Dimitrios Poutias

Lipopolysaccharide Internalization Activates Endotoxin-Dependent Signal Transduction in Cardiomyocytes

We tested the hypothesis that bacterial lipopolysaccharide (LPS) must be internalized to facilitate endotoxin-dependent signal activation in cardiac myocytes. Fluorescently labeled LPS was used to treat primary cardiomyocyte cultures, perfused heart preparations, and the RAW264.7 macrophage cell line. Using confocal microscopy and spectrofluorometry, we found that LPS was rapidly internalized in cardiomyocyte cultures and Langendorff-perfused hearts. Although LPS uptake was also observed in macrophages, only a fraction of these cells were found to internalize endotoxin to the extent seen in cardiomyocytes. Colocalization experiments with organelle or structure-specific fluorophores showed that LPS was concentrated in the Golgi apparatus, lysosomes, and sarcomeres. Similar intracellular localization was demonstrated in cardiomyocytes by transmission electron microscopy using gold-labeled LPS. The internalization of LPS was dependent on endosomal trafficking, because an inhibitor of microfilament reorganization prevented uptake in both cardiomyocytes and whole hearts. Inhibition of endocytosis specifically restricted early activation of extracellular signal-regulated kinase proteins and nuclear factor-&kgr;B as well as later tumor necrosis factor-&agr; production and inducible nitric oxide synthase expression. In conclusion, we have demonstrated that bacterial endotoxin is internalized and transported to specific intracellular sites in heart cells and that these events are obligatory for activation of LPS-dependent signal transduction.

Vesnarinone and amrinone reduce the systemic inflammatory response syndrome.

OBJECTIVE The systemic inflammatory response is an important cause of organ dysfunction. The present study tested the hypothesis that 2 clinically used agents, amrinone and vesnarinone, would decrease inflammation and cardiac dysfunction in a relevant model of systemic inflammatory response activation. METHODS Rabbits received intravenous endotoxin, alone or in conjunction with amrinone or vesnarinone. Systemic effects were assessed by death, fever, behavior, and acidosis. Measures of inflammatory signaling were (1) plasma tumor necrosis factor-alpha and interleukin-1 beta production, (2) lung tissue myeloperoxidase activity, and (3) myocardial inducible nitric oxide synthase activity. Indices of systolic and diastolic myocardial function were measured in Langendorff-perfused hearts. RESULTS Vesnarinone, in particular, reduced mortality rates (19% vs 61% for lipopolysaccharide alone, P =.01) and acidosis in lipopolysaccharide-treated rabbits. Both agents markedly reduced systemic tumor necrosis factor and interleukin-1 concentrations, lipopolysaccharide-mediated effects on myocardial systolic and diastolic function and on myocardial inducible nitric oxide synthase activity. Vesnarinone, but not amrinone, (1) decreased fever and lethargy, consistent with decreased central nervous system effects of endotoxin, and (2) decreased lung leukocyte infiltration. CONCLUSIONS Vesnarinone and amrinone, which are used clinically for their inotropic and vasodilating properties, may be useful to limit inflammatory activation and consequent organ dysfunction. Structure-activity and/or pharmacokinetic between the compounds may be important, particularly in preventing inflammatory signaling within certain tissues.

Increased myocardial calcium cycling and reduced myofilament calcium sensitivity in early endotoxemia

BACKGROUND Mechanisms of cardiac dysfunction during endotoxemia are multiple and their targets uncertain. This study tested the hypothesis that endotoxin (LPS) induces abnormal calcium-activated contractile force in the heart. METHODS Adult rabbits were given LPS intravenously; 2 hours later hearts were studied in the Langendorff mode. Measurements included peak developed pressure (PDP), myocardial oxygen consumption (MVO2), high-energy phosphates by 31P-NMR, and beat-to-beat intracellular calcium (Cai) by fluorescence spectroscopy. Myofibrillar calcium sensitivity was assessed from the relationship of PDP to Cai and the rate of diastolic Cai removal (tau Ca) was quantified. RESULTS Force-calcium relationships were markedly depressed in LPS hearts despite increased Cai. MVO2 was increased in parallel with increased Cai. Taken together, these data denote myofilament calcium insensitivity and mechanical inefficiency. tau Ca was markedly prolonged in LPS hearts, indicating impaired calcium reuptake and/or extrusion. High-energy phosphates and intracellular pH were unaffected by LPS; however, inorganic phosphate (Pi) was significantly increased. Dobutamine further increased Cai and MVO2 in LPS hearts without significantly improving calcium-activated force. Pyruvate, an inotrope that reduces Pi, significantly improved contractility in LPS hearts. CONCLUSIONS Endotoxemia rapidly induced futile calcium cycling and reduced myofibrillar calcium sensitivity. This state was resistant to beta-agonist inotropic stimulation; inotropes that normalize the calcium-force relationship may be more effective.

