Linking epithelial-mesenchymal transition to the well-known polarity protein Par6

From shape-shifting embryonic cells to oncology: The fascinating history of epithelial mesenchymal transition

Epithelial-to-mesenchymal transition or transformation (EMT) is a cell shape-changing process that is utilized repeatedly throughout embryogenesis and is critical to the attainment of a precise body plan. In the adult, EMT is observed under both normal and pathological conditions, such as during normal wounding healing, during development of certain fibrotic states and vascular anomalies, as well as in some cancers when malignant cells progress to become more aggressive, invasive, and metastatic. Epithelia derived from any of the three embryonic germ layers can undergo EMT, including those derived from mesoderm, such as endothelial cells (sometimes termed Endo-MT) and those derived from endoderm such as fetal liver stroma. At the cellular level, EMT is defined as the transformation of epithelial cells towards a mesenchymal phenotype and is marked by attenuation of expression of epithelial markers and de novo expression of mesenchymal markers. This process is induced by extracellular factors and can be reversible, resulting in mesenchymal-to-epithelial transformation (MET). It is now clear that a cell can simultaneously express properties of both epithelia and mesenchyme, and that such transitional cell-types drive tumor cell heterogeneity, an important aspect of cancer progression, development of a stem-like cell state, and drug resistance. Here we review some of the earliest studies demonstrating the existence of EMT during embryogenesis and discuss the discovery of the extracellular factors and intracellular signaling pathways that contribute to this process, with components of the TGFβ signaling superfamily playing a prominent role. We mention early controversies surrounding in vivo EMT during embryonic development and in adult diseased states, and the maturation of the field to a stage wherein targeting EMT to control disease states is an aspirational goal.

Understanding and targeting anoikis in metastasis for cancer therapies

The development of effective treatments for cancers requires investigations for a more detailed and comprehensive understanding of the basic cellular mechanisms involved in carcinogenesis, cancer progression, and metastasis. One of those driving mechanisms is anoikis, a special type of apoptosis, which is induced by losing anchorage from the extracellular matrix (ECM). In other words, resisting death in detached cells (cells without ECM) forms an anoikis-resistant phenotype. Since the anoikis-resistance state compensates for the initial steps of cancer metastasis, this review aimed to discuss mechanisms of gaining anoikis/anoikis resistance phenotype in tumor cells. Finally, we highlighted the significance of anoikis in malignancies so as to provide clear insight into cancer diagnosis and therapy development.

Pathogenesis of periodontitis - A potential role for epithelial-mesenchymal transition

Epithelial mesenchymal transition (EMT) is a process comprising cellular and molecular events which result in cells shifting from an epithelial to a mesenchymal phenotype. Periodontitis is a destructive chronic disease of the periodontium initiated in response to a dysbiotic microbiome, and dominated by Gram-negative bacteria in the subgingival niches accompanied by an aberrant immune response in susceptible subjects. Both EMT and periodontitis share common risk factors and drivers, including Gram-negative bacteria, excess inflammatory cytokine production, smoking, oxidative stress and diabetes mellitus. In addition, periodontitis is characterized by down-regulation of key epithelial markers such as E-cadherin together with up-regulation of transcriptional factors and mesenchymal proteins, including Snail1, vimentin and N-cadherin, which also occur in the EMT program. Clinically, these phenotypic changes may be reflected by increases in microulceration of the pocket epithelial lining, granulation tissue formation, and fibrosis. Both in vitro and in vivo data now support the potential involvement of EMT as a pathogenic mechanism in periodontal diseases which may facilitate bacterial invasion into the underlying gingival tissues and propagation of inflammation. This review surveys the available literature and provides evidence linking EMT to periodontitis pathogenesis.

A Computational Protocol for Regulating Protein Binding Reactions with a Light-Sensitive Protein Dimer

Light-sensitive proteins can be used to perturb signaling networks in living cells and animals with high spatiotemporal resolution. We recently engineered a protein heterodimer that dissociates when irradiated with blue light and demonstrated that by fusing each half of the dimer to termini of a protein that it is possible to selectively block binding surfaces on the protein when in the dark. On activation with light, the dimer dissociates and exposes the binding surface, allowing the protein to bind its partner. Critical to the success of this system, called Z-lock, is that the linkers connecting the dimer components to the termini are engineered so that the dimer forms over the appropriate binding surface. Here, we develop and test a protocol in the Rosetta molecular modeling program for designing linkers for Z-lock. We show that the protocol can predict the most effective linker sets for three different light-sensitive switches, including a newly designed switch that binds the Rho-family GTPase Cdc42 on stimulation with blue light. This protocol represents a generalized computational approach to placing a wide variety of proteins under optogenetic control with Z-lock.

