Characterization of apoB, E receptor function in the luteinized ovary

Ovarian granulosa cells utilize scavenger receptor SR-BI to evade cellular cholesterol homeostatic control for steroid synthesis

Cellular cholesterol is known to be under homeostatic control in nonsteroidogenic cells, and this intrigued us to understand how such control works in steroidogenic cells that additionally use cholesterol for steroid hormone synthesis. We employed primary culture of rat ovarian granulosa cells to study how steroidogenic cells adapt to acquire sufficient cholesterol to meet the demand of active steroidogenesis under the stimulation of gonadotropin follicle-stimulating hormone (FSH) and cytokine transforming growth factor (TGF)β1. We found that TGFβ1 potentiated FSH to upregulate scavenger receptor class B member I (SR-BI) and LDL receptor (LDLR), both functional in uptaking cholesterol as hHDL(3) and hLDL supplementation enhanced progesterone production, and the effect of each lipoprotein was completely or partially blocked by SR-BI selective inhibitor BLT-1. Uptaken cholesterol could also be stored in lipid droplets. Importantly, LDLR and SR-BI responded to sterol with different sensitivity. Giving cells lipoproteins or 25-hydroxycholesterol downregulated Ldlr but not Scarb1; Scarb1 was ultimately downregulated by excessive sterol accumulation under 25-hydroxycholesterol and aminoglutethimide (inhibitor of steroidogenesis) cotreatment. Furthermore, transcription factors sterol regulatory element-binding protein (SREBP)-2 and liver receptor homolog (LRH)-1 crucially mediated Ldlr and Scarb1 differential response to sterol challenge. This study reveals that ovarian granulosa cells retain the cholesterol homeostatic control machinery like nonsteroidogenic cells, although during active steroidogenesis, they utilize SR-BI to evade such feedback control.

Conversion of low density lipoprotein-associated phosphatidylcholine to triacylglycerol by primary hepatocytes

We have studied the uptake and metabolism of phosphatidylcholine (PC), the major phospholipid of low density lipoproteins (LDL), by cultures of primary hepatocytes. Strikingly, in the absence of the LDL receptor, PC incorporation into hepatocytes was inhibited by only 30%, whereas cholesteryl ether uptake was inhibited by 60-70%. On the other hand, scavenger receptor class B, type I, the other important receptor for LDL in the liver, was found to be responsible for the uptake of the remaining 30-40% of LDL-cholesteryl ether. PC uptake was, however, only partially inhibited (30%) in scavenger receptor class B, type I, knock-out hepatocytes. Once LDL-PC was taken up by hepatocytes, approximately 50% of LDL-[(3)H]oleate-PC was converted to triacylglycerol rather than degraded in lysosomes as occurs for LDL-derived cholesteryl esters. The remainder of the LDL-derived PC was not significantly metabolized to other products. Triacylglycerol synthesis from LDL-PC requires a PC-phospholipase C activity as demonstrated by inhibition with the phospholipase C inhibitor D609 or activation with rattlesnake venom. Small interfering RNA-mediated suppression of acyl-CoA:diacylglycerol acyltransferase 2 (DGAT2), but not DGAT1, decreased the acylation of the LDL-derived diacylglycerol. These findings show that PC in LDL particles is taken up not only by the classical receptors but also by additional mechanism(s) followed by metabolism that is completely different from the cholesteryl esters or apoB100, the other main components of LDL.

Role of the nuclear receptors for oxysterols LXRs in steroidogenic tissues: beyond the "foie gras", the steroids and sex?

Various physiological functions have been ascribed to the liver X receptors (LXRs). Recently, we have identified new functions of these nuclear receptors in steroidogenic tissues. In adrenal, LXRalpha prevents accumulation of free cholesterol in mouse by controlling expression of genes involved in all aspects of cholesterol utilization. Under chronic dietary stress, adrenals from LXR-deficient mice accumulate free cholesterol while wild-type animals maintain cholesterol homeostasis through basal regulation of cholesterol efflux and storage. Hence, LXRalpha provides a safety valve to limit free cholesterol levels as a basal protective mechanism in the adrenal. Beside, mice lacking LXRalpha show lower levels of testicular testosterone while wild-type mice treated with the specific LXR agonist present an increase of testosterone production. Altogether, these data identify new roles for LXRs, in the regulation of cholesterol homeostasis in steroidogenic tissues and hormone synthesis.

