Overexpression of pp60c-src elicits invasive behavior in rat colon epithelial cells

An mRNA-specific tRNAi carrier eIF2A plays a pivotal role in cell proliferation under stress conditions: stress-resistant translation of c-Src mRNA is mediated by eIF2A

c-Src, a non-receptor protein tyrosine kinase, activates NF-κB and STAT3, which in turn triggers the transcription of anti-apoptosis- and cell cycle-related genes. c-Src protein regulates cell proliferation, cell motility and programmed cell death. And the elevated level of activated c-Src protein is related with solid tumor generation. Translation of c-Src mRNA is directed by an IRES element which mediates persistent translation under stress conditions when translation of most mRNAs is inhibited by a phosphorylation of the alpha subunit of eIF2 carrying the initiator tRNA (tRNA_i) to 40S ribosomal subunit under normal conditions. The molecular basis of the stress-resistant translation of c-Src mRNA remained to be elucidated. Here, we report that eIF2A, an alternative tRNA_i carrier, is responsible for the stress-resistant translation of c-Src mRNA. eIF2A facilitates tRNA_i loading onto the 40S ribosomal subunit in a c-Src mRNA-dependent manner. And a direct interaction between eIF2A and a stem-loop structure (SL I) in the c-Src IRES is required for the c-Src IRES-dependent translation under stress conditions but not under normal conditions. Finally, we showed that the eIF2A-dependent translation of c-Src mRNA plays a pivotal role in cell proliferation under stress conditions.

Reduced surface expression of epithelial E-cadherin evoked by interferon-gamma is Fyn kinase-dependent

Interferon gamma (IFNγ) is an important regulatory cytokine that can exert a pro-inflammatory effect in the gut, where it has been shown to increase epithelial permeability via disruption of the tight junctions. Here we investigated the potential for IFNγ to regulate the adherens junction protein E-cadherin, an important mediator of normal epithelial tissue function, using the model T84 human colonic epithelial cell line. IFNγ (10 ng/ml) stimulated increased internalization of E-cadherin as assessed by immunofluorescence microscopy; internalization was reversed when cells were treated with PP1 (125 nM), a Src kinase-selective inhibitor. Immunoprecipitation studies demonstrated loss of E-cadherin from membrane fractions following IFNγ treatment and a corresponding increase in cytosolic E-cadherin and its binding partners, p120-catenin and beta-catenin: effects that were Src-kinase dependent. E-cadherin and p120-catenin phosphorylation was increased by IFNγ treatment and siRNA studies showed this was dependent upon the Src-kinase isoform Fyn. E-cadherin ubiquitinylation and subsequent proteasomal degradation stimulated by IFNγ was found to be dependent upon Fyn and the E-cadherin-selective ubiquitin ligase, Hakai. Use of Fyn and Hakai siRNA inhibited the internalization of E-cadherin as shown by immunoblotting and confocal fluorescence microscopy. Finally, IFNγ treatment resulted in a more fragile T84 cell monolayer with increased cell detachment in response to physical stress, which was prevented by PP1 and siRNA targeting Fyn or Hakai. Collectively, these results demonstrate a Fyn kinase-dependent mechanism through which IFNγ regulates E-cadherin stability and suggest a novel mechanism of disruption of epithelial cell contact, which could contribute to perturbed epithelial barrier function.

Phosphorylation of the SRC epithelial substrate Trask is tightly regulated in normal epithelia but widespread in many human epithelial cancers

PURPOSE

The frequently elevated activities of the c-src and c-yes products in human epithelial tumors suggest that these activated tyrosine kinases have tumorigenic functions analogous to the v-src and v-yes oncogene products. Studies of v-src-transformed fibroblasts have identified many of the effectors of this potent oncogene; however, because c-src and c-yes lack the mutational and promiscuous activities of their retroviral oncogene homologues, their presumptive tumorigenic functions in human epithelial tumors are more subtle, less well-defined, and await identification of possible effectors more directly relevant to epithelial cells.

