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INTRODUCTION |
Lipoprotein lipase
(LpL)1 is rate-limiting in
hydrolysis of triglycerides in circulating lipoproteins, and reduced
levels or activity of the active dimeric species therefore lead to
increased triglyceride levels. Even small changes in LpL function in
man may have important consequences. Thus, common mutations in the LpL
gene with marginal effects on the activity are associated with
dyslipidemias (1, 2) and may enhance the development of atherosclerosis
(3).
The mechanisms for synthesis, folding, and secretion of LpL to the
lumenal side of the vascular epithelium are not known in detail. A
large part of the synthesized LpL is normally degraded within the cell
(4, 5), and trimming of sugar residues from oligosaccharide chains of
LpL appears necessary for proper folding and expression of LpL activity
(6). Moreover, dimerization is important for regulation of catalytic
activity (7). At the endothelium, LpL is bound to heparan sulfate
proteoglycans. LpL easily dissociates from these sites (8) and
reassociates to other sites, including receptors that mediate its
uptake into cells. Clearance occurs via multifunctional endocytic
receptors of the LDL receptor family, notably LDL receptor-related
protein/
2-macroglobulin receptor (LRP) (9, 10), aided by
accumulation of LpL on cell surface proteoglycans (10, 11) as well as
directly via transmembrane proteoglycans (12-15).
Sortilin is a ~95-kDa type-I receptor first isolated from human brain
(16) and recently shown to be identical with the neurotensin receptor-3
(17). It consists of a lumenal domain homologous to yeast vacuolar
protein-sorting 10 protein, a single transmembrane domain, and a short
cytoplasmic tail with a C terminus strongly homologous to that of the
mannose 6-phosphate/insulin-like growth factor-II (IGF-II) receptor
(16). In mature sortilin, an N-terminal 44-residue propeptide has been
cleaved off, and recent results show that furin-mediated propeptide
cleavage conditions sortilin for ligand binding (18). Besides in
neurones, sortilin is abundant in several cell types including skeletal
muscle, heart, and adipocytes (16, 19). Although sortilin, like the
IGF-II receptor, is mainly located in the Golgi compartment and
vesicles (16, 17), it is also expressed on the cell surface. In
differentiated 3T3-L1 adipocytes, sortilin colocalizes with the IGF-II
receptor, and insulin causes a ~2-fold increase in the expression of
both receptors on the plasma membrane (19). Thus, like the IGF-II
receptor, sortilin has the potential of functioning both as a sorting
receptor in the Golgi compartment and as a clearance receptor on the
cell surface.
Mature sortilin binds the receptor-associated protein, RAP (16), a
specialized chaperone that interacts with LDL receptor family members,
notably the multifunctional receptors LRP, megalin, and the very low
density lipoprotein receptor (for reviews, see Refs. 20-22). RAP
consists of three homologous domains of which domain D3 binds to
sortilin (23). We have previously shown that LpL and segments of RAP
containing D3 cross-compete for binding to LRP, and these ligands are
therefore thought to bind to the same or overlapping sites on the
receptor (24). As an approach to elucidate the function of sortilin as
a putative endocytic and sorting receptor, we therefore investigated
whether sortilin binds LpL. We show that the soluble lumenal domain of
mature sortilin (s-sortilin) binds LpL with an affinity comparable with
that of LRP and that the binding is inhibited by RAP and by
neurotensin. Moreover, LpL is associated with sortilin in 3T3
adipocytes and is specifically degraded in stably transfected CHO cells
that express mature full-length sortilin on the cell surface. We
propose that sortilin, in addition to performing not yet elucidated
sorting functions in the Golgi compartment, scavenges a diverse set of extracellular ligands such as LpL and neurotensin.
