1. Introduction
Fucoidan is a sulfated polysaccharide found in the cell wall matrix of brown seaweed, such as
Ascophyllum nodosum,
Cladosiphon okamuranus,
Ecklonia kurome,
Fucus evanescens,
Fucus vesiculosus,
Hizikia fusiforme,
Laminaria angustata and
Undaria pinnatifida [
1,
2,
3]. Structurally, fucoidan is a heparin-like molecule with a substantial percentage of
l-fucose, sulfated ester groups, as well as small proportions of
d-xylose,
d-galactose,
d-mannose, and glucuronic acid [
4]. Among the several kinds of fucoidans, the main one is a sulfated polysaccharide of fucodian from
Undaria pinnatifida, described as sulfated galactofucan [
5]. Fucoidan has various biological activities, such as anti-cancer [
6], anti-inflammatory, anti-angiogenic [
7], anti-coagulant [
8] and anti-HIV [
9] activities. However, the action mechanism of fucoidan as an anti-cancer agent has not been fully elucidated.
Prostate cancer is the most commonly diagnosed cancer and second leading cause of mortality in males in industrialized countries [
10]. The incidence of prostate cancer in Asian countries is lower than that in Western countries; however, the mortality of prostate cancer is increasing rapidly in Asian males due to westernization of dietary life style [
11]. Early-stage prostate cancer requires androgens for growth but eventually regresses to an androgen-independent stage, and progresses despite androgen ablation [
12]. The molecular mechanisms for androgen-independent cancer progression are poorly understood. Among prostate cancer cell lines, the PC-3 cell line is known to be analogous to androgen-independent cancer cells [
13].
The up-regulation of the Wnt/β-catenin pathway has been found in a large portion of prostate cancer patients in several reports [
14]. The Wnt ligands that belong to a family of secreted cysteine-rich glycoproteins, have been described to play various roles during early development and tumorigenesis [
15]. The increased expression of β-catenin, a key component of the canonical Wnt signaling pathway, plays a pivotal role in many cancers. The level of free β-catenin is strongly regulated by a β-catenin degradation complex. In the absence of a Wnt signal, the β-catenin level is constitutively decreased by a β-catenin degradation complex, including axin, adenomatous polyposis coli (APC), casein kinase I and GSK-3β [
16]. In the presence of the Wnt signal, the degradation of β-catenin is prevented, which results in the accumulation of β-catenin in the nucleus, a characteristic of the Wnt signaling pathway activation. The nuclear accumulation of β-catenin promotes transcriptional activity of lymphoid enhancer-binding factor (LEF)/T-cell factor (TCF) transcription factors in the nucleus, thereby Wnt target genes (c-myc, cyclin D1, and MMP-7,
etc.) are activated. Among the proteins of the β-catenin destruction complex, GSK-3β plays a pivotal role in the Wnt pathway. GSK-3β is also prevented via Wnt signaling, which may contribute to the progression of prostate cancer [
17].
The present study demonstrates the anticancer effect of fucoidan on apoptosis induction by the down-regulation of the Wnt/β-catenin pathway in PC-3 human prostate cancer cells.
3. Discussion
In the present study, it was observed that fucoidan treatment could induce the apoptosis of PC-3 human prostate cancer cells via the activation of the ERK1/2 MAPK signaling pathway, the inactivation of p38 MAPK and PI3K/Akt, and inhibition of the Wnt/β-catenin signaling pathway.
Previous studies have also indicated that fucoidan from several brown algae directly inhibite the proliferation of various cancer cells, including HS-Sultan [
6], U937 [
25], MCF-7 [
2], HCT-15 [
26] and A549 [
27] and SMMC-7721 [
28] cells in a specific manner without cytotoxicity against the normal cells, such as RAW 264.7 [
29] and HEL-299 [
26] cells. Although no comparative analysis by an experiment was done, these reports suggest that the anti-cancer effects of fucoidan may have nothing to do with its sources. Indeed, this study confirms that fucoidan could directly inhibit the proliferation of PC-3 cells in a dose-dependent manner (
Figure 1).