EXPERIMENTAL DESIGN

We recently identified a transmembrane glycoprotein named Trask that is expressed in epithelial tissues but not fibroblasts and is phosphorylated by SRC kinases in mitotic epithelial cells. In this study, we have surveyed the expression and phosphorylation of Trask in many human epithelial cancer cell lines and surgical tissues and tumors.

RESULTS

Trask is widely expressed in human epithelial tissues, but its phosphorylation is tightly regulated and restricted to detached mitotic cells or cells undergoing physiologic shedding. However, abberant Trask phosphorylation is seen in many epithelial tumors from all stages including preinvasive, invasive, and metastatic tumors. Trask phosphorylation requires SRC kinases, and is also aberrantly hyperphosphorylated in the SRC-activated PyMT mouse epithelial tumors and dephosphorylated by the SRC inhibitor treatment of these tumors.

CONCLUSIONS

The widespread phosphorylation of Trask in many human epithlelial cancers identifies a new potential effector of SRC kinases in human epithelial tumorigenesis.

Src kinase induces tumor formation in the c-SRC C57BL/6 mouse

Src kinase has been linked as a causative agent in the progression of a number of cancers including colon, breast, lung and melanoma. Src protein and activity levels are increased in colorectal cancer and liver metastases arising secondary to colon cancer. However, although Src protein is increased in colon cancer as early as the adenomatous polyp stage, a role for Src in carcinogenesis has not been established. We developed the c-SRC transgenic mouse in the C57BL/6 strain to address the issue of carcinogenesis in cells with high levels of Src expression. The transgene was constructed with the human c-SRC gene downstream of the mouse metallothionein promoter to create zinc inducible gene expression. In these C57BL/6 mice, Src protein was increased in a number of tissues both with and without zinc induction. No additional carcinogenic agent was administered. After 20 months, mice were assessed for tumor development in the liver and GI tract, as well as other organs. Of the mice with the transgene, 15% developed tumors in the liver while no tumors were detected in wild type C57BL/6 mice. A further study was conducted by crossing c-SRC C57BL/6 mice with p21 nullizygous mice to determine the effect of oncogene expression combined with inactivation of the tumor suppressor gene, p21. Addition of the c-SRC transgene to the p21-/- background increased tumor formation almost 3-fold, while it increased metastasis 6-fold. The data from our study show, for the first time, that Src kinase may play a role in carcinogenesis.

Transcriptional profile of Rous Sarcoma Virus transformed chicken embryo fibroblasts reveals new signaling targets of viral-src

Transformation of chicken fibroblasts in vitro by Rous Sarcoma Virus represents a model of cancer in which a single oncogene, viral src, uniformly and rapidly transforms primary cells in culture. We experimentally surveyed the transcriptional program affected by Rous Sarcoma Virus (RSV) in primary culture of chicken embryo fibroblasts. As a control, we used cells infected with non-transforming RSV mutant td106, in which the src gene was deleted. Using Affymetrix GeneChip Chicken Genome Arrays, we report 811 genes that were modulated more than 2.5 fold in the virus transformed cells. Among these, 409 genes were induced and 402 genes were repressed by viral src. From the repertoire of modulated genes, we selected 20 genes that were robustly changed. We then validated and quantified the transcriptional changes of most of the 20 selected genes by real-time PCR. The set of strongly induced genes contains vasoactive intestinal polypeptide, MAP kinase phosphatase 2 and follistatin, among others. The set of strongly repressed genes contains TGF beta 3, TGF beta-induced gene, and deiodinase. The function of several robustly modulated genes sheds new light on the molecular mechanism of oncogenic transformation.

Elevated Src activity promotes cellular invasion and motility in tamoxifen resistant breast cancer cells

