Intracellular cholesterol transport

Peripheral benzodiazepine receptor ligands in rat liver mitochondria: effect on 27-hydroxylation of cholesterol

The effect of peripheral benzodiazepine receptor ligands: PK11195 (1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)isoquinoline-3-carboxamid e), Ro 5-4864 (4-chlorodiazepam), hemin, N-methyl protoporphyrin IX and protoporphyrin IX on liver mitochondrial 27-hydroxylation of cholesterol was studied by adding them together with [4-14C]cholesterol. N-Methyl protoporphyrin IX, PK11195 and protoporphyrin IX stimulated mitochondrial 27-hydroxylation of [4-14C] cholesterol in vitro, the first two being the most potent (2-3-fold increase). Ro 5-4864 and hemin were not active. 27-Hydroxylation of [4-14C]cholesterol was reduced to below control levels (respectively 40 and 56% decrease compared to control, P < 0.01) when PK11195, N-methyl protoporphyrin IX or protoporphyrin IX were allowed to equilibrate in vitro with mitochondria for 20 min at 37 degrees C. Hepatic protoporphyria was induced using 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) (100 mg/kg, i.p.) to study the effect of in vivo accumulation of large amounts of dicarboxylic porphyrins, i.e. endogenous peripheral benzodiazepine receptor ligands, on cholesterol 27-hydroxylation. DDC treatment caused an increase in total porphyrin content in liver homogenate (10-fold) and mitochondria (2-fold). Mitochondrial 27-hydroxylation of [4-14C]cholesterol was depressed after treatment (60% decrease, P < 0.01). We suggest that peripheral benzodiazepine receptor ligands act on liver mitochondrial 27-hydroxylation of cholesterol by a mechanism coupled to these receptors and that the time of exposure of peripheral benzodiazepine receptors to ligands is a major factor. The modulation of 27-hydroxycholesterol production may have a physiological role in liver and possibly in other tissues.

Compartmental isolation of cholesterol participating in the cytoplasmic cholesteryl ester cycle in Chinese hamster ovary 25-RA cells

Using the Chinese hamster ovary cell line, 25-RA, we have demonstrated that lipoprotein-derived cholesterol and endogenously synthesized cholesterol are selectively differentiated with respect to their cellular locations. These cells lack sterol-mediated regulation, spontaneously storing large amounts of esterified cholesterol, which turns over with a half-time of 7.5 h. When [3H]cholesterol was provided to the cells in serum to trace cellular cholesterol, the specific activities of cellular free and esterified cholesterol (6238 +/- 273 and 5128 +/- 277 cpm/ microg, respectively) failed to equilibrate, indicating that bulk cellular free cholesterol is isolated from that participating in the cholesteryl ester cycle. Using [3H]acetate to trace the fate of endogenously synthesized cholesterol, a failure of equilibration was also observed (specific activities of free and esterified cholesterol = 280 +/- 37 and 458 +/- 8 cpm/ microg, respectively). The lower specific activity of the precursor indicates that endogenously synthesized cholesterol is preferentially esterified. When cells radiolabeled with [3H]acetate were post-incubated in the absence of radiolabel, the specific activity of the esterified cholesterol pool remained significantly higher than that of the free cholesterol, suggesting that cholesterol derived from hydrolysis of esterified cholesterol is preferentially re-esterified.

Regulation of sterol 27-hydroxylase and an alternative pathway of bile acid biosynthesis in primary cultures of rat hepatocytes

In man, hepatic mitochondrial sterol 27-hydroxylase and microsomal cholesterol 7alpha-hydroxylase initiate distinct pathways of bile acid biosynthesis from cholesterol, the "acidic" and "neutral" pathways, respectively. A similar acidic pathway in the rat has been hypothesized, but its quantitative importance and ability to be regulated at the level of sterol 27-hydroxylase are uncertain. In this study, we explored the molecular regulation of sterol 27-hydroxylase and the acidic pathway of bile acid biosynthesis in primary cultures of adult rat hepatocytes. mRNA and protein turnover rates were approximately 10-fold slower for sterol 27-hydroxylase than for cholesterol 7alpha-hydroxylase. Sterol 27-hydroxylase mRNA was not spontaneously expressed in culture. The sole requirement for preserving sterol 27-hydroxylase mRNA at the level of freshly isolated hepatocytes (0 h) after 72 h was the addition of dexamethasone (0.1 microM; > 7-fold induction). Sterol 27-hydroxylase mRNA, mass and specific activity were not affected by thyroxine (1.0 microM), dibutyryl-cAMP (5O microM), nor squalestatin 1 (15O nM-1.0 microM), an inhibitor of cholesterol biosynthesis. Taurocholate (50 microM), however, repressed sterol 27-hydroxylase mRNA levels by 55%. Sterol 27-hydroxylase specific activity in isolated mitochondria was increased > 10-fold by the addition of 2-hydroxypropyl-beta-cyclodextrin. Under culture conditions designed to maximally repress cholesterol 7alpha-hydroxylase and bile acid synthesis from the neutral pathway but maintain sterol 27-hydroxylase mRNA and activity near 0 h levels, bile acid synthesis from [14C]cholesterol remained relatively high and consisted of beta-muricholate, the product of chenodeoxycholate in the rat. We conclude that rat liver harbors a quantitatively important alternative pathway of bile acid biosynthesis and that its initiating enzyme, sterol 27-hydroxylase, may be slowly regulated by glucocorticoids and bile acids.

