This review discusses advances in understanding how the controlled delivery of cholesterol between subcellular compartments is achieved and what novel experimental strategies are being employed to ...address this fundamental question. Recent work has focused on cholesterol-binding proteins that can facilitate directional cholesterol transfer between contacts of the ER and Golgi or late endosomal membranes. Increasing structural information on cholesterol-binding proteins, new modules engineered from them as well as improved imaging and gene editing techniques are providing valuable insights. There is also mounting information on how the crosstalk between cholesterol transport and nutrient signaling is orchestrated and how cellular fatty acid metabolism and cholesterol homeostasis are intertwined.
Institute of Biomedicine/Anatomy, University of Helsinki, Helsinki, Finland
This review summarizes the mechanisms of cellular cholesterol transport and monogenic human diseases caused by defects in ...intracellular cholesterol processing. In addition, selected mouse models of disturbed cholesterol trafficking are discussed. Current pharmacological strategies to prevent atherosclerosis are largely based on altering cellular cholesterol balance and are introduced in this context. Finally, because of the organizing potential of cholesterol in membranes, disturbances in cellular cholesterol transport have implications for a wide variety of human diseases, of which selected examples are given.
Cholesterol represents the most abundant single lipid in mammalian cells. How its asymmetric distribution between subcellular membranes is achieved and maintained attracts considerable interest. One ...of the challenges is that cholesterol rarely is transported alone, but rather is coupled with heterotypic transport and metabolism of other lipids, in particular phosphoinositides, phosphatidylserine, and sphingolipids. This perspective summarizes the major exo- and endocytic cholesterol transport routes and how lipid transfer proteins at membrane contacts and membrane transport intersect along these routes. It discusses the co-transport of cholesterol with other lipids in mammalian cells and reviews emerging evidence related to the physiological relevance of this process.
Cholesterol is the most abundant single lipid in mammalian cells. Ikonen and Zhou discuss major exo-and endocytic cholesterol transport routes and how lipid transfer proteins at membrane contacts and membrane transport intersect along these routes. They discuss co-transport of cholesterol with other lipids as well as physiological relevance.
Seipin is an endoplasmic reticulum (ER) membrane protein implicated in lipid droplet (LD) biogenesis and mutated in severe congenital lipodystrophy (BSCL2). Here, we show that seipin is stably ...associated with nascent ER–LD contacts in human cells, typically via one mobile focal point per LD. Seipin appears critical for such contacts since ER–LD contacts were completely missing or morphologically aberrant in seipin knockout and BSCL2 patient cells. In parallel, LD mobility was increased and protein delivery from the ER to LDs to promote LD growth was decreased. Moreover, while growing LDs normally acquire lipid and protein constituents from the ER, this process was compromised in seipin‐deficient cells. In the absence of seipin, the initial synthesis of neutral lipids from exogenous fatty acid was normal, but fatty acid incorporation into neutral lipids in cells with pre‐existing LDs was impaired. Together, our data suggest that seipin helps to connect newly formed LDs to the ER and that by stabilizing ER–LD contacts seipin facilitates the incorporation of protein and lipid cargo into growing LDs in human cells.
Synopsis
Seipin, mutated in severe congenital lipodystrophy (BSCL2), is an ER–lipid droplet (LD) contact protein that regulates the extent of ER–LD contacts and facilitates incorporation of lipid and protein cargo into maturing LDs.
Seipin deficiency increases the heterogeneity of ER–LD contacts, resulting in completely missing, rudimentary, or very extensive contacts.
Seipin is required during LD formation for the targeting of ER‐derived fatty acid‐activating enzyme ACSL3 to LDs.
An ER‐to‐LD targeting model peptide and a fluorescent fatty acid analog initially reach newly formed LDs, but their subsequent incorporation into LDs is impaired in the absence of seipin.
In seipin deficiency, the fatty acid flux to neutral lipids becomes compromised when LD formation has been initiated.
Seipin, mutated in severe congenital lipodystrophy (BSCL2), is an ER–lipid droplet (LD) contact protein that regulates the extent of ER–LD contacts and facilitates incorporation of lipid and protein cargo into maturing LDs.
Seipin is an oligomeric integral endoplasmic reticulum (ER) protein involved in lipid droplet (LD) biogenesis. To study the role of seipin in LD formation, we relocalized it to the nuclear envelope ...and found that LDs formed at these new seipin-defined sites. The sites were characterized by uniform seipin-mediated ER-LD necks. At low seipin content, LDs only grew at seipin sites, and tiny, growth-incompetent LDs appeared in a Rab18-dependent manner. When seipin was removed from ER-LD contacts within 1 h, no lipid metabolic defects were observed, but LDs became heterogeneous in size. Studies in seipin-ablated cells and model membranes revealed that this heterogeneity arises via a biophysical ripening process, with triglycerides partitioning from smaller to larger LDs through droplet-bilayer contacts. These results suggest that seipin supports the formation of structurally uniform ER-LD contacts and facilitates the delivery of triglycerides from ER to LDs. This counteracts ripening-induced shrinkage of small LDs.
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•Seipin can determine the site of LD formation•Seipin-mediated ER-LD membrane contacts display a uniform neck-like architecture•Seipin at ER-LD contact facilitates continuous triglyceride transfer to LD•Acute removal of seipin reveals a principle of LD ripening via the ER
Salo et al. show that seipin determines where a lipid droplet (LD) forms and establishes a neck between ER and LD. LDs continuously exchange lipids via ER, and biophysically, larger LDs acquire lipids more readily than smaller ones. Seipin facilitates lipid transfer to droplets, helping small LDs to grow.
