Background
Colorectal cancer (CRC) is the third most common cancers expected to occur and the third most common cause of cancer deaths both in men and women in the United States in 2017 [
1]. Even multiple modules of treatment including surgical resection and systemic infusional chemotherapy have been applied, the overall 5 year death rate for CRC patients is still about 34% [
2].
Apoptosis includes two main pathways which are termed “extrinsic pathway” and “intrinsic pathway” that involve cell surface death receptors or the mitochondria respectively [
3]. The pro-apoptotic proteins Bax or Bak of Bcl-2 family constitute the central effector of the intrinsic pathway [
4]. Mitochondria have key roles in intrinsic apoptosis execution. Mitochondrial reactive oxygen species (ROS) signaling increases longevity through the intrinsic apoptosis pathway in the nematode
C. elegans [
5]. Chemotherapy promotes survival in CRC patients through ROS mediated apoptosis [
6,
7]. Bax is a key regulatory role in inducing the mitochondrial outer membrane permeabilization (MOMP) [
8]. Mitochondria, the convergence of pro-apoptotic proteins and redox, orchestrate the sequential events of MOMP and intrinsic apoptosis.
Serine-threonine kinase receptor-associated protein (STRAP) is a TGF-β receptor-interacting protein that participates in the regulation of cell proliferation and cell death [
9]. Maternal embryonic leucine zipper kinase (MELK) is a member of the AMP-activated protein kinase-related kinase family and controls a variety of biological processes, including cell cycle, cell proliferation, carcinogenesis, and apoptosis [
10]. STRAP is a positive regulators of MELK and MELK phosphorylates STRAP at Ser188 via direct interaction [
9]. Althrough STRAP [
11,
12] and MELK [
10,
13‐
15] are highly expressed in multiple human cancers, their association in tumor progression and as a therapeutic target is largely unknown.
Macleaya cordata is a kind of commonly used traditional medicinal plants that first described in Ben Cao Shi Yi (“Supplement to the Materia Medica”) in Tang dynasty [
16]. Sanguinarine is a major bio-active component that belongs to benzylisoquinoline alkaloid from
Macleaya cordata. It has been demonstrated that sanguinarine could induce apoptosis in head and neck cancer cells [
17], human colorectal cancer [
18] and drug-resistant non-small-cell lung cancer cells [
19]. Sanguinarine increased the generation of reactive oxygen species (ROS) and activation of c-Jun-N-terminal kinase (JNK) and nuclear factor-kappa B (NF-kappa B) [
17]. We suppose that sanguinarine dictates the apoptosis-related collapse of the mitochondrial membrane potential to suppress the growth of CRC [
20]. The role of STRAP and MELK in regulating MOMP induced by sanguinarine would be documented in our research.
Methods
Regeants
Antibodies against Caspase-3 (No. 9662), PARP (No. 9532), Pro-caspase 9 (No. 9508), Cleaved Caspase-9 (No. 7237), Cytochrome C (No. 4272), Bcl-2 (No. 2870), Bax (No. 5023) and PCNA (No. 13110) were from Cell Signaling Technology (CST), USA. Anti-maternal embryonic leucine zipper kinase (MELK) (No. ab155767) was obtained from Abcam, UK. Anti-serine-threonine kinase receptor-associated protein (STRAP) (No. sc-377,345) and Caspase 8 (No. sc-56,070) antibody was purchased from Santa Cruz Biotechnology, USA. Anti-phospho-MELK (Thr167, Ser171) (No. WG-00203P) and Anti-phospho- STRAP (Thr175, Ser179) (No. WG-00204P) were ordered from ABclonal Technology, China. Alexa fluor® 488 goat anti-rabbit IgG(H + L) (No. CA11008s) and Alexa fluor®546 goat anti-mouse IgG(H + L) (No. A11003) were obtained from Molecular Probes (Invitrogen). Beta-actin antibody (No. E021020–01) was from EarthOx, LLC, San Francisco, USA. Sanguinarine was purchased from National Institutes of Food and Drug Control (Beijing, China). Sanguinarine was dissolved in DMSO (MP, France) and diluted in culture medium for each experiment. The final concentration of DMSO didn’t exceed 0.1%.