The molecular control of corpus luteum formation, function, and regression

The corpus luteum (CL) is one of the few endocrine glands that forms from the remains of another organ and whose function and survival are limited in scope and time. The CL is the site of rapid remodeling, growth, differentiation, and death of cells originating from granulosa, theca, capillaries, and fibroblasts. The apparent raison d'etre of the CL is the production of progesterone, and all the structural and functional features of this gland are geared toward this end. Because of its unique importance for successful pregnancies, the mammals have evolved a complex series of checks and balances that maintains progesterone at appropriate levels throughout gestation. The formation, maintenance, regression, and steroidogenesis of the CL are among the most significant and closely regulated events in mammalian reproduction. During pregnancy, the fate of the CL depends on the interplay of ovarian, pituitary, and placental regulators. At the end of its life span, the CL undergoes a process of regression leading to its disappearance from the ovary and allowing the initiation of a new cycle. The generation of transgenic, knockout and knockin mice and the development of innovative technologies have revealed a novel role of several molecules in the reprogramming of granulosa cells into luteal cells and in the hormonal and molecular control of the function and demise of the CL. The current review highlights our knowledge on these key molecular events in rodents.

Endocytosis is not required for the selective lipid uptake mediated by murine SR-BI

The scavenger receptor class B, type I (SR-BI) mediates the cellular selective uptake of cholesteryl esters and other lipids from high-density lipoproteins (HDL) and low-density lipoproteins (LDL). This process, unlike classical receptor-mediated endocytosis, does not result in lipoprotein degradation. Instead, the lipid depleted particles are released into the medium. Here we show that selective lipid uptake mediated by murine SR-BI can be uncoupled from the endocytosis of HDL or LDL particles. We found that blocking selective lipid uptake by incubating cells with the small chemical inhibitors BLT-1 or BLT-4 did not affect endocytosis of HDL. Similarly, blocking endocytosis by hyperosmotic sucrose or K+ depletion did not prevent selective lipid uptake from HDL or LDL. These findings suggest that mSR-BI-mediated selective uptake occurs at the cell surface upon the association of lipoproteins with mSR-BI and does not require endocytosis of HDL or LDL particles.

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.

The apolipoprotein E-dependent low density lipoprotein cholesteryl ester selective uptake pathway in murine adrenocortical cells involves chondroitin sulfate proteoglycans and an alpha 2-macroglobulin receptor

Cells acquire lipoprotein cholesterol by receptor-mediated endocytosis and selective uptake pathways. In the latter case, lipoprotein cholesteryl ester (CE) is transferred to the plasma membrane without endocytosis and degradation of the lipoprotein particle. Previous studies with Y1/E/tet/2/3 murine adrenocortical cells that were engineered to express apolipoprotein (apo) E demonstrated that apoE expression enhances low density lipoprotein (LDL) CE uptake by both selective and endocytic pathways. The present experiments test the hypothesis that apoE-dependent LDL CE selective uptake is mediated by scavenger receptor, class B, type I (SR-BI). Surprisingly, SR-BI expression was not detected in the Y1/E/tet/2/3 clone of Y1 adrenocortical cells, indicating the presence of a distinct apoE-dependent pathway for LDL CE selective uptake. ApoE-dependent LDL CE selective uptake in Y1/E/tet/2/3 cells was inhibited by receptor-associated protein and by activated alpha(2)-macroglobulin (alpha(2)M), suggesting the participation of the LDL receptor-related protein/alpha(2)M receptor. Reagents that inhibited proteoglycan synthesis or removed cell surface chondroitin sulfate proteoglycan completely blocked apoE-dependent LDL CE selective uptake. None of these reagents inhibited SR-BI-mediated LDL CE selective uptake in the Y1-BS1 clone of Y1 cells in which LDL CE selective uptake is mediated by SR-BI. We conclude that LDL CE selective uptake in adrenocortical cells occurs via SR-BI-independent and SR-BI-dependent pathways. The SR-BI-independent pathway is an apoE-dependent process that involves both chondroitin sulfate proteoglycans and an alpha(2)M receptor.

