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Ligand-induced internalisation and subsequent downregulation of receptor tyrosine kinases (RTKs) serve to determine biological outputs of their signalling. Intrinsically kinase-deficient RTKs control a variety of biological responses, however, the mechanism of their downregulation is not well understood and its analysis is focused exclusively on the ErbB3 receptor. The Eph group of RTKs is represented by the EphA and EphB subclasses. Each bears one kinase-inactive member, EphA10 and EphB6, respectively, suggesting an important role for these molecules in the Eph signalling network. While EphB6 effects on cell behaviour have been assessed, the mechanism of its downregulation remains elusive. Our work reveals that EphB6 and its kinase-active relative, and signaling partner, EphB4, are downregulated in a similar manner in response to their common ligand, ephrin-B2. Following stimulation, both receptors are internalised through clathrin-coated pits and are degraded in lysosomes. Their targeting for lysosomal degradation relies on the activity of an early endosome regulator, the Rab5 GTPase, as this process is inhibited in the presence of a Rab5 dominant-negative variant. EphB6 also interacts with the Hsp90 chaperone and EphB6 downregulation is preceded by their rapid dissociation. Moreover, the inhibition of Hsp90 results in EphB6 degradation, mimicking its ligand-induced downregulation. These processes appear to rely on overlapping mechanisms, since Hsp90 inhibition does not significantly enhance ligand-induced EphB6 elimination. Taken together, our observations define a novel mechanism for intrinsically kinase-deficient RTK downregulation and support an intriguing model, where Hsp90 dissociation acts as a trigger for ligand-induced receptor removal.

Anaplastic lymphoma kinase (ALK) is a receptor tyrosine kinase that was first reported as a fusion partner of nucleophosmin in Anaplastic large cell lymphoma in 1994. ALK is involved in myriad of cancers including neuroblastoma which is the most common extracranial solid tumor affecting young children. It arises in the neural crest cells of sympathetic nervous system origin and is responsible for 12% of all childhood cancer deaths. Several point mutations in ALK have been described in both familial and sporadic neuroblastoma. With the aim to understand the role of ALK in neuroblastoma further, we investigated the point mutations in ALK reported in patients. Using cell culture based methods and Drosophila as a model organism; we first characterized these mutations under three broad categories: 1) Ligand independent mutations that were constitutively active, 2) Kinase dead mutation and 3) Ligand dependent mutations that behaved as inducible wild type. Further, to understand the activation mechanism of ALK, we constructed mutations that could potentially alter ALK’s conformation based on the available crystal structure. From the data generated, we were able to provide a new perspective to the activation of full length ALK receptor. This was more in line with activation mechanism of insulin receptor and different from that suggested for ALK fusion protein. From a clinical point of view, all the mutations in the study were blocked to different degrees using the ALK inhibitor, crizotinib. Lastly, we identified potential downstream targets of ALK using phosphoproteomics. From the various targets identified, we focused on STAT3 and confirmed its role as a mediator in ALK initiated MYCN transcription. We showed that STAT3 inhibition led to reduction of MYCN levels and thereby identifying it as a potential therapeutic target in neuroblastoma. Overall, our study highlights clinical relevance of ALK mutations in neuroblastoma and from a basic biology viewpoint; it reveals important mechanistic insight into receptor activation.

Thesis (Ph.D.)--East Carolina University, 2009. / Presented to the faculty of the Department of Anatomy and Cell Biology. Advisor: Ann O. Sperry. Title from PDF t.p. (viewed June 12, 2010). Includes bibliographical references.

