pp60c-src activation in human colon carcinoma

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

BACKGROUND & AIMS

Src activation is reported as an early event found in preneoplastic colonic adenomas and in 70% of colon carcinomas. The aim of this study was to identify the biological consequences of c-src overexpression in rat colon epithelial cells.

METHODS

Introduction and overexpression of c-src in an immortalized rat colon epithelial cell line was achieved using lipofection. Transfectants were tested for changes in growth and cell behavior using different in vitro assay systems.

RESULTS

Colon epithelial cells overexpressing c-src showed the ability to form microcolonies in soft agar without acquiring tumorigenic potential. In in vitro assays, c-src transfectants displayed a gain of invasive potential through Matrigel without an accompanying change in migrational ability. No discernible qualitative changes were observed in the phosphotyrosyl protein profile between c-src and v-src transfectants. Assessment of the cadherin/catenin status in these cells revealed an intact, functional complex with no detectable tyrosine phosphorylation of different components of the complex.

CONCLUSIONS

Overexpression of c-src in an immortalized rat colon epithelial cell line does not elicit full neoplastic transformation but enhances anchorage-independent growth and confers invasion capability. Increased invasion through Matrigel was not linked to inactivation of the cadherin complex in c-src transfectants.

Potent dipeptide inhibitors of the pp60c-src SH2 domain

The design, synthesis, and evaluation of dipeptide analogues as ligands for the pp60c-src SH2 domain are described. The critical binding interactions between Ac-Tyr-Glu-N(n-C5H11)2 (2) and the protein are established and form the basis for our structure-based drug design efforts. The effects of changes in both the C-terminal (11-27) and N-terminal (51-69) portions of the dipeptide are explored. Analogues with reduced overall charge (92-95) are also investigated. We demonstrate the feasibility of pairing structurally diverse subunits in a modest dipeptide framework with the goal of increasing the druglike attributes without sacrificing binding affinity.

pp60c-src activation in hepatocellular carcinoma of humans and LEC rats

For the related Src kinases, a close correlation exists between elevated tyrosine kinase activity and cell transformation. However, the involvement of pp60c-src in hepatocellular carcinoma (HCC) remains obscure. The aim of this study was to evaluate whether pp60c-src tyrosine kinase activity is elevated in HCC. We analyzed the kinase activity of pp60c-src in normal liver tissue, chronic hepatitis liver tissue, and tumorous and adjacent nontumorous portions of HCC tissue from patients and Long-Evans cinnamon (LEC) rats that are known to develop liver cancer spontaneously. The kinase activity of pp60c-src was rarely detected in the normal human liver tissue and chronic hepatitis liver tissue, but it was elevated in tumorous and nontumorous portions of HCC tissue. Furthermore, the kinase activity of pp60c-src was significantly elevated in tumorous tissues compared with nontumorous tissues. The kinase activity of pp60c-src was also higher in poorly differentiated HCC. In addition, the kinase activity of pp60c-src increased proportionately with the development of HCC of LEC rats. Our results suggest that activation of the protooncogene product pp60c-src may play an important role in the malignant transformation of hepatocytes in human and LEC rats, and that it may be closely related to the histopathological grading of human HCC.

Characterization of human epidermal growth factor receptor and c-Src interactions in human breast tumor cells

In C3H/10T1/2 murine fibroblasts, overexpression of both c-Src and the human epidermal growth factor (EGF) receptor 1 (HER1) is required for detection of stable complexes between the two molecules and results in hyperactivation of the receptor and synergistic increases in tumor formation in nude mice, as compared with cells that overexpress only one of the pair. Elevated levels or activities of c-Src and HER1 also occur in a subset of later-stage breast cancers, suggesting that interactions between these two molecules could contribute to a more aggressive clinical course. To determine whether stable complexes between c-Src and HER1 occur in human breast cancers under the same conditions as in murine fibroblasts and whether the appearance of such complexes correlates with enhanced signaling through the EGF receptor and increased tumor growth, human breast tumor cell lines and tumor tissues were analyzed for a number of c-Src/HER1-mediated signaling events and tumorigenicity. In a panel of 14 cell lines, 10 overexpressed c-Src, and of these, five contained elevated levels of HER1 and exhibited an EGF-dependent association between HER1 and c-Src. This association was also present in a HER1/c-Src-overexpressing tumor sample from a breast cancer patient. Further analysis of signaling events revealed that phosphorylation of the HER1 substrate, Shc, and its downstream effector, mitogen-activated protein kinase, was increased in EGF-stimulated MDA-MB-468, MDA-MB-231, and BT-549 cells (which overexpress both c-Src and HER1) as compared with MCF7 and ZR-75-1 cells (which only overexpress c-Src). Furthermore, MDA-MB-468 and MDA-MB-231 cells displayed increased tumorigenicity in nude mice. These results support the hypothesis that c-Src/HER1 interactions contribute to tumor progression in certain late-stage breast tumor cells.

The integrity of the SH3 binding motif of AFAP-110 is required to facilitate tyrosine phosphorylation by, and stable complex formation with, Src

The actin filament-associated protein AFAP-110 forms a stable complex with activated variants of Src in chick embryo fibroblast cells. Stable complex formation requires the integrity of the Src SH2 and SH3 domains. In addition, AFAP-110 encodes two adjacent SH3 binding motifs and six candidate SH2 binding motifs. These data indicate that both SH2 and SH3 domains may work cooperatively to facilitate Src/AFAP-110 stable complex formation. As a test for this hypothesis, we sought to understand whether one or both SH3 binding motifs in AFAP-110 modulate interactions with the Src SH3 domain and if this interaction was required to present AFAP-110 for tyrosine phosphorylation by, and stable complex formation with, Src. A proline to alanine site-directed mutation in the amino terminal SH3 binding motif (SH3bm I) was sufficient to abrogate absorption of AFAP-110 with GST-SH3STC. Co-expression of activated Src (pp60(527F)) with AFAP-110 in Cos-1 cells permit tyrosine phosphorylation of AFAP-110 and stable complex formation with pp60(527F). However, co-expression of the SH3 null-binding mutant (AFAP71A) with pp60(527F) revealed a 2.7 fold decrease in steady-state levels of tyrosine phosphorylation, compared to AFAP-110. Although a lower but detectable level of AFAP71A was phosphorylated on tyrosine, AFAP71A could not be detected in stable complex with pp60(527F), unlike AFAP-110. These data indicate that SH3 interactions facilitate presentation of AFAP-110 for tyrosine phosphorylation and are also required for stable complex formation with pp60(527F).

