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Biomedical subjects

E T Liu

Publications and source records attributed to E T Liu.

At least 55 records · Page 3Linked to original sources
PubMed1996

Differential activation of the Ras/extracellular-signal-regulated protein kinase pathway is responsible for the biological consequences induced by the Axl receptor tyrosine kinase.

To understand the mechanism of Axl signaling, we have initiated studies to delineate downstream components in interleukin-3-dependent 32D cells by using a chimeric receptor containing the recombinant epidermal growth factor (EGF) receptor extracellular and transmembrane domains and the Axl kinase domain (EAK [for EGF receptor-Axl kinase]). We have previously shown that upon exogenous EGF stimulation, 32D-EAK cells are capable of proliferation in the absence of interleukin-3. With this system, we determined that EAK-induced cell survival and mitogenesis are dependent upon the Ras/extracellular-signal-regulated protein kinase (ERK) cascade. Although the phosphatidylinositol-3 kinase pathway is activated upon EAK signaling, it appears to be dispensable for the biological actions of the Axl kinase. Furthermore, we demonstrated that different threshold levels of Ras/ERK activation are needed to induce a block to apoptosis or proliferation in 32D cells. Recently, we have identified an Axl ligand, GAS6. Surprisingly, GAS6-stimulated 32D-Axl cells exhibited no blockage to apoptosis or mitogenic response which is correlated with the absence of Ras/ERK activation. Taken together, these data suggest that different extracellular domains dramatically alter the intracellular response of the Axl kinase. Furthermore, our data suggest that the GAS6-Axl interaction does not induce mitogenesis and that its exact role remains to be determined.

Adaptor Proteins, Signal Transducing↗
PubMed1996

Attenuation of the expression of the focal adhesion kinase induces apoptosis in tumor cells.

The focal adhesion kinase (FAK) is a nonreceptor protein tyrosine kinase implicated in integrin-mediated signal transduction pathways, oncogenic transformation by v-src, and the invasion of human tumors. The overexpression of p125FAK in a variety of human tumors and tumor cell lines in comparison to their nontransformed counterparts suggested that attenuation of p125FAK expression might have an effect on tumor cell proliferation. In this study, we have treated tumor cell lines that expressed high levels of p125FAK with different antisense oligonucleotides to FAK, and have specifically attenuated p125FAK expression. The cells treated with antisense oligonucleotides not only lost their attachment, but also underwent apoptosis. Extensive control oligonucleotide experiments suggested that this attenuation was highly FAK specific. Furthermore, normal human fibroblasts, which did not express high levels of p125FAK, did not lose their attachment or become apoptotic with FAK antisense treatment. These results suggested that FAK is involved in adhesion-mediated growth in tumor cells and that FAK may be a rational gene-directed target for disrupting tumor cell growth.

Apoptosis↗
PubMed1995

A kinase-deficient splice variant of the human JAK3 is expressed in hematopoietic and epithelial cancer cells.

Signal transduction of cytokine receptors is mediated by the JAK family of tyrosine kinases. Recently, the kinase partners for the interleukin (IL)-2 receptor have been identified as JAK1 and JAK3. In this study, we report the identification of splice variants that may modulate JAK3 signaling. Three splice variants were isolated from different mRNA sources: breast (B), spleen (S), and activated monocytes (M). Sequence analysis revealed that the splice variants contain identical NH2-terminal regions but diverge at the COOH termini. Analyses of expression of the JAK3 splice isoforms by reverse transcriptase-polymerase chain reaction on a panel of cell lines show splice preferences in different cell lines: the S-form is more commonly seen in hematopoietic lines, whereas the B- and M-forms are detected in cells both of hematopoietic and epithelial origins. Antibodies raised against peptides to the B-form splice variant confirmed that the 125-kDa JAK3B protein product is found abundantly in hematopoietic as well as epithelial cells, including primary breast cancers. The lack of subdomain XI in the tyrosine kinase core of the B-form JAK3 protein suggests that it is a defective kinase. This is supported by the lack of detected autokinase activity of the B-form JAK3. Intriguingly, both the S and B splice isoforms of JAK3 appear to co-immunoprecipitate with the IL-2 receptor from HUT-78 cell lysates. This and the presence of multiple COOH-terminal splice variants coexpressed in the same cells suggest that the JAK3 splice isoforms are functional in JAK3 signaling and may enrich the complexity of the intracellular responses functional in IL-2 or cytokine signaling.

