HDL3-retroendocytosis in cultured small intestinal crypt cells: a novel mechanism of cholesterol efflux

Dietary cholesterol and apolipoprotein A-I are trafficked in endosomes and lysosomes in the live zebrafish intestine

Difficulty in imaging the vertebrate intestine in vivo has hindered our ability to model nutrient and protein trafficking from both the lumenal and basolateral aspects of enterocytes. Our goal was to use live confocal imaging to increase understanding of intestinal trafficking of dietary cholesterol and apolipoprotein A-I (APOA-I), the main structural component of high-density lipoproteins. We developed a novel assay to visualize live dietary cholesterol trafficking in the zebrafish intestine by feeding TopFluor-cholesterol (TF-cholesterol), a fluorescent cholesterol analog, in a lipid-rich, chicken egg yolk feed. Quantitative microscopy of transgenic zebrafish expressing fluorescently tagged protein markers of early, recycling, and late endosomes/lysosomes provided the first evidence, to our knowledge, of cholesterol transport in the intestinal endosomal-lysosomal trafficking system. To study APOA-I dynamics, transgenic zebrafish expressing an APOA-I fluorescent fusion protein (APOA-I-mCherry) from tissue-specific promoters were created. These zebrafish demonstrated that APOA-I-mCherry derived from the intestine accumulated in the liver and vice versa. Additionally, intracellular APOA-I-mCherry localized to endosomes and lysosomes in the intestine and liver. Moreover, live imaging demonstrated that APOA-I-mCherry colocalized with dietary TF-cholesterol in enterocytes, and this colocalization increased with feeding time. This study provides a new set of tools for the study of cellular lipid biology and elucidates a key role for endosomal-lysosomal trafficking of intestinal cholesterol and APOA-I. NEW & NOTEWORTHY A fluorescent cholesterol analog was fed to live, translucent larval zebrafish to visualize intracellular cholesterol and apolipoprotein A-I (APOA-I) trafficking. With this model intestinal endosomal-lysosomal cholesterol trafficking was observed for the first time. A new APOA-I fusion protein (APOA-I-mCherry) expressed from tissue-specific promoters was secreted into the circulation and revealed that liver-derived APOA-I-mCherry accumulates in the intestine and vice versa. Intestinal, intracellular APOA-I-mCherry was observed in endosomes and lysosomes and colocalized with dietary cholesterol.

Endocytosis of lipoproteins

During their metabolism, all lipoproteins undergo endocytosis, either to be degraded intracellularly, for example in hepatocytes or macrophages, or to be re-secreted, for example in the course of transcytosis by endothelial cells. Moreover, there are several examples of internalized lipoproteins sequestered intracellularly, possibly to exert intracellular functions, for example the cytolysis of trypanosoma. Endocytosis and the subsequent intracellular itinerary of lipoproteins hence are key areas for understanding the regulation of plasma lipid levels as well as the biological functions of lipoproteins. Indeed, the identification of the low-density lipoprotein (LDL)-receptor and the unraveling of its transcriptional regulation led to the elucidation of familial hypercholesterolemia as well as to the development of statins, the most successful therapeutics for lowering of cholesterol levels and risk of atherosclerotic cardiovascular diseases. Novel limiting factors of intracellular trafficking of LDL and the LDL receptor continue to be discovered and to provide drug targets such as PCSK9. Surprisingly, the receptors mediating endocytosis of high-density lipoproteins or lipoprotein(a) are still a matter of controversy or even new discovery. Finally, the receptors and mechanisms, which mediate the uptake of lipoproteins into non-degrading intracellular itineraries for re-secretion (transcytosis, retroendocytosis), storage, or execution of intracellular functions, are largely unknown.

