Regulation of cholesterol responsive genes in ovary cells: impact of cholesterol delivery systems

Mono-(2-ethylhexyl) phthalate stimulates basal steroidogenesis by a cAMP-independent mechanism in mouse gonadal cells of both sexes

Phthalates are widely used as plasticizers in a number of daily-life products. In this study, we investigated the influence of mono-(2-ethylhexyl) phthalate (MEHP), the active metabolite of the frequently used plasticizer di-(2-ethylhexyl) phthalate (DEHP), on gonadal steroidogenesisin vitro. MEHP (25–100 μM) stimulated basal steroid synthesis in a concentration-dependent manner in immortalized mouse Leydig tumor cells (MLTC-1). The stimulatory effect was also detected in KK-1 granulosa tumor cells. MEHP exposure did not influence cAMP or StAR protein levels and induced a gene expression profile of key steroidogenic proteins different from the one induced by human chorionic gonadotropin (hCG). Simultaneous treatment with MEHP and a p450scc inhibitor (aminoglutethimide) indicated that MEHP exerts its main stimulatory effect prior to pregnenolone formation. MEHP (10–100 μM) up-regulated hormone-sensitive lipase and 3-hydroxy-3-methylglutaryl coenzyme A reductase, suggesting that MEHP increases the amount of cholesterol available for steroidogenesis. Our data suggest that MEHP, besides its known inhibitory effect on hCG action, can directly stimulate gonadal steroidogenesis in both sexes through a cAMP- and StAR-independent mechanism. The anti-steroidogenic effect of DEHP has been proposed to cause developmental disorders such as hypospadias and cryptorchidism, whereas a stimulation of steroid synthesis may prematurely initiate the onset of puberty and theoretically affect the hypothalamic–pituitary–gonadal axis.

Lipoprotein receptor expression during luteinization of the ovarian follicle

Ovarian follicles luteinize after ovulation, requiring structural and molecular remodeling along with exponential increases in steroidogenesis. Cholesterol substrates for luteal steroidogenesis are imported via scavenger receptor-BI (SR-BI) and the low-density lipoprotein (LDL) receptor from circulating high-density lipoproteins and LDL. SR-BI mRNA is expressed in pig ovaries at all stages of folliculogenesis and in the corpus luteum (CL). An 82-kDa form of SR-BI predominates throughout, is weakly present in granulosa cells, and is robustly expressed in the CL, along with the less abundant 57-kDa form. Digestion of N-linked carbohydrates substantially reduced the SR-BI mass in luteal cells, indicating that differences between forms is attributable to glycosylation. Immunohistochemistry revealed SR-BI to be concentrated in the cytoplasm of follicular granulosa cells, although found mostly at the periphery of luteal cells. To examine receptor dynamics during gonadotropin-induced luteinization, pigs were treated with an ovulatory stimulus, and ovaries were collected at intervals to ovulation. SR-BI in granulosa cell cytoplasm increased through the periovulatory period, with migration to the cell periphery as the CL matured. In vitro culture of follicles with human chorionic gonadotropin induced time-dependent upregulation of 82-kDa SR-BI in granulosa cells. SR-BI and LDL receptor were reciprocally expressed, with the latter highest in follicular granulosa cells, declining precipitously with CL formation. We conclude that luteinization causes upregulation of SR-BI expression, its posttranslational maturation by glycosylation, and insertion into luteal cell membranes. Expression of the LDL receptor is extinguished during luteinization, indicating dynamic regulation of cholesterol importation to maintain elevated steroid output by the CL.

LDL and cAMP cooperate to regulate the functional expression of the LRP in rat ovarian granulosa cells

Rat ovarian granulosa rely heavily on lipoprotein-derived cholesterol for steroidogenesis, which is principally supplied by the LDL receptor- and scavenger receptor class B type I (SR-BI)-mediated pathways. In this study, we characterized the hormonal and cholesterol regulation of another member of the LDL receptor superfamily, low density lipoprotein receptor-related protein (LRP), and its role in granulosa cell steroidogenesis. Coincubation of cultured granulosa cells with LDL and N6,O2'-dibutyryl adenosine 3',5'-cyclic monophosphate (Bt2cAMP) greatly increased the mRNA/protein levels of LRP. Bt2cAMP and Bt2cAMP plus human hLDL also enhanced SR-BI mRNA levels. However, there was no change in the expression of receptor-associated protein, a chaperone for LRP, or another lipoprotein receptor, LRP8/apoER2, in response to Bt2cAMP plus hLDL, whereas the mRNA expression of LDL receptor was reduced significantly. The induced LRP was fully functional, mediating increased uptake of its ligand, alpha2-macroglobulin. The level of binding of another LRP ligand, chylomicron remnants, did not increase, although the extent of remnant degradation that could be attributed to the LRP doubled in cells with increased levels of LRP. The addition of lipoprotein-type LRP ligands such as chylomicron remnants and VLDL to the incubation medium significantly increased the progestin production under both basal and stimulated conditions. In summary, our studies demonstrate a role for LRP in lipoprotein-supported ovarian granulosa cell steroidogenesis.

