Discovering and Developing Successful Cardiovascular Therapeutics: A Conversation With James N. Topper, MD, PhD

Dr James (also known as Jamie) N. Topper, MD, PhD, serves as Managing General Partner at Frazier Healthcare Partners, where he leads the Life Science Venture practice. In 2011, and 2016, he was named to the Midas List of leading venture capitalists, and, in 2013, he was recognized by Forbes as one of the top 10 healthcare investors. He has >25 years of experience working with entrepreneurs to found and build successful therapeutics-focused companies. Dr Topper holds a BS from the University of Michigan. He received an MD and PhD (in biophysics) from Stanford University School of Medicine. He completed postgraduate training in internal medicine and cardiovascular disease at the Brigham and Women's Hospital in Boston and is board certified in both disciplines.

Conditional expression of Smad7 in pancreatic beta cells disrupts TGF-beta signaling and induces reversible diabetes mellitus

Identification of signaling pathways that maintain and promote adult pancreatic islet functions will accelerate our understanding of organogenesis and improve strategies for treating diseases like diabetes mellitus. Previous work has implicated transforming growth factor-β (TGF-β) signaling as an important regulator of pancreatic islet development, but has not established whether this signaling pathway is required for essential islet functions in the adult pancreas. Here we describe a conditional system for expressing Smad7, a potent inhibitor of TGF-β signaling, to identify distinct roles for this pathway in adult and embryonic β cells. Smad7 expression in Pdx1⁺ embryonic pancreas cells resulted in striking embryonic β cell hypoplasia and neonatal lethality. Conditional expression of Smad7 in adult Pdx1⁺ cells reduced detectable β cell expression of MafA, menin, and other factors that regulate β cell function. Reduced pancreatic insulin content and hypoinsulinemia produced overt diabetes that was fully reversed upon resumption of islet TGF-β signaling. Thus, our studies reveal that TGF-β signaling is crucial for establishing and maintaining defining features of mature pancreatic β cells.

TGF-β signaling is known to regulate the development of pancreatic β cells; here the authors show that TGF-β is also required for the maintenance of β cell identity in the adult.

Adaptation of the endothelium to fluid flow: in vitro analyses of gene expression and in vivo implications

Biomechanical forces generated by blood flow play an important role in the pathogenesis of vascular disease. For example, regions exposed to non-uniform shear stresses develop early atherosclerotic lesions while areas exposed to uniform shear stresses are protected. A variety of in vitro flow apparatuses have been created to apply well-characterized flow patterns to endothelial cells in an effort to dissect the cellular and molecular pathways involved in these distinct processes. Recent advances in biotechnology have permitted large-scale transcriptional profiling techniques to replace candidate gene screens and have allowed the genome-wide examination of biomechanical force-induced endothelial gene expression profiles. This review provides an overview of biomechanical force-induced modulation of endothelial phenotype. It examines the effect of sustained laminar shear stress (LSS), a type of uniform shear stress, on in vitro endothelial gene expression by synthesizing data from the early candidate gene and differential display polymerase chain reaction (PCR) approaches to the numerous, recent, high throughput functional genomic analyses. These studies demonstrate that prolonged LSS regulates the expression of only a small percentage (approximately 1-5%) of endothelial genes, and this transcriptional profile produces an endothelial phenotype that is quiescent, being protected from apoptosis, inflammation and oxidative stress. These observations provide a possible molecular mechanism for the strong correlation between patterns of blood flow and the occurrence of vascular pathologies, such as atherosclerosis, in vivo.

A novel secreted, cell-surface glycoprotein containing multiple epidermal growth factor-like repeats and one CUB domain is highly expressed in primary osteoblasts and bones

We have previously identified a novel family of secreted, cell-surface proteins expressed in human vascular endothelium. To date, two family members have been described, sharing a characteristic domain structure including an amino-terminal signal peptide followed by multiple copies of epidermal growth factor (EGF)-like repeats, a spacer region, and one CUB domain at the carboxyl terminus. Thus, this family was termed SCUBE for signal peptide-CUB-EGF-like domain containing proteins. Here we described the identification and characterization of one additional member of the SCUBE family named SCUBE3 in humans, sharing an overall 60% protein identity and a similar domain structure with other family members. Real-time quantitative reverse transcriptase-PCR and Northern blot analyses revealed that this gene is highly expressed in primary osteoblasts and the long bones (humerus and femur), followed by a low level of expression in human umbilical vein endothelial cells and in heart. When overexpressed in human embryonic kidney 293T cells, the recombinant hSCUBE3 protein is a secreted glycoprotein that can form oligomers tethered to the cell surface. Interestingly, the secreted hSCUBE3 protein can be further proteolytically processed by a serum-associated protease to release the EGF-like repeats from the CUB domain. The SCUBE3 gene is mapped to human chromosome 6p21.3, a region that has been linked with the locus for a rare form of metabolic bone disease, Paget's disease of bone 1. Together, this novel secreted, cell-surface osteoblast protein may act locally and/or distantly through a proteolytic mechanism, and may play an important role in bone cell biology.

Common genomic response in different mouse models of beta-adrenergic-induced cardiomyopathy

BACKGROUND

Although beta-adrenergic receptor (AR) blockade therapy is beneficial in the treatment of heart failure, little is known regarding the transcriptional mechanisms underlying this salutary action.

