Antitumor activity of plant cannabinoids with emphasis on the effect of cannabidiol on human breast carcinoma.

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1. Alessia Ligresti,
2. Aniello Schiano Moriello,
3. Katarzyna Starowicz,
4. Isabel Matias,
5. Simona Pisanti,
6. Luciano De Petrocellis,
7. Chiara Laezza,
8. Giuseppe Portella,
9. Maurizio Bifulco and
10. Vincenzo Di Marzo

+Author Affiliations

1. 
   
   Endocannabinoid Research Group, Istituto di Chimica Biomolecolare (A.L., A.S.M., K.S., I.M., V.D.M.), and Istituto di Cibernetica (A.S.M., L.D.P.), Consiglio Nazionale delle Ricerche Pozzuoli, Italy; Dipartimento di Biologia e Patologia Cellulare e Molecolare “L. Califano”, Università di Napoli “Federico II”, Napoli, Italy (S.P., C.L., G.P., M.B.); and Dipartimento di Scienze Farmaceutiche, Università degli Studi di Salerno, Fisciano, Italy (S.P., M.B.)

1. Address correspondence to:
   Dr. Vincenzo Di Marzo, Istituto di Chimica Biomolecolare, Consiglio Nazionale delle Ricerche, Via Campi Flegrei 34, 80078 Pozzuoli, Napoli, Italy. E-mail:vdimarzo@icmib.na.cnr.it

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Abstract

Δ⁹-Tetrahydrocannabinol (THC) exhibits antitumor effects on various cancer cell types, but its use in chemotherapy is limited by its psychotropic activity. We investigated the antitumor activities of other plant cannabinoids, i.e., cannabidiol, cannabigerol, cannabichromene, cannabidiol acid and THC acid, and assessed whether there is any advantage in using Cannabis extracts (enriched in either cannabidiol or THC) over pure cannabinoids. Results obtained in a panel of tumor cell lines clearly indicate that, of the five natural compounds tested, cannabidiol is the most potent inhibitor of cancer cell growth (IC₅₀ between 6.0 and 10.6 μM), with significantly lower potency in noncancer cells. The cannabidiol-rich extract was equipotent to cannabidiol, whereas cannabigerol and cannabichromene followed in the rank of potency. Both cannabidiol and the cannabidiol-rich extract inhibited the growth of xenograft tumors obtained by s.c. injection into athymic mice of human MDA-MB-231 breast carcinoma or rat v-K-ras-transformed thyroid epithelial cells and reduced lung metastases deriving from intrapaw injection of MDA-MB-231 cells. Judging from several experiments on its possible cellular and molecular mechanisms of action, we propose that cannabidiol lacks a unique mode of action in the cell lines investigated. At least for MDA-MB-231 cells, however, our experiments indicate that cannabidiol effect is due to its capability of inducing apoptosis via: direct or indirect activation of cannabinoid CB₂ and vanilloid transient receptor potential vanilloid type-1 receptors and cannabinoid/vanilloid receptor-independent elevation of intracellular Ca²⁺ and reactive oxygen species. Our data support the further testing of cannabidiol and cannabidiol-rich extracts for the potential treatment of cancer.

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The therapeutic properties of the hemp plant, Cannabis sativa, have been known since antiquity, but the recreational use of its euphoric and other psychoactive effects has restricted for a long time research on its possible pharmaceutical application. The isolation of Δ⁹-tetrahydrocannabinol (THC), the main psychoactive component of Cannabis (Gaoni and Mechoulam, 1964), opened the way to further investigations. After the discovery of the two specific molecular targets for THC, CB₁, and CB₂ (for review, see Pertwee, 1997), it became clear that most of the effects of marijuana in the brain and peripheral tissues were due to activation of these two G-protein-coupled cannabinoid receptors. However, evidence is also accumulating that some pharmacological effects of marijuana are due to Cannabis components different from THC. Indeed, C. sativacontains at least 400 chemical components, of which 66 have been identified to belong to the class of the cannabinoids (Pertwee, 1997).

To date, cannabinoids have been successfully used in the treatment of nausea and vomiting (for review, see Robson, 2005), two common side effects that accompany chemotherapy in cancer patients. Nevertheless, the use of cannabinoids in oncology might be somehow underestimated since increasing evidence exist that plant, synthetic, and endogenous cannabinoids (endocannabinoids) are able to exert a growth-inhibitory action on various cancer cell types. However, the precise pathways through which these molecules produce an antitumor effect has not been yet fully characterized, also because their mechanism of action appears to be dependent on the type of tumor cell under study. It has been reported that cannabinoids can act through different cellular mechanisms, e.g., by inducing apoptosis, cell-cycle arrest, or cell growth inhibition, but also by targeting angiogenesis and cell migration (for review, seeBifulco and Di Marzo, 2002; Guzman, 2003; Kogan, 2005). Furthermore, the antitumoral effects of plant, synthetic and endocannabinoids can be mediated by activation of either CB₁ (Melck et al., 2000; Bifulco et al., 2001; Ligresti et al., 2003; Mimeault et al., 2003) or CB₂ receptors or both (Sanchez et al., 2001;Casanova et al., 2003; McKallip et al., 2005), and, at least in the case of the endocannabinoid anandamide, by transient receptor potential vanilloid type-1 (TRPV1) receptors (Maccarrone et al., 2000; Jacobsson et al., 2001; Contassot et al., 2004) as well as by noncannabinoid, nonvanilloid receptors (Ruiz et al., 1999). Additionally, cannabidiol has been suggested to inhibit glioma cell growth in vitro and in vivo independently from cannabinoid and vanilloid receptors (Massi et al., 2004; Vaccani et al., 2005).

