Sirtuin 1 retards hyperphosphatemia-induced calcification of vascular smooth muscle cells

Fibrocalcific aortic valve disease: opportunity to understand disease mechanisms using mouse models

Studies in vitro and in vivo continue to identify complex-regulated mechanisms leading to overt fibrocalcific aortic valve disease (FCAVD). Assessment of the functional impact of those processes requires careful studies of models of FCAVD in vivo. Although the genetic basis for FCAVD is unknown for most patients with FCAVD, several disease-associated genes have been identified in humans and mice. Some gene products which regulate valve development in utero also protect against fibrocalcific disease during postnatal aging. Valve calcification can occur via processes that resemble bone formation. But valve calcification can also occur by nonosteogenic mechanisms, such as formation of calcific apoptotic nodules. Anticalcific interventions might preferentially target either osteogenic or nonosteogenic calcification. Although FCAVD and atherosclerosis share several risk factors and mechanisms, there are fundamental differences between arteries and the aortic valve, with respect to disease mechanisms and responses to therapeutic interventions. Both innate and acquired immunity are likely to contribute to FCAVD. Angiogenesis is a feature of inflammation, but may also contribute independently to progression of FCAVD, possibly by actions of pericytes that are associated with new blood vessels. Several therapeutic interventions seem to be effective in attenuating the development of FCAVD in mice. Therapies which are effective early in the course of FCAVD, however, are not necessarily effective in established disease.

Mechanisms of vascular calcification in CKD-evidence for premature ageing?

Ageing is a potent, independent risk factor for cardiovascular disease. Calcification of the vascular smooth muscle cell (VSMC) layer of the vessel media is a hallmark of vascular ageing. Young patients with chronic kidney disease (CKD) exhibit an extremely high cardiovascular mortality, equivalent to that seen in octogenarians in the general population. Even children on dialysis develop accelerated medial vascular calcification and arterial stiffening, leading to the suggestion that patients with CKD exhibit a 'premature ageing' phenotype. It is now well documented that uraemic toxins, particularly those associated with dysregulated mineral metabolism, can drive VSMC damage and phenotypic changes that promote vascular calcification; epidemiological data suggest that some of these same risk factors associate with cardiovascular mortality in the aged general population. Importantly, emerging evidence suggests that uraemic toxins may promote DNA damage, a key factor driving cellular ageing, and moreover, that these ageing mechanisms may reiterate some of those seen in patients with genetically induced progeric syndromes caused by nuclear lamina disruption. This new knowledge should pave the way for the development of novel therapies that target tissue-specific ageing mechanisms to treat vascular decline in CKD.

Epigenetic regulation of vascular smooth muscle cell function in atherosclerosis

Epigenetics involve heritable and acquired changes in gene transcription that occur independently of the DNA sequence. Epigenetic mechanisms constitute a hierarchic upper-level of transcriptional control through complex modifications of chromosomal components and nuclear structures. These modifications include, for example, DNA methylation or post-translational modifications of core histones; they are mediated by various chromatin-modifying enzymes; and ultimately they define the accessibility of a transcriptional complex to its target DNA. Integrating epigenetic mechanisms into the pathophysiologic concept of complex and multifactorial diseases such as atherosclerosis may significantly enhance our understanding of related mechanisms and provide promising therapeutic approaches. Although still in its infancy, intriguing scientific progress has begun to elucidate the role of epigenetic mechanisms in vascular biology, particularly in the control of smooth muscle cell phenotypes. In this review, we will summarize epigenetic pathways in smooth muscle cells, focusing on mechanisms involved in the regulation of vascular remodeling.

