The emerging importance of lymphatics in health and disease: an NIH workshop report

The lymphatic system (LS) is composed of lymphoid organs and a network of vessels that transport interstitial fluid, antigens, lipids, cholesterol, immune cells, and other materials in the body. Abnormal development or malfunction of the LS has been shown to play a key role in the pathophysiology of many disease states. Thus, improved understanding of the anatomical and molecular characteristics of the LS may provide approaches for disease prevention or treatment. Recent advances harnessing single-cell technologies, clinical imaging, discovery of biomarkers, and computational tools have led to the development of strategies to study the LS. This Review summarizes the outcomes of the NIH workshop entitled "Yet to be Charted: Lymphatic System in Health and Disease," held in September 2022, with emphasis on major areas for advancement. International experts showcased the current state of knowledge regarding the LS and highlighted remaining challenges and opportunities to advance the field.

Mapping the lymphatic system across body scales and expertise domains: A report from the 2021 National Heart, Lung, and Blood Institute workshop at the Boston Lymphatic Symposium

Enhancing our understanding of lymphatic anatomy from the microscopic to the anatomical scale is essential to discern how the structure and function of the lymphatic system interacts with different tissues and organs within the body and contributes to health and disease. The knowledge of molecular aspects of the lymphatic network is fundamental to understand the mechanisms of disease progression and prevention. Recent advances in mapping components of the lymphatic system using state of the art single cell technologies, the identification of novel biomarkers, new clinical imaging efforts, and computational tools which attempt to identify connections between these diverse technologies hold the potential to catalyze new strategies to address lymphatic diseases such as lymphedema and lipedema. This manuscript summarizes current knowledge of the lymphatic system and identifies prevailing challenges and opportunities to advance the field of lymphatic research as discussed by the experts in the workshop.

TFEB-driven lysosomal biogenesis is pivotal for PGC1α-dependent renal stress resistance

Because injured mitochondria can accelerate cell death through the elaboration of oxidative free radicals and other mediators, it is striking that proliferator γ coactivator 1-α (PGC1α), a stimulator of increased mitochondrial abundance, protects stressed renal cells instead of potentiating injury. Here, we report that PGC1α’s induction of lysosomes via transcription factor EB (TFEB) may be pivotal for kidney protection. CRISPR and stable gene transfer showed that PGC1α-KO tubular cells were sensitized to the genotoxic stressor cisplatin, whereas Tg cells were protected. The biosensor mitochondrial-targeted Keima (mtKeima) unexpectedly revealed that cisplatin blunts mitophagy both in cells and mice. PGC1α and its downstream mediator NAD⁺ counteracted this effect. PGC1α did not consistently affect known autophagy pathways modulated by cisplatin. Instead RNA sequencing identified coordinated regulation of lysosomal biogenesis via TFEB. This effector pathway was sufficiently important that inhibition of TFEB or lysosomes unveiled a striking harmful effect of excess PGC1α in cells and conditional mice. These results uncover an unexpected effect of cisplatin on mitophagy and PGC1α’s reliance on lysosomes for kidney protection. Finally, the data illuminate TFEB as a potentially novel target for renal tubular stress resistance.

PGC1α is renoprotective in the setting of platinum-based chemotherapy through the induction of mitophagy and lysosomal biogenesis via transcription factor EB.

Macrophage fatty acid oxidation inhibits atherosclerosis progression

Atherosclerosis is a chronic disorder of the vessel wall. One key regulator of disease progression is lipid handling in macrophages. However, the role of macrophage mitochondrial-dependent fatty acid β-oxidation (FAO) in atherosclerosis is not well defined. To address this, we focused on carnitine palmitoyltransferase (CPT) 1 and 2, which play an essential role in the transport of long chain fatty acids (FAs) into the mitochondria. Using conditional alleles of these mitochondrial enzymes, we have generated myeloid-specific Cpt1a and Cpt2 knockout mutants (CPT1a M-KO and CPT2 M-KO). In culture, macrophages derived from CPT1a and CPT2 M-KO mice have impaired FAO, enhanced expression of the CD36 scavenger receptor, increased uptake of oxidized low-density lipoprotein (oxLDL), and augmented transformation into cholesterol-rich foam cells. In line with these in vitro observations, in the atherosclerosis-susceptible apolipoprotein E (ApoE) KO background, CPT2 M-KO mice demonstrated augmented atherosclerosis, accompanied by increased accumulation of aortic macrophages with elevated CD36 expression. These data suggest that macrophage FAO is athero-protective and that augmenting FAO may potentially slow atherosclerotic progression.

