Aberrant differentiation of fibroblast progenitors contributes to fibrosis in the aged murine heart: role of elevated circulating insulin levels

The Senescent Heart-"Age Doth Wither Its Infinite Variety"

Cardiovascular diseases are a leading cause of morbidity and mortality world-wide. While many factors like smoking, hypertension, diabetes, dyslipidaemia, a sedentary lifestyle, and genetic factors can predispose to cardiovascular diseases, the natural process of aging is by itself a major determinant of the risk. Cardiac aging is marked by a conglomerate of cellular and molecular changes, exacerbated by age-driven decline in cardiac regeneration capacity. Although the phenotypes of cardiac aging are well characterised, the underlying molecular mechanisms are far less explored. Recent advances unequivocally link cardiovascular aging to the dysregulation of critical signalling pathways in cardiac fibroblasts, which compromises the critical role of these cells in maintaining the structural and functional integrity of the myocardium. Clearly, the identification of cardiac fibroblast-specific factors and mechanisms that regulate cardiac fibroblast function in the senescent myocardium is of immense importance. In this regard, recent studies show that Discoidin domain receptor 2 (DDR2), a collagen-activated receptor tyrosine kinase predominantly located in cardiac fibroblasts, has an obligate role in cardiac fibroblast function and cardiovascular fibrosis. Incisive studies on the molecular basis of cardiovascular aging and dysregulated fibroblast function in the senescent heart would pave the way for effective strategies to mitigate cardiovascular diseases in a rapidly growing elderly population.

Sex-specific Phenotypes in the Aging Mouse Heart and Consequences for Chronic Fibrosis

The incidence of diastolic dysfunction increases with age in both humans and mice. This is characterized by increased passive stiffness and slower relaxation of the left ventricle. The stiffness arises at least partially from progressively increased interstitial collagen deposition due to highly secretory fibroblasts. In the past, we demonstrated that AMPK activation via the drug Aicar in middle-aged mice reduced adverse remodeling after myocardial infarction. Therefore as an attempt to normalize the fibroblast phenotype, we used 21 month-old male and female mice and treated them with Aicar (0.166 mg/g of body weight) where each mouse was followed in a functional study over a 3-month period. We found sex-related differences in extracellular matrix (ECM) composition as well as heart function indices at baseline, which were further accentuated by Aicar treatment. Aicar attenuated the age-related increase in left atrial volume (LAV, an indicator of diastolic dysfunction) in female but not in male hearts which was associated with reduced collagen deposition in the old female heart, and reduced the transcription factor Gli1 expression in cardiac fibroblasts. We further demonstrated that collagen synthesis was dependent on Gli1, which is a target of AMPK-mediated degradation. By contrast, Aicar had a minor impact on cardiac fibroblasts in the old male heart due to blunted AMPK phosphorylation. Hence it did not significantly improve old male heart function indices. In conclusion, we demonstrated that male and female hearts are phenotypically different, and sex-specific differences need to be considered when analyzing the response to pharmacological intervention.

Cleavage stimulating factor 64 depletion mitigates cardiac fibrosis through alternative polyadenylation

Alternative polyadenylation (APA) regulates gene expression by cleavage and addition of poly(A) sequence at different polyadenylation sites (PAS) in 3'UTR, thus, generating transcript isoforms with different lengths. Cleavage stimulating factor 64 (CstF64) is an APA regulator which plays a role in PAS selection and determines the length of 3'UTR. CstF64 favors the use of proximal PAS, resulting in 3'UTR shortening, which enhances the protein expression by increasing the stability of the target genes. The aim of this study is to investigate the role of CstF64 in cardiac fibrosis, a key event leading to heart failure (HF). We determined the expression of CstF64, key profibrotic genes, and their 3'UTR changes by calculating distal PAS (dPAS) usage in left ventricular (LV) tissues and cardiac fibroblasts from HF patients. CstF64 was upregulated in HF LV tissues and cardiac fibroblasts along with increased deposition of fibrosis genes such as COL1A and FN1 and significant shortening in their 3'UTR. In addition, HF cardiac fibroblasts showed increased transforming growth factor receptor β1 (TGFβR1) expression consistent with significant shortening in 3'UTR of TGFβR1. Upon knockdown of CstF64 from HF fibroblasts, downregulation in pro-fibrotic genes corresponding to lengthening in their 3'UTR was observed. Our finding suggests an important role of CstF64 in myofibroblast activation and promotion of cardiac fibrosis during HF through APA. Therefore, targeting CstF64 mediated RNA processing approach in human HF could provide a new therapeutic treatment strategy for limiting fibrotic remodeling.

Relation of serum heart type fatty acid binding protein to left ventricular diastolic dysfunction in patients with type 2 diabetes and early diabetic kidney disease

AIMS

We aimed to investigate the serum level of heart type fatty acid binding protein (H-FABP) and its relation to left ventricular (LV) diastolic dysfunction in patients with type 2 diabetes (T2DM) and early diabetic kidney disease (DKD).

METHODS

This study was conducted on 100 T2DM patients divided into 50 patients with early DKD and 50 patients without DKD. Doppler echocardiography was used to assess LV function and serum H-FABP levels were measured using ELISA technique.

RESULTS

78% of patients with DKD and 12% of patients without DKD had LV diastolic dysfunction. Among patients with DKD, those with diastolic dysfunction had significantly higher urinary albumin to creatinine ratio (UACR) (p = 0.041). H-FABP levels were significantly higher in patients with DKD (p˂0.001) and it had significant positive correlation with UACR (p = 0.009). No significant difference was found regarding serum H-FABP levels between patients with normal LV function and those with diastolic dysfunction in both study groups.

CONCLUSION

Diastolic dysfunction is a common finding among patients with T2DM. UACR, but not serum H-FABP, is significantly associated with diastolic dysfunction in patients with early DKD. Serum H-FABP level is significantly higher in early DKD and positively correlated with the level of albuminuria.

