TGF-beta1-mediated activations of c-Src and Rac1 modulate levels of cyclins and p27(Kip1) CDK inhibitor in hepatoma cells replated on fibronectin

The Small GTPase RAC1B: A Potent Negative Regulator of-and Useful Tool to Study-TGFβ Signaling

RAC1 and its alternatively spliced isoform, RAC1B, are members of the Rho family of GTPases. Both isoforms are involved in the regulation of actin cytoskeleton remodeling, cell motility, cell proliferation, and epithelial-mesenchymal transition (EMT). Compared to RAC1, RAC1B exhibits a number of distinctive features with respect to tissue distribution, downstream signaling and a role in disease conditions like inflammation and cancer. The subcellular locations and interaction partners of RAC1 and RAC1B vary depending on their activation state, which makes RAC1 and RAC1B ideal candidates to establish cross-talk with cancer-associated signaling pathways-for instance, interactions with signaling by transforming growth factor β (TGFβ), a known tumor promoter. Although RAC1 has been found to promote TGFβ-driven tumor progression, recent observations in pancreatic carcinoma cells surprisingly revealed that RAC1B confers anti-oncogenic properties, i.e., through inhibiting TGFβ-induced EMT. Since then, an unexpected array of mechanisms through which RAC1B cross-talks with TGFβ signaling has been demonstrated. However, rather than being uniformly inhibitory, RAC1B interacts with TGFβ signaling in a way that results in the selective blockade of tumor-promoting pathways, while concomitantly allowing tumor-suppressive pathways to proceed. In this review article, we are going to discuss the specific interactions between RAC1B and TGFβ signaling, which occur at multiple levels and include various components such as ligands, receptors, cytosolic mediators, transcription factors, and extracellular inhibitors of TGFβ ligands.

Dasatinib inhibits TGFβ-induced myofibroblast differentiation through Src-SRF Pathway

Persistent myofibroblast differentiation is a hallmark of fibrotic diseases. Myofibroblasts are characterized by de novo expression of alpha smooth muscle actin (αSMA) and excess fibronectin assembly. Recent studies provide conflicting reports on the effects of tyrosine kinase inhibitor dasatinib on myofibroblast differentiation and fibrosis. Also, it is not fully understood whether dasatinib modulates myofibroblast differentiation by targeting Src kinase. Herein, we investigated the effect of dasatinib on cSrc and transforming growth factor-β (TGFβ)-induced myofibroblast differentiation in vitro. Our results indicated that selective Src kinase inhibition using PP2 mimicked the effect of dasatinib in attenuating myofibroblast differentiation as evident by blunted αSMA expression and modest, but significant inhibition of fibronectin assembly in both NIH 3T3 and fibrotic human lung fibroblasts. Mechanistically, our data showed that dasatinib modulates αSMA synthesis through Src kinase-mediated modulation of serum response factor expression. Collectively, our results demonstrate that dasatinib modulates myofibroblast differentiation through Src-SRF pathway. Thus, dasatinib could potentially be a therapeutic option in fibrotic diseases.

AFAP1 Is a Novel Downstream Mediator of TGF-β1 for CCN2 Induction in Osteoblasts

Background

CCN2 acts as an anabolic growth factor to regulate osteoblast differentiation and function. CCN2 is induced by TGF-β1 and acts as a mediator of TGF-β1 induced matrix production in osteoblasts and Src is required for CCN2 induction by TGF-β1; however, the molecular mechanisms that control CCN2 induction in osteoblasts are poorly understood. AFAP1 binds activated forms of Src and can direct the activation of Src in certain cell types, however a role for AFAP1 downstream of TGF-β1 or in osteoblats is undefined. In this study, we investigated the role of AFAP1 for CCN2 induction by TGF-β1 in primary osteoblasts.

Results

We demonstrated that AFAP1 expression in osteoblasts occurs in a biphasic pattern with maximal expression levels occurring during osteoblast proliferation (~day 3), reduced expression during matrix production/maturation (~day 14–21), an a further increase in expression during mineralization (~day 21). AFAP1 expression is induced by TGF-β1 treatment in osteoblasts during days 7, 14 and 21. In osteoblasts, AFAP1 binds to Src and is required for Src activation by TGF-β1 and CCN2 promoter activity and protein induction by TGF-β1 treatment was impaired using AFAP1 siRNA, indicating the requirement of AFAP1 for CCN2 induction by TGF-β1. We also demonstrated that TGF-β1 induction of extracellular matrix protein collagen XIIa occurs in an AFAP1 dependent fashion.

