Autophagy in the intestinal epithelium reduces endotoxin-induced inflammatory responses by inhibiting NF-κB activation

Autophagy is a lysosomal degradation pathway that is essential for survival, differentiation, development and homeostasis. There is growing evidence that impaired autophagy leads to the pathogenesis of diverse diseases. However, the role of autophagy in intestinal epithelium is not clearly understood, although previous studies have pointed out the possibility for the relationships of autophagy with bowel inflammation. In this study, we investigated the involvement of autophagy in intestinal epithelium with inflammatory responses. We generated the mice with a conditional deletion of Atg7, which is one of the autophagy regulated gene, in intestinal epithelium. In Atg7-deficient small intestinal epithelium, LPS-induced production of TNF-α and IL-1β mRNA was enhanced in comparison to the control small intestinal tissues. In addition, the degree of LPS-induced activation of NF-κB was promoted in Atg7-deficient intestinal epithelium. These results demonstrate that autophagy can attenuate endotoxin-induced inflammatory responses in intestinal epithelium resulting in the maintenance of intestinal homeostasis.     

Human apolipoprotein L1 (ApoL1) in cancer and chronic kidney disease

Human apolipoprotein L1 (ApoL1) possesses both extra- and intra-cellular functions crucial in host defense and cellular homeostatic mechanisms. Alterations in ApoL1 function due to genetic, environmental, and lifestyle factors have been associated with African sleeping sickness, atherosclerosis, lipid disorders, obesity, schizophrenia, cancer, and chronic kidney disease (CKD). Importantly, two alleles of APOL1 carrying three coding-sequence variants have been linked to CKD, particularly in Sub-Saharan Africans and African Americans. Intracellularly, elevated ApoL1 can induce autophagy and autophagy-associated cell death, which may be critical in the maintenance of cellular homeostasis in the kidney. Similarly, ApoL1 may protect kidney cells against renal cell carcinoma (RCC). We summarize the role of ApoL1 in RCC and CKD, highlighting the critical function of ApoL1 in autophagy.     

The roles and functions of Paneth cells in Crohn’s disease: A critical review

Paneth cells (PCs) are located at the base of small intestinal crypts and secrete the α‐defensins, human α‐defensin 5 (HD‐5) and human α‐defensin 6 (HD‐6) in response to bacterial, cholinergic and other stimuli. The α‐defensins are broad‐spectrum microbicides that play critical roles in controlling gut microbiota and maintaining intestinal homeostasis. Inflammatory bowel disease, including ulcerative colitis and Crohn's disease (CD), is a complicated autoimmune disorder. The pathogenesis of CD involves genetic factors, environmental factors and microflora. Surprisingly, with regard to genetic factors, many susceptible genes and pathogenic pathways of CD, including nucleotide‐binding oligomerization domain 2 (NOD2), autophagy‐related 16‐like 1 (ATG16L1), immunity‐related guanosine triphosphatase family M (IRGM), wingless‐related integration site (Wnt), leucine‐rich repeat kinase 2 (LRRK2), histone deacetylases (HDACs), caspase‐8 (Casp8) and X‐box‐binding protein‐1 (XBP1), are relevant to PCs. As the underlying mechanisms are being unravelled, PCs are identified as the central element of CD pathogenesis, integrating factors among microbiota, intestinal epithelial barrier dysfunction and the immune system. In the present review, we demonstrate how these genes and pathways regulate CD pathogenesis via their action on PCs and what treatment modalities can be applied to deal with these PC‐mediated pathogenic processes.     

Molecular genetics of human obesity: A comprehensive review

Obesity and its related health complications is a major problem worldwide. Hypothalamus and their signalling molecules play a critical role in the intervening and coordination with energy balance and homeostasis. Genetic factors play a crucial role in determining an individual's predisposition to the weight gain and being obese. In the past few years, several genetic variants were identified as monogenic forms of human obesity having success over common polygenic forms. In the context of molecular genetics, genome-wide association studies (GWAS) approach and their findings signified a number of genetic variants predisposing to obesity. However, the last couple of years, it has also been noticed that alterations in the environmental and epigenetic factors are one of the key causes of obesity. Hence, this review might be helpful in the current scenario of molecular genetics of human obesity, obesity-related health complications (ORHC), and energy homeostasis. Future work based on the clinical discoveries may play a role in the molecular dissection of genetic approaches to find more obesity-susceptible gene loci.     

