High density lipoprotein and low density lipoprotein utilization by human granulosa cells for progesterone synthesis in serum-free culture: respective contributions of free and esterified cholesterol

Targeting NPC1 in Renal Cell Carcinoma

Rapidly proliferating cancer cells have a greater requirement for cholesterol than normal cells. Tumor cells are largely dependent on exogenous lipids given that their growth requirements are not fully met by endogenous pathways. Our current study shows that ccRCC cells have redundant mechanisms of cholesterol acquisition. We demonstrate that all major lipoproteins (i.e., LDL, HDL, and VLDL) have a comparable ability to support the growth of ccRCC cells and are equally effective in counteracting the antitumor activities of TKIs. The intracellular trafficking of exogenous lipoprotein-derived cholesterol appears to be distinct from the movement of endogenously synthesized cholesterol. De novo synthetized cholesterol is transported from the endoplasmic reticulum directly to the plasma membrane and to the acyl-CoA: cholesterol acyltransferase, whereas lipoprotein-derived cholesterol is distributed through the NPC1-dependent endosomal trafficking system. Expression of NPC1 is increased in ccRCC at mRNA and protein levels, and high expression of NPC1 is associated with poor prognosis. Our current findings show that ccRCC cells are particularly sensitive to the inhibition of endolysosomal cholesterol export and underline the therapeutic potential of targeting NPC1 in ccRCC.

Maturation and culture affect the metabolomic profile of oocytes and follicular cells in young and old mares

Introduction: Oocytes and follicular somatic cells within the ovarian follicle are altered during maturation and after exposure to culture in vitro. In the present study, we used a nontargeted metabolomics approach to assess changes in oocytes, cumulus cells, and granulosa cells from dominant, follicular-phase follicles in young and old mares. Methods: Samples were collected at three stages associated with oocyte maturation: (1) GV, germinal vesicle stage, prior to the induction of follicle/oocyte maturation in vivo; (2) MI, metaphase I, maturing, collected 24 h after induction of maturation in vivo; and (3) MIIC, metaphase II, mature with collection 24 h after induction of maturation in vivo plus 18 h of culture in vitro. Samples were analyzed using gas and liquid chromatography coupled to mass spectrometry only when all three stages of a specific cell type were obtained from the same mare. Results and Discussion: Significant differences in metabolite abundance were most often associated with MIIC, with some of the differences appearing to be linked to the final stage of maturation and others to exposure to culture medium. While differences occurred for many metabolite groups, some of the most notable were detected for energy and lipid metabolism and amino acid abundance. The study demonstrated that metabolomics has potential to aid in optimizing culture methods and evaluating cell culture additives to support differences in COCs associated with maternal factors.

Implications of High-Density Cholesterol Metabolism for Oocyte Biology and Female Fertility

Cholesterol is an essential component of animal cells. Different regulatory mechanisms converge to maintain adequate levels of this lipid because both its deficiency and excess are unfavorable. Low cell cholesterol content promotes its synthesis and uptake from circulating lipoproteins. In contrast, its excess induces the efflux to high-density lipoproteins (HDL) and their transport to the liver for excretion, a process known as reverse cholesterol transport. Different studies suggest that an abnormal HDL metabolism hinders female fertility. HDL are the only lipoproteins detected in substantial amounts in follicular fluid (FF), and their size and composition correlate with embryo quality. Oocytes obtain cholesterol from cumulus cells via gap junctions because they cannot synthesize cholesterol de novo and lack HDL receptors. Recent evidence has supported the possibility that FF HDL play a major role in taking up excess unesterified cholesterol (UC) from the oocyte. Indeed, genetically modified mouse models with disruptions in reverse cholesterol transport, some of which show excessive circulating UC levels, exhibit female infertility. Cholesterol accumulation can affect the egg´s viability, as reported in other cell types, and activate the plasma membrane structure and activity of membrane proteins. Indeed, in mice deficient for the HDL receptor Scavenger Class B Type I (SR-B1), excess circulating HDL cholesterol and UC accumulation in oocytes impairs meiosis arrest and hinders the developmental capacity of the egg. In other cells, the addition of cholesterol activates calcium channels and dysregulates cell death/survival signaling pathways, suggesting that these mechanisms may link altered HDL cholesterol metabolism and infertility. Although cholesterol, and lipids in general, are usually not evaluated in infertile patients, one study reported high circulating UC levels in women showing longer time to pregnancy as an outcome of fertility. Based on the evidence described above, we propose the existence of a well-regulated and largely unexplored system of cholesterol homeostasis controlling traffic between FF HDL and oocytes, with significant implications for female fertility.

The function of high-density lipoprotein and low-density lipoprotein in the maintenance of mouse ovarian steroid balance

The membrane proteins, low-density lipoprotein receptor (LDLR) and scavenger receptor class B member 1 (SR-BI, gene name Scarb1), are lipoprotein receptors that play central roles in lipoprotein metabolism. Cholesterol bound in high-density lipoprotein (HDL) and LDL is transported into cells mainly by SR-BI and LDLR. The relative contribution of LDL and HDL to the steroidogenic cholesterol pool varies among species and may vary among tissues within one species. To investigate which of these pathways is more important in the supply of cholesterol in mouse ovary, we utilized immunohistochemistry, western blotting, RNAi, and RT-PCR as well as Ldlr-/- mice to explore the uptake of HDL and LDL in the ovary. Our data demonstrate that both SR-BI and LDLR are present in the interstitial cells, thecal cells, and corpora lutea (CLs), and their expression fluctuates with the development of follicles and CLs. The intracellular cholesterol concentration was significantly decreased when Ldlr or Scarb1 was silenced in luteal cells. Furthermore, Ldlr-/- mice had lower progesterone and estrogen levels compared to wild-type mice, and when Ldlr-/- mice were treated with the inhibitor of de novo cholesterol synthesis, lovastatin, serum progesterone, and estrogen concentrations were further reduced. These results demonstrate that both LDLR and SR-BI play important roles in importing cholesterol and that both HDL and LDL are crucial in steroidogenesis in mouse ovaries.