Regulation of vascular endothelial growth factor expression in human colon carcinoma cells by activity of src kinase

BACKGROUND

The c-src protooncogene encodes a protein tyrosine kinase, pp60c-src, that is a mediator in many signal transduction pathways. One pathway in which pp60c-src protein tyrosine kinase activity is implicated involves regulation of vascular endothelial growth factor (VEGF), an angiogenic factor important to neovascularization of growing tumors. Recently we demonstrated that decreased activity of pp60c-src in colon tumor cells contributes to decreased expression of VEGF. This study examined the relationship between pp60c-src activation, cell density, and VEGF production in a colon tumor cell line.

METHODS

Parental HT-29 colon adenocarcinoma cells and stable subclones created by transfection with c-src antisense and sense (control) expression vectors were plated under sparse (2 x 10(4) cells/cm2) and confluent (20 x 10(4) cells/cm2) conditions and grown for 36 hours. Protein and RNA were extracted from cells to determine pp60c-src levels, c-Src tyrosine kinase activity, and VEGF mRNA expression.

RESULTS

The pp60c-src kinase activity of HT-29 cells and control sense-transfected clones grown under confluent conditions was increased threefold to fivefold compared with cells grown under sparse conditions. In contrast, the ability of confluent culture conditions to increase pp60c-src activity was blunted in antisense transfectants. By regression analysis, VEGF expression was found to vary directly with pp60c-src levels (r2 = 0.886).

CONCLUSIONS

Cell density contributes to the regulation of c-src kinase activity and VEGF expression in HT-29 cells. When the steady-state level of pp60c-src is reduced in antisense transfectants, not only is the steady-state level of VEGF reduced, but the ability of confluence to stimulate pp60c-src activity and VEGF production is too. These data suggest that c-src may be an intermediary of both constitutive and inducible pathways for VEGF production in colon tumor cells.

Molecular pathogenesis of colorectal cancer

The enormous progress made in the identification of genes that are involved in colon carcinogenesis has provided the foundation for further understanding the biology of both normal and cancer cells and for targeted therapeutic strategies. In one sense, the genes described in this review are only the building blocks of a larger puzzle that constitutes the integrated metabolic function of a cell. The current challenge is to understand the functional role of these genes in normal cellular physiology and make the connections between pathways that knit together integrated cellular homeostasis. A complete understanding of the regulatory pathways, and the synthesis and modifications of the proteins involved, will provide novel targets for therapeutic agents.

Adhesion of HT-29 colon carcinoma cells to E-selectin results in increased tyrosine phosphorylation and decreased activity of c-src

Adhesion of metastatic cancer cells at secondary sites is known to be regulated by several families of adhesion proteins, including selectins and integrins. Colon carcinoma cells have been shown to tether to and roll on both stimulated endothelial cells and purified E-selectin. We have demonstrated that HT-29 human colon carcinoma cells adhere specifically to an E-selectin-IgG chimera. Upon adhesion to E-selectin, the amount of tyrosine phosphorylation of several proteins in HT-29 cell lysates increases compared with cells in bovine serum albumin-coated wells on phosphotyrosine Western blots; this increase is statistically significant. This effect is specific for adhesion to E-selectin, since addition of an E-selectin blocking monoclonal antibody (MAb), E3, to the wells causes a statistically significant decrease in tyrosine phosphorylation relative to E-selectin alone on phosphotyrosine Western blots. One protein that is affected this way has been identified as c-src. Kinase assays show a dose-dependent and statistically significant decrease in c-src activity upon adhesion to E-selectin, which correlates with an increase in phosphorylation of Tyr 527, the negative regulatory tyrosine. CnBr digestion of 32P-labeled c-src shows an increase in phosphorylation of tyrosine 527 after adhesion to E-selectin. Our results may identify a signaling pathway involving the E-selectin ligand on HT-29 cells and c-src.

pp60v-src transformation of rat cells but not chicken cells strongly correlates with low-affinity phosphopeptide binding by the SH2 domain

Substrates critical for transformation by pp60v-src remain unknown, as does the precise role of the src homology 2 (SH2) domain in this process. To continue exploring the role of the SH2 domain in pp60v-src-mediated transformation, site-directed mutagenesis was used to create mutant v-src alleles predicted to encode proteins with overall structural integrity intact but with reduced ability to bind phosphotyrosine-containing peptides. Arginine-175, which makes critical contacts in the phosphotyrosine-binding pocket, was mutated to lysine or alanine. Unexpectedly, both mutations created v-src alleles that transform chicken cells with wild-type (wt) efficiency and are reduced for transformation of rat cells; these alleles are host dependent for transformation. Additionally, these alleles resulted in a round morphological transformation of chicken cells, unlike 12 of the 13 known host-dependent src SH2 mutations that result in a fusiform morphology. Analysis of phosphopeptide binding by the mutant SH2 domains reveal that the in vitro ability to bind phosphopeptides known to have a high affinity for wt src SH2 correlates with wt (round) morphological transformation in chicken cells and in vitro ability to bind phosphopeptides known to have a low affinity for wt src SH2 correlates with rat cell transformation. These results suggest that the search for critical substrates in rat cells should be among proteins that interact with pp60v-src with low affinity.

