Phytosterol supplements lower low density lipoprotein (LDL) cholesterol, but accumulate in vascular lesions of patients and limit the anti-atherosclerotic effects of LDL lowering in apolipoprotein E deficient mice, suggesting that the cholesterol lowering benefit of phytosterol supplementation may not be fully realized. Individual phytosterols have cell-type specific effects that may either be beneficial or deleterious with respect to atherosclerosis, but little is known concerning their effects on macrophage function. The effects of phytosterols on ABCA1 and ABCG1 abundance, cholesterol efflux, and inflammatory cytokine secretion were determined in cultured macrophage foam cells. Among the commonly consumed phytosterols, stigmasterol increased expression of ABCA1 and ABCG1 and increased efflux of cholesterol to apolipoprotein (Apo) AI and high density lipoprotein (HDL). Campesterol and sitosterol had no effect on ABCA1 or ABCG1 levels. Sitosterol had no effect of cholesterol efflux to Apo AI or HDL, whereas campesterol had a modest, but significant reduction in cholesterol efflux to HDL in THP-1 macrophages. Whereas stigmasterol blunted aggregated LDL-induced increases in tumor necrosis factor (TNF)-α, interleukin (IL)-6 and IL-1β secretion, sitosterol exacerbated these effects. The presence of campesterol had no effect on agLDL-induced inflammatory cytokine secretion from THP-1 macrophages. In conclusion, the presence of stigmasterol in modified lipoproteins promoted cholesterol efflux and suppressed inflammatory cytokine secretion in response to lipid loading in macrophage foam cells. While campesterol was largely inert, the presence of sitosterol increased the proinflammatory cytokine secretion.
This article is available online at http://www.jlr.orgPeroxisomes play essential roles in the metabolism of both dietary and endogenously synthesized lipids ( 1, 2 ). The synthesis of the etherphospholipids (platelet activating factor and plasmalogens), partial  -oxidation of verylong-chain fatty acids (VLCFAs; C>22), and the ␣ -oxidation of phytanic acid occur within this organelle. A comprehensive integrative map of peroxisomal metabolic pathways can be found at www.peroxisomedb.org ( 3 ). The importance of the organelle is best exemplifi ed by the occur rence of peroxisome biogenesis disorders and singleenzyme defi ciencies that result in severe metabolic diseases that are often lethal in early childhood. X-linked adrenoleukodystrophy (X-ALD) is the most common peroxisomal disorder characterized by the accumulation of VLCFA in tissues, progressive demyelination, and adrenocortical dysfunction ( 4 ). X-ALD is caused by mutations in ABCD1 (D1), an ATP binding cassette (ABC) half transporter that presumably mediates the transport of very-long-chain acylCoAs into peroxisomes ( 5 ). D1 is the founding member of a quartet of ABC half-transporters that contain peroxisomal targeting sequences and form heterodimers in vitro ( 6 ). However, homodimers appear to be preferred in vivo, and each subfamily member has a unique tissue distribution ( 7,8 ).The closest paralog to D1 is D2, which shares 66% amino acid identity and is expressed in the adrenal, brain, and liver ( 9 ). Expression of a D2 transgene in D1-defi cient mice partially corrected the phenotype in this mouse Abstract The ATP binding cassette transporter, ABCD2 (D2), is a peroxisomal protein whose mRNA has been detect ed in the adrenal, brain, liver, and fat. Although the role of this transporter in neural tissues has been studied, its function in adipose tissue remains unexplored. The level of immunoreactive D2 in epididymal fat is >50-fold of that found in brain or adrenal. D2 is highly enriched in adipocytes and is upregulated during adipogenesis but is not essen tial for adipocyte differentiation or lipid accumulation in day 13.5 mouse embryonic fi broblasts isolated from D2-defi cient (D2 ؊ / ؊ ) mice. Although no differences were appreci ated in differentiation percentage, total lipid accumulation was greater in D2 ؊ / ؊ adipocytes compared with the wild type. These results were consistent with in vivo observa tions in which no signifi cant differences in adiposity or adipocyte diameter between wild-type and D2؊ / ؊ mice were observed. D2
Background: G5G8 promotes elimination of cholesterol. Results: The absence of G5G8 accelerates the loss of glycemic control and exacerbates the development of steatosis. Conclusion: G5G8 mitigates the impact of a high fat diet on hepatic lipid accumulation and glucose intolerance. Significance: Increased G5G8 in insulin resistance may be an adaptive mechanism that opposes steatosis but increases the risk of cholesterol gallstones.