Expression and significance of c-kit and epithelial-mesenchymal transition (EMT) molecules in thymic epithelial tumors (TETs)

Background

To investigate the expression and significance of c-kit and epithelial-mesenchymal transition (EMT) molecules (E-cadherin, N-cadherin, Twist, Snail) in thymic epithelial tumors (TETs).

Methods

The tissue microarray technology and immunohistochemistry MaxVisionTM-use kit were used to detect the expression of c-kit and EMT molecular markers in 150 cases of paraffin sections of TET tissue and analysis the correlation between c-kit and EMT molecules and explore the malignancy and the relationship of clinicopathological parameters between c-kit, EMT molecules and TETs.

Results

The expression difference of c-kit and EMT molecular markers (E-cadherin, N-cadherin, Snail, Twist) in TETs subtypes was statistically significant (P<0.01) and their positive expression rate of thymic carcinoma was significantly higher than that in thymoma, and the difference was statistically significant, respectively (P<0.01). There is a negative correlation between the expression of c-kit and E-cadherin as well as a positive correlation between the expression level of c-kit, N-cadherin, Twist, and Snail. Furthermore, E-cadherin was negatively correlated with N-cadherin, Twist, and Snail while N-cadherin expression was positively correlated with Twist, Snail.

Conclusions

Five indicators (c-kit, E-cadherin, N-cadherin, Twist, and Snail) may determine the malignancy of TETs, especially for distinguishing thymoma and thymic carcinoma.

Alpha-tubulin acetyltransferase/MEC-17 regulates cancer cell migration and invasion through epithelial-mesenchymal transition suppression and cell polarity disruption

MEC-17, a newly identified alpha-tubulin-N-acetyltransferase 1, serves as the major α-tubulin acetyltransferase to promote α-tubulin acetylation in vitro and in vivo. Alteration of α-tubulin acetylation may be involved in morphology regulation, cell migration, and tumour metastasis. However, MEC-17’s role in cell physiology and its effect on epithelial–mesenchymal transition (EMT) and cell polarity remain elusive. In the present study, we characterized the overexpressed or downregulated cell models through gene targeting as MEC-17 gain- or loss-of-function. Overexpression of MEC-17 enhanced the cell spreading area, suppressed pseudopods formation in a three-dimensional (3D) culture system, and inhibited cancer cell migratory and invasive ability and tumour metastasis by orthotopic lung cancer animal model. Furthermore, morphological change and migration inhibition of cancer cells were accompanied by EMT repression, Golgi reorientation, and polarity disruption caused by alteration of cdc42 activity via a decrease in Rho-GAP, ARHGAP21. By contrast, a reduction in endogenous MEC-17 accelerated the pseudopods formation and EMT, and facilitated cell migration and invasion. These results demonstrated the crucial role of MEC-17 in the modulation of intrinsic cell morphogenesis, migration, and invasive function through regulation of EMT and cell polarity.

Roles of partitioning-defective protein 6 (Par6) and its complexes in the proliferation, migration and invasion of cancer cells

A pivotal regulator of cell polarity and homeostasis, partitioning-defective protein 6 (Par6) forms multicomponent complexes that not only regulate cell polarity and stabilize cell morphology, but have also been demonstrated to participate in the proliferation, migration and invasion of cancer cells. The transforming growth factor (TGF)-β and extracellular signal-regulated kinase (Erk) 1/2 pathways are the most thoroughly studied pathways involving Par6 in many cancers. Aurothiomalate has been used to disrupt the interaction between Par6 and atypical protein kinase C within the multicomponent complexes, and has been shown to effectively block transformed growth and metastasis in vitro and/or in vivo in a variety of cancers, including pancreatic, prostate and lung cancers, as well as alveolar rhabdomyosarcoma. It is likely that with further revelations regarding the critical roles of Par6 in cancer initiation, progression and metastasis, targeted therapies against Par6 will be discovered and prove effective preclinically, and hopefully clinically, in cancer treatment.

MMP-14 and CD44 in Epithelial-to-Mesenchymal Transition (EMT) in ovarian cancer

Background

To investigate the expression of MMP-14 and CD44 as well as epithelial-to-mesenchymal transition(EMT)-like changes in ovarian cancer and to determine correlations with clinical outcome.

