Characterization of cytoplasmic lipid droplets in each region of the small intestine of lean and diet-induced obese mice in response to dietary fat

GLP-2 regulation of intestinal lipid handling

Lipid handling in the intestine is important for maintaining energy homeostasis and overall health. Mishandling of lipids in the intestine contributes to dyslipidemia and atherosclerotic cardiovascular diseases. Despite advances in this field over the past few decades, significant gaps remain. The gut hormone glucagon-like peptide-2 (GLP-2) has been shown to play pleotropic roles in the regulation of lipid handling in the intestine. Of note, GLP-2 exhibits unique actions on post-prandial lipid absorption and post-absorptive release of intestinally stored lipids. This review aims to summarize current knowledge in how GLP-2 regulates lipid processing in the intestine. Elucidating the mechanisms of GLP-2 regulation of intestinal lipid handling not only improves our understanding of GLP-2 biology, but also provides insights into how lipids are processed in the intestine, which offers opportunities for developing novel strategies towards prevention and treatment of dyslipidemia and atherosclerotic cardiovascular diseases.

An early-life microbiota metabolite protects against obesity by regulating intestinal lipid metabolism

The mechanisms by which the early-life microbiota protects against environmental factors that promote childhood obesity remain largely unknown. Using a mouse model in which young mice are simultaneously exposed to antibiotics and a high-fat (HF) diet, we show that Lactobacillus species, predominant members of the small intestine (SI) microbiota, regulate intestinal epithelial cells (IECs) to limit diet-induced obesity during early life. A Lactobacillus-derived metabolite, phenyllactic acid (PLA), protects against metabolic dysfunction caused by early-life exposure to antibiotics and a HF diet by increasing the abundance of peroxisome proliferator-activated receptor γ (PPAR-γ) in SI IECs. Therefore, PLA is a microbiota-derived metabolite that activates protective pathways in the small intestinal epithelium to regulate intestinal lipid metabolism and prevent antibiotic-associated obesity during early life.

Glucagon-like peptide-2 acutely enhances chylomicron secretion in humans without mobilizing cytoplasmic lipid droplets

CONTEXT

A portion of ingested fats are retained in the intestine for many hours before they are mobilized and secreted in chylomicron (CM) particles. Factors like glucagon-like peptide-2 (GLP-2) and glucose can mobilize these stored intestinal lipids and enhance CM secretion. We have recently demonstrated in rodents that GLP-2 acutely enhances CM secretion by mechanisms that do not involve the canonical CM synthetic assembly and secretory pathways.

OBJECTIVE

To further investigate the mechanism of GLP-2's potent intestinal lipid mobilizing effect, we examined intracellular cytoplasmic lipid droplets (CLDs) in intestinal biopsies of humans administered GLP-2 or placebo.

DESIGN, SETTING, PATIENTS, AND INTERVENTIONS

A single dose of placebo or GLP-2 was administered subcutaneously five hours after ingesting a high-fat bolus. In one subset of participants, plasma samples were collected to quantify lipid and lipoprotein concentrations for 3 hours post-placebo or GLP-2. In another subset, a duodenal biopsy was obtained one-hour post-placebo or GLP-2 administration for transmission electron microscopy (TEM) and proteomic analysis.

RESULTS

GLP-2 significantly increased plasma-TG by 46% (P = 0.009), mainly in CM-sized particles (CM-TG) by 133% (P = 0.003), without reducing duodenal CLD size or number. Several proteins of interest were identified that require further investigation to elucidate their potential role in GLP-2-mediated CM secretion.

CONCLUSIONS

Unlike glucose that mobilizes enterocyte CLDs and enhances CM secretion, GLP-2 acutely increased plasma CMs without significant mobilization of CLDs, supporting our previous findings that GLP-2 does not act directly on enterocytes to enhance CM secretion and most likely mobilizes secreted CMs in the lamina propria and lymphatics.