Regulation of luteinizing hormone/chorionic gonadotropin receptor messenger ribonucleic acid expression in the rat ovary: relationship to cholesterol metabolism

Down-regulation of LH/human chorionic gonadotropin (hCG) receptor (LHR) mRNA in the ovary after the preovulatory LH surge or the administration of a pharmacological dose of LH/hCG occurs through a posttranscriptional mechanism. A LHR mRNA-binding protein was identified as the LHR mRNA destabilizing factor, and its identity was established as mevalonate kinase (Mvk). In the present study, we determined that, in the pseudopregnant rat ovary, LHR mRNA levels began to fall 4 h after hCG injection, at which time Mvk protein levels were elevated, and this elevation was preceded by an increase in Mvk mRNA levels. When the cytosolic fractions of hCG-treated ovaries were subjected to RNA EMSA, an increase in LHR mRNA-LHR mRNA-binding protein complex formation was observed, in parallel with the increase of Mvk expression. We also found that hCG coordinately up-regulated the expression of Mvk and other sterol-responsive elements containing cholesterol biosynthesis enzymes, such as 3-hydroxy-3-methylglutaryl-coenzyme A synthase, 3-hydroxy-3-methylglutaryl-coenzyme A reductase, and farnesyl pyrophosphate synthase. This up-regulation was transient, but the hCG-induced ovarian cholesterol depletion lasted for more than 24 h. Taken together, our results suggest that, in the ovary, LH/hCG up-regulates the expression of cholesterol biosynthesis enzymes and lipoprotein receptors to replenish cellular cholesterol, and the up-regulation of Mvk leads to a down-regulation of LHR and suppresses the LH/hCG signal cascade transiently. Thus Mvk, an enzyme involved in cholesterol biosynthesis, serves as a link between LHR mRNA expression and cellular cholesterol metabolism.

Aging alters the functional expression of enzymatic and non-enzymatic anti-oxidant defense systems in testicular rat Leydig cells

In aged rats, trophic hormone-stimulated testosterone secretion by isolated Leydig cells is greatly reduced. The current studies were initiated to establish a functional link between excess oxidative stress and the age-related decline in steroidogenesis. Highly purified Leydig cell preparations obtained from 5-month (young mature) and 24-month (old) Sprague-Dawley rats were employed to measure and compare levels of lipid peroxidation, non-enzymatic (alpha-tocopherol, ascorbic acid, and reduced/oxidized glutathione) and enzymatic (Cu, Zn-superoxide dismutase, Cu, Zn-SOD; Mn-superoxide dismutase, Mn-SOD; glutathione peroxidase-1, GPX-1, and catalase, CAT) anti-oxidants. The extent of lipid peroxidation (oxidative damage) in isolated membrane fractions was quantified by measuring the content of thiobarbituric acid-reactive substances (TBARS) under basal conditions, or in the presence of non-enzymatic or enzymatic pro-oxidants. Membrane preparations isolated from Leydig cells from old rats exhibited two- to three-fold enhancement of basal TBARS formation. However, aging had no significant effect on TBARS formation in response to either non-enzymatic or enzymatic pro-oxidants. Among the non-enzymatic anti-oxidants, the levels of reduced glutathione were drastically reduced during aging, while levels of alpha-tocopherol and ascorbic acid remained unchanged. Both steady-state mRNA levels and catalytic activities of Cu, Zn-SOD, Mn-SOD, and GPX-1 were also significantly lower in Leydig cells from 24-month-old rats as compared with 5-month-old control rats. In contrast, neither mRNA levels nor enzyme activity of catalase was sensitive to aging. From these data we conclude that aging is accompanied by reduced expression of key enzymatic and non-enzymatic anti-oxidants in Leydig cells leading to excessive oxidative stress and enhanced oxidative damage (lipid peroxidation). It is postulated that such excessive oxidative insult may contribute to the observed age-related decline in testosterone secretion by testicular Leydig cells.

