Research Article
Angiogenin Is Translocated to the Nucleus of HeLa Cells and Is
Involved in Ribosomal RNA Transcription and Cell Proliferation
Takanori Tsuji, Yeqing Sun, Koji Kishimoto, Karen A. Olson, Shumei Liu, Saori Hirukawa,
and Guo-fu Hu
Center for Biochemical and Biophysical Sciences and Medicine, Department of Pathology, Harvard Medical School, Boston, Massachusetts
Abstract
Angiogenin is an angiogenic protein known to play a role in
rRNA transcription in endothelial cells. Nuclear translocation
of angiogenin in endothelial cells decreases as cell density
increases and ceases when cells are confluent. Here we report
that angiogenin is constantly translocated to the nucleus of
HeLa cells in a cell density–independent manner. Downregulation of angiogenin expression by antisense and RNA
interference results in a decrease in rRNA transcription,
ribosome biogenesis, proliferation, and tumorigenesis both
in vitro and in vivo. Exogenous angiogenin rescues the cells
from antisense and RNA interference inhibition. The results
showed that angiogenin is constitutively translocated into the
nucleus of HeLa cells where it stimulates rRNA transcription.
Thus, besides its angiogenic activity, angiogenin also plays a
role in cancer cell proliferation. (Cancer Res 2005; 65(4): 1352-60)
Introduction
Angiogenin is a 14-kDa angiogenic protein originally isolated
from the conditioned medium of HT-29 human colon adenocarcinoma cells based on its angiogenic activity (1). Angiogenin has
been shown to play a role in tumor angiogenesis (2, 3). Its
expression is up-regulated in many types of cancers including
breast (4), cervical (5), colon (6), colorectal (7), endometrial (8),
gastric (9), kidney (10), ovarian (11), pancreatic (12), prostate (13),
and urothelial (14) cancers, as well as astrocytoma (15), leukemia
(16), lymphangioma (17), melanoma (18), Wilms tumor (19), and
others.
Angiogenin undergoes nuclear translocation in endothelial cells,
which has been shown to be necessary for angiogenesis. Inhibition of nuclear translocation of angiogenin (20) or mutagenesis at
its nuclear localization sequence (21) both abolished the angiogenic activity. Nuclear translocation of angiogenin in endothelial cells is rapid (22) and independent of microtubules and
lysosomes (23), but is strictly dependent on cell density (22).
It decreases as cell density increases and ceases when cells are
confluent. No nuclear angiogenin can be detected in normal,
non–blood vessel cells regardless of the cell density. Consistently,
angiogenin does not induce any detectable cellular response in
Note: T. Tsuji and Y. Sun contributed equally. Y. Sun is currently in the Institute of
Environmental Science and Systems Biology, Dalian Maritime University, Dalian
116026, China. K. Kishimoto is currently in the Department of Oral and Maxillofacial
Surgery, Okayama University Graduate Schools, Okayama 700-8525, Japan. K.A. Olson
is currently in Archemix Corp., 1 Hampshire Street, Cambridge, MA 02139.
Requests for reprints: Guo-fu Hu, Department of Pathology, Harvard Medical
School, 77 Avenue Louis Pasteur, MA 02115. Phone: 617-432-6582; Fax: 617-432-6580;
E-mail: [email protected].
#2005 American Association for Cancer Research.
Cancer Res 2005; 65: (4). February 15, 2005
epithelial cells and fibroblasts. These results suggested that the
nuclear function of angiogenin in normal cells is specific for endothelial cells and occurs only when cells are not confluent.
Angiogenin interacts with endothelial cells to induce a wide
range of cellular response including migration (24), proliferation
(25), and tube formation (26). All these are necessary steps in the
process of angiogenesis. The activity of angiogenin is relatively low
when compared with that of classic endothelial cell mitogens such
as vascular endothelial growth factor and basic fibroblast growth
factor. However, angiogenin has a comparable angiogenic activity
in various in vivo angiogenesis assays (1, 27). Moreover,
angiogenin antagonists potently inhibited the establishment,
progression, and metastasis of human cancer cells inoculated in
athymic mice (2, 3). These results suggested that angiogenin has a
distinct, yet to be uncovered role in angiogenesis and tumorigenesis. Recently, we discovered that angiogenin is able to bind to the
rRNA gene and stimulate rRNA transcription (28). An angiogeninbinding element has been identified from rDNA and we have
shown that this DNA sequence has angiogenin-dependent
promoter activity (29), suggesting that the function of nuclear
angiogenin is related to rRNA transcription.