Conclusions

This study demonstrates that AFAP1 is an essential downstream signaling component of TGF-β1 for Src activation, CCN2 induction and collagen XIIa in osteoblasts.

Interaction of tetraspan(in) TM4SF5 with CD44 promotes self-renewal and circulating capacities of hepatocarcinoma cells

Tumor metastasis involves circulating and tumor-initiating capacities of metastatic cancer cells. Epithelial-mesenchymal transition (EMT) is related to self-renewal capacity and circulating tumor cell (CTC) characteristics for tumor metastasis. Although tumor metastasis is a life-threatening, complicated process that occurs through circulation of tumor cells, mechanistic aspects of self-renewal and circulating capacities have been largely unknown. Hepatic transmembrane 4 L six family member 5 (TM4SF5) promotes EMT for malignant growth and migration, so it was rationalized that TM4SF5, as a hepatocellular carcinoma (HCC) biomarker, might be important for metastatic potential. Here, self-renewal capacity by TM4SF5 was mechanistically explored using hepatocarcinoma cells with or without TM4SF5 expression, and we explored whether they became CTCs using mouse liver-orthotopic model systems. We found that TM4SF5-dependent sphere growth correlated with CD24(-) , aldehyde dehydrogenase (ALDH) activity, as well as a physical association between CD44 and TM4SF5. Interaction between TM4SF5 and CD44 was through their extracellular domains with N-glycosylation modifications. TM4SF5/CD44 interaction activated proto-oncogene tyrosine-protein kinase Src (c-Src)/signal transducer and activator of transcription 3 (STAT3)/Twist-related protein 1 (Twist1)/B-cell-specific Moloney murine leukemia virus integration site 1 (Bmi1) signaling for spheroid formation, whereas disturbing the interaction, expression, or activity of any component in this signaling pathway inhibited spheroid formation. In serial xenografts using 200∼5,000 cells per injection, TM4SF5-positive tumors exhibited subpopulations with locally increased CD44 expressions, supporting for tumor cell differentiation. TM4SF5-positive, but not TM4SF5- or CD44-knocked-down, cells were identified circulating in blood 4-6 weeks after orthotopic liver injection using in vivo laser scanning endomicroscopy. Anti-TM4SF5 reagent blocked their metastasis to distal intestinal organs.

CONCLUSION

TM4SF5 promotes self-renewal and CTC properties supported by TM4SF5(+) /CD44(+(TM4SF5-bound)) /ALDH(+) /CD24(-) markers during HCC metastasis.

Hepatic stellate cells: central modulators of hepatic carcinogenesis

Hepatocellular carcinoma (HCC) represents the second most common cause of cancer-related death worldwide, and is increasing in incidence. Currently, our therapeutic repertoire for the treatment of HCC is severely limited, and therefore effective new therapies are urgently required. Recently, there has been increasing interest focusing on the cellular and molecular interactions between cancer cells and their microenvironment. HCC represents a unique opportunity to study the relationship between a diseased stroma and promotion of carcinogenesis, as 90 % of HCCs arise in a cirrhotic liver. Hepatic stellate cells (HSC) are the major source of extracellular proteins during fibrogenesis, and may directly, or via secreted products, contribute to tumour initiation and progression. In this review we explore the complex cellular and molecular interplay between HSC biology and hepatocarcinogenesis. We focus on the molecular mechanisms by which HSC modulate HCC growth, immune cell evasion and angiogenesis. This is followed by a discussion of recent progress in the field in understanding the mechanistic crosstalk between HSC and HCC, and the pathways that are potentially amenable to therapeutic intervention. Furthermore, we summarise the exciting recent developments in strategies to target HSC specifically, and novel techniques to deliver pharmaceutical agents directly to HSC, potentially allowing tailored, cell-specific therapy for HCC.

