Cannabinoids inhibit tumor angiogenesis in
mice, but the mechanism of their antiangiogenic action is still
unknown. Because
the vascular endothelial growth factor (VEGF)
pathway plays a critical role in tumor angiogenesis, here we studied
whether
cannabinoids affect it. As a first approach, cDNA
array analysis showed that cannabinoid administration to mice bearing
s.c.
gliomas lowered the expression of various VEGF
pathway-related genes. The use of other methods (ELISA, Western
blotting, and
confocal microscopy) provided additional evidence
that cannabinoids depressed the VEGF pathway by decreasing the
production
of VEGF and the activation of VEGF receptor
(VEGFR)-2, the most prominent VEGF receptor, in cultured glioma cells
and in mouse
gliomas. Cannabinoid-induced inhibition of VEGF
production and VEGFR-2 activation was abrogated both in vitro and in vivo by pharmacological blockade of ceramide biosynthesis. These changes in the VEGF pathway were paralleled by changes in tumor
size. Moreover, intratumoral administration of the cannabinoid Δ9-tetrahydrocannabinol
to two patients with glioblastoma multiforme (grade IV astrocytoma)
decreased VEGF levels and VEGFR-2
activation in the tumors. Because blockade of the
VEGF pathway constitutes one of the most promising antitumoral
approaches
currently available, the present findings provide a
novel pharmacological target for cannabinoid-based therapies.
INTRODUCTION
To grow beyond minimal size, tumors must generate a new vascular supply for purposes of gas exchange, cell nutrition, and
waste disposal
(1,
2,
3,
4)
. They do so by secreting proangiogenic cytokines
that promote the formation of blood vessels. Vascular endothelial growth
factor (VEGF; also known as VEGF-A) is considered
the most important proangiogenic molecule because it is expressed
abundantly
by a wide variety of animal and human tumors and
because of its potency, selectivity, and ability to regulate most and
perhaps
all of the steps in the angiogenic cascade
(1,
2,
3,
4)
. The best characterized VEGF receptors are two
related receptor tyrosine kinases termed VEGF receptor (VEGFR)-1 (also
known
as Flt-1) and VEGFR-2 (also known as kinase domain
region or Flk-1). Although VEGF binds to VEGFR-1 with higher affinity,
numerous studies in cultured cells and laboratory
animals have provided evidence that VEGFR-2 is the major mediator of the
mitogenic, antiapoptotic, angiogenic, and
permeability-enhancing effects of VEGF
(1,
2,
3,
4)
. Because overexpression of VEGF and VEGFR-2 is
causally involved in the progression of many solid tumors, several
strategies
to inhibit VEGF signaling have been translated into
clinical trials in cancer patients, including anti-VEGF and
anti-VEGFR-2
antibodies, small VEGFR-2 inhibitors, and a soluble
decoy VEGFR
(5,
6,
7,
8)
. In addition, clinical trials are being performed
with a number of promising anticancer compounds such as Iressa and
Herceptin
that block proteins involved in the induction of
the VEGF pathway
(5
, 8)
.
Cannabinoids, the active components of
Cannabis sativa L. (marijuana), and their derivatives exert a wide array of effects by activating their specific G protein-coupled receptors
CB
1 and CB
2, which are normally engaged by a family of endogenous ligands–the endocannabinoids
(9
, 10)
. Marijuana and its derivatives have been used in
medicine for many centuries, and there is currently a renaissance in the
study of the therapeutic effects of cannabinoids.
Today, cannabinoids are approved to palliate the wasting and emesis
associated
with cancer and AIDS chemotherapy
(11)
, and ongoing clinical trials are determining
whether cannabinoids are effective agents in the treatment of pain
(12)
, neurodegenerative disorders such as multiple sclerosis
(13)
, and traumatic brain injury
(14)
. In addition, cannabinoid administration to mice
and/or rats induces the regression of lung adenocarcinomas
(15)
, gliomas
(16)
, thyroid epitheliomas
(17)
, lymphomas
(18)
, and skin carcinomas
(19)
. These studies have also evidenced that
cannabinoids display a fair drug safety profile and do not produce the
generalized
cytotoxic effects of conventional chemotherapies,
making them potential antitumoral agents
(20
, 21)
.
Little is known, however, about the mechanism of cannabinoid antitumoral action
in vivo.