Oocyte environment: follicular fluid and cumulus cells are critical for oocyte health

Bidirectional somatic cell-oocyte signaling is essential to create a changing intrafollicular microenvironment that controls primordial follicle growth into a cohort of growing follicles, from which one antral follicle is selected to ovulate a healthy oocyte. Such intercellular communications allow the oocyte to determine its own fate by influencing the intrafollicular microenvironment, which in turn provides the necessary cellular functions for oocyte developmental competence, which is defined as the ability of the oocyte to complete meiosis and undergo fertilization, embryogenesis, and term development. These coordinated somatic cell-oocyte interactions attempt to balance cellular metabolism with energy requirements during folliculogenesis, including changing energy utilization during meiotic resumption. If these cellular mechanisms are perturbed by metabolic disease and/or maternal aging, molecular damage of the oocyte can alter macromolecules, induce mitochondrial mutations, and reduce adenosine triphosphate production, all of which can harm the oocyte. Recent technologies are now exploring transcriptional, translational, and post-translational events within the human follicle with the goal of identifying biomarkers that reliably predict oocyte quality in the clinical setting.

Genetic alterations affecting cholesterol metabolism and human fertility

Single nucleotide polymorphisms (SNPs) represent genetic variations among individuals in a population. In medicine, these small variations in the DNA sequence may significantly impact an individual's response to certain drugs or influence the risk of developing certain diseases. In the field of reproductive medicine, a significant amount of research has been devoted to identifying polymorphisms which may impact steroidogenesis and fertility. This review discusses current understanding of the effects of genetic variations in cholesterol metabolic pathways on human fertility that bridge novel linkages between cholesterol metabolism and reproductive health. For example, the role of the low-density lipoprotein receptor (LDLR) in cellular metabolism and human reproduction has been well studied, whereas there is now an emerging body of research on the role of the high-density lipoprotein (HDL) receptor scavenger receptor class B type I (SR-BI) in human lipid metabolism and female reproduction. Identifying and understanding how polymorphisms in the SCARB1 gene or other genes related to lipid metabolism impact human physiology is essential and will play a major role in the development of personalized medicine for improved diagnosis and treatment of infertility.

21-Hydroxylase-derived steroids in follicles of nonobese women undergoing ovarian stimulation for in vitro fertilization (IVF) positively correlate with lipid content of luteinized granulosa cells (LGCs) as a source of cholesterol for steroid synthesis

CONTEXT

Mineralocorticoid synthesis by the nonhuman primate periovulatory follicle enhances luteinization. Whether a similar event occurs in women undergoing in vitro fertilization (IVF) is unknown.

OBJECTIVE

The objective of the study was to determine whether human luteinized granulosa cells (LGCs) produce mineralocorticoids derived from 21-hydroxylase activity and also express mRNA for 21-hydroxylase and the mineralocorticoid receptor.

DESIGN

This was a prospective cohort study.

SETTING

The study was conducted at an academic center.

PATIENTS

LGC lipid content and follicle fluid (FF) hormone analysis was performed on 27 nonobese IVF women. LGCs from six additional nonobese IVF women were used for gene expression studies.

INTERVENTION

At oocyte retrieval, FF was aspirated from the first follicle (≥16 mm in size) of each ovary and pooled LGCs were collected.

MAIN OUTCOME MEASURES

FF steroid analysis was performed by liquid chromatography-tandem mass spectrometry. LGCs were stained with lipid fluorescent dye BODIPY FL C16 to estimate lipid content by confocal microscopy as a cholesterol source for steroidogenesis in vivo. Quantitative real-time PCR was performed using LGCs to detect 21-hydroxylase and mineralocorticoid receptor mRNA expression. Pearson correlation coefficients determined associations between FF steroid levels and LGC lipid content.

RESULTS

FF levels of the 21-hydroxylase-derived steroids, 11-deoxycorticosterone [DOC, 39.97, median (13.94-63.02) ng/mL] and 11-deoxycortisol [11DOC, 2.07 (0.69-5.01) ng/mL], along with the 21-hydroxylase precursor 17-hydroxyprogesterone [1268.21 (493.26-3558.39) ng/mL], positively correlated with LGC lipid content (84 ± 43 fluorescent units/sample) (P ≤ .05, all steroids). 21-Hydroxylase and mineralocorticoid receptor mRNA expression was detected in LGCs.

CONCLUSIONS

Human LGCs likely synthesize 21-hydroxylase-derived mineralocorticoids from cholesterol-containing lipid in vivo to promote postovulatory luteinization via mineralocorticoid receptor-mediated events.