Role of multidrug resistance P-glycoproteins in cholesterol biosynthesis

Multidrug resistance (MDR) P-glycoproteins were first recognized for their ability to catalyze ATP-dependent efflux of cytotoxic agents from tumor cells when overexpressed. Despite extensive study, little is known about the normal substrate(s) and normal cellular function of these proteins. In the accompanying manuscript (Metherall, J. E., Waugh, K., and Li, H. (1996) J. Biol. Chem. 271, 2627-2633), we demonstrate that progesterone inhibits cholesterol biosynthesis, causing the accumulation of a number of cholesterol precursors. In the current manuscript, we use several criteria to show that the progesterone receptor is not involved in this inhibition. Rather, we demonstrate that progesterone inhibits cholesterol biosynthesis by interfering with MDR activity. We show that a steroid hormone's ability to inhibit cholesterol biosynthesis is correlated with: 1) its general hydrophobicity and 2) its ability to inhibit MDR activity. The only exception to this finding is beta-estradiol, which is a more potent inhibitor of cholesterol biosynthesis than expected based solely on hydrophobicity and MDR inhibition. We further demonstrate that nonsteroidal inhibitors of MDR also inhibit cholesterol biosynthesis. Since MDR activity is required for esterification of LDL-derived cholesterol (P. DeBry and J. E. Metherall, submitted for publication), we investigated the relationship between these phenomena and show that inhibition of cholesterol esterification does not cause inhibition of cholesterol biosynthesis and that inhibition of cholesterol biosynthesis does not cause inhibition of cholesterol esterification. We propose a model in which MDR is required for transport of sterols from the plasma membrane to the endoplasmic reticulum (ER). Inhibiting this transport prevents cholesterol esterification and cholesterol biosynthesis by preventing sterol substrates from reaching ER-resident enzymes.

Peripheral benzodiazepine receptor ligands in rat liver mitochondria: effect on cholesterol translocation

Peripheral benzodiazepine receptors mediate cholesterol translocation between the outer and inner mitochondrial membranes in steroidogenic tissues. They are found in many other tissues too, including liver. We studied the effect of the peripheral benzodiazepine receptor ligands PK11195 [1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)isoquinoline-3-carboxa mid e], Ro 5-4864 (4-chlorodiazepam), hemin, protoporphyrin IX and N-methyl protoporphyrin IX on cholesterol mitochondrial intermembrane transport of cholesterol in vitro in rat liver. Endogenous cholesterol translocation from outer to inner mitochondrial membranes was significantly increased by PK11195 and N-methyl protoporphyrin IX (140% and 150% increase, respectively, at 1 microM, P<0.01). 5 microM protoporphyrin IX, 1 microM Ro 5-4864 and 5 microM hemin was ineffective. When mitochondria were labeled with exogenous [4-14C]cholesterol, PK11195 and N-methyl protoporphyrin IX were the most effective in increasing total cholesterol incorporation and cholesterol translocation into inner membranes, and their effect was dose-dependent. These data suggest that in liver the binding to peripheral benzodiazepine receptors is related to cholesterol translocation and the interaction of ligands with these receptors may play a role in the complex mechanism of regulation of cholesterol traffic between liver mitochondrial membranes.

Intrinsic sterol- and phosphatidylcholine transfer activities of 17 beta-hydroxysteroid dehydrogenase type IV

Previous studies have shown that the 80 kDa 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) type IV comprises distinct domains, including an N-terminal region related to the short chain alcohol dehydrogenase multigene family and a C-terminal part related to the lipid transfer protein sterol carrier protein 2 (SCP2). In this study, we have investigated whether the SCP2-related part of the 80 kDa protein leads to an intrinsic sterol and phospholipid transfer activity, as shown earlier for the 60 kDa SCP2-related peroxisomal 3-ketoacyl CoA thiolase with intrinsic sterol and phospholipid transfer activity called sterol carrier protein x (SCPx). Our results indicate that a fraction rich in the 80 kDa form of 17 beta-HSD type IV exhibits high transfer activities for 7-dehydrocholesterol and phosphatidylcholine. In addition, a purified recombinant peptide derived from the SCP2-related domain of the 17 beta-HSD type IV has about 30% of the transfer activities for 7-dehydrocholesterol and phosphatidylcholine seen with purified recombinant human SCP2. We conclude that the 80 kDa type IV 17 beta-HSD represents a potentially multifunctional protein with intrinsic in vitro sterol and phospholipid transfer activity in addition to its enzymatic activity.

