PKC zeta-mediated phosphorylation controls budding of the pre-chylomicron transport vesicle

Biosynthesis and Metabolism of ApoB-Containing Lipoproteins

Recent advances in human genetics, together with a substantial body of epidemiological, preclinical and clinical trial evidence, strongly support a causal relationship between triglyceride-rich lipoproteins (TRLs) and atherosclerotic cardiovascular disease. Consequently, the secretion and metabolism of TRLs have a significant impact on cardiovascular health. This knowledge underscores the importance of understanding the molecular mechanisms and regulation of very-low-density lipoprotein (VLDL) and chylomicron biogenesis. Fortunately, there has been a resurgence of interest in the intracellular assembly, trafficking, degradation, and secretion of VLDL, leading to many ground-breaking molecular insights. Furthermore, the identification of molecular control mechanisms related to triglyceride metabolism has greatly advanced our understanding of the complex metabolism of TRLs. In this review, we explore recent advances in the assembly, secretion, and metabolism of TRLs. We also discuss available treatment strategies for hypertriglyceridemia.

PKC-ζ mediated reduction of the extracellular vesicles-associated TGF-β1 overcomes radiotherapy resistance in breast cancer

BACKGROUND

Radiotherapy is widely applied in breast cancer treatment, while radiotherapy resistance is inevitable. TGF-β1 has been considered to be an endogenous factor for the development of radiotherapy resistance. As a large portion of TGF-β1 is secreted in an extracellular vesicles-associated form (TGF-β1_EV), particularly in radiated tumors. Thus, the understanding of the regulation mechanisms and the immunosuppressive functions of TGF-β1_EV will pave a way for overcoming the radiotherapy resistance in cancer treatment.

METHODS

The superoxide-Zinc-PKC-ζ-TGF-β1_EV pathway in breast cancer cells was identified through sequence alignments of different PKC isoforms, speculation and experimental confirmation. A series of functional and molecular studies were performed by quantitative real-time PCR, western blot and flow cytometry analysis. Mice survival and tumor growth were recorded. Student's t test or two-way ANOVA with correction was used for comparisons of groups.

RESULTS

The radiotherapy resulted in an increased expression of the intratumoral TGF-β1 and an enhanced infiltration of the Tregs in the breast cancer tissues. The intratumoral TGF-β1 was found mainly in the extracellular vesicles associated form both in the murine breast cancer model and in the human lung cancer tissues. Furthermore, radiation induced more TGF-β1_EV secretion and higher percentage of Tregs by promoting the expression and phosphorylation of protein kinase C zeta (PKC-ζ). Importantly, we found that naringenin rather than 1D11 significantly improved radiotherapy efficacy with less side effects. Distinct from TGF-β1 neutralizing antibody 1D11, the mechanism of naringenin was to downregulate the radiation-activated superoxide-Zinc-PKC-ζ-TGF-β1_EV pathway.

CONCLUSIONS

The superoxide-zinc-PKC-ζ-TGF-β1_EV release pathway was elucidated to induce the accumulation of Tregs, resulting in radiotherapy resistance in the TME. Therefore, targeting PKC-ζ to counteract TGF-β1_EV function could represent a novel strategy to overcome radiotherapy resistance in the treatment of breast cancer or other cancers.

TRIAL REGISTRATION

The using of patient tissues with malignant Non-Small Cell Lung Cancer (NSCLC) was approved by the ethics committees at Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China (NCC2022C-702, from June 8th, 2022).

Genetics of Triglyceride-Rich Lipoproteins Guide Identification of Pharmacotherapy for Cardiovascular Risk Reduction

OBJECTIVE

Despite aggressive reduction of low-density lipoprotein cholesterol (LDL-C), there is a residual risk of cardiovascular disease (CVD). Hypertriglyceridemia is known to be associated with increased CVD risk, independently of LDL-C. Triglycerides are one component of the heterogenous class of triglyceride-rich lipoproteins (TGRLs).

METHODS/RESULTS

Growing evidence from biology, epidemiology, and genetics supports the contribution of TGRLs to the development of CVD via a number of mechanisms, including through proinflammatory, proapoptotic, and procoagulant pathways.

CONCLUSION

New genetics-guided pharmacotherapies to reduce levels of triglycerides and TGRLs and thus reduce risk of CVD have been developed and will be discussed here.