The main limitation of the possible future use of THC in oncology might be represented by adverse effects principally at the level of the central nervous system, consisting mostly of perceptual abnormalities, occasionally hallucinations, dysphoria, abnormal thinking, depersonalization, and somnolence (Walsh et al., 2003). However, most non-THC plant cannabinoids seem to be devoid of direct psychotropic proprieties. In particular, it has been ascertained that cannabidiol is nonpsychotropic (for review, see Mechoulam et al., 2002;Pertwee, 2004) and may even mitigate THC psychoactivity by blocking its conversion to the more psychoactive 11-hydroxy-THC (Bornheim and Grillo, 1998; Russo and Guy, 2006). Moreover, it has been recently found that systematic variations in its constituents (i.e., cannabidiol and cannabichromene) do not affect the behavioral or neurophysiological responses to marijuana (Ilan et al., 2005). Finally, it has been also shown that, unlike THC, systemic administration to rats of cannabigerol does not provoke poly-spike discharges in the cortical electroencephalogram during wakefulness and behavioral depression (Colasanti, 1990). These and other observations reinforce the concept that at least cannabidiol, cannabigerol, and cannabichromene lack psychotropic activity and indicate that for a promising medical profile in cancer therapy, research should focus on these compounds, which instead have been poorly studied with regard to their potential antitumor effects. By keeping this goal in mind, we decided to investigate the antitumor properties of cannabigerol and cannabichromene. We also screened THC acid and cannabidiol acid and two distinct Cannabis extracts (enriched in either cannabidiol or THC), where the presence of nonpsychotropic cannabinoids along with THC has been reported to mitigate the potential side effects of the latter compound in clinical trials (Russo and Guy, 2006).

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Materials and Methods

Drugs. All plant cannabinoids, the two cannabinoid acids, and the two Cannabisextracts were kindly provided by GW Pharmaceuticals (Wiltshire, UK; Fig. 1). Cannabidiol- and THC-rich extracts contained approximately 70% cannabidiol or THC, respectively, together with lesser amounts of other cannabinoids. The two cannabinoid receptor antagonists, SR141716A and SR144528, were a kind gift from Sanofi-Aventis (Paris, France), whereas methyl-β-cyclodextrin, all of the antioxidant drugs (α-tocopherol, vitamin C, astaxantine), N-Acetyl-Asp-Glu-Val-Asp-aldehyde, and BAPTA-AM were purchased from Sigma-Aldrich (St. Louis, MO). The endocannabinoid uptake inhibitor (S)-1′-(4-hydroxybenzyl)-N-ethyl-oleoylamide was synthesized as previously described in Ortar et al. (2003). Finally, all of the TRPV1 or cannabinoid receptor agonists and antagonists (capsaicin, resiniferatoxin, arachidonoyl-2-chloro-ethylamide, JWH-133, AM251, AM630) were obtained from Tocris Cookson (Bristol, UK).

Cell Cultures. Cell lines from various origins (MCF-7 and MDA-MB-231 human breast carcinoma cells, DU-145 human prostate carcinoma cells, CaCo-2 human colorectal carcinoma cells, AGS human gastric adenocarcinoma cells, C₆ rat glioma cells, KiMol rat thyroid cells transformed with the v-K-ras oncogene, and rat basophilic leukemia cells) were maintained at 37°C in a humidified atmosphere containing 5% CO₂. Media, sera, and subculturing procedures differed from line to line and were according to the information provided in each case by the supplier company (DSMZ, Braunschweig, Germany). Primary cells derived from normal human mammary glands were purchased from Cell Applications, Inc. (San Diego, CA) and cultured as described in the data sheet from the supplier.

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Fig. 1.

Chemical structures of the plant-derived cannabinoids used in this study.

Cell Proliferation Assay. Six-well culture plates were incubated at 37°C at a cell density of 5 × 10⁴ cells/well in a humidified atmosphere containing 5% CO₂. Three hours after seeding, vehicle or cannabinoids at different concentrations were added to the medium and then daily with each change of medium for 4 days, and the effect of compounds on cell growth was measured by Crystal Violet vital staining. After staining, cells were lysated in 0.01% acetic acid and analyzed by spectrophotometer analysis (PerkinElmer Lambda 12, λ = 595 nm; PerkinElmer Life and Analytical Sciences, Boston, MA). Optical density values from vehicle-treated cells were considered as 100% of proliferation. Statistical analysis was performed using ANOVA followed by Bonferroni test.

