Leaking Capillaries and White Lung in Sepsis—Is Angiopoietin 2 the Culprit?
Authors: Parikh S.M., Mammoto T., Schultz A., Yuan H.T., Christiani D., Karumanchi S.A., Sukhatme V.P.
Journal: Journal of the American Society of Nephrology (2006)

Sepsis is a frequent medical emergency. Despite much recent progress in patient management, it still carries a devastating prognosis (1). Sepsis with multiorgan failure is well known to nephrologists because of its frequent association with acute renal failure (2,3). The pathomechanisms involved are becoming ever more complex and have not been completely elucidated (4,5). The extravasation of fluid into lung tissue causing the respiratory distress syndrome ("white lung") is one dire facet of multiorgan failure and continues to be a frequent and major cause of death (6,7). There is a desperate need for innovative interventions and the search goes on (8). The work of Parikh et al. (9), if confirmed, promises to be a major breakthrough that may open new perspectives for focused and rational therapies. In a well-organized series of studies from bedside to bench, the authors examined septic patients with one major complication of sepsis (i.e., impaired oxygenation as a result of pulmonary vascular leak), identified angiopoietin-2 as a potential culprit, then went to the bench conducting in vitro and in vivo studies to verify that angiopoietin-2 did indeed cause vascular leak, and finally attempted to identify the underlying molecular mechanism(s). The results of this logical sequence are analogous to fulfilling Koch's postulates. What is angiopoietin-2? For the nonexpert, some background information may be useful (10). The formation of vessels in the embryo begins when angioblasts differentiate into endothelial cells and assemble into tubes in response to the signals of a large family of vascular endothelial growth factor (VEGF) isoforms from surrounding tissues (11). Tissues "speak" to endothelial cells by secreting VEGF (12). The importance of VEGF in angiogenesis is illustrated by the fact that the loss of two alleles with VEGF is lethal and the loss of one allele causes vascular defects. VEGF interacts with two receptor kinases: VEGF-R1 or flt, and VEGF-R2 or flk (13). The former is well known to nephrologists, because a soluble isoform acts in the circulation as a decoy receptor scavenging and inhibiting the action of proangiogenic VEGF. It has recently been identified as one cause of preeclampsia (14). Once VEGF has caused endothelial cell induction, the further downstream steps in angiogenesis are a highly complex, coordinated process through the sequential action of a series of receptors and numerous ligands (10,11). In microvessels, PDGF is involved in recruiting pericytes. In larger vessels, it is primarily the angiopoietin-1/Tie2 (tyrosine kinase with Ig and EGF homology domains) ligand/receptor pair that is involved in recruiting smooth muscle cells. An important recent concept is the notion that maintenance of vascular integrity is the result of a balance between stabilization and regression. Although endothelial cells have half-lives of several years, without the input of survival signals that stabilize endothelial cells and smooth muscle cells, vascular regression and disintegration will occur. It is here that the angiopoietins enter the story: the Tie2 receptor binds angiopoietin-1 and angiopoietin-2. Angiopoietin-1 antagonizes the proangiogenic and permeability-promoting cytokine VEGF. Angiopoietin-1 is an endothelial survival factor. It causes vascular maturation and quiescence. It is ubiquitously expressed and preserves cell-to-cell contacts (15). It phosphorylates the Tie2 receptor and is usually antiangiogenic, since it stabilizes and tightens up the vessel wall. In contrast, the antagonistic ligand angiopoietin-2 is not ubiquitously expressed. Notably, however, it is strongly expressed in the lung (16) together with the Tie2 receptor (17). Angiopoietin-2 initiates vascular remodeling by loosening up the vessel wall, a precondition for vascular remodeling—and, as one could predict, such loosening up might also increase vascular permeability. Furthermore, angiopoietin-2 has functions beyond vascular morphogenesis: it is stored in Weibel-Palade bodies, from where it is released rapidly upon stimulation, e.g., by proinflammatory signals, thus initiating rapid vascular responses (18). The involvement