Andrew O. Kadlec

The Human Microcirculation: Regulation of Flow and Beyond

The microcirculation is responsible for orchestrating adjustments in vascular tone to match local tissue perfusion with oxygen demand. Beyond this metabolic dilation, the microvasculature plays a critical role in modulating vascular tone by endothelial release of an unusually diverse family of compounds including nitric oxide, other reactive oxygen species, and arachidonic acid metabolites. Animal models have provided excellent insight into mechanisms of vasoregulation in health and disease. However, there are unique aspects of the human microcirculation that serve as the focus of this review. The concept is put forth that vasculoparenchymal communication is multimodal, with vascular release of nitric oxide eliciting dilation and preserving normal parenchymal function by inhibiting inflammation and proliferation. Likewise, in disease or stress, endothelial release of reactive oxygen species mediates both dilation and parenchymal inflammation leading to cellular dysfunction, thrombosis, and fibrosis. Some pathways responsible for this stress-induced shift in mediator of vasodilation are proposed. This paradigm may help explain why microvascular dysfunction is such a powerful predictor of cardiovascular events and help identify new approaches to treatment and prevention.

Mitochondrial signaling in the vascular endothelium: beyond reactive oxygen species

Traditionally, the mitochondria have been viewed as the cell’s powerhouse, producing energy in the form of ATP. As a byproduct of ATP formation, the mitochondrial electron transport chain produces substantial amounts of reactive oxygen species (ROS). First thought to be toxic, recent literature indicates an important signaling function for mitochondria-derived ROS, especially in relation to cardiovascular disease pathogenesis. This has spawned an evolution to a more contemporary view of mitochondrial function as a dynamic organelle involved in key regulatory and cell survival processes. Beyond ROS, recent studies have identified a host of mitochondria-linked factors that influence the cellular and extracellular environments, including mitochondria-derived peptides, mitochondria-localized proteins, and the mitochondrial genome itself. Interestingly, many of these factors help orchestrate ROS homeostasis and ROS-related signaling. The paradigm defining the role of mitochondria in the vasculature needs to be updated yet again to include these key signaling factors, which serves as the focus of the current review. In describing these novel signaling factors, we pay specific attention to their influence on endothelial homeostasis. Therapies targeting these pathways are discussed, as are emerging research directions.

PGC-1α (Peroxisome Proliferator–Activated Receptor γ Coactivator 1-α) Overexpression in Coronary Artery Disease Recruits NO and Hydrogen Peroxide During Flow-Mediated Dilation and Protects Against Increased Intraluminal Pressure

Blood flow through healthy human vessels releases NO to produce vasodilation, whereas in patients with coronary artery disease (CAD), the mediator of dilation transitions to mitochondria-derived hydrogen peroxide (mtH2O2). Excessive mtH2O2 production contributes to a proatherosclerotic vascular milieu. Loss of PGC-1&agr; (peroxisome proliferator–activated receptor &ggr; coactivator 1&agr;) is implicated in the pathogenesis of CAD. We hypothesized that PGC-1&agr; suppresses mtH2O2 production to reestablish NO-mediated dilation in isolated vessels from patients with CAD. Isolated human adipose arterioles were cannulated, and changes in lumen diameter in response to graded increases in flow were recorded in the presence of PEG (polyethylene glycol)–catalase (H2O2 scavenger) or L-NAME (NG-nitro-L-arginine methyl ester; NOS inhibitor). In contrast to the exclusively NO- or H2O2-mediated dilation seen in either non-CAD or CAD conditions, respectively, flow-mediated dilation in CAD vessels was sensitive to both L-NAME and PEG-catalase after PGC-1&agr; upregulation using ZLN005 and &agr;-lipoic acid. PGC-1&agr; overexpression in CAD vessels protected against the vascular dysfunction induced by an acute increase in intraluminal pressure. In contrast, downregulation of PGC-1&agr; in non-CAD vessels produces a CAD-like phenotype characterized by mtH2O2-mediated dilation (no contribution of NO). Loss of PGC-1&agr; may contribute to the shift toward the mtH2O2-mediated dilation observed in vessels from subjects with CAD. Strategies to boost PGC-1&agr; levels may provide a therapeutic option in patients with CAD by shifting away from mtH2O2-mediated dilation, increasing NO bioavailability, and reducing levels of mtH2O2. Furthermore, increased expression of PGC-1&agr; allows for simultaneous contributions of both NO and H2O2 to flow-mediated dilation.