By modulating key cell signaling pathways, cannabinoids directly induce
apoptosis or cell cycle arrest in different transformed
cells
in vitro
(20)
. However, the involvement of these events in their antitumoral action
in vivo is as yet unknown. More recently, immunohistochemical and functional analyses of the vasculature of gliomas
(22)
and skin carcinomas
(19)
have shown that cannabinoid administration to mice
inhibits tumor angiogenesis. These findings prompted us to explore the
mechanism by which cannabinoids impair angiogenesis
of gliomas and, particularly, the possible impact of cannabinoids on
the
VEGF pathway. Here, we report that cannabinoid
administration inhibits the VEGF pathway in cultured glioma cells, in
glioma-bearing
mice, and in two patients with glioblastoma
multiforme. In addition, this effect may be mediated by ceramide, a
sphingolipid
second messenger implicated previously in
cannabinoid signaling in glioma cells
(23)
.
MATERIALS AND METHODS
Cannabinoids.
The Δ
9-tetrahydrocannabinol was kindly given by Alfredo Dupetit (The Health Concept, Richelbach, Germany). JWH-133 was kindly given
by Dr. John Huffman (Department of Chemistry, Clemson University, Clemson, SC; Ref.
24
). WIN-55,212-2 and anandamide were from Sigma (St.
Louis, MO). SR141716 and SR144528 were kindly given by
Sanofi-Synthelabo
(Montpellier, France). For
in vitro incubations, cannabinoid agonists and antagonists were directly applied at a final DMSO concentration of 0.1–0.2% (v/v).
For
in vivo experiments, ligands were prepared at 1% (v/v) DMSO in 100 μl PBS supplemented with 5 mg/ml BSA. No significant influence
of the vehicle was observed on any of the parameters determined.
Cell Culture.
The rat C6 glioma
(25)
, the human U373 MG astrocytoma
(25)
, the mouse PDV.C57 epidermal carcinoma
(19)
, and the human ECV304 bladder cancer epithelioma
(22)
were cultured as described previously. Human glioma
cells were prepared from a glioblastoma multiforme (grade IV
astrocytoma;
Ref.
26
). The biopsy was digested with collagenase (type
Ia; Sigma) in DMEM at 37°C for 90 min, the supernatant was seeded in
DMEM
containing 15% FCS and 1 m
m glutamine, cells were grown for 2 passages, and 24 h before the experiments, cells were transferred to 0.5%-serum DMEM. Cell
viability was determined by trypan blue exclusion. Rat recombinant VEGF and
N-acetylsphingosine (C
2-ceramide) were from Sigma.
Tumor Induction in Mice.
Tumors were induced in mice deficient in recombination activating gene 2 by s.c. flank inoculation of 5 × 10
6 C6 glioma cells in 100 μl PBS supplemented with 0.1% glucose
(16)
. When tumors had reached a volume of 350–450 mm
3,
animals were assigned randomly to the various groups and injected
intratumorally for up to 8 days with 50 μg/day JWH-133
and/or 60 μg/day fumonisin B1 (Alexis, San Diego,
CA). Control animals were injected with vehicle. Tumors were measured
with
external caliper, and volume was calculated as
(4π/3) × (width/2)
2 × (length/2).
Human Tumor Samples.
Tumor biopsies were obtained from two of the patients enrolled in an ongoing Phase I/II clinical trial (at the Neurosurgery
Department of Tenerife University Hospital, Spain) aimed at investigating the effect of Δ9-tetrahydrocannabinol
administration on the growth of recurrent glioblastoma multiforme. The
patients had failed standard
therapy, which included surgery, radiotherapy (60
Gy), and temozolomide chemotherapy (4 cycles). Patients had clear
evidence
of tumor progression on sequential magnetic
resonance scanning before enrollment in the study, had received no
anticancer
therapy for ∼1 year, and had a fair health status
(Karnofski performance score = 90). The patients provided written
informed
consent. The protocol was approved by the Clinical
Trials Committee of Tenerife University Hospital and by the Spanish
Ministry
of Health.