Detection of Reactive Oxygen Species. Intracellular reactive oxygen species (ROS) generation was determined by spectrofluorometric analysis. MDA-MB-231 cells were plated (16 × 10³ cells/well) in Porvair PS-White Microplate 96-well (PerkinElmer Life and Analytical Sciences) for 12 h. The day of the experiment, cells were rinsed once with Tyrode’s buffer, then loaded (1 h at 37°C in darkness) with 10 μM 2′,7′-dichlorofluorescein diacetate (fluorescent probe; Molecular Probes, Eugene, OR) in the presence of 0.05% Pluronic F-127. Reactive oxygen species (ROS)-induced fluorescence of intracellular 2′,7′-dichlorofluorescein diacetate was measured with a microplate reader (PerkinElmer LS50B, λ_Ex, 495 nm; λ_Em, 521 nm). Fluorescence detections were carried out after the incubation of 100 μM H₂O₂ and/or increasing concentrations of cannabidiol at room temperature in the darkness for different times (0-30-60-120 min). The fluorescence measured at time 0 was considered as basal ROS production and subtracted from the fluorescence at different times (Δ₁). Data are reported as mean ± S.E. of Δ₂, i.e., fluorescence Δ₁ values at different doses subtracted of the Δ₁ values of cells incubated with vehicle. In some experiments, a buffer containing MgCl₂ in amounts equivalent to CaCl₂ and 0.1 mM EGTA and cells preloaded for 30 min with BAPTA-AM (40 μM) were used instead.

Reverse Transcription-Polymerase Chain Reaction Analysis. Total RNAs from these cells were extracted using the Trizol reagent according to the manufacturer’s recommendations (Invitrogen, Carlsbad, CA). Following extraction, RNA was precipitated using ice-cold isopropanol, resuspended in diethyl pyrocarbonate (Sigma-Aldrich)-treated water, and its integrity was verified following separation by electrophoresis on a 1% agarose gel containing ethidium bromide. RNA was further treated with RNase-free DNase I (Ambion DNA-free kit; Ambion, Austin, TX) according to the manufacturer’s recommendations to digest contaminating genomic DNA and to subsequently remove the DNase and divalent cations.

The expression of mRNAs for CB₁, CB₂, TRPV1, and GAPDH were examined by semiquantitative RT-PCR. Total RNA was reverse-transcribed using random primers. DNA amplifications were carried out in PCR buffer (Invitrogen) containing 2 μl of cDNA, 500 μM dNTP, 2 mM MgCl2, 0.8 μM of each primer, and 0.5 U of Taqpolymerase platinum (Invitrogen). The thermal reaction profile consisted of a denaturation step at 94°C for 1 min, annealing at 55°C (GAPDH) or 57°C (CB₂ and TRPV1) or 60°C (CB₁) for 1 min, and an extension step at 72°C for 1 min. A final extension step of 10 min was carried out at 72°C. The PCR cycles observed to be optimal and in the linear portion of the amplification curve were 24 for GAPDH, 29 for CB₁ and CB₂, and 28 for TRPV1 (data not shown). Reaction was performed in a PE Gene Amp PCR System 9600 (PerkinElmer Life and Analytical Sciences). After reaction, the PCR products were electrophoresed on a 2% agarose gel containing ethidium bromide for UV visualization.

Specific rat and human oligonucleotides were synthesized on the basis of cloned rat and human cDNA sequences of CB₁ (GenBank accession nos. NM_012784.3 and X81120 for rat and human, respectively), CB₂ (GenBank accession nos. NM_0205433 and X74328 for rat and human, respectively), TRPV1 (GenBank accession nos. NM_031982 and NM_080706.2 for rat and human, respectively), and GAPDH (GenBank accession nos. NM_017008.2 and BT006893.1 for rat and human, respectively).

For rat and human CB₁, the primers sequences were 5′-GAT GTC TTT GGG AAG ATG AAC AAG C-3′ (nt 1250-1274 for rat and nt 1187-1211 for human; sense) and 5′-AGA CGT GTC TGT GGA CAC AGA CAT GG-3′ (nt 1558-1534 for rat and nt 1495-1470 for human; antisense). The rat CB₂ sense and antisense primers were 5′-TA(C/T) CC(G/A) CCT (A/T)CC TAC AAA GCT C-3′ (nt 407-428) and 5′-C (A/T)GG CAC CTG CCT GTC CTG GTG-3′ (nt 698-676), respectively. For human CB₂, the primers sequences were 5′-TTT CCC ACT GAT CCC CAA TG-3′ (nt 672-691; sense) and 5′-AGT TGA TGA GGC ACA GCA-3′ (nt 1000-983; antisense). For rat TRPV1, the primers sequences were 5′-GAC ATG CCA CCC AGC AGG-3′ (nt 2491-2508; sense) and 5′-TCA ATT CCC ACA CAC CTC CC-3′ (nt 2752-2733; antisense). The human TRPV1 sense and antisense primers were 5′-TGG ACG AGG TGA ACT GGA C-3′ (nt 2761-2779) and 5′-ACT CTT GAA GAC CTC AGC GTC-3′ (nt 3023-3003), respectively. For rat and human GAPDH, the primers sequences were 5′-CCC TTC ATT GAC CTC AAC TAC ATG GT-3′ (nt 949-974 for rat and nt 106-131 for human; sense) and 5′-GAG GGG CCA TCC ACA GTC TTC TG-3′ (nt 1418-1396 for rat and nt 575-553 for human; antisense).

The expected sizes of the amplicons were 309 bp for rat and human CB₁, 291 bp for rat CB₂, 329 bp for human CB₂, 263 bp for rat TRPV1, 262 bp for human TRPV1, and 470 bp for rat and human GAPDH. In the presence of contaminant genomic DNA, the expected size of the amplicons would be 1062 bp for GAPDH (data not shown). No PCR product was detected when the reverse transcriptase step was omitted.

