Review Article
Current Clinical Challenges and Emerging Multi-Modal Approaches for the Treatment of Pancreatic Cancer
Fiona Haxho1#, Manpreet Sambi1#, Bessi Qorri1#, William Harless2* and Myron R. Szewczuk1*
1Department of Biomedical and Molecular Sciences, Queen’s University, Kingston, ON, Canada
2ENCYT Technologies, Inc. Membertou, NS, Canada
#These authors have contributed equally
*Corresponding author: Myron R Szewczuk, Department of Biomedical and Molecular Sciences, Queen’s University, Canada, Fax: +1 613 533 6796; Tel: +1 613 533 2457; E-mail: [email protected] William Harless, MD, PhD, ENCYT Technologies, Inc. Membertou, NS B1S 0H1, Canada
Published: 16 May 2017
Cite this article as: Haxho F, Sambi M, Qorri B, Harless
W, Szewczuk MR. Current Clinical
Challenges and Emerging Multi-Modal
Approaches for the Treatment of
Pancreatic Cancer. Clin Oncol. 2017;
2: 1296.
Abstract
Today, emerging therapies must effectively shut down multiple enabling characteristics that drive
pancreatic cancer invasion and progression. These therapies include the concomitant suppression of
growth factor signaling and anti-apoptotic pathways, immune-derived promoters of tumorigenesis,
mechanisms of acquired drug resistance, as well as pro-metastatic signals that facilitate cancer cell
migration and successful homing of disseminated tumor cells. Here, we provide an overview of
some of the current treatment options available for patients with pancreatic cancer, as well as their
limitations. Finally, we review some alternative multi-target strategies that may provide increased
efficacy in cancer therapy.
Keywords: Pancreatic cancer; PDAC; Cancer therapy; Drug resistance; Cancer stem cells; Alternative therapy
Introduction
Despite major advancements in cancer treatment, survival rates for patients with pancreatic
cancer have shown minimal improvement over the last forty years [1]. Pancreatic ductal
adenocarcinoma (PDAC) accounts for the majority of pancreatic cancers and is one of the most
lethal malignancies worldwide [2]. The difficulty in diagnosing PDAC at early stages further
contributes to low survival rates. Located in the retro-peritoneum of patients who present with
non-specific symptoms, PDAC is not diagnosed until it has reached an advanced clinical stage in
over 80% of patients [3], with only a 5% five year survival rate [4]. Furthermore, lack of effective
screening and early biomarker detection has prevented clinicians from identifying this cancer in a
pre-malignant stage.
Once diagnosed, a number of different interventions are used to treat disease progression,
including surgical resection, neoadjuvant and adjuvant chemotherapy and radiation. Unfortunately,
the inherent and acquired heterogeneity of primary tumors and secondary metastases renders them
highly malignant and gradually resistant to the majority of such therapies. Today, it is understood
that in order to overcome these treatment challenges, emerging therapies must effectively shut
down multiple enabling characteristics that drive pancreatic cancer invasion and progression.
These include the concomitant suppression of growth factor signaling and anti-apoptotic pathways,
immune-derived promoters of tumorigenesis, mechanisms of acquired drug resistance, as well as
pro-metastatic signals that facilitate cancer cell migration and successful homing of disseminated
tumor cells. Here, we provide an overview of some of the current treatment options available for
patients with pancreatic cancer, as well as their limitations. Finally, we review some alternative
multi-target strategies that may provide increased efficacy in cancer therapy.
Current Treatment Strategies and Limitations
Although surgery offers hope for curative therapy, less than 20% of patients present with
potentially operable tumors [5]. A number of poor predictors for successful resection has been
identified, including lymph node involvement [6], high tumor grade [7], large tumor size [8], elevated CA 19-9 levels [9], and positive tumor margins post-surgery [5]. Unfortunately, surgical
patients with pancreatic cancer remain at a high risk for relapse. On average, surgery has been
shown to prolong patient survival by only 10 months [10]. Patients with advanced disease do not
meet the criteria for surgery as their cancer can demonstrate distant metastasis, pancreatic lymph
node involvement, encasement or occlusion of the superior mesenteric vein (SMV) or SMV/portal vein confluence and/or involvement of the celiac axis, aorta, inferior vena cava or superior mesenteric artery [5]. As a result, radiation and
chemotherapy is recommended for these patients.
Radiation therapy is used to eliminate rapidly dividing cells
located in a specific area of the body. It can be delivered as highenergy
rays or as a radioactive agent and is able to selectively induce
DNA damage in proliferating tumor cells. Neoadjuvant radiation
therapy may be used prior to surgery to shrink an operable tumor,
or used after surgery (adjuvant radiation) to treat residual disease.
However, the efficacy of radiation therapy is suboptimal due to its
limited tolerance in normal tissue.
Neoadjuvant chemotherapy is used as an induction treatment to
shrink a tumor before the primary treatment which is usually surgery.
Pre-operative chemotherapy with radiation has also been shown to
improve survival, but not the cancer stage of locally invasive tumors
[11]. Post-operative chemotherapy and/or chemo-radiation are often
incorporated in the therapeutic regimen. Indeed, adjuvant chemoradiation
has become the most frequently used adjuvant treatment
for resectable pancreatic cancer in the United States [5]. Since 1997,
gemcitabine (20,20-difluoro-20-deoxycytidine or dFdC) has been
the first-line therapy for patients with PDAC [12]. This is due to its
lower toxicity when compared to other chemotherapeutic agents
and increased progression-free survival. Gemcitabine is a nucleoside
pyrimidine analog with multiple modes of action inside cancer cells,
the most important being the inhibition of DNA synthesis [13]. When
gemcitabine triphosphate (dFdCTP) is incorporated into DNA, only
a single deoxynucleotide can be incorporated afterwards, ultimately
preventing chain elongation [14]. The induction of apoptosis through
caspase signalling is another important mechanism of action, in
which gemcitabine activates p38 mitogen-activated protein kinase
(MAPK) to trigger apoptosis in response to cellular stress in tumor
cells, but not in normal cells [15-18]. Indeed, activity of MAPKactivated
protein kinase (MK2), a p38-MAPK effector, was shown
to be required for gemcitabine-induced cell death in vitro. However,
in these patients, gemcitabine treatment has been shown to prolong
the average survival rate by only 4 months and is primarily used in
palliative care. In phase II and III studies, gemcitabine has also been
administered in combination with platinum analogues, including
cisplatin [19,20] and oxaliplatin [21]. Other effective chemotherapy
agents used today in the neoadjuvant and adjuvant setting include
folfirinox (5-fluorouracil, leucovorin, irinotecan, oxaliplatin), or
a combination of gemcitabine with nanoparticle-bound paclitaxel
(abraxane). Paclitaxel is a chemotherapeutic agent which interacts
with microtubules, specifically the β-subunit on tubulin to prevent
the formation of functional mitotic spindles during cell division [22].
In one phase III trial, combined treatment with gemcitabine and
paclitaxel demonstrated significantly improved patient outcomes,
when compared to treatment with either agent alone [23].
These regimens can demonstrate minor tumor shrinkage in
20-30% of patients and can slow the progression of the disease
for approximately six months in patients with metastatic cancer
[24]. Developments of other complementary agents to enhance
chemotherapeutic effects are currently under review in pre-clinical
and clinical trials [5]. Conceptually, any treatments that are better
able to shrink primary tumors from borderline operable to potentially
curative, or to eradicate remaining micrometastatic disease after
surgery, would represent a huge advancement in our ability to
treat pancreatic cancer. Such treatments would also be predicted to
improve progression-free survival in patients with metastatic disease, most of whom will die within one year of diagnosis.
Ongoing Challenges for Drug Development
Acquired drug resistance
One of the greatest difficulties in curing metastatic disease is the
inability to prevent or reverse the acquired resistance to drug therapy.
The high frequency of acquired chemoresistance in PDAC has been
linked to an accumulation of highly penetrant genetic mutations at
various loci, including K-ras, p53, cdkn2a and smad4/DPC4 [25].
