Introduction
The process of malignant transformation involves the sequential acquisition of a number of genetic and epigenetic alterations as a result of increasing genomic instability caused by defects in checkpoint controls [
1,
2]. These alterations allow cancer cells to acquire the capabilities to become self-sufficient in mitogenic signals, deregulate the control of cell cycle, escape from apoptosis, and obtain unlimited replication potential [
3‐
5]. Within a growing tumor mass, the genetic changes during tumor progression also enable cancer cells to gain the ability to induce angiogenesis, invade neighboring tissues, and metastasize to distinct organs [
6]. The new chemopreventive agents or therapeutic strategies that inhibit angiogenesis, metastasis and invasion can be considered for future clinical development.
Epidemiological data have demonstrated that curcumin is safe, non-toxic, and has long lasting beneficial effects on human health. Curcumin [1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-hepatadiene-3,5-dione; diferulolylmethane], a major constituent of the yellow spice turmeric, is derived from the rhizomes of
Curcuma spp. [
7]. It possesses antitumor, anti-inflammatory and anti-oxidant properties [
7,
8]. In addition, curcumin has been shown to inhibit tumor metastasis, invasion and angiogenesis [
9‐
12]. We have recently shown that Bax and Bak genes completely inhibited curcumin-induced apoptosis in Bax
-/- and Bax
-/- mouse embryonic fibroblasts [
13], and curcumin induced apoptosis in prostate cancer cells by inhibiting Akt activity upstream of mitochondria [
14]. These data suggest that curcumin regulates multiple signaling pathways and possesses several therapeutic benefits.
Nuclear factor (NFκB) is a dimeric DNA binding protein consisting of members of the NFκB/Rel family [
15]. Its expression is ubiquitous in mammalian cells. Normally, NFκB resides in the cytoplasm in an inactive form in association with inhibitory proteins. These inhibitory proteins, which belong to a family of proteins named inhibitor of NFκB [
15], prevent NFκB nuclear translocation by masking the NFκB nuclear localization signal and thus, inhibit NFκB DNA binding and transactivational function [
15,
16]. Various stimuli activate a large number of distinct signaling pathways that eventually result in the phosphorylation of inhibitor of NFκB and its subsequent degradation by the proteasome or its dissociation from NFκB without additional degradation [
15‐
17]. The released NFκB then translocates to the nucleus and binds to κB DNA motifs to initiate gene transcription. The putative target genes of NFκB are involved in immune and inflammatory responses, and in the control of cell proliferation, apoptosis, metastasis and angiogenesis [
15,
16]. Tumor cells usually express high levels of constitutively active NFκB [
16,
18]. Furthermore, curcumin inhibited NFκB activity in cancer cells [
9,
19] and sensitized cancer cells to chemotherapy and radiotherapy [
20‐
25].
TNF-related apoptosis-inducing ligand (TRAIL) binds to TRAIL-R1/DR4 and TRAIL-R2/DR5. TRAIL induces apoptosis in cancer cells of various origins [
26‐
30]. Data on experimental animals and primates led us to believe that TRAIL has great promise as a selective anticancer agent [
27,
28,
31]. We have recently demonstrated that TRAIL induces apoptosis in several prostate cancer cells lines, but it was ineffective in inducing apoptosis in LNCaP cells [
27,
28,
32]. Furthermore, curcumin sensitizes TRAIL-resistant prostate cancer cells to growth inhibition by TRAIL
in vitro [
33‐
35]. However, the ability of curcumin to sensitize TRAIL-resistant prostate cancer cells
in vivo has not yet been demonstrated.
The purpose of our studies was to investigate the molecular mechanisms by which curcumin sensitized TRAIL-resistant prostate cancer cells in vivo. Our results indicated that curcumin inhibited growth, metastasis, and angiogenesis of TRAIL-resistant LNCaP xenografts in nude mice through regulation of NFκB and its gene products, and sensitized these xenografts to TRAIL treatment. Thus, curcumin can be used alone or combined with TRAIL for prostate cancer prevention and/or therapy.
