Sirt3 prevents maternal obesity-associated oxidative stress and meiotic defects in mouse oocytes

Maternal obesity enhances oocyte chromosome abnormalities associated with aging

Obesity is increasing globally, and maternal obesity has adverse effects on pregnancy outcomes and the long-term health of offspring. Maternal obesity has been associated with pregnancy failure through impaired oogenesis and embryogenesis. However, whether maternal obesity causes chromosome abnormalities in oocytes has remained unclear. Here we show that chromosome abnormalities are increased in the oocytes of obese mice fed a high-fat diet and identify weakened sister-chromatid cohesion as the likely cause. Numbers of full-grown follicles retrieved from obese mice were the same as controls and the efficiency of in vitro oocyte maturation remained high. However, chromosome abnormalities presenting in both metaphase-I and metaphase-II were elevated, most prominently the premature separation of sister chromatids. Weakened sister-chromatid cohesion in oocytes from obese mice was manifested both as the terminalization of chiasmata in metaphase-I and as increased separation of sister centromeres in metaphase II. Obesity-associated abnormalities were elevated in older mice implying that maternal obesity exacerbates the deterioration of cohesion seen with advancing age.

Placental Ageing in Adverse Pregnancy Outcomes: Telomere Shortening, Cell Senescence, and Mitochondrial Dysfunction

Preeclampsia is a multisystemic pregnancy disorder and a major cause of maternal and neonatal morbidity and mortality worldwide. The exact pathophysiology of preeclampsia remains unclear; however, it is speculated that the various pathologies can be attributed to impaired vascular remodelling and elevated oxidative stress within the placenta. Oxidative stress plays a key role in cell ageing, and the persistent presence of elevated oxidative stress precipitates cellular senescence and mitochondrial dysfunction, resulting in premature ageing of the placenta. Premature ageing of the placenta is associated with placental insufficiency, which reduces the functional capacity of this critical organ and leads to abnormal pregnancy outcomes. The changes brought about by oxidative insults are irreversible and often lead to deleterious modifications in macromolecules such as lipids and proteins, DNA mutations, and alteration of mitochondrial functioning and dynamics. In this review, we have summarized the current knowledge of placental ageing in the aetiology of adverse pregnancy outcomes and discussed the hallmarks of ageing which could be potential markers for preeclampsia and fetal growth restriction.

Advances in research into gamete and embryo-fetal origins of adult diseases

The fetal and infant origins of adult disease hypothesis proposed that the roots of adult chronic disease lie in the effects of adverse environments in fetal life and early infancy. In addition to the fetal period, fertilization and early embryonic stages, the critical time windows of epigenetic reprogramming, rapid cell differentiation and organogenesis, are the most sensitive stages to environmental disturbances. Compared with embryo and fetal development, gametogenesis and maturation take decades and are more vulnerable to potential damage for a longer exposure period. Therefore, we should shift the focus of adult disease occurrence and pathogenesis further back to gametogenesis and embryonic development events, which may result in intergenerational, even transgenerational, epigenetic re-programming with transmission of adverse traits and characteristics to offspring. Here, we focus on the research progress relating to diseases that originated from events in the gametes and early embryos and the potential epigenetic mechanisms involved.

Preovulatory exposure to a protein-restricted diet disrupts amino acid kinetics and alters mitochondrial structure and function in the rat oocyte and is partially rescued by folic acid

BACKGROUND

Detrimental exposures during pregnancy have been implicated in programming offspring to develop permanent changes in physiology and metabolism, increasing the risk for developing diseases in adulthood such as hypertension, diabetes, heart disease and obesity. This study investigated the effects of protein restriction on the metabolism of amino acids within the oocyte, liver, and whole organism in a rat model as well as effects on mitochondrial ultrastructure and function in the cumulus oocyte complex.

METHODS

Wistar outbred female rats 8-11 weeks of age (n = 24) were assigned to three isocaloric dietary groups, including control (C), low protein (LP) and low protein supplemented with folate (LPF). Animals were superovulated and 48 h later underwent central catheterization. Isotopic tracers of 1-¹³C-5C²H₃-methionine, ²H₂-cysteine, U-¹³C₃-cysteine and U-¹³C₃-serine were administered by a 4 h prime-constant rate infusion. After sacrifice, oocytes were denuded of cumulus cells and liver specimens were obtained.