Src kinase plays a central role in growth factor signalling, regulating a diverse array of cellular functions including proliferation, migration and invasion. Recent studies have demonstrated that Src activity is frequently elevated in human tumours and correlates with disease stage. We have previously demonstrated that, upon acquisition of tamoxifen resistance, MCF7 cells display increased epidermal growth factor receptor (EGFR) activation and a more aggressive phenotype in vitro. Since tumours exhibiting elevated EGFR signalling may possess elevated levels of Src activity, we wished to investigate the role of Src in our MCF7 model of endocrine resistance. Src kinase activity was significantly elevated in tamoxifen-resistant (TamR) cells in comparison to wild type MCF7 cells. This increase was not due to elevated Src protein or gene expression. Treatment of TamR cells with the novel Src inhibitor, AZD0530, significantly reduced the amount of activated Src detectable in both cell types whilst having no effect on total Src levels. AZD0530 significantly suppressed the motile and invasive nature of TamR cells in vitro, reduced basal levels of activated focal adhesion kinase (FAK) and paxillin and promoted elongation of focal adhesions. Furthermore, the use of this compound in conjunction with the EGFR inhibitor, gefitinib, was markedly additive towards inhibition of TamR cell motility and invasion. These observations suggest that Src plays a pivotal role in mediating the motile and invasive phenotype observed in endocrine-resistant breast cancer cells. The use of Src inhibitors in conjunction with EGFR inhibitors such as gefitinib may provide an effective method with which to prevent cancer progression and metastasis.

Number 1Epithelial biology

The oral mucous membrane has features similar to skin but also differs in several ways. This paper reviews the aspects of epithelial biology necessary for an understanding of the vesiculoerosive disorders.

K+ channel inhibition accelerates intestinal epithelial cell wound healing

Restitution is the process by which superficial interruptions in the gastrointestinal mucosa are repaired by the flattening and spreading of epithelial cells surrounding the damage. During this process, mucosal epithelial cells undergo extensive reshaping and cytoskeletal remodeling. K(+) channels, located primarily on the basolateral surface of gut epithelial cells, are central to both actin polymerization, via their control of membrane potential, and cell volume regulation. We questioned whether K(+) channels are involved in restitution using an in vitro model of intestinal epithelium, monolayers of the human colon carcinoma cell line T84. We report that pharmacologic K(+) channel inhibition accelerates wound healing in T84 cell monolayers. Both Ca(++)-dependent and constitutively active channels are involved, as indicated by the sensitivity to clotrimazole, charybdotoxin, and barium. The ability of clotrimazole to accelerate wound resealing was also observed in Caco-2 cell sheets. Pharmacologic stimulation of K(+) channel activity had no effect on the repair rate. Analysis of the resealing process by time lapse and confocal microscopy revealed that K(+) channel inhibitors abolished the initial wound retraction, briefly accelerated the repair rate, and altered the shape of the cell sheet abutting the injury during the early phase of resealing. We hypothesize that K(+) channel inactivation interrupts the coregulation of f-actin polymerization and volume control that is initiated by the healing process.

Inhibition of SRC tyrosine kinase impairs inherent and acquired gemcitabine resistance in human pancreatic adenocarcinoma cells

PURPOSE

We tested the hypotheses that Src tyrosine kinase overactivity represents a chemoresistance mechanism and that Src inhibition may enhance gemcitabine cytotoxicity in pancreatic adenocarcinoma cells.

EXPERIMENTAL DESIGN

Pancreatic adenocarcinoma cells PANC1, MiaPaCa2, Capan2, BxPC3, and PANC1(GemRes), a stably gemcitabine-resistant subline of PANC1, were exposed to combinations of gemcitabine and Src tyrosine kinase inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2). Src expression, phosphorylation (Tyr-416), and activity were analyzed by immunoblotting and in vitro kinase assay. Expression of the M2 subunit of ribonucleotide reductase (RRM2), a putative chemoresistance enzyme, was quantified by Northern and Western blot. Cellular proliferation was quantified by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Apoptosis was characterized by YO-PRO-1/propidium iodide staining, fluorometric caspase profiling, and caspase inhibition (Z-Val-Ala-Asp-fluoromethyl ketone). The effects of constitutively active and dominant negative Src were determined. The therapeutic efficacy of PP2 in combination with gemcitabine was tested in nude mice orthotopically xenografted with PANC1(GemRes).

