Cholesterol, a unique lipid molecule biosynthesized by all animal
cells, is an essential structural constituent in cell membranes to
maintain their structural integrity and fluidity. Cholesterol is critical
to synthesis of hormones, vitamin D and bile acid, multiple cellular
signaling, intracellular transport and a variety of cell functions [1,2].
Cholesterol is necessary to the structure and function of caveolae
and clathrin-coated pits, enabling the endocytotic activity [3,4]. In
recent decades, increasing lines of evidence show that lipid rafts,
which are membrane micro domains assemble glycosphingolipids,
protein receptors and kinases, preferentially associate with
cholesterol and saturated lipids in driving a wide variety of
cellular signaling pathways [5,6]. Not only in normal cellular and
physiological functions, intracellular cholesterol homoeostasis
once deregulated can be responsible for the development of
malignancies through decreasing chemotherapeutic susceptibility
and increasing resistance in cancer cells, inhibiting the release of
mitochondrial cell death-promoting molecules, activating survival
kinases and many other mechanisms [7-9]. Epidemiological
studies also have suggested a positive correlation between serum
cholesterol levels and cancer risk [9-11]. These studies support the
notion that the development of cholesterol-lowering agents can be
a potential anticancer strategy.
Abbreviations: PL3K: Phosphatidylinositol 3 Kinase; mTOR: mammalian Target of Rapamycin; SREBP: Sterol Regulatory Element Binding
Protein
Phosphatidylinositol 3 kinase (PI3K), protein kinase B (Akt)
and mammalian target of rapamycin (mTOR) are three serine/
threonine-specific protein kinases that coordinately regulate
cell cycle, quiescence, transformation, proliferation and survival
[12]. Accumulating lines of evidence suggest that cholesterol
homeostasis and biosynthesis usually appear to be altered in
cancer cells and, once inhibited, the tumor genesis can be blocked
indicating that cholesterol content tightly regulates cancer cell fate
[13,14]. Recent studies have reported that constitutive activation
of PI3K/Akt signaling pathway induces an increase in intracellular
cholesterol content in cancer cells through multiple mechanisms,
including induction of LDL receptor-related cholesterol import,
activation of sterol regulatory element binding protein (SREBP)-
dependent cholesterol synthesis and inhibition of ATP-binding
cassette transporter ABCA1-regulated mTORC1-dependent
cholesterol export [8,15-17]. Further studies reveal that the
increase of cholesterol levels is responsible to cancer cell growth,
cell survival and cancer aggressiveness and bone metastases
[15, 16,18-21]. The mechanism that PI3K/Akt/mTOR pathway
regulates intracellular cholesterol levels has been identified,
suggesting Niemann-Pick disease type C1 (NPC1) protein serves
as a crucial target. NPC1 is a membrane protein which controls
intracellular cholesterol trafficking in mammals [22]. Cholesterol
binds to NPC1 with hydroxyl group in the binding pocket, leading
to the export from the limiting membrane of late endosomes/
lysosomes to the endoplasmic reticulum and plasma membrane
[23]. Recently, the link between NPC1 degradation and Akt/mTOR
pathway has been addressed in several types of cancer [24-26].
It has been demonstrated that inhibition of Akt/mTOR pathway
induces a decrease in NPC1 ubiquitination, suggesting a role of Akt/
mTOR pathway in NPC1 proteasomal degradation. These studies
reveal Akt serving as a key regulator on NPC1 degradation and
connect this protein with cancer cell proliferation and migration
[24,25]. Moreover, Naren and the colleagues have used U18666A,
an inhibitor of NPC1 function, to inhibit cholesterol trafficking
to mimic the condition of NPC1 defect in cells, leading to higher
NPC1 expression and higher resistance against imatinib mesylate,
a chemotherapy medication used to treat chronic myelogenous
leukemia [27]. The study suggests that cells with highly expressed
NPC1 may have higher resistance to cancer chemotherapeutic
agents.