METHODS AND RESULTS

In the present study, we screened mice overexpressing Gsalpha, beta1AR, beta2AR, or protein kinase A to test if a common genomic pathway exists in different models with enhanced beta-adrenergic signaling. In mice overexpressing Gsalpha, differentially expressed genes were identified by mRNA profiling. In addition to well-known markers of cardiac hypertrophy (atrial natriuretic factor, CARP, and beta-myosin heavy chain), uncoupling protein 2 (UCP2), a protein involved in the control of mitochondrial membrane potential, and four-and-a-half LIM domain protein-1 (FHL1), a member of the LIM protein family, were predicted to be upregulated. Upregulation of these genes was confirmed by quantitative reverse transcriptase-polymerase chain reaction at all time points tested during the development of cardiomyopathy in mice overexpressing Gsalpha. In mice overexpressing beta1AR, beta2AR, or protein kinase A, increased UCP2 and FHL1 expression was also observed at the onset of cardiomyopathy. BetaAR blockade treatment reversed the cardiomyopathy and suppressed the increased expression of UCP2 and FHL1 in mice overexpressing Gsalpha.

CONCLUSIONS

UCP2 and FHL1 are important candidate genes that correlate with the development of betaAR-induced cardiomyopathy in different mouse models with enhanced betaAR signaling. In addition to preserving cardiac function, betaAR blockade treatment also prevents the genomic regulation that correlates with the onset of heart failure.

A novel interleukin-17 receptor-like protein identified in human umbilical vein endothelial cells antagonizes basic fibroblast growth factor-induced signaling

We have previously utilized a combination of high throughput sequencing and genome-wide microarray profiling analyses to identify novel cell-surface proteins expressed in human umbilical vein endothelial cells. One gene identified by this approach encodes a type I transmembrane receptor that shares sequence homology with the intracellular domain of members of the interleukin-17 (IL-17) receptor family. Real-time quantitative PCR and Northern analyses revealed that this gene is highly expressed in human umbilical vein endothelial cells and in several highly vascularized tissues such as kidney, colon, skeletal muscle, heart, and small intestine. In addition, we also found that it is also highly expressed in the ductal epithelial cells of human salivary glands, seminal vesicles, and the collecting tubules of the kidney by in situ hybridization. This putative receptor, which we have termed human SEF (hSEF), is also expressed in a variety of breast cancer tissues. In co-immunoprecipitation assays, this receptor is capable of forming homomeric complexes and can interact with fibroblast growth factor (FGF) receptor 1. Overexpression of this receptor inhibits FGF induction of an FGF-responsive reporter gene in human 293T cells. This appears to occur as a result of specific inhibition of p42/p44 ERK in the absence of upstream MEK inhibition. This inhibitory effect is dependent upon a functional intracellular domain since deletion mutants missing the IL-17 receptor-like domain lack this inhibitory effect. These findings are consistent with the recent discovery of the zebrafish homologue, Sef (similar expression to fgf genes), which specifically antagonizes FGF signaling when ectopically expressed in zebrafish or Xenopus laevis embryos. Based on sequence and functional similarities, this novel IL-17 receptor homologue represents a potential human SEF and is likely to play critical roles in endothelial or epithelial functions such as proliferation, migration, and angiogenesis.

New insights into endothelial diversity

It has long been appreciated that endothelium from distinct vascular beds manifest diverse structural and functional properties. With current technologies, we have begun to systematically define the unique molecular profiles that orchestrate and sustain this diversity, and to assess the influences of various environmental and developmental stimuli on these processes. This review focuses on recent advances and findings that provide insights into both the extent and causes of endothelial cell diversity.

Characterization of pDJA1, a cardiac-specific chaperone found by genomic profiling of the post-ischemic swine heart

BACKGROUND

Previously, we showed by subtractive hybridization in a swine model of ischemia/reperfusion that an upregulation of genes participating in mechanisms of cell survival is a potential genomic mechanism to tilt the balance from necrosis to functional reversibility.

METHODS AND RESULTS

We present here the full-length sequencing and characterization of a novel gene that was found in this subtraction, encoding a cardiac-specific DnaJ-like co-chaperone that we call Pig DnaJ-like protein A1 (pDJA1). The expression of pDJA1 was found to be restricted to the heart, as opposed to skeletal muscle, liver, lung, kidney, aorta, stomach and spleen. Expression of pDJA1 is restricted to cardiac myocytes, as determined by in situ hybridization. The transcript is expressed more in the left ventricle than in the other cardiac chambers. Remarkably, expression of pDJA1 follows a transmural gradient in the left ventricle, with the highest level of expression in the subendocardium. Expression of pDJA1 slightly increased during an episode of ischemia, but increased by 4-fold during the following period of reperfusion. Adenovirus-mediated transduction of pDJA1 in isolated rat neonatal cardiac myocytes decreased by 65% the rate of apoptosis induced by staurosporine.

CONCLUSION

Therefore, pDJA1 is a novel heart-specific, ventricle-enriched cardioprotective co-chaperone, which participates in the program of cell survival that limits irreversible damage in post-ischemic myocardium.

