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Originally published In Press as doi:10.1074/jbc.M502392200 on July 20, 2005

J. Biol. Chem., Vol. 280, Issue 39, 33536-33540, September 30, 2005
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Insulin and Angiotensin II Induce the Translocation of Scavenger Receptor Class B, Type I from Intracellular Sites to the Plasma Membrane of Adipocytes*

Anne-Laure Tondu, Céline Robichon, Laurent Yvan-Charvet, Nathalie Donne, Xavier Le Liepvre, Eric Hajduch, Pascal Ferré, Isabelle Dugail, and Georges Dagher1

From the INSERM U671, Université Pierre et Marie Curie, Institut Biomédical des Cordeliers, 15 Rue de l'École de Médecine, 75006 Paris, France

Received for publication, March 3, 2005 , and in revised form, July 19, 2005.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Scavenger receptor class B, type I (SR-BI) mediates the selective uptake of lipids from high density lipoproteins and is expressed in several types of tissues. However, to date little is known about its role in adipocytes. In this study, we investigated the cellular distribution of SR-BI in 3T3-L1 adipocytes and its regulation by hormones known to increase lipid storage such as angiotensin II (Ang II) and insulin. SR-BI was mainly distributed in the cytoplasm as determined by laser-scanning confocal analysis of the immunofluorescence labeling of SR-BI or the study of an enhanced green fluorescent protein-tagged SR-BI fusion protein. Exposure of cells to either insulin or Ang II (1-2 h) induced the mobilization of SR-BI from intracellular pools to the plasma membrane. This was further confirmed by Western blotting on purified plasma membrane and by fluorescence-activated cell sorter analysis of the SR-BI receptor. Similar results were also observed in primary adipocytes. We also demonstrated that, in the presence of either insulin or Ang II, SR-BI translocation to the cell membrane is functional, because insulin and Ang II induced a significant increase in the high density lipoprotein-delivered 22-(N-7-nitrobenz-2-oxa-1,3-diazo-4-yl)-amino-23,24-bisnor-5-cholen-3-ol uptake and in total cholesterol content. These data demonstrate that SR-BI can be acutely mobilized from intracellular stores to the cell surface by insulin or Ang II, two hormones that exert lipogenic effects in adipocytes. This suggests that SR-BI might participate in the storage of lipids in the adipose tissue.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Scavenger receptor class B, type I (SR-BI)2 is a membrane protein that can bind various classes of modified and unmodified lipoproteins. High density lipoproteins (HDLs), to which SR-BI binds with high affinity, are now considered as its physiological ligands (1, 2). As a consequence of HDL binding, it has been shown that SR-BI mediates the selective uptake of cholesteryl esters, a process that clearly differs from the low density lipoprotein receptor endocytic pathway (3, 4).

SR-BI is particularly abundant in several tissues, including the liver and steroidogenic tissue (4). Several studies have reported a determinant role for hepatic SR-BI in the control of HDL cholesterol. Thus, mice lacking a functional SR-BI gene have increased levels of plasma cholesterol, whereas overexpression of SR-BI in mouse liver by adeno-virus injection lowers HDL levels and increases biliary cholesterol (5-8). Modulation of SR-BI expression in vivo has also revealed that in steroidogenic tissues this receptor plays a major role in delivering cholesterol to be used for the synthesis of steroid hormones (4).

These studies have left unresolved the role of this receptor in adipose tissue known to express SR-BI (9). SR-BI is also expressed in adipocyte cell lines and is strongly induced during the course of adipose differentiation, suggesting a role for SR-BI in the fully differentiated adipocyte (9).

To get some insight into the function of SR-BI in adipocytes, we studied the regulation of SR-BI expression and its intracellular distribution in 3T3-L1 cells exposed to different hormonal conditions favoring either triglyceride storage or lipid mobilization. Our results show that angiotensin II (Ang II) and insulin translocate SR-BI from cytoplasmic compartments to the cell membrane of adipocytes. This translocation is concomitant with an increase in HDL-delivered cholesterol uptake and cell cholesterol content.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Human plasma HDLs were obtained from Calbiochem (La Jolla, CA). A monoclonal antibody (Ab) to GLUT4 clone 1F8, was obtained from R&D Systems. 22-(N-7-nitrobenz-2-oxa-1,3-diazo-4-yl)-amino-23,24-bisnor-5-cholen-3-ol (NBD-cholesterol or NBD-chol), and Alexa Fluor 488 and 546, goat anti-mouse or goat anti-rabbit IgG conjugates, were supplied by Molecular Probes (Eugene, OR). Rabbit anti-SR-BI IgGs were from Novus Biologicals (NB 400-104 and NB 400-113, Novus). Anti-GRP75 IgG was from BD Transduction Laboratories. Dulbecco's modified Eagle's medium (DMEM), penicillin/streptomycin, and trypsin-EDTA were from Invitrogen. Fetal bovine serum was from Perbio (PAA Laboratories, Pasching, Austria). Lipoprotein-deficient serum (LPDS) was prepared from the same batches of fetal bovine serum used for the differentiation of 3T3-L1, as described previously (10). All other chemicals were from Sigma.

