All-trans-retinoic acid attenuates neointima formation with acceleration of reendothelialization in balloon-injured rat aorta

Vitamin A as a Transcriptional Regulator of Cardiovascular Disease

Vitamin A is a micronutrient and signaling molecule that regulates transcription, cellular differentiation, and organ homeostasis. Additionally, metabolites of Vitamin A are utilized as differentiation agents in the treatment of hematological cancers and skin disorders, necessitating further study into the effects of both nutrient deficiency and the exogenous delivery of Vitamin A and its metabolites on cardiovascular phenotypes. Though vitamin A/retinoids are well-known regulators of cardiac formation, recent evidence has emerged that supports their role as regulators of cardiac regeneration, postnatal cardiac function, and cardiovascular disease progression. We here review findings from genetic and pharmacological studies describing the regulation of both myocyte- and vascular-driven cardiac phenotypes by vitamin A signaling. We identify the relationship between retinoids and maladaptive processes during the pathological hypertrophy of the heart, with a focus on the activation of neurohormonal signaling and fetal transcription factors (Gata4, Tbx5). Finally, we assess how this information might be leveraged to develop novel therapeutic avenues.

All-Trans Retinoic Acid Promotes Osteogenic Differentiation and Bone Consolidation in a Rat Distraction Osteogenesis Model

Distraction osteogenesis (DO) is used to treat specific disorders associated with growth abnormalities and/or loss of bone stock secondary to trauma or disease. However, a high rate of complications and discomfort hamper its further application in clinical practice. Here, we investigated the effects of all-trans retinoic acid (ATRA) on osteogenic differentiation of rat bone marrow-derived mesenchymal stem cells (rBMSCs) and bone consolidation in a rat DO model. Different doses of ATRA were used to treat rBMSCs. Cell viability and osteogenic differentiation were assessed using CCK-8 and alkaline phosphatase staining, respectively. The mRNA expression of osteogenic differentiation-genes (including ALP, Runx2, OCN, OPN, OSX, and BMP2) and angiogenic genes (including VEGF, HIF-1, FLK-2, ANG-2, and ANG-4) were determined by quantitative real-time PCR analysis. Further, we locally injected ATRA or PBS into the gap in the rat DO model every 3 days until termination. X-rays, micro-computed tomography (Micro-CT), mechanical testing, and immunohistochemistry stains were used to evaluate the quality of the regenerates. ATRA promoted osteogenic differentiation of rBMSCs. Moreover, ATRA elevated the mRNA expression levels of osteogenic differentiation-genes and angiogenic genes. In the rat model, new bone properties of bone volume/total tissue volume and mechanical strength were significantly higher in the ATRA-treatment group. Micro-CT examination showed more mineralized bone after the ATRA-treatment, and immunohistochemistry demonstrated more new bone formation after ATRA-treatment than that in the PBS group. In conclusion, as a readily available and very cost effective bio-source, ATRA may be a novel therapeutic method to enhance bone consolidation in the clinical setting.

Early inhibition of endothelial retinoid uptake upon myocardial infarction restores cardiac function and prevents cell, tissue, and animal death

Physiologically, following myocardial infarction (MI), retinoid levels elevate locally in the infarcted area. Whereas therapeutic systemic application of retinoids was shown to reduce the progression of ventricular dilatation and the onset of heart failure, the role of acute physiologically increased retinoids in the infarction zone is unknown to date. To reveal the role of local retinoids in the MI zone is the central aim of this study. Using human cell culture and co-culture models for hypoxia as well as various assays systems, lentivirus-based transgene expression, in silico molecular docking studies, and an MI model in rats, we analysed the impact of the retinoid all-trans retinoic acid (ATRA) on cell signalling, cell viability, tissue survival, heart function, and MI-induced death in rats. Based on our results, ATRA-mediated signalling does aggravate the MI phenotype (e.g. 2.5-fold increased mortality compared to control), whereas 5'-methoxyleoligin (5ML), a new agent which interferes with ATRA-signalling rescues the ATRA-dependent phenotype. On the molecular level, ATRA signalling causes induction of TXNIP, a potent inhibitor of the physiological antioxidant thioredoxin (TRX1) and sensitizes cells to necrotic cell death upon hypoxia. 5ML-mediated prevention of ATRA effects were shown to be based on the inhibition of cellular ATRA uptake by interference with the cholesterol (and retinol) binding motif of the transmembrane protein STRA6. 5ML-mediated inhibition of ATRA uptake led to a strong reduction of ATRA-dependent gene expression, reduced ROS formation, and protection from necrotic cell death. As 5ML exerted a cardioprotective effect, also independent of its inhibition of cellular ATRA uptake, the agent likely has another cardioprotective property, which may rely on the induction of TRX1 activity. In summary, this is the first study to show i) that local retinoids in the early MI zone may worsen disease outcome, ii) that inhibition of endothelial retinoid uptake using 5ML may constitute a novel treatment strategy, and iii) that targeting endothelial and myocardial retinoid uptake (e.g. via STRA6 inhibition) may constitute a novel treatment target in acute MI.