Intestinal CD36 and Other Key Proteins of Lipid Utilization: Role in Absorption and Gut Homeostasis

Several proteins have been implicated in fatty acid (FA) transport by enterocytes including the scavenger receptor CD36 (SR-B2), the scavenger receptor B1 (SR-B1) a member of the CD36 family and the FA transport protein 4 (FATP4). Here, we review the regulation of enterocyte FA uptake and its function in lipid absorption including prechylomicron formation, assembly and transport. Emphasis is given to CD36, which is abundantly expressed along the digestive tract of rodents and humans and has been the most studied. We also address the pleiotropic functions of CD36 that go beyond lipid absorption and metabolism to include recent evidence of its impact on intestinal homeostasis and barrier maintenance. Areas of progress involving contribution of membrane phospholipid remodeling and of cytosolic FA-binding proteins, FABP1 and FABP2 to fat absorption will be covered. © 2018 American Physiological Society. Compr Physiol 8:493-507, 2018.

Recent discoveries on absorption of dietary fat: Presence, synthesis, and metabolism of cytoplasmic lipid droplets within enterocytes

Dietary fat provides essential nutrients, contributes to energy balance, and regulates blood lipid concentrations. These functions are important to health, but can also become dysregulated and contribute to diseases such as obesity, diabetes, cardiovascular disease, and cancer. Within enterocytes, the digestive products of dietary fat are re-synthesized into triacylglycerol, which is either secreted on chylomicrons or stored within cytoplasmic lipid droplets (CLDs). CLDs were originally thought to be inert stores of neutral lipids, but are now recognized as dynamic organelles that function in multiple cellular processes in addition to lipid metabolism. This review will highlight recent discoveries related to dietary fat absorption with an emphasis on the presence, synthesis, and metabolism of CLDs within this process.

Regulation of intestinal lipid absorption by clock genes

Plasma levels of triacylglycerols and diacylglycerols, the lipoproteins that transport them, and proteins involved in their absorption from the intestinal lumen fluctuate in a circadian manner. These changes are likely controlled by clock genes expressed in the intestine that are probably synchronized by neuronal and humoral signals from the suprachiasmatic nuclei, which constitute a master clock entrained by light signals from the eyes and from the environment, e.g., food availability. Acute changes in circadian rhythms--e.g., due to nonsynchronous work schedules or a transcontinental flight--may trigger intestinal discomfort. Chronic disruptions in circadian control mechanisms may predispose the individual to irritable bowel syndrome, gastroesophageal reflux disease, and peptic ulcer disease. A more detailed understanding of the molecular mechanisms underlying temporal changes in intestinal activity might allow us to identify novel targets for developing therapeutic approaches to these disorders.

Dietary and biliary phosphatidylcholine activates PKCζ in rat intestine

Chylomicron output by the intestine is proportional to intestinal phosphatidylcholine (PC) delivery. Using five different variations of PC delivery to the intestine, we found that lyso-phosphatidylcholine (lyso-PC), the absorbed form of PC, concentrations in the cytosol (0 to 0.45 nM) were proportional to the input rate. The activity of protein kinase C (PKC)ζ, which controls prechylomicron output rate by the endoplasmic reticulum (ER), correlated with the lyso-PC concentration suggesting that it may be a PKCζ activator. Using recombinant PKCζ, the Km for lyso-PC activation was 1.49 nM and the Vmax 1.12 nM, more than the maximal lyso-PC concentration in cytosol, 0.45 nM. Among the phospholipids and their lyso derivatives, lyso-PC was the most potent activator of PKCζ and the only one whose cytosolic concentration suggested that it could be a physiological activator because other phospholipid concentrations were negligible. PKCζ was on the surface of the dietary fatty acid transport vesicle, the caveolin-1-containing endocytic vesicle. Once activated, PKCζ, eluted off the vesicle. A conformational change in PKCζ on activation was suggested by limited proteolysis. We conclude that PKCζ on activation changes its conformation resulting in elution from its vesicle. The downstream effect of dietary PC is to activate PKCζ, resulting in greater chylomicron output by the ER.

Genetics and causality of triglyceride-rich lipoproteins in atherosclerotic cardiovascular disease

Triglycerides represent 1 component of a heterogeneous pool of triglyceride-rich lipoproteins (TGRLs). The reliance on triglycerides or TGRLs as cardiovascular disease (CVD) risk biomarkers prompted investigations into therapies that lower plasma triglycerides as a means to reduce CVD events. Genetic studies identified TGRL components and pathways involved in their synthesis and metabolism. We advocate that only a subset of genetic mechanisms regulating TGRLs contribute to the risk of CVD events. This "omic" approach recently resulted in new targets for reducing CVD events.