Correlations between transmembrane 4 L6 family member 5 (TM4SF5), CD151, and CD63 in liver fibrotic phenotypes and hepatic migration and invasive capacities

Transmembrane 4 L6 family member 5 (TM4SF5) is overexpressed during CCl₄-mediated murine liver fibrosis and in human hepatocellular carcinomas. The tetraspanins form tetraspanin-enriched microdomains (TEMs) consisting of large membrane protein complexes on the cell surface. Thus, TM4SF5 may be involved in the signal coordination that controls liver malignancy. We investigated the relationship between TM4SF5-positive TEMs with liver fibrosis and tumorigenesis, using normal Chang hepatocytes that lack TM4SF5 expression and chronically TGFβ1-treated Chang cells that express TM4SF5. TM4SF5 expression is positively correlated with tumorigenic CD151 expression, but is negatively correlated with tumor-suppressive CD63 expression in mouse fibrotic and human hepatic carcinoma tissues, indicating cooperative roles of the tetraspanins in liver malignancies. Although CD151 did not control the expression of TM4SF5, TM4SF5 appeared to control the expression levels of CD151 and CD63. TM4SF5 interacted with CD151, and caused the internalization of CD63 from the cell surface into late lysosomal membranes, presumably leading to terminating the tumor-suppressive functions of CD63. TM4SF5 could overcome the tumorigenic effects of CD151, especially cell migration and extracellular matrix (ECM)-degradation. Taken together, TM4SF5 appears to play a role in liver malignancy by controlling the levels of tetraspanins on the cell surface, and could provide a promising therapeutic target for the treatment of liver malignancies.

Endothelial cell-derived fibronectin extra domain A promotes colorectal cancer metastasis via inducing epithelial-mesenchymal transition

Recent evidence has been suggesting the important roles of endothelial cells (ECs) involved in the pathogenesis of several cancers, including colorectal carcinomas (CRCs), but the underlying mechanism remains elusive. We have demonstrated previously that CRC-derived fibronectin extra domain A (EDA) promotes vasculogenesis, tumorigenesis and metastasis of CRCs. At the current study, we showed that EC-secreted EDA promotes the metastatic capacity CRC cells via inducing an epithelial-mesenchymal transition. In vitro and in vivo experiments showed that EC-secreted EDA, via the interaction with integrin α9β1 on neighboring CRC cells, leads to the activation of focal adhesion kinase as well as Rac signalings, thus strengthens the polarity of cytoskeleton and promotes the invasion capacity of CRC cells. Furthermore, Erk signaling pathway was revealed to critically mediate the effect of EC-derived EDA on CRC cells. Our findings reveal a novel oncogenic role of ECs in promoting CRC malignancy through secreting EDA.

Transforming growth factor-β regulates the growth of valve interstitial cells in vitro

Although valve interstitial cell (VIC) growth is an essential feature of injured and diseased valves, the regulation of VIC growth is poorly understood. Transforming growth factor (TGF)-β promotes VIC proliferation in early-stage wound repair; thus, herein, we tested the hypothesis that TGF-β regulates VIC proliferation under normal nonwound conditions using low-density porcine VIC monolayers. Cell numbers were counted during a 10-day period, whereas proliferation and apoptosis were quantified by bromodeoxyuridine staining and TUNEL, respectively. The extent of retinoblastoma protein phosphorylation and expression of cyclin D1, CDK 4, and p27 were compared using Western blot analysis. Adhesion was quantified using a trypsin adhesion assay, and morphological change was demonstrated by immunofluorescence localization of α-smooth muscle actin and vinculin. TGF-β-treated VICs were rhomboid; significantly decreased in number, proliferation, and retinoblastoma protein phosphorylation; and concomitantly had decreased expression of cyclin D1/CDK4 and increased expression of p27. TGF-β-treated VICs adhered better to substratum and had more vinculin plaques and α-smooth muscle actin stress fibers than did controls. Thus, the regulation of VIC growth by TGF-β is context dependent. TGF-β prevents excessive heart valve growth under normal physiological conditions while it promotes cell proliferation in the early stages of repair, when increased VICs are required.

Targeting integrins in hepatocellular carcinoma

INTRODUCTION

Integrins, which are heterodimeric membrane glycoproteins, consist of a family of cell-surface receptors mediating cell-matrix and cell-cell adhesion. Analysis of tumor-associated integrins has revealed an important relationship between integrins and tumor development, bringing new insights into integrin-based cancer therapies. Hepatocellular carcinoma (HCC) is one of the most malignant tumors worldwide and integrins appeal to be a novel group of potential therapeutic targets for HCC.