Apoptosis is a highly regulated physiologic mechanism of cell death during homeostasis, disease and development [
30]; morphologically characterized by chromatin condensation, membrane blebbing, cell shrinkage, and an increased population of sub-G
1 hypodiploid cells [
31]. Fucoidan induced the nuclear morphologic changes of PC-3 cells in line with the physiologic apoptotic process (
Figure 2A); treatment with fucoidan (100 μg/mL; approximately 0.75 μM) also increased the sub-G
1 fraction by 34.72% at 48 h (
Figure 2B,C) compared with the control. On the other hand, resveratrol is known to induce apoptosis of prostate cancer cells. When PC-3 cells were treated with resveratrol (25 μM for 96 h), the sub-G
1 hypodiploid cell population increased by 4.3% as compared to that of the control [
32]. Compared with resveratrol, fucoidan appears to be more effective in the induction of apoptosis in the PC-3 cells.
Two key molecular signaling pathways are implicated in the induction of apoptotic cell death. The one is the extrinsic pathway, which is activated by a death receptor from outside the cell; the other is the intrinsic pathway, which is activated by a Bcl-2 protein family and downstream mitochondrial signals from inside the cell [
18]. Fucoidan treatment led to the activation of DR5 and cleavage of caspase-8, which are critical in the extrinsic pathway; fucoidan also led to the down-regulation of Bcl-2, up-regulation of Bax, and activation of caspase-9, which are essential in intrinsic pathway (
Figure 3A–D). Extrinsic and intrinsic apoptosis pathways induce apoptosis via interaction of MAPK and PI3K/Akt signaling pathways; in other words, these pathways affect each other.
MAPK pathways are known to regulate apoptosis [
33]. Among MAPK proteins, ERK1/2 MAPK is known to promote differentiation, survival and proliferation of cells [
34], but several reports have indicated that the activation of ERK1/2 MAPK can induce apoptosis [
35]. Cisplatin is known to induce apoptosis in the HeLa cells via activation of the ERK pathway [
36]. p38 MAPK is known to be activated by stress to modulate cell differentiation, cell cycle, cell growth, inflammation, and cell death [
37]; whereas some reports have suggested that p38 MAPK can promote cancer cell growth and survival. Docosahexaenoic acid is reported to induce apoptosis of the A549 cells by down-regulation of p38 MAPK [
38]. This study confirmed that fucoidan treatment could activate ERK1/2 MAPK, whereas p38 MAPK was inactivated (
Figure 4A–D). Fucoidan treatment decreased the phosphorylation of Akt as expected (
Figure 5A,B). Phosphorylation of Akt is reported to be regulated by p38; the results in the present study suggest that fucoidan treatment could inactivate p38 MAPK signaling pathway, followed by the inactivation of PI3K/Akt signaling pathway.
Fucoidan increased the cell fraction of the G
0/G
1 phase, whereas the cell percentage of the S phase was decreased (
Figure 6A). Among the cell-cycle-related proteins, E2F-1 and p21
Cip1/Waf are known to play an important role in the cell cycle progression from the G
1 to S phase. Fucoidan decreased expression of E2F-1 and increased expression of p21, followed by the inhibition of proceeding from G
1 to S phase in the PC-3 cells (
Figure 6B,C). E2F-1 and p21 are regulated by β-catenin, an essential component of Wnt/β-catenin pathway.
