Morphological evidence of lysosomal uptake of high-density lipoproteins by rat adrenocortical cells in vitro

Roles of NS1 Protein in Flavivirus Pathogenesis

Flaviviruses such as dengue, Zika, and West Nile viruses are highly concerning pathogens that pose significant risks to public health. The NS1 protein is conserved among flaviviruses and is synthesized as a part of the flavivirus polyprotein. It plays a critical role in viral replication, disease progression, and immune evasion. Post-translational modifications influence NS1's stability, secretion, antigenicity, and interactions with host factors. NS1 protein forms extensive interactions with host cellular proteins allowing it to affect vital processes such as RNA processing, gene expression regulation, and cellular homeostasis, which in turn influence viral replication, disease pathogenesis, and immune responses. NS1 acts as an immune evasion factor by delaying complement-dependent lysis of infected cells and contributes to disease pathogenesis by inducing endothelial cell damage and vascular leakage and triggering autoimmune responses. Anti-NS1 antibodies have been shown to cross-react with host endothelial cells and platelets, causing autoimmune destruction that is hypothesized to contribute to disease pathogenesis. However, in contrast, immunization of animal models with the NS1 protein confers protection against lethal challenges from flaviviruses such as dengue and Zika viruses. Understanding the multifaceted roles of NS1 in flavivirus pathogenesis is crucial for effective disease management and control. Therefore, further research into NS1 biology, including its host protein interactions and additional roles in disease pathology, is imperative for the development of strategies and therapeutics to combat flavivirus infections successfully. This Review provides an in-depth exploration of the current available knowledge on the multifaceted roles of the NS1 protein in the pathogenesis of flaviviruses.

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.

Lipoproteins, lipid droplets, lysosomes, and adrenocortical steroid hormone synthesis: morphological studies

Recent studies concerning cellular cholesterol homeostasis suggest that there is a relationship between the serum lipoproteins (low density and high density lipoproteins: LDL and HDL), the intracellular storage of cholesterol (lipid droplets), lysosomes, and the steroidogenic activity of adrenocortical cells. This review surveys the current knowledge on cholesterol import from LDL/HDL by adrenocortical cells, its regulation, and the participation of lipid droplets and lysosomes in this process. The possible role of adrenocortical cell microvilli in the uptake of LDL/HDL is discussed. Under certain physiological, experimental, and pathological circumstances lysosomes accumulate unesterified and/or esterified cholesterol in the form of lipid-lysosome complexes. As suggested by the data presented in this review, lipid-lysosome complexes appear to be involved in cholesterol homeostasis, via altering lipid compartmentalization. Since previous reports do not clearly demonstrate a positive correlation between the volume of lipid- and lysosome-compartments and the rate of steroid hormone synthesis [for review, see Nussdorfer (1986) Int. Rev. Cytol., 98:1-405], the objective of this review is to provide a better understanding of the interactions of plasma lipoproteins, lipid droplets, lysosomes, and steroidogenesis.

Effect of lipoproteins on cholesterol synthesis in rat Sertoli cells

Lipoprotein metabolism has been investigated in cultured rat Sertoli cells. Cells incubated with low-density lipoproteins (LDLs) or high-density lipoproteins (HDLs) showed a concentration-dependent decrease of sterol synthesis, indicating a net cholesterol delivery to the Sertoli cells. At 50 micrograms/mL, lipoproteins inhibited the incorporation of [14C]acetate into free cholesterol by 83% for the LDL and 47% for the HDL. Electron microscopic examinations of the Sertoli cells provide evidence of the internalization of gold-labelled HDL into coated pits and coated vesicles. Competitive studies between human LDL and rat HDL indicate that Sertoli cells take up cholesterol from LDL and HDL containing apolipoprotein (apo) E by common pathways. These results suggest that Sertoli cells possess apo B and E receptors for the uptake and degradation of LDL and HDL, although the basement membrane excludes the passage of LDL from blood capillaries to the Sertoli cells. At 50 micrograms/mL, apo-E-depleted HDL inhibited the incorporation of [14C]acetate into free cholesterol by 34%. Thus, this study shows that Sertoli cells are capable of taking up apo-E-depleted HDL cholesterol for cell metabolism.