All-trans retinoic acid improves the viability of ischemic skin flaps in diabetic rat models

AIMS

Endothelial progenitor cells (EPCs) play a critical role in neovascularization, which enhances proliferation under all-trans retinoic acid (ATRA) treatment. However, the effects of ATRA on the skin flap survival in diabetic flap ischemia remains unknown.

METHODS

Ischemic random skin flaps were made in 40 diabetic Sprague-Dawley rats with 20 normal rats used as control in this study. At 7 days postoperatively, the surviving area of each skin flap was measured. Immunofluorescence staining was used to analyze capillary density and EPCs recruited to the flaps. The expression of ANG2 and VEGF was determined by Western blotting. Circulating EPC number was determined by flow cytometry. In vitro tube formation experiment was used to analyze the function of EPCs.

RESULTS

The flap survival rate and capillary density of ATRA-treated flap were significantly increased. Fluorescence-activated cell sorting (FACS) analysis demonstrated a marked increase in systemic CD34+/Flk-1+ EPCs in ATRA-treated rat. The expression of ANG2 and VEGF was increased in diabetic flap tissues under ATRA administration. Furthermore, ATRA administration restored the impaired function of diabetic EPCs in tube formation.

CONCLUSION

ATRA could notably exert preventive effects against skin flap necrosis and promote neovascularization in diabetic rats, which may partially through elevating the expression of ANG2 and VEGF, and augmenting EPC mobilization.

Inhibiting intimal hyperplasia in prosthetic vascular grafts via immobilized all-trans retinoic acid

Peripheral arterial disease is a leading cause of morbidity and mortality. The most commonly utilized prosthetic material for peripheral bypass grafting is expanded polytetrafluoroethylene (ePTFE) yet it continues to exhibit poor performance from restenosis due to neointimal hyperplasia, especially in femoral distal bypass procedures. Recently, we demonstrated that periadventitial delivery of all-trans retinoic acid (atRA) immobilized throughout porous poly(1,8 octamethylene citrate) (POC) membranes inhibited neointimal formation in a rat arterial injury model. Thus, the objective of this study was to investigate whether atRA immobilized throughout the lumen of ePTFE vascular grafts would inhibit intimal formation following arterial bypass grafting. Utilizing standard ePTFE, two types of atRA-containing ePTFE vascular grafts were fabricated and evaluated: grafts whereby all-trans retinoic acid was directly immobilized on ePTFE (atRA-ePTFE) and grafts where all-trans retinoic acid was immobilized onto ePTFE grafts coated with POC (atRA-POC-ePTFE). All grafts were characterized by SEM, HPLC, and FTIR and physical characteristics were evaluated in vitro. Modification of these grafts, did not significantly alter their physical characteristics or biocompatibility, and resulted in inhibition of intimal formation in a rat aortic bypass model, with atRA-POC-ePTFE inhibiting intimal formation at both the proximal and distal graft sections. In addition, treatment with atRA-POC-ePTFE resulted in increased graft endothelialization and decreased inflammation when compared to the other treatment groups. This work further confirms the biocompatibility and efficacy of locally delivered atRA to inhibit intimal formation in a bypass setting. Thus, atRA-POC-ePTFE grafts have the potential to improve patency rates in small diameter bypass grafts and warrant further investigation.