Patient 1 (a 48-year-old man) had a
right-occipital-lobe tumor (7.5 × 6 cm maximum diameters), and Patient 2
(a 57-year-old-man)
had a right-temporal-lobe tumor (6 × 5 cm maximum
diameters). Both tumors were diagnosed by the Pathology Department of
Tenerife
University Hospital as glioblastoma multiforme and
showed the hallmarks of this type of tumor (high vascularization,
necrotic
areas, abundant palisading and mitotic cells, and
so on). The tumors were removed extensively by surgery, biopsies were
taken,
and the tip (∼5 cm) of a silastic infusion
cathether (9.6 French; 3.2 mm diameter) was placed into the resection
cavity. The
infusion cathether was connected to a Nuport
subclavicular s.c. reservoir. Each day 0.5–1.5 (median 1.0) mg of Δ
9-tetrahydrocannabinol
(100 μg/μl in ethanol solution) were dissolved in 30 ml of
physiological saline solution supplemented
with 0.5% (w/v) human serum albumin, and the
resulting solution was filtered and subsequently administered at a rate
of 0.3
ml/min with a syringe pump connected to the s.c.
reservoir. Patient 1 started the treatment 4 days after the surgery and
received
a total amount of 24.5 mg of Δ
9-tetrahydrocannabinol for 19 days. The posttreatment biopsy was taken 19 days after the cessation of Δ
9-tetrahydrocannabinol administration. Patient 2 started the treatment 4 days after the surgery and received a total amount
of 13.5 mg of Δ
9-tetrahydrocannabinol for 16 days. The posttreatment biopsy was taken 43 days after the cessation of Δ
9-tetrahydrocannabinol administration. Samples were either transferred to DMEM containing 15% FCS and 1 m
m glutamine (for tumor-cell isolation, see above; Fig. 2
B
⇓
) and frozen (for VEGF determination, Patients 1
and 2; and for VEGFR-2 Western blotting, Patient 1; Fig. 6,
A and
C
⇓
) or fixed in formalin and embedded in paraffin (for VEGFR-2 confocal microscopy, Patients 1 and 2; Fig. 6
B
⇓
).
The cDNA Arrays.
Total RNA was extracted
(27)
from tumors of vehicle- or JWH-133-treated mice (see above), and poly(A)
+ RNA was isolated with oligotex resin (Qiagen Inc., Valencia, CA) and reverse-transcribed with Moloney murine leukemia virus
reverse transcriptase in the presence of 50 μCi [α-
33P]dATP
for the generation of radiolabeled cDNA probes. Purified radiolabeled
probes were hybridized to angiogenesis, hypoxia,
and metastasis gene array membranes (GEArray Q
Series; Superarray Bioscience Corporation, Frederick, MD) according to
the
manufacturer’s instructions.
5
Hybridization signals were detected by
phosphorimager and analyzed by Phoretix housekeeping genes in the blots
as internal
controls for normalization. The selection criteria
were set conservatively throughout the process, and the genes selected
were required to exhibit at least a 2-fold change
of expression and a
P < 0.01.
ELISA.
VEGF levels were determined in cell culture media and in tumor extracts, obtained by homogenization as described previously
(16)
, by solid-phase ELISA using the Quantikine mouse
VEGF Immunoassay (R&D Systems, Abingdon, United Kingdom; 70%
cross-reactivity
with rat VEGF) for rat and mouse samples and the
Quantikine human VEGF Immunoassay (R&D Systems) for human samples.
Western Blot.
Particulate cell or tissue fractions were
subjected to SDS-PAGE, and proteins were transferred from the gels onto
polyvinylidene
fluoride membranes. Blots were incubated with
antibodies against total VEGFR-2 (1:1000; Santa Cruz Biotechnology,
Santa Cruz,
CA), VEGFR-2 phosphotyrosine 996 (1:250; Cell
Signaling, Beverly, MA), VEGFR-2 phosphotyrosine 1214 (1:250; kindly
given by
Dr. Francesco Pezzella, Nuffield Department of
Clinical Laboratory Science, University of Oxford, United Kingdom), and
α-tubulin
(1:4000, Sigma). The latter was used as a loading
control. In all of the cases, samples were subjected to luminography
with
an enhanced chemiluminescence detection kit
(Amersham Life Sciences, Arlington Heights, IL). Densitometric analysis
of the
blots was performed with the Multianalyst software
(Bio-Rad Laboratories, Hercules, CA).
Confocal Microscopy.
Glioma cells were cultured in coverslips
and fixed in acetone for 10 min. Mouse tumors were dissected and frozen,
and 5-μm
sections were fixed in acetone for 10 min. Human
tumors were fixed in 10% buffered formalin and then paraffin-embedded,
5-μm
sections were deparaffinized and rehydrated, and
antigen retrieval was carried out by immersing the slides in 10 mm
citrate (pH 6.0) and boiling for 3 min. All of the samples were
incubated with 10% goat serum in PBS for 30 min at room temperature
to block nonspecific binding. Slices were incubated
for 1.5 h with the aforementioned primary antibodies against total
VEGFR-2
(1:50) and VEGFR-2 phosphotyrosine 1214 (1:20).
