The transforming growth factor pathway plays a central role in cellular proliferation, recognition, differentiation, apoptosis and specification of developmental fate, during embryogenesis as well as in mature tissues 12. Nearly thirty members have been described in human, many orthologs are known in other vertebrates. Four are known in Caenorhabditis elegans and seven in Drosophila melanogaster. The family is divided into three subfamilies. One subfamily contains of the bone morphogenetic proteins (BMP) plus the growth and differentiaion factors (GDF). The second family consists of the transforming growth factor β and activin and nodal proteins. The last family comprises of ligand antagonists such as inhibin-α 3.

TGFβ signaling generally has a negative effect on cell growth. Inactivation of this pathway contributes to tumorigenesis. The TGFβ protein is released as an inactive 'latent' complex, comprising a TGFβ dimer in a non-covalent complex with two prosegments, to which one of several 'latent TGFβ-binding proteins' is often linked. This latent complex represents an important safeguard against 'inadvertent' activation, and may stabilize and target latent TGFβ to the extracellular matrix, where it is sequestered. The matrix thus acts as a reservoir from which TGFβ can readily be recruited without the need for new synthesis.

The secretion of TGFβ as a latent complex necessitates the existence of a regulated activation process, which is most probably mediated through the activities of proteases that preferentially degrade the TGFβ prosegments and thereby release the highly stable, active TGFβ dimer. Because plasmin activates latent TGFβ and plasminogen is converted into plasmin at sites of cell migration and invasion; it is assumed that endothelial and tumor cells are exposed to active TGFβ.

TGFβ signaling is mediated by a heterotetrameric complex of two trans-membrane receptor serine/threonine kinases, containing a type II ligand binding receptor (TGFβ-RII) and a type I signaling receptor (TGFβ-RI). Smads 2 and 3 are direct substrates of TGFβ-RI and, together with the common mediator, Smad4, play key roles as cytoplasmic signaling mediators. Although the Smad pathway has received much attention in the past years, it is now appreciated that the activated receptor complex can also signal through other pathways, such as those involving the mitogen-activated protein kinases (MAPKs), phosphoinositol-3 kinase (PI3K), and PP2A/p70s6K, though the molecular details of this coupling are still obscure. The relative importance and interplay of these various pathways in the changing responses of cells to TGFβ are just beginning to be probed 4.

In mammal cells, there are three TGFβs, TGFβ₁, TGFβ₂ and TGFβ₃, which are encoded by different genes and which all function through the same receptor system. Of these, TGFβ₁ is most frequently upregulated in tumor cells and is the focus of most studies on the role of TGFβ in tumorigenesis.

The integrin family of cell adhesion molecules mediates cellular contacts to the extracellular matrix (ECM) or cell counter receptors, thereby regulating development, cell motility, cell polarity, cell growth and survival 5. Ligand binding to integrins leads to integrin clustering and recruitment of actin filaments and signaling proteins to the cytoplasmic domain of integrins, referred to as focal complexes when they are still nascent and in the process of forming, or focal adhesions (FAs) when they have matured into larger complexes. The formation of cell adhesion complexes assures substrate adhesion as well as targeted location of Actin filaments and signaling components. Cell adhesion complexes are also essential for establishing cell polarity, directed cell migration, and maintaining cell growth and survival 67.

The integrin-actin cytoskeleton connection is highly dynamic and is differentially regulated in different locations of the cell. At the leading edge of migrating cells, integrins bind the ECM, recruit the actin cytoskeleton and initiate local reorganization of the actin network, promoting different types of membrane protrusion. At the rear of the cell, integrins detach from the ECM, dissolve the link to the cytoskeleton and are, at least partially, recycled to the front of the cell.

Signaling pathways, which depend on localized integrin activation have also been reported; this is essential for the reorientation of the microtubular network and the directed movement of cells. Complexity is added by the fact that integrin-associated molecules are multifunctional. Integrin-linked actin binding proteins attach to signaling molecules and function as platforms, bringing kinases and substrates together. Integrin-bound signaling molecules, on the other hand, bind to actin binding proteins, enforcing the integrin-cytoskeleton connection. As it turns out for integrin-linked kinase (ILK), such adapter function might be even more important in vivo than the kinase function demonstrated in vitro.