Master of Science in Biomedical Sciences / Department of Clinical Sciences / Mary Lynn Higginbotham / This study evaluated sixteen canine nasal carcinoma and five normal nasal epithelium samples for expression and phosphorylation of known targets of toceranib [vascular endothelial growth factor receptor-2 (VEGR2), platelet derived growth factor alpha (PDGFR-[alpha]), platelet derived growth factor receptor beta (PDGFR-[beta]), and stem cell factor receptor (c-KIT)] and epidermal growth factor receptor 1 (EGFR1) using immunohistochemistry, RT-PCR and a receptor tyrosine kinase (RTK) phosphorylation panel. Protein for VEGFR2 was expressed in neoplastic cells of all carcinomas, PDGFR-[alpha] was noted in 15/16, whereas PDGFR-[beta] was detected in 3/16 samples, but showed primarily stromal staining. Protein expression for c-KIT was present in 4/16 and EGFR1 was noted in 14/16 samples. Normal tissue showed variable protein expression of the RTKs. Messenger RNA for VEGFR2, PDGFR-[beta], and c-KIT were noted in all samples. Messenger RNA for PDGFR-[alpha] and EGFR1 were detected in 15/16 samples. All normal nasal tissue detected messenger RNA for all RTKs of interest. Constitutive phosphorylation of VEGFR2, PDGFR-[alpha], PDGFR-[beta] and c-KIT was not observed in any carcinoma or normal nasal sample, but phosphorylation of EGFR1 was noted in 10/16 carcinoma and 3/5 normal samples. The absence of major phosphorylated RTK targets of toceranib suggests the clinical effect of toceranib may occur through inhibition of alternative and currently unidentified RTK pathways in canine nasal carcinomas. The observed protein and message expression and phosphorylation of EGFR1 in the nasal carcinoma samples merits further inquiry into EGFR1 as a therapeutic target for this cancer.

Pathological angiogenesis is an essential component of tumor growth, development, and metastasis for which few effective therapeutic options exist. Though many cancer therapies target the function of cell surface receptors, mechanisms regulating membrane receptor crosstalk remain unclear. Two important families of receptors in angiogenesis, the Ties and Integrins, respond to the extracellular environment via outside-in and, in the case of Integrins, also inside- out signaling. Recent reports showed that the endothelial specific tyrosine kinase receptor, Tie2, forms complexes with two of the endothelial Integrin heterodimers, α5β1 and αVβ3, providing a convenient mechanism for the integration of extracellular stimuli. Our data confirm the interaction between Integrins and Tie2 and additionally indicate an interaction with the orphan co-receptor Tie1. To elucidate the biological role of these macromolecular complexes, biochemical and biophysical methods including co-immunoprecipitation, FRET microscopy, and cellular based assays were used to follow receptor/Integrin association in response to the Tie2 ligands Angiopoietin-1 and -2 as well as the Integrin ligand fibronectin. Furthermore, structural analysis by small angle x-ray scattering of Tie2-ligand complexes and specific Integrin and Tie complexes are being used to identify the basis for growth factor receptor and Integrin signal transduction.