Overexpression of SAP-1, a transmembrane-type protein tyrosine phosphatase, in human colorectal cancers

SAP-1 is a human transmembrane-type protein tyrosine phosphatase that is abundant in colon and pancreatic cancer cell lines. The expression of SAP-1 in surgically excised human colorectal cancer specimens has now been investigated by immunohistochemical staining and in situ hybridization. Normal colon tissue or adenomas with mild dysplasia showed no detectable expression of SAP-1. In contrast, 2 of 17 adenomas with moderate or severe dysplasia and 19 of 48 (40%) adenocarcinomas expressed SAP-1, SAP-1 was localized predominantly to the apical surface of colonic cells in adenoma tissue showing moderate dysplasia, but it was also observed in the cytoplasm and at the basal surface of both adenoma cells showing severe dysplasia and adenocarcinoma cells. In situ hybridization with a SAP-1 antisense RNA probe detected SAP-1 mRNA in 6 of 9 colorectal cancers, but not in the surrounding mesenchymal tissue or normal mucosa. Sequencing of the K-RAS gene revealed that 10 of 15 (67%) SAP-1-positive cancers contained a mutation in codon 12. The serum concentration of carcinoembryonic antigen was normal in approximately half of the patients with SAP-1-positive colorectal cancers. These results suggest that SAP-1 is frequently overexpressed in human colorectal cancers and that such overexpression may occur relatively late in the adenoma-carcinoma sequence.

c-Src protein expression is increased in human breast cancer. An immunohistochemical and biochemical analysis

In human breast cancer, c-Src activity is elevated compared to normal breast tissue. It is not yet known whether this increase in c-Src activity is accompanied by an increase in c-Src protein expression. In this study, c-Src activity and protein expression were determined in a series of human breast cancers and in normal breast tissue, using immune complex kinase assays and immunoblotting. As the heterogeneity of breast cancer is not taken into account in these biochemical experiments, immunohistochemistry was also used to distinguish between normal and malignant cells. In human breast cancers, the c-Src activity is increased 4- to 30-fold, compared with normal breast tissue. This enhanced activity is accompanied by an increase in c-Src protein expression, as shown by both immunoblotting and immunohistochemistry. Immunohistochemistry indicates that the majority of c-Src appears to be concentrated around the nucleus in malignant cells, whereas in normal cells, it is distributed more evenly in the cytoplasm. These data confirm that c-Src activity is increased in human breast cancer. In addition, this study provides strong immunohistochemical evidence that the c-Src protein is also overexpressed, enabling a distinction to be made between normal and malignant cells.

The role of Src family kinases in the normal and neoplastic gastrointestinal tract

Src family kinases are a group of non-receptor tyrosine kinases that mediate signal transduction pathways involved in the growth and differentiation of normal tissues. Considerable evidence exists for a role of these proteins in neoplastic progression in various organ systems including the nervous, hematopoietic and skeletal systems. In addition, the role of the Src kinase family has been characterized for colon cancer, but only limited progress has been made in delineating the role of Src kinases in the normal gastrointestinal (GI) tract and extracolonic GI cancers. In this review, we provide an up-to-date assessment of the Src family kinases in the normal and neoplastic GI tract.

Augmented expression of LCK message directed from the downstream promoter in human colorectal cancer specimens

Lck protein is expressed in some colon carcinoma cell lines but its expression in colon cancer cells in vivo has not been clarified. LCK transcription is regulated from two distinct promoters and initiated exclusively from the downstream promoter in colon carcinoma cell lines in contrast to peripheral lymphocytes. We investigated the expression of the downstream promoter-initiated LCK transcript in 18 colorectal primary cancer and normal mucosae, and two hepatic metastases, using a RNase protection assay with the EcoRI-BglII fragment of human LCK cDNA, YT16. In normal tissues, only traces of the LCK transcript were detected. The expression of the LCK transcript was augmented in 3/18 cancer specimens. The relative level of the LCK transcript in the cancer tissue compared to the average value of normal adjacent tissue was 10-60 in 3 cases, and 3-10 in 7 cases. One hepatic metastasis expressed more LCK message than the primary lesion. Our results indicate that the LCK message is strongly expressed in some colorectal cancers.

Potentiation of epidermal growth factor receptor-mediated oncogenesis by c-Src: implications for the etiology of multiple human cancers

c-Src is a nontransforming tyrosine kinase that participates in signaling events mediated by a variety of polypeptide growth factor receptors, including the epidermal growth factor receptor (EGFR). Overexpression and continual ligand stimulation of the EGFR results in morphological transformation of cells in vitro and tumor development in vivo. Elevated levels of c-Src and the EGFR are found in a variety of human malignancies, raising the question of whether c-Src can functionally cooperate with the EGFR during tumorigenesis. To address this issue, we generated c-Src/EGFR double overexpressors and compared their proliferative and biochemical characteristics to those of single overexpressors and control cells. We found that in cells expressing high levels of receptor, c-Src potentiated DNA synthesis, growth in soft agar, and tumor formation in nude mice. Growth potentiation was associated with the formation of a heterocomplex between c-Src and activated EGFR, the appearance of a distinct tyrosyl phosphorylation on the receptor, and an enhancement of receptor substrate phosphorylation. These findings indicate that c-Src is capable of potentiating receptor-mediated tumorigenesis and suggest that synergism between c-Src and the EGFR may contribute to a more aggressive phenotype in multiple human tumors.

Increased mRNA expression of the receptor-like protein tyrosine phosphatase alpha in late stage colon carcinomas

The protein tyrosine phosphatase alpha (PTP alpha) mRNA level in paired samples of late stage (Dukes' D) colorectal tumors and adjacent normal colon mucosa was quantified by RNase protection assays. After normalization against 18S RNA or beta-actin mRNA level, a 2-10-fold increase in PTP alpha mRNA was detected in 10 of 14 tumors (approximately 70%) compared to mucosa. In situ hybridization of digoxigenin-labelled antisense PTP alpha RNA to tumor and mucosa sections produced a signal only in neoplastic cells of the tumor sample, consistent with the high increase in PTP alpha mRNA detected by RNase protection assays of some of the tumors. This is the first report suggesting an association of a protein tyrosine phosphatase with colorectal carcinoma. PTP alpha is a receptor-like PTP thought to be involved in regulating cell proliferation. Its oncogenic properties when overexpressed in cultured fibroblasts suggest that PTP alpha overexpression could contribute to the tumorigenic process in colon carcinoma.