Originating in the ductal epithelium, pancreatic cancer can quickly
evolve from a pre-malignant lesion to an aggressive and invasive
metastatic disease [25]. 90% of PDACs have point mutations within
the KRAS2 oncogene, resulting in constitutive expression of Ras
[26]. These genetic alterations can sustain the malignant phenotype
because once activated, Ras initiates a signal transduction cascade
that activates proliferation and cell survival pathways and increases
cancer cell invasion [27]. These point mutations are of clinical interest
because they may result in the expression of pancreatic tumor-specific
neo-antigens, capable of being recognized by helper and cytotoxic T
lymphocytes, and may therefore represent novel vaccine targets [27].
In contrast to the constitutive expression of the KRAS2 oncogene,
the p53 tumor suppressor gene is inactivated in approximately
80% of pancreatic tumors [28]. As a result, there is impaired DNA
damage recognition and repair in pancreatic epithelial cells, impaired
apoptosis and deregulated cell cycle control [29]. Two other tumor
suppressor genes, p16Ink4a and p15ARF are encoded by the cdkn2a
locus. Inactivation mutations in these genes are present in about 90%
of human pancreatic cancers [30], and are implicated in the drugresistant
mechanisms of the cancer. The number and combination of
these mutations correlates with patient prognosis, such that patients
with 3-4 mutations will have a poorer diagnosis than those with only
1-2 mutations [30].
The upregulated or downregulated activities of specific drug
transporters also play crucial roles in the efficacy of chemotherapy.
For example, human equilibrative nucleoside transporter-1 (hENT1)
is a membrane facilitative transporter responsible for the direct entry
of gemcitabine into cancer cells [31]. Gemcitabine is a hydrophilic
drug, and its rate of cellular entry through the hydrophobic plasma
membrane is negligible without hENT1 transporter activity.
Indeed, a lower expression level of hENT1 has been correlated with
gemcitabine resistance [31]. As major drivers of drug resistance,
these genetic and cellular components represent important targets for
drug development as well as patient-specific predictors of treatment
response.
Pancreatic tumor heterogeneity and cancer stem cells
In an effort to improve personalized cancer therapy, studies
have begun to elucidate the genetic heterogeneity among pancreatic
cancer patients. Tumor heterogeneity, a concept proposed over 30
years ago, refers to the presence of multiple subpopulations within
a single neoplasm each of which are postulated to originate from a
unique lineage [32]. These lineages may differentiate subpopulations
by their ability to metastasize, self-renew, proliferate and acquire
chemoresistance, among other processes observed in tumorigenesis
[32]. As described above, current cytotoxic therapies for PDAC are
designed with the intention of arresting cell proliferation and target processes in cell division such as DNA synthesis [12]. Although single
agent and combination therapies show some efficacy in increasing
patient survivorship, cancerous lesions continue to be treated as
homogenous growths. As a result, almost no treatment options have
been developed to target this intratumoral heterogeneity.
The concept of clonal expansion and subsequent development
of unique subpopulations within tumors was first described by
Peter Nowell [33], and its clinical implications have been extensively
described for over four decades [34,35]. In order to effectively cure
PDAC, emerging therapies must target the variation in tumor cell
composition and their unique susceptibility to different anti-cancer
agents [36]. This variation can include differences at the genetic level
(i.e. different mutations that subpopulations may possess) and/or
at the phenotypic level (i.e. the unique cell surface protein targets
on a subpopulation of cells that can be therapeutically exploited).
Therefore, PDAC must be conceptualized as a dynamic growth that
is influenced by the surrounding microenvironment and is made
up of a diverse population of cells. Two different subpopulations
can co-exist or they can be separated by a physical barrier, such as
blood vessels, or by a difference in their microenvironment. Both of
these factors may generate differences in how these subpopulations
respond to therapy and difficulties arise in detecting these mosaic
phenotypes in heterogeneous tumors, further complicating the
development of a personalized treatment regimen [37]. For example,
inflammatory cells have been shown to secrete both pro-angiogenic
and anti-angiogenic cytokines [38]. Therefore, the tumor cells that
are able to respond to pro-angiogenic cytokines, as opposed to those
that do not, would be able to promote tumor neovascularization
and comprise a distinct subpopulation within the tumor. This
has been shown in bladder cancer, in which a subpopulation of
cells with increased expression of CD14, a glycoprotein involved
in the signaling pathways of Toll-like receptors (TLRs) were able
to facilitate neovascularization of bladder tumors by recruiting
endothelial cells with greater efficiency [39]. Interestingly, cells
that expressed low levels of CD14 had greater proliferative capacity
and represented another distinct subpopulation. Notably, both of
these cell populations were postulated to behave synergistically in
promoting overall tumor growth [39]. This suggests that targeting
one particular cancer cell subpopulation might not be effective as
other surrounding subpopulations can compensate to facilitate their
regrowth. Moreover, one study performed a comprehensive genome
assessment on 24 different pancreatic cancers [40]. Results revealed
an average of 63 genetic mutations per cancer, spanning 12 separate
signal transduction pathways. This study supports the notion of
pancreatic cancer being a genetically heterogeneous malignancy,
partially accounting for its notable resistance to therapy as well as
varied responses to treatment. Moreover, this finding likely explains
why no candidate gene has yet been identified. This cancer cell
heterogeneity will likely dictate an individualized, unique approach
for each particular case.
Another major obstacle in the treatment of pancreatic cancer is
the selective targeting and killing of cancer stem-like cells (CSCs).
Tumor initiating populations, or CSCs, have been identified in a
number of cancers, all of which began with the seminal discovery of
these tumor progenitor cells in leukemia [41], followed by verification
of these populations in breast [42], brain [43] and pancreatic cancer
[44]. In PDAC, tumor initiating cells (TICs) are characterized as
being CD44+/CD24+/ESA+ [44]. 0.2-0.8% of pancreatic cancer cells
possess this unique phenotype and have the capacity to re-establish
progeny with a nearly identical phenotype when compared with the
primary tumor [44]. Not only do these cells possess unlimited selfrenewal
potential, they are also capable of giving rise to differentiated
progeny [44]. The identification and targeting of this subpopulation
is of particular clinical importance as these cells are also resistant to
chemotherapy [45].
Hypoxia is another important hallmark of tumor growth and can
select for a unique subset of cancer cells that are capable of survival
in an oxygen-depleted environment, including CSCs [46]. These cells
can upregulate their expression of cytokeratin 19 (an epithelial stem
cell marker for breast cancer) [47] and/or CD34 (a hematopoietic
stem cell marker expressed in leukemic cancer). Unless these cells
are activated to proliferate, conventional chemotherapeutic drugs
will not affect TIC expansion. Thus, while it may appear that a tumor
is shrinking, in reality chemotherapeutic drugs are targeting the
differentiated cell population within the tumor allowing for the tumor
to regenerate itself and potentially metastasize and home to distant
organs [48]. Recent findings demonstrate that these cells are capable
of epithelial-to-mesenchymal transition (EMT), contributing to their
motility and metastatic behavior due to phenotypic changes [49].
Current therapies have already been designed to target CSC-specific
antigens in order to inhibit their roles in cell survival, adhesion, selfrenewal
and invasion.
A greater understanding of individual CSC populations and how
they interact with one another will advance progress in the treatment
of pancreatic cancer. Therapeutic targets of pancreatic CSCs include
genes located in developmental pathways such as hedgehog, Wnt,
Notch, CXCR4 and Met. In addition, targeting apoptotic pathways
such as DR5 and nodal-activin may also provide significant
therapeutic benefit [44,49].
Desmoplasia, the tumor microenvironment and immuneregulated
tumorigenesis
Paracrine signals from pancreatic cancer cells stimulate the
extracellular proliferation of leukocytes, fibroblasts, endothelial
cells, neuronal cells, collagen type I and hyaluron. This extracellular
proliferation of cells is known as a desmoplastic reaction forming a
thick stromal environment around the pancreatic cancer cells [50],
providing a mechanical barrier to the tumor cells and also thought to
contribute to the anti-angiogenic environment that is characteristic
of PDAC. Both properties directly affect therapeutic efficacy as the
dense microenvironment limits drug delivery to the primary tumor.