Dysregulated Epicardial Adipose Tissue as a Risk Factor and Potential Therapeutic Target of Heart Failure with Preserved Ejection Fraction in Diabetes

Cardiovascular (CV) disease and heart failure (HF) are the leading cause of mortality in type 2 diabetes (T2DM), a metabolic disease which represents a fast-growing health challenge worldwide. Specifically, T2DM induces a cluster of systemic metabolic and non-metabolic signaling which may promote myocardium derangements such as inflammation, fibrosis, and myocyte stiffness, which represent the hallmarks of heart failure with preserved ejection fraction (HFpEF). On the other hand, several observational studies have reported that patients with T2DM have an abnormally enlarged and biologically transformed epicardial adipose tissue (EAT) compared with non-diabetic controls. This expanded EAT not only causes a mechanical constriction of the diastolic filling but is also a source of pro-inflammatory mediators capable of causing inflammation, microcirculatory dysfunction and fibrosis of the underlying myocardium, thus impairing the relaxability of the left ventricle and increasing its filling pressure. In addition to representing a potential CV risk factor, emerging evidence shows that EAT may guide the therapeutic decision in diabetic patients as drugs such as metformin, glucagon-like peptide‑1 (GLP-1) receptor agonists and sodium-glucose cotransporter 2 inhibitors (SGLT2-Is), have been associated with attenuation of EAT enlargement.

Fibrosis of the diabetic heart: Clinical significance, molecular mechanisms, and therapeutic opportunities

In patients with diabetes, myocardial fibrosis may contribute to the pathogenesis of heart failure and arrhythmogenesis, increasing ventricular stiffness and delaying conduction. Diabetic myocardial fibrosis involves effects of hyperglycemia, lipotoxicity and insulin resistance on cardiac fibroblasts, directly resulting in increased matrix secretion, and activation of paracrine signaling in cardiomyocytes, immune and vascular cells, that release fibroblast-activating mediators. Neurohumoral pathways, cytokines, growth factors, oxidative stress, advanced glycation end-products (AGEs), and matricellular proteins have been implicated in diabetic fibrosis; however, the molecular links between the metabolic perturbations and activation of a fibrogenic program remain poorly understood. Although existing therapies using glucose- and lipid-lowering agents and neurohumoral inhibition may act in part by attenuating myocardial collagen deposition, specific therapies targeting the fibrotic response are lacking. This review manuscript discusses the clinical significance, molecular mechanisms and cell biology of diabetic cardiac fibrosis and proposes therapeutic targets that may attenuate the fibrotic response, preventing heart failure progression.

Heart failure with preserved ejection fraction based on aging and comorbidities

Heart failure (HF) with preserved ejection fraction (HFpEF) is a leading cause of hospitalizations and mortality when diagnosed at the age of ≥ 65 years. HFpEF represents multifactorial and multisystemic syndrome and has different pathophysiology and phenotypes. Its diagnosis is difficult to be established based on left ventricular ejection fraction and may benefit from individually tailored approaches, underlying age-related changes and frequent comorbidities. Compared with the rapid development in the treatment of heart failure with reduced ejection fraction, HFpEF presents a great challenge and needs to be addressed considering the failure of HF drugs to improve its outcomes. Further extensive studies on the relationships between HFpEF, aging, and comorbidities in carefully phenotyped HFpEF subgroups may help understand the biology, diagnosis, and treatment of HFpEF. The current review summarized the diagnostic and therapeutic development of HFpEF based on the complex relationships between aging, comorbidities, and HFpEF.

Mitochondria-Associated Membranes (MAMs): A Novel Therapeutic Target for Treating Metabolic Syndrome

Mitochondria-associated Endoplasmic Reticulum (ER) Membranes (MAMs) are the cellular structures that connect the ER and mitochondria and mediate communication between these two organelles. MAMs have been demonstrated to be involved in calcium signaling, lipid transfer, mitochondrial dynamic change, mitophagy, and the ER stress response. In addition, MAMs are critical for metabolic regulation, and their dysfunction has been reported to be associated with metabolic syndrome, including the downregulation of insulin signaling and the accelerated progression of hyperlipidemia, obesity, and hypertension. This review covers the roles of MAMs in regulating insulin sensitivity and the molecular mechanism underlying MAM-regulated cellular metabolism and reveals the potential of MAMs as a therapeutic target in treating metabolic syndrome.

Klotho Deficiency Causes Heart Aging via Impairing the Nrf2-GR Pathway

RATIONALE

Cardiac aging is an important contributing factor for heart failure, which affects a large population but remains poorly understood.

OBJECTIVE

The purpose of this study is to investigate whether Klotho plays a role in cardiac aging.

METHODS AND RESULTS

Heart function declined in old mice (24 months), as evidenced by decreases in fractional shortening, ejection fraction, and cardiac output. Heart size and weight, cardiomyocyte size, and cardiac fibrosis were increased in old mice, indicating that aging causes cardiac hypertrophy and remodeling. Circulating Klotho levels were dramatically decreased in old mice, which prompted us to investigate whether the Klotho decline may cause heart aging. We found that Klotho gene mutation (KL-/-) largely decreased serum klotho levels and impaired heart function. Interestingly, supplement of exogenous secreted Klotho prevented heart failure, hypertrophy, and remodeling in both old mice and KL (-/-) mice. Secreted Klotho treatment inhibited excessive cardiac oxidative stress, senescence and apoptosis in old mice and KL (-/-) mice. Serum phosphate levels in KL (-/-) mice were kept in the normal range, suggesting that Klotho deficiency-induced heart aging is independent of phosphate metabolism. Mechanistically, Klotho deficiency suppressed GR (glutathione reductase) expression and activity in the heart via inhibition of transcription factor Nrf2 (nuclear factor-erythroid 2 p45-related factor 2). Furthermore, cardiac-specific overexpression of GR prevented excessive oxidative stress, apoptosis, and heart failure in both old and KL (-/-) mice.

CONCLUSIONS

Klotho deficiency causes cardiac aging via impairing the Nrf2-GR pathway. Supplement of exogenous secreted Klotho represents a promising therapeutic strategy for aging-associated cardiomyopathy and heart failure.

Mechanosensing dysregulation in the fibroblast: A hallmark of the aging heart

The myofibroblast is a specialized fibroblast that expresses α-smooth muscle actin (α-SMA) and participates in wound contraction and fibrosis. The fibroblast to myofibroblast transition depends on chemical and mechanical signals. A fibroblast senses the changes in the environment (extracellular matrix (ECM)) and transduces these changes to the cytoskeleton and the nucleus, resulting in activation or inhibition of α-SMA transcription in a process called mechanosensing. A stiff matrix greatly facilitates the transition from fibroblast to myofibroblast, and although the aging heart is much stiffer than the young one, the aging fibroblast has difficulties in transitioning into the contractile phenotype. This suggests that the events occurring downstream of the matrix, such as activation or changes in expression levels of various proteins participating in mechanotransduction can negatively alter the ability of the aging fibroblast to become a myofibroblast. In this review, we will discuss in detail the changes in ECM, receptors (integrin or non-integrin), focal adhesions, cytoskeleton, and transcription factors involved in mechanosensing that occur with aging.