Cellular uptake and catabolism of high-density-lipoprotein triacylglycerols in human cultured fibroblasts: degradation block in neutral lipid storage disease

High-density lipoprotein (HDL)-[3H]triolein (i.e. [3H]triolein incorporated into reconstituted HDL) was taken up by cultured fibroblasts through an apparently saturable process, competitively inhibited by non-labelled HDL and independent of the LDL receptor. Using 125I-HDL and HDL-[3H]triolein, binding experiments (at 0 degrees C) followed by a short-time 'chase' at 37 degrees C showed that 125I radioactivity was rapidly released in the culture medium (as trichloroacetic acid-precipitable material), whereas 3H radioactivity remained associated with the cell. The cell-associated HDL-[3H]triolein was rapidly degraded in normal fibroblasts, and the liberated [3H]oleic acid was incorporated into newly biosynthesized phospholipids. In Wolman-disease fibroblasts HDL-[3H]triolein was degraded at a normal rate, and thus independently of the lysosomal compartment. In contrast, the degradation of HDL-[3H]triolein was blocked in fibroblasts from Neutral Lipid Storage Disease (NLSD), similarly to that of endogenously biosynthesized triacylglycerols [Radom, Salvayre, Nègre, Maret and Douste-Blazy (1987) Eur. J. Biochem. 164, 703-708]. Trypsin-treated HDL-[3H]triolein was also taken up by cells and degraded quite similarly to HDL-[3H]triolein. In conclusion, all these data taken together suggest that HDL-[3H]triolein is: (i) associated with the cell through a process independent of intact apolipoprotein (apo) As, thus probably independent of an apoA-receptor-mediated uptake; (ii) internalized by cells, whereas 125I-apoAs are released in the culture medium; (iii) directed to the same non-lysosomal catabolic pool (blocked in NLSD) as for endogenously biosynthesized triacylglycerols.

Cytoplasmic triacylglycerols and cholesteryl esters are degraded in two separate catabolic pools in cultured human fibroblasts

The sources and the catabolic pathways of cytoplasmic pools of triacylglycerols and cholesteryl esters have been comparatively investigated in cultured fibroblasts from normal subjects and from patients affected with neutral lipid storage disease (NLSD) and Wolman disease (WD). (i) Endogenously biosynthesized triacylglycerols and cholesteryl esters were degraded extra-lysosomally since they were catabolized at similar rates in normal and in WD fibroblasts. In NLSD fibroblasts, the degradation of endogenous triacylglycerols was severely deficient, whereas that of endogenous cholesteryl esters was in the normal range. (ii) Reconstituted high density lipoproteins (HDL) containing radiolabelled [3H]triolein and cholesteryl [14C]oleate were taken up by cultured fibroblasts and rapidly degraded in a non-lysosomal compartment. In NLSD fibroblasts the degradation of HDL-[3H]triolein was blocked whereas that of HDL-[14C]cholesteryl oleate was in the normal range. These data suggest that: (i) the cytoplasmic pools of triacylglycerols and cholesteryl esters originate from HDL uptake and from endogenous biosynthesis as well; (ii) cytoplasmic (non-lysosomal) triacylglycerols and cholesteryl esters are degraded by two separate catabolic pathways.

Characterization and isolation of a high-density-lipoprotein-binding protein from bovine corpus luteum plasma membrane

The ovary uses the cholesterol from high-density lipoproteins (HDL) as a substrate source for steroid hormone production. It is not clear, however, how ovarian cells acquire the lipoprotein cholesterol. This study describes the characterization and isolation of a high-affinity-binding protein for apolipoprotein E-free HDL from the plasma-membrane fraction of bovine corpora lutea. Plasma membranes were prepared by differential centrifugation with 5-6-fold enrichment of 5'-nucleotidase activity. The binding of 125I-HDL to the plasma membranes was time-dependent, and there appeared to be a single high-affinity site with a Kd of 6.7 micrograms of HDL/ml of assay buffer. The binding was not affected by high concentrations of low-density lipoproteins or the Ca2+ chelator EDTA, nor by changes in pH in the range 6.5-9.0. The binding was affected by the salt concentration in the buffer, with a dose-dependent increase that reached a maximum at 150-250 mM-NaCl. Binding was increased in the presence of high concentrations of KCl and KBr, and most significantly increased by high concentrations of bivalent metal ions. Ligand-blot analysis under reducing conditions revealed that the binding protein was a single polypeptide of about 108 kDa that was associated with the plasma-membrane fraction. This HDL-binding protein was purified to homogeneity by solubilization with Triton X-100, poly(ethylene glycol) precipitation, DEAE-Sephadex chromatography, and preparative SDS/PAGE. The purified binding protein is a single polypeptide of 108 kDa that retains high affinity and specificity for HDL as assayed by ligand blotting.