Protein kinases in human breast cancer

The family of protein kinases includes many oncogenes and growth factor receptors, many of which have been linked to the pathogenesis and progression of cancer. Protein tyrosine kinases such as HER-2/c-erbB-2 and the epidermal growth factor receptor (EGFR) have been linked specifically to breast cancer, and perturbations of HER-2 affect response to chemotherapy. We have reviewed the biology of protein kinases in human breast cancer, as well as their translational applications to breast cancer patients. We have studied the spectrum of protein kinases expressed in human breast cancer cells and have identified four protein kinases with potentially important functions in breast cancer: rak (src-related), TK5 (which we now designate JAK3), the focal adhesion kinase (FAK), and STK1 (human M015/CAK). We describe the potential significance of these genes in breast cancer, as well as our methodology for identifying and characterizing novel genes in breast cancer.

Influence of protein tyrosine phosphorylation on the expression of the c-myc oncogene in cancer of the large bowel

We tested the potential impact of tyrosine phosphorylation on the expression of the c-myc gene in two colon cancer cell lines, HCT8 and SW837. We found that the protein tyrosine kinase inhibitor genistein causes a decrease in the abundance of c-myc RNA and an inhibition of proliferation with a similar dose response. Geldanamycin, a mechanistically different tyrosine kinase inhibitor, also causes a decrease in both the expression of c-myc RNA and proliferation. Genistein has also been found to inhibit topoisomerase II, but the topoisomerase II inhibitor novobiocin did not lower the expression of c-myc. The most likely interpretation is that inhibition of protein tyrosine kinase activity caused a decrease in c-myc expression in these cells. The impact of tyrosine phosphorylation on the expression of the c-myc gene is further supported by the finding that inhibition of phosphotyrosine phosphatase using orthovanadate causes an increase in the level of c-myc RNA. The effect of genistein on HCT8 cells is not dependent on the synthesis of new protein and does not involve an alteration in the stability of the message. Analysis of transcription in the c-myc gene reveals a more complicated picture with a decrease in initiation and an increase in elongation but no net change in transcription. We speculate that the genistein induced reduction in myc expression is the result of a posttranscriptional intranuclear event(s).

Src activity increases and Yes activity decreases during mitosis of human colon carcinoma cells

Src and Yes protein-tyrosine kinase activities are elevated in malignant and premalignant tumors of the colon. To determine whether Src activity is elevated throughout the human colon carcinoma cell cycle as it is in polyomavirus middle T antigen- or F527 Src-transformed cells, and whether Yes activity, which is lower than that of Src in the carcinoma cells, is regulated differently, we measured their activities in cycling cells. We observed that the activities of both kinases were higher throughout all phases of the HT-29 colon carcinoma cell cycle than in corresponding phases of the fibroblast cycle. In addition, during mitosis of HT-29 cells, Src specific activity increased two- to threefold more, while Yes activity and abundance decreased threefold. The decreased steady-state protein levels of Yes during mitosis appeared to be due to both decreased synthesis and increased degradation of the protein. Inhibition of tyrosine but not serine/threonine phosphatases abolished the mitotic activation of Src. Mitotic Src was phosphorylated at novel serine and threonine sites and dephosphorylated at Tyr-527. Two cellular proteins (p160 and p180) were phosphorylated on tyrosine only during mitosis. Tyrosine phosphorylation of several other proteins decreased during mitosis. Thus, Src in HT-29 colon carcinoma cells, similar to Src complexed to polyomavirus middle T antigen or activated by mutation at Tyr-527, is highly active in all phases of the cell cycle. Moreover, Src activity further increases during mitosis, whereas Yes activity and abundance decrease. Thus, Src and Yes appear to be regulated differently during mitosis of HT-29 colon carcinoma cells.

Direct tumorigenic conversion of human gallbladder carcinoma cells by v-src but not by activated c-H-ras oncogene

The roles of activated ras and src oncogene products in the acquisition of fully neoplastic phenotype by human gallbladder adenocarcinoma cells were investigated by co-transfecting non-tumorigenic HAG-I human gallbladder carcinoma cells with the pSV2neo plasmid and a plasmid carrying either activated c-H-ras or v-src oncogene. G418-resistant clones were isolated and assessed for the acquisition of anchorage-independent growth potential. Neither the 10 established clones transfected with pSV2neo alone nor the 17 clones transfected with activated c-H-ras, including 4 clones expressing the mutated p21H-ras protein, could form colonies in soft agar. By contrast, out of 10 clones transfected with v-src, 2 formed colonies in soft agar and produced tumors in athymic nude mice, the resulting progressive neoplasms being poorly differentiated adenocarcinomas. These tumorigenic clones were shown to have v-src DNA and mRNA levels with p60v-src protein, but there were no significant chromosomal alterations following tumorigenic conversion. Moreover, herbimycin A, a selective src-kinase inhibitor, markedly reduced clonogenic growth of these cells in soft agar rather than monolayer growth, suggesting that anchorage-independent growth of the v-src-transformed HAG-I cells might be driven directly by p60v-src kinase activity. Taken together, our data suggest that the fully neoplastic conversion of HAG-I cells depends on src-related tyrosine-kinase activity, but not solely on the function mediated by activated ras, thus providing evidence of an src-related signaling pathway for the acquisition of tumorigenic potential by human gallbladder adenocarcinoma cells.