RESULTS

Greater gemcitabine resistance was associated with higher Src phosphorylation and activity, both of which were higher in PANC1(GemRes), relative to PANC1; total Src levels were alike. PANC1(GemRes) overexpressed RRM2. PP2 enhanced inherent gemcitabine chemosensitivity and attenuated gemcitabine resistance in PANC1(GemRes). Constitutively active Src increased gemcitabine chemoresistance; dominant negative Src impaired gemcitabine chemoresistance. PP2 augmented gemcitabine-induced caspase-mediated apoptosis, suppressed RRM2 expression, and decreased activity of the RRM2-regulating transcription factor E2F1 in PANC1(GemRes). PP2 and gemcitabine in combination substantially decreased tumor growth and inhibited metastasis in vivo.

CONCLUSIONS

Increased Src tyrosine kinase activity represents a potential chemoresistance mechanism and a promising therapeutic target warranting further investigation in gemcitabine-resistant pancreatic adenocarcinoma.

Regulation of cell motility by tyrosine phosphorylated villin

Temporal and spatial regulation of the actin cytoskeleton is vital for cell migration. Here, we show that an epithelial cell actin-binding protein, villin, plays a crucial role in this process. Overexpression of villin in doxycyline-regulated HeLa cells enhanced cell migration. Villin-induced cell migration was modestly augmented by growth factors. In contrast, tyrosine phosphorylation of villin and villin-induced cell migration was significantly inhibited by the src kinase inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) as well as by overexpression of a dominant negative mutant of c-src. These data suggest that phosphorylation of villin by c-src is involved in the actin cytoskeleton remodeling necessary for cell migration. We have previously shown that villin is tyrosine phosphorylated at four major sites. To further investigate the role of tyrosine phosphorylated villin in cell migration, we used phosphorylation site mutants (tyrosine to phenylalanine or tyrosine to glutamic acid) in HeLa cells. We determined that tyrosine phosphorylation at residues 60, 81, and 256 of human villin played an essential role in cell migration as well as in the reorganization of the actin cytoskeleton. Collectively, these studies define how biophysical events such as cell migration are actuated by biochemical signaling pathways involving tyrosine phosphorylation of actin binding proteins, in this case villin.

siRNA directed against c-Src enhances pancreatic adenocarcinoma cell gemcitabine chemosensitivity

BACKGROUND

The c-Src tyrosine kinase is a determinant of malignant cellular behavior in a variety of human cancers. We sought to determine the effect of suppressing c-Src expression on pancreatic adenocarcinoma chemosensitivity to gemcitabine.

STUDY DESIGN

PANC1, MIAPaCa2, BxPC3, and Capan2 pancreatic adenocarcinoma cell lines were studied. Expression of c-Src was determined by Western blot analysis. c-Src kinase activity was determined by in vitro kinase assay. RNA interference was used to suppress c-Src expression. Gemcitabine-induced cytotoxicity was determined by tetrazolium reduction assay and caspase profiling was performed. The effect of Src-specific siRNA on Akt activity was quantified.

RESULTS

Src expression and kinase activity in cell lines were directly correlated with gemcitabine chemoresistance. c-Src-specific siRNA suppressed c-Src expression and kinase activity. c-Src-specific siRNA increased gemcitabine-induced, caspase-mediated apoptosis. Akt activity was decreased by suppression of c-Src expression.

CONCLUSIONS

c-Src is a determinant of pancreatic adenocarcinoma chemoresistance and represents a potential target for therapeutic intervention.

Colon cancer cell adhesion in response to Src kinase activation and actin-cytoskeleton by non-laminar shear stress

Malignant cells shed from tumors during surgical resection or spontaneous metastasis experience physical forces such as shear stress and turbulence within the peritoneal cavity during irrigation, laparoscopic air insufflation, or surgical manipulation, and within the venous or lymphatic system. Since physical forces can activate intracellular signals that modulate the biology of various cell types in vitro, we hypothesized that shear stress and turbulence might increase colon cancer cell adhesion to extracellular matrix, potentiating metastatic implantation. Primary human malignant colon cancer cells isolated from resected tumors and SW620 were subjected to shear stress and turbulence by stirring cells in suspension at 600 rpm for 10 min. Shear stress for 10 min increased subsequent SW620 colon cancer cell adhesion by 40.0 +/- 3.0% (n = 3; P < 0.001) and primary cancer cells by 41.0 +/- 3.0% to collagen I when compared to control cells. In vitro kinase assay (1.5 +/- 0.13 fold) and Western analysis (1.34 +/- 0.04 fold) demonstrated a significant increase in Src kinase activity in cells exposed shear stress. Src kinase inhibitors PP1 (0.1 microM), PP2 (20 microM), and actin-cytoskeleton stabilizer phalloidin (10 microM) prevented the shear stress stimulated cell adhesion to collagen I. Furthermore, PP2 inhibited basal (50.0 +/- 2.8%) and prevented shear stress induced src activation but phalloidin pretreatment did not. These results raise the possibility that shear stress and turbulence may stimulate the adhesion of malignant cells shed from colon cancers by a mechanism that requires both actin-cytoskeletal reorganization an independent physical force activation of Src kinase. Blocking this pathway might reduce tumor metastasis during surgical resection.