Cell lines and cell culture
Human colon carcinoma cell lines SW480 (cat. No. TCHu172) and HCT116 (WT) (cat. No. TCHu 99) were purchased in 2014 from the Chinese Academy of Science Committee Type Culture Collection Cell Bank (Shanghai, China). HCT116 Bax
−/−
human colon carcinoma cell line was a generous gift from Dr. Bert Vogelstein (Howard Hughes Medical Institute, MD). SW480 and HCT116 (WT) cells were cultured in RPMI 1640 medium (Gibco) and HCT116 Bax
−/−
cells in McCoy’s 5A medium (Invitrogen). The medium was supplemented with 10% fetal bovine serum (Gibco) and 1% penicillin-streptomycin (Invitrogen). All cells were maintained at 37 °C in a humidified incubator of 5% CO2-containing atmosphere.
Cell viability assay
Cells were seeded in 96 wells plates at density of 0.8 × 104 cells per well. Sanguinarine was dissolved in DMSO as a stock solution and was stored in aliquots at − 20 °C. After treatment with or without sanguinarine, MTT (3-[4, 5-dimethylthiazolyl-2]-2, 5-diphenyl tetrazolium bromide) (MP, France) was added into each well. Cells were incubated for 4 h. The formazan crystals were dissolved in DMSO. The absorbance value was measured at 490 nm.
Hoechst 33,342 staining
Hoechst 33,342 (MP, France) staining was performed according to the manufacturer’s instructions. Morphological changes in nuclei were observed under a fluorescence microscope using a blue filter (NIKON ECLIPSE Ti-S).
PE-Annexin V/7-AAD staining assay
Cells were stained with PE-Annexin V/7-AAD (BD Bioscience pharmingen) according to the manufacturer’s instructions. Cells were resuspended in 500 μL 1 x binding buffer, 5 μl PE-Annexin V and 5 μl 7-AAD were added to the sample and incubated at room temperature for 15 min in the dark. The stained samples were then detected by flow cytometry (BD FACSCalibur).
Western blotting
Total cells were lysed in RIPA buffer. Mitochondrial and cytoplasmic fractions were isolated according to the manufacturer’s instructions of Cell Mitochondria Isolation Kit (Beyotime, China). Mitochondria were lysed in RIPA buffer. The protein concentrations of the mitochondria, cytoplasm and whole cells were determined by the BCA (Thermo scientific) method using the Thermo protein assay kit. Equal amount of proteins from each group were subjected to SDS-PAGE on 12% gel, transferred to a PVDF membrane (Millipore) by electroblotting, blocked with 5% nonfat milk for 1 h at room temperature and incubated with primary antibodies overnight at 4 °C. Next, the membrane was incubated by HRP-conjugated appropriate secondary antibodies, visualized by enhanced chemiluminescence (Millipore) and exposed by KODAK Image Station 4000MM Digital Imaging System [
21,
22].
JC-1 staining method
Cells were stained with JC-1 according to the manufacturer’s instructions of Mitochondrial membrane potential assay kit (Beyotime, China). Cells were then detected using flow cytometry (BD FACSCalibur). The mitochondrial membrane potential was calculated based on the following equation: mitochondrial membrane potential = red fluorescence intensity / green fluorescence intensity.
Detection of ROS generation
After treatment with sanguinarine of different concentration for indicated time, cells were stained with 5-(and 6)-carboxy-2′,7′-dichlorodihydrofluorescein diacetate (DCFH-DA) according to the manufacturer’s instructions of Reactive Oxygen Species Assay Kit (Beyotime, China). Intracellular production of ROS was evaluated by flow cytometry. Relative Ros levels = DCH fluorescence intensity(treatment group) / DCH fluorescence intensity (control group).
QRT-PCR
Total RNA was extracted with TRIzol (Takara, Dalian, China). 2 μg of the total RNA was reversely transcribed using a reverse transcription kit (Takara) according to the manufacturer’s protocols and cDNA was obtained. The primers for STRAP were 5’-AAGGGACACTTTGGTCCTATTC-3′ (fwd), 5’-CCTACCACAGTTTGCCATAGT-3′ (rev). The primers for MELK were 5′- ACTTGCCTGCCATATCCTTAC-3′ (fwd), 5’-GGTTCTTCAAGGCCTCAATCT -3′ (rev).The primers for GAPDH were 5’-ATTGTCAGCAATGCATCCTG-3′ (fwd), and 5′-ATGGACTGTGGTCATGAGCC-3′ (rev). All reactions were performed in triplicate for 40 cycles in a Stratagene Mx3005P system as previously described [
23]. Relative expression was calculated with GAPDH using the 2
-ΔΔCt.