Expression of retinoic acid target genes in coronary artery disease

Coronary atherosclerosis can lead to myocardial infarction, and secondarily to post-infarct remodelling and heart failure. Retinoic acid (RA) influences cell proliferation. We hypothesized that RA could influence gene expression and proliferation of cardiovascular cells. Left ventricular biopsies from patients with end-stage heart failure due to coronary artery disease (CAD) or dilated cardiomyopathy were investigated for the content of RA metabolites using liquid chromatography mass spectrometry (LC-MS/MS), and compared with healthy donors. All-trans retinoic acid (ATRA) was increased in the hearts of CAD patients. Gene expression (quantitative PCR) of RA target genes was not influenced in failing hearts, but was increased in the hearts of patients with CAD undergoing open heart surgery. The expression of RA target genes was increased in atherosclerotic lesions from carotid arteries compared to healthy arteries. Stimulation of cardiomyocytes, cardiofibroblasts, smooth muscle cells and endothelial cells with ATRA increased the gene expression of the key enzymes. Cardiofibroblast and smooth muscle cell proliferation were reduced by ATRA, which increased endothelial cell proliferation. Coronary artery disease leads to increased expression of RA target genes. ATRA accumulated in the failing human heart. All investigated cell types present in the heart had induced expression of RA target genes when stimulated with ATRA, which also influenced cell proliferation.

Retinoid metabolism and its effects on the vasculature

Retinoids, the metabolically-active structural derivatives of vitamin A, are critical signaling molecules in many fundamental biological processes including cell survival, proliferation and differentiation. Emerging evidence, both clinical and molecular, implicates retinoids in atherosclerosis and other vasculoproliferative disorders such as restenosis. Although the data from clinical trials examining effect of vitamin A and vitamin precursors on cardiac events have been contradictory, this data does suggest that retinoids do influence fundamental processes relevant to atherosclerosis. Preclinical animal model and cellular studies support these concepts. Retinoids exhibit complex effects on proliferation, growth, differentiation and migration of vascular smooth muscle cells (VSMC), including responses to injury and atherosclerosis. Retinoids also appear to exert important inhibitory effects on thrombosis and inflammatory responses relevant to atherogenesis. Recent studies suggest retinoids may also be involved in vascular calcification and endothelial function, for example, by modulating nitric oxide pathways. In addition, established retinoid effects on lipid metabolism and adipogenesis may indirectly influence inflammation and atherosclerosis. Collectively, these observations underscore the scope and complexity of retinoid effects relevant to vascular disease. Additional studies are needed to elucidate how context and metabolite-specific retinoid effects affect atherosclerosis. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

Vitamin A: a drug for prevention of restenosis/reocclusion after percutaneous coronary intervention?

The re-establishment of adequate blood flow in a vessel with a reduced lumen due to an atherosclerotic plaque by percutaneous vascular intervention is a well established procedure. However, the long-term outcome of such interventions is negatively influenced by the development of intimal hyperplasia/restenosis. Although extensively researched, this still represents a significant clinical problem. Retinoids, i.e. natural and synthetic derivates of vitamin A, represent a potential therapeutic compound, since they have been shown to influence the vast majority of processes that ultimately lead to reocclusion of the injured vessel. Retinoids exert their effects at the transcriptional level through their nuclear receptors. Targeting multiple processes, i.e. proliferation, migration, extracellular matrix composition and cell differentiation, as well as coagulation/fibrinolysis, should increase their future role in the prevention of restenosis. The purpose of this review is to summarize the diverse effects of retinoids on pathobiological and biological processes activated at sites of vascular injury with particular emphasis on intimal hyperplasia/restenosis after endovascular interventions.

beta-Cryptoxanthin, a novel natural RAR ligand, induces ATP-binding cassette transporters in macrophages