Friend or foe? Telomerase as a pharmacological target in cancer and cardiovascular disease

Aging, cancer, and chronic disease have remained at the forefront of basic biological research for decades. Within this context, significant attention has been paid to the role of telomerase, the enzyme responsible for lengthening telomeres, the nucleotide sequences located at the end of chromosomes found in the nucleus. Alterations in telomere length and telomerase activity are a common denominator to the underlying pathology of these diseases. While nuclear-specific, telomere-lengthening effects of telomerase impact cellular/organismal aging and cancer development, non-canonical, extra-nuclear, and non-telomere-lengthening contributions of telomerase have only recently been described and their exact physiological implications are ill defined. Although the mechanism remains unclear, recent reports reveal that the catalytic subunit of telomerase, telomerase reverse transcriptase (TERT), regulates levels of mitochondrial-derived reactive oxygen species (mtROS), independent of its established role in the nucleus. Telomerase inhibition has been the target of chemotherapy (directed or indirectly) for over a decade now, yet no telomerase inhibitor is FDA approved and few are currently in late-stage clinical trials, possibly due to underappreciation of the distinct extra-nuclear functions of telomerase. Moreover, evaluation of telomerase-specific therapies is largely limited to the context of chemotherapy, despite reports of the beneficial effects of telomerase activation in the cardiovascular system in relation to such processes as endothelial dysfunction and myocardial infarction. Thus, there is a need for better understanding of telomerase-focused cell and organism physiology, as well as development of telomerase-specific therapies in relation to cancer and extension of these therapies to cardiovascular pathologies. This review will detail findings related to telomerase and evaluate its potential to serve as a therapeutic target.

Role of PGC-1α in Vascular Regulation Implications for Atherosclerosis

Mitochondrial dysfunction results in high levels of oxidative stress and mitochondrial damage, leading to disruption of endothelial homeostasis. Recent discoveries have clarified several pathways, whereby mitochondrial dysregulation contributes to endothelial dysfunction and vascular disease burden. One such pathway centers around peroxisome proliferator receptor-&ggr; coactivator 1&agr; (PGC-1&agr;), a transcriptional coactivator linked to mitochondrial biogenesis and antioxidant defense, among other functions. Although primarily investigated for its therapeutic potential in obesity and skeletal muscle differentiation, the ability of PGC-1&agr; to alter a multitude of cellular functions has sparked interest in its role in the vasculature. Within this context, recent studies demonstrate that PGC-1&agr; plays a key role in endothelial cell and smooth muscle cell regulation through effects on oxidative stress, apoptosis, inflammation, and cell proliferation. The ability of PGC-1&agr; to affect these parameters is relevant to vascular disease progression, particularly in relation to atherosclerosis. Upregulation of PGC-1&agr; can prevent the development of, and even encourage regression of, atherosclerotic lesions. Therefore, PGC-1&agr; is poised to serve as a promising target in vascular disease. This review details recent findings related to PGC-1&agr; in vascular regulation, regulation of PGC-1&agr; itself, the role of PGC-1&agr; in atherosclerosis, and therapies that target this key protein.Mitochondrial dysfunction results in high levels of oxidative stress and mitochondrial damage, leading to disruption of endothelial homeostasis. Recent discoveries have clarified several pathways, whereby mitochondrial dysregulation contributes to endothelial dysfunction and vascular disease burden. One such pathway centers around peroxisome proliferator receptor-γ coactivator 1α (PGC-1α), a transcriptional coactivator linked to mitochondrial biogenesis and antioxidant defense, among other functions. Although primarily investigated for its therapeutic potential in obesity and skeletal muscle differentiation, the ability of PGC-1α to alter a multitude of cellular functions has sparked interest in its role in the vasculature. Within this context, recent studies demonstrate that PGC-1α plays a key role in endothelial cell and smooth muscle cell regulation through effects on oxidative stress, apoptosis, inflammation, and cell proliferation. The ability of PGC-1α to affect these parameters is relevant to vascular disease progression, particularly in relation to atherosclerosis. Upregulation of PGC-1α can prevent the development of, and even encourage regression of, atherosclerotic lesions. Therefore, PGC-1α is poised to serve as a promising target in vascular disease. This review details recent findings related to PGC-1α in vascular regulation, regulation of PGC-1α itself, the role of PGC-1α in atherosclerosis, and therapies that target this key protein. # Highlights {#article-title-101}

