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Geriatric Research, Education and Clinical Center, Veterans Administration Palo Alto Health Care System (E.R., A.N., S.L.-S., S.A.), Palo Alto, California 94304; and the Department of Pharmacological Sciences, University Medical Center, State University of New York (R.T., D.L.W.), Stony Brook, New York 11794
Address all correspondence and requests for reprints to: Eve Reaven, Ph.D., Veterans Administration Palo Alto Health Care System (GRECC, 182B), 3801 Miranda Avenue, Palo Alto, California 94304. E-mail: eve{at}icon.palo-alto.med.va.gov
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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A number of observations have led to the association between SR-BI expression and the selective cholesterol pathway. Acton et al. (1) initially showed that SR-BI-transfected Chinese hamster ovary cells bind HDL with high affinity and take up both radiolabeled and fluorescent lipid markers, and this idea was reinforced by subsequent observations showing that SR-BI itself is specifically associated with steroidogenic tissues (20, 21, 22, 23, 24), liver (20, 22), and a variety of cell models known to use large quantities of HDL cholesterol (1, 21, 22, 25, 26). Furthermore, mice lacking a functional SR-BI gene have reduced adrenal CE accumulation, implicating SR-BI as necessary for HDL cholesterol uptake in vivo (27). Although selective CE uptake data were not actually provided in these studies, it was assumed that changes in levels of SR-BI have led to similar changes in selective HDL-CE uptake in the examined cells and tissues (20, 21, 22, 23, 24). However, the actual link between SR-BI and selective uptake in cultured adrenal cells is strengthened by recent results showing that antibody to SR-BI can, in fact, block 50% of normal HDL-CE selective uptake and reduce the delivery of HDL-CE to the steroidogenic pathway (28).
In rat ovary, most HDL-CE is taken up by the selective pathway (5, 7, 8, 9), and this is correlated with high expression of SR-BI in ovarian tissue, particularly in cells of the corpus luteum (20, 24). More recently, the link between SR-BI and the selective HDL-CE pathway has been examined in cultured nonluteinized (basal) granulosa cells prepared from 17ß-estradiol-treated rats (29). These cells showed no selective uptake of HDL-CEs, no progestin production, and no expression of SR-BI until stimulated (and luteinized) by tropic hormones or adenylate cyclase stimulators. After luteinization, selective HDL-CE uptake, SR-BI protein levels, and progestin production were dramatically up-regulated, indicating a tight coupling between SR-BI expression and the uptake and utilization of HDL cholesterol for steroid production (29).
In the current report we examined the relationship between HDL-CE selective uptake and SR-BI expression in a superovulated (luteinized) rat ovary in vivo. This relationship is further challenged with tropic hormone-induced desensitization and formation of a dysfunctional ovary. The desensitization model is one in which an additional injection of hCG into rats already primed with PMSG and hCG leads within 4–24 h to a decrease in ovarian gonadotropin receptors, a decrease in gonadotropin-stimulated adenylate cylase activity, and a marked loss of hCG-stimulated steroidogenesis (30, 31, 32, 33, 34, 35, 36, 37). Previous studies show that the reduction in progesterone secretion after desensitization is related to a reduction of substrate cholesterol for steroidogenesis (38, 39), and that luteal tissue CE content falls dramatically immediately after the administration of the desensitizing dose of hCG, suggesting that CE provides substrate for steroidogenesis (38, 39). In the present study we asked whether the resulting reduction in CE storage and progestin production in the desensitized ovary is due to less HDL-CE selective uptake and, if so, is this related to reduced SR-BI expression? The results showed, surprisingly, that HDL-CE selective uptake and SR-BI expression were dramatically up-regulated in the desensitized ovary, suggesting that the cellular cholesterol status is a potent regulator of SR-BI expression. Furthermore, SR-BI was localized to the cell surface microvillar compartment, providing support for the hypothesis that microvilli and microvillar channels represent a specialized subcellular compartment in which HDL-CE selective uptake occurs.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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-32P]CTP [SA, 29.6 tetrabecquerels
(TBq)/mmol; 800 Ci/mmol], [4-14C]cholesterol (SA, 2.1
gigabecquerels/mmol; 56.6 mCi/mmol), and
[1,2,6,7-N-3H]cholesteryl oleate (2.6 TBq/mmol; 71
Ci/mmol) were purchased from New England Nuclear Life Science Products
(Boston, MA). [1,2-N-3H]Cholesteryl oleolyl ether
(SA, 1.78 TBq/mmol; 48.0 Ci/mmol) and ECL Western blotting kit were
obtained from Amersham Corp. (Arlington Heights, IL). The following
chemicals were supplied by Sigma Chemical Co. (St. Louis, MO): PMSG,
hCG, cholesterol, cholesteryl oleate, and horseradish
peroxidase-conjugated goat antirabbit IgG. Purified hCG (CR-121;
biological potency, 13,450 IU/mg) was provided by Dr. R. E.
