1.1. Colorectal cancer
Colorectal cancer is one of the leading causes of cancer mortality in the world.
Epidemiological studies have linked increased risk of colorectal cancer with a diet
high in red meat and animal fat, low-fiber diet, and low overall intake of fruits and
vegetables [1]. Colorectal cancer comes in many forms, including adenocarcinoma,
leiomyosarcoma, lymphoma, melanoma, and neuroendocrine tumors.
Adenocarcinoma is the most common type of colorectal cancer [2]. In the past 10
years, an unprecedented advance in systemic therapy for colorectal cancer has
dramatically improved outcome for patients with metastatic disease [3]. Until the
mid-1990s, the only approved agent for colorectal cancer was 5-fluorouracil [4]. New
agents that became available in the past 10 years include cytotoxic agents such as
irinotecan and oxaliplatin, oral fluoropyrimidines, and biologic agents such as
bevacizumab, cetuximab, and panitumuma [5]. Development of novel anticancer
drugs for colon cancer therapy is highly desired.
1.2. Anticancer activities of chalcones
Chalcones, a group of aromatic enones, forms the central core of a variety of
important biological compounds, which belong to the flavonoid family and are often
responsible for the yellow pigmentation in plants [6]. These phenolic compounds all
bear a 1,3-diphenyl-2-en-1-one framework. Chalcones have a variety of biological
activities, including antifungal [7], antibacterial, antiprotozoal [8, 9], antimutagenic,
antitumorogenic [10] and anti-inflammatory properties [8, 11, 12]. Various of
chalcones, such as flavokawain B [13-15], isoliquiritigenin [16], and isobavachalcone
[17, 18] have been shown to induce apoptosis in different types of cancer cells. These
triterpenoids have a common target, Bcl-2 protein, which can induce apoptosis in
cancer cells [6, 19]. Chalcones also have potential to block the NF-κB activation and
inhibit proliferation, invasion, metastasis and angiogenesis [20-24].
1.3. Butein and its anticancer activities
Butein (3,4,2′,4′-tetrahydroxychalcon), a plant polyphenol flavonoid extracted
from the stembark of cashews and Rhus verniciflua stokes, has traditionally been used
for the treatment of pain, parasitic, and thrombotic diseases [25]. Butein has
anticancer activities against cancers, including leukemia [26], melanoma [27], breast
carcinoma [28, 29], colon carcinoma [30, 31], osteosarcoma [32], and hepatic stellate
cells [33]. Butein can induce apoptosis in different types of cancer cells [23, 34, 35].
It has been shown that the apoptotic effect of butein is due to its inhibition of the
expression of such NF-κB-regulated gene products as IAP2, Bcl-2, and Bcl-xL [23].
Also, butein can increase caspase-3 activity and expression of death receptor DR5
[36]. Moreover, butein induced cell cycle arrest and apoptosis in human hepatoma
cancer cells [34]. Butein also down-regulated MMP-9 in human leukemia cells [23].
In addition, butein is a tyrosine kinase inhibiter to cause inhibition of EGF-induced
tyrosine phosphorylation of EGFR in cancer cells [35].
1.4. Apoptosis
Characteristic apoptotic features include membrane blebbing, cell shrinkage,
chromatin condensation, and formation of a DNA ladder with multiple fragments
caused by internucleosomal DNA cleavage [37]. There are two major apoptosis pathways, including intrinsic (also called „mitochondrial‟ or „Bcl-2-regulated‟) and
extrinsic (also called „death receptor‟) apoptosis signaling, in cells responsive to
apoptotic stimuli [38, 39]. The extrinsic pathway is initiated by binding of the
transmembrane death receptors such as Fas, tumor necrosis factor (TNF) receptor,
DR3, DR4, or DR5 with their specific ligands, which is followed by activation of
initiator caspase-8 to induce apoptosis [40]. The intrinsic pathway is activated by
intrinsic death stimuli such as reactive oxygen species (ROS), DNA-damaging
reagents, resulting in the release of cytochrome-c and the activation of caspase-9
which in turn activates caspase-3 [41]. Both extrinsic and intrinsic pathways lead to
activation caspase-3 for apoptotic induction [42, 43]. Activation of caspases can
cleave specific cellular substrates, including cytoplasmic structural proteins such as
actin and nuclear proteins such as poly (ADP-ribose) polymerase (PARP) for
inducing cell death [19, 44, 45]. Failure of apoptosis regulation results in pathological
conditions including cancer development [46].
1.5. Cell cycle progression
Cell cycle progression is an important biological event to control normal cells,
which almost universally becomes aberrant or deregulated in transformed and
neoplastic cells. The cell cycle consists of four orderly and tightly regulated phases,
including G1, S, G2 and M [47-49]. The regulation of cell cycle progression is
regulated by cyclin dependent kinases (CDKs) and cyclins [50]. CDKs participate in
cell cycle by binding with cyclins and negatively regulated by CDK inhibitors
(CDKIs) [48]. In order to regulate cell cycle progression, different checkpoints are set
at various stages of the cell cycle [51]. Entry into mitosis is controlled by CDC2, also
known as CDK1 (Cyclin Dependent Kinase 1), which is regulated by the cell
cycle-dependent synthesis and degradation of cyclin B1, which accumulates during
G2/M and disappears at the end of mitosis. CDC2 is also regulated by phosphorylation
at different three sites [52]. Phosphorylation of threonine 161 by CDK-activating
kinase (CAK) is required for CDC2 activity [53], whereas phosphorylation of tyrosine
15 by Wee1 [51, 54, 55] and threonine 14 by Myt1 [56] inhibits CDC2 activity. At the
onset of mitosis, the phosphatase CDC25C dephosphorylates tyrosine 15 and
threonine 14 to activate CDC2 [57].
1.6. Securin and apoptosis
Securin consists of a homologous family of proteins expressed in different
species [58-61]. Securin participates in DNA repair after radiation [62]. Securin
overexpression has been reported in a variety of endocrine-related tumors [63-66] and
nonendocrine-related cancers [67-69]. It has been shown that securin can promote the
cell proliferation and tumorigenesis [70, 71]. Securin levels correlate with tumor
invasiveness, and it has been identified as a key signature gene associated with tumor
metastasis [72]. It has been reported that overexpression of securin induces apoptosis
[70, 73], aneuploidy [74], genomic instability [75, 76], angiogenesis [77, 78], and
senescence [79].
1.7. Securin and cell division
Securin has a well-established role in binding and inhibiting separase, an enzyme
that cleaves the chromosomal cohesion, and thereby ensuring the appropriate timing
of sister chromatid separation to prevent abnormal sister chromatid separation in the
mitosis progression [14, 80-84]. Securin can prevent aberrant chromosomal
segregation when cellular DNA or spindles are damaged [59, 60, 85]. Securin
accumulates during G2 and prophase and is destroyed at the onset of anaphase [14,
86].
1.8. Purpose of this study
The regulation of securin in the butein-induced mitotic arrest and apoptosis was
still unclear. In this study, the anticancer abilities of butein on mitotic arrest and
apoptosis are investigated in the human colon cancer cells. Understanding the
mechanism by which securin regulates butein-induced mitotic arrest and apoptosis
may provide the identification of novel strategies for colon cancer therapy.