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MATERIALS AND METHODS |
Sortilin, LpL, and RAP--
The extracellular domain of mature
sortilin (s-sortilin), comprising residues 45-725 of full-length
sortilin, was expressed in stably transfected Chinese hamster ovary
(CHO-K1) cells and purified from the culture medium by RAP affinity
chromatography as described (23). The propeptide (residues 1-44)
was expressed in Escherichia coli BL21(DE3) cells as a
glutathione S-transferase fusion protein using the pGEX4T
vector (Amersham Pharmacia Biotech) and purified on a
glutathione-agarose column (18). Full-length sortilin was subcloned
from the cloning pBK-CMV vector (16) into the
pCDNA3.1/Zeo(
) vector using XbaI and
SmaI restriction enzymes and transfected into CHO-K1 cells
(18). LpL was purified from bovine milk as the enzymatically active
dimeric enzyme (~600 units/mg) as described previously (25). LpL and
s-sortilin were 125I-iodinated to specific activities of
about 0.5 mol of 125I/mol of protein using chloramine T as
the oxidizing agent. The iodinated LpL preparation was applied to a
Sepharose C16B column (Amersham Pharmacia Biotech). Labeled material
that eluted at 0.9 M NaCl was discarded, and the fraction
that eluted at 1.5 M NaCl, representing the dimeric form of
the lipase, was used for experiments. In some cases, LpL was made
monomeric by incubation with 1 M guanidinium hydrochloride
for 3 h at 20 °C and dialyzed immediately before use (26). The
construct LpL-(347-389/394-448 was produced in E. coli as
described (27). The construct consists of the hexahistidine-Factor X
substrate sequence Met-Gly-Ser-(His)6-Ser-Ile-Glu-Gly-Arg and amino acid residues 347-389 and 394-448 of human LpL. Deletion of
the sequence Trp390-Ser-Asp-Trp393 from the
construct LpL-(347-448) increases the solubility of the peptide. The
construct contains the sites for binding to LRP and has about the same
affinity as the uninterrupted stretch LpL-(347-448) and
LpL-(313-448), which constitutes the C-terminal folding domain of LpL
(27). Human RAP and RAP constructs containing domain D1 (residues
18-112) or domains 2 and 3 (D23) (residues 113-323) were produced in
E. coli as hexahistidine-tagged peptides (28).
Antibodies--
Rabbit anti-s-sortilin IgG used for
immunoprecipitation of sortilin and for Western blotting and rabbit
antiserum against a synthetic peptide containing residues
Pro15-Arg28 of the sortilin propeptide have
been described (18). For detection of LpL in mouse 3T3-L1 adipocytes,
we used IgG isolated from egg yolk of chickens immunized with bovine
LpL (29).
Cross-linking of Soluble Components and Solid Phase
Assay--
125I-Labeled s-sortilin (~105
cpm) was incubated with LpL in 140 mM NaCl, 10 mM CaCl2, 10 mM Hepes, pH 7.8, for
16 h at 4 °C followed by incubation with the bifunctional
reagent BS3 (Pierce) at a final concentration of 100 µM for 30 min at 20 °C. The reaction was quenched by
the addition of SDS sample buffer with 20 mM Tris followed
by SDS-PAGE and autoradiography. For solid phase assay, LpL was
immobilized in Polysorp microtiter wells (NUNC, Denmark), the wells
were blocked with 2% Tween 20 for 2 h at 20 °C (24), and
incubations with 125I-s-sortilin were in the above buffer.
After 16 h at 4 °C, the wells were washed, and bound
radioactivity was eluted with 10% SDS and counted.
Surface Plasmon Resonance Analysis--
All measurements were
performed on a BIAcore 2000 instrument (Biosensor, Uppsala, Sweden)
equipped with CM5 sensor chips. The carboxylated dextran matrix of the
sensor chip (flow cells 1 and 2) was activated by the injection of 240 µl of solution containing 0.2 M
N-ethyl-N'-(3-dimethylaminopropyl)carbodiimide and 0.05 M N-hydroxysuccimide in water, and
s-sortilin was immobilized to an estimated density of 64 fmol/mm2. Samples for binding (40 µl) were injected at 5 µl/min (4 °C) in 10 mM Hepes, 150 mM NaCl,
1.5 mM CaCl2, 1 mM EGTA, 0.005%
Tween 20, pH 7.4 (running buffer), and binding was expressed in terms of relative response units (RU), e.g. the response
obtained from the flow cell with immobilized sortilin minus the
response obtained using an activated but uncoupled flow cell.
Regeneration of the chip was performed by injecting 20 µl of 10 mM Tris, 50 mM NaCl, 4 M urea,
0.005% Tween 20, pH 8.0. Kinetic parameters were determined using the
BIAevaluation 3.0 software. The number of LpL molecules bound/mol of
immobilized sortilin was estimated by dividing the ratio
RUligand/massligand with
RUsortilin/masssortilin.
