Autophagy in Intestinal Injury Caused by Severe Acute Pancreatitis

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Autophagy in Intestinal Injury Caused by Severe Acute Pancreatitis

Severe acute pancreatitis (SAP) is a life-threatening condition characterized by significant morbidity and mortality. It is frequently associated with systemic inflammatory response syndrome, sepsis, and multiple organ dysfunction. A critical aspect of SAP is the dysfunction of the intestinal barrier, which leads to bacterial translocation (BT) and subsequent systemic inflammation and sepsis. Recent research has highlighted the pivotal role of autophagy in maintaining intestinal homeostasis and protecting the intestinal mucosal barrier during SAP. Autophagy, a conserved catabolic process in eukaryotic organisms, degrades cytoplasmic content, removes damaged organelles, and eliminates invading microorganisms, thereby contributing to cellular homeostasis under stress conditions such as starvation, hypoxia, infection, and endoplasmic reticulum stress. This review explores the mechanisms by which autophagy influences intestinal injury in SAP and its potential as a therapeutic target.

Autophagy: An Overview

Autophagy is broadly categorized into three types: macroautophagy, microautophagy, and chaperone-mediated autophagy (CMA). CMA, specific to mammals, involves the selective degradation of proteins. The process of autophagy is divided into two stages: signal transmission and execution. Signal transmission involves molecular switches such as protein kinase A, mitogen-activated protein kinase, and mammalian target of rapamycin (mTOR), which regulate autophagy induction or inhibition. The execution stage includes initiation, nucleation, extension, closure, and cycling, mediated by complexes such as Unc-51 like autophagy activating kinase 1, BECLIN1-PtdIns3KC3-ATG14L, and Autophagy protein 12 (Atg12)-Atg5.

The Intestinal Mucosal Barrier in SAP

The intestinal mucosal barrier is a critical defense mechanism that prevents the translocation of luminal bacteria and toxins into the systemic circulation. It comprises biological barriers (intestinal microbiota), immune barriers, and mechanical barriers (intestinal epithelial cells, gap junctions, and tight junctions). Tight junctions (TJs) act as gates between cells, preventing the passage of hydrophilic molecules, while gap junctions facilitate intercellular communication. In SAP, the intestinal barrier is compromised due to microcirculation disorders, hypovolemia, ischemia-reperfusion injury, and the generation of reactive oxygen species (ROS). These factors lead to increased intestinal permeability, allowing bacteria and endotoxins to enter the bloodstream, triggering systemic inflammation and secondary infections.

Autophagy in Intestinal Mucosal Homeostasis

Autophagy plays a multifaceted role in maintaining intestinal homeostasis and protecting the mucosal barrier during SAP. Its functions include:

  1. Elimination of Invading Microorganisms and Toxins: Autophagy is activated during the uptake of bacteria by host cells or macrophages. It contributes to antibiosis by controlling the dissemination of pathogens such as Listeria monocytogenes, Mycobacterium tuberculosis, and Salmonella. Additionally, autophagy facilitates antigen presentation by binding endogenous antigens to major histocompatibility complex-II molecules, recognized by CD4+ T cells. It also senses viral RNA and DNA, enhancing intestinal resistance to viral infections.

  2. Protection of Tight Junctions and Gap Junctions: Autophagy enhances the barrier function of TJs by promoting the lysosomal degradation of Claudin-2, a cation-selective pore-forming protein. It also degrades abnormal TJ proteins, preventing the release of intestinal toxins and pro-inflammatory cytokines. However, excessive autophagy can lead to the destruction of TJ proteins, resulting in apoptosis.

  3. Maintenance of Paneth and Goblet Cell Function: Autophagy supports the secretory function of Paneth cells, which are essential for antimicrobial defense. Autophagy-related genes such as ATG16L1 influence Paneth cell function. Similarly, autophagy regulates the development and function of goblet cells, with ATG16L1 polymorphisms altering goblet cell morphology. Autophagy deficiency in goblet cells reduces mucin production by affecting ROS generation and calcium release from the endoplasmic reticulum.

  4. Immune Response Regulation: Autophagy modulates both innate and adaptive immune responses in the intestine. It affects dendritic cells, T cells, B cells, and natural killer cells, influencing antigen presentation, cytokine secretion, and antimicrobial peptide production. Reduced autophagy levels lead to decreased antigen sampling and interleukin-10 secretion, promoting the overgrowth of intestinal bacteria and increasing the risk of BT.

  5. ROS and Inflammation Regulation: Autophagy, particularly mitophagy, limits ROS accumulation by eliminating damaged mitochondria. Autophagy deficiency increases ROS levels, contributing to intestinal mucosal injury. Autophagy also modulates cytokine-induced programmed cell death, limiting intestinal inflammation. High-mobility group box-1 (HMGB1), a key inflammatory mediator in SAP, is regulated by autophagy, which helps maintain the internal environment by adjusting oxidative stress.

  6. Antifibrotic Effects: Autophagy promotes the degradation of fibroblast collagen, exerting antifibrotic effects. Inhibition of autophagy exacerbates fibrosis, while autophagy stimulation may inhibit intestinal fibrosis by modulating innate immune responses and mesenchymal activity.

  7. Intestinal Stem Cell Regeneration: Autophagy is crucial for the maintenance and regeneration of intestinal stem cells (ISCs). Deletion of autophagy-related genes such as Atg5 and Atg7 impairs ISC function, leading to the accumulation of mitochondria and ROS, and reduced regenerative capacity. Autophagy supports ISC maintenance and promotes the recovery of intestinal epithelial cells after injury.

Therapeutic Potential of Autophagy Modulation in SAP

Given the critical role of autophagy in intestinal homeostasis and its involvement in SAP, researchers are exploring autophagy modulation as a therapeutic strategy. Several compounds have shown promise in preclinical studies:

  1. Trehalose: This disaccharide enhances autophagy efficiency, reducing pancreatic injury and SAP severity in animal models. It holds potential as a therapeutic agent in SAP.

  2. Chloroquine (CQ): CQ and its derivatives inhibit autophagy and have been used to mitigate inflammation in models of colitis. However, their application in SAP requires further investigation.

  3. Glutamine: Glutamine enhances autophagy in intestinal epithelial cells under stress conditions by regulating mTOR and mitogen-activated protein kinase/p38 pathways, limiting cellular apoptosis.

  4. Bone Marrow-Derived Mesenchymal Stem Cells: These cells suppress autophagy in multiple organs, including the pancreas, small intestine, and lungs, protecting against SAP-induced multiple-organ injury.

Conclusion

Autophagy plays a dual role in intestinal injury caused by SAP, acting as both a protective mechanism and a potential contributor to pathology. Its involvement in maintaining intestinal homeostasis, regulating immune responses, and mitigating oxidative stress underscores its importance in SAP. However, the precise mechanisms of autophagy in SAP-induced intestinal injury remain incompletely understood. Further research is needed to elucidate these mechanisms and develop targeted therapies that harness the protective effects of autophagy while minimizing its potential detrimental impacts. The exploration of autophagy modulators such as trehalose, chloroquine, glutamine, and mesenchymal stem cells offers promising avenues for the treatment of SAP and its associated complications.

doi.org/10.1097/CM9.0000000000001594

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