After washing with PBS, slices were additionally incubated (1 h, room
temperature,
darkness) with a mixture of the secondary goat
antimouse antibodies Alexa Fluor 488 and Alexa Fluor 546 (both at 1:400;
Molecular
Probes, Leyden, The Netherlands). After washing
with PBS, sections were fixed in 1% paraformaldehyde for 10 min and
mounted
with DAKO fluorescence mounting medium containing
TOTO-3 iodide (1:1000; Molecular Probes) to stain cell nuclei. Confocal
fluorescence images were acquired using a Laser
Sharp 2000 software (Bio-Rad) and a Confocal Radiance 2000 coupled to
Axiovert
S100 TV microscope (Carl Zeiss, Oberkochen,
Germany). Pixel quantification and colocalization were determined with
Metamorph-Offline
software (Universal Imaging, Downingtown, PA).
Ceramide Synthesis.
C6 glioma cells were cultured for 48 h in serum-free medium with the additions indicated together with 1 μCi of
l-U-[
14C]serine/well, lipids were extracted, and ceramide resolved by thin-layer chromatography as described previously
(28)
.
Statistics.
Results shown represent mean ± SD. Statistical analysis was performed by ANOVA with a post hoc analysis by the Student-Neuman-Keuls test or by unpaired Student’s t test.
RESULTS
Changes in Gene Expression Profile in Mouse Gliomas.
The cDNA array analysis was used as a
first approach to test whether cannabinoid administration affects the
VEGF pathway in
mouse gliomas. Because cannabinoid-based
therapeutic strategies should be as devoid as possible of psychotropic
side effects
and glioma cells express functional CB
2 receptors, which do not mediate psychoactivity
(16
, 26)
, mice bearing s.c. gliomas were injected with the selective CB
2 agonist JWH-133
(26)
.
A total of 267 genes related to
angiogenesis, hypoxia (perhaps the most potent stimulus for the onset of
tumor angiogenesis),
and metastasis (a characteristic of actively
growing tumors related closely to angiogenesis) were analyzed, of which
126 were
considered to be expressed in reliable amounts.
JWH-133 administration altered the expression of 10 genes, all of which
are
directly or indirectly related to the VEGF pathway
(Fig. 1)
⇓
. Thus, cannabinoid treatment lowered the expression of the following: (
a) VEGF-A [confirming our previous Northern blot data
(22)]
and its relative VEGF-B
(3
, 4)
; (
b) hypoxia-inducible factor-1α [one of the subunits of hypoxia-inducible factor-1, the major transcription factor involved
in VEGF gene expression
(29)]
; (
c) two genes known to be under the control of VEGF, namely those encoding connective tissue growth factor [a mitogen involved
in extracellular matrix production and angiogenesis
(30)]
, and heme oxygenase-1 [an enzyme highly expressed during hypoxia and inflammation
(31)]
; and (
d) four genes known to encode proteins functionally related to VEGF, namely Id3 [a transcription factor inhibitor involved
in angiogenesis and tumor progression
(32)]
, midkine [a proangiogenic and tumorigenic growth factor
(33)]
, angiopoietin-2 [a prominent proangiogenic factor that cooperates with VEGF
(3
, 19
, 22)]
, and Tie-1 [an angiopoietin receptor
(34)]
. In addition, cannabinoid treatment increased the
expression of the gene encoding type I procollagen α1 chain (a
metalloproteinase
substrate related to matrix remodeling during
angiogenesis; Ref.
35
).
Fig. 1.
Changes in gene expression profile in mouse gliomas after cannabinoid treatment. Animals bearing gliomas were treated with
either vehicle (Control) or JWH-133 (JWH) for 8 days as described in “Materials and Methods.” Equal amounts of poly(A)+ RNA from tumors of 2 animals/group were pooled and hybridized to angiogenesis, hypoxia, and metastasis cDNA array membranes.
Genes affected by cannabinoid treatment are listed. Examples of affected genes are pointed with arrows. Angiogenesis membrane, angiopoietin-2 (top), midkine (middle), and VEGF-A (bottom); Hypoxia membrane, procollagen Iα1 (top), heme oxygenase-1 (middle), and VEGF-A (bottom); and Metastasis membrane, VEGF-A.
