**5.1 Rho GTPases and cytoskeleton organization**

Eukaryotic cellular morphology and attachment to the substratum in response to extracellular signals are largely dependent on rearrangements of the actin cytoskeleton. Cell motility, cytokinesis and phagocytosis all rely on coordinated regulation of the actin cytoskeleton (Small 1994; Zigmond 1996). Filamentous actin can be organized into several discrete structures: (a) filopodia, finger-like protrusions that contain a tight bundle of long actin filaments in the direction of the protrusion. These are found primarily in motile cells and neuronal growth cones. (b) lamellipodia, thin protrusive actin sheets that dominate the edges of cultured fibroblasts and many other motile cells. Membrane ruffles observed at the leading edge of the cell result from lamellipodia that lift up off the substrate and fold backward. (c) actin stress fibers, bundles of actin filaments that traverse the cell and are linked to the ECM through focal adhesion (Van Aelst and D'Souza-Schorey 1997). The actin polymerization is tightly regulated by Rho GTPases.

Rho activation in fibroblasts is known to stimulate the assembly of contractile actin/myosin filaments, the formation of stress fibers, and the clustering of integrins involved in the formation of focal adhesion complexes. Activation of Rac facilitates actin polymerization at the cell periphery to generate protrusive actin-rich lamellipodia and membrane ruffling. And activation of Cdc42 results in actin polymerization to form peripheral actin microspikes and filopidia. As described previously, a number of proteins have been identified as targets

Akt pathways (Mazieres, Antonia et al. 2004). Moreover, Rac1 enhances estrogen receptor α (ERα) transcriptional activity, resulting in increased proliferation in breast cancer cells

Cdc42 is overexpressed in some breast cancers and there is accumulating evidence that activated Cdc42 contributes to the accumulation of ErbB1 in cells through the regulation of c-Cbl function (Abraham, Kuriakose et al. 2001; Marionnet, Lalou et al. 2003). The view that Cdc42 is involved in human breast carcinogenesis is supported by a rodents model of breast carcinoma where the expression of a dominant negative mutant of Cdc42 reduced the number of focal contacts, inhibited colony formation in soft agar and affected cell growth *in vivo* (Fritz, Just et al. 1999). The dominant negative Cdc42 also reduced intravasation of tumor cells into peripheral blood and ability to form lung metastasis. In addition, through the activation of Cdc42, transforming growth factor α (TGF-α) mediates the invasion of MDA-MB-231 cells into 3-D collagen matrices by initiating the formation of protrusions into collagen. (Kamai, Tsujii et al. 2003; Fisher, Sacharidou et al. 2009). Further, another study has shown that membrane-type-1 matrix metalloproteinase (MT1- MMP) and Cdc42 are fundamental components of a co-associated invasion-signaling complex that controls directed single-cell invasion of 3D collagen matrices (Fisher,

**5. Multiple functions mediated by Rho GTPases in breast cancer** 

**5.1 Rho GTPases and cytoskeleton organization** 

polymerization is tightly regulated by Rho GTPases.

Rho GTPases mediate housekeeping aspects of cell biology including cell growth, cell polarity, cell adhesion, membrane trafficking and motility. They function as signaling switches that regulate lipid metabolism, microtubules- and actin-based structures, epithelial

Eukaryotic cellular morphology and attachment to the substratum in response to extracellular signals are largely dependent on rearrangements of the actin cytoskeleton. Cell motility, cytokinesis and phagocytosis all rely on coordinated regulation of the actin cytoskeleton (Small 1994; Zigmond 1996). Filamentous actin can be organized into several discrete structures: (a) filopodia, finger-like protrusions that contain a tight bundle of long actin filaments in the direction of the protrusion. These are found primarily in motile cells and neuronal growth cones. (b) lamellipodia, thin protrusive actin sheets that dominate the edges of cultured fibroblasts and many other motile cells. Membrane ruffles observed at the leading edge of the cell result from lamellipodia that lift up off the substrate and fold backward. (c) actin stress fibers, bundles of actin filaments that traverse the cell and are linked to the ECM through focal adhesion (Van Aelst and D'Souza-Schorey 1997). The actin

Rho activation in fibroblasts is known to stimulate the assembly of contractile actin/myosin filaments, the formation of stress fibers, and the clustering of integrins involved in the formation of focal adhesion complexes. Activation of Rac facilitates actin polymerization at the cell periphery to generate protrusive actin-rich lamellipodia and membrane ruffling. And activation of Cdc42 results in actin polymerization to form peripheral actin microspikes and filopidia. As described previously, a number of proteins have been identified as targets

cell-junctions, cell cycle and apoptosis regulatory proteins, and transcription factors.

(Rosenblatt, Garcia et al. ; Folkman 1972).

**4.3 Cdc42 family in breast tumors** 

Sacharidou et al. 2009).

of Rho, Rac and Cdc42 (Fig. 3). Most of them are involved in Rho GTPases mediated cytoskeletal rearrangements (Tang, Olufemi et al. 2008).

Fig. 3. A model of the cellular migratory process. See text for detailed explanation of motility phases.