Intestinal lipid absorption and lipoprotein formation

PURPOSE OF REVIEW

To summarize the evidence for the presence of two lipid absorption pathways and their regulation.

RECENT FINDINGS

Lipid absorption involves hydrolysis of dietary fat in the lumen of the intestine, followed by the uptake of hydrolyzed products by enterocytes. Lipids are resynthesized in the endoplasmic reticulum and are either secreted with chylomicrons and HDLs or stored as cytoplasmic lipid droplets. Lipids in the droplets are hydrolyzed and are secreted at a later time. Secretion of lipids by the chylomicron and HDL pathways are dependent on microsomal triglyceride transfer protein (MTP) and ATP-binding cassette family A protein 1, respectively, and are regulated independently. Gene-ablation studies showed that MTP function and chylomicron assembly is essential for the absorption of triglycerides. Ablation of MTP abolishes triglyceride absorption and results in massive triglyceride accumulation in enterocytes. Although the majority of phospholipid, cholesterol, and vitamin E are absorbed through the chylomicron pathway, a significant amount of these lipids are also absorbed via the HDL pathway. Chylomicron assembly and secretion is increased by the enhanced availability of fatty acids, whereas the HDL pathway is upregulated by liver X receptor agonists.

SUMMARY

Triglycerides are exclusively transported with chylomicrons and this process is critically dependent on MTP. In addition to chylomicrons, absorption of phospholipids, free cholesterol, retinol, and vitamin E also involves HDLs. These two pathways are complementary and are regulated independently. They may be targeted to lower lipid absorption in order to control hyperlipidemia, obesity, metabolic syndrome, steatosis, insulin resistance, atherosclerosis, and other disorders.

From fatty-acid sensing to chylomicron synthesis: role of intestinal lipid-binding proteins

Today, it is well established that the development of obesity and associated diseases results, in part, from excessive lipid intake associated with a qualitative imbalance. Among the organs involved in lipid homeostasis, the small intestine is the least studied even though it determines lipid bioavailability and largely contributes to the regulation of postprandial hyperlipemia (triacylglycerols (TG) and free fatty acids (FFA)). Several Lipid-Binding Proteins (LBP) are expressed in the small intestine. Their supposed intestinal functions were initially based on what was reported in other tissues, and took no account of the physiological specificity of the small intestine. Progressively, the identification of regulating factors of intestinal LBP and the description of the phenotype of their deletion have provided new insights into cellular and molecular mechanisms involved in fat absorption. This review will discuss the physiological contribution of each LBP in the main steps of intestinal absorption of long-chain fatty acids (LCFA): uptake, trafficking and reassembly into chylomicrons (CM). Moreover, current data indicate that the small intestine is able to adapt its lipid absorption capacity to the fat content of the diet, especially through the coordinated induction of LBP. This adaptation requires the existence of a mechanism of intestinal lipid sensing. Emerging data suggest that the membrane LBP CD36 may operate as a lipid receptor that triggers an intracellular signal leading to the modulation of the expression of LBP involved in CM formation. This event could be the starting point for the optimized synthesis of large CM, which are efficiently degraded in blood. Better understanding of this intestinal lipid sensing might provide new approaches to decrease the prevalence of postprandial hypertriglyceridemia, which is associated with cardiovascular diseases, insulin resistance and obesity.

CideB protein is required for the biogenesis of very low density lipoprotein (VLDL) transport vesicle

Nascent very low density lipoprotein (VLDL) exits the endoplasmic reticulum (ER) in a specialized ER-derived vesicle, the VLDL transport vesicle (VTV). Similar to protein transport vesicles (PTVs), VTVs require coat complex II (COPII) proteins for their biogenesis from the ER membranes. Because the size of the VTV is large, we hypothesized that protein(s) in addition to COPII components might be required for VTV biogenesis. Our proteomic analysis, supported by Western blotting data, shows that a 26-kDa protein, CideB, is present in the VTV but not in other ER-derived vesicles such as PTV and pre-chylomicron transport vesicle. Western blotting and immunoelectron microscopy analyses suggest that CideB is concentrated in the VTV. Our co-immunoprecipitation data revealed that CideB specifically interacts with VLDL structural protein, apolipoprotein B100 (apoB100), but not with albumin, a PTV cargo protein. Confocal microscopic data indicate that CideB co-localizes with apoB100 in the ER. Additionally, CideB interacts with COPII components, Sar1 and Sec24. To investigate the role of CideB in VTV biogenesis, we performed an in vitro ER budding assay. We show that the blocking of CideB inhibits VTV budding, indicating a direct requirement of CideB in VTV formation. To confirm our findings, we knocked down CideB in primary hepatocytes and isolated ER and cytosol to examine whether they support VTV budding. Our data suggest that CideB knockdown significantly reduces VTV biogenesis. These findings suggest that CideB forms an intricate COPII coat and regulates the VTV biogenesis.