Methods

In 97 patients with ovarian cancer, MMP-14 and CD44 expression as determined by immunohistochemistry was investigated in relation to EMT-like changes. To determine this, immunohistochemical staining of E-cadherin and vimentin was performed.

Results

Patients with expression of both MMP-14 and CD44 in their tumors had a poor prognosis despite complete debulking. Serous histology in advanced-stage tumors (FIGO IIB-IV) correlated with CD44 (rho .286, p < 0.01). Also, CD44 correlated with percentage vimentin expression (rho .217, p < 0.05).

In logistic regression analysis with complete debulking as the outcome parameter, CD44 expression was found to be significant (OR 3,571 (95 % Confidence Interval 1,112–11,468) p = 0.032), though this was not the case for MMP-14 and EMT parameters.

Conclusion

The subgroup of patients with double expression of MMP-14 and CD44 had a poor prognosis despite complete debulking. Serous subtype in advanced-stage patients and CD44 expression were found to be correlated with vimentin expression, and CD44 expression was found to be significantly correlated with complete debulking. However, a significant correlation between EMT and clinical parameters was not found.

Clinical significance of epithelial-mesenchymal transition

The concept of epithelial-mesenchymal transition (EMT), a process where cells change their epithelial towards a mesenchymal phenotype, has gained overwhelming attention especially in the cancer research community. Thousands of scientific reports investigated changes in gene, mRNA and protein expression compatible with EMT and their possible correlation with tumor invasion, metastatic spread or patient prognosis; however, up to now, a proof of clinical significance of the concept is still missing. This review, with a main focus on the role of EMT in tumors, will summarize the basic molecular events underlying EMT including the signaling pathways capable of its induction as well as changes in EMT-associated protein expression and will very briefly touch the role of microRNAs in EMT. We then outline protein markers that are used most frequently for the assessment of EMT in research and diagnostic evaluation of tumor specimens and depict the link between EMT, a cancer stem cell (CSC) phenotype and resistance to conventional antineoplastic therapies. Furthermore, we evaluate a possible correlation between EMT marker expression and patient prognosis as well as current therapeutic concepts targeting the EMT process to slow down or prevent metastatic spread of malignant tumors.

Atypical protein kinase C phosphorylates Par6 and facilitates transforming growth factor β-induced epithelial-to-mesenchymal transition

Epithelial-to-mesenchymal transition (EMT) is controlled by cellular signaling pathways that trigger the loss of cell-cell adhesion and lead to the restructuring of the cell cytoskeleton. Transforming growth factor β (TGF-β) has been shown to regulate cell plasticity through the phosphorylation of Par6 on a conserved serine residue (S345) by the type II TGF-β receptor. We show here that atypical protein kinase C (aPKC) is an essential component to this signaling pathway in non-small-cell lung cancer (NSCLC) cells. We show that the aPKC, PKCι, interacts with TGF-β receptors through Par6 and that these proteins localize to the leading edge of migrating cells. Furthermore, Par6 phosphorylation on serine 345 by TGF-β receptors is enhanced in the presence of aPKC. aPKC kinase activity, as well as an association with Par6, were found to be important for Par6 phosphorylation. In effect, small interfering RNA-targeting aPKC reduces TGF-β-induced RhoA and E-cadherin loss, cell morphology changes, stress fiber production, and the migration of NSCLC cells. Interestingly, reintroduction of a phosphomimetic Par6 (Par6-S345E) into aPKC-silenced cells rescues both RhoA and E-cadherin loss with TGF-β stimulation. In conclusion, our results suggest that aPKCs cooperate with TGF-β receptors to regulate phospho-Par6-dependent EMT and cell migration.

Derlin-dependent retrograde transport from endosomes to the Golgi apparatus

Cells have to maintain stable plasma membrane protein and lipid compositions under normal conditions and to remodel their plasma membranes in response to stimuli. This maintenance and remodeling require that integral membrane proteins at the plasma membrane that become misfolded, because of the relatively harsher extracellular milieu or carbohydrate and amino acid sequence changes, are degraded. We had previously shown that Derlin proteins, required for quality control mechanisms in the endoplasmic reticulum, also localize to endosomes and function in the degradation of misfolded integral membrane proteins at the plasma membrane. In this study, we show that Derlin proteins physically associate with sorting nexins that function in retrograde membrane transport from endosomes to the Golgi apparatus. Using genetic studies in Caenorhabditis elegans and ricin pulse-chase analyses in murine RAW264.7 macrophages, we show that the Derlin-sorting nexin interaction is physiologically relevant. Our studies suggest that at least some integral membrane proteins that are misfolded at the plasma membrane are retrogradely transported to the Golgi apparatus and ultimately to the endoplasmic reticulum for degradation via resident quality control mechanisms.

