Regulation of Chylomicron Secretion: Focus on Post-Assembly Mechanisms

Rapid and efficient digestion and absorption of dietary triglycerides and other lipids by the intestine, the packaging of those lipids into lipoprotein chylomicron (CM) particles, and their secretion via the lymphatic duct into the blood circulation are essential in maintaining whole-body lipid and energy homeostasis. Biosynthesis and assembly of CMs in enterocytes is a complex multistep process that is subject to regulation by intracellular signaling pathways as well as by hormones, nutrients, and neural factors extrinsic to the enterocyte. Dysregulation of this process has implications for health and disease, contributing to dyslipidemia and a potentially increased risk of atherosclerotic cardiovascular disease. There is increasing recognition that, besides intracellular regulation of CM assembly and secretion, regulation of postassembly pathways also plays important roles in CM secretion. This review examines recent advances in our understanding of the regulation of CM secretion in relation to mobilization of intestinal lipid stores, drawing particular attention to post-assembly regulatory mechanisms, including intracellular trafficking of triglycerides in enterocytes, CM mobilization from the lamina propria, and regulated transport of CM by intestinal lymphatics. Rapid and efficient digestion and absorption of dietary triglycerides and other lipids by the intestine, the packaging of those lipids into lipoprotein chylomicron (CM) particles, and their secretion via the lymphatic duct into the blood circulation are essential in maintaining whole-body lipid and energy homeostasis. Biosynthesis and assembly of CMs in enterocytes is a complex multistep process that is subject to regulation by intracellular signaling pathways as well as by hormones, nutrients, and neural factors extrinsic to the enterocyte. Dysregulation of this process has implications for health and disease, contributing to dyslipidemia and a potentially increased risk of atherosclerotic cardiovascular disease. There is increasing recognition that, besides intracellular regulation of CM assembly and secretion, regulation of postassembly pathways also plays important roles in CM secretion. This review examines recent advances in our understanding of the regulation of CM secretion in relation to mobilization of intestinal lipid stores, drawing particular attention to post-assembly regulatory mechanisms, including intracellular trafficking of triglycerides in enterocytes, CM mobilization from the lamina propria, and regulated transport of CM by intestinal lymphatics. SummaryDietary fat absorption in the intestine consists of multiple steps, including absorption into enterocytes and intracellular assembly of chylomicrons, chylomicron transport and exit from the enterocytes, movement across the lamina propria, entry into lacteals, and regulated transport in intestinal lymphatics. This review draws attention to postassembly regulation of chylomicron secretion that may present opportunities for improving health. Dietary fat absorption in the intestine consists of multiple steps, including absorption into enterocytes and intracellular assembly of chylomicrons, chylomicron transport and exit from the enterocytes, movement across the lamina propria, entry into lacteals, and regulated transport in intestinal lymphatics. This review draws attention to postassembly regulation of chylomicron secretion that may present opportunities for improving health. Intestinal digestion, absorption, and secretion of dietary fats are important steps in maintaining whole-body lipid and energy homeostasis.1Abumrad N.A. Davidson N.O. Role of the gut in lipid homeostasis.Physiol Rev. 2012; 92: 1061-1085Crossref PubMed Scopus (181) Google Scholar Compromised lipid handling by the gut has implications for health and disease, potentially contributing to dyslipidemia and atherosclerotic cardiovascular disease. Understanding lipid processing by the gut is essential for developing novel therapeutic strategies to improve cardiac and metabolic health.2Lewis G.F. Xiao C. Hegele R.A. Hypertriglyceridemia in the genomic era: a new paradigm.Endocr Rev. 2015; 36: 131-147Crossref PubMed Google Scholar The majority of