PKC-dependent activation of FAK and src induces tyrosine phosphorylation of Cas and formation of Cas-Crk complexes

PKCθ-mediated serine/threonine phosphorylations of FAK govern adhesion and protrusion dynamics within the lamellipodia of migrating breast cancer cells

The cytoskeleton and cell-matrix adhesions constitute a dynamic network that controls cellular behavior during development and cancer. The Focal Adhesion Kinase (FAK) is a central actor of these cell dynamics, promoting cell-matrix adhesion turnover and active membrane fluctuations. However, the initial steps leading to FAK activation and subsequent promotion of cell dynamics remain elusive. Here, we report that the serine/threonine kinase PKCθ participates in the initial steps of FAK activation. PKCθ, which is strongly expressed in aggressive human breast cancers, controls the dynamics of cell-matrix adhesions and active protrusions through direct FAK activation, thereby promoting cell invasion and lung metastases. Using various tools for in vitro and live cell studies, we precisely decipher the molecular mechanisms of FAK activation. PKCθ directly interacts with the FAK FERM domain to open FAK conformation through PKCθ's specific V3 domain, while phosphorylating FAK at newly identified serine/threonine residues within nascent adhesions, inducing cell dynamics and aggressive behavior. This study thus places PKCθ-directed FAK opening and phosphorylations as an original mechanism controlling dynamic, migratory, and invasive abilities of aggressive breast cancer cells, further strengthening the emerging oncogenic function of PKCθ.

Priming stem cells with protein kinase C activator enhances early stem cell-chondrocyte interaction by increasing adhesion molecules

BACKGROUND

Osteoarthritis (OA) can be defined as degradation of articular cartilage of the joint, and is the most common degenerative disease. To regenerate the damaged cartilage, different experimental approaches including stem cell therapy have been tried. One of the major limitations of stem cell therapy is the poor post-transplantation survival of the stem cells. Anoikis, where insufficient matrix support and adhesion to extracellular matrix causes apoptotic cell death, is one of the main causes of the low post-transplantation survival rate of stem cells. Therefore, enhancing the initial interaction of the transplanted stem cells with chondrocytes could improve the therapeutic efficacy of stem cell therapy for OA. Previously, protein kinase C activator phorbol 12-myristate 13-acetate (PMA)-induced increase of mesenchymal stem cell adhesion via activation of focal adhesion kinase (FAK) has been reported. In the present study, we examine the effect PMA on the adipose-derived stem cells (ADSCs) adhesion and spreading to culture substrates, and further on the initial interaction between ADSC and chondrocytes.

RESULTS

PMA treatment increased the initial adhesion of ADSC to culture substrate and cellular spreading with increased expression of adhesion molecules, such as FAK, vinculin, talin, and paxillin, at both RNA and protein level. Priming of ADSC with PMA increased the number of ADSCs attached to confluent layer of cultured chondrocytes compared to that of untreated ADSCs at early time point (4 h after seeding).

CONCLUSION

Taken together, the results of this study suggest that priming ADSCs with PMA can increase the initial interaction with chondrocytes, and this proof of concept can be used to develop a non-invasive therapeutic approach for treating OA. It may also accelerate the regeneration process so that it can relieve the accompanied pain faster in OA patients. Further in vivo studies examining the therapeutic effect of PMA pretreatment of ADSCs for articular cartilage damage are required.

CD99 inhibits CD98-mediated β1 integrin signaling through SHP2-mediated FAK dephosphorylation

The human CD99 protein is a 32-kDa type I transmembrane glycoprotein, while CD98 is a disulfide-linked 125-kDa heterodimeric type II transmembrane glycoprotein. It has been previously shown that CD99 and CD98 oppositely regulate β1 integrin signaling, though the mechanisms by which this regulation occurs are not known. Our results revealed that antibody-mediated crosslinking of CD98 induced FAK phosphorylation at Y397 and facilitated the formation of the protein kinase Cα (PKCα)-syntenin-focal adhesion kinase (FAK), focal adhesions (FAs), and IPP-Akt1-syntenin complex, which mediates β1 integrin signaling. In contrast, crosslinking of CD99 disrupted the formation of the PKCα-syntenin-FAK complex as well as FA via FAK dephosphorylation. The CD99-induced dephosphorylation of FAK was apparently mediated by the recruitment of Src homology region 2 domain-containing phosphatase-2 (SHP2) to the plasma membrane and subsequent activation of its phosphatase activity. Further consequences of the activation of SHP2 included the disruption of FAK-talin and talin-β1 integrin interactions and attenuation in the formation of the IPP-Akt1-syntenin complex at the plasma membrane, which resulted in reduced cell-ECM adhesion. This report uncovers the molecular mechanisms underlying the inverse regulation of β1 integrin signaling by CD99 and CD98 and may provide a novel therapeutic approach to treat inflammation and cancer.

Activation of protein kinase C-α and Src kinase increases urea transporter A1 α-2, 6 sialylation

The urea transporter A1 (UT-A1) is a glycosylated protein with two glycoforms: 117 and 97 kD. In diabetes, the increased abundance of the heavily glycosylated 117-kD UT-A1 corresponds to an increase of kidney tubule urea permeability. We previously reported that diabetes not only causes an increase of UT-A1 protein abundance but also, results in UT-A1 glycan changes, including an increase of sialic acid content. Because activation of the diacylglycerol (DAG)-protein kinase C (PKC) pathway is elevated in diabetes and PKC-α regulates UT-A1 urea transport activity, we explored the role of PKC in UT-A1 glycan sialylation. We found that activation of PKC specifically promotes UT-A1 glycan sialylation in both UT-A1-MDCK cells and rat kidney inner medullary collecting duct suspensions, and inhibition of PKC activity blocks high glucose-induced UT-A1 sialylation. Overexpression of PKC-α promoted UT-A1 sialylation and membrane surface expression. Conversely, PKC-α-deficient mice had significantly less sialylated UT-A1 compared with wild-type mice. Furthermore, the effect of PKC-α-induced UT-A1 sialylation was mainly mediated by Src kinase but not Raf-1 kinase. Functionally, increased UT-A1 sialylation corresponded with enhanced urea transport activity. Thus, our results reveal a novel mechanism by which PKC regulates UT-A1 function by increasing glycan sialylation through Src kinase pathways, which may have an important role in preventing the osmotic diuresis caused by glucosuria under diabetic conditions.