Inhibition of VEGF Production in Cultured Glioma Cells and in Mouse Gliomas.
We focused next on the two main components of the VEGF pathway, namely VEGF and VEGFR-2, in both cultured glioma cells and
gliomas
in vivo. Incubation of C6 glioma cells with the synthetic cannabinoid WIN-55,212-2 (100 n
m), a mixed CB
1/CB
2 receptor agonist, inhibited VEGF release into the medium in a time-dependent manner (Fig. 2
A)
⇓
. The cannabinoid did not affect cell viability
throughout the time interval in which VEGF determinations were performed
(up
to 48 h; data not shown). Cannabinoid-induced
attenuation of VEGF production was evident in another glioma cell line
(the
human astrocytoma U373 MG) and, more importantly,
in tumor cells obtained directly from a human glioblastoma multiforme
biopsy
(Fig. 2
B)
⇓
. The cannabinoid effect was also observed in the
mouse skin carcinoma PDV.C57 and in the human bladder cancer epithelioma
ECV304 (Fig. 2
B)
⇓
.
Fig. 2.
Inhibition of VEGF production by cannabinoids in cultured glioma cells and in mouse gliomas. A, C6 glioma cells were cultured for the times indicated with vehicle (□) or 100 nm WIN-55,212-2 (▪), and VEGF levels in the medium were determined (n = 4). B, U373 MG astrocytoma cells, tumor cells obtained from a patient with glioblastoma multiforme (GBM), PDV.C57 epidermal carcinoma cells, and ECV304 bladder cancer epithelioma cells were cultured for 48 h with vehicle (□)
or 100 nm
WIN-55,212-2 (▪), and VEGF levels in the medium were determined. Data
represent the percentage of VEGF in cannabinoid incubations
versus the respective controls (n = 3–4). C, C6 glioma cells were cultured for 48 h with vehicle (Control), 100 nm WIN-55,212-2 (WIN), 100 nm JWH-133 (JWH), 2 μm anandamide (AEA), 0.5 μm SR141716 (SR1), and/or 0.5 μm SR144528 (SR2), and VEGF levels in the medium were determined (n = 4–6). D, C6 glioma cells were cultured for 48 h with vehicle (Control), 100 μm WIN-55, 212-2 (WIN), 1 μm C2-ceramide (CER), and/or 0.5 μm fumonisin B1 (FB1), and VEGF levels in the medium were determined (n = 4). E, animals bearing gliomas were treated with either vehicle (Control), JWH-133 (JWH), fumonisin B1 (FB1),
or JWH-133 plus fumonisin B1 for 8 days as described in “Materials and
Methods,” and VEGF levels in the tumors were determined
(n = 4–6 for each experimental group). Significantly different (∗, P < 0.01; ∗ ∗, P < 0.05) from control incubations or control animals. Bars, ±SD.
To prove the specificity of WIN-55,212-2
action on VEGF release, we used other cannabinoid receptor agonists as
well as selective
cannabinoid receptor antagonists (Fig. 2
C)
⇓
. The inhibitory effect of WIN-55,212-2 was mimicked by the endocannabinoid anandamide (2 μ
m), another mixed CB
1/CB
2 agonist, and by the synthetic cannabinoid JWH-133 (100 n
m), a selective CB
2 agonist. In addition, the CB
1 antagonist SR141716 (0.5 μ
m) and the CB
2 antagonist SR144528 (0.5 μ
m)
prevented WIN-55,212-2 action, pointing to the involvement of CB
receptors in cannabinoid-induced inhibition of VEGF production.
The sphingolipid messenger ceramide has been implicated in the regulation of tumor cell function by cannabinoids
(16
, 23
, 36)
. The involvement of ceramide in
cannabinoid-induced inhibition of VEGF production was tested by the use
of
N-acetylsphingosine (C
2-ceramide), a cell-permeable ceramide analog, and fumonisin B1, a selective inhibitor of ceramide synthesis
de novo. In line with our previous data in primary cultures of rat astrocytes
(28)
, fumonisin B1 was able to prevent cannabinoid-induced ceramide biosynthesis (relative values of [
14C]serine incorporation into ceramide,
n = 3: vehicle, 100; 100 n
m WIN-55,212-2, 140 ± 1; 100 n
m WIN-55,212-2 plus 0.5 μ
m fumonisin B1, 86 ± 9). C
2-ceramide (1 μ
m) depressed VEGF production, whereas pharmacological blockade of ceramide synthesis
de novo with fumonisin B1 (0.5 μ
m) prevented the inhibitory effect of WIN-55,212-2 (Fig. 2
D)
⇓
. We subsequently evaluated whether fumonisin B1 action was also evident
in vivo. The decrease in tumor VEGF levels induced by cannabinoid administration
(19
, 22
, 37)
was prevented by cotreatment of the animals with fumonisin B1 (Fig. 2
E)
⇓
.
