Background
Nucleolin is an abundant DNA-, RNA- and protein-binding protein ubiquitously expressed in exponentially growing eukaryotic cells. It is found at several locations in cells: in the nucleolus it controls many aspects of DNA and RNA metabolism; in the cytoplasm it shuttles proteins into the nucleus and provides a posttranscriptional regulation of strategic mRNAs; and on the cell surface it serves as an attachment protein for several ligands from growth factors to microorganisms [
1‐
7]. Surface and cytoplasmic nucleolin are differentiated from nuclear nucleolin by a slight shift in their isoelectric point, which could reflect glycosylation of surface/cytoplasmic nucleolin [
3,
8,
9]. Moreover, surface/cytoplasmic nucleolin is regulated independently of its nuclear counterpart, since marked reduction of surface/cytoplasmic nucleolin could occur without any apparent effect on the level or nucleolar localization of nuclear nucleolin [
10].
Emerging evidences highlight the importance of the cell-surface expressed nucleolin in cell proliferation, tumor cell growth and angiogenesis [
3,
10‐
14]. The enhanced expression of surface nucleolin is observed
in vitro and
in vivo in lymphoid organs containing activated lymphocytes, on the surface of tumor cells and activated endothelial cells, or in angiogenic endothelial cells within the tumor vasculature [
11,
14,
15]. Among surface nucleolin binding growth factors and proteins, midkine and pleiotrophin can transform cells, whereas on endothelial cells they exert both mitogenic and angiogenic effect [
16]. Urokinase that is implicated in mechanisms regulating pericellular proteolysis, cell-surface adhesion, and mitogenesis binds and is co-internalized with surface nucleolin [
17,
18]. Other surface nucleolin binding proteins such as laminin-1, factor J, L- and P-selectins, and hepatocyte growth factor are involved in tumor development, induce cell differentiation, regulate cell adhesion, leukocyte trafficking, inflammation and angiogenesis [
19‐
23]. The tumor homing peptide F3 that binds both endothelial and tumor cells is internalized via surface nucleolin, while endostatin that inhibits angiogenesis binds nucleolin on the surface of endothelial cells before translocation to the nucleus [
11,
13]. Accordingly, the functional blockade or down-regulation of surface nucleolin in endothelial cells inhibits migration of endothelial cells and prevents capillary-tubule formation [
10,
12]. Ligand binding results in clustering of cell-surface nucleolin in lipid raft membrane microdomains before endocytosis of the ligand-nucleolin complex by an active process [
5,
24,
25].
We recently reported that both tumor growth and angiogenesis could be suppressed by targeting surface nucleolin using the HB-19 pseudopeptide, which binds the RGG domain located at the C-terminal tail of nucleolin [
10,
26]. HB-19 reduced markedly colony-forming capacity of several human carcinoma cell lines in soft agar, impaired migration of endothelial cells and formation of capillary-like structures in collagen gel, and reduced vessel arborization in the chick embryo chorioallantoic membrane. Significantly, HB-19 treatment markedly suppressed the progression of established human breast tumor cell xenografts in athymic nude mice, and in some cases eliminated measurable tumors while displaying no toxicity to normal tissue [
10].
In a more relevant tumor model, now we provide evidence that HB-19 can also interfere with the spontaneous development of cancer in RET mice. Such mice express constitutively an active form of the
ret oncogene leading to development of spontaneous melanoma, thus providing a genetically driven model of tumors [
27]. In this model the severity grade of melanoma is associated with the location of skin tumors in which the onset of dorsal nodules corresponds to a more aggressive disease [
28]. The skin primary tumors eventually metastasize mainly to lymph nodes, mediastinum or lungs [
27]. Moreover, our recent data in this model suggests that the growth of melanoma involved several tolerance mechanisms [
29]. Here we show that HB-19 treatment delays significantly the onset and frequency of spontaneous melanoma in RET mice. In addition, the frequency of visceral metastasis and tumor vascularization are significantly reduced in HB-19 treated compared to control mice, thus indicating inhibitory effects on both metastasis and angiogenesis. Using a melanoma cell line derived from a cutaneous nodule of a RET mouse, we show that HB-19 can affect several criteria implicated in the tumorigenic potential of melanoma cells, such as restoration of contact inhibition in culture, reduction of colony formation in soft agar, and impairment of tumorigenicity and lung metastasis in mice. Interestingly, these changes are associated with a specific inhibitory action of HB-19 on expression of genes implicated in tumorigenesis.