Cancer is characterized by sustained cell growth requiring
continuous protein synthesis that depends on a constant supply of
ribosomes (30). Ribosome biogenesis is a multistep process
involving assembly of ribosomal proteins and rRNA in an equal
molar ratio. During tumorigenesis, the transcription of ribosomal
proteins is known to be up-regulated through the Akt-PI3K-mTORS6K pathway. However, it is less clear how rRNA transcription is
up-regulated. Here we report that angiogenin is continuously
translocated into the nucleus of HeLa cells in a cell density–
independent manner. Down-regulation of angiogenin expression
inhibited rRNA transcription, ribosome biogenesis, cell proliferation, and tumorigenesis.
Materials and Methods
Cell Culture. Human umbilical vein endothelial cells (HUVEC) were
cultured in human endothelial serum-free basal growth medium + 5% fetal
bovine serum (FBS) and 5 ng/mL basic fibroblast growth factor. HeLa cells
were cultured in DMEM + 10% FBS. Cell viability was determined by trypan
blue exclusion method. Cell number was determined with a Coulter
counter.
Nuclear Translocation of Angiogenin. HUVEC and HeLa cells were
seeded at various densities on a coverslip for 24 hours, washed with serumfree medium and incubated with 1 Ag/mL angiogenin at 37jC for 1 hour.
Cells were washed with PBS, fixed with methanol at 20jC for 10 minutes,
and washed again with PBS containing 30 mg/mL bovine serum albumin.
The fixed cells were then incubated with 50 Ag/mL of anti-angiogenin
monoclonal antibody 26-2F for 1 hour, washed, and incubated with Alexa
488–labeled goat F(abV)2 anti-mouse IgG at 1:100 dilution for 1 hour. The
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Angiogenin Plays a Role in HeLa Cell Proliferation
cells were then washed, mounted in 50% glycerol, and monitored with
a Nikon Labphot epifluorescent microscope.
[3H]Thymidine and [3H]Uridine Incorporation. Cells were cultured
for 24 hours, serum starved for another 24 hours, stimulated with 10% FBS,
and continuously labeled with 1 ACi/mL [3H]thymidine or [3H]uridine for
24 hours. The cells were washed with PBS, precipitated with 10%
trichloroacetic acid, and solubilized with 0.2 N NaOH plus 0.2% SDS. After
neutralization with 0.2 volume of 1 N HCl, the radioactivity was determined
by liquid scintillation counting.
Northern Hybridization. The probe used for 45S rRNA (5V-GGTCGCCAGAGGACAGCGTGTCAG-3V) hybridizes with the first 25 nucleotides of the
45S rRNA (31). The probe used for actin (5V-CTCTGTGCTCGCGGGGCGGAC-3V) hybridizes with the 5V end of cytoplasmic h-actin
mRNA. The probe used for angiogenin was a 569-bp DNA fragment
containing the complete 441-bp angiogenin coding sequence and 44 bp
from the 5V and 15 bp from the 3V noncoding regions.
ELISA Detection of Angiogenin. ELISA plates were coated with 1 Ag
26-2F and blocked with 5 mg/mL bovine serum albumin in PBS. Samples
were added in triplicates and the plates were incubated at 4jC overnight,
washed with PBS, and incubated with 100 AL/well anti-angiogenin
polyclonal antibody R112 (1:4,000) at room temperature for 2 hours. After
washing with PBS, an alkaline phosphatase–labeled goat anti-rabbit
antibody (1.25 Ag/mL) was added and incubated at room temperature
for 1 hour. The plate was washed with PBS and p-nitrophenyl phosphate
(5 mg/mL, 100 AL/well) dissolved in 0.1 mol/L diethanolamine containing
10 mmol/L MgCl2 (pH 9.8) was added. After 1-hour incubation at room
temperature, the absorbance was measured at 410 nm with a turbidity
reference at 630 nm. A standard curve (50-1,000 pg angiogenin per well)
was done each time on every plate.