Tumor suppression by autophagy through the management of metabolic stress

Autophagy plays a critical protective role maintaining energy homeostasis and protein and organelle quality control. These functions are particularly important in times of metabolic stress and in cells with high energy demand such as cancer cells. In emerging cancer cells, autophagy defect may cause failure of energy homeostasis and protein and organelle quality control, leading to the accumulation of cellular damage in metabolic stress. Some manifestations of this damage, such as activation of the DNA damage response and generation of genome instability may promote tumor initiation and drive cell-autonomous tumor progression. In addition, in solid tumors, autophagy localizes to regions that are metabolically stressed. Defects in autophagy impair the survival of tumor cells in these areas, which is associated with increased cell death and inflammation. The cytokine response from inflammation may promote tumor growth and accelerate cell non-autonomous tumor progression. The overreaching theme is that autophagy protects cells from damage accumulation under conditions of metabolic stress allowing efficient tolerance and recovery from stress, and that this is a critical and novel tumor suppression mechanism. The challenge now is to define the precise aspects of autophagy, including energy homeostasis and protein and organelle turnover, that are required for the proper management of metabolic stress that suppress tumorigenesis. Furthermore, we need to be able to identify human tumors with deficient autophagy, and to develop rational cancer therapies that take advantage of the altered metabolic state and stress responses inherent to this autophagy defect.     

Autophagy proteins control goblet cell function by potentiating reactive oxygen species production

Delivery of granule contents to epithelial surfaces by secretory cells is a critical physiologic process. In the intestine, goblet cells secrete mucus that is required for homeostasis. Autophagy proteins are required for secretion in some cases, though the mechanism and cell biological basis for this requirement remain unknown. We found that in colonic goblet cells, proteins involved in initiation and elongation of autophagosomes were required for efficient mucus secretion. The autophagy protein LC3 localized to intracellular multi‐vesicular vacuoles that were consistent with a fusion of autophagosomes and endosomes. Using cultured intestinal epithelial cells, we found that NADPH oxidases localized to and enhanced the formation of these LC3‐positive vacuoles. Both autophagy proteins and endosome formation were required for maximal production of reactive oxygen species (ROS) derived from NADPH oxidases. Importantly, generation of ROS was critical to control mucin granule accumulation in colonic goblet cells. Thus, autophagy proteins can control secretory function through ROS, which is in part generated by LC3‐positive vacuole‐associated NADPH oxidases. These findings provide a novel mechanism by which autophagy proteins can control secretion.     

Tumor suppression by autophagy through the management of metabolic stress

Autophagy plays a critical protective role maintaining energy homeostasis and protein and organelle quality control. These functions are particularly important in times of metabolic stress and in cells with high energy demand such as cancer cells. In emerging cancer cells, autophagy defect may cause failure of energy homeostasis and protein and organelle quality control, leading to the accumulation of cellular damage in metabolic stress. Some manifestations of this damage, such as activation of the DNA damage response and generation of genome instability may promote tumor initiation and drive cell-autonomous tumor progression. In addition, in solid tumors, autophagy localizes to regions that are metabolically stressed. Defects in autophagy impair the survival of tumor cells in these areas, which is associated with increased cell death and inflammation. The cytokine response from inflammation may promote tumor growth and accelerate cell non-autonomous tumor progression. The overreaching theme is that autophagy protects cells from damage accumulation under conditions of metabolic stress allowing efficient tolerance and recovery from stress, and that this is a critical and novel tumor suppression mechanism. The challenge now is to define the precise aspects of autophagy, including energy homeostasis, and protein and organelle turnover, that are required for the proper management of metabolic stress that suppress tumorigenesis. Furthermore, we need to be able to identify human tumors with deficient autophagy, and to develop rational cancer therapies that take advantage of the altered metabolic state and stress responses inherent to this autophagy defect.     

