New Member of Endothelial Arsenal Against Inflammation

• Editorials
• 5-methoxytryptophan
• capillary permeability
• endothelium
• inflammation

The endothelium evolved from a conceptual cellophane-like structure involved in blood–tissue exchange of solutes to an active set of vascular cells with a diverse and broad array of functions in health and disease.^1,2 Because of its strategic immediate contact with blood and tissues, the endothelium regulates transport barrier properties, contributes to control of blood flow, prevents hemorrhage through fine tuning of platelet adhesion, and is an important player in controlling immunologic responses. Several articles and reviews have pointed out that endothelial dysfunction is the fundamental basis of inflammation and vascular disease.^3–6

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Research in vascular biology has identified several key endothelial proteins along with factors that cause the onset of inflammation. Tissue necrosis factor-α is a major cytokine that activates endothelial cells and promotes inflammation. Proteins involved in regulatory functions include intercellular adhesion molecule-1, vascular cell adhesion molecule-1, and vascular endothelial cadherin. Intercellular adhesion molecule-1-1 and vascular cell adhesion molecule-1-1 play an important role in adhesion and migration of leukocytes in inflammation,⁷ whereas vascular endothelial cadherin participates in the regulation of microvascular transport.^8,9

The proteins mentioned in the preceding paragraph, as well as the proteins that serve as coagulation factors, are largely located on the cell membrane and exert their main function by reacting to external stimuli. Interest in the biology of endothelial cells was immensely enhanced by the realization that they produce endogenous factors, leading to signaling cascades that control vascular function at the cellular and organ levels. Endothelium-derived relaxing factor, subsequently identified as nitric oxide (NO), is the most revolutionary among these endogenous factors.^10–14 Indeed, deficit in NO availability is considered as a clinical sign for endothelial dysfunction.¹⁵ NO derived from endothelial NO synthase (eNOS) serves as the main signal for vasodilation in vascular smooth muscle. Importantly, an endogenous function (in regard to endothelium) of eNOS-derived NO is to serve as a fundamental signal for the onset of hyperpermeability in response to inflammatory agents, a function implemented mainly via S-nitrosylation, that is, binding of NO^· to cysteine.^9,16,17 Prostacyclin, endothelin, carbon monoxide, and endothelial-hyperpolarizing factor share with NO the vascular function of controlling blood flow. Prostacyclin, or at least its analog iloprost, has been linked to the function of restoring microvascular permeability after inflammation-induced hyperpermeability.^18,19 Because they regulate the vascular functions of tissue perfusion and blood–tissue transport, all the endothelium-derived molecules play a significant role in the response to inflammatory stimuli.

Wang et al²⁰ add a new molecule of endothelial origin as an anti-inflammatory agent in their article in this issue of Circulation Research. The authors extracted and identified 5-methoxytryptophan (5-MTP) from endothelial cell–conditioned medium. Although it is known that other cells make 5-MTP, one of the merits of the work by Wang et al²⁰ is the demonstration that endothelial cells produce 5-MTP. They conducted a series of logically designed experiments to ascertain the role of 5-MTP in inflammation using accepted in vitro and in vivo models of endotoxemia (lipopolysaccharide, in vitro—human umbilical vein endothelial cells) and sepsis (cecal ligation and puncture, in vivo—in mice), as well as in a translational manner in patients. Overall, the results support the authors’ main conclusion that 5-MTP plays a role as an anti-inflammatory agent.

Wang et al²⁰ showed the translational potential of the their research through results revealing a decrease in 5-MTP concentration in the blood of septic patients, which suggest that an increase in 5-MTP may yield benefits to improve health. Through comparison of 5-MTP concentrations in the blood of different groups of patients, the authors concluded that 5-MTP is a marker of systemic inflammation—but it does not report the stage of progression of sepsis. Wang et al²⁰ tested in vivo whether administration of exogenous 5-MTP prevents lipopolysaccharide-induced extravasation of albumin in mouse lungs. Although the results are encouraging, the mechanisms through which 5-MTP prevents pulmonary hyperpermeability to proteins remain to be determined. The authors tested microvascular permeability using an all or none approach. A combination of more sophisticated intravital (in vivo) methods and cultured endothelial cells should provide mechanistic insights to explain the beneficial actions of 5-MTP. Similarly, it will be important to assess whether 5-MTP can reverse extravasation of macromolecules induced by inflammatory processes.

Wang et al²⁰ evaluated the source of 5-MTP by immunohistochemistry and found it to be associated with endothelium and vascular smooth muscle. Appropriately, the authors tested the endothelial presence of 5-MTP in vivo in the lung, a tissue endowed with high microvascular endothelial mass, in addition to testing for its presence in aorta. This observation is significant inasmuch as human umbilical vein endothelial cell and aorta represent endothelial cells derived from large vessels and may incompletely recapitulate the behavior of microvascular endothelial cells. The presence of 5-MTP in conduit large vessels and in microvessels suggests that this chemical may be an element of endothelial defense against several inflammatory vascular diseases; some of which may be manifested primarily in large vessels (atherosclerosis) and others that have deep roots in the microcirculation (hypertension and diabetes mellitus) where the regulation of blood flow and microvascular permeability resides.

In terms of future directions, this report²⁰ opens the door for investigations of the specific stimuli leading to production of 5-MTP, the cellular mechanisms of action of 5-MTP, and the pathways through which it is exported. Because the authors determined the location of 5-MTP in the cytoplasm and because eNOS translocation from the cell membrane to the cytosol is a requirement for the development of hyperpermeability,^21–24 it is tempting to speculate that 5-MTP may reduce the transport of macromolecules across microvascular walls by returning eNOS from the cytoplasm to the cell membrane or by inhibiting the synthesis of eNOS-derived NO in the cytoplasm.

Figure 1 shows a simplified account of pro- and anti-inflammatory mediators acting on the vasculature. The state of our understanding of the factors that play a role in vascular inflammation has been the subject of extensive reviews.^5,25 The balance of our knowledge shows the predominance of proinflammatory agents and relative paucity of endothelial anti-inflammatory molecules. The report by Wang et al²⁰ rekindles the interest in identifying and exploring regulatory factors produced by endothelial and vascular cells.

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Figure 1.

Simplified scheme displaying a selection of representative pro- and anti-inflammatory molecules. Several cells and plasma-based biochemical reactions produce an abundance of proinflammatory molecules. Endothelial cells (ECs) counteract inflammation through synthesis of anti-inflammatory factors. Cytoplasmic nitric oxide (NO) synthase (eNOS)–NO functions as proinflammatory mediator, whereas caveolar-cell membrane eNOS–NO functions as an anti-inflammatory molecule. 5-methoxytryptophan (5-MTP), which is found in EC and in vascular smooth muscle cells, joins the EC anti-inflammatory arsenal. The swing position is intended to stimulate further search for novel endothelial (vascular) anti-inflammatory molecules. IL indicates interleukin; PAF, platelet-activating factor; TNF-α, tumor necrosis factor-a; and VEGF, vascular endothelial growth factor.

• © 2016 American Heart Association, Inc.