Molecular regulation of urokinase-receptor gene expression as one potential concept for molecular staging and therapy

The urokinase-receptor (u-PAR) is a central molecule of invasion and metastasis promoting plasminogen-dependent extracellular matrix degradation in diverse carcinoma types such as gastric or colon cancer. Overexpression of u-PAR has been reported to occur mainly at the transcriptional level in malignant cells, and has been shown to indicate a poor clinical prognosis of cancer patients. This review will give an overview on experimental findings on u-PAR and its function, molecular mechanisms of its regulation, and its impact for future clinical decision planning and potential therapeutic concepts.

Epidermal growth factor stimulation can substitute for c-Src overexpression in promoting breast carcinoma invasion

BACKGROUND AND AIMS

Cancer progression is in large part dependent on the complex process of cell invasion, involving adhesion, motility, and enzymatic proteolysis. Overexpression of the Src proto-oncogene (c-Src), a nonreceptor tyrosine kinase, has been implicated in the progression of both colon and breast cancer. Our group has previously reported that overexpression of c-Src leads to a significant gain in invasive cell behavior in vitro. In this study, we sought to assess the relative importance of epidermal growth factor (EGF) stimulation and c-Src overexpression in conferring an invasive phenotype.

METHODS

Breast carcinoma cells and colon epithelial cells which naturally express low levels of c-Src were used for these studies. The cells were transfected so that they overexpressed c-Src; the mock-transfected parent lines were used as controls. Transfectants were tested for changes in invasion patterns after Src inhibition and EGF stimulation.

RESULTS

Invasion assays in both cell systems confirmed the importance of c-Src in determining invasive potential. A significant correlation was shown between c-Src kinase protein and cell invasion. Furthermore, Src inhibition significantly inhibited invasion in a dose-dependent manner. To clarify the relative contribution of EGF and c-Src to cell invasion, the ability of cells to invade through growth-factor-reduced matrigel, with or without EGF added, was compared to invasion through intact matrigel. The breast and colon cell lines behave quite differently in this regard. In the colon model, overexpressed c-Src is critical for cell invasion and stimulation with EGF is synergistic with c-Src overexpression. Conversely, the breast carcinoma cells transfected with c-Src were unable to invade without EGF stimulation and did not demonstrate the same synergistic relationship between c-Src and EGF. Instead, our results indicate that in BT474 breast carcinoma cells, EGF can substitute for c-Src in promoting breast cancer cell invasion.

CONCLUSIONS

Because most breast carcinomas overexpress c-Src, it behooves one to question the extent to which reducing the amount of EGF and consequent EGFR activity will decrease invasion. In this study, the effects of EGF on cell invasion were determined in light of a single alteration in c-Src expression. Our results show that EGF enhances the impact of c-Src overexpression on invasion. In breast cancer cells, EGF is capable of inducing invasion to the same extent as c-Src overexpression. This suggests that anti-EGFR therapies will be efficacious in retarding breast carcinoma invasion and metastasis.