Methods
Mice
MT/
ret
+/- transgenic mice (C57BL/6 background, called RET mice, litter one) expressing the
rfp-ret oncogene develop a spontaneous melanoma [
27,
30]. Constitutively activated
rfp-ret enhances cRET protein expression in the process of melanomagenesis in RET mice [
31]. Non-transgenic littermates (MT/
ret
-/-) were used for transplantation experiments. Mice were maintained in our own animal facilities corresponding to a pathogen free environment. All experiments were performed in compliance with French Ministry of Agriculture regulations for animal experimentation (number 75-510).
HB-19 treatment of RET mice
HB-19 was synthesized as described previously [
10,
26]. Although it is readily soluble in water, HB-19 was dissolved in PBS for the purpose of treatment of mice. Ten days old RET mice were treated intraperitoneally with HB-19 at 5 injections/week during week 1-3 and 2 injections/week during week 4-42. The dose of HB-19 was 50, 100, and 200 μg for the first, second and the rest of the weeks, respectively. Control mice were injected with PBS at the different time points. Clinical signs of mice treated (n = 9) and untreated (n = 11) were assessed once a week on vigil mice and once a month on anesthetized mice. Development of facial or dorsal tumor nodules was recorded. At the end of the treatment (day 300), all mice were sacrificed and autopsied to monitor for distant metastasis.
Flow cytometry. Cutaneous tumors were pooled, mechanically dissociated and digested with 1 mg/mL collagenase A and 0.1 mg/mL DNase I (Roche, Mannheim, Germany) for 25 min at 37°C. Single cell suspensions were filtered, washed in PBS, 5% FCS, 0.5 mM EDTA and resuspended in RPMI 1640. After incubation with anti-FcgII/IIIR antibody (clone 2.4G2), cell suspensions were stained at 4°C, for 15 min with the following combinations of monoclonal antibodies (all from Pharmingen): PerCP-conjugated anti-CD45.2/APC-conjugated anti-CD11b and APC-conjugated anti-CD45.2/PE-conjugated anti-TcRab. Flow cytometric analyses were performed on a FacsCalibur cytofluorometer (BD Biosciences) and data were analyzed using CellQuestPro Software (BD).
Murine melanoma cells and transplantation
The TIII cell-line was derived from the neck cutaneous nodule that developed in a RET mouse. Cells were cultured in RPMI 1640 with 10% FCS, 2 mM L-glutamine, 100 U/ml penicillin, 100 μg/ml streptomycin. In addition to the nucleus, nucleolin is also expressed in the cytoplasm and at the cell surface of TIII cells, as it is the case in different types of tumor cells and other melanoma cell lines [
3,
32].
TIII (1×106) cells cultured in the presence or not of HB-19 (10 μM; 12 passages) were transplanted subcutaneously in 10 weeks old MT/ret
-/- mice. Fourteen days later, mice were sacrificed and tumor mass was determined with a caliper. Alternatively, mice were injected in the tail vein with TIII cells (5×105) in the absence or presence of 10 μM of HB-19. Twenty hours later and then daily during two weeks, mice were treated by intraperitoneal injections of PBS or PBS containing HB-19 (5 mg/kg). Mice were then sacrificed, and the number of black macro-metastases on the lung surface was counted.
Immunofluorescence and confocal microscopy
Cells were plated 24 hours before the experiment in eight-well glass slides (Lab-Tek Brand; Nalge Nunc International, Naperville, IL). Cells were fixed with paraformaldehyde (PFA; 3.7%, 10 min), permeabilized by Triton X-100 (0.5%, 15 min) and stained for the intracellular actin cytoskeleton using FITC-conjugated phalloidin (Sigma) [
10,
26]. The nuclei were stained with 4',6-diamidino-2-phenylindole (DAPI).
TIII cells (2×10
4) were mixed in 0.35% top agar diluted in RPMI containing 10% FCS in the absence or presence of different concentrations of HB-19 before plating onto 0.8% bottom agar in 12-multiwell plates. Cells were treated every two days during 14 days. Colonies with diameter superior to 100 μm were scored as positive using a phase contrast microscope equipped with a measuring grid at magnification 50×. The number of colonies was determined by analyzing 5 fields/well from 3 wells [
10].