Autophagy in the intestinal epithelium regulates Citrobacter rodentium infection

Autophagy, a ubiquitous degradation pathway, is important for the survival and homeostasis of cells. Previous studies have demonstrated the role of autophagy in host defense against bacterial infection, but the importance of autophagy in the intestinal epithelium for the regulation of bacterial infection has not been fully elucidated. In this study, we showed that the essential autophagy protein Atg7 is required for resistance to Citrobacter rodentium infection in the intestinal epithelium. Infected mice in which Atg7 had been conditionally deleted from the intestinal epithelium exhibited greater clinical evidence of disease and higher expression levels of pro-inflammatory cytokine mRNA in the large intestine. Moreover, C. rodentium clearance was reduced in the Atg7 conditional knockout mice. These results demonstrate that autophagy in intestinal epithelial cells plays an important role in host defense against C. rodentium infection and the regulation of C. rodentium infectious colitis.     

Genome-wide association scanning highlights two autophagy genes, ATG16L1 and IRGM, as being significantly associated with Crohn's disease

The era of genome-wide association (GWA) scanning has shed new light on the genetic basis of common disease and nowhere is this better illustrated than Crohn's disease (CD). CD is a chronic debilitating inflammatory bowel disease characterized by stricturing and fistula formation. Mainstays of current therapy are immune suppression and surgery. The pathogenesis of CD is poorly understood, but it has long been recognized that both genetic susceptibility and bacterial antigens play important roles. A variety of intracellular bacteria have been postulated to trigger CD, but the evidence for any one organism is equivocal. The current consensus is that commensal gut bacteria provide the drive for CD-related inflammation. Three GWA scans undertaken in the last 6 months have identified 10 new loci demonstrating highly significant and replicated association with CD. Two of the strongest hits implicate genes IRGM and ATG16L1, which encode proteins thought to be critical to the autophagy pathway. The critical next step is functional characterization of the CD-associated genetic variants in IRGM and ATG16L. It seems highly plausible that variation in these genes holds the key to understanding exactly which bacteria drive the intestinal inflammation of CD and the mechanism by which they do this.     

Autophagosomes contribute to intracellular lipid distribution in enterocytes

Enterocytes, the intestinal absorptive cells, have to deal with massive alimentary lipids upon food consumption. They orchestrate complex lipid-trafficking events that lead to the secretion of triglyceride-rich lipoproteins and/or the intracellular transient storage of lipids as lipid droplets (LDs). LDs originate from the endoplasmic reticulum (ER) membrane and are mainly composed of a triglyceride (TG) and cholesterol-ester core surrounded by a phospholipid and cholesterol monolayer and specific coat proteins. The pivotal role of LDs in cellular lipid homeostasis is clearly established, but processes regulating LD dynamics in enterocytes are poorly understood. Here we show that delivery of alimentary lipid micelles to polarized human enterocytes induces an immediate autophagic response, accompanied by phosphatidylinositol-3-phosphate appearance at the ER membrane. We observe a specific and rapid capture of newly synthesized LD at the ER membrane by nascent autophagosomal structures. By combining pharmacological and genetic approaches, we demonstrate that autophagy is a key player in TG targeting to lysosomes. Our results highlight the yet-unraveled role of autophagy in the regulation of TG distribution, trafficking, and turnover in human enterocytes.     

Autophagy in major human diseases

Autophagy is a core molecular pathway for the preservation of cellular and organismal homeostasis. Pharmacological and genetic interventions impairing autophagy responses promote or aggravate disease in a plethora of experimental models. Consistently, mutations in autophagy‐related processes cause severe human pathologies. Here, we review and discuss preclinical data linking autophagy dysfunction to the pathogenesis of major human disorders including cancer as well as cardiovascular, neurodegenerative, metabolic, pulmonary, renal, infectious, musculoskeletal, and ocular disorders.     

Lifespan extension by preserving proliferative homeostasis in Drosophila

Regenerative processes are critical to maintain tissue homeostasis in high-turnover tissues. At the same time, proliferation of stem and progenitor cells has to be carefully controlled to prevent hyper-proliferative diseases. Mechanisms that ensure this balance, thus promoting proliferative homeostasis, are expected to be critical for longevity in metazoans. The intestinal epithelium of Drosophila provides an accessible model in which to test this prediction. In aging flies, the intestinal epithelium degenerates due to over-proliferation of intestinal stem cells (ISCs) and mis-differentiation of ISC daughter cells, resulting in intestinal dysplasia. Here we show that conditions that impair tissue renewal lead to lifespan shortening, whereas genetic manipulations that improve proliferative homeostasis extend lifespan. These include reduced Insulin/IGF or Jun-N-terminal Kinase (JNK) signaling activities, as well as over-expression of stress-protective genes in somatic stem cell lineages. Interestingly, proliferative activity in aging intestinal epithelia correlates with longevity over a range of genotypes, with maximal lifespan when intestinal proliferation is reduced but not completely inhibited. Our results highlight the importance of the balance between regenerative processes and strategies to prevent hyperproliferative disorders and demonstrate that promoting proliferative homeostasis in aging metazoans is a viable strategy to extend lifespan.     