Silver Staining of Nucleolar Organizer Region. Cells cultured on
a coverslip were fixed with methanol/acetic acid (9:1, v/v) at room
temperature for 10 minutes and incubated in 60 mmol/L NaAc (pH 4.1)
containing 0.8 g/mL AgNO3, 15% formaldehyde, and 3% methanol at 37jC
in the dark for 30 minutes. The slip was washed with water, mounted on
a glass slide, and observed at 1,000 magnification. nucleolar organizer
region (NOR) dots were counted in 30 randomly selected nuclei.
[32P] Labeling of Newly Synthesized RNA. Cells were pulse-labeled
with 25 ACi/mL [32P]Pi for 2 hours and washed with PBS. Total cellular RNA
was extracted with Trizol. Equal amounts of RNA were applied for agarose/
formaldehyde gel electrophoresis and transferred onto a nylon membrane.
Radiolabeled RNA was visualized by autoradiography.
Stable Transfection of HeLa Cells with Angiogenin Antisense Vector.
The entire coding region of the human angiogenin cDNA was amplified
from pAngC plasmid and cloned into the pCI-neo vector in the antisense
orientation. The pCI-Ang( ) plasmid was transfected into HeLa cells using
Lipofectin and stable transfectants were selected with 2 mg/mL G418.
Integration of the transfected gene into chromosome was confirmed by
genomic DNA PCR with the forward primer (5V-AGTACTTAATACGACTCACTATAGGC-3V) from the T7 sequence of the pCI vector and the reverse
primer (5V-ATGCAGGATAACTCCAGGTACAC-3V) from the inserted angiogenin sequence. Transcription of the antisense human angiogenin mRNA
was confirmed by reverse transcription–PCR using the same set of
primers.
Stable Transfection of HeLa Cells with a Plasmid Containing
Angiogenin RNA Interference Cassette. Three regions corresponding
to the nucleotides 106 to 126, 122 to 142, 381 to 401, respectively, of the
angiogenin mRNA were originally selected for small interfering RNA
targeting. Double-stranded, 21-nucleotide-long RNA with 2-nucleotide
overhang at the 3V end were synthesized and transfected into HeLa cells
to test the efficiency of these small interfering RNA in inhibiting angiogenin
expression. ELISA showed that the third region (381-401) with the sequence
of 5V-GGTTCAGAAACGTTGTTGTTA-3V was most effective in reducing
angiogenin expression. This sequence was therefore used to construct an
angiogenin RNA interference (RNAi) plasmid in the pBS/U6 vector
according to the method of Sui et al. (32). This plasmid (pAng-RNAi) and
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pBabe-puro were cotransfected into HeLa cells in the presence of Lipofectin
and the stable transfectants were selected with 0.5 Ag/mL puromycin for
2 weeks.
Soft Agar Assay. Cells were seeded at a density of 4 103 cells per
35-mm cell culture dishes in 0.33% agar and cultured for 7 days. Dishes
were stained with crystal violet solution (0.05%) overnight at 4jC. Colonies
were counted in 10 fields at 25 magnification. Two-tailed Student’s t test
was used to verify the differences between the groups.
Xenografic Growth of HeLa Cell Tumors in Athymic Mice. pCI or pCIAng( ) transfectants, 8 105 cells per mouse, were injected s.c. into the
left shoulders of male athymic mice (eight mice per group). Tumor sizes
were measured with a microcaliper and recorded in cubic millimeters
(length width2). Mice were sacrificed on day 13 and the wet weight of the
tumor was recorded.
Immunohistochemistry. Dako’s (Carpinteria, CA) Envision system was
used. Neovesssels were stained with an anti–von Willebrand’s factor IgG at
a 1:200 dilution. von Willebrand’s factor–possitive vessels in each tumor
were counted in five most vascularized areas at 200 magnification.
Proliferating cells were stained with an anti–proliferating cell nuclear
antigen (PCNA) IgG. Angiogenin was stained with 26-2F as the primary
antibody at the concentration of 10 Ag/mL.
Results
Angiogenin Is Constitutively Translocated to the Nucleus of
HeLa Cells. Immunofluorescent staining showed that nuclear
translocation of exogenous angiogenin in HUVECs occurred only
when cells were not confluent. Bright nuclear/nucleolar staining
was detected when cells were cultured at 5 103 cells/cm2.