Treatment with a DC-SIGN ligand reduces macrophage polarization and diastolic dysfunction in the aging female but not male mouse hearts

Cardiac diastolic dysfunction in aging arises from increased ventricular stiffness caused by inflammation and interstitial fibrosis. The diastolic dysfunction contributes to heart failure with preserved ejection fraction (HFpEF), which in the aging population is more common in women. This report examines its progression over 12 weeks in aging C57BL/6J mice and correlates its development with changes in macrophage polarization and collagen deposition.

Aged C57BL/6J mice were injected with dendritic cell–specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN) ligand 1 (DCSL1, an anti-inflammatory agent) or saline for 12 weeks. Echo and Doppler measurements were performed before and after 4 and 12 weeks of treatment. DCSL1 prevented the worsening of diastolic dysfunction over time in females but not in males. Cardiac single cell suspensions analyzed by flow cytometry revealed changes in the inflammatory infiltrate: (1) in males, there was an increased total number of leukocytes with an increased pro-inflammatory profile compared with females and they did not respond to DCSL1; (2) by contrast, DCSL1 treatment resulted in a shift in macrophage polarization to an anti-inflammatory phenotype in females. Notably, DCSL1 preferentially targeted tumor necrosis factor-α (TNFα⁺) pro-inflammatory macrophages. The reduction in pro-inflammatory macrophage polarization was accompanied by a decrease in collagen content in the heart.

Age-associated diastolic dysfunction in mice is more severe in females and is associated with unique changes in macrophage polarization in cardiac tissue. Treatment with DCSL1 mitigates the changes in inflammation, cardiac function, and fibrosis. The characteristics of diastolic dysfunction in aging female mice mimic similar changes in aging women.

Differential Gender-Dependent Patterns of Cardiac Fibrosis and Fibroblast Phenotypes in Aging Mice

Aging is characterized by physiological changes within the heart leading to fibrosis and dysfunction even in individuals without underlying pathologies. Gender has been shown to influence the characteristics of cardiac aging; however, gender-dependent cardiac fibrosis occurring with age remains largely not elucidated. Thus, broadening our understanding of this phenomenon proves necessary in order to develop novel anti-fibrotic strategies in the elderly. In this study, we aim to characterize cardiac fibrosis and cardiac fibroblast (CF) populations in aged male and female mice. Echocardiography revealed eccentric hypertrophy with left ventricular dilatation in the aged male versus concentric hypertrophy with left posterior wall thickening in the female, with preserved cardiac function in both groups. Reactive fibrosis was evidenced in the myocardium and epicardium of the aged female mice hearts whereas perivascular and replacement ones where present in the male heart. Collagen I was predominant in the aged male heart whereas collagen III was the main component in the female heart. CFs in the aged male heart were mainly recruited from resident PDGFRα⁺ populations but not derived from epicardium as evidenced by the absence of epicardial progenitor transcription factors Tcf21, Tbx18 and Wt1. Our results present a paradigm for gender-dependent cardiac fibrosis and the origins of CFs with age. This sets forth to revisit cardiac anti-fibrotic management according to the gender in the elderly and to explore novel therapeutic targets.

The Blood Circulating Rare Cell Population. What is it and What is it Good For?

Blood contains a diverse cell population of low concentration hematopoietic as well as non-hematopoietic cells. The majority of such rare cells may be bone marrow-derived progenitor and stem cells. This paucity of circulating rare cells, in particular in the peripheral circulation, has led many to believe that bone marrow as well as other organ-related cell egress into the circulation is a response to pathological conditions. Little is known about this, though an increasing body of literature can be found suggesting commonness of certain rare cell types in the peripheral blood under physiological conditions. Thus, the isolation and detection of circulating rare cells appears to be merely a technological problem. Knowledge about rare cell types that may circulate the blood stream will help to advance the field of cell-based liquid biopsy by supporting inter-platform comparability, making use of biological correct cutoffs and “mining” new biomarkers and combinations thereof in clinical diagnosis and therapy. Therefore, this review intends to lay ground for a comprehensive analysis of the peripheral blood rare cell population given the necessity to target a broader range of cell types for improved biomarker performance in cell-based liquid biopsy.

IL-18/IL-18R1 promotes circulating fibrocyte differentiation in the aging population

BACKGROUND

Fibrosis in multiple organs increases with age. Circulating fibrocytes are bone-marrow-derived mesenchymal progenitors that contribute to heart, lung, and kidney fibrosis under the diseased conditions. Whether circulating fibrocytes contribute to aging-related fibrosis is very limited.

METHODS AND RESULTS

We measured the proportion and differentiation of circulating fibrocytes (CD45⁺/CD34⁺/collagen I⁺) from elders (n = 12) and adults (n = 12) using flow cytometry. Differentiated fibrocytes in the culture dishes were isolated and microarray was performed. The percentage of circulating fibrocytes in elders (1.95 ± 0.43%) was comparable to that in the adults (1.71 ± 0.38%). Cultured fibrocytes displayed enhanced potential of differentiation in the elder group (67.91 ± 5.88%) vs the adult group (44.03 ± 7.98%). In addition, expression of fibroblast activation markers and cell migratory ability were also increased in differentiated fibrocytes from elders. Microarray analysis revealed that differentiated fibrocytes from elders expressed high level of interleukin-18 (IL-18) receptor 1 (IL-18R1). Furthermore, we found IL-18 was elevated in the plasma of elders and IL-18/IL-18R1 was shown to promote fibrocyte differentiation.

CONCLUSION

Circulating fibrocytes from elders had an enhanced capacity to differentiate into myofibroblasts, and might contribute to age-dependent fibrosis. Age-dependent increment of differentiation at least in part arose from their enhanced expression of IL-18R1. Inhibiting fibrocyte differentiation might be useful as an adjuvant treatment to delay the fibrosis process in aging population.

