Magnetoferritin nanoparticles for targeting and visualizing tumour tissues

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Magnetoferritin nanoparticles for targeting and
visualizing tumour tissues
Kelong Fan1, Changqian Cao2, Yongxin Pan2,DiLu
1,DonglingYang
1, Jing Feng1,LinaSong
1,
Minmin Liang1*and Xiyun Yan1*
Engineered nanoparticles have been used to provide diagnos-
tic1–3, therapeutic4,5 and prognostic information6,7 about the
status of disease. Nanoparticles developed for these purposes
are typically modified with targeting ligands (such as anti-
bodies8–10, peptides11,12 or small molecules13) or contrast
agents14–16 using complicated processes and expensive
reagents. Moreover, this approach can lead to an excess of
ligands on the nanoparticle surface, and this causes non-
specific binding17–20 and aggregation of nanoparticles18–20,
which decreases detection sensitivity17–20. Here, we show that
magnetoferritin nanoparticles (M-HFn) can be used to target
and visualize tumour tissues without the use of any targeting
ligands or contrast agents. Iron oxide nanoparticles are encap-
sulated inside a recombinant human heavy-chain ferritin (HFn)
protein shell, which binds to tumour cells that overexpress
transferrin receptor 1 (TfR1). The iron oxide core catalyses
the oxidation of peroxidase substrates in the presence of hydro-
gen peroxide to produce a colour reaction that is used to visu-
alize tumour tissues. We examined 474 clinical specimens from
patients with nine types of cancer and verified that these nano-
particles can distinguish cancerous cells from normal cells with
a sensitivity of 98% and specificity of 95%.
Ferritin is an iron storage protein composed of 24 subunits made
up of the heavy-chain ferritin (HFn) and the light-chain ferritin.
Ferritin is spherical, with an outer diameter of 12 nm and interior
cavity diameter of 8 nm (ref. 21). The cavity has been used as a reac-
tion chamber to synthesize highly crystalline and monodisperse
nanoparticles through biomimetic mineralization within the
protein shell22–24. Recently, it was shown that HFn binds to
human cells via transferrin receptor 1 (TfR1)25. Because TfR1 is
overexpressed in tumour cells, this receptor has been used as a tar-
geting marker for tumour diagnosis and therapy26–29.
Current HFn-based tumour detection methods rely on functiona-
lization of HFn with recognition ligands30,31 and signal molecules31,32.
Previously, we have shown that iron oxide nanoparticles can catalyse
the oxidation of peroxidase substrates in the presence of hydrogen
peroxide to produce a colour reaction similar to that of natural per-
oxidases33. We hypothesize that magnetoferritin (M-HFn) nanopar-
ticles generated by encapsulating iron oxide nanoparticles inside a
HFn shell should be able to target TfR1 without any additional rec-
ognition ligands on their surface, and visualize tumour tissues
through the peroxidase activity of the iron oxide core (Fig. 1a).
We expressed recombinant human HFn in Escherichia coli. After
purification, the proteins were analysed by transmission electron
microscopy (TEM). The HFn protein shells had a well-defined mor-
phology and were monodisperse in size (Fig. 1b,c). After iron
loading and oxidation, a well-defined iron oxide core with an
average diameter of 4.7 nm was synthesized within the HFn
protein shell (Fig. 1d,e). The iron mineral core of M-HFn is com-
posed of magnetite or maghemite, as characterized in our previous
publication34. Cryoelectron transmission microscopy (cryoTEM)
analysis showed that the mineral cores were clearly encapsulated
within the HFn protein shells (Fig. 1b). Dynamic light scattering
(DLS) and size-exclusion chromatography (SEC) results further
confirmed that the M-HFn nanoparticles were monodispersed
with an outer diameter of 12–16 nm (Supplementary Fig. S1a),
and the iron loading did not significantly perturb the overall
protein cage architecture of HFn (Supplementary Fig. S1b,c).