Sonic hedgehog signaling regulates the mammalian cardiac regenerative response

Certain organisms, including zebrafish, are capable of complete cardiac regeneration in response to injury. This response has also been observed in newborn mice, although in this case, the regenerative capacity is lost at approximately one week of age. The mechanisms regulating this short temporal window of cardiac regeneration in mice are not well understood. Here, we show that sonic hedgehog (Shh) signaling modulates the neonatal mouse regenerative response. In particular, we demonstrate that following apical resection of the heart on postnatal day 1, mice activate Shh ligand expression and downstream signaling. This response is largely absent when surgery is performed on non-regenerative, postnatal day 7 pups. Furthermore, an enhanced cardiac regeneration response was detected in ptch heterozygous mice which have a genetically-based constitutive increase in Shh signaling. We further show that Shh ligand is produced in the myocardium by non-myocytes and appears to regulate cardiomyocyte proliferation, as well as the recruitment of monocytes/macrophages to the regenerating area. Finally, we demonstrate that a small molecule activator of Shh signaling promotes heart regeneration, whereas an inhibitor of Shh signaling impairs the regenerative response. Together, these results implicate Shh signaling as a regulator of mammalian heart regeneration and suggest that modulating this pathway may lead to new potential therapies for cardiovascular diseases.

A Metabolic Basis for Endothelial-to-Mesenchymal Transition

Endothelial-to-mesenchymal transition (EndoMT) is a cellular process often initiated by the transforming growth factor β (TGF-β) family of ligands. Although required for normal heart valve development, deregulated EndoMT is linked to a wide range of pathological conditions. Here, we demonstrate that endothelial fatty acid oxidation (FAO) is a critical in vitro and in vivo regulator of EndoMT. We further show that this FAO-dependent metabolic regulation of EndoMT occurs through alterations in intracellular acetyl-CoA levels. Disruption of FAO via conditional deletion of endothelial carnitine palmitoyltransferase II (Cpt2^E-KO) augments the magnitude of embryonic EndoMT, resulting in thickening of cardiac valves. Consistent with the known pathological effects of EndoMT, adult Cpt2^E-KO mice demonstrate increased permeability in multiple vascular beds. Taken together, these results demonstrate that endothelial FAO is required to maintain endothelial cell fate and that therapeutic manipulation of endothelial metabolism could provide the basis for treating a growing number of EndoMT-linked pathological conditions.

The In Vivo Biology of the Mitochondrial Calcium Uniporter

The identification of the molecular composition of the mitochondrial calcium uniporter has allowed for the genetic manipulation of its components and the creation of various in vivo genetic models. Here, we review the initial attempts to modulate the expression of components of the calcium uniporter in a range of organisms from plants to mammals. This analysis has confirmed the strict requirement for the uniporter for in vivo mitochondrial calcium uptake and for maintaining mitochondrial calcium homeostasis. We further discuss the physiological effects following genetic manipulation of the uniporter on tissue bioenergetics and the threshold for cell death. Finally, we analyze the limited information regarding the role of various uniporter components in human disease.