Gene expression profile of human endothelial cells exposed to sustained fluid shear stress

Biomechanical forces can modulate endothelial phenotype through changes in gene expression. We hypothesized that physiological laminar shear stresses (LSS) act as differentiative stimuli on endothelial cells (EC) to alter gene expression, creating an antioxidant, anti-apoptotic and anti-proliferative environment. The transcriptional profile of cultured human umbilical vein endothelial cells (HUVEC) exposed to LSS was evaluated by GeneCalling; 107 genes demonstrated at least a twofold change in expression at 24 h (LSS vs. static). These flow-responsive genes represent a limited number of functional clusters that include transcription factors, antioxidants, signaling molecules, cell cycle regulators, and genes involved in cellular differentiation. Immunohistochemistry and in situ hybridization confirmed that many of these flow-responsive genes, including the novel basic helix-loop-helix transcription factor Hath6, are expressed in EC in vivo. Thus these data identify a limited set of flow-responsive genes expressed in the endothelium that may be responsible for the establishment and maintenance of the flow-adapted endothelial phenotype in vivo.

Flow loading induces macrophage antioxidative gene expression in experimental aneurysms

OBJECTIVE

Reactive oxygen species may act as proinflammatory mediators in abdominal aortic aneurysm (AAA) disease. Flow loading increases antioxidative enzyme expression and limits reactive oxygen species production in vascular smooth muscle cells in vitro, limits experimental AAA enlargement in rodent models, and is indirectly associated with reduced clinical AAA risk. We attempted to determine the mechanism or mechanisms by which flow loading limits AAA enlargement.

METHODS AND RESULTS

Rodent AAAs were flow loaded via femoral arteriovenous fistula creation. Aortic wall shear stress and relative wall strain were significantly higher in flow-loaded rodents. Flow loading reduced AAA diameter by 26% despite evidence of flow-mediated aortic enlargement proximal to the aneurysmal segment. Messenger RNA from AAA tissue was harvested for cDNA labeling and hybridization to a 384-clone DNA microarray. Twenty-nine genes were differentially expressed (relative intensity/relative intensity of control ratio >1.5 and <0.67) in flow-loaded compared with normal flow AAA tissue, including heme oxygenase 1 (HO-1). Increased HO-1 expression was confirmed via reverse transcriptase-polymerase chain reaction. Immunohistochemistry localized HO-1 expression to infiltrative macrophages. alpha-Tocopherol was found to be as effective as flow loading in limiting AAA enlargement. Flow loading and alpha-tocopherol therapy reduced AAA reactive oxygen species production.

CONCLUSIONS

Flow loading may attenuate AAA enlargement via wall shear or strain-related reductions in oxidative stress.

Inducible and selective transgene expression in murine vascular endothelium

We have developed a system utilizing the murine Tie2 promoter/enhancer coupled with the "tetracycline-on" regulatory elements to create a model that allows regulated and selective expression of a beta-galactosidase (betaGal) reporter transgene in the adult murine vascular endothelium. Two independent lines of viable and fertile mice were characterized, and they exhibit minimal betaGal expression under basal conditions. In response to exogenous doxycycline (Dox), selective expression of betaGal was demonstrated in the vascular endothelium of all tissues examined. En face analyses of the aorta and its principle branches indicate that the vast majority of lumenal endothelial cells express the transgene. Inducible betaGal expression also extends to the endocardium and the microvasculature of all organs. There is no evidence of specific transgene expression in nonendothelial cell types. Induction of the betaGal was effectively achieved after 3 days of oral Dox treatment and persisted for over 3 mo with continuous administration. This model can now be widely applied to study the role of specific genes in the phenotype of adult murine vasculature.

Identification of a novel family of cell-surface proteins expressed in human vascular endothelium

Vascular endothelial cells (EC) play a key role in a variety of pathophysiologic processes, such as angiogenesis, inflammation, cancer metastasis, and vascular diseases. As part of a strategy to identify all genes expressed in human EC, a full-length cDNA encoding a potential secreted protein harboring 10 epidermal growth factor (EGF)-like domains and one CUB domain at the carboxyl terminus (termed, SCUBE1 for Signal peptide-CUB-EGF-like domain containing protein 1) was identified. SCUBE1 shares homology with several protein families, including members of the fibrillin and Notch families, and the anticoagulant proteins, thrombomodulin and protein C. SCUBE1 mRNA is found in several highly vascularized tissues such as liver, kidney, lung, spleen, and brain and is selectively expressed in EC by in situ hybridization. SCUBE1 is a secreted glycoprotein that can form oligomers and manifests a stable association with the cell surface. A second gene encoding a homologue (designated SCUBE2) was also identified and is expressed in EC as well as other cell types. SCUBE2 is also a cell-surface protein and can form a heteromeric complex with SCUBE1. Both SCUBE1 and SCUBE2 are rapidly down-regulated in EC after interleukin-1beta and tumor necrosis factor-alpha treatment in vitro and after lipopolysaccharide injection in vivo. Thus, SCUBE1 and SCUBE2 define an emerging family of human secreted proteins that are expressed in vascular endothelium and may play important roles in development, inflammation, and thrombosis.