M-HFn nanoparticles catalyses the oxidation of peroxidase sub-
strates 3,3,5,5-tetramethylbenzidine (TMB) and di-azo-aminoben-
zene (DAB) in the presence of H
2
O
2
to give a blue colour
(Fig. 1f) and brown colour (Fig. 1g), respectively, confirming that
M-HFn nanoparticles have peroxidase activity towards typical per-
oxidase substrates. The mineral phase composition of the iron core
determines the peroxidase activity of the M-HFn, as is evident from
Supplementary Fig. S2. M-HFn with mineral cores consisting of
magnetite or maghemite34 exhibited a much higher peroxidase
activity when compared with natural holoferritin. The natural
cores in holoferritin consist mainly of the hydrated iron oxide
mineral ferrihydrite (5Fe
2
O
3
.9H
2
O)35, which exhibits little peroxi-
dase activity. Apoferritin, without a mineral core, exhibited no
peroxidase activity.
The specificity of HFn binding to living cancer cells was investi-
gated using human breast, colon and liver cancer cell lines and
their corresponding xenograft tumours. HT-29 human colon
cancer cells and SMMC-7721 human liver cancer cells express TfR1
at high levels, but MX-1 human breast cancer cells do not express
this receptor (Fig. 2a). HFn bound to TfR1-positive HT-29 and
SMMC-7721 cells, and their xenograft tumours, but not to TfR1-
negative MX-1 or its xenograft tumour (Fig. 2b,c). The binding of
HFn to TfR1-positive cells was saturable, and could be inhibited by
adding an excess of unconjugated HFn (Supplementary Fig. S3a),
showing that HFn binding is specific. The saturation binding curve
and Scatchard analysis demonstrate that the K
d
value for HFn is
50 nM (Supplementary Fig. S3b), indicating that HFn has a high affi-
nity for TfR1. In addition, HFn showed significant binding to A375
melanoma cells, MDA-MB-231 breast cancer cells, K562 erythroleu-
kemia cells, HeLa cervical cancer cells, SKOV-3 ovarian cancer cells,
PC-3 prostate cancer cells, U251 glioblastoma cells, U937 histiocytic
lymphoma cells, SW1990 pancreatic cancer cells and Jurkat T-cell
leukemia cells (Supplementary Fig. S4), indicating that HFn has a
universal capability for recognizing cancer cells.
1Key Laboratory of Protein and Peptide Pharmaceutical, National Laboratory of Biomacromolecules, CAS-University of Tokyo Joint Laboratory of Structural
Virology and Immunology, Institute of Biophysics, Chinese Academy of Sciences, 15 Datun Road, Beijing 100101, China, 2Paleomagnetism and
Geochronology Laboratory, Key Laboratory of the Earth’s Deep Interior, Institute of Geology and Geophysics, Chinese Academy of Sciences,
Beijing 100029, China. *e-mail: mmliang@moon.ibp.ac.cn; yanxy@sun5.ibp.ac.cn
LETTERS
PUBLISHED ONLINE: 17 JUNE 2012 | DOI: 10.1038/NNANO.2012.90
NATURE NANOTECHNOLOGY | VOL 7 | JULY 2012 | www.nature.com/naturenanotechnology 459
© 2012 Macmillan Publishers Limited. All rights reserved.

To investigate whether TfR1 mediates the specific binding of
HFn to cancer cells, TfR1 was immunoprecipitated from HT-29
cell lysates. HFn reacted with the precipitated TfR1 (Fig. 2d), indi-
cating that HFn binds to cancer cells via TfR1. The next far-
Western blotting analysis showed that HFn bound to TfR1 in
HT-29 and SMMC-7721 cells, but not in MX-1 cells (Fig. 2e), con-
sistent with their TfR1 expression patterns (Fig. 2a) and confirming
that TfR1 is the receptor of HFn and mediates its specific binding to
cancer cells. Further flow cytometry and confocal results showed
that anti-TfR1 monoclonal antibody (mAb) could completely
inhibit the binding of FITC-labelled HFn to TfR1 on SMMC-
7721 cancer cells (Supplementary Fig. S5), indicating that TfR1 is
the only receptor for HFn present on these cancer cells. In addition,
although transferrin competed with HFn for binding to TfR1, excess
transferrin only partially inhibited HFn binding (Supplementary
Fig. S6), consistent with observations reported by Li and
Seaman
25
. These results suggest that HFn and transferrin share
receptor TfR1, but may bind to different epitopes on TfR1.