Furthermore, the increased deposition of collagen and fibronectin
results in decreased elasticity of tumor tissue accompanied with
an increase in tumor interstitial fluid pressure (IFP). Increased IFP
results in a lower perfusion rate of therapeutic agents, ultimately
diminishing their efficacy [51]. Studies have demonstrated that the
signals that influence the proliferation of the desmoplastic reaction
originate from the K-ras mutant oncogene in the epithelium of the
tumor [52]. Sonic hedgehog (SHH) functions similarly to the K-ras
mutant. Although it is overexpressed in pancreatic cancer cells during
the early stages of their development, SHH does not act on the SHH
pathway in these cells 52]. Instead, it acts in a paracrine fashion in
extracellular fibroblasts, resulting in their growth and differentiation.
The key players in the formation and turnover of this dense stroma
are pancreatic stellate cells. Certain growth factors, including TGF-β1,
PDGF and FGF, are able to activate these cells into myofibroblasts
which can then secrete components of the extracellular matrix to
further reduce the vascularization of the primary PDAC tumor [53].
In addition to forming a mechanical barrier around the pancreatic
cancer cells, the stroma has an important role in tumor formation,
progression and metastasis [54]. Many proteins expressed by stromal
cells have been directly correlated with poor prognosis and resistance
to current therapies, including COX-2, PDGF receptor, VEGF,
stromal-derived factor, chemokines, integrins, secreted protein acidic
and rich in cysteine (SPARC) and SHH elements.
Notably, the dense stroma is characterized by a tumor-promoting
immunosuppressive environment. Using a CD40 antibody combined
with gemcitabine chemotherapy, researchers have attempted to
reverse immune suppression and drive anti-tumor T cell responses in
patients with non-resectable pancreatic cancer. Studies have shown
that this dual combination results in tumor regression by stimulating
tumor-associated macrophages (TAMs) to attack and deplete the
pancreatic cancer stroma [55]. To date, treatment of PDAC has
proved most effective in patients with locally advanced disease,
especially in patients with tumors characterized by wild-type tumor
suppressor Smad4 (DPC4). These tumors are known to be less prone
to metastasis and possess higher stromal content. However, primary
tumors that have already metastasized cannot be effectively treated
with current stromal-targeting agents. This is due to the fact that
although PDAC has a rich and hypovascularized stroma, metastases
arising from this cancer do not, making them more similar to other
highly vascular tumors [56]. Other studies have also suggested a role
for the tumor stroma in the T cell-depleted microenvironment of
pancreatic cancers [57]. Several cell types found in the desmoplastic
reaction are involved in localized tumorigenesis, including TAMs,
cancer associated fibroblasts, regulatory T-cells (Tregs) and myeloid
derived suppressor cells. In addition, K-ras dependent signaling
molecules have been shown to upregulate granulocyte-macrophage
colony stimulating factor (GM-CSF) when activated, thus promoting
the maturation of immature myeloid progenitor cells into myeloid
derived suppressor cells.
Unlike the immunosuppressive nature of the stroma, the primary
pancreatic tumor and/or distant micrometastases can be exposed
to a highly inflammatory microenvironment. Tumor-derived proinflammatory
signalling can contribute to chemoresistance , selection
of cancer stem-like cells and the desmoplastic reaction. Nuclear factor
kappa B (NF-κB) signaling, critical for the inducible expression of
cellular and viral genes involved in inflammation, has been found
to be constitutively activated in pancreatic cancer. Tumor-derived
inflammation is also associated with cyclooxygenase (COX) activity.
COX-1 is constitutively expressed in most tissues, while COX-2,
the inducible form, is not normally expressed, but upregulated by
cytokines, growth factors, and tumor-promoter genes. COX-2 has
been found to be upregulated in PDAC, localized to the cytoplasm of
the tumor cells and not in the surrounding stromal or inflammatory
cells [58]. COX-2 is one of the downstream target genes of NF-κB,
and is involved in mechanisms such as prostaglandin synthesis,
promotion of angiogenesis, inhibition of immune surveillance and
inhibition of apoptosis.
Cancer-Associated Hypercoagulation and Angiogenesis
Thromboembolic disease is a common complication and can be
the presenting feature of pancreatic cancer, usually associated with a
poorer prognosis [59]. Pancreatic cancer cells activate platelets and
express several pro-coagulant factors, including tissue factor and
thrombin [59]. Tissue factor, a transmembrane-receptor protein
that initiates the extrinsic pathway of coagulation, can promote
an angiogenic phenotype by upregulation of vascular endothelial
growth factor (VEGF) and downregulation of the angiogenic
inhibitor thrombospondin [60,61]. Tissue factor also seems to
control angiogenic tumor signals through the production of growthregulatory
molecules from the endothelium [61]. Furthermore,
mutated or activated KRAS2 (found in 95% of pancreatic cancers) can
directly or indirectly affect angiogenesis (through increased VEGF
expression), thrombosis (through increased expression of urokinase
plasminogen activator), and metastasis (through increased expression
of matrix metalloproteinases) [62]. Thrombin is another key enzyme
involved in coagulation, and can convert circulating fibrinogen to
fibrin, activate platelets, and amplify initial signals in the coagulation
cascade. Thrombin generates a fibrin scaffold that attracts endothelial
cells, activates various protease-activated receptors on endothelial
cells, increases expression of VEGF receptors, and activates hypoxia
inducible factor 1α (HIF-1α), leading to the production of several
angiogenic molecules [63]. Functional thrombin receptors have
been identified in human pancreatic cancer cells [64,65], but not
in healthy pancreatic cells [66]. Thrombin enhances adhesion of
PDAC cells to ECM proteins and to endothelial cells, suggesting an
important role for tumor growth and invasion [67]. Fibrin, the end
product of the coagulation cascade, also plays an important role in
the prothrombotic and proangiogenic state of cancer, especially in
pancreatic cancer. The fibrin matrix functions as a scaffold and as a
reservoir for proangiogenic growth factors such as heparin binding
growth factor-2 and VEGF. It enhances the activity of heparin
binding growth factor-2 [68], and sequesters several growth factors
from proteolytic degradation [69].
The activation of coagulation is not simply a phenomenon,
but has also been shown to enhance tumor growth, angiogenesis
and metastasis. Treatment options include warfarin and lowmolecular-
weight heparins (LMWH); however, studies over the past
decade indicate that the use of LMWH in the prevention of venous
thromboembolic disease improves outcomes for cancer patients, in
comparison with warfarin and other anticoagulants [59]. A review
by Khorana and Fine summarizes the promising clinical studies
employing anti-coagulant therapy in cancer [59]. Strong preclinical
data suggest that heparin, or LMWH, offer advantages over warfarin
in terms of efficacy of anti-coagulation, as well as anti-cancer effects
(including inhibition of angiogenesis). Emerging prospective clinical
data support this finding by showing improved outcomes with
protracted use of LMWH [59].
A Multi-Modal Approach To Optimizing Treatment
Suppression of cancer cell metabolism and growth
In addition to the use of proliferation-targeted interventions such
as chemotherapy and radiation, major metabolic pathways in cancer
cells may also be exploited. This may include: 1) the disabled/reduced
supply of glucose and glutamine to the tumor; 2) interruption of the
mechanisms that enable survival in a hypoxic environment [70];
and/or 3) prevention of the cancer cell’s ability to digest intracellular
organelles for energy [71]. Since aberrant metabolic pathways have
become a hallmark of cancer, investigators have identified several
key metabolic enzymes to target, including hexokinase, pyruvate
kinase, lactate dehydrogenase A (LDHA) and ampicillin-activated
proteinkinase (AMPK). Several pre-clinical trials have demonstrated
the anti-tumor effects of agents directed against these enzymes. Two of these drugs, rapamycin and metformin, have shown promising
results when used alone, or in combination with other anti-cancer
therapies. Rapamycin, an inhibitor of the mammalian target of
rapamycin (mTOR), is able to decrease glucose uptake by reducing
levels of Glut1 in pancreatic cancer [72]. Metformin, an oral
hypoglycemic agent for the treatment of type 2 diabetes, has a glucoselowering
effect and is able to reduce hyperinsulinemia by improving
insulin sensitivity in peripheral tissues [73,74]. Metformin reduces
gluconeogenesis in the liver, an effect mediated by AMP-activated
serine-threonine protein kinase (AMPK). AMPK is an intracellular
sensor of energy and nutrient levels, and is a regulator of cell ATP
and lipid, cholesterol and glucose metabolism and homeostasis
[75,76]. Recent studies have shown that diabetic patients treated with
metformin have a lower incidence of cancer. The phosphorylation of
the AMPK catalytic subunit is regulated by liver kinase B1 (LKB1).