Despite its serious adverse effects, recent accumulating evidence suggests that a physiological retinoic acid receptor (RAR) agonist, all-trans retinoic acid (atRA), exhibits preventive effects on atherogenesis. Therefore, the present study was designed to explore novel natural RAR ligands with anti-atherogenic effects in order to identify and develop a drug without severe side effects. Among xanthophylls and carotenoids studied, beta-cryptoxanthin and lutein exhibited RAR ligand activity in yeast two-hybrid system that was found to be completely abolished by the RAR pan-antagonist LE540. Furthermore, these molecules can bind the RAR ligand-binding domain in the CoA-BAP system but not RXR ligand-binding domain. These results indicate that both beta-cryptoxanthin and lutein serve as ligands for RAR, but not RXR, although their binding affinity was three orders of magnitude lower than that of atRA. Additionally, when applied to macrophages, beta-cryptoxanthin indeed was found to induce the ATP-binding cassette transporter A1 (ABCA1) and ABCG1 mRNAs, which exert anti-atherosclerotic effects by preventing cholesteryl ester accumulation in macrophages. The induction of ABCA1 proteins by beta-cryptoxanthin as well as atRA was abrogated by LE540. In summary, beta-cryptoxanthin appears to be more an efficient provitamin A source than other carotenoids and xanthophylls including beta-carotene, since beta-cryptoxanthin can act not only as a RAR agonist but also a source of vitamin A. Taking into account that the pharmacodynamics difference between beta-cryptoxanthin and atRA, beta-cryptoxanthin appears to exert beneficial effects on atherogenesis through RAR activation in the manner different from atRA.

Upregulation of nitric oxide production in vascular endothelial cells by all-trans retinoic acid through the phosphoinositide 3-kinase/Akt pathway

BACKGROUND

A natural retinoid all-trans retinoic acid (ATRA) contains various beneficial effects on vasculature, including suppression of neointima formation after balloon injury. However, little is known about the effects of ATRA on vascular endothelial function. We therefore studied its role in nitric oxide (NO) production of vascular endothelial cells (ECs).

METHODS AND RESULTS

Human dermal microvascular ECs, human umbilical vein ECs, and SV40-transformed rat lung vascular ECs were incubated with or without ATRA (1 micromol/L) for 48 hours. Their NO production was determined with the use of a fluorescent NO indicator, diaminofluorescein-2 diacetate. ATRA significantly increased their basal as well as acetylcholine-induced NO production. Treatment with Nomega-nitro-L-arginine methyl ester or carboxy-PTIO suppressed their fluorescence. Increase of NO production was also observed by incubation with retinoic acid receptor (RAR) agonist Am580. ATRA-induced NO increase was abolished by coincubation with RAR antagonist LE540. Moreover, the NO increase was completely inhibited by the phosphoinositide 3-kinase (PI3K) inhibitor wortmannin and LY294002. ATRA as well as Am580 enhanced endothelial NO synthase (eNOS) phosphorylation at Ser-1177 as well as Akt phosphorylation at Ser-473 without changing their protein expression. Overexpression of dominant-negative Akt inhibited the eNOS phosphorylation. Moreover, ATRA increased PI3K activity as well as PI3K catalytic subunit p110beta protein expression, which was completely inhibited by LE540 treatment. Real-time polymerase chain reaction analyses demonstrated that ATRA increased PI3K catalytic subunit p110beta mRNA expression without affecting its stability. Finally, ATRA-induced NO increase was observed in COS-1 cells transfected with wild-type eNOS and RARalpha, but not with mutated eNOS whose Ser-1177 was substituted.

CONCLUSIONS

ATRA increases NO production by eNOS phosphorylation through RAR-mediated PI3K/Akt pathway activation in vascular ECs and possibly plays beneficial roles in vascular endothelium. Retinoids may therefore be candidates as novel therapeutic agents against vascular disorders with endothelial damage.