To establish the validity of the M-HFn nanoparticle-based cancer
diagnostic method, we carried out the following histological staining
experiments in xenograft tumours. FITC-conjugated HFn showed
strong fluorescence staining in HT-29, SKOV-3 and SMMC-7721
xenograft tumours (Fig. 3, top row), confirming the tumour-
binding reactivity of the HFn protein. After iron loading and
oxidation, M-HFn nanoparticles displayed an intensive brown per-
oxidase activity that visualized the tumour cells after adding DAB
substrate and H
2
O
2
(Fig. 3, middle row), verifying the feasibility
of our M-HFn nanoparticle-based cancer diagnostic method. The
fluorescence staining co-localized with mineral-peroxidase staining
in tumour cells (Supplementary Fig. S7), indicating that iron loading
and fluorescence labelling do not affect the tumour-binding activity
of the HFn protein. This again shows the feasibility of our M-HFn-
based assay for tumour detection.
Traditional immunohistochemical staining using anti-TfR1
antibodies (Abs) was next performed to compare its tumour-
binding specificity and staining quality with our M-HFn nanoparti-
cle-based method in xenograft tumour tissues. The intensity and the
pattern of M-HFn nanoparticle-based staining were almost the
same as that of immunohistochemical staining (Fig. 3, bottom
row), demonstrating the accuracy of tumour detection by the M-
HFn nanoparticles. TfR1-negative MX-1 tumour xenograft tissues
consistently showed negative staining for M-HFn nanoparticles,
FITC-conjugated HFn and TfR1 Abs (Fig. 3, right column),
further confirming that HFn targets tumour cells via TfR1.
To evaluate the potential clinical application of M-HFn nanopar-
ticles as a diagnostic agent for tumours in tissue specimens, we
screened 247 clinical tumour tissue samples and 227 corresponding
normal tissue samples by histological staining. Staining was con-
sidered positive when 10% or more of the tumour cells were
stained (cutoff, 10%). M-HFn did not stain, or only slightly
stained, normal or lesion tissues, with a staining frequency of only
4.8% (11/227, Table 1, Fig. 4). In tumour tissues, M-HFn
cd
b
TMB
2
0
40
80
120
160
200
3456
n = 700
4.7 ± 0.8 nm
78
Diameter (nm)
Number of particles
TMB/H2O2/M-HFn
fg
DAB DAB/H2O2/M-HFn
e
20 nm 20 nm 20 nm
a
HFn
protein shell
H2O2
M-HFn
nanoparticle
Iron
core
Fe2+
Figure 1 | Preparation and characterization of M-HFn nanoparticles. a, Schematic showing the preparation of M-HFn nanoparticles and their structure.
b, CryoTEM image of M-HFn nanoparticles. c,d, TEM images of HFn protein shells (c) and iron oxide cores (d). HFn protein shells were negatively stained
with uranyl acetate for TEM observations and iron oxide cores in HFn were unstained. e, Size distribution of iron oxide cores, with a median diameter of
4.7+0.8 nm. f,g, Characterization of peroxidase activity of M-HFn nanoparticles. M-HFn catalysed the oxidation of peroxidase substrates TMB (f)andDAB
(g) in the presence of H
2
O
2
to give a coloured product.
LETTERS NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2012.90
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© 2012 Macmillan Publishers Limited. All rights reserved.

nanoparticles strongly stained tumour cells, and a clear distinction
was seen between cancerous cells and adjacent normal cells in repre-
sentative sections (Fig. 4). M-HFn staining showed a sensitivity of
98% across nine types of cancer represented by 247 tumour
tissues (Table 1). These clinical tissue specimens were further
stained with FITC-conjugated HFn protein shells. As expected, con-
sistent staining patterns were observed (Fig. 4), verifying the diagno-
sis based on M-HFn nanoparticles. Importantly, HFn showed a
distinct staining reaction in different grades and growth patterns
of hepatocellular carcinoma, lung squamous cell carcinoma, cervical
squamous cell carcinoma, prostate adenocarcinoma, ovarian serous
papillary carcinoma and colonic adenocarcinoma (Supplementary
Fig. S8–S13), demonstrating that HFn has an impressive ability to
discriminate tumour cells from normal cells, and thus has clinical
potential in cancer diagnosis.