Notably, LKB1 is the protein product of a corresponding tumor
suppressor gene. The activation of the LKB1–AMPK pathway inhibits
the mammalian target of rapamycin complex-1 (mTORC1), a kinase
activated in the majority of human cancers [77]. In prostate cancer
cell lines, metformin demonstrated an anti-proliferative effect via the
induction of a p53 target gene (REDD1) [78]. Notably, metformin
was also found to play a role in NF-κB signaling, a pro-inflammatory
pathway implicated in the enhanced proliferation, anti-apoptotic
mechanisms, and invasiveness of cancer cells, as well as in the immune
surveillance of tumors [79]. Hattori and colleagues showed that
metformin blocked NF-κB activation induced by TNF-α in vascular
endothelial cells [80]. In smooth muscle cells, metformin was found
to suppress the phosphorylation of key signaling molecules involved
in NF-κB activity, including p38, JNK, Erk and Akt [81]. Metforminmediated
inhibition of NF-κB activity in mouse pancreatic tumors
was also found to downregulate the mRNA expression of MCP-
1, TGF-β1, TNF-α, and IL-1β, each of which play unique roles in
tumor development [82]. Metformin has also been shown to inhibit
the TNF-α-induced secretion of CXCL8, a chemokine with wellestablished
pro-tumorigenic actions [83].
A number of other studies have demonstrated an anti-cancer
effect of metformin on the cell cycle, apoptosis and glioblastoma
[86], colon [87], ovary [88], pancreas [89], lung [90], and prostate
tumors [91]. Metformin also seems to have an affect on CSCs. Bao
et al. showed that metformin attenuates CSC phenotypes, functions
and mediators [92]. The drug reduces the expansions of CSC clones
by inducing apoptosis and by inhibiting CSC mediators and markers.
Other lines of research suggest that metformin regulates the EMT
status, an essential differentiation program in early embryonic
development that is modified in cancer to mediate acquisition of
malignant and stem-like cell properties [93]. Metformin decreases the
expression of key drivers of EMT including the transcription factors
ZEB1, TWIST1 and SNAI2 (Slug), and the pleiotropic cytokines
TGFβs in several cell types [94]. The inhibition of these components
of EMT by metformin causes an inhibition of cell invasiveness
without affecting cell migration [95]. Metformin was also shown to
reduce the expression of miR-34a and its direct EMT targets Notch,
Slug, and Snail [96].
Targeting Tumor-Associated Inflammation and Apoptosis-Resistant Cancer Cells
Due to the inflammatory nature of the disease, non-steroidal antiinflammatory
drugs (NSAIDs) such as aspirin have been proposed
to provide therapeutic effects to manage inflammation and sensitize
malignant cells to chemotherapy [97]. NSAIDs such as aspirin inhibit
NF-κB activation by binding to IKK-2. Inhibition of NF-κB in PANC-
1 cells by aspirin was found to be dose-dependent. Notably, MTT
assays on PANC-1 cells treated with 18 mM aspirin resulted in no
significant inhibition in the growth of cells [98]. A different study also
investigated the effect of aspirin on the proliferation of four pancreatic
cancer cell lines. It found a negative correlation between the intensity
of COX-2 expression and the IC50 of aspirin [99]|. A dose-dependent
growth inhibition was seen across all cell lines following 72 hours of
aspirin treatment. However, cell lines with low COX-2 expression
(KP-2 and PNS-1) demonstrated a significantly lower IC50 than those
with high COX-2 expression (MiaPaca-2 and PANC-1).
Anti-inflammatory agents have also been proposed to
counteract acquired resistance to drug therapy. One study used
four chemoresistant PDAC cell lines and treated the cells with
gemcitabine, different concentrations of aspirin, or a combination of
these drugs [97]. Aspirin was found to significantly induce apoptosis
in vitro and reduce the viability, self-renewal potential, expression of
inflammatory mediators and CSC signaling. Specifically, treatment
with aspirin for 48 hours decreased the expression of TNF-α and
downregulated the self-renewal stem cell markers Oct4, Nanog and
SOX2. Aspirin also blocked the growth and invasion of orthotopic
pancreatic tumor xenografts and significantly prolonged the survival
of mice when co-administered with gemcitabine. Furthermore, aspirin
treatment resulted in decreased inflammation and desmoplasia in
vivo, as well as reduced expression of tumor progression markers
Ki67, c-Met, CSCR4, CD44 and TNF-α [97]. Due to its wellestablished
safety profile, as well as its promising application in preclinical
cancer studies, aspirin represents a novel and well-tolerated
chemo-sensitizing agent for the treatment of pancreatic cancer.
Celecoxib is another COX-2 inhibitor that has demonstrated
potent anti-tumor activity in a wide variety of tumor types, including
prostate [100], colorectal [101], breast [102], and non-small cell
lung cancers [103]. Today, several pre-clinical trials are assessing
the use of celecoxib in the prevention and treatment of pancreatic,
breast, ovarian, non-small cell lung cancer and other advanced
human epithelial cancers [104]. Among the COXIB-family members,
celecoxib has the unique capacity to induce apoptotic cell death in
tumor and endothelial cells. Although inhibition of COX-2 can
contribute to its cytotoxic effects, celecoxib is a prototype of drugs that
induce cell death independently from COX-2 mainly by activation
of an intrinsic, mitochondria-dependent apoptosis pathway [104].
COX-2-independent drug targets include the survival kinase Akt, the
Ca2+ ATPase SERCA, GSK-3b/b-catenin, and anti-apoptotic proteins
of the IAP and the Bcl-2 families [104]. Studies have also shown that
celecoxib can significantly trigger cell death in Bcl-2 overexpressing
cells and downregulate the anti-apoptotic factors Mcl-1 and survivin
[104]. Thus, the pro-apoptotic activity of celecoxib differs from that
of standard chemo-radiation and provides promising evidence for
the use of celecoxib in apoptosis-resistant tumors. Furthermore,
neoplastic disease that depends on Bcl-2, Mcl-1 or survivin for cell
survival seems to be an ideal target for the use of celecoxib alone or
in combination with chemotherapy, radiation or other anti-cancer
agents.
Blocking Upregulated Receptor Signaling via Glycan Modification
The abnormal expression of cell surface glycosylation has become
a key hallmark of cancer and provides a new dimension for targeting tumor cells. Specifically, the terminal sialylation of several receptors
that are upregulated or constitutively active in cancer cells is known
to regulate their structure, ligand affinities, as well as downstream
signaling cascades. The sialidase activity of neuraminidase-1 (Neu1)
has been previously shown to regulate the activation of EGFR, insulin
receptor (IR), and a number of TLRs [105]. Notably, these receptors
each play unique and profound roles in tumor development via
promotion of cell proliferation and survival pathways, cell growth
and metabolism, and immune-mediated tumorigenesis, respectively
[106]. It is important to note that the regulatory role of Neu1 is
dependent upon a cell-surface signaling platform that induces
neuromedin B G protein-coupled receptor (NMBR) activation and
MMP9 activity. Abdulkhalek et al. [105], have reported an extensive
review describing this cell-surface molecular mechanism and its role
in regulating receptor activation and downstream signaling.
