Introduction
Galectin-1 is a β-galactoside binding protein with a highly conserved carbohydrate-recognition domain (CRD) [
1,
2]. A wide range of tumor cells over-express galectin-1, which has been shown in separate studies to play important roles in tumor progression, including tumor cell proliferation and adhesion as well as immune cell inhibition by inducing apoptosis of activated T cells [
3‐
6]. The outcome of the immune inhibitory influences is a reduced immune response by the host, thereby facilitating tumor growth. Recent reports document that galectin-1 is also important for tumor angiogenesis [
3,
7]. In addition, hypoxia inducible factor-1 (HIF-1), activated during the cellular response to hypoxia commonly associated with tumor growth induces galectin-1 expression. Ironically, hypoxia is associated with increased levels of oxidative stress, activating signaling pathways leading to HIF family protein stabilization and activation (reviewed in [
8]).
Tumor angiogenesis refers to formation of new blood vessels induced by tumor-secreted factors and is involved in tumor progression [
9]. Growth of solid tumors strictly depends on the formation of new blood vessels and this tumorigenic neovascularization requires several steps including stimulation of vascular endothelial cells (ECs) and destabilization of the basal membrane and extracellular matrix [
10]. Hypoxia, due to poor blood supply, is associated with oxidative stress and increased tumor angiogenesis [
11] and is also recognized as a major obstacle to successful immunotherapeutic outcomes, jeopardizing cancer therapy. More specifically, the aberrant new blood vessel formation hampers infiltration and recruitment of immune effector cells into tumors because of the chaotic vascular morphology and reduced expression of adhesion molecules [
12]. In fact, it has been shown that normalization of the tumor vasculature improved immunotherapeutic outcomes in a mouse tumor model [
13].
High levels of galectin-1 expression have been observed in growth stimulated ECs as well as in tumor cells, thereby promoting the proliferation and migration of ECs into the tumor [
5,
7,
14]. In addition, galectin-1 has been linked with reduction in the migration of lymphocytes, their trafficking to ECs and transendothelial infiltration into tumors [
15,
16]. Previous studies have shown that by reducing galectin-1 expression or using a peptide such as anginex to inhibit CRD binding could inhibit endothelial function, as well as tumor growth and metastasis [
4,
17,
18]. TDG has been more closely examined here as a small drug inhibitor of galectin-1 that increases the number of CD8
+ lymphocytes infiltrating into tumors as well as suppressing tumor angiogenesis, most likely by preventing EC survival under conditions of oxidative stress.
Materials and methods
Cell lines and culture conditions
The murine melanoma B16F10 cell line, murine mammary carcinoma 4T1 cell line and human umbilical vein ECs (HUVECs) were maintained in DMEM supplemented with 10% heat-inactivated FBS, 50 IU/ml penicillin and streptomycin, 20 mM Hepes buffer, and 1.6 mM
l-glutamate (Sigma). Human EAhy926 ECs were grown in DMEM containing HAT (hypoxanthine/aminopterin/thymidine) as previously described [
19].
ELISA for detecting galectin-1 binding
96-well plates were pre-coated with mouse laminin (Invitrogen) in 0.2 M NaHCO
3 at 1 μg/well. EAhy926 cells or HUVECs were fixed onto the wells by overnight incubation in 4%
p-formaldehyde/PBS. After washing with PBS containing 0.05% Tween-20, plates were blocked with 3% BSA/PBS at 37°C for 1 h. Recombinant galectin-1 protein (see [
20] and Supplemental data for method of preparation and Supplementary Fig. 1 for purity analysis) was pre-incubated with serial dilutions of TDG or sucrose (as a control disaccharide not binding the galectin-1 CRD) at 37°C for 1 h before testing for protein binding to the plates of laminin or cells. Samples were incubated at RT for 2 h, then washed before adding biotinylated anti-galectin-1 IgG. After incubating the plates at RT for 1 h, the unbound antibody was removed by washing. HRP streptavidin was added at RT for 1 h before final washing and addition of Lumigen
® TMA-6 (GE Healthcare) for chemiluminescence and photon counting in a plate reader (FLUOstar Optima; BMG Labtech).
Ice-cold Matrigel™ (BD Biosciences) was added at 300 μl per well to pre-cooled 24-well plates. 50 μl of additional DMEM containing galectin-1 protein (wild type or P79R mutant; see Supplemental data for details of preparation and purity, Supplemental Fig. 1), TDG, or sucrose control were added to samples as indicated and the plates were kept at 37°C for 30 min. 5 × 105 EAhy926 cells or HUVECs were plated onto the gel surface in 200 μl of DMEM. After 18 h of culture, the cell rearrangement and tube formation were visualized by microscopy. The numbers of tubes and the interconnecting nodes were counted and individual tube lengths recorded.