Reciprocal regulation of acetyl-CoA carboxylase 1 and senescence in human fibroblasts involves oxidant mediated p38 MAPK activation

We sought to explore the fate of the fatty acid synthesis pathway in human fibroblasts exposed to DNA damaging agents capable of inducing senescence, a state of irreversible growth arrest. Induction of premature senescence by doxorubicin or hydrogen peroxide led to a decrease in protein and mRNA levels of acetyl-CoA carboxylase 1 (ACC1), the enzyme that catalyzes the rate-limiting step in fatty-acid biosynthesis. ACC1 decay accompanied the activation of the DNA damage response (DDR), and resulted in decreased lipid synthesis. A reduction in protein and mRNA levels of ACC1 and in lipid synthesis was also observed in human primary fibroblasts that underwent replicative senescence. We also explored the consequences of inhibiting fatty acid synthesis in proliferating non-transformed cells. Using shRNA technology, we knocked down ACC1 in human fibroblasts. Interestingly, this metabolic perturbation was sufficient to arrest proliferation and trigger the appearance of several markers of the DDR and increase senescence associated β-galactosidase activity. Reactive oxygen species and p38 mitogen activated protein kinase phosphorylation participated in the induction of senescence. Similar results were obtained upon silencing of fatty acid synthase (FAS) expression. Together our results point towards a tight coordination of fatty acid synthesis and cell proliferation in human fibroblasts.

Measuring In Vivo Mitophagy

Alterations in mitophagy have been increasingly linked to aging and age-related diseases. There are, however, no convenient methods to analyze mitophagy in vivo. Here, we describe a transgenic mouse model in which we expressed a mitochondrial-targeted form of the fluorescent reporter Keima (mt-Keima). Keima is a coral-derived protein that exhibits both pH-dependent excitation and resistance to lysosomal proteases. Comparison of a wide range of primary cells and tissues generated from the mt-Keima mouse revealed significant variations in basal mitophagy. In addition, we have employed the mt-Keima mice to analyze how mitophagy is altered by conditions including diet, oxygen availability, Huntingtin transgene expression, the absence of macroautophagy (ATG5 or ATG7 expression), an increase in mitochondrial mutational load, the presence of metastatic tumors, and normal aging. The ability to assess mitophagy under a host of varying environmental and genetic perturbations suggests that the mt-Keima mouse should be a valuable resource.

Autophagy regulates endothelial cell processing, maturation and secretion of von Willebrand factor

Endothelial secretion of von Willebrand factor (VWF) from intracellular organelles known as Weibel-Palade bodies (WPBs) is required for platelet adhesion to the injured vessel wall. Here we demonstrate that WPBs are often found near or within autophagosomes and that endothelial autophagosomes contain abundant VWF protein. Pharmacological inhibitors of autophagy or knockdown of the essential autophagy genes Atg5 or Atg7 inhibits the in vitro secretion of VWF. Furthermore, although mice with endothelial-specific deletion of Atg7 have normal vessel architecture and capillary density, they exhibit impaired epinephrine-stimulated VWF release, reduced levels of high-molecular weight VWF multimers and a corresponding prolongation of bleeding times. Endothelial-specific deletion of Atg5 or pharmacological inhibition of autophagic flux results in a similar in vivo alteration of hemostasis. Thus, autophagy regulates endothelial VWF secretion, and transient pharmacological inhibition of autophagic flux may be a useful strategy to prevent thrombotic events.

Increased mammalian lifespan and a segmental and tissue-specific slowing of aging after genetic reduction of mTOR expression

We analyzed aging parameters using a mechanistic target of rapamycin (mTOR) hypomorphic mouse model. Mice with two hypomorphic (mTOR(Δ/Δ)) alleles are viable but express mTOR at approximately 25% of wild-type levels. These animals demonstrate reduced mTORC1 and mTORC2 activity and exhibit an approximately 20% increase in median survival. While mTOR(Δ/Δ) mice are smaller than wild-type mice, these animals do not demonstrate any alterations in normalized food intake, glucose homeostasis, or metabolic rate. Consistent with their increased lifespan, mTOR(Δ/Δ) mice exhibited a reduction in a number of aging tissue biomarkers. Functional assessment suggested that, as mTOR(Δ/Δ) mice age, they exhibit a marked functional preservation in many, but not all, organ systems. Thus, in a mammalian model, while reducing mTOR expression markedly increases overall lifespan, it affects the age-dependent decline in tissue and organ function in a segmental fashion.