Discussion
We have recently shown that curcumin induces apoptosis in TRAIL-sensitive PC-3 cells, and sensitizes TRAIL-resistant LNCaP cells
in vitro through activation of multiple signaling pathways [
35]. Curcumin-induced apoptosis engages mitochondria, which was evident by drop in mitochondrial membrane potential and activation of caspase-3 and caspase-9 in both prostate cancer PC-3 and LNCaP cells [
35]. Curcumin induced expression of proapoptotic proteins (Bax, Bak, PUMA, Noxa and Bim), death receptors (TRAIL-R1/DR4 and TRAIL-R2/DR5), and inhibited expression of antiapoptotic proteins (Bcl-2 and Bcl-X
L) and IAPs (XIAP and survivin) [
35]. Since these proteins regulate cell-intrinsic and/or cell-extrinsic pathways of apoptosis, and they may be responsible for sensitization of TRAIL-resistant LNCaP cells. In the present study, we have demonstrated that curcumin inhibited the growth of LNCaP xenografts, metastasis and angiogenesis. Although the TRAIL was ineffective alone, the combination of curcumin and TRAIL had greater effect on tumor growth inhibition, metastasis and angiogenesis than curcumin.
In vitro curcumin downregulated the expression of Bcl-2, and Bcl-X
L and upregulated the expression of p53, Bax, Bak, PUMA, Noxa, and Bim at mRNA and protein levels in prostate cancer cells [
14]. We have also demonstrated that curcumin upregulated the expression, phosphorylation, and acetylation of p53 in androgen-dependent LNCaP cells [
14]. The ability of curcumin to regulate gene transcription was also evident as it caused acetylation of histone H3 and H4 in LNCaP cells [
14]. Furthermore, treatment of LNCaP cells with curcumin resulted in translocation of Bax and p53 to mitochondria, production of reactive oxygen species, drop in mitochondrial membrane potential, release of mitochondrial proteins (cytochrome c, Smac/DIABLO and Omi/HtrA2), and activation of caspase-3 leading to apoptosis [
14]. Furthermore, deletion of Bax and Bak genes completely inhibited curcumin-induced cytochrome c and Smac/DIABLO release in mouse embryonic fibroblasts [
13]. In the present study, tumor tissues derived from curcumin treated mice showed that curcumin inhibited the exprerssion of Bcl-2 and Bcl-X
L, and induced the expression of Bax and Bak. The combinatioin of curcumin and TRAIL was more effective in regulating Bcl-2 family members than single agent alone. Our
in vitro and
in vivo studies demonstrate that curcumin can engage cell-intrinsic pathway of apoptosis by regulating the expression of Bcl-2 family of proteins.
We and others have recently shown that curcumin caused a growth arrest at G1/S stage in several cancers including prostate [
43‐
45]. The G1/S phase arrest by curcumin was associated with the induction of p21
/WAF1, p27
/KIP1, and p16, and inhibition of cyclin D1, cyclin E, Cdk4 and cdk 6 [
45]. The ability of curcumin to induce cdk inhibitors p21
/CIP1 and p27
/KIP1 and inhibit cyclin D1 expression was also confirmed in our xenograft experiment. In a recent study, we have demonstrated that inhibition of p21
/CIP1 inhibited curcumin-induced cell cycle arrest and apoptosis [
46]. We and others have also demonstrated that curcumin induces the degradation of cyclin E expression through ubiquitin-dependent pathway in several cancer cell lines [
44,
45]. Interestingly, deregulated expression of cyclin E correlated with chromosome instability [
47], malignant trasformation [
48], tumor progression [
49], and patient survival [
50]. Overall, our data suggest that curcumin induces growth arrest at G1/S stage of cell cycle.
