Platelet-derived growth factor-BB mediates cell migration through induction of activating transcription factor 4 and tenascin-C.

The acute response to vascular cell injury, which underpins vasculo-occlusive pathologies such as atherogenesis and restenosis after percutaneous coronary intervention, involves a complex series of molecular events that alter patterns of gene expression and favor a synthetic phenotype. One transcription factor that has been implicated in this process is the evolutionarily conserved mammalian stress response pathway regulator activating transcription factor 4 (ATF-4). Here, we show for the first time that both mRNA and protein levels of ATF-4 are induced in smooth muscle cells (SMCs) by the potent migratory factor PDGF-BB through PDGFR-ß. PDGF-BB also stimulates the expression of tenascin-C (TN-C), an extracellular matrix glycoprotein that regulates the activity of focal adhesion complexes, facilitating the SMC migration that underlies negative vascular remodeling in response to injury. Overexpression of ATF-4 increased transcript levels of the four TN-C isoforms in rat vascular SMCs, and ATF-4 knockdown inhibited PDGF-BB-inducible TN-C expression in vitro and injury-inducible TN-C protein expression in the balloon-injured rat artery wall. Furthermore, we show that ATF-4 is required for PDGF-BB-inducible SMC migration in response to injury. PDGF-BB-induced migration was also compromised in ATF-4 null mEFs, and this effect was rescued by the addition of TN-C. Our findings thus demonstrate the role of ATF-4 in both injury- and PDGF-BB-inducible TN-C expression and cell migration.

Interplay between heme oxygenase-1 and the multifunctional transcription factor yin yang 1 in the inhibition of intimal hyperplasia.

RATIONALE: induction of heme oxygenase (HO)-1 protects against experimental atherosclerotic diseases, and certain pharmacological HO-1 inducers, like probucol, inhibit the proliferation of vascular smooth muscle cells and, at the same time, promote the growth of endothelial cells in vivo and in vitro. OBJECTIVE: because such cell-specific effects are reminiscent of the action of the transcription factor Yin Yang (YY)1, we tested the hypothesis that there is a functional relationship between HO-1 and YY1. METHODS AND RESULTS: we report that probucol increases the number of YY1(+) cells in rat carotid artery following balloon injury at a time coinciding with increased HO-1 expression. The drug also induces the expression of YY1 mRNA and protein in rat aortic smooth muscle cells (RASMCs) in vitro, as do other known HO-1 inducers (tert-butylhydroquinone and hemin) and overexpression of HO-1 using a human HMOX1 cDNA plasmid. Conversely, overexpression of YY1 induces expression of HO-1 in RASMCs. Induction of YY1 expression is dependent on HO-1 enzyme activity and its reaction product CO, because pharmacological inhibition of heme oxygenase activity or CO scavenging block, whereas exposure of RASMCs to a CO-releasing molecule increases, YY1 expression. Furthermore, RNA interference knockdown of YY1 prevents probucol or adeno-HO-1 from inhibiting RASMC proliferation in vitro and neointimal formation in vivo. CONCLUSIONS: our findings show, for the first time, that HO-1 functionally interplays with the multifunctional transcription factor YY1 and that this interplay explains some of the protective activities of HO-1.

Activation transcription factor-4 induced by fibroblast growth factor-2 regulates vascular endothelial growth factor-A transcription in vascular smooth muscle cells and mediates intimal thickening in rat arteries following balloon injury.

Activation transcription factor (ATF)-4 is a member of the ATF/CREB family of basic leucine zipper transcription factors that regulates cellular responses to a variety of stresses. The role of ATF-4 in smooth muscle cells of the vessel wall is completely unknown. Here, we show that ATF-4 expression is induced in smooth muscle cells in response to injury, both in vitro using a model of mechanical injury and in the media of balloon-injured rat carotid arteries. We demonstrate that ATF-4 is activated by fibroblast growth factor (FGF)-2, an injury-induced mitogen, through the phosphatidylinositol 3-kinase pathway. Injury also activates vascular endothelial growth factor (VEGF)-A, whose expression is stimulated by ATF-4 overexpression and exposure to FGF-2. FGF-2 induces ATF-4 binding to a recognition element located in the VEGF-A gene at +1767 bp and luciferase reporter gene expression dependent on this site. Moreover, ATF-4 knockdown with small interfering RNA or ATF-4 deficiency ameliorates FGF-2-inducible VEGF-A expression. Intraluminal delivery of ATF-4 small interfering RNA in rat carotid arteries blocks balloon injury-inducible ATF-4 and VEGF-A expression after 4 hours and intimal thickening after 14 days. These findings reveal, for the first time, the induction of ATF-4 by both vascular injury and FGF-2. ATF-4 serves as a conduit for the inducible expression of 1 growth factor by another during the process of intimal thickening.

Activation transcription factor-4 and the acute vascular response to injury.

Atherosclerosis is a complex fibroproliferative-inflammatory process triggered by vascular injury. Transcription factors play an important role in the control of genes that effect critical changes in the vessel wall. Recent evidence indicates an emerging role for activation transcription factor 4 (ATF4), a master regulator for evolutionarily conserved mammalian stress response pathways, in cardiovascular pathologic settings. For example, in endothelial cells, ATF4 is induced by atherogenic factors such as oxidised phospholipids and homocysteine, and in monocytes, ATF4 is activated by hypoxia. In this context, ATF4 is thought to regulate pro-inflammatory signalling cascades and subsequent apoptosis. ATF4 is induced in aortic smooth muscle cells by fibroblast growth factor 2 and in the intact vessel wall following balloon angioplasty. Our own work indicates that ATF4 knockdown blocks injury-inducible intimal proliferation. Furthermore, studies in ATF4-deficient mice have established a role for ATF4 in diet-induced diabetes and hyperlipidaemia. In this article, we will review recent developments on the regulation of this intriguing nuclear protein and its transcriptional roles in the context of vascular injury and related disease.