Cell Culture Condition—3T3-L1 mouse preadipocytes were grown to confluence on either glass coverslips or 8-well Labtek plates (Polylabo) in high glucose DMEM supplemented with 10% fetal bovine serum, 20 mM HEPES, 25 mM sodium bicarbonate, and 100 units/ml penicillin/streptomycin. Differentiation of confluent preadipocytes was induced by adding methyl isobutylxanthine (100 µM), dexamethasone (0.25 µM), and insulin (1 µg/ml) to the above solution. After 48 h, the culture medium was replaced with DMEM supplemented with 10% fetal bovine serum and 1 µg/ml insulin for 8 days. At this stage, cells were fully differentiated and displayed the characteristic lipid droplet accumulation of mature adipocytes. Adipocytes were isolated from the epididymal fat pads of 2-month-old Wistar rats by collagenase treatment as described previously (10).



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  		FIGURE 1.
Immunoblot analysis of SR-BI during the course of adipocyte differentiation. A, induction of SR-BI protein during 3T3-L1 differentiation. Total cell lysates were prepared at different days after the addition of methyl isobutylxanthine-dexamethasone (day 0). 30 µg of protein were run on SDS-PAGE and probed for SR-BI as described under "Materials and Methods." A commercial antibody against GRP75 was used as a control for equal loading in each lane. B, absence of regulation of SR-BI protein content by insulin or Ang II. Differentiated 3T3-L1 adipocytes (at day 8) were incubated for 24 h in a serum-free medium containing 0.2% bovine serum albumin in the presence or absence of either insulin (100 nM) or Ang II (100 nM). Cell lysates were then prepared and analyzed for the presence of SR-B1.

 
Fluorescent Immunochemistry—Cells were grown on coverslips and differentiated as described above. They were then washed three times with PBS and fixed with formaldehyde in PBS (3%; 45 min), followed by exposure to PBS plus glycine (50 mM; 30 min) and then PBS plus bovine serum albumin (2%; 30 min). Cells were then permeabilized with saponin (0.01%; 5 min) in PBS. Immunolabeling was performed in PBS with the antibody (SR-BI (1:200)) and then visualized with the appropriate Alexa Fluor secondary antibodies at a dilution of 1:200. Coverslips were mounted onto slides with either Vectashield or SlowFade and observed with a Zeiss laser-scanning confocal microscope (LSM 500).

EGFP-SR-BI Construct—A full-length mouse SR-BI cDNA sequence (a kind gift from X. Collet) in pRc/CMV was excised with Hind III and XbaI. After purification, it was ligated into the HindIII XbaI sites of pEGFP-C3. The resulting vector thus encoded the green fluorescent protein tag fused at the N-terminal part of the SR-BI protein. This construct was introduced into differentiated 3T3-L1 adipocytes by electroporation as described previously (11). Enhanced green fluorescent protein (EGFP) fluorescence was imaged in living cells 2 days after transfection by confocal microscopy.

Western Blot Analysis—3T3-L1 adipocyte total membranes or purified plasma membrane fractions were prepared as described previously (12, 13). The protein contents of each isolated membrane fraction were determined using the Bradford assay with bovine serum albumin as a standard (14). Western blot analysis was performed after SDS-PAGE and electrotransfer onto nitrocellulose sheets (ECL, Amersham Biosciences) using standard procedures. Mouse SR-BI or GLUT4 label was revealed using the ECL system.