Melatonin enhances the human mesenchymal stem cells motility via melatonin receptor 2 coupling with Gαq in skin wound healing

Melatonin, a circadian rhythm-promoting molecule, has a variety of biological functions, but the functional role of melatonin in the motility of mesenchymal stem cells (MSCs) has yet to be studied. In a mouse skin excisional wound model, we found that transplantation of umbilical cord blood (UCB)-MSCs pretreated with melatonin enhanced wound closure, granulation, and re-epithelialization at mouse skin wound sites, where relatively more UCB-MSCs which were engrafted onto the wound site were detected. Thus, we identified the signaling pathway of melatonin, which affects the motility of UCB-MSCs. Melatonin (1 μm) significantly increased the motility of UCB-MSCs, which had been inhibited by the knockdown of melatonin receptor 2 (MT2). We found that Gαq coupled with MT2 and that the binding of Gαq to MT2 uniquely stimulated an atypical PKC isoform, PKCζ. Melatonin induced the phosphorylation of FAK and paxillin, which were concurrently downregulated by blocking of the PKC activity. Melatonin increased the levels of active Cdc42 and Arp2/3, and it has the ability to stimulate cytoskeletal reorganization-related proteins such as profilin-1, cofilin-1, and F-actin in UCB-MSCs. Finally, a lack of MT2 expression in UCB-MSCs during a mouse skin transplantation experiment resulted in impaired wound healing and less engraftment of stem cells at the wound site. These results demonstrate that melatonin signaling via MT2 triggers FAK/paxillin phosphorylation to stimulate reorganization of the actin cytoskeleton, which is responsible for Cdc42/Arp2/3 activation to promote UCB-MSCs motility.

SRChing for the substrates of Src

By the mid 1980's, it was clear that the transforming activity of oncogenic Src was linked to the activity of its tyrosine kinase domain and attention turned to identifying substrates, the putative next level of control in the pathway to transformation. Among the first to recognize the potential of phosphotyrosine-specific antibodies, Parsons and colleagues launched a risky shotgun-based approach that led ultimately to the cDNA cloning and functional characterization of many of today's best-known Src substrates (for example, p85-Cortactin, p110-AFAP1, p130Cas, p125FAK and p120-catenin). Two decades and over 6000 citations later, the original goals of the project may be seen as secondary to the enormous impact of these protein substrates in many areas of biology. At the request of the editors, this review is not restricted to the current status of the substrates, but reflects also on the anatomy of the project itself and some of the challenges and decisions encountered along the way.

uPAR and cathepsin B knockdown inhibits radiation-induced PKC integrated integrin signaling to the cytoskeleton of glioma-initiating cells

Despite advances in radiotherapeutic and chemotherapeutic techniques and aggressive surgical resection, the prognosis of glioblastoma patients is dismal. Accumulation of evidence indicates that some cancer cells survive even the most aggressive treatments, and these surviving cells, which are resistant to therapy and are perhaps essential for the malignancy, may be cancer stem cells. The CD133 surface marker is commonly used to isolate these extremely resistant glioma-initiating cells (GICs). In the present study, GICs which tested positive for the CD133 marker (CD133⁺) were isolated from both the established U251 cell line and the 5310 xenograft glioma cell line to study the events related to the molecular pathogenesis of these cells. Simultaneous down-regulation of uPAR and cathepsin B by shRNA (pUC) treatment caused the disruption of radiation-induced complex formation of pPKC θ/δ, integrin β1 and PKC ζ, integrin β1 in glioma cells. Further, pUC treatment inhibited PKC/integrin signaling via FAK by causing disassociation of FAK and the cytoskeletal molecules vinculin and α-actinin. Also, we observed the inhibition of ERK phosphorylation. This inhibition was mediated by pUC and directed a negative feedback mechanism over the FAK signaling molecules, which led to an extensive reduction in the signal for cytoskeletal organization generating migratory arrest. Altogether, it can be hypothesized that knockdown of uPAR and cathepsin B using shRNA is an effective strategy for controlling highly invasive glioma cells and extremely resistant glioma-initiating cells.

Neutrophil transmigration, focal adhesion kinase and endothelial barrier function

Neutrophil activation is an essential component of innate immune defense against infection and injury. In response to inflammatory stimulation, circulating neutrophils undergo a series of dynamic and metabolic changes characterized by β2-intergrin mediated adhesion to microvascular endothelium and subsequent transendothelial migration. During this process, neutrophils release granular contents containing digestive enzymes and produce cytotoxic agents such as reactive oxygen species and cytokines. These products target endothelial barriers inducing phosphorylation-triggered junction dissociation, actin stress fiber formation, and actomyosin contraction, manifest as paracellular hyperpermeability. Endothelial cell-matrix focal adhesions play an integral role in this process by providing structural support for endothelial conformational changes that facilitate neutrophil transmigration, as well as by recruiting intracellular molecules that constitute the hyperpermeability signaling cascades. As a central connector of the complex signaling network, focal adhesion kinase (FAK) is activated following neutrophil adhesion, and further mediates the reorganization of endothelial integrin-matrix attachments in a pattern coordinating with cytoskeleton contraction and junction opening. In this review, we present recent experimental evidence supporting the importance of FAK in neutrophil-dependent regulation of endothelial permeability. The discussion focuses on the mechanisms by which neutrophils activate FAK and its downstream effects on endothelial barriers.

Formation of endothelial lumens requires a coordinated PKCepsilon-, Src-, Pak- and Raf-kinase-dependent signaling cascade downstream of Cdc42 activation

In this study, we present data showing that Cdc42-dependent lumen formation by endothelial cells (ECs) in three-dimensional (3D) collagen matrices involves coordinated signaling by PKCepsilon in conjunction with the Src-family kinases (SFKs) Src and Yes. Activated SFKs interact with Cdc42 in multiprotein signaling complexes that require PKCepsilon during this process. Src and Yes are differentially expressed during EC lumen formation and siRNA suppression of either kinase, but not Fyn or Lyn, results in significant inhibition of EC lumen formation. Concurrent with Cdc42 activation, PKCepsilon- and SFK-dependent signaling converge to activate p21-activated kinase (Pak)2 and Pak4 in steps that are also required for EC lumen formation. Pak2 and Pak4 further activate two Raf kinases, B-Raf and C-Raf, leading to ERK1 and ERK2 (ERK1/2) activation, which all seem to be necessary for EC lumen formation. This work reveals a multicomponent kinase signaling pathway downstream of integrin-matrix interactions and Cdc42 activation involving PKCepsilon, Src, Yes, Pak2, Pak4, B-Raf, C-Raf and ERK1/2 to control EC lumen formation in 3D collagen matrices.