AREAS COVERED

This review summarizes the current knowledge of integrins involved in HCC and the potential of integrin-targeted drugs in HCC therapy. A brief introduction on the structure, biological function and regulatory mechanism of integrins is given. The distinct expression patterns and biological functions of HCC-associated integrins are described. Finally, the current situation of integrin-based therapies in HCC and other tumor types are extensively discussed in the light of their implications in preclinical and clinical trials.

EXPERT OPINION

To date, increasing numbers of integrin-targeted drugs are undergoing development and they exhibit diverse effects in cancer clinical trials. Tumor heterogeneity should be emphasized in developing effective integrin-targeted drugs specific for HCC. A better understanding of how integrins cooperatively function in HCC will assist in designing more successful integrin-targeted therapeutic drugs and corresponding approaches.

Src is a major signaling component for CTGF induction by TGF-beta1 in osteoblasts

Connective tissue growth factor (CTGF/CCN2) is induced by transforming growth factor beta1 (TGF-beta1) where it acts as a downstream mediator of TGF-beta1 induced matrix production in osteoblasts. We have shown the requirement of Src, Erk, and Smad signaling for CTGF induction by TGF-beta1 in osteoblasts; however, the potential interaction among these signaling pathways remains undetermined. In this study we demonstrate that TGF-beta1 activates Src kinase in ROS17/2.8 cells and that treatment with the Src family kinase inhibitor PP2 prevents Src activation and CTGF induction by TGF-beta1. Additionally, inhibiting Src activation prevented Erk activation, Smads 2 and 3 activation and nuclear translocation by TGF-beta1, demonstrating that Src is an essential upstream signaling partner of both Erk and Smads in osteoblasts. MAPKs such as Erk can modulate the Smad pathway directly by mediating the phosphorylation of Smads or indirectly through activation/inactivation of required nuclear co-activators that mediate Smad DNA binding. When we treated cells with the Erk inhibitor, PD98059, it inhibited TGF-beta1-induced CTGF protein expression but had no effect on Src activation, Smad activation or Smad nuclear translocation. However PD98059 impaired transcriptional complex formation on the Smad binding element (SBE) of the CTGF promoter, demonstrating that Erk activation was required for SBE transactivation. These data demonstrate that Src is an essential upstream signaling transducer of Erk and Smad signaling with respect to TGF-beta1 in osteoblasts and that Smads and Erk function independently but are both essential for forming a transcriptionally active complex on the CTGF promoter in osteoblasts.

TMPRSS4 induces invasion and epithelial-mesenchymal transition through upregulation of integrin alpha5 and its signaling pathways

TMPRSS4 is a novel type II transmembrane serine protease that is highly expressed on the cell surface in pancreatic, thyroid and other cancer tissues, although its oncogenic significance and molecular mechanisms are unknown. Previously, we have shown that TMPRSS4 promotes invasion, migration and metastasis of human tumor cells by facilitating an epithelial-mesenchymal transition (EMT). In this study, we explored the molecular basis underlying TMPRSS4-mediated effects. We show that multiple downstream signaling pathways, including focal adhesion kinase (FAK), extracellular signal-regulated kinase (ERK), Akt, Src and Rac1, are activated by TMPRSS4 expression and that FAK signaling and ERK activation are required for TMPRSS4-induced invasiveness and EMT, including cadherin switch. Inhibition of PI3K or Src reduced invasiveness and actin rearrangement mediated by TMPRSS4 without restoring E-cadherin expression. Downregulation of E-cadherin was required for TMPRSS4-mediated effects but was not sufficient to induce EMT and invasion. TMPRSS4 induced integrin alpha5 expression and its signal transduction, leading to invasiveness and EMT accompanied by downregulation of E-cadherin. Functional blocking confirmed that integrin alpha5beta1 is a critical signaling molecule that is sufficient to induce TMPRSS4-mediated effects. Immunohistochemical analysis showed that TMPRSS4 expression was significantly higher in human colorectal cancer tissues from advanced stages than in that of early stage. Furthermore, upregulation of TMPRSS4 was correlated with enhanced integrin alpha5 expression. These observations implicate integrin alpha5 upregulation as a molecular mechanism by which TMPRSS4 induces invasion and contributes to cancer progression.