Wnt/β-catenin signaling plays a pivotal role in the development and progression of prostate cancer. Furthermore, previous a study has suggested that highly invasive androgen-independent prostate cancer cell lines, such as PC-3 and DU-145, display higher levels of Wnt/β-catenin signaling compared with the androgen-dependent prostate cancer cell line, LNCaP, and non-cancerous PWR-1E and PZ-HPV-7 prostate cells [
23]. The activation of β-catenin is inhibited by GSK-3β; GSK-3β promotes the phosphorylation on the serine and threonine residues in the amino-terminal region of β-catenin, and thereby targets it for ubiquitination and degradation via the ubiquitin proteasome pathway by βTrCP or Siah [
16]. In the present study, fucoidan treatment was able decrease β-catenin levels through the activation of GSK-3β (
Figure 7A,B). The activity of GSK-3β is diminished through the phosphorylation of serine 9, which is known to be done by Akt [
17]. Fucoidan treatment decreased phospho-Akt level, followed by the inhibition of GSK-3β phosphorylation (
Figure 5A,B). These results suggest that fucoidan could decrease β-catenin level through the inactivation of Akt and the activation of GSK-3β. When pretreated with LiCl, a GSK-3β inhibitor, and then treated with fucoidan, the level of β-catenin was restored to vehicle-treated level. The results support the hypothesis that fucoidan could regulate the level of β-catenin via the Wnt/β-catenin signaling pathway (
Figure 7E,F). Fucoidan treatment led to a down-regulation of c-myc and cyclin D1, which are known to be β-catenin target proteins (
Figure 7C,D). These results might demonstrate that induction of apoptosis and cell cycle arrest by fucoidan were accompanied by a down-regulation of the Wnt/β-catenin signaling pathway.
Fucoidan is likely to be administered parenterally, because of its high-molecular weight. In recent reports, intraperitoneal administration of fucoidan was able to inhibit breast cancer cell growth in a mouse model [
39,
40]. On the other hand, effects of fucoidan are known to be mediated by class A and class B scavenger receptors (SR-A and SR-B) [
41,
42,
43], and prostate cancer cells including PC-3 express SR-B1 [
44,
45]. Therefore, fucoidan may have therapeutic potential for prostate cancer treatment.
4. Experimental Section
4.1. Materials
Fucoidan (from Undaria pinnatifida) was purchased from Sigma-Aldrich Korea (Sigma-Aldrich Korea, Kyunggi-do, Korea), 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), Hoechst 33342, propidium iodide (PI) and lithium chloride (LiCl) were purchased from Sigma (Sigma Chemical Co., St. Louis, MO, USA). The anti-Bcl-2, anti-c-myc, anti-Bax, anti-procaspase-3, anti-β-catenin, anti-E2F-1, anti-DR5, anti-GSK-3β and anti-phospho-GSK-3β were purchased from Santa Cruz Biotechnology (Santa Cruz Biotech, Paso Robles, CA, USA); anti-p38, anti-phospho-p38, anti-Akt, anti-phospho-Akt, anti-ERK1/2, anti-phospho-ERK1/2, anti-cleaved poly(ADP-ribose)polymerase (PARP) and anti-cleaved caspase-9 were purchased from Cell Signaling Technology (Cell Signaling Technology, Beverly, MA, USA); anti-cleaved caspase-8, anti-p21 and cyclin D1 were purchased from BD Biosciences (BD Biosciences, San Diego, CA, USA).
4.2. Cell Culture
PC-3, a human prostate cancer cell line, was obtained from the Korean Cell Line Bank (KCLB) and cultured in Roswell Park Memorial Institute medium (RPMI) 1640 (Hyclone, Logan, UT, USA) supplemented with 10% fetal bovine serum (Hyclone, UT, USA), 100 U/mL penicillin and 100 mg/mL streptomycin (GIBCO Inc., Grand Island, NY, USA) at 37 °C in a humidified atmosphere with 5% CO2.
4.3. Cell Viability Assay
The effect of fucoidan on the growth of PC-3 cells was evaluated using the MTT assay [
46]. The cells (1 × 10
5 cells/mL) were seeded in 200 μL on 96-well microplates. After 18 h incubation to allow cell attachment, the cells were treated with fucoidan (10, 50, 100 and 200 μg/mL) for 72 h. The cells were treated with 50 μL (5 mg/mL) MTT dye and incubated 37 °C for 4 h. The medium was aspirated and 150 μL/well dimethyl sulfoxide was added to dissolve the formazan precipitate. Cell viabilities were determined by measuring the absorbance at 540 nm using a microplate enzyme-linked immunosorbent assay (ELISA) reader (BioTek Instruments, Inc., Winooski, VT, USA). Each experiment was repeated at least three times.