Elevated c-yes tyrosine kinase activity in premalignant lesions of the colon

BACKGROUND/AIMS

The cellular oncogene c-yes and its viral homologue v-yes (the transforming gene of Yamaguchi 73 and Esh avian sarcoma viruses) encode 62-kilodalton, cytoplasmic, membrane-associated, protein-tyrosine kinases. For the related Src kinase, a close correlation exists between elevated kinase activity and cell transformation. Previously, we observed elevated Yes activity in many human colon carcinomas. Colonic neoplasia provides an opportunity to study tumor progression because most carcinomas arise from adenomas, which in turn arise from normal epithelia. The malignant potential of adenomas varies with size, histology, and degree of dysplasia. Large adenomas (> or = 2 cm) with villous architecture and severe dysplasia are most likely to develop carcinoma.

METHODS

To determine whether Yes is activated in premalignant lesions of the colon, we measured its in vitro protein-tyrosine kinase activity in 21 colonic adenomas from 17 patients.

RESULTS

Activity of Yes in adenomas at greatest risk for cancer was significantly greater (12- or 14-fold as measured by enolase or autophosphorylation, respectively) than activity in adjacent normal mucosa. Moreover, villous structure, large size (> or = 2 cm), or severe dysplasia correlated with elevated Yes activity.

CONCLUSIONS

The activity of Yes is elevated in adenomas that are at greatest risk for developing cancer.

Inhibition by two lavendustins of the tyrosine kinase activity of pp60F527 in vitro and in intact cells

The mutant pp60F527 protein possesses an activated protein-tyrosine kinase (PTK) activity correlated with a transforming activity. We have studied the inhibition of the pp60F527 PTK activity by two EGF-R tyrosine kinase inhibitors, lavendustin A and one of its derivatives, lavendustin C6. In vitro, both molecules were non-competitive inhibitors for the ATP binding site and uncompetitive inhibitors for the peptide binding site. The determined IC50S of the inhibition of pp60F527 kinase activity were 18 microM for lavendustin A and 5 microM for lavendustin C6, as determined on the exogenous substrate enolase, showing that lavendustin C6 was more potent than lavendustin A. Lavendustin C6, but not lavendustin A, inhibited the tyrosine phosphorylation of pp60F527 cellular substrates (the GAP-associated p190, pp125FAK and cortactin) in intact cells. However, this in situ inhibitory effect did not result in a reversion of the morphological changes induced by pp60F527 in cells. On the other hand, lavendustin C6 and lavendustin A exerted antiproliferative effects on cells, suggesting that inhibition of cellular targets related or not to the kinase was also possible.

Clinical and pathological characteristics of sporadic colorectal carcinomas with DNA replication errors in microsatellite sequences

DNA replication errors (RERs) in repeated nucleotide sequences due to defective mismatch repair genes have been reported in a subset of sporadic colorectal carcinomas and in the majority of tumors from patients with hereditary nonpolyposis colorectal cancer syndrome (HNPCC). We detected RER in 18 cases (13%) in a prospective series of 137 sporadic stage II and III (Dukes' B and C) colorectal carcinomas. The clinical and pathological features of the RER-positive cases differed from those without RER. The patients with RER-positive cancers tended to be somewhat younger (60 +/- 5 years, range 22-83, versus 66 +/- 1, range 27-90, P = 0.2 with unequal variances) and had a marked preponderance of tumors proximal to the splenic flexure (17/18, 94%, versus 41/119, 34%, P < 0.0001). Only two RER-positive patients (11%) had a family history of colorectal cancer. In comparison to the 41 RER-negative proximal colonic cancers, RER-positive cancers had more frequent exophytic growth (P = 0.04), large size (P = 0.03), poor differentiation (P = 0.0004), extracellular mucin production (P = 0.003) and Crohn's-like lymphoid reaction (P = 0.003), and a trend toward less frequent p53 gene product overexpression by immunohistochemistry (3/17, 18%, versus 18/41, 44%, P = 0.06). We conclude that a subset of sporadic colorectal carcinomas has unique biological features that may indicate inherited germline mutation, de novo germline mutation, or somatic mutations of the mismatch repair genes involved in HNPCC.

pp60v-src kinase overexpression leads to cellular resistance to the antiproliferative effects of tumor necrosis factor

While some tumor cells are sensitive to the antiproliferative effects of tumor necrosis factor (TNF), others are resistant. The molecular basis for cellular resistance to TNF is not completely understood. Previously we have shown that transfection of cells with an oncogene HER2/neu/erb B2, a receptor tyrosine kinase, leads to resistance to the anticellular effects of TNF [(1988) Proc. Natl. Acad. Sci. USA 85, 5102-5106]. In the present study, we demonstrate that the overexpression of another oncogenic tyrosine kinase, pp60v-src also induces resistance to TNF. In contrast to HER2, however, pp60v-src transfection of cells did not lead to down-modulation of TNF receptors but rather to decreased intracellular glutathione levels. The pp60v-src-induced cellular resistance to TNF could be abrogated by interferon-gamma. Thus, these results indicate that the resistance of certain tumors to TNF may also be due in part to the overexpression of pp60v-src oncogene.

Oncogenic activation of the Lck protein accompanies translocation of the LCK gene in the human HSB2 T-cell leukemia

The tyrosine protein kinase p56lck transduces signals important for antigen-induced T-cell activation. In transgenic mice, p56lck is oncogenic when overexpressed or expressed as a mutant, catalytically activated enzyme. In humans, the LCK gene is located at the breakpoint of the t(1;7)(p34;q34) chromosomal translocation. This translocation positions the beta T-cell receptor constant region enhancer upstream of the LCK gene without interrupting the LCK coding sequences, and a translocation of this sort occurs in both the HSB2 and the SUP-T-12 T-cell lines. We have found that, although the level of the p56lck protein in HSB2 cells is elevated approximately 2-fold in comparison with that in normal T-cell lines, total cellular tyrosine protein phosphorylation is elevated approximately 10-fold. Increased levels of phosphotyrosine in HSB2 cells resulted from mutations in the LCK gene that activated its function as a phosphotransferase and converted it into a dominant transforming oncogene. The oncogenic p56lck in HSB2 cells contained one amino acid substitution within the CD4/CD8-binding domain, two substitutions in the kinase domain, and an insertion of Gln-Lys-Pro (QKP) between the SH2 and kinase domains. In NIH 3T3 fibroblasts, three of these mutations cooperated to produce the fully oncogenic form of this p56lck variant. These results suggest that mutation of LCK may contribute to some human T-cell leukemias.