Fluorescence-activated Cell Sorter Analysis of Surface SR-B1—Differentiated 3T3-L1 cells were resuspended in DMEM containing a 1:100 dilution of a blocking SR-BI antibody that recognized the extracellular loop of SR-B1. After 1 h at 37°C the cells were pelleted, and a second Alexa-Fluor 488 antibody (1:100) was added for 1 h at 37°C. The cell suspension was washed twice in PBS and used for fluorescence-activated cell sorter analysis (Epics Altra).

High Density Lipoprotein Labeling—HDL loaded with either NBDchol) or BODIPY-cholesterol ester were reconstituted as described previously (15, 16).

Uptake of HDL-derived NBD-chol—To assess the uptake of HDL-derived NBD-chol, 3T3-L1 adipocytes were incubated in DMEM and 10% lipoprotein-deficient serum at 37 °C in the presence of HDL loaded with NBD-chol. NBD-chol uptake kinetics were determined using laser confocal fluorescence microscopy on single living cells as described previously (16). Briefly, cholesterol influx was initiated by the addition of NBD-chol and followed by the acquisition of digital images at different time intervals using a time course module (Zeiss AIM software). The effect of various hormonal agents was assessed by preincubating cells in the presence of the hormones prior to NBD-chol addition. The average pixel intensity (region of interest) of each single cell as a function of time was determined using Zeiss software and expressed as mean ± S.E. The kinetic of NBD-chol uptake was analyzed by the Marquardt algorithm using Ultrafit Biosoft software (Cambridge, UK). The calculated time constants were analyzed statistically using Student's t test.



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  		FIGURE 2.
A, expression of the EGFP-SR-BI construct in COS cells. An empty EGFP vector, a plasmid encoding full-length SR-BI, or the EGFP-SR-BI fusion construct was tranfected in COS cells. An SDS-PAGE of cell lysates is shown. B-D, immunofluorescence of the EGFP-SR-BI construct in adipocytes. Laser-scanning confocal immunofluorescence analysis of differentiated 3T3-L1 cells expressing EGFP-SR-BI is shown in panel B, and that of cells labeled with a specific antibody recognizing both the endogenous and the tagged form of SR-BI is presented in panel C. A merged image is shown in panel D. Bar, 5 µm.

 
Cholesterol Determinations—Differentiated cells on 100-mm dishes were lysed in PBS, and total lipids were extracted as described by Folch et al. (17). Cholesterol was determined using a cholesterol assay kit from Randox Laboratories.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
SR-BI Protein Expression during 3T3-L1 Differentiation into Adipocytes—The presence of SR-BI proteins was analyzed during differentiation of 3T3-L1 from fibroblasts to mature adipocytes. Western blot analysis using a polyclonal antibody detected two immunoreactive bands in 3T3L1 cells (Fig. 1). The lower band of ~55 kDa corresponds to the predicted molecular mass of SR-BI deduced from the cDNA sequence, whereas the upper band of ~80kDa is classically observed in an adrenocortical cell line and identified as the glycosylated form of the protein (18). Fig. 1A indicates that the SR-BI protein followed the same pattern as that reported previously for SR-BI mRNA, namely a strong induction (~10-fold) during the differentiation of 3T3-L1 cells into adipocytes. Thus, 3T3-L1 differentiated adipocytes express high levels of the SR-BI protein. We next examined whether the total amount of the SR-BI protein could be modulated in adipocytes. We tested the effect of the addition of various hormones known to favor lipid storage or mobilization. After 24 h in the presence of lipogenic hormones such as insulin (100 nM) or Ang II (100 nM) we detected no change in the SR-BI protein content of adipocytes (Fig. 1B). Thus, hormones that physiologically regulate the adipocyte function by inducing fat storage do not significantly change the total amount of the SR-BI protein in 3T3L1 adipocytes.



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  		FIGURE 3.
Intracellular distribution of SR-BI in 3T3L1. A, immunofluorescent labeling of SR-BI receptor in differentiated 3T3L1 adipocytes, N, location of the nucleus; bar, 2 µm. B, phase-contrast microscopy of single cell with fully formed lipid droplets. N, nucleus. C, background fluorescence was found to be minimal as evaluated by labeling cells with secondary antibody alone. D-F, labeling of the cell periphery was observed in cells treated for 2 h with 100 nM Ang II (D) or for 1 h with 100 nM insulin (Ins) (E), but not when wortmannin (Wortm.) was added in insulin-treated cells (F). G-K, 3T3-L1 differentiated adipocytes (G-H) or undifferentiated preadipocytes at day 0 (I-K) were transfected with an EGFP-SR-B1 construct. Fluorescence of EGFP-SR-BI is shown in unfixed control (Cont.) (G and I), Ang II (H and K), or insulin-treated cells (C and J).