Lipoprotein receptors and steroidogenesis in adrenocortical cells

Steroid-producing tissues require a continuous supply of cholesterol for hormone synthesis. In the majority of the steroidogenic tissues the cholesterol is imported via the receptor-mediated uptake of lipoproteins, and therefore the influence on the lipoprotein receptors provides an additional level for the regulation of hormone synthesis. Hormones regulating the adrenocortical activity exert both short- and long-term action, and thus they may control the interactions of the major cholesterol delivery particles--low- (LDLs) and high-density lipoproteins (HDLs)--and their receptors in short- and long-term action, possibly modulating the signal transduction in the former case and the number and distribution in the latter. The LDL and HDL pathway and the signal transduction mechanism is briefly reviewed. Data are discussed concerning short- and long-term action of hormones (alpha-MSH and ACTH, respectively) on the HDL3 receptors of isolated adrenocortical cells. Short-term treatment with alpha-MSH and long-term treatment with ACTH increased the binding of HDL3 to zona glomerulosa and fasciculata cells, respectively, while both treatments increased the hormone production in the presence of HDL. The lipoprotein receptors were frequently found on the microvilli of adrenocortical cell membranes.

Effect of a cholesterol synthesis inhibitor (BM 15.766) in the presence and absence of HDL on corticosteroidogenesis of isolated zona glomerulosa and fasciculata cells

The effect of the cholesterol synthesis inhibitor BM 15.766, 4-[2-[1-(4-chlorocinamyl)piperazin-4-yl]ethyl]-benzoic acid on the corticosteroid production was studied in order to reveal the importance of endogenous cholesterol synthesis in the function of zona glomerulosa and zona fasciculata cells of rats. Attempts were made to compensate the effect of BM 15.766 through the application of high-density lipoproteins (HDL). Electron microscopy was used to trace the binding and intracellular accumulation of colloidal gold-labelled HDL (HDL-Au, a cholesterol carrier), in the presence of the cholesterol biosynthesis inhibitor. The stimulation of both types of cells with ACTH was less effective in the presence of 2 x 10(-5) M BM 15.766. The inhibitory effect of BM 15.766 was most marked on the aldosterone production of the zona glomerulosa cells, and could not be reversed by addition of a small amount of HDL-Au. Corticosterone-aldosterone conversion was inhibited by 2 x 10(-5) M BM 15.766. ACTH-stimulated, short-term HDL uptake and internalization was not affected by the cholesterol synthesis inhibitor. The results suggest that certain metabolites of de novo cholesterol biosynthesis may participate in the control of aldosterone production.

Hepatic metabolism of colloidal gold-low-density lipoprotein complexes in the rat: evidence for bulk excretion of lysosomal contents into bile

Rats were treated with 17 alpha-ethinyl estradiol to induce high levels of low-density lipoprotein receptors in hepatocytes. When these rats were given intravenous injections of low-density lipoprotein-colloidal gold complexes, most of the gold (labeled with 195Au) appeared to be taken up by Kupffer cells, as were complexes of colloidal gold with albumin or polyvinylpyrrolidone. However, when these rats were also administered gadolinium chloride, which blocks Kupffer cell activity, most of the low-density lipoprotein-gold (but not gold complexed with albumin or polyvinylpyrrolidone) was taken up into hepatocytes by receptor-mediated endocytosis and concentrated in peribiliary lysosomes, as determined by electron microscopy. Colloidal gold taken up as a complex with low-density lipoprotein was excreted into the feces via the common bile duct at a maximal rate of about 5% daily, 4 to 12 days after injection. Thereafter, the rate of gold excretion fell off until reaching a plateau after 3 weeks. At this late time, most of the colloidal gold was shown by electron microscopy to be in Kupffer cells, whereas earlier (6 days after injection) it was contained mainly in older hepatocytic lysosomes, identified by lipofuscin granules. It is concluded that, in rats, hepatocytic lysosomes empty most of their contents into bile every week or two, apparently by exocytosis.

Colloidal gold-labeled lipoprotein binding and internalization in adrenocortical cells in vitro

Human (normal and adenomatous) and rat (normal) adrenocortical cells were incubated in vitro with colloidal gold labeled low-density (LDL-Au) and high-density (HDL-Au) lipoproteins, respectively, in order to visualize lipoprotein binding and internalization at an electron microscopic level. Both normal and adenomatous human adrenocortical cells accumulated LDL-Au by receptor-mediated endocytosis via coated pits, coated vesicles, noncoated vesicles, and lysosomes. HDL-Au was not internalized. In rat adrenocortical cells, both HDL-Au and to a lesser extent LDL-Au were internalized. It is concluded that LDL-Au and HDL-Au conjugates can be used to identify lipoprotein receptors and to follow lipoprotein internalization in adrenocortical cells.