To understand the mechanism of the M-HFn-based peroxidase-
like reaction, the formation of OH
†
during the reaction was
measured using electron spin resonance. OH
†
was produced
during the peroxidase-like reaction in the presence of both
M-HFn nanoparticles and H
2
O
2
(Supplementary Fig. S14a,b).
With the addition of an OH
†
scavenger (ethanol), the formed
OH
†
disappeared (Supplementary Fig. S14 c) and the peroxidase
activity of the M-HFn nanoparticles decreased to 20% of the original
activity (Supplementary Fig. S14d,e), indicating that the OH
†
formed during the peroxidase-like reaction is responsible for the
catalytic oxidation of peroxidase substrate that gives the coloured
precipitate at the site of its target.
Based on these results, we propose the following reaction mech-
anism. With the addition of H
2
O
2
and peroxidase substrate to the
M-HFn reaction solution, H
2
O
2
diffuses into the ferritin cavity
through its hydrophilic channels and interacts with the iron oxide
core of M-HFn to generate OH
†
on the surface of the iron core.
The generated OH
†
then oxidizes nearby peroxidase substrates
(for example, DAB) to form an insoluble coloured precipitate at
the site of the M-HFn, which is targeted to cancer cells. The
coloured precipitates are only formed at the site of the M-HFn
because OH
†
radicals are highly reactive and short-lived, and can
only oxidize nearby substrates. The clear boundary between
tumour and normal tissues on M-HFn-stained tissue slides
(Supplementary Fig. S15) also demonstrates that the coloured pre-
cipitates are generated right at the site of the M-HFn-targeted
cancer cells and do not diffuse away from their targets.
Achieving rapid, low-cost and sensitive cancer diagnosis remains
a challenge due to the complexities of this disease. Our studies show
that one-step tumour targeting and visualization with low-cost and
mass-produced M-HFn nanoparticles is feasible for convenient and
sensitive monitoring and analysis of tumour cells in tissue speci-
mens. The recombinant human HFn protein shell has tumour-
specific binding properties, and the encapsulated iron oxide core
has strong peroxidase activity, which allows us to combine effective
tumour cell recognition in clinical tissues with highly sensitive stain-
ing of the targeted cells. Staining results for 247 clinical tumour
tissue samples from patients with ovarian, liver, prostate, lung,
breast, cervical, thymus colorectal or oesophageal cancers, as well
as for 227 normal and lesion tissue control samples (Table 1,
Fig. 4), clearly demonstrate the capacity of M-HFn nanoparticles
to distinguish cancer cells from normal ones in tissue specimens.
Compared with conventional antibody-based histological
methods for cancer detection in clinics, our novel M-HFn nanopar-
ticle-based method has the following advantages. First, it has high
a
TfR1
β-actin
IP mIgGα-TfR1
TfR1
HFn
WB
d
c
b
Counts
HT-29
Counts
SMMC-7721
SMMC-7721
HT-29
MX-1
SMMC-7721
HT-29
MX-1
Counts
MX-1
150
120
90
60
30
0
100
150
120
90
60
30
0
101102103104100101102103104
150
120
90
60
30
0
100101102103104
MX-1 HT-29 SMMC-7721
Fluorescence intensity Fluorescence intensity Fluorescence intensity
95 kDa
17 kDa
55 kDa
43 kDa
130 kDa
170 kDa
72 kDa
e
TfR1
Figure 2 | HFn binds specifically to TfR1 in cancer cells. a, Western blot of TfR1 expression in HT-29 colon cancer cells, SMMC-7721 liver cancer cells and
MX-1 breast cancer cells. b-actin was used as a loading control. b, Flow cytometric analysis of the specific binding of HFn to HT-29, SMMC-7721 and MX-1
cancer cells. c, Fluorescence staining of HT-29, SMMC-7721 and MX-1 xenograft tumours incubated with FITC-conjugated HFn (scale bars, 50
m
m). d,TfR1
immunoprecipitated from HT-29 cell lysates by anti-TfR1 mAbs (a-TfR1) was recognized by HFn, detected by mouse anti-HFn mAbs and visualized with
HRP-coupled anti–mouse IgG. e, Far-Western blotting of HT-29, SMMC-7721 and MX-1 cell line lysates was performed using HFn, detected by mouse
anti-HFn mAbs and visualized with HRP-coupled anti-mouse IgG. TfR1 represented an 95 kDa band.