Deficiency of scavenger receptor class B type I negatively affects progesterone secretion in human granulosa cells

Our goal was to examine the effect of deficiency of the lipoprotein receptor, scavenger receptor class B type I (SR-BI), on progesterone secretion in human granulosa cells (HGL5). Scrambled or SR-BI small interfering RNA [knockdown (KD)] cells were exposed to dimethylsulfoxide [DMSO, vehicle for forskolin (Fo)], Fo, serum, high-density lipoprotein, low-density lipoprotein (LDL), or Fo plus lipoproteins or serum for 24 h. Progesterone secretion was lower in all of the SR-BI KD cells regardless of treatment. We examined progesterone secretion in SR-BI KD, LDL receptor KD, and double KD cells incubated with DMSO, Fo, LDL, or Fo + LDL for 6-24 h. As compared with scrambled cells, progesterone secretion was lower in SR-BI and double KD cells regardless of treatment; whereas progesterone secretion was only lower in LDL receptor KD cells incubated with LDL and Fo + LDL. We measured phosphorylation of hormone-sensitive lipase (pHSL) expression, intracellular total cholesterol (TC) mass, and progesterone secretion in scrambled and SR-BI KD cells incubated with DMSO or Fo for 2-24 h. The expression of pHSL was similar between the cells and conditions. The mean change in TC mass and progesterone secretion was lower in SR-BI KD cells exposed to DMSO and Fo. Incubating SR-BI KD cells with 22-hydroxy cholesterol did not overcome the reduction in progesterone secretion. At different time points, RNA expression of steroidogenic acute regulatory protein, side-chain cleavage, and 3β-hydroxysteroid dehydrogenase was significantly lower in SR-BI KD cells incubated with Fo. In conclusion, SR-BI protein deficiency, in part, might explain progesterone deficiency in some infertile women.

Effects of human follicular fluid and high-density lipoproteins on early spermatozoa hyperactivation and cholesterol efflux

The preovulatory human follicular fluid contains only HDLs as a lipoprotein class with a typically high proportion of prebeta HDL. We first examined the role of follicular fluid and HDL subfractions on human spermatozoa capacitation, a process characterized by a hyperactivation of the flagellar movement and a depletion of plasma membrane cholesterol. Whole follicular fluid and isolated HDL, used at constant free cholesterol concentration, were both able to promote an early flagellar hyperactivation. Moreover, incubation of [(3)H]cholesterol-labeled spermatozoa with follicular fluid induced a rapid cholesterol efflux from spermatozoa that was confirmed by mass measurements of cholesterol transfer. Using isolated HDL, the cholesterol efflux had a similar time course and represented 70% of that mediated by whole follicular fluid. We then analyzed the time course of radioactive labeling of HDL subfractions. In the first minute of incubation, we found that the prebeta HDL fraction incorporated the main part of the radioactivity (60%), with the rest being found in alpha-HDL, but strikingly, the labeling of alpha-HDL increased with time at the expense of prebeta HDL.Thus, our results indicate that HDLs are involved in both spermatozoa hyperactivation and cholesterol effl ux and suggest the role of prebeta-HDL particles as fi rst cellular cholesterol acceptors.

Expression of scavenger receptor-BI and low-density lipoprotein receptor and differential use of lipoproteins to support early steroidogenesis in luteinizing macaque granulosa cells

An ovulatory hCG stimulus to rhesus macaques undergoing controlled ovarian stimulation protocols results in a rapid and sustained increase in progesterone synthesis. The use of lipoproteins as a substrate for progesterone synthesis remains unclear, and the expression of lipoprotein receptors [very-low-density lipoprotein receptor (VLDLR), low-density lipoprotein receptor (LDLR), and scavenger receptor-BI (SR-BI)] soon after human chorionic gonadotropin (hCG) (<12 h) has not been characterized. This study investigated lipoprotein receptor expression and lipoprotein (VLDL, LDL, and HDL) support of steroidogenesis during luteinization of macaque granulosa cells. Granulosa cells were aspirated from rhesus monkeys undergoing controlled ovarian stimulation before or up to 24 h after an ovulatory hCG stimulus. The expression of VLDLR decreased within 3 h of hCG, whereas LDLR and SR-BI increased at 3 and 12 h, respectively. Granulosa cells isolated before hCG were cultured for 24 h in the presence of FSH or FSH plus hCG with or without VLDL, LDL, or HDL. Progesterone levels increased in the presence of hCG regardless of lipoprotein addition, although LDL, but not HDL, further augmented hCG-induced progesterone. Other cells were cultured with FSH or FSH plus hCG without an exogenous source of lipoprotein for 24 h, followed by an additional 24 h culture with or without lipoproteins. Cells treated with hCG in the absence of any lipoprotein were unable to maintain progesterone levels through 48 h, whereas LDL (but not HDL) sustained progesterone synthesis. These data suggest that an ovulatory stimulus rapidly mobilizes stored cholesterol esters for use as a progesterone substrate and that as these are depleted, new cholesterol esters are obtained through an LDLR- and/or SR-BI-mediated mechanism.

Roles for prostaglandins in the steroidogenic response of human granulosa cells to high-density lipoproteins

In human granulosa-lutein cells, high-density lipoproteins (HDL) can stimulate progesterone synthesis. The objective of the present study was to establish whether prostaglandins (PGs) participate in the steroidogenic response to HDL. Both HDL and apolipoprotein AI (ApoAI) stimulated concentration-dependent increases in PGE2, cAMP and progesterone accumulation. The minimum concentrations of HDL and ApoAI required to elevate PGE2 production were the same as those required to stimulate cAMP accumulation and progesterone synthesis. Concentrations of PGE2 were elevated within 10 min in cells exposed to HDL and rose progressively over 24 h, whereas cAMP and progesterone were only increased significantly after 24 h of treatment with HDL. Co-treatment with prostaglandin H synthase inhibitors (meclofenamic acid and indomethacin) abolished the cAMP and progesterone responses to both HDL and ApoAI. Hence, the ability of HDL to stimulate progesterone synthesis can be mimicked by ApoAI and appears to involve increased generation of one or more luteotrophic PGs, possibly acting via cAMP.