Proteomic analysis of the very low density lipoprotein (VLDL) transport vesicles

The VLDL transport vesicle (VTV) mediates the transport of nascent VLDL particles from the ER to the Golgi and plays a key role in VLDL-secretion from the liver. The functionality of VTV is controlled by specific proteins; however, full characterization and proteomic profiling of VTV remain to be carried out. Here, we report the first proteomic profile of VTVs. VTVs were purified to their homogeneity and characterized biochemically and morphologically. Thin section transmission electron microscopy suggests that the size of VTV ranges between 100 nm to 120 nm and each vesicle contains only one VLDL particle. Immunoblotting data indicate VTV concentrate apoB100, apoB48 and apoAIV but exclude apoAI. Proteomic analysis based on 2D-gel coupled with MALDI-TOF identified a number of vesicle-related proteins, however, many important VTV proteins could only be identified using LC-MS/MS methodology. Our data strongly indicate that VTVs greatly differ in their proteome with their counterparts of intestinal origin, the PCTVs. For example, VTV contains Sec22b, SVIP, ApoC-I, reticulon 3, cideB, LPCAT3 etc. which are not present in PCTV. The VTV proteome reported here will provide a basic tool to study the mechanisms underlying VLDL biogenesis, maturation, intracellular trafficking and secretion from the liver.

Phosphorylation of Sar1b protein releases liver fatty acid-binding protein from multiprotein complex in intestinal cytosol enabling it to bind to endoplasmic reticulum (ER) and bud the pre-chylomicron transport vesicle

Native cytosol requires ATP to initiate the budding of the pre-chylomicron transport vesicle from intestinal endoplasmic reticulum (ER). When FABP1 alone is used, no ATP is needed. Here, we test the hypothesis that in native cytosol FABP1 is present in a multiprotein complex that prevents FABP1 binding to the ER unless the complex is phosphorylated. We found on chromatography of native intestinal cytosol over a Sephacryl S-100 HR column that FABP1 (14 kDa) eluted in a volume suggesting a 75-kDa protein complex that contained four proteins on an anti-FABP1 antibody pulldown. The FABP1-containing column fractions were chromatographed over an anti-FABP1 antibody adsorption column. Proteins co-eluted from the column were identified as FABP1, Sar1b, Sec13, and small VCP/p97-interactive protein by immunoblot, LC-MS/MS, and MALDI-TOF. The four proteins of the complex had a total mass of 77 kDa and migrated on native PAGE at 75 kDa. When the complex was incubated with intestinal ER, there was no increase in FABP1-ER binding. However, when the complex member Sar1b was phosphorylated by PKCζ and ATP, the complex completely disassembled into its component proteins that migrated at their monomer molecular weight on native PAGE. FABP1, freed from the complex, was now able to bind to intestinal ER and generate the pre-chylomicron transport vesicle (PCTV). No increase in ER binding or PCTV generation was observed in the absence of PKCζ or ATP. We conclude that phosphorylation of Sar1b disrupts the FABP1-containing four-membered 75-kDa protein complex in cytosol enabling it to bind to the ER and generate PCTV.

Gut triglyceride production

Our knowledge of how the body absorbs triacylglycerols (TAG) from the diet and how this process is regulated has increased at a rapid rate in recent years. Dietary TAG are hydrolyzed in the intestinal lumen to free fatty acids (FFA) and monoacylglycerols (MAG), which are taken up by enterocytes from their apical side, transported to the endoplasmic reticulum (ER) and resynthesized into TAG. TAG are assembled into chylomicrons (CM) in the ER, transported to the Golgi via pre-chylomicron transport vesicles and secreted towards the basolateral side. In this review, we mainly focus on the roles of key proteins involved in uptake and intracellular transport of fatty acids, their conversion to TAG and packaging into CM. We will also discuss intracellular transport and secretion of CM. Moreover, we will bring to light few factors that regulate gut triglyceride production. Furthermore, we briefly summarize pathways involved in cholesterol absorption. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