Epidermal growth factor induces Egr-1 promoter activity in hepatocytes in vitro and in vivo

Early growth response-1 (Egr-1) is a transcription factor that couples short-term changes in the extracellular milieu to long-term changes in gene expression. Under in vitro conditions, the Egr-1 gene has been shown to respond to many extracellular signals. In most cases, these findings have not been extended to the in vivo setting. The goal of the present study was to explore the role of epidermal growth factor (EGF) in mediating Egr-1 expression in hepatocytes under both in vitro and in vivo conditions. In HepG2 cells, Egr-1 protein and mRNA were upregulated in the presence of EGF. In stable transfections of HepG2 cells, a 1,200-bp Egr-1 promoter contained information for EGF response via a protein kinase C-independent, mitogen-activated protein kinase-dependent signaling pathway. A promoter region containing the two most proximal serum response elements was sufficient to transduce the EGF signal. In transgenic mice that carry the Egr-1 promoter coupled to the LacZ reporter gene, systemic delivery of EGF by intraperitoneal injection resulted in an induction of the endogenous Egr-1 gene and the Egr-1-lacZ transgene in hepatocytes. Together, these results suggest that the 1,200-bp promoter contains information for EGF response in hepatocytes both in vitro and in intact animals.

Gene program for cardiac cell survival induced by transient ischemia in conscious pigs

Therapy for ischemic heart disease has been directed traditionally at limiting cell necrosis. We determined by genome profiling whether ischemic myocardium can trigger a genetic program promoting cardiac cell survival, which would be a novel and potentially equally important mechanism of salvage. Although cardiac genomics is usually performed in rodents, we used a swine model of ischemia/reperfusion followed by ventricular dysfunction (stunning), which more closely resembles clinical conditions. Gene expression profiles were compared by subtractive hybridization between ischemic and normal tissue of the same hearts. About one-third (23/74) of the nuclear-encoded genes that were up-regulated in ischemic myocardium participate in survival mechanisms (inhibition of apoptosis, cytoprotection, cell growth, and stimulation of translation). The specificity of this response was confirmed by Northern blot and quantitative PCR. Unexpectedly, this program also included genes not previously described in cardiomyocytes. Up-regulation of survival genes was more profound in subendocardium over subepicardium, reflecting that this response in stunned myocardium was proportional to the severity of the ischemic insult. Thus, in a swine model that recapitulates human heart disease, nonlethal ischemia activates a genomic program of cell survival that relates to the time course of myocardial stunning and differs transmurally in relation to ischemic stress, which induced the stunning. Understanding the genes up-regulated during myocardial stunning, including those not previously described in the heart, and developing strategies that activate this program may open new avenues for therapy in ischemic heart disease.

Transforming growth factor-beta 1 inhibition of macrophage activation is mediated via Smad3

Activated macrophages are critical cellular participants in inflammatory disease states. Transforming growth factor (TGF)-beta1 is a growth factor with pleiotropic effects including inhibition of immune cell activation. Although the pathway of gene activation by TGF-beta1 via Smad proteins has recently been elucidated, suppression of gene expression by TGF-beta1 remains poorly understood. We found that of Smad1-Smad7, Smad3 alone was able to inhibit expression of markers of macrophage activation (inducible nitric-oxide synthase and matrix metalloproteinase-12) following lipopolysaccharide treatment in gene reporter assays. Transient and constitutive overexpression of a dominant negative Smad3 opposed the inhibitory effect of TGF-beta1. Domain swapping experiments suggest that both the Smad MH-1 and MH-2 domains are required for inhibition. Mutation of a critical amino acid residue required for DNA binding in the MH-1 of Smad3 (R74A) resulted in the loss of inhibition. Transient overexpression of p300, an interactor of the Smad MH-2 domain, partially alleviated the inhibition by TGF-beta1/Smad3, suggesting that inhibition of gene expression may be due to increased competition for limiting amounts of this coactivator. Our results have implications for the understanding of gene suppression by TGF-beta1 and for the regulation of activated macrophages by TGF-beta1.

The Egr-1 promoter contains information for constitutive and inducible expression in transgenic mice

Egr-1 is an immediate early gene that couples short-term changes in the extracellular milieu to long-term changes in gene expression. Under in vitro conditions, the Egr-1 gene is expressed in many cell types and is induced by a wide variety of extracellular signals. The mechanisms by which the Egr-1 gene is regulated in vivo remain poorly understood. In this study, we have generated transgenic mice with a construct containing 1200 bp of the mouse Egr-1 promoter coupled to nuclear localized LacZ. In multiple independent lines of mice, reporter gene expression was detected in subsets of endothelial cells, vascular smooth-muscle cells, cardiomyocytes, neurons, and hepatocytes. This pattern closely resembled that of the endogenous gene. After partial hepatectomy, reporter gene activity was upregulated between two- and fivefold in regenerating livers. Taken together, these findings suggest that the Egr-1 promoter contains information for appropriate spatial and temporal expression in vivo.

Inhibition of E-selectin gene expression by transforming growth factor beta in endothelial cells involves coactivator integration of Smad and nuclear factor kappaB-mediated signals

Transforming growth factor (TGF)-beta(1) is a pleiotropic cytokine/growth factor that is thought to play a critical role in the modulation of inflammatory events. We demonstrate that exogenous TGF-beta(1) can inhibit the expression of the proinflammatory adhesion molecule, E-selectin, in vascular endothelium exposed to inflammatory stimuli both in vitro and in vivo. This inhibitory effect occurs at the level of transcription of the E-selectin gene and is dependent on the action of Smad proteins, a class of intracellular signaling proteins involved in mediating the cellular effects of TGF-beta(1). Furthermore, we demonstrate that these Smad-mediated effects in endothelial cells result from a novel competitive interaction between Smad proteins activated by TGF-beta(1) and nuclear factor kappaB (NFkappaB) proteins activated by inflammatory stimuli (such as cytokines or bacterial lipopolysaccharide) that is mediated by the transcriptional coactivator cyclic AMP response element-binding protein (CREB)-binding protein (CBP). Augmentation of the limited amount of CBP present in endothelial cells (via overexpression) or selective disruption of Smad-CBP interactions (via a dominant negative strategy) effectively antagonizes the ability of TGF-beta(1) to block proinflammatory E-selectin expression. These data thus demonstrate a novel mechanism of interaction between TGF-beta(1)-regulated Smad proteins and NFkappaB proteins regulated by inflammatory stimuli in vascular endothelial cells. This type of signaling mechanism may play an important role in the immunomodulatory actions of this cytokine/growth factor in the cardiovascular system.