Lactic acid bacteria-derived γ-linolenic acid metabolites are PPARδ ligands that reduce lipid accumulation in human intestinal organoids

Gut microbiota regulate physiological functions in various hosts, such as energy metabolism and immunity. Lactic acid bacteria, including Lactobacillus plantarum, have a specific polyunsaturated fatty acid saturation metabolism that generates multiple fatty acid species, such as hydroxy fatty acids, oxo fatty acids, conjugated fatty acids, and trans-fatty acids. How these bacterial metabolites impact host physiology is not fully understood. Here, we investigated the ligand activity of lactic acid bacteria–produced fatty acids in relation to nuclear hormone receptors expressed in the small intestine. Our reporter assays revealed two bacterial metabolites of γ-linolenic acid (GLA), 13-hydroxy-cis-6,cis-9-octadecadienoic acid (γHYD), and 13-oxo-cis-6,cis-9-octadecadienoic acid (γKetoD) activated peroxisome proliferator-activated receptor delta (PPARδ) more potently than GLA. We demonstrate that both γHYD and γKetoD bound directly to the ligand-binding domain of human PPARδ. A docking simulation indicated that four polar residues (T289, H323, H449, and Y473) of PPARδ donate hydrogen bonds to these fatty acids. Interestingly, T289 does not donate a hydrogen bond to GLA, suggesting that bacterial modification of GLA introducing hydroxy and oxo group determines ligand selectivity. In human intestinal organoids, we determined γHYD and γKetoD increased the expression of PPARδ target genes, enhanced fatty acid β-oxidation, and reduced intracellular triglyceride accumulation. These findings suggest that γHYD and γKetoD, which gut lactic acid bacteria could generate, are naturally occurring PPARδ ligands in the intestinal tract and may improve lipid metabolism in the human intestine.

Reovirus uses temporospatial compartmentalization to orchestrate core versus outercapsid assembly

Reoviridae virus family members, such as mammalian orthoreovirus (reovirus), encounter a unique challenge during replication. To hide the dsRNA from host recognition, the genome remains encapsidated in transcriptionally active proteinaceous core capsids that transcribe and release +RNA. De novo +RNAs and core proteins must repeatedly assemble into new progeny cores in order to logarithmically amplify replication. Reoviruses also produce outercapsid (OC) proteins μ1, σ3 and σ1 that assemble onto cores to create highly stable infectious full virions. Current models of reovirus replication position amplification of transcriptionally-active cores and assembly of infectious virions in shared factories, but we hypothesized that since assembly of OC proteins would halt core amplification, OC assembly is somehow regulated. Kinetic analysis of virus +RNA production, core versus OC protein expression, and core particles versus whole virus particle accumulation, indicated that assembly of OC proteins onto core particles was temporally delayed. All viral RNAs and proteins were made simultaneously, eliminating the possibility that delayed OC RNAs or proteins account for delayed OC assembly. High resolution fluorescence and electron microscopy revealed that core amplification occurred early during infection at peripheral core-only factories, while all OC proteins associated with lipid droplets (LDs) that coalesced near the nucleus in a μ1–dependent manner. Core-only factories transitioned towards the nucleus despite cycloheximide-mediated halting of new protein expression, while new core-only factories developed in the periphery. As infection progressed, OC assembly occurred at LD-and nuclear-proximal factories. Silencing of OC μ1 expression with siRNAs led to large factories that remained further from the nucleus, implicating μ1 in the transition to perinuclear factories. Moreover, late during infection, +RNA pools largely contributed to the production of de-novo viral proteins and fully-assembled infectious viruses. Altogether the results suggest an advanced model of reovirus replication with spatiotemporal segregation of core amplification, OC complexes and fully assembled virions.