RESULTS

Oocytes demonstrated reduced serine flux in LP vs. LPF (p < 0.05), reduced cysteine flux in LP and LPF vs. C (p < 0.05), and a trend toward reduced transsulfuration in LP vs. C and LPF. Folic acid supplementation reversed observed effects on serine flux and transsulfuration. Preovulatory protein restriction increased whole-body methionine transmethylation, methionine transsulfuration and the flux of serine in LP and LPF vs. C (p = 0.003, p = 0.002, p = 0.005). The concentration of glutathione was increased in erythrocytes and liver in LP and LPF vs. C (p = 0.003 and p = 0.0003). Oocyte mitochondrial ultrastructure in LP and LPF had increased proportions of abnormal mitochondria vs. C (p < 0.01 and p < 0.05). Cumulus cell mitochondrial ultrastructure in LP and LPF groups had increased proportions of abnormal mitochondria vs. C (p < 0.001 and p < 0.05). Preovulatory protein restriction altered oocyte expression of Drp1, Opa-1, Mfn1/2, Parl and Ndufb6 (p < 0.05) and Hk2 (p < 0.01), which are genes involved in mitochondrial fission (division) and fusion, mitochondrial apoptotic mechanisms, respiratory electron transport and glucose metabolism.

CONCLUSIONS

Preovulatory protein restriction resulted in altered amino acid metabolism, abnormal cumulus oocyte complex mitochondrial ultrastructure and differential oocyte expression of genes related to mitochondrial biogenesis.

Intersectin-Cdc42 interaction is required for orderly meiosis in porcine oocytes

Intersectins (ITSNs) have been shown to act as adaptor proteins that govern multiple cellular events via regulating Cdc42 activity. However, it remains to be determined whether the ITSN-Cdc42 pathway is functional in porcine oocytes. To address this question, we used a small molecule, ZCL278, to selectively disrupt the ITSN2-Cdc42 interaction. In the present study, we find that porcine oocytes exposed to ZCL278 are unable to completely progress through meiosis. Meanwhile, the spindle defects and chromosomal congression failure are frequently detected in these oocytes. In support of this, we observed the accumulated distribution of vesicle-like ITSN2 signals around the chromosome/spindle region during porcine oocyte maturation. In addition, our results also showed that inhibition of the ITSN-Cdc42 interaction impairs the actin polymerization in porcine oocytes. In summary, the findings support a model where ITSNs, through the interaction with Cdc42, modulates the assembly of meiotic apparatus and actin polymerization, consequently ensuring the orderly meiotic progression during porcine oocyte maturation.

Kif18a regulates Sirt2-mediated tubulin acetylation for spindle organization during mouse oocyte meiosis

Background

During oocyte meiosis, the cytoskeleton dynamics, especially spindle organization, are critical for chromosome congression and segregation. However, the roles of the kinesin superfamily in this process are still largely unknown.

Results

In the present study, Kif18a, a member of the kinesin-8 family, regulated spindle organization through its effects on tubulin acetylation in mouse oocyte meiosis. Our results showed that Kif18a is expressed and mainly localized in the spindle region. Knock down of Kif18a caused the failure of first polar body extrusion, dramatically affecting spindle organization and resulting in severe chromosome misalignment. Further analysis showed that the disruption of Kif18a caused an increase in acetylated tubulin level, which might be the reason for the spindle organization defects after Kif18a knock down in oocyte meiosis, and the decreased expression of deacetylase Sirt2 was found after Kif18a knock down. Moreover, microinjections of tubulin K40R mRNA, which could induce tubulin deacetylation, protected the oocytes from the effects of Kif18a downregulation, resulting in normal spindle morphology in Kif18a-knock down oocytes.

Conclusions

Taken together, our results showed that Kif18a affected Sirt2-mediated tubulin acetylation level for spindle organization during mouse oocyte meiosis. Our results not only revealed the critical effect of Kif18a on microtubule stability, but also extended our understanding of kinesin activity in meiosis.