Transforming growth factor-beta 1 inhibits cytokine-mediated induction of human metalloelastase in macrophages

Matrix metalloproteinases (MMP) have been identified in vulnerable areas of atherosclerotic plaques and may contribute to plaque instability through extracellular matrix degradation. Human metalloelastase (MMP-12) is a macrophage-specific MMP with broad substrate specificity and is capable of degrading proteins found in the extracellular matrix of atheromas. Despite its potential importance, little is known about the regulation of MMP-12 expression in the context of atherosclerosis. In this study, we report that in human peripheral blood-derived macrophages, MMP-12 mRNA was markedly up-regulated by several pro-atherosclerotic cytokines and growth factors including interleukin-1beta, tumor necrosis factor-alpha, macrophage colony-stimulating factor, vascular endothelial growth factor, and platelet-derived growth factor-BB. In contrast, the pleiotropic anti-inflammatory growth factor transforming growth factor-beta1 (TGF-beta1) inhibited cytokine-mediated induction of MMP-12 mRNA, protein, and enzymatic activity. Analyses of MMP-12 promoter through transient transfections and electrophoretic mobility shift assays indicated that both its induction by cytokines and its inhibition by TGF-beta1 depended on signaling through an AP-1 site at -81 base pairs. Moreover, the inhibitory effect of TGF-beta1 on MMP-12 was dependent on Smad3. Taken together, MMP-12 is induced by several factors implicated in atherosclerosis. The inhibition of MMP-12 expression by TGF-beta1 suggests that TGF-beta1, acting via Smad3, may promote plaque stability.

Endothelial dysfunction, hemodynamic forces, and atherogenesis

Phenotypic modulation of endothelium to a dysfunctional state contributes to the pathogenesis of cardiovascular diseases such as atherosclerosis. The localization of atherosclerotic lesions to arterial geometries associated with disturbed flow patterns suggests an important role for local hemodynamic forces in atherogenesis. There is increasing evidence that the vascular endothelium, which is directly exposed to various fluid mechanical forces generated by pulsatile blood flow, can discriminate among these stimuli and transduce them into genetic regulatory events. At the level of individual genes, this regulation is accomplished via the binding of certain transcription factors, such as NF kappa B and Egr-1, to shear-stress response elements (SSREs) that are present in the promoters of biomechanically inducible genes. At the level of multiple genes, distinct patterns of up- and downregulation appear to be elicited by exposure to steady laminar shear stresses versus comparable levels of non-laminar (e.g., turbulent) shear stresses or cytokine stimulation (e.g., IL-1 beta). Certain genes upregulated by steady laminar shear stress stimulation (such as eNOS, COX-2, and Mn-SOD) support vasoprotective (i.e., anti-inflammatory, anti-thrombotic, anti-oxidant) functions in the endothelium. We hypothesize that the selective and sustained expression of these and related "atheroprotective genes" in the endothelial lining of lesion-protected areas represents a mechanism whereby hemodynamic forces can influence lesion formation and progression.

TGF-beta in the cardiovascular system: molecular mechanisms of a context-specific growth factor

Transforming growth factor beta-1 is the prototypical member of a class of growth factors whose actions have been strongly implicated in a number of pathophysiologic processes including chronic vascular diseases such as atherosclerosis and hypertension. One of the hall-marks of this class of growth factors is the diverse nature of their actions; a characteristic that is thought to arise from the fact that the effects of these factors are very dependent upon the particular cellular context in which they operate. There has been substantial progress in understanding the molecular signaling mechanisms utilized by these factors. These findings are beginning to provide a mechanistic framework with which to understand the complex and pleiotropic actions of these factors on cells and tissues of the cardiovascular system.

MEKK-1, a component of the stress (stress-activated protein kinase/c-Jun N-terminal kinase) pathway, can selectively activate Smad2-mediated transcriptional activation in endothelial cells

Smad proteins are essential components of the intracellular signaling pathways utilized by members of the transforming growth factor-beta (TGF-beta) superfamily of growth factors. Certain Smad proteins (e.g. Smad1, -2, and -3) can act as regulated transcriptional activators, a process that involves phosphorylation of these proteins by activated TGF-beta superfamily receptors. We demonstrate that the intracellular kinase mitogen-activated protein kinase kinase kinase-1 (MEKK-1), an upstream activator of the stress-activated protein kinase/c-Jun N-terminal kinase pathway, can participate in Smad2-dependent transcriptional events in cultured endothelial cells. A constitutively active form of MEKK-1 but not mitogen-activated protein kinase kinase-1 (MEK-1) or TGF-beta-activated kinase-1, two distinct intracellular kinases, can specifically activate a Gal4-Smad2 fusion protein, and this effect correlates with an increase in the phosphorylation state of the Smad2 protein. These effects do not require the presence of the C-terminal SSXS motif of Smad2 that is the site of TGF-beta type 1 receptor-mediated phosphorylation. Activation of Smad2 by active MEKK-1 results in enhanced Smad2-Smad4 interactions, nuclear localization of Smad2 and Smad4, and the stimulation of Smad protein-transcriptional coactivator interactions in endothelial cells. Overexpression of Smad7 can inhibit the MEKK-1-mediated stimulation of Smad2 transcriptional activity. A physiological level of fluid shear stress, a known activator of endogenous MEKK-1 activity in endothelial cells, can stimulate Smad2-mediated transcriptional activity. These data demonstrate a novel mechanism for activation of Smad protein-mediated signaling in endothelial cells and suggest that Smad2 may act as an integrator of diverse stimuli in these cells.

