Gary H. Gibbons

Vascular smooth muscle cell hypertrophy vs. hyperplasia. Autocrine transforming growth factor-beta 1 expression determines growth response to angiotensin II.

Recent observations in our laboratory suggest that angiotensin II (Ang II) is a bifunctional vascular smooth muscle cell (VSMC) growth modulator capable of inducing hypertrophy or inhibiting mitogen-stimulated DNA synthesis. Because transforming growth factor-beta 1 (TGF beta 1) has similar bifunctional effects on VSMC growth, we hypothesized that autocrine production of TGF beta 1 may mediate the growth modulatory effects of Ang II. Indeed, this study demonstrates that Ang II induces a severalfold increase in TGF beta 1 mRNA levels within 4 h that is dependent on de novo protein synthesis and appears to be mediated by activation of protein kinase C (PKC). Ang II not only stimulates the synthesis of latent TGF beta 1, but also promotes its conversion to the biologically active form as measured by bioassay. The coincubation of VSMCs with Ang II and control IgG has no significant mitogenic effect. However, the co-administration of Ang II and the anti-TGF beta 1 antibody stimulates significantly DNA synthesis and cell proliferation. We conclude that: (a) Ang II induces increased TGF beta 1 gene expression via a PKC dependent pathway involving de novo protein synthesis; (b) Ang II promotes the conversion of latent TGF beta 1 to its biologically active form; (c) Ang II modulates VSMC growth by activating both proliferative and antiproliferative pathways; and (d) Autocrine active TGF beta 1 appears to be an important determinant of VSMC growth by hypertrophy or hyperplasia.

Multiple autocrine growth factors modulate vascular smooth muscle cell growth response to angiotensin II.

Angiotensin (Ang) II stimulates hypertrophic growth of vascular smooth muscle cells (VSMC). Accompanying this growth is the induction of the expression of growth-related protooncogenes (c-fos, c-jun, and c-myc), as well as the synthesis of the autocrine growth factors, such as PDGF-A and TGF-beta 1. In this study, we demonstrate further that Ang II also induces the synthesis of basic fibroblast growth factor (bFGF), a potent mitogen for VSMC. To examine how these factors interact to modulate the growth response of VSMC to Ang II, we used antisense oligomers to determine the relative contribution of these three factors. Treatment of confluent, quiescent smooth muscle cells with specific antisense oligomers complementary to bFGF, PDGF-A, and TGF-beta 1 efficiently inhibited the syntheses of these factors. Our results demonstrate that in these VSMC, TGF-beta 1 affects a key antiproliferative action, modulating the mitogenic properties of bFGF. Autocrine PDGF exerts only a minimal effect on DNA synthesis. An imbalance in these signals activated by Ang II may result in abnormal VSMC growth leading to the development of vascular disease.

Increased Accumulation of Tissue ACE in Human Atherosclerotic Coronary Artery Disease

BACKGROUND Angiotensin may play a pathophysiological role in experimental and human cardiovascular disease. Clinical studies have shown that ACE inhibitors reduce mortality, recurrent myocardial infarction, and ischemic events in patients with left ventricular dysfunction. Animal studies suggest that tissue ACE, particularly within blood vessels, may be an important target. METHODS AND RESULTS To study tissue ACE in human coronary artery disease and to identify potential mechanisms of ACE inhibitor action, we examined ACE expression immunohistochemically in nonatherosclerotic and diseased human coronary arteries. In nonatherosclerotic arteries, ACE immunoreactivity was found in luminal and adventitial vasa vasorum endothelium. In early- and intermediate-stage atherosclerotic lesions, ACE was detected prominently in regions of fat-laden macrophages and in association with T lymphocytes. In advanced lesions, ACE immunoreactivity was also localized to the endothelium of the microvasculature throughout the plaques. Immunoreactive angiotensin II was also detected in these areas. ACE expression in macrophages was further examined by in vitro experiments with a monocytoid cell line. ACE activity was induced threefold after differentiation of the cells into macrophages and was further increased after stimulation with acetylated LDL. CONCLUSIONS These observations demonstrate that significant sources of tissue ACE in human atherosclerotic plaques are regions of inflammatory cells, especially areas of clustered macrophages as well as microvessel endothelial cells. These results suggest that ACE accumulation within the plaque may contribute to an increased production of local angiotensin that may participate in the pathobiology of coronary artery disease. Plaque ACE probably is an important target of drug action.

