Enhanced expression of the Flt-1 and Flk-1 receptor tyrosine kinases in a newborn piglet model of ischemic tolerance

Authors

  • Jahan Ara,

    Corresponding author
    • Department of Pediatrics, Drexel University College of Medicine and Saint Christopher's Hospital for Children, Philadelphia, PA, USA
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  • Panchanan Shukla,

    1. Department of Pediatrics, Drexel University College of Medicine and Saint Christopher's Hospital for Children, Philadelphia, PA, USA
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  • Melissa Frank

    1. Department of Pediatrics, Drexel University College of Medicine and Saint Christopher's Hospital for Children, Philadelphia, PA, USA
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Address correspondence and reprint requests to Jahan Ara, Department of Pediatrics, Drexel University College of Medicine and Saint Christopher's Hospital for Children, 245 N. 15th Street, New College Building, Room 7408, Philadelphia, PA 19102, USA. E-mail: jahan.ara@drexelmed.edu

Abstract

Vascular endothelial growth factor (VEGF), an angiogenic factor induced by hypoxia, also exerts direct effects on neural tissues. VEGF up-regulation after hypoxia coincides with expression of its two tyrosine kinase receptors Flt-1(VEGFR-1) and Flk-1 (KDR/VEGFR-2), which are the key mediators of physiological angiogenesis. We have recently shown that hypoxic-preconditioning (PC) leading to tolerance to hypoxia–ischemia in neonatal piglet brain resulted in increased expression of VEGF. In this study, we used a hypoxic-preconditioning model of ischemic tolerance to analyze the expression and cellular distribution of VEGF receptors and phosphorylation of cAMP-response element-binding protein (CREB) in newborn piglet brain. The response of Flt-1 and Flk-1 mRNA to PC alone was biphasic with peaks early (6 h) and late (1 week) after PC. The mRNA expression of Flt-1 and Flk-1 in piglets preconditioned 24 h prior to hypoxia–ischemia was significantly higher than non-preconditioned piglets and remained up-regulated up to 7 days. Furthermore, PC prior to hypoxia–ischemia significantly increased the protein levels of Flt-1 and Flk-1 compared with hypoxia–ischemia in a time-dependent manner. Double-immunolabeling indicated that both Flt-1 and Flk-1 are expressed in neurons and endothelial cells with a similar time course of expression following PC and that PC leads to the growth of new vessels. Finally, our data demonstrate that PC significantly phosphorylated and activated cAMP-response element-binding protein in nucleus. These results suggest that mechanism(s) initiated by PC can induce VEGF receptor up-regulation in newborn brain and that VEGF–VEGF receptor-coupled signal transduction pathways could contribute to the establishment of tolerance following hypoxia–ischemia.

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