Western Immunoblotting Analysis for Caspase-3. Immunoblotting analysis was performed on the cytosolic fraction of cells treated as described above and according to previous published work (Iuvone et al., 2004). Cytosolic fraction proteins were mixed with gel loading buffer (50 mM Tris/10% SDS/10% glycerol 2-mercaptoethanol/2 mg of bromphenol/ml) in a ratio of 1:1, boiled for 5 min and centrifuged at 10,000g for 10 min. Protein concentration was determined, and equivalent amounts (50 μg) of each sample were separated under reducing conditions in 12% SDS-polyacrylamide minigel. The proteins were transferred onto nitrocellulose membrane, according to the manufacturer’s instructions (Bio-Rad, Hercules, CA). The membranes were blocked by incubation at 4°C overnight in high-salt buffer (50 mM Trizma base, 500 mM NaCl, 0.05% Tween 20) containing 5% bovine serum albumin and then incubated for 2 h with anticaspase 3 (1:2000, v/v) at room temperature, followed by incubation for 2 h with horseradish peroxidase-conjugate secondary antibody (Dako, Glostrup, Denmark). The immune complexes were developed using enhanced chemiluminescence detection reagents (GE Healthcare, Little Chalfont, Buckinghamshire, UK) according to the manufacturer’s instructions and exposed to Kodak X-OMAT film (Eastman Kodak, Rochester, NY). The bands of τ protein on X-ray film were scanned and densitometrically analyzed with a GS-700 imaging densitometer.

Immunofluorescence. For immunoreaction, MDA-MB-231 cells were seeded on sterile coverslips (22 × 22 mm; Menzel, Braunschweig, Germany) in six-well culture plates and incubated under standard conditions until they were at least 70% confluent. Cultured cells were processed for immunofluorescence. After three washes with PBS, cells were fixed by incubating them in 4% (v/v) paraformaldehyde in PBS for 20 min at room temperature, rinsed with PBS, permeabilized for 15 min in 0.5% Triton X-100 in PBS, and incubated overnight at 4°C with rabbit polyclonal rabbit anti-CB₁ or anti-CB₂ antibody (Cayman Chemical, Ann Arbor, MI), both diluted 1:50 in 0.5% Triton X-100 in PBS, or goat anti-TRPV1 antibody (Santa Cruz Biochemicals, Santa Cruz, CA) diluted 1:100 in 0.5% Triton X-100 in PBS. After three washes in PBS, fluorescence was revealed by incubation for 2 h in an Alexa Fluor 488-labeled secondary anti-rabbit antibody (Invitrogen) diluted 1:100 in 0.5% Triton X-100 in PBS or Alexa Fluor 546-labeled secondary anti-goat antibody (Invitrogen) diluted 1:200 in 0.5% Triton X-100 in PBS. The preabsorption of antibodies with the respective blocking peptides as well as omission of primary antibodies (control immunoreaction) resulted in much weaker or negative staining, respectively. Sections processed for immunofluorescence were studied with an epifluorescence microscope equipped with the appropriate filter (Leica DM IRB; Leica, Wetzlar, Germany). Images were acquired using a digital Leica DFC 320 camera connected to the microscope and the image analysis software Leica IM500. Images were processed in Adobe Photoshop (Adobe Systems, Mountain View, CA), with brightness and contrast being the only adjustments made.

In Vivo Studies: Effect on Xenograft Models of Carcinoma. All of the experiments were performed by using Charles River 6-week-old male athymic mice (Charles River, Margate, Kent, UK) as described previously (Bifulco et al., 2001). Two different mouse xenograft models of tumor growth were induced by s.c. injection (5 × 10⁵ cells) of two distinct highly invasive tumoral cell lines (KiMol or MBA-MD-231 cells) into the dorsal right side of athymic mice. Starting from the appearance of tumoral mass, pure compounds or Cannabis extracts were injected intratumor in the same inoculation region twice per week for 20 (KiMol cell-induced tumors) or 16 (MBA-MD-231 cell-induced tumors) days. THC and cannabidiol were administered at the dose of 5 mg/kg, whereas THC-rich and cannabidiol-rich were administrated at the dose of 6.5 mg/kg, which contains 5 mg/kg THC and cannabidiol, respectively. Tumor diameters were measured with calipers every other day until the animals were killed. Tumor volumes (V) were calculated by the formula of rotational ellipsoid (V = AxB2/2; A = axial diameter, B = rotational diameter). Results were reported as means ± S.E. Statistical analysis was performed using ANOVA followed by the Bonferroni’s test.

In Vivo Analysis: Effect on Experimental Lung Metastasis. Monocellular suspension of MDA-MB-231 cells containing 2.5 × 10⁵ cells was injected into the left paw of 30-day-old BalB/c male mice. Animals were divided into three groups: vehicle (n = 11), cannabidiol (5 mg/kg/dose, n = 14), or cannabidiol-rich (6.5 mg/kg/dose, n = 14). The drugs were injected i.p. every 72 h. Experimental metastases were evaluated 21 days after the injection. To contrast lung nodules, lungs were fixed in Bouin’s fluid, and metastatic nodes were scored on dissected lung under a stereoscopic microscope. All animal studies were conducted in accordance with the Italian regulation for the welfare of animals in experimental neoplasia. All data were presented as means ± S.D. Statistical analysis was performed using one-way ANOVA.

