Role of PLSCR in Cell Response to Growth Factors

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DESCRIPTION (provided by applicant): Phospholipid scramblase 1 (PLSCR1) is a multiply palmitoylated, endofacial plasma membrane protein first identified in red cells, platelets, leukocytes, and other blood cells, which is thought to mediate the transbilayer movement of membrane phospholipids in response to cell activation, injury, or apoptosis. PLSCR1 has been found associated with several growth factor receptors, including epidermal growth factor receptor (EGF-R). PLSCR1 expression is transcriptionally induced by EGF and related growth factors, and it is phosphorylated by tyrosine kinases that are involved in the signal transduction pathways initiated through EGF-R and related receptors. We recently observed that newly synthesized PLSCR1 can localize to the nucleus, and we found nuclear import of PLSCR1 to occur whenever the polypeptide fails to palmitoylate. By contrast, palmitoylated PLSCR1 is a component of plasma membrane lipid "rafts", cholesterol- and sphingomyelin-rich membrane microdomains that are believed to be involved in both assembly of receptor signaling platforms and the endocytic trafficking of activated EGF-R and related receptors from the plasma membrane. We also observed that neutrophil production and maturation in response to select growth factors is impaired in PLSCR1-/-mice, whereas fibroblasts and epithelial cells from these animals exhibit attenuated responses to EGF and related growth factors, with particular effect on EGF-dependent activation of c-Src. In this Project, we aim to elucidate the role of PLSCR1 in the signaling pathways of EGF-R and related growth factor receptors. Our Specific Aims are: Aim1 -- To determine whether the level of expression of plasma membrane PLSCR1 influences cell surface expression, topology, sensitivity to ligand, and/or receptor-initiated activation of signaling kinases by EGF and related growth factors. AIM2 -- To determine whether PLSCR1 influences the localization of EGF-R to membrane lipid rafts, its interactions with key adaptor proteins and signaling kinases, or its endocytic uptake, endosomal trafficking, and degradation following receptor activation. AIM3 - To identify the mechanism by which PLSCR1 traffics into the nucleus, and to determine what role nuclear PLSCR1 might play in the cellular transcription response to cytokine stimulation. We believe that this research will provide new insights into the regulatory control mechanisms that affect signaling by activated growth factor receptors.