Entry of malignant cells into the vasculature (i.e. intravasation) requires proteolytic remodeling of the extracellular matrix so that tumor cells may pass through the local stroma and penetrate the vessel wall. The circulatory system then provides a means of transporting tumor cells to distant sites where they extravasate and establish metastatic lesions. Matrix metalloproteinase (MMP) is up-regulated in many tumor types and has been implicated in tumor progression and metastasis. MMP is critical for pericellular degradation of the extracellular matrix, thereby promoting tumor cell invasion and dissemination. To grow efficiently
in vivo, tumor cells induce angiogenesis in both primary solid tumors and metastatic foci. Our results showed that curcumin significantly inhibited the growth of TRAIL-resistant LNCaP xenografts and sensitized these xenografts to undergo apoptosis by TRAIL. Tumor tissues derived from curcumin treated mice showed that curcumin inhibibited proliferation (PCNA and Ki67 staining), induced apoptosis (TUNEL staining), metastasis (uPA, MMP-2 and MMP-9 staining), and angiogenesis (CD31 and VEGF staining). Curcumin also inhibited VEGFR2-positive circulating endothelial cells. Treatment of LNCaP xenografted mice with TRAIL alone had no effect on tumor growth, apoptosis, metastasis and angiogenesis. Our recent
in vitro studies demonstrated that curcumin inhibits capillary tube formation and endothelial cell migration, and the inhibitory effects of curcumin were enhanced in the presence of ERK MAP kinase inhibitor [
35]. These data suggest that curcumin can inhibit tumor growth by inhibiting apoptosis, metastasis and angiogenesis.
TRAIL induces apoptosis in cancer cells which express TRAIL-R1/DR4 and TRAIL-R2/DR5. We have shown that the upregulation of death receptors by chemotherapeutic drugs, irradiation and chemopreventive agents enhance or sensitize cancer cells to TRAIL treatment [
28,
30,
35,
51‐
58]. Specifically, TRAIL-resistant LNCaP cells can be sensitized by chemotherapeutic drugs and irradiation
in vitro and
in vivo through upregulation of death receptors DR4 and/or DR5 [
27,
28]. Similarly, our
in vitro study has demonstrated the upregulation of DR4 and DR5 in PC-3 and LNCaP cells by curcumin [
35]. Interestigly, curcumin sensitized TRAIL-resistant LNCaP xenografts by inhibiting tumor cell proliferation and inducing apoptosis which were correlated with induction of death receptors DR4 and DR5. Death receptor (DR4 and/or DR5) regulation has been shown to be under the control of transcription factor NFκB, SP1 and p53 [
59‐
64]. Inducible silencing of KILLER/DR5
in vivo promoted bioluminescent colon tumor xenograft growth and confers resistance to chemotherapeutic agent 5-fluorouracil [
65]. These finding suggest that upregulation of death receptors DR4 and DR5 by curcumin may be one of the mechanisms by which curcumin enhances the therapeutic potnetial of TRAIL.
The NFκB family of transcription factors has been shown to be constitutively activated in various human malignancies, including a number of solid tumors, leukemias, and lymphomas [
66]. NFκB is shown to contribute to development and/or progression of malignancy by regulating the expression of genes involved in cell growth, differentiation, apoptosis, angiogenesis and metastasis [
66]. Prostate cancer cells have been reported to have constitutive NFκB activity due to increased activity of the IκB kinase complex [
67]. Furthermore, an inverse correlation between androgen receptor (AR) status and NFκB activity was observed in prostate cancer cell lines [
68]. In prostate cancer cells, NFκB may promote cell growth and proliferation by regulating expression of genes such as c-myc, cyclin D1, and IL-6 [
66,
69], and inhibit apoptosis through activation of expression of anti-apoptotic genes, such as Bcl-2 and Bcl-X
L. NFκB-mediated expression of genes, involved in angiogenesis (IL-8, VEGF), invasion and metastasis (MMP-9, uPA, uPA receptor), may further contribute to the progression of prostate cancer. Constitutive NFκB activity has also been demonstrated in primary prostate cancer tissue samples and suggested to have prognostic importance for a subset of primary tumors. In the present study, curcumin inhibited the activation of NFκB and its gene products such as VEGF, Bcl-2, Bcl-X
L, uPA, cyclin D1, MMP-2, MMP-9, COX-2 and IL-8 in LNCaP xenografted tumors. These findings suggest that NFκB may play a role in human prostate cancer development, and/or progression, and curcumin can inhibit these processes through regulation of NFκB-regulated gene products.