OBJECTIVE: Glioblastoma Multiforme (GBM), one of the most malignant forms of primary brain tumors, is characterized by its highly heterogenous genetic composition, aggressive infiltration of surrounding tissue, and resistance to current treatments. Gene expression analysis has characterized GBM into four main types, with a significant portion belonging to the Classical subtype, typified by overexpression and/or mutation of the epidermal growth factor receptor (EGFR). Also common to this subtype of GBM is the loss of crucial tumor suppressor genes Ink4A/ARF and PTEN, which contribute to the invasive nature and unregulated proliferation that underlie the GBM pathology. The high rate of tumor recurrence post treatment with surgical resection, chemotherapy, and radiation has driven the pursuit of more effective molecularly targeted therapies. This study was undertaken to determine the effects of two types of small molecule tyrosine kinase inhibitors on cells overexpressing wild-type EGFR in the context of their respective complements of tumor suppressor genes. METHODS: Several cell lines were established from mouse models of EGFR wild-type (EGFRWT) driven gliomagenesis and treated with 10 μM of type I tyrosine kinase inhibitors Gefitinib (Iressa®, Astra Zeneca), CI-1033 (Canertinib, Pfizer), or Dimethyl Sulfoxide vehicle. Cells were exposed to each drug treatment as part of a time course ranging from 0 to 24 hours and then evaluated by trypan blue exclusion and Western blot analysis for cell viability and molecular and biochemical effects respectively. RESULTS: Evaluation of cell viability indicated that CI-1033 caused a greater increase in cell death than gefitinib when compared to control treated cells regardless of the tumor suppressors lost. Gefitinib was found to cause cell death only in cells expressing the PTEN tumor suppressor whereas CI-1033 showed similar levels of cell death for cells deficient in Ink4A/ARF or both Ink4A/ARF and PTEN tumor suppressors. Western blot analysis revealed that CI-1033 more effectively inhibited EGFR compared to gefitinib. Treatment with both gefitinib and CI-1033 effectively blocked phosphorylation of EGFR, but this effect was less pronounced with gefitinib treatment. Further analysis of downstream signaling molecules showed a greater presence of cleaved caspase 3, a hallmark of apoptosis, in gefitinib treated cells expressing PTEN than in those cells treated with CI-1033. Cells deficient in both Ink4A/ARF and PTEN did not demonstrate any induction of cleaved caspase 3 following either treatment. CONCLUSIONS: Based on the significant differences in cell viability between treatments, CI-1033 is an overall more effective inhibitor of EGFRWT expressing cells lacking PTEN, while gefitinib and CI-1033 were found to be similarly effective in cells expressing PTEN. The results of western blot analysis indicate that total and irreversible EGFR inhibition may be necessary to induce cell death in a manner that effectively terminates downstream cell signaling. It is likely that CI-1033, unlike gefitinib, induces apoptosis in a caspase-independent manner, which may be one of the many differences in downstream effects produced by these two drugs. Further research is necessary to determine the extent to which each inhibitor shuts down proliferative cell signaling pathways such as PI3K-AKT and MEK-ERK signaling pathways downstream of EGFR. Overall, these data indicate that genotype plays an important role in the determination of therapeutic response and may aid in the evaluation of clinical prognoses.

The Receptor Tyrosine Kinase (RTK) Met, overexpressed or mutated in cancer, plays a major role in cancer progression and represents an attractive target for cancer therapy. This study aimed to investigate whether PI3K plays a role in Met oncogenicity. Three cell models were used: (i) NIH3T3 cells expressing WT Met or the constitutively active mutant M1268T Met; (ii) U87MG glioblastoma cells, with endogenous WT Met constitutively activated due to an autocrine loop; (iii) A549 lung cancer cells expressing endogenous WT Met, activated upon binding exogenous HGF. Met dependent Rac1 translocation to the plasma membrane, actin cytoskeleton organisation, cell migration, anchorage independent growth in soft agar and tumour growth were studied in the presence of inhibitors of pan-PI3K / mTOR, various PI3K Class I isoforms, mTOR or Akt, or following siRNA knock-down of PI3K isoforms. We report that PI3K class I (but not class III) regulates Met dependent cell migration. The PI3K class I isoforms required varies among the cell models. Interestingly, the combined inhibition of all p110 Class I isoforms lead to the strongest reduction of Met dependent cell migration. Met dependent phosphorylation of Akt, an effector of PI3K class I, is reduced upon endocytosis inhibition, suggesting that Met signals to PI3K Class I on endosomes. Our results indicate that mTOR is responsible for Met dependent anchorage independent growth and tumour growth in vivo. Surprisingly, PI3K class I (and class III) are not required. Moreover, Rac1 is required for Met dependent mTOR activation, (phosphorylation of mTORC1's effector, p70 S6K) subcellular translocation of mTOR and anchorage independent growth. Finally, our results suggest that this Met-Rac1- mTOR pathway occurs on endosomes. Thus while PI3K class I regulates Met dependent cell migration, mTOR regulates Met driven anchorage independent growth and in vivo tumorigenesis. Thus PI3K Class I / mTOR may be targeted in Met driven cancers.

