Internalized plasma membrane cholesterol passes through an endosome compartment that is distinct from the acid vesicle-lysosome compartment

Dynamic Remodeling of Membranes and Their Lipids during Acute Hormone-Induced Steroidogenesis in MA-10 Mouse Leydig Tumor Cells

Lipids play essential roles in numerous cellular processes, including membrane remodeling, signal transduction, the modulation of hormone activity, and steroidogenesis. We chose steroidogenic MA-10 mouse tumor Leydig cells to investigate subcellular lipid localization during steroidogenesis. Electron microscopy showed that cAMP stimulation increased associations between the plasma membrane (PM) and the endoplasmic reticulum (ER) and between the ER and mitochondria. cAMP stimulation also increased the movement of cholesterol from the PM compared to untreated cells, which was partially inhibited when ATPase family AAA-domain containing protein 3 A (ATAD3A), which functions in ER and mitochondria interactions, was knocked down. Mitochondria, ER, cytoplasm, PM, PM-associated membranes (PAMs), and mitochondria-associated membranes (MAMs) were isolated from control and hormone-stimulated cells. Lipidomic analyses revealed that each isolated compartment had a unique lipid composition, and the induction of steroidogenesis caused the significant remodeling of its lipidome. cAMP-induced changes in lipid composition included an increase in phosphatidylserine and cardiolipin levels in PAM and PM compartments, respectively; an increase in phosphatidylinositol in the ER, mitochondria, and MAMs; and a reorganization of phosphatidic acid, cholesterol ester, ceramide, and phosphatidylethanolamine. Abundant lipids, such as phosphatidylcholine, were not affected by hormone treatment. Our data suggested that PM–ER–mitochondria tethering may be involved in lipid trafficking between organelles and indicated that hormone-induced acute steroid production involves extensive organelle remodeling.

Potential COVID-19 therapeutics from a rare disease: weaponizing lipid dysregulation to combat viral infectivity

The coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus (SARS-CoV)-2 has resulted in the death of more than 328,000 persons worldwide in the first 5 months of 2020. Herculean efforts to rapidly design and produce vaccines and other antiviral interventions are ongoing. However, newly evolving viral mutations, the prospect of only temporary immunity, and a long path to regulatory approval pose significant challenges and call for a common, readily available, and inexpensive treatment. Strategic drug repurposing combined with rapid testing of established molecular targets could provide a pause in disease progression. SARS-CoV-2 shares extensive structural and functional conservation with SARS-CoV-1, including engagement of the same host cell receptor (angiotensin-converting enzyme 2) localized in cholesterol-rich microdomains. These lipid-enveloped viruses encounter the endosomal/lysosomal host compartment in a critical step of infection and maturation. Niemann-Pick type C (NP-C) disease is a rare monogenic neurodegenerative disease caused by deficient efflux of lipids from the late endosome/lysosome (LE/L). The NP-C disease-causing gene (NPC1) has been strongly associated with viral infection, both as a filovirus receptor (e.g., Ebola) and through LE/L lipid trafficking. This suggests that NPC1 inhibitors or NP-C disease mimetics could serve as anti-SARS-CoV-2 agents. Fortunately, there are such clinically approved molecules that elicit antiviral activity in preclinical studies, without causing NP-C disease. Inhibition of NPC1 may impair viral SARS-CoV-2 infectivity via several lipid-dependent mechanisms, which disturb the microenvironment optimum for viral infectivity. We suggest that known mechanistic information on NPC1 could be utilized to identify existing and future drugs to treat COVID-19.

Plasma Membrane Origin of the Steroidogenic Pool of Cholesterol Used in Hormone-induced Acute Steroid Formation in Leydig Cells