The prognostic impact of TGF-β1, fascin, NF-κB and PKC-ζ expression in soft tissue sarcomas

Aims

Transforming growth factor-β (TGF-β), fascin, nuclear factor-kappa B (NF-κB) p105, protein-kinase C-zeta (PKC-ζ), partioning-defective protein-6 (Par-6), E-cadherin and vimentin are tumor promoting molecules through mechanisms involved in cell dedifferentiation. In soft tissue sarcomas, their expression profile is poorly defined and their significance is uncertain. We aimed to investigate the prognostic impact of TGF-β1, NF-κB p105, PKC-ζ, Par-6α, E-cadherin and vimentin in non-gastrointestinal stromal tumor soft tissue sarcomas (non-GIST STSs).

Patients and Methods

Tumor samples and clinical data from 249 patients with non-GIST STS were obtained, and tissue microarrays (TMAs) were constructed for each specimen. Immunohistochemistry (IHC) was used to evaluate marker expression in tumor cells.

Results

In univariate analysis, the expression levels of TGF-β1 (P = 0.016), fascin (P = 0.006), NF-κB p105 (P = 0.022) and PKC-ζ, (P = 0.042) were significant indicators for disease specific survival (DSS). In the multivariate analysis, high TGF-β1 expression was an independent negative prognostic factor for DSS (HR = 1.6, 95% CI = 1.1–2.4, P = 0.019) in addition to tumor depth, malignancy grade, metastasis at diagnosis, surgery and positive resection margins.

Conclusion

Expression of TGF-β1 was significantly associated with aggressive behavior and shorter DSS in non-GIST STSs.

Analysis of epithelial and mesenchymal markers in ovarian cancer reveals phenotypic heterogeneity and plasticity

In our studies of ovarian cancer cells we have identified subpopulations of cells that are in a transitory E/M hybrid stage, i.e. cells that simultaneously express epithelial and mesenchymal markers. E/M cells are not homogenous but, in vitro and in vivo, contain subsets that can be distinguished based on a number of phenotypic features, including the subcellular localization of E-cadherin, and the expression levels of Tie2, CD133, and CD44. A cellular subset (E/M-MP) (membrane E-cadherin(low)/cytoplasmic E-cadherin(high)/CD133(high), CD44(high), Tie2(low)) is highly enriched for tumor-forming cells and displays features which are generally associated with cancer stem cells. Our data suggest that E/M-MP cells are able to differentiate into different lineages under certain conditions, and have the capacity for self-renewal, i.e. to maintain a subset of undifferentiated E/M-MP cells during differentiation. Trans-differentiation of E/M-MP cells into mesenchymal or epithelial cells is associated with a loss of stem cell markers and tumorigenicity. In vivo xenograft tumor growth is driven by E/M-MP cells, which give rise to epithelial ovarian cancer cells. In contrast, in vitro, we found that E/M-MP cells differentiate into mesenchymal cells, in a process that involves pathways associated with an epithelial-to-mesenchymal transition. We also detected phenotypic plasticity that was dependent on external factors such as stress created by starvation or contact with either epithelial or mesenchymal cells in co-cultures. Our study provides a better understanding of the phenotypic complexity of ovarian cancer and has implications for ovarian cancer therapy.

Cell polarity in motion: redefining mammary tissue organization through EMT and cell polarity transitions