Virtually no nuclear angiogenin was observed when cells were
cultured at a density exceeding 1 105 cells/cm2 (Fig. 1A).
However, angiogenin was detected in the nuclei of HeLa cells
cultured under the densities ranging from 5 103 to 2 105 cells/
cm2 (Fig. 1A).
The primary antibody (26-2F) used in this experiment is specific
for human angiogenin (33). It does not recognize angiogenin from
other species including bovine, porcine, rabbit, and mouse. X-ray
structural analysis of angiogenin-antibody complex has shown that
26-2F interacts with two segments consisting of residues 34 to 41
and 85 to 91, respectively (34). These two regions are apart in the
primary but close in the 3-dimentional structures. No fluorescence
was observed when 26-2F was replaced by a nonimmune IgG (data
not shown). Western blotting analysis confirmed that comparable
amounts of angiogenin protein were detected when equal amounts
of nuclear proteins, extracted from HeLa cells cultured under
various densities, were applied. However, in HUVECs, angiogenin
was detectable only in the nuclear proteins extracted from
nonconfluent cells (Fig. 1B). Fig. 1C shows that the nuclei isolated
from HeLa cells cultured under various densities responded to
angiogenin in transcribing 45S rRNA in a nuclear run-on assay.
We have also found that angiogenin is translocated to the nucleus
in other types of cancer cells including MB231 breast, PC-3
prostate, and HT-29 colon cancer cells (data not shown).
These results suggested that nuclear translocation of angiogenin
occurs in HeLa cells regardless of the cell confluence status and
that angiogenin stimulates rRNA synthesis in HeLa nuclei. They
also indicated that HeLa cell nuclei are not yet saturated with
endogenous angiogenin because they can still be stimulated to
synthesize rRNA by exogenous angiogenin. However, downregulation of endogenous angiogenin expression did reduce the
transcription potential of HeLa nuclei. As shown in Fig. 1D, the
rRNA transcription activity of the nuclei isolated from angiogeninunderexpressing HeLa cells after angiogenin RNAi transfection was
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lower than that from vector control transfectants. These results
showed that the rRNA transcription activity of the nuclei is
correlated with angiogenin level.
Transient Transfection of an Angiogenin Antisense Compound Inhibited Cell Proliferation. To confirm that endogenous
angiogenin is indeed involved in rRNA transcription, we transiently
transfected HeLa cells with an angiogenin antisense oligo to inhibit
angiogenin expression and examined the resultant change in rRNA
transcription. Fig. 2A shows that transfection of HeLa cells with
CT-1, a second-generation antisense oligo for angiogenin (28),
decreased the level of 45S rRNA but had no effect on that of
glyceraldehyde-3-phosphate dehydrogenase. Transfection with
CT-2, a scrambled control oligo, had no effect. A decrease in
angiogenin mRNA and protein levels was confirmed by Northern
blotting and ELISA, respectively (Fig. 2A and B).
HeLa cell proliferation was inhibited by CT-1 transfection
(Fig. 2C). CT-2 had no effect on cell proliferation indicating that
the inhibitory activity of CT-1 was not due to a nonspecific cytotoxic
effect. These results showed a positive correlation between
angiogenin level, rRNA transcription, and cell proliferation. CT-1
also significantly inhibited the proliferation of MB231, PC-3, and
HT-29 cells, whereas CT-2 had no effect (data not shown).
Stable Transfection of an Angiogenin Antisense Plasmid
Inhibited rRNA Transcription. To further show that endogenous
angiogenin is required for rRNA transcription and cancer cell
growth, we transfected HeLa cells with pCI-Ang( ), a plasmid
containing the antisense sequence of angiogenin cDNA, and
selected stable transfectants with G418. Angiogenin level in the
antisense transfectants was reduced as determined by ELISA and
Western blotting analysis (Fig. 3A).
The morphology of the cells changed dramatically in the
antisense transfectants (Fig. 3B). The steady-state level of the 45S
rRNA was significantly lower in the antisense transfectants as
determined by Northern blotting (Fig. 3C, left ). Ribosome
biogenesis was measured by silver staining of NOR (Fig. 3C, right).