Angiotensin II promotes selective uptake of high density lipoprotein cholesterol esters in bovine adrenal glomerulosa and human adrenocortical carcinoma cells through induction of scavenger receptor class B type I

Angiotensin II is one of the main physiological regulators of aldosterone biosynthesis in the zona glomerulosa of the adrenal cortex. The hormone stimulates intracellular cholesterol mobilization to the mitochondrion for steroid biosynthesis. Here we have examined whether angiotensin II also modulates exogenous lipoprotein cholesterol ester supply to the steroidogenic machinery and whether this control is exerted on the selective transport of high density lipoprotein-derived cholesterol ester to intracellular lipid droplets through the scavenger receptor class B type I. In bovine adrenal glomerulosa and human NCI H295R adrenocortical carcinoma cells, high density lipoprotein stimulated steroid production. Angiotensin II pretreatment for 24 h potentiated this response. Fluorescence microscopy of cellular uptake of reconstituted high density lipoprotein containing a fluorescent cholesterol ester revealed an initial, time-dependent narrow labeling of the cell membrane followed by an intense accumulation of the fluorescent cholesterol ester within lipid droplets. At all time points, labeling was more pronounced in cells that had been treated for 24 h with angiotensin II. Fluorescence incorporation into cells was prevented by a monoclonal antibody directed against apolipoprotein A-I. Upon quantitative fluorometric determination, cholesterol ester uptake in angiotensin II-treated bovine cells was increased to 175 +/- 15% of controls after 2 h and to 260 +/- 10% after 4 h of exposure to fluorescent high density lipoprotein. The amount of scavenger receptor class B type I protein detected in cells treated with angiotensin II for 24 h reached 203 +/- 12% of that measured in control cells (n = 3, P < 0.01). In contrast, low density lipoprotein receptors were only minimally affected by angiotensin II treatment. This increase in scavenger receptor class B type I protein was associated with a 3-fold induction of scavenger receptor class B type I mRNA, which could be prevented by actinomycin D but not by cycloheximide. Similar results were obtained in the human adenocarcinoma cell line H295R. These observations show that angiotensin II regulates the scavenger receptor class B type I-mediated selective transport of lipoprotein cholesterol ester across the cell membrane as a major source of precursor for mineralocorticoid biosynthesis in both human and bovine adrenal cells.

Masoprocol decreases rat lipolytic activity by decreasing the phosphorylation of HSL

Masoprocol (nordihydroguaiaretic acid), a lipoxygenase inhibitor isolated from the creosote bush, has been shown to decrease adipose tissue lipolytic activity both in vivo and in vitro. The present study was initiated to test the hypothesis that the decrease in lipolytic activity by masoprocol resulted from modulation of adipose tissue hormone-sensitive lipase (HSL) activity. The results indicate that oral administration of masoprocol to rats with fructose-induced hypertriglyceridemia significantly decreased their serum free fatty acid (FFA; P < 0.05), triglyceride (TG; P < 0.001), and insulin (P < 0.05) concentrations. In addition, isoproterenol-induced lipolytic rate and HSL activity were significantly lower (P < 0.001) in adipocytes isolated from masoprocol compared with vehicle-treated rats and was associated with a decrease in HSL protein. Incubation of masoprocol with adipocytes from chow-fed rats significantly inhibited isoproterenol-induced lipolytic activity and HSL activity, associated with a decrease in the ability of isoproterenol to phosphorylate HSL. Masoprocol had no apparent effect on adipose tissue phosphatidylinositol 3-kinase activity, but okadaic acid, a serine/threonine phosphatase inhibitor, blocked the antilipolytic effect of masoprocol. The results of these in vitro and in vivo experiments suggest that the antilipolytic activity of masoprocol is secondary to its ability to inhibit HSL phosphorylation, possibly by increasing phosphatase activity. As a consequence, masoprocol administration results in lower serum FFA and TG concentrations in hypertriglyceridemic rodents.

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.

Selective uptake of cholesteryl ester from low density lipoprotein is involved in HepG2 cell cholesterol homeostasis