4.4. Flow Cytometric Analysis of Apoptosis
The effect of fucoidan on cell cycle distribution was analyzed by flow cytometry after staining the cells with PI [
47]. PC-3 cells (1 × 10
5 cells/mL) were treated with 100 μg/mL of fucoidan and cultured for 12, 24 and 48 h. The treated cells were trypsinized, washed twice with phosphate-buffered saline (PBS) and fixed with 70% ethanol for 30 min at −20 °C. The fixed cells were washed twice with cold PBS, incubated with 50 μg/mL RNase A at 37 °C for 30 min, and stained with 50 μg/mL PI in the dark for 30 min at 37 °C. The stained cells were analyzed using fluorescence activated cell sorter (FACS) caliber flow cytometry (Becton Dickinson, San Jose, CA, USA). The proportion of cells in G
0/G
1, S and G
2/M phases was represented as DNA histograms. Apoptotic cells with hypodiploid DNA were measured by quantifying the sub-G
1 peak in the cell cycle pattern. For each experiment, 10,000 events per sample were analyzed, and experiments were repeated three times.
4.5. Morphological Analysis of Apoptosis by Hoechst 33342 Staining
For the detection of apoptosis, the PC-3 cells were seeded at 1 × 105 cells/mL on 24-well microplates. After 18 h incubation to allow cell attachment, the cells were treated with 100 μg/mL of fucoidan and cultured for 12, 24 and 48 h. The cells were incubated in a Hoechst 33342 staining solution in a final concentration of 10 μg/mL at 37 °C for 20 min. The stained cells were observed with an inverted fluorescent microscope equipped with an IX-71 Olympus camera and photographed (magnification ×200).
4.6. Western Blot Analyses
The PC-3 cells were treated with 100 μg/mL of fucoidan and incubated for 12, 24 and 48 h. The cells were harvested and washed twice with cold PBS. The cells were lysed with lysis buffer (50 mM Tris-HCl (pH 7.5), 150 mM NaCl, 2 mM EDTA, 1 mM EGTA, 1 mM NaVO
3, 10 mM NaF, 1 mM dithiothreitol (DTT), 1 mM phenylmethylsulfonylfluoride (PMSF), 25 μg/mL aprotinin, 25 μg/mL leupeptin, and 1% nonidet P-40) and kept on ice for 30 min at 4 °C. The lysates were centrifuged at 15,000 rpm at 4 °C for 15 min. The supernatants were stored at −20 °C until use. Protein content was determined by Bradford assay [
48]. The same amount of lysates were separated on 6%–10% sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) gels and then transferred onto a polyvinylidene fluoride (PVDF) membrane (BIO-RAD, Hercules, CA, USA) by glycine transfer buffer (192 mM glycine, 25 mM Tris-HCl (pH 8.8), and 20% MeOH (v/v)) at 150 V for 90 min. After blocking with 5% nonfat dried milk, the membrane was incubated with primary antibody and then with a secondary horseradish peroxidase (HRP) antibody (1:10,000) at room temperature. The membrane was exposed on X-ray films (Agfa-Gaevert, Antwerp, Belgium), and protein bands were detected using a WEST-ZOL
® plus Western Blot Detection System (iNtRON, Gyeonggi-do, Korea). An anti-β-actin (Sigma Chemical Co., St. Louis, MO, USA) was used as a loading control.
4.7. Statistical Analyses
All results were expressed as the means ± standard deviation (SD) of at least three independent experiments. Student’s t-test was used to evaluate the data with the following significance levels: * p < 0.05, ** p < 0.01 and *** p < 0.001.