Cholesterol also can be an upstream effectors to regulate PI3K/
Akt activities. Several studies have reported that the depletion of
cholesterol from plasma membranes with beta-cyclodextrins is able
to disrupt PI3K/Akt signal transduction [28-30]. The studies also
reveal the importance of lipid raft integrity. Lipid rafts are dynamic
plasma membrane micro domains which have been implicated in cell
survival, proliferation, cell adhesion and invasion and cholesterol
metabolism. Lipid rafts can form unique domains with diverse
compositions and assist signal transduction through recruiting
target proteins in response to intracellular and extracellular stimuli
[31-33]. A wide variety of proteins related to cancer development
are associated with lipid rafts, including growth factor receptors,
serine/threonine protein kinases (PI3K/Akt/mTOR) and integrins
[34-36]. Despite lipid rafts are hubs of many critical survival
proteins, recent studies have provided evidence suggesting that
lipid rafts can also orchestrate death receptor-mediated extrinsic
apoptotic signaling [37-39]. The synthetic alkyllysophospholipid
edelfosine and derivatives have a high affinity for cholesterol and
are trapped in lipid rafts in a number of solid tumors and malignant
hematological cells, inducing translocation of death receptors
and downstream signaling molecules to these membrane micro
domains and eventually leading to apoptosis of cancer cells [39-
43]. Edelfosine also can displace PI3K/Akt signal transduction from
lipid rafts, inducing PI3K/Akt inhibition. Therefore, lipid rafts can
serve as hubs where separation between pro-apoptotic and prosurvival
cellular targets can take place [34].
It has been suggested that cancer cells have higher levels of
cholesterol-rich lipid rafts compared to those in normal cells.
Li and the colleagues have studied and compared the raft levels
and effect of methyl-beta cyclodextrin-mediated raft disruption
on cell viability of human cancer cell lines versus their normal
counterparts. The cholesterol depletion caused apoptosis in
human epidermoid carcinoma A431 cells involving decreased raft
levels, Akt inactivation, and Bcl-xL down-regulation and caspase-3
activation regardless of epidermal growth factor receptor activation.
The Akt activation and cell viability can be rescued by cholesterol
replenishment [44]. Notably, they have reported that both breast
and prostate cancer cell lines have more lipid rafts which lead to
their higher susceptibility to apoptotic stimuli caused by cholesterol
depletion [44]. These studies also suggest a potential use of lipid
raft-modulating agents in cancer cells those have increased levels
of lipid rafts.
Recent studies have addressed the alterations of specific lipid
molecules found in cancer cells as well as in tumor microenvironment
[45-47]. Moreover, lipid rafts are considered as a center to couple
between membrane microenvironment and drug resistance since
membrane lipid composition is tightly relevant to the function of
ATP-binding cassette transporter P-glycoprotein (Pgp). It has been
evident that the Pgp activity is highly sensitive to the presence of
cholesterol. However, the membrane fluidity does not solely explain
cholesterol-dependent alterations of Pgp-activity. In contrast,
accurate lipid raft properties may predominantly be responsible to the Pgp-transport capacity [48,49]. These studies also support the
notion that cholesterol depletion may sensitize chemotherapeutic
agents in killing cancer cells through the inhibition of drug
resistance. It has been supported by the observations that
melittin, a Chinese traditional medicine, sensitizes gemcitabineinduced
apoptosis in pancreatic ductal adenocarcinoma cells
by down-regulating cholesterol pathway and decreasing drug
resistance [50]. Similar study has demonstrated that a ginsenoside
derivative, which appears to redistribute lipid rafts and Pgp,
results in an accumulation of doxorubicin by decreasing Pgp
activity in doxorubicin-resistant cells, leading to chemotherapeutic
amplification [51]. These studies suggest that lipid raft-modulating
agents may have potential in reducing multidrug resistance activity
for chemotherapeutic sensitization.