Blood flow and vascular gene expression: fluid shear stress as a modulator of endothelial phenotype

Vascular endothelium, the cellular monolayer lining the entire cardiovascular system, is exposed to a variety of biochemical and biomechanical stimuli. Fluid shear stresses generated by blood flow in the vasculature can profoundly influence the phenotype of the endothelium by regulating the activity of certain flow-sensitive proteins (for example, enzymes), as well as by modulating gene expression. The finding that specific fluid mechanical forces can alter endothelial structure and function has provided a framework for a mechanistic understanding of flow-dependent processes, ranging from vascular remodeling in response to hemodynamic changes, to the initiation and localization of chronic vascular diseases such as atherosclerosis.

Human prostaglandin transporter gene (hPGT) is regulated by fluid mechanical stimuli in cultured endothelial cells and expressed in vascular endothelium in vivo

BACKGROUND

biomechanical forces generated by blood flow within the cardiovascular system have been proposed as important modulators of regional endothelial phenotype and function. This process is thought to involve the regulation of vascular gene expression by physiological fluid mechanical stimuli such as fluid shear stresses.

METHODS AND RESULTS

We demonstrate sustained upregulation of a recently identified gene encoding a human prostaglandin transporter (hPGT) in cultured human vascular endothelium exposed to a physiological fluid mechanical stimulus in vitro. This biomechanical induction is selective in that steady laminar shear stress is sufficient to upregulate the hPGT gene at the level of transcriptional activation, whereas a comparable level of turbulent shear stress (a nonphysiological stimulus) is not. Various biochemical stimuli, such as bacterial endotoxin and the inflammatory cytokines recombinant human interleukin 1beta cytokines (rhIL-1beta) and tumor necrosis factor-alpha (TNF-alpha), did not significantly induce hPGT. Using a specific antiserum to hPGT, we demonstrate endothelial expression within the arterial vasculature and the microcirculation of highly vascularized tissues such as the heart.

CONCLUSIONS

Our results identify hPGT as an inducible gene in vascular endothelium and suggest that biomechanical stimuli generated by blood flow in vivo may be important determinants of hPGT expression. Furthermore, this demonstration of regulated endothelial expression of hPGT implicates this molecule in the regional metabolism of prostanoids within the cardiovascular system.

CREB binding protein is a required coactivator for Smad-dependent, transforming growth factor beta transcriptional responses in endothelial cells

The transforming growth factor-beta (TGF-beta) superfamily of growth factors and cytokines has been implicated in a variety of physiological and developmental processes within the cardiovascular system. Smad proteins are a recently described family of intracellular signaling proteins that transduce signals in response to TGF-beta superfamily ligands. We demonstrate by both a mammalian two-hybrid and a biochemical approach that human Smad2 and Smad4, two essential Smad proteins involved in mediating TGF-beta transcriptional responses in endothelial and other cell types, can functionally interact with the transcriptional coactivator CREB binding protein (CBP). This interaction is specific in that it requires ligand (TGF-beta) activation and is mediated by the transcriptional activation domains of the Smad proteins. A closely related, but distinct endothelial-expressed Smad protein, Smad7, which does not activate transcription in endothelial cells, does not interact with CBP. Furthermore, Smad2,4-CBP interactions involve the COOH terminus of CBP, a region that interacts with other regulated transcription factors such as certain signal transduction and transcription proteins and nuclear receptors. Smad-CBP interactions are required for Smad-dependent TGF-beta-induced transcriptional responses in endothelial cells, as evidenced by inhibition with overexpressed 12S E1A protein and reversal of this inhibition with exogenous CBP. This report demonstrates a functional interaction between Smad proteins and an essential component of the mammalian transcriptional apparatus (CBP) and extends our insight into how Smad proteins may regulate transcriptional responses in many cell types. Thus, functional Smad-coactivator interactions may be an important locus of signal integration in endothelial cells.

Vascular MADs: two novel MAD-related genes selectively inducible by flow in human vascular endothelium

Vascular endothelium is an important transducer and integrator of both humoral and biomechanical stimuli within the cardiovascular system. Utilizing a differential display approach, we have identified two genes, Smad6 and Smad7, encoding members of the MAD-related family of molecules, selectively induced in cultured human vascular endothelial cells by steady laminar shear stress, a physiologic fluid mechanical stimulus. MAD-related proteins are a recently identified family of intracellular proteins that are thought to be essential components in the signaling pathways of the serine/threonine kinase receptors of the transforming growth factor beta superfamily. Smad6 and Smad7 possess unique structural features (compared with previously described MADs), and they can physically interact with each other, and, in the case of Smad6, with other known human MAD species, in endothelial cells. Transient expression of Smad6 or Smad7 in vascular endothelial cells inhibits the activation of a transfected reporter gene in response to both TGF-beta and fluid mechanical stimulation. Both Smad6 and Smad7 exhibit a selective pattern of expression in human vascular endothelium in vivo as detected by immunohistochemistry and in situ hybridization. Thus, Smad6 and Smad7 constitute a novel class of MAD-related proteins, termed vascular MADs, that are induced by fluid mechanical forces and can modulate gene expression in response to both humoral and biomechanical stimulation in vascular endothelium.