Tissue preparation, immunohistochemical staining and image analysis
Tumors were fixed in FineFix (Milestone, Bazainville, France) for paraffin inclusion. Sections of 8 μm thickness were re-hydrated and saturated in PBS containing 5% goat serum. Sections were incubated with 1:20 dilution of a rat anti-mouse CD34 monoclonal antibody (Abcam, Cambridge, UK) for 2 hours at room temperature. After two washes in PBS, sections were incubated for 1 hour at room temperature with biotin-conjugated goat anti-rat IgG (Chemicon International Inc., Temecula, CA) diluted at 1:500, followed by three washes in PBS and incubation with avidin/peroxidase complex (Vector Laboratories, Burlingame, CA). The horseradish-Peroxidase activity was revealed by incubating the sections with 3,3'-Diaminobenzidine substrate kit (Vector Laboratories). Finally, the sections were counterstained with haematoxylin, followed by water wash and cover slipped with Mowiol medium. Five microscopic fields (at 200-fold magnification) were selected randomly for analysis using the Image analysis. The density of endothelial cells in each field was expressed as the ratio of cell area/total area examined × 100 (%). These values were then averaged for the tumors recovered from control and HB-19 treated mouse.
mRNA expression monitored by RT-PCR
TIII cells were cultured in RPMI medium containing 10% FCS in the absence or presence of HB-19. Total RNA was prepared from cells (5×105) and fresh tumors isolated from control and HB-19 treated RET mice using RNeasy Mini Kit (Qiagen) according to the manufacturer's instructions. RT was carried out with oligo(dT) and 1-4 μg of total RNA using Superscript II RNase H- Reverse Transcriptase (Gibco BRL). The expression of specific mRNAs was investigated by RT-PCR using primers for matrix metalloproteinase-2 and -9 (MMP-2, MMP-9), vascular endothelial growth factor (VEGF-A), tumor necrosis factor alpha (TNF-α), signal transducer and activator of transcription 1 (STAT-1), melanoma inhibitory activity (MIA), and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). PCR was performed in a RoboCycler 96 (Stratagene) with the following primers: MMP-2 sense 5'-GAGTTGGCAGTGCAATACCT-3' and antisense 5'-GCCGTCCTTCTCAAAGTTGT-3'; MMP-9 sense 5'-AGTTTGGTGTCGCGGAGCAC-3' and antisense 5'-TACATGAGCGCTTCCGGCAC-3'; VEGF-A sense 5'-AGAGCAACATCACCATGCAG-3' and antisense 5'-AGGAATCCCAGAAACAACCC-3'; TNF-α sense 5'-ACTCCCAGAAAAGCAAGCAA-3' and antisense 5'-TGGAAGACTCCTCCCAGGTA-3'; STAT-1 sense 5'-CGTGGGAACGGAAGCATTTG-3 and antisense 5'-GAGACATCATAGGCAGCGTG-3'; MIA sense 5'-ATCCTATCTCCATGGCTGT-3' and antisense 5'-ACTGGCAGTAGAAATCCCA-3'; GAPDH sense 5'-CGTCCCGTAGACAAAATGGT-3' and antisense 5'-CCTTCCACAATGCCAAAGTT-3'. PCR amplification conditions were 95°C for 2 min, 30 cycles at 95°C for 30 sec, 53°C (for MMP-2, VEGF, Mia-1 and GAPDH) or 55°C (for STAT-1) or 57°C (MMP-9) for 30 sec and 72°C for 45 sec, and this was followed by 5-min incubation at 72°C. The expected RT-PCR product for MMP-2, MMP-9, VEGF-A, TNF-α, STAT-1, MIA-1 and GAPDH was 666, 754, 663, 688, 425, 267 and 527 base pairs, respectively.
Statistical analysis
Statistical significance was determined by ANOVA unpaired T test or the Wilcoxon log-rank test using the GraphPad Prism 4.0 software (San Diego, CA). Values of p < 0.05 were considered significant.