Cell Cycle and Apoptosis Detection. Different cell lines were exposed to 10 μM of cannabidiol or cannabigerol for 48 h at 37°C in a humidified atmosphere containing 5% CO₂. The distribution of cells among the different phases of the cell cycle and apoptosis rate were evaluated by flow cytometric analysis of the DNA content. Cells (5 × 10⁵) were collected, washed twice with PBS, fixed by ethanol 70%, and kept at -20°C for at least 4 h. Propidium iodide (10 μg/ml) in PBS containing 100 U/ml DNase-free RNase was added to the cells for 15 min at room temperature. Cells were acquired by a FACScalibur flow cytometer (BD Biosciences, San Jose, CA), and then analysis was performed using ModFit LT version 3.0 from Verity Software House, Inc. (Topsham, ME); 10,000 events were collected and corrected for debris and aggregate populations.

Anandamide Cellular Reuptake and Intracellular Hydrolysis. The effect of compounds on anandamide cellular reuptake was analyzed on rat basophilic leukemia cells or MDA-MB-231 cells by using 2.5 μM (10,000 cpm) [¹⁴C]anandamide as described previously (De Petrocellis et al., 2000). Briefly, cells were incubated with [¹⁴C]anandamide for 5 min at 37°C, in the presence or absence of varying concentrations of the compounds. Residual [¹⁴C]anandamide in the incubation medium after extraction with CHCl₃/CH₃OH 2:1 (by volume), determined by scintillation counting of the lyophilized organic phase, was used as a measure of the anandamide that was taken up by cells (De Petrocellis et al., 2000). Nonspecific binding of [¹⁴C]anandamide to cells and plastic dishes was determined in the presence of 100 μM anandamide and was never higher than 30%. Data are expressed as the concentration exerting 50% inhibition of anandamide uptake (IC₅₀) calculated by GraphPad software (GraphPad Software Inc., San Diego, CA). The effect of compounds on the enzymatic hydrolysis of anandamide was studied using membranes prepared from N18TG2 cells, incubated with the test compounds and [¹⁴C]anandamide (20,000 cpm; 5 μM) in 50 mM Tris-HCl, pH 9, for 30 min at 37°C. [¹⁴C]Ethanolamine produced from [¹⁴C]anandamide hydrolysis was measured by scintillation counting of the aqueous phase after extraction of the incubation mixture with 2 volumes of CHCl₃/CH₃OH 1:1 (by volume). Data are expressed as the concentration exerting 50% inhibition of [¹⁴C]anandamide hydrolysis (IC₅₀), calculated by GraphPad software.

Activity at Human Recombinant TRPV1. The effect of the substances on [Ca²⁺]_i was determined by using Fluo-3 (Molecular Probes), a selective intracellular fluorescent probe for Ca²⁺ (De Petrocellis et al., 2000). Human embryonic kidney (HEK) 293 cells stably overexpressing human TRPV1 receptor or MDA-MB-231 cells were transferred into six-well dishes coated with poly-L-lysine (Sigma-Aldrich) 1 day prior to experiments and grown in the culture medium mentioned above. On the day of the experiment, the cells (50-60,000 cells/well) were loaded for 2 h at 25°C with 4 μM Flu-3-methylester (Invitrogen) in dimethyl sulfoxide containing 0.04% Pluronic F-127 (Invitrogen). After loading, cells were washed with Tyrode’s solution, pH 7.4, trypsinized, resuspended in Tyrode’s solution, and transferred to the cuvette of the fluorescence detector (PerkinElmer LS50B) under continuous stirring. Experiments were carried out by measuring cell fluorescence at 25°C (λ_EX = 488 nm, λ_EM = 540 nm) before and after the addition of the test compounds at various concentrations. Data are expressed as the concentration exerting a half-maximal effect (EC₅₀). The efficacy of the effect was determined by comparing it with the analogous effect observed with 4 μM ionomycin. In some experiments with MDA-MB-231 cells, the effect of cannabidiol was measured also in the absence of extracellular Ca²⁺ (i.e., in a Tyrode’s solution containing Mg²⁺ instead of Ca²⁺ and 0.1 mM EGTA) and in cells preloaded with BAPTA-AM (20 μM).

Results

Effect on Cancer Cell Growth: In Vitro Studies. For in vitro studies, the cannabinoids under investigation were screened for their ability to reduce cell proliferation on a collection of tumoral cell lines. Cannabidiol always exhibited the highest potency with IC₅₀ values ranging between 6.0 ± 3.0 and 10.6 ± 1.8 μM (Table 1). Cannabidiol acid was the least potent compound. Among the other plant cannabinoids, cannabigerol was almost always the second most potent compound, followed by cannabichromene (Table 1). The effect of the twoCannabis extracts (enriched in cannabidiol or THC) was next investigated, and in some circumstances, the cannabidiol-rich extract appeared slightly more potent than pure cannabidiol (Table 1). In the case of MCF-7 cells, both compounds exhibited quite similar potency, as indicated by the IC₅₀ values of 8.2 ± 0.3 and 6.0 ± 1.0 μM, respectively, for cannabidiol and cannabidiol-rich extract (Fig. 2A), on the contrary, in the case of C₆ glioma cells, cannabidiol-rich extract also exhibited significantly higher potency than pure cannabidiol (IC₅₀, 4.7 ± 0.6 and 8.5 ± 0.8 μM, respectively, p < 0.05, Fig. 2B). Only in the case of human DU-145 prostate carcinoma cells, plant cannabinoids induced a stimulatory effect on cancer growth at the lowest doses tested and an inhibitory effect only at the highest concentration tested (25 μM) (as also found by Sanchez et al., 2003 in another prostate carcinoma cell line). In this case, however, the cannabidiolrich extract lacked the pro-proliferative effect even at the lowest concentration tested of 2 μM (Fig. 2, C and D).