Scavenger receptor class BI and selective cholesteryl ester uptake: partners in the regulation of steroidogenesis

The steroidogenic tissues have a special requirement for cholesterol, which is used as a substrate for steroid hormone biosynthesis. In many species this cholesterol is obtained from plasma lipoproteins by a unique pathway in which circulating lipoproteins bind to the surface of the steroidogenic cells and contribute their cholesteryl esters to the cells by a 'selective' process in which the whole lipoprotein particle does not enter the cell. This review describes the lipoprotein selective cholesteryl ester uptake process and its specific partnership with the HDL receptor, scavenger receptor class BI (SR-BI). It describes the characteristics of the selective pathway, and the molecular properties, localization, regulation, anchoring sites and potential mechanisms of action of SR-BI in facilitating cholesteryl ester uptake by steroidogenic cells.

Human granulosa cells use high density lipoprotein cholesterol for steroidogenesis

This study examines the ability of human high density lipoproteins (HDL3) to deliver cholesteryl esters to human granulosa cells and describes the selective cholesterol pathway by which this occurs. Luteinized cells obtained from subjects undergoing in vitro fertilization-embryo transfer procedures were incubated with native HDL3 (or radiolabeled or fluorescently labeled HDL cholesteryl esters) to determine whether cells from humans (in which HDL is not the primary circulating lipoprotein species) can nevertheless interiorize and appropriately process cholesteryl esters for steroidogenesis. The results indicate that hormone-stimulated granulosa cells actively and efficiently use human HDL-derived cholesterol for progesterone production. More than 95% of the mass of HDL cholesteryl esters entering cells does so through the nonlysosomal (selective) pathway, i.e. cholesteryl esters released from HDL are taken up directly by the cells without internalization of apoproteins. Once internalized, the cholesteryl esters are either hydrolyzed and directly used for steroidogenesis or stored in the cells as cholesteryl esters until needed. The utilization of the internalized cholesteryl esters is a hormone-regulated event; i.e. luteinized human granulosa cells internalize and store large quantities of HDL-donated cholesteryl esters when available, but further processing of the cholesteryl esters (hydrolysis, re-esterification, or use in steroidogenesis) does not occur unless the cells are further stimulated to increase progesterone secretion.

Increasing progesterone secretion and 3 beta-hydroxysteroid dehydrogenase activity of human cumulus cells and granulosa-lutein cells concurrent with successful fertilization of the corresponding oocyte

In many studies it has been documented that the induction of multiple follicular growth in humans results in an asynchrony between the degree of cumulus mucification, oocyte meiotic maturation, fertilizability, and follicular cell progesterone (P4) secretion. The present study was carried out on oocytes enclosed in fully mucified cumulus. Thus, oocyte fertilizability was correlated to human cumulus cell (hCC) and human granulosa-lutein (G-L) cell competence for P4 secretion in culture. In the G-L cells, P4 secretion and percentage of cells manifesting 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) activity increased concurrently with the period of culture. In the hCC, however, P4 secretion decreased concurrently with elongation of the culture period, whereas the percentage of 3 beta-HSD-positive cells increased. In hCC corresponding to the fertilized oocytes, P4 accumulation in culture medium was 1.9-fold (P < 0.001) and 1.6-fold (P < 0.02) higher on days 0-3 and 3-5 of culture, respectively, as compared to P4 accumulation in hCC of unfertilized oocytes. Also, in hCC corresponding to the fertilized oocytes, the degree of 3 beta-HSD activity was found to be significantly higher shortly after aspiration and after either 3 or 5 days, compared to hCC of unfertilized oocytes. In the G-L cells pooled from all follicles yielding mature cumulus-oocyte complexes, P4 accumulation and percentage of 3 beta-HSD-positive cells increased concurrently with the increase in percentage of fertilized eggs of each individual woman. These results indicate that in stimulated cycles, follicles yielding mature cumulus-oocyte complex, oocyte fertilizability, and G-L cell or hCC competence for P4 secretion are correlated and synchronous.

Increase of high-density lipoprotein cholesterol at ovulation in healthy women

Plasma cholesterol is believed to vary more in women than in men, with the menstrual cycle, yet our review of the literature found no consistent pattern. We examined variations in plasma lipoproteins in relation to ovarian hormones in 12 healthy, menstruating women. Twenty fasting blood samples were obtained on alternate days over one menstrual cycle; ovulation was timed by hormone measurements. Plasma was analysed enzymatically for total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C) and triacylglycerol (TAG). Low-density lipoprotein cholesterol (LDL-C) was estimated by the Friedewald formula. The greatest effect was seen in HDL-C. Concentrations increased by 12% (P < 0.001) between the times of menstruation and ovulation and remained elevated until the following premenstrual phase. The height of peak oestradiol concentrations at ovulation was significantly associated with HDL-C in that phase (r = +0.75, P < 0.01), and with mean HDL-C concentrations over the whole cycle (r = +0.65, P < 0.05). TC and LDL-C also increased at ovulation, by 9% (P < 0.005) and 11% (P < 0.025) respectively, although the effect was more transient. This study demonstrates that consistent changes in plasma lipoproteins do occur during the menstrual cycle.

Human ovarian granulosa cell culture: determination of blood cell contamination and evaluation of possible culture purification steps

OBJECTIVE

To determine the degree of blood cell contamination in GC preparations; to assess techniques of GC purification; and to determine possible effects of contaminating cells and purification techniques on cultured GC in terms of steroid hormone production.