Discussion
The results presented herein demonstrate the antitumoral action of HB-19 in the transgenic RET mouse model against development of spontaneous cutaneous melanoma and visceral metastasis. We show that HB-19 treatment of 10 day old mice for a period of 10 months causes a significant delay in the onset and the frequency of large cutaneous tumors compared to untreated control mice (Figure
1). In addition, HB-19 treatment exerts an inhibitory effect on distant metastasis, since the frequency of visceral metastasis was 72% and 33% in the control and HB-19 treated mice, respectively (Table
1). Furthermore, tumor vascularization is reduced significantly in tumors recovered from HB-19 treated compared to untreated control mice (Figure
2). These observations illustrate the dual inhibitory action of HB-19 on tumor cell growth and tumor angiogenesis, consistent with our previous report using athymic nude mice with established human breast tumor cell xenografts [
10]. Fogal et al [
14] have recently reported that targeting surface nucleolin with an antibody preparation against nucleolin has no significant effect on tumor size or progression while mediating a significant reduction of blood vessel density. This difference in the mechanism of the antitumoral action of anti-nucleolin antibody compared to HB-19 could be due to their mode of interaction with surface nucleolin, since HB-19 binds the C-terminal RGG domain [
26] whereas the epitope of the anti-nucleolin antibody is in the N-terminal acidic domain of nucleolin [
14].
In the melanoma derived TIII cells, we show that HB-19 treated cells proliferate under contact inhibition and loose partially their tumorigenic potential as demonstrated by impaired colony formation in soft agar, and reduced tumorigenicity and lung metastasis in MT/
ret
-/- mice (Figures
3 and
4). Interestingly, HB-19 treatment induces a specific down regulation of transcripts coding MMP-2, MMP-9, and TNF-α
in vitro in the TIII cells and
in vivo in tumors of HB-19 treated RET mice (Figure
5 and
6). Strikingly, the expression of these genes remains down regulated in HB-19 pretreated TIII cells even after seven passages in the absence of HB-19, thus suggesting that HB-19 treatment could trigger differentiation of cultured melanoma cells into a sub-population with somewhat stably reduced malignant phenotype. Consistent with the results observed in melanoma TIII cells, we have shown that HB-19 treatment impairs the tumorigenic potential of several human epithelial tumor cells of different origin, such as breast (MDA-MB-231, MDA-MB-435), prostate (LNCaP), renal (G401), and colon (SW480, SW620) carcinoma (manuscript in preparation). For example, HB-19 treatment of G401 cells leads to restoration of contact inhibition, specific down regulation of several genes associated with tumorigenesis, and marked reduction of tumorigenicity in the nude mice. Therefore, the observed inhibitory action of HB-19 on the melanoma TIII cells is not specific for the rearranged RET-driven melanoma [
27,
40]. The RET mice express
rfp-ret hybrid oncogene identified from a recombination event in transfection assays carried on murine NIH 3T3 cells [
40]. Consequently, the potential role of RET as a human oncogene remains to be demonstrated. Nevertheless, it is interesting to note that activation of the intrinsic
c-ret proto-oncogene has recently been correlated with melanoma cell proliferation. Accordingly, the c-RET protein is reported to be expressed in human melanomas while in human malignant melanoma cell lines and in the RET mice there is increased expression of c-RET transcripts [
31,
41].
We recently reported that HB-19 can trigger rapid and intense membrane Ca
2+ fluxes in various types of tumor cells by a mechanism that involves store-operated Ca
2+ entry [
8]. Consequently, HB-19 could be involved in the activation of signaling pathways leading to regulation of gene transcription. However, the mechanism responsible for selective inhibition of MMP-2, MMP-9 and TNF-α expression in HB-19 treated melanoma cells and tumors remains to be elucidated. Matrix metalloproteinases are extracellular proteinases associated with cancer invasion and metastasis by virtue of degrading components of the extracellular matrix [
34], whereas proinflammatory cytokines are indispensable participants in the neoplastic process by orchestrating a tumor-supporting microenvironment [
42]. Expression of MMP-2, MMP-9 and TNF-α is strongly linked with malignant tumor progression, angiogenesis and metastasis of various types of cancers [
43,
44]. Consequently, the selective down regulation of such strategic genes could account, at least in part, for the mechanism of the antitumoral action of HB-19 in RET mice. This is in accord with the results presented in Figure
1 and Table
1 showing a significant delay for the development of spontaneous tumors and the reduced incidence of visceral metastasis in HB-19 treated RET mice compared to the corresponding controls. Although the expression of transcripts coding various isoforms of VEGF-A is not affected in HB-19 treated TIII cells (Figure
5), we have previously shown that HB-19 impairs several VEGF induced endothelial functions involved in angiogenesis by targeting surface nucleolin [
10]. By its capacity to block activation of endothelial cells therefore, HB-19 could impair tumor vascularization in RET mice (Figure
2). Taken together, these
in vitro and
in vivo studies provide new insights into the mechanism of antitumor action of HB-19, and suggest that several inhibitory pathways could be operating in order to coordinate the delay in the development of melanoma in HB-19 treated RET mice. Recently, it was reported that infiltration of functionally impaired CD8
+ T cells, regulatory T cells, tolerogenic dendritic cells and macrophages can occur within metastatic melanoma lesions in patients [
45‐
47]. We found no profound alteration in the proportion of myeloid and T cell populations infiltrating tumors in HB-19 treated and related control RET mice. Consequently, it is unlikely that the inhibitory effect of HB19 treatment is due to the quantitative modulation of the tumor infiltrating immune cells. However, we cannot rule out the potential implication of such hematopoietic cells in the overall antitumoral action of HB-19.