Expression of scavenger receptor class B type 1 (SR-BI) promotes microvillar channel formation and selective cholesteryl ester transport in a heterologous reconstituted system

In the "selective" cholesteryl ester (CE) uptake process, surface-associated lipoproteins [high density lipoprotein (HDL) and low density lipoprotein] are trapped in the space formed between closely apposed surface microvilli (microvillar channels) in hormone-stimulated steroidogenic cells. This is the same location where an HDL receptor (SR-BI) is found. In the current study, we sought to understand the relationship between SR-BI and selective CE uptake in a heterologous insect cell system. Sf9 (Spodoptera frugiperda) cells overexpressing recombinant SR-BI were examined for (i) SR-BI protein by Western blot analysis and light or electron immunomicroscopy, and (ii) selective lipoprotein CE uptake by the use of radiolabeled or fluorescent (BODIPY-CE)-labeled HDL. Noninfected or infected control Sf9 cells do not express SR-BI, show microvillar channels, or internalize CEs. An unexpected finding was the induction of a complex channel system in Sf9 cells expressing SR-BI. SR-BI-expressing cells showed many cell surface double-membraned channels, immunogold SR-BI, apolipoprotein (HDL) labeling of the channels, and high levels of selective HDL-CE uptake. Thus, double-membraned channels can be induced by expression of recombinant SR-BI in a heterologous system, and these specialized structures facilitate both the binding of HDL and selective HDL-CE uptake.

Transport of lipids from high and low density lipoproteins via scavenger receptor-BI

The scavenger receptor-BI (SR-BI) delivers sterols from circulating lipoproteins to tissues, but the relative potency of individual lipoproteins and the transported cholesterol has not been studied in detail. In this study, we used Chinese hamster ovary cells that express recombinant mouse SR-BI but have no functional low density lipoprotein (LDL) receptors (ldlA7-SRBI cells) to compare the fate of lipids transferred from high or low density lipoproteins to cells by SR-BI. HDL and LDL were equally effective in mediating the transfer of [(3)H]cholesterol to cells. Only 5% of the free cholesterol transferred to cells was esterified, in direct contrast to the findings in the cells that express LDL receptors in which 50% of the transported cholesterol was esterified. Almost all the free cholesterol transferred from lipoproteins to cells was rapidly excreted when the ldlA7-SRBI cells were switched to media containing unlabeled lipoproteins. SR-BI expression was associated with an increase in selective cholesteryl ester uptake from both lipoproteins, but HDL was a more effective donor. HDL and LDL were equally effective in delivering cholesterol to the intracellular regulatory pool via SR-BI. These data indicate that SR-BI is able to exchange cholesterol rapidly between lipoproteins and cell membranes and can mediate the uptake of cholesteryl esters from both classes of lipoproteins.

Upregulation of selective cholesteryl ester uptake pathway in mice with deletion of low-density lipoprotein receptor function

This study examines the effect of mutation of the low-density lipoprotein receptor (LDLR) on cholesterol metabolism, and especially lipoprotein-derived cholesteryl ester uptake, in murine ovarian granulosa cells. Although the tests were conducted on cells prepared by two different procedures, the results are similar. Deletion of LDLR function did not noticeably affect key enzymes of the steroidogenic pathway or affect progestin production and secretion in granulosa cells. No change was found in expression of LDL-related protein (LRP). These data suggested that cholesterol turnover in cells from the knockout animals is within normal limits and that the cells are not stressed to acquire more cholesterol. Both biochemical and morphological data indicate that unstimulated granulosa cells from LDLR-/- mice are nonetheless programmed to take in double the amount of lipoprotein-derived cholesteryl ester (via the selective cholesteryl ester uptake pathway) and to process (hydrolyze, re-esterify, or utilize) more than twofold the cholesteryl ester processed by cells from wildtype (LDLR+/+) animals. Bt2cAMP stimulation of the murine granulosa cells increases the mass of cholesteryl ester taken up by the selective pathway by an additional 38%. To determine to what extent this increase is related to high-density lipoprotein (HDL) scavenger receptor protein (SR-BI) or caveolin function, Western blots and immunohistochemical studies were performed under a variety of conditions. SR-BI levels are found to be low in unstimulated cells of both LDLR+/+ and LDLR-/- animals, but highly expressed (approximately 20-fold increase over basal levels) in stimulated (Bt2cAMP) cells of both animal models. Thus, the functional relationship between selective cholesteryl ester uptake and SR-BI receptor protein is not as tight as in previously reported studies, suggesting a requirement for other tissue factors. Caveolin expression did not change under any of the conditions tested and appears not to be functionally involved in this process.