Vasoactive Substances Regulate Vascular Smooth Muscle Cell Apoptosis: Countervailing Influences of Nitric Oxide and Angiotensin II

This study tests the hypothesis that the control of vascular smooth muscle cell (VSMC) apoptosis is regulated by the antagonistic balance between vasoactive substances such as NO and angiotensin II (Ang II). Moreover, it is postulated that the cellular signaling pathways involved in regulating vessel tone are also coupled to the regulation of programmed cell death. Using an in vitro model system, we documented that the addition of NO donor molecules S-nitroso-N-acetylpenicillamine or sodium nitroprusside to VSMC dose-dependently induced apoptosis as documented by DNA laddering and quantified by analysis of cellular chromatin morphology. The mediator role of the guanylate cyclase signaling pathway in NO-induced apoptosis was evidenced by (1) induction of apoptosis by the 8-bromo-cGMP analogue, (2) potentiation of NO-induced apoptosis by cGMP-specific phosphodiesterase inhibition, and (3) the prevention of NO-induced apoptosis by the inhibition of the cGMP-dependent protein kinase 1 alpha. In contrast, Ang II directly antagonized NO donor- and cGMP analogue-induced apoptosis via activation of the type I Ang II receptor. These findings suggest that the countervailing balance between NO and Ang II may determine the overall cell population within the vessel wall by regulating genetic programs determining cell death as well as cell growth.

Inhibition of neointimal cell bcl-x expression induces apoptosis and regression of vascular disease

We postulated that activation of a genetic program that tonicaiiy inhibits intimal cell death is a necessary condition for the pathogenesis of vascular disease. Studies of vascular lesions in humans and animal models documented increased expression of the anti-apoptotic gene product Bcl-xL within intimal cells. Downregulation of intimal cell bcl-xL expression with the use of antisense oligonucleotides induced apoptosis and acute regression of vascular lesions. These findings indicate that apoptosis regulatory genes such as bcl-xL are critical determinants of intimal lesion formation and that targeted apoptosis may be a novel therapy for intimal vascular disease.

Fluid shear stress induces endothelial transforming growth factor beta-1 transcription and production. Modulation by potassium channel blockade.

The endothelium has the capacity to modulate vascular structure in response to hemodynamic stimuli. We tested the hypothesis that exposure of the endothelium to increased laminar shear stress induces the expression of TGF beta 1 via a signal transduction pathway modulated by K+ channel currents. Although TGF beta 1 is normally secreted in a latent, inactive form, exposure of cultured endothelial cells to steady laminar shear stress (20 dynes/cm2) induced increased generation of biologically active TGF beta 1. This increase in active TGF beta 1 was associated with a sustained increase in TGF beta 1 mRNA expression within 2 h of stimulation. TGF beta 1 mRNA levels increased in direct proportion to the intensity of the shear stress within the physiologic range. The effect of shear stress on TGF beta 1 mRNA expression was regulated at the transcriptional level as defined by nuclear run-off studies and transient transfection of a TGF beta 1 promoter-reporter gene construct. Blockade of endothelial K+ channels with tetraethylammonium significantly inhibited: activation of TGF beta 1 gene transcription; increase in steady state mRNA levels; and generation of active TGF beta 1 in response to shear stress. These data suggest that endothelial K+ channels and autocrine-paracrine TGF beta 1 may be involved in the mechanotransduction mechanisms mediating flow-induced vascular remodeling.

Fluid shear stress differentially modulates expression of genes encoding basic fibroblast growth factor and platelet-derived growth factor B chain in vascular endothelium.

Fluid shear stress has been shown to be an important regulator of vascular structure and function through its effect on the endothelial cell. We have explored the effect of shear stress on the expression of the heparin-binding growth factors platelet-derived growth factor B chain (PDGF-B) and basic fibroblast growth factor (bFGF) in bovine aortic endothelial cells using a purpose-built cone-plate viscometer. Using morphometric analysis, we have mimicked the endothelial cell shape changes encountered in vivo in response to shear stress and correlated these with changes in gene expression. Steady laminar shear stress of 15 and 36 dyn/cm2 both resulted in endothelial cell shape change, but the higher shear stress induced greater and more uniform alignment in the direction of flow and nuclear protrusion after 24 h. Steady laminar shear stress of both 15 and 36 dyn/cm2 induced a significant 3.9- and 4.2-fold decrease, respectively, in PDGF-B mRNA at 9 h. In contrast, steady laminar shear of 15 dyn/cm2 induced a mild and transient 1.5-fold increase in bFGF mRNA while shear of 36 dyn/cm2 induced a significant 4.8-fold increase at 6 h of shear which remained at 2.9-fold at 9 h. Pulsatile and turbulent shear stress showed the same effect as steady laminar shear stress (all at 15 dyn/cm2 time-average magnitude) on PDGF-B and bFGF mRNA content. Cyclic stretch (20% strain, 20/min) of cells grown on silicone substrate did not significantly affect either PDGF-B or bFGF mRNA levels. These results suggest that expression of each peptide growth factor gene is differentially regulated by fluid shear stress in the vascular endothelial cell. These results may have implications on vascular structure and function in response to hemodynamic forces and present a model for the study of transduction of mechanical stimuli into altered gene expression.

Intimal hyperplasia after vascular injury is inhibited by antisense cdk 2 kinase oligonucleotides.