SIRT7 functions in redox homeostasis and cytoskeletal organization during oocyte maturation

SIRT7, a member of the sirtuin family, with coenzyme NAD catalyzes protein deacetylation and has been implicated in multiple biologic processes; however, its function in mammalian oocytes remains to be explored. Here, we report disrupted meiotic maturation upon specific knockdown of SIRT7 in mouse oocytes. In particular, disorganized spindle/chromosomes and the loss of the cortical actin cap are readily observed in SIRT7-depleted oocytes, generating aneuploid eggs. Furthermore, we found that SIRT7 depletion markedly elevated reactive oxygen species levels in oocytes, thereby compromising the developmental competence of early embryos. Of note, SIRT7 protein level is significantly decreased in oocytes from obese mice, and the forced expression of exogenous SIRT7 ameliorates maternal obesity-associated meiotic defects and oxidative stress in oocytes. In summary, our data suggest that SIRT7 is an essential factor in the determination of oocyte quality and may mediate the effects of obesity on female reproduction.-Gao, M., Li, X., He, Y., Han, L., Qiu, D., Ling, L., Liu, H., Liu, J., Gu, L. SIRT7 functions in redox homeostasis and cytoskeletal organization during oocyte maturation.

Loss of TIGAR Induces Oxidative Stress and Meiotic Defects in Oocytes from Obese Mice

Maternal obesity has been reported to impair oocyte quality in mice, however, the underlying mechanisms remain unclear. In the present study, by conducting a comparative proteomic analysis, we identified a reduced expression of TIGAR (TP53-induced glycolysis and apoptosis regulator) protein in ovulated oocytes from high-fat diet (HFD)-fed mice. Specific depletion of TIGAR in mouse oocytes results in the marked elevation of reactive oxygen species (ROS) levels and the failure of meiotic apparatus assembly. Importantly, forced expression of TIGAR in HFD oocytes not only attenuates ROS production, but also partly prevents spindle disorganization and chromosome misalignment during meiosis. Meantime, we noted that TIGAR knockdown in oocytes induces a strong activation of autophagy, whereas overexpression of TIGAR significantly reduces the LC3 accumulation in HFD oocytes. By anti-oxidant treatment, we further demonstrated that such an autophagic response is dependent on the TIGAR-controlled ROS production. In summary, our data indicate a role for TIGAR in modulating redox homeostasis during oocyte maturation, and uncover that loss of TIGAR is a critical pathway mediating the effects of maternal obesity on oocyte quality.

Alpha-linolenic acid protects the developmental capacity of bovine cumulus-oocyte complexes matured under lipotoxic conditions in vitro

Elevated concentrations of free fatty acids (FFAs), predominantly palmitic, stearic, and oleic acids (PSO), exert detrimental effects on oocyte developmental competence. This study examined the effects of omega-3 alpha-linolenic acid (ALA) during in vitro oocyte maturation (IVM) in the presence of PSO on subsequent embryo development and quality, and the cellular mechanisms that might be involved. Bovine cumulus-oocyte complexes (COCs) were supplemented during IVM with ALA (50 μM), PSO (425 μM), or PSO+ALA. Compared with FFA-free controls (P < 0.05), PSO increased embryo fragmentation and decreased good quality embryos on day 2 postfertilization. Day 7 blastocyst rate was also reduced. Day 8 blastocysts had lower cell counts and higher apoptosis but normal metabolic profile. In the PSO group, cumulus cell (CC) expansion was inhibited with an increased CC apoptosis while COC metabolism was not affected. Mitochondrial inner membrane potential (MMP; JC-1 staining) was reduced in the CCs and oocytes. Heat shock protein 70 (HSP70) but not glucose-regulated protein 78 kDa (GRP78, known as BiP; an endoplasmic reticulum stress marker) was upregulated in the CCs. Higher reactive oxygen species levels (DCHFDA staining) were detected in the oocytes. In contrast, adding ALA in the presence of PSO normalized embryo fragmentation, cleavage, blastocyst rates, and blastocyst quality compared to controls (P > 0.05). Combined treatment with ALA also reduced CC apoptosis, partially recovered CC expansion, abrogated the reduction in MMP in the CCs but not in the oocytes, and reduced BiP and HSP70 expression in CCs, compared with PSO only (P < 0.05). In conclusion, ALA supplementation protected oocyte developmental capacity under lipotoxic conditions mainly by protecting cumulus cell viability.