 
Intracellular Distribution of SR-BI in 3T3-L1 Adipocytes—The intra-cellular distribution of SR-BI in fat cells was studied by immunofluorescence. Staining control adipocytes with the SR-BI antibody revealed a punctuated label distributed all over the cell cytoplasm that excluded the nucleus (Fig. 2C). The intracellular SR-BI was further confirmed by a z-scanning (0.3 µm) of the cells, revealing a scarce labeling of the cell surface and an important intracytoplasmic presence of the receptor (results not shown).

The distribution and trafficking of SR-BI using a EGFP-SR-BI construct in living cells was then assessed. For this purpose, we developed a plasmid encoding EGFP fused to murine SR-BI. Control analysis by Western blot in transfected COS cells revealed that the EGFP-SR-BI fusion protein migrated at the expected position relative to native SR-BI, i.e. 110 kDa (Fig. 2A). When cells were transfected, only 1-2% of cells expressed EGFP-SR-BI, which was distributed in a punctuate pattern all over the cytoplasm (Fig. 2B) similarly to that observed with SR-BI Ab (Fig. 2C). Immunofluorescence microscopy of EGFP-SR-BI transfected cells, using an SR-BI Ab recognizing both the endogenous and the green fluorescent protein-tagged receptor (Fig. 2, B and C), showed that the overall distribution of SR-BI was not altered by the expression of EGFP-SR-BI, because a significant but not complete overlap of endogenous and EGFP-tagged receptors was observed (Fig. 2D). On the other hand, the response of EGFP-SR-B1 to Ang II is similar to that observed in non-transfected cells (see below and Fig. 3). We concluded from these results that EGFP-SR-BI is a reliable reporter of SR-BI.

When we examine the dynamics of EGFP-SR-BI trafficking in live cells (see supplemental data available in the on-line version of this article), fluorescent spots were stationary except for 3-4% of the spots that moved laterally and also along the plasma membrane. The traffic rate of SR-BI varied from 0.48 to 0.10 µm/sec with a mean ± S.E. of 0.25 ± 0.12 µm, a magnitude similar to that observed for protein traffic along microtubules.

Angiotensin II and Insulin Induce the Translocation of SR-BI at the Cell Surface—When adipocytes (3T3-L1) were pretreated with either Ang II for 2 h (Fig. 3D) or insulin for 1 h (Fig. 3E) and then fixed and labeled with SR-B1Ab, we observed a substantial change in SR-BI distribution. Labeling of the cell membrane was markedly increased. This was associated with a dramatic decrease in labeling of cytoplasm (com-pare with control cells; Fig. 3A). Similar results were observed for Ang II in EGFP-SR-BI-transfected cells (Fig. 3, G and H). Together, these results suggest that the treatment of adipocytes with Ang II or insulin induce the translocation of SR-BI from intracellular stores to the cell surface.

Phosphatidylinositol 3-kinases are required for a wide range of cellular processes, including protein traffic to the plasma membrane and recycling (19, 20). To test whether SR-BI mobilization to plasma membrane requires phosphatidylinositol 3-kinase activity, we performed experiments in which cells were pretreated with 100 nM wortmannin (10 min). In these conditions, insulin (Fig. 3F) or Ang II (results not shown) failed to mobilize SR-BI to the cell surface. We also observed that SR-BI remained located intracellularly if introduced as an EGFP construct in undifferentiated preadipocytes (Fig. 3I) which did not yet express endogenous SR-BI at that early stage (see day 0 in Fig. 1A). In such undifferentiated cells, neither insulin nor Ang II (Fig. 3, J and K) modified SR-BI intracellular distribution, indicating that the translocation of the receptor to the plasma membrane was dependent on the fully mature adipocyte differentiated phenotype.