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sensitivity and specificity. Screening 474 clinical specimens from
patients with nine types of cancer shows that the M-HFn nanopar-
ticle-based method has a sensitivity of 98% and a specificity of 95%
(Table 1), which is much better than most antibody-based
histological detection methods reported in the literature
(Supplementary Table S1). In addition, a side-by-side comparison
of the standard antibody-based immunohistochemistry and our
M-HFn-based approach in two hepatocellular carcinoma cases ident-
ified by pathologists also shows that our M-HFn-based approach
performs much better than anti-TfR1 antibody-based immunohisto-
chemistry when distinguishing tumours from normal tissues
(Supplementary Fig. S16). The second advantage of our method is
its high accuracy, credibility and repeatability. Traditional immuno-
histochemistry involves multiple manipulation steps. The results are
easily affected by the proficiency and subjectivityof the manipulators.
By using a one-step incubation of one reagent in our M-HFn nano-
particle-based method, the accuracy, credibility and repeatability of
the results are improved greatly. Our method also has the advantage
of a rapid examination time, taking 1 h, rather than the 4 h required
for immunohistochemistry, which generally involves multistep incu-
bation of primary antibody, secondary antibody or enzyme-labelled
third antibody. Finally, it is low in cost, avoiding the use of expensive
and unstable antibodies, and the HFn can be produced in Escherichia
coli at high yield. M-HFn nanoparticles can be low in cost and mass-
produced by simply oxidizing Fe
2þ
within HFn by H
2
O
2
.
M-HFn-based peroxidase staining and FITC-HFn-based fluor-
escence staining target tumour cells via the same HFn protein, but
visualize them using chromogenic and fluorescence signals, respect-
ively. Peroxidase staining is clearly better for clinical diagnostics
because it is compatible with haematoxylin counterstains, which
allows visualization of the context of a tumour’s expression
pattern and provides more detailed histopathological information,
allowing diagnostic features to be easily discerned. Fluorescence
staining, as with most fluorescence techniques, has two significant
FITC-conjugated
HFn
HT-29 SKOV-3
M-HFn
nanoparticles
Anti-TfR1 Abs
SMMC-7721 MX-1
Figure 3 | M-HFn nanoparticle staining of tumour tissues. FITC-conjugated HFn-based fluorescence staining (top row), M-HFn nanoparticle-based
peroxidase staining (middle row) and anti-TfR1 Abs-based immunohistochemical staining (bottom row) of paraffin-embedded HT-29 colon cancer, SKOV-3
ovarian cancer, SMMC-7721 liver cancer and MX-1 breast cancer xenograft tumours. TfR1-positive xenograft tumours showed strong positive staining for
FITC-conjugated HFn (green fluorescence), M-HFn nanoparticles (brown) and anti-TfR1 Abs (brown), whereas TfR1-negative xenograft tumours showed no
staining for FITC-conjugated HFn, M-HFn nanoparticles and anti-TfR1 Abs (scale bars, 100
m
m).Abs,antibodies.
Table1 | Histological analysis of M-HFn nanoparticle staining
of tumours in clinical tissue specimens.