DESIGN

Contamination of GC by white blood cells was assessed by Wright's stain and immunohistochemistry. Purification was attempted by: (1) Ficoll density centrifugation (to remove polymorphonuclear leukocytes [PML]); (2) incubation in tissue culture plastic dishes (to remove adherent monocyte/macrophages); and (3) incubation in the presence of high salt (to remove lymphocytes).

RESULTS

Ficoll density centrifugation reduced PML to 2% of total cells, incubation in plastic dishes reduced monocyte/macrophage contamination from 6% to 7% down to less than 1%, and high-salt incubation reduced lymphocyte contamination from 10% to 12% down to 4%. Granulosa cells plated after preparation with addition of high salt showed increased progesterone production, which could not be entirely explained by the removal of contaminating lymphocytes.

CONCLUSION

These results indicate that the human GC culture system is more complex than is often assumed, and methods that remove a majority of these white blood cells are presented.

Cyclic fluctuations in human serum lipid and apolipoprotein levels during the normal menstrual cycle: comparison with changes occurring during oral contraceptive therapy

The influence of menstrual cycle phases and hormonal contraception on serum lipid and apolipoprotein (apo) levels was investigated in a group of normally menstruating young women. The study period covered a normal menstrual cycle (pretherapy), the fourth cycle of treatment with a triphasic oral contraceptive (OC) preparation, and the cycle immediately following interruption of therapy (cycle 5, posttherapy). Cycle phases were defined on the basis of serum hormone levels and basal body temperature determinations. Significant differences in cholesterol (free and esterified) levels were observed during the menstrual phase of both the normal menstrual cycle (lower) and the OC cycle (higher), when compared with the other phases. Triglycerides, which were higher under OCs, fluctuated similarly throughout the two cycles, but phase differences did not reach statistical significance. Apo AI and apo B were both higher under OCs, and apo B followed a trend similar to cholesterol during the two cycles. During the first month after discontinuation of OCs, cholesterol levels returned progressively to baseline values, while triglycerides were only partially decreased. We conclude that cyclic fluctuations in lipid levels do occur under the influence of both endogenous and exogenous sex hormones.

Hepatic lipase-mediated hydrolysis versus liver uptake of HDL phospholipids and triacylglycerols by the perfused rat liver

Hepatic triacylglycerol-lipase-mediated hydrolysis and liver uptake of high-density lipoprotein (HDL) lipid components were studied in a recirculating rat liver perfusion, a situation where the enzyme is physiologically expressed and active at the vascular bed. Human native HDL were labelled with tri-[3H]oleoylglycerol, [N-methyl-3H]dipalmitoylphosphatidylcholine (DPPC), 1-palmitoyl,2-[14C]linoleoylphosphatidylcholine (PLPC), 1-palmitoyl,2-[14C]linoleoylphosphatidyl-ethanolamine (PLPE) and 1-palmitoyl,2-[14C]palmitoylphosphatidylethanolamine (DPPE). (1) Relative degradation rates of phosphatidylethanolamine molecular species were 2- to 10-fold higher than those of phosphatidylcholine. Considering [14C] PLPC and [14C] PLPE as representative of HDL phosphatidylcholine and phosphatidylethanolamine, respectively, the amounts of lysophosphatidylcholine and lysophosphatidylethanolamine generated after a 60 min perfusion were comparable. The enzyme showed a clear preference for the molecular species bearing an unsaturated fatty acid at the 2 position of glycerol; this was the most pronounced in the case of phosphatidylethanolamine molecular species. (2) Relative liver uptake of HDL-phosphatidylethanolamine was 4- to 5-fold higher than that of HDL-phosphatidylcholine, irrespective of the constitutive fatty acids. Nevertheless, mass estimation indicated that 3 times more molecules of phosphatidylcholine than of phosphatidylethanolamine were transferred. No correlation could be found between the relative degradation rates of phospholipids and their relative liver uptake, indicating a dissociation between the two processes. (3) Perfusate decay and relative liver uptake of labelled HDL-triacylglycerol were higher than that of any phospholipid class. No circulating radiolabelled free fatty acids accumulated in the perfusate, but they were found acylated into liver cell phospholipids and triacylglycerols. (4) A prior 10-12-min washout of the liver vascular bed with heparin removed over 80% of the hepatic lipase activity, as assessed by specific immunoinhibition. Hepatic lipase-depleted liver displayed impaired phospholipid hydrolysis and triacyglycerol uptake, whereas the transfer of HDL phospholipids to liver tissue was unaffected.

Behaviour of phospholipase modified-HDL towards cultured hepatocytes. II. Increased cell cholesterol storage and bile acid synthesis

Human total HDL (hydrated density 1.070-1.210), HDL2 (1.070-1.125), HDL3 (1.125-1.210) or HDL separated by heparin affinity chromatography were treated with or without purified phospholipase A2 from Crotalus adamanteus. Control and treated HDL were reisolated and were then incubated with cultured hepatocytes. 1. Mass measurements evidenced a time-dependent cholesterol enrichment in hepatocytes cultured in the absence of lipoproteins. Addition of HDL2 still enhanced by 25% the cell cholesterol content and down-regulated endogenous sterol synthesis in similar proportions. Conversely, HDL3 slightly decreased the amount of free cholesterol in hepatocytes (-12%). 2. Incubations with phospholipase A2-treated HDL resulted in a 35%-50% increase of both the cellular cholesterol esterification and the cholesterylester accumulation, when compared to cells cultured in the presence of control-HDL. This effect was observed with HDL2, HDL3 and combining the data with all subfractions. 3. Cultured hepatocytes secreted cholic and beta-muricholic acids as major bile acids and HDL2 showed a tendency to stimulate their secretion. Phospholipase treatment of HDL again induced an increased production by hepatocytes of those two bile acids. Thus, whereas HDL2 and HDL3 display different behaviours with respect to cell cholesterol content, neosynthesis and bile acid secretion, their modifications by phospholipases always orientate the cell sterol metabolism in the same direction: increased cholesterylester accumulation and bile acid production.