ABGG5 (G5) and ABCG8 (G8) are ABC half-transporters that dimerize within the endoplasmic reticulum, traffic to the cell surface, and mediate cholesterol excretion into bile. Mice harboring defects in the leptin axis (db/db and ob/ob) have reduced biliary cholesterol concentrations. Rapid weight loss brought about by administration of leptin or dietary restriction increases biliary cholesterol excretion. We hypothesized that the reduction in biliary cholesterol in mice harboring defects in the leptin axis is associated with a reduction in G5G8 transporters and that levels of the transporter would increase with leptin administration and dietary restriction. We examined mRNA and protein levels for G5 and G8 in db/db and ob/ob mice. In both models G5 and G8 protein levels were reduced. In ob/ob mice, both leptin administration and dietary restriction increased G5 and G8 protein and biliary cholesterol concentrations. Finally, we examined the effects of tauroursodeoxycholate, which has been shown to increase biliary cholesterol excretion and function as a molecular chaperone. Tauroursodeoxycholate increased G5 and G8 protein and biliary cholesterol concentrations in both wild-type and db/db mice. Our results indicate that the mechanism for reduced biliary cholesterol excretion in db/db and ob/ob mice involves reductions in G5 and G8 protein levels and that this may occur at the level of G5G8 heterodimer assembly within the endoplasmic reticulum. ABCG5 (G5) and ABCG8 (G8) play a major role in the elimination of dietary and endogenously synthesized sterols in humans and mice (1-5). G5 and G8 are ATP binding cassette (ABC) 2 half-transporters that dimerize in the endoplasmic reticulum (ER) before trafficking of the functional G5G8 sterol transporter to the apical surface where it promotes the excretion of sterols from hepatocytes and enterocytes (3, 6 -8). In the liver, the G5G8 sterol transporter appears to be the major route for cholesterol excretion into bile. Mice deficient in G5, G8, or both transporters have 80 -90% reductions in biliary cholesterol concentrations (3-5). Conversely, expression of a human transgene in mice results in a 6 -8-fold increase in biliary cholesterol concentration and supersaturation of bile (7).In the absence of dietary or pharmacological perturbations, biliary cholesterol concentrations correlate with Abcg5/Abcg8 genocopy (9). Activation of the liver X receptor (LXR␣, NR1H3) by cholesterol feeding or administration of an agonist increases G5 and G8 mRNA as well as biliary cholesterol concentrations (10). In addition, these effects are absent in mice lacking LXR ␣/, suggesting that LXR is the principal regulator of G5 and G8 expression in response to dietary cholesterol (11). Although biliary cholesterol excretion generally correlates with expression levels of G5 and G8 mRNAs, exceptions include increases in biliary cholesterol excretion after treatment with diosgenin or tauroursodeoxycholate (TUDCA) and in liver transplant patients after surgery (12-14). The uncoupling of biliary chole...
Purpose of review This review summarizes recent developments in the activity, regulation, and physiology of the ABCG5 ABCG8 (G5G8) transporter and the use of its xenobiotic substrates, phytosterols, as cholesterol lowering agents in the treatment of cardiovascular disease. Recent progress has significant implications for the role of G5G8 and its substrates in complications associated with features of the metabolic syndrome. Recent findings Recent reports expand the clinical presentation of sitosterolemia to include platelet and adrenal dysfunction. The G5G8 sterol transporter is critical to hepatobiliary excretion of cholesterol under nonpathological conditions and has been linked to the cholesterol gallstone susceptibility. Finally, the cardiovascular benefits of cholesterol lowering through the use of phytosterol supplements were offset by vascular dysfunction, suggesting that alternative strategies to reduced cholesterol absorption offer greater benefit. Summary Insulin resistance elevates G5G8 and increases susceptibility to cholesterol gallstones. However, this transporter is critical for the exclusion of phytosterols from the absorptive pathways in the intestine. Challenging the limits of this protective mechanism through phytosterol supplementation diminishes the cardioprotective benefits of cholesterol lowering in mouse models of cardiovascular disease.