Generation of galectin-1 knockdown (G1KD) tumor cell lines
pLKO.1-puro (Sigma–Aldrich) encoding a short hairpin RNA (shRNA) as an RNAi expressing vector was used with the targeting sequence for galectin-1: 5′-CCTGACCATCAAGCTGCCAGA-3′. The sequence of the non-target control shRNA was 5′-CAACAAGATGAAGAGCACCAA-3′. Viral supernatants were prepared by transient co-transfection of 293T cells (80% confluence) using CaCl2 and 20 μg of transfer plasmid (pLKO.1-puro, anti-galectin-1 or non-targeting control), 15 μg of packaging plasmid (pCMV-dR8.74) and 6 μg of envelop plasmid (pMD2.G). Media was replaced with supplemented DMEM after 8 h. Viral supernatants were collected after a further 48 h incubation and stored at −80°C until use. 5 μl of the viral stock was added to target cells in 96-well plates in 100 μl of DMEM containing 8 μg/ml of polybrene. After culturing for 24 h, cells were treated with puromycin to select for stable transfectants which were cloned as single colonies and galectin-1 expression analyzed by RT–PCR and immunoblotting.
Tumor mouse models and TDG treatment
Male C57BL/6, female Balb/c and female Balb/c nu/nu mice at 5–6 weeks of age were obtained from the Animal Resources Centre (Perth, WA, Australia). For all studies, aged, sex-matched groups of mice were used for the control and test cohorts. To establish primary solid tumors, B16F10 or 4T1 cells (7 × 105 cells/mouse in 150 μl of PBS) were implanted subcutaneously (s.c.) into the left chest region of C57BL/6 or Balb/c mice. Tumor cells were incubated with TDG (Carbosynth, UK) at 37°C for 1 h before s.c. injection as tumor challenge. After 3 days, mice received TDG (40, 80 and 120 mg/kg) by intratumoral injection which was repeated every 3 days. Tumor volumes were measured using the following formula: tumor volume = [longest diameter × (shortest diameter)2]/2 or by volumetric measurement using ultrasound (with an RMV704 scan head; mean frequency, 25 MHz; resolution, 40 μm; Vevo 770, VisualSonics). After 3 weeks, mice were sacrificed. All animal experiments were approved by the Griffith University animal ethics committee, and followed the procedures and guidelines set out by the Australian NHMRC and ANZCAART regarding animal welfare.
Cell preparation from excised tissues
Tumors were removed, cut into small pieces and incubated in DMEM containing 70 U/ml of collagenase (Sigma) at 37°C for 1 h before cells were washed and resuspended in PBS. Tumor infiltrating lymphocytes (TILs) were collected using Ficoll-Paque Plus (GE Healthcare). Blood was collected by cardiac puncture and peripheral blood mononuclear cells (PBMCs) isolated using Ficoll-Paque Plus. Spleens were removed and treated using 0.83% NH4Cl-Tris hemolysis buffer. The residual white blood cells were then analyzed by flow cytometry.
Three-dimensional power Doppler ultrasonography of tumors
Power Doppler ultrasound imaging of tumors was performed according to a previously published method [
19]. Tumor bearing nude mice were anesthetized using isoflurane and the heart rate and body temperature were monitored during imaging by ultrasound. Two-dimensional images (2-D) were taken at 200 μm intervals and reconstituted to produce the final three-dimensional (3-D) volumetric images.
Statistical analysis
All values are expressed as mean ± SE, and the number (n) of samples used was as indicated. The statistical significance of differences between experimental and control groups was determined by Student’s t test with p-values < 0.05 considered significant. Two-way ANOVA with Bonferroni post-test analysis was performed for tumor growth experiments. All statistical analyses were performed using GraphPad Prism v4.03.
Discussion
Galectins bind β-galactosides such as lactose, lactulose and
N-acetyllactosamine (Galβ1,3/4GlcNAc) “LacNAc” with low inhibitory potency and
K
d values of ~1.0 and ~0.2 mM, respectively whereas natural disaccharides that are extended from the C3′ position of galactose, replacing the 3′-OH, showed enhanced affinity for some galectins [
31]. Compared to the natural ligands such as those above, TDG (which we have used extensively here and elsewhere [
1]), has advantages because it is readily available, has a higher affinity and
Κ
d of ~78 μM for galectin-1 and is a simple, synthetic and non-metabolizable disaccharide. TDG was very effective at suppressing tumor growth by inhibiting multiple tumor promoting and protective activities of tumor-derived galectin-1, including immune cell dysregulation, angiogenesis, and protection against oxidative stress. Indeed, intratumoral treatment with TDG significantly suppressed both melanoma and mammary carcinoma growth in mouse models, with much less effect observed for the G1KD tumors in which the protective functions of tumor-derived galectin-1 would be greatly reduced (Fig.