The selective pathway and a high-density lipoprotein receptor (SR-BI) in ovarian granulosa cells of the mouse

We recently reported that rat luteinized ovary tissue and primary cultures of rat ovarian granulosa cells reveal a remarkably tight functional correlation between expressed selective uptake of lipoprotein cholesteryl esters and the expression of an HDL receptor protein, scavenger receptor, class B, type I (SR-BI). In the current study, we examine these same processes in C57 mouse granulosa cells and report a different correlation. Unlike the rat cells, non-hormone stimulated mouse granulosa cells are able to effectively carry out their selective pathway functions and secrete HDL-derived progestins despite low levels of SR-BI and barely detectable levels of SR-BII (an isoform of SR-BI). Once stimulated with trophic hormones or Bt2cAMP, small (30-40%) increases are observed in selective pathway functions, but major (approximately 20-fold) increases are seen in SR-BI and SR-BII expression: thus, relatively little is gained in selective cholesteryl ester uptake by mouse granulosa cells even though SR-BI and SR-BII levels are greatly increased. The importance of the HDL receptor proteins to the selective pathway remains clear, however, since a significant portion of the selective process in both basal and stimulated granulosa cells is inhibitable by the use of blocking antibody. Another surface protein, caveolin, previously reported to co-localize with SR-BI in mouse cells shows no change in expression during periods when SR-BI/BII levels are undergoing major shifts.

Scavenger receptor, class B, type I-dependent stimulation of cholesterol esterification by high density lipoproteins, low density lipoproteins, and nonlipoprotein cholesterol

Scavenger receptor, class B, type I (SR-BI) is a cell surface glycoprotein that mediates selective uptake and efflux of sterols from high density lipoproteins (HDL) to cells. A Chinese hamster ovary cell line that is deficient in functional LDL receptors, but has high expression levels of recombinant SR-BI (ldlA7-SR-BI), was used to examine the effect of SR-BI on the trafficking of sterols between lipoproteins and cells. To monitor the fate of sterols transported by SR-BI into cells, we measured the incorporation of [14C]oleate into cholesterol esters by acyl-CoA:cholesteryl acyltransferase in the endoplasmic reticulum. We show that incubation of ldlA7-SRBI cells with either LDL or HDL resulted in an equally dramatic increase in the formation of [14C]oleate-labeled cholesterol esters. The lipoprotein-stimulated, SR-BI-dependent increase in cholesterol esterification was inhibited by chloroquine. The uptake of sterols and their incorporation into cholesterol esters by SR-BI from LDL was largely a selective process. The addition of free cholesterol to ldlA7-SRBI cells also stimulated cholesterol ester formation in a chloroquine-sensitive fashion. We also show that SR-BI mediates the efflux of endogenously synthesized sterols from the cell membrane. From these studies we conclude that, in the absence of the LDL receptor, overexpression of SR-BI can mediate significant transport of sterols between lipoproteins and the endoplasmic reticulum of cells.

Expression and microvillar localization of scavenger receptor, class B, type I (a high density lipoprotein receptor) in luteinized and hormone-desensitized rat ovarian models

