ISREC - Research




Cancer research

Cancer development

Radtke Freddy, Senior scientist

Freddy Radtke obtained his Ph.D. in Molecular Biology from the University of Zürich in 1994. In 1995, he started his postdoctoral research in the laboratory of Michel Aguet at Genentech, Inc. (San Francisco, USA). In 1997, he returned to Switzerland with Michel Aguet and finished his postdoctoral fellowship at the Swiss Institute for Experimental Cancer Research (ISREC) in Lausanne. From 1999‑2005, he was a group leader and Associate Member at the Ludwig Institute for Cancer Research. Freddy Radtke then joined ISREC in January 2006 as a senior scientist and in July was appointed Associate Professor in the School of Life Sciences at EPFL.

Molecular mechanisms controlling homeostasis of self-renewing organs and their dysregulation during tumorigenesis

The molecular mechanisms and signaling pathways controlling stem cell maintenance, lineage commitment and differentiation in self-renewing systems such as the hematopoietic system, skin and intestine are a main focus of our research. The basic principle of self-renewing tissues is that they constantly produce cells from a stem cell reservoir that gives rise to proliferating transient amplifying cells, which subsequently differentiate and home to their proper compartments. These processes have to be under stringent control to ensure life-long homeostasis. In recent years a substantial body of evidence has accumulated to support the notion that signaling pathways known to be important during embryonic development (such as Shh, wnt and Notch) play also key roles in regulating self-renewing tissues. Moreover, these same pathways are often dysregulated during tumorigenesis due to mutations of crucial signaling elements within these pathways.
The general concept is that a better understanding of the mechanisms controlling stem cell maintenance versus differentiation may lead to the identification of novel therapeutic targets, as well as to improving strategies to specifically manipulate these players during tumorigenesis. Currently, our attention focuses on the evolutionarily conserved Notch signaling pathway (Fig. 1), which plays pleiotropic roles in different self-renewing tissues and cancer.

Notch signaling pathway

Figure 1: Mammals possess 4 Notch receptors (Notch1‑4) and five ligands (Jagged1 and 2, and Delta-like 1, 3 and 4). Notch receptors are expressed on the cell surface as heterodimeric proteins. They are composed of an extracellular domain containing up to 36 EGF‑like repeats followed by 3 cysteine rich LIN repeats and an intracellular domain containing multiple protein-protein interaction domains as well as a transactivation domain. Notch signaling is triggered upon ligand-receptor interaction which induces two sequential proteolytic cleavages, the first in the extracellular domain mediated by metalloproteases of the ADAM family, and the second within the transmembrane domain mediated by a g‑secretase activity of presenilins (PS). This second cleavage allows the release and translocation of the intracellular domain of Notch (NICD) into the nucleus where it associates with CSL. Binding of NICD to CSL leads to transcriptional activation by displacement of co-repressors (CoRs) and simultaneous recruitment of different coactivators (CoA) including mastermind like proteins (MAML) and p300.

Past and ongoing work

Notch signaling in hematopoiesis and T cell leukemias

We have established an essential role for Notch1 in specifying the T cell lineage employing conditional gene targeting strategies. Inducible inactivation of Notch1 in bone marrow progenitors results in a block in T cell development and ectopic B cell development in the thymus suggesting that Notch1 instructs an early lymphoid progenitor to adopt a T cell fate. In the absence of a Notch1 signal an early lymphoid progenitor chooses the B cell fate by default (Fig. 2). In addition, we uncovered a function for Notch1 at a later stage of T cell development being involved in the control of VDJ rearrangement of the T cell receptor (TCR) beta locus.

Aberrant Notch1 signaling within the hematopoietic system results in the development of acute lymphoblastic T cell leukemia. Thus, Notch1 functions as an oncogene in the hematopoietic system. We are currently investigating the molecular mechanisms by which uncontrolled Notch1 signaling exerts its oncogenic functions.

Notch signaling in the skin

In contrast to the previously established role of Notch1 as an oncogene and lineage specifier in the hematopoietic system, we unexpectedly identified a novel role for Notch1 as tumor suppressor. Skin specific inactivation of Notch1 leads first to epidermal hyperproliferation followed by development of skin tumors. Mechanisms that contribute to tumorigenesis as a consequence of Notch1 deficiency in the epidermis are sustained expression of Gli2 (a downstream effector of SHH signaling), downregulation of the cell cycle regulator CDKI p21 and de-repressed ß‑catenin mediated signaling in cells that should normally undergo differentiation (Fig. 2). We are presently exploring the roles of additional Notch receptor and ligand family members within the epidermis and skin appendages under normal and cancer promoting situations.

Notch signaling in the intestine

Aberrant Wnt signaling in the intestine is one of the most frequent events during the development of colorectal cancer. The inhibitory interaction between Notch and Wnt signaling in our skin research project prompted us to investigate Notch function in the gastrointestinal tract. If Notch signaling plays a similar role in the intestine as in the skin, then mice in which Notch signaling is inhibited are expected to develop tumors as a consequence of increased ß‑catenin mediated wnt signaling. Surprisingly, conditional inactivation of the CSL (which mediates Notch signaling of all receptors) within the crypt compartment results in the complete loss of transient amplifying cells followed by their conversion into mucus secreting goblet cells. Thus, Notch functions as progenitor gate-keeper in the intestine, and seems to cooperate with wnt signaling in order to maintain the undifferentiated proliferative crypt compartment (Fig. 2). This project was done in collaboration with the group of Hans Clevers (from the Hubrecht Laboratory, Utrecht, Netherlands). Current studies aim to identify the crucial Notch receptors and ligand family members, as well as to uncover the molecular mechanisms that contribute to the gate-keeper function of Notch.

Pleiotropic effects of Notch signaling

Figure 2: The four major roles of the Notch cascade that are relevant within self-renewing tissues or during tumorigenesis are schematically illustrated. A. Gate-keeper function: Notch maintains stem and/or transient amplifying cells (TA) in an undifferentiated state. In the intestine for example, Notch prevents crypt progenitor cells (TA) from differentiating. B. Binary cell fate decisions: In the lymphoid system Notch specifies the T cell lineage at the expense of the B cell lineage from an (at least) bi-potent early thymocyte progenitor. C. Induction of differentiation: In the skin, Notch induces terminal differentiation events of TA cells, and during thymocyte differentiation Notch1 promotes differentiation of pro‑T‑cells into pre‑T cells. D. Tumorigenesis: Overexpression of Notch within hematopoietic bone marrow cells or in T cell progenitors results in T cell leukemias, and as such, Notch functions as an oncogene. However in the skin, Notch functions as a tumor suppressor since loss of Notch signaling results in the development of basal cell carcinoma-like tumors.

Therefore, the question arises how the Notch signaling pathway, which is not only evolutionarily but also mechanistically conserved (Fig. 1)can lead to so many different and sometimes opposing outcomes (Fig. 2). One of the major goals in the future will be to gain further insights into Notch target genes, and cross talk between Notch and other signaling pathways. This will lead to a better understanding on how one and the same pathway can function context dependently either as an oncogene or a tumor suppressor.

Keywords

Experimental systems: mouse and human tumor samples
Self-renewing organs: hematopoietic system, epidermis, skin appendages, and intestine
Tumor models: T cell leukemia and skin derived tumors