Up-regulation of soluble vascular endothelial growth factor receptor-1 prevents angiogenesis in hypertrophied myocardium.

AIMS Inadequate capillary growth in pressure-overload hypertrophy impairs myocardial perfusion and substrate delivery, contributing to progression to failure. Capillary growth is tightly regulated by angiogenesis growth factors like vascular endothelial growth factor (VEGF) and endogenous inhibitors such as the splice variant of VEGF receptor-1, sVEGFR-1. We hypothesized that inadequate expression of VEGF and up-regulation of VEGFR-1 and its soluble splice variant, sVEGFR-1, restrict capillary growth in pressure-overload hypertrophy. METHODS AND RESULTS Neonatal New Zealand White rabbits underwent aortic banding. mRNA (qRT-PCR) and protein levels (immunoblotting) were determined in hypertrophied and control myocardium (7/group) for total VEGF, VEGFR-1, sVEGFR-1, VEGFR-2, and phospho-VEGFR-1 and -R-2. Free VEGF was determined by enzyme-linked immunoassay (ELISA) in hypertrophied myocardium, controls, and hypertrophied hearts following inhibition of sVEGFR-1 with placental growth factor (PlGF). VEGFR-1 and sVEGFR-1 mRNA (seven-fold up-regulation, P = 0.001) and protein levels were significantly up-regulated in hypertrophied hearts vs. controls (VEGFR-1: 44 ± 8 vs. 23 ± 1, P = 0.031; sVEGFR-1: 71 ± 13 vs. 31 ± 3, P = 0.016). There was no change in VEGF and VEGFR-2 mRNA or protein levels in hypertrophied compared with controls hearts. A significant decline in free, unbound VEGF was found in hypertrophied myocardium which was reversed following inhibition of sVEGFR-1 with PlGF, which was accompanied by phosphorylation of VEGFR-1 and VEGFR-2. CONCLUSION Up-regulation of the soluble VEGFR-1 in pressure-loaded myocardium prevents capillary growth by trapping VEGF. Inhibition of sVEGFR-1 released sufficient VEGF to induce angiogenesis and preserved contractile function. These data suggest sVEGFR-1 as possible therapeutic targets to prevent heart failure.

Electron transport chain dysfunction in neonatal pressure-overload hypertrophy precedes cardiomyocyte apoptosis independent of oxidative stress.

OBJECTIVES We have previously shown in a model of pressure-overload hypertrophy that there is increased cardiomyocyte apoptosis during the transition from peak hypertrophy to ventricular decompensation. Electron transport chain dysfunction is believed to play a role in this process through the production of excessive reactive oxygen species. In this study we sought to determine electron transport chain function in pressure-overload hypertrophy and the role of oxidative stress in myocyte apoptosis. METHODS AND RESULTS Neonatal rabbits underwent thoracic aortic banding at 10 days of age. Compensated hypertrophy (4 weeks of age), decompensated hypertrophy (6 weeks of age), and age-matched controls (n = 4-8 per group) as identified by serial echocardiography were studied. Electron transport chain complex activities were determined by spectophotometry in isolated mitochondria. Complex I was significantly decreased (P = .005) at 4 weeks and further decreased at 6 weeks (P = .001). Complex II was significantly decreased at both time points (4 weeks, P = .003; 6 weeks, P = .009). However, hyddrogen peroxide production, measured in isolated mitochondria by fluorescence spectroscopy, was significantly decreased at 4 weeks of age in banded animals compared with controls (P = .038), and mitochondrial DNA oxidative damage (measurement of 8- hydroxydeoxyguanosine by enzyme-linked immunosorbent assay) was also significantly decreased at 4 weeks of age (P = .031). Mitochondrial activated apoptosis was determined by Bax/Bcl-2 ratios (immunoblotting). Bax/Bcl-2 levels were significantly increased in banded animals at 6 weeks. CONCLUSIONS In pressure-overload hypertrophy, the transition from compensated left ventricular hypertrophy to failure and cardiomyocyte apoptosis is preceded by mitochondrial complex I and II dysfunction followed by an increase in Bax/Bcl-2 ratios. The mechanism of apoptosis initiation is independent of increased oxidative stress.