MiR-199a-5p promotes migration and tube formation of human cytomegalovirus-infected endothelial cells through downregulation of SIRT1 and eNOS

Human cytomegalovirus (HCMV) infection has been shown to contribute to vascular disease through the induction of angiogenesis. However, the role of microRNA in angiogenesis induced by HCMV infection remains unclear. The present study was thus designed to explore the potential effect of miR-199a-5p on angiogenesis and to investigate the underlying mechanism in endothelial cells. We found that HCMV infection of endothelial cells (ECs) enhanced expression of miR-199a-5p and reduced the SIRT1 protein level at 24 h postinfection (hpi). Transfection with miR-199a-5p mimics significantly suppressed SIRT1 protein expression and promoted cellular migration and tube formation induced by HCMV infection, which could be reversed by transfection with an miR-199a-5p inhibitor. Furthermore, pretreatment with resveratrol depressed motility and tube formation of HCMV-infected ECs, which could be reversed by SIRT1 siRNA. Finally, overexpression of miR-199a-5p decreased the level of eNOS modulated by SIRT1, an effect repressed by transfection with an miR-199a-5p inhibitor. In summary, HCMV infection of endothelial cells upregulates miR-199a-5p expression and enhances cell migration and tube formation through downregulation of SIRT1/eNOS by miR-199a-5p.

Phosphate is a vascular toxin

Elevated phosphate (P) levels are seen in advanced renal failure and, together with dysregulated calcium, parathyroid hormone and vitamin D levels, contribute to the complex of chronic kidney disease-mineral and bone disease (CKD-MBD). Converging evidence from in vitro, clinical and epidemiological studies suggest that increased P is associated with vascular calcification and mortality. When vessels are exposed to high P conditions in vitro, they develop apoptosis, convert to bone-like cells and develop extensive calcification. Clinical studies in children on dialysis show that high P is associated with increased vessel wall thickness, arterial stiffness and coronary calcification. Epidemiological studies in adult dialysis patients demonstrate a significant and independent association between raised P and mortality. Importantly, raised P is associated with cardiovascular changes even in pre-dialysis CKD, and also in subjects with normal renal function but high P. All P binders can effectively reduce serum P, and this decrease is linked to improved survival. Raised serum P triggers the release of fibroblast growth factor 23 (FGF-23), which has the beneficial effect of increasing P excretion in early CKD, but is increased several 1,000-fold in dialysis, and may be an independent cardiovascular risk factor. Both FGF-23 and its co-receptor Klotho may have direct effects on the vasculature leading to calcification. Fascinatingly, disturbances in FGF-23-Klotho and raised P have also been associated with premature aging. These data suggest that high P levels have adverse vascular effects and that maintaining the serum P levels in the normal range reduces cardiovascular risk and mortality.

Azelnidipine suppresses the progression of aortic aneurysm in wild mice model through anti-inflammatory effects

BACKGROUND

Although systemic hypertension is closely associated with aortic aneurysm (AA) formation, there are many patients with AA without hypertension. In these patients, an inflammation-mediated progression of aneurysmal disease is likely responsible for AA growth and eventual rupture. Unfortunately, there remains no reproducible and durable small animal model of aortic aneurysmal disease, the development of which would enable the investigation of the pathophysiology of this vexing condition. The first aim was to establish a useful wild-type mouse model of AA with low mortality. The second aim was to use this model to assess the protective effect of azelnidipine, a new calcium channel blocker, against the progression of the AA independent of its antihypertensive effect.

METHODS

Angiotensin II and β-aminopropionitrile (a lysyl oxidase inhibitor) were administrated subcutaneously in 7-week-old C57BL/6J mice using an osmotic minipump for 4 weeks to generate a wild-type mouse model of AA. Concurrently, azelnidipine (a calcium channel blocker) or a placebo was administrated orally for 4 weeks. Mice were humanely killed and assessed at the end of the 4 weeks of pharmacologic manipulation.