Atg7 modulates p53 activity to regulate cell cycle and survival during metabolic stress

Withdrawal of nutrients triggers an exit from the cell division cycle, the induction of autophagy, and eventually the activation of cell death pathways. The relation, if any, among these events is not well characterized. We found that starved mouse embryonic fibroblasts lacking the essential autophagy gene product Atg7 failed to undergo cell cycle arrest. Independent of its E1-like enzymatic activity, Atg7 could bind to the tumor suppressor p53 to regulate the transcription of the gene encoding the cell cycle inhibitor p21(CDKN1A). With prolonged metabolic stress, the absence of Atg7 resulted in augmented DNA damage with increased p53-dependent apoptosis. Inhibition of the DNA damage response by deletion of the protein kinase Chk2 partially rescued postnatal lethality in Atg7(-/-) mice. Thus, when nutrients are limited, Atg7 regulates p53-dependent cell cycle and cell death pathways.

Oncogene-induced senescence results in marked metabolic and bioenergetic alterations

Oncogene-induced senescence (OIS) is characterized by permanent growth arrest and the acquisition of a secretory, pro-inflammatory state. Increasingly, OIS is viewed as an important barrier to tumorgenesis. Surprisingly, relatively little is known about the metabolic changes that accompany and therefore may contribute to OIS. Here, we have performed a metabolomic and bioenergetic analysis of Ras-induced senescence. Profiling approximately 300 different intracellular metabolites reveals that cells that have undergone OIS develop a unique metabolic signature that differs markedly from cells undergoing replicative senescence. A number of lipid metabolites appear uniquely increased in OIS cells, including a marked increase in the level of certain intracellular long chain fatty acids. Functional studies reveal that this alteration in the metabolome reflects substantial changes in overall lipid metabolism. In particular, Ras-induced senescent cells manifest a decline in lipid synthesis and a significant increase in fatty acid oxidation. Increased fatty acid oxidation results in an unexpectedly high rate of basal oxygen consumption in cells that have undergone OIS. Pharmacological or genetic inhibition of carnitine palmitoyltransferase 1, the rate-limiting step in mitochondrial fatty acid oxidation, restores a pre-senescent metabolic rate and, surprisingly, selectively inhibits the secretory, pro-inflammatory state that accompanies OIS. Thus, Ras-induced senescent cells demonstrate profound alterations in their metabolic and bioenergetic profiles, particularly with regards to the levels, synthesis and oxidation of free fatty acids. Furthermore, the inflammatory phenotype that accompanies OIS appears to be related to these underlying changes in cellular metabolism.

Wnt signaling regulates hepatic metabolism

The contribution of the Wnt pathway has been extensively characterized in embryogenesis, differentiation, and stem cell biology but not in mammalian metabolism. Here, using in vivo gain- and loss-of-function models, we demonstrate an important role for Wnt signaling in hepatic metabolism. In particular, β-catenin, the downstream mediator of canonical Wnt signaling, altered serum glucose concentrations and regulated hepatic glucose production. β-Catenin also modulated hepatic insulin signaling. Furthermore, β-catenin interacted with the transcription factor FoxO1 in livers from mice under starved conditions. The interaction of FoxO1 with β-catenin regulated the transcriptional activation of the genes encoding glucose-6-phosphatase (G6Pase) and phosphoenolpyruvate carboxykinase (PEPCK), the two rate-limiting enzymes in hepatic gluconeogenesis. Moreover, starvation induced the hepatic expression of mRNAs encoding different Wnt isoforms. In addition, nutrient deprivation appeared to favor the association of β-catenin with FoxO family members, rather than with members of the T cell factor of transcriptional activators. Notably, in a model of diet-induced obesity, hepatic deletion of β-catenin improved overall metabolic homeostasis. These observations implicate Wnt signaling in the modulation of hepatic metabolism and raise the possibility that Wnt signaling may play a similar role in the metabolic regulation of other tissues.