It is important to understand how viruses replicate and assemble to discover antiviral therapies and to modify viruses for applications like gene therapy or cancer therapy. Reovirus is a harmless virus being tested as a cancer therapy. Reovirus has two coats of proteins, an inner coat and an outer coat. To replicate, reovirus particles need only the inner coat, but to become infectious they require the outer coat. Strangely, inner and outer coat proteins are all made by the virus at once, so it was unknown what determines whether newly made viruses will contain just the inner coat to continue to replicate, or both coats to transmit to new hosts. Our experiments reveal that the inner coat proteins are located in a different area of an infected cell versus the outer coat proteins. The location therefore determines if the newly made viruses contain just the inner coat versus both coats. Reoviruses have evolved extravagant mechanisms to be able to efficiently take on the best composition required for replication and transmission.

Adaptation to short-term extreme fat consumption alters intestinal lipid handling in male and female mice

The small intestine is a highly adaptable organ serving as both a barrier to the external environment and a conduit for nutrient absorption. Enterocytes package dietary triglycerides (TG) into chylomicrons for transport into circulation; the remaining TGs are stored in cytosolic lipid droplets (CLDs). The current study aimed to characterize the impact of diet composition on intestinal lipid handling in male and female wild-type mice. Mice were continued on their grain-based diet (GBD) and switched to a high-fat, high cholesterol Western-style diet (WD) or a ketogenic diet (KD) for 3 or 5 weeks. KD-fed mice displayed significantly higher plasma TG levels in response to an olive oil gavage than WD- and GBD-fed mice; TG levels were ~2-fold higher in male KD-fed mice than female KD-fed mice. Poloxamer-407 experiments revealed enhanced intestinal-TG secretion rates in male mice fed a KD upon olive oil gavage, whereas secretion rates were unchanged in female mice. Surprisingly, jejunal CLD size and TG mass after oil gavage were similar among the groups. At fasting, TG mass was significantly higher in the jejunum of male KD-fed mice and the duodenum of female KD-fed mice, providing increased substrate for chylomicron formation. In addition to greater fasting intestinal TG stores, KD-fed male mice displayed longer small intestinal lengths, while female mice displayed markedly longer jejunal villi lengths. After 5 week of diet, 12 h fasting-2 h refeeding experiments revealed jejunal TG levels were similar between diet groups in male mice; however, in female mice, jejunal TG mass was significantly higher in KD-fed mice compared to GBD- and WD-fed mice. These experiments reveal that KD feeding promotes distinct morphological and functional changes to the small intestine compared to the WD diet. Moreover, changes to intestinal lipid handling in response to carbohydrate and protein restriction manifest differently in male and female mice.

Recent advances in cytoplasmic lipid droplet metabolism in intestinal enterocyte

Processing of dietary fats in the intestine is a highly regulated process that influences whole-body energy homeostasis and multiple physiological functions. Dysregulated lipid handling in the intestine leads to dyslipidemia and atherosclerotic cardiovascular disease. In intestinal enterocytes, lipids are incorporated into lipoproteins and cytoplasmic lipid droplets (CLDs). Lipoprotein synthesis and CLD metabolism are inter-connected pathways with multiple points of regulation. This review aims to highlight recent advances in the regulatory mechanisms of lipid processing in the enterocyte, with particular focus on CLDs. In-depth understanding of the regulation of lipid metabolism in the enterocyte may help identify therapeutic targets for the treatment and prevention of metabolic disorders.

Mitochondrial function in intestinal epithelium homeostasis and modulation in diet-induced obesity

Background

Systemic low-grade inflammation observed in diet-induced obesity has been associated with dysbiosis and disturbance of intestinal homeostasis. This latter relies on an efficient epithelial barrier and coordinated intestinal epithelial cell (IEC) renewal that are supported by their mitochondrial function. However, IEC mitochondrial function might be impaired by high fat diet (HFD) consumption, notably through gut-derived metabolite production and fatty acids, that may act as metabolic perturbators of IEC.

