A journey with Tony Hugli up the inflammatory cascade towards the auto-digestion hypothesis

The autodigestion hypothesis: Proteolytic receptor cleavage in rheological and cardiovascular cell dysfunction1

Transformation of circulating leukocytes from a dormant into an activated state with changing rheological properties leads to a major shift of their behavior in the microcirculation. Low levels of pseudopod formation or expression of adhesion molecules facilitate relatively free passage through microvessels while activated leukocytes with pseudopods and enhanced levels of adhesion membrane proteins become trapped in microvessels, attach to the endothelium and migrate into the tissue. The transformation of leukocytes into an activated state is seen in many diseases. While mechanisms for activation due to infections, tissue trauma, as well as non-physiological biochemical or biophysical exposures are well recognized, the mechanisms for activation in many diseases have not been conclusively liked to these traditional mechanisms and remain unknown. We summarize our recent evidence suggesting a major and surprising role of digestive enzymes in the small intestine as root causes for leukocyte activation and microvascular disturbances. During normal digestion of food digestive enzymes are compartmentalized in the lumen of the intestine by the mucosal epithelial barrier. When permeability of this barrier increases, these powerful degrading enzymes leak into the wall of the intestine and into the systemic circulation. Leakage of digestive enzymes occurs for example in physiological shock and multi-organ failure. Entry of digestive enzymes into the wall of the small intestine leads to degradation of the intestinal tissue in an autodigestion process. The digestive enzymes and tissue/food fragments generate not only activate leukocytes but also cause numerous cell dysfunctions. For example, proteolytic destruction of membrane receptors, plasma proteins and other biomolecules occurs. We conclude that escape of digestive enzymes from the intestinal track serves as a major source of cell dysfunction, morbidity and even mortality, including abnormal leukocyte activation seen in rheological studies.

Transmural intestinal wall permeability in severe ischemia after enteral protease inhibition

In intestinal ischemia, inflammatory mediators in the small intestine's lumen such as food byproducts, bacteria, and digestive enzymes leak into the peritoneal space, lymph, and circulation, but the mechanisms by which the intestinal wall permeability initially increases are not well defined. We hypothesize that wall protease activity (independent of luminal proteases) and apoptosis contribute to the increased transmural permeability of the intestine's wall in an acutely ischemic small intestine. To model intestinal ischemia, the proximal jejunum to the distal ileum in the rat was excised, the lumen was rapidly flushed with saline to remove luminal contents, sectioned into equal length segments, and filled with a tracer (fluorescein) in saline, glucose, or protease inhibitors. The transmural fluorescein transport was determined over 2 hours. Villi structure and epithelial junctional proteins were analyzed. After ischemia, there was increased transmural permeability, loss of villi structure, and destruction of epithelial proteins. Supplementation with luminal glucose preserved the epithelium and significantly attenuated permeability and villi damage. Matrix metalloproteinase (MMP) inhibitors (doxycycline, GM 6001), and serine protease inhibitor (tranexamic acid) in the lumen, significantly reduced the fluorescein transport compared to saline for 90 min of ischemia. Based on these results, we tested in an in-vivo model of hemorrhagic shock (90 min 30 mmHg, 3 hours observation) for intestinal lesion formation. Single enteral interventions (saline, glucose, tranexamic acid) did not prevent intestinal lesions, while the combination of enteral glucose and tranexamic acid prevented lesion formation after hemorrhagic shock. The results suggest that apoptotic and protease mediated breakdown cause increased permeability and damage to the intestinal wall. Metabolic support in the lumen of an ischemic intestine with glucose reduces the transport from the lumen across the wall and enteral proteolytic inhibition attenuates tissue breakdown. These combined interventions ameliorate lesion formation in the small intestine after hemorrhagic shock.