Steroidogenic cells in rats and mice obtain most of their cholesterol for steroid production and cholesteryl ester (CE) storage via the selective uptake pathway in which high density lipoprotein CE (HDL-CE) is taken into the cell without the uptake and degradation of the HDL particle. A number of recent studies show that the scavenger receptor, class B, type I (SR-BI) can mediate HDL-CE selective uptake in cultured cells and suggest that this receptor may be responsible for HDL-CE selective uptake in steroidogenic cells in vivo. In the current study we examine the relationship between SR-BI expression and HDL-CE selective uptake in the gonadotropin-primed, luteinized rat ovary and in the ovary that is desensitized by multiple gonadotropin treatments. Results from this study demonstrate a tight association between expression of SR-BI and measurements of HDL-CE selective uptake regardless of the steroidogenic state of the ovary. Thus, in the luteinized ovary (which is actively producing progestins), HDL-CE selective uptake is high, as is the expression of SR-BI. In the desensitized ovary (where CE content is reduced by 90% and progestin production is virtually absent), HDL-CE selective uptake and SR-BI are induced 2- to 3-fold compared with those in the luteinized ovary. These data argue that SR-BI can be regulated by the cholesterol status of the luteal cell independently of gonadotropic stimulation. Immunostaining at the light microscopic level showed strong expression of SR-BI specifically on the surface of luteal cells in the luteinized and desensitized ovary. Immunolocalization at the electron microscopic level showed that SR-BI was associated with microvilli and microvillar channels of the luteal cell surface. This result supports the hypothesis that microvilli and microvillar channels represent a cell surface compartment that is specialized for the selective uptake of lipoprotein cholesterol into steroidogenic cells.

The role of hepatic lipase in lipoprotein metabolism and atherosclerosis

In addition to its traditional role in the hydrolysis of lipoprotein triglycerides and phospholipids, recent studies have implicated hepatic lipase in other aspects of cellular lipid and/or lipoprotein metabolism and atherosclerosis. Hepatic lipase may serve as a ligand that mediates the interaction of lipoproteins to cell surface receptors and/or proteoglycans as well as modulating aortic lesion development in different animal models. Over the past several years significant advances have been made in our understanding of new, alternative mechanisms by which hepatic lipase may modulate lipoprotein metabolism and the development of atherosclerosis in vivo.

Selective uptake of low density lipoprotein-cholesteryl ester is enhanced by inducible apolipoprotein E expression in cultured mouse adrenocortical cells

Apolipoprotein (apo) E is expressed at high levels by steroidogenic cells of the adrenal gland, ovary, and testis. The cell surface location of apoE in adrenocortical cells suggests that apoE may facilitate the uptake of lipoprotein cholesterol by either the endocytic or the selective uptake pathways, or both. To examine these possibilities, the human apoE gene was expressed in murine Y1 adrenocortical cells under control of an inducible tetracycline-regulated promoter. The results show that induction of apoE yielded a 2-2.5-fold increase in the uptake of low density lipoprotein-cholesteryl ester (LDL-CE) but had little effect on high density lipoprotein-CE uptake. Analysis of lipoprotein uptake pathways showed that apoE increased LDL-CE uptake by both endocytic and selective uptake pathways. In terms of cholesterol delivery to the adrenal cell, the apoE-mediated enhancement of LDL-CE selective uptake was quantitatively more important. Furthermore, the predominant effect of apoE expression was on the low affinity component of LDL-CE selective uptake. LDL particles incubated with apoE-expressing cells contained 0.92 +/- 0.11 apoE molecules/apoB after gel filtration chromatography, indicating stable complex formation between apoE and LDL. ApoE expression by Y1 cells was necessary for enhanced LDL-CE selective uptake. This result may indicate an interaction between apoE-containing LDL and cell surface apoE. These data suggest that apoE produced locally by steroidogenic cells facilitates cholesterol acquisition by the LDL selective uptake pathway.

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.

Scavenger receptor class B, type I (SR-BI) is the major route for the delivery of high density lipoprotein cholesterol to the steroidogenic pathway in cultured mouse adrenocortical cells

The class B, type I scavenger receptor, SR-BI, binds high density lipoprotein (HDL) and mediates the selective uptake of HDL cholesteryl ester (CE) by cultured transfected cells. The high levels of SR-BI expression in steroidogenic cells in vivo and its regulation by tropic hormones provides support for the hypothesis that SR-BI is a physiologically relevant HDL receptor that supplies substrate cholesterol for steroid hormone synthesis. This hypothesis was tested by determining the ability of antibody directed against murine (m) SR-BI to inhibit the selective uptake of HDL CE in Y1-BS1 adrenocortical cells. Anti-mSR-BI IgG inhibited HDL CE-selective uptake by 70% and cell association of HDL particles by 50% in a dose-dependent manner. The secretion of [3H]steroids derived from HDL containing [3H]CE was inhibited by 78% by anti-mSR-BI IgG. These results establish mSR-BI as the major route for the selective uptake of HDL CE and the delivery of HDL cholesterol to the steroidogenic pathway in cultured mouse adrenal cells.