To further ascertain these observations, we performed an immunoblot analysis of SR-BI in isolated plasma membranes from adipocytes. As depicted in Fig. 4A, we observed an increase in SR-BI content of plasma membranes of cells exposed to Ang II. Similarly, in a time course study insulin increased plasma membrane SR-BI, an effect detectable after 30 min of exposure to the hormone (Fig. 4B). It is noteworthy that the SR-BI content of plasma membranes increased with a similar magnitude (3-fold) upon the addition of insulin or Ang II. We compared the effect of insulin on SR-B1 translocation with that on GLUT4, a protein known to be translocated to the cell surface by this hormone. We observed that GLUT4 was recruited to the plasma membrane more rapidly than SR-BI. Indeed, as shown by others (21) the recruitment of GLUT4 was maximal after 15 min of insulin treatment, whereas for SR-BI 45-60 min were required (Fig. 4B). On the other hand, both proteins behaved similarly upon insulin removal (Fig. 4C). Thus, the redistribution to internal pools of either protein occurred within 30-60 min, in accord with previous results reported for GLUT4 (22). We also performed fluorescence flow cytometric analysis of SR-BI receptor cell surface expression. Control or AngII-treated cell suspensions were exposed for 1 h at 37 °C to a blocking SR-BI Ab recognizing an extracellular epitope and then labeled with an Alexa 488 secondary antibody. Fig. 4D depicts the cell distribution of the relative fluorescence intensity in control cell population (trace b) or that pretreated with Ang II (trace c). An increase in cell surface receptors was observed in cells pretreated with Ang II. Finally, we used rat isolated fat cells instead of 3T3-L1 adipocytes to test the effect of insulin and AngII. We observed that both hormones increased the SR-BI content of plasma membranes (Fig. 4E). In these experiments GLUT4 was used as a control for the insulin effect. Ang II had no effect on GLUT4, as shown previously (23). Altogether, these results strongly suggest that insulin and Ang II induce the trans-location of SR-BI from cytoplasmic compartments to cell membrane.



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  		FIGURE 4.
SR-BI content of purified plasma membrane preparations. Plasma membrane proteins were prepared from differentiated 3T3-L1 adipocytes treated as described under "Materials and Methods." A and B, a typical immunoblot of SR-BI is shown in the upper sections, and the quantitative changes in SR-BI content obtained from the densitometric scanning of blots obtained in separated experiments are shown in the lower sections (*, p ≤ 0.01 as compared with control). Effect of Ang II is shown in panel A, and the time course effect of insulin is shown in panel B. C and D, the effect of insulin removal is shown in panel C. For comparison, the insulin effect on plasma membrane GLUT4 is also shown in panel C. The fluorescence flow cytometric analysis of receptor cell surface expression is shown in panel D. Cells were exposed to Ang II as described under "Materials and Methods," labeled with a blocking anti-SR-BI antibody recognizing the extracellular epitope, and then labeled with Alexa 488 secondary Ab. The number of cells (vertical axis) as a function of the relative fluorescence intensity for fluorescence (log scale) is depicted for control (trace b) and Ang II-treated cells (trace c) in panel D. Autofluorescence of cells is depicted by trace a in panel D. Mean arbitrary fluorescence intensity for control was 157, and for Ang II it was 972. E, the effect of insulin and Ang II in primary adipocytes. Plasma membranes were purified from isolated rat fat cells (epidydimal fat pads of Wistar rats). After collagenase digestion, isolated adipocytes were stimulated or not stimulated with insulin (100 nM; 1 h) or angiotensin II (100 nM; 2 h), and plasma membranes were probed for SR-BI. GLUT4 was used as a control for hormone action.

 



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  		FIGURE 5.
Effect of angiotensin II and insulin on cholesterol influx and cell cholesterol content. A, the effect of AngII and insulin on NBD-chol uptake in differentiated adipocytes. Bars represent the mean ± S.D. of time constants calculated as described under "Materials and Methods." Where indicated, the AT1 antagonist losartan (Los) (100 nM) or the SR-BI blocking Ab (SR-BIAb) (1:100) was added 1 h prior to uptake. *, p < 0.05 compared with control (cont). B, effect of insulin and Ang II (AII) on cell cholesterol content. Cholesterol content was assessed in control cells incubated in lipid-depleted medium in the presence of HDL (3 h), insulin plus HDL, or Ang II plus HDL. *, p < 0.05 compared with control.

