The GTPases Rac and Cdc42 play a pivotal role in the establishment of cell polarity by stimulating biogenesis of tight junctions (TJs). In this study, we show that the Rac-specific guanine nucleotide exchange factor Tiam1 (T-lymphoma invasion and metastasis) controls the cell polarity of epidermal keratinocytes. Similar to wild-type (WT) keratinocytes, Tiam1-deficient cells establish primordial E-cadherin–based adhesions, but subsequent junction maturation and membrane sealing are severely impaired. Tiam1 and V12Rac1 can rescue the TJ maturation defect in Tiam1-deficient cells, indicating that this defect is the result of impaired Tiam1–Rac signaling. Tiam1 interacts with Par3 and aPKCζ, which are two components of the conserved Par3–Par6–aPKC polarity complex, and triggers biogenesis of the TJ through the activation of Rac and aPKCζ, which is independent of Cdc42. Rac is activated upon the formation of primordial adhesions (PAs) in WT but not in Tiam1-deficient cells. Our data indicate that Tiam1-mediated activation of Rac in PAs controls TJ biogenesis and polarity in epithelial cells by association with and activation of the Par3–Par6–aPKC polarity complex.

We conclude that the Par-Tiam1 complex stabilizes front-rear polarization of noncontacting migratory cells, thereby stimulating persistent and chemotactic migration, whereas in contacting keratinocytes, the same complex controls the establishment of long-lasting apical-basal polarity. These findings underscore a remarkable flexibility of the Par polarity complex that, depending on the biological context, controls distinct forms of cellular polarity.

The GTPases of the Rho family are molecular switches that play an important role in a wide range of cellular processes and are increasingly implicated in tumourigenesis. Unlike what was found for the Ras oncogenes in tumours, hardly any activating mutations have been found in the genes encoding Rho proteins. In the past, we have identi¢ed Tiam1 (T-lymphoma invasion and metastasis) as a speci¢c activator for the Rholike GTPase Rac. In vivo, Tiam1 de¢ciency protects against Ras-induced skin carcinogenesis, underscoring the consequences of deregulated signalling for the onset and progression of tumours. Thus, an important level of regulation of signalling via the Rho-like GTPases comes from the speci¢c control of their activators. In this paper, we review what is known on the speci¢c regulation of Tiam1 signalling towards Rac.

Cell polarization is required for virtually all functions of T cells, including transendothelial migration in response to chemokines. However, the molecular pathways that establish T cell polarity are poorly understood. We show that the activation of the partitioning defective (Par) polarity complex is a key event during Rap1- and chemokine-induced T cell polarization. Intracellular localization and activation of the Par complex are initiated by Rap1 and require Cdc42 activity. The Rac activator Tiam1 associates with both Rap1 and components of the Par complex, and thereby may function to connect the Par polarity complex to Rap1 and to regulate the Rac-mediated actin remodelling required for T cell polarization. Consistent with these findings, Tiam1-deficient T cells are impaired in Rap1- and chemokine-induced polarization and chemotaxis. Our studies implicate Tiam1 and the Par polarity complex in polarization of T cells, and provide a mechanism by which chemokines and Rap1 regulate T cell polarization and chemotaxis.

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The Rho-like guanosine triphosphatase Rac1 regulates various signaling pathways, including integrin-mediated adhesion and migration of cells. However, the mechanisms by which integrins signal toward Rac are poorly understood. We show that the Rac-specific guanine nucleotide exchange factor Tiam1 (T-lymphoma invasion and metastasis 1) is required for the integrin-mediated laminin (LN)-5 deposition, spreading, and migration of keratinocytes. In contrast to wild-type keratinocytes, Tiam1-deficient (Tiam1−/−) keratinocytes are unable to adhere to and spread on a glass substrate because they are unable to deposit their own LN5 substrate. Both Tiam1 and V12Rac1 can rescue the defects of Tiam1−/− keratinocytes, indicating that these deficiencies are caused by impaired Tiam1-mediated Rac activation. Tiam1−/− cells are unable to activate Rac upon α3β1-mediated adhesion to an exogenous LN5 substrate. Moreover, Tiam1 deficiency impairs keratinocyte migration in vitro and reepithelialization of excision wounds in mouse skin. Our studies indicate that Tiam1 is a key molecule in α3β1-mediated activation of Rac, which is essential for proper production and secretion of LN5, a requirement for the spreading and migration of keratinocytes.

Tiam1 is a ubiquitously expressed activator of the small GTPase Rac. Previously, we found that Tiam1 knockout (KO) mice are resistant to DMBA-induced skin tumorigenicity, which correlated with increased apoptosis in keratinocytes of the skin epidermis. Here, we have studied the mechanisms by which Tiam1 protects against apoptosis. We found that Tiam1-KO keratinocytes show increased apoptosis in response to apoptotic stimuli, including growth factor deprivation and heat-shock treatment. Expression of catalytically active Tiam1, but not inactive Tiam1, rescues the apoptosis susceptibility of Tiam1-KO keratinocytes, indicating that this defect is caused by impaired Tiam1-mediated Rac activation. Apoptosis induced by growth factor starvation correlates with impaired ERK phosphorylation in Tiam1-KO keratinocytes. Moreover, Tiam1-KO keratinocytes contain lower levels of intracellular reactive oxygen species (ROS) when compared with wild-type cells. The ROS content of keratinocytes is dependent on both Tiam1 and the activity of NADPH oxidase (Nox), and is required for ERKmediated survival signaling. Indeed, Tiam1 deficiency or the inhibition of intracellular ROS production blocks ERK phosphorylation and sensitizes wild-type keratinocytes to apoptotic stimuli. Our results indicate that the Rac activator Tiam1 controls the intracellular redox balance by Nox-mediated ROS production, which regulates ERK phosphorylation and the susceptibility of keratinocytes to apoptotic signaling.