Telomerase reverse transcriptase protects against angiotensin II-induced microvascular endothelial dysfunction

A rise in reactive oxygen species (ROS) may contribute to cardiovascular disease by reducing nitric oxide (NO) levels, leading to loss of NOs vasodilator and anti-inflammatory effects. Although primarily studied in larger conduit arteries, excess ROS release and a corresponding loss of NO also occur in smaller resistance arteries of the microcirculation, but the underlying mechanisms and therapeutic targets have not been fully characterized. We examined whether either of the two subunits of telomerase, telomerase reverse transcriptase (TERT) or telomerase RNA component (TERC), affect microvascular ROS production and peak vasodilation at baseline and in response to in vivo administration to angiotensin II (ANG II). We report that genetic loss of TERT [maximal dilation: 52.0 ± 6.1% with vehicle, 60.4 ± 12.9% with Nω-nitro-l-arginine methyl ester (l-NAME), and 32.2 ± 12.2% with polyethylene glycol-catalase (PEG-Cat) ( P < 0.05), means ± SD, n = 9-19] but not TERC [maximal dilation: 79 ± 5% with vehicle, 10.7 ± 9.8% with l-NAME ( P < 0.05), and 86.4 ± 8.4% with PEG-Cat, n = 4-7] promotes flow-induced ROS formation. Moreover, TERT knockout exacerbates the microvascular dysfunction resulting from in vivo ANG II treatment, whereas TERT overexpression is protective [maximal dilation: 88.22 ± 4.6% with vehicle vs. 74.0 ± 7.3% with ANG II (1,000 ng·kg-1·min-1) ( P = not significant), n = 4]. Therefore, loss of TERT but not TERC may be a key contributor to the elevated microvascular ROS levels and reduced peak dilation observed in several cardiovascular disease pathologies. NEW & NOTEWORTHY This study identifies telomerase reverse transcriptase (TERT) but not telomerase RNA component as a key factor regulating endothelium-dependent dilation in the microcirculation. Loss of TERT activity leads to microvascular dysfunction but not conduit vessel dysfunction in first-generation mice. In contrast, TERT is protective in the microcirculation in the presence of prolonged vascular stress. Understanding the mechanism of how TERT protects against vascular stress represents a novel target for the treatment of vascular disorders.

Microvascular Adaptations to Exercise: Protective Effect of PGC-1 Alpha

BACKGROUND Sedentary behavior and obesity are major risk factors for cardiovascular disease. Regular physical activity has independent protective effects on the cardiovascular system, but the mechanisms responsible remain elusive. Recent studies suggest that the protein peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) participates in the response to exercise training. We hypothesized that the arterioles of athletes maintain dilation to flow despite combined inhibition of multiple vasodilators, but loss of PGC-1α renders these vessels susceptible to inhibition of a single vasodilator pathway. In addition, arterioles from overweight and obese individuals will display an an exercise-like phenotype when PGC-1α is activated. METHODS Isolated arterioles from exercise-trained (ET) and from mildly overweight or obese subjects (body mass index >25) were cannulated, and changes in lumen diameter in response to graded increases in flow were recorded in the absence and presence of compounds that inhibit various endothelium-dependent vasodilators. RESULTS Microvessels of ET subjects displayed robust dilation that could not be inhibited through targeting the combination of nitric oxide, prostaglandins, and hydrogen peroxide, but were inhibited via interference with membrane hyperpolarization. Loss of PGC-1α (siRNA) in the microcirculation of ET subjects eliminates this vasodilatory robustness rendering vessels susceptible to blockade of H2O2 alone. Pharmacological activation of PGC-1α with alpha-lipoic acid in isolated microvessels from sedentary, overweight, and obese subjects increases arteriolar resistance to vasodilator blockade and protects against acute increases in intraluminal pressure. CONCLUSIONS These findings suggest that the microvascular adaptations to exercise training, and the exercise-induced protection against acute vascular stress in overweight/obese subjects, are mediated by PGC-1α.