Immunoprecipitation
Immunoprecipitation was performed as previously described [
24]. Briefly, cell extract was mixed with anti-STRAP, anti-MELK antibody at 4 °C overnight. Agarose beads were added at a ratio of 1 mg of extract per 120 μl of agarose at 4 °C for 3 h. The beads were then pelleted at 2500×g for 3 min and washed with lysis buffer five times. The beads were subjected to elution with 5 vol of 0.5 mg/ml peptide for 4 h or directly boiled in loading buffer.
Immunofluorescence
Cells were fixing with 4% paraformaldehyde at room temperature for 25 min and permeabilized using 0.2% Triton-100X and then blocked by 5% BSA at room temperature for 1 h. Next, they were incubated with primary antibody overnight at 4 °C according to the manufacturer’s instructions. Then, the cells were followed by staining with secondary antibodies (Alexa Fluor 488 and 546, Invitrogen). Cell nuclei were stained with DAPI at 37 °C in darkness for 8 min. Pictures were taken under confocal microscope (NIKON C2+).
Kinase activity assay
The in vitro kinase activity of MELK was analyzed by using HTRF® KinEASE™-STK kit. It is a generic method for measuring Serine/Treonine kinase activities. The assay was performed according to manufacturer’s protocol. MELK Kinase was incubated with sanguinarine and the STK Substrate-biotin for 50 min at room temperature in the presence of ultrapure ATP. STK Antibody labeled with Eu3+-Cryptate STK Substrate-biotin and streptavidin-XL665 were added to the mixture in a single addition with EDTA. They were incubated for 60 min at room temperature to stop the kinase activity. The kinase activity of MELK were detected by Tecan Infinite® 200 PRO microplate spectrophotometer.
Orthotopic CRC model
6 to 8 week-old BALB/c-nu male nude mice (NO. 44002100007706) were purchased from Experimental Animal Centre of Southern Medical University. The mice were housed in a specific pathogen-free facility and maintained under 12 h light/dark cycles. All animal studies were performed according to protocols of the Care and Use of Laboratory Animals published by the National Institutes of Health and approved by the Laboratory Animals Care and Use Committee of Southern Medical University. Orthotopic CRC model on nude mice was established according to our previously described methods [
24]. Twenty-Four tumor-bearing male nude mice were randomly divided into four groups (
n = 6 each group) treated with (1) Control diluents; (2) sanguinarine 4 mg/kg/d by administeration via oral gavage; (3) sanguinarine 8 mg/kg/d by administeration via oral gavage; (4) cisplatin 1 mg/kg/d by intraperitoneal injection for 21 days
. Nude mice were weighted every 5 days. The mice were anesthetized with Sodium Pentobarbital 100 mg/kg and then sacrificed at the end of the experiment according to the euthanasia guidelines for experimental animals of Southern Medical University. The tumor tissues were dissected and fixed in 4% paraformaldehyde for histologic examination or stored at − 80 °C for western blotting.
Tumor tissue samples
The collections of tumor and adjacent tissue samples were approved by the Ethics Committee of Guangdong General Hospital and all aspects of the study comply with the Declaration of Helsinki. Informed consents were signed voluntarily by all patients that included in this study. Fresh primary CRC specimens and paired noncancerous colorectal tissue were provided by the Tumor Tissue Bank of Guangdong General Hospital. In each case, a diagnosis of primary CRC had been made, and the patient had undergone elective surgery for CRC in Guangdong General Hospital between 2015 and 2016. The pathological diagnosis was made in the Department of Pathology of Guangdong General Hospital.
Immunohistochemistry
Tumour tissues from control and treated mice were obtained after sacrificing the mice and then fixed in 4% paraformaldehyde and embedded in paraffin. Sections were deparaffinized, rehydrated. Endogenous peroxidase activity was blocked with 3% (
v/v) hydrogen peroxide solution. Heat-induced antigen retrieval was performed. Sections were incubated with 5% BSA to block no-specific staining. After incubation with the primary antibodies (anti-MELK, anti-STRAP, anti-Bax) overnight at 4 °C in a humid chamber, sections were incubated with mouse/rabbit-labelled polymer from GTVision™ III kit (Gene Tech, China) for 1 h at 37°C. Positive signals were visualized by DAB kit. The slides were reviewed by two or three pathologists blind to the study. To evaluate the expression levels, immunostained slides were evaluated using a method described previously [
25]. TUNEL assay was performed according to manufacturer’s protocol of In Situ Cell Death Detction Kit, TMR red (Roche, Mannhem, Germany).
