# RAC1B: A Rho GTPase with Versatile Functions in Malignant Transformation and Tumor Progression
## 5 Biological Functions of RAC1B
### Epithelial-Mesenchymal Transition (EMT)  
Primary steps of metastases formation include cell detachment from the original primary tumor which in many cases involves EMT although previous work has demonstrated that EMT is not necessarily required for all kinds of metastatic behavior [29; 30].  
As outlined above, RAC1B is a key mediator of MMP3-induced EMT in breast, lung, and pancreas carcinomas (Figure 2). The first evidence that RAC1B can promote EMT came from a study upon exposure of cultured murine mammary epithelial cells to MMP3 which induces the expression of Rac1b and causes an increase in cellular ROS and ROS-dependent stimulation of the expression of the EMT-associated transcription factor Snail [28; 31]. MMP3/Rac1b-mediated EMT requires cell spreading to cover a larger surface, a process which is induced in cells by treatment with MMP3 or in cells with ectopic expression of Rac1b [32]. The EMT phenotype is not stable but is shaped by environmental cues and is promoted by stiff/rigid substrata, with compliances characteristic of breast tumors, while soft substrata, with compliances comparable to that of normal mammary tissue, are protective against EMT. Strikingly, in cells cultured on stiff substrata or in collagen-rich regions of human breast tumors, Rac1b localizes to the plasma membrane where it complexes with NADPH oxidase and promotes the production of ROS, expression of Snail, and activation of the EMT program. In contrast, soft microenvironments inhibit the membrane localization of Rac1b and subsequent redox changes. Hence, Rac1b represents a crucial component of a mechanotransduction pathway regulating epithelial plasticity [17]. Also in mammary epithelial cells sequestering of Rac1b from the membrane by \(\alpha\)6-integrin is required for inhibition of EMT by laminin. In contrast, \(\alpha\)5-integrin-mediated is maintenance of Rac1b at the membrane is required for promotion of EMT by fibronectin [18]. Moreover, interference of cell contact-regulated communication by MMP3 and further cooperation with activated Rac1b is involved in EMT and plays a key role in early stages of breast cancer development. Specifically, treatment of cells with MMP3 via Rac1b interrupts a defined subset of cell contact-regulated genes, including genes that encode RNA splicing proteins known to regulate the expression of Rac1b [33]. A RAC1B-mediated release of ROS is also elicited in prostate carcinoma cells in response to metalloproteases secreted by cancer-associated fibroblasts (CAFs) [34]. RAC1B also seems to regulate the expression of other EMT markers in addition to Snail. RNAi-mediated suppression of RAC1B resulted in a concomitant increase in E-cadherin transcript levels [15], suggesting that RAC1B negatively regulates E-cadherin expression and positively controls expression of Snail2/Slug, a specific transcriptional repressor of E-cadherin. Interestingly, ARHGAP11A utilizes RAC1B to facilitate EMT in HCC cells while upregulation of RAC1B was capable of protecting cells from undergoing the reverse process, mesenchymal-to-epithelial transition (MET) that resulted from ARHGAP11A knockdown [22].  
The studies cited so far focused on the role of RAC1B in MMP-induced EMT. However, a recent report investigated EMT induced by TGF-f1 in PDAC-derived cells. Intriguingly, the authors observed an inhibitory role of RAC1B (Figure 2). RNAi-mediated, exon 3b-targeted knockdown of RAC1B increased an EMT phenotype as evidenced by cell morphology, gene expression of EMT markers, cell migration (see Section 5.6), and growth inhibition [35]. The suppressive effect of RAC1B on EMT was further substantiated by the observation that the activities of SMAD and non-SMAD (MKK3/6-p38 MAPK and MEK-ERK1/2) signaling pathways, all of which are involved in TGF-\(\beta\)-induced EMT in pancreatic cells, were increased upon RAC1B knockdown [35].  
The role of RAC1B in EMT appears thus inconsistent in that it can be both an inducer or an inhibitor of EMT (Figure 2).