Hypertension during Weight Lifting Reduces Flow-Mediated Dilation in Nonathletes

Purpose The purpose of this study was to determine whether increased intraluminal pressure is the damaging factor that reduces flow-mediated dilation (FMD) in young, healthy subjects after resistance exercise to maximal exertion. Hypothesis Attenuating the rise in brachial artery pressure during weight lifting by placing a blood pressure cuff on the upper arm prevents postexercise impairment of brachial artery FMD in sedentary individuals. Methods Nine sedentary individuals who exercise once a week or less and six exercise-trained individuals who exercise three times a week or more performed leg press exercise to maximal exertion on two separate occasions. During one visit, a blood pressure cuff, proximal to the site of brachial artery measurement, was inflated to 100 mm Hg to protect the distal vasculature from the rise in intraluminal pressure, which occurs during resistance exercise. Brachial artery FMD was determined using ultrasonography before and 30 min after weight lifting. Results Without the protective cuff, brachial artery FMD in sedentary individuals was reduced after weight lifting (9.0% ± 1.2% prelift vs 6.6% ± 0.8% postlift; P = 0.005), whereas in exercise-trained individuals, FMD was unchanged (7.4% ± 0.7% prelift vs 8.0% ± 0.9% postlift; P = 0.543). With the protective cuff, FMD no longer decreased but rather increased in sedentary individuals (8.7% ± 1.2% prelift vs 10.5% ± 1.0% postlift, P = 0.025). An increase in FMD was also seen in exercise-trained subjects when the cuff was present (6.6% ± 0.7% prelift vs 10.9% ± 1.5% postlift, P < 0.001). Conclusion Protecting the brachial artery from exercise-induced hypertension enhances FMD in sedentary and exercise-trained individuals. These results indicate that increased intraluminal pressure in the artery contributes to the reduced FMD after heavy resistance exercise in sedentary individuals.

Physiological Consequences of Coronary Arteriolar Dysfunction and Its Influence on Cardiovascular Disease

To date, the major focus of diagnostic modalities and interventions to treat coronary artery disease has been the large epicardial vessels. Despite substantial data showing that microcirculatory dysfunction is a strong predictor of future adverse cardiovascular events, very little research has gone into developing techniques for in vivo diagnosis and therapeutic interventions to improve microcirculatory function. In this review, we will discuss the pathophysiology of coronary arteriolar dysfunction, define its prognostic implications, evaluate the diagnostic modalities available, and provide speculation on current and potential therapeutic opportunities.

YAP1-TEAD1 signaling controls angiogenesis and mitochondrial biogenesis through PGC1α

Mitochondria contribute to key processes of cellular function, while mitochondrial dysfunction is implicated in metabolic disorders, neurodegenerative diseases, and cardiovascular diseases, in which angiogenesis - the formation of new blood capillaries - is dysregulated. The Hippo signaling transducer, Yes-associated protein (YAP1) binds to the TEA domain (TEAD1) transcription factor and controls angiogenesis. YAP1 also regulates glucose metabolism through peroxisome proliferator-activated receptor gamma co-activator 1-alpha (PGC1α), a major player controlling mitochondrial biogenesis. However, the role of YAP1-TEAD1-PGC1α signaling in mitochondrial structure, cellular metabolism, and angiogenesis in endothelial cells (ECs) remains unclear. We now find that knockdown of TEAD1 decreases the expression of PGC1α and suppresses mitochondrial biogenesis, glycolysis, and oxygen consumption in ECs. A YAP1 mutant construct, YAP1S127A, which stimulates binding of YAP1 to TEAD1, upregulates the expression of PGC1α, induces mitochondrial biogenesis, and increases oxygen consumption and glycolytic flux in ECs; in contrast, YAP1S94A, which fails to bind to TEAD1, attenuates these effects. PGC1α knockdown inhibits YAP1S127A-induced EC sprouting in vitro and vascular morphogenesis in the fibrin gel subcutaneously implanted on mice, while overexpression of PGC1α reverses vascular morphogenesis suppressed by YAP1S94A. These results suggest that YAP1-TEAD1 signaling induces mitochondrial biogenesis in ECs and stimulates angiogenesis through PGC1α. Modulation of YAP1-TEAD1-PGC1α signaling in ECs may provide a novel intervention for angiogenesis-related diseases.