Proteolytic Activity Attenuates the Response of Endothelial Cells to Fluid Shear Stress

Digestive Enzyme Activity and Protein Degradation in Plasma of Heart Failure Patients

Introduction

Heart failure is associated with degradation of cell functions and extracellular matrix proteins, but the trigger mechanisms are uncertain. Our recent evidence shows that active digestive enzymes can leak out of the small intestine into the systemic circulation and cause cell dysfunctions and organ failure.

Methods

Accordingly, we investigated in morning fasting plasma of heart failure (HF) patients the presence of pancreatic trypsin, a major enzyme responsible for digestion.

Results

Western analysis shows that trypsin in plasma is significantly elevated in HF compared to matched controls and their concentrations correlate with the cardiac dysfunction biomarker BNP and inflammatory biomarkers CRP and TNF-α. The plasma trypsin levels in HF are accompanied by elevated pancreatic lipase concentrations. The trypsin has a significantly elevated activity as determined by substrate cleavage. Mass spectrometry shows that the number of plasma proteins in the HF patients is similar to controls while the number of peptides was increased about 20% in HF patients. The peptides are derived from extracellular and intracellular protein sources and exhibit cleavage sites by trypsin as well as other degrading proteases (data are available via ProteomeXchange with identifier PXD026332).

Connclusions

These results provide the first evidence that active digestive enzymes leak into the systemic circulation and may participate in myocardial cell dysfunctions and tissue destruction in HF patients.

Conclusions

These results provide the first evidence that active digestive enzymes leak into the systemic circulation and may participate in myocardial cell dysfunctions and tissue destruction in HF patients.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-021-00693-w.

Fluid shear stress-mediated mechanotransduction in circulating leukocytes and its defect in microvascular dysfunction

Leukocytes (neutrophils, monocytes) in the active circulation exhibit multiple phenotypic indicators for a low level of cellular activity, like lack of pseudopods and minimal amounts of activated, cell-adhesive integrins on their surfaces. In contrast, before these cells enter the circulation in the bone marrow or when they recross the endothelium into extravascular tissues of peripheral organs they are fully activated. We review here a multifaceted mechanism mediated by fluid shear stress that can serve to deactivate leukocytes in the circulation. The fluid shear stress controls pseudopod formation via the FPR receptor, the same receptor responsible for pseudopod projection by localized actin polymerization. The bioactivity of macromolecular factors in the blood plasma that interfere with receptor stimulation by fluid flow, such as proteolytic cleavage in the extracellular domain of the receptor or the membrane actions of cholesterol, leads to a defective ability to respond to fluid shear stress by actin depolymerization. The cell reaction to fluid shear involves CD18 integrins, nitric oxide, cGMP and Rho GTPases, is attenuated in the presence of inflammatory mediators and modified by glucocorticoids. The mechanism is abolished in disease models (genetic hypertension and hypercholesterolemia) leading to an increased number of activated leukocytes in the circulation with enhanced microvascular resistance and cell entrapment. In addition to their role in binding to biochemical agonists/antagonists, membrane receptors appear to play a second role: to monitor local fluid shear stress levels. The fluid shear stress control of many circulating cell types such as lymphocytes, stem cells, tumor cells remains to be elucidated.

Aldosterone induces albuminuria via matrix metalloproteinase-dependent damage of the endothelial glycocalyx

Aldosterone contributes to end-organ damage in heart failure and chronic kidney disease. Mineralocorticoid-receptor inhibitors limit activation of the receptor by aldosterone and slow disease progression, but side effects, including hyperkalemia, limit their clinical use. Damage to the endothelial glycocalyx (a luminal biopolymer layer) has been implicated in the pathogenesis of endothelial dysfunction and albuminuria, but to date no one has investigated whether the glomerular endothelial glycocalyx is affected by aldosterone. In vitro, human glomerular endothelial cells exposed to 0.1 nM aldosterone and 145 mMol NaCl exhibited reduced cell surface glycocalyx components (heparan sulfate and syndecan-4) and disrupted shear sensing consistent with damage of the glycocalyx. In vivo, administration of 0.6 μg/g/d of aldosterone (subcutaneous minipump) and 1% NaCl drinking water increased glomerular matrix metalloproteinase 2 activity, reduced syndecan 4 expression, and caused albuminuria. Intravital multiphoton imaging confirmed that aldosterone caused damage of the glomerular endothelial glycocalyx and increased the glomerular sieving coefficient for albumin. Targeting matrix metalloproteinases 2 and 9 with a specific gelatinase inhibitor preserved the glycocalyx, blocked the rise in glomerular sieving coefficient, and prevented albuminuria. Together these data suggest that preservation of the glomerular endothelial glycocalyx may represent a novel strategy for limiting the pathological effects of aldosterone.

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.

Proteolytic receptor cleavage in the pathogenesis of blood rheology and co-morbidities in metabolic syndrome. Early forms of autodigestion

Abnormal blood rheological properties seldom occur in isolation and instead are accompanied by other complications, often designated as co-morbidities. In the metabolic syndrome with complications like hypertension, diabetes and lack of normal microvascular blood flow, the underlying molecular mechanisms that simultaneously lead to elevated blood pressure and diabetes as well as abnormal microvascular rheology and other cell dysfunctions have remained largely unknown. In this review, we propose a new hypothesis for the origin of abnormal cell functions as well as multiple co-morbidities. Utilizing experimental models for the metabolic disease with diverse co-morbidities we summarize evidence for the presence of an uncontrolled extracellular proteolytic activity that causes ectodomain receptor cleavage and loss of their associated cell function. We summarize evidence for unchecked degrading proteinase activity, e.g. due to matrix metalloproteases, in patients with hypertension, Type II diabetes and obesity, in addition to evidence for receptor cleavage in the form of receptor fragments and decreased extracellular membrane expression levels. The evidence suggest that a shift in blood rheological properties and other co-morbidities may in fact be derived from a common mechanism that is due to uncontrolled proteolytic activity, i.e. an early form of autodigestion. Identification of the particular proteases involved and the mechanisms of their activation may open the door to treatment that simultaneously targets multiple co-morbidities in the metabolic syndrome.

The autodigestion hypothesis for shock and multi-organ failure

An important medical problem with high mortality is shock, sepsis and multi-organ failure. They have currently no treatments other than alleviation of symptoms. Shock is accompanied by strong markers for inflammation and involves a cascade of events that leads to failure in organs even if they are not involved in the initial insult. Recent evidence indicates that pancreatic digestive enzymes carried in the small intestine after mixing with ingested food are a major cause for multi-organ failure. These concentrated and relatively non-specific enzymes are usually compartmentalized inside the intestinal lumen as requirement for normal digestion. But after breakdown of the mucosal barrier they leak into the wall of the intestine and start an autodigestion process that includes destruction of villi in the intestine. Digestive enzymes also generate cytotoxic mediators, which together are transported into the systemic circulation via the portal venous system, the intestinal lymphatics and via the peritoneum. They cause various degrees of cell and organ dysfunction that can reach the point of complete organ failure. Blockade of digestive enzymes in the lumen of the intestine in experimental forms of shock serves to reduce breakdown of the mucosal barrier and autodigestion of the intestine, organ dysfunctions and mortality.