Epithelial to mesenchymal transition (EMT) and its reversion via mesenchymal to epithelial transition (MET), represent a stepwise cycle of epithelial plasticity that allows for normal tissue remodelling and diversification during development. In particular, epithelial-mesenchymal plasticity is central to many aspects of mammary development and has been proposed to be a key process in breast cancer progression. Such epithelial-mesenchymal plasticity requires complex cellular reprogramming to orchestrate a change in cell shape to an alternate morphology more conducive to migration. During this process, epithelial characteristics, including apical-basal polarity and specialised cell-cell junctions are lost and mesenchymal properties, such as a front-rear polarity associated with weak cell-cell contacts, increased motility, resistance to apoptosis and invasiveness are gained. The ability of epithelial cells to undergo transitions through cell polarity states is a central feature of epithelial-mesenchymal plasticity. These cell polarity states comprise a set of distinct asymmetric distributions of cellular constituents that are fashioned to allow specialized cellular functions, such as the regulated homeostasis of molecules across epithelial barriers, cell migration or cell diversification via asymmetric cell divisions. Each polarity state is engineered using a molecular toolbox that is highly conserved between organisms and cell types which can direct the initiation, establishment and continued maintenance of each asymmetry. Here we discuss how EMT pathways target cell polarity mediators, and how this EMT-dependent change in polarity states impact on the various stages of breast cancer. Emerging evidence places cell polarity at the interface of proliferation and morphology control and as such the changing dynamics within polarity networks play a critical role in normal mammary gland development and breast cancer progression.

Numb: A new player in EMT

Epithelial to mesenchymal transition (EMT) is a critical event in embryogenesis and plays a fundamental role in cancer progression and metastasis. Numb has been shown to play an important role in the proper functions of Par protein complex and in cell-cell junctions, both of which are associated with EMT. However, the role of Numb in EMT has not been fully elucidated. Recently, we showed that Numb is capable of binding to both Par3 and E-cadherin. Intriguingly, the interaction of Numb with E-cadherin or the Par protein complex is dynamically regulated by tyrosine phosphorylation induced by HGF or Src. Knockdown of Numb by shRNA in MDCK cells led to a lateral to apical translocation of E-cadherin and beta-catenin, active F-actin polymerization, mis-localization of Par3 and aPKC, a decrease in cell-cell adhesion and an increase in cell migration and proliferation. These data suggest a diverse role for Numb in regulating cell-cell adhesion, polarity and migration during EMT.

The basics of epithelial-mesenchymal transition

The origins of the mesenchymal cells participating in tissue repair and pathological processes, notably tissue fibrosis, tumor invasiveness, and metastasis, are poorly understood. However, emerging evidence suggests that epithelial-mesenchymal transitions (EMTs) represent one important source of these cells. As we discuss here, processes similar to the EMTs associated with embryo implantation, embryogenesis, and organ development are appropriated and subverted by chronically inflamed tissues and neoplasias. The identification of the signaling pathways that lead to activation of EMT programs during these disease processes is providing new insights into the plasticity of cellular phenotypes and possible therapeutic interventions.

The basics of epithelial-mesenchymal transition

The origins of the mesenchymal cells participating in tissue repair and pathological processes, notably tissue fibrosis, tumor invasiveness, and metastasis, are poorly understood. However, emerging evidence suggests that epithelial-mesenchymal transitions (EMTs) represent one important source of these cells. As we discuss here, processes similar to the EMTs associated with embryo implantation, embryogenesis, and organ development are appropriated and subverted by chronically inflamed tissues and neoplasias. The identification of the signaling pathways that lead to activation of EMT programs during these disease processes is providing new insights into the plasticity of cellular phenotypes and possible therapeutic interventions.

The basics of epithelial-mesenchymal transition

The origins of the mesenchymal cells participating in tissue repair and pathological processes, notably tissue fibrosis, tumor invasiveness, and metastasis, are poorly understood. However, emerging evidence suggests that epithelial-mesenchymal transitions (EMTs) represent one important source of these cells. As we discuss here, processes similar to the EMTs associated with embryo implantation, embryogenesis, and organ development are appropriated and subverted by chronically inflamed tissues and neoplasias. The identification of the signaling pathways that lead to activation of EMT programs during these disease processes is providing new insights into the plasticity of cellular phenotypes and possible therapeutic interventions.

Polarity proteins in migration and invasion

Cancer is the result of the deregulation of cell proliferation and cell migration. In advanced tumors, cells invade the surrounding tissue and eventually form metastases. This is particularly evident in carcinomas in which epithelial cells have undergone epithelial-mesenchymal transition. Increased cell migration often correlates with a weakening of intercellular interactions. Junctions between neighboring epithelial cells are required to establish and maintain baso-apical polarity, suggesting that not only loss of cell-cell adhesion but also alteration of cell polarity is involved during invasion. Accordingly, perturbation of cell polarity is an important hallmark of advanced invasive tumors. Cell polarity is also essential for cell migration. Indeed, a front-rear polarity axis has first to be generated to allow a cell to migrate. Because cells migrate during invasion, cell polarity is not completely lost. Instead, polarity is modified. From a nonmigrating baso-apically polarized epithelial phenotype, cells acquire a polarized migrating mesenchymal phenotype. The aim of this review is to highlight the molecular relationship between the control of cell polarity and the regulation of cell motility during oncogenesis.