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Effect of cannabinoids andCannabis extracts on cancer cell growth

Various epithelial cell lines of various tumoral origin were treated with different concentrations of drugs, and after 4 days, the cell number was measured with Crystal Violet Vital staining (see Materials and Methods). Data are reported as mean ± S.E. of IC₅₀ values (micromolar) calculated from three independent experiments. CBG, cannabigerol; CBC, cannabichromene; CBD-A, cannabidiol-acid; THC-A, THC-acid; CBD-rich, cannabidiol-enriched cannabis extract; THC-rich, THC-enriched cannabis extract.

Determination of cell cycle arrest, apoptosis, and mortality in the various cell lines exposed for 48 h to 10 μM of cannabidiol before flow cytometry analysis (see Fig. 4 and Materials and Methods); each experiment was repeated three times

Schematic and qualitative representation of the results of the RT-PCR analyses of mRNAs for cannabinoid and vanilloid receptors in the cell lines under study

Total RNA from cells was extracted, and its integrity was verified. RNA was further treated with RNase-free DNase I (Ambion DNA-free kit) to digest contaminating genomic DNA and to subsequently remove the DNase and divalent cations. The expression of mRNAs was examined by RT-PCR. Transcripts for fatty acid amide hydrolase (FAAH) and CB₁and CB₂ receptors were analyzed and are classified as: a, abundant; m, medium; w, weak; and nd, not detected, based on the intensity of the band normalized to the band corresponding to glyceraldehyde-3-phosphate dehydrogenase as the housekeeping gene and on the number of cycles necessary to obtain a visible band. Results are based on n = 3 separate determinations.

Effect of plant cannabinoids on anandamide inactivation

Membranes from N18TG2 cells were incubated with [¹⁴C]anandamide in the presence of compounds for 30 min at 37°C (see Material and Methods) to determine the effect on the enzymatic hydrolysis by fatty acid amide hydrolase (FAAH). Intact RBL-2H3 cells were incubated with [¹⁴C]anandamide in the presence of compounds for 5 min at 37°C (seeMaterial and Methods) to determine the effect on anandamide cellular uptake (ACU). Data represent mean ± S.E. of three different experiments.

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Effect of cannabinoids andCannabis extracts on the proliferation of some of the cell lines investigated in this study. MCF-7 cells (A), C₆ cells (B), and DU-145 cells (C and D) were treated with increasing concentrations of cannabidiol, cannabinoids (C), and cannabidiol-rich extracts (daily added with each change of medium for 4 days). Effect on cell proliferation was measured by Crystal Violet vital staining. After staining, cells were lysated in 0.01% acetic acid and analyzed by spectrophotometric analysis (PerkinElmer Lambda 12, λ = 595 nm). Results are reported as percentage of inhibition of proliferation where optical density value from vehicle-treated cells was considered as 100% of proliferation and represent the mean ± S.E. of three different experiments. ^*, p < 0.05 versus cannabidiol pure by ANOVA followed by Bonferroni’s test. E and F, effect of cannabidiol on nontumoral cell lines. Cells were treated with two different concentrations of cannabidiol for 4 (E) or 3 days (F), and vitality was evaluated by using trypan blue dye exclusion assay (see Materials and Methods). In cells treated with vehicle, mortality was always lower than 2%. Data are expressed as percentage of control and represent the mean ± S.E. of three different experiments. Statistical analysis was carried out by ANOVA followed by Bonferroni’s test (^**, p < 0.01; ^***, p < 0.001 versus the same concentration of cannabidiol on MDA-MB-231 cells). CBG, cannabigerol; CBC, cannabichromene; CBD-A, cannabidiol acid; THC-A, THC acid; CBD-rich, cannabidiol-enriched cannabis extract; THC-rich, THC-enriched cannabis extract.

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In vivo actions of cannabidiol on tumor growth and metastasis. A and B, effect of cannabidiol (5 mg/kg) and cannabidiol-rich extract (6.5 mg/kg) on two different xenograft tumor models in athymic mice. KiMol cells (A) or MBA-MD-231 cells (B) were injected s.c. (day 0 of treatment) into the dorsal right side of athymic mice, and the intratumor treatments were administered twice per week. Results represent mean ± S.E. (^*, p < 0.05 versus vehicle; n = 6 by ANOVA followed by Bonferroni’s test). C, effect of cannabidiol and cannabidiol-rich extract on breast cancer cell metastasis. MDA-MB-231 cells were injected into the left paw of 30-day-old BalB/c male mice. Animals were divided into three groups (n = 11 for vehicle; n = 14 for treated) and treated with vehicle (CTR), cannabidiol (5 mg/kg/dose), or cannabidiol-rich extract (6.5 mg/kg/dose). The drugs were injected i.p. every 72 h. Lung metastatic nodules were evaluated 21 days after the injection. Data represent mean ± S.E. of number of nodules (^*, p < 0.05;^**, p < 0.01 versus CTR). Statistical analysis was performed by ANOVA followed by Bonferroni’s test. CBD-rich, cannabidiol-enriched cannabis extract.