Xanthine oxidoreductase depletion induces renal interstitial fibrosis through aberrant lipid and purine accumulation in renal tubules

Xanthine oxidoreductase (XOR) is an enzyme responsible for purine degradation, reactive oxygen species production, and adipogenesis. XOR gene-disrupted (XOR(-/-)) mice demonstrate renal failure and early death within several months. The aim of this study was to elucidate the mechanism of renal damage in XOR(-/-) mice and to determine the physiological role of XOR in the kidney. Histological analysis revealed that renal tubular damage in XOR(-/-) mice was accompanied by deposition of crystals and lipid-rich substances. Triglyceride content in renal homogenates was significantly increased in XOR(-/-) mice. The level of lipogenesis-related gene expression was comparable in XOR(+/+) and XOR(-/-) mice, whereas the expression of adipogenesis-related gene expression was significantly elevated in XOR(-/-) mice. Urinary excretions of xanthine and hypoxanthine were markedly elevated in XOR(-/-) mice. Immunohistochemical analysis, Western blotting, and real time RT-PCR revealed that various markers of fibrosis, inflammation, ischemia, and oxidative stress were increased in XOR(-/-) mice. Finally, we demonstrate that primary renal epithelial cells from XOR(-/-) mice are more readily transformed to myofibroblasts, which is a marker of increased epithelial mesenchymal transition. These results suggest that XOR gene disruption induced the depletion of uric acid and the accumulation of triglyceride-rich substances, xanthine, and hypoxanthine in the renal tubules. We believe that these changes contribute to a complex cellular milieu characterized by inflammation, tissue hypoxia, and reactive oxygen species production, ultimately resulting in renal failure through increased renal interstitial fibrosis.

Mitochondrial dysfunction and oxidative stress mediate the physiological impairment induced by the disruption of autophagy

Impaired or deficient autophagy is believed to cause or contribute to aging, as well as a number of age-related pathologies. The exact mechanism through which alterations in autophagy induce these various pathologies is not well understood. Here we describe the creation of two in vivo mouse models that allow for the characterization of the alteration in mitochondrial function and the contribution of the corresponding oxidative stress following deletion of Atg7. Using these models we demonstrate that isolated mitochondria obtained from Atg7^-/- skeletal muscle exhibit a significant defect in mitochondrial respiration. We further show that cells derived from Atg7^-/- mice have an altered metabolic profile characterized by decreased resting mitochondrial oxygen consumption and a compensatory increase in basal glycolytic rates. Atg7^-/-cells also exhibit evidence for increased steady state levels of reactive oxygen species. The observed mitochondrial dysfunction and oxidative stress is also evident in a mouse model where Atg7 is deleted within the pancreatic β cell. In this model, the simple administration of an antioxidant can significantly ameliorate the physiological impairment in glucose-stimulated insulin secretion. Taken together, these results demonstrate the potential role of mitochondrial dysfunction and oxidative stress in autophagy related pathology.

Augmented Wnt signaling in a mammalian model of accelerated aging

The contribution of stem and progenitor cell dysfunction and depletion in normal aging remains incompletely understood. We explored this concept in the Klotho mouse model of accelerated aging. Analysis of various tissues and organs from young Klotho mice revealed a decrease in stem cell number and an increase in progenitor cell senescence. Because klotho is a secreted protein, we postulated that klotho might interact with other soluble mediators of stem cells. We found that klotho bound to various Wnt family members. In a cell culture model, the Wnt-klotho interaction resulted in the suppression of Wnt biological activity. Tissues and organs from klotho-deficient animals showed evidence of increased Wnt signaling, and ectopic expression of klotho antagonized the activity of endogenous and exogenous Wnt. Both in vitro and in vivo, continuous Wnt exposure triggered accelerated cellular senescence. Thus, klotho appears to be a secreted Wnt antagonist and Wnt proteins have an unexpected role in mammalian aging.