Phosphorylation of AFAP-110 affects podosome lifespan in A7r5 cells

AFAP-110 is an actin-binding and -crosslinking protein that is enriched in Src and phorbol ester (PE)-induced podosomes. In vascular smooth muscle cells endogenous AFAP-110 localized to actin stress fibers and, in response to treatment with phorbol-12,13-dibutyrate (PDBu), to actin-rich podosomes. Since PEs can activate PKCalpha, AFAP-110 is a substrate of PKCalpha and PKCalpha-AFAP-110 interactions direct podosome formation, we sought to identify a PE-induced phosphorylation site in AFAP-110 and determine whether phosphorylation is linked to the formation of podosomes. Mutational analysis revealed Ser277 of AFAP-110 to be phosphorylated in PE-treated cells. The use of a newly generated, phospho-specific antibody directed against phosphorylated Ser277 revealed that PKCalpha activation is associated with PE-induced AFAP-110 phosphorylation. In PDBu-treated A7r5 rat vascular smooth muscle cells, immunolabeling using the phospho-specific antibody showed that phospho-AFAP-110 is primarily associated with actin in podosomes. Although mutation of Ser at position 277 to Ala (AFAP-110(S277A)) did not alter the ability of AFAP-110 to localize to podosomes, overexpression of AFAP-110(S277A) in treated and untreated A7r5 cells resulted in an increased number of cells that display podosomes. Video microscopy demonstrated that AFAP-110(S277A) expression correlates with an increased number of long-lived podosomes. Therefore, we hypothesize that AFAP-110 phosphorylation and/or dephosphorylation is involved in the regulation of podosome stability and lifespan.

Tumor formation due to abnormalities in the beta-catenin-independent pathway of Wnt signaling

Wnt signaling is a complex pathway in which beta-catenin is typically viewed as a central mediator in regulating cell proliferation and differentiation. The significance of Wnt signaling in human cancer has been elucidated by the identification of mutations in genes coding for the beta-catenin-dependent pathway components, adenomatous polyposis coli gene product, beta-catenin, and Axin. Within the past 15 years, evidence has been growing of a beta-catenin-independent pathway in Wnt signaling. It is likely that this pathway activates several intracellular signaling systems to regulate cell migration, adhesion, and polarity. The beta-catenin-independent pathway has also been shown to play an important role in tumor biology. In contrast to the beta-catenin-dependent pathway, which is upregulated in many cancers and serves as a tumor promoter, the role of the beta-catenin-independent pathway is still controversial. Here we review recent developments in both the functions and mechanisms of the beta-catenin-independent pathway, with an emphasis on its functional contribution to human tumor progression.

Post-translational palmitoylation and glycosylation of Wnt-5a are necessary for its signalling

Wnt-5a is a representative ligand that activates a beta-catenin-independent pathway in Wnt signalling. In the present paper, the roles of the post-translational modifications in the actions of Wnt-5a were investigated. We found that Wnt-5a is modified with palmitate at Cys104 and glycans at Asn114, Asn120, Asn311 and Asn325. The palmitoylation was not essential for the secretion of Wnt-5a, but was necessary for its ability to suppress Wnt-3a-dependent T-cell factor transcriptional activity and to stimulate cell migration. Wnt-5a activated focal adhesion kinase and this activation also required palmitoylation. Wild-type Wnt-5a induced the internalization of Fz (Frizzled) 5, but a Wnt-5a mutant that lacks the palmitoylation site did not. Furthermore, the binding of Wnt-5a to the extracellular domain of Fz5 required palmitoylation of Wnt-5a. These results indicate that palmitoylation of Wnt-5a is important for the triggering of signalling at the cell surface level and, therefore, that the lipid-unmodified form of Wnt-5a cannot activate intracellular signal cascades. In contrast, glycosylation was necessary for the secretion of Wnt-5a, but not essential for the actions of Wnt-5a. Thus the post-translational palmitoylation and glycosylation of Wnt-5a are important for the actions and secretion of Wnt-5a.

PI3K activation is required for PMA-directed activation of cSrc by AFAP-110

Activation of PKCalpha will induce the cSrc binding partner AFAP-110 to colocalize with and activate cSrc. The ability of AFAP-110 to colocalize with cSrc is contingent on the integrity of the amino-terminal pleckstrin homology (PH1) domain, while the ability to activate cSrc is dependent on the integrity of its SH3 binding motif, which engages the cSrc SH3 domain. The outcome of AFAP-110-directed cSrc activation is a change in actin filament integrity and the formation of podosomes. Here, we address what cellular signals promote AFAP-110 to colocalize with and activate cSrc, in response to PKCalpha activation or PMA treatment. Because PH domain integrity in AFAP-110 is required for colocalization, and PH domains are known to interact with both protein and lipid binding partners, we sought to determine whether phosphatidylinositol 3-kinase (PI3K) activation played a role in PMA-induced colocalization between AFAP-110 and cSrc. We show that PMA treatment is able to direct activation of PI3K. Treatment of mouse embryo fibroblast with PI3K inhibitors blocked PMA-directed colocalization between AFAP-110 and cSrc and subsequent cSrc activation. PMA also was unable to induce colocalization or cSrc activation in cells that lacked the p85alpha and -beta regulatory subunits of PI3K. This signaling pathway was required for migration in a wound healing assay. Cells that were null for cSrc or the p85 regulatory subunits or expressed a dominant-negative AFAP-110 also displayed a reduction in migration. Thus PI3K activity is required for PMA-induced colocalization between AFAP-110 and cSrc and subsequent cSrc activation, and this signaling pathway promotes cell migration.