of the angiopoietin-1/angiopoietin-2 system in septic shock had been suggested by the observation of Witzenbichler et al. (19) that angiopoietin-1 protected mice with endotoxin shock: it prevented hemodynamic instability, reduced lung water content, and diminished myeloperoxidase activity. Baffert et al. showed that this was mediated by a reduction in the number and the size of endothelial gaps (20). Conversely, it has been shown that angiopoietin-2 is crucial for the inflammatory vascular response to chemicals or bacterial infection sensitizing endothelial cells to TNFα (18). In the study highlighted here, Parikh et al. (9) examined whether angiopoietin-2 played a role in the respiratory distress syndrome of septicemia. In a small sample of 22 patients with sepsis and 29 control patient in the general medical service, the authors prospectively sampled serum and measured angiopoietin-2 concentrations, which were significantly higher in patients with sepsis and multiorgan dysfunction (23.2 ± 9.1 ng/ml) than in nonseptic controls (3.5 ± 0.6 ng/ml) or septic patients without multiorgan dysfunction (4.8 ± 1.5 ng/ml). Next, Parikh et al. postulated that the high expression of angiopoietin-2 (16) and Tie2 receptor (17) in the lung made it a plausible target organ for angiopoietin-2–mediated organ dysfunction. Therefore they examined the relation between angiopoietin-2 concentrations and indicators of poor oxygenation (PaO2/FIO2). Indeed, a strong correlation was found. Advancing from bedside to bench, the authors next studied whether the serum of septic patients disrupted the architecture of human microvascular endothelial cells obtained from neonatal dermis. This tested the hypothesis that actin-myosin–driven cell contraction opens gaps between endothelial cells, thus permitting paracellular escape of molecules across the endothelial barrier. The results showed that patient serum with high angiopoietin-2 concentrations increased (and addition of angiopoitein-1 reversed) gap formation of endothelial test cells. The in vivo relevance of this finding was documented by measuring the escape of intravenously-administered Evans Blue out of the pulmonary vascular bed in animals pretretated with angiopoietin-2; enhanced leakage was found, though the mice did not show the signs of sickness that one typically encounters in sepsis. Finally, the authors addressed the molecular mechanisms involved in the angiopoietin-2–induced endothelial cell hyperpermeability. It has been known for some time that myosin-driven (21) cell contractions caused by myosin light chain phosphorylation (22) via myosin light chain kinase (23) underlie endothelial cell gap formation (24). The authors showed that, in their test system, angiopoietin-2 dose-dependently caused myosin light chain phosphorylation. This was inhibited by angiopoietin-1. Upregulation of myosin light chain phosphorylation is known to require activation of the small GTPase RhoA. It was therefore not surprising that in this preparation the specific inhibitor of Rho-kinase Y27632 completely abolished angiopoietin-2–induced myosin light chain phosphorylation. That the effects of angiopoietin-2 are mediated via inactivation of the Tie2 receptor was finally proven by a knock-down experiment. The result suggested that the Rho-kinase/myosin light chain phosphorylation cascade is triggered when the antagonistic angiopoietin-2 abrogates the phosphorylation of the Tie-2 receptor, which is normally caused by the agonist angiopoietin-1. These findings strongly suggest a causal role of angiopoietin-2 in the genesis of the pulmonary vascular leak of sepsis. Whether the effects of angiopoietin-2 on the pulmonary microcirculation are the only mechanism or whether additional upstream actions on larger vessels contribute requires further study. Of course, it is too early to tell whether these impressive mechanistic insights will ultimately translate into novel interventions, but the appeal of modulating the above cascade would be that instead of relatively broadly acting, unfocused immunomodulatory and anti-inflammatory interventions fraught with an array of potential side effects, the angiopoietin-2–based strategy would be narrowly focused. Unless unforeseen effects show up, one would anticipate fewer side effects. Of course, confirmation of these stimulating findings and controlled clinical trials are required.Eberhard Ritz: Feature Editor