Changes in free cholesterol content, measured by filipin fluorescence and flow cytometry, correlate with changes in cholesterol biosynthesis in THP-1 macrophages

The free cholesterol content of cells can be monitored by the intensity of fluorescence emissions from the polyene antibiotic filipin. In a previous study (Hassall: Cytometry 13:381-388, 1992) using THP-1 macrophages, a decrease in filipin fluorescence in response to increasing concentrations of modified lipoprotein was observed, suggesting a reduction in the free cholesterol content of the cells. In this study, THP-1 macrophages were treated with a number of agents known to modulate cholesterol biosynthesis and cholesterol esterification. Changes in filipin fluorescence emissions were measured by flow cytometry, and correlated with changes in cholesterol biosynthesis measured by incorporation of [14C]acetate into cholesterol. A correlation between decreases in filipin fluorescence and reductions in cholesterol biosynthesis was apparent, even when cholesterol esterification was inhibited. These results suggest that the decreases in filipin fluorescence observed may be due, at least in part, to reduction in cholesterol biosynthesis.

Independent regulation of cholesterol incorporation into free apolipoprotein-mediated cellular lipid efflux in rat vascular smooth muscle cells

Cholesterol was poorly available to free apolipoprotein (apo)A-I-mediated cellular lipid efflux from cholesterol-loaded rat vascular smooth muscle cells generating cholesterol-poorer pre-beta-HDL particles than those generated from macrophages by the same reaction (Li, Q., Komaba, A., and Yokoyama, S. (1993) Biochemistry 32, 4597-4603). The factors known to induce transformation of the smooth muscle cells into a macrophage-like stage were used in order to modulate this reaction, such as human platelet-derived growth factor, macrophage colony-stimulating factor, and phorbol 12-myristate-13-acetate (PMA). When the cells were stimulated by PMA following the pretreatment with platelet-derived growth factor plus macrophage colony-stimulating factor, cholesterol efflux mediated by free apoA-I increased 3-fold without changing phospholipid efflux, resulting in generation of pre-beta-HDL particles more rich in cholesterol. This treatment had only a little or no effect on apparent cellular cholesterol efflux to HDL or lipid microemulsion, respectively. Overall cellular free cholesterol pool size was unaffected by the treatment, and probing by extracellular cholesterol oxidase did not detect gross change in the cellular surface cholesterol. This specific enrichment of cholesterol in the apoA-I-mediated cellular lipid efflux was reversed by protein kinase C inhibitors. Measurement of intracellular cholesterol esterification suggested that PMA induced translocation of intracellular cholesterol to a specific pool for apoA-I-mediated efflux, and a protein kinase C inhibitor reversed this effect.

Amiodarone induced lipidosis similar to Niemann-Pick C disease. Biochemical and morphological study

Amiodarone is effective in the treatment of supraventricular and ventricular cardiac arrhythmia, however a high incidence of toxic side effects has been observed in various organs and tissues during chronic treatment. Ultrastructural observation of affected tissues reveals myelinoid inclusion bodies. The exact pathogenetic mechanism of these changes is still unknown. In this study we investigated the biochemical effects of this drug on lysosomal hydrolases and the alterations induced in subcellular organelles of fibroblasts cultured for 24 h with different concentrations of amiodarone in the medium. Of the enzyme activities assayed, we only observed a significant reduction in sphingomyelinase. Ultrastructural observation of fibroblasts showed swollen lysosomes and a few onionoid inclusion bodies at lower concentrations of the drug; at higher concentrations the lysosomal system was severely impaired. Cytochemical staining of unesterified cholesterol with filipin showed accumulation of cholesterol. We conclude that chronic amiodarone treatment in experimental conditions induces inhibition in sphingomyelinase activity through interaction with membrane lipids and modification of bilayer structure. Higher concentrations of the drug impair cholesterol transport and induce lipid accumulation. These results may be useful for understanding the pathogenesis of induced lipidosis in patients in chronic treatment with amiodarone.