Seipin is a disk-like oligomeric endoplasmic reticulum (ER) protein important for lipid droplet (LD) biogenesis and triacylglycerol (TAG) delivery to growing LDs. Here we show through biomolecular ...simulations bridged to experiments that seipin can trap TAGs in the ER bilayer via the luminal hydrophobic helices of the protomers delineating the inner opening of the seipin disk. This promotes the nanoscale sequestration of TAGs at a concentration that by itself is insufficient to induce TAG clustering in a lipid membrane. We identify Ser166 in the α3 helix as a favored TAG occupancy site and show that mutating it compromises the ability of seipin complexes to sequester TAG in silico and to promote TAG transfer to LDs in cells. While the S166D-seipin mutant colocalizes poorly with promethin, the association of nascent wild-type seipin complexes with promethin is promoted by TAGs. Together, these results suggest that seipin traps TAGs via its luminal hydrophobic helices, serving as a catalyst for seeding the TAG cluster from dissolved monomers inside the seipin ring, thereby generating a favorable promethin binding interface.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Tamoxifen treatment of estrogen receptor (ER)-positive breast cancer reduces mortality by 31%. However, over half of advanced ER-positive breast cancers are intrinsically resistant to tamoxifen and ...about 40% will acquire the resistance during the treatment.
In order to explore mechanisms underlying endocrine therapy resistance in breast cancer and to identify new therapeutic opportunities, we created tamoxifen-resistant breast cancer cell lines that represent the luminal A or the luminal B. Gene expression patterns revealed by RNA-sequencing in seven tamoxifen-resistant variants were compared with their isogenic parental cells. We further examined those transcriptomic alterations in a publicly available patient cohort.
We show that tamoxifen resistance cannot simply be explained by altered expression of individual genes, common mechanism across all resistant variants, or the appearance of new fusion genes. Instead, the resistant cell lines shared altered gene expression patterns associated with cell cycle, protein modification and metabolism, especially with the cholesterol pathway. In the tamoxifen-resistant T-47D cell variants we observed a striking increase of neutral lipids in lipid droplets as well as an accumulation of free cholesterol in the lysosomes. Tamoxifen-resistant cells were also less prone to lysosomal membrane permeabilization (LMP) and not vulnerable to compounds targeting the lipid metabolism. However, the cells were sensitive to disulfiram, LCS-1, and dasatinib.
Altogether, our findings highlight a major role of LMP prevention in tamoxifen resistance, and suggest novel drug vulnerabilities associated with this phenotype.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Cholesterol is an essential structural component in the cell membranes of most vertebrates. The biophysical properties of cholesterol and the enzymology of cholesterol metabolism provide the basis ...for how cells handle cholesterol and exchange it with one another. A tightly controlled--but only partially characterized--network of cellular signalling and lipid transfer systems orchestrates the functional compartmentalization of this lipid within and between organellar membranes. This largely dictates the exchange of cholesterol between tissues at the whole body level. Increased understanding of these processes and their integration at the organ systems level provides fundamental insights into the physiology of cholesterol trafficking.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Low‐density lipoprotein (LDL)‐cholesterol delivery from late endosomes to the plasma membrane regulates focal adhesion dynamics and cell migration, but the mechanisms controlling it are poorly ...characterized. Here, we employed auxin‐inducible rapid degradation of oxysterol‐binding protein‐related protein 2 (ORP2/OSBPL2) to show that endogenous ORP2 mediates the transfer of LDL‐derived cholesterol from late endosomes to focal adhesion kinase (FAK)‐/integrin‐positive recycling endosomes in human cells. In vitro, cholesterol enhances membrane association of FAK to PI(4,5)P2‐containing lipid bilayers. In cells, ORP2 stimulates FAK activation and PI(4,5)P2 generation in endomembranes, enhancing cell adhesion. Moreover, ORP2 increases PI(4,5)P2 in NPC1‐containing late endosomes in a FAK‐dependent manner, controlling their tubulovesicular trafficking. Together, these results provide evidence that ORP2 controls FAK activation and LDL‐cholesterol plasma membrane delivery by promoting bidirectional cholesterol/PI(4,5)P2 exchange between late and recycling endosomes.
Synopsis
How LDL‐cholesterol is delivered from late endosomes to the plasma membrane and enhances cell motility is unknown. Here, this is found to depend on ORP2‐mediated lipid exchange between late endosomes and focal adhesion kinase (FAK)‐positive recycling endosomes promoting FAK activation.
Loss of ORP2 abrogates LDL‐cholesterol delivery from late to recycling endosomes.
LDL‐cholesterol activates FAK pending ORP2‐mediated cholesterol transfer.
FAK activity promotes PI(4,5)P2 generation in endosomes, fueling the delivery of LDL‐cholesterol to the plasma membrane.
ORP2 controls endomembrane distribution of PI(4,5)P2 and tubulation of NPC1‐positive endosomes.
Cholesterol effects on cell motility depend on ORP2‐mediated lipid transfer from late endosomes to focal adhesion kinase‐/integrin‐positive recycling endosomes.
How Cells Handle Cholesterol Simons, Kai; Ikonen, Elina
Science (American Association for the Advancement of Science),
12/2000, Letnik:
290, Številka:
5497
Journal Article
Recenzirano
Cholesterol plays an indispensable role in regulating the properties of cell membranes in mammalian cells. Recent advances suggest that cholesterol exerts many of its actions mainly by maintaining ...sphingolipid rafts in a functional state. How rafts contribute to cholesterol metabolism and transport in the cell is still an open issue. It has long been known that cellular cholesterol levels are precisely controlled by biosynthesis, efflux from cells, and influx of lipoprotein cholesterol into cells. The regulation of cholesterol homeostasis is now receiving a new focus, and this changed perspective may throw light on diseases caused by cholesterol excess, the prime example being atherosclerosis.
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