NORs are actively transcribing rDNA loops and reflect the ribosome
biogenesis status (35). They are associated with argyrophilic proteins
and can be visualized by silver staining. The size and number of
NORs reflect the capacities of the cell to transcribe rRNA and are
therefore indices for cell proliferation, transformation, and even
malignancy (36). The decrease in 45S rRNA level and in NOR
numbers in pCI-Ang( ) transfectants indicated that rRNA transcription and ribosome biogenesis were decreased when angiogenin
expression was inhibited.
Figure 1. Nuclear translocation and nuclear function of angiogenin in HeLa cells is independent of cell density. A, HUVEC and HeLa cells, seeded at the
densities indicated, were incubated with 1 Ag/mL angiogenin at 37jC for 1 hour. The cells were washed with PBS and fixed with methanol at 20jC for 10 minutes.
Angiogenin was visualized with 10 Ag/mL 26-2F and Alexa 488–labeled goat anti-mouse IgG. B, nuclear proteins were extracted from HUVEC and HeLa cells
cultured under various densities and analyzed by Western blotting (150 Ag per lane) with an anti-angiogenin polyclonal antibody (R112). C, HeLa cells were cultured
for 24 hours and nuclei were isolated by subcellular fractionation. Nuclear run-on transcription was carried out with 5 104 nuclei isolated from HeLa cells cultured
under various densities in the presence or absence of 1 Ag/mL angiogenin. D, nuclei were isolated from HeLa cells transfected with an angiogenin
RNAi plasmid (pAng-RNAi) and the vector control (pBS/U6) and nuclear run-on assay was done.
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Angiogenin Plays a Role in HeLa Cell Proliferation
Figure 2. The effect of angiogenin antisense oligonucleotide CT-1 on HeLa
cell growth. A, HeLa cells were treated with 1 Amol/L CT-1 or CT-2 in the
presence of 5 AL/mL Lipofectin. Total RNA from 2 106 cells was
isolated and analyzed by Northern blotting for 45S rRNA, glyceraldehyde-3phosphate dehydrogenase, and angiogenin. B, HeLa cells, 5 104 cells
per 35-mm dish, were cultured in DMEM + 10% FBS for 24 hours. The cells
were washed by OptiMEM and transfected with 1 Amol/L CT-1 or CT-2
in the presence of 5 AL/mL Lipofectin. The culture media were collected at 72
hours and angiogenin levels were determined by ELISA and normalized
to cell numbers. C, cell numbers were determined at 48, 72, and 96 hours.
Points, means of three experiments; bars, SD.
RNAi was also used to down-regulate angiogenin expression in
HeLa cells. Very similar results were obtained as in antisense
transfection except that they were more significant due to a
greater degree of inhibition in angiogenin expression (Fig. 3E).
We used a DNA vector-based RNAi method (32) to construct a
pAng-RNAi plasmid, with the insertion DNA template
corresponding to the nucleotides 381 to 401 of angiogenin
mRNA. It was transfected into HeLa cells together with pBabepuro (37) and those cells that stably express angiogenin RNAi
were selected by puromycin resistance. Stable transfection of
pAng-RNAi resulted in an 80% reduction of angiogenin
expression, as shown by ELISA and Western Blotting (Fig. 3D).
The 45S rRNA level and NOR numbers were also decreased more
significantly (Fig. 3F).
The effect of down-regulating angiogenin expression on the
de novo synthesis of rRNA was determined by metabolic labeling
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with 32P. Cells were pulse-labeled with [32P]Pi for 2 hours and the
total cellular RNA was isolated with Trizol. Fig. 4A shows that the
newly synthesized 45S rRNA was significantly decreased in pAngRNAi transfectants than that in pBS/U6 control transfectants
when equal amounts of total RNA was loaded (Fig. 4B). In the
2-hour period, some of the newly synthesized 45S rRNA has been
processed to 32S rRNA whose level was significantly lower in
pAng-RNAi transfectants (Fig. 4A). Because of the large cytoplasmic pool of stable ribosomes, the cellular levels of 28S and 18S
rRNA were relatively stable (38). Thus, 28S and 18S rRNA still
serve as the best loading control. However, it should be noted
that the RNAi transfectants do have a reduced steady-state level
of 45S rRNA, which results in a decrease in the cellular level of 28S
and 18S rRNA. Therefore, use of 28S rRNA as a loading control
actually underestimated the difference in newly synthesized 45S
rRNA per cell after antisense and RNAi transfection. In any event,
these results indicated that the de novo synthesis of 45S rRNA
decreased when angiogenin expression was inhibited, confirming
that the nuclear function of angiogenin is related to rRNA
transcription.