Culture and Incubation of Cells--
3T3-L1 fibroblasts were
grown in Dulbecco's modified Eagle's medium/F-12 (1:1 mix;
Bio-Wittaker, Belgium), 10% donor calf serum, and 50 µg/ml
gentamycin. To induce their differentiation to adipocytes, donor calf
serum was replaced with fetal calf serum, and the medium was
supplemented with 10 µM dexamethasone for 48 h and
subsequently with insulin (10 µg/ml) for 8 days. Wild-type CHO-K1
cells and cells stably transfected with sortilin (18) were grown to
near confluency in HyQ-CCM5 medium (HyClone, Utah; about 3 × 105 cells/well). Degradation of 125I-LpL at was
measured as 12% trichloroacetic acid-soluble radioactivity in the medium.
Metabolic Labeling--
3T3-L1 adipocytes were washed,
preincubated for 10 min in cysteine- and methionine-free modified
Eagle's medium (Sigma) before overnight incubation in the same medium
supplemented with 200 µCi/ml [35S]cysteine and
[35S]methionine (pro-mix, Amersham Pharmacia Biotech),
5% full medium, and 10 µg/ml insulin. For identification of
LpL-sortilin complexes, labeled 3T3-L1 cells were incubated with 1 mM cell-permeable and reducible cross-linker
dithiobis(succinimidyl propionate) (Pierce). After 30 min at 4 °C,
the reaction was quenched in 20 mM Tris, and following
washes, the cells were lysed in 400 µl of 1.0% Triton X-100
containing 20 mM Tris and proteinase inhibitors (Complete Mini, Boehringer Mannheim). After centrifugation, the lysate
supernatant was diluted in 2 ml of a Tris-balanced salt solution and
incubated with Sepharose-coupled anti-s-sortilin IgG for 16 h at
4 °C. The beads were washed, and bound material was eluted in 300 µl of 100 mM glycine, pH 2.7, neutralized with Tris
buffer and subjected to reducing SDS-PAGE either before or after an
additional step of precipitation using chicken anti-LpL IgG.
Diphenyloxazole-fluorographed gels were exposed at
70 °C.
Quantitation of Cell Surface-expressed Sortilin--
The
transfected CHO cells were washed three times in ice-cold
phosphate-buffered saline, pH 8.0, and incubated with 0.5 mg/ml membrane-impermeable reagent
sulfo-N-hydroxysuccinimidobiotin (Pierce, IL) for 90 min at
4 °C. After washes in phosphate-buffered saline with 50 mM Tris to quench any unreacted reagent, the cells were
lysed for 10 min at 4 °C in 1% Triton X-100, 20 mM
Tris, 10 mM EDTA, 150 mM NaCl, pH 8.0, with
protease inhibitors, and biotinylated proteins were precipitated with
streptavidin-coupled Sepharose 4B beads (Zymed Laboratories
Inc., CA). The fractions of streptavidin-bound (i.e.
surface-biotinylated) and unbound sortilin were detected by Western
blotting using horseradish peroxidase-conjugated swine anti-rabbit IgG
as secondary antibody (Dako, Denmark) and enhanced chemiluminescence
(ECL; Amersham Pharmacia Biotech). Quantitation was performed by laser
scanning densitometry using a 2202 Ultroscan instrument (LKB, Sweden).
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RESULTS |
Soluble Sortilin Binds LpL--
To test for binding, we first
incubated LpL with 125I-labeled s-sortilin followed by the
addition of the cross-linker BS3. As shown in Fig.
1A, a complex was formed when
the labeled receptor was incubated with 125 nM LpL
(lane 2) but not without LpL (lane 1). The
labeled complex was increased by unlabeled soluble sortilin at low
concentration (9 nM) (lane 3). However, the
formation of complex was inhibited by 180-540 nM unlabeled
soluble sortilin (lanes 4 and 5) and by
heparin (lane 6). In addition, 500 nM LRP or megalin, which are both known to bind LpL (10, 11, 30), abolished
the formation of complex between sortilin and LpL (not shown).

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Fig. 1.
Cross-linking of sortilin to LpL.
A, 125I-labeled s-sortilin (100 pM,
~105 cpm) was incubated for 16 h at 4 °C without
additions (lane 1) or with 125 nM LpL in the
absence (lane 2) or presence of 9, 180, or 540 nM unlabeled sortilin (lanes 3-5), or heparin
(100 units/ml) (lane 6), followed by incubation with the
cross-linker BS3 for 30 min. The reaction was then
quenched, and the samples were applied to reducing SDS-PAGE (4-16%).