Signaling pathway underlying stimulation of L-type Ca2+ channels in rabbit portal vein myocytes by recombinant Gbetagamma subunits

In previous studies, we (Callaghan B, Koh SD, and Keef KD, Circ Res 94: 626-633, 2004) have shown that voltage-dependent L-type Ca(2+) channels (Cav) in portal vein myocytes are enhanced when muscarinic M2 receptors are activated with ACh. Current stimulation was coupled to the G protein subunit Gbetagamma along with the downstream mediators phosphatidylinositol-3-kinase (PI3K), protein kinase C (PKC), and c-Src. The present study was designed to determine whether the same second messenger pathway could be identified when exogenous recombinant Gbetagamma subunits are introduced into cells. Smooth muscle myocytes were freshly isolated from rabbit portal vein, and Cav currents were recorded by using the patch-clamp technique. Dialysis of cells with recombinant Gbetagamma (50 nM) significantly increased Cav currents (141%). Nifedipine (1 microM) reduced both control and stimulated currents by approximately 90%. The enhancement of current by Gbetagamma was equivalent to that produced by ACh (142%), whereas the PKC activator phorbol 12,13-dibutyrate (PdBu) gave rise to greater current stimulation (192%). Current stimulation with Gbetagamma, ACh, and PdBu were not associated with changes in the voltage dependence of activation or inactivation. The PI3K inhibitor LY-294002 (20 microM) reduced peak currents by 32% in cells dialyzed with Gbetagamma, whereas the inactive analog LY-303511 resulted in a small but significant reduction in current (12%). The c-Src inhibitor PP2 (1 microM) also significantly reduced currents (34%), whereas the inactive analog PP3 was without effect. These data provide further evidence for the hypothesis that Gbetagamma leads to stimulation of Cav currents in rabbit portal vein myocytes via a signaling pathway that includes PI3K, PKC, and c-Src.

Effects of tyrosine phosphorylation of cortactin on podosome formation in A7r5 vascular smooth muscle cells

Cortactin, a predominant substrate of Src family kinases, plays an important role in Arp2/3-dependent actin polymerization in lamellipodia and membrane ruffles and was recently shown to be enriched in podosomes induced by either c-Src or phorbol ester. However, the mechanisms by which cortactin regulates podosome formation have not been determined. In this study, we showed that cortactin is required for podosome formation, using siRNA knockdown of cortactin expression in smooth muscle A7r5 cells. Treatment with phorbol ester or expression of constitutively active c-Src induced genesis of cortactin-containing podosomes as well as increase in phosphorylation of cortactin at Y421 and Y466, the Src phosphorylation sites on cortactin. The Src kinase inhibitor SU-6656 significantly inhibited formation of podosomes induced by phorbol ester and phosphorylation of cortactin, whereas PKCalpha inhibitor did not affect podosome formation in c-Src-transfected cells. Unexpectedly, expression of cortactin mutants containing Y421F, Y421D, Y466F, or Y466D mutated sites did not affect podosome formation or cortactin translocation to podosomes, although endogenous tyrosine-phosphorylated cortactin at Y421 and Y466 was present in podosomes. Our data indicate that 1) PKCalpha acts upstream of Src in phosphorylation of cortactin and podosome formation in smooth muscle cells; 2) expression of cortactin is essential for genesis of podosomes; 3) phosphorylation at Y421 and Y466 is not required for translocation of cortactin to podosomes, although phosphorylation at these sites appears to be enriched in podosomes; and 4) tyrosine phosphorylation of cortactin may be involved in regulation of stability and turnover of podosomes, rather than targeting this protein to the site of podosome formation.

The signaling network of transforming growth factor beta1, protein kinase Cdelta, and integrin underlies the spreading and invasiveness of gastric carcinoma cells

Integrin-mediated cell adhesion and spreading enables cells to respond to extracellular stimuli for cellular functions. Using a gastric carcinoma cell line that is usually round in adhesion, we explored the mechanisms underlying the cell spreading process, separate from adhesion, and the biological consequences of the process. The cells exhibited spreading behavior through the collaboration of integrin-extracellular matrix interaction with a Smad-mediated transforming growth factor beta1 (TGFbeta1) pathway that is mediated by protein kinase Cdelta (PKCdelta). TGFbeta1 treatment of the cells replated on extracellular matrix caused the expression and phosphorylation of PKCdelta, which is required for expression and activation of integrins. Increased expression of integrins alpha2 and alpha3 correlated with the spreading, functioning in activation of focal adhesion molecules. Smad3, but not Smad2, overexpression enhanced the TGFbeta1 effects. Furthermore, TGFbeta1 treatment and PKCdelta activity were required for increased motility on fibronectin and invasion through matrigel, indicating their correlation with the spreading behavior. Altogether, this study clearly evidenced that the signaling network, involving the Smad-dependent TGFbeta pathway, PKCdelta expression and phosphorylation, and integrin expression and activation, regulates cell spreading, motility, and invasion of the SNU16mAd gastric carcinoma cell variant.

Cellular mechanisms of bacterial internalization counteracted by Yersinia

Upon host-cell contact, human pathogenic Yersinia species inject Yop virulence effectors into the host through a Type III secretion-and-translocation system. These virulence effectors cause a block in phagocytosis (YopE, YopT, YpkA, and YopH) and suppression of inflammatory mediators (YopJ). The Yops that block phagocytosis either interfere with the host cell actin regulation of Rho GTPases (YopE, YopT, and YpkA) or specifically and rapidly inactivate host proteins involved in signaling from the receptor to actin (YopH). The block in uptake has been shown to be activated following binding to Fc, Complement, and beta1-integrin receptors in virtually any kind of host cell. Thus, the use of Yersinia as a model system to study Yersinia-host cell interactions provides a good tool to explore signaling pathways involved in phagocytosis.

