Fine-tuning vascular fate during endothelial-mesenchymal transition

Abstract

In the heart and other organs, endothelial-mesenchymal transition (EndMT) has emerged as an important developmental process that involves coordinated migration, differentiation, and proliferation of the endothelium. In multiple disease states including cancer angiogenesis and cardiovascular disease, the processes that regulate EndMT are recapitulated, albeit in an uncoordinated and dysregulated manner. Members of the transforming growth factor beta (TGFβ) superfamily are well known to impart cellular plasticity during EndMT by the timely activation (or repression) of transcription factors and miRNAs in addition to epigenetic regulation of gene expression. On the other hand, fibroblast growth factors (FGFs) are reported to augment or oppose TGFβ-driven EndMT in specific contexts. Here, we have synthesized the currently understood roles of TGFβ and FGF signalling during EndMT and have provided a new, comprehensive paradigm that delineates how an autocrine and paracrine TGFβ/FGF axis coordinates endothelial cell specification and plasticity. We also provide new guidelines and nomenclature that considers factors such as endothelial cell heterogeneity to better define EndMT across different vascular beds. This perspective should therefore help to clarify why TGFβ and FGF can both cooperate with or oppose one another during the complex process of EndMT in both health and disease. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Keywords: TGFβ; bFGF; cardiovascular disease; endothelial heterogeneity; endothelial plasticity; endothelial-to-mesenchymal transition; tumour microenvironment.

Fig. 1. bFGF and TGFβ signalling converge to regulate EndMT

Heterogeneous EC populations consisting of at least two different subpopulations undergo distinct forms of TGFβ-stimulated EndMT. One EC subpopulation gives rise to myofibroblast-like cells (SMA⁺) while the other are SMA⁻ “fibroblast-like” cells. It is unknown if the two EC subtypes interconvert in vivo. TGFβ stimulates EndMT and the conversion of SMA⁻ fibroblasts to myofibroblasts (SMA⁺), whereas bFGF counteracts TGFβ signalling during these processes. Acquisition of a stable mesenchymal phenotype requires overcoming epigenetic barriers that maintain endothelial specification/identity. During EndMT, bFGF can also play a cooperative role by stimulating proliferation of differentiated myofibroblasts and fibroblasts.

Fig. 2. Molecular mechanisms underlying bFGF’s antagonistic effect on TGFβ signalling during EndMT

A) During EndMT, ECs up-regulate bFGF in response to TGFβ stimulation. bFGF secreted by these transitioning ECs counteracts TGFβ and maintains EC identity through autocrine/paracrine loops. B) TGFβ signals through a TGFβR1 and TGFβR2 receptor complex, which activates SMAD2/3 (and non-SMAD pathways) to induce EndMT. bFGF neutralizes TGFβ signalling by targeting multiple nodes of the TGFβ pathway. bFGF increases “anti-EndMT” microRNAs including let-7 and miR-20a leading to the down-regulation of TGFβ receptors and a receptor-associated protein SARA. Another pathway activated by bFGF is the Ras/MEK/ERK branch, which can oppose TGFβ signalling by suppressing SMAD2 phosphorylation. Additional auxiliary regulatory mechanisms through MEK/ERK may also contribute to bFGF’s antagonistic activities.

Fig. 3. Endothelial homeostasis and phenotype switching are coordinated by bFGF and TGFβ

A balance of bFGF and TGFβ signalling is required for endothelial homeostasis and EndMT. High bFGF signalling alone preserves EC specification and gene expression, whereas high TGFβ and low bFGF signalling drives ECs to undergo EndMT. During EndMT, ECs transition through a spectrum of reversible intermediate stages before differentiating into a stable and committed (myo)fibroblast phenotype.