Canfield, College of Physicians and Surgeons of Columbia University
(New York, NY), through the Center of Population Research of the
NICHHD, NIH (Bethesda, MD). All other reagents used were of analytical
grade. The complementary DNA probes for rat LDL (B/E) receptor, rat HMG
CoA reductase, and rat 18S ribosomal RNA were obtained as described
previously from this laboratory (40, 41). An 608-bp, PCR-generated
(bases 441–849), complementary DNA fragment of rat LH/hCG receptor,
cloned into the EcoRI site of pGEM4z (42) was provided by
Dr. Aaron Hsueh, Stanford University (Stanford, CA). For uptake and
internalization studies, human (h) HDL3 and hLDL
preparations were conjugated with residualizing labels, i.e.125I-labeled dilactitol tyramine (DLT) and
[3H]cholesteryl oleolyl ether (COE) (11).
Animals
Sexually immature, female Sprague-Dawley rats, 22–24 days old,
were injected sc with 50 IU PMSG, followed 56 h later with 25 IU
hCG (5, 7); day 0 was taken as the day of hCG injection. The procedure
results in superovulated (highly luteinized) ovaries by days 6–7; the
luteal cells isolated from these ovaries are very responsive and
synthesize and secrete high levels of progesterone when stimulated with
Bt2cAMP and lipoproteins (43, 44). Previous studies from
this laboratory show that the luteinized ovary obtains CEs from
lipoproteins mainly through the selective cholesterol uptake process
(5, 7, 9), uses this CE for steroid production (5, 7), and stores large
quantities of the lipoprotein-derived CE (7, 9).
For desensitized ovaries, the animals were injected again with hCG between 0900–1000 h on day 6 (post-hCG) (38, 39), and ovaries were removed 24 h later (day 7). Control animals received vehicle only. The desensitized ovaries show no hormonal response after stimulation (even in the presence of HDL) (31, 38), and are associated with low tissue levels of stored CE (38, 39).
To study recovery from the desensitization state, some desensitized animals were examined 24, 48, and 72 h after the second (desensitizing) hCG injection; ovary tissue samples were assayed directly for CE uptake, or animals were given radiolabeled lipoproteins for 4-h intervals before tissue sampling.
Mature, 220- to 240-g Sprague-Dawley rats were donors for the adrenal, liver, and kidney tissues used to test the tissue distribution of SR-BI.
Selective CE uptake
Organ perfusions of the luteinized and desensitized ovaries with
reconstituted double radiolabeled lipoproteins containing nonreleasable
tags (11, 13) were routinely carried out to assay selective CE uptake
(5, 7, 9). The standard perfusion sequence involved a 2-min washout
with DMEM-Ham’s F-12-HEPES medium, followed by a 90-min
nonrecirculating (flow through) perfusion of
[125I]DLT-[3H]COE hHDL3 or
[125I]DLT-[3H]COE hHDL3 hLDL
(100 µg protein/ml) at a flow rate of 1.1 ml/min and a precise 2-min
washout with medium at the end of perfusion (5, 7, 9, 11). The ovaries
were homogenized, and trapped radioactivity was released by repeated
freezing and thawing (five to seven times) of the homogenate. A
suitable aliquot of this homogenate was treated with trichloroacetic
acid to determine both insoluble (precipitable) and soluble
125I radioactivity. A second aliquot was extracted with
organic solvents (11, 13) to determine 3H
radioactivity.