Human mesenchymal stem cells exert potent antitumorigenic effects in a model of Kaposi's sarcoma

Emerging evidence suggests that both human stem cells and mature stromal cells can play an important role in the development and growth of human malignancies. In contrast to these tumor-promoting properties, we observed that in an in vivo model of Kaposi's sarcoma (KS), intravenously (i.v.) injected human mesenchymal stem cells (MSCs) home to sites of tumorigenesis and potently inhibit tumor growth. We further show that human MSCs can inhibit the in vitro activation of the Akt protein kinase within some but not all tumor and primary cell lines. The inhibition of Akt activity requires the MSCs to make direct cell-cell contact and can be inhibited by a neutralizing antibody against E-cadherin. We further demonstrate that in vivo, Akt activation within KS cells is potently down-regulated in areas adjacent to MSC infiltration. Finally, the in vivo tumor-suppressive effects of MSCs correlates with their ability to inhibit target cell Akt activity, and KS tumors engineered to express a constitutively activated Akt construct are no longer sensitive to i.v. MSC administration. These results suggest that in contrast to other stem cells or normal stromal cells, MSCs possess intrinsic antineoplastic properties and that this stem cell population might be of particular utility for treating those human malignancies characterized by dysregulated Akt.

Redox-dependent transcriptional regulation

Reactive oxygen species contribute to the pathogenesis of a number of disparate disorders including tissue inflammation, heart failure, hypertension, and atherosclerosis. In response to oxidative stress, cells activate expression of a number of genes, including those required for the detoxification of reactive molecules as well as for the repair and maintenance of cellular homeostasis. In many cases, these induced genes are regulated by transcription factors whose structure, subcellular localization, or affinity for DNA is directly or indirectly regulated by the level of oxidative stress. This review summarizes the recent progress on how cellular redox status can regulate transcription-factor activity and the implications of this regulation for cardiovascular disease.

Xanthine oxidoreductase is an endogenous regulator of cyclooxygenase-2

Xanthine oxidoreductase (XOR) is the enzyme responsible for the final step in purine degradation resulting in the generation of uric acid. Here we have generated mice deficient in XOR. As expected, these animals lack tissue XOR activity and have low to undetectable serum levels of uric acid. Although normal at birth, XOR-/- mice fail to thrive after 10 to 14 days, and most die within the first month. The cause of death appears to be a form of severe renal dysplasia, a phenotype that closely resembles what has been observed previously in cyclooxygenase-2 (COX-2)-deficient mice. We further demonstrate that in the first month of life, a period in which the mouse kidney is undergoing rapid maturation and remodeling, wild-type mice exhibit an approximately 30-fold increase in renal XOR activity, with a corresponding induction of COX-2 expression. In contrast, during this same period, XOR-/- animals fail to augment renal COX-2 expression. Finally, we show that in vitro and in vivo, uric acid can stimulate basal COX-2 expression. These results demonstrate that XOR activity is an endogenous physiological regulator of COX-2 expression and thereby provide insight into previous epidemiological evidence linking elevated serum uric levels with systemic hypertension and increased mortality from cardiovascular diseases. In addition, these results suggest a novel molecular link between cellular injury and the inflammatory response.

Regulation of the Werner helicase through a direct interaction with a subunit of protein kinase A

Werner syndrome is a hereditary disease characterized by cancer predisposition, genetic instability, and the premature appearance of features associated with normal aging. At the molecular level this syndrome has been related to mutations in the Werner helicase, a member of the RecQ family of DNA helicases which are required to maintain genomic stability in cells. Here we show by a yeast two-hybrid screen that the Werner helicase can directly interact with the regulatory subunit (RIbeta) of cAMP protein kinase A (PKA). We confirm that this interaction occurs in vivo. Interestingly, serum withdrawal causes a redistribution of the Werner helicase within the nucleus of mammalian cells. Raising intracellular cAMP levels or increased expression of the regulatory but not the catalytic subunit of PKA inhibits this nuclear redistribution stimulated by serum deprivation. These results suggest that similar to lower organisms, gene products linked to genomic instability and aging may be directly regulated by growth factor-sensitive, PKA-dependent pathways.

Oxidants painting the cysteine chapel: redox regulation of PTPs

Growth factors and cytokines appear to stimulate the intracellular production of reactive oxygen species (ROS). Evidence suggests that this alteration in the cellular redox state is essential for downstream signaling, but the precise mechanism has remained elusive. A new study now demonstrates that ligand-stimulated intracellular hydrogen peroxide can specifically and reversibly regulate the activity of protein tyrosine phosphatases.