Amino Acid Sequence↗
PubMed1995

The nuclear tyrosine kinase Rak associates with the retinoblastoma protein pRb.

Rak is a nuclear tyrosine kinase containing Src homology 2 and 3 domains at its NH2 terminus. We report here that the retinoblastoma tumor susceptibility gene product pRb associates with Rak in vivo and in vitro. Rak binds in the A/B pocket region of pRb, a region that is frequently mutated in human cancer, during the G1 and S phases of the cell cycle. Furthermore, Rak expression is elevated in G1, and transfection of Rak into NIH 3T3 cells results in a significant decrease in the number of emerging colonies. Thus, Rak is a tyrosine kinase with growth suppressing activity that may function, in part, through its interaction with pRb.

3T3 Cells↗
PubMed1995

Both p16 and p21 families of cyclin-dependent kinase (CDK) inhibitors block the phosphorylation of cyclin-dependent kinases by the CDK-activating kinase.

Phosphorylation of cyclin-dependent kinases (CDKs) by the CDK-activating kinase is required for the activation of CDK enzymes. Members of two families of CDK inhibitors, p16/p18 and p21/p27, become physically associated with and inhibit the activity of CDKs in response to a variety of growth-modulating signals. Here, we show that the representative members of both families of CDK inhibitors, p21waf1,cip1, p27kip1, and p18, can prevent the phosphorylation of their CDK partners, CDK2 and CDK6, by CDK-activating kinase. No direct interaction between CDK-activating kinase and the CDK inhibitors could be detected, suggesting that binding of these CDK inhibitors to CDK subunits renders CDK inaccessible to the CDK-activating kinase phosphorylation. These findings suggest that a general mechanism of CDK inhibitor function is to block the phosphorylation of CDK enzymes by CDK-activating kinase.

Base Sequence↗
PubMed1995

Overexpression of the focal adhesion kinase (p125FAK) in invasive human tumors.

The focal adhesion kinase (FAK) gene encodes a tyrosine kinase (p125FAK) thought to be involved in signal transduction pathways used in cell adhesion, motility, and anchorage-independent growth. Because alterations in these cellular processes occur in tumor invasion and metastasis, we studied the protein expression of FAK in a variety of human tumors and found that in the 119 samples studied, increased levels of p125FAK correlated with the invasive potential of a tumor. By comparing FAK expression in tumors with normal tissue from the same patient, we found that p125FAK was significantly elevated in 17 (100%) of 17 invasive and metastatic colonic lesions and in 22 (88%) of 25 invasive and metastatic breast tumors. Additional studies of FAK expression in 13 high grade sarcomas showed high levels in all samples compared to benign, noninvasive mesenchymal specimens. Furthermore, FAK protein levels were elevated in preinvasive lesions, such as large (> 2 cm) colonic villous adenomas, whereas noninvasive, yet hypercellular, neoplastic tissues such as parathyroid and hepatocellular adenomas did not overexpress FAK. These data provide evidence that both epithelial and mesenchymal tumor progression are accompanied by increased p125FAK expression and suggest that the level of FAK expression might be a marker for the invasive potential of a tumor.

Adenofibroma↗
PubMed1995

Receptor tyrosine kinases expressed in metastatic colon cancer.