Lipid Metabolism in Zebrafish

Forward genetics is an unbiased methodology to discover new genes or functions of genes. At the present, the zebrafish is one of the few vertebrate systems where large-scale forward genetic studies are practical. Fluorescent lipid labeling of zebrafish larvae derived from families created from ENU-mutagenized fish enabled us to perform a large scale in vivo screen to identify mutants with perturbed lipid processing. With the aid of the zebrafish genome project, positional cloning of mutated genes with abnormal lipid metabolism can be accelerated. MO- and gripNA-based transient gene silencing is feasible in zebrafish embryos and provides a reverse genetic screening strategy to search for important lipid regulators. The advantages of using zebrafish as a vertebrate model to study lipid metabolism include its rapid external development and its optical clarity that enables the monitoring of biological processes. Large scale, high-throughput drug screening in vivo, especially for drugs that inhibit lipid absorption, can be easily achieved in this model. These zebrafish-based assays are important tools to understand aspects of lipid biology with significant clinical implications.

Recycling of apoprotein E is associated with cholesterol efflux and high density lipoprotein internalization

After receptor-mediated endocytosis of triglyceride-rich lipoproteins (TRL) into the liver, TRL particles are immediately disintegrated in peripheral endosomal compartments. Whereas core lipids and apoprotein B are delivered for degradation into lysosomes, TRL-derived apoE is efficiently recycled back to the plasma membrane. This is followed by apoE re-secretion and association of apoE with high density lipoproteins (HDL). Because HDL and apoE can independently promote cholesterol efflux, we investigated whether recycling of TRL-derived apoE in human hepatoma cells and fibroblasts could be linked to intracellular cholesterol transport. In this study we demonstrate that HDL(3) does not only act as an extracellular acceptor for recycled apoE but also stimulates the recycling of internalized TRL-derived apoE. Furthermore, radioactive pulse-chase experiments indicate that apoE recycling is accompanied by cholesterol efflux. Confocal imaging reveals co-localization of apoE and cholesterol in early endosome antigen 1-positive endosomes. During apoE re-secretion, HDL(3)-derived apoA-I is found in these early endosome antigen 1, cholesterol-containing endosomes. As shown by time-lapse fluorescence microscopy, apoE recycling involves the intracellular trafficking of apoA-I to pre-existing and TRL-derived apoE/cholesterol-containing endosomes in the periphery. Thus, these studies provide evidence for a new intracellular link between TRL-derived apoE, cellular cholesterol transport, and HDL metabolism.

Cholesterol efflux to apolipoprotein AI involves endocytosis and resecretion in a calcium-dependent pathway

We previously have described the cAMP-mediated induction of cholesterol and phospholipid efflux from the murine macrophage RAW264 cell line to lipid-free apolipoprotein acceptors. This induction of cholesterol efflux is associated with increased binding and association of apolipoprotein to the cells. In the present study, using primarily apolipoprotein AI (apoAI) as the acceptor, cAMP-dependent cholesterol efflux to apolipoprotein acceptors was associated with apoAI binding to coated pits, cellular uptake, and resecretion. After cell association and washing, 58% of the apoAI was resecreted during a 90-min chase period. In addition, after apoAI uptake and washing, cholesterol efflux was observed during a chase period without additional acceptors. Cholesterol efflux was partially blocked by chlorpromazine and hypertonic media, two inhibitors of coated pit endocytosis. Cholesterol efflux to apoAI was found to depend on extracellular calcium. By temporally separating the cAMP induction phase from the apoAI chase phase, calcium was found to be required during the apoAI chase phase rather than during the cAMP induction period. In the absence of calcium the 8-Br-cAMP-mediated induction of apoAI binding was maintained, but the specific apoAI cellular association was inhibited. The data are consistent with a model for cholesterol efflux to apolipoproteins that involves a calcium-dependent endocytic pathway, followed by recycling and the subsequent release of the nascent lipoprotein particle from the cell.

