De novo expression of pp125FAK in human macrophages regulates CSK distribution and MAP kinase activation but does not affect focal contact structure

Differential expression of FAK and Pyk2 in metastatic and non-metastatic EL4 lymphoma cell lines

The murine EL4 lymphoma cell line exists in variants that are either sensitive or resistant to phorbol 12-myristate 13-acetate (PMA). In sensitive cells, PMA causes Erk MAPK activation and Erk-mediated growth arrest. In resistant cells, PMA induces a low level of Erk activation, without growth arrest. A relatively unexplored aspect of the phenotypes is that resistant cells are more adherent to culture substrate than are sensitive cells. In this study, the roles of the protein tyrosine kinases FAK and Pyk2 in EL4 phenotype were examined, with a particular emphasis on the role of these proteins in metastasis. FAK is expressed only in PMA-resistant (or intermediate phenotype) EL4 cells, correlating with enhanced cell-substrate adherence, while Pyk2 is more highly expressed in non-adherent PMA-sensitive cells. PMA treatment causes modulation of mRNA for FAK (up-regulation) and Pyk2 (down-regulation) in PMA-sensitive but not PMA-resistant EL4 cells. The increase in Pyk2 mRNA is correlated with an increase in Pyk2 protein expression. The roles of FAK in cell phenotype were further explored using transfection and knockdown experiments. The results showed that FAK does not play a major role in modulating PMA-induced Erk activation in EL4 cells. However, the knockdown studies demonstrated that FAK expression is required for proliferation and migration of PMA-resistant cells. In an experimental metastasis model using syngeneic mice, only FAK-expressing (PMA-resistant) EL4 cells form liver tumors. Taken together, these studies suggest that FAK expression promotes metastasis of EL4 lymphoma cells.

PKCdelta regulates endothelial basal barrier function through modulation of RhoA GTPase activity

We have shown that PKCdelta enhanced microvascular endothelial basal barrier function, correlating with elevated RhoA GTPase activity and increased focal contact formation. In the current study, we investigated signaling pathways important in PKCdelta modulation of barrier function in unstimulated endothelial cell monolayers by assessing the effects of PKCdelta inhibition in endothelial cells (EC) derived from rat pulmonary artery (PAEC) and epididymus (FPEC). Rottlerin exposure or Ad PKCdeltadn infection significantly enhanced monolayer permeability in both EC. Immunofluorescence analyses demonstrated fewer stress fibers and focal contacts in rottlerin-treated or Ad PKCdeltadn-infected EC; yet, PKCdelta inhibition caused no significant changes in microtubule structures. These changes correlated with a reduction in both focal adhesion kinase (FAK) and RhoA GTPase activities. Microfilament stabilization significantly attenuated the focal contact and barrier disruptive effects of rottlerin. FAK overexpression did not blunt the effects of rottlerin-induced barrier dysfunction or stress fiber and focal contact disruption. Conversely, GFP-linked dominant active RhoA overexpression protected EC from stress fiber and focal contact disruption induced by both rottlerin exposure and overexpression of PKCdelta dominant negative protein. Additionally, PKCdelta immunoprecipitated with p190RhoGAP and p120RasGAP, modulators of RhoA activity. Thus, PKCdelta may regulate basal endothelial barrier function by stabilizing microfilaments and focal contacts by regulating RhoA GTPase activity through upstream modulators, p190RhoGAP and p120RasGAP.

Direct transmembrane clustering and cytoplasmic dimerization of focal adhesion kinase initiates its tyrosine phosphorylation

We investigated mechanisms for inducing focal adhesion kinase (FAK) tyrosine phosphorylation and their ability to trigger MAP kinase signaling using transmembrane chimeras that localize FAK and its mutants to the plasma membrane. We tested whether tyrosine phosphorylation was triggered by FAK transmembrane aggregation using antibodies against the chimeric extracellular domain. Experimental clustering of chimeras containing integrin beta cytoplasmic domains or FAK induced FAK tyrosine phosphorylation and trans-phosphorylation of endogenous FAK, as well as strong ERK activation. Next, we examined whether lower-order molecular proximity, namely dimerization, could regulate FAK tyrosine phosphorylation. We found that even relatively low-affinity FAK dimerization (K(d)=3.9 x 10(-5) M), in either of two different orientations, could induce FAK tyrosine phosphorylation. However, this cytoplasmic FAK dimerization could not induce MAP kinase activation or trans-phosphorylation of endogenous FAK. We conclude that dimerization of FAK is sufficient to induce its tyrosine phosphorylation, but that higher-order molecular proximity (clustering) at the cell membrane is apparently needed for additional biochemical events. This study identifies a proximity mechanism for regulating the initiation of FAK-mediated biochemical signaling.

Regulation of c-SRC activity and function by the adapter protein CAS

SRC family kinases play essential roles in a variety of cellular functions, including proliferation, survival, differentiation, and apoptosis. The activities of these kinases are regulated by intramolecular interactions and by heterologous binding partners that modulate the transition between active and inactive structural conformations. p130(CAS) (CAS) binds directly to both the SH2 and SH3 domains of c-SRC and therefore has the potential to structurally alter and activate this kinase. In this report, we demonstrate that overexpression of full-length CAS in COS-1 cells induces c-SRC-dependent tyrosine phosphorylation of multiple endogenous cellular proteins. A carboxy-terminal fragment of CAS (CAS-CT), which contains the c-SRC binding site, was sufficient to induce c-SRC-dependent protein tyrosine kinase activity, as measured by tyrosine phosphorylation of cortactin, paxillin, and, to a lesser extent, focal adhesion kinase. A single amino acid substitution located in the binding site for the SRC SH3 domain of CAS-CT disrupted CAS-CT's interaction with c-SRC and inhibited its ability to induce tyrosine phosphorylation of cortactin and paxillin. Murine C3H10T1/2 fibroblasts that expressed elevated levels of tyrosine phosphorylated CAS and c-SRC-CAS complexes exhibited an enhanced ability to form colonies in soft agar and to proliferate in the absence of serum or growth factors. CAS-CT fully substituted for CAS in mediating growth in soft agar but was less effective in promoting serum-independent growth. These data suggest that CAS plays an important role in regulating specific signaling pathways governing cell growth and/or survival, in part through its ability to interact with and modulate the activity of c-SRC.

Tensin can induce JNK and p38 activation

Cells organize diverse types of specialized adhesion sites upon attachment to extracellular matrix (ECM) components. One of the physiological roles of such cell-ECM interactions is to initiate and regulate adhesion-mediated signal transduction responses. The association of cells with fibronectin fibrils has been shown to regulate the JNK and p38 signaling pathways. We tested whether tensin, a cytoskeletal component localized to both focal contacts and fibronectin-associated fibrillar adhesions, can induce these signaling pathways. We found that tensin overexpression resulted in activation of both the c-Jun amino-terminal kinase (JNK) and p38 pathways. Tensin-mediated JNK activation was independent of the activities of the small GTP binding proteins Rac and Cdc42, but did depend on SEK, a kinase involved in the JNK pathway. We suggest that tensin may directly activate the JNK and p38 pathways, acting downstream or independent of the activities of the small GTP binding proteins Rac and Cdc42.