Creating a spectrum of fibrocartilages through different cell sources and biochemical stimuli

In this study a scaffoldless approach was employed with two different cell sources and biochemical stimuli to engineer a spectrum of fibrocartilages representative of the different regions of the knee meniscus. Constructs composed of 100% fibrochondrocytes (FC) or a 50:50 co-culture of fibrochondrocytes and chondrocytes (CC) were cultured in 10% fetal bovine serum medium or serum-free "chondrogenic" medium, each +/-10 ng/mL TGF-beta1 (+T). Constructs from these two cell groups and four culture conditions were cultured for 6 weeks. By varying the cell type and presence of the growth factor, GAG per dry weight of the constructs spanned that of native tissue, ranging 16-45% and 1-7% in the CC and FC groups, respectively. Collagen density was most dependent on cell type and was significantly lower than tissue values. The collagen I/II ratio could be manipulated by cell type and serum presence to span the native range, from 3.5 in the serum-free CC group to over 1,000 in the FC groups treated with serum-containing medium. Using the CC cell group in the presence of serum-free medium dramatically increased the compressive stiffness to 128 +/- 34 kPa, similar to native tissue. Similarly, serum-free medium or TGF-beta1 treatment enhanced the tensile modulus by an order of magnitude, up to 3,000 kPa. Using two cell sources and manipulating biochemical stimuli, a range of fibrocartilaginous neotissues was engineered. Fibrocartilages such as the knee meniscus are characterized by heterogeneity in matrix and functional properties, and this work demonstrates a strategy for recreating these heterogeneous tissues.

Tetraspanin TM4SF5 mediates loss of contact inhibition through epithelial-mesenchymal transition in human hepatocarcinoma

The growth of normal cells is arrested when they come in contact with each other, a process known as contact inhibition. Contact inhibition is lost during tumorigenesis, resulting in uncontrolled cell growth. Here, we investigated the role of the tetraspanin transmembrane 4 superfamily member 5 (TM4SF5) in contact inhibition and tumorigenesis. We found that TM4SF5 was overexpressed in human hepatocarcinoma tissue. TM4SF5 expression in clinical samples and in human hepatocellular carcinoma cell lines correlated with enhanced p27Kip1 expression and cytosolic stabilization as well as morphological elongation mediated by RhoA inactivation. These TM4SF5-mediated effects resulted in epithelial-mesenchymal transition (EMT) via loss of E-cadherin expression. The consequence of this was aberrant cell growth, as assessed by S-phase transition in confluent conditions, anchorage-independent growth, and tumor formation in nude mice. The TM4SF5-mediated effects were abolished by suppressing the expression of either TM4SF5 or cytosolic p27Kip1, as well as by reconstituting the expression of E-cadherin. Our observations have revealed a role for TM4SF5 in causing uncontrolled growth of human hepatocarcinoma cells through EMT.

O-GlcNAc modulation at Akt1 Ser473 correlates with apoptosis of murine pancreatic beta cells

O-GlcNAc transferase (OGT)-mediated modification of protein Ser/Thr residues with O-GlcNAc influences protein activity, similar to the effects of phosphorylation. The anti-apoptotic Akt1 is both activated by phosphorylation and modified with O-GlcNAc. However, the nature and significance of the Akt1 O-GlcNAc modification is unknown. The relationship of O-GlcNAc modification and phosphorylation at Akt1 Ser473 was examined with respect to apoptosis of murine beta-pancreatic cells. Glucosamine treatment induced apoptosis, which correlated with enhanced O-GlcNAc modification of Akt1 and concomitant reduction in Ser473 phosphorylation. Pharmacological inhibition of OGT or O-GlcNAcase revealed an inverse correlation between O-GlcNAc modification and Ser473 phosphorylation of Akt1. MALDI-TOF/TOF mass spectrometry analysis of Akt1 immunoprecipitates from glucosamine-treated cells, but not untreated controls, showed a peptide containing S473/T479 that was presumably modified with O-GlcNAc. Furthermore, in vitro O-GlcNAc-modification analysis of wildtype and mutant Akt1 revealed that S473 was targeted by recombinant OGT. A S473A Akt1 mutant demonstrated reduced basal and glucosamine-induced Akt1 O-GlcNAc modification compared with wildtype Akt1. Furthermore, wildtype Akt1, but not the S473A mutant, appeared to be associated with OGT following glucosamine treatment. Together, these observations suggest that Akt1 Ser473 may undergo both phosphorylation and O-GlcNAc modification, and the balance between these may regulate murine beta-pancreatic cell fate.