HB-19 treatment significantly delays the development of cancer in RET mice while displaying no toxicity to normal tissue. After binding surface nucleolin, HB-19 enters cells by an active process but it does not cross the nuclear membrane. Consequently, the effect of HB-19 is exerted differentially and specifically via the cell surface expressed nucleolin [
10,
26]. Although nuclear nucleolin is involved in many aspects of gene expression [
1,
2], the lack of translocation of HB-19 to the nucleus and nucleolus could account, at least in part, for its lack of toxicity in cultured cells and in animals. By studies on the pharmacokinetic and biodistribution properties of HB-19 in rats, we have demonstrated that after preferential uptake of HB-19 by specific tissues it is eliminated by renal glomerular filtration in the form of HB-19 metabolites [
15]. This and the threshold for tissue uptake of HB-19 could prevent prolonged accumulation of HB-19
in vivo, which otherwise would lead to toxic effects [
15]. Consistent with this, no apparent toxicity was observed in the RET mice that were treated over a period of 10 months with HB-19. The molecular target of HB-19
in vitro and
in vivo is surface nucleolin that is expressed by activated and proliferating cells [
2,
3,
15,
48]. In contrast to normal cells however, nucleolin is constantly and abundantly expressed on the surface of tumor cells making them a preferential target for the inhibitory action of HB-19 (Hovanessian et al., submitted).
Conclusion
The growth and metastasis of solid tumors are dependent on neovascularization in order to provide an appropriate blood supply necessary for tumor cell proliferation and tumor invasion [
49]. Although HB-19 treatment failed to prevent the development of spontaneous melanoma in the RET mice, it delayed significantly the onset and frequency of cutaneous tumors, and reduced visceral metastasis and tumor vascularization. Therefore, the inhibitory action of HB-19 on tumor and endothelial cells as well as on metastasis ([
10] and the results herein) fulfills the criteria as an efficient and a nontoxic drug for therapeutic intervention in cancer. HB-19 could also be used as an alternative therapy in cancer patients that develop resistance to chemotherapy. Another advantage of HB-19 over traditional anti-cancer drugs is its capacity to bind surface nucleolin in an irreversible manner under physiological conditions [
26], making the half-life of tissue associated HB-19 much longer compared to that of any other cancer drug [
15]. Finally, its reproducible synthesis, stability in serum and
in vivo lack of toxicity make HB-19 a unique drug against tumor growth and angiogenesis, thus providing novel therapeutic opportunities in cancer therapy by itself or as an adjuvant therapy in association with current therapeutic interventions on a virulent cancer like melanoma.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
DEK performed experiments. DD performed experiments. RL performed in vivo treatments and related statistical analysis. BK performed experiments. YHK performed in situ experiments. SN performed in vivo treatments. MG performed in vivo treatments. MK provided the RET mice. JPB synthesized HB-19 and helped to draft the manuscript. JC designed the experiments performed by D.D. and Y.H.K. and helped to draft the manuscript. AGH conceived the overall research plan, designed experiments, and wrote the paper. APB conceived the overall research plan, designed experiments, and wrote the paper. All authors read and approved the final manuscript.