Under the conditions used, trichloroacetic acid-insoluble 125I radioactivity was assumed to represent 125I-labeled protein remaining bound to the cell surface as part of intact lipoprotein (11, 13); trichloroacetic acid-soluble 125I radioactivity was taken to be internalized, degraded, and accumulated residualizing protein 125I label. As the 125I and 3H labels are on the same lipoprotein particles, it follows that the relative amounts of surface-bound 125I and 3H radioactivities must be equal. Thus, the amount of CE (3H radioactivity) internalized can be computed as the difference between total CE uptake and trichloroacetic acid-insoluble (i.e. surface-bound) 125I (3H) radioactivity. The uptake is expressed as nanograms of 125I-labeled (endocytic uptake) or 3H-labeled (selective uptake) protein internalized (11, 13). To determine the net mass of CE internalized, 125I- and 3H-labeled protein values are divided by the protein/cholesterol ratio of each lipoprotein (e.g. for hHDL3 and hLDL, respective protein/cholesterol ratios are 2.53 and 0.48).
In recovery studies evaluating HDL-CE uptake, luteinized animals were given hCG or saline on day 6; subsequently, two rats from each experimental group were given the double labeled HDL described above iv at 24, 48, or 72 h post-hCG. The ligands were allowed to circulate for 4 h before the animals were killed, and ovary tissue was taken for determination of total, endocytic, and selective uptake of cholesteryl ester as described above.
SR-BI antibodies
Polyclonal antibodies raised against peptide corresponding to
the carboxyl-terminus of mouse SR-BI [amino acids 489–509;
AYSESLMSPAAKGTVLEQEAKL (1)] and against the
glutathione-S-transferase fusion protein containing mouse
SR-BI amino acid residues 174–356 [proposed extracellular domain of
mouse SR-BI (1, 28)] were prepared in rabbits using standard
procedures. These antibodies are fully functional in immunoblotting and
immunohistochemical assays.
Morphological techniques
For immunohistochemistry, ovaries were perfused with fixative
[4% paraformaldehyde (PF) in PBS] overnight, then left in 1% PF
fixative until processed for embedment. Paraffin-prepared sections were
blocked with 5% goat serum and 5% nonfat milk (1 h, 37 C), incubated
with primary antibody or preimmune serum (1:1000) overnight at 4 C, and
labeled with a standard biotinylated horseradish peroxidase procedure
(Vector Laboratories, Burlingame, CA).
For immunocytochemical experiments at the electron microscope level, ovaries were perfused with 4% PF and 0.5% glutaraldehyde and fixed overnight. Tissue blocks were processed and embedded in LR gold resin [London Resin Co., Berkshire, UK (purchased from Ted Pella Co., Redding, CA)] using techniques described by Berryman et al. (45). Ultrathin sections were blocked with 5% normal goat serum (1 h, 22 C), incubated with primary antisera (1:100 to 1:150 dilution) or preimmune serum overnight at 4 C, and labeled with goat antirabbit IgG-10 nm gold (EM Sciences, Fort Washington, PA) for 1 h at 22 C. Counterstains were osmium vapor and lead citrate.
Assay for HDL receptor protein (SR-BI)
The presence and quantity of SR-BI were assessed by Western blot
analysis using methods previously described (40, 41). Briefly, luteal
tissue was homogenized in 10 vol buffer [20 mM Tris-HCl
(pH 7.5), 2 mM MgCl2, 0.25 M
sucrose, 1 mM phenylmethylsulfonylfluoride, 10 µg/ml
leupeptin, 20 µg/ml aprotinin, and 5 µg/ml pepstatin], and
centrifuged (800 x g) for 10 min, and the supernatant
was centrifuged for 60 min at 100,000 x g. The
resulting pellet was washed with buffer to remove lipids, and membranes
were used for immunoblotting of SR-BI. A constant amount of membrane
protein (20–60 µg, or as indicated) was solubilized in Laemmli
buffer, separated on 10% SDS-PAGE, and transferred to Immobilon
polyvinylidene difluoride membrane (Millipore, Corp., Bedford, MA). The
membranes were blocked in PBS-0.02% Tween-20 containing 5% each of
powdered milk and FBS, incubated with 1:1,000 diluted rabbit polyclonal
antipeptide antibody to mouse SR-BI, washed, and then incubated with a
goat antirabbit IgG antibody (1:20,000) conjugated with horseradish
peroxidase. After extensive washing, chemiluminescent substrate was
added (enhanced chemiluminescence detection system, Amersham, Arlington
Heights, IL), and the membranes were subjected to autoradiography
(3–10 min) followed by densitometric scanning.