Sterol carrier protein-2 is involved in cholesterol transfer from the endoplasmic reticulum to the plasma membrane in human fibroblasts

The cellular mechanism of cholesterol transport from the endoplasmic reticulum to the plasma membrane is currently unknown. To assess the possibility that sterol carrier protein-2 (SCP-2) is involved in this transport, we studied the time course of newly synthesized cholesterol incorporation in the plasma membrane of normal and SCP-2-deficient (Zellweger syndrome) human fibroblasts. Cholesterol transfer was rapid, cytoskeleton-independent, and Golgi-independent in normal cells, but it was slower, cytoskeleton-dependent, and Golgi-dependent in SCP-2-deficient cells. After SCP-2 antisense oligonucleotides treatment of normal fibroblasts, the rapid transport was reduced by 81% with a simultaneous increase of the slower one. These results suggest that in normal fibroblasts the major fraction of newly synthesized cholesterol is transported to the plasma membrane by a SCP-2-dependent mechanism. In contrast, in SCP-2-deficient cells, newly synthesized cholesterol leaves the endoplasmic reticulum by a cytoskeleton/Golgi-dependent mechanism.

Severe familial HDL deficiency in French-Canadian kindreds. Clinical, biochemical, and molecular characterization

A decreased level of HDL cholesterol (HDL-C) is the most common lipoprotein abnormality seen in people with premature coronary artery disease (CAD). In many cases, HDL-C reduction in patients with CAD may be the result of increased apo B-containing lipoprotein production by the liver with secondary hypoalphalipoproteinemia. Primary hypoalphalipoproteinemia is seen in approximately 4% of people with CAD. We report findings in four subjects with severe familial HDL deficiency (HDL-C << 5th percentile for age and sex; 0.08 to 0.38 mmol/L) in three French-Canadian kindreds with autosomal codominant inheritance. By inclusion criteria, all four subjects had normal fasting triglycerides and none were diabetic. HDL particle size by gradient gel electrophoresis revealed small HDL particles (estimated Stokes' diameter, 8.14 to 8.30 nm). Apo AI analysis by polyacrylamide gel electrophoresis and use of isoelectrofocusing gels in affected subjects revealed normal molecular weight (28.3 kD) and normal isoelectrofocusing point but a relative increase in proapoliprotein AI, with near-normal levels of proapolipoprotein AI in plasma, suggesting normal secretion of apo AI. Quantitative Southern blot analysis of the apo AI-CIII-AIV gene cluster reveals no gene rearrangements or allele deletion. Haplotypes of the apo AI gene, determined by use of the restriction enzymes Pst I, Xmn I, and Sst I and of the apo AII gene by use of the enzyme Msp I, did not reveal segregation of the low HDL-C trait with either the apo AI or the AII gene. Sequence analysis of the promoter region of the apo AI gene reveals heterozygosity for guanine-to-adenine substitution at position 76 in two kindreds with no evidence of segregation with the low HDL trait. None of the patients had mutations of the lipoprotein lipase gene common in subjects of French-Canadian descent. Haplotype analysis of the lipoprotein lipase gene did not show segregation with the low HDL trait. Plasma lecithin: cholesterol acyltransferase (LCAT) activity was found to be within normal levels in affected subjects and in nonaffected first-degree relatives. None of the affected subjects had clinical manifestations of Tangier disease. Two of the four cases examined, both men, had severe CAD and had undergone revascularization procedures. The third is a younger brother of one of these probands and the fourth is a 30-year-old woman, and both were free of clinical CAD. However, in none of the families did the low HDL trait unequivocally cosegregate with CAD.(ABSTRACT TRUNCATED AT 400 WORDS)

Side-chain structure is critical for the transport of sterols from lysosomes to cytoplasm

Macrophages take up and metabolize negatively charged liposomes containing free cholesterol efficiently, resulting in a massive accumulation of cholesteryl esters and triacylglycerols in their cytoplasm (Nishikawa, K., Arai, H. and Inoue, K. (1990) J. Biol. Chem. 265, 5226-5231). This system was used to assess the effects of structural variation of sterol on the intracellular transport and the metabolism of endocytosed sterols by the cells. Liposomes containing phytosterols with an extra one (campesterol) or two (beta-sitosterol, stigmasterol, fucosterol) carbons at the C-24 position of the cholesterol side-chain were endocytosed as efficiently as those containing cholesterol without exhibiting any apparent toxicity on the cells. Esterification of endocyotosed phytosterols was, however, extremely low; campesterol esterification was only 20% that of cholesterol and either beta-sitosterol or stigmasterol was not esterified appreciably. A morphological study showed that the endocytosed phytosterols were accumulated in the phagolysosomes of the cells. Blocking of esterification of endocytosed cholesterol by an acyl-CoA:cholesterol acyltransferase (ACAT) inhibitor did not lead to cholesterol accumulation in the phagolysosomes. These data suggest that accumulation of endocytosed phytosterols in phagolysosomes is not a consequence of the inability of the cell to esterify sterols in the endoplasmic reticulum. In the light of these observations, we conclude that cultured macrophages can discriminate between sterols that differ only by a methyl or ethyl group at the C-24 position at their lysosomal compartment.