Intracellular events in the "selective" transport of lipoprotein-derived cholesteryl esters

The current study utilizes human, apoE-free high density lipoprotein reconstituted with a highly specific fluorescent-cholesteryl ester probe to define the initial steps and regulatory sites associated with the "selective" uptake and intracellular itinerary of lipoprotein-derived cholesteryl esters. Bt2cAMP-stimulated ovarian granulosa cells were used as the experimental model, and both morphological and biochemical fluorescence data were obtained. The data show that cholesteryl ester provided through the selective pathway is a process which begins with a temperature-independent transfer of cholesteryl ester to the cell's plasma membrane. Thereafter transfer of the lipid proceeds rapidly and accumulates prominently in a perinuclear region (presumed to be the Golgi/membrane sorting compartment) and in lipid storage droplets of the cells. The data suggest that lipid transfer proteins (or other small soluble proteins) are not required for the intracellular transport of the cholesteryl esters, nor is an intact Golgi complex or an intact cell cytoskeleton (although the transfer is less efficient in the presence of certain microtubule-disrupting agents). The intracellular transfer of the cholesteryl esters is also somewhat dependent on an energy source in that a glucose-deficient culture medium or a combination of metabolic inhibitors reduces the efficiency of the transfer. A protein-mediated event may be required for cholesteryl ester internalization from the plasma membrane, in that N-ethylmaleimide dramatically blocks the internalization phase of the selective uptake process. Taken together these data suggest that the selective pathway is a factor-dependent, energy-requiring cholesteryl ester transport system, in which lipoprotein-donated cholesteryl esters probably flow through vesicles or intracellular membrane sheets and their connections, rather than through the cell cytosol.

Selective uptake of low-density lipoprotein-associated cholesteryl esters by human fibroblasts, human HepG2 hepatoma cells and J774 macrophages in culture

High-density lipoprotein-(HDL) associated cholesteryl esters (CE) are taken up by hepatic and extrahepatic cells at a higher rate than HDL apolipoproteins. This selective uptake of HDL CE is independent from HDL particle uptake. For low-density lipoprotein (LDL), receptor-mediated endocytosis by cells is well established. In this study, the question was addressed if LDL-associated CE are also taken up by cells independently from LDL particles, i.e., selectively. Human LDL (d = 1.02-1.05 g/ml) was doubly radiolabeled with intracellularly trapped tracers: [125I]Tyramine-Cellobiose ([125I]TC) traced apolipoprotein B, [3H]cholesteryl oleyl ether ([3H]CEt) traced CE. The uptake of doubly radiolabeled LDL by normal and LDL receptor-negative human skin fibroblasts, human HepG2 hepatoma cells and murine J774 macrophages was investigated. Each cell type took up LDL particles as indicated by [125I]TC. However, in fibroblasts, HepG2 cells and J774 macrophages the rate of uptake for LDL-associated [3H]CEt was greater than that according to [125I]TC. These results indicate that extrahepatic and hepatic cells selectively take up LDL CE and this uptake is independent from LDL receptor-mediated endocytosis. Loading cells with cholesterol down-regulated selective uptake of LDL CE. In summary, human skin fibroblasts, human HepG2 cells and murine J774 macrophages selectively take up LDL CE, i.e., CE are taken up independently from LDL particles.

Enhanced expression of granulosa cell low density lipoprotein receptor activity in response to in vitro culture conditions

Previous studies have shown that the B/E (low density lipoprotein [LDL]) receptor pathway plays a minor role in cholesterol uptake in the intact rat ovary, but when granulosa cells are isolated and maintained in culture, the cells develop a fully functional B/E receptor system. In the current study we examined the development of the B/E receptor over time (96 h) in culture and compared its physiological function, expression of mRNA and protein levels, and morphological events to the upregulation induced in 24 h by hormone (human chorionic gonadotropin [hCG] or Bt2cAMP). With both protocols, increased progestin production occurs and is associated with elevated binding, uptake, and degradation of LDL in the medium although the impact of Bt2cAMP stimulation on all these measurements is several times that observed with time alone. Only the hormone-stimulated LDL receptor response was associated with an increase in receptor protein (Western blot) or mRNA levels (RNase protection assay). We conclude that unstimulated granulosa cells show posttranslational increases in B/E receptor activity with time in culture, but transcriptional changes in B/E receptor follow stimulation with trophic hormone or its second messenger, cAMP.