Using a PCR-based cloning technique, we have isolated a series of DNA fragments coding for tyrosine kinases that are expressed in a metastatic human colon tumor, and have subsequently analyzed their expression pattern at the protein level in human tumors. We identified both the alpha and the beta forms of the platelet-derived growth factor receptor (PDGFR), axl and 8 other genes, including 3 cytoplasmic tyrosine kinases. To study their expression in human colon cancer, we performed Western blots of matched sets of normal tissues and of carcinomas from the same patient. These revealed that the alpha-PDGFR migrates predominantly as a 200-kDa band in 8/8 normal tissues, and as a 170-kDa band in 17/17 malignant tissues, as well as in colonic polyps, suggesting that expression of an isoform of this receptor may be a marker for the progression of colon cancer. Additional studies showed that the Axl receptor tyrosine kinase was expressed at 10-fold higher levels in a peritoneal metastatic nodule than in other normal and malignant tissues. Immunohistochemistry revealed Axl over-expression specifically in the malignant cells of the tumor. This indicates that over-expression and possibly a differential processing event of tyrosine kinase receptors may be involved in colon cancer, and that they are potential markers for the progression of this disease.

Amino Acid Sequence↗
PubMed1995

The transforming receptor tyrosine kinase, Axl, is post-translationally regulated by proteolytic cleavage.

Several receptor tyrosine kinases generate soluble ligand binding domains either by differential splicing resulting in a truncated RNA transcript, or by proteolytic cleavage. Although the exact role in vivo of these soluble extracellular domains is unclear, proteolysis may function to down-regulate the receptor, and soluble extracellular domains (ECD) may compete with the intact receptor binding to ligand. Axl is a member of a new class of receptor tyrosine kinases characterized by an ECD resembling cell adhesion molecules and unique sequences in the kinase domain. In addition, Axl is transforming in both fibroblast and hematopoietic cells, and appears to be involved in mesenchymal development. We now find that Axl is post-translationally processed by cleavage in a 14 amino acid region immediately NH2-terminal to the transmembrane domain resulting in a soluble ECD and a membrane bound kinase domain. The sequence of this putative cleavage site shares no homology with recognition sites of known proteases. Characterization of this proteolytic processing shows that it does not require protein synthesis or transport but is augmented by phorbol ester treatment. Since the cleavage of Axl enhances turnover of the kinase on the cell surface, we suggest that proteolytic processing down-regulates Axl kinase activity.

3T3 Cells↗
PubMed1995

Interactions of Wnt-1 and Wnt-3a are essential for neural tube patterning.

Wnt-1 and Wnt-3a have been postulated to share functional redundancy in spinal cord morphogenesis due to their homologies in protein structure and overlapping expression patterns. In this study, antisense oligonucleotides and a murine whole embryo culture system were used to examine functional interactions of Wnt-1 and Wnt-3a in late gastrulation and neurulation. Early somite mouse embryos were injected with combinations of Wnt-1 and Wnt-3a antisense oligonucleotides and then grown in vitro for up to 48 hr. Simultaneous inhibition of Wnt-1 and Wnt-3a expression resulted in pattern loss in the presumptive spinal cord, which was apparent within 4 hr following antisense treatment. The neural tube was wavy, there was a reduction in the number of nuclear layers in the walls of the neural tube, and evidence of decreased cell adhesion between neuroepithelial cells by 12 hr postinjection. In addition, notochord and primitive streak abnormalities accompanied neural tube abnormalities. The existence of regulatory interactions between Wnt-1, Wnt-3a, and engrailed genes was also examined in this study. Antisense inhibition of Wnt-1 or Wnt-3a expression resulted in reduction of engrailed protein levels in the brain, somites, and spinal cord. However, simultaneous inhibition of both Wnt genes resulted in more complete loss of engrailed protein in these regions. Herein, we present data suggesting functional redundancy of Wnt-1 and Wnt-3a in neural tube patterning and in regulation of engrailed expression.

Animals↗
PubMed1995

Antisense targeting of engrailed-1 causes abnormal axis formation in mouse embryos.