Attenuation of mouse mesangial cell contractility by high glucose and mannitol: involvement of protein kinase C and focal adhesion kinase

Hyperglycemia and mannitol activate protein kinase C (PKC) and induce mesangial cell hypocontractility that subsequently may modulate renal function. Since focal adhesion kinase (FAK) activation is known to be linked with PKC activity, FAK may also be involved in mesangial cell contraction. To facilitate our understanding of the PKC- and FAK-modulating mechanism, we developed an in vitro model of mouse mesangial cell hypocontractility induced by hyperglycemia or mannitol. Mouse mesangial cells (CRL-1927) were exposed to: normal D-glucose (group N), high D-glucose (group H), and control groups at the same osmolality as H plus L-glucose (group L) and mannitol (group M). Changes in the planar surface area of cells in response to 1 microM phorbol 12-myristate 13-acetate (PMA) were determined. Western blot analyses for PKC, phosphorylated (p)-PKC, tyrosine phosphorylation, FAK, and p-FAK were done on each of these four groups. The effects of mannitol in various doses on cell contraction and activation of PKC and FAK were also assayed. The planar surface areas of groups H and M both showed an attenuated change in response to PMA stimulation. Before PMA stimulation, the baseline PKC expression of groups H and M showed a higher expression of p-PKC alpha and delta than that seen in group N (p < 0.05). Results of tyrosine phosphorylation and immunoprecipitation showed that FAK may be involved in this contraction process. The total amount of FAK showed no significant difference among the four experimental groups; however, p-FAK was found to have significantly increased in group M (p < 0.05). The use of PKC and tyrosine kinase inhibitors reduced PMA-induced mesangial cell contraction in all four groups. Activation of PKC alpha, delta, and FAK with the resultant inhibition of mesangial cell contraction by mannitol was found to be dose-dependent. These results may provide a correlation between increased expression of several PKC isoforms and, in particular, increased phosphorylation levels of PKC alpha and delta and hypocontractility induced by high glucose and mannitol treatment. Furthermore, the mannitol-induced hypocontractility involving PKC and FAK occurred in a dose-dependent manner.

Protein kinase Calpha activates c-Src and induces podosome formation via AFAP-110

We report that the actin filament-associated protein AFAP-110 is required to mediate protein kinase Calpha (PKCalpha) activation of the nonreceptor tyrosine kinase c-Src and the subsequent formation of podosomes. Immunofluorescence analysis demonstrated that activation of PKCalpha by phorbol 12-myristate 13-acetate (PMA), or ectopic expression of constitutively activated PKCalpha, directs AFAP-110 to colocalize with and bind to the c-Src SH3 domain, resulting in activation of the tyrosine kinase. Activation of c-Src then directs the formation of podosomes, which contain cortactin, AFAP-110, actin, and c-Src. In a cell line (CaOV3) that has very little or no detectable AFAP-110, PMA treatment was unable to activate c-Src or effect podosome formation. Ectopic expression of AFAP-110 in CaOV3 cells rescued PKCalpha-mediated activation of c-Src and elevated tyrosine phosphorylation levels and subsequent formation of podosomes. Neither expression of activated PKCalpha nor treatment with PMA was able to induce these changes in CAOV3 cells expressing mutant forms of AFAP-110 that are unable to bind to, or colocalize with, c-Src. We hypothesize that one major function of AFAP-110 is to relay signals from PKCalpha that direct the activation of c-Src and the formation of podosomes.

Muscarinic M2 Receptor Stimulation of Cav1.2b Requires Phosphatidylinositol 3-Kinase, Protein Kinase C, and c-Src

This study investigated regulation of L-type calcium channels (Cav1.2b) by acetylcholine (ACh) in rabbit portal vein myocytes. Whole-cell currents were recorded using 5 mmol/L barium as charge carrier. ACh (10 μmol/L) increased peak currents by 40%. This effect was not reversed by the selective muscarinic M3 receptor antagonist 4-DAMP (100 nmol/L) but was blocked by the M2 receptor antagonist methoctramine (5 μmol/L). The classical and novel protein kinase C (PKC) antagonist calphostin C (50 nmol/L) abolished ACh responses, whereas the classical PKC antagonist Gö6976 (200 nmol/L) had no effect. ACh responses were also abolished by the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 (20 μmol/L), by the c-Src inhibitor PP2 (10 μmol/L) (but not the inactive analogue PP3), and by dialyzing cells with an antibody to the G-protein subunit Gβγ. Cells dialyzed with c-Src had significantly greater currents than control cells. Current enhancement persisted in the presence of LY294002, suggesting that c-Src is downstream of PI3K. Phorbol 12,13-dibutyrate (PDBu, 0.1 μmol/L) increased currents by 74%. This effect was abolished by calphostin C and reduced by Gö6976. The PDBu response was also reduced by PP2, and the PP2-insensitive component was blocked by Gö6976. In summary, these data suggest that ACh enhances Cav1.2b currents via M2 receptors that couple sequentially to Gβγ, PI3K, a novel PKC, and c-Src. PDBu stimulates the novel PKC/c-Src pathway along with a second pathway that is independent of c-Src and involves a classical PKC.

Induction of cell-cell detachment during glucose starvation through F-actin conversion by SNARK, the fourth member of the AMP-activated protein kinase catalytic subunit family

SNARK, the fourth member of the AMPK catalytic subunit family, was originally identified in a rat kidney cDNA library, and in this study we isolated its human homologue. A BLAST search analysis using rat SNARK protein yielded a single high homology clone, DKFZp434J037, isolated from human testis, and since its hypothetical protein showed 84% homology to rat SNARK protein, we assumed DKFZp434J037 to be the human SNARK cDNA. The human SNARK cDNA is 3443bp long and encodes a 628 amino acid protein having an estimated molecular weight of 69kDa, and its chromosomal localization had been assigned to 1q32.1. The same as other members of AMPK catalytic subunit family, human SNARK showed AMP-dependent GST-SAMS phosphorylation activity and enhanced HepG2 cell survival during glucose starvation. Human SNARK-overexpressing HepG2 cells (H/SNK) showed acute cell-cell detachment when exposed to glucose-free medium and the cell-cell detachment correlated well with the detection of G-actin. Deletion mutant analysis strongly suggested that the putative catalytic domain of SNARK is necessary for the cell-cell detachment, and Western blotting analysis showed that phosphorylation of FAK and PKC, which were dramatically increased by glucose starvation in HepG2 cells, was markedly suppressed by SNARK.