Effect of age on cholesterol uptake and utilization by rat adrenals: I. Internalization of lipoprotein-derived cholesteryl esters

Previous studies from this laboratory have documented a progressive age-related decline in trophic hormone (or second messenger cAMP) stimulated corticosterone production in isolated adrenocortical cells. In the current study, we examined the possibility that the aging process exerts this effect by interfering with an early step in the delivery of lipoprotein-derived cholesteryl esters to the cell. As such, we monitored the ability of two different rat adrenocortical cell model systems (intact perfused adrenal glands and primary cultures of adrenocortical cells from 5- and 18- to 20-month-old rats) to internalize lipoprotein cholesteryl esters, and to convert the newly internalized cholesteryl esters to corticosterone production. The results indicate that lipoprotein (hHDL3 and rHDL) cholesteryl ester internalization (by both the endocytic and 'selective' pathways) is comparable in adrenocortical cells of the young and old rats. However, despite this, both the mass of corticosterone produced and the ratio of newly internalized (radiolabeled) cholesteryl ester incorporated into corticosterone is dramatically reduced in cells of the older animals. Thus, the lipoprotein uptake pathway appears to be intact in adrenals of older rats, but the intracellular processing of internalized cholesteryl ester is defective.

Effect of age on cholesterol uptake and utilization by rat adrenals: II. Lipoproteins from young and old rats

The current study examines whether age-related changes in high density lipoproteins (HDL) influences how these particles are handled by adrenal cells. It appears that HDL from 18- to 20-month-old Sprague-Dawley rats show a seven- to eightfold increase in content of apolipoprotein E compared to HDL from 2- to 5-month-old rats. The 'aged' particles show increased binding to susceptible hepatic membranes, and show a doubling in whole particle endocytosis by cortical cells of the perfused adrenal gland and by isolated adrenal cells from all rats regardless of age. Despite this twofold increase in particle uptake, the increase in total cholesteryl ester uptake by either the perfused adrenal or incubated adrenal cells is minor, amounting to less than 10% of the total cholesteryl ester internalized. This discrepancy occurs since the high apo E content of the 'aged' HDL only affects cholesteryl ester uptake by the 'endocytic' pathway; uptake via the 'selective' pathway (where cholesteryl ester is separated from the rest of the particle at the cell surface and directly internalized) is not altered.

Okadaic acid interferes with lipoprotein-supported corticosterone production in adrenal cells

Rat adrenocortical cells in culture respond to stimulation by ACTH alone (15 fold over basal) and to ACTH + added lipoproteins (as an exogeneous source of cholesterol), with an additional 25-30 fold rise in steroidogenesis. With the addition of okadaic acid (OKA, 100 nM), a potent protein phosphatase inhibitor, the lipoprotein-induced rise in steroidogenesis is blocked. If 20 alpha-hydroxycholesterol is provided instead of lipoprotein-cholesterol, OKA has no effect suggesting that OKA affects only actively transported cholesterol. Since the OKA block is preceded by specific morphological changes in the cell (i.e., the loss of Golgi-associated microtubules followed by the disruption of the Golgi apparatus itself), it is hypothesized that some OKA-sensitive phosphoprotein associated with the microtubule/Golgi network of adrenocortical cells is critical for lipoprotein-derived cholesterol uptake and/or transport during steroidogenesis.