Hormone-sensitive acute steroid biosynthesis requires trafficking of cholesterol from intracellular sources to the inner mitochondrial membrane. The precise location of the intracellular cholesterol and its transport mechanism are uncertain. Perfringolysin O, produced by Clostridium perfringens, binds cholesterol. Its fourth domain (D4) retains cholesterol-binding properties but not cytotoxicity. We transfected steroidogenic MA-10 cells of mouse Leydig cell tumors with the mCherry-D4 plasmid. Tagged D4 with fluorescent proteins enabled us to track cholesterol. The staining was primarily localized to the inner leaflet of the plasma membrane and was partially released upon treatment with dibutyryl-cAMP (Bt₂cAMP), a cAMP analog. Inhibitors of cholesterol import into mitochondria blocked steroidogenesis and prevented release of D4 (and presumably cholesterol) from the plasma membrane. We conclude that the bulk of the steroidogenic pool of cholesterol, mobilized by Bt₂cAMP for acute steroidogenesis, originates from the plasma membrane. Treatment of the cells with steroid metabolites, 22(R)-hydroxycholesterol and pregnenolone, also reduced D4 release from the plasma membrane, perhaps evidence for a feedback effect of elevated steroid formation on cholesterol release. Interestingly, D4 staining was localized to endosomes during Bt₂cAMP stimulation suggesting that these organelles are on the route of cholesterol trafficking from the plasma membrane to mitochondria. Finally, D4 was expressed in primary rat Leydig cells with a lentivirus and was released from the plasma membrane following Bt₂cAMP treatment. We conclude that the plasma membrane is the source of cholesterol for steroidogenesis in these cells as well as in MA-10 cells.

Dysregulation of testicular cholesterol metabolism following spontaneous mutation of the niemann-pick c1 gene in mice

The Niemann-Pick-type C1 (Npc1) protein mobilizes LDL-derived cholesterol from lysosomes. Npc1 deficiency disease is a panethnic autosomal recessive disorder of intracellular cholesterol trafficking, leading to accumulation of cholesterol in endosomes/lysosomes. This report assesses the effects of a spontaneous inactivating mutation of the Npc1 gene on spermatogenesis and cholesterol homeostasis in mice. We quantified 1) free and esterified cholesterol levels by enzymatic analysis, 2) cholesterol enzymes and transporter protein expression by Western blotting, and 3) the number of Apostain-labeled apoptotic germ cells and apoptosis levels by ELISA in seminiferous tubule-enriched fractions. In wild-type (WT) mice, esterified cholesterol was elevated when Npc1 expression was low during puberty, while in adulthood, the levels were low (P < 0.05) when Npc1 expression was high (P < 0.01). In Npc1-/- mice, free and esterified cholesterol were significantly elevated. The abundance of cholesterol regulatory proteins, HMGR ACAT1, ACAT2, SR-BI, and ABCA1 was significantly higher in Npc1-/- than in WT mice. The level of apoptosis determined by ELISA and the number of Apostain-labeled cells/tubule were higher in Npc1-/- than in WT mice. Circulating testosterone levels in the Npc1-/- males were threefold lower than those observed in the WT. Deleting the Npc1 gene is accompanied by an increase in germ cell apoptosis and compensatory imbalances in the expression of cholesterol enzymatic and transporter factors and is associated with esterified cholesterol accumulation in seminiferous tubules.

Lipids on trial: the search for the offending metabolite in Niemann-Pick type C disease

Niemann-Pick disease type C is a complex lysosomal storage disorder caused by mutations in either the NPC1 or NPC2 genes that is characterized at the cellular level by the storage of multiple lipids, defective lysosomal calcium homeostasis and unique trafficking defects. We review the potential role of each of the individual storage lipids in initiating the pathogenic cascade and propose a model of NPC1 and NPC2 function based on the current knowledge.

Chloroquine-mediated radiosensitization is due to the destabilization of the lysosomal membrane and subsequent induction of cell death by necrosis

The anti-malarial drug chloroquine (CQ) is also thought to be a potential radiation sensitizer. To gain a better understanding of how the lysomotropic CQ can potentiate the effects of ionizing radiation, we investigated the effects of CQ on lysosomal and mitochondrial membrane stability, the subcellular localization of ceramide, plasma membrane permeability, and the mode of cell death in response to irradiation. We found that CQ accumulated in the lysosomes and thus lysosomal volumes increased. As a result, both the lysosomal and plasma membranes were destabilized. After 7 Gy irradiation, most ceramide was associated with the lysosomes in the cells treated with CQ but not in the CQ-untreated control. The elevated levels of ceramide in the lysosomes of the CQ-treated cells appeared to further destabilize the lysosomal and plasma membranes of the cell. Both CQ-treated and -untreated cells had approximately the same rate of cell death by apoptosis after 7 Gy irradiation (P > 0.05, ns). However, in contrast to the CQ-untreated control, the CQ-treated cells underwent massive cell death by necrosis at 24-48 h after irradiation (P < 0.05). Taken together, our data support the idea that the increase in cytotoxic effects by the combination of CQ and radiation is due to radiation-mediated apoptosis and CQ-mediated necrosis.

