in murine and human intestinal cancers. USP28 and c-MYC form a positive feedback loop that maintains high c-MYC protein levels in tumours.

As deubiquitinases similar to USP28 have been successfully inhibited by small molecules, inhibition of USP28's enzymatic activity may be a promising strategy for cancer therapy.

UBR5-mediated ubiquitination of ATMIN is required for IR-induced ATM signalling and function

The checkpoint kinase ATM directs the cellular response to ionising radiation (IR) by localising to DNA damage sites and actively phosphorylating proteins involved in repair and survival.

ATM is recruited and activated at damage sites via an interaction with the Mre11/Rad50/NBS1 (MRN) complex. An alternative ATM binding partner, ATMIN, is not involved in the response to IR but mediates ATM kinase signalling in response to chromatin changes. The molecular mechanism favouring either MRN or ATMIN in response to specific stimuli is enigmatic. We have previously shown that ATMIN competitively inhibits ATM's interaction with the NBS1 subunit of MRN, suggesting that there must be a mechanism preventing ATMIN from disrupting ATM signaling in IR conditions.

We have now identified the E3 ubiquitin ligase UBR5 as a key component of ATM activation in response to IR. We discovered that UBR5 ubiquitinates ATMIN, which favours its dissociation from ATM, freeing ATM to interact with NBS1. This mechanism allows efficient ATM activation at damage sites and promotes cell survival after irradiation. UBR5 interacts with ATMIN and catalyses ubiquitination of ATMIN at lysine 238 in an IR-stimulated manner.

We showed that UBR5 deficiency, or mutation of ATMIN lysine 238, prevents ATMIN dissociation from ATM and inhibits ATM and NBS1 foci formation after IR. This reduction in ATM signalling impairs checkpoint activation and increases radiosensitivity. Thus, UBR5-mediated ATMIN ubiquitination is a vital event for ATM pathway selection and activation in response to DNA damage.

 

Axel Behrens

axel.behrens@crick.ac.uk
+44 (0)20 379 61194

  • Qualifications and history
  • 1998 PhD in Biology, Institute of Molecular Pathology, Austria
  • 1998 Postdoctoral Fellow, Institute of Molecular Pathology, Austria
  • 1999 Postdoctoral Fellow, Institute of Neuropathology, Switzerland
  • 2001 Established lab at the Imperial Cancer Research Fund, UK (in 2002 the Imperial Cancer Research Fund became Cancer Research UK)
  • 2015 Group Leader, the Francis Crick Institute, London, UK