We have previously shown that Neu1 inhibitor oseltamivir
phosphate (OP) demonstrates potent anti-cancer effects in vitro and
in mouse models of pancreatic [107-109], breast [110], and ovarian
cancer [111]. Briefly, Neu1 inhibition by OP is able to suppress
oncogenic downstream cellular pathways that are associated with
EGF and insulin signaling [109,112], as well as TLR-mediated proinflammatory
signaling [113-115]. We have previously reported
that OP treatment in mice bearing PANC-1 and MiaPaCa-2 tumor
xenografts significantly improved animal health and survival,
decreased tumor volume and angiogenesis, and prevented metastasis
to the liver and lungs [107-109]. Tumor neovascularization was
significantly decreased in OP-treated mice, as indicated by H&E
analysis and immunostaining for tumor CD31 (murine endothelial
marker). It is proposed that the ability of OP to downregulate a
number of signaling pathways simultaneously may be responsible for
its broad therapeutic effect.
Gilmour et al. [109] found that OP could directly target
Neu1 desialylation of EGFR, and prevent receptor activation and
subsequent auto-phosphorylation in vitro. In mouse models,
immunohistochemistry and western blot analysis showed that OPtreated
mice expressed significantly decreased phospho-EGFR levels
from intact pancreatic tumor xenografts and lysed tumor samples,
when compared to the untreated cohort. One study by O’Shea et
al. [108], analyzed the effect of OP on chemoresistant pancreatic
cancer cell lines. They found that these cells, although resistant to
gemcitabine, cisplatin, tamoxifen and other chemotherapy agents,
were sensitive to OP treatment which resulted in reduced cell
proliferation and viability. Other markers of tumor progression
were also analyzed in these studies, including the relative levels
of cell adhesion molecules, E- and N-cadherin. Normally, more
malignant cancer cell phenotypes will display an increase in their
surface expression of N-cadherin and a corresponding decrease in
relative E-cadherin. This expression paradigm will later dictate EMT
pathways that facilitate cancer cell motility and metastasis. Like most
membrane glycoproteins, cadherins are terminally sialylated and
may act as Neu1 substrates. O’Shea et al. [108], showed that OPtreatment
of parental and chemoresistant pancreatic cancer cells
was able to modulate the expression levels of E- vs. N-cadherin,
such that OP-treated cells demonstrated relatively higher expression
levels of E-cadherin and reduced expression of N-cadherin, when
compared to the respective expression levels of untreated cells. These
findings suggest that OP may be exerting its effects by targeting the
glycan modification of adhesion molecules that play critical roles in
cancer cell migration and invasion. This is consistent with the wellestablished
positive regulatory role of Neu1 on the structure and
function of cell surface integrins, other major cell surface recognition
and adhesion molecules [116]. Future studies should build upon
these promising findings and aim to improve OP-based protocols for
the treatment of pancreatic cancer.
Conclusion
Several clinical challenges remain in the treatment of pancreatic cancer. In addition to its late detection, there have been no significant advancements in patient outcomes and drug development. Today, it is understood that in order to face these challenges, future studies must not rely on targeting a single oncogenic pathway, but must suppress the multiple enabling hallmark capabilities of pancreatic tumor cells. This is due to the fact that the cancer cell program is adaptive and invasive, such that more aggressive phenotypes will survive and metastasize, despite therapeutic intervention. Future studies should investigate the potential of multi-modal regimens that can suppress tumor growth and malignancy, immune-regulated tumorigenesis, stromal-derived promoters of tumor progression and desmoplasia, as well as the genetic and cellular components that drive drug resistance.
Acknowledgment
This work was supported F Haxho was the recipient of the Queen’s Graduate Award (QGA), the Graduate Entrance Tuition Award (GETA), the Natural Sciences and Engineering Research Council of Canada (NSERC) Alexander Graham Bell Canada Graduate Scholarship-Master’s (CGS M), and now the Vanier Canada Graduate Scholarship. M Sambi is a recipient of the QGA. B Qorri is the recipient of the QGA and the 2017 Terry Fox Research Institute Transdisciplinary Training Program in Cancer Research. The authors report no other conflicts of interest in this work.
Author Contributions
All authors contributed toward data analysis, drafting and critically revising the paper and agree to be accountable for all aspects of the work.
References
- Sener SF, Fremgen A, Menck HR, Winchester DP. Pancreatic cancer: a report of treatment and survival trends for 100,313 patients diagnosed from 1985-1995, using the National Cancer Database. J Am Coll Surg. 1999; 189(1): 1-7.
- Jemal A, Bray F, Center MM, Ferlay J, Ward E, Forman D. Global cancer statistics. CA Cancer J Clin. 2011; 61(2): 69-90.
- Stathis A, Moore MJ. Advanced pancreatic carcinoma: current treatment and future challenges. Nat Rev Clin Oncol. 2010; 7(3): 163-72.
- Matsuda T, Matsuda A. Five-year relative survival rate of pancreas cancer in the USA, Europe and Japan. Jpn J Clin Oncol. 2014; 44(4): 398-399.
- Rossi ML, Rehman AA, Gondi CS. Therapeutic options for the management of pancreatic cancer. World J Gastroenterol. 2014; 20(32): 11142-11159.
- Riediger H, Keck T, Wellner U, zur Hausen A, Adam U, Hopt UT, et al. The lymph node ratio is the strongest prognostic factor after resection of pancreatic cancer. J Gastrointest Surg. 2009; 13(7): 1337-44.
- Wasif N, Ko CY, Farrell J, Wainberg Z, Hines OJ, Reber H, et al. Impact of tumor grade on prognosis in pancreatic cancer: should we include grade in AJCC staging? Ann Surg Oncol. 2010; 17(9): 2312-2320.
- Fortner JG, Klimstra DS, Senie RT, Maclean BJ. Tumor size is the primary prognosticator for pancreatic cancer after regional pancreatectomy. Ann Surg. 1996; 223(2): 147-153.
- Goonetilleke KS, Siriwardena AK. Systematic review of carbohydrate antigen (CA 19-9) as a biochemical marker in the diagnosis of pancreatic cancer. Eur J Surg Oncol. 2007; 33(3): 266-270.
- Bilimoria KY, Bentrem DJ, Ko CY, Ritchey J, Stewart AK, Winchester DP, et al. Validation of the 6th edition AJCC Pancreatic Cancer Staging System: report from the National Cancer Database. Cancer. 2007; 110(4): 738-744.
- Massucco P, Capussotti L, Magnino A, Sperti E, Gatti M, Muratore A, et al. Pancreatic resections after chemoradiotherapy for locally advanced ductal adenocarcinoma: analysis of perioperative outcome and survival. Ann Surg Oncol. 2006; 13(9): 1201-1208.
- Burris HA 3rd, Moore MJ, Andersen J, Green MR, Rothenberg ML, Modiano MR, et al. Improvements in survival and clinical benefit with gemcitabine as first-line therapy for patients with advanced pancreas cancer: a randomized trial. J Clin Oncol. 1997; 15(6): 2403-2413.
- Huang MT1, Lysz T, Ferraro T, Abidi TF, Laskin JD, Conney AH. Inhibitory effects of curcumin on in vitro lipoxygenase and cyclooxygenase activities in mouse epidermis. Cancer Res. 1991; 51(3): 813-819.
- Gandhi V, Legha J, Chen F, Hertel LW, Plunkett W. Excision of 2',2'-difluorodeoxycytidine (gemcitabine) monophosphate residues from DNA. Cancer Res. 1996; 56(19): 4453-4459.
- Ferreira CG, Span SW, Peters GJ, Kruyt FA, Giaccone G. Chemotherapy triggers apoptosis in a caspase-8-dependent and mitochondria-controlled manner in the non-small cell lung cancer cell line NCI-H460. Cancer Res. 2000; 60(24): 7133-7141.
- Habiro A, Tanno S, Koizumi K, Izawa T, Nakano Y, Osanai M, et al. Involvement of p38 mitogen-activated protein kinase in gemcitabineinduced apoptosis in human pancreatic cancer cells. Biochem Biophys Res Commun. 2004; 316(1): 71-77.
- Chandler NM, Canete JJ, Callery MP. Caspase-3 drives apoptosis in pancreatic cancer cells after treatment with gemcitabine. J Gastrointest Surg. 2004; 8(8): 1072-1078.