HDL-mediated efflux of intracellular cholesterol is impaired in fibroblasts from Tangier disease patients

To further elucidate the cellular mechanisms leading to HDL deficiency in Tangier disease, HDL-mediated cholesterol efflux was studied in cultured skin fibroblasts from Tangier patients. Both Tangier and control fibroblasts show specific saturable binding of HDL3 to the cell membrane (Bmax = 70 and 52 ng/mg protein, respectively; Kd = 8.8 and 10.6 micrograms/mL, respectively). There was no appreciable uptake of HDL3 by Tangier and control fibroblasts, indicating that cholesterol efflux from fibroblasts occurs at the cell membrane. When cellular cholesterol was labeled to equilibrium by [14C]cholesterol incubation for 48 hours at 37 degrees C, HDL3-mediated cholesterol efflux from Tangier fibroblasts was only 50% of control fibroblasts. To define this abnormality in HDL3-mediated cholesterol efflux more precisely, several additional experiments were performed. First, membrane desorption of cholesterol was determined after cell membranes were labeled with [14C]cholesterol for 3 hours at 15 degrees C. With this labeling protocol, there was no difference in HDL3-mediated cholesterol efflux between control and Tangier fibroblasts. Second, efflux of newly synthesized sterols was determined after incorporation of the precursor [14C]mevalonolactone. Under these conditions, specific HDL3-mediated efflux of sterols was almost absent in Tangier fibroblasts. Third, cells were labeled by incubation with reconstituted [3H]cholesteryl-linoleate-LDL. Efflux of LDL-derived cholesterol was only slightly reduced for the first 4 hours of incubation. After 12 hours, there was no difference between control and Tangier cells. The combined data indicate that the reduced efflux of cholesterol from Tangier fibroblasts observed after homogeneous labeling is due to severely reduced efflux of newly synthesized sterol.(ABSTRACT TRUNCATED AT 250 WORDS)

Reverse cholesterol transport in plasma of patients with different forms of familial HDL deficiency

HDLs encompass structurally heterogenous lipoproteins that fulfill specific functions in reverse cholesterol transport. Two-dimensional nondenaturing gradient gel electrophoresis (2D-PAGGE) of normoalphalipoproteinemic plasma and subsequent immunoblotting with anti-apoA-I-antibodies differentiates pre-beta 1-LpA-I, pre-beta 2-LpA-I, pre-beta 3-LpA-I, alpha-LpA-I2, and alpha-LpA-I3. Immunodetection with anti-apoE antibodies differentiates gamma-LpE and alpha-LpE. Pulse-chase incubations of plasma with [3H]unesterified cholesterol ([3H]UC)-labeled fibroblasts and subsequent 2D-PAGGE revealed that cell-derived [3H]UC is taken up by pre-beta 1-LpA-I and gamma-LpE. From these initial acceptors, [3H]UC is transferred to LDL via pre-beta 2-LpA-I-->pre-beta 3-LpA-I-->alpha-LpA-I. Some UC is esterified in pre-beta 3-LpA-I, and some is esterified in alpha-LpA-I after its retransfer from LDL. In this study we investigated the effect of various forms of familial HDL deficiency on reverse cholesterol transport. Plasma samples of patients with various forms of HDL deficiency are characterized by the lack of specific HDL subclasses. ApoE-containing HDLs, including gamma-LpE, are present in all kinds of HDL deficiency. However, all forms of LpA-I are absent in apoA-I-deficient plasma, pre-beta 3-LpA-I and alpha-LpA-I from the plasma of patients with Tangier disease (TD), and pre-beta 3-LpA-I and large alpha-LpA-I from the plasma of patients with lecithin:cholesterol acyltransferase (LCAT) deficiency and fish-eye disease (FED). After a 1-minute pulse with labeled fibroblasts, efflux of [3H]UC into HDL-deficient plasmas decreased, compared with normal plasma, by 49% (apoA-I deficiency), 36% (TD), 21% (LCAT deficiency), and 28% (FED). In apoA-I deficiency, only gamma-LpE takes up cell-derived [3H]UC. In the three other HDL-deficiency states, cell-derived [3H]UC is initially taken up by both pre-beta 1-LpA-I and gamma-LpE. The four HDL deficiencies are also characterized by differences in the esterification of cell-derived [3H]UC. No esterification occurs in LCAT-deficient plasma. In FED plasma, [3H]UC is esterified in LDL. In apoA-I deficiency and TD, however, [3H]UC is esterified in lipoproteins free of apoA-I and apoB. In the two latter cases, the transfer of [3H]cholesteryl ester to LDL is enhanced compared with normal plasma. The lack of specific HDL subclasses and the consequent changes in reverse cholesterol transport pathways differently affect net mass efflux of cholesterol from fibroblasts into HDL-deficient plasma.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of steroidogenesis in crayfish molting glands: involvement of protein synthesis