Inhibition of VEGFR-2 in Cultured Glioma Cells and in Mouse Gliomas.
VEGFR-2 activation was determined by
measuring the extent of phosphorylation of two of its essential tyrosine
autophosphorylation
residues, namely 996 and 1214
(3
, 4)
. Western blot experiments showed that C6 glioma
cells express highly phosphorylated VEGFR-2 in the absence of ligand,
indicating
that the receptor may be constitutively active.
Incubation of C6 glioma cells with WIN-55,212-2 or JWH-133 decreased
VEGFR-2
activation without affecting total VEGFR-2 levels
(Fig. 3
A)
⇓
. Confocal microscopy experiments confirmed the
decrease in VEGFR-2 immunoreactivity by cannabinoid challenge when
fluorescence
was expressed per cell nucleus (Fig. 3
B)
⇓
or per total-VEGFR-2 fluorescence (data not shown).
Moreover, fumonisin B1 prevented cannabinoid inhibitory action, and C
2-ceramide reduced VEGFR-2 activation (Fig. 3,
A and
B)
⇓
. Interestingly, on cannabinoid exposure the
receptor seemed to be preferentially condensed in the perinuclear
region, and
this relocalization was prevented by fumonisin B1
(Fig. 3
B)
⇓
. The functional impact of VEGF on C6 glioma cells
was supported by the finding that VEGF induced a prosurvival action by
preventing the loss of cell viability on prolonged
(72 h) cannabinoid or C
2-ceramide challenge (Fig. 3
C)
⇓
.
Fig. 3.
Inhibition of VEGFR-2 by cannabinoids in cultured glioma cells. A, C6 glioma cells were cultured for 4 h with vehicle (Control), 100 nm WIN-55,212-2 (WIN), 100 nm JWH-133 (JWH), 10 μm C2-ceramide (CER), and/or 0.5 μm fumonisin B1 (FB1), and VEGFR-2 activation (anti-VEGFR-2 PY996 and anti-VEGR2 PY1214 antibodies) and expression (antitotal VEGFR-2 antibody) were determined by Western blot. Absorbance values relative to those of total VEGFR-2 are given in arbitrary units.
Significantly different (∗, P < 0.01) from control incubations (n = 3). B, C6 glioma cells were cultured as in panel A, and VEGFR-2 activation (anti-VEGFR-2 PY1214 antibody, green) and expression (antitotal VEGFR-2 antibody, red) were determined by confocal microscopy. Cell nuclei are stained in blue.
One representative experiment of 3 is shown. Relative values of
activated-VEGFR-2 pixels/cell nucleus are given in parentheses.
C, C6 glioma cells were cultured for 72 h with vehicle (Control), 100 nm WIN-55,212-2 (WIN), 100 nm JWH-133 (JWH), or 1 μm C2-ceramide (CER) with (▪) or without (□) 50 ng/ml VEGF, and the number of viable cells was determined. Significantly different (∗, P < 0.01) from control incubations (n = 3–4). Bars, ±SD.
The effect of cannabinoid administration
on VEGFR-2 activation was subsequently tested in tumor-bearing mice. The
ceramide-dependent
cannabinoid-induced inhibition of VEGFR-2
activation found in cultured cells was also observed by Western blot
(Fig. 4
A)
⇓
and confocal microscopy (Fig. 4
B)
⇓
in mouse gliomas. Like in the cultured-cell
experiments and in line with the cDNA array experiments (data not
shown), total
VEGFR-2 expression in the tumors was unaffected by
cannabinoid treatment (Fig. 4,
A and
B)
⇓
.
Fig. 4.