Do most patients with obesity or type 2 diabetes, and atrial fibrillation, also have undiagnosed heart failure? A critical conceptual framework for understanding mechanisms and improving diagnosis and treatment

Obesity and diabetes can lead to heart failure with preserved ejection fraction (HFpEF), potentially because they both cause expansion and inflammation of epicardial adipose tissue and thus lead to microvascular dysfunction and fibrosis of the underlying left ventricle. The same process also causes an atrial myopathy, which is clinically evident as atrial fibrillation (AF); thus, AF may be the first manifestation of HFpEF. Many patients with apparently isolated AF have latent HFpEF or subsequently develop HFpEF. Most patients with obesity or diabetes who have AF and exercise intolerance have increased left atrial pressures at rest or during exercise, even in the absence of diagnosed HFpEF. Among patients with AF, those who also have latent HFpEF have increased risk for systemic thromboembolism and death. The identification of HFpEF in patients with obesity or diabetes alters the risk-to-benefit relationship of commonly prescribed treatments. Bariatric surgery and statins can ameliorate AF and reduce the risk for HFpEF. Conversely, antihyperglycaemic drugs that promote adipogenesis or cause sodium retention (insulin and thiazolidinediones) may increase the risk for heart failure in patients with an underlying ventricular myopathy. Patients with obesity and diabetes who undergo catheter ablation for AF are at increased risk for AF recurrence and for post-ablation increases in pulmonary venous pressures and worsening heart failure, especially if HFpEF coexists. Therefore, AF may be the earliest indicator of HFpEF in patients with obesity or type 2 diabetes, and recognition of HFpEF alters the management of these patients.

Disease-treatment interactions in the management of patients with obesity and diabetes who have atrial fibrillation: the potential mediating influence of epicardial adipose tissue

Both obesity and type 2 diabetes are important risk factors for atrial fibrillation (AF), possibly because they both cause an expansion of epicardial adipose tissue, which is the source of proinflammatory adipocytokines that can lead to microvascular dysfunction and fibrosis of the underlying myocardium. If the derangement of epicardial fat adjoins the left atrium, the result is an atrial myopathy, which is clinically manifest as AF. In patients with AF, there is a close relationship between epicardial fat volume and the severity of electrophysiological abnormalities in the adjacent myocardial tissues, and epicardial fat mass predicts AF in the general population. The expansion of epicardial adipose tissue in obesity and type 2 diabetes may also affect the left ventricle, impairing its distensibility and leading to heart failure with a preserved ejection fraction (HFpEF). Patients with obesity or type 2 diabetes with AF often have HFpEF, but the diagnosis may be missed, if dyspnea is attributed to increased body mass or to the arrhythmia. The expected response to the treatment for obesity, diabetes or AF may be influenced by their effects on epicardial inflammation and the underlying atrial and ventricular myopathy. Bariatric surgery and metformin reduce epicardial fat mass and ameliorate AF, whereas insulin promotes adipogenesis and cardiac fibrosis, and its use is accompanied by an increased risk of AF. Rate control strategies for AF may impair exercise tolerance, because they allow for greater time for ventricular filling in patients who cannot tolerate volume loading because of cardiac fibrosis and HFpEF. At the same time, both obesity and diabetes decrease the expected success rate of rhythm control strategies for AF (e.g., electrical cardioversion or catheter ablation), because increased epicardial adipose tissue volumes and cardiac fibrosis are important determinants of AF recurrence following these procedures.

Targeting pro-senescence mitogen activated protein kinase (Mapk) enzymes with bioactive natural compounds

Aging is a multifactorial universal process characterized by a gradual decrease in physiological and biochemical functions. Given that life expectancy is on the rise, a better understanding of molecular mechanisms of the aging process is necessary in order to develop anti-aging interventions. Uncontrolled cellular senescence promotes persistent inflammation and accelerates the aging process by decreasing tissue renewal, repair and regeneration. Senescence of immune cells, immunesenescence, is another hallmark of aging. Targeting pro-senescent enzymes increases survival and therefore the lifespan. Although the upregulation of Mitogen Activated Protein Kinases (MAPK) enzymes in aging is still controversial, increasing evidence shows that dysregulation of those enzymes are associated with biological processes that contribute to aging such as irreversible senescence. In this manuscript components of the MAPK pathway will be summarized, including extracellular signal-regulated kinase 1 and 2 (ERK1/2), c-Jun N-terminal kinase (JNK) and p38, as well as natural flavonoids, phenolic and diterpenoids with anti-senescence activity that shows positive effects on longevity and MAPK inhibition. Although more studies using additional aging models are needed, we suggest that these selected natural bioactive compounds that regulate MAPK enzymes and reduce senescent cells can be potentially used to improve longevity and prevent/treat age-related diseases.

Changes in cardiac resident fibroblast physiology and phenotype in aging

The cardiac fibroblast plays a central role in tissue homeostasis and in repair after injury. With aging, dysregulated cardiac fibroblasts have a reduced capacity to activate a canonical transforming growth factor-β-Smad pathway and differentiate poorly into contractile myofibroblasts. That results in the formation of an insufficient scar after myocardial infarction. In contrast, in the uninjured aged heart, fibroblasts are activated and acquire a profibrotic phenotype that leads to interstitial fibrosis, ventricular stiffness, and diastolic dysfunction, all conditions that may lead to heart failure. There is an apparent paradox in aging, wherein reparative fibrosis is impaired but interstitial, adverse fibrosis is augmented. This could be explained by analyzing the effectiveness of signaling pathways in resident fibroblasts from young versus aged hearts. Whereas defective signaling by transforming growth factor-β leads to insufficient scar formation by myofibroblasts, enhanced activation of the ERK1/2 pathway may be responsible for interstitial fibrosis mediated by activated fibroblasts. Listen to this article's corresponding podcast at https://ajpheart.podbean.com/e/fibroblast-phenotypic-changes-in-the-aging-heart/ .

Antioxidant and Antifibrotic Effect of a Herbal Formulation In Vitro and in the Experimental Andropause via Nrf2/HO-1 Signaling Pathway

The Korean herbal formulation Ojayeonjonghwan is used for improving late-onset hypogonadism (LOH) symptoms such as erectile dysfunction (ED). A previous research suggested that a modified Ojayeonjonghwan (KH-204) could be used as an alternative to the treatment for ED. The pharmacological effects were examined in different conditions, including in vitro and in vivo. We measured the survival rate of TM3 Leydig cells under the oxidative stress condition. The s.c. injection of leuprorelin was used to induce androgen deprivation. We measured serum testosterone levels, oxidative stress, and apoptosis. The results of the treatment by KH-204 (1) preserved TM3 cells from oxidative stress by improving the expression of nuclear factor erythroid 2-related factor 2 (Nrf2)/heme oxygenase-1 (HO-1); (2) lowered the expression of transforming growth factor-beta (TGF-β) 1/SMAD; (3) increased the average of serum testosterone in androgen-deprived male rats; (4) kept the activation of spermatogenesis; (5) upgraded the contents of 8-hydroxy-20-deoxyguanosine (8-OHdG) and degraded the contents of superoxide dismutase (SOD); and (6) reduced apoptosis. We studied that KH-204 improved testicular dysfunction in LOH. It is likely, at least in part, to degrade oxidative stress through the Nrf2/HO-1 pathway. These findings may offer credible evidences for the use of new alternative therapies to treat LOH.