Gut-liver interaction in triglyceride-rich lipoprotein metabolism

The liver and intestine have complementary and coordinated roles in lipoprotein metabolism. Despite their highly specialized functions, assembly and secretion of triglyceride-rich lipoproteins (TRL; apoB-100-containing VLDL in the liver and apoB-48-containing chylomicrons in the intestine) are regulated by many of the same hormonal, inflammatory, nutritional, and metabolic factors. Furthermore, lipoprotein metabolism in these two organs may be affected in a similar fashion by certain disorders. In insulin resistance, for example, overproduction of TRL by both liver and intestine is a prominent component of and underlies other features of a complex dyslipidemia and increased risk of atherosclerosis. The intestine is gaining increasing recognition for its importance in affecting whole body lipid homeostasis, in part through its interaction with the liver. This review aims to integrate recent advances in our understanding of these processes and attempts to provide insight into the factors that coordinate lipid homeostasis in these two organs in health and disease.

Molecular analysis and intestinal expression of SAR1 genes and proteins in Anderson's disease (Chylomicron retention disease)

BACKGROUND

Anderson's disease (AD) or chylomicron retention disease (CMRD) is a very rare hereditary lipid malabsorption syndrome. In order to discover novel mutations in the SAR1B gene and to evaluate the expression, as compared to healthy subjects, of the Sar1 gene and protein paralogues in the intestine, we investigated three previously undescribed individuals with the disease.

METHODS

The SAR1B, SAR1A and PCSK9 genes were sequenced. The expression of the SAR1B and SAR1A genes in intestinal biopsies of both normal individuals and patients was measured by RTqPCR. Immunohistochemistry using antibodies to recombinant Sar1 protein was used to evaluate the expression and localization of the Sar1 paralogues in the duodenal biopsies.

RESULTS

Two patients had a novel SAR1B mutation (p.Asp48ThrfsX17). The third patient, who had a previously described SAR1B mutation (p.Leu28ArgfsX7), also had a p.Leu21dup variant of the PCSK9 gene. The expression of the SAR1B gene in duodenal biopsies from an AD/CMRD patient was significantly decreased whereas the expression of the SAR1A gene was significantly increased, as compared to healthy individuals. The Sar1 proteins were present in decreased amounts in enterocytes in duodenal biopsies from the patients as compared to those from healthy subjects.

CONCLUSIONS

Although the proteins encoded by the SAR1A and SAR1B genes are 90% identical, the increased expression of the SAR1A gene in AD/CMRD does not appear to compensate for the lack of the SAR1B protein. The PCSK9 variant, although reported to be associated with low levels of cholesterol, does not appear to exert any additional effect in this patient. The results provide further insight into the tissue-specific nature of AD/CMRD.

The identification of the SNARE complex required for the fusion of VLDL-transport vesicle with hepatic cis-Golgi

VLDLs (very-low-density lipoproteins) are synthesized in the liver and play an important role in the pathogenesis of atherosclerosis. Following their biogenesis in hepatic ER (endoplasmic reticulum), nascent VLDLs are exported to the Golgi which is a physiologically regulatable event. We have previously shown that a unique ER-derived vesicle, the VTV (VLDL-transport vesicle), mediates the targeted delivery of VLDL to the Golgi lumen. Because VTVs are different from other ER-derived transport vesicles in their morphology and biochemical composition, we speculated that a distinct set of SNARE (soluble N-ethylmaleimide-sensitive factor-attachment protein receptor) proteins would form a SNARE complex which would eventually facilitate the docking/fusion of VTVs with Golgi. Our results show that Sec22b is concentrated in VTVs as compared with the ER. Electron microscopic results show that Sec22b co-localizes with p58 and Sar1 on the VTV surface. Pre-treatment of VTV with antibodies against Sec22b inhibited VTV-Golgi fusion, indicating its role as a v-SNARE (vesicle SNARE). To isolate the SNARE complex, we developed an in vitro docking assay in which VTVs were allowed to dock with the Golgi, but fusion was prevented to stabilize the SNARE complex. After the docking reaction, VTV-Golgi complexes were collected, solubilized in 2% Triton X-100 and the SNARE complex was co-immunoprecipitated using anti-Sec22b or GOS28 antibodies. A approximately 110 kDa complex was identified in non-boiled samples that was dissociated upon boiling. The components of the complex were identified as Sec22b, syntaxin 5, rBet1 and GOS28. Antibodies against each SNARE component significantly inhibited VTV-Golgi fusion. We conclude that the SNARE complex required for VTV-Golgi fusion is composed of Sec22b, syntaxin 5, rBet1 and GOS28.