Cell Proliferation Was Inhibited by Down-regulating Angiogenin Expression. Because of the central importance of rRNA
transcription in cell growth, decreased rRNA transcription should
slow cell proliferation. This is confirmed by three different cell
proliferation assays (Fig. 5). Direct cell number counting (Fig. 5A
and B) showed that both antisense and RNAi transfectants have
reduced cell proliferation rates than that of their corresponding
vector control transfectants. However, the extent of inhibition was
greater in RNAi (Fig. 5B) than in antisense transfectants (Fig. 5A),
probably reflecting the fact that RNAi is more effective in
inhibiting angiogenin expression.
The rate of DNA and RNA synthesis was determined by
[3H]thymidine and [3H]uridine incorporation, respectively. Cells
were continuously labeled for 24 hours and the incorporated
radioactivity was normalized to cell numbers. pCI-Ang( ) and
pAng-RNAi transfectants had slower DNA (Fig. 5C and D) and RNA
(Fig. 5E and F) synthesis rates than do the respective vector
control transfectants.
These results showed that endogenous angiogenin in HeLa cells
plays an important role in rRNA transcription, ribosome
biogenesis, and cell proliferation.
Colony Formation in Soft Agar Was Inhibited by Angiogenin
RNAi. The anchorage-independent growth of the pBS/U6 and
pAng-RNAi transfectants was analyzed by colony formation assay
in soft agar (Fig. 6A-C). RNAi transfection decreased both colony
number and size significantly. Colony number per 35-mm dish was
1,631 F 52 and 908 F 42, and the average colony size was 115 F
8 and 65 F 8 Am in vector control and in pAng-RNAi transfectants,
respectively (Fig. 6A and B). A complete recovery was obtained
when exogenous angiogenin (0.1 Ag/mL) was added (Fig. 6C),
indicating that the decrease in colony formation in pAng-RNAi
transfectants was caused by reduced angiogenin expression. These
results showed that the tumorigenicity of HeLa cells is decreased
by down-regulating angiogenin expression.
Angiogenin Down-regulation Inhibited Ecotopic Growth
of HeLa Cell Tumors in Athymic Mice. The effect of angiogenin
antisense transfection on tumor growth was examined in athymic
mice. Figure 7 shows that the appearance, establishment, and
growth of HeLa cells inoculated in athymic mice were all significantly inhibited after angiogenin antisense transfection. There
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was a delay in tumor occurrence (Fig. 7A) and the progression was
slower (Fig. 7B) in angiogenin antisense-transfected HeLa cells.
When tumors did develop, their average size was about half of
those in the control group (Fig. 7B). These data confirmed the
results obtained in soft agar assays that the tumorigenicity of HeLa
cells decreased when angiogenin expression was down-regulated.
Immunohistochemical staining with an anti–von Willebrand’s
factor antibody (Fig. 7C) showed that the HeLa tumor tissue grown
from pCI-Ang( ) transfectants had about half of the blood vessel
density (11 versus 21 vessels per field at 200, peripheral regions
of the tumors) as compared with those grown from vector control
transfectants, indicating that angiogenesis was inhibited in
angiogenin antisense transfectants, in agreement with the known
role that angiogenin plays in tumor angiogenesis. More importantly, PCNA staining showed that cell proliferation in the pCIAng( ) tumor was significantly lower than that in the control
(Fig. 7D), indicating that not only tumor angiogenesis but also
tumor cell proliferation per se was inhibited by angiogenin downregulation. Staining with 26-2F confirmed that angiogenin in the
nuclei of tumor cells decreased after antisense transfection
(Fig. 7E). These results further showed that angiogenin plays
a dual role in tumor growth by contributing both in angiogenesis
and in cancer cell proliferation.
Discussion
Angiogenin was isolated as a tumor angiogenic factor based
solely on its angiogenic activity (1). Therefore, subsequent studies
had mainly focused on how it induces angiogenesis and how its
angiogenic activity can be intervened. Angiogenin has been
considered to interact only with endothelial cells and vascular
smooth muscle cells. Results presented in this article indicated
that cancer cells are also targets for angiogenin.