Scope of review

This review presents the current general knowledge on mitochondria, before focusing on IEC mitochondrial function and its role in the control of intestinal homeostasis, and featuring the known effects of nutrients and metabolites, originating from the diet or gut bacterial metabolism, on IEC mitochondrial function. It then summarizes the impact of HFD on mitochondrial function in IEC of both small intestine and colon and discusses the possible link between mitochondrial dysfunction and altered intestinal homeostasis in diet-induced obesity.

Major conclusions

HFD consumption provokes a metabolic shift toward fatty acid β-oxidation in the small intestine epithelial cells and impairs colonocyte mitochondrial function, possibly through downstream consequences of excessive fatty acid β-oxidation and/or the presence of deleterious metabolites produced by the gut microbiota. Decreased levels of ATP and concomitant O₂ leaks into the intestinal lumen could explain the alterations of intestinal epithelium dynamics, barrier disruption and dysbiosis that contribute to the loss of epithelial homeostasis in diet-induced obesity. However, the effect of HFD on IEC mitochondrial function in the small intestine remains unknown and the precise mechanisms by which HFD induces mitochondrial dysfunction in the colon have not been elucidated so far.

Lymphatics - not just a chylomicron conduit

PURPOSE OF REVIEW

Lymphatics are known to have active, regulated pumping by smooth muscle cells that enhance lymph flow, but whether active regulation of lymphatic pumping contributes significantly to the rate of appearance of chylomicrons (CMs) in the blood circulation (i.e., CM production rate) is not currently known. In this review, we highlight some of the potential mechanisms by which lymphatics may regulate CM production.

RECENT FINDINGS

Recent data from our lab and others are beginning to provide clues that suggest a more active role of lymphatics in regulating CM appearance in the circulation through various mechanisms. Potential contributors include apolipoproteins, glucose, glucagon-like peptide-2, and vascular endothelial growth factor-C, but there are likely to be many more.

SUMMARY

The digested products of dietary fats absorbed by the small intestine are re-esterified and packaged by enterocytes into large, triglyceride-rich CM particles or stored temporarily in intracellular cytoplasmic lipid droplets. Secreted CMs traverse the lamina propria and are transported via lymphatics and then the blood circulation to liver and extrahepatic tissues, where they are stored or metabolized as a rich energy source. Although indirect data suggest a relationship between lymphatic pumping and CM production, this concept requires more experimental evidence before we can be sure that lymphatic pumping contributes significantly to the rate of CM appearance in the blood circulation.

Regulation of Apelin-13 on Bcl-2 and Caspase-3 and Its Effects on Adipocyte Apoptosis

OBJECTIVE

The effects of apelin-13 on the expression of Bcl-2 and caspase-3 factors and the apoptosis of adipocytes were studied at the cellular and animal levels.

METHODS

3T3-L1 preadipocytes were cultured and grouped. The third-generation cells were added to the control DMSO solvent and amidation-modified apelin-13. The expression of Bcl-2 and caspase-3 were detected. The cell growth viability and cell apoptosis were detected. DOI model rats were established. The effects of apelin-13 on DOI rat biochemical indicators, the expression of Bcl-2, caspase-3, and cell apoptosis were investigated by injecting amidation-modified apelin-13 through the tail vein.

RESULT

In in vitro experiments, amidation-modified apelin-13 can significantly reduce the growth viability of adipocytes and the expression of Bcl-2, increase the expression of caspase-3, and promote the apoptosis of adipocytes. Animal experiments also show that apelin-13 modified by amidation can adjust the abnormal biochemical indicators of DOI rats, decrease the expression of Bcl-2 in adipose tissue, increase the expression of caspase-3, and promote the apoptosis of adipocytes.

CONCLUSION

Amidation of apelin-13 can promote fat cell apoptosis and reduce the incidence of obesity. The mechanism may be accomplished by inhibiting Bcl-2 and caspase-3 factors. This study helps us understand the effect of apelin-13 on fat cell apoptosis and hopes to provide a basis for the development of antiobesity drugs.