RESULTS

The combined infusion of angiotensin II and β-aminopropionitrile induced degenerative aneurysm of the thoracic and/or abdominal aorta (11/12; 92%). The majority of aneurysms were located in the distal aortic arch and suprarenal abdominal aorta. Although there was no difference in systolic blood pressure between the control and azelnidipine-treated groups, azelnidipine significantly reduced the incidence of AA (2/11; 18%). Azelnidipine treatment reduced the pathologic findings normally associated with aneurysm formation within the aortic wall. Azelnidipine also reduced the number of macrophage antigen-3 (MAC-3)-positive cells in the periaortic adipose tissue and reduced the gene expression levels of tumor necrosis factor-alpha and matrix metalloproteinase-2 and -9 within the aortic wall.

CONCLUSIONS

This study demonstrates that combined treatment with angiotensin II and β-aminopropionitrile induces degenerative AAs in wild-type mice, and azelnidipine prevents aneurysm progression via its anti-inflammatory effect.

Trichostatin A, an HDAC class I/II inhibitor, promotes Pi-induced vascular calcification via up-regulation of the expression of alkaline phosphatase

AIM

Vascular calcification, a major complication of chronic kidney disease (CKD), refers to the mineralization of vascular smooth muscle cells (VSMCs), resulting from a phenotypic change towards osteoblast-like cells. Histone deacetylase inhibitors (HDIs), potential therapeutic agents for CKD, are known to promote the differentiation and mineralization of osteoblasts. In this study, we aimed to determine the effects of an HDI on the phenotypic change of VSMCs and the development of vascular calcification.

METHODS

The effect of trichostatin A (TSA), an HDI, on human aortic smooth muscle cells (HASMCs) was determined. The mineralization of HASMCs was induced by inorganic phosphorus (Pi), and was confirmed by quantitation of Ca levels and by von Kossa staining. Furthermore, we examined the effect of alkaline phosphatase (ALP) suppression using siRNA on Pi-induced vascular calcification in the presence or absence of TSA.

RESULTS

TSA increased the expression and activity of ALP in HASMCs at a concentration which showed an inhibitory effect of histone deacetylase (HDAC) activity but not on cell viability. Moreover, TSA promoted the Pi-induced mineralization of HASMCs. In addition, both phosphonoformic acid (PFA), which is a sodium-dependent phosphate transporter inhibitor, and suppression of ALP expression by siRNA markedly inhibited the TSA-promoted mineralization of HASMCs.

CONCLUSION

These data show that inhibition of HDAC activity promotes Pi-induced vascular calcification via the up-regulation of ALP expression. Taken together, HDIs may increase the risk of vascular calcification in CKD patients.

Advanced glycation end products accelerate rat vascular calcification through RAGE/oxidative stress

Background

Arterial media calcification (AMC) is highly prevalent and is a major cause of morbidity, mortality, stroke and amputation in patients with diabetes mellitus (DM). Previous research suggests that advanced glycation end products (AGEs) are responsible for vascular calcification in diabetic patients. The potential link between oxidative stress and AGEs-induced vascular calcification, however, has not been examined.

Methods

Male Wistar rats received a high fat diet for 8 weeks followed by a single dose of streptozotocin to induce DM (DM). Calcification was induced with Vitamin D3 and nicotine (VDN). We started VDN treatment at 1 week after the initial streptozotocin injection (DM+VDN). Age-matched rats were used as controls (CON). Metabolic parameters, aortic calcium content, alkaline phosphatase (ALP) protein, malondialdehyde (MDA) content, Cu/Zn superoxide dismutase (SOD) activity, aorta receptor for advanced glycation end products (RAGE) and aorta AGEs levels were measured. In vitro, vascular smooth muscle cells (VSMCs) were cultured with AGEs in DMEM containing 10 mmol·L^-1 ß -glycerophosphate (ß-GP). Calcium content and ALP activity were used to identify osteoblastic differentiation and mineralization. Western blots were used to examine protein expression of Cu/Zn SOD, NADPH oxidase Nox1 and RAGE. In addition, the intracellular reactive oxygen species (ROS) generation was evaluated using fluorescent techniques with dihydroethidine (DHE) method.