The integrin α_Vβ₆ is expressed principally on epithelial cells 8 where it has been shown to be a receptor for RGD and non-RGD sites in ligands 9. It is demonstrated that the mature form of TGFβ₁ binds to and activates of α_Vβ₆ integrin 10. Among others, binding of mature TGFβ₁ to α_Vβ₆ integrin resulted in an enhanced immobilization and phosphorylation of proteins, which are associated with focal adhesions 1011. Surprisingly, stimulation with mature TGFβ₁ leads to upregulation of c-Jun in TGFβ₁ sensitive cells 10. We therefore sought to determine if mature TGFβ₁ activates c-Jun via MEKK1/p38 and if this activation may influence cytoskeletal reorganization.

Here, we show that binding of mature TGFβ₁ recruits the mitogen-activated protein kinase kinase kinase 1 (MEKK1), a mediator of c-Jun activation, and the extracellular signaling-regulated kinase-1 (Erk1) to focal adhesions. In addition, the p21-activated kinase 1 (PAK1) is associated with focal adhesions and differentially phosphorylated upon stimulation with mature TGFβ₁. We conclude that TGFβ₁ activates c-Jun via the MEKK1/p38 MAP kinase pathway and influences cytoskeletal organization. These finding may provide a link between TGFβ₁ and metastatic behavior of cancers.

Crosstalk between integrins and growth factor receptors are an important signaling mechanism to provide specificity during normal development and pathological processes in vascular biology. Evidence from several model systems demonstrates the physiological importance of the coordination of signals from growth factors and the extracellular matrix to support cell proliferation, migration, and invasion in vivo 121314. Several examples of crosstalk between these two important classes of receptors indicate that integrin ligation is required for growth factor-induced biological processes 15. Furthermore, integrins can directly associate with growth factor receptors, thereby regulating the capacity of integrin/growth factor receptor complexes to propagate downstream signaling 101116.

We have demonstrated that mature TGFβ₁ colocalizes with α_Vβ₆ integrin in Panc-1 cells 10. To further support our initial findings, we assayed for colocalization between TGFβ₁, α_Vβ₆ integrin and the F-actin filaments of the cytoskeleton in SW48, DLD1, HeLa cells as well as keratinocytes. Cells were stimulated with 10 nM of mature TGFβ₁. After preparation of the cytoskeletal fraction by Triton-X100 extraction 101718, slides were stained using Alexa Fluor 680 goat anti-mouse IgG (Molecular Probes, Eugene, OR) for Actin, Alexa Fluor 488 donkey anti-rabbit IgG (Molecular Probes) for α_Vβ₆ integrin, and Alexa Fluor 350 donkey anti-rabbit IgG (Molecular Probes) for TGFβ₁ labeling 10. As shown in Figure 1, in TGFβ₁, α_Vβ₆ integrin and the F-Actin filaments of the cytoskeleton colocalize after stimulation with mature TGFβ₁ in all cell lines used. These results further support our initial findings that mature TGFβ₁ is a ligand for α_Vβ₆ integrin.

Adhesion to the extracellular matrix is an important process that controls cell shape change, migration, proliferation, survival, and differentiation. Upon adhesion, integrins and a selective group of cytoskeletal and signaling proteins are recruited to cell matrix contact sites where they link the actin cytoskeleton to the ECM and mediate signal transduction between the intracellular and extracellular compartments 18.

We demonstrated that mature TGFβ₁ induces an enhanced immobilization and phosphorylation of proteins, which are associated with focal adhesions, in TGFβ₁ sensitive cells 10. In order to provide further evidence for the specificity of this interaction, we preincubated cells with antibodies against α_Vβ₈, α_Vβ₆, α_Vβ₅, α_Vβ₃, and α_Vβ₁ integrins prior to stimulation with mature TGFβ₁. Consistent with our initial report, preincubation with α_Vβ₆ antibodies abolished the TGFβ₁-induced tyrosine phosphorylation of proteins, which are associated with focal adhesions (Figure 2). In sum, we could confirm our initial findings that mature TGFβ₁ is a ligand for α_Vβ₆ integrin and that this association results in immobilization and tyrosine phosphorylation of proteins, which are associated with focal adhesions, in TGFβ₁ sensitive cells.

