TGFβ, smooth muscle cells and coronary artery disease: a review

Abstract

Excessive vascular smooth muscle cell (SMC) proliferation, migration and extracellular matrix (ECM) synthesis are key events in the development of intimal hyperplasia, a pathophysiological response to acute or chronic sources of vascular damage that can lead to occlusive narrowing of the vessel lumen. Atherosclerosis, the primary cause of coronary artery disease, is characterised by chronic vascular inflammation and dyslipidemia, while revascularisation surgeries such as coronary stenting and bypass grafting represent acute forms of vascular injury. Gene knockouts of transforming growth factor-beta (TGFβ), its receptors and downstream signalling proteins have demonstrated the importance of this pleiotropic cytokine during vasculogenesis and in the maintenance of vascular homeostasis. Dysregulated TGFβ signalling is a hallmark of many vascular diseases, and has been associated with the induction of pathological vascular cell phenotypes, fibrosis and ECM remodelling. Here we present an overview of TGFβ signalling in SMCs, highlighting the ways in which this multifaceted cytokine regulates SMC behaviour and phenotype in cardiovascular diseases driven by intimal hyperplasia.

Keywords: Cardiovascular disease; Revascularisation surgery; Smads; Smooth muscle cells; Transforming growth factor-beta; Vascular cells.

Fig. 1

- Canonical TGFβ signalling pathway. Active TGFβ homodimers signal via binding to specific transmembrane receptor complexes comprised of two type I (TβRI) and two type II (TβRII) serine/threonine kinase receptors. TβRI and TβRII are structurally similar with small cysteine-rich extracellular domains (ECD), single transmembrane domains (TMD) and highly conserved intracellular serine/threonine domains (S/TKD). TGFβ binding to TβRII induces the assembly of TβRII and TβRI receptors into a heteromeric complex, within which constitutively active TβRII phosphorylates TβRI at several serine and threonine residues within its conserved glycine-serine domain (GSD). R-Smads become phosphorylated by the activated TβRI at their C-terminal SSXS motif. The L45 loop of TβRI and the L3 loop of the R-Smad MH2 domain determine R-Smad receptor specificity, with ALK5 specifically phosphorylating Smads 2 and 3. The adaptor protein Smad anchor for receptor activation (SARA) can also facilitate recognition of R-Smads by the receptors. I-Smads contain MH2 domains and can act to turn off Smad TGFβ signalling by interfering with Smad-receptor or Smad-Smad interactions. Phosphorylated R-Smads form a heteromeric complex with Co-Smad, accumulate in the nucleus and directly regulate the transcription of specific target genes.

Fig. 2

– Vascular remodelling during atherosclerosis (A) and after revascularisation surgery (B) (A) Atherosclerosis is initiated by the activation of the endothelium in response to oxidative, haemodynamic or biochemical stimuli. Activated endothelial cells (ECs) upregulate surface adhesion molecules and secrete growth factors and cytokines, promoting rolling adhesion of circulating leukocytes as well as activation of the underlying smooth muscle cells (SMCs). Activated SMCs dedifferentiate and start proliferating and migrating, contributing to the growing neointima. Leukocytes adhering to the endothelium migrate into the intima through diapedesis, maturing into macrophages and phagocytosing low density lipoproteins to become foam cells, characteristic of the ‘fatty streak’ lesions that can be observed from adolescence onwards. Fibroatheromas form from areas of intimal thickening, which consist of foam cells, remnants of apoptotic SMC and a lipid rich ECM pool. Early fibroatheromas are characterised by an acellular necrotic core and a thick fibrous cap, composed of collagen fibrils interspersed with SMCs. Advancing fibroatheromas contain cholesterol crystals, neovessels and lymphocytes, and have thin fibrous caps due to proteolytic ECM degradation, making these lesions particularly susceptible to rupture and thrombosis. Rupture and thrombosis frequently occurs at the shoulder regions of plaques, where mast cells accumulate and secrete pro-angiogenic factors and enzymes to further promote microvessel formation. (B) Vein graft implantation or coronary stent deployment induces endothelial damage and denudation. Within hours, platelets and red blood cells adhere to the endothelial layer, initiating a coagulation cascade that results in the deposition of fibrin-rich layers. In the weeks following surgery, circulating leukocytes attach and infiltrate the vascular endothelium, while SMCs in the media are activated and start migrating into the growing neointima. Growth factors and cytokines released by cells in the vessel wall induce SMC proliferation and ECM deposition, resulting in further intimal thickening and inward vascular remodelling. Intimal thickening can act as a substrate for superimposed atherosclerosis or neoatherosclerosis, which is frequently observed between 2-5 years following revascularisation surgery. The pathogenesis of superimposed atherosclerosis/neoatherosclerosis bears many similarities with native coronary artery atherosclerosis (A), albeit within a much shorter timeframe.