The biogenesis of chylomicrons

The absorption of dietary fat is of increasing concern given the rise of obesity not only in the United States but throughout the developed world. This review explores what happens to dietary fat within the enterocyte. Absorbed fatty acids and monoacylglycerols are required to be bound to intracellular proteins and/or to be rapidly converted to triacylglycerols to prevent cellular membrane disruption. The triacylglycerol produced at the level of the endoplasmic reticulum (ER) is either incorporated into prechylomicrons within the ER lumen or shunted to triacylglycerol storage pools. The prechylomicrons exit the ER in a specialized transport vesicle in the rate-limiting step in the intracellular transit of triacylglycerol across the enterocyte. The prechylomicrons are further processed in the Golgi and are transported to the basolateral membrane via a separate vesicular system for exocytosis into the intestinal lamina propria. Fatty acids and monoacylglycerols entering the enterocyte via the basolateral membrane are also incorporated into triacylglycerol, but the basolaterally entering lipid is much more likely to enter the triacylglycerol storage pool than the lipid entering via the apical membrane.

A novel multiprotein complex is required to generate the prechylomicron transport vesicle from intestinal ER

Dietary lipid absorption is dependent on chylomicron production whose rate-limiting step across the intestinal absorptive cell is the exit of chylomicrons from the endoplasmic reticulum (ER) in its ER-to-Golgi transport vesicle, the prechylomicron transport vesicle (PCTV). This study addresses the composition of the budding complex for PCTV. Immunoprecipitation (IP) studies from rat intestinal ER solubilized in Triton X-100 suggested that vesicle-associated membrane protein 7 (VAMP7), apolipoprotein B48 (apoB48), liver fatty acid-binding protein (L-FABP), CD36, and the COPII proteins were associated on incubation of the ER with cytosol and ATP. This association was confirmed by chromatography of the solubilized ER over Sephacryl S400-HR in which these constituents cochromatographed with an apparent kDa of 630. No multiprotein complex was detected when the ER was chromatographed in the absence of PCTV budding activity (resting ER or PKCzeta depletion of ER and cytosol). Treatment of the ER with anti-apoB48 or anti-VAMP7 antibodies or using gene disrupted L-FABP or CD36 mice all significantly inhibited PCTV generation. A smaller complex (no COPII proteins) was formed when only rL-FABP was used to bud PCTV. The data support the conclusion that the PCTV budding complex in intestinal ER is composed of VAMP7, apoB48, CD36, and L-FABP, plus the COPII proteins.

Intestinal absorption of long-chain fatty acids: evidence and uncertainties

Over the two last decades, cloning of proteins responsible for trafficking and metabolic fate of long-chain fatty acids (LCFA) in gut has provided new insights on cellular and molecular mechanisms involved in fat absorption. To this systematic cloning period, functional genomics has succeeded in providing a new set of surprises. Disruption of several genes, thought to play a crucial role in LCFA absorption, did not lead to clear phenotypes. This observation raises the question of the real physiological role of lipid-binding proteins and lipid-metabolizing enzymes expressed in enterocytes. The goal of this review is to analyze present knowledge concerning the main steps of intestinal fat absorption from LCFA uptake to lipoprotein release and to assess their impact on health.

Intestinal lipid absorption

Our knowledge of the uptake and transport of dietary fat and fat-soluble vitamins has advanced considerably. Researchers have identified several new mechanisms by which lipids are taken up by enterocytes and packaged as chylomicrons for export into the lymphatic system or clarified the actions of mechanisms previously known to participate in these processes. Fatty acids are taken up by enterocytes involving protein-mediated as well as protein-independent processes. Net cholesterol uptake depends on the competing activities of NPC1L1, ABCG5, and ABCG8 present in the apical membrane. We have considerably more detailed information about the uptake of products of lipid hydrolysis, the active transport systems by which they reach the endoplasmic reticulum, the mechanisms by which they are resynthesized into neutral lipids and utilized within the endoplasmic reticulum to form lipoproteins, and the mechanisms by which lipoproteins are secreted from the basolateral side of the enterocyte. apoB and MTP are known to be central to the efficient assembly and secretion of lipoproteins. In recent studies, investigators found that cholesterol, phospholipids, and vitamin E can also be secreted from enterocytes as components of high-density apoB-free/apoAI-containing lipoproteins. Several of these advances will probably be investigated further for their potential as targets for the development of drugs that can suppress cholesterol absorption, thereby reducing the risk of hypercholesterolemia and cardiovascular disease.