Results

The DM+VDN group showed a significant increase in aortic calcium content, levels of aorta AGEs, MDA content, ALP protein levels and RAGE expression, although Cu/Zn SOD activity decreased significantly. In vitro, enhanced Nox1, RAGE expression as well as the production of intracellular superoxide anions, and reduced expression of Cu/Zn SOD induced by AGEs were attenuated by the anti-RAGE antibody or a ROS inhibitor. Furthermore, the AGEs-stimulated ROS increase was also significantly inhibited by a SOD mimetic. Increased ALP activity and calcium deposition were also inhibited markedly by the ROS inhibitor and the anti-RAGE antibody.

Conclusions

These results suggest that AGEs enhance vascular calcification partly through a RAGE/oxidative stress pathway.

Methods to investigate the role of SIRT1 in endothelial senescence

Sirtuins are a family of proteins with NAD(+)-dependent deacetylase or mono-ADP-ribosyltransferase activity. SIRT1, the mammalian ortholog most closely related to Sir2 (the first gene of this family discovered in yeast), exhibits anti-senescence activity in a wide range of mammalian cells. Here, we describe the use of an ex vivo senescence model to study SIRT1 function in primary endothelial cells isolated from the porcine aorta. The methods can be applied to the investigation of the role of SIRT1 in the development of endothelial senescence and atherosclerosis.

Vascular smooth muscle cell sirtuin 1 protects against DNA damage and inhibits atherosclerosis

BACKGROUND

Vascular smooth muscle cells (VSMCs) in human atherosclerosis manifest extensive DNA damage and activation of the DNA damage response, a pathway that coordinates cell cycle arrest and DNA repair, or can trigger apoptosis or cell senescence. Sirtuin 1 deacetylase (SIRT1) regulates cell ageing and energy metabolism and regulates the DNA damage response through multiple targets. However, the direct role of SIRT1 in atherosclerosis and how SIRT1 in VSMCs might regulate atherosclerosis are unknown.

METHODS AND RESULTS

SIRT1 expression was reduced in human atherosclerotic plaques and VSMCs both derived from plaques and undergoing replicative senescence. SIRT1 inhibition reduced DNA repair and induced apoptosis, in part, through reduced activation of the repair protein Nijmegen Breakage Syndrome-1 but not p53. Fat feeding reduced SIRT1 and induced DNA damage in VSMCs. VSMCs from mice expressing inactive truncated SIRT1 (Δex4) showed increased oxidized low-density lipoprotein-induced DNA damage and senescence. ApoE(-/-) mice expressing SIRT1(Δex4) only in smooth muscle cells demonstrated increased DNA damage response activation and apoptosis, increased atherosclerosis, reduced relative fibrous cap thickness, and medial degeneration.

CONCLUSIONS

SIRT1 is reduced in human atherosclerosis and is a critical regulator of the DNA damage response and survival in VSMCs. VSMC SIRT1 protects against DNA damage, medial degeneration, and atherosclerosis.

Mechanistic insights into vascular calcification in CKD

Cardiovascular disease begins early in the course of renal decline and is a life-limiting problem in patients with CKD. The increased burden of cardiovascular disease is due, at least in part, to calcification of the vessel wall. The uremic milieu provides a perfect storm of risk factors for accelerated calcification, but elevated calcium and phosphate levels remain key to the initiation and progression of vascular smooth muscle cell calcification in CKD. Vascular calcification is a highly regulated process that involves a complex interplay between promoters and inhibitors of calcification and has many similarities to bone ossification. Here, we discuss current understanding of the process of vascular calcification, focusing specifically on the discrete and synergistic effects of calcium and phosphate in mediating vascular smooth muscle cell apoptosis, osteochondrocytic differentiation, vesicle release, calcification inhibitor expression, senescence, and death. Using our model of intact human vessels, factors initiating vascular calcification in vivo and the role of calcium and phosphate in driving accelerated calcification ex vivo are described. This work allows us to link clinical and basic research into a working theoretical model to explain the pathway of development of vascular calcification in CKD.