Paxillin is a multi-domain protein that localizes in cultured cells primarily to sites of cell adhesion to the extracellular matrix (ECM) called focal adhesions 6. Focal adhesion proteins including paxillin also serve as a point of convergence for signals resulting from stimulation of various classes of growth factor receptors 1920. It binds to many proteins that are involved in effecting changes in the organization of the actin cytoskeleton, which are necessary for cell motility events associated with embryonic development, wound repair and tumor metastasis. These range from structural proteins such as vinculin and actopaxin, that bind actin directly to regulators of actin cytoskeletal dynamics, such as the ARF GAP, PKL, the exchange factor PIX and the p21-activated kinase, PAK. These proteins serve as modulators/effectors of the ARF and RHO GTPase families 67. In our assays, Paxillin was found to be phosphorylated at Y¹¹⁸ after stimulation and associated with focal adhesions after stimulation of BxPC-3 cells with mature TGFβ₁ (Figure 3). This phosphorylation was dependent on an intact cytoskeleton as well as free intracellular calcium (Figure 3).

Both paxillin and focal adhesion kinase (FAK) undergo phosphorylation during integrin-mediated cell adhesion and during stimulation by a variety of mitogens and growth factors 67. There is growing evidence that integrins function as mechanotransducers and that the regulation of cellular responses to mechanical stimuli is coordinated by the complex of cytoskeletal proteins that associate with the cytoplasmic domains of integrin molecules 17. In cultured cells, FAK and paxillin undergo tyrosine phosphorylation in response to periods of repetitive mechanical strain 1718. In a number of cultured cell types, including endothelial cells and airway smooth muscle cells, cyclic mechanical strain has been shown to induce the alignment of actin filaments along the axis perpendicular to the force vector 1718. Paxillin and FAK were identified as integration points in signaling proximal to integrins and growth-factor receptors. We speculate that the conversion of growth-factor and adhesion signaling occurs on the actin filaments. The cytoplasmic domains of integrins and receptors for growth factors and cytokines are closely associated with the ends of the cytoskeleton; reorganization of microfilaments may influence the environment of these integrins and receptors, thereby facilitating the triggering of a signaling pathway. The mechanisms are obscure.

Activation of paxillin/FAK by integrins and growth factors is important for efficient signal propagation by pathways including paxillin that lead to the regulation growth and differentiation. A similar role for FAK was described in the control of growth factor- and integrin-mediated cell migration in fibroblasts 21. This suggests that paxillin/FAK have a general role in linking integrins/growth-factor receptors with the regulation of cytoskeletal changes that controls various biological processes in different cells. Paxillin and FAK have been proposed to play an integral role in these strain-induced morphological changes.

Reorganization of the cytoarchitecture regulates signaling pathways including the mobilization of intracellular calcium, activation of tyrosine kinases, Ras, extracellular signal-regulated kinase (ERK), and c-Jun NH₂-terminal kinase (JNK). Consistent with the activation of signaling pathways, specific transcription factors are activated by cytoskeletal restructuring 101117. We stimulated BxPC-3 cells with 10 nM of mature TGFβ₁, prepared the Triton-X100 nonsuluble fraction, precipitated with α_Vβ₆ integrin antibodies followed by re-precipitation with FAK antibodies. Our assays revealed that MEKK1 and c-Jun are phosphorylated and associated with focal adhesions after stimulation of BxPC-3 cells with mature TGFβ₁ (Figures 3 and 4). Our findings are in part supported by Verrecchia et al showing that TGFβ induces c-Jun and JunB 22. In addition, our finding that MEKK1 associates with focal adhesions finds support in the report from Cuevas et al 23. Cuevas et al show evidence that MEKK1 colocalizes with focal adhesions in adhering MEKK1^-/- fibroblasts reconstituted with EGFP-MEKK1 23. However, this is the first report that c-Jun is associated with the cytoskeleton, whilst phosphorylated.

MEKK1 is absolutely required for cellular responses, which alter the integrity of the microtubule cytoskeleton and cell shape; MEKK1 is the MAPK kinase kinase regulating the JNK pathway 2425. The significance for a cytoskeletal immobilization of c-Jun is not known. We also detected Erk1 associated with focal adhesions in BxPC-3 cells, stimulated with 10 nM of mature TGFβ₁ (Figure 5). Most strikingly, we observed two bands for the phosphorylated form of Erk1. These bands are not visible after incubation with phosphatases (data not shown).