Oncogenic activation of the Lck protein accompanies translocation of the LCK gene in the human HSB2 T-cell leukemia

The tyrosine protein kinase p56lck transduces signals important for antigen-induced T-cell activation. In transgenic mice, p56lck is oncogenic when overexpressed or expressed as a mutant, catalytically activated enzyme. In humans, the LCK gene is located at the breakpoint of the t(1;7)(p34;q34) chromosomal translocation. This translocation positions the beta T-cell receptor constant region enhancer upstream of the LCK gene without interrupting the LCK coding sequences, and a translocation of this sort occurs in both the HSB2 and the SUP-T-12 T-cell lines. We have found that, although the level of the p56lck protein in HSB2 cells is elevated approximately 2-fold in comparison with that in normal T-cell lines, total cellular tyrosine protein phosphorylation is elevated approximately 10-fold. Increased levels of phosphotyrosine in HSB2 cells resulted from mutations in the LCK gene that activated its function as a phosphotransferase and converted it into a dominant transforming oncogene. The oncogenic p56lck in HSB2 cells contained one amino acid substitution within the CD4/CD8-binding domain, two substitutions in the kinase domain, and an insertion of Gln-Lys-Pro (QKP) between the SH2 and kinase domains. In NIH 3T3 fibroblasts, three of these mutations cooperated to produce the fully oncogenic form of this p56lck variant. These results suggest that mutation of LCK may contribute to some human T-cell leukemias.

Solid-phase synthesis of (tyrosyl-alanyl-glutamyl)n by segment condensation

(Tyr-Ala-Glu)n, n = 1-9, were synthesized by segment condensation using the Fmoc/tert-butyl protection strategy and solid-phase techniques. The C-terminal residue was coupled to the resin and the peptides were built out by adding Fmoc-Glu(O-t-Bu)-Tyr(t-Bu)-Ala-OH units. When the desired lengths were reached the peptides were capped with Fmoc-Tyr(t-Bu)-Ala-OH units. Fmoc-Tyr(t-Bu)-Ala-OH and Fmoc-Glu(O-t-Bu)-Tyr(t-Bu)-Ala-OH were synthesized in aqueous solution by the successive addition of N-hydroxysuccinimide esters of Fmoc-Tyr(t-Bu) and Fmoc-Glu(O-t-Bu) to the growing chain. Neither sequential amino acid addition or segment condensation techniques were successful on polystyrene supports. However, the segment condensations were highly successful on kieselguhr-supported polydimethylacrylamide based resins. (Tyr-Ala-Glu)n, n = 1-9, were tested as inhibitors of the protein tyrosine kinase, pp60c-src. Inhibition, as measured by IC50 values, increased with increasing size of the peptide.

Elevated c-Src tyrosine kinase activity in premalignant epithelia of ulcerative colitis

Ulcerative colitis (UC) is a chronic inflammatory disease of the colon with a high incidence of colon cancer. Dysplasia is a precursor to carcinoma and a predictor of malignant potential; epithelia containing high-grade or severe dysplasia is most likely to develop cancer. The cellular oncogene c-src and its viral homologue v-src (the transforming gene of Rous sarcoma virus) encode 60-kD cytoplasmic, membrane-associated protein tyrosine kinases. For the viral protein or transforming mutants of the cellular protein (Src), a close correlation exists between elevated tyrosine kinase activity and malignant transformation of cells. Previously, we and others observed elevated Src activity in sporadic colon carcinomas and benign adenomas at greatest risk for developing cancer (those with large size, villous architecture, and/or severe dysplasia). Here we report that Src activity and protein abundance are also elevated in neoplastic UC epithelia. Activity is highest in malignant and severely dysplastic epithelia, and 6-10-fold higher in mildly dysplastic than in nondysplastic epithelia. Thus, Src activity is elevated in premalignant UC epithelia, which is at greatest risk for developing cancer. The data suggest that activation of the src proto-oncogene is an early event in the genesis of UC colon cancer.

Variable activity of protein tyrosine kinase in apparently normal thyroid glands

We examined protein tyrosine kinase (PTK) activity in apparently normal thyroid tissue (n = 22) obtained from patients undergoing thyroid surgery for papillary thyroid cancer or benign disease. The PTK activity in apparently normal thyroid tissue from patients with papillary thyroid cancer progressively was elevated in the cytosolic fraction compared with that from patients without cancer (p < 0.05). The cytosolic proportion of PTK activity was also significantly increased in the normal thyroid tissue of patients with cancer (62% vs. 51%, p < 0.05). These findings suggest that alterations in PTK activity may indicate apparently normal thyroid tissue at increased risk of developing malignancy.

Neoplastic progression of human and rat intestinal cell lines after transfer of the ras and polyoma middle T oncogenes

BACKGROUND

Activation of the p21ras and pp60c-src oncoproteins occurred at high incidence in the early stage of human colorectal carcinogenesis. Our study aimed to investigate the role of these signal-transduction pathways in the process of initiation and promotion of the malignant phenotype in intestinal cells.

METHODS

The human Ha-ras and the polyoma middle T (Py-MT) viral oncogenes were transferred into large T oncogene of simian virus 40 immortalized rat intestinal epithelial SLC-44 cells and human colonic adenocarcinoma Caco-2 cells.