Myocardial Hypertrophy Overrides the Angiogenic Response to Hypoxia

Background Cyanosis and myocardial hypertrophy frequently occur in combination. Hypoxia or cyanosis can be potent inducers of angiogenesis, regulating the expression of hypoxia-inducible factors (HIF), vascular endothelial growth factors (VEGF), and VEGF receptors (VEGFR-1 and 2); in contrast, pressure overload hypertrophy is often associated with impaired pro-angiogenic signaling and decreased myocardial capillary density. We hypothesized that the physiological pro-angiogenic response to cyanosis in the hypertrophied myocardium is blunted through differential HIF and VEGF-associated signaling. Methods and Results Newborn rabbits underwent aortic banding and, together with sham-operated littermates, were transferred into a hypoxic chamber (FiO2 = 0.12) at 3 weeks of age. Control banded or sham-operated rabbits were housed in normoxia. Systemic cyanosis was confirmed (hematocrit, arterial oxygen saturation, and serum erythropoietin). Myocardial tissue was assayed for low oxygen concentrations using a pimonidazole adduct. At 4 weeks of age, HIF-1α and HIF-2α protein levels, HIF-1α DNA-binding activity, and expression of VEGFR-1, VEGFR-2, and VEGF were determined in hypoxic and normoxic rabbits. At 6 weeks of age, left-ventricular capillary density was assessed by immunohistochemistry. Under normoxia, capillary density was decreased in the banded rabbits compared to non-banded littermates. As expected, non-hypertrophied hearts responded to hypoxia with increased capillary density; however, banded hypoxic rabbits demonstrated no increase in angiogenesis. This blunted pro-angiogenic response to hypoxia in the hypertrophied myocardium was associated with lower HIF-2α and VEGFR-2 levels and increased HIF-1α activity and VEGFR-1 expression. In contrast, non-hypertrophied hearts responded to hypoxia with increased HIF-2α and VEGFR-2 expression with lower VEGFR-1 expression. Conclusion The participation of HIF-2α and VEGFR-2 appear to be required for hypoxia-stimulated myocardial angiogenesis. In infant rabbit hearts with pressure overload hypertrophy, this pro-angiogenic response to hypoxia is effectively uncoupled, apparently in part due to altered HIF-mediated signaling and VEGFR subtype expression.

Endogenous Angiogenesis Inhibitors Prevent Adaptive Capillary Growth in Left Ventricular Pressure Overload Hypertrophy

BACKGROUND In left ventricular (LV) pressure-overload hypertrophy, lack of adaptive capillary growth contributes to progression to failure. Remodeling of the hypertrophied myocardium requires proteolysis of the extracellular matrix (ECM) carried out by matrix metalloproteinases (MMPs). MMPs, specifically MMP-9, are known to cleave ECM components to generate angiogenesis inhibitors (angiostatin, endostatin, tumstatin). We hypothesize that MMP-9 releases antiangiogenic factors during compensated and decompensated hypertrophy, which results in lack of adaptive capillary growth. METHODS Newborn rabbits underwent aortic banding. Myocardial tissue from age-matched and banded animals at compensated (4 weeks) and decompensated hypertrophy (7 weeks), as identified by serial echocardiography, was analyzed by immunoblotting for angiostatin, endostatin, and tumstatin. MMP-9 activity was determined by zymography. A cell-permeable, potent, selective MMP-9 inhibitor was administered intrapericardially to animals with hypertrophied hearts and tissue was analyzed. RESULTS MMP-9 is activated in hypertrophied myocardium versus in control hearts (22 ± 2 versus 16 ± 1; p = 0.04), which results in significantly increased levels of angiostatin (115 ± 10 versus 86 ± 7; p = 0.02), endostatin (33 ± 1 versus 28 ± 1; p = 0.006), and tumstatin (35 ± 6 versus 17 ± 4; p = 0.04). Zymography confirms inhibition of MMP-9 (hypertrophy + MMP-9 inhibitor, 14 ± 0.6 versus hypertrophy + vehicle, 17 ± 1; p = 0.01) and angiostatin, endostatin, and tumstatin are down-regulated, accompanied by up-regulation of capillary density (hypertrophy + MMP-9 inhibitor, 2.99 ± 0.07 versus hypertrophy + vehicle, 2.7 ± 0.05; p = 0.002). CONCLUSIONS Up-regulation of angiogenesis inhibitors prevents adaptive capillary growth in pressure-overload hypertrophied myocardium. Therapeutic interventions aimed at inhibition of angiogenesis inhibitors are useful in maintaining capillary density and thereby preventing heart failure.