The MAD-related protein Smad7 associates with the TGFbeta receptor and functions as an antagonist of TGFbeta signaling

TGFbeta signaling is initiated when the type I receptor phosphorylates the MAD-related protein, Smad2, on C-terminal serine residues. This leads to Smad2 association with Smad4, translocation to the nucleus, and regulation of transcriptional responses. Here we demonstrate that Smad7 is an inhibitor of TGFbeta signaling. Smad7 prevents TGFbeta-dependent formation of Smad2/Smad4 complexes and inhibits the nuclear accumulation of Smad2. Smad7 interacts stably with the activated TGFbeta type I receptor, thereby blocking the association, phosphorylation, and activation of Smad2. Furthermore, mutations in Smad7 that interfere with receptor binding disrupt its inhibitory activity. These studies thus define a novel function for MAD-related proteins as intracellular antagonists of the type I kinase domain of TGFbeta family receptors.

Expression of the bumetanide-sensitive Na-K-Cl cotransporter BSC2 is differentially regulated by fluid mechanical and inflammatory cytokine stimuli in vascular endothelium

In vascular endothelium, the electroneutral Na-K-Cl cotransport system is thought to function in the maintenance of a selective permeability barrier in certain vascular beds (e.g., brain), as well as in the preservation of endothelial homeostasis in the face of fluctuating osmotic conditions that may accompany certain pathophysiological conditions (e.g., diabetes mellitus). Here we demonstrate that the gene encoding the bumetanide-sensitive cotransporter BSC2, one of the two major isoforms of Na-K-Cl cotransporters present in mammalian cells, can be differentially regulated by inflammatory cytokines and fluid mechanical forces in cultured endothelium. Interleukin-1beta and tumor necrosis factor-alpha significantly upregulate expression of BSC2 mRNA and protein in human umbilical vein endothelial cells, a response that is inhibited by pretreatment with interferon-gamma. Steady laminar fluid shear stress, at a physiologic magnitude (10 dyn/cm2), is also able to induce and maintain elevated expression of BSC2 in cultured human umbilical vein endothelial cells, while a comparable time-averaged magnitude of turbulent fluid shear stress is not. In vivo, BSC2 mRNA is upregulated after intraperitoneal administration of bacterial endotoxin (LPS) in murine lung and kidney, but not in cardiac tissue. These results provide the first experimental evidence that the BSC2 gene can be selectively regulated by different inflammatory cytokine and fluid mechanical stimuli in endothelium, and support a role for BSC2 in vascular homeostasis and inflammation.

Identification of vascular endothelial genes differentially responsive to fluid mechanical stimuli: cyclooxygenase-2, manganese superoxide dismutase, and endothelial cell nitric oxide synthase are selectively up-regulated by steady laminar shear stress

Early atherosclerotic lesions develop in a topographical pattern that strongly suggests involvement of hemodynamic forces in their pathogenesis. We hypothesized that certain endothelial genes, which exhibit differential responsiveness to distinct fluid mechanical stimuli, may participate in the atherogenic process by modulating, on a local level within the arterial wall, the effects of systemic risk factors. A differential display strategy using cultured human endothelial cells has identified two genes, manganese superoxide dismutase and cyclooxygenase-2, that exhibit selective and sustained up-regulation by steady laminar shear stress (LSS). Turbulent shear stress, a nonlaminar fluid mechanical stimulus, does not induce these genes. The endothelial form of nitric oxide synthase also demonstrates a similar LSS-selective pattern of induction. Thus, three genes with potential atheroprotective (antioxidant, antithrombotic, and antiadhesive) activities manifest a differential response to distinct fluid mechanical stimuli, providing a possible mechanistic link between endothelial gene expression and early events in atherogenesis. The activities of these and other LSS-responsive genes may have important implications for the pathogenesis and prevention of atherosclerosis.

Secondary structure of the RNA component of a nuclear/mitochondrial ribonucleoprotein

RNase mitochondrial RNA processing (MRP) is a site-specific endoribonuclease located in both the nucleus and mitochondria of vertebrate cells. The enzyme is a ribonucleoprotein whose RNA component has been shown to be encoded by a nuclear gene. Because RNase MRP is particular in its substrate requirement, RNA-RNA interaction has been proposed as important for the cleavage reaction. A secondary structure of this RNA from mouse cells has been derived by chemical modification of in vivo MRP RNA in ribonucleoprotein form, as isolated free RNA, and as RNA synthesized in vitro. Full-length MRP RNA appears to adopt a conformation containing a significant number of single-stranded residues and may form a pseudoknot. The data are consistent with both the RNA within the ribonucleoprotein and the free RNA possessing comparable secondary structures and suggest a possible site of interaction between enzyme and substrate. The human MRP RNA can be folded into a conformation very similar to that predicted for the mouse MRP RNA. A more limited analysis of human MRP RNA is consistent with the structure proposed for the mouse species.