The prognostic impact of NF-kappaB p105, vimentin, E-cadherin and Par6 expression in epithelial and stromal compartment in non-small-cell lung cancer

Vimentin, nuclear factor-κB (NF-κB) p105, fascin, E-cadherin, TGF-β, Par6 and atypical PKC are molecular markers that play an important role in cell differentiation. Herein, we investigate their prognostic impact in primary non-small-cell carcinoma (NSCLC). Tumour tissue samples from 335 resected patients with stage I–IIIA were used. Tissue microarrays were constructed from duplicate cores of both neoplastic cells and stromal cells and were immunohistochemically evaluated. In univariate analyses, high tumour epithelial cell expressions of NF-κB p105 (P=0.02) and E-cadherin (P=0.03) were positive prognostic indicators for disease-specific survival (DSS), whereas high tumour epithelial cell expression of vimentin (P=0.001) was a negative prognostic indicator. High expression of NF-κB p105 (P=0.001) and Par6 (P=0.0001) in the stromal compartment correlated with a good prognosis. In multivariate analyses, the tumour epithelial cell expression of NF-κB p105 (P=0.0001) and vimentin (P=0.005) and the stromal cell expression of NF-κB p105 (P=0.007) and Par6 (P=0.0001) were independent prognostic factors for DSS. High expression of NF-κB p105 and low expression of vimentin in tumour epithelial cells are independent predictors of better survival in primary NSCLC. In stromal cells, high expressions of NF-κB p105 and Par6 are both favourable independent prognostic indicators.

The epithelial-mesenchymal transition: new insights in signaling, development, and disease

The conversion of an epithelial cell to a mesenchymal cell is critical to metazoan embryogenesis and a defining structural feature of organ development. Current interest in this process, which is described as an epithelial–mesenchymal transition (EMT), stems from its developmental importance and its involvement in several adult pathologies. Interest and research in EMT are currently at a high level, as seen by the attendance at the recent EMT meeting in Vancouver, Canada (October 1–3, 2005). The meeting, which was hosted by The EMT International Association, was the second international EMT meeting, the first being held in Port Douglas, Queensland, Australia in October 2003. The EMT International Association was formed in 2002 to provide an international body for those interested in EMT and the reverse process, mesenchymal–epithelial transition, and, most importantly, to bring together those working on EMT in development, cancer, fibrosis, and pathology. These themes continued during the recent meeting in Vancouver.

Discussion at the Vancouver meeting spanned several areas of research, including signaling pathway activation of EMT and the transcription factors and gene targets involved. Also covered in detail was the basic cell biology of EMT and its role in cancer and fibrosis, as well as the identification of new markers to facilitate the observation of EMT in vivo. This is particularly important because the potential contribution of EMT during neoplasia is the subject of vigorous scientific debate (Tarin, D., E.W. Thompson, and D.F. Newgreen. 2005. Cancer Res. 65:5996–6000; Thompson, E.W., D.F. Newgreen, and D. Tarin. 2005. Cancer Res. 65:5991–5995).

TGF-beta receptor-binding proteins: complex interactions

Members of the Smad protein family are fundamental downstream mediators of TGF-beta signals. However, the basic, linear Smad signaling pathway is unlikely to be the sole contributor to the plethora of cell type-specific TGF-beta responses. Investigators have identified a number of molecules that interact with the TGF-beta receptors (TbetaRs) and may explain, at least in part, the tight regulation of TGF-beta effects. Understanding these TbetaR-interacting molecules is thus a matter of great potential significance for elucidating TGF-beta-family signal transduction. The present article reviews our current understanding of the roles and mechanisms of action of this relatively understudied group of molecules.

Tiam1 takes PARt in cell polarity

Cell polarity is an essential requirement for the proper tissue development of complex organisms. This is underscored by in vivo studies showing that loss of cell polarity contributes to the formation and progression of tumours. Evolutionary conserved multiprotein complexes, such as the Par3-Par6-aPKC or, in short, the Par polarity complex, regulate the establishment of cell polarity. The small Rho GTPases CDC42 and Rac control the activation of the Par polarity complex. Evidence now implicates the Rac activator Tiam1 as a crucial component of the Par complex in regulating neuronal (axonal) and epithelial (apical-basal) polarity. Our current knowledge places Tiam1 at the centre of a pivotal biological process, the establishment and maintenance of cell polarity, and suggests that deregulation of the Tiam1-Par complex contributes to tumourigenicity.