Regulation of TM4SF5-mediated tumorigenesis through induction of cell detachment and death by tiarellic acid

mRNA for four-transmembrane L6 family member 5 (TM4SF5), a homolog of tumor antigen L6, was previously shown to be highly expressed in diverse tumors. We recently found that human hepatocarcinoma tissues also overexpressed TM4SF5 protein, in comparison to normal liver tissues. We also found that tiarellic acid (TA) caused cell detachment-related apoptosis in cells expressing endogenous or stably-overexpressing TM4SF5. When cells expressing TM4SF5 were treated with TA, we observed reduced phosphorylation of focal adhesion kinase, paxillin, and p130Cas, but not c-Src. TA treatment also caused focal adhesion loss and reduced cell adhesion, and increased the numbers of floating cells and apoptotic cells. These effects were blocked by overexpression of focal adhesion molecules, suggesting that treatment with TA mediates anoikis of TM4SF5-expressing cells. However, TM4SF5-null cells were not affected by TA, indicating that these effects occur specifically in TM4SF5-positive cells. TA administration reduced tumor formation in nude mice injected with TM4SF5-expressing cells, presumably through increased apoptosis in TM4SF5-positive tumors. These observations indicate that TM4SF5-positive tumorigenesis can be inhibited by TA via induction of cell detachment-related apoptosis, and suggest that TA may be developed as a putative therapeutic reagent against TM4SF5-positive tumorigenesis.

Endosomal NADPH oxidase regulates c-Src activation following hypoxia/reoxygenation injury

c-Src has been shown to activate NF-kappaB (nuclear factor kappaB) following H/R (hypoxia/reoxygenation) by acting as a redox-dependent IkappaBalpha (inhibitory kappaB) tyrosine kinase. In the present study, we have investigated the redox-dependent mechanism of c-Src activation following H/R injury and found that ROS (reactive oxygen species) generated by endosomal Noxs (NADPH oxidases) are critical for this process. Endocytosis following H/R was required for the activation of endosomal Noxs, c-Src activation, and the ability of c-Src to tyrosine-phosphorylate IkappaBalpha. Quenching intra-endosomal ROS during reoxygenation inhibited c-Src activation without affecting c-Src recruitment from the plasma membrane to endosomes. However, siRNA (small interfering RNA)-mediated knockdown of Rac1 prevented c-Src recruitment into the endosomal compartment following H/R. Given that Rac1 is a known activator of Nox1 and Nox2, we investigated whether these two proteins were required for c-Src activation in Nox-deficient primary fibroblasts. Findings from these studies suggest that both Nox1 and Nox2 participate in the initial redox activation of c-Src following H/R. In summary, our results suggest that Rac1-dependent Noxs play a critical role in activating c-Src following H/R injury. This signalling pathway may be a useful therapeutic target for ischaemia/reperfusion-related diseases.

Targeting alpha-fetoprotein represses the proliferation of hepatoma cells via regulation of the cell cycle

BACKGROUND

AFP is a biomarker for primary liver cancer, yet little is known about its effect in the pathogenesis of hepatoma. We examined how AFP modulates the proliferation of hepatoma cells.

METHODS

Recombinant adenovirus expressing siRNA against AFP was created. The repression of cell proliferation in vitro and growth of hepatoma in vivo were examined by colony formation assay and tumor xenograft in SCID mice, respectively. Cell cycle was assayed by flow cytometry. Expression profile was determined by microarrays.

RESULTS

siRNA targeting reduced expression of AFP specifically and markedly inhibited the proliferation of hepatoma cells. Local treatment using Adv-AFPsiRNA caused significant repression of the growth of hepatoma derived HepG2 cells in xenograft in SCID mice. Knockdown of AFP resulted in an obvious delay in the G(1)/S transition of cell cycle, but did not affect apoptosis in HepG2 cells. Some genes related to the cell cycle, including SKP2, Cyclin D1, Csk and EBAG9 were identified.