Src phosphorylates Cas on tyrosine 253 to promote migration of transformed cells

Cas is a member of the focal adhesion complex. Phosphorylation of Cas by Src is an important event leading to cell transformation. Using mass spectrometry, we have mapped 11 sites in Cas that are phosphorylated by Src. These sites are all located between residues 132 and 414 of Cas, in a region that is required for binding to a number of other proteins including Crk. We tested synthetic peptides modeled on Cas phosphorylation sites, and found that the sequence containing tyrosine 253 was phosphorylated by Src most efficiently. Using cells derived from Cas-deficient mice, we confirmed that Cas greatly enhanced the ability of Src to transform cells. Phosphorylation of Cas on tyrosine 253 was not required for Src to increase growth rate, suppress contact inhibition, or suppress anchorage dependence. Yet, in contrast to these growth characteristics, phosphorylation of Cas on tyrosine 253 was required for Src to promote cell migration. Thus, a single phosphorylation site on this focal adhesion adaptor protein can effectively separate cell migration from other transformed growth characteristics.

Protein kinase C delta induces Src kinase activity via activation of the protein tyrosine phosphatase PTP alpha

Previously we have shown that protein kinase C (PKC)-mediated reorganization of the actin cytoskeleton in smooth muscle cells is transmitted by the non-receptor tyrosine kinase, Src. Several authors have described how 12-O-tetradecanoylphorbol-13-acetate (TPA) stimulation of cells results in an increase of Src activity, but the mechanism of the PKC-mediated Src activation is unknown. Using PKC isozymes purified from Spodoptera frugiperda insect cells, we show here that PKC is not able to activate Src directly. Our data reveal that the PKC-dependent Src activation occurs via the activation of the protein tyrosine phosphatase (PTP) PTP alpha. PTP alpha becomes activated in vivo after TPA stimulation. Further, we show that PKC delta phosphorylates and activates only PTP alpha in vitro but not any other of the TPA-responsive PKC isozymes that are expressed in A7r5 rat aortic smooth muscle cells. To further substantiate our data, we show that cells lacking PKC delta have a markedly reduced PTP alpha and Src activity after 12-O-tetradecanoylphorbol-13-acetate stimulation. These data support a model in which the main mechanism of 12-O-tetradecanoylphorbol-13-acetate-induced Src activation is the direct phosphorylation and activation of PTP alpha by PKC delta, which in turn dephosphorylates and activates Src.

Glucose stimulates the tyrosine phosphorylation of Crk-associated substrate in pancreatic beta-cells

Several years ago, we demonstrated that glucose induced tyrosine phosphorylation of a 125-kDa protein (p125) in pancreatic beta-cells (Konrad, R. J., Dean, R. M., Young, R. A., Bilings, P. C., and Wolf, B. A. (1996) J. Biol. Chem. 271, 24179-24186). Glucose induced p125 tyrosine phosphorylation in beta-TC3 insulinoma cells, beta-HC9 cells, and in freshly isolated rat islets, whereas increased tyrosine phosphorylation was not observed with other fuel secretagogues. Initial efforts to identify p125 were unsuccessful, so a new approach was taken. The protein was purified from betaTC6,F7 cells via an immunodepletion method. After electrophoresis and colloidal Coomassie Blue staining, the area of the gel corresponding to p125 was excised and subjected to tryptic digestion. Afterward, mass spectrometry was performed and the presence of Crk-associated substrate (Cas) was detected. Commercially available antibodies against Cas were obtained and tested directly in beta-cells, confirming glucose-induced tyrosine phosphorylation of Cas. Further experiments demonstrated that in beta-cells the glucose-induced increase in Cas tyrosine phosphorylation occurs immediately and is not accompanied by increased focal adhesion kinase tyrosine phosphorylation. Finally, it is also demonstrated via Western blotting that Cas is present in normal isolated rat islets. Together, these results show that the identity of the previously described p125 beta-cell protein is Cas and that Cas undergoes rapid glucose-induced tyrosine phosphorylation in beta-cells.

[Focal adhesion kinase (FAK), a multifunctional protein]

Focal adhesion kinase (FAK) is a cytoplasmic protein tyrosine kinase localized to regions called focal adhesions. Many stimuli can induce tyrosine phosphorylation and activation of FAK, including integrins and growth factors. The major site of autophosphorylation, tyrosine 397, is a docking site for the SH2 domains of Src family proteins. The other sites of phosphorylation are phosphorylated by Src kinases. Phosphorylated FAK binds proteins of focal adhesion and can activate them directly or indirectly by phosphorylation. These activated proteins forming the FAK complex facilitate the generation of downstream signals necessary to regulate cell functions, like motility, survival and proliferation. Dysregulation of FAK could participate in the development of cancer. This review will focus upon the mechanisms by which FAK transmits biochemical signals and elicits biological effects.

Tyrosine phosphorylation of I-kappa B kinase alpha/beta by protein kinase C-dependent c-Src activation is involved in TNF-alpha-induced cyclooxygenase-2 expression

The signaling pathway involved in TNF-alpha-induced cyclooxygenase-2 (COX-2) expression was further studied in human NCI-H292 epithelial cells. A protein kinase C (PKC) inhibitor (staurosporine), tyrosine kinase inhibitors (genistein and herbimycin A), or a Src kinase inhibitor (PP2) attenuated TNF-alpha- or 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced COX-2 promoter activity. TNF-alpha- or TPA-induced I-kappaB kinase (IKK) activation was also blocked by these inhibitors, which reversed I-kappaBalpha degradation. Activation of c-Src and Lyn kinases, two Src family members, was inhibited by the PKC, tyrosine kinase, or Src kinase inhibitors. The dominant-negative c-Src (KM) mutant inhibited induction of COX-2 promoter activity by TNF-alpha or TPA. Overexpression of the constitutively active PKCalpha (PKCalpha A/E) or wild-type c-Src plasmids induced COX-2 promoter activity, and these effects were inhibited by the dominant-negative c-Src (KM), NF-kappaB-inducing kinase (NIK) (KA), or IKKbeta (KM) mutant. The dominant-negative PKCalpha (K/R) or c-Src (KM) mutant failed to block induction of COX-2 promoter activity caused by wild-type NIK overexpression. In coimmunoprecipitation experiments, IKKalpha/beta was found to be associated with c-Src and to be phosphorylated on its tyrosine residues after TNF-alpha or TPA treatment. Two tyrosine residues, Tyr(188) and Tyr(199), near the activation loop of IKKbeta, were identified to be crucial for NF-kappaB activation. Substitution of these residues with phenylalanines attenuated COX-2 promoter activity and c-Src-dependent phosphorylation of IKKbeta induced by TNF-alpha or TPA. These data suggest that, in addition to activating NIK, TNF-alpha also activates PKC-dependent c-Src. These two pathways cross-link between c-Src and NIK and converge at IKKalpha/beta, and go on to activate NF-kappaB, via serine phosphorylation and degradation of IkappaB-alpha, and, finally, to initiate COX-2 expression.