TGF-beta 1-induced PAI-1 expression is E box/USF-dependent and requires EGFR signaling

Transforming growth factor-beta1 (TGF-beta1) transcriptionally regulates the expression of genes that encode specific proteins (e.g., plasminogen activator inhibitor-1; PAI-1) important in stromal remodeling and cellular invasion. Definition of molecular events underlying TGF-beta1-initiated PAI-1 transcription, therefore, may lead to the identification of new therapeutic targets for diseases associated with elevated PAI-1 synthesis (e.g., tissue fibrosis, vascular disorders, tumor progression). An intact upstream stimulatory factor (USF)-binding E box motif (5'-(-165)CACGTG(-160)-3') at the HRE-2 site in the rat PAI-1 gene was required for PAI-1 transcription in TGF-beta1-treated cells. Mutation of the CA dinucleotide to TC at position -165/-164 in a reporter construct driven by 764 bp of PAI-1 promoter sequence decreased TGF-beta1-dependent CAT activity by >80% indicating the necessity for a consensus hexanucleotide E box motif in induced expression. The same CA --> TC substitution eliminated USF binding to an 18-bp HRE-2 DNA target highlighting the importance of site occupancy to transcriptional activation. Transfection of a dominant-negative USF construct, moreover, completely inhibited formation of USF/HRE-2 probe complexes, attenuated PAI-1 promoter-driven luciferase activity and reduced the response of the endogenous PAI-1 gene to TGF-beta1 (to that approximating quiescent controls). Maximal immediate-early PAI-1 induction upon exposure to TGF-beta1 required EGFR, p21ras, MEK and pp60(c-src) signaling as pharmacologic or dominant-negative inhibition of any of the four intermediates (EGFR, p21ras, MEK, pp60(c-src)) virtually eliminated TGF-beta1-augmented PAI-1 levels. U0126 titering experiments, furthermore, revealed that the same MEK inhibitor concentration that blocked the TGF-beta1 increase in ERK1/2 phosphorylation (20 microM) also effectively attenuated the PAI-1 inductive response suggesting a requirement for stimulated ERK signaling in TGF-beta1-mediated PAI-1 expression. These data suggest a model whereby TGF-beta1 activates a complex signaling cascade to affect PAI-1 gene control and involves USF occupancy of a critical E box motif at the HRE-2 site in the PAI-1 gene.

Tight junction biology and kidney dysfunction

The epithelial tight junction (TJ) has three major functions. As a "gate," it serves as a regulatory barrier separating and maintaining biological fluid compartments of different composition. As a "fence," it generates and maintains the apicobasal polarity of cells that form the confluent epithelium. Finally, the TJ proteins form a trafficking and signaling platform that regulates cell growth, proliferation, differentiation, and dedifferentiation. Six examples are selected that illustrate the emerging link between TJ dysfunction and kidney disease. First, the glomerular slit diaphragm (GSD) is evolved, in part, from the TJ and, on maturation, exhibits all three functions of the TJ. GSD dysfunction leads to proteinuria and, in some instances, podocyte dedifferentiation and proliferation. Second, accumulating evidence supports epithelial-mesenchymal transformation (EMT) as a major player in renal fibrosis, the final common pathway that leads to end-stage renal failure. EMT is characterized by a loss of cell-cell contact and apicobasal polarity, which are hallmarks of TJ dysfunction. Third, in autosomal dominant polycystic kidney disease, mutations of the polycystins may disrupt their known interactions with the apical junction complex, of which the TJ is a major component. This can lead to disturbances in epithelial polarity regulation with consequent abnormal tubulogenesis and cyst formation. Fourth, evidence for epithelial barrier and polarity dysregulation in the pathogenesis of ischemic acute renal failure will be summarized. Fifth, the association between mutations of paracellin-1, the first TJ channel identified, and clinical disorders of magnesium and calcium wasting and bovine renal fibrosis will be used to highlight an integral TJ protein that can serve multiple TJ functions. Finally, the role of WNK4 protein kinase in shunting chloride across the TJ of the distal nephron will be addressed.