Tumour tissues Positive/cases
(sensitivity)
Normal
tissues
Negative/cases
(specificity)
Hepatocellular
carcinoma
54/55 (98%) Liver 39/45 (87%)
Lung squamous
cell carcinoma
50/52 (96%) Lung 55/56 (98%)
Colonic
adenocarcinoma
23/23 (100%) Colon 40/41 (98%)
Cervical
squamous cell
carcinoma
28/28 (100%) Cervix 52/52 (100%)
Prostate
adenocarcinoma
22/22 (100%) Prostate 14/16 (88%)
Ovarian serous
papillary
carcinoma
55/56 (98%) Ovary 4/4(100%)
Breast ductal
carcinoma
3/4 (75%) Breast 4/4(75%)
Thymic carcinoma 4/4 (100%) Thymus 4/4(100%)
Oesophagus
squamous cell
carcinoma
2/3 (67%) Oesophagus 5/5(100%)
To t a l 2 4 1 /247 (98%) Total 216/227 (95%)
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problems—photobleaching and autofluorescence—which severely
limits the detection sensitivity of FITC-HFn-based fluorescence
staining. This study suggests that the easily synthesized M-HFn
nanoparticles have the potential to become a diagnostic tool for
rapid, low-cost and universal assessment of cell cancerization.
Methods
Conjugation of HFn and M-HFn. HFn and M-HFn were labelled by fluorescein
isothiocyanate (FITC, Sigma) using the following procedure. FITC was dissolved in
dry dimethylformamide and added to an HFn or M-HFn solution in 0.1 M
NaHCO
3
, pH 8.5, at an FITC to HFn or M-HFn molar ratio of 10:1. The reaction
solution was gently stirred for 3 h at room temperature in the dark and then purified
on a polyacrylamide column (Thermo Scientific, MWCO 6000) using 0.1 M PBS,
pH 7.5, as an eluant to remove free dyes. The labelled FITC concentration was
determined by measuring the absorbance at 492 nm, and the HFn or M-HFn
concentration was determined using a BCA protein assay reagent kit.
Cell binding studies. The reactivity of HFn with cancer cell lines was assessed by
flow cytometry. Briefly, 100 ml detached cell suspensions (1 ×10
6
cells per ml) were
stained with 20 mgml
21
of FITC-conjugated HFn for 2 h at 4 8C in PBS containing
0.3% bovine serum albumin. After three washes in cold PBS, cells were analysed
immediately using a FACSCalibur flow cytometry system (Becton Dickinson).
Western blotting, far-Western blotting and immunopreciptation. TfR1
expression was assessed by Western blotting. Cell lysates of each type were run on a
10% SDS–polyacrylamide gel and transferred to a nitrocellulose membrane blocked
with 5% non-fat milk, 0.1% Tween 20 in PBS for 30 min, and then incubated
overnight at 4 8C with a 1:2,000 dilution of mouse anti-human TfR1 monoclonal
antibody (mAbs, BD Bioscience). The TfR1 mAbs was detected using a 1:6,000
dilution of goat anti-mouse IgG conjugated to HRP (Pierce), and developed with
ECL substrate (Pierce).
Immunopreciptation was performed to confirm that TfR1 is the binding
receptor of HFn. Cell lysates were pre-cleared by incubation with 15 ml protein A/G
agarose (Santa Cruz Biotech), centrifuged to remove the beads, and then incubated
at 4 8C overnight with 4 ml of 0.5 mg ml
21
mouse anti-human TfR1 mAbs or 1 mlof
2mgml
21
normal mouse IgG (Sigma) as a control, followed by incubation with
15 ml protein A/G agarose for 1 h at room temperature. The precipitated complexes
were boiled for 15 min, analysed by 10% SDS–polyacrylamide gel and then
transferred to a nitrocellulose membrane. After blocking with non-fat dry milk, the
nitrocellulose membranes were probed with a 1:2,000 dilution of anti-TfR1 mAbs or
with 3.8 mgml
21
HFn protein followed by a 1:2,000 dilution of mouse anti-HFn
mAbs, and developed with HRP-conjugated anti-mouse IgG.