Assays for cholesterol and CE content
Luteal tissue free cholesterol and CEs were extracted and
separated by silicic acid/Celite column chromatography as previously
described (38, 39). The isolated CEs were hydrolyzed in ethanolic KOH,
and the derived cholesterol and tissue free cholesterol were quantified
by the micromethod of Glick et al. (46).
Messenger RNA (mRNA) quantitation by ribonuclease (RNase)
protection assay
The mRNA concentrations of LH/hCG receptor, LDL (B/E) receptor,
and HMG CoA reductase were measured by RNase protection assays as
described previously (40, 44). Total RNA was isolated from luteal
tissues using the procedure of Chomczynski and Sacchi (47). The
antisense [32P]complementary RNA probes were synthesized
using [-32P]rCTP, restriction endonuclease linearized
plasmids (XhoI for LDL receptor, HindIII for HMG
CoA reductase, BamHI for 18S ribosomal RNA, and
BglII for LH/hCG receptor) and appropriate T3 or T7
polymerase following the method supplied in Stratagene’s in
vitro transcription kit (La Jolla, CA). Because of their high
lability, the riboprobes were always freshly prepared before
hybridization. Aliquots of total luteal RNA (10 µg) or control
transfer RNA (10 µg) were hybridized with 100,000 cpm specific probe
for 18 h at 42 C. The unprotected probe was digested with RNase A
(40 µg/ml) and RNase T1 (2 µg/ml) for 1 h at 30 C, followed by
the addition of proteinase K (50 µg) and SDS (2 mg) for 15 min at 37
C. After phenol-chloroform extraction and precipitation, the protected
RNA-RNA hybrids were resolved on 6% acrylamide-urea denaturing gels.
After electrophoresis, gels were exposed to Kodak XAR-5 film at -70 C
with intensifying screens. For strong signals, gels were usually
exposed for 6–12 h; for weaker signals, they were exposed for up to
48 h.
For quantification, the films were analyzed by densitometry. The data are expressed as the fraction of LDL(B/E) receptor, HMG CoA reductase, or LH/hCG receptor signal compared to that of 18S ribosomal RNA to correct for differences in RNA loading (41). In these studies, the steady state levels of 18S ribosomal RNA remained constant in response to hCG treatment.
Miscellaneous techniques
The procedure of Markwell et al. (48) was used to
quantify protein content of hHDL3, hLDL, and double labeled
lipoprotein preparations. Protein in the membrane fractions was
determined by a modification of the procedure of Lowry et
al. (49) as described by Peterson (50). Measurement of lipoprotein
binding (HDL receptor activity) was carried out as described previously
using partially purified luteal plasma membranes and
[125I]rat HDL as tracer (38, 51). Gonadotropin receptor
activity ([125I]hCG binding) in partially purified plasma
membranes was determined as described by Azhar et al. (38, 51).
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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show that
hCG-induced desensitization of the luteinized ovary leads to a 90%
reduction in tissue CE, with little or no change in free cholesterol.
To test whether this reduction in ovarian CE was due to reduced HDL-CE
selective uptake, luteinized and 24-h desensitized ovaries were
perfused for 4 h with double radiolabeled (nondegradable) HDL and
assayed for CE uptake via the selective and endocytic (Fig. 1B)
pathways as described in Materials and Methods. The results
(Fig. 2) showed that HDL-CE selective
uptake was increased 4-fold in the desensitized ovary. Note that
endocytic uptake of HDL-CE was also increased in the desensitized
ovary, but CE uptake via this pathway was negligible.
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demonstrate that hCG-induced desensitization of luteinized ovary led to
a 2-fold increase in rat HDL binding in partially purified luteal
membrane preparations. Under identical conditions,
[125I]hCG receptor activity was reduced by more than 90%
after treatment of animals with a desensitizing dose of hCG. To
determine ovarian SR-BI protein levels, the same membrane preparations
were analyzed by Western blotting with a polyclonal antipeptide
antibody raised to the C-terminal domain of mouse SR-BI. This antibody
detected high levels of SR-BI in mature rat ovaries and adrenal glands,
low levels in liver, and no detectable SR-BI in kidney, as was
previously shown by others (20) (data not shown).
