Abstract
IntroductionT-cell lymphoma invasion and metastasis-inducing protein (Tiam1) is an Ras-related C3 botulinum toxin substrate (Rac)-specific guanine nucleotide exchange factor that was isolated based on its ability to induce a metastatic phenotype. In polarized migrating keratinocytes, Tiam1 is found at the leading edge, where it cooperates with the protease-activated receptor 1 (Par1) complex to establish front-to-rear polarity. Although a positive correlation has been observed between Tiam1 expression and tumor grade in a variety of human malignancies, including breast, its role in breast cancer cells has not yet been examined.MethodsTiam1 expression and Rac activity were examined in a panel of human breast cancer cell lines that exhibit different degrees of cell motility. The contribution of Tiam1 to cell motility was directly examined by using transwell motility and wound-healing assays.ResultsAlthough we observed a striking, positive correlation between Tiam1 expression and cell motility in the panel of breast cancer cell lines, we did not observe a correlation between Tiam1 expression and overall levels of Rac activity. Consistent with this, small interfering ribonucleic acid (siRNA)-mediated suppression of Tiam1 expression limits the motility of cell lines in which Tiam1 expression is high (MDA-MB-231 and MDA-MB-453) but does not substantially alter the overall levels of activated Rac. Tiam1 overexpression is also not sufficient to increase the motility of more poorly motile cells (T-47D) or to increase Rac activity. Immunofluorescence and cellular fractionations indicate that Tiam1 is found predominantly in the Golgi of breast cancer cells, and in the latter case, Tiam1 was shown to co-fractionate with a limited pool of Rac1. Consistent with this Golgi localization, Tiam1 supports cell motility and Golgi reorientation in response to serum in a wound-healing assay using MDA-MB-231 and MDA-MB-435S cells.ConclusionsTiam1 expression correlates with cell motility in human breast cancer cells and is required to support the motile phenotype. Localization of endogenous Tiam1 to the Golgi, and its demonstrated role in Golgi reorientation, suggest that it may support motility through a mechanism that is discrete from its known function in leading-edge dynamics.
Highlights
T-cell lymphoma invasion and metastasis-inducing protein (Tiam1) is an Ras-related C3 botulinum toxin substrate (Rac)-specific guanine nucleotide exchange factor that was isolated based on its ability to induce a metastatic phenotype
T-cell lymphoma invasion and metastasisinducing protein (Tiam1) is differentially expressed in human breast epithelial cell lines To determine whether Tiam1 is expressed in tumorderived human breast epithelial cells, lysates were collected from a panel of cell lines and examined with Western blot with an antibody specific for Tiam1 (Figure 1)
Tiam1 has been well characterized as a Racspecific guanine nucleotide exchange factor (GEF) [20], we do not observe a correlation between Tiam1 expression and overall Rac activity, and reduced expression of Tiam1 does not have an appreciable effect on Rac-GTP levels
Summary
T-cell lymphoma invasion and metastasis-inducing protein (Tiam1) is an Ras-related C3 botulinum toxin substrate (Rac)-specific guanine nucleotide exchange factor that was isolated based on its ability to induce a metastatic phenotype. Motile cell include the formation of filopodia and lamellipodia at the leading edge, stabilization and localized capture of microtubules at the leading edge, and reorientation of the microtubule organizing center and Golgi complex toward the direction of migration [8]. The Rho family members RhoA, Rac, and Cdc have been identified as key regulators of cell motility, initially based on their ability to regulate the actin cytoskeleton [9]. Subsequent studies revealed an important role for these three Rho family members in virtually all aspects of the motile phenotype, including actin reorganization, microtubule stabilization and capture, centrosome reorientation, and vesicle trafficking from the Golgi [8]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