Antisense oligonucleotide targeting of engrailed-1 (En-1) in early somite mouse embryos resulted in reduced En protein levels and produced abnormalities of the brain, face, and heart and shortening of the embryonic axis (caudal dysgenesis). Defects of the brain and limbs were consistent with abnormalities observed in null mutant mice described by other investigators. Abnormalities of the face and heart may be related to alterations in neural crest cells. Caudal dysgenesis suggested a role for En-1 in axis formation and this hypothesis was supported by results showing that En-1 protein and mRNA are present in the primitive streak. Thus, in addition to participating in the signaling pathway for brain and limb development, En-1 appears to play a role in patterning the embryonic axis.

Animals↗
PubMed1995

Antisense inhibition of engrailed genes in mouse embryos reveals roles for these genes in craniofacial and neural tube development.

The roles of engrailed-1 (En-1) and engrailed-2 (En-2) have been investigated during gastrulation and neurulation in mouse embryos. Using antisense oligonucleotides and murine whole embryo culture, early somite embryos were injected with antisense phosphorothioated oligonucleotides directed against En-1 or En-2 transcripts and then grown in vitro for 48 hr. Inhibition of En-1 by antisense targeting during this period resulted in embryos with craniofacial abnormalities, specifically loss of mid- and hindbrain tissue and hypoplasia in associated neural crest derived areas such as the face and first and second pharyngeal arches. In addition, En-1 appeared to be essential in early patterning of the neural tube. Embryos removed from culture as early as 8 hr following injection exhibited undulations in the presumptive spinal cord. Histological analysis of the affected neural tubes at 48 hr showed disrupted cytoarchitecture and in some cases, apparent dorsal-ventral duplication of the neural tube and underlying notochord. Using S100 beta as a notochord marker, embryos removed from culture at 8 or 12 hr following injection exhibited loss of S100 beta expression in the notochord region subjacent to affected neural tube segments, suggesting that S100 beta, or other notochordal genes, may be downstream components of the En-1 regulatory cascade. Furthermore, antisense inhibition of En-1 induced caudal dysgenesis, suggesting disruption in primitive streak function. Antisense targeting of En-2 expression of early somite stages produced few alterations in development, although approximately one third of the embryos exhibited a series of lateral folds in the spinal cord at the level of the forelimb-bud.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1995

With a different voice: integrating the psychosocial perspective into routine oncology care.

We have established a psychosocial support system that is highly integrated with physician activities and that is tailored to the special needs of our patient population. The core of this system is the close interaction between the physician and the patient counselor, who is a clinical social worker and psychotherapist. Viewed as a permanent member of the breast cancer team, the patient counselor provides psychosocial care in the oncology clinic using normal clinic events as vehicles for interaction. Thus, time spent waiting in the clinic and follow-up phone calls can be converted to sessions for psychological counselling. This approach has been effective in our patient population, where social biases and financial constraints limit the use of out-of-clinic psychiatric services.

Breast Neoplasms↗
PubMed1995

The Carolina Breast Cancer Study: integrating population-based epidemiology and molecular biology.

The integration of epidemiology and molecular biology provides a new strategy to identify additional risk factors for breast cancer and to better understand the role played by traditionally recognized risk factors. The Carolina Breast Cancer Study (CBCS) is a population-based, case-control study designed to identify causes of breast cancer among Caucasian and African-American women who are residents of a 24-county area of central and eastern North Carolina. Information on established and potential breast cancer risk factors is obtained by personal interviews. Blood samples are collected from all consenting participants. Medical record documentation and paraffin-embedded tumor specimens are obtained for all breast cancer patients. DNA from tumor tissue is tested for a variety of molecular alterations characteristic of breast cancer. Germline DNA from blood lymphocytes is evaluated for presence of alleles increasing susceptibility to breast cancer. Statistical analyses evaluate gene-environment interaction by exploring the associations between environmental/behavioral factors and breast cancer in relation to specific molecular alterations (germline and tumor). Results will help identify high-risk women, clarify causal pathways, and hopefully contribute to the prevention of breast cancer.

