Retinoids in the modulation of vascular inflammation

Abstract: Vascular disease is multifactorial. Smooth muscle cells, the major constituent of the normal vessel wall, play a pivotal role. The pathogenesis includes cellular differentiation, proliferation and inflammatory activation. Retinoids have been shown to influence all these processes and have therefore been identified as potential therapeutic agents in vascular pathology. However, knowledge about the role of retinoids in vascular disease is limited. The aim of this thesis was to investigate the effects of retinoids on vascular inflammation and vascular injury with special focus on vascular SMCs. In manifest atherosclerosis with impared blood flow due to reduced vessel diameter, therapeutic endovascular interventions including angioplasty and stent implantations are performed. The long-term outcome of these interventions is negatively influenced by the development of restenosis, in which proliferation of vascular SMCs is a key process. Retinoids are known regulators of cellular proliferation. We explored this mechanism and identified a retinoic acid receptor-cc mediated inhibition of SMC growth. We also showed that retinoids inhibit neointima formation after vascular angioplasty, resulting in increased luminal diameter. Inflammation is a significant component of many forms of vascular pathology. In atherosclerosis, the inflammation is chronic, localized, low-grade and restricted to large arteries. In septic shock, the inflammation is acute, intense and generalized. Although clinically diverse, these processes share properties at the molecular level. Nitric Oxide (NO) is an important regulator of the homeostasis in the vessel wall and offer protection against an early phase of atherogenesis. However, high concentrations, produced by the inducible nitric oxide synthase (iNOS) in activated SMCs, are pro-inflammatory. The high local NO concentrations seen in atherosclerosis may cause cell- and tissue damage, whereas the high systemic levels in septic shock may contribute to vasoplegia and multiple organ failure. We hypothesized that retinoids exert some of their modulatory effects on inflammation through the iNOS pathway. Our results showed that all-trans retinoic acid, the biologically active retinoid ligand, inhibits iNOS transcription and thereby NO production in cytokinestimulated vascular SMCs through the nuclear Retinoic Acid Receptor-alpha. In addition, we showed increased survival in endotoxemic rats when treated with synthetic retinoid agonists. Retinoid receptors act as ligand-activated transcription factors, which require active retinoid ligands intracellularly. Ligands may originate from intracellular synthesis or uptake of preformed active retinoid ligands from extracellular sources. Vascular SMCs are naturally exposed to the plasma content of circulating retinoids. Since the plasma concentration ratio of all-trans RA to all-trans ROH is about 1: 1000, the availability of the active ligand is very limited compared to the inactive pre-form. Thus, cellular biosynthesis of all-trans RA may strongly influence intracellular concentrations of active retinoid ligands and hence the transcriptional activity. Nonetheless, the role of endogenous retinoid ligands in the regulation of genes of vascular importance has not, so far, retained much interest. The modulatory effect of retinoids on pathological processes in the vascular wall has almost exclusively been studied in models of exogenous administrated active retinoid ligands. We therefore aimed to investigate the metabolism of retinoids and the generation of active retinoid ligands in vascular SMCs. These cells were shown to express several metabolizing enzymes and were competent producers of active retinoid ligands. Interestingly, we found that pro-inflammatory cytokines increase the production of active retinoid ligands in vascular SMCs. Hence, a link between endogenous retinoid metabolism and vascular inflammation was identified. Realizing the potential importance of retinoid metabolism for the regulation of vascular inflammation, we compared the ability of biosynthesis of active retinoid ligands between intimal and medial vascular SMCs. Since intimal and medial SMCs are phenotypically different, we hypothesized that they differ in their capacity to synthesize active retinoid ligands. Intimal SMCs displayed increased retinoid metabolism and subsequent increased production of active retinoid ligands compared to medial SMCs. Thus, retinoid metabolism is linked to the phenotype of SMCs. In summary, our studies suggest that the availability of active retinoid ligands in vascular smooth muscle cells influences the vascular response to inflammation and damage. The recognition of retinoids as important modulators in prevalent vascular pathology implies a potential therapeutic role for these agents in the treatment of certain vascular diseases.