Signaling hierarchy downstream of retinoic acid that independently regulates vascular remodeling and endothelial cell proliferation

Abstract

We previously demonstrated that during vascular morphogenesis, retinoic acid (RA) is required for the control of endothelial cell proliferation and capillary plexus remodeling. Herein, we investigate the mechanisms by which RA regulates these processes in the yolk sac. We found that although the enzyme required for RA production during early embryogenesis, retinaldehyde dehydrogenase-2 (Raldh2), was expressed in the visceral endoderm, RA receptors ␣1 and ␣2 were expressed in endothelial cells in the mesoderm, indicating that they are direct targets of RA. In Raldh2 −/− embryos, there was down-regulation of TGF-␤1, fibronectin (Fn) and integrin ␣5, which was associated with decreased visceral endoderm survival and production of VEGF-A, Indian hedgehog (IHH), and bFGF. Exogenous provision of RA or Fn to Raldh2 −/− explants in whole mouse embryo culture restored vascular remodeling, visceral endoderm survival, as well as integrin ␣5 expression and its downstream signaling that controls endothelial growth. Exogenous provision of visceral endoderm-derived factors (VEGF-A, IHH, and bFGF) failed to rescue endothelial cell proliferative control but collectively promoted vascular remodeling, suggesting that these processes are independently regulated via a signaling hierarchy downstream of RA.

Retinoic acid induces TGFβ-dependent autocrine fibroblast growth

“…However, it has been shown that T-RA reduces SMAD phosphorylation in HL60 cells, thus negatively regulating TGFb-signalling and differentiation into monocytes (Cao et al, 2003). T-RA has also been shown to regulate TGFb expression and proliferation in embryonic palate mesenchymal cells (Nugent et al, 1998) and more recently, to regulate TGFb in the embryonal yolk sac to ensure visceral endoderm survival (Bohnsack et al, 2004). Together with our present results, these observations shown that T-RATGFb cross talk occurs in a variety of tissues and can have distinct functional consequences depending on the cell type.…”

The oxidizing enzyme CYP26a1 tightly regulates the availability of retinoic acid in the gastrulating mouse embryo to ensure proper head development and vasculogenesis

“…We have been further studying Por Ϫ/Ϫ mutants, and used various approaches to demonstrate that both their vascular and other phenotypic traits are the result of elevated endogenous RA activity (Ribes et al, 2006). Other work performed on Raldh2 Ϫ/Ϫ mouse mutants has led to the conclusion that RA may act during the early steps of vascular development to regulate the proliferation and survival of endothelial precursors, most likely through the TGF␤ and fibronectin pathways (Lai et al, 2003;Bohnsack et al, 2004). In the chick embryo, both Cyp26a1 and Cyp26b1 gene expression is strongly upregulated in the vascular endothelium following excess RA application (Reijntjes et al, 2005).…”

Molecular and developmental biology of the hemangioblast

“…It is also possible that combination of several factors may be required for the formation of the hemangioblast, endothelial, and hematopoietic lineages. Hedgehog, BMP4, retinoid acid, VEGF, and unknown factors may play synergistic roles in this process, in a tightly regulated spatial and temporal manner, since each of these signaling molecules was shown to be expressed in the extraembryonic yolk sac and be essential for both lineage development in mice and/or zebrafish (Winnier et al, 1995;Carmeliet et al, 1996;Ferrara et al, 1996;Dyer et al, 2001;Lawson et al, 2001;Byrd et al, 2002;Bohnsack et al, 2004;Covassin et al, 2006;Gupta et al, 2006;Bahary et al, 2007). Future genetic and molecular analyses of compound mutant embryos in mice and zebrafish may elucidate major signals in this process.…”

“…Molecular identity and plasticity of hemangioblasts are not fully characterized. Signals including hedgehogs, retinoid acid, fibroblast growth factors, BMP4, and VEGFs from the extraembryonic visceral endoderm and mesoderm and/or notochord are involved in the generation of endothelial and hematopoietic lineages (Winnier et al, 1995;Lawson et al, 1999;Dyer et al, 2001;Byrd et al, 2002;Damert et al, 2002;Bohnsack et al, 2004;Park et al, 2004). Inactivation of a single VEGF allele, of VEGFR-2 (flk1) or of phospholipase C-gamma 1 (Plcg1) in mice has reduced angioblasts and hematopoietic stem cells (HSCs; Shalaby et al, 1995Shalaby et al, , 1997Carmeliet et al, 1996;Ferrara et al, 1996;Damert et al, 2002;Liao et al, 2002).…”