Behaviour of phospholipase-modified HDL towards cultured hepatocytes. I. Enhanced transfers of HDL sterols and apoproteins

Human HDL subfractions (HDL2, HDL3, or HDL separated by heparin affinity chromatography) were labelled either on their apolipoprotein moiety with 125I or on their sterols: unesterified [14C]cholesterol and [3H]cholesteryl linoleyl ether, a non-hydrolysable analog of esterified cholesterol. HDL subfractions were then treated with or without phospholipase A2 from Crotalus adamanteus in presence of albumin leading to a 72-82% phosphatidylcholine degradation. Control and treated HDL were reisolated and then addressed to cultured rat hepatocytes. (A) During incubations, unesterified [14C]cholesterol from HDL3 readily appeared in hepatocytes. The specific uptake of HDL esterified cholesterol calculated from [3H]cholesteryl ether was 2-4-times less important. Uptake of HDL cholesterol tended to saturate at 150-200 micrograms/ml HDL protein. A prior phospholipase treatment of HDL3 stimulated by 2-5-fold the uptake of [3H]cholesteryl ether, whereas the transfer of free [14C]cholesterol was minimally increased. The uptake of 3H/14C-labelled sterols from HDL2 was 2-3-times higher than from HDL3. (B) Parallel experiments were conducted with 125I-labelled HDL subfractions. At 37 degrees C, the specific uptake and degradation of HDL3 125I-apolipoprotein were about 2-fold enhanced following treatment of HDL3 with phospholipase A2. Uptakes of apolipoprotein and of esterified cholesterol were compared, indicating a preferential delivery of the sterol over apoprotein (X5). The dissociation was still more pronounced with phospholipase-treated HDL3. Competition experiments showed that 12-times more unlabelled HDL3 were required to half reduce the uptake of HDL3 [3H]cholesteryl ether than to impede similarly the HDL 125I-apolipoprotein recovered in cells. Uptake of 125I-labelled apolipoprotein from HDL2 was quantitatively comparable to that from HDL3. (C) Binding of 125I-HDL subfractions was followed at 4 degrees C. A specific binding was observed for HDL2 and HDL3, although kinetic parameters were quite different (KD of 9 and 25 micrograms/ml, respectively). Following phospholipolysis, both the specific and non-specific contributions to total binding were increased. Hence, hepatocytes take up more 125I-labelled apolipoprotein and 3H/14C-labelled sterols from lipolysed HDL than from unmodified particles. This is associated to changes in the binding characteristics.

Reactivity of HDL subfractions towards lecithin-cholesterol acyltransferase. Modulation by their content in free cholesterol

(1) Human HDL2 (d 1.070-1.125) and HDL3 (d 1.125-1.21) labelled with unesterified [14C]cholesterol, were incubated with a source of lecithin-cholesterol acyltransferase. For optimal activity, the reaction required the addition of albumin in excess, at least 3-times greater than the concentration of HDL-free cholesterol. Under such conditions, the reaction appeared saturable. HDL3 was found the most efficient substrate and the Vmax values expressed for 1.5 IU LCAT/ml and with an albumin/free cholesterol ratio of 3, were 8.3 nmol free cholesterol esterified/ml per h and 4.1 nmol/ml per h for HDL3 and HDL2, respectively. (2) HDL3 were modified in the presence of VLDL by inducing triacylglycerol lipolysis with a semipurified lipoprotein lipase from bovine milk. The newly formed HDL had gained free cholesterol and phospholipids, so that about 50% of these modified HDL, referred to as light-LIP-HDL3, were reisolated in the HDL2 density range. Light-LIP-HDL3 were enriched mostly in free cholesterol (+ 160%) and in phospholipid (+ 40%). Their reactivity towards LCAT was half-reduced compared to parent HDL3, which correlated well with a decrease in their phospholipid/free cholesterol molar ratio. Moreover, HDL3 artificially enriched in free cholesterol and exhibiting a comparable PL/FC behaved like lipolysis-modified HDL in their reactivity towards LCAT. (3) HDL3 were also modified by co-incubation with VLDL (post-VLDL-HDL3), or with VLDL and a source of lipid transfer protein (CET-HDL3). The latter treatment greatly affected the lipid composition of the core particle (-25% esterified cholesterol, +190% TG). In both cases, the moderate decreasing LCAT reactivity observed could be related to the phospholipid/free cholesterol ratio. Thus, like in artificial substrates, the lipid composition of the HDL surface may control the rate of LCAT-mediated cholesterol esterification.

Transfer of free and esterified cholesterol from low-density lipoproteins and high-density lipoproteins to human adipocytes