Shuttling of information between the mucosal and luminal environment drives intestinal homeostasis

The gastrointestinal tract is a passageway for dietary nutrients, microorganisms and xenobiotics. The gut is home to diverse bacterial communities forming the microbiota. While bacteria and their metabolites maintain gut homeostasis, the host uses innate and adaptive immune mechanisms to cope with the microbiota and luminal environment. In recent years, multiple bi-directional instructive mechanisms between microbiota, luminal content and mucosal immune systems have been uncovered. Indeed, epithelial and immune cell-derived mucosal signals shape microbiota composition, while microbiota and their by-products shape the mucosal immune system. Genetic and environmental perturbations alter gut mucosal responses which impact on microbial ecology structures. On the other hand, changes in microbiota alter intestinal mucosal responses. In this review, we discuss how intestinal epithelial Paneth and goblet cells interact with the microbiota, how environmental and genetic disorders are sensed by endoplasmic reticulum stress and autophagy responses, how specific bacteria, bacterial- and diet-derived products determine the function and activation of the mucosal immune system. We will also discuss the critical role of HDAC activity as a regulator of immune and epithelial cell homeostatic responses.     

Lipid droplet–mediated ER homeostasis regulates autophagy and cell survival during starvation

Lipid droplets (LDs) are conserved organelles for intracellular neutral lipid storage. Recent studies suggest that LDs function as direct lipid sources for autophagy, a central catabolic process in homeostasis and stress response. Here, we demonstrate that LDs are dispensable as a membrane source for autophagy, but fulfill critical functions for endoplasmic reticulum (ER) homeostasis linked to autophagy regulation. In the absence of LDs, yeast cells display alterations in their phospholipid composition and fail to buffer de novo fatty acid (FA) synthesis causing chronic stress and morphologic changes in the ER. These defects compromise regulation of autophagy, including formation of multiple aberrant Atg8 puncta and drastically impaired autophagosome biogenesis, leading to severe defects in nutrient stress survival. Importantly, metabolically corrected phospholipid composition and improved FA resistance of LD-deficient cells cure autophagy and cell survival. Together, our findings provide novel insight into the complex interrelation between LD-mediated lipid homeostasis and the regulation of autophagy potentially relevant for neurodegenerative and metabolic diseases.     

The role of short-chain fatty acids in orchestrating two types of programmed cell death in colon cancer

Autophagy serves a critical function in cellular homeostasis by prolonging survival during nutrient deprivation. Although primarily characterized as a cell survival mechanism, the relationship between autophagy and cell death pathways remains incompletely understood. Autophagy has heretofore not been studied in the context of human pulmonary disease. We have recently observed increased morphological and biochemical markers of autophagy in human lung tissue from patients with chronic obstructive pulmonary disease (COPD). Similar observations of increased autophagy were also made in mouse lung tissue subjected to chronic cigarette smoke exposure, a primary causative agent in COPD, and in pulmonary cells exposed to aqueous cigarette smoke extract. Since knockdown of autophagic regulator proteins inhibited apoptosis in response to cigarette smoke exposure in vitro, we concluded that increased autophagy was associated with increased cell death in this model. We hypothesize that increased autophagy contributes to COPD pathogenesis by promoting epithelial cell death. Further research will examine whether autophagy plays a causative, correlative, or protective role in specific lung pathologies.     

Interplay Between Autophagy and Zinc

Autophagy is a conserved catabolic process that plays an important role in cellular homeostasis. The study of the interplay between autophagy and zinc has gained interest over the last years. Multiple studies have indicated that zinc stimulates autophagy and is critical for basal and induced autophagy in mammalian cells. Conversely, autophagy is induced by zinc starvation in yeast. There are no studies analyzing the role of zinc in either Microautophagy or Chaperone-Mediated-Autophagy. The mechanisms by which zinc modulates autophagy are still poorly understood. Studies examining loss of function of genes involved in cellular zinc homeostasis have provided novel insights into the role of zinc in autophagy. Autophagy may help cells adapt to changes in zinc availability in medium by controlling zinc mobilization, recycling, and secretion. Zinc is a key player in toxic and protective autophagy.     