absorbed lipids are packaged into chylomicron (CM) particles in the intestinal enterocyte, secreted into and transported through the lymphatic system to enter the blood circulation, and delivered to various tissues for storage or energy utilization. Numerous studies have elucidated the elegant cellular and molecular control of dietary fat absorption and CM biosynthesis, which has been reviewed extensively elsewhere.3Dash S. Xiao C. Morgantini C. Lewis G.F. New insights into the regulation of chylomicron production.Annu Rev Nutr. 2015; 35: 265-294Crossref PubMed Scopus (59) Google Scholar, 4Hussain M.M. Intestinal lipid absorption and lipoprotein formation.Curr Opin Lipidol. 2014; 25: 200-206Crossref PubMed Scopus (104) Google Scholar, 5Mansbach C.M. Siddiqi S.A. The biogenesis of chylomicrons.Annu Rev Physiol. 2010; 72: 315-333Crossref PubMed Scopus (103) Google Scholar Recently, there has been increasing recognition that regulation of the movement of lipids and CM, retained at multiple sites in the intestinal structure, from enterocyte to circulation also contributes to the overall rate of secretion of CMs into the circulation.6Mansbach C.M. Siddiqi S. Control of chylomicron export from the intestine.Am J Physiol Gastrointest Liver Physiol. 2016; 310: G659-G668Crossref PubMed Scopus (15) Google Scholar, 7Xiao C. Stahel P. Carreiro A.L. Buhman K.K. Lewis G.F. Recent advances in triacylglycerol mobilization by the gut.Trends Endocrinol Metab. 2018; 29: 151-163Abstract Full Text Full Text PDF PubMed Scopus (13) Google Scholar This review examines recent advances in our understanding of the regulation of CM secretion in relation to mobilization of intestinal lipid stores, with a focus on post-assembly regulatory mechanisms, including enterocyte intracellular triglyceride (TG) trafficking, CM movement in the lamina propria, and CM transport by intestinal lymphatics. Our focus is on TGs and CM particles rather than other lipid moieties. Dietary TG digestion starts with TG hydrolysis by lingual lipase in the mouth. In the stomach, gastric lipase and lingual lipase both contribute to TG hydrolysis, especially in the digestion of milk fat in newborns.8Fink C.S. Hamosh P. Hamosh M. Fat digestion in the stomach: stability of lingual lipase in the gastric environment.Pediatr Res. 1984; 18: 248-254Crossref PubMed Google Scholar, 9Hamosh M. Scanlon J.W. Ganot D. Likel M. Scanlon K.B. Hamosh P. Fat digestion in the newborn. Characterization of lipase in gastric aspirates of premature and term infants.J Clin Invest. 1981; 67: 838-846Crossref PubMed Google Scholar Pancreatic lipase is the major enzyme to catalyze TG hydrolysis in the small intestine. The digestion products monoglycerides (MGs) and fatty acids (FAs) form micelles with bile salts, which facilitates their absorption at the brush border across the apical membrane of enterocytes. Absorption of MGs and FAs is achieved through both passive diffusion and active transport. Facilitated diffusion is predominant at low concentrations, while simple diffusion is predominant at high concentrations for linoleic acid absorption by rat jejunum.10Chow S.L. Hollander D. A dual, concentration-dependent absorption mechanism of linoleic acid by rat jejunum in vitro.J Lipid Res. 1979; 20: 349-356PubMed Google Scholar Various transport proteins facilitate the transport process, including but not limited to CD36 and lipid binding proteins.11Cifarelli V. Abumrad N.A. Intestinal CD36 and other key proteins of lipid utilization: role in absorption and gut homeostasis.Compr Physiol. 2018; 8: 493-507Crossref PubMed Scopus (1) Google Scholar, 12Buttet M. Traynard V. Tran T.T.T. Besnard P. Poirier H. Niot I. From fatty-acid sensing to chylomicron synthesis: role of intestinal lipid-binding proteins.Biochimie. 2014; 96: 37-47Crossref PubMed Google Scholar Intestinal lipid binding proteins are involved not only in luminal absorption of long-chain FAs (LCFAs), but also in modulating intracellular trafficking of FAs, TG resynthesis, and CM formation.12Buttet M. Traynard V. Tran T.T.T. Besnard P. Poirier H. Niot I. From fatty-acid sensing to chylomicron synthesis: role of intestinal lipid-binding proteins.Biochimie. 