Statistical analysis
Parallel experiments were repeated three times. Data was analyzed using SPSS 22.0 statistical software. And results are expressed as: mean ± standard deviation (SD). Mean was compared between groups using one Way-ANOVA. When the variance was homogeneous, LSD was used for multiple comparisons. Dunnett T3 method was applied for the unequal variances. Mean of two groups was compared with independent samples t test. A value of p < 0.05 indicated statistically significant difference.
Discussion
Our experimental data provide direct evidence that sanguinarine suppresses the growth of CRC through inducing intrinsic apoptosis in vivo. It induced Bax-dependent apoptosis in multiple CRC cell lines both in dose- and time-dependent manner. It is consistent with the reports that sanguinarine promotes apoptosis in human CRC, bladder cancer, oral squamous cell carcinoma and primary effusion lymphoma cell lines, et al. [
31‐
34]. However, these studies simply broached the subject of the effect of sanguinarine on Bax expression but did not define the underlying mechanisms. Our current study focused on these unaddressed mechanisms and found that MOMP is activated by sanguinarine and Bax is a key regulator in sanguinarine induced MOMP. Cytochrome
c is thus released from mitochondria into cytosol and subsequently triggers hierarchical activation of caspase-9, caspase-3 and caspase-7. Furthermore, we investigated that sanguinarine induced extrinsic apoptosis in CRC cells (Additional File
9: Figure S9).
Our results demonstrate that sanguinarine downregulates the expression of STRAP and MELK. Increased MELK expression has been identified in multiple human cancers: prostate [
14], breast [
13], brain [
35,
36], colorectal [
37], gastric [
38] and lung cancer [
15]. The high expression of MELK is correlated aggressive subtypes and poor prognosis of breast cancer [
13] and poorly differentiated phenotypes in human astrocytoma [
35] and prostate cancers [
14]. Upregulation of MELK drives cell cycle progression and tumor formation and it has been shown to be differentially expressed in cancer stem cells or tumor-initiating cells [
39]. Its expression localizes to colorectal carcinoma and in particular the basal regions of crypts of normal gastrointestinal epithelium where the location of stem cells in normal colonic tissue [
37]. These data suggests that sanguinarine might suppress malignant transformation and proliferation of CRC through downregulating expression of MELK. It has been reported that MELK was associated with increased resistance of colorectal cancer cells against radiation and 5-FU [
40], sanguinarine mightbe a candidate therapy for apoptosis resistant patients.
Serine-threonine kinase receptor-associated protein (STRAP) is a novel tryptophan-aspartate 40 (WD40) domain-containing protein [
11]. It is an oncogenic protein that is up-regulated in 60% of colorectal cancers and 78% of lung cancers and increases proliferate potential of tumor cells [
10,
11]. STRAP acts as a negative regulator of apoptosis signal-regulating kinase 1 and suppresses apoptosis in a dose-dependent manner [
41]. It is consistent with our results that sanguinarine downregulates STRAP and induces apoptosis in CRC cell lines.
The orthotopical models showed the efficacy of sanguinarine with less body weight loss, suggested its translational potential as an anti-CRC agent with less toxicity. Sanguinarine downregulates the expression of MELK and STRAP in CRC tissues. Furthermore, it attenuates the association between MELK and STRAP. Sanguinarine induces intrinsic apoptosis in the presence of Bax. The weakened interaction of MELK and STRAP is necessary for the transactivation of Bax from cytosol to mitochondria. The accumulation of Bax in the mitochondria induces MOMP which cause the release of cytochrome
c into the cytosol from mitochondria [
24].
STRAP and MELK are highly expressed and phosphorylated in tissues of CRC patients. The strengthened association between them might be a marker for the formation of CRC. The attenuated formation of MELK and STRAP complex can be observed in cells treated with sanguinarine, and can also be induced by sanguinarine in mice bearing CRC tumors. These data provide evidence that sanguinarine induces intrinsic apoptosis through downregulation and disassociation of MELK and STRAP. MELK and STRAP are potential therapeutic targets that for triggering MOMP in CRC.
Acknowledgements
We thank Dr. Bert Vogelstein of Howard Hughes Medical Institute for HCT116 Bax−/− cell lines.
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