Cholesterol stored in human adipose tissue is derived from circulating lipoproteins. To delineate the cholesterol transport function of LDL and HDL, the movement of radiolabelled esterified cholesterol and free cholesterol from labelled LDL and HDL to human adipocytes was examined in the present study. LDL and HDL were enriched and labelled in esterified cholesterol with [14C]cholesterol by the action of plasma lipid transfer proteins and lecithin-cholesterol acyltransferase. Doubly labelled (3H,14C) LDL and HDL were prepared by exchanging free [3H]cholesterol into the 14C-labelled lipoproteins. 14C-labelled lipoprotein and 3H-labelled lipoprotein were also prepared separately and mixed to yield a mixed doubly labelled lipoprotein. Relative to the total amount added, proportionally more free than esterified cholesterol was transferred to the adipocytes upon incubation with any doubly labelled LDL and HDL. The calculated mass of free and esterified cholesterol transferred, however, varied with different labelled lipoproteins. 3H- and 14C-labelled LDL or HDL transferred 2-3-fold more esterified than free cholesterol while the reverse occurred with the mixed doubly labelled LDL or HDL. Thus, free cholesterol-depleted particles preferentially transferred cholesterol ester to the fat cells. In the presence of the homologous unlabelled native lipoprotein, the transfers of free and esterified cholesterol from labelled LDL or HDL were specifically inhibited. Selective transfer of esterified cholesterol relative to apoprotein was also observed when esterified cholesterol uptake from both LDL and HDL was assayed along with the binding of 125I-labelled lipoprotein. The cellular accumulation of cholesterol ether-labelled HDL (a non-hydrolyzable analogue of cholesterol ester) exceeded that of cholesterol ester consistent with significant hydrolysis of the latter physiological substrate. These results demonstrate preferential transfer of free cholesterol and esterified cholesterol over apoprotein for both LDL and HDL in human adipocytes. Furthermore, the data suggest that the cholesterol ester transport function of LDL and HDL can be enhanced by free cholesterol depletion and cholesterol ester enrichment of the particles, and affirms a role for adipose tissue in the metabolism of lipid-modified lipoproteins.

Phosphatidylcholine and triacylglycerol hydrolysis in HDL as induced by hepatic lipase: modulation of the phospholipase activity by changes in the particle surface or in the lipid core

(1) Human HDL2 (d 1.063-1.125) and HDL3 (d 1.125-1.210), labelled with 2-[14C]oleoylphosphatidylcholine (PC), and with/without tri[3H]oleoylglycerol, were incubated with a partially purified human hepatic triacylglycerol lipase, at pH 8.5. PC hydrolysis was linear up to 90-120 min incubation and within a range of lipase activities, from 50 to 500 mIU/ml. At low degrees of lipolysis, the hydrolysis of triacylglycerol was linearly related to that of PC, but the relative degradation rate was 10-fold higher for the former, which was thus very rapidly consumed. HDL subfractions were then differentiated in terms of PC hydrolysis. Km values were 0.32 and 0.43 mM for HDL2 PC and HDL3 PC, respectively. The corresponding Vmax values expressed for 200 mIU/ml hepatic lipase activity were 41.0 nmol PC hydrolysed/ml per h (HDL2) and 28.6 nmol PC/ml per h (HDL3). (2) HDL3 were modified in the presence of VLDL by inducing triacylglycerol lipolysis in VLDL with a semi-purified human plasma or bovine milk lipoprotein lipase (LPL). Lipolysis-modified HDL3 (LIP-HDL3) were mostly enriched in free cholesterol (+80%, P less than 0.05) and to a lesser extent in triacylglycerol (+33%). As a consequence, 45% of the LIP-HDL3 was reisolated in the HDL2-density interval, and is referred to as light LIP-HDL3. LIP-HDL3 displayed a 65% increase in its reactivity towards hepatic lipase compared to control HDL3. The light LIP-HDL3 showed the lowest Km (0.19 mM PC) and the highest Vmax (69 nmol/ml per h) of all HDL tested. Coincubation of HDL3 with VLDL and albumin did not alter the further reactivity of HDL3 towards hepatic lipase. Cholesterol loading of HDL3 by celite-cholesterol dispersions also led to an enhanced reactivity, though less important than with the lipolysis modification. (3) HDL3 were also modified by coincubation with VLDL and the lecithin-cholesterol acyltransferase-inhibited plasma fraction of d greater than 1.21 g/ml, thus allowing the cholesteryl ester transfer reaction to occur. The modified HDL3 (CET-HDL3) were depleted in esterified cholesterol (-25%, P less than 0.05) and enriched in triacylglycerol (+70%, P less than 0.05). However, these particles behaved like control HDL3 in their reactivity towards hepatic triacylglycerol lipase. Thus, the hydrolysis of HDL PC mediated by hepatic triacylglycerol lipase appears to be influenced by changes occurring in the particle's surface rather than in the lipid core.

Cultivation of rat granulosa cells in a serum-free chemically defined medium--a useful model to study lipoprotein metabolism

We have developed a chemically defined, serum-free medium for the culture of rat granulosa cells. This medium contains Dulbecco's modified Eagle's medium/Ham's nutrient F12 (DME:F12) (1:1) plus insulin (2 micrograms/ml), hydrocortisone (100 ng/ml), transferrin (5 micrograms/ml) and fibronectin (2 micrograms/cm2). Granulosa cells grown in this medium have an absolute requirement for added cholesterol-rich lipoproteins for steroidogenesis. When cells are cultured in basal medium, progestin production is low; when cells are cultured in the presence of follicle-stimulating hormone (FSH) or dibutyryl cAMP [Bu)2 cAMP), progestin secretion is increased 10-100-fold. Both heterologous and homologous lipoproteins synergistically increased the effects of (Bu)2 cAMP or FSH: e.g., addition to the medium of human (h)-HDL3 produced a significant increase in both basal (approx. 15-fold) and (Bu)2 cAMP-stimulated (approx. 1000-2000-fold) progestin production. LDL were less effective than HDL at equivalent concentrations of lipoprotein cholesterol. FSH invoked changes similar to that of (Bu)2 cAMP, although the magnitude of the FSH-induced change was less dramatic than that seen with (Bu)2 cAMP. The effect of h-HDL3 and h-LDL on both basal and hormone-stimulated progestin production was concentration- and time-dependent. The maximum effect of h-HDL3 was achieved at a protein concentration of 500 micrograms/ml, with an ED50 of approx. 90 micrograms/ml. In contrast, h-LDL was most effective at a concentration of 30-40 micrograms protein/ml. Likewise, rat (r-)HDL and r-LDL supported steroidogenesis in a concentration-dependent manner. Maximal responses to all additions were observed after 72 h of treatment. Granulosa cells secreted 20 alpha-hydroxypregn-4-ene-3-one as the predominant steroid in response to (Bu)2 cAMP. However, with the addition of h-HDL3, the major secreted product was progesterone. In conclusion, rat granulosa cells maintained in the described serum-free medium are exquisitely sensitive to supplied cholesterol-rich lipoproteins. When cultured in the presence of both lipoproteins and stimulatory agents, they produce from 1000-2000-times the progestins made by comparable cells maintained in medium alone. This responsiveness of the cells to both lipoprotein and hormone stimulation makes them uniquely suitable for studies involving the uptake and metabolism of lipoproteins during steroidogenesis.