We have showed in this article that angiogenin is constitutively
translocated to the nucleus of HeLa cells where it plays a role in
rRNA transcription. Because rRNA transcription regulates ribosome production and, consequently, the translation potential of a
cell, it is conceivable that deregulation of rRNA transcription may
be an important determinant in neoplastic transformation.
Continuous nuclear translocation of angiogenin in cancer cells
can certainly be one of the contributing factors. Indeed, inhibiting
angiogenin expression reduced tumorigenicity and reversed the
malignant phenotype of HeLa cells.
Figure 3. Down-regulation of angiogenin inhibits HeLa cell growth. HeLa cells were transfected with pCI-Ang( ) or pAng-RNAi and their corresponding control
vectors, pCI and pBS/U6. Stable transfectants were selected with 2 mg/mL G418 and 0.5 Ag/mL puromycin for pCI-Ang( ) and pAng-RNAi, respectively. Similar
results were obtained from all the individual clones and from the pooled populations. A-C , pCI-Ang( ) and pCI transfectants. D-F , pAng-RNAi and pBS/U6
transfectants. A and D , angiogenin secretion levels determined by ELISA (left), and cellular angiogenin levels detected by Western blotting with actin as the
loading control (right ). B and E, cell morphology changes in angiogenin-underexpressing cells. C and F, the steady-state level of 45S rRNA detected by
Northern blotting with 18S rRNA as the loading control (left ) and the numbers of AgNOR (right ).
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Angiogenin Plays a Role in HeLa Cell Proliferation
Figure 4. Angiogenin RNAi transfection inhibits the de novo synthesis of
45S rRNA. pBS/U6 or pAng-RNAi transfectants were cultured in DMEM
+ 10% FBS. [32P]Pi (25 ACi/mL) was added to each dish and incubated for
2 hours. Total cellular RNA was isolated with Trizol and the amount determined
by absorbance at 260 nm. Equal amounts of RNA were loaded to each lane
and subjected to agarose/formaldehyde electrophoresis. The newly synthesized
RNA were visualized by autoradiography (A ). 28S and 18S rRNA were
stained by ethidium bromide (B). Representative experiment of three repeats.
Several animal models have been used to examine the role
angiogenin plays in tumor growth (2, 39). Whereas the antitumor
activity of angiogenin antagonists was obvious, the mechanism by
which they prevented or delayed tumor appearance was not fully
understood. Based on the long-held assumption that angiogenin is
a tumor angiogenic protein, these data seemed to support the
proposition that the observed antitumor effects were due
to inhibition of angiogenin-induced tumor angiogenesis. However,
the tumors that eventually developed in angiogenin antagonist–
treated mice were substantially smaller than those in the control
group. It is therefore possible that other effects such as repression
of rRNA transcription of cancer cells also contributed to the
marked anticancer activity of angiogenin antagonists. Our results
shown in Fig. 7 showed that this is truly the case. Down-regulating
angiogenin expression in HeLa cells not only reduced tumor
angiogenesis (Fig. 7D) but also inhibited tumor cell proliferation
(Fig. 7E).
The finding that angiogenin is involved in rRNA transcription in
cancer cells is significant for our understanding of cancer biology
and neoplastic transformation. One of the hallmarks of cancer is
sustained cell growth and this can only be achieved by increased
protein synthesis. To accommodate this need, there must be an
increase in ribosome biogenesis. Up-regulation of ribosomal
proteins and rRNA transcription is an important factor in cancer
transformation (40). It has been reported that the proliferation
effects of estrogen in the rat induces pituitary tumors (41).
However, the susceptibility to formation of such tumors is highly
strain dependent. The particularly susceptible Fisher 344 strain
Figure 5. Angiogenin-underexpressing
HeLa cells have reduced growth rate. A,
C, and E, antisense transfectants. B, D,
and F, RNAi transfectants. A and B, cell
proliferation was determined by direct cell
number counting. C and D, DNA synthesis
determined by [3H]thymidine incorporation.
E and F, RNA synthesis determined by
[3H]uridine incorporation. Cells were
cultured for 24 hours, serum-starved for
another 24 hours, stimulated with 10% FBS,
and continuously incubated with 1 ACi/mL
[3H]thymidine (C and D ) or 1 ACi/mL
[3H]uridine (E and F ) for 24 hours.