Angiogenesis inhibitor vasohibin-1 enhances stress resistance of endothelial cells via induction of SOD2 and SIRT1

Vasohibin-1 (VASH1) is isolated as an endothelial cell (EC)-produced angiogenesis inhibitor. We questioned whether VASH1 plays any role besides angiogenesis inhibition, knocked-down or overexpressed VASH1 in ECs, and examined the changes of EC property. Knock-down of VASH1 induced premature senescence of ECs, and those ECs were easily killed by cellular stresses. In contrast, overexpression of VASH1 made ECs resistant to premature senescence and cell death caused by cellular stresses. The synthesis of VASH1 was regulated by HuR-mediated post-transcriptional regulation. We sought to define the underlying mechanism. VASH1 increased the expression of (superoxide dismutase 2) SOD2, an enzyme known to quench reactive oxygen species (ROS). Simultaneously, VASH1 augmented the synthesis of sirtuin 1 (SIRT1), an anti-aging protein, which improved stress tolerance. Paraquat generates ROS and causes organ damage when administered in vivo. More VASH1 (+/-) mice died due to acute lung injury caused by paraquat. Intratracheal administration of an adenovirus vector encoding human VASH1 augmented SOD2 and SIRT1 expression in the lungs and prevented acute lung injury caused by paraquat. Thus, VASH1 is a critical factor that improves the stress tolerance of ECs via the induction of SOD2 and SIRT1.

Are sirtuins viable targets for improving healthspan and lifespan?

Although the increased lifespan of our populations illustrates the success of modern medicine, the risk of developing many diseases increases exponentially with old age. Caloric restriction is known to retard ageing and delay functional decline as well as the onset of disease in most organisms. Studies have implicated the sirtuins (SIRT1-SIRT7) as mediators of key effects of caloric restriction during ageing. Two unrelated molecules that have been shown to increase SIRT1 activity in some settings, resveratrol and SRT1720, are excellent protectors against metabolic stress in mammals, making SIRT1 a potentially appealing target for therapeutic interventions. This Review covers the current status and controversies surrounding the potential of sirtuins as novel pharmacological targets, with a focus on SIRT1.

The intersection between aging and cardiovascular disease

The average lifespan of humans is increasing, and with it the percentage of people entering the 65 and older age group is growing rapidly and will continue to do so in the next 20 years. Within this age group, cardiovascular disease will remain the leading cause of death, and the cost associated with treatment will continue to increase. Aging is an inevitable part of life and unfortunately poses the largest risk factor for cardiovascular disease. Although numerous studies in the cardiovascular field have considered both young and aged humans, there are still many unanswered questions as to how the genetic pathways that regulate aging in model organisms influence cardiovascular aging. Likewise, in the molecular biology of aging field, few studies fully assess the role of these aging pathways in cardiovascular health. Fortunately, this gap is beginning to close, and these two fields are merging together. We provide an overview of some of the key genes involved in regulating lifespan and health span, including sirtuins, AMP-activated protein kinase, mammalian target of rapamycin, and insulin-like growth factor 1 and their roles regulating cardiovascular health. We then discuss a series of review articles that will appear in succession and provide a more comprehensive analysis of studies carried out linking genes of aging and cardiovascular health, and perspectives of future directions of these two intimately linked fields.