Effect of a luteinizing hormone-releasing hormone agonist (Buserelin) on steroidogenesis of cultured human preovulatory granulosa cells

In order to ascertain whether there is a direct effect of luteinizing hormone-releasing hormone (LH-RH) agonists on ovarian steroidogenesis, human preovulatory granulosa cells were cultured in the presence of an LH-RH agonist (Buserelin, Hoechst Roussel Pharmaceuticals, Paris, France). Cultures were performed without serum but with precursors of estrogens and progestins. In basal conditions, Buserelin, at 1 ng/ml, increased estradiol and progesterone (P) secretion, while at concentrations of 10 and 100 ng/ml, no effect was observed. In the presence of follicle-stimulating hormone (FSH) or LH, Buserelin at 1 and 10 ng/ml did not modify the cell's steroidogenesis. However, at a concentration of 100 ng/ml, Buserelin decreased LH stimulation of P secretion. These results suggest that LH-RH agonists, at concentrations in the range of those obtained in clinical use, can modulate ovarian steroidogenesis by direct action.

Lipid and lipoprotein contents of human follicular fluid

The concentrations of total cholesterol, phospholipids, triacylglycerols and lipoproteins were measured in 87 follicular fluids obtained from 35 women undergoing in vitro fertilization and embryo transfer. The results were correlated with the levels of progesterone in follicular fluid. Two different types of ovarian stimulation were used. High density lipoproteins were the dominant lipoproteins found in the preovulatory follicular fluid. Low density lipoproteins were absent or appeared in trace amounts. Significantly higher triacylglycerol and high density lipoprotein levels were found when stimulation with human menopausal gonadotropins and chorionic gonadotropin was applied, as compared to the clomiphene citrate-menopausal gonadotropin-chorionic gonadotropin menstrual cycle. In both groups, extracorporal fertilization resulted in cleavage of oocytes and embryo transfer. No significant correlation between any follicular fluid lipid and progesterone concentration was found. The lipids estimated in the follicular fluid appeared to have no influence on the oocyte fertilizability. The presence of triacylglycerols and high density lipoproteins in the follicular fluid may indicate follicular wall permeability under the treatment with menopausal gonadotropins.

Uptake of HDL unesterified and esterified cholesterol by human endothelial cells. Modulation by HDL phospholipolysis and cell cholesterol content

Human HDL (1.070-1.210), doubly labelled with 3H/14C-labelled unesterified cholesterol and 3H-labelled esterified cholesterol were incubated for 1-5 h with monolayer cultures of human endothelial cells. HDL were preincubated for 60-120 min the presence of albumin and with/without purified phospholipase A2 (control HDL, phospholipase A2 HDL) before dilution in the cell culture medium. Average phosphatidylcholine (PC) degradation was 62.10% +/- 2.57% (range 45-80%). A purified lipase/phospholipase A1 from guinea pig pancreas was used in some experiments (range of PC hydrolysis: 16-70%). (1) 3H/14C-labelled unesterified cholesterol and 3H-labelled esterified cholesterol appeared in cells during 0-5 h incubations. Trypsin treatment allowed a simple adsorption of HDL onto the cell surface to be avoided, and most of the 3H-labelled esterified cholesterol transferred to cells was hydrolysed. Cell uptake of radioactive cholesterol increased as a function of HDL concentration but no saturation was achieved at the highest lipoprotein concentration used (200 micrograms cholesterol/ml). Flux of 3H/14C-labelled unesterified cholesterol was related to the cell cholesterol content, suggesting that it might partly represent an exchange process. The cell cholesterol content was slightly increased after 5 h incubation with HDL (+16%). (2) Pretreatment of HDL with purified phospholipase A2 doubled on average the amount of cell recovered 3H-labelled esterified cholesterol, while the flux of 3H/14C-labelled unesterified cholesterol was enhanced by 15-25%. Both transfer and cell hydrolysis of 3H-labelled esterified cholesterol were increased. A stimulation was also observed using purified lipase/phospholipase A1, provided that a threshold phospholipid degradation was achieved (between 27 and 45%). (3) Endothelial cells were conditioned in different media so as to modulate their charge in cholesterol. The uptake of 3H-labelled esterified cholesterol was found to be significantly higher in cholesterol-enriched cells compared to the sterol-depleted state. Finally, movements of 3H-labelled esterified cholesterol from HDL to endothelial cells were essentially unaffected by cell density or by the presence of partially purified cholesterol ester transfer protein. The possible roles of the transfer of HDL esterified cholesterol to endothelial cells and its modulation by phospholipases are discussed.