2014; 96: 37-47Crossref PubMed Google Scholar CD36 may not play a quantitatively significant role in FA uptake, but it may induce key proteins in CM formation, thereby playing an important regulatory role in CM secretion.13Tran T.T.T. Poirier H. Clément L. Nassir F. Pelsers M.M.A.L. Petit V. Degrace P. Monnot M.-C. Glatz J.F.C. Abumrad N.A. Besnard P. Niot I. Luminal lipid regulates CD36 levels and downstream signaling to stimulate chylomicron synthesis.J Biol Chem. 2011; 286: 25201-25210Crossref PubMed Scopus (75) Google Scholar In a mouse model of diet-induced metabolic syndrome, down-regulation of CD36 by lipids was abolished and lipid sensing by CD36 was impaired, resulting in delayed induction of microsomal triglyceride transfer protein (MTP), liver-type-fatty acid binding protein (FABP), and apolipoprotein C-II, and aberrant TG-rich lipoprotein formation.14Buttet M. Poirier H. Traynard V. Gaire K. Tran T.T.T. Sundaresan S. Besnard P. Abumrad N.A. Niot I. Deregulated lipid sensing by intestinal CD36 in diet-induced hyperinsulinemic obese mouse model.PLoS One. 2016; 11: e0145626Crossref PubMed Scopus (0) Google Scholar CD36-/- mice have decreased lymph flow,15Drover V.A. Ajmal M. Nassir F. Davidson N.O. Nauli A.M. Sahoo D. Tso P. Abumrad N.A. CD36 deficiency impairs intestinal lipid secretion and clearance of chylomicrons from the blood.J Clin Invest. 2005; 115: 1290-1297Crossref PubMed Google Scholar, 16Nauli A.M. Nassir F. Zheng S. Yang Q. Lo C.-M. Vonlehmden S.B. Lee D. Jandacek R.J. Abumrad N.A. Tso P. CD36 is important for chylomicron formation and secretion and may mediate cholesterol uptake in the proximal intestine.Gastroenterology. 2006; 131: 1197-1207Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar suggesting CD36 involvement in post-assembly transport of CMs. Polymorphism in FABP2 (encoding intestinal FABP) is associated with exaggerated postprandial plasma and CM TG response in human beings.17Agren J.J. Valve R. Vidgren H. Laakso M. Uusitupa M. Postprandial lipemic response is modified by the polymorphism at codon 54 of the fatty acid-binding protein 2 gene.Arterioscler Thromb Vasc Biol. 1998; 18: 1606-1610Crossref PubMed Google Scholar Mice deficient in intestinal Fabp affects weight gain in a sex-dependent manner,18Vassileva G. Huwyler L. Poirier K. Agellon L.B. Toth M.J. The intestinal fatty acid binding protein is not essential for dietary fat absorption in mice.FASEB J. 2000; 14: 2040-2046Crossref PubMed Scopus (145) Google Scholar which may underlie sexual dimorphism in lipid and lipoprotein metabolism, although gender differences in CM synthesis and secretion has not been studied extensively. Upon entry into enterocytes, MGs and FAs are re-esterified to form TGs in the endoplasmic reticulum (ER) membrane leaflet. TG resynthesis occurs primarily (approximately 80%) through the monoacylglycerol pathway. MGs first combine with a FA to form diglycerides catalyzed by acyl CoA:monoacylglycerol acyltransferase, followed by TG formation, with the addition of a second FA catalyzed by acyl-CoA:diacylglycerol acyltransferase. The glycerol-3-phosphate pathway using glycerol-phosphate acyltransferase contributes approximately 20% to intestinal TG synthesis.19Khatun I. Clark R.W. Vera N.B. Kou K. Erion D.M. Coskran T. Bobrowski W.F. Okerberg C. Goodwin B. Characterization of a novel intestinal glycerol-3-phosphate acyltransferase pathway and its role in lipid homeostasis.J Biol Chem. 2016; 291: 2602-2615Crossref PubMed Scopus (11) Google Scholar TGs synthesized in the ER membrane form lipid droplets that either bud off to form cytoplasmic lipid droplets (CLDs) encased in a phospholipid monolayer, or secreted into the ER lumen for CM synthesis. Formation of pre-CM occurs at the inner ER membrane where apolipoprotein (apo) B48 is lipidated, facilitated by MTP. The prevailing model of CM lipidation depicts the formation of a poorly lipidated, dense, apoB48-containing particle and an apoB48-free lipid droplet, with subsequent fusion of the 2 to form the pre-CM. ApoAIV also is added to pre-CM in the ER and plays an important role in CM size and metabolism in the circulation.20Kohan A.B. Wang F. Lo C.-M. Liu M. Tso P. ApoA-IV: current and emerging roles in intestinal lipid metabolism, glucose homeostasis, and satiety.Am J Physiol Gastrointest Liver Physiol. 