RESULTS

These transfers conferred the tumorigenic and invasive phenotypes on immortalized SLC-44 cells and potentiated the tumorigenicity of Caco-2 cells and markedly repressed the terminal differentiation of this cell line. In SLC-44T cells, induction of the invasive phenotype by the activated Ha-ras oncogene correlated with weak expression of E-cadherin and reduced accumulation of the transcripts encoding the basement membrane components alpha 1 (IV) collagen, nidogen, and BM40, which might result partly from the inactivation of the transforming growth factor beta signaling pathway. The down-regulation of the alpha 1 (IV) collagen messenger RNA in SLC-44T cells was not due to the protein kinase C-dependent pathways or the secretion of autocrine factor(s).

CONCLUSIONS

These results suggest that the activation of the p21ras and Py-MT/pp60c-src oncogenic pathways are critical effectors at different stages of colorectal carcinogenesis and in Caco-2 cells interferes with the program of enterocyte differentiation.

Cyclic peptide substrates of pp60c-src. Synthesis and evaluation

To study the effects of constrained conformation and amino acid sequence on their kinetic parameters, a series of cyclic peptides were synthesized and each was tested as both a substrate and an inhibitor of pp60c-src, the product of the src proto-oncogene. The amino acid sequences were derived from Glu-Leu-Pro-Tyr-Ala-Gly and from the autophosphorylation site of pp60c-src (Ile-Glu-Asp-Asn-Glu-Tyr-Ala-Ala-Arg-Gln-Gly). Linear precursor peptides were synthesized by SPPS on aminomethylated polystyrene resin using the Fmoc-tert-butyl protection scheme with 4-hydroxymethyl-3-methoxyphenoxyacetic acid as the linkage agent. The peptides were cleaved from the support with 1% TFA in dichloromethane with the N-terminal Fmoc and the side-chain protecting groups in place. Removal of the Fmoc group with diethylamine and cyclization with BOP afforded cyclic peptides in 55-78% yield. Side-chain deprotection and further purification gave the final products in 25-48% yields based on their linear precursors. Based on the activities of the linear analogues, cyclization had little effect on the binding (Ki and Km) and rate of phosphorylation (Vmax) of cyclo(Glu-Leu-Pro-Tyr-Ala-Gly) and cyclo(Ile-Glu-Asp-Asn-Glu-Tyr-Ala-Ala-Arg-Gln). A series of cyclic decapeptides that contained the dipeptide D-Phe-Pro inserted in various positions in the autophosphorylation sequence showed marked differences in Ki, Km and Vmax. Compared to the well characterized linear substrate Val-5 angiotensin II, the D-Phe-Pro-containing cyclic peptides have higher Vmax values but differ little in Km, with values in the millimolar range.

Novel protein kinases expressed in human breast cancer

The family of protein kinases includes many oncogenes and growth-factor receptors, as well as genes that are involved in cell-cycle regulation. We have identified protein kinases expressed in a human breast-cancer cell line, 600PEI, and a primary human breast carcinoma, using PCR cloning techniques based on consensus sequences in the kinase domain. Twenty-five different protein kinases were isolated, including 3 novel putative tyrosine kinases (designated TK1, TK2, and TK5), and 2 novel putative cell-cycle-associated serine/threonine kinases (designated STK1 and STK2). TK1 is a new member of the src family of kinases that is expressed predominantly in epithelial cells. TK2 is homologous to the receptor kinase, HEK, and TK5 appears to be another member of the JAK family of kinases. The novel serine/threonine kinases, designated STK1 and STK2, were homologous to the human cdc2 and the Aspergillus nimA genes. We subsequently analyzed the levels of expression of all of these protein kinases in a panel of human breast carcinomas, using PCR-based methods. This analysis revealed different expression profiles in different primary breast carcinomas and, therefore, may determine new molecular sub-sets of human breast cancer.

A product of phosphatidylinositol-3 kinase is elevated in dividing HT29 colonic epithelial cells

Phosphatidylinositol 3 kinase (PI-3 kinase) activity has been linked to cell proliferation and growth regulation. Therefore, we studied changes in phosphoinositide metabolism during the cell cycle of HT-29 cells, a colonic epithelial cell line. HT29 cells were treated with the microtubule disrupter, nocodozole, separated into mitotic and quiescent populations and their phospholipid composition was analyzed. Radiolabelled phospholipids from cells labelled with 32PO4 or [3H]myoinositol were analyzed by TLC and/or deacylated and analyzed by HPLC. In all cases, levels of phosphatidylinositol 3 phosphate from mitotic phase cells were double that in resting populations. Therefore, levels of a product of PI-3 kinase are elevated and may play a role in cell division.

Protein tyrosine kinase in colorectal adenoma

To determine whether protein tyrosine kinase (PTK) could be utilized as a biological indicator of colonic tumorigenesis, we examined the PTK activity in the cytosolic and particulate fractions of homogenates from 25 adenomas (polyps) and from adjacent normal mucosa. The particulate fraction PTK activity in the colorectal adenomas was significantly higher than in the normal mucosa. The adenomas were then analyzed according to size, histological type (tubular and tubulovillous), and degree of dysplasia (mild, moderate and severe). The average particulate fraction PTK activity ratio (adenoma/normal mucosa) of tubulovillous adenomas or adenomas with moderate to severe dysplasia was significantly elevated in comparison with that of tubular adenomas or adenomas with mild dysplasia. The particulate PTK activity ratio increased significantly with increasing adenoma size, while the cytosolic ratio did not. The cytosolic PTK activity ratio in tubulovillous adenomas or adenomas with severe dysplasia decreased significantly with increasing size. These findings suggest that colonic carcinogenesis might be associated with alterations in the cellular level of PTK activity and that the PTK activity ratio (adenoma/normal mucosa) in the particulate and/or cytosolic fractions may possibly correlate with the risk of malignant transformation.

