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Lack of binding between the ephrin A1 ligand and the EphA2 receptor, 130 kDa tyrosine kinase receptor found in adult human epithelial cells, causes unstoppable cell growth, and subsequently, development of tumors associated with epithelial cancers (Kinch, 2005; Ansuini et al, 2009). In a normal cell, EphA2 receptor kinase activity is inhibited when the cell-membrane EphA2 receptor can bind to ephrin A ligands, as shown in figure 1. Conversely, in a cancerous cell, EphA2 receptor is damaged and cannot bind correctly to ligands causing EphA2 to continually phosphorylate thus increasing number of malignant tumor cells (Walker-Daniels et al, 2002). Inhibiting the kinase activity and phosphorylation of EphA2 protein receptor has been associated with a decrease in the growth of malignant cells (Ansuini et al, 2009).
Figure 1: In diagram A, the EphA2 receptor is modified, preventing ephrinA1 from binding. The lack of binding causes kinase activity in the cell and subsequent growth of tumor cells. However, a normally functioning EphA2 receptor will correctly bind to ephrinA1 inhibiting phosphorylation within the cell and normal cell activity, adapted by Larsen (2007).
Specific Aim 1: Selection of RNA aptamers against EphA2
Increased levels of EphA2 phosphorylation due to decreased amount of EphA2/ephrinA1 binding has been linked to increased tumor growth. Thus, selection of RNA aptamers conjugated with a bacterial toxin can be used as a therapeutic tool, degrading the malfunctioning EphA2 protein and thus decreasing kinase activity and tumor proliferation. Such an aptamer would be much more useful than its antibody equivalent due to modifications that can make the aptamer less resistant to enzyme degradation which will increase therapeutic delivery time and potentially decrease number of treatments.
Ansuini, H., et al (2009) “Anti-EphA2 Antibodies with Distinct In Vitro Properties Have Equal In Vivo Efficacy in Pancreatic Cancer.” Journal of Oncology 2009: 1-10.
Kinch, M. S. (2005). Targeted drug delivery using EphA2 or EphA4 binding moieties. Patent No. 20050153923. Laytonsville, MD, US.
Larsen, A. B., Pedersen, M. W., et al (2007) “Activation of the EGFR Gene Target EphA2 Inhibits Epidermal Growth Factor–Induced Cancer Cell Motility.” Molecular Cancer Research 5: 283.
Phillips, J. A., et al (2008) “Applications of aptamers in cancer cell biology.” Analytica Chimica Acta 621 Review: 101-108.
Walker-Daniels, J., et al (2003) “Differential Regulation of EphA2 in Normal and Malignant Cells.”American Journal of Pathology 162: 1037-1042.
Wykosky, Jill. "The EphA2 Receptor and EphrinA1 Ligand in Solid Tumors: Function and Therapeutic Targeting." Molecular Cancer Research 2008.