2015; 308: G472-G481Crossref PubMed Scopus (24) Google Scholar Pre-CMs are transported in special transport vesicles (pre-CM transport vesicle [PCTV]) from the ER to the Golgi apparatus, where they are processed further into mature CMs. Mature CMs exit enterocytes at the basolateral membrane, enter the lamina propria and then the lacteals, and move through the mesenteric lymphatic ducts to the thoracic duct where they enter the venous circulation at the left subclavian vein. The following discussion focuses on TG trafficking, storage, and mobilization in the small intestine, with an emphasis on nodes of regulation in CM secretion postassembly of nascent lipoproteins (Figure 1). Pre-CMs are packaged into PCTVs, which bud off the ER membrane and move to the cis-Golgi. Pre-CM exiting the ER is considered the rate-limiting step for intracellular trafficking of TGs and is a multistep process.6Mansbach C.M. Siddiqi S. Control of chylomicron export from the intestine.Am J Physiol Gastrointest Liver Physiol. 2016; 310: G659-G668Crossref PubMed Scopus (15) Google Scholar FABP1 binds to the ER to initiate PCTV formation, and together with CD36, VAMP7, and apoB48, facilitates PCTV budding. Transport of pre-CMs in PCTVs requires the soluble NSF attachment protein receptor (SNARE) protein complex. Transport vesicles use their vesicle SNARE (vSNARE) to direct the vesicles to their target, where vSNAREs pair with the target SNARE of the target membrane to form a SNARE complex to deliver the vesicle contents to target lumen via membrane fusion. The SNARE complex is composed of 4 helices, 1 from the transport vesicle and 3 from the target membrane. After budding from the ER, PCTVs are directed by vSNARE toward the Golgi. VAMP7 of vSNARE joins with syntaxin-5, rbet1, and vti1a of target SNARE to form the SNARE complex, which facilitates docking and fusion of PCTV with the Golgi membrane to release CM cargo into the Golgi lumen.21Siddiqi S.A. Siddiqi S. Mahan J. Peggs K. Gorelick F.S. Mansbach C.M. The identification of a novel endoplasmic reticulum to Golgi SNARE complex used by the prechylomicron transport vesicle.J Biol Chem. 2006; 281: 20974-20982Crossref PubMed Scopus (0) Google Scholar The regulation of this machinery has not been fully defined. Our recent study suggests that syntaxin-binding protein 5 may be involved in oral glucose mobilization of intestinal lipid stores.22Xiao C. Stahel P. Carreiro A.L. Hung Y.-H. Dash S. Bookman I. Buhman K.K. Lewis G.F. Oral glucose mobilizes triglyceride stores from the human intestine.Cell Mol Gastroenterol Hepatol. 2019; 7: 313-337Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar A plausible mechanism is that binding of syntaxin-binding protein 5 closes syntaxin-5 and prevents the formation of the SNARE complex, PCTV fusion with the Golgi membrane, and uptake into the Golgi. Other proteins (eg, apoAI) are transported by cytoplasmic coat protein complex II vesicles from the ER to the Golgi. cytoplasmic coat protein complex II is not needed for pre-CM to exit the ER but is needed for Golgi fusion of PCTVs. In the Golgi, maturation of CMs occurs with acquisition of additional apolipoproteins (eg, apoAI and apoAIV), and apoB48 glycosylation. Mature CMs exit the Golgi into the cytosol in a large vesicle by an unknown mechanism, before being exocytosed at the basolateral membrane. The basement membrane beneath the enterocytes may become leaky during active lipid absorption to facilitate the movement of CMs from the intercellular space to the lamina propria.23Tso P. Balint J.A. Formation and transport of chylomicrons by enterocytes to the lymphatics.Am J Physiol. 1986; 250: G715-G726PubMed Google Scholar Despite many similarities in the synthesis of CMs in enterocytes and very-low-density lipoprotein (VLDL) in hepatocytes, their intracellular transport differs in several aspects. VLDL synthesis in hepatocytes starts with MTP-dependent lipidation of nascent apoB100 to generate lipid-poor, primordial VLDL particles in the ER lumen. Lipidation of apoB100 depends on the availability of TGs, which may be derived from several sources, including free fatty acids from adipose hydrolysis, CM remnant uptake, and de novo lipogenesis. Further lipid enrichment and maturation may occur via fusion with preformed lipid