Polarized membrane traffic and cell polarity development is dependent on dihydroceramide synthase-regulated sphinganine turnover

Sphingoid bases have been implicated in various cellular processes including cell growth, apoptosis and cell differentiation. Here, we show that the regulated turnover of sphingoid bases is crucial for cell polarity development, i.e., the biogenesis of apical plasma membrane domains, in well-differentiated hepatic cells. Thus, inhibition of dihydroceramide synthase or sphinganine kinase activity with fumonisin B1 or N,N-dimethylsphingosine, respectively, dramatically perturbs cell polarity development, which is due to increased levels of sphinganine. Consistently, reduction of free sphinganine levels stimulates cell polarity development. Moreover, dihydroceramide synthase, the predominant enzyme responsible for sphinganine turnover, is a target for cell polarity stimulating cAMP/protein kinase A (PKA) signaling cascades. Indeed, electrospray ionization tandem mass spectrometry analyses revealed a significant reduction in sphinganine levels in cAMP/PKA-stimulated cells. These data suggest that sphinganine turnover is critical for and is actively regulated during HepG2 cell polarity development. Previously, we have identified an apical plasma membrane-directed trafficking pathway from the subapical compartment. This transport pathway, which is part of the basolateral-to-apical transcytotic itinerary, plays a crucial role in apical plasma membrane biogenesis. Here, we show that, as a part of the underlying mechanism, the inhibition of dihydroceramide synthase activity and ensuing increased sphinganine levels specifically perturb the activation of this particular pathway in the de novo apical membrane biogenesis.

Analysis of post-lysosomal compartments

Lysosomes are acidic intracellular compartments and are regarded as degradative and the end point, of the endocytic pathway. Here we provide evidence for the generation of acid hydrolase poor and non-acidic post-lysosomal compartments in NRK cells that have accumulated non-digestible macromolecules, Texas red-dextran (TR-Dex), within lysosomes. When TR-Dex was fed to the cells for 6h, most of the internalized TR-Dex colocalized with a lysosomal enzyme, cathepsin D. With an increase in the chase period, however, the internalized TR-Dex gradually accumulated in cathepsin D-negative vesicles. These vesicles were positive for a lysosomal membrane protein, LGP85, and their formation was inhibited by treatment of the cells with U18666A, which impairs membrane transport out of late endosomal/lysosomal compartments, thereby suggesting that the vesicles are derived from lysosomes. Interestingly, these compartments are non-acidic as judged for the DAMP staining. The results, therefore, suggest that the excess accumulation of non-digestible macromolecules within lysosomes induces the formation of acid hydrolase poor and non-acidic post-lysosomal compartments. The fact that treatment of the cells with lysosomotropic amines or a microtubule-depolymerization agent resulted in extensive colocalization of TR-Dex with cathepsin D further indicates that the formation of the post-lysosomal compartments depends on the lysosomal acidification and microtubule organization. Furthermore, these results suggest bi-directional membrane transport between lysosomes and the post-lysosomal compartments, which implies that the latter are not resting compartments.