Inhibition of VEGFR-2 by cannabinoids in mouse gliomas. A, animals bearing gliomas were treated with either vehicle (Control), JWH-133 (JWH), fumonisin B1 (FB1), or JWH-133 plus fumonisin B1 for 8 days as described in “Materials and Methods,” and VEGFR-2 activation (anti-VEGFR-2 PY996 and anti-VEGR2 PY1214 antibodies) and expression (antitotal VEGFR-2 antibody) were determined by Western blot. Absorbance values relative to those of total VEGFR-2 (phosphorylated VEGFR-2 blots)
or of α-tubulin (total VEGFR-2 blots) are given in arbitrary units. Significantly different (∗, P < 0.01) from control animals (n = 3–4 for each experimental group). B, animals bearing gliomas were treated as in panel A, and VEGFR-2 activation (anti-VEGFR-2 1214 antibody, green) and expression (antitotal VEGFR-2 antibody, red) were determined by confocal microscopy. Cell nuclei are stained in blue. Low- and high-magnification pictures are shown. One representative tumor of 3–4 for each experimental group is shown. Relative
values of activated-VEGFR-2 pixels/cell nucleus are given in parentheses.
Phosphorylated VEGFR-2 has been found previously in the cell nucleus, and it has been postulated that this translocation process
might play a role in VEGFR-2 signaling
(38,
39,
40)
. However, by confocal microscopy, we found a
rather variable fraction of phosphorylated VEGFR-2 in the nuclei of C6
glioma
cells in culture and on inoculation in mice, and
this fraction of nuclear VEGFR-2 was unaltered after treatment with
cannabinoids
and/or fumonisin B1
in vitro and
in vivo (data not shown).
Changes in the Size of Mouse Gliomas.
To test whether the aforementioned ceramide-dependent changes in the VEGF pathway are functionally relevant, we measured tumor
size along cannabinoid and fumonisin B1 treatment. In agreement with previous observations
(26)
, JWH-133 administration blocked the growth of s.c.
gliomas in mice. Of importance, cotreatment of the animals with
fumonisin
B1 prevented cannabinoid antitumoral action (Fig.
5)
⇓
.
Fig. 5.
Changes in the size of mouse gliomas after cannabinoid and fumonisin B1 treatment. Animals bearing gliomas (n = 4–6 for each experimental group) were treated with either vehicle (Control, ○), JWH-133 (JWH, •), fumonisin B1 (FB1, □), or JWH-133 plus fumonisin B1 (▪) for up to 8 days as described in “Materials and Methods.” Examples of formaldehyde-fixed
dissected tumors after 8 days of treatment are shown. Bars, ±SD.
Inhibition of the VEGF Pathway in Two Patients with Glioblastoma Multiforme.
To obtain additional support for the potential therapeutic implication of cannabinoid-induced inhibition of the VEGF pathway,
we analyzed the tumors of two patients enrolled in a clinical trial aimed at investigating the effect of Δ
9-tetrahydrocannabinol, a mixed CB
1/CB
2 agonist, on recurrent glioblastoma multiforme. The patients were subjected to local Δ
9-tetrahydrocannabinol administration, and biopsies were taken before and after the treatment. In both patients, VEGF levels
in tumor extracts were lower after cannabinoid inoculation (Fig. 6
A)
⇓
. The Δ
9-tetrahydrocannabinol also
lowered the expression of phosphorylated VEGFR-2 in the tumors of the
two patients, and this was
accompanied (in contrast to the mouse glioma
experiments shown above) by a decrease in total VEGFR-2 levels (Fig. 6
B)
⇓
. This was confirmed by Western blot analysis in Patient 1 (Fig. 6
C)
⇓
. Unfortunately, we were unable to obtain appropriate samples for Western blot from Patient 2.
Fig. 6.
Inhibition of the VEGF pathway in two patients with glioblastoma multiforme after cannabinoid treatment. The patients were
subjected to Δ9-tetrahydrocannabinol (THC) administration as described in “Materials and Methods.” A, VEGF levels in the tumors before (□) and after (▪) THC treatment. B, VEGFR-2 activation (anti-VEGFR-2 PY1214 antibody, green) and expression (antitotal VEGFR-2 antibody, red) in the tumors before and after THC treatment as determined by confocal microscopy. Cell nuclei are stained in blue. Relative values of activated-VEGFR-2 pixels (parentheses) and of total-VEGFR-2 pixels (square brackets) per cell nucleus are given for the two patients. C, VEGFR-2 activation (anti-VEGFR-2 PY996 and anti-VEGR2 PY1214 antibodies) and expression (antitotal VEGFR-2 antibody) in the tumor of Patient 1 before and after THC treatment, as determined by Western blot. Absorbance values relative
to those of loading controls (α-tubulin) are given in arbitrary units.