Receptor tyrosine kinases (RTKs) are integral membrane proteins that regulate many functions including cell proliferation, cell survival, and cell death. They have been shown to be responsible for the uncontrolled growth of several cancers. RTKs phosphorylate downstream targets such as phosphatidylinositol 3 kinase (PI3K), a lipid kinase that is made up of two major subunitsp85 and p110. Receptor-mediated endocytosis delivers RTKs from the plasma membrane to late endosomes and lysosomes for degradation. This process is controlled by ESCRT proteins and Rab7. PI3K associates with PDGFR during endocytosis, and PI3K binding sites are necessary for the lysosomal trafficking of PDGFR. The smaller isoforms of the ankyrin 3 (Ank3) proteins bind p85. Ank3 overexpression was shown to increase PDGFR degradation, perhaps by controlling the targeting of PDGFR to late endosomes and lysosomes. Ank3 overexpression also reduced the RTK levels and cell proliferation rates of NIH 3T3 cells. We sought to investigate if cancer cells with RTK overexpression might be deficient in Ank3, and if overexpression of ankyrin 105 (Ank105), one of the smaller isoforms of Ank3, would reduce RTK levels and the tumorigenic properties of cancer cells. Two brain cancer cell lines showed reduced Ank105 levels associated with high RTK levels, while high levels of Ank105 associated with low RTK levels were found in normal brain cells. This suggested a loss of Ank105 in the cancer cells, which may have played a role in the cancer development process. We observed reduced RTK levels and anchorage-independent growth in cancer cells overexpressing HA-Ank105, however, most cells overexpressing a blank vector also showed the same results. An independent effect of the overexpression process was thought to play a role in influencing cell behavior. In the lung cancer cell line HCC827, however, there was significant reduction of anchorage-independent growth that was specific for HA-Ank105. There also appeared to be a significant reduction in the cell proliferation rate of T98G brain cancer cells following transfection with HA-Ank105. Furthermore, those cells overexpressing HA-Ank105 tended to die early in tissue culture, with those that survived losing their HA-Ank105 expression. Overall our results suggest a possible role for Ank105 in downregulating RTK levels and growth properties of cancer cells.

Receptor tyrosine kinases (RTKs) are integral membrane proteins that regulate many functions including cell proliferation, cell survival, and cell death. They have been shown to be responsible for the uncontrolled growth of several cancers. RTKs phosphorylate downstream targets such as phosphatidylinositol 3 kinase (PI3K), a lipid kinase that is made up of two major subunitsp85 and p110. Receptor-mediated endocytosis delivers RTKs from the plasma membrane to late endosomes and lysosomes for degradation. This process is controlled by ESCRT proteins and Rab7. PI3K associates with PDGFR during endocytosis, and PI3K binding sites are necessary for the lysosomal trafficking of PDGFR. The smaller isoforms of the ankyrin 3 (Ank3) proteins bind p85. Ank3 overexpression was shown to increase PDGFR degradation, perhaps by controlling the targeting of PDGFR to late endosomes and lysosomes. Ank3 overexpression also reduced the RTK levels and cell proliferation rates of NIH 3T3 cells. We sought to investigate if cancer cells with RTK overexpression might be deficient in Ank3, and if overexpression of ankyrin 105 (Ank105), one of the smaller isoforms of Ank3, would reduce RTK levels and the tumorigenic properties of cancer cells. Two brain cancer cell lines showed reduced Ank105 levels associated with high RTK levels, while high levels of Ank105 associated with low RTK levels were found in normal brain cells. This suggested a loss of Ank105 in the cancer cells, which may have played a role in the cancer development process. We observed reduced RTK levels and anchorage-independent growth in cancer cells overexpressing HA-Ank105, however, most cells overexpressing a blank vector also showed the same results. An independent effect of the overexpression process was thought to play a role in influencing cell behavior. In the lung cancer cell line HCC827, however, there was significant reduction of anchorage-independent growth that was specific for HA-Ank105. There also appeared to be a significant reduction in the cell proliferation rate of T98G brain cancer cells following transfection with HA-Ank105. Furthermore, those cells overexpressing HA-Ank105 tended to die early in tissue culture, with those that survived losing their HA-Ank105 expression. Overall our results suggest a possible role for Ank105 in downregulating RTK levels and growth properties of cancer cells.