droplets in the ER and in the Golgi before secretion.24Tiwari S. Siddiqi S.A. Intracellular trafficking and secretion of VLDL.Arterioscler Thromb Vasc Biol. 2012; 32: 1079-1086Crossref PubMed Scopus (71) Google Scholar, 25Fisher E.A. Ginsberg H.N. Complexity in the secretory pathway: the assembly and secretion of apolipoprotein B-containing lipoproteins.J Biol Chem. 2002; 277: 17377-17380Crossref PubMed Scopus (343) Google Scholar ApoB100 degradation is a major regulatory mechanism, thus lipidation is necessary for stabilization of apoB100 and a reduction in fatty acid supply promotes apoB100 degradation.25Fisher E.A. Ginsberg H.N. Complexity in the secretory pathway: the assembly and secretion of apolipoprotein B-containing lipoproteins.J Biol Chem. 2002; 277: 17377-17380Crossref PubMed Scopus (343) Google Scholar, 26Davidson N.O. Shelness G.S. Apolipoprotein B: mRNA editing, lipoprotein assembly, and presecretory degradation.Annu Rev Nutr. 2000; 20: 169-193Crossref PubMed Scopus (199) Google Scholar Whether apoB48 protein degradation plays a major regulatory role in CM secretion is unresolved. Abundant apoB48 was observed in Caco-2 cells even without fatty acid supply, implying the lack of significant apoB48 degradation.27Liao W. Chan L. Apolipoprotein B, a paradigm for proteins regulated by intracellular degradation, does not undergo intracellular degradation in CaCo2 cells.J Biol Chem. 2000; 275: 3950-3956Crossref PubMed Scopus (32) Google Scholar On the other hand, apoB48 degradation was reported in Caco-2 cells28Morel E. Demignot S. Chateau D. Chambaz J. Rousset M. Delers F. Lipid-dependent bidirectional traffic of apolipoprotein B in polarized enterocytes.Mol Biol Cell. 2004; 15: 132-141Crossref PubMed Scopus (0) Google Scholar and in primary hamster enterocytes.29Haidari M. Leung N. Mahbub F. Uffelman K.D. Kohen-Avramoglu R. Lewis G.F. Adeli K. Fasting and postprandial overproduction of intestinally derived lipoproteins in an animal model of insulin resistance. Evidence that chronic fructose feeding in the hamster is accompanied by enhanced intestinal de novo lipogenesis and ApoB48-containing lipoprotein overproduction.J Biol Chem. 2002; 277: 31646-31655Crossref PubMed Scopus (192) Google Scholar It is known that intestinal lipoprotein secretion occurs in rats and hamsters even in the fasted state, where a more dense, high-density lipoprotein-like or VLDL-like particle is secreted.29Haidari M. Leung N. Mahbub F. Uffelman K.D. Kohen-Avramoglu R. Lewis G.F. Adeli K. Fasting and postprandial overproduction of intestinally derived lipoproteins in an animal model of insulin resistance. Evidence that chronic fructose feeding in the hamster is accompanied by enhanced intestinal de novo lipogenesis and ApoB48-containing lipoprotein overproduction.J Biol Chem. 2002; 277: 31646-31655Crossref PubMed Scopus (192) Google Scholar, 30Ockner R.K. Hughes F.B. Isselbacher K.J. Very low density lipoproteins in intestinal lymph: origin, composition, and role in lipid transport in the fasting state.J Clin Invest. 1969; 48: 2079-2088Crossref PubMed Scopus (0) Google Scholar Transport of VLDL from the ER to the Golgi uses a distinct transport vesicle (VLDL transport vesicle), which differs from PCTV in biogenesis, size, and SNARE components in vesicle–Golgi membrane fusion.24Tiwari S. Siddiqi S.A. Intracellular trafficking and secretion of VLDL.Arterioscler Thromb Vasc Biol. 2012; 32: 1079-1086Crossref PubMed Scopus (71) Google Scholar A relatively recent development in the field is the recognition of postprandial lipid retention in the intestine that extends well beyond the prandial and postprandial periods. Plasma TG concentration in a normolipidemic individual after a high-fat meal usually peaks after 3–5 hours and returns to baseline by approximately 6–8 hours. Studies in human beings documented the early appearance of CM TGs after a fat-rich meal, the origin of which was attributed to lipids derived from a previous meal.31Evans K. Kuusela P.J. Cruz M.L. Wilhelmova I. Fielding B.A. Frayn K.N. Rapid chylomicron appearance following sequential meals: effects of second meal composition.Br J Nutr. 1998; 79: 425-429Crossref PubMed Scopus (41) Google Scholar, 32Jackson K.G. Robertson M.D. Fielding B.A. Frayn K.N. Williams