Biological properties of cardiac mesenchymal stem cells in rats with diabetic cardiomyopathy

Cardiomyopathy is a major outcome in patients with diabetes mellitus (DM) and contributes to the high morbidity/mortality observed in this disease.

AIMS

To evaluate several biological properties of cardiac mesenchymal stem cells (cMSCs) in a rat model of streptozotocin-induced DM with concomitant diabetic cardiomyopathy.

MAIN METHODS

After 10weeks of DM induction, diabetic and control rats were assessed using ECG and ventricular hemodynamics monitoring. Then, the hearts were excised and processed for histology and for extracting non-cardiomyocytic cells. A pool of these cells was plated for a colony forming units-fibroblasts (CFU-F) assay in order to estimate the number of cMSCs. The remaining cells were expanded to assess their proliferation rate as well as their osteogenic and adipogenic differentiation ability.

KEY FINDINGS

DM rats presented intense hyperglycemia and changes in ECG, LV hemodynamic, cardiac mass index and fibrosis, indicating presence of DCM. The CFU-F assay revealed a higher number of cardiac CFU-Fs in DM rats (10.4±1.1CFU-F/10⁵ total cells versus 7.6±0.7CFU-F/10⁵ total cells in control rats, p<0.05), which was associated with a significantly higher proliferative rate of cMSCs in DM rats. In contrast, cMSCs from DM rats presented a lower capacity to differentiate into both osteogenic (20.8±4.2% versus 10.1±1.0% in control rats, p<0.05) and adipogenic lineages (4.6±1.0% versus 1.3±0.5% in control rats, p<0.05).

SIGNIFICANCE

The findings suggest, for the first time, that in chronic DM rats with overt DCM, cMSCs increase in number and exhibit changes in several functional properties, which could be implicated in the pathogenesis of diabetic cardiomyopathy.

Aicar treatment reduces interstitial fibrosis in aging mice: Suppression of the inflammatory fibroblast

In 2030, elderly people will represent 20% of the United States population. Even now, chronic cardiac diseases, especially heart failure with preserved systolic function (HFpEF), are the most expensive DRGs for Medicare. Progressive interstitial fibrosis in the aging heart is well recognized as an important component of HFpEF. Our recent studies suggested an important pathophysiologic role for reduced TGF-β receptor 1 (TGFβR1) signaling in mesenchymal stem cells (MSCs) and their mesenchymal fibroblast progeny in the development of interstitial fibrosis. This report arises from our previous studies, which suggest that an inflammatory phenotype exists in these mesenchymal fibroblasts as a result of a reduced TGF-β-Smad-dependent pathway but upregulated farnesyltransferase (FTase)-Ras-Erk signaling. In this report we provide evidence for a therapeutic approach that downregulates Erk activation through an adenosine monophosphate-activated kinase (AMPK) pathway. Aging C57BL/6J mice were treated with AICAR (an AMPK activator) for a 30-day period. This treatment suppressed excessive monocyte chemoattractant protein-1 (MCP-1) generation, which diminished leukocyte infiltration and in consequence suppressed the formation of macrophage-derived myeloid fibroblasts. Interestingly, the number of mesenchymal fibroblasts was also reduced. In addition, we observed changes in extracellular matrix (ECM) deposition, specifically that collagen type I and the alternatively spliced variant of fibronectin (EDA) expressions were reduced. These data suggest that the upregulation of AMPK activity is a potential therapeutic approach to fibrosis in the aging heart.

Dissecting the role of myeloid and mesenchymal fibroblasts in age-dependent cardiac fibrosis

Aging is associated with increased cardiac interstitial fibrosis and diastolic dysfunction. Our previous study has shown that mesenchymal fibroblasts in the C57BL/6J (B6J) aging mouse heart acquire an inflammatory phenotype and produce higher levels of chemokines. Monocyte chemoattractant protein-1 (MCP-1) secreted by these aged fibroblasts promotes leukocyte uptake into the heart. Some of the monocytes that migrate into the heart polarize into M2a macrophages/myeloid fibroblasts. The number of activated mesenchymal fibroblasts also increases with age, and consequently, both sources of fibroblasts contribute to fibrosis. Here, we further investigate mechanisms by which inflammation influences activation of myeloid and mesenchymal fibroblasts and their collagen synthesis. We examined cardiac fibrosis and heart function in three aged mouse strains; we compared C57BL/6J (B6J) with two other strains that have reduced inflammation via different mechanisms. Aged C57BL/6N (B6N) hearts are protected from oxidative stress and fibroblasts derived from them do not develop an inflammatory phenotype. Likewise, these mice have preserved diastolic function. Aged MCP-1 null mice on the B6J background (MCP-1KO) are protected from elevated leukocyte infiltration; they develop moderate but reduced fibrosis and diastolic dysfunction. Based on these studies, we further delineated the role of resident versus monocyte-derived M2a macrophages in myeloid-dependent fibrosis and found that the number of monocyte-derived M2a (but not resident) macrophages correlates with age-related fibrosis and diastolic dysfunction. In conclusion, we have found that ROS and inflammatory mediators are necessary for activation of fibroblasts of both developmental origins, and prevention of either led to better functional outcomes.

