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-2α

Abstract

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.

Validation of the type 1 insulin-like growth factor receptor as a therapeutic target in renal cancer

“…1D). This finding is consistent with reports that expression of HIF-2α by breast cancer cells was unaffected by PI3K inhibition (21), although in other cell types, HIF-2α was reported to be IGF-inducible (22,23).…”

EPAS1 Promotes Adipose Differentiation in 3T3-L1 Cells

“…Previous studies have shown that insulin-like growth factor activates EPAS1 in osteoblast-like cells (28). Thus, to understand the molecular basis by which EPAS1 promotes adipose differentiation, we next examined the effects of EPAS1-(1-485) on the expression of genes for insulin signaling and glucose transport.…”

“…In addition to these roles, a contribution of EPAS1 to the regulation of adipogenesis has been suggested for the following reasons. First, several growth factors, including insulin and insulin-like growth factor, induce and activate EPAS1 activity in a phosphatidylinositol 3-kinase-dependent or related manner (28). Also, HIF-1␣, which is closely related to EPAS1, mediates insulin signaling in hepatocytes (29).…”

Hypoxia-inducible Factor-1α Protein Negatively Regulates Load-induced Bone Formation

“…A number of growth factors (21,22) and paracrine factors (23)(24)(25)(26) activate the Hif-1 pathway in several cell types under normoxia. In osteoblasts, insulin-like growth factor-1 stabilizes Hif␣ via a mechanism that involves PI3-kinase/Akt signaling (27), and the expression of Hif-1␣ is necessary for normal osteoblast proliferation (28). Additionally, mechanical signals prevent the degradation of Hif ␣-subunits in both skeletal and cardiac muscle (29,30).…”