Mito-TEMPO Alleviates Renal Fibrosis by Reducing Inflammation, Mitochondrial Dysfunction, and Endoplasmic Reticulum Stress

Background

Renal fibrosis is a common pathological symptom of chronic kidney disease (CKD). Many studies support that mitochondrial dysfunction and endoplasmic reticulum (ER) stress are implicated in the pathogenesis of CKD. In our study, we investigated the benefits and underlying mechanisms of Mito-TEMPO on renal fibrosis in 5/6 nephrectomy mice.

Methods

Mice were randomly divided into five groups as follows: control group, CKD group, CKD + Mito-TEMPO (1 mg·kg^-1·day^-1) group, CKD + Mito-TEMPO (3 mg·kg^-1·day^-1) group, and Mito-TEMPO group (3 mg·kg^-1·day^-1). Renal fibrosis was evaluated by PAS, Masson staining, immunohistochemistry, and real-time PCR. Oxidative stress markers such as SOD2 activity and MDA level in serum and isolated mitochondria from renal tissue were measured by assay kits. Mitochondrial superoxide production was evaluated by MitoSOX staining and Western blot. Mitochondrial dysfunction was assessed by electron microscopy and real-time PCR. ER stress-associated protein was measured by Western blot.

Results

Impaired renal function and renal fibrosis were significantly improved by Mito-TEMPO treatment. Furthermore, inflammation cytokines, profibrotic factors, oxidative stress markers, mitochondrial dysfunction, and ER stress were all increased in the CKD group. However, these effects were significantly ameliorated in the Mito-TEMPO treatment group.

Conclusions

Mito-TEMPO ameliorates renal fibrosis by alleviating mitochondrial dysfunction and endoplasmic reticulum stress possibly through the Sirt3-SOD2 pathway, which sheds new light on prevention of renal fibrosis in chronic kidney disease.

Sirtuins in the Cardiovascular System: Potential Targets in Pediatric Cardiology

Cardiovascular diseases represent a major cause of death and morbidity. Cardiac and vascular pathologies develop predominantly in the aged population in part due to lifelong exposure to numerous risk factors but are also found in children and during adolescence. In comparison to adults, much has to be learned about the molecular pathways driving cardiovascular diseases in the pediatric population. Sirtuins are highly conserved enzymes that play pivotal roles in ensuring cardiac homeostasis under physiological and stress conditions. In this review, we discuss novel findings about the biological functions of these molecules in the cardiovascular system and their possible involvement in pediatric cardiovascular diseases.

Melatonin protects against maternal obesity-associated oxidative stress and meiotic defects in oocytes via the SIRT3-SOD2-dependent pathway

Maternal obesity in humans is associated with poor outcomes across the reproductive spectrum. Emerging evidence indicates that these defects are likely attributed to factors within the oocyte. Although various molecules and pathways may contribute to impaired oocyte quality, prevention of fertility issues associated with maternal obesity is a challenge. Using mice fed a high-fat diet (HFD) as an obesity model, we document spindle disorganization, chromosome misalignment, and elevated reactive oxygen species (ROS) levels in oocytes from obese mice. Oral administration of melatonin to HFD mice not only reduces ROS generation, but also prevents spindle/chromosome anomalies in oocytes, consequently promoting the developmental potential of early embryos. Consistent with this finding, we find that melatonin supplement during in vitro maturation also markedly attenuates oxidative stress and meiotic defects in HFD oocytes. Finally, by performing morpholino knockdown and acetylation-mimetic mutant overexpression assays, we reveal that melatonin ameliorates maternal obesity-induced defective phenotypes in oocytes through the SIRT3-SOD2-dependent mechanism. In sum, our data uncover the marked beneficial effects of melatonin on oocyte quality from obese females; this opens a new area for optimizing culture system as well as fertility management.

HDAC3 promotes meiotic apparatus assembly in mouse oocytes by modulating tubulin acetylation

Histone deacetylases (HDACs) have been shown to deacetylate numerous cellular substrates that govern a wide array of biological processes. HDAC3, a member of the Class I HDACs, is a highly conserved and ubiquitously expressed protein. However, its roles in meiotic oocytes are not known. In the present study, we find that mouse oocytes depleted of HDAC3 are unable to completely progress through meiosis, and are blocked at metaphase I. These HDAC3 knockdown oocytes show spindle/chromosome organization failure, with severely impaired kinetochore-microtubule attachments. Consistent with this, the level of BubR1, a central component of the spindle assembly checkpoint, at kinetochores is dramatically increased in metaphase oocytes following HDAC3 depletion. Knockdown and overexpression experiments reveal that HDAC3 modulates the acetylation status of α-tubulin in mouse oocytes. Importantly, the deacetylation mimetic mutant tubulin-K40R can partly rescue the defective phenotypes of HDAC3 knockdown oocytes. Our data support a model whereby HDAC3, through deacetylating tubulin, promotes microtubule stability and the establishment of kinetochore-microtubule interaction, consequently ensuring proper spindle morphology, accurate chromosome movement and orderly meiotic progression during oocyte maturation.