Objective Recent studies support a role for cholesterol in the development of obesity and nonalcoholic fatty liver disease. Mice lacking the ABCG5 ABCG8 (G5G8) sterol transporter have reduced biliary cholesterol secretion and are more susceptible to steatosis, hepatic insulin resistance, and loss of glycemic control when challenged with a high-fat diet. We hypothesized that accelerating G5G8-mediated biliary cholesterol secretion would correct these phenotypes in obese mice. Approach and Results Obese (db/db) male and their lean littermates were administered a cocktail of control adenovirus or adenoviral vectors encoding ABCG5 and ABCG8 (AdG5G8). Three days after viral administration, measures of lipid and glucose homeostasis were determined, and tissues were collected for biochemical analyses. AdG5G8 increased biliary cholesterol and fecal sterol elimination. Fasting glucose and triglycerides declined, and glucose tolerance improved in obese mice expressing G5G8 compared with mice receiving control adenovirus. These changes were associated with a reduction in phosphorylated eukaryotic initiation factor 2α and c-Jun N-terminal kinase in liver, suggesting alleviation of endoplasmic reticulum stress. Phosphorylated insulin receptor and protein kinase B were increased, indicating restored hepatic insulin signaling. However, there was no reduction in hepatic triglycerides after the 3-day treatment period. Conclusions Accelerating biliary cholesterol secretion restores glycemic control and reduces plasma triglycerides in obese db/db mice.
Synapse formation can be promoted by intense activity. At the Drosophila larval neuromuscular junction (NMJ), new synaptic boutons can grow acutely in response to patterned stimulation. We combined confocal imaging with electron microscopy and tomography to investigate the initial stages of growth and differentiation of new presynaptic boutons at the Drosophila NMJ. We found that the new boutons can form rapidly in intact larva in Significance Statement Neurons can grow and form new synapses in response to intense activity. We investigated the stages of synapse formation at intact and dissected Drosophila larvae and identified a very rapid initial step, which is especially sensitive to nerve stimulation. Specifically, we demonstrated that intense activity triggers budding of new synaptic boutons filled with vesicles, and this pathway becomes very prominent under the conditions of pathologic activity, such as seizures. We found that this pathway depends on protein kinase A and its target synapsin, the protein regulating clustering of synaptic vesicles. These findings suggest a new function for dynamic vesicle clustering in neuronal development and demonstrate that this mechanism can create a positive feedback loop during seizure activity. New Research July/August 2019, 6(4) ENEURO.0060-19.2019 1-15 response to intense crawling activity, and we observed two different patterns of bouton formation and maturation. The first pathway involves the growth of filopodia followed by a formation of boutons that are initially devoid of synaptic vesicles (SVs) but filled with filamentous matrix. The second pathway involves rapid budding of synaptic boutons packed with SVs, and these more mature boutons are sometimes capable of exocytosis/endocytosis. We demonstrated that intense activity predominantly promotes the second pathway, i.e., budding of more mature boutons filled with SVs. We also showed that this pathway depends on synapsin (Syn), a neuronal protein which reversibly associates with SVs and mediates their clustering via a protein kinase A (PKA)-dependent mechanism. Finally, we took advantage of the temperature-sensitive mutant sei to demonstrate that seizure activity can promote very rapid budding of new boutons filled with SVs, and this process occurs at scale of minutes. Altogether, these results demonstrate that intense activity acutely and selectively promotes rapid budding of new relatively mature presynaptic boutons filled with SVs, and that this process is regulated via a PKA/Syn-dependent pathway.
Synaptic vesicles fuse at morphological specializations in the presynaptic terminal termed active zones (AZs). Vesicle fusion can occur spontaneously or in response to an action potential. Following fusion, vesicles are retrieved and recycled within nerve terminals. It is still unclear whether vesicles that fuse spontaneously or following evoked release share similar recycling mechanisms. Genetic deletion of the SNARE-binding protein complexin dramatically increases spontaneous fusion, with the protein serving as the synaptic vesicle fusion clamp at Drosophila synapses. We examined synaptic vesicle recycling pathways at complexin null neuromuscular junctions, where spontaneous release is dramatically enhanced. We combined loading of the lipophilic dye FM1-43 with photoconversion, electron microscopy, and electrophysiology to monitor evoked and spontaneous recycling vesicle pools. We found that the total number of recycling vesicles was equal to those retrieved through spontaneous and evoked pools, suggesting that retrieval following fusion is partially segregated for spontaneous and evoked release. In addition, the kinetics of FM1-43 destaining and synaptic depression measured in the presence of the vesicle-refilling blocker bafilomycin indicated that spontaneous and evoked recycling pools partially intermix during the release process. Finally, FM1-43 photoconversion combined with electron microscopy analysis indicated that spontaneous recycling preferentially involves synaptic vesicles in the vicinity of AZs, whereas vesicles recycled following evoked release involve a larger intraterminal pool. Together, these results suggest that spontaneous and evoked vesicles use separable recycling pathways and then partially intermix during subsequent rounds of fusion.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.