Gene structure and expression in colorectal cancer

Colorectal cancer provides a unique model for the study of molecular changes that are associated with tumorigenesis. The cancer evolves as an apparent ordered sequence from a benign to a malignant lesion in histopathological recognizable stages. Since it is relatively easy to isolate tissue representing each of these stages, studies of molecular events associated with tumor progression are feasible. Such studies have shown that multiple changes in gene structure, expression and activity occur during tumorigenesis.

Increase in activity and level of pp60c-src in progressive stages of human colorectal cancer

Activation of the tyrosine kinase of the c-src gene product, pp60c-src, has been shown to occur in nearly every primary colorectal carcinoma, and is found as early as in polyps of high malignant potential. However, no studies have addressed potential pp60c-src changes which occur during progression. To examine this question, we have studied kinase activity and protein levels in 7 colonic polyps, 19 primary lesions, and 19 liver metastases relative to normal colonic mucosa. Significant increases in tyrosine kinase activity were seen as early as in colonic polyps of high malignant potential. Further increases were observed in activity and level in primary tumors. However, the greatest increases in activity and protein levels were observed in liver metastases. Additionally, six metastatic lesions were obtained in which synchronous primary tumor was resected. In each of these liver metastases, pp60c-src activity and level were significantly increased relative to the corresponding primary tumor, as well as to normal colonic mucosa. Our results demonstrate that progression of colon primary tumors to liver metastases correlates with increased pp60c-src kinase activity and protein level.

Differential pp60c-src activity in well and poorly differentiated human colon carcinomas and cell lines

The results presented in this report demonstrate increased pp60c-src kinase activity associated with moderate to well differentiated colon tumors, corroborating previous observations by other groups. Extension of this analysis to include a small number of poorly differentiated colon carcinomas revealed src kinase activity comparable to that observed in normal colonic mucosa, considerably less than that observed in moderate/well differentiated lesions. Correlations of src kinase activity with differentiation was confirmed within a panel of colon cell lines where increased activity, associated with moderate/well differentiated lines, was accompanied by increased expression of pp60c-src protein. Use of an antiphosphotyrosine antibody in immunoprecipitation revealed the presence of novel phosphotyrosyl cellular substrates in human colon cell lines displaying elevated pp60c-src kinase activity. These observations suggest a role for the src protooncogene in colonic differentiation pathways.

Molecular genetics of colorectal carcinoma

The molecular genetics of colorectal carcinoma are among the best understood of any common human cancer. Reported molecular genetic abnormalities involve tumor-suppressor genes that undergo inactivation (e.g., apc, mcc, dcc, p53, and possibly genes on chromosomes 8p, 1p, and 22q) and dominant-acting oncogenes (e.g., ras, src, and myc). Multiple clonal genetic abnormalities accumulate during the development of colorectal carcinoma in adenomas. Altered DNA methylation is an early event, and the specific genetic alterations occur in a preferential order. However, the clinical application of molecular genetics in patients who are at risk for or have colorectal carcinoma is in its infancy. Patients with a predisposition to colorectal carcinoma caused by inheritance of familial adenomatous polyposis can be identified by genetic analysis of the apc gene on chromosome 5q21. In patients who undergo curative resection of colorectal cancer, deletion of the p53 gene on chromosome 17p, deletion of the dcc gene on 18q, and high fractional allelic loss (fraction of nonacrocentric autosomal arms with deletion) in the primary tumor appear to indicate an increased likelihood of occult disseminated disease and thus a poor prognosis. Additional studies are needed to establish the role of the molecular genetics of colorectal carcinoma in the management of patients who are at risk for or already have neoplasia of the large bowel.

The relationship of DNA aneuploidy to molecular genetic alterations in colorectal carcinoma

Altered total nuclear DNA content is frequent in colorectal carcinomas, but the mechanisms producing aneuploidy are unknown. Therefore, DNA ploidy by flow cytometry was correlated with molecular genetic alterations and tumor characteristics in 50 colorectal carcinomas. The prognostic value of these alterations was also evaluated because aneuploidy has been associated with poor prognosis. Thirty-nine of the carcinomas (78%) were DNA aneuploid. When compared with diploid carcinomas, aneuploid tumors had greater mean fractional allelic loss, defined as the fraction of evaluable nonacrocentric autosomal arms with deletion (0.25 +/- 0.15, range 0-0.667, vs. 0.12 +/- 0.10, range 0-0.345; P = 0.006). DNA index by flow cytometry correlated with fractional allelic loss (r = 0.38, P = 0.006). Aneuploid tumors also had more frequent allelic loss on chromosome 17p (87% vs. 55%; P = 0.017), but less frequent ras gene mutation (44% vs. 82%; P = 0.025). Among the 25 right-sided and 25 left-sided tumors, DNA diploid tumors were more frequent on the right side (P = 0.002), whereas deletion of 17p was found predominantly on the left side. Aneuploidy was associated with moderate and poor differentiation of the carcinomas but not with distant metastasis. By contrast, high fractional allelic loss, deletion of 17p, and deletion of 18q were associated with distant metastasis. In survival analysis of patients with Dukes' B or C carcinoma, DNA aneuploidy was not a significant discriminator, but patients whose tumor had deletion of 17p or deletion of both 17p and 18q had poorer survival (P = 0.045 and 0.022, respectively). The results suggest that DNA aneuploidy is associated with some of the molecular genetic alterations and phenotypic characteristics of colorectal carcinomas but is not a reliable indicator of metastatic potential.

Protein tyrosine kinases in nervous system development

Protein tyrosine kinases are important mediators of intracellular signaling during nervous system development. Activation of receptor protein tyrosine kinases by neurotrophic factors are initial events in the development of discrete cell populations. The patterns of expression and characterization of substrates for nonreceptor protein tyrosine kinases indicates that they also play a crucial role in neuronal development. The observed functional redundancy among protein tyrosine kinases and their associated intracellular signaling pathways underscores the need for further characterization of these novel interactions to elucidate the mechanisms regulating nervous system development.