PAK is required for the disruption of E-cadherin adhesion by the small GTPase Rac
Encarnacin Lozano
1
2
3
4
Marieke A. M. Frasa
2
4
Katarzyna Smolarczyk
2
4
Ulla G. Knaus
0
4
Vania M. M. Braga
)
2
4
0
Department of Immunology, The Scripps Research Institute
,
La Jolla, CA 92037
,
USA
1
Ecology and Evolution Research Section, Faculty of Life Sciences, Imperial College London
,
London, SW7 2AZ
,
UK
2
Molecular Medicine Section, NHLI, Faculty of Medicine
3
Present address: Museo Nacional de Ciencias Naturales
,
CSIC, 28006 Madrid
,
Spain
4
cDNA contructs Full-length constitutively active Rac1
5
kinase-dead in pRK5-myc were a gift from Alan Hall (Memorial Sloane-Kettering Cancer Center
,
New York, NY). PAK1
Summary E-cadherin cell-cell adhesion plays a major role in the maintenance of the morphology and function of epithelial tissues. Modulation of E-cadherin function is an important process in morphogenesis and tumour de-differentiation. We have previously shown that constitutively active Rac1 induces the disassembly of E-cadherin complexes from junctions in human keratinocytes. Here, we compare this activity in three ce members of the Rac subfamily (Rac1, Rac3 and Rac1b) and en investigate the molecular mechanisms underlying Rac1ic induced destabilization of junctions. We demonstrate that Rac3 lleS smheadrieastewdiathdhResaiocn1. tRhaec1abbiilsityantoalteinrtneartfievree spwliitche vcaardiahnertinofC Rac1 but, surprisingly, Rac1b cannot induce junction fo disassembly. Thus, Rac family members differ on their lrnau cpmaoedtcehhneatrinianils-mdteoptehnprdeoreutnguthrabdwhkheeiscriohantinRdooacecystepnrootcmeilnlo-vtcoeesllvledcaisonanstisnaeccmtrseb.alsyTehionef o J
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Introduction
E-cadherin is one of the most important cell-cell adhesion receptors
involved in tissue morphogenesis and maintenance of epithelial
tissue integrity (Gumbiner, 2005). Loss of function of E-cadherin
has emerged as an important event for local invasion and metastasis
(Christofori and Semb, 1999). Furthermore, E-cadherin expression
becomes down-regulated when epithelial cells acquire motile and
invasive characteristics (i.e. during epithelial wound healing and
embryonic development) (Thiery, 2003).
The small GTPase Rac1 is an important modulator of the actin
cytoskeleton and a key regulator of E-cadherin function
(Gumbiner, 2005). Activation of Rac1 perturbs the distribution of
E-cadherin at junctions in different cell types (Akhtar and Hotchin,
2001; Braga, 2000; Edme et al., 2002; Keely et al., 1997; Quinlan,
1999; Shintani et al., 2006; Yagi et al., 2007). This effect is specific
because other surface receptors, such as integrins, are not removed
from cell-cell contacts with the same time frame as cadherins
(Braga et al., 2000).
In vivo, Rac1-dependent disruption of junctions has been shown
during intestinal epithelia differentiation (Stappenbeck and
Gordon, 2000), salivary gland morphogenesis (Pirraglia et al.,
2006) and tracheal tubulogenesis (Chihara et al., 2003).
Furthermore, Rac activation is also required to perturb E-cadherin
junctions after Ras activation (Braga et al., 2000; Edme et al.,
contractility. Instead, activation of the Rac target PAK1 is
necessary for destabilization of cell-cell contacts. Inhibition of
PAK1 by dominant-negative constructs or depletion of
endogenous PAK1 by RNA interference efficiently blocked
Rac1-induced perturbation of junctions. Interestingly, PAK1
cannot be activated by Rac1b, suggesting that this may
contribute to the inability of Rac1b to disrupt cell-cell contacts
in keratinocytes. As PAK1 also plays a crucial role in
lamellipodia formation, our data indicate that PAK1 is at the
interface between junction destabilization and increased
motility during morphogenetic events.
Supplementary material available online at
http://jcs.biologists.org/cgi/content/full/121/7/933/DC1
2002), Csk kinase expression (Yagi et al., 2007) and HGF/scatter
factor treatment (Ridley et al., 1995; Takaishi et al., 1994). Yet,
these effects may be cell-type specific (Braga et al., 1999; Lozano
et al., 2003).
Upregulation of Rac1 protein and mRNA levels has been
demonstrated in human tumours of epithelial origin, implicating
Rac1 in cellular transformation (Lozano et al., 2003; Sahai and
Marshall, 2002a). Two other family members, Rac3 and Rac1b,
have also been found overexpressed or activated in several
epithelial tumours. Rac3 is divergent from Rac1 at the C-terminal
region (Haataja et al., 1997; Mira et al., 2000). Rac1b is an
alternative splice variant of Rac1 (Jordan et al., 1999; Schnelzer et
al., 2000) with an in-frame insertion of 19 amino acids in a region
important for Rac1 interaction with regulators and effectors. Rac1b
can neither induce lamellipodia nor activate the two known Rac1
effectors p21 activated kinase (PAK) and the downstream signaling
pathway of Jun N-terminal kinase 1 (JNK1) (Fiegen et al., 2004;
Matos et al., 2003). Yet, the functional consequences of Rac1, Rac3
and Rac1b activation are only now beginning to be unravelled
(Chan et al., 2005; Matos et al., 2003). F (...truncated)