CONCLUSIONS

The endogenous AFP is a critical determinant of the growth of hepatoma cells, which functions by regulating the cell cycle. This study suggests that targeting of AFP with siRNA could be a potential therapeutic approach for hepatoma.

Cell adhesion-dependent cofilin serine 3 phosphorylation by the integrin-linked kinase.c-Src complex

Integrin-linked kinase (ILK) is involved in signal transduction by integrin-mediated cell adhesion that leads to dynamic actin reorganization. Actin (de)polymerization is regulated by cofilin, the Ser(3) phosphorylation (pS(3)cofilin) of which inhibits its actin-severing activity. To determine how ILK regulates pS(3)cofilin, we examined the effects of ILK on pS(3)cofilin using normal RIE1 cells. Compared with suspended cells, fibronectin-adherent cells showed enhanced pS(3)cofilin, depending on ILK expression and c-Src activity. The ILK-mediated pS(3)cofilin in RIE1 cells did not involve Rho-associated kinase, LIM kinase, or testicular protein kinases, which are known to be upstream of cofilin. The kinase domain of ILK, including proline-rich regions, appeared to interact physically with the Src homology 3 domain of c-Src. In vitro kinase assay revealed that ILK immunoprecipitates phosphorylated the recombinant glutathione S-transferase-cofilin, which was abolished by c-Src inhibition. Interestingly, epidermal growth factor treatment abolished the ILK effects, indicating that the linkage from ILK to cofilin is biologically responsive to extracellular cues. Altogether, this study provides evidence for a new signaling connection from ILK to cofilin for dynamic actin polymerization during cell adhesion, depending on the activity of ILK-associated c-Src.

TGF-beta-regulated collagen type I accumulation: role of Src-based signals

Transforming growth factor-beta (TGF-beta) stimulates myofibroblast transdifferentiation, leading to type I collagen accumulation and fibrosis. We investigated the function of Src in TGF-beta-induced collagen I accumulation. In human mesangial cells, PTyr416 Src (activated Src) was 3.3-fold higher in TGF-beta-treated cells than in controls. Src activation by TGF-beta was blocked by rottlerin and by a dominant negative mutant of protein kinase Cdelta (PKCdelta), showing that TGF-beta activates Src by a PKCdelta-based mechanism. Pharmacological inhibitors and a dominant negative Src mutant prevented the increase in collagen type I secretion in cells exposed to TGF-beta. Similarly, on-target Src small interference RNA (siRNA) prevented type I collagen secretion in response to TGF-beta, but off-target siRNA complexes had no effect. It is well established in mesangial cells that upregulation of type I collagen by TGF-beta requires extracellular signal-regulated kinase 1/2 (ERK1/2), and we found that activation of ERK1/2 by TGF-beta requires Src. In conclusion, these results suggest that stimulation of collagen type I secretion by TGF-beta requires a PKCdelta-Src-ERK1/2 signaling motif.

Inhibition of in vitro chondrogenesis in RGD-modified three-dimensional alginate gels

The goal of this study was to investigate the effects of adhesion to the arginine-glycine-aspartic acid (RGD) sequence on the chondrogenesis of bone marrow stromal cells (BMSCs). Synthetic RGE- and RGD-containing peptides were conjugated to sodium alginate, and bovine BMSCs were seeded onto 2D alginate surfaces or encapsulated in 3D gels. BMSCs spread specifically on RGD-modified surfaces, and spreading was inhibited by a soluble RGD peptide and by anti-beta1 and anti-alpha(v)beta3 integrin blocking antibodies. After 7 days in 3D gel culture, the chondrogenic supplements (TGF-beta1 and dexamethasone) significantly stimulated chondrocytic gene expression (collagen II, aggrecan, and Sox-9) and matrix accumulation (collagen II and sGAG) in RGE-modified gels, but this response was inhibited in the RGD-modified gels. Inhibition of sGAG synthesis increased with increasing RGD density, and synthesis was partially rescued by adding a soluble RGD peptide. Addition of an anti-alpha(v)beta3 integrin blocking antibody had no effect on chondrogenesis, while an anti-alpha5 antibody reduced sGAG accumulation. Overall, this study demonstrates that interaction with the RGD motif significantly inhibits the initial chondrogenesis of BMSCs within 3D alginate gels. These results provide new insights into the role of cell-matrix interactions in regulating chondrogenesis and highlight the importance of choosing appropriate biomaterials for tissue engineering therapies.