- Kummer JL, Rao PK, Heidenreich KA. Apoptosis induced by withdrawal of trophic factors is mediated by p38 mitogen-activated protein kinase. J Biol Chem. 1997; 272(33): 20490-20494.
- Colucci G, Giuliani F, Gebbia V, Biglietto M, Rabitti P, Uomo G, et al. Gemcitabine alone or with cisplatin for the treatment of patients with locally advanced and/or metastatic pancreatic carcinoma: a prospective, randomized phase III study of the Gruppo Oncologia dell'Italia Meridionale. Cancer. 2002; 94(4): 902-910.
- Inal A, Kos FT, Algin E, Yildiz R, Dikiltas M, Unek IT, et al. Gemcitabine alone versus combination of gemcitabine and cisplatin for the treatment of patients with locally advanced and/or metastatic pancreatic carcinoma: a retrospective analysis of multicenter study. Neoplasma. 2012; 59(3): 297-301.
- Louvet C, Labianca R, Hammel P, Lledo G, Zampino MG, André T, et al. Gemcitabine in combination with oxaliplatin compared with gemcitabine alone in locally advanced or metastatic pancreatic cancer: results of a GERCOR and GISCAD phase III trial. J Clin Oncol. 2005; 23(15): 3509- 3516.
- Horwitz SB. Taxol (paclitaxel): mechanisms of action. Ann Oncol. 1994; 6: S3-S6.
- Von Hoff DD, Ervin T, Arena FP, Chiorean EG, Infante J, Moore M, et al. Increased survival in pancreatic cancer with nab-paclitaxel plus gemcitabine. N Engl J Med. 2013; 369(18): 1691-703.
- Marsh Rde W, Talamonti MS, Katz MH, Herman JM. Pancreatic cancer and FOLFIRINOX: a new era and new questions. Cancer Med. 2015; 4(6): 853-863.
- Almoguera C, Shibata D, Forrester K, Martin J, Arnheim N, Perucho M. Most human carcinomas of the exocrine pancreas contain mutant c-K-ras genes. Cell. 1988; 53(4): 549-554.
- Hruban RH, Maitra A, Goggins M. Update on pancreatic intraepithelial neoplasia. International journal of clinical and experimental pathology. 2008; 1(4): 306-316.
- Gedde-Dahl T, 3rd, Eriksen JA, Thorsby E, Gaudernack G. T-cell responses against products of oncogenes: generation and characterization of human T-cell clones specific for p21 ras-derived synthetic peptides. Hum Immunol. 1992; 33(4): 266-274.
- Olive KP, Tuveson DA, Ruhe ZC, Yin B, Willis NA, Bronson RT, et al. Mutant p53 gain of function in two mouse models of Li-Fraumeni syndrome. Cell. 2004; 119(6): 847-860.
- Caldas C, Hahn SA, da Costa LT, Redston MS, Schutte M, Seymour AB, et al. Frequent somatic mutations and homozygous deletions of the p16 (MTS1) gene in pancreatic adenocarcinoma. Nat Genet. 1994; 8(1): 27-32.
- Hahn SA, Hoque AT, Moskaluk CA, da Costa LT, Schutte M, Rozenblum E, et al. Homozygous deletion map at 18q21.1 in pancreatic cancer. Cancer Res. 1996; 56(3): 490-494.
- Nordh S, Ansari D, Andersson R. hENT1 expression is predictive of gemcitabine outcome in pancreatic cancer: a systematic review. World J Gastroenterol. 2014; 20(26): 8482-8490.
- Heppner GH. Tumor heterogeneity. Cancer Res. 1984; 44(6): 2259-2265.
- Nowell PC. The clonal evolution of tumor cell populations. Science. 1976; 194(4260): 23-28.
- Heppner GH, Dexter DL, DeNucci T, Miller FR, Calabresi P. Heterogeneity in drug sensitivity among tumor cell subpopulations of a single mammary tumor. Cancer Res. 1978; 38: 3758-3763.
- Heppner G, Yamashina K, Miller B, Miller F. Tumor heterogeneity in metastasis. Prog Clin Biol Res. 1986; 212: 45-59.
- Samuel N, Hudson TJ. The molecular and cellular heterogeneity of pancreatic ductal adenocarcinoma. Nat Rev Gastroenterol Hepatol. 2011; 9(2): 77-87.
- Burrell RA, McGranahan N, Bartek J, Swanton C. The causes and consequences of genetic heterogeneity in cancer evolution. Nature. 2013; 501(7467): 338-345.
- Lorusso G, Ruegg C. The tumor microenvironment and its contribution to tumor evolution toward metastasis. Histochem Cell Biol. 2008; 130(6): 1091-1103.
- Cheah MT, Chen JY, Sahoo D, Contreras-Trujillo H, Volkmer AK, Scheeren FA, et al. CD14-expressing cancer cells establish the inflammatory and proliferative tumor microenvironment in bladder cancer. Proc Natl Acad Sci U S A. 2015; 112(15): 4725-4730.
- Jones S, Zhang X, Parsons DW, Lin JC, Leary RJ, Angenendt P, et al. Core signaling pathways in human pancreatic cancers revealed by global genomic analyses. Science. 2008; 321(5897): 1801-1806.
- Bonnet D, Dick JE. Human acute myeloid leukemia is organized as a hierarchy that originates from a primitive hematopoietic cell. Nat Med. 1997; 3(7): 730-737.
- Al-Hajj M, Wicha MS, Benito-Hernandez A, Morrison SJ, Clarke MF. Prospective identification of tumorigenic breast cancer cells. Proc Natl Acad Sci U S A. 2003;100(7):3983-8.
- Singh SK, Clarke ID, Terasaki M, Bonn VE, Hawkins C, Squire J, et al. Identification of a cancer stem cell in human brain tumors. Cancer Res. 2003; 63(18): 5821-5828.
- Li C, Heidt DG, Dalerba P, Burant CF, Zhang L, Adsay V, et al. Identification of pancreatic cancer stem cells. Cancer Res. 2007; 67(3): 1030-1037.
- Dean M, Fojo T, Bates S. Tumour stem cells and drug resistance. Nature Rev Cancer. 2005; 5(4): 275-284.
- Kim Y, Lin Q, Glazer PM, Yun Z. Hypoxic tumor microenvironment and cancer cell differentiation. Curr Mol Med. 2009; 9: 425-434.
- Helczynska K, Kronblad A, Jögi A, Nilsson E, Beckman S, Landberg G, et al. Hypoxia promotes a dedifferentiated phenotype in ductal breast carcinoma in situ. Cancer Res. 2003; 63(7): 1441-1444.
- Kreso A, van Galen P, Pedley NM, Lima-Fernandes E, Frelin C, Davis T, et al. Self-renewal as a therapeutic target in human colorectal cancer. Nat Med. 2014; 20(1): 29.
- Rhim AD, Mirek ET, Aiello NM, Maitra A, Bailey JM, McAllister F, et al. EMT and dissemination precede pancreatic tumor formation. Cell. 2012; 148(1-2): 349-361.
- Neesse A, Algül H, Tuveson DA3, Gress TM4. Stromal biology and therapy in pancreatic cancer: a changing paradigm. Gut. 2015; 64(9): 1476-1484.
- Whatcott CJ, Posner RG, Von Hoff DD, Han H. Desmoplasia and chemoresistance in pancreatic cancer. In: Grippo PJ, Munshi HG, editors. Pancreatic Cancer and Tumor Microenvironment. Trivandrum (India). 2012.
- Tian H, Callahan CA, DuPree KJ, Darbonne WC, Ahn CP, Scales SJ, et al. Hedgehog signaling is restricted to the stromal compartment during pancreatic carcinogenesis. Proc Natl Acad Sci U S A. 2009; 106(11): 4254- 4259.
- Jimeno A, Feldmann G, Suarez-Gauthier A, Rasheed Z, Solomon A, Zou GM, et al. A direct pancreatic cancer xenograft model as a platform for cancer stem cell therapeutic development. Mol Cancer Ther. 2009; 8(2): 310-314.