The gene for the RNA component of the mitochondrial RNA-processing endoribonuclease is located on human chromosome 9p and on mouse chromosome 4

Mitochondrial RNA-processing endoribonuclease (RNAase MRP) has the capacity to cleave mitochondrial RNA complementary to the light strand of the displacement loop at a unique site. The enzyme is a ribonucleoprotein whose RNA component is a nuclear gene product. The 5' flanking region of the primary transcript has control elements characteristic of RNA polymerase II transcription, and the coding region has features of RNA polymerase III transcription signals. The RNA associated with RNAase MRP is the first known RNA encoded by a single-copy gene in the nucleus and believed to be imported into mitochondria. The gene (RMRP) for this RNA component of RNAase MRP was assigned to human chromosome 9 and mouse chromosome 4 by Southern blot analyses of 11 human X rodent hybrids and 11 mouse X rodent hybrids with probe pHM1.0 and probe pSP270, respectively. In situ hybridization of probe pHSTU300 to normal human chromosomes revealed 29 of 100 cells with label on 9p and 9.6% of 302 silver grains located at 9p21--p12.

Characterization of human MRP/Th RNA and its nuclear gene: full length MRP/Th RNA is an active endoribonuclease when assembled as an RNP

Vertebrate cells contain a site-specific endoribonuclease (RNase MRP) that cleaves mitochondrial RNA transcribed from the origin of leading-strand mitochondrial DNA replication. This report presents the characterization of the human enzyme and its essential RNA component. Human RNase MRP is a ribonucleoprotein with a nucleus-encoded RNA of 265 nucleotides. As expected, the single-copy RNA coding region is homologous (84%) to the corresponding mouse gene; surprisingly, at least 700 nucleotides of the immediate 5'-flanking region are conserved. The 265-nucleotide MRP RNA and an MRP RNA cleavage product representing the 3'-terminal 108 nucleotides exist in nuclear and mitochondrial RNA isolates; the larger MRP RNA is present in greatest abundance in the nucleus. The putative processing site within the 265-nucleotide MRP RNA is offset from that of mouse MRP RNA, but in each case cleavage is precise and occurs at the sequence ANCCCGC. Oligonucleotide-mediated inhibition experiments reveal that both the 5' and 3' portions of the MRP RNA are involved in cleavage by RNase MRP; this implies that full length MRP RNA complexed with proteins is an active species in vertebrate cells.

The RNA processing enzyme RNase MRP is identical to the Th RNP and related to RNase P

Sera from patients with autoimmune diseases often contain antibodies that bind ribonucleoproteins (RNPs). Sera from 30 such patients were found to immunoprecipitate ribonuclease P (RNase P), an RNP enzyme required to process the 5' termini of transfer RNA transcripts in nuclei and mitochondria of eukaryotic cells. All 30 sera also immunoprecipitated the nucleolar Th RNP, indicating that the two RNPs are structurally related. Nucleotide sequence analysis of the Th RNP revealed it was identical to the RNA component of the mitochondrial RNA processing enzyme known as RNase MRP. Antibodies that immunoprecipitated the Th RNP selectively depleted murine and human cell extracts of RNase MRP activity, indicating that the Th and RNase MRP RNPs are identical. Since RNase P and RNase MRP are not associated with each other during biochemical purification, we suggest that these two RNA processing enzymes share a common autoantigenic polypeptide.

Identification of transcriptional regulatory elements in human mitochondrial DNA by linker substitution analysis

Human mitochondrial DNA contains two major promoters, one for transcription of each strand of the helix. Previous mapping and mutagenesis data have localized these regulatory elements and have suggested regions important to their function. In order to define, at high resolution, the sequences critical for accurate and efficient transcriptional initiation, a linker substitution analysis of the entire promoter region was performed. Each promoter was shown to consist of approximately 50 base pairs comprising two functionally distinct elements. These and previous data strongly support a mode of transcription initiation requiring minimal sequences surrounding the initiation sites that are likely interactive with core polymerase and upstream regulatory domains capable of binding a transcription factor that modulates the efficiency of transcription initiation. Furthermore, in at least one case, this upstream regulatory domain is capable of operating bidirectionally.

Promoter selection in human mitochondria involves binding of a transcription factor to orientation-independent upstream regulatory elements

Selective transcription of human mitochondrial DNA requires a transcription factor (mtTF) in addition to an essentially nonselective RNA polymerase. Partially purified mtTF is able to sequester promoter-containing DNA in preinitiation complexes in the absence of mitochondrial RNA polymerase, suggesting a DNA-binding mechanism for factor activity. Functional domains, required for positive transcriptional regulation by mtTF, are identified within both major promoters of human mtDNA through transcription of mutant promoter templates in a reconstituted in vitro system. These domains are essentially coextensive with DNA sequences protected from nuclease digestion by mtTF-binding. Comparison of the sequences of the two mtTF-responsive elements reveals significant homology only when one sequence is inverted; the binding sites are in opposite orientations with respect to the predominant direction of transcription. Thus mtTF may function bidirectionally, requiring additional protein-DNA interactions to dictate transcriptional polarity. The mtTF-responsive elements are arrayed as direct repeats, separated by approximately 80 bp within the displacement-loop region of human mitochondrial DNA; this arrangement may reflect duplication of an ancestral bidirectional promoter, giving rise to separate, unidirectional promoters for each strand.