The c-Jun N-terminal Kinase (JNK)-binding Protein (JNKBP1) Acts as a Negative Regulator of NOD2 Protein Signaling by Inhibiting Its Oligomerization Process

NOD2 is one of the best characterized members of the cytosolic NOD-like receptor family. NOD2 is able to sense muramyl dipeptide, a specific bacterial cell wall component, and to subsequently induce various signaling pathways leading to NF-κB activation and autophagy, both events contributing to an efficient innate and adaptive immune response. Interestingly, loss-of-function NOD2 variants were associated with a higher susceptibility for Crohn disease, which highlights the physiological importance of proper regulation of NOD2 activity. We performed a biochemical screen to search for new NOD2 regulators. We identified a new NOD2 partner, c-Jun N-terminal kinase-binding protein 1 (JNKBP1), a scaffold protein characterized by an N-terminal WD-40 domain. JNKBP1, through its WD-40 domain, binds to NOD2 following muramyl dipeptide activation. This interaction attenuates NOD2-mediated NF-κB activation and IL-8 secretion as well as NOD2 antibacterial activity. JNKBP1 exerts its repressor effect by disturbing NOD2 oligomerization and RIP2 tyrosine phosphorylation, both steps required for downstream NOD2 signaling. We furthermore showed that JNKBP1 and NOD2 are co-expressed in the human intestinal epithelium and in immune cells recruited in the lamina propria, which suggests that JNKBP1 contributes to maintain NOD2-mediated intestinal immune homeostasis.     

Autophagy protects intestinal epithelial Cells against Deoxynivalenol toxicity by alleviating oxidative stress via IKK signaling pathway

Autophagy is an intracellular process of homeostatic degradation that promotes cell survival under various stressors. Deoxynivalenol (DON), a fungal toxin, often causes diarrhea and disturbs the homeostasis of the intestinal system. To investigate the function of intestinal autophagy in response to DON and associated mechanisms, we firstly knocked out ATG5 (autophagy-related gene 5) in porcine intestinal epithelial cells (IPEC-J2) using CRISPR-Cas9 technology. When treated with DON, autophagy was induced in IPEC-J2 cells but not in IPEC-J2.Atg5ko cells. The deficiency in autophagy increased DON-induced apoptosis in IPEC-J2.atg5ko cells, in part, through the generation of reactive oxygen species (ROS). The cellular stress response can be restored in IPEC-J2.atg5ko cells by overexpressing proteins involved in protein folding. Interestingly, we found that autophagy deficiency downregulated the expression of endoplasmic reticulum folding proteins BiP and PDI when IPEC-J2.atg5ko cells were treated with DON. In addition, we investigated the molecular mechanism of autophagy involved in the IKK, AMPK, and mTOR signaling pathway and found that Bay-117082 and Compound C, specific inhibitors for IKK and AMPK, respectively, inhibited the induction of autophagy. Taken together, our results suggest that autophagy is pivotal for protection against DON in pig intestinal cells.     

脂肪组织中自噬信号通路及其功能

细胞自噬是一种真核生物中高度保守的代谢过程,包括巨自噬、微自噬以及分子伴侣介导的自噬等。自噬过程可以清除受损的细胞器,降解糖原、脂类和蛋白质等生物大分子物质,供细胞重新利用,维持细胞内代谢平衡。自噬障碍与多种疾病的病理发生过程息息相关,包括肿瘤、2型糖尿病、肥胖、骨骼肌病以及神经退行性疾病等。脂肪组织是人体脂质储存的重要场所,广泛分布于全身各处,如内脏和皮下等。脂肪组织通过储存冗余脂肪并分泌脂肪因子,防止脂肪的异位堆积和脂毒性的发生,维持机体的脂质稳态。近期的许多研究表明,自噬进程深度参与脂肪细胞的细胞分化与能量代谢。因此,深入探究脂肪组织自噬过程与机体脂质稳态的调控关系,有利于揭示机体脂质平衡的内在机制,为新型药物靶点的开发提供扎实的理论依据和数据支持。本文就近年来关于自噬影响脂肪组织脂质代谢的最新研究进展作一综述。