Integrin signaling and cell spreading mediated by phorbol 12-myristate 13-acetate treatment

Spreading of SNU16mAd gastric carcinoma cells was previously shown to be regulated via a signaling network from transforming growth factor beta1 (TGFbeta1) to integrins signaling, through a mediation of protein kinase C delta (PKCdelta). However, in the previous study, the roles of PKCdelta appeared complicated. In this study to clarify the roles of PKCdelta in the spreading of the gastric carcinoma cells, we questioned if PKC activation via phorbol 12-myristate 13-acetate (PMA) treatment could mimic the TGFbeta1 effects. An acute PMA treatment increased phosphorylations of focal adhesion (FA) kinase, paxillin, c-Src, and cofilin, just as TGFbeta1 did. Furthermore, cell spreading mediated by TGFbeta1- or acute PMA treatment correlated with activation of RhoA, which regulates actin reorganization and FA formation. However, stress fiber formation was prominent in TGFbeta1-treated cells, compared to cortical actin organization in PMA-treated cells. Altogether, these observations indicate that acute PMA treatment could mimic the TGFbeta1 mechanisms for cell spreading through subtly different effects on actin reorganization.

Transforming growth factor-β1-induced endothelial barrier dysfunction involves Smad2-dependent p38 activation and subsequent RhoA activation

Lung edema due to increased vascular permeability is a hallmark of acute lung injury and acute respiratory distress syndrome. Both p38 and RhoA signaling events are involved in transforming growth factor (TGF)-β1-increased endothelial permeability; however, the mechanism by which these pathways cooperate is not clear. In this study, we hypothesized that TGF-β1-induced changes in endothelial monolayer permeability and in p38 and RhoA activation are dependent on Smad2 signaling. We assessed the role of Smad2 in p38 activation and the role of p38 in RhoA activation by TGF-β1. We found that TGF-β1caused Smad2 phosphorylation between 0.5 and 1 h of exposure in endothelial cells. Knockdown of Smad2 protein prevented TGF-β1-induced p38 activation and endothelial barrier dysfunction. Furthermore, TGF-β1-enhanced RhoA activation was dependent on p38 activation. Inhibition of the RhoA-Rho kinase signaling pathway blunted TGF-β1-induced adherens junction disruption and focal adhesion complex formation. In addition, depletion of heat shock protein 27, a downstream signaling molecule of p38, did not prevent TGF-β1-induced endothelial barrier dysfunction. Finally, inhibition of de novo protein expression blunted TGF-β1-induced RhoA activation and endothelial barrier dysfunction. Our data indicate that TGF-β1induces endothelial barrier dysfunction involving Smad2-dependent p38 activation, resulting in RhoA activation by possible transcriptional regulation.

Role of c-Src in Human MCF7 Breast Cancer Cell Tumorigenesis

To study the role of c-Src in breast cancer tumorigenesis, we generated a cell line derived from MCF7 carrying an inducible dominant negative c-Src (c-SrcDN: K295M/Y527F) under tetracycline control (Tet-On system). c-SrcDN expression caused phenotypic changes, relocation of c-Src, Fak, and paxillin, and loss of correct actin fiber assembly. These alterations were coupled to increased Fak-Tyr(397) autophosphorylation and to inhibition of Fak-Tyr(925), p130(CAS), and paxillin phosphorylation. An increased association of total Src with Fak and a decreased interaction of p130(CAS) and p85-PI3K with Fak were also observed. SrcDN inhibited cell attachment, spreading, and migration. Serum and EGF-induced stimulation of cell proliferation and Akt phosphorylation were also significantly reduced by SrcDN, whereas p27(Kip1) expression was increased. Consistently, silencing c-Src expression by siRNA in MCF7 cells significantly reduced cell migration, attachment, spreading and proliferation. Inoculation of MCF7 cells carrying inducible SrcDN to nude mice generated tumors. However, doxycycline administration to mice significantly reduced tumorigenesis, and when doxycycline treatment was installed after tumor development, a significant tumor regression was observed. In both situations, inhibition of tumorigenesis was associated with decreased Ki67 staining and increased apoptosis in tumors. These data undoubtedly demonstrate the relevance of the Src/Fak complex in breast cancer tumorigenesis.

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.