c-Src-dependent tyrosine phosphorylation of IKKbeta is involved in tumor necrosis factor-alpha-induced intercellular adhesion molecule-1 expression

The signaling pathway involved in tumor necrosis factor-alpha (TNF-alpha)-induced intercellular adhesion molecule-1 (ICAM-1) expression was further studied in human A549 epithelial cells. TNF-alpha- or 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced ICAM-1 promoter activity was inhibited by a protein kinase C (PKC) inhibitor (staurosporine), tyrosine kinase inhibitors (genistein and herbimycin A), or an Src-specific tyrosine kinase inhibitor (PP2). TNF-alpha- or TPA-induced IkappaBalpha kinase (IKK) activation was also blocked by these inhibitors, which slightly reversed TNF-alpha-induced but completely reversed TPA-induced IkappaBalpha degradation. c-Src and Lyn, two members of the Src kinase family, were abundantly expressed in A549 cells, and their activation by TNF-alpha or TPA was inhibited by the same inhibitors. Furthermore, the dominant-negative c-Src (KM) mutant inhibited induction of ICAM-1 promoter activity by TNF-alpha or TPA. Overexpression of the constitutively active PKC or wild-type c-Src plasmids induced ICAM-1 promoter activity, this effect being inhibited by the dominant-negative c-Src (KM) or IKKbeta (KM) mutant but not by the nuclear factor-kappaB-inducing kinase (NIK) (KA) mutant. The c-Src (KM) mutant failed to block induction of ICAM-1 promoter activity caused by overexpression of wild-type NIK. In co-immunoprecipitation and immunoblot experiments, IKK was found to be associated with c-Src and to be phosphorylated on tyrosine residues after TNF-alpha or TPA treatment. Two tyrosine residues, Tyr188 and Tyr199, near the activation loop of IKKbeta, were identified as being important for NF-kappaB activation. Substitution of these residues with phenylalanines abolished ICAM-1 promoter activity and c-Src-dependent phosphorylation of IKKbeta induced by TNF-alpha or TPA. These data suggest that, in addition to activating NIK, TNF-alpha also activates PKC-dependent c-Src. These two pathways converge at IKKbeta and go on to activate NF-kappaB, via serine phosphorylation and degradation of IkappaB-alpha, and, finally, to initiate ICAM-1 expression.

Cyclic strain activates redox-sensitive proline-rich tyrosine kinase 2 (PYK2) in endothelial cells

Proline-rich tyrosine kinase 2 (PYK2), structurally related to focal adhesion kinase, has been shown to play a role in signaling cascades. Endothelial cells (ECs) under hemodynamic forces increase reactive oxygen species (ROS) that modulate signaling pathways and gene expression. In the present study, we found that bovine ECs subjected to cyclic strain rapidly induced phosphorylation of PYK2 and Src kinase. This strain-induced PYK2 and Src phosphorylation was inhibited by pretreating ECs with an antioxidant N-acetylcysteine. Similarly, ECs exposed to H(2)O(2) increased both PYK2 and Src phosphorylation. An increased association of Src to PYK2 was observed in ECs after cyclic strain or H(2)O(2) exposure. ECs treated with an inhibitor to Src (PPI) greatly reduced Src and PYK2 phosphorylation, indicating that Src mediated PYK2 activation. Whereas the protein kinase C (PKC) inhibitor (calphostin C) pretreatment was shown to inhibit strain-induced NADPH oxidase activity, ECs treated with either calphostin C or the inhibitor to NADPH oxidase (DPI) reduced strain-induced ROS levels and then greatly inhibited the Src and PYK2 activation. In contrast to the activation of PYK2 and Src with calcium ionophore (ionomycin), ECs treated with a Ca(2+) chelator inhibited both phosphorylation, indicating that PYK2 and Src activation requires Ca(2+). ECs transfected with antisense to PKCalpha, but not antisense to PKCepsilon(,) reduced cyclic strain-induced PYK2 activation. These data suggest that cyclic strain-induced PYK2 activity is mediated via Ca(2+)-dependent PKCalpha that increases NADPH oxidase activity to produce ROS crucial for Src and PYK2 activation. ECs under cyclic strain thus activate redox-sensitive PYK2 via Src and PKC, and this PYK2 activation may play a key role in the signaling responses in ECs under hemodynamic influence.

Syndecan-4 modulates focal adhesion kinase phosphorylation

The cell-surface heparan sulfate proteoglycan syndecan-4 acts in conjunction with the alpha(5)beta(1) integrin to promote the formation of actin stress fibers and focal adhesions in fibronectin (FN)-adherent cells. Fibroblasts seeded onto the cell-binding domain (CBD) fragment of FN attach but do not fully spread or form focal adhesions. Activation of Rho, with lysophosphatidic acid (LPA), or protein kinase C, using the phorbol ester phorbol 12-myristate 13-acetate, or clustering of syndecan-4 with antibodies directed against its extracellular domain will stimulate formation of focal adhesions and stress fibers in CBD-adherent fibroblasts. The distinct morphological differences between the cells adherent to the CBD and to full-length FN suggest that syndecan-4 may influence the organization of the focal adhesion or the activation state of the proteins that comprise it. FN-null fibroblasts (which express syndecan-4) exhibit reduced phosphorylation of focal adhesion kinase (FAK) tyrosine 397 (Tyr(397)) when adherent to CBD compared with FN-adherent cells. Treating the CBD-adherent fibroblasts with LPA, to activate Rho, or the tyrosine phosphatase inhibitor sodium vanadate increased the level of phosphorylation of Tyr(397) to match that of cells plated on FN. Treatment of the fibroblasts with PMA did not elicit such an effect. To confirm that this regulatory pathway includes syndecan-4 specifically, we examined fibroblasts derived from syndecan-4-null mice. The phosphorylation levels of FAK Tyr(397) were lower in FN-adherent syndecan-4-null fibroblasts compared with syndecan-4-wild type and these levels were rescued by the addition of LPA or re-expression of syndecan-4. These data indicate that syndecan-4 ligation regulates the phosphorylation of FAK Tyr(397) and that this mechanism is dependent on Rho but not protein kinase C activation. In addition, the data suggest that this pathway includes the negative regulation of a protein-tyrosine phosphatase. Our results implicate syndecan-4 activation in a direct role in focal adhesion regulation.