Sirt1 inhibits resistin expression in aortic stenosis

The development of human calcified aortic stenosis (AS) includes age-dependent processes that have been involved in atherosclerosis, such as infiltration of macrophages in aortic valves, which then promote production of many pro-inflammatory cytokines, including resistin. However, the molecular mechanisms contributing to these processes are not established. Since Sirt1 has been shown to modulate macrophage biology and inflammation, we examined its levels in human AS and tested its impact on resistin expression. Sirt1 mRNA (p = 0.01) and protein (p<0.05) levels were reduced in explanted valves from AS patients (n = 51) compared to those from control (n = 11) patients. Sirt1 mRNA levels were negatively associated with resistin mRNA levels quantified in AS valves (p = 0.02). Stimulation of Sirt1 by resveratrol or virus-driven overexpression robustly diminished resistin mRNA and protein expression in macrophages, whereas down-regulation of Sirt1 triggered a large increase in resistin expression. These effects were direct, as chromatin immunoprecipitation assays showed that Sirt1 physically interacted with the resistin promoter region at an AP-1 response element. Moreover, Sirt1 blocked c-jun-induced resistin transactivation in gene reporter assays. These findings demonstrate that, in calcified AS, levels of Sirt1 are reduced whereas those of resistin are increased within aortic valve leaflets. Our results also suggest that this loss of Sirt1 expression alleviates its inhibition of resistin transcription in macrophages. Although the overall contribution of this process to the underlying mechanisms for AS disease development remains unresolved, these observations suggest that modification of Sirt1 expression and/or activity could represent a novel approach against inflammation in AS.

The role of vitamin D in the FGF23, klotho, and phosphate bone-kidney endocrine axis

1,25-dihydroxyvitamin D (1,25D), through association with the nuclear vitamin D receptor (VDR), exerts control over a novel endocrine axis consisting of the bone-derived hormone FGF23, and the kidney-expressed klotho, CYP27B1, and CYP24A1 genes, which together prevent hyperphosphatemia/ectopic calcification and govern the levels of 1,25D to maintain bone mineral integrity while promoting optimal function of other vital tissues. When occupied by 1,25D, VDR interacts with RXR to form a heterodimer that binds to VDREs in the region of genes directly controlled by 1,25D (e.g., FGF23, klotho, Npt2c, CYP27B1 and CYP24A1). By recruiting complexes of comodulators, activated VDR initiates a series of events that induces or represses the transcription of genes encoding proteins such as: the osteocyte-derived hormone, FGF23; the renal anti-senescence factor and protein co-receptor for FGF23, klotho; other mediators of phosphate transport including Npt2a/c; and vitamin D hormone metabolic enzymes, CYP27B1 and CYP24A1. The mechanism whereby osteocytes are triggered to release FGF23 is yet to be fully defined, but 1,25D, phosphate, and leptin appear to play major roles. The kidney responds to FGF23 to elicit CYP24A1-catalyzed detoxification of the 1,25D hormone while also repressing both Npt2a/c to mediate phosphate elimination and CYP27B1 to limit de novo 1,25D synthesis. Comprehension of these skeletal and renal actions of 1,25D should facilitate the development of novel mimetics to prevent ectopic calcification, chronic renal and vascular disease, and promote healthful aging.

Arterial calcification in chronic kidney disease: key roles for calcium and phosphate

Vascular calcification contributes to the high risk of cardiovascular mortality in chronic kidney disease (CKD) patients. Dysregulation of calcium (Ca) and phosphate (P) metabolism is common in CKD patients and drives vascular calcification. In this article, we review the physiological regulatory mechanisms for Ca and P homeostasis and the basis for their dysregulation in CKD. In addition, we highlight recent findings indicating that elevated Ca and P have direct effects on vascular smooth muscle cells (VSMCs) that promote vascular calcification, including stimulation of osteogenic/chondrogenic differentiation, vesicle release, apoptosis, loss of inhibitors, and extracellular matrix degradation. These studies suggest a major role for elevated P in promoting osteogenic/chondrogenic differentiation of VSMC, whereas elevated Ca has a predominant role in promoting VSMC apoptosis and vesicle release. Furthermore, the effects of elevated Ca and P are synergistic, providing a major stimulus for vascular calcification in CKD. Unraveling the complex regulatory pathways that mediate the effects of both Ca and P on VSMCs will ultimately provide novel targets and therapies to limit the destructive effects of vascular calcification in CKD patients.