Educational Forum with Clinical Studies Current Science and Research

June 17, 2011

Wnt signalling in stem cells and cancer. Xanya Sofra Weiss

The canonical Wnt cascade has emerged as a critical regulator of stem cells. In many tissues, activation of Wnt signalling has also been associated with cancer. This has raised the possibility that the tightly regulated self-renewal mediated by Wnt signalling in stem and progenitor cells is subverted in cancer cells to allow malignant proliferation. Insights gained from understanding how the Wnt pathway is integrally involved in both stem cell and cancer cell maintenance and growth in the intestinal, epidermal and haematopoietic systems may serve as a paradigm for understanding the dual nature of self-renewal signals.

Stem cells are cells that have the unique ability to self- renew as well as to generate more differentiated progeny. The most primitive stem cell is the embryonic stem cell, which is derived from the inner cell mass of the blasto- cyst. This cell is pluripotent and can thus generate all the tissues of the body. Following the pioneering work on haemato- poietic stem cells over the last five decades, a multitude of recent studies have indicated that most other adult tissues also harbour stem cells. These adult stem cells are normally involved in homeo- static self-renewal processes but can also be rapidly recruited to repair tissues upon injury. With the study of adult stem cell biology, recurring roles of a limited set of signalling cascades are rapidly being uncovered. One of these is the canonical Wnt cascade. Notably, in many of the same tissues where the Wnt cascade controls stem cells, cancer ensues upon dysregulated activation of this pathway. Conceptually, this indicates that an efficient road to cancer involves the hijacking of physiological regulators of stem cell function in these particular tissues. Below, we first outline the canonical Wnt cascade, and then describe its mirror image roles in the biology of stem cells and cancer.

vertebrate genome encodes four highly similar Tcf/Lef proteins. In the absence of a Wnt signal, Tcf/Lef proteins repress target genes through a direct association with co-repressors such as Groucho. The interaction with b-catenin transiently converts Tcf/Lef factors into transcriptional activators. Drosophila genetics has recently identified two additional nuclear components, Pygopus and Bcl9 (also known as legless), conserved in vertebrates. Pygopus is essential for transcriptional activation of Tcf/Lef target genes, whereas Bcl9 seems to bridge Pygopus to Tcf-bound b-catenin. In sum, the canonical pathway translates a Wnt signal into the transient transcription of a Tcf/Lef target gene programme.

Xanya Sofra Weiss

Xanya Sofra Weiss

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