B, incubation was with 125I-s-sortilin without
additions (lane 1), with 125 nM LpL alone
(lane 2), or with 125 nM LpL plus 5 µM RAP (lane 3), 5 µM RAP D23
(lane 4), or 125 nM RAP D23 (lane 5),
followed by cross-linking and SDS-PAGE. The bands were
visualized by autoradiography.
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We next examined if RAP inhibits the binding. Because the cross-linked
complex with RAP is not easily distinguished from that with LpL (Fig.
1B, lanes 3 versus 2), we used the
smaller construct comprising RAP D23, which binds to sortilin (23) and
shares binding sites with LpL on LRP (24). At 5 µM, D23
markedly inhibited sortilin-LpL complex and caused the formation of a
smaller complex (lane 4), whereas 125 nM D23
caused partial inhibition (lane 5). To further analyze the
inhibition, 125I-sortilin was incubated with LpL
immobilized in microtiter wells. Fig. 2
shows that RAP and RAP D23, but not D1, inhibited the binding reaction.
This agrees with the previous result that sortilin binds RAP D3 and D23
but not the individual domains D1 or D2 (23).

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Fig. 2.
Inhibition of sortilin binding to LpL by RAP
constructs. 125I-s-sortilin (~100 pM)
was incubated in microtiter wells with immobilized LpL in the presence
of RAP, RAP D1, or RAP D23 as indicated. The results are the mean
values ±1 S.D. of three replicate values. The binding of
125I-sortilin in the absence of inhibitors was
approximately 5% that of the added tracer and was normalized to 100%.
The background values for binding (about 0.5% of the added tracer)
have been subtracted.
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The binding of LpL was further substantiated by plasmon resonance
analysis using s-sortilin immobilized on the sensor chip. Fig.
3A shows that LpL binds
readily and demonstrates a marked decrease in binding when LpL was made
monomeric by treatment with guanidinium hydrochloride. The calculated
capacity was about 0.4 mol of LpL/mol of s-sortilin, suggesting that
the binding stoichiometry might be 1:1. The Kd for
binding of LpL to s-sortilin was calculated at about 26 nM
from these and a series of similar curves obtained at LpL
concentrations ranging from 9 nM to 454 nM. The
affinity for binding of monomeric LpL was about 100-fold lower. For
comparison, LpL binding was also analyzed using LRP immobilized to the
chip. The Kd value was calculated at about 11 nM for binding of LpL to LRP and was in the micromolar range when LpL had been made monomeric (not shown), which is in broad
agreement with previous observations using LRP immobilized in
microtiter wells (11). To further test the specificity of LpL binding,
the chip with immobilized sortilin was perfused with the fragment
LpL-(347-389/394-448), which contains the two LRP binding segments in
the C-terminal folding domain of human LpL (27). Fig. 3B
shows that this peptide (20 µM) binds to sortilin, suggesting that the same segments in LpL are required for interaction with the two receptors. We also probed whether the 13-residue neuropeptide neurotensin, recently identified as a sortilin ligand (17), interferes with LpL binding. As shown in Fig. 3B,
neurotensin at 20 µM essentially abolished the binding of
LpL, suggesting that the two ligands bind to overlapping sites on
sortilin. Finally, we tested the sortilin propeptide (as glutathione
S-transferase fusion protein), which binds to s-sortilin as
well as to mature full-length sortilin and inhibits binding of RAP and
neurotensin (18). The results demonstrated that s-sortilin, when first
incubated with LpL at a near-saturating concentration (200 nM, cf. panel A), was unable to bind propeptide
(not shown).

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Fig. 3.
Surface plasmon resonance analysis of LpL
binding to sortilin. A, the sensor chip was coupled
covalently with s-sortilin and perfused with LpL at the concentrations
indicated or with LpL made monomeric by treatment with guanidinium
hydrochloride. The perfusion was shifted to buffer alone at 700 s.
B, the chip was perfused with 100 nM LpL or 20 µM of the peptide LpL-(347-389/394-448). Perfusion was
also performed with 20 µM neurotensin (NT),
which itself gives a minor signal because of its small size, and 100 nM LpL was added at 740 s (arrow).
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Thus, LpL binds to soluble sortilin with an affinity comparable with
that for binding to LRP, presumably via its C-terminal folding domain.
Moreover, the binding is competed for by the sortilin ligands, RAP (via
D3) and neurotensin and by the sortilin propeptide.