NPC1-containing compartment of human granulosa-lutein cells: a role in the intracellular trafficking of cholesterol supporting steroidogenesis

Steroidogenic cells represent unique systems for the exploration of intracellular cholesterol trafficking. We employed cytochemical and biochemical methods to explore the expression, regulation, and function of the Niemann-Pick C1 protein (NPC1) in human granulosa-lutein cells. NPC1 was localized in a subset of lysosome-associated membrane glycoprotein 2 (LAMP-2)-positive vesicles. By analyzing the sensitivity of NPC1 N-linked oligosaccharide chains to glycosidases and neuraminidase, evidence was obtained for movement of nascent NPC1 from the endoplasmic reticulum through the medial and trans compartments of the Golgi apparatus prior to its appearance in cytoplasmic vesicles. NPC1 protein content and the morphology and cellular distribution of NPC1-containing vesicles were not affected by treatment of the granulosa-lutein cells with 8-Br-cAMP, which stimulates cholesterol metabolism into progesterone. In contrast, steroidogenic acute regulatory (StAR) protein levels were increased by 8-Br-cAMP. Incubation of granulosa-lutein cells with low-density lipoprotein (LDL) in the presence of the hydrophobic amine, U18666A, caused accumulation of free cholesterol in granules, identified by filipin staining, that contained LAMP-2 and NPC1. These granules also stained for neutral lipid with Nile red, reflecting accumulation of LDL-derived cholesterol esters. LDL-stimulated progesterone synthesis was completely blocked by U18666A, leaving steroid output at levels similar to those of cells incubated in the absence of LDL. The hydrophobic amine also blocked the LDL augmentation of 8-Br-cAMP-stimulated progesterone synthesis, reducing steroid production to levels seen in cells stimulated with 8-Br-cAMP in the absence of LDL. Steroidogenesis recovered after U18666A was removed from the culture medium. U18666A treatment caused a 2-fold or more increase in NPC1 protein and mRNA levels, suggesting that disruption of NPC1's function activates a compensatory mechanism resulting in increased NPC1 synthesis. We conclude that the NPC1 compartment plays an important role in the trafficking of LDL-derived substrate in steroidogenic cells; that NPC1 expression is up-regulated when NPC1 action is blocked; and that the NPC1 compartment can be functionally separated from other intracellular pathways contributing substrate for steroidogenesis.

Niemann-Pick C1 is a late endosome-resident protein that transiently associates with lysosomes and the trans-Golgi network

Niemann-Pick type C (NPC) disease is a severe cell lipidosis characterized by the accumulation of unesterified cholesterol in the endosomal/lysosomal system. Recently the primary disease-causing gene, NPC1, was identified, but few clues regarding its potential function(s) could be derived from its predicted amino acid sequence. Therefore, efforts were directed at characterizing the subcellular location of the NPC1 protein. Initial studies with a FLAG-tagged NPC1 cDNA demonstrated that NPC1 is a glycoprotein that associates with the membranes of a population of cytoplasmic vesicles. Immunofluorescence microscopy using anti-NPC1 polyclonal antibodies confirmed this analysis. Double-label immunofluorescence microscopy and subcellular fractionation studies indicated that NPC1 associates predominantly with late endosomes (Rab9 GTPase-positive vesicles) and, to a lesser extent, with lysosomes and the trans-Golgi network. When cholesterol egress from lysosomes was blocked by treatment of cells with U18666A, the NPC1 location shifted from late endosomes to the trans-Golgi network and lysosomes. Subcellular fractionation of liver homogenates from U18666A-treated mice confirmed these observations. These data suggest that U18666A may inhibit the retrograde transport of NPC1 from lysosomes to late endosomes for subsequent transfer to the trans-Golgi network.

Dominant-negative caveolin inhibits H-Ras function by disrupting cholesterol-rich plasma membrane domains

The plasma membrane pits known as caveolae have been implicated both in cholesterol homeostasis and in signal transduction. CavDGV and CavKSY, two dominant-negative amino-terminal truncation mutants of caveolin, the major structural protein of caveolae, significantly inhibited caveola-mediated SV40 infection, and were assayed for effects on Ras function. We find that CavDGV completely blocked Raf activation mediated by H-Ras, but not that mediated by K-Ras. Strikingly, the inhibitory effect of CavDGV on H-Ras signalling was completely reversed by replenishing cell membranes with cholesterol and was mimicked by cyclodextrin treatment, which depletes membrane cholesterol. These results provide a crucial link between the cholesterol-trafficking role of caveolin and its postulated role in signal transduction through cholesterol-rich surface domains. They also provide direct evidence that H-Ras and K-Ras, which are targeted to the plasma membrane by different carboxy-terminal anchors, operate in functionally distinct microdomains of the plasma membrane.