The involvement of continuous protein synthesis in the mechanisms of crustacean steroidogenesis was investigated using crayfish molting glands (Y-organs). During intermolt, Y-organ steroidogenic activity is low. Eyestalk ablation initiates premolt which is characterized by a rapid increase in the production of ecdysteroids. In vitro incorporation of [14C]leucine into TCA-precipitable proteins was measured in Y-organs. A significant increase of de novo protein synthesis within 2 h and simultaneously led to a strong inhibition of the ecdysteroid synthesis. Sinus gland extracts (containing molt inhibiting hormone) also induced both a limited but reproducible inhibition of Y-organ protein synthesis and a pronounced inhibition of ecdysteroid production within 2 h. The results suggest a functional link between protein synthesis in the Y-organ and sustained ecdysteroid production. The analysis of autoradiographs from one-dimensional gel electrophoreses revealed an overall increase in de novo synthesis of glandular proteins in early premolt but also a more specific effect on distinct proteins (increase of 150, 140, 50-60, 22 and 15-18 kDa proteins) which may be more directly involved in the regulation of ecdysteroidogenesis.

Translocation of both lysosomal LDL-derived cholesterol and plasma membrane cholesterol to the endoplasmic reticulum for esterification may require common cellular factors involved in cholesterol egress from the acidic compartments (lysosomes/endosomes)

Using a stable cell line 25-RA derived from wild-type Chinese hamster ovary (CHO) cells as the parental cell, this laboratory previously reported the isolation and characterization of CHO cell mutants (cholesterol-trafficking or CT) defective in transporting LDL-derived cholesterol out of the acidic compartment(s) (lysosomes/endosomes) to the endoplasmic reticulum (ER) for esterification. In this report, we show that the CT mutation can be complemented by fusion with human cells; however, attempts to complement the CT defect through DNA transfection have resulted in a collection of stable cell lines designated as ST cells. Under cholesterol starvation condition, the ST cells exhibit an elevated rate of cholesterol ester biosynthesis (by 3- to 5-fold) compared to both the parental CHO cells and the CT cells. The phenotypes of the ST cells are stable. ST cells are thus new cell lines arisen from the CT cells. When the plasma membranes of the parental, CT, and ST cells are labelled with [3H]cholesterol, ST cells show rates of [3H]cholesterol esterification much higher than that observed in CT cells but lower than that observed in the parental CHO cells. This result shows that translocation of plasma membrane cholesterol to the ER for esterification is defective in the CT cells. This result also suggests that ST cells acquire increased cholesterol trafficking activity between the lysosome and the ER without mixing the plasma membrane cholesterol pool. The characteristics of CT cells and ST cells reported here suggest that translocation of both lysosomal LDL-derived cholesterol and plasma membrane cholesterol to the ER for esterification may require common cellular factors involved in cholesterol egress from the acidic compartment(s) (lysosomes/endosomes).

Cholesterol domains in biological membranes

Membrane cholesterol is distributed asymmetrically both within the cell or within cellular membranes. Elaboration of intracellular cholesterol trafficking, targeting and intramembrane distribution has been spurred by both molecular and structural approaches. The expression of recombinant sterol carrier proteins in L-cell fibroblasts has been especially useful in demonstrating for the first time that such proteins actually elicit intracellular and intraplasma membrane redistribution of sterol. Additional advances in the use of native fluorescent sterols allowed resolution of transbilayer and lateral cholesterol domains in plasma membranes from cultured fibroblasts, brain synaptosomes and erythrocytes. In all three cell surface membranes, cholesterol is enriched in the inner, cytofacial leaflet. Up to three different cholesterol domains have been identified in the lateral plane of the plasma membrane: a fast exchanging domain comprising less than 10% of cholesterol, a slowly exchanging domain comprising about 30% of cholesterol, and a very slowly or non-exchangeable sterol domain comprising 50-60% of plasma membrane cholesterol. Factors modulating plasma membrane cholesterol domains include polyunsaturated fatty acids, expression of intracellular sterol carrier proteins, drugs such as ethanol, and several membrane pathologies (systemic lupus erythematosus, sickle cell anaemia and aging). Disturbances in plasma membrane cholesterol domains alter transbilayer fluidity gradients in plasma membranes. Such changes are associated with decreased Ca(2+)-ATPase and Na+, K(+)-ATPase activity. Thus, the size, dynamics and distribution of cholesterol domains within membranes not only regulate cholesterol efflux/influx but also modulate plasma membrane protein functions and receptor-effector coupled systems.