Uptake and utilization of lipoprotein cholesteryl esters by rat granulosa cells

Earlier studies have shown that rat granulosa cells grown in serum-free medium are exquisitely responsive to exogenously provided lipoprotein cholesterol. In this study we compare the amount of cholesterol (cholesteryl ester) actually delivered from various homologous and heterologous cholesterol-rich lipoproteins and examine the intracellular pathways used in the delivery system. Granulosa cells were incubated for 5 or 24 h with 125I-labeled human (h) HDL3, rat (r) HDL or hLDL equipped with non-releasable apoprotein and cholesteryl ether tags which accumulate within cells, even after degradation. We show that all the tested lipoproteins were similarly efficient in cholesteryl ester delivery; i.e., based on cholesterol: protein ratios of the starting ligands, each delivered approximately the same cholesteryl ester mass and evoked a similar progestin response. However, each lipoprotein was processed quite differently by the granulosa cells: hHDL3-cholesteryl ester was taken up almost exclusively by an non-endocytic pathway, hLDL-cholesteryl ester almost exclusively by an endocytic pathway and rHDL-cholesteryl ester by both pathways. In general, there was no correlation between the total amount of lipoprotein bound or apoprotein internalized and/or degraded by the cells with the amount of cholesteryl ester received or the level of the progestin response. Hormone stimulation upregulated the preferred pathway for each lipoprotein.

Cholesterol esters selectively taken up from high-density lipoproteins are hydrolyzed extralysosomally

High-density lipoprotein (HDL) cholesterol esters (CE) are taken up by many cells without parallel uptake of HDL apoproteins. This selective uptake is mediated by reversible incorporation of HDL CE into a plasma membrane pool, from which the CE are internalized. We now show that selectively taken up CE are directed to an extralysosomal destination where they are hydrolyzed and available to the steroidogenic pathway. Cultured human fibroblasts take up HDL CE predominantly by selective uptake. Wolman's disease fibroblasts, which are deficient in lysosomal cholesterol esterase, effectively hydrolyzed CE from HDL, but not CE taken up in low density lipoproteins (LDL); normal fibroblasts hydrolyzed both effectively. Analogously, the lysosomotropic agent chloroquine effectively blocked hydrolysis of LDL CE but not HDL CE. A similar effect of chloroquine was seen in primary cultures of rat adrenal cells, which are very active in selective uptake. More than 50% of HDL CE taken up by adrenal cells appeared in the medium as corticosterone. To examine the subcellular destination of selectively taken up CE, non-hydrolyzable tracers of HDL and LDL CE were simultaneously injected into rats. On fractionation of adrenal glands 24 h after injection, 83% of the HDL CE tracer and 48% of the LDL CE tracer were recovered in cytoplasmic lipid droplets; that LDL tracer in the lipid droplets was accounted for by selective uptake of CE from LDL. Thus, selectively taken up cholesterol esters are processed by a mechanism distinct from the classical endosomal/lysosomal pathway, and are delivered to a cytoplasmic compartment.

Differences in uptake of high-density lipoproteins by rat adrenals using in vivo vs. in situ perfusion techniques

This study describes the effect of the delivery route of high-density lipoproteins (HDL) on the ultimate fate of the lipoprotein in the intact rat adrenal. Equal amounts of human (h)-derived affinity-purified apoE-free 125I-labeled HDL3 was given to ethinyl estradiol-treated (i.e., lipoprotein-deficient) rats either intravenously (in vivo route) or by non-recycling perfusion (in situ perfusion route). After 60-90 min, the adrenals were either excised and assessed for uptake of radioactivity, or perfusion-fixed with glutaraldehyde and prepared for autoradiograms at the electron microscope level. The results show that hHDL3 circulated in vivo binds 9-times more readily to adrenal tissues than the same quantity of ligand delivered by perfusion. Also, when the lipoprotein is administered in vivo, it is 5-times more likely to be interiorized as an intact particle by zona fasciculata (corticosterone-secreting) cells via an endocytic pathway than when delivered by perfusion. Similar differences between the in vivo and in situ routes were not seen when 125I-labeled rat HDL was the ligand delivered. Whereas the starting hHDL3 ligand was free of apoE, there was a substantial (7-fold) conversion of the HDL3 to apoE-containing HDL3 following in vivo circulation of the ligand, as shown by sodium phosphotungstate-MgCl2 precipitation or heparin-Sepharose column chromatography. These results show that the route of lipoprotein delivery to specific tissues can play a major role in determining both the binding and the processing of the ligand by the tissue in question. With hHDL3, acquisition of apoE during only 1 h of recirculation in lipoprotein-deficient rats was sufficient to totally alter the fate of the ligand in the adrenal cortex.