Because of the crucial roles played by cholesterol in cancer
development, the interruption of cholesterol supplementation and/
or lipid raft integrity can be a potential strategy in the development
of cancer chemotherapeutic agents. Statins, well known competitive
inhibitors of hydroxymethylglutaryl-CoA reductase enzyme (HMGCoA
reductase), are widely used as cholesterol-lowering agents. In
recent decades, much attention has been directed toward the use
of statins in oncological therapy. Accumulated cellular and animal
studies show an adequate anticancer effect of statins including
inhibition of cell proliferation and invasion, and induction of
apoptosis and differentiation. Among the statin family, lovastatin,
simvastatin, atorvastatin, cerivastatin and fluvastatin have been
extensively elucidated and the signaling pathways have been
reported regarding the inhibition of PI3K/Akt/mTOR/p70S6K
pathway, deregulating cell cycle proteins, blocking MAPK/Erk
signaling, activation of JNK pathway, inhibition of RhoA membrane
translocation from cytosol, F-actin depolymerization and inhibition
of actin stress fiber assembly [52-61]. Statins may also induce a
decrease of cholesterol content in lipid raft, suppress caveolin-1
expression in lipid rafts, and induce Fas translocation into lipid rafts,
suggesting that statins may trigger apoptotic cell death through
the modulation of death receptors in lipid rafts [62]. Furthermore,
statins have been studied to inhibit angiogenesis through downregulation
of VEGF, inhibition of endothelial cell proliferation and
block of adhesion to extracellular matrix. However, it has been noted
that different statin may exert dual and concentration-dependent
impact on regulating angiogenic activities of human primary
macrovascular endothelial cells [52,63]. Altogether, statins can
induce different effects depending on the concentration, duration
of exposure of cells to statins, cell lines and the type of statin being
used [64].
In cancer patients, the efficacy of statins as chemotherapeutic
drugs has been evaluated both in monotherapy and in combinatory
therapy with clinical chemotherapeutic drugs [52]. Some clinical
studies have demonstrated a positive outcome. Kawata and the
colleagues have evaluated the efficacy of pravastatin combined
with 5-Fu in patients with unrespectable hepatocellular carcinoma.
Results show a significant prolonged survival in statin-treated
group [65]. Similar positive effects have been demonstrated in the report by Graf and the colleagues on the treatment of patients
with hepatocellular carcinoma by transarterial chemoembolization
combined with pravastatin [66]. Several epidemiologic studies also
suggest a positive correlation between increased serum cholesterol
content and risk for some cancer types including prostate cancer,
melanoma and non-metastatic rectal cancer [15,52,67-69]. On the
contrary, some studies show that statins have failed to improve
the median survival of patients with certain types of cancer. The
meta-analysis of large randomized clinical trials also suggests no
association between cholesterol and cancer [15,52,70,71]. Because
of the controversy, additional studies are required to connect the
mechanism evidence, clinical studies and epidemiological data to
solve these problems.
Omega-3 polyunsaturated fatty acids, such as eicosapentaenoic
acid (EPA) and docosahexaenoic acid (DHA), control some key
cellular mechanisms and play a beneficial role in inflammatory
diseases. However, the evidence connecting the consumption
of omega-3 polyunsaturated fatty acids to a lower cancer risk
is insufficient [72] exception possibly of breast cancer [73].
Several studies have reported that EPA and DHA can inhibit cell
proliferation and induce apoptosis of MDA-MB-231 human breast
cancer cells through the incorporation of these fatty acids into lipid
rafts, leading to an activation of p38MAPK and a decrease in EGFR
levels in lipid rafts in spite of the accompanied phosphorylation
of EGFR [74]. Moreover, both EPA and DHA can reduce surface
expression of CXCR4, leading to a decrease of CXCR4-mediated cell
migration of MDA-MB-231 cells [75]. These studies have provided
evidence that omega-3 polyunsaturated fatty acids can modify lipid
raft in both biochemical and biophysical features, decreasing the
content of cholesterol and distribution of key proteins [76]. These
effects can ultimately induce a decrease of cell proliferation and
metastasis, and an increase of apoptosis in breast cancer cells.
From a large body of evidence, lipid raft modifying/cholesterol
lowering agents can decrease lipid raft associated pro-survival
protein (e.g., growth factor receptors and PI3K, Akt and mTOR
kinases) and in induce translocation of death receptors. These
impacts can eventually lead to the inhibition of cell proliferation
and metastasis, and induction of cell death. However, the solubility,
pharmacokinetics and delivery of the high lipophilic agents are key
issues to solve. Therefore, several statin-loaded nanoparticles have
been developed such as solutol-based lipid nanocapsules and cholic
acid core, star-shaped polymer consisting of poly (D,L-lactideco-
glycolide) nanoparticles are able to display good anticancer
activities in breast cancer [77,78]. It has been, therefore, suggested
that the development of lipid raft modifying/cholesterol lowering
agents is a potential anticancer strategy if the solubility and drug
delivery can be appropriately solved.