Vitamin A and infancy. Biochemical, functional, and clinical aspects

Vitamin A is a very intriguing natural compound. The molecule not only has a complex array of physiological functions, but also represents the precursor of promising and powerful new pharmacological agents. Although several aspects of human retinol metabolism, including absorption and tissue delivery, have been clarified, the type and amounts of vitamin A derivatives that are intracellularly produced remain quite elusive. In addition, their precise function and targets still need to be identified. Retinoic acids, undoubtedly, play a major role in explaining activities of retinol, but, recently, a large number of physiological functions have been attributed to different retinoids and to vitamin A itself. One of the primary roles this vitamin plays is in embryogenesis. Almost all steps in organogenesis are controlled by retinoic acids, thus suggesting that retinol is necessary for proper development of embryonic tissues. These considerations point to the dramatic importance of a sufficient intake of vitamin A and explain the consequences if intake of retinol is deficient. However, hypervitaminosis A also has a number of remarkable negative consequences, which, in same cases, could be fatal. Thus, the use of large doses of retinol in the treatment of some human diseases and the use of megavitamin therapy for certain chronic disorders as well as the growing tendency toward vitamin faddism should alert physicians to the possibility of vitamin overdose.

all-trans-Retinoic acid increases nitric oxide synthesis by endothelial cells: a role for the induction of dimethylarginine dimethylaminohydrolase

all-trans-Retinoic acid (atRA) has important effects on the developing and mature cardiovascular system. Nitric oxide (NO) production has been associated with the atRA-induced differentiation of neuronal cells, and we hypothesized that NO may also mediate certain actions of atRA in the cardiovascular system. We studied the effects of atRA on NO production by endothelial cells and determined whether regulation of enzymes responsible for metabolism of asymmetric dimethylarginine (ADMA) contributed to the effects seen. Murine endothelioma (sEnd.1) cells were incubated with or without atRA. Nitrite production was determined using the Griess reaction. The expression of NO synthase (NOS) and dimethylarginine dimethylaminohydrolase (DDAH) genes was determined by Northern blotting. A reporter gene assay was also used to study the effect of atRA on the DDAH II promoter. atRA significantly increased nitrite production by sEnd.1 cells despite no increase in eNOS expression. atRA also increased DDAH II gene expression and promoter activity and reduced the ratio of ADMA to symmetric dimethylarginine (SDMA) in culture medium. The DDAH inhibitor 4124W significantly reduced the induction of NO synthesis by atRA. The present study demonstrates that atRA increases NO synthesis in endothelial cells without increasing eNOS expression. atRA also increases the expression of DDAH II, the predominant DDAH isoform in endothelial cells. Our data suggests that the induction of NO synthesis by atRA may be facilitated by DDAH II. This pathway may help to explain some of the effects of atRA on the cardiovascular system.

Effect of vitamin A deficiency on cardiovascular function in the rat

Selected parameters of cardiovascular function were evaluated in vitamin A-deficient rats at 70 days of age. Resting heart rate was increased by an average of 100 bpm (21.4+/-2.7%), whereas resting systolic blood pressure was normal in vitamin A-deficient animals. The maximal contractile force developed per milligram weight of tissue by aortic rings excised from vitamin A-deficient animals was reduced in response to high potassium (-25.0+/-8.7%) and phorbol 12,13-dibutyrate (-36.8+/-8.4%) but was only slightly reduced in response to norepinephrine (-17.8+/-11.1%). Intimal rubbing to remove the endothelium had no effect on the loss in contractile responsiveness, and the relaxant response to acetylcholine was similar between control and vitamin A-deficient tissue groups. This suggests that the decrease in contractility of vascular smooth muscle from the vitamin A-deficient rats did not involve altered release of endothelium-derived vasoactive factors. Western blot analysis suggested a reduction in the protein levels of several differentiation markers including alpha-actin (-22%), calponin (-37%), desmin (-37%), and vinculin (-40%), whereas the level of PKCalpha was unchanged from control values. Our findings indicate a significant decrease in contractile responsiveness of aortic smooth muscle of the vitamin A-deficient rat that may be associated with a down regulation in the expression of contractile-related proteins.