Increased Bcl-xL expression mediates v-Src-induced resistance to anoikis in intestinal epithelial cells

Acquisition of resistance to anoikis (detachment-induced apoptosis) is considered to be a requirement for transformed intestinal epithelial cells to invade surrounding tissues and metastasize to distant organs. Increased Src kinase activity, which is a feature of a large proportion of colorectal cancers, has been identified as one of the factors that can contribute to anoikis resistance. However, the molecular mechanism by which high levels of Src activity contribute to anoikis resistance in intestinal epithelial cells is unknown. Here we show that high Src activity confers resistance to anoikis in intestinal epithelial cells, at least in part, by inducing Bcl-xL overexpression, and that this induction is mediated by the MEK/MAPK pathway. Based on the findings reported here, and on our previous study showing that Bcl-xL plays a critical role in ras-induced resistance to anoikis, we propose that the increased Bcl-xL levels found in colorectal cancers play a significant role in the induction of resistance to anoikis during the progression of this disease.

Activation of Src kinase in primary colorectal carcinoma: an indicator of poor clinical prognosis

BACKGROUND

The specific activity of the non-receptor protein tyrosine kinase, Src, is increased in the majority of colon and rectal adenocarcinomas compared to normal mucosa. However, the prognostic significance of this difference is unknown. The objective of the current study was to determine if Src activity is a marker for poor clinical prognosis in colon carcinoma patients. As Src activation leads to expression of urokinase/plasminogen activator receptor (u-PAR), expression of Src and u-PAR were correlated with patient survival.

METHODS

Tumors and adjacent normal colonic mucosae from 45 patients with colorectal carcinoma were screened for Src activity by the immune complex kinase assay. Expression of u-PAR was determined by enzyme linked immunoabsorbent assay. The primary tumor-to-normal mucosa ratios of activity were compared following classification and regression tree (CART) analysis to determine the prognostic significance of elevated specific Src activity. Expression of u-PAR was correlated with Src activity.

RESULTS

By CART analysis, Src activity in tumors elevated more than twofold over normal mucosa was significant. Increased Src activity significantly correlated with Dukes stage, pT and pN classification, and increased u-PAR levels (P < 0.001). Kaplan Meier analysis showed a significant association between elevated Src activity and shorter overall survival of all patients (P = 0.0004) and of Dukes Stage A-C patients (P = 0.0037). In patients who underwent curative resection, a significant correlation with a decreased disease-free survival rate was found (P < 0.0001). Multivariate analysis revealed that elevated Src activity was a prognostic parameter independent of M classification (P = 0.0125, relative risk 3.54, 95% confidence interval 1.31 - 9.76).

CONCLUSIONS

Src activity is an independent indicator of poor clinical prognosis in all stages of human colon carcinoma. These data suggest that Src-specific inhibitors may have a therapeutic role in inhibiting tumor progression and metastasis, and that measurement of Src activity may aid in selection of early stage patients for adjuvant therapy.

A possible role for the WNT-1 pathway in oral carcinogenesis

Reductions in cell-cell adhesion and stromal and vascular invasion are essential steps in the progression from localized malignancy to metastatic disease for all cancers. Proteins involved in intercellular adhesion, such as E-cadherin and catenin, probably play an important role in metastatic processes and cellular differentiation. While E-cadherin and beta-catenin expression has been extensively studied in many forms of human cancers, less is known about the role of the Wingless-Type-1 (WNT-1) pathway in human tumors. A large body of genetic and biochemical evidence has identified beta-catenin as a key downstream component of the WNT signaling pathway, and recent studies of colorectal tumors have shown a functional link among beta-catenin, adenomatous polyposis coli gene product (APC), and other components of the WNT-1 pathway. WNT-1 pathway signaling is thought to be mediated via interactions between beta-catenin and members of the LEF-1/TCF family of transcription factors. The WNT signal stabilizes beta-catenin protein and promotes its accumulation in the cytoplasm and nucleus. In the nucleus, beta-catenin associates with TCF to form a functional transcription factor which mediates the transactivation of target genes involved in the promotion of tumor progression, invasion, and metastasis, such as C-Myc, cyclin D1, c-jun, fra-1, and u-PAR. There is a strong correlation between the ability of the WNT-1 gene to induce beta-catenin accumulation and its transforming potential in vivo, suggesting that the WNT-1 gene activates an intracellular signaling pathway that can induce the morphological transformation of cells. For these reasons, data obtained from the study of the WNT-1 pathway could be important in our understanding of the mechanisms of epithelial tumors, in general, and probably also of oral squamous cell carcinoma, in particular.