C.M. Olive oil increases the number of triacylglycerol-rich chylomicron particles compared with other oils: an effect retained when a second standard meal is fed.Am J Clin Nutr. 2002; 76: 942-949Crossref PubMed Google Scholar, 33Fielding B.A. Callow J. Owen R.M. Samra J.S. Matthews D.R. Frayn K.N. Postprandial lipemia: the origin of an early peak studied by specific dietary fatty acid intake during sequential meals.Am J Clin Nutr. 1996; 63: 36-41Crossref PubMed Scopus (188) Google Scholar, 34Silva K.D.R.R. Wright J.W. Williams C.M. Lovegrove J.A. Meal ingestion provokes entry of lipoproteins containing fat from the previous meal: possible metabolic implications.Eur J Nutr. 2005; 44: 377-383Crossref PubMed Scopus (0) Google Scholar This time frame is earlier than that required for dietary fat digestion and absorption and CM biogenesis to occur.35Mansbach C.M. Nevin P. Intracellular movement of triacylglycerols in the intestine.J Lipid Res. 1998; 39: 963-968Abstract Full Text Full Text PDF PubMed Google Scholar Studies with stable isotope labeling of dietary fat showed that 10%–12% of TGs from the previous meal appear in new CM within the first 20 minutes of a second meal ingestion, and that lipids from earlier meals can contribute to CM TG secretion more than 18 hours after the last meal.36Chavez-Jauregui R.N. Mattes R.D. Parks E.J. Dynamics of fat absorption and effect of sham feeding on postprandial lipema.Gastroenterology. 2010; 139: 1538-1548Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar Postprandial CM TG concentrations are higher than those after an earlier meal, even when the 2 sequential meals contain exactly the same macronutrient composition, supporting a contribution of earlier dietary lipids to the appearance of CM TGs after the second meal.37Mattes R.D. Oral fat exposure increases the first phase triacylglycerol concentration due to release of stored lipid in humans.J Nutr. 2002; 132: 3656-3662Crossref PubMed Google Scholar In addition, abundant lipid droplets were visible in jejunal enterocyte cytoplasm 6 hours after a high-fat liquid meal in 1 study,38Robertson M.D. Parkes M. Warren B.F. Ferguson D.J. Jackson K.G. Jewell D.P. Frayn K.N. Mobilisation of enterocyte fat stores by oral glucose in humans.Gut. 2003; 52: 834-839Crossref PubMed Scopus (101) Google Scholar and we recently showed CLDs present in duodenal enterocyte cytoplasm as long as 10 hours after a high-fat meal.22Xiao C. Stahel P. Carreiro A.L. Hung Y.-H. Dash S. Bookman I. Buhman K.K. Lewis G.F. Oral glucose mobilizes triglyceride stores from the human intestine.Cell Mol Gastroenterol Hepatol. 2019; 7: 313-337Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar In mice, enterocyte cytoplasm contains lipid depots up to 12 hours after a high-fat meal.39Zhu J. Lee B. Buhman K.K. Cheng J.-X. A dynamic, cytoplasmic triacylglycerol pool in enterocytes revealed by ex vivo and in vivo coherent anti-Stokes Raman scattering imaging.J Lipid Res. 2009; 50: 1080-1089Crossref PubMed Scopus (97) Google Scholar These observations support the existence of an enteral source of lipid stores in the postprandial period that is derived from previous dietary fat intake. This stored enteral lipid is released in response to a second meal and other stimulatory cues (discussed in greater detail later), contributing to circulating CM TGs. Although the exact source of these intestinal lipid stores is unknown, candidate pools include lipid droplets in intracellular spaces (eg, cytoplasm, organelles, and secretory pathways), and CMs in extracellular spaces (eg, lamina propria and lacteals of the mesenteric lymphatic system). In addition to the retention of TGs and their subsequent release in response to various stimuli, we have shown that preformed apoB48 is rapidly released from the intestine in response to the gut peptide glucagon-like peptide-2 (GLP-2).40Dash S. Xiao C. Morgantini C. Connelly P.W. Patterson B.W. Lewis G.F. Glucagon-like peptide-2 regulates release of chylomicrons from the intestine.Gastroenterology. 