Optimization of Reference Genes for Normalization of Reverse Transcription Quantitative Real-Time Polymerase Chain Reaction Results in Senescence Study of Mesenchymal Stem Cells

Recently, it has been suggested that cellular senescence is associated with stem cell exhaustion, which reduces the regenerative potential of tissues and contributes to aging and age-related diseases. Mesenchymal stem cells (MSCs) attract a large amount of attention in stem cell research and regeneration medicine because they possess multiple advantages and senescent MSCs could be one of the most useful stem cell models in aging studies. It is important to quantitatively evaluate senescence markers to both identify and study the mechanisms involved in MSC senescence. Reverse transcription quantitative real-time polymerase chain reaction (RT-qPCR) is currently the most widely used tool to quantify the mRNA levels of markers. However, no report has demonstrated the optimal reference genes that should be used to normalize RT-qPCR in senescence studies of MSCs. In this study, we compared 16 commonly used reference genes (GAPDH, ACTB, RPL13A, TBP, B2M, GUSB, RPLPO, YWHAZ, RPS18, EEF1A1, ATP5F1, HPRT1, PGK1, TFRC, UBC, and PPIA) in proliferating or replicative-senescent human adipose-derived MSCs (hAD-MSCs) that were isolated from seven healthy donors aged 29-59 years old. Three algorithms (geNorm, NormFinder, and BestKeeper) were used to determine the most optimal reference gene. The results showed that PPIA exhibited the most stable expression during senescence, while the widely used ACTB exhibited the lowest stability. We also confirmed that different reference genes lead to different evaluations of senescence markers. Our work ensures that results obtained from senescence studies of hAD-MSCs will be appropriately evaluated in both basic research and clinical trials.

Insulin Cannot Induce Adipogenic Differentiation in Primary Cardiac Cultures

Cardiac tissue contains a heterogeneous population of cardiomyocytes and nonmyocyte population especially fibroblasts. Fibroblast differentiation into adipogenic lineage is important for fat accumulation around the heart which is important in cardiac pathology. The differentiation in fibroblast has been observed both spontaneously and due to increased insulin stimulation. The present study aims to observe the effect of insulin in adipogenic differentiation of cardiac cells present in primary murine cardiomyocyte cultures. Oil Red O (ORO) staining has been used for observing the lipid accumulations formed due to adipogenic differentiation in murine cardiomyocyte cultures. The accumulated lipids were quantified by ORO assay and normalized using protein estimation. The lipid accumulation in cardiac cultures did not increase in presence of insulin. However, addition of other growth factors like insulin-like growth factor 1 and epidermal growth factor promoted adipogenic differentiation even in the presence of insulin and other inhibitory molecules such as vitamins. Lipid accumulation also increased in cells grown in media without insulin after an initial exposure to insulin-containing growth media. The current study adds to the existing knowledge that the insulin by itself cannot induce adipogenic induction in the cardiac cultures. The data have significance in the understanding of cardiovascular health especially in diabetic patients.

Involvement of fibrocytes in asthma and clinical implications

Bloodborne fibrocytes are bone marrow-derived cells that participate in immune responses and exhibit pro-inflammatory and matrix remodelling properties. In patients with asthma receiving an adequate treatment, the blood fibrocyte count is very low and comparable to that obtained in healthy individuals. In these patients, a transient increase in fibrocyte numbers in the peripheral blood and in the airways occurs in concomitance with increased bronchial inflammation and reflects disease worsening and the need for more intensive treatment. Persistently elevated numbers of fibrocytes in the peripheral blood and in the bronchial mucosa are observed in chronically undertreated or corticosteroid-resistant asthma and are associated with persistent airway inflammation and ongoing remodelling of the bronchial wall. The asthmatic bronchial epithelium is the main source of fibrocyte chemoattractants in asthma and contributes with T helper type 2 lymphocytes and eosinophils to promote the proliferation and pro-remodelling function of recruited fibrocytes. The presence of elevated numbers of fibrocytes in the bronchial mucosa of allergic patients with undertreated or treatment-resistant asthma may also increase the risk of acute exacerbations because these cells can amplify T helper type 2 lymphocyte-driven inflammation on every exposure to the clinically relevant allergen and can promote further inflammation on rhinovirus infections by allowing viral replication and releasing additional pro-inflammatory factors. Improved methods for the isolation and functional analysis of pure populations of viable circulating fibrocytes have allowed a better understanding of the effector role of these cells. A reliable and clinically applicable assay has been developed to measure blood fibrocyte counts as outcome measure in future clinical trials. New therapeutic agents are needed to block both persistent inflammation and fibrocytosis in corticosteroid-resistant asthma.

Mesenchymal stem cell-derived inflammatory fibroblasts mediate interstitial fibrosis in the aging heart

Pathologic fibrosis in the aging mouse heart is associated with dysregulated resident mesenchymal stem cells (MSC) arising from reduced stemness and aberrant differentiation into dysfunctional inflammatory fibroblasts. Fibroblasts derived from aging MSC secrete higher levels of 1) collagen type 1 (Col1) that directly contributes to fibrosis, 2) monocyte chemoattractant protein-1 (MCP-1) that attracts leukocytes from the blood and 3) interleukin-6 (IL-6) that facilitates transition of monocytes into myeloid fibroblasts. The transcriptional activation of these proteins is controlled via the farnesyltransferase (FTase)-Ras-Erk pathway. The intrinsic change in the MSC phenotype acquired by advanced age is specific for the heart since MSC originating from bone wall (BW-MSC) or fibroblasts derived from them were free of these defects. The potential therapeutic interventions other than clinically approved strategies based on findings presented in this review are discussed as well. This article is a part of a Special Issue entitled "Fibrosis and Myocardial Remodeling".

Heart Failure as a Comorbidity of Diabetes: Role of Dipeptidyl Peptidase 4

Heart failure is a primary cause of death worldwide, and it is notable that heart failure patients exhibit a high incidence of diabetes. On the other hand, comorbid diabetes significantly worsens the prognosis of heart failure, even independently of complicated coronary artery disease.To date, heart failure caused by diabetes has been designated as "diabetic cardiomyopathy (DMC)," and a recent cohort study of the large-scale (1.9 million people) research platform of linked electronic medical records in UK (CALIBER registry) demonstrated that heart failure and peripheral arterial disease are the most common initial manifestations of cardiovascular disease in type 2 diabetes. The underlying pathophysiology has been characterized as microvasculopathy, myocardial hypertrophy, and cardiac fibrosis; however, these evidences are mostly obtained under a preclinical setting, and its clinical application on DMC in terms of its diagnosis and therapeutic intervention yet has reached practical. Our group has focused on and clarified the molecular mechanisms underlying DMC both in preclinical and clinical settings and has found the primary role of "dipeptidyl peptidase-4 (DPP4)" in the pathogenesis of diabetic microvasculopathy in the heart. Moreover, there are evidences implicating the potent role of circulating DPP4 activity in the diagnosis of diastolic heart failure. The present review aimed to review the current comprehension regarding diabetes and heart failure and discuss the therapeutic and diagnostic roles of DPP4.