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). This result is in close
agreement with the 2-fold increase in HDL binding noted above (Table 1
). These data suggest that SR-BI expression and HDL binding are both
up-regulated in the ovary in response to the CE depletion that occurs
after hCG-induced ovarian desensitization. To test whether this CE
depletion is indicative of an altered regulatory cholesterol status in
the ovary, mRNAs encoding HMG CoA reductase and the LDL (B/E) receptor
were measured by RNase protection assay. These mRNAs are under feedback
control by cholesterol and are strongly induced when cellular
cholesterol is depleted (52, 53). As shown in Fig. 4, concentrations of mRNA for HMG CoA
reductase and the LDL (B/E) receptor are markedly up-regulated in
desensitized ovary.
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). Each luteal cell in Fig. 5A was
prominently outlined with reaction product, indicating the presence of
SR-BI on all faces of the steroidogenic luteal cell. This pattern
is similar to that seen on luteal cells of ovaries from ethinyl
estradiol-treated rats (20) and on zona fasciculata cells of the
ACTH-treated mouse adrenal gland (21). Note, that connective tissue
lying between the corpora lutea of this ovary is not reactive for
SR-BI, and as such, the corpus luteum shown in Fig. 5A is surrounded by
unstained tissue that appears white in the photograph.
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) was similar, but the cell surface expression of SR-BI (under
identical immunostaining conditions) was substantially increased, in
agreement with the Western blots. Figure 5C is of interest. On
the right, it shows luteal cells from a desensitized ovary
heavily stained for SR-BI. On the left, it shows a small
follicle with the central ovum surrounded by many layers of basal
(nonluteinized) granulosa cells and two or more layers of cells
(presumed to be thecal cells) comprising the surface of the follicle;
whereas the granulosa cells of the follicle do not express SR-BI, some
expression was found in the ovum and theca. Although such follicles are
common in the artificial (desensitized) ovarian model studied here, the
cellular distribution and the expression of SR-BI are probably
different in the adult cycling ovary (20).
Electron microscopy. In an effort to better understand the
localization of SR-BI in the luteinized ovary, sections from
perfusion-fixed ovaries were immunostained with two different
polyclonal antibodies to SR-BI: one against the cytoplasmic C-terminal
domain and the other against the extracellular domain of the SR-BI
molecule. Both antibodies showed that SR-BI was highly expressed on the
microvilli and in microvillar channels that cover the surface of luteal
cells (Fig. 6). Microvillar channels form
by juxtaposition of adjacent microvilli to create channels that fill
with HDL particles; such channels are part of a subcellular compartment
in which HDL-CE selective uptake occurs (54, 55). Figure 6 shows
abundant immunogold labeling of this microvillar compartment formed by
the junction of two luteal cells. The cell fragment to the
right of the microvillar compartment appears to be an
endothelial cell and has no apparent cell surface gold particles.
Scattered gold particles also exist in the cytoplasm of luteal cells;
these may represent newly synthesized SR-BI in transport to the cell
surface.
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provides magnified views of the
luteal cell surface showing microvilli packed together in bundles.
Figure 7A represents a preimmune serum-stained section to be compared
with the micrograph in Fig. 7B, where staining was carried out with the
antibody to the extracellular domain of SR-BI. The latter shows that
SR-BI is highly expressed in the microvillar compartment, with
particular concentration in and around microvillar channel areas. The
double arrowheads identify representative channels and
illustrate their association with SR-BI.
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Ovarian cholesterol metabolism during recovery from hCG-induced
desensitization
To monitor the recovery of cholesterol metabolism in desensitized
ovaries, rat ovaries were assayed for CE content, HDL-CE selective
uptake, and SR-BI content 24, 48, and 72 h after the last hCG
injection, and the results were compared with those obtained with
luteinized ovaries from animals that did not receive the desensitizing
dose of hCG. The data in Fig. 8, top panel, shows that the CE content of the desensitized
ovary increased to the 50% level by 48 h and was fully restored
by 72 h after the desensitizing dose of hCG.