2014; 147: 1275-1284Abstract Full Text Full Text PDF PubMed Scopus (44) Google Scholar The apoB48 released into the circulation is predominantly in the form of CMs, but whether it is derived from a pool of unlipidated/poorly lipidated apoB48 or fully preformed CMs is not fully known. Apical distribution of apoB has been reported in rat jejunal enterocytes and Caco-2 cells.41Levy E. Bendayan M. Use of immunoelectron microscopy and intestinal models to explore the elaboration of apolipoproteins required for intraenterocyte lipid transport.Microsc Res Tech. 2000; 49: 374-382Crossref PubMed Scopus (0) Google Scholar A small amount of apoB is reported to be released from the apical side of pig intestinal explants, which was abolished by a fat meal.42Danielsen E.M. Hansen G.H. Poulsen M.D. Apical secretion of apolipoproteins from enterocytes.J Cell Biol. 1993; 120: 1347-1356Crossref PubMed Scopus (26) Google Scholar In differentiated Caco-2 cells, apoB showed apical distribution, located within the brush-border microvilli and in the subapical region.28Morel E. Demignot S. Chateau D. Chambaz J. Rousset M. Delers F. Lipid-dependent bidirectional traffic of apolipoprotein B in polarized enterocytes.Mol Biol Cell. 2004; 15: 132-141Crossref PubMed Scopus (0) Google Scholar This apical pool of apoB was derived from the trans-Golgi network, with lipid supply driving the export of apoB from the ER and post-Golgi targeting to the apex. Apical supply of lipid micelles rapidly depleted the apical pool of apoB, mobilizing it toward the basolateral area, whereas a continuous supply of lipids replenished it. It is important to note that, in this model, the supply of exogenous lipids did not significantly increase apoB synthesis; therefore, apoB degradation served as part of the regulation and mobilization of the pre-existing ready-to-use store of apical pool of apoB, which constitutes an additional contribution to the increased apoB secretion in this condition. Synthesis of TGs and lipid droplets, from the lipid supply on the apical side, in the ER in the subapical compartment triggers the trafficking of apical apoB toward the basolateral side. The apical pool of apoB may be in the form of primordial lipoproteins, ready to be greatly expanded in size within the secretory pathway after an influx of dietary lipids. This repeated replenishment and recruitment of an apical pool of apoB48 during the feeding-fasting cycle is very interesting. The presence of an apical pool of apoB (ie, polarized distribution) is believed to be cell-specific and has not been described for hepatocytes. It remains to be established whether this process is altered in different metabolic conditions, such as the metabolic syndrome and type 2 diabetes. It is not known whether the basolateral supply of lipids also promotes the mobilization of the apical apoB pool. It also remains to be examined whether other stimuli (eg, glucose, both an apical and basolateral supply; a cephalic phase response; or GLP-2 in vivo) mobilizes this apical pool of apoB in a similar fashion. As we have shown previously, GLP-2 promotes the rapid release of preformed CM particles in which apoB48 was not labeled with stable isotope.40Dash S. Xiao C. Morgantini C. Connelly P.W. Patterson B.W. Lewis G.F. Glucagon-like peptide-2 regulates release of chylomicrons from the intestine.Gastroenterology. 2014; 147: 1275-1284Abstract Full Text Full Text PDF PubMed Scopus (44) Google Scholar It is intriguing to speculate that the apical pool of apoB48 was mobilized by GLP-2. Lipid droplets that are formed in the ER membrane undergo 1 of 2 fates: CM synthesis in the ER lumen or formation of CLDs. CLDs consist of a neutral lipid core containing mostly TGs and some cholesteryl ester, surrounded by a phospholipid monolayer and associated proteins. CLDs undergo dynamic synthesis, metabolism, and catabolism.43D'Aquila T. Hung Y.-H. Carreiro A. Buhman K.K. Recent discoveries on absorption of dietary fat: presence, synthesis, and metabolism of cytoplasmic lipid droplets within enterocytes.Biochim Biophys Acta. 2016; 1861: 730-747Crossref PubMed Scopus (0) Google Scholar Newly synthesized TGs accumulate within the ER membrane and promote the budding of a CLD, with CLDs further expanding in size via either fusion or TG synthesis at the CLD surface. Several enzymes that facilitate CLD synthesis are associated with the CLD membrane.43D'Aquila T. Hung Y.-H. Carreiro A. Buhman K.K. Recent discoveries on absorption of dietary fat: presence, synt