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A to D, representative fluorescence-activated cell sorter analyses showing the effect of 2 days of treatment of 10 μM cannabidiol (CBD) on apoptosis rate in various cell lines calculated as the percentage of cells showing a subdiploid DNA peak (subG1). Graphs are representative of three independent experiments with similar results. Graphs on the left are from cells treated with vehicle and those on the right from cells treated with cannabidiol. Line bar shows where the subdiploid DNA peak is calculated. CTR, vehicle-treated cells. E, effect of cannabidiol on caspase 3 release from procaspase. Western immunoblotting analysis was performed to detect the levels of caspase-3 expression. Proteins were extracted from DU-145 cells (lanes 1 and 2) or MDA-MB-231 cells (lanes 3 and 4) treated with vehicle (CTR, lanes 1 and 3) or 10 μM cannabidiol (cannabidiol, lanes 2 and 4) for 48 h. Determination of relative band intensity was carried out using a GS700 densitometer, and the results are presented in arbitrary scanning units. DU-145, CTR = 5.7 ± 0.81; cannabidiol = 4.2 ± 0.74; MDA-MB-231, CTR = 3.11 ± 0.67; cannabidiol = 2.64 ± 0.26 (Procaspase), 2.89 ± 0.51 (Caspase), mean ± S.E. of n = 3 separate experiments.

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Effect of plant cannabinoids and Cannabis extracts on vanilloid TRPV1 receptor activation. HEK293 cells overexpressing the human recombinant TRPV1 receptor were loaded with a selective fluorescent probe (see Materials and Methods). The TRPV1-mediated effect on [Ca²⁺]_i was determined by measuring cell fluorescence before and after the addition of the test compounds at increasing concentrations. Data are reported as percentage of the maximal effect obtained with Ionomycin 4 μM and are means of n = 3 separate experiments. Error bars are not shown for the sake of clarity and were never higher than 5% of the means. CBG, cannabigerol; CBC, cannabichromene; CBD-A, cannabidiol-acid; THC-A, THC acid; CBD-rich, cannabidiol-enriched cannabis extract; THC-rich, THC-enriched cannabis extract.

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Influence of selective receptor antagonists on CBD antiproliferative action. MDA-MB-231 cells were treated with 10 μM cannabidiol in presence or in absence of selective antagonist for CB₂ receptors [0.5 μM SR144528 (SR₂)], TRPV1 receptors [100 nM 5′-I-resiniferatoxin (I-RTX)], or a mixture of both compounds (mix). Data are shown as percent inhibition of proliferation. Cells vehicle-treated were used as 100% of proliferation. ^*, p < 0.05 versus cannabidiol only, by ANOVA followed by Bonferroni’s test.

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Representative photomicrographs demonstrating localization of CB₁, CB₂, and TRPV1 receptors in human breast adenocarcinoma (MDA-MB-231) cells as determined by the immunofluorescence technique described under Materials and Methods. A, CB₁ receptor immunoreactivity. B, CB₂ receptor immunoreactivity. C, TRPV1-immunoreactivity was performed using rabbit polyclonal anti-CB₁, anti-CB₂ (both diluted 1:50), and Alexa Fluor 488-conjugated secondary antibody (1: 100) and goat polyclonal anti-TRPV1 diluted 1:100 and Alexa Fluor 546-conjugated secondary antibody (1:200). Magnification, 63×. Scale bar, 40 μm. Immunofluorescence was almost undetectable when preincubating antibodies with the corresponding blocking peptides (data not shown).

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Effect of CBD on intracellular Ca²⁺ in MDA-MB-231 cells. A, dose-related effect of cannabidiol in the presence of extracellular Ca²⁺, as determined with Fluo-4. Data are mean ± S.E. of n = 4 experiments and are expressed as percentage of the effect obtained with 4 μM ionomycin. B, time-related effect of cannabidiol (10 μM) in the presence of extracellular Ca²⁺. Trace is representative of n = 4 experiments. C, dose-related effect of cannabidiol in the absence of extracellular Ca²⁺, as determined with Fura-2. Data are mean ± S.E. of n = 4 experiments. Maximal Δ_fluorescence was 0.235 ± 0.031 at 10 μM cannabidiol and was usually attained after 200 s (D). Effect of various antagonists (the CB₁antagonist AM251, 1 μM; the CB₂ antagonist AM630, 1 μM; the TRPV1 antagonist I-RTX, 0.1 μM; 5-min pretreatment) and the intracellular calcium chelating agent BAPTA-AM (20 μM, loaded onto the cells before stimulation) on cannabidiol (1 μM) action on intracellular Ca²⁺. Similar results were obtained with SR141716A and SR144528.