1). Therefore, our evidence strongly implicates TDG acting mainly by blocking the CRD of galectin-1 to exert its effects and interfere with the tumor-promoting and protective functions of the tumor-derived galectin-1 protein. Although a contribution of other galectins such as galectin-3 remains a possibility, our evidence, particularly based on the galectin-1 knockdown tumor studies, supports a key role for tumor derived galectin-1 in tumor development and makes the contributions of other members of the galectin family much less likely [
32].
Perhaps most interestingly, one of the significant effects demonstrated here with intratumoral TDG treatment was the large increase in numbers of tumor-infiltrating CD4
+ and CD8
+ lymphocytes (Figs.
2,
3). Similarly, the populations of CD4
+ and CD8
+ lymphocytes in blood and spleen as immune organs were also increased following this treatment (Fig.
3).
We attempted to resolve the relative contribution mediated by the increased infiltration of the immune effector T cell populations versus the anti-angiogenic activity and which is the most important outcome that results from TDG inhibition of galectin-1 function. Thus, the comparison revealed that intratumoral TDG treatment suppressed tumor volume in nude mice by 47% versus the untreated control (Fig.
4), which was much less than the 67% reduction obtained when TDG was used to treat tumors in immunocompetent mice. These differences highlight the importance of T cell immunity and tumor infiltration by TILs to maximize the combined effects of TDG-mediated inhibition of galectin-1 function on both angiogenesis and immune activation. Supporting this viewpoint, our previous results [
21] showed that TDG treatment promoted the induction of splenic-derived CD8
+ CTL responses raised against breast cancer cells and slowed tumor progression. Hence, blocking galectin-1 induces greater anti-tumor effects by a combination of effects of modulating tumor neovascularization and promoting the infiltration of CD8
+ T cells into the tumors.
The inhibitory effects of TDG on the pro-angiogenic function of galectin-1 were shown to be highly significant, both in vitro and in vivo, reducing binding to laminin and the tube-forming activity of ECs, as well as EC proliferation (Fig.
6). More importantly, our data provides a possible explanation for how TDG could act to inhibit the tumor protective function of galectin-1 in promoting survival of ECs subjected to tumor derived oxidative stress, with the result that TDG blocking galectin-1 would mean greater exposure to the tumor oxidative stress, enhancing the loss of EC by apoptosis (Fig.
7). This could explain the marked changes observed in the tumor vascular networks and the CD31
+ EC populations within tumors, which were considerably reduced by intratumoral TDG treatment (Figs.
1,
2). In addition, we have for the first time shown considerable reductions in blood flow occurring within the TDG treated tumors, particularly in the highly vascularised periphery. These observations confirmed that TDG, as a single agent, inhibited tumor angiogenesis by blocking the functions of galectin-1 in promoting EC growth and survival and blood flow in our tumor models and presumably would do so in other tumors.
Anti-angiogenic therapy and immunotherapy are currently being used to improve the treatment of cancer patients. Enhancing immune responses by using cancer vaccines or adoptive transfer of ex vivo expanded immune cells has significantly progressed recently and several strategies are already being tested. Although tumor-specific T cells in the circulation can be effectively generated using these protocols, they have met with limited success with complete responses being uncommon [
33]. Our results highlight that inhibiting galectin-1 function with TDG has multiple effects including promoting increased numbers of immune cells and their infiltration into tumors, apparently involving modifications to the tumor vasculature. Accumulating evidence suggests that the lack of correlation between the presence of circulating immunotherapy-induced lymphocytes and tumor regression is probably due to the restricted lymphocyte infiltration and recruitment caused by physical barriers associated with biologically aberrant tumor neovasculature [
12,
34]. In order to evade immune cell infiltration, tumors decrease EC expression of adhesion molecules, such as ICAM-1, VCAM-1 and E-selectins, thereby preventing lymphocytes from trafficking into tumors [
35,
36]. It has been shown that normalization of the aberrant tumor vasculature can increase tumor infiltration of cytotoxic T cells leading to enhanced survival of experimental animals [
13]. In addition, administration of anti-vascular endothelial growth factor (VEGF) antibody to disrupt the VEGF/VEGFR-2 signaling axis increases the effects of adoptive T cell transfer therapy in the B16 murine melanoma model by normalizing the vasculature to allow T cell extravasation into the tumor stroma [
37]. Therefore, for tumor immunotherapeutic strategies to be more effective, it may be beneficial to normalize their abnormal tumor blood vessels, perhaps using a drug such as anti-VEGF or TDG.