The mitogen-activated protein kinase, referred to as MAP kinase or extracellular signal-regulated kinase (ERK), is a serine/threonine protein kinase whose activity is rapidly stimulated by a number of external stimuli through mechanisms mediated by tyrosine kinase-encoded receptors, non-receptor type tyrosine kinases, and G protein-coupled receptors. MAP kinase is activated by phosphorylation on its threonine and tyrosine residues, a process, which is carried out by a dual-specificity protein kinase, MAP kinase kinase. MAP kinases have been shown to phosphorylate and thereby activate many well studied regulatory proteins located in diverse cellular compartments, including nuclear transcriptional factors 26. A function of MAP kinases may therefore be to provide a link between transmembrane signaling and the nucleus. There is much evidence that MAP kinases may be involved in cell growth and differentiation by phosphorylating and thereby activating nuclear transcriptional factors 27.

Meanwhile, there are many compelling examples of gene expression induced by adhesive interactions with ECM 282930. Thus, MAP kinase may play a pivotal role in adhesion-dependent gene activation through the integrin-mediated signaling pathways. The fact that MAP kinase is activated by both, growth factor- and integrin-mediated signals, suggests that these two signaling pathways converge at this or upstream points. However, resolving the nature and degree of interactions between integrin- and growth factor-mediated signaling pathways, and their relative contributions, must await more complete definition and understanding of integrin-mediated growth-factor-induced signal transduction pathways.

Hyperactive Erk may phosphorylate Tau in an abnormal fashion. Tau hyperphosphorylation by abnormaly active Erk is demonstrated in neuroblastoma cells, suggesting that abnormak Erk activity function as apoptosis inducing kinase 31. Additional factors may compensate this effect, leading to resistance to apoptosis, multidrug resistance, or metastatic behavior of cancers 3233. Support for this speculation comes from another report showing that the family of p21 activated kinases (PAK) is involved in actin cytoskeleton organization 3435.

The p21-activated kinases (PAKs) are serine/threonine protein kinases that bind to and, in some cases, are stimulated by activated forms of the small GTPases, Cdc42 and Rac 36. The PAK family of kinases has been implicated in control of actin filaments and in cell motility. Targets for PAK are likely to be involved in migration, including myosin light chain kinase (MLCK) and LIM kinase. MLCK phosphorylates MLC, and this phosphorylation has been shown to be important in regulating actin cytoskeletal dynamics. In addition, PAK phosphorylates and activates LIMK, which in turn phosphorylates and inhibits the actin severing protein cofilin, thus promoting filament assembly. The PAK interacting guanine nucleotide exchange factor PIX, the ARF GTPase activating protein PKL, and the adaptor protein Nck, are also mediators of Rho GTPase signaling; they form a complex with PAK to regulate the actin cytoskeleton and stimulate focal complex formation through paxillin interactions. G protein coupled receptor kinase interacting protein (GIT1) links PAK to FAK and results in focal contact turnover, while another protein, p95 APP 1, of the GIT family, has been implicated in membrane recycling during locomotion, suggesting a role of GIT family members in cell motility 37. Thus, PAK plays an important role in Rac-driven cell motility through several different signaling cascades.

The Erk-MAP kinase, including PAK, PI3K, and MEKK1, have been linked to motility 38. Rac can regulate several different pathways through different effectors. Rac activates PI3K and AKT independent of JNK, and activates the MEKK-JNK cascade independent of PAK. In addition to the Rac-MEKK-JNK and the ERK cascades, PI3K-AKT has also been shown to promote cell migration.

We found that Pak1 is associated with focal adhesions upon stimulation of BxPC-3 cells with 10 nM of mature TGFβ₁ (Figure 6). The Src family kinase inhibitor 4-amino-5-(4-chloro-phenyl)-7-(t-butyl)pyrazolo- [3,4-d]-pyrimidine (PP2) significantly reduced the phosphorylation level of Pak1 compared with the MEK1 inhibitor PD98059. The effect of the MEK1 inhibitor on the Pak1 phosphorylation is striking because Pak1 is considered upstream of MEK1/Erk 39. It is possible that other signaling molecules mediate a crosstalk between the two pathways. Notably, the src-inhibitor PP2 partially reduced the Pak1 phosphorylation level. Blocking of Pak activity is considered a so called "signal therapy for Ras-induced cancers" 4041.

As is clear from this study, there is a lot that remains to be understood about the integrin-mediated signal transduction elicited by mature TGFβ₁ 4243. Further studies will provide us with answers in the near future that will shed light on many of the remaining mysteries of the TGFβ puzzle.

CS performed all assays and drafted the manuscript. KL provided suggestions for its finalization. Both authors read and approved the final manuscript.