Endothelial progenitor cells as putative targets for angiostatin

Angiostatin, a product of the proteolytic cleavage of plasminogen, possesses potent antitumor and antiangiogenic properties in vivo. Studies with cultured endothelial cells suggest that under certain conditions, angiostatin inhibits the migration and proliferation of these cells or, alternatively, increases their rate of apoptosis. In general, the effects of angiostatin have been considerably less potent in vitro than in vivo. One potential explanation for this disparity is that the in vivo target of angiostatin is not the mature endothelial cell. Recently, evidence has accumulated to show that circulating endothelial progenitor cells (EPCs) contribute to neovascularization. In this study, we have isolated EPCs from human subjects and demonstrated that, in contrast to that of mature endothelial cells, the growth of EPCs is exquisitely sensitive to angiostatin. These results suggest that angiostatin and related compounds may exert their biological effects by inhibiting the contribution of EPCs to angiogenesis and not by altering the growth of mature endothelial cells.

A role for reactive oxygen species in endothelial cell anoikis

When adherent cells, such as epithelial or endothelial cells, are detached and continuously maintained in suspension, they undergo a form of programmed cell death termed anoikis. We demonstrate that coincident with endothelial cell detachment, there is a dramatic rise in the intracellular level of reactive oxygen species (ROS). Reattachment to a solid surface rapidly attenuates the level of ROS. The mitochondria appear to be the major source of the detachment-induced rise in ROS. The change in the intracellular redox state appears to contribute to endothelial anoikis, because treatment with either the cell-permeant antioxidant N-acetylcysteine or the flavin protein inhibitor diphenylene iodonium is demonstrated to reduce oxidant levels and protect against subsequent cell death. Similarly, the endogenous intracellular level of ROS is shown to correlate with the extent of cell death. Finally, we demonstrate that the activities of both caspases and of the c-Jun N-terminal kinases are modulated by the rise in intracellular ROS levels. These results suggest that oxidants serve as signaling molecules and regulators of anoikis.

Regulation of endothelial cell adherens junctions by a Ras-dependent signal transduction pathway

Adherens junctions, consisting of transmembrane cadherin molecules and their associated cytoplasmic alpha-, beta-, and gamma-catenin proteins, are thought to be critical for the development of stable cell adhesion and subsequent 3-dimensional tissue organization. In human endothelial cells there is a marked induction of gamma-catenin levels when cells reach confluence. We demonstrate that expression of a dominant negative ras gene product (N17ras) via adenoviral mediated gene transfer inhibits the confluent-dependent rise in gamma-catenin mRNA and protein levels. Consistent with its effects on overall gamma-catenin levels, expression of N17ras also reduces the amount of gamma-catenin associated with the adherens junction. Finally, although expression of N17ras under normal culture conditions produces no clear morphological phenotype, endothelial cells expressing a dominant negative ras gene product fail to form 3-dimensional, vascular-like structures when plated on reconstituted extracellular matrix.

A requirement for the rac1 GTPase in the signal transduction pathway leading to cardiac myocyte hypertrophy

We have used adenoviral-mediated gene transfer of a constitutively active (V12rac1) and dominant negative (N17rac1) isoform of rac1 to assess the role of this small GTPase in cardiac myocyte hypertrophy. Expression of V12rac1 in neonatal cardiac myocytes results in sarcomeric reorganization and an increase in cell size that is indistinguishable from ligand-stimulated hypertrophy. In addition, V12rac1 expression leads to an increase in atrial natriuretic peptide secretion. In contrast, expression of N17rac1, but not a truncated form of Raf-1, attenuated the morphological hypertrophy associated with phenylephrine stimulation. Consistent with the observed effects on morphology, expression of V12rac1 resulted in an increase in new protein synthesis, while N17rac1 expression inhibited phenylephrine-induced leucine incorporation. These results suggest rac1 is an essential element of the signaling pathway leading to cardiac myocyte hypertrophy.