Isoprenoid lipid metabolism in the retina: dynamics of squalene and cholesterol incorporation and turnover in frog rod outer segment membranes

Frogs were injected intravitreally with [3H]acetate, and the formation of [3H]-labeled squalene and cholesterol in the retina and their incorporation into rod outer segment (ROS) membranes were evaluated biochemically over a 60-day time course. ROS [3H]squalene specific activity was maximal by 1-3 days, then declined with a half-time of approximately 20-30 days. In contrast, the specific activity of ROS [3H]cholesterol initially increased to a level substantially less than that of [3H]squalene, and then remained constant. Thus, ROS squalene appears to turn over without obligatory conversion to, or coturnover with, ROS cholesterol. When [3H]acetate was injected into one eye, radiolabel in non-saponifiable lipids of the contralateral retina represented < 1% of those recovered from the ipsilateral retina; hence, systemic contributions to de novo synthesis were obviated. Long-term (> or = 8 hr) in vitro incubations of isolated retinas with [3H]acetate resulted in incorporation of [3H]-labeled sterols and squalene into ROS, at levels comparable to those observed in ROS from companion incubated eyecup preparations and from retinas 8 hr after intravitreal injection of [3H]acetate. These results demonstrate that the in vitro system faithfully reflects the in vivo biosynthetic capacity with respect to isoprenoid lipid metabolism, and suggest that de novo synthesis within the neural retina is responsible for generating most, if not all, of the [3H]squalene and [3H]cholesterol formed under the given conditions. Treatment of retinas in vitro with brefeldin A or energy poisons blocked transport of newly synthesized opsin, but not squalene, to the ROS. Furthermore, frogs maintained at 8 degrees C exhibited marked suppression of incorporation of newly synthesized protein into the ROS, while [3H]squalene incorporation was only minimally reduced, compared with frogs maintained at 22 degrees C. These results are consistent with prior findings that suggest that lipids are transported to the ROS by a mechanism distinct and independent from that employed for intracellular trafficking of opsin and other ROS-destined membrane proteins.

Species difference in cholesteryl ester cycle and HDL-induced cholesterol efflux from macrophage foam cells

The species difference in the turnover rates of the cholesteryl ester (CE) cycle in macrophage foam cells (MFC) was examined in mice and rats. MFC were induced by acetyl-LDL and pulsed with [3H]oleate, followed by a chase with [14C]oleate. The replacement of the initial amount of cholesteryl [3H]oleate by cholesteryl [14C]oleate within 24 hours was 63% in mouse MFC, whereas it was 33% in rat MFC. The corresponding replacement in rabbit MFC was < 10%. In addition, HDL removed 41% of the CE mass from mouse MFC but only 22% from rat MFC. HDL-induced CE reduction from mouse MFC was enhanced by 40% by the inhibitor for acyl-coenzyme A:cholesterol acyltransferase (58-035), whereas the enhancing effect was not observed with rat MFC. These results indicate that the rate of CE turnover may serve as a critical factor to determine the capacity of MFC to respond to HDL-induced CE reduction, suggesting the possibility that the species difference in the turnover rates of the CE cycle in MFC might explain, in part, the species difference in susceptibility to experimental atherosclerosis.

Intermediate filaments and lipoprotein cholesterol

The ability of cells to utilize cholesterol derived from lipoprotein is important in plasma membrane biosynthesis, steroidogenesis and the regulation of sterol synthesis. While the endocytosis of lipoprotein-derived cholesterol has been well characterized, the subsequent events that mediate its post-lysosomal intracellular transport are not understood. Recent studies have suggested that vimentin-type intermediate filaments may have a role in cholesterol transport. The mechanism by which vimentin filaments affect this process is not known, but future studies promise to provide new insights into both the post-lysosomal transport of cholesterol and the intracellular functions of intermediate filaments.

Dexamethasone-dependent modulation of cholesterol levels in human lymphoblastoid B cell line through sphingosine production

The effect of dexamethasone on lipid composition of Epstein-Barr virus transformed human B lymphocytes have been investigated by 31P- and 1H-NMR spectroscopy and compared to the effects due to exogenous sphingosine treatment. Furthermore, the effects of dexamethasone and sphingosine on membrane structure was evaluated by fluorimetry. No significant changes were evidenced in phospholipid composition and in the ratio of unsaturated to total fatty-acid chains. A significant increase in total cholesterol levels was evident at 30 min incubation with dexamethasone or sphingosine; a parallel increase in DPH polarization at 30 min was also demonstrated. TMA-DPH intensity measurements suggest a slowing of vesicular intracellular traffic due to the treatment. The results suggest a dexamethasone- and sphingosine-dependent inhibition of intracellular cholesterol transport.