Binding of LpL to Sortilin in 3T3-L1 Cells--
To elucidate
interaction in cells, we used 3T3-L1 adipocytes because they express
LpL as well as sortilin (31, 19). Initially we observed coexpression of
LpL and sortilin upon differentiation, i.e. after 3-4 days
in culture (not shown). Fully differentiated 35S-labeled
3T3-L1 adipocytes were incubated with the permeable and thiol-cleavable
cross-linker dithiobis(succinimidyl propionate) and lysed, followed by
incubation with Sepharose-coupled rabbit anti-sortilin IgG. Complexes
were then released from the Sepharose beads and subjected to reducing
SDS-PAGE. As shown in Fig. 4, lane
1, a dominating band corresponds to sortilin itself, and a marked
band is compatible with LpL. Other labeled peptides may represent
nonspecific binding reactions. To confirm the presence of LpL, we
immunoprecipitated the complexes released from the anti-sortilin
Sepharose beads using chicken anti-LpL IgG. Fig. 4, lane 2,
shows that reducing SDS-PAGE identified labeled peptides compatible
with LpL and sortilin. Thus, a fraction of LpL appears associated with
sortilin in the 3T3-L1 cells.

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Fig. 4.
Binding of LpL to sortilin in 3T3-L1
adipocytes. 35S-Labeled 3T3 cells were treated with
the cell-permeable and thiol-cleavable cross-linker,
dithiobis(succinimidyl propionate). After quenching of the reaction,
the cells were lysed followed by incubation with Sepharose-coupled
anti-sortilin. Lane 1 shows SDS-PAGE (8-16%) followed by
autoradiography of acid-eluted labeled proteins after cleavage of the
cross-linker by reduction. Some of the eluted material (before
reduction) was immunoprecipitated with chicken anti-LpL. Lane
2 shows the precipitated labeled proteins after cleavage of the
cross-linker. The bands were visualized by autoradiography of
2,5-diphenyloxazole-impregnated gels.
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Sortilin Can Mediate LpL Degradation--
We used stably
transfected CHO cells to elucidate functional consequences of LpL
binding to sortilin. The transfectants expressed sortilin in contrast
to the wild-type cells as seen from Western blotting of total lysates
(Fig. 5A, inset,
lane 2 versus lane 1). The receptor was
predominantly in the mature form because no reaction was observed when
using anti-serum directed against the N-terminal propeptide (not
shown). This is in agreement with the results of pulse-chase
experiments, demonstrating that all newly synthesized sortilin is
cleaved (18). To assess the fraction of sortilin expressed on the cell
surface, the transfectants were treated with the nonpermeable reagent
sulfo-N-hydroxysuccinimidobiotin at 4 °C and lysed, and
biotinylated proteins were recovered on streptavidin-Sepharose beads.
Sortilin was then detected by SDS-PAGE followed by Western blotting of
the bound material as compared with the sortilin content in the
fraction of the lysate not bound to streptavidin-Sepharose. Fig. 5,
inset, shows that biotinylated (lane 3) as well
as nonbiotinylated sortilin (lane 4) were readily detected.
Scanning densitometry of the Western blots (cf. legend to
Fig. 5) revealed that about 8% sortilin in the transfected cells had
been accessible to biotinylation and, thus, represents the fraction
expressed on the cell surface.

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Fig. 5.
Degradation of LpL in sortilin-transfected
CHO cells. A, the cells (1 × 105/well) were incubated at 37 °C in 300 µl of medium
with 125I-LpL (1.5 × 104 cpm/ml). The
ordinate shows the percent of the added tracer soluble in
trichloroacetic acid. A, the filled symbols
represent sortilin transfectants, and the open symbols
represent wild-type cells without (circles) and with
(triangles) 500 nM unlabeled LpL. The
inset shows Western blots of total lysates of wild-type
(lane 1) and transfected cells (lane 2).
Lane 3 shows the Western blot of biotinylated sortilin
recovered on streptavidin-Sepharose from the lysate of
surface-biotinylated cells. Lane 4 shows Western blot of the
remaining nonbiotinylated sortilin in the same lysate. The band shown
in lane 3 represents lysate from 11 times more cells than
the band shown in lane 4. B, degradation of
125I-LpL by the transfected cells incubated for 3 h
without additions, with RAP (6 µM), or with chloroquine
(100 µM). All values are means of triplicate
determinations ±1 S.D.
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We next aimed at determining LpL binding to the cell surface expressed
sortilin. However, 125I-labeled LpL bound readily and
equally well to wild-type and transfected CHO cells (not shown),
presumably to cell surface proteoglycans as shown previously (12).