Sirt3 Impairment and SOD2 Hyperacetylation in Vascular Oxidative Stress and Hypertension

RATIONALE

Clinical studies have shown that Sirt3 (Sirtuin 3) expression declines by 40% by 65 years of age paralleling the increased incidence of hypertension and metabolic conditions further inactivate Sirt3 because of increased NADH (nicotinamide adenine dinucleotide, reduced form) and acetyl-CoA levels. Sirt3 impairment reduces the activity of a key mitochondrial antioxidant enzyme, superoxide dismutase 2 (SOD2) because of hyperacetylation.

OBJECTIVE

In this study, we examined whether the loss of Sirt3 activity increases vascular oxidative stress because of SOD2 hyperacetylation and promotes endothelial dysfunction and hypertension.

METHODS AND RESULTS

Hypertension was markedly increased in Sirt3-knockout (Sirt3^-/-) and SOD2-depleted (SOD2^+/-) mice in response to low dose of angiotensin II (0.3 mg/kg per day) compared with wild-type C57Bl/6J mice. Sirt3 depletion increased SOD2 acetylation, elevated mitochondrial O₂^· -, and diminished endothelial nitric oxide. Angiotensin II-induced hypertension was associated with Sirt3 S-glutathionylation, acetylation of vascular SOD2, and reduced SOD2 activity. Scavenging of mitochondrial H₂O₂ in mCAT mice expressing mitochondria-targeted catalase prevented Sirt3 and SOD2 impairment and attenuated hypertension. Treatment of mice after onset of hypertension with a mitochondria-targeted H₂O₂ scavenger, mitochondria-targeted hydrogen peroxide scavenger ebselen, reduced Sirt3 S-glutathionylation, diminished SOD2 acetylation, and reduced blood pressure in wild-type but not in Sirt3^-/- mice, whereas an SOD2 mimetic, (2-[2,2,6,6-tetramethylpiperidin-1-oxyl-4-ylamino]-2-oxoethyl) triphenylphosphonium (mitoTEMPO), reduced blood pressure and improved vasorelaxation both in Sirt3^-/- and wild-type mice. SOD2 acetylation had an inverse correlation with SOD2 activity and a direct correlation with the severity of hypertension. Analysis of human subjects with essential hypertension showed 2.6-fold increase in SOD2 acetylation and 1.4-fold decrease in Sirt3 levels, whereas SOD2 expression was not affected.

CONCLUSIONS

Our data suggest that diminished Sirt3 expression and redox inactivation of Sirt3 lead to SOD2 inactivation and contributes to the pathogenesis of hypertension.

Sirt3-dependent deacetylation of SOD2 plays a protective role against oxidative stress in oocytes from diabetic mice

Maternal diabetes has been demonstrated to adversely affect oocyte quality in mouse oocytes. However, the potential molecular mechanisms are poorly understood. Here, we established a type I diabetic mouse model and detected the increased reactive oxygen species (ROS) levels and decreased Sirt3 expression in oocytes from diabetic mice. Furthermore, we found that forced expression of Sirt3 in diabetic oocytes significantly attenuates such an excessive production of ROS. The acetylation status of lysine 68 of superoxide dismutase (SOD2K68) is dependent on Sirt3 in oocytes. In line with this, SOD2K68 acetylation levels were markedly increased in diabetic oocytes, and Sirt3 overexpression could effectively suppress this tendency. Importantly, the deacetylation-mimetic mutant SOD2K68R is capable of partly preventing the oxidative stress in oocytes from diabetic mice. In conclusion, our findings support a model where Sirt3 plays a protective role against oxidative stress in oocytes exposed to maternal diabetes through deacetylating SOD2K68.