The cell cycle regulatory enzyme, cdk (cyclin-dependent kinase) 2 kinase, is activated in the rat carotid artery after balloon angioplasty injury, and may mediate smooth muscle proliferation. To test the hypothesis that inhibition of the expression of this key enzyme can inhibit intimal hyperplasia, we studied the effect of antisense phosphorothioate oligodeoxynucleotides (ODN) against cdk 2 kinase administered by intraluminal delivery using hemagglutinating virus of Japan (HVJ)-liposome-mediated transfer. The specificity of antisense cdk 2 ODN was confirmed by the observation that mRNA level of cdk 2 kinase in injured vessels was markedly diminished by the antisense ODN treatment. At 2 wk after transfection, antisense cdk 2 ODN treatment (15 microM) resulted in a significant inhibition (60%) in neointima formation, compared with sense ODN-treated and untreated vessels. Since we have previously observed that cell division cycle 2 kinase mRNA was also activated after vascular injury, we administered the combination of antisense cdc 2 and cdk 2 ODN in this study. Antisense cdc 2 ODN alone (15 microM) only reduced intimal formation by 40%. Combined antisense treatment resulted in near complete inhibition of neointima formation. To understand the mechanism of the sustained effect of a single antisense ODN administration, we examined kinetics of ODN in the vessel wall. Using phosphorothioate FITC-labeled ODN, we transfected carotid artery using the HVJ-liposome method. Fluorescence localized immediately to the medial layer, and persisted up to 2 wk after transfection. These results demonstrate that a single intraluminal administration of antisense ODN directed to cell cycle regulatory genes (e.g., cdk 2 kinase) using the HVJ method can result in a sustained inhibition of neointima formation after balloon angioplasty in rat carotid injury model.

Shear stress elevates endothelial cGMP. Role of a potassium channel and G protein coupling.

BackgroundThe endothelium acts as the sensor of shear stress and as the mediator of flow-induced changes in vessel tone and structure. The purpose of this study was to delineate the signal transduction pathway of flow-induced release of endothelium-derived relaxing factor (EDRF). Methods and ResultsWe used a shear stress apparatus (a modified cone-plate viscometer) to expose cultured endothelial cells to a well-defined laminar fluid flow. Confluent bovine aortic endothelial cells (BAECs) were subjected to varying levels of shear stress, and intracellular cyclic GMP (cGMP) in the BAECs was measured by radioimmunoassay. After 60 seconds of laminar fluid flow, BAEC cGMP increased by 300% from basal levels (from 0.54 to 1.70 pmol/mg protein, p<0.05). The elevation in intracellular cGMP was proportional to the intensity of shear stress within a physiological range up to 40 dynes/cm2. This increase in cGMP was abrogated by L-N-methyl-arginine (the competitive antagonist of nitric oxide [NO] synthase), indicating that the flow-induced activation of soluble guanylate cyclase was mediated by autocrine NO production. Furthermore, a potassium channel antagonist, tetraethylammonium ion (TEA [3 mmol/L]) and a GI or G. protein inhibitor, pertussis toxin (100 ng/mL) also blocked the flow-induced increase in cGMP. By contrast, calcium ionophore or atrial natriuretic peptide caused elevations of cGMP that were not affected by TEA or pertussis toxin. ConclusionsThese findings indicate that shear stress elevates endothelial cGMP via an NO-dependent mechanism. The effect of shear stress is mediated by a unique signal transduction pathway that is coupled to a pertussis toxin-sensitive G protein and that requires the activity of an endothelial potassium channel.

Evidence for direct local effect of angiotensin in vascular hypertrophy. In vivo gene transfer of angiotensin converting enzyme.

In vitro studies have demonstrated that angiotensin (Ang) II directly stimulates vascular smooth muscle cell (VSMC) growth. However, it is still unclear if Ang II exerts a direct effect on vascular hypertrophy in vivo independent of its effect on blood pressure. In vivo gene transfer provides the opportunity to assess the effects of increased activity of the vascular angiotensin system in the intact animal while avoiding an increase in circulating angiotensin or in blood pressure. Accordingly, we transfected the human angiotensin converting enzyme (ACE) vector into intact rat carotid arteries by the hemagglutinating virus of Japan-liposome method. 3 d after transfection, we detected increased ACE activity in the transfected artery. Immunohistochemistry localized immunoreactive ACE in the medial VSMC as well as in the intimal endothelial cells. The increase in vascular ACE activity was associated with a parallel increase in DNA synthesis as assessed by BrdU (bromo-deoxyuridine) index and vascular DNA content. This increase in DNA synthesis was abolished by the in vivo administration of an Ang II receptor-specific antagonist (DuP 753). Morphometry at 2 wk after transfection revealed an increase in the wall to lumen ratio of the ACE-transfected blood vessel as compared with control vector transfected vessels. This was accompanied by increases in protein and DNA contents without an increase in cell number. Local transfection of ACE vector did not result in systemic effects such as increased blood pressure, heart rate, or serum ACE activity. These morphological changes were abolished by the administration of the Ang II receptor antagonist. In this study, we used in vivo gene transfer to increase local expression of vascular angiotensin converting enzyme and provided proof that increased autocrine/paracrine angiotensin can directly cause vascular hypertrophy independent of systemic factors and hemodynamic effects. This approach has important potentials for defining the role of autocrine/paracrine substances in vascular biology and hypertension.