Significant down-regulation of alpha-albumin in human hepatoma and its implication

The potential association of alpha-albumin (ALF) with hepatocellular carcinoma (HCC) was investigated. Expression of ALF was significantly reduced in HCC tumor tissue as compared with the paired peritumor tissue from 16 patients and in four HCC cell lines as compared with normal hepatocytes. ALF mRNA was also down-expressed in circulating HCC cells compared to circulating normal hepatocytes. The proliferation of Hep3B cells was inhibited by over-expression of ALF. Taken together, ALF is significantly down-regulated in HCC, and this might facilitate the proliferation of HCC. Thus, detection of ALF mRNA, in addition to that of alpha-fetoprotein (AFP) mRNA, might help to distinguish normal or malignant hepatocytes in peripheral blood.

Disruption of cell-substrate adhesion activates the protein tyrosine kinase pp60(c-src)

Treatment of confluent chicken embryo fibroblasts (CEFs) with trypsin results in a dose- and time-dependent increase in c-Src protein tyrosine kinase (PTK) activity. A similar, but less marked, increase in c-Src PTK activity occurs upon incubation of CEFs in calcium-free phosphate-buffered saline, which also causes a decrease in cell-substrate adhesion. The increase in c-Src PTK activity following disruption of cell-substrate adhesion correlates with a decrease in the phosphorylation of c-Src at the regulatory site, Tyr527. The phosphotyrosine phosphatase inhibitor phenylarsine oxide blocks the increase in c-Src PTK activity seen following treatment with trypsin and the morphological changes associated with the disruption of cell-substrate adhesion. In contrast, disruption of cell-substrate adhesion causes a decrease in FAK PTK activity that rapidly returns to control levels when the cells are plated on fibronection-coated dishes. Treatment of cells with cytochalasin D, which disrupts actin filaments but not cell-substrate adhesion, causes only a slight increase in c-Src PTK activity. Thus, these studies demonstrate a ligand-independent mechanism for the activation of c-Src that is consistent with its role in both cell adhesion and cell motility. Furthermore, these data suggest that similar to adhesion, loss of adhesion is not a passive process but can activate specific signaling pathways that may have significant effects on cellular function.

Adhesion-linked kinases in cancer; emphasis on src, focal adhesion kinase and PI 3-kinase

Our understanding of the complex signal transduction pathways involved in signalling within cancer cells, between cancer cells and between cancer cells and their environment has increased dramatically in recent years. Here we concentrate on three non-receptor kinases: Src, focal adhesion kinase (FAK) and phosphatidylinositol 3-kinase (PI 3-kinase). These form part of a complex network of intracellular signals which is thought to be important in regulating cancer cells.

Transcriptional induction of the urokinase receptor gene by a constitutively active Src. Requirement of an upstream motif (-152/-135) bound with Sp1

Since c-src overexpression increases colonic cell invasiveness and because both Src activity and urokinase receptor protein are elevated in invasive colon cancers, the present study was undertaken: 1) to determine if a constitutively active Src regulates urokinase receptor expression and 2) to identify required cis-elements and trans-acting factors. SW480 colon cancer cells transfected with an expression plasmid (c-srcY527F) encoding a constitutively active Src protein manifested increased urokinase receptor gene expression and Src activity. Treatment of the src transfectants with a Src-inhibitor (PD173955) reduced urokinase receptor protein levels and laminin degradation. Inasmuch as we recently implicated an upstream region of the urokinase receptor promoter (-152/-135) in constitutive urokinase receptor expression, we determined its role for the induction by src. Whereas the activity of a CAT reporter driven by this region was stimulated by c-srcY527F, the u-PAR promoter mutated at the Sp1-binding motif in the -152/-135 region was not. Nuclear extracts from the src transfectants demonstrated increased Sp1 binding to region -152/-135 compared with those from SW480 cells. Finally, endogenous urokinase receptor protein amounts in 10 colon cancers and corresponding normal colon correlated with Src specific activity. These data suggest that urokinase receptor gene expression is regulated by Src partly via increased Sp1 binding.