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, middle panel), ovary samples showed that HDL-CE selective
uptake remained high for 48 h, but was reduced to the level of the
luteinized ovary by 72 h (see also Fig. 2B). Interestingly, SR-BI
levels continued to increase up to 48 h and remained elevated at
72 h (Fig. 8, bottom panel and Western blot). |
Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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An important result in the present study is the dissociation of SR-BI expression and HDL-CE selective uptake from tropic hormone stimulation in the desensitized ovary. In cultured granulosa cells, SR-BI expression and HDL-CE selective uptake are dependent on gonadotropin stimulation (13, 15). This also appears to be the case in vivo when ovaries are first exposed to gonadotropins (24, 29). However, upon administration of a second desensitizing dose of hCG, luteal cells lose LH/hCG receptors and hCG-induced steroid production. Despite the loss of gonadotropin responsiveness, SR-BI and HDL-CE selective uptake are up-regulated in the desensitized ovary, suggesting that factors other than gonadotropins are potent regulators of SR-BI. The most likely candidate for SR-BI regulation in the desensitized ovary is the cellular cholesterol status. The increased expression of SR-BI in the desensitized ovary was paralleled by the increased expression of HMG CoA reductase and LDL (B/E) receptor mRNAs, presumably reflecting feedback regulation due to reduced levels of cellular cholesterol, as reflected by the depletion of cellular CE stores. These data argue that SR-BI is also subject to regulation by the cholesterol status of the steroidogenic cell. This interpretation is consistent with the elevated levels of SR-BI expression reported in the CE-depleted adrenal glands of the apoA-I-deficient mouse (22) and the lecithin cholesterol acyl transferase-deficient mouse (56). An important point not yet resolved is whether gonadotropin-induced SR-BI expression in ovarian cells and ACTH-induced SR-BI expression in adrenal cells are secondary to hormone-mediated changes in cellular cholesterol or are due to direct effects of the hormones on SR-BI gene expression.
hCG-induced desensitization of the rat ovary revealed two additional features relevant to the regulation of luteal cell cholesterol metabolism. First, within 24 h after the desensitizing dose of hCG, CE stores were lost from the luteal cell despite elevated levels of SR-BI, increased uptake of CE from HDL, and increased expression of HMG CoA reductase, which are suggestive of increased cholesterol synthesis. These results argue that in the absence of gonadotropin stimulation, cellular CE is hydrolyzed, and free cholesterol effluxes from the luteal cell. This may occur because of a failure to esterify cholesterol, because CE hydrolysis is greatly accelerated, or both in the desensitized ovary. An interesting question is whether rapid efflux of luteal cell cholesterol may occur after desensitization, and whether such efflux may be mediated by SR-BI. Recent experiments in Chinese hamster ovary cells stably transfected with SR-BI show that SR-BI can mediate free cholesterol efflux (57), raising the possibility that the elevated levels of SR-BI in the luteal cells of the desensitized ovary are responsible for a rapid efflux of cholesterol after hCG treatment.
A second feature relevant to the regulation of luteal cell cholesterol
metabolism was seen during the late recovery phase from
desensitization, when some dissociation between the expression of SR-BI
and the selective CE pathway occurred. SR-BI levels remained elevated
at 72 h, but HDL-CE selective uptake declined (Fig. 7). This
result indicates that the activity of SR-BI in the HDL-CE selective
uptake pathway may be regulated independently of the level of SR-BI
protein.
Immunolocalization of SR-BI at the light microscopic level showed that SR-BI is localized to luteal cells in luteinized and desensitized ovaries. Strong cell surface staining was seen on all faces of the luteal cell. Indeed, the very thickness of the immunoreactive portion of the cell surface is unusual and suggests the involvement of surface-related structures beyond the simple labeling of the luteal cell plasma membrane. Immunolocalization at the electron microscopic level showed that, in fact, the immunoreactivity was due to SR-BI on microvilli that coat the surfaces of these cells (54, 55). The microvilli are often juxtaposed to form channels that fill with serum-derived lipoproteins (51, 54, 55). Such microvillar channels have been proposed to be a subcellular compartment in which HDL-CE selective uptake occurs (54, 55). The presence of high levels of SR-BI on the membranes of microvillar channels supports the idea that this cell surface compartment is a major site of selective uptake.
In summary, this study provides strong support for the idea that HDL binding, SR-BI protein, and HDL-CE selective uptake are tightly linked in the rat luteinized ovary model, even when the tissue is in a desensitized state. The up-regulation of luteal cell SR-BI and HDL-CE selective uptake in the desensitized state suggests that SR-BI expression is sensitive to the cholesterol status of the cell. The localization of SR-BI within the microvillar domain provides additional evidence that microvillar channels are an active site of HDL-CE selective uptake.
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Footnotes |
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Received January 8, 1998.
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References |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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