- Zhang GN, Liang Y, Zhou LJ, Chen SP, Chen G, Zhang TP, et al. Combination of salinomycin and gemcitabine eliminates pancreatic cancer cells. Cancer Lett. 2011; 313(2): 137-144.
- Beatty GL, Chiorean EG, Fishman MP, Saboury B, Teitelbaum UR, Sun W, et al. CD40 agonists alter tumor stroma and show efficacy against pancreatic carcinoma in mice and humans. Science. 2011; 331(6024): 1612-1616.
- Schrader H, Wiese M, Ellrichmann M, Belyaev O, Uhl W, Tannapfel A, et al. Diagnostic value of quantitative EUS elastography for malignant pancreatic tumors: relationship with pancreatic fibrosis. Ultraschall in der Medizin. 2012; 33(7): E196-201.
- Bayne LJ, Beatty GL, Jhala N, Clark CE, Rhim AD, Stanger BZ, et al. Tumor-derived granulocyte-macrophage colony-stimulating factor regulates myeloid inflammation and T cell immunity in pancreatic cancer. Cancer cell. 2012; 21(6): 822-835.
- Tucker ON1, Dannenberg AJ, Yang EK, Zhang F, Teng L, Daly JM, et al. Cyclooxygenase-2 expression is up-regulated in human pancreatic cancer. Cancer Res. 1999; 59(5): 987-990.
- Khorana AA, Fine RL. Pancreatic cancer and thromboembolic disease. The Lancet Oncology. 2004; 5(11): 655-663.
- Rao LV, Pendurthi UR. Factor VIIIa-induced gene expression: potential implications in pathophysiology. Trends in cardiovascular medicine. 2001; 11(1): 14-21.
- Zhang Y, Deng Y, Luther T, Müller M, Ziegler R, Waldherr R, et al. Tissue factor controls the balance of angiogenic and antiangiogenic properties of tumor cells in mice. J Clin Invest. 1994; 94(3): 1320-1327.
- Kranenburg O, Gebbink MF, Voest EE. Stimulation of angiogenesis by Ras proteins. Biochim Biophys Acta. 2004; 1654(1): 23-37.
- Maragoudakis ME, Tsopanoglou NE, Andriopoulou P. Mechanism of thrombin-induced angiogenesis. Biochem Soc Trans. 2002; 30(2): 173- 177.
- Ikeda O, Egami H, Ishiko T, Ishikawa S, Kamohara H, Hidaka H, et al. Expression of proteinase-activated receptor-2 in human pancreatic cancer: a possible relation to cancer invasion and induction of fibrosis. Int J Oncol. 2003; 22(2): 295-300.
- Rudroff C, Schafberg H, Nowak G, Weinel R, Scheele J, Kaufmann R. Characterization of functional thrombin receptors in human pancreatic tumor cells (MIA PACA-2). Pancreas. 1998; 16(2): 189-194.
- Rudroff C, Seibold S, Kaufmann R, Zetina CC, Reise K, Schafer U, et al. Expression of the thrombin receptor PAR-1 correlates with tumour cell differentiation of pancreatic adenocarcinoma in vitro. Clin Exp Metastasis. 2002; 19(2): 181-189.
- Rudroff C, Striegler S, Schilli M, Scheele J. Thrombin enhances adhesion in pancreatic cancer in vitro through the activation of the thrombin receptor PAR 1. Eur J Surg Oncol. 2001; 27(5): 472-476.
- Sahni A, Sporn LA, Francis CW. Potentiation of endothelial cell proliferation by fibrin(ogen)-bound fibroblast growth factor-2. J Biol Chem. 1999; 274(21): 14936-14941.
- Sahni A1, Baker CA, Sporn LA, Francis CW. Fibrinogen and fibrin protect fibroblast growth factor-2 from proteolytic degradation. Thromb Haemost. 2000; 83(5): 736-741.
- Von Hoff DD, Korn R, Mousses S. Pancreatic cancer--could it be that simple? A different context of vulnerability. Cancer cell. 2009; 16(1): 7-8.
- Le A, Rajeshkumar NV, Maitra A, Dang CV. Conceptual framework for cutting the pancreatic cancer fuel supply. Clin Cancer Res. 2012; 18(16): 4285-4290.
- Ma WW, Jacene H, Song D, Vilardell F, Messersmith WA, Laheru D, et al. [18F]fluorodeoxyglucose positron emission tomography correlates with Akt pathway activity but is not predictive of clinical outcome during mTOR inhibitor therapy. J Clin Oncol. 2009; 27(16): 2697-2704.
- Stumvoll M, Nurjhan N, Perriello G, Dailey G, Gerich JE. Metabolic effects of metformin in non-insulin-dependent diabetes mellitus. N Engl J Med. 1995; 333(9): 550-554.
- Inzucchi SE, Maggs DG, Spollett GR, Page SL, Rife FS, Walton V, et al. Efficacy and metabolic effects of metformin and troglitazone in type II diabetes mellitus. N Engl J Med. 1998; 338(13): 867-872.
- Zhou G, Myers R, Li Y, Chen Y, Shen X, Fenyk-Melody J, et al. Role of AMP-activated protein kinase in mechanism of metformin action. J Clin Invest. 2001; 108(8): 1167-1174.
- Shackelford DB1, Shaw RJ. The LKB1-AMPK pathway: metabolism and growth control in tumour suppression. Nat Rev Cancer. 2009; 9(8): 563- 575.
- Chiang GG, Abraham RT. Targeting the mTOR signaling network in cancer. Trends Mol Med. 2007; 13(10): 433-442.
- Ben Sahra I, Regazzetti C, Robert G, Laurent K, Le Marchand-Brustel Y, Auberger P, et al. Metformin, independent of AMPK, induces mTOR inhibition and cell-cycle arrest through REDD1. Cancer Res. 2011; 71(13): 4366-4372.
- Pacifico F, Leonardi A. NF-kappaB in solid tumors. Biochem Pharmacol. 2006; 72(9): 1142-1152.
- Hattori Y, Suzuki K, Hattori S, Kasai K. Metformin inhibits cytokineinduced nuclear factor kappaB activation via AMP-activated protein kinase activation in vascular endothelial cells. Hypertension. 2006; 47(6): 1183-1188.
- Isoda K, Young JL, Zirlik A, MacFarlane LA, Tsuboi N, Gerdes N, et al. Metformin inhibits proinflammatory responses and nuclear factorkappaB in human vascular wall cells. Arterioscler Thromb Vasc Biol. 2006; 26(3): 611-617.
- Tan XL, Bhattacharyya KK, Dutta SK, Bamlet WR, Rabe KG, Wang E, et al. Metformin suppresses pancreatic tumor growth with inhibition of NFκB/STAT3 inflammatory signaling. Pancreas. 2015; 44(4): 636-647.
- Rotondi M, Coperchini F, Pignatti P, Magri F, Chiovato L. Metformin reverts the secretion of CXCL8 induced by TNF-α in primary cultures of human thyroid cells: an additional indirect anti-tumor effect of the drug. J Clin Endocrinol Metab. 2015; 100(3): E427-432.
- Zakikhani M, Dowling R, Fantus IG, Sonenberg N, Pollak M. Metformin is an AMP kinase-dependent growth inhibitor for breast cancer cells. Cancer Res. 2006; 66(21): 10269-10273.
- Queiroz EA, Puukila S, Eichler R, Sampaio SC, Forsyth HL, Lees SJ, et al. Metformin induces apoptosis and cell cycle arrest mediated by oxidative stress, AMPK and FOXO3a in MCF-7 breast cancer cells. Plos One. 2014; 9(5): e98207.
- Carmignani M, Volpe AR, Aldea M, Soritau O, Irimie A, Florian IS, et al. Glioblastoma stem cells: a new target for metformin and arsenic trioxide. Journal of biological regulators and homeostatic agents. 2014; 28(1): 1-15.
- Buzzai M, Jones RG, Amaravadi RK, Lum JJ, DeBerardinis RJ, Zhao F, et al. Systemic treatment with the antidiabetic drug metformin selectively impairs p53-deficient tumor cell growth. Cancer Res. 2007; 67(14): 6745- 6752.