Clathrin-mediated endocytosis of high density lipoprotein3 in human intestinal Caco-2 cells. A post-embedding immunocytochemical study

The mechanism by which high density lipoprotein (HDL) removes excess cholesterol from intracellular sites has been the subject of much controversy. There is some evidence that HDL binds to specific cell surface receptors without internalization. Other evidence suggests that HDL is taken up by endocytosis, enters a pathway of endosomal trafficking and is resecreted from the cells (retroendocytsosis). In the present study, we investigated the distribution of apolipoprotein AI, the major protein constituent of HDL, in cultured intestinal Caco-2 cells employing post-embedding immunocytochemistry on LR White-embedded material. Cells grown under control conditions showed label for apolipoprotein AI in the endoplasmic reticulum. After incubation with native apolipoprotein E-free high density lipoprotein3 (HDL3) additional label for apolipoprotein AI was found in endosomes. These endosomes were observed near lipid droplets and in the basolateral cytoplasm. Further, it was demonstrated that label for apolipoprotein AI was colocalized with label for clathrin on the basolateral membrane. Our results support the concept that HDL3 is internalized and subsequently processed in an endosomal pathway in Caco-2 cells besides de novo synthesis of apolipoprotein AI.

HDL3-mediated cholesterol efflux from cultured enterocytes: the role of apoproteins A-I and A-II

High density lipoproteins (HDL) were recently demonstrated in an enterocyte model (CaCo-2 cells) to mediate reverse cholesterol transport by retroendocytosis. The present study was carried out to define the role of the major HDL apoproteins (apo) A-I and apo A-II in this pathway. HDL3 was fractionated by heparin affinity chromatography into the two main fractions containing either apo A-I only (fraction A) or both apo A-I and apo A-II (fraction B). In addition, liposomes were reconstituted from purified apo A-I or apo A-II and dimyristoyl phosphatidylcholine. The cell binding properties and cholesterol efflux potential were studied in the lipoprotein fractions and the liposomes. Both fractions exhibited similar maximal binding capacities of 4427 (A) and 5041 (B) ng/mg cell protein, but their dissociation constants differed (40.5 and 167.7 micrograms/mL, respectively). Fraction A induced cholesterol efflux and stimulated cholesterol synthesis more than did fraction B. Fraction A mobilized both cellular free and esterified cholesterol, whereas fraction B preferentially mobilized cholesteryl esters. Liposomes, containing either apo A-I or apo A-II, showed specific binding, endocytosis and endosomal transport, and were released as intact particles. Apo A-I liposomes also mediated cholesterol efflux. In conclusion, there is evidence that the HDL3 subfractions A and B, as well as reconstituted liposomes containing either apo A-I or apo A-II, were specifically bound and entered a retroendocytosis pathway which was directly linked to cholesterol efflux. Quantitatively, the apo A-I subfraction appeared to play the dominant role in normal enterocytes. The apo A-II content of fraction B was related to the mobilization of cholesteryl esters.

High-density lipoprotein 3 retroendocytosis: a new lipoprotein pathway in the enterocyte (Caco-2)

The present study in Caco-2 cells, derived from a human colon carcinoma and capable of enterocyte differentiation in culture, describes a retroendocytotic pathway for high-density lipoprotein 3 (HDL3). These cells exhibit specific binding of apolipoprotein E-free HDL3 which was competed for by HDL3 but not by low-density lipoproteins. At 37 degrees C, degradation was negligible and intact particles were internalized and resecreted into the medium within 2 hours. Electron microscopy showed binding and internalization of gold-labeled HDL3 in coated pit regions and transport in endosomes distinct from lysosomes to lipid droplets. The fusion of these endosomes with lipid droplets was followed by their dissolution and the subsequent extrusion of HDL particles from the cells. Fluorescence labeling studies of HDL3 supported cytosolic transport in vesicles. Specific binding showed negative feedback regulation by HDL3, was modulated by alterations in cellular cholesterol content, and increased with the cellular state of differentiation. HDL3 mediated efflux of endogenously labeled cholesterol. It is concluded that intact HDL3 is bound specifically by Caco-2 cells, leading to a subsequent intracellular passage and resecretion through a process of retroendocytosis effecting the efflux of cellular cholesterol.