Advanced glycation end-products accelerate the cardiac aging process through the receptor for advanced glycation end-products/transforming growth factor-β-Smad signaling pathway in cardiac fibroblasts

AIMS

The current study was carried out to evaluate the effect of advanced glycation end-products (AGE) on cardiac aging and to explore its underlying mechanisms.

METHODS

Neonatal rat cardiac fibroblasts were cultured and divided into four groups: control; AGE; AGE + receptor for AGE antibody and AGE + SB431542 (transforming growth factor-β [TGF-β]/Smad signaling pathway inhibitor, 10 μmol/L) group. After being cultured for 48 h, the cells were harvested and the senescence-associated beta-galactosidase expression was analyzed. Then the level of p16, TGF-β, Smad/p-smad and matrix metalloproteinases-2 was evaluated by western blot.

RESULTS

Significantly increased senescence-associated beta-galactosidase activity as well as p16 level was observed in the AGE group. Furthermore, AGE also significantly increased the TGF-β1, p-smad2/3 and metalloproteinases-2 expression in cardiac fibroblasts (all P < 0.01). Meanwhile, either pretreatment with receptor for AGE-Ab or SB431542 significantly inhibited the upregulated cardiac senescence (beta-galactosidase activity and P16) and fibrosis-associated (TGF-β1, p-smad2/3 and metalloproteinases-2) markers induced by AGE.

CONCLUSIONS

Taken together, all these results suggested that AGE are an important factor for cardiac aging and fibrosis, whereas the receptor for AGE and TGF-β/Smad signaling pathway might be involved in the AGE-induced cardiac aging process.

Multipotent stem cells of the heart-do they have therapeutic promise?

The last decade has brought a comprehensive change in our view of cardiac remodeling processes under both physiological and pathological conditions, and cardiac stem cells have become important new players in the general mainframe of cardiac homeostasis. Different types of cardiac stem cells show different capacities for differentiation into the three major cardiac lineages: myocytes, endothelial cells and smooth muscle cells. Physiologically, cardiac stem cells contribute to cardiac homeostasis through continual cellular turnover. Pathologically, these cells exhibit a high level of proliferative activity in an apparent attempt to repair acute cardiac injury, indicating that these cells possess (albeit limited) regenerative potential. In addition to cardiac stem cells, mesenchymal stem cells represent another multipotent cell population in the heart; these cells are located in regions near pericytes and exhibit regenerative, angiogenic, antiapoptotic, and immunosuppressive properties. The discovery of these resident cardiac stem cells was followed by a number of experimental studies in animal models of cardiomyopathies, in which cardiac stem cells were tested as a therapeutic option to overcome the limited transdifferentiating potential of hematopoietic or mesenchymal stem cells derived from bone marrow. The promising results of these studies prompted clinical studies of the role of these cells, which have demonstrated the safety and practicability of cellular therapies for the treatment of heart disease. However, questions remain regarding this new therapeutic approach. Thus, the aim of the present review was to discuss the multitude of different cardiac stem cells that have been identified, their possible functional roles in the cardiac regenerative process, and their potential therapeutic uses in treating cardiac diseases.

Mesenchymal stem cell-derived inflammatory fibroblasts promote monocyte transition into myeloid fibroblasts via an IL-6-dependent mechanism in the aging mouse heart

Fibrosis in the old mouse heart arises partly as a result of aberrant mesenchymal fibroblast activation. We have previously shown that endogenous mesenchymal stem cells (MSCs) in the aged heart are markedly resistant to TGF-β signaling. Fibroblasts originating from these MSCs retain their TGF-β unresponsiveness and become inflammatory. In current studies, we found that these inflammatory fibroblasts secreted higher levels of IL-6 (3-fold increase, P < 0.05) when compared with fibroblasts derived from the young hearts. Elevated IL-6 levels in fibroblasts derived from old hearts arose from up-regulated expression of Ras protein-specific guanine nucleotide releasing factor 1 (RasGrf1), a Ras activator (5-fold, P < 0.01). Knockdown of RasGrf1 by gene silencing or pharmacologic inhibition of farnesyltransferase (FTase) or ERK caused reduction of IL-6 mRNA (more than 65%, P < 0.01) and decreased levels of secreted IL-6 (by 44%, P < 0.01). In vitro, IL-6 markedly increased monocyte chemoattractant protein-1-driven monocyte-to-myeloid fibroblast formation after transendothelial migration (TEM; 3-fold, P < 0.01). In conclusion, abnormal expression of RasGrf1 promoted production of IL-6 by mesenchymal fibroblasts in the old heart. Secreted IL-6 supported conversion of monocyte into myeloid fibroblasts. This process promotes fibrosis and contributes to the diastolic dysfunction in the aging heart.

Clinical features of subclinical left ventricular systolic dysfunction in patients with diabetes mellitus

Background

Left ventricular (LV) longitudinal systolic dysfunction has been identified even in asymptomatic patients with diabetes mellitus (DM) and preserved LV ejection fraction (LVEF). However, its relevant clinical features have not been fully evaluated.

Methods

We studied 144 asymptomatic DM patients without coronary artery disease. Their mean age was 57 ± 15 years, 79 (55%) were female, and mean LVEF was 66 ± 4% (all ≥50%). Global longitudinal strain (GLS) was determined as the average peak strain of 18 segments from the three standard apical views, and was expressed as an absolute value. With the pre-defined cutoff for subclinical LV systolic dysfunction in DM patients with preserved LVEF set at GLS < 18%, this dysfunction was detected in 53 patients (37%).

Results

Multivariate logistic regression analysis revealed that type 2 DM, hypertriglyceridemia, overweight/obesity, nephropathy and neuropathy were independently associated with GLS < 18%, with nephropathy being the highest risk factor (OR: 5.26; 95% CI 2.111-13.12, p < 0.001). For sequential logistic regression models, a model based on clinical variables including gender, type 2 DM and DM duration (χ² = 24.1) was improved by addition of overweight/obesity and hypertriglyceridemia (χ² = 45.6, p < 0.001), and further improved by addition of nephropathy and neuropathy (χ² = 70.2, p < 0.001) as variables. Furthermore, albuminuria significantly correlated with GLS (r = −0.51, p < 0.001), and a multivariate regression model showed it to be the factor most closely associated with GLS (β = −0.33, p < 0.001).