Experiments were therefore performed to determine whether sortilin
mediates uptake and subsequent degradation of the surface-associated
LpL. As shown in Fig. 5A, the transfected cells degraded
125I-LpL much faster than wild-type cells. The
cell-mediated degradation was inhibited by 100 units/ml heparin (not
shown) and by 500 nM unlabeled LpL, indicating a saturable
mechanism. As shown in Fig. 5B, the degradation of LpL in
sortilin transfectants was inhibited by RAP and by the weak base
chloroquine that raises pH in intracellular compartments. We conclude
that sortilin expressed on the cell surface can interact with LpL and
mediate its degradation.
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DISCUSSION |
The results show that sortilin is multifunctional because it binds
LpL in addition to neurotensin, and they establish sortilin as an
endocytic receptor on the cell surface because it can mediate LpL
degradation. The affinity of LpL for binding to sortilin is similar to
that for binding to LRP. As also shown for LRP (11), the LpL monomer
has markedly reduced affinity, possibly because the dimeric state
provides the right conformation for receptor binding segments in the
C-terminal folding domain. Alternatively, it cannot be excluded that
the LpL dimer can interact with two sortilin molecules. We consistently
observed that small amounts of unlabeled s-sortilin facilitated
cross-linking of labeled sortilin to LpL (Fig. 1). Although this
phenomenon remains unexplained, it is possible that sortilin, like
heparin, can help stabilizing LpL in the dimeric form.
Sortilin is mainly located intracellularly in several cell types. In
transiently transfected COS-1 cells, full-length sortilin is mainly in
the Golgi compartment, and chimeric receptors consisting of the
C-terminal cytoplasmic tail of sortilin and the lumenal domain of the
interleukin-2 receptor colocalize with the IGF-II receptor (16). The
molecular basis for the predominant retrieval of sortilin to the Golgi
compartment is most likely the acidic cluster at the C terminus,
because an identical cluster in the IGF-II receptor, containing a
phosphorylatable serine residue, is important for the retrieval via
interaction with the newly identified protein PACS-1 (32). In neurons,
however, sortilin is up-regulated on the cell membrane following
neurotensin-induced sequestration of the neurotensin receptor-1 (17,
33), and in rat adipocytes and 3T3-L1-cultured adipocytes, the fraction of sortilin expressed on the surface is increased 1.7-fold by insulin
(19). Interestingly, both the localization and the insulin responsiveness of sortilin in adipocytes resembles that of the IGF-II
receptor, presumably because of the homology of important signal
sequences in their cytoplasmic tails (16). The two receptors are
therefore likely to be similar in terms of cycling among the Golgi,
endosomal, and plasma membrane compartments. The transfected CHO cells
described in this report express about 8% of sortilin on the cell
surface, in broad agreement with the fractional surface expressions of
sortilin and IGF-II receptors in adipocytes and of IGF-II receptors in
normal rat kidney cells (19, 34).
LpL is taken up by members of the LDL receptor family, particularly LRP
(9, 11), through contacts in the the C-terminal folding domain of the
LpL (27). The molecular basis for the interaction with sortilin appears
to be similar because a construct of LpL containing the segments
important for binding to LRP also bound to sortilin. In addition, RAP
domains D23 and LpL cross-compete for binding to sortilin, indicating
that the two ligands have the same or overlapping sites for binding
like on LRP (24). Previous studies using cultured cells have shown that
LpL, independent of its catalytic activity, can enhance lipoprotein
uptake by providing a bridge between lipoproteins and endocytic
receptors of the LDL receptor family (Refs. 11, 35, and 36; for review,
see Ref. 22), and LpL-facilitated lipoprotein uptake has recently been confirmed in vivo (37). Future studies should show whether
sortilin, via interaction with LpL, participates in receptor-mediated
lipoprotein uptake.
The result that cell surface-expressed sortilin can mediate degradation
of LpL may apply to other ligands, e.g. neurotensin (17,
33). However sortilin may also exhibit important functions in the Golgi
compartment in analogy with the established dual functions of the
IGF-II receptor as an endocytic and a sorting receptor. Because
sortilin is abundantly expressed in cell types that secrete LpL, future
studies should elucidate whether it is involved in sorting of newly
synthesized LpL to specialized compartments.
In conclusion, we find that sortilin binds LpL with an affinity
comparable with that of LRP and that the receptor can mediate uptake
and degradation of the ligand when expressed on the cell surface.