Low density lipoprotein (LDL) can follow either a holoparticle uptake pathway, initiated by the LDL receptor (LDLr), and be completely degraded, or it can deliver its cholesteryl esters (CE) selectively to HepG2 cells. Although high density lipoprotein-CE selective uptake has been shown to be linked to cell cholesterol homeostasis in nonhepatic cells, there is no available information on the effect of LDL-CE selective uptake on hepatic cell cholesterol homeostasis. In order to define the role of the LDL-CE selective uptake pathway in hepatic cell cholesterol homeostasis, we used a cellular model that expresses constitutively a LDLr antisense mRNA and that shows LDLr activity at 31% the normal level (HepG2-all cells). The addition of a specific antibody anti-LDLr (IgG-C7) reduces LDL protein degradation (LDLr activity) to 7%. This cellular model therefore reflects, above all, LDL-CE selective uptake activity when incubated with LDL. The inactivation of LDLr reduces LDL-protein association by 78% and LDL-CE association by only 43%. The LDL-CE selective uptake was not reduced by the inactivation of LDLr. The activities of the various enzymes involved in cell cholesterol homeostasis were measured in normal and LDLr-deficient cells during incubation in the absence or presence of LDL as a cholesterol source. Essentially, 3-hydroxy-3-methylglutaryl coenzyme A reductase and acyl coenzyme A:cholesterol acyltransferase (ACAT) activities responded to LDL in LDLr-deficient cells as well as in normal HepG2 cells. Inhibition of lysosomal hydrolysis with chloroquine abolished the effect measured on ACAT activity in the presence of LDL, suggesting that CE of LDL, but not free cholesterol, maintains cell cholesterol homeostasis. Thus, in HepG2 cells, when LDLr function is virtually abolished, LDL-CE selective uptake is coupled to cell cholesterol homeostasis.

Enhancement of mdr2 gene transcription mediates the biliary transfer of phosphatidylcholine supplied by an increased biosynthesis in the pravastatin-treated rat

An increase of biliary lipid secretion is known to occur in the rat under sustained administration of statin-type 3-hydroxy-3-methylglutaryl (HMG) coenzyme A (CoA) reductase inhibitors. The present study has addressed critical mechanisms of hepatic lipid synthesis and phosphatidylcholine (PC) biliary transport in the rat fed with a 0.075% pravastatin diet for 3 weeks. After treatment, biliary secretion of PC and cholesterol increased to 233% and 249% of controls, while that of bile salts was unchanged. Activity of cytidylyltransferase (CT), a major regulatory enzyme in the CDP-choline pathway of PC synthesis, was raised in both microsomal and cytosolic fractions (226% and 150% of controls), and there was an increase to 187% in the mass of active enzyme as determined by Western blot of microsomal protein using an antibody specific to CT. Cytosolic activity of choline kinase, another enzyme of the CDP-choline pathway, also increased to 175% of controls. In addition, there was an over eightfold increase in the HMG CoA reductase activity and mRNA. Thus, an increased PC and cholesterol synthetic supply to hepatocytes appeared as a basic mechanism for the biliary hypersecretion of these lipids. Notwithstanding the increased synthesis, hepatic PC content was unchanged, suggesting an enhanced transfer of this lipid into bile. Indeed, there was a sevenfold increase of multidrug resistance gene 2 (mdr2) gene mRNA coding for a main PC canalicular translocase. Thus, hypersecretion of biliary PC in the model studied can be explained by an up-regulation of mdr2 gene transcription and its P-glycoprotein product mediating the biliary transfer of PC supplied by an increased biosynthesis.

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.

Effect of reduced low-density lipoprotein receptor level on HepG2 cell cholesterol metabolism

Low-density lipoproteins (LDL) are taken up by both LDL receptor (LDLr)-dependent and -independent pathways. In order to determine the importance of these pathways in the activity of the various enzymes that are important in maintaining the cellular cholesterol level in hepatic cells, we created HepG2 cells expressing lower levels of LDLr. Thus HepG2 cells were transfected with a constitutive expression vector (pRc/CMV) containing a fragment of LDLr cDNA inserted in an antisense manner. Stable transformants were obtained that showed significant reductions of 42, 72 and 85% of LDLr protein levels compared with the control, as demonstrated by immunoblotting and confirmed by the LDL binding assay. The best inactivation was achieved with the construct containing the first 0.7 kb of LDLr cDNA. Incubating the different HepG2 cell subtypes with LDL showed similar association of apolipoprotein B (apo B) or cholesteryl esters from LDL with the cells, indicating that the LDLr deficiency did not significantly affect LDL uptake by the cell. However, apoB degradation was reduced significantly by 71-82% in the most LDLr-deficient HepG2 cells. We also found that 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCoA red) activity is significantly increased by 32-35% in HepG2 cells expressing very low levels of LDLr that also demonstrate a significant decrease of 20% in acyl-CoA:cholesterol acyltransferase (ACAT) activity. However, these effects are moderate compared with those observed when cells were incubated in lipoprotein-depleted medium, where a >900% increase in HMGCoA red activity and a loss of 60% of ACAT activity was observed. Thus, in HepG2 cells, different levels of LDLr affect LDL-apoB degradation, but have very little effect on LDL association, HMGCoA red and ACAT activities, revealing that LDLr is more important in the clearance of LDL-apoB than in HepG2 cell cholesterol homoeostasis, a role that should be attributable to both LDLr-dependent and -independent pathways.