Protective effect of antioxidants on the pre-maturation aging of mouse oocytes

Pre-maturation aging of immature oocytes may adversely affect the fate of an oocyte. Oxidative stress is one of the most detrimental factors affecting oocyte developmental competence and maturation during aging. In this study, experiments were designed to examine whether supplementation of antioxidants in a culture medium could protect immature mouse oocytes from damages caused by oxidative stress. Mouse oocytes at germinal vesicle stage were prevented from meiosis resumption and cultured in a medium with or without antioxidants for 12–36 h to allow oocytes to undergo aging. After aging, oocytes were cultured for maturation. Nuclear maturation, mitochondria activity, spindle morphology and DNA integrity were examined after maturation. It was found that antioxidants had protective effects on the oocytes in terms of nuclear maturation, functional mitochondria, spindle morphology and DNA integrity. As aging time was prolonged from 12 to 36 h, the protective effect of antioxidants became more obvious. However, as compared with oocytes without aging, it was found that aging significantly inhibited nuclear maturation, impaired mitochondria function, and damaged the spindle and DNA. These results indicate that pre-maturation aging is detrimental to oocytes’ competence to undergo maturation and other cellular activities, and antioxidants can protect oocytes from damages caused by aging.

SIRT1, 2, 3 protect mouse oocytes from postovulatory aging

The quality of metaphase II oocytes will undergo a time-dependent deterioration following ovulation as the result of the oocyte aging process. In this study, we determined that the expression of sirtuin family members (SIRT1, 2, 3) was dramatically reduced in mouse oocytes aged in vivo or in vitro. Increased intracellular ROS was observed when SIRT1, 2, 3 activity was inhibited. Increased frequency of spindle defects and disturbed distribution of mitochondria were also observed in MII oocytes aged in vitro after treatment with Nicotinamide (NAM), indicating that inhibition of SIRT1, 2, 3 may accelerate postovulatory oocyte aging. Interestingly, when MII oocytes were exposed to caffeine, the decline of SIRT1, 2, 3 mRNA levels was delayed and the aging-associated defective phenotypes could be improved. The results suggest that the SIRT1, 2, 3 pathway may play a potential protective role against postovulatory oocyte aging by controlling ROS generation.

Effects of obesity on hormonally driven cancer in women

Obesity increases the risk of numerous poor health outcomes, including cancer. Obesity is especially problematic in women because both they and their offspring may be at increased risk of cancer. Studying transmission of obesity-induced cancer risk is challenging in humans, but animal studies are beginning to reveal the underlying mechanisms.

Sirtuins-mediators of maternal obesity-induced complications in offspring?

Obesity is a complex metabolic disease, attributed to diverse and interactive genetic and environmental factors. The associated health consequences of obesity are pleiotropic, with individuals being more susceptible to chronic diseases such as type 2 diabetes mellitus, hypertension, and lipotoxicity-related chronic diseases. The contribution of maternal obesity to the offspring's predisposition to both obesity and its complications is increasingly recognized. Understanding the mechanisms underlying these "transmissible" effects is critical to develop therapeutic interventions to reduce the risk for "programmed" obesity. Sirtuins (SIRTs), particularly SIRT1 and SIRT3, are NAD(+)-dependent deacetylases that regulate metabolic balance and stress responses in both central and peripheral tissues, of which dysregulation is a well-established mediator for the development and effects of obesity. Nevertheless, their implication in the transmissible effects of maternal obesity across generations remains largely elusive. In this review, we examine multiple pathways and systems that are likely to mediate such effects, with particular emphasis on the role of SIRTs.-Nguyen, L. T., Chen, H., Pollock, C. A., Saad, S. Sirtuins-mediators of maternal obesity-induced complications in offspring?

Sirtuins, aging, and cardiovascular risks

The sirtuins comprise a highly conserved family proteins present in virtually all species from bacteria to mammals. Sirtuins are members of the highly conserved class III histone deacetylases, and seven sirtuin genes (sirtuins 1-7) have been identified and characterized in mammals. Sirtuin activity is linked to metabolic control, apoptosis, cell survival, development, inflammation, and healthy aging. In this review, we summarize and discuss the potential mutual relations between each sirtuin and cardiovascular health and the impact of sirtuins on oxidative stress and so age-related cardiovascular disorders, underlining the possibility that sirtuins will be novel targets to contrast cardiovascular risks induced by aging.