PKC-regulated myogenesis is associated with increased tyrosine phosphorylation of FAK, Cas, and paxillin, formation of Cas-CRK complex, and JNK activation

Previous reports suggest that PKC plays an important role in regulating myogenesis. However, the regulatory signaling pathways are not fully understood. We examined the effects of PKC downregulation on signaling events during skeletal muscle differentiation. We found that downregulation of PKC results in increased myogenesis in C2C12 cells as measured by creatine kinase activity and myogenin expression. We showed that, during differentiation, downregulation of PKC expression results in increased tyrosine phosphorylation of FAK, Cas, and paxillin, concomitant with enhanced Cas-CrkII complex formation, which leads to activation of JNK2. But in proliferated muscle cells, PKC inhibition results in FAK and Cas tyrosine dephosphorylation. Further, disruption of actin cytoskeleton by cytochalasin D prevents the activation of FAK and Cas as well as the formation of Cas-CrkII complex stimulated by PKC downregulation during muscle cell differentiation. Finally, we observed that PKC downregulation increases the tyrosine phosphorylation of focal adhesion associated proteins. Based on the above data, we propose that PKC downregulation results in enhanced tyrosine phosphorylation of FAK, Cas, and paxillin, thus promoting the establishment of Cas-CrkII complex, leading to activation of JNK and that these interactions are dependent upon the integrity of actin cytoskeleton during muscle cell differentiation. Data presented here significantly contribute to elucidating the regulatory role of PKC in myogenesis possibly through integrin signaling pathway.

Protein kinase C induces actin reorganization via a Src- and Rho-dependent pathway

We have investigated the mechanism of PKC-induced actin reorganization in A7r5 vascular smooth muscle cells. PKC activation by 12-O-tetradecanoylphorbol-13-acetate induces the disassembly of actin stress fibers concomitant with the appearance of membrane ruffles. PKC also induces rapid tyrosine phosphorylation in these cells. As we could show, utilizing the Src-specific inhibitor PP2 and a kinase-deficient c-Src mutant, actin reorganization is dependent on PKC-induced Src activation. Subsequently, the activity of the small G-protein RhoA is decreased, whereas Rac and Cdc42 activities remain unchanged. Disassembly of actin stress fibers could also be observed using the Rho kinase-specific inhibitor Y-27632, indicating that the decrease in RhoA activity on its own is responsible for actin reorganization. In addition, we show that tyrosine phosphorylation of p190RhoGAP is increased upon 12-O-tetradecanoylphorbol-13-acetate stimulation, directly linking Src activation to a decrease in RhoA activity. Our data provide substantial evidence for a model elucidating the molecular mechanisms of PKC-induced actin rearrangements.

Cell Junction Dynamics in the Testis: Sertoli-Germ Cell Interactions and Male Contraceptive Development

Spermatogenesis is an intriguing but complicated biological process. However, many studies since the 1960s have focused either on the hormonal events of the hypothalamus-pituitary-testicular axis or morphological events that take place in the seminiferous epithelium. Recent advances in biochemistry, cell biology, and molecular biology have shifted attention to understanding some of the key events that regulate spermatogenesis, such as germ cell apoptosis, cell cycle regulation, Sertoli-germ cell communication, and junction dynamics. In this review, we discuss the physiology and biology of junction dynamics in the testis, in particular how these events affect interactions of Sertoli and germ cells in the seminiferous epithelium behind the blood-testis barrier. We also discuss how these events regulate the opening and closing of the blood-testis barrier to permit the timely passage of preleptotene and leptotene spermatocytes across the blood-testis barrier. This is physiologically important since developing germ cells must translocate across the blood-testis barrier as well as traverse the seminiferous epithelium during their development. We also discuss several available in vitro and in vivo models that can be used to study Sertoli-germ cell anchoring junctions and Sertoli-Sertoli tight junctions. An in-depth survey in this subject has also identified several potential targets to be tackled to perturb spermatogenesis, which will likely lead to the development of novel male contraceptives.

Brain metastases: biology and the role of the brain microenvironment

Metastatic lesions constitute the most frequently occurring malignancy in the brain, and their detection portends a grim prognosis. Efforts to treat these lesions have failed partly because the biologic processes that govern their development are poorly understood. In recent years, it has become evident that metastases occur as a result of a multistep process involving a rigorous natural selection of cells in the primary tumor that bear molecular and biologic characteristics permitting brain metastasis. In addition, recent studies have uncovered the importance of the brain microenvironment and its contribution to the metastatic process. The development of targeted therapies against brain metastases demands a better understanding of these molecular processes and the factors that influence them. This review examines the interplay between tumor cells and host brain tissue in the context of our current understanding of the role of various molecules involved in the metastatic process.

Functions of the adapter protein Cas: signal convergence and the determination of cellular responses

Since Cas was first identified as a highly phosphorylated 130 kilodalton protein that associated with the v-Src and v-Crk-oncoproteins, considerable effort has been made to determine its function. Its predicted role as a scaffolding molecule based on its domain structure has been largely confirmed. Through its ability to undergo rapid changes in phosphorylation, subcellular localization and association with heterologous proteins, Cas may spatially and temporally regulate the function of its binding partners. Numerous proteins have been identified that bind to Cas in vitro and/or in vivo, but in only a few cases is there an understanding of how Cas may function in these protein complexes. To date, Cas-Crk and Cas-Src complexes have been most frequently implicated in Cas function, particularly in regards to processes involving regulation of the actin cytoskeleton and proliferation. These and other Cas protein complexes contribute to the critical role of Cas in cell adhesion, migration, proliferation and survival of normal cycling cells. However, under conditions in which these processes are deregulated, Cas appears to play a role in oncogenic transformation and perhaps metastasis. Therefore, in its capacity as an adapter protein, Cas serves as a point of convergence for many distinct signaling inputs, ultimately contributing to the generation of specific cellular responses.