Discussion

The aim of this study was to identify natural cannabinoids with antitumor activities at least similar to those of THC and devoid of the potential central effects of this compound. Given that the efficacy of cannabinoids as antitumoral agents appears to be strictly correlated to the cell type under investigation, we screened a panel of plant cannabinoids in a wide range of tumoral cell lines distinct in origin and typology. We found that, surprisingly, cannabidiol acted as a more potent inhibitor of cancer cell growth than THC and that cannabigerol and cannabichromene usually followed cannabidiol in the rank of potency. The cell growth-inhibitory effect of cannabidiol depended on its chemical structure since the addition of a carboxylic acid group (as in cannabidiol acid) dramatically reduced its activity. This is unlikely due to simple modification of the lipophilicity of the compound and subsequent decrease of its capability to penetrate the cell membrane since THC-A was often more efficacious than THC. We also found that the cannabidiol-rich Cannabis extract was as potent as pure cannabidiol in most cases or even more potent in some cell lines. These results suggest the use in cancer therapy for cannabidiol, a compound lacking the psychotropic effects typical of THC. Indeed, the efficacy of cannabidiol and of the cannabidiol-rich extract were confirmed in vivo in two different models of xenograft tumors obtained by inoculation in athymic mice of either v-K-ras-transformed thyroid epithelial cells or of the highly invasive MDA-MB-231 breast cancer cells. Furthermore, cannabidiol and the cannabidiolrich extract also inhibited the formation of lung metastases subsequent to inoculation of MDA-MB-231 cells, in agreement with the inhibitory actions on cancer cell migration previously described for this compound (Vaccani et al., 2005).

Study of the involvement of oxidative stress in the effect of CBD. A, antiproliferative effect of 10 μM cannabidiol on MDA-MB-231 cells was measured after 4 days of treatment in absence or in presence of increasing concentrations of α-tocopherol. Data represent mean ± S.E. of percent inhibition of proliferation (^*, p < 0.05 by ANOVA followed by Bonferroni’s test). B, time course of ROS production by MDA-MB-231 cells (16 × 10³ cells/well) as measured by spectrofluorometric analysis. Cells were loaded 1 h with 10 μM fluorescent probe in the presence of 0.05% Pluronic; fluorescence was measured in a 96-well microplate reader (PerkinElmer LS50B, λ_Ex, 495 nm; λ_Em, 521 nm). Fluorescence detection was carried out after the incubation of either 100 μMH₂O₂ or increasing concentrations of cannabidiol at different times (0-30-60-120 min). 100 μMH₂O₂ was used as a positive control in these experiments. The fluorescence measured at time 0 was considered as basal ROS production and subtracted from the fluorescence at different times (Δ₁). Data are reported as Δ₂, i.e., Δ₁ values at different doses subtracted of the Δ₁values of cells incubated with vehicle, and are mean ± S.E. of n = 3 experiments. The effects of H₂O₂ and of all doses of cannabidiol were significantly different from control values as determined by ANOVA followed by Bonferroni’s test. Inset, lack of effect of cannabidiol 10 μM on ROS production (after 60 min) in the absence of both extracellular and intracellular Ca²⁺ is shown. ^***, p < 0.005 by ANOVA, n = 5.

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Study of the involvement of oxidative stress in the effect of CBD. A and B, antiproliferative effect of cannabidiol (24-h incubation) was evaluated in standard growth conditions or after 12 h of serum deprivation to induce oxidative stress. Poststarvation, cells were treated with 0.5 or 10 μM cannabidiol for 24 h, and the effect on proliferation was evaluated by Crystal Violet staining. Data are reported as mean ± S.E. of percent inhibition of proliferation, n = 3. C, ROS production after 2 h of incubations with cannabidiol or H₂O₂ was measured in MDA-MB-231 cells (16 × 10³cells/well) by spectrofluorometric analysis. The effect of cannabidiol per se (0.5 and 10 μM) is reported in Fig. 9. Cells were loaded 1 h with 10 μM fluorescent probe in presence of 0.05% Pluronic, and fluorescence was measured in a 96-well microplate reader (PerkinElmer LS50B, λ_Ex, 495 nm; λ_Em, 521 nm). Fluorescence was measured at t = 0 and after 2 h of incubation with H₂O₂ in the presence or absence of increasing concentration of cannabidiol. Data are expressed as explained in the legend to Fig. 9. Cannabidiol inhibited ROS production by H₂O₂ only at the lowest concentration (0.5 μM, p < 0.05 by ANOVA followed by Bonferroni’s test).

Acknowledgments

We thank Teresa Iuvone and Daniele De Filippis for valuable help with caspase-3 data and Tiziana Bisogno for critical comments and general contribution to the work.

Footnotes

• This study was supported by GW Pharmaceuticals (research grant to V.D.M.).

• doi:10.1124/jpet.106.105247.

• ABBREVIATIONS: THC, Δ⁹-tetrahydrocannabinol; CB₁, cannabinoid receptor type-1; CB₂, cannabinoid receptor type-2; TRPV1, transient receptor potential vanilloid type-1; SR141716A, N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide HCl; SR144528, N-[(1S)-endo-1,3,3-trimethylbicyclo[2.2.1]heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-1-pyrazole-3-carboxamide; JWH-133, 1,1-dimethylbutyl-1-deoxy-Δ⁹-tetrahydrocannabinol; AM251, N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide; AM630, 6-iodo-2-methyl-1-[2-(4-morpholinyl)-ethyl]-1H-indol-3-yl](4-methoxyphenyl)-methanone; ANOVA, analysis of variance; ROS, reactive oxygen species; RT, reverse transcription; PCR, polymerase chain reaction; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; nt, nucleotide; PBS, phosphate-buffered saline; I-RTX, 5′-iodo-resiniferatoxin; HEK, human embryonic kidney; CBD, cannabidiol; BAPTA-AM, 1,2-Bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid tetrakis (acetoxymethyl ester).

• • Received March 25, 2006.
  • Accepted May 23, 2006.

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