Methods
Reagents
Antibodies against CD31, VEGF, VEGFR2, Bcl-2, Bcl-X
L, Bax, Bak, TRAIL-R1/DR4, TRAIL-R2/DR5 and β-actin were purchased from Santa Cruz Biotechnology Inc. (Santa Cruz, CA). Antibodies against p21, p27, phospho-p65-NFκB, Cox-2, IL-8, cyclin D1, uPA, MMP-2, and MMP-9 were purchased from Cell Signaling Technology, Inc. (Danvers, MA). Enhanced chemiluminescence (ECL) Western blot detection reagents were from Amersham Life Sciences Inc. (Arlington Heights, IL). Terminal Deoxynucleotidyl Transferase Biotin-dUTP Nick End Labeling (TUNEL) assay kit was purchased from EMD Biosciences/Calbiochem (San Diego, CA). TRAIL was purified as described elsewhere [
71]. Curcumin was purchased from LKT Laboratories, Inc. (St. Paul, MN).
Western blot analysis
Western blot analysis was performed as we described earlier [
13]. Protein bands were visualized on X-ray film using an enhanced chemiluminescence system.
Xenograft assays in nude mice
Athymic nude mice (Balb c nu/nu, 4–6 weeks old) were purchased from the National Cancer Institute (Frederick, MD). LNCaP cells (2 × 106cells as a 50% suspension in matrigel, Becton Dickinson, Bedford, MA) in a final volume of 0.1 ml were injected subcutaneously at right flank of Balb c nude mice. When the average tumor volume reached about 100 mm3, mice were randomized into four groups of 10 mice/group, and the following treatment protocol was implemented: Group 1, vehicle control (0.1 ml normal saline containing 0.5 % DMSO) administered by oral injection, three times/week (Monday, Wednesday and Friday) beginning the day of tumor cell implantation through out the duration of experiment; Group 2, TRAIL (15 mg/kg) administered i.v. on day 1, 7, 14, and 21; Group 3, curcumin (30 mg/kg, in 0.1 ml normal saline containing 0.5 % DMSO) administered by oral injection, three times/week (Monday, Wednesday and Friday) beginning the day of tumor cell implantation through out the duration of experiment; Group 4, curcumin and TRAIL, curcumin administered through oral injection, and TRAIL administered i.v. Mice were housed under pathogen-free conditions and maintained on a 12 h light/12 h dark cycle, with food and water supplied ad libitum. Tumor volume was calculated using the equation: (volume = length × width × depth × 0.5236 mm3). The in vivo experiment was performed under IACUC's approved protocol.
Immunohistochemistry
Immunohistochemistry was performed as described earlier (28, 29). In brief, tumor tissues were collected on week 6, excised and fixed with 10% formalin, embedded in paraffin and sectioned. Tissue sections were stained with primary antibodies against Bax, Bak, Bcl-2, Bcl-XL, DR4, DR5, Ki-67, PCNA, p21/WAF1/CIP1, p27/Kip1, IL-8, Cox-2, phospho-p65-NFkB, CD31, VEGF, VEGFR2, MMP-2, MMP-9 and uPA or TUNEL reaction mixture. For immunohistochemistry, sections were fixed in cold 100% acetone for 3 min, air-dried, and incubated with various primary antibodies at room temperature for 4 h. Subsequently, slides were washed three times in PBS and incubated with secondary antibody at room temperature for 1 h. Finally, alkaline phosphatase or hydrogen peroxide polymer-AEC chromagen substrate kits were used as per manufacturer' instructions (Lab Vision Corporation). After washing with PBS, Vectashield (Vector Laboratories) mounting medium was applied and sections were coverslipped and imaged.
Electrophoretic mobility shift assay
EMSA was performed as we described elsewhere [
59].
Statistical analysis
The mean and SD were calculated for each experimental group. Differences between groups were analyzed by one or two way ANOVA. The non-parametric Mann-Whitney U test was performed to assess the difference of tumor volume between control and treatment group. To assess the difference between two groups under multiple conditions, one-way ANOWA followed by Bonferoni's multiple comparison tests were performed using PRISM statistical analysis software (GrafPad Software, Inc., San Diego, CA). Significant differences among groups were calculated at P < 0.05.
Competing interests
The author(s) declare that they have no competing interests.
Authors' contributions
SS, SG, and QC have performed the experiments and drafted the manuscript. RS has directed the project and edited the manuscript. All authors read and approved the final manuscript.