Intracellular pathways triggered by the selective FLT‐1‐agonist placental growth factor in vascular smooth muscle cells exposed to hypoxia

Abstract

We have previously shown that hypoxia makes vascular smooth muscle cells (VSMCs) responsive to placental growth factor (PlGF) through the induction of functional fms-like tyrosine kinase (Flt-1) receptors. The aim of this study was to investigate the molecular mechanisms involved in the PlGF effects on proliferation and contraction of VSMCs previously exposed to hypoxia (3% O2). In cultured rat VSMCs exposed to hypoxia, PlGF increased the phosphorylation of protein kinase B (Akt), p38 and STAT3; activation of STAT3 was higher than that of other kinases. In agreement with this finding, the proliferation of hypoxia-treated VSMCs in response to PlGF was significantly impaired by the p38 and the phosphatidylinositol 3-kinase inhibitors SB202190 and LY294002, respectively, and was almost completely prevented by AG490, a janus tyrosine kinase (JAK)/signal transducer and activator of transcription (STAT) inhibitor. Since hypoxia was able to reverse the vasorelaxant effect of PlGF into a vasoconstrictor response, the mechanism of this latter effect was also investigated. Significant Flt-1 activity was measured in isolated preparations from rat aorta exposed to hypoxia. Inhibitors of mitogen-activated protein kinase kinase, Akt and STAT3 induced a modest inhibition of the vasoconstrictor response to PlGF, while the p38 inhibitor SB202190 markedly impaired the PlGF-induced contractile response. These effects were selectively mediated by Flt-1 without any involvement of foetal liver kinase-1 receptors. These data are the first evidence that different intracellular pathways activated by Flt-1 receptor in VSMCs are involved in diverse biological effects of PlGF: while mitogen activated protein kinase kinase/extracellular signal regulated kinase(1/2) and JAK/STAT play a role in VSMC proliferation, p38 is involved in VSMC contraction. These findings may highlight the role of PlGF in vascular pathology.

PlGF: A Multitasking Cytokine with Disease-Restricted Activity

“…PlGF enhances the proliferation, migration, and survival of endothelial cells (Ziche et al 1997;Carmeliet et al 2001;Adini et al 2002;Fischer et al 2007;Schmidt et al 2011), although some of these effects remain debated (see below). This cytokine also stimulates proliferation of mesenchymal fibroblasts and regulates the contractile response of mural cells, organized around the endothelium during collateral vessel growth (Yonekura et al 1999;Bellik et al 2005). In addition, PlGF recruits myeloid progenitors to growing sprouts and collateral vessels (Hattori et al 2002;Luttun et al 2002;Pipp et al 2003;Rafii et al 2003;Scholz et al 2003).…”

Vascular Endothelial Growth Factor Receptor 1 Contributes toEscherichia coliK1 Invasion of Human Brain Microvascular Endothelial Cells through the Phosphatidylinositol 3-Kinase/Akt Signaling Pathway

“…To date, VEGFR1 remains the elusive member of the VEGFR family. Although the signaling potential of VEGFR1 remains controversial, it has seemed that VEGFR1 plays a role in inducing signaling responses, especially in the cell types that do not express or express low levels of VEGFR2 (4,6). We found that PI3K/Akt signaling mediated by VEGFR1, but not VEGFR2, is involved in E. coli K1 invasion of HBMEC, and our results showed that the expression level of VEGFR1 is higher than that of VEGFR2 in HBMEC.…”

VEGF receptor signaling in vertebrate development

“…51 It is also possible that PGF binding to FLT1 promotes the transphosphorylation of KDR by FLT1 in FLT1/KDR heterodimers to increase VEGF/KDR signaling. 48 Lastly, PGF activation of FLT1 may stimulate vessel formation and maturation indirectly by acting on non-endothelial cell types, for example smooth muscle cells 24,52 or bone marrow derived cells that are recruited to sites of neovascularization. 25,53,54 It is presently debated whether proangiogenic bone marrow derived cells support tumor angiogenesis by differentiating into endothelial cells 55 or by providing perivascular support cells.…”

“…23 Thus, Flt1 is upregulated in vascular smooth muscle cells experiencing hypoxic stress, perhaps to control vascular remodelling or tone. 24 However, further studies are required to fully understand the physiological significance of the transcriptional regulation of FLT1 by hypoxia, and how it may complement the regulation of VEGF by hypoxia. In contrast to Vegfa and Flt1, Kdr has no HIF1A binding sites in its promoter region and is therefore not regulated by hypoxia.…”