- Erices R, Bravo ML, Gonzalez P, Oliva B, Racordon D, Garrido M, et al. Metformin, at concentrations corresponding to the treatment of diabetes, potentiates the cytotoxic effects of carboplatin in cultures of ovarian cancer cells. Reproductive sciences. 2013; 20(12): 1433-1446.
- Wang LW, Li ZS, Zou DW, Jin ZD, Gao J, Xu GM. Metformin induces apoptosis of pancreatic cancer cells. World J Gastroenterol. 2008; 14(47): 7192-718.
- Ashinuma H, Takiguchi Y, Kitazono S, Kitazono-Saitoh M, Kitamura A, Chiba T, et al. Antiproliferative action of metformin in human lung cancer cell lines. Oncol Rep. 2012; 28(1): 8-14.
- Ben Sahra I, Laurent K, Loubat A, Giorgetti-Peraldi S, Colosetti P, Auberger P, et al. The antidiabetic drug metformin exerts an antitumoral effect in vitro and in vivo through a decrease of cyclin D1 level. Oncogene. 2008; 27(25): 3576-86.
- Bao B, Wang Z, Ali S, Ahmad A, Azmi AS, Sarkar SH, et al. Metformin inhibits cell proliferation, migration and invasion by attenuating CSC function mediated by deregulating miRNAs in pancreatic cancer cells. Cancer Prev Res (Phila). 2012; 5(3): 355-364.
- Mani SA, Guo W, Liao MJ, Eaton EN, Ayyanan A, Zhou AY, et al. The epithelial-mesenchymal transition generates cells with properties of stem cells. Cell. 2008; 133(4): 704-715.
- Vazquez-Martin A, Oliveras-Ferraros C, Cufi S, Del Barco S. MartinCastillo B, Menendez JA. Metformin regulates breast cancer stem cell ontogeny by transcriptional regulation of the epithelial-mesenchymal transition (EMT) status. Cell Cycle. 2010; 9(18): 3807-3814.
- Cerezo M, Tichet M, Abbe P, Ohanna M, Lehraiki A, Rouaud F, et al. Metformin blocks melanoma invasion and metastasis development in AMPK/p53-dependent manner. Mol Cancer Ther. 2013; 12(8): 1605- 1615.
- Cifarelli V, Lashinger LM, Devlin KL, Dunlap SM, Huang J, Kaaks R, et al. Metformin and Rapamycin Reduce Pancreatic Cancer Growth in Obese Prediabetic Mice by Distinct MicroRNA-Regulated Mechanisms. Diabetes. 2015; 64(5): 1632-1642.
- Yiyao Zhang LL, Pei Fan, Nathalie Bauer, Jury Gladkich, Eduard Ryschich, Alexandr V,et al. Aspirin counteracts cancer stem cell features, desmoplasia and gemcitabine resistance in pancreatic cancer. Oncotarget. 2015; 6(12): 9999-10015.
- Sclabas GM, Uwagawa T, Schmidt C, Hess KR, Evans DB, Abbruzzese JL, et al. Nuclear factor kappa B activation is a potential target for preventing pancreatic carcinoma by aspirin. Cancer. 2005; 103(12): 2485-2490.
- Kokawa A, Kondo H, Gotoda T, Ono H, Saito D, Nakadaira S, et al. Increased expression of cyclooxygenase-2 in human pancreatic neoplasms and potential for chemoprevention by cyclooxygenase inhibitors. Cancer. 2001; 91(2): 333-338.
- Mathew P. Inflammatory pathogenesis of prostate cancer and celecoxib. J Clin Oncol. 2010;28(12):e197.
- Atari-Hajipirloo S, Nikanfar S, Heydari A, Noori F, Kheradmand F. The effect of celecoxib and its combination with imatinib on human HT-29 colorectal cancer cells: Involvement of COX-2, Caspase-3, VEGF and NFkappaB genes expression. Cell Mol Biol (Noisy-le-grand). 2016; 62(2): 68-74.
- Dai ZJ, Ma XB, Kang HF, Gao J, Min WL, Guan HT, et al. Antitumor activity of the selective cyclooxygenase-2 inhibitor, celecoxib, on breast cancer in Vitro and in Vivo. Cancer Cell Int. 2012; 12(1): 53.
- Hou LC, Huang F, Xu HB. Does celecoxib improve the efficacy of chemotherapy for advanced non-small cell lung cancer? British journal of clinical pharmacology. 2016; 81(1): 23-32.
- Jendrossek V. Targeting apoptosis pathways by Celecoxib in cancer. Cancer Lett. 2013; 332(2): 313-324.
- Abdulkhalek S, Hrynyk M, Szewczuk MR. A novel G-protein-coupled receptor-signaling platform and its targeted translation in human disease. Research and Reports in Biochemistry. 2013; 3: 17-30.
- Haxho F, Neufeld RJ, Szewczuk MR1. Neuraminidase-1: a novel therapeutic target in multistage tumorigenesis. Oncotarget. 2016; 7(26): 40860-40881.
- Hrynyk M, Ellis JP, Haxho F, Allison S, Steele JA, Abdulkhalek S, et al. Therapeutic designed poly (lactic-co-glycolic acid) cylindrical oseltamivir phosphate-loaded implants impede tumor neovascularization, growth and metastasis in mouse model of human pancreatic carcinoma. Drug design, development and therapy. 2015; 9: 4573-4586.
- O'Shea LK, Abdulkhalek S, Allison S, Neufeld RJ, Szewczuk MR. Therapeutic targeting of Neu1 sialidase with oseltamivir phosphate (Tamiflu(R)) disables cancer cell survival in human pancreatic cancer with acquired chemoresistance. OncoTargets and therapy. 2014; 7: 117- 134.
- Gilmour AM, Abdulkhalek S, Cheng TS, Alghamdi F, Jayanth P, O'Shea LK, et al. A novel epidermal growth factor receptor-signaling platform and its targeted translation in pancreatic cancer. Cell Signal. 2013; 25(12): 2587-2603.
- Haxho F, Allison S, Alghamdi F, Brodhagen L, Kuta VE, Abdulkhalek S, et al. Oseltamivir phosphate monotherapy ablates tumor neovascularization, growth, and metastasis in mouse model of human triple-negative breast adenocarcinoma. Breast cancer. 2014; 6: 191-203.
- Abdulkhalek S, Geen OD, Brodhagen L, Haxho F, Alghamdi F, Allison S, et al. Transcriptional factor snail controls tumor neovascularization, growth and metastasis in mouse model of human ovarian carcinoma. Clin Transl Med. 2014; 3(1): 28.
- Alghamdi F, Guo M, Abdulkhalek S, Crawford N, Amith SR, Szewczuk MR. A novel insulin receptor-signaling platform and its link to insulin resistance and type 2 diabetes. Cellular Signalling. 2014; 26(6): 1355-1368.
- Abdulkhalek S, Szewczuk MR. Neu1 sialidase and matrix metalloproteinase-9 cross-talk regulates nucleic acid-induced endosomal TOLL-like receptor-7 and -9 activation, cellular signaling and proinflammatory responses. Cellular Signalling. 2013; 25(11): 2093-2105.
- Abdulkhalek S, Guo M, Amith SR, Jayanth P, Szewczuk MR. G-protein coupled receptor agonists mediate Neu1 sialidase and matrix metalloproteinase-9 cross-talk to induce transactivation of TOLL-like receptors and cellular signaling. Cellular signalling. 2012; 24(11): 2035- 2042.
- Abdulkhalek S, Amith SR, Franchuk SL, Jayanth P, Guo M, Finlay T, et al. Neu1 sialidase and matrix metalloproteinase-9 cross-talk is essential for Toll-like receptor activation and cellular signaling. J Biol Chem. 2011; 286(42): 36532-36549.
- Pshezhetsky AV, Ashmarina LI. Desialylation of surface receptors as a new dimension in cell signaling. Biochemistry (Mosc). 2013; 78(7): 736- 745.