Far-Western blotting was performed to analyse the binding pattern of HFn to
cancer cells. Lysates from theHT-29, SMMC-7721 and MX-1 cell lines were run ona
10% SDS–polyacrylamide gel and transferred to a nitrocellulose membrane, blocked
in non-fat dry milk for 30 min, and then incubated overnight at 4 8C with HFn
(3.8 mgml
21
). HFn was detected using a 1:2,000 dilution of mouse anti-HFn mAbs
(Santa Cruz Biotech), and developed with HRP-conjugated anti-mouse IgG.
Staining of tumour xenografts and clinical specimens. Paraffin-embedded tissue
sections were deparaffinized by washing twice in xylene for 10 min and then
hydrated progressively using an ethanol gradient. Endogenous peroxidase activity
was quenched by incubation with 0.3% H
2
O
2
in methanol for 30 min. After rinsing,
the tissue sections were boiled in 10 mM citrate buffer (pH 6.0) at 100 8C for 30 min,
cooled to room temperature, blocked with 5% goat serum in PBS for 1 h at 37 8C,
washed and then incubated with M-HFn nanoparticles (1.8 mM) for 1 h at 37 8C,
and then rinsed in PBS. Freshly prepared DAB was added for colour development.
All samples were counterstained with haematoxylin (blue stain). The stained
sections were analysed under a microscope and the results were expressed in terms of
the percentage of stained tumour cells (0 to 100%). If 10% or more of the tumour
cells were stained, the slide was scored as positive. Two independent pathologists
who were blind to all clinical information scored all specimens.
Fluorescence staining of tissue sections was performed to confirm the binding
specificity of HFn to cells. After blocking in serum, tissue sections were incubated
with FITC-conjugated HFn (1 mM) at 4 8C overnight. The stained tissues were
examined under a confocal laser scanning microscope (Olympus).
Immunohistochemical staining of tissue sections by anti-TfR1 Abs was
performed to compare tumour-binding specificity and staining quality with M-HFn
nanoparticles. Briefly, after blocking in serum, tissue sections were incubated at 4 8C
overnight with a 1:300 dilution of polyclonal rabbit anti-TfR1 antibody (Abcam).
The bound antibody was detected by incubating the tissues with a 1:1,000 dilution of
biotinylated anti-rabbit antibody (Santa Cruz Biotech) at 37 8C for 1 h, then with a
1:200 dilution of HRP-conjugated streptavidin (Pierce) for 40 min.
Received 27 February 2012; accepted 7 May 2012;
published online 17 June 2012; corrected after print 22 October 2012;
corrected after print 27 November 2012
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Acknowledgements
This work was partially supported by grants from the National Science and Technology
Major Project (2012ZX10002009-016), the Knowledge Innovation Program of the Chinese
Academy of Sciences (KJCX2-YW-M15), 973 Program (2011CB933500, 2012CB934003),
and the National Defense Science and Technology Innovation Fund of Chinese Academy of
Sciences (CXJJ-11-M61).
Author contributions
M.L. conceived and designed the experiments. K.F. and M.L. performed the experiments.
M.L. and X.Y. reviewed, analysed and interpreted the data. C.C. and Y.P. synthesized the
nanoparticles. D.L., D.Y., J.F. and L.S. cultured the cancer cells. M.L. wrote the paper.
All authors discussed the results and commented on the manuscript.
Additional information
The authors declare no competing financial interests. Supplementary information
accompanies this paper at www.nature.com/naturenanotechnology.
Reprints and
permission information is available online at http://www.nature.com/reprints. Correspondence
and requests for materials should be addressed to M.L. and X.Y.
LETTERS NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2012.90
NATURE NANOTECHNOLOGY | VOL 7 | JULY 2012 | www.nature.com/naturenanotechnology464
© 2012 Macmillan Publishers Limited. All rights reserved.

Editorial note: A potential breach of our editorial policies has emerged following the publication of this paper. Yongxin Pan and
Changqian Cao, who are listed as contributing authors, have declared no knowledge of the submission of the paper, and their
disagreement with its publication. ey have pointed out several errors in the description of the synthesis and characterization of the
magnetoferritin nanoparticles used in the study. We take breaches of editorial policies very seriously and have therefore informed the
listed authors’ institutions and the Chinese Academy of Sciences.
