Involvement of Neuropilin-1 in the autocrine mechanism of Vascular Endothelial Growth Factor-165 in breast cancer progression
Matheny, Shannon Lynn
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Vascular Endothelial Growth Factor (VEGF) is a potent inducer of angiogenesis through chemotatic, mitogenic and anti-apoptotic effects on endothelial cells in cancer progression. The autocrine effect of VEGF on epithelial breast cancer cells is addressed in the following studies, which focus on the VEGF receptor, Neuropilin-1 (NRP-1). Although Neuropilin-1 is a specific receptor for the VEGF-165 isoform, NRP-1 requires interaction with other cell surface proteins in order to transduce signals intracellularly. Due to the lack of typical receptors responsible for NRP-1 mediated VEGF signaling on the surface of breast epithelial cells, expression of NRP-1 coreceptors was assessed in vitro in breast carcinoma cell lines and in vivo using human breast tissue samples. NRP-1 co-receptors (Plexin-A1 and L1 Cellular Adhesion Molecule- L1CAM) were previously believed to be functionally isolated to neuronal development. Using RNA and protein analysis, the expression of the NRP-1 co-receptors in breast carcinoma cell lines was shown. Co-expression and colocalization of Plexin-A1 and NRP-1, and L1-CAM and NRP-1, was shown in human breast tissues. We also determined that Plexin-A1 expressing breast cancer cells respond to VEGF stimulation in culture by actin rearrangement. Mechanistic studies revealed the involvement of LIM Kinase in this process. In addition, the role of L1-CAM in antiapoptotic effect of VEGF on MCF-7 cells in hypoxic conditions was determined. Mechanistic studies revealed an increased phosphorylation of Akt is involved in this process. A novel isoform of NRP-1 was isolated in our laboratory, and the effect of this isoform in vivo was characterized. We created a transgenic mouse designed to overexpress the novel NRP-1 isoform under the direction of the muscle creatine kinase promoter. In this study the generation of the transgenic animals to assess the autocrine effect of VEGF in mammary tumor formation and progression is described. These studies suggest new mechanisms of action for the autocrine effect of VEGF-165 on breast cancer cells and a novel method for assessing these findings in vivo. Through the discovery of Plexin-A1 as a factor affecting VEGF-mediated migration of tumor cells, and L1-CAM as a receptor involved in VEGF mediated suppression of apoptosis, our understanding of the potency of VEGF as a cancer promoting molecule is greatly increased. By generating a transgenic mouse model that expresses high levels of a soluble VEGF- binding NRP-1 receptor, a powerful model for the study the autocrine effect of VEGF in tumor progression has been developed.