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Figure 6. Angiogenin RNAi transfectants have reduced
tumorigenicity in soft agar. Anchorage-independent growth of pBS/
U6 control (A ) and pAng-RNAi transfectants in the absence (B)
or presence of 0.1 Ag/mL angiogenin (C ) was determined in soft
agar. The numbers of colonies in the entire 35-mm dish were
counted. Average colony size was determined by measuring the
diameters of colonies in 10 microscope fields with a microcaliber.
Pictures from representative areas.
develops tumors after 30 to 55 days of estrogen treatment.
In contrast, the Sprague-Dawley strain is resistant to such tumors.
The major difference between the two strains is the marked
increase (250%) of rRNA in the pituitaries of Fisher 344 rats
(42). It is notable that the increase in rRNA accumulation
occurs within 3 days of estrogen treatment when the increase in
the number of lactotrophs is minimal or nonexistent. These
results indicated that estrogen induces rRNA synthesis in the
early stages of tumorigenesis before any tumor growth is
detectable.
Androgen-dependent growth of the prostate has been well
documented in prostatic hyperplasia and prostatic carcinoma
(43). It has been shown that androgens regulate the accumulation of rRNA during androgen-dependent cell growth (44) and
that androgen-stimulated rRNA synthesis is the mechanism by
which androgens affect growth (45). Recently, it has been
Figure 7. Angiogenin antisense HeLa transfectants have reduced tumorigenicity in athymic mice. pCI vector control and pCI-Ang( ) antisense transfectants, 8 105
cells per mouse, were injected s.c. into the left shoulder of the mice. Eight mice were used per group. A, tumor appearance. Mice were examined for tumor growth thrice
per week. B, tumor volume. Tumor sizes were measured with a caliper and recorded in cubic millimeters (length width2). C, neovessels were stained with an anti–von
Willebrand’s factor IgG. Vessel density (vessels/field) was shown as mean F SD for each group. D, proliferating cells were stained by an anti-PCNA monoclonal
antibody. PCNA-positive and total numbers of cells were counted in 10 randomly selected areas at 200 magnification. E, 26-2F was used to reveal nuclear angiogenin
in HeLa tumors derived from pCI and pCI-Ang( ) transfectants. No staining was observed when a nonimmune IgG was used to replace 26-2F. Pictures from a
representative animal of each group.
Cancer Res 2005; 65: (4). February 15, 2005
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Angiogenin Plays a Role in HeLa Cell Proliferation
reported that angiogenin is the most up-regulated gene in
prostate intraepithelial neoplasia in the murine prostate
restricted Akt kinase transgenic mice (13). In these mice,
expression of Akt1 in the prostate results in activation of the
p70S6K pathway that up-regulates ribosomal proteins through the
Akt-p70S6K-mTOR pathway. Because rRNA production has to
increase in an equimolar ratio to that of ribosomal proteins for
ribosome biogenesis, we are currently investigating whether
rRNA transcription in the prostate is stimulated by angiogenin
and whether inhibition of angiogenin expression will inhibit
prostate intraepithelial neoplasia development.
The role angiogenin plays in rRNA transcription in cancer
cells suggests that up-regulation of angiogenin expression in
various cancer cells not only induces tumor angiogenesis but
also directly contributes to cell proliferation. Thus, inhibitors
targeting angiogenin will be more effective than those that
inhibit either angiogenesis or cancer cell proliferation alone. We
have found that neomycin, an aminoglycoside antibiotic that
blocks nuclear translocation of angiogenin (20), significantly
inhibits both tumor angiogenesis and cancer cell proliferation in
an ecotopic human tumor model in athymic mice.1
The mechanism by which angiogenin stimulates rRNA transcription is unclear at present. We believe that angiogenin acts in
cancer cell nucleus in a similar manner as it does in endothelial
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Acknowledgments
Received 6/7/2004; revised 11/9/2004; accepted 11/22/2004.
Grant support: NIH grant CA91086 (G-F. Hu) and the Endowment for Research in
Human Biology, Inc.
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.
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Research.
Angiogenin Is Translocated to the Nucleus of HeLa Cells and
Is Involved in Ribosomal RNA Transcription and Cell
Proliferation
Takanori Tsuji, Yeqing Sun, Koji Kishimoto, et al.
Cancer Res 2005;65:1352-1360.
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