Cloning of a novel retinoid-inducible serine carboxypeptidase from vascular smooth muscle cells

Retinoids block smooth muscle cell (SMC) proliferation and attenuate neointimal formation after vascular injury, presumably through retinoid receptor-mediated changes in gene expression. To identify target genes in SMC whose encoded proteins could contribute to such favorable biological effects, we performed a subtractive screen for retinoid-inducible genes in cultured SMC. Here, we report on the cloning and initial characterization of a novel retinoid-inducible serine carboxypeptidase (RISC). Expression of RISC is low in cultured SMC but progressively increases over a 5-day time-course treatment with all-trans-retinoic acid. A near full-length rat RISC cDNA was cloned and found to have a 452-amino acid open reading frame containing an amino-terminal signal sequence, followed by several conserved domains comprising the catalytic triad common to members of the serine carboxypeptidase family. In vitro transcription and translation experiments showed that the rat RISC cDNA generates an approximately 51-kDa protein. Confocal immunofluorescence microscopy of COS-7 cells transiently transfected with a RISC-His tag plasmid revealed cytosolic localization of the fusion protein. Western blotting studies using conditioned medium from transfected COS-7 cells suggest that RISC is a secreted protein. Tissue Northern blotting studies demonstrated robust expression of RISC in rat aorta, bladder, and kidney with much lower levels in all other tissues analyzed; high level RISC expression was also observed in human kidney. In situ hybridization verified the localization of RISC to medial SMC of the adult rat aorta. Interestingly, expression in kidney was restricted to proximal convoluted tubules; little or no expression was observed in glomerular cells, distal convoluted and collecting tubules, or medullary cells. Radiation hybrid mapping studies placed the rat RISC locus on chromosome 10q. These studies reveal a novel retinoid-inducible protease whose activity may be involved in vascular wall and kidney homeostasis.

A novel retinoid-response gene set in vascular smooth muscle cells

A modified suppression subtractive hybridization assay was performed to uncover genes induced by all-trans retinoic acid in cultured smooth muscle cells (SMC). Northern blotting studies confirmed the induction of 14 genes, many of which have heretofore been unrecognized as retinoid-inducible. Temporal expression and cycloheximide studies allowed us to categorize these genes as either immediate-early (LOX-1, endolyn, Stoned B/TFIIA alpha/beta-like factor, Src Suppressed C Kinase Substrate, and tissue transglutaminase) or delayed (cathepsin-L, ceruloplasmin, epithelin, importin alpha, alpha(8)-integrin, lactate dehydrogenase B, retinol dehydrogenase, spermidine/spermine N(1)-acetyltransferase, and VCAM-1) retinoid-response genes. A survey of rat tissues showed two of the genes (tissue transglutaminase and alpha(8)-integrin) to be highly restricted to vascular tissue. In situ hybridization verified expression of both tissue transglutaminase and alpha(8)-integrin to SMC in balloon-injured rat carotid artery. These findings unveil a new retinoid-response gene set that should be exploited to define molecular pathways involved in the antagonistic effects of retinoids on SMC growth and neointimal formation.

Retinoic acid-induced tissue transglutaminase and apoptosis in vascular smooth muscle cells

Retinoids exert antiproliferative and prodifferentiating effects in vascular smooth muscle cells (SMCs) and reduce neointimal mass in balloon-injured blood vessels. The mechanisms through which retinoids carry out these effects are unknown but likely involve retinoid receptor-mediated changes in gene expression. Here we report the cloning, chromosomal mapping, and biological activity of the retinoid-response gene rat tissue transglutaminase (tTG). Northern blotting studies showed that tTG is rapidly and dose-dependently induced in a protein synthesis-independent manner after stimulation with the natural retinoid all-trans retinoic acid (atRA). The induction of tTG was selective for atRA and its stereoisomers 9-cis and 13-cis RA, because little or no elevation in mRNA expression was observed with a panel of growth factors. Western blotting and immunofluorescence confocal microscopy showed an accumulation of cytosolic tTG protein after atRA stimulation. Radiolabeled cross-linking studies revealed a corresponding elevation in in vitro tTG activity. The increase in tTG activity was reduced in the presence of 2 distinct inhibitors of tTG (monodansylcadaverine and cystamine). atRA-induced tTG mRNA and protein expression were followed by a significant elevation in SMC apoptosis. Such retinoid-induced programmed cell death could be partially inhibited with each tTG inhibitor and was completely blocked when both inhibitors were used simultaneously. These results establish a role for atRA in the sequential stimulation of tTG and apoptosis in cultured SMCs. atRA-mediated apoptosis in SMCs seems to require the participation of active tTG, suggesting a potential mechanistic link between this retinoid-inducible gene and programmed cell death.