Induction of vascular endothelial growth factor by IGF-I in osteoblast-like cells is mediated by the PI3K signaling pathway through the hypoxia-inducible factor-2alpha.
IGF-I is known to stimulate the expression of oxygen- and nutrient-sensitive genes in several cell types. In this study we investigated the signaling pathways and transcriptional mechanisms that mediate IGF-I induction of vascular endothelial growth factor (VEGF) expression in human osteoblast-like cells. IGF-I (50 ng/ml) induced a rapid increase (3-fold) in VEGF mRNA in osteoblasts that was accompanied by an increase in the level of hypoxia-inducible factor-2alpha (HIF-2alpha) protein without changes in HIF-2alpha mRNA expression. These effects were mimicked by chemical inhibition of proteosomal degradation of HIF-2alpha. Transcriptional activation of a proximal VEGF promoter-luciferase construct was greatly enhanced by cotransfection with an HIF-2alpha, but not an HIF-1alpha, construct. IGF-I acutely stimulated Akt phosphorylation, which was abolished by pretreatment of cells with the PI3K inhibitor LY294002. Pretreatment of the cells with LY294002 also greatly attenuated IGF-I induction of HIF-2alpha and blunted IGF-I-induced VEGF promoter activity. Finally, forced expression of a constitutively active PI3K expression construct induced VEGF promoter to levels similar to those observed with IGF-I alone. These data indicate that IGF-I, by activation of the PI3K pathway, induces VEGF expression in osteoblasts through a transcriptional control mechanism common to those that activate VEGF and other hypoxia response genes.
Akeno
Nagako
N
Department of Medicine, University of Cincinnati College of Medicine, Cincinnati, Ohio 45267, USA.
Robins
Jared
J
Zhang
Mei
M
Czyzyk-Krzeska
Maria F
MF
Clemens
Thomas L
TL
D051792:Basic Helix-Loop-Helix Proteins; D015152:Blotting, Northern; D015153:Blotting, Western; D002478:Cells, Cultured; D016228:Endothelial Growth Factors / Q000096:biosynthesis*; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects; D017930:Genes, Reporter / Q000235:genetics; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D008222:Lymphokines / Q000096:biosynthesis*; D010006:Osteoblasts / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D019869:Phosphatidylinositol 3-Kinases / Q000235:genetics / Q000502:physiology*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D012333:RNA, Messenger / Q000096:biosynthesis / Q000302:isolation & purification; D018336:Receptors, Aryl Hydrocarbon / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D015534:Trans-Activators / Q000502:physiology*; D015533:Transcriptional Activation / Q000187:drug effects; D042461:Vascular Endothelial Growth Factor A; D042442:Vascular Endothelial Growth Factors; D000099162:Endothelial PAS Domain-Containing Protein 1
D051792:Basic Helix-Loop-Helix Proteins; D016228:Endothelial Growth Factors; D007334:Insulin-Like Growth Factor I; D008222:Lymphokines; D019869:Phosphatidylinositol 3-Kinases; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D012333:RNA, Messenger; D018336:Receptors, Aryl Hydrocarbon; D015534:Trans-Activators; D042461:Vascular Endothelial Growth Factor A; D042442:Vascular Endothelial Growth Factors; D000099162:Endothelial PAS Domain-Containing Protein 1; C494918:AKT1 protein, human