Magnetoferritin nanoparticles for targeting and visualizing tumour tissues
Kelong Fan, Changqian Cao, Yongxin Pan, Di Lu, Dongling Yang, Jing Feng, Lina Song, Minmin Liang and Xiyun Yan
Nature Nanotechnology 7, 459–464 (2012); published online 17 June 2012; corrected aer print 22 October 2012.
ADDENDUM
© 2012 Macmillan Publishers Limited. All rights reserved

Editorial note: Following an investigation by the Chinese Academy of Sciences under the directive of Professor Chunli Bai, the investi-
gating committee consisting of Professor Tao Xu, Professor Rixiang Zhu, Professor Jinghui Guo and Professor Ruiming Xu concluded
that,owing to lack of communication between the authors, the original paper published online on 17 June 2012 contained errors but
the central ndings of the paper remain valid. e authors have reconciled their opinions and relevant sections of the paper have now
been corrected through a Corrigendum.
Magnetoferritin nanoparticles for targeting and visualizing tumour tissues
Kelong Fan, Changqian Cao, Yongxin Pan, Di Lu, Dongling Yang, Jing Feng, Lina Song, Minmin Liang and Xiyun Yan
Nature Nanotechnology 7, 459–464 (2012); published online 17 June 2012; corrected aer print 22 October 2012;
corrected aer print 27 November 2012.
ADDENDUM
In the version of this Letter originally published, Fig.1a was incorrect and in Fig.1c the wrong TEM image was used, they should have
appeared as shown below.
In the Supplementary Information, one of the authors was not mentioned in the author list: Lina Song has now been added. In the sec-
tion ‘Preparation and characterization of M-HFn particles’ the column used for size-exclusion chromatography was incorrect: it should
have been ‘Sepharose 6B’. e synthesis procedure for M-HFn nanoparticles was incorrect: it should have read ‘HFn protein shells were
used as a reaction template to synthesize iron oxide nanoparticles according to the method reported by Cao etal.2 with some modi-
cation. e solution of 50ml 100mM NaCl with HFn (1mgml−1) was added to the reaction vessel, synthesized at 65°C and pH8.5.
Fe(II) (25mM (NH4)2Fe(SO4)2•6H2O) and stoichiometric equivalents (1:3 H2O2:Fe2+) of freshly prepared H2O2 (8.33mM) were added.
Fe(II) was added in a rate of 100Fe/(protein min) using a dosing device (800Dosino) connected with 842 Titrando. Aer theoretical
5000Fe/ protein cage were added to the reaction vessel, the reaction was continued for another 5min. Finally, 200μl of 300mM sodium
citrate was added to chelate any free iron. e synthesized magnetite-containing HFn (M-HFn) nanoparticles were centrifuged and
puried through size exclusion chromatography to remove the aggregated nanoparticles. e concentration of M-HFn nanoparticles
was assumed to be the same as that of HFn protein and was determined using a BCA protein assay kit (Pierce). Puried M-HFn nano-
particles were obtained with a yield of about 75%.’ Reference2was incorrect and should have read Cao, C. Q. etal. Magnetic characteri-
zation of noninteracting, randomly oriented, nanometer-scale ferrimagnetic particles. J.Geophys. Res. 115, B07103 (2010).
e aforementioned errors did not aect the main conclusions of the paper. ese errors have now been corrected in the HTML and
PDF versions.
Magnetoferritin nanoparticles for targeting and visualizing tumour tissues
Kelong Fan, Changqian Cao, Yongxin Pan, Di Lu, Dongling Yang, Jing Feng, Lina Song, Minmin Liang and Xiyun Yan
Nature Nanotechnology 7, 459–464 (2012); published online 17 June 2012; corrected aer print 22 October 2012;
corrected aer print 27 November 2012.
CORRIGENDUM
HFn
protein shell
H2O2
M-HFn
nanoparticle
Iron
core
Fe2+
Fig.1a. Schematic showing the preparation of M-HFn nanoparti-
cles and their structure.
Fig.1c. TEM image of HFn protein shells.
20 nm
© 2012 Macmillan Publishers Limited. All rights reserved.