Paterson Institute for Cancer Research SCIENTIFIC REPORT 2005

CELL SIGNALLING
One probable mechanism by which Tiam1/Rac
antagonizes malignant progression is through their
positive effect on cell-cell adhesion. In vitro studies
have shown that over-expression of activated Rac
or Tiam1 can promote the formation of adherens
junctions and the accompanying induction of an
epithelioid phenotype in a number of cell lines
(Malliri & Collard, Curr Opin Cell Biol 2003; 15:
583). Moreover, using both RNA interference and
cells derived from Tiam1-deficient mice, it was
recently shown that endogenous Tiam1 is required
for both the formation as well as the maintenance
of cadherin-based adhesions (Malliri et al., J Biol
Chem 2004; 279: 30092). Intriguingly, Asef, another
Rac specific exchange factor, promotes intestinal
tumour cell migration and invasiveness in in vitro
models through down-regulating cadherin-mediated
adhesion (Kawasaki et al., Nature Cell Biol 2003;
5: 211).
Tiam1/Rac signalling and intestinal
tumourigenesis
Recently we have shown that Tiam1 is a potent
modifier of intestinal tumourigenesis. The majority
of intestinal tumours are caused by mutations in
the canonical Wnt signaling pathway, leading to its
activation. However, few genes targeted by this
pathway have been demonstrated to affect tumour
development in vivo. We determined that Tiam1 is a
Wnt-responsive gene. It is expressed in the proliferative
compartments (crypts) of the adult mammalian
intestine where the Wnt pathway is normally
active. It is also up-regulated in adenomas from
patients with either sporadic colorectal polyps or
familial adenomatous polyposis (FAP), as well as in
adenomatous polyps in Min (multiple intestinal
neoplasia) mice. In each instance, the Wnt pathway
is hyperactivated due to a mutation in the apc
tumour suppressor gene. Further, by comparing
tumour development in Min mice expressing or
lacking Tiam1, we found that Tiam1 deficiency significantly
reduces the formation as well as growth
of polyps in vivo. However, invasion of malignant
intestinal tumours is enhanced by a lack of Tiam1.
In line with this, knock-down of Tiam1 reduced the
growth potential of human colorectal cancer (CRC)
cells and their ability to form E-cadherin-based
adhesions, a prerequisite for local invasion of
tumour cells (Malliri et al., J Biol Chem epub Oct
05). The two studies on tumourigenesis in vivo
demonstrate that two independent oncogenic signalling
pathways of major clinical significance (Ras
and Wnt) recruit the Tiam1-Rac signalling pathway
by specific, albeit distinct mechanisms. In the context
of oncogenesis, activation of this signalling
module promotes tumour initiation and growth.
Thus, Tiam1 appears to make a unique contribution
to the development of the tumour types studied so
far, since its loss cannot be compensated for by
other Rho GEFs. Moreover, the diverse roles of
Rho GEFs in certain processes clearly indicate that
Rho GEFs do more than simply activate Rho molecules,
and several studies now point to their role in
influencing the choice of biological response elicited
by a given Rho protein. GEFs have been shown
to bind to effectors directly or to scaffold proteins
that complex with components of effector pathways.
Tiam1 contributes to the signalling specificity
downstream of Rac via associating with
IB2/JIP2, a scaffold that promotes Rac activation
of p38 kinase cascade over JNK MAP kinase cascade
(Buchsbaum et al., Mol Cell Biol 2002; 22:
4073). Tiam1 can also influence Rac signalling
specificity through its interaction with spinophilin, a
scaffold that binds to p70 S6K, another kinase regulated
by Rac. Spinophilin binding supresses the
ability of Tiam1 to activate Pak1, a different Rac
effector (Buchsbaum et al., J Biol Chem 2003; 278:
18833). In our lab we are taking a biochemical
approach to identify Rac effectors involved in different
aspects of transformation including malignant
progression. We have also started investigating
the potential role of the Tiam1 homologue, Stef, in
tumourigenesis and the impact of down-regulating
more than one GEF simultaneously in different
aspects of the transformed phenotype.
The Rho GTPase cycle. Rho-like GTPases cycle
between an active GTP-bound and an inactive GDPbound
form. This is regulated by guanine nucleotide
exchange factors (GEFs) and GTPase-activating proteins
(GAPs). Guanine nucleotide dissociation
inhibitors (GDIs) inhibit nucleotide dissociation and
control cycling of Rho GTPases between membrane
and cytosol. Active GTPases interact with effector
molecules to elicit various cellular responses including
cytoskeletal reorganisation, transcription, cell-cycle
progression, apoptosis, membrane trafficking.
Additionally GEFs could work as scaffold proteins by
either binding directly to Rac effectors or other scaffold
proteins that bind to effectors.
P A T E R S O N I N S T I T U T E S C I E N T I F I C R E P O R T 2 0 0 5
Publications listed
on page 57
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