DISCUSSION
Angiogenesis is a prerequisite for the
progression of most solid tumors. In particular, gliomas first acquire
their blood
supply by co-opting existing normal brain vessels
to form a well-vascularized tumor mass without the necessity to initiate
angiogenesis
(41,
42,
43)
. When gliomas progress, they become hypoxic as the
co-opted vasculature regresses and malignant cells rapidly proliferate.
These hypoxic conditions, in turn, induce robust
angiogenesis via the VEGF pathway and angiopoietin-2, and in fact, this
angiogenic
sprouting distinguishes a grade IV astrocytoma
(glioblastoma multiforme) from lower-grade astrocytomas
(41,
42,
43)
. Here, we show that cannabinoid treatment impairs
the VEGF pathway in mouse gliomas by blunting VEGF production and
signaling.
Cannabinoid-induced inhibition of VEGF expression
and VEGFR-2 activation also occurred in cultured glioma cells,
indicating
that the changes observed
in vivo may
reflect the direct impact of cannabinoids on tumor cells. Moreover, a
depression of the VEGF pathway was also evident
in two patients with glioblastoma multiforme.
Although the changes in VEGFR-2 expression observed in these two
patients do
not fully mirror the cultured-cell and mouse data,
they clearly follow the same direction. The molecular basis of this
discrepancy
is, however, unknown.
Our observations do not exclude that
cannabinoids may also blunt tumor VEGF signaling indirectly by targeting
other receptor-mediated
processes that stimulate the VEGF pathway. For
example, it is known that engagement of epidermal growh factor
(44)
and nerve growth factor
(45)
receptors induces the VEGF pathway, and
cannabinoids have been reported to inhibit the epidermal growth factor
receptor in
skin carcinoma
(19)
and prostate carcinoma cells
(46)
as well as the TrkA neurotrophin receptor in breast carcinoma
(47)
and pheochromocytoma cells
(20)
. However, the molecular mechanisms by which
cannabinoid receptor activation impact these growth factor receptors
remain obscure.
Recent work has shown that cannabinoids can modulate sphingolipid-metabolizing pathways by increasing the intracellular levels
of ceramide
(23)
, a lipid second messenger that controls cell fate in different systems
(48
, 49)
. After cannabinoid receptor activation, two peaks
of ceramide generation are observed in glioma cells that have different
mechanistic origin: (
a) the first peak comes from sphingomyelin hydrolysis
(50)
; and (
b) the second peak originates from ceramide synthesis
de novo
(36)
. The findings reported here expand the role of
de novo-synthesized
ceramide in cannabinoid action. Moreover, as far as we know, this is
also the first report showing that ceramide
depresses the VEGF pathway by interfering with VEGF
production and VEGFR-2 activation, a notion that is in line with the
observation
that ceramide analogs prevent VEGF-induced cell
survival
(51
, 52)
. In the context of the “sphingolipid rheostat” theory
(48
, 49)
, the mitogenic sphingolipid sphingosine
1-phosphate would shift the balance toward angiogenesis and
tumorigenesis
(5
, 53)
, whereas the antiproliferative sphingolipid
ceramide would blunt angiogenesis and tumorigenesis (present study).
The use of cannabinoids in medicine is limited by their psychoactive effects mediated by neuronal CB
1 receptors
(9
, 10)
. Although these adverse effects are within the
range of those accepted for other medications, especially in cancer
treatment,
and tend to disappear with tolerance on continuous
use
(20)
, it is obvious that cannabinoid-based therapies
devoid of side-effects would be desirable. As glioma cells express
functional
CB
2 receptors
(26)
, we used a selective CB
2 ligand to target the VEGF pathway. Selective CB
2 receptor activation in mice also inhibits the growth and angiogenesis of skin carcinomas
(19)
. Unfortunately, very little is known about the pharmacokinetics and toxicology of the selective CB
2 ligands synthesized to date, making them as yet unavailable for clinical trials.
Gliomas are one of the most malignant
forms of cancer, resulting in the death of affected patients within 1–2
two years after
diagnosis. Current therapies for glioma treatment
are usually ineffective or just palliative. Therefore, it is essential
to
develop new therapeutic strategies for the
management of glioblastoma multiforme, which will most likely require a
combination
of therapies to obtain significant clinical
results. In line with the idea that anti-VEGF treatments constitute one
of the
most promising antitumoral approaches currently
available
(5,
6,
7)
, the present laboratory and clinical findings
provide a novel pharmacological target for cannabinoid-based therapies.