Adult↗
PubMed1995

Ha-ras rare alleles in breast cancer susceptibility.

Over the last several years, evidence has accumulated to support the idea that rare Ha-ras polymorphisms are associated with inherited susceptibility to certain human cancers. A recent epidemiologic study conducted at our institution found a significant association specifically with breast cancer, although the mechanism underlying this relationship remains unclear. We have proposed that rare Ha-ras alleles are markers of a genomic instability that predisposes to breast cancer. To address this hypothesis, we are investigating the relationship between the presence of rare alleles and another form of instability, gene amplification, and are developing new methodologies both to improve VNTR allele length detection and to characterize the internal repeat sequence variations of the various alleles. These studies should enable us to more clearly define the role of this region in cancer development by delineating VNTR structure and function and the mechanisms of rare allele generation. Ultimately, we hope to identify VNTR characteristics that will permit more accurate cancer risk assessment.

Alleles↗
PubMed1995

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.

Breast Neoplasms↗
PubMed1995

Developing strategies for intervention and prevention in hereditary breast cancer.

Prophylactic mastectomy, intensified breast cancer screening, and the use of chemopreventive agents have all been recommended to reduce breast cancer risk in women with a family history of breast cancer. Yet, little is currently known about the efficacy of these approaches in reducing breast cancer mortality. The recent identification of BRCA1 and the localization of BRCA2 lend urgency to the need to assess breast cancer intervention and prevention strategies for women likely to carry germline mutations at these loci. At present, families with a history consistent with a BRCA1 or BRCA2 mutation should be tested within the confines of a research protocol and encouraged to participate in intervention and prevention trials. Both retrospective studies and prospective clinical trials are critically needed. While randomized clinical trials would be the optimal mechanism to assess the relative efficacy of these potential interventions, no consensus was obtained as to whether such a trial would be feasible because of strong patient preference for intervention type. It is likely that optimal intervention and prevention strategies will consist of a combined approach to risk reduction. Participants must be appropriately informed of the potential risks as well as the potential benefits of such testing. The potential risks of testing for genetic susceptibility include not only potential psychosocial harm that may result from learning one's carrier status, but also the potential for altered family relationships and insurance and job discrimination. Participants and their family members must be counseled concerning the implication of their test results.

Breast Neoplasms↗
PubMed1994

Steroid hormones and risk of breast cancer.

BACKGROUND: More than three decades of epidemiologic studies have identified numerous risk factors for breast cancer. These factors have been estimated to account for only 20-25% of disease occurrence. However, among these factors, several are related to sex steroid hormones: sex of the affected individuals (women), early age of menarche and late age of menopause, parity, late age at first pregnancy, and obesity in postmenopausal women. METHODS: Theoretical models and laboratory data support hormonal mechanisms of carcinogenesis, particularly as they relate to proliferation of breast ductal epithelium and terminal end bud growth and differentiation in the lobules of the breast. The recent introduction of biologic markers and molecular epidemiology allows for studies that use laboratory technology in the context of epidemiologic research. RESULTS: This paper summarizes the epidemiologic literature on exogenous hormones, addresses the issue of endogenous steroid hormone levels and estrogen metabolism in serum and breast tissue in premenopausal and postmenopausal women with and without cancer, speaks to the cellular mechanisms of action of estrogen and progesterone, and highlights some of the biologic markers relevant to studies of breast cancer and precursor lesions, with particular emphasis on those that may be hormonally induced or altered. CONCLUSIONS: These markers must be better defined in terms of breast cancer pathogenesis. Studies are needed to evaluate the direct effects of behavioral/environmental risk factors on relevant biomarkers as well as to assess the interactions of epidemiologic factors and biomarkers on risk of breast cancer.

Breast Neoplasms↗