Cholesterol transport from plasma membranes to intracellular membranes is inhibited by 3 beta-[2-(diethylamino)ethoxy]androst-5-en-17-one

The compound U1866A (3 beta-[2-(diethylamino)ethoxy]androst-5-en-17-one) has been shown to inhibit the cellular transfer of low-density lipoprotein-derived cholesterol from lysosomes to plasma membranes (Liscum and Faust (1989) J. Biol. Chem. 264, 11796-806). We have in this study examined the effects of U18666A on cholesterol translocation from plasma membranes to intracellular membranes. Translocation of plasma membrane cholesterol was induced by degradation of plasma membrane sphingomyelin. The sphingomyelinase-induced activation of the acyl-CoA cholesterol acyl transferase (ACAT) reaction was completely inhibited in a dose-dependent manner by U18666A, both in cultured human skin fibroblasts and baby hamster kidney cells. Half-maximal inhibition (within 60 min) was obtained with 0.5-1 microgram/ml of U18666A. A time-course study indicated that the onset of inhibition was rapid (within 10-15 min), and reversible if U18666A was removed from the incubation mixture. Using a cholesterol oxidase assay, we observed that the extent of plasma membrane cholesterol translocation in sphingomyelinase-treated HSF cells was significantly lowered in the presence of U18666A (at 3 micrograms/ml). The effect of U18666A on cholesterol translocation was also fully reversible when the drug was withdrawn. In mouse Leydig tumor cells, labeled to constant specific activity with [3H]cholesterol, the compound U18666A inhibited in a dose-dependent manner the cyclic AMP-stimulated secretion of [3H]steroid hormones. The effects seen with compound U18666A appeared to be specific for this molecule, since another hydrophobic amine, imipramine, did not in our experiments affect cholesterol translocation or ACAT activation. Since different cell types display sensitivity to U18666A in various intracellular cholesterol transfer processes, they appear to have a common U18666A-sensitive regulatory mechanism.

Changes in general and specific protein synthesis that accompany ecdysteroid synthesis in stimulated prothoracic glands of Manduca sexta

The prothoracic glands of fifth instar Manduca sexta larvae respond to stimulation by the brain neuropeptide, prothoracicotropic hormone (PTTH), with a several-fold increase in the rate of ecdysteroid synthesis. Previous studies have shown that this response requires protein synthesis and that the action of PTTH can be mimicked by dibutyryl cAMP (dbcAMP) and the Ca2+ ionophore, A23187. To further understand the role of protein synthesis in the response of prothoracic glands to PTTH, patterns of protein synthesis in stimulated glands were examined using glands incubated in vitro with [35S]methionine. All three agents caused an increase in the rate of ecdysteroid synthesis as well as an increase of up to 300% in the synthesis and/or accumulation of three proteins (p100, p70, and p"50") within 2 h of stimulation. Changes in these three proteins were specific to the prothoracic gland, were not elicited by non-brain peptides and were not simply a result of increased general protein synthesis in the gland. Exposure of the glands to A23187 alone, or concurrently with dbcAMP, resulted in increased synthesis of p100, p70, p"50" and ecdysteroids but decreased general protein synthesis. Increased synthesis of these proteins could be detected within 15 min after initiating PTTH stimulation. The behavior of these three proteins makes them candidates for modulators of ecdysteroid synthesis in the prothoracic gland. The results suggest also that PTTH may activate two biochemical pathways in the gland: one path leading to increased synthesis of the p100, p70, and p"50" proteins and increased ecdysteroid synthesis, and the second leading to increased general protein synthesis. This second trophic effect is vulnerable to intracellular Ca2+ changes that do not inhibit the first pathway.

Transport and sorting of membrane lipids

The lipid composition of cellular membranes may seem unnecessarily complex. However, the lipid composition of each membrane is carefully regulated by local metabolism and specificity in transport, marking the functional significance for the cell. Recent research has revealed unexpected discoveries concerning the topology of lipid synthesis, specificity in lipid transport, and the function of lipid and protein microdomains in sorting.

Substrate delivery as a determinant of hepatic apoB secretion

The evidence that apoB particles secreted by the liver can differ in number and composition has been reviewed. No evidence has yet emerged that changes in apoB100 itself affect the rate of its secretion from the liver. The metabolic role of apoB appears to be the prevention of lipid accumulation within the liver cell: when delivery of lipid to the liver increases, apoB secretion will increase pari passu. This reality in no way detracts from the critical role played by the LDL receptor in determining the number of LDL particles in plasma, nor does it diminish the potential importance of intracellular processes such as 7 alpha-hydroxylase activity to also mediate LDL receptor activity. However, it should be obvious that variation in catabolism by itself cannot explain all that has been observed in physiological and pathological studies. On the contrary, the whole process must be taken into account--the rate at which apoB particles are added to the circulation, the rate at which they are converted to LDL, and the rate at which they are irreversibly removed from plasma--if we are to understand and appreciate this most peculiar and most important of transport systems.