 
SR-BI Mobilized at the Cell Surface Is Functional—We next asked the question of whether the SR-BI present on the plasma membrane after insulin or Ang II stimulation is functional. Because SR-BI receptor was reported to bind HDL and to mediate selective delivery of cholesterol and cholesterol esters, we assessed the uptake of NBD-chol from HDL as a functional test. As shown in Fig. 5A, Ang II and insulin significantly stimulates NBD cholesterol influx. Furthermore, a blocking SR-BI Ab hampers the stimulation of NBD-chol uptake by Ang II (Fig. 5A), suggesting that the effect of the hormone on the entry of NBD-chol requires SR-BI. Similar results were observed with insulin (not shown). Pretreatment of cells with wortmannin significantly decreased basal NBD-chol uptake, and no stimulation could be observed in the presence of insulin (Fig. 5A).

We also assessed the effect of 100 nmol/liter Losartan (Parke-Davis), a specific antagonist of angiotensin receptors, type I (AT1). A significant reduction in Ang II-stimulated NBD-chol influx was observed, suggesting the involvement of AT1 receptors in this effect.

Finally, we evaluated the changes in cholesterol contents of cells that had been incubated in lipid depleted medium for 16 h and then provided with cholesterol containing HDL for 3 h in the presence or absence of insulin and Ang II (Fig. 5B). We observed that the addition of HDL significantly increased cell cholesterol content by 40% within 3 h and that the intracellular accumulation of cholesterol from HDL was almost doubled in the presence of insulin or Ang II. Together, these data indicate that the Ang II or insulin-induced translocation of SR-BI to the cell membrane is associated with enhanced cholesterol uptake and intracellular content.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Previous reports have revealed the role of SR-BI in mediating HDL-derived cholesterol ester-selective uptake in different cell types, and several authors reported its presence in adipocytes and 3T3-L1 cells (Refs. 24-26 and the present results). However, to date little is known about SR-BI regulation and function in fat cells. In the present study we show for the first time that insulin and Ang II translocates SR-BI from its cytoplasmic location to the plasma membrane of adipocytes. Evidence for such a translocation stems from confocal analysis of SR-BI and EGFP-SR-BI distribution in cells, immunoblot analysis of plasma membrane extracts, and fluorescence-activated cell sorter analysis of SR-BI receptors at cell membranes. This translocation process is rapid and implicates a mobilization of intracellular pools, as no significant increase in SR-B1 protein level was observed. This short term regulatory effect seems to be specific to 3T3L1 and primary adipocytes because, in other cell types such as the adrenal gland and the gonads, trophic hormones were reported to stimulate SRB1 expression after 24 h. This was associated with enhanced uptake of HDL-delivered cholesterol esters and steroidogenesis (25, 27-30).

In adipocytes, mobilization of transport proteins to the cell membrane is a key event in the cellular response to insulin. Thus, the intracellularly stored GLUT4 glucose transporter and the fatty acid transport proteins are rapidly redistributed to the plasma membrane in presence of insulin (31-33). Our observation goes along with these findings. Indeed, insulin and Ang II, both known to promote lipid storage in adipose tissue (34), induce the translocation of SR-BI to the plasma membrane. The uptake of fatty acids by adipose tissue and muscle is facilitated by CD36, a closely related member of the class B scavenger receptor family. Interestingly, SR-BI but not CD36 is mobilized to the cell membrane in conditions favoring fatty acid storage. Indeed, a stable plasma membrane localization of CD36 was observed in 3T3-L1 adipocytes and was not modified by insulin (33). Thus, among the members of the scavenger receptor family expressed in adipocytes, SR-B1 is involved in trafficking to and from the cell membrane. This suggests that it might participate in the process of lipid storage in fat cells. This redistribution, associated with an increase in both cholesterol influx and cell content, suggests that SR-BI translocation could play a crucial role in regulation of the lipid metabolism of the cell.


   FOOTNOTES
 
* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. Back

The on-line version of this article (available at http://www.jbc.org) contains supplemental data in the form of a video presentation of EGFP-SR-BI traffic in living 3T3-L1 adipocytes. Back

1 To whom correspondence should be addressed. Tel.: 33-1-42346923; Fax: 33-1-40518586; E-mail: dagher{at}ccr.jussieu.fr.

2 The abbreviations used are: SR-BI, scavenger receptor class B, type I; Ab, antibody; Ang II, angiotensin II; DMEM, Dulbecco's modified Eagle's medium; EGFP, enhanced green fluorescent protein; HDL, high density lipoprotein; NBD-chol, 22-(N-7-nitrobenz-2-oxa-1,3-diazo-4-yl)-amino-23,24-bisnor-5-cholen-3-ol; PBS, phosphate-buffered saline. Back



   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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