Galectin-1 is becoming widely recognized as an important lectin protein with significant roles in tumor progression and one of the roles of galectin-1 is its contribution to tumor cell evasion of immune cell surveillance [
38,
39]. Galectin-1 has been shown to directly suppress T cell immunity by inducing T cell apoptosis [
38], inhibiting T cell activation [
40] and promoting regulatory T cell function [
41]. In addition, we previously showed that blocking galectin-1 with TDG increased the induction of tumor-specific CD8
+ T cell cytotoxicity, thereby suppressing growth of breast tumors in a murine model [
21]. Therefore, blocking galectin-1 with metabolically stable disaccharides such as TDG should protect T cells from the negative impact of tumor derived galectin-1, and enhance T cell activity against many different types of cancers.
Another reason why galectin-1 is a promising new cancer target is because of its angiogenic promoting activities and its highly increased expression by tumors and tumor endothelium where elevated galectin-1 promotes EC proliferation and migration [
7,
30]. Adding to this area, our results from the tube-forming assay using Matrigel and the MTT assay further advance understanding of the mechanisms whereby exogenous galectin-1 promotes angiogenesis by enhancing the tube-forming activity of ECs and their cell proliferation as observed with both EAhy926 and HUVECs (Fig.
6). These studies suggest that galectin-1 has an important function in cell to cell adhesion of ECs that is mediated by the CRD. Consistent with these reports and our results it appears that galectin-1 produced by tumors directly contributes to the formation of abnormal tumor vascular endothelium typifying the blood vessels in solid tumors as another of its tumor promoting functions. Thus, it was shown recently that tumor-derived and secreted galectin-1 is transported inside ECs where it specifically promotes H-Ras signaling, activating the Raf/mitogen-activated protein kinase/extracellular signal-regulated kinase (Erk) kinase (MEK)/Erk pathway to promote EC proliferation [
14].
Hypoxia and oxidative stress are other important features of tumor microenvironments [
11], where galectin-1 also plays a role as a well described hypoxia-inducible protein, mediated by the HIF-1 transcription factor [
6,
29]. A poor blood supply due to the abnormal outgrowth of chaotically arranged blood vessels frequently results in regions of oxygen insufficiency within tumors. This promotes production of reactive oxygen species (ROS) [
11] causing HIF-1 stabilization which in turn induces transcription of VEGF, the glucose transporter-1 (GLUT-1) and galectin-1 [
6]. On the basis of our results, it is now clear that galectin-1 secretion by tumor cells will protect EC survival and promote their growth, reducing the levels of apoptosis caused by tumor-associated oxidative stress. Blocking the CRD of galectin-1 with TDG suppressed its protective effects against hydrogen peroxide-induced oxidative stress to ECs, and would promote vascular EC function within the tumor microenvironment. Given that the galectin-1 molecule contains six cysteine residues and is secreted from cells in the reduced form, perhaps galectin-1 could act as an antioxidant by conversion of its cysteines to sulfenic or sulfonic acids thereby absorbing ROS produced by tumors under oxidative stress [
20,
42]. Certainly, our results showed that blocking the CRD of galectin-1 suppressed its protective effects against hydrogen peroxide-induced oxidative stress to ECs. However, the precise contributions of galectin-1 to the pro-angiogenic activities within tumor regions undergoing oxidative stress, including the relationship between the structures of reduced versus oxidized galectin-1 and biological actions of the galectin-1 CRD remain to be resolved.
Intensive chemical structural studies have been aimed at developing more effective and selective galectin inhibitors or their conjugates. Such compounds may produce stronger anti-tumor effects than the TDG disaccharide used here as proof-of-principle. For example, Nilsson et al. [
43,
44] have developed TDG derivatives bearing aromatic amide substituents and aromatic lactose 2-
O-ester derivatives with much lower
K
d values than the prototypic disaccharide used in our study. In addition, a β-sheet forming peptide-based galectin-1 antagonist has been shown to significantly suppress tumor angiogenesis [
4,
45,
46]. Whether these compounds will prove to be effective and free of side effects in vivo remains to be demonstrated.
To conclude, our results emphasize the substantial promise of drugs such as TDG as inhibitors of galectin-1, an emerging target against cancer. Since galectin-1 is a multi-functional protein promoting tumor progression, such inhibitors should now be tested in clinical trials. In addition, the discovery that blocking galectin-1 function promotes T-cell infiltration into tumors supports the therapeutic potential for combining galectin-1 inhibitors as adjuvants to be used with immunotherapy and/or chemotherapy to improve existing cancer treatments.