Conclusions

Diabetic complications, hypertriglyceridemia and overweight/obesity were closely associated with early stage of LV systolic longitudinal myocardial dysfunction in asymptomatic DM patients with preserved LVEF. Our findings can be clinically noticeable for the management of DM patients.

Role of adenosine A2B receptor signaling in contribution of cardiac mesenchymal stem-like cells to myocardial scar formation

Adenosine levels increase in ischemic hearts and contribute to the modulation of that pathological environment. We previously showed that A2B adenosine receptors on mouse cardiac Sca1(+)CD31(-) mesenchymal stromal cells upregulate secretion of paracrine factors that may contribute to the improvement in cardiac recovery seen when these cells are transplanted in infarcted hearts. In this study, we tested the hypothesis that A2B receptor signaling regulates the transition of Sca1(+)CD31(-) cells, which occurs after myocardial injury, into a myofibroblast phenotype that promotes myocardial repair and remodeling. In vitro, TGFβ1 induced the expression of the myofibroblast marker α-smooth muscle actin (αSMA) and increased collagen I generation in Sca1(+)CD31(-) cells. Stimulation of A2B receptors attenuated TGFβ1-induced collagen I secretion but had no effect on αSMA expression. In vivo, myocardial infarction resulted in a rapid increase in the numbers of αSMA-positive cardiac stromal cells by day 5 followed by a gradual decline. Genetic deletion of A2B receptors had no effect on the initial accumulation of αSMA-expressing stromal cells but hastened their subsequent decline; the numbers of αSMA-positive cells including Sca1(+)CD31(-) cells remained significantly higher in wild type compared with A2B knockout hearts. Thus, our study revealed a significant contribution of cardiac Sca1(+)CD31(-) cells to the accumulation of αSMA-expressing cells after infarction and implicated A2B receptor signaling in regulation of myocardial repair and remodeling by delaying deactivation of these cells. It is plausible that this phenomenon may contribute to the beneficial effects of transplantation of these cells to the injured heart.

Diabetes-related heart failure

As the link between heart failure (HF) and diabetes mellitus (DM) becomes unignorable, so the need is further increasing for pathological comprehension: What is "diabetic cardiomyopathy (DMC)?" In response to current concern, the most updated guidelines stated by the ACCF/AHA and by the ESC/EASD take one step further, including the definition of DMC, although it is a matter yet to be completed. For more than 40 years, coronary artery disease and hypertension have been considered as the main causes of diabetes-related cardiac dysfunction. HF was originally considered as a result of reduced left ventricular ejection fraction (HF-REF); however, it has been recognized that HF symptoms are often observed in patients with preserved EF (HF-PEF). DMC includes HF with both reduced and preserved entities independent of coronary stenosis and hypertension. Cardiologists are thus facing a sort of chaos without clear guidelines for the "deadly intersection" of DM and HF. Today, the increasing interest and concern have caused DMC to be revisited and the first step in controlling the chaos around DMC is to organize and analyze all of the available evidence from preclinical and clinical studies. This review aims to illustrate the current concepts of DMC by shedding light on the new molecular mechanisms.  (Circ J 2014; 78: 576-583).

Adverse fibrosis in the aging heart depends on signaling between myeloid and mesenchymal cells; role of inflammatory fibroblasts

Aging has been associated with adverse fibrosis. Here we formulate a new hypothesis and present new evidence that unresponsiveness of mesenchymal stem cells (MSC) and fibroblasts to transforming growth factor beta (TGF-β), due to reduced expression of TGF-β receptor I (TβRI), provides a foundation for cardiac fibrosis in the aging heart via two mechanisms. 1) TGF-β promotes expression of Nanog, a transcription factor that retains MSC in a primitive state. In MSC derived from the aging heart, Nanog expression is reduced and therefore MSC gradually differentiate and the number of mesenchymal fibroblasts expressing collagen increases. 2) As TGF-β signaling pathway components negatively regulate transcription of monocyte chemoattractant protein-1 (MCP-1), a reduced expression of TβRI prevents aging mesenchymal cells from shutting down their own MCP-1 expression. Elevated MCP-1 levels that originated from MSC attract transendothelial migration of mononuclear leukocytes from blood to the tissue. MCP-1 expressed by mesenchymal fibroblasts promotes further migration of monocytes and T lymphocytes away from the endothelial barrier and supports the monocyte transition into macrophages and finally into myeloid fibroblasts. Both myeloid and mesenchymal fibroblasts contribute to fibrosis in the aging heart via collagen synthesis. This article is part of a Special Issue entitled "Myocyte-Fibroblast Signalling in Myocardium ".

Aging Impairs the Proliferative Capacity of Cardiospheres, Cardiac Progenitor Cells and Cardiac Fibroblasts: Implications for Cell Therapy

INTRODUCTION

Cardiospheres (CS) are self-assembling clusters of cells that can be grown from cardiac tissue. They contain a heterogeneous cell population that includes cardiac progenitor cells (CPCs) and cardiac fibroblasts. CS and CPCs have been shown to improve cardiac function after myocardial infarction (MI) in experimental models and are now being studied in clinical trials. The effects of aging on the proliferative capacity of CS and CPCs, and the paracrine signaling between cell types, remain incompletely understood.

METHODS AND RESULTS

We compared the growth of CS from young and aging murine hearts at baseline and following MI. The number of CS from young and aging hearts was similar at baseline. However, after MI, young hearts had a dramatic increase in the number of CS that grew, but this proliferative response to MI was virtually abolished in the aging heart. Further, the proportion of cells within the CS that were CPCs (defined as Sca-1(stem cell antigen-1)(+)/CD45(-)) was significantly lower in aging hearts than young hearts. Thus the number of available CPCs after culture from aging hearts was substantially lower than from young hearts. Cardiac fibroblasts from aging hearts proliferated more slowly in culture than those from young hearts. We then investigated the interaction between aging cardiac fibroblasts and CPCs. We found no significant paracrine effects on proliferation between these cell types, suggesting the impaired proliferation is a cell-autonomous problem.

CONCLUSIONS

Aging hearts generate fewer CPCs, and aging CPCs have significantly reduced proliferative potential following MI. Aging cardiac fibroblasts also have reduced proliferative capacity, but these appear to be cell-autonomous problems, not caused by paracrine signaling between cell types.