Oral melatonin attenuates lung inflammation and airway hyperreactivity induced by inhalation of aerosolized pancreatic fluid in rats

Melatonin is a free radical scavenger with potent antioxidant properties and immunomodulatory effects. The purpose of this study was to determine the effects of orally administered melatonin in a pancreatic fluid (PF)-induced lung inflammation and airway hyperreactivity model. Aerosolized PF was introduced into airways to induce inflammation in rats. Animals were randomized into three experimental groups: sham treated; PF treated (200 μL/kg); and PF with melatonin (10 mg/kg) pretreatment. Airway reactivity to methacholine, airflow and airway resistance, bronchoalveolar lavage (BAL) cellular differential, the tumor necrosis factor α (TNFα) level, lavage nitric oxide, hydroxyl radical, and lactic dehydrogenase (LDH) were compared among groups. mRNA expressions of inducible nitric oxide synthase (iNOS) and TNFα in lung tissues were determined by real-time polymerase chain reaction. Protein expressions of iNOS and nitrotyrosine and lung tissue myeloperoxidase (MPO) activity were determined using an ELISA assay. Oral melatonin treatment indicated anti-inflammatory efficacy as evidenced by decreased methacholine sensitivity by 24% and airway obstruction by 28%, reduction in BAL eosinophil (P < 0.01) and neutrophil counts (P < 0.05), LDH (P < 0.05), and TNFα concentrations (P < 0.05) when compared to levels in sham-treated rats. Melatonin-treated animals also had reduced nitric oxide and hydroxyl radical concentrations (P < 0.05) in lavage fluid. Oral melatonin significantly reduced mRNA and protein expression of iNOS (P < 0.05 and P < 0.01, respectively), TNFα (P < 0.05), nitrotyrosine (P < 0.05), and MPO activity (P < 0.05) in lung tissues when compared with the sham-treated animals. These results suggest that oral treatment with melatonin had a beneficial effect on PF-induced obstructive ventilatory insufficiency by attenuating nitrosative and oxidative stress.

Assessing shock resuscitation strategies by oxygen debt repayment

Identification of occult shock is a major clinical problem compounded by inadequate criteria for assessing the efficacy of fluid resuscitation. We suggest that these problems may be resolved in part by understanding both the physiological mechanisms underlying oxygen debt accumulation and, more importantly, the debt repayment schedule during resuscitation. We present a simplified tutorial that incorporates the concept of the oxygen supply-delivery relationship with that of oxygen debt and show how this is relevant to the understanding of shock and resuscitation. Use of oxygen debt metrics as end points for shock have been controversial; however, much of the controversy may have been due to incomplete understanding of basic physiology of shock and semantic confusion between the various metrics proposed as end points. Here, we provide working definitions for the frequently misunderstood concepts of oxygen deficit and oxygen debt and discuss the relatively novel concept of oxygen debt repayment schedule. We introduce predictions made on the basis of data derived from animal models of hemorrhagic shock. Our calculations suggest that the amount of debt repaid in the first 2 h of resuscitation, rather than the restoration of volume per se, influences the likelihood of organ damage. Because of difficulties inherent in measuring oxygen debt in the prehospital and emergency settings, various metabolic end points such as lactate and base deficit have been proposed as surrogates. We demonstrate the heuristic value of this model in providing a predictive framework for both the optimum therapeutic time window and optimum fluid loadings before critical transitions to an irreversible shock state can occur. The model also provides an unambiguous and objective standard for quantifying the behavior of various postulated shock "markers".

Biomechanical aspects of the auto-digestion theory

Increasing evidence suggests that most cardiovascular diseases, tumors and other ailments are associated with an inflammatory cascade. The inflammation is accompanied by activation of cells in the circulation and fundamental changes in the mechanics of the microcirculation, expression of pro-inflammatory genes and downregulation of anti-inflammatory genes, attachment of leukocytes to the endothelium, elevated permeability of the endothelium, and many other events. The evidence has opened great opportunities for medicine to develop new anti-inflammatory interventions. But it also raises a fundamental question: What is the origin of inflammation? I will discuss a basic series of studies that was designed to explore trigger mechanisms for inflammation in shock and multi-organ failure, an important clinical problem associated with high mortality. We traced the source of the inflammatory mediators to the powerful digestive enzymes in the intestine. Synthesized in the pancreas as part of normal digestion, they have the ability to degrade almost all biological tissues and molecules. In the lumen of the intestine, digestive enzymes are fully activated and self-digestion of the intestine is prevented by compartmentalization in the lumen of the intestine facilitated by the mucosal epithelial barrier. Under conditions of intestinal ischemia, however, the mucosal barrier becomes permeable to pancreatic enzymes allowing their entry into the wall of the intestine. The process leads to auto-digestion of the intestinal wall and production of inflammatory mediators. The hypothesis that multi-organ failure in shock may be due an auto-digestion process by pancreatic enzymes is ready to be tested in a variety of shock conditions.