Despite extensive research into the biology of CRC (colorectal cancer), and recent advances in surgical techniques and chemotherapy, CRC continues to be a major cause of death throughout the world. Therefore it is important to develop novel chemopreventive/chemotherapeutic agents for CRC. Cannabinoids are a class of compounds that are currently used in the treatment of chemotherapy-induced nausea and vomiting, and in the stimulation of appetite. However, there is accumulating evidence that they could also be useful for the inhibition of tumour cell growth by modulating key survival signalling pathways. The chemotherapeutic potential for plant-derived and endogenous cannabinoids in CRC therapy is reviewed.
- arachidonoyl ethanolamide (anandamide)
- colorectal carcinoma
CRC (Colorectal cancer) is one of the major causes of cancer death in the industrialized world, and the incidence is likely to rise even further with the increasing trend towards obesity. It is therefore essential to develop more prevention strategies and novel agents for CRC chemotherapy. One class of compounds that have produced promising results are non-steroidal anti-inflammatory drugs, including aspirin and sulindac, which are thought to produce their chemopreventive action by inhibiting the COX (cyclo-oxygenase) enzymes. COX enzymes are responsible for the rate-limiting step in the conversion of the membrane fatty acid arachidonic acid into prostaglandins and thromboxane. Evidence from clinical, animal and in vitro studies have established that COX-2 is associated with promoting tumorigenesis (reviewed in ).
Recently there has been a great deal of interest in the possible use of cannabinoids as novel anticancer agents . Cannabinoids are a class of compounds that have the ability to activate CB (cannabinoid) receptors, of which there are two subtypes: CB1 and CB2. CB1 receptors are primarily located in the brain and are thought to be responsible for the psychotropic effects of cannabinoids. They have also been detected in peripheral tissues including reproductive tissues, heart, urinary bladder and gastrointestinal system (reviewed in ). CB2 receptors are unlikely to mediate the psychotropic effects of cannabinoids since they are predominantly expressed on immune cells, particularly on B-cells and natural killer cells . CB1 receptors are thought to mediate the psychotropic effects of cannabinoids due to their profuse location in regions of the brain controlling memory and cognition, and motor activity . Furthermore, CB1−/− mice did not produce the classical effects associated with cannabinoids, such as antinociception, catalepsy, hypomotility and hypothermia when exposed to Δ9-THC (Δ9-tetrahydrocannabinol) . Furthermore, Massa et al.  demonstrated that in response to dextran sulphate sodium, CB1−/− mice produced a stronger inflammatory response in the colon than wild-type mice, suggesting that the endogenous cannabinoid system protects against colonic inflammation. In addition, colitis was reduced in mice when administered with a CB receptor dual agonist (HU210) . Therefore the signalling of endogenous cannabinoid system is likely to provide intrinsic protection against colonic inflammation. These results could have important implications in terms of colon cancer prevention, since patients with inflammatory bowel disease (Crohn's disease or ulcerative colitis) have an increased risk of developing colorectal cancer.
CB receptors are cell membrane G-protein-coupled receptors that are linked through Gi/o family of proteins to signal transduction pathways including inhibition of adenylate cyclase, activation of mitogen-activated protein kinase and regulation of calcium and potassium channels (reviewed in ). Therefore the cumulative effects of CB receptor signalling are likely to have important implications in the control of cell survival and cell death with the potential for regulating tumour cell growth.
Both plant-derived and endogenous cannabinoids exist. There are at least 60 different cannabinoids that are unique to the Cannabis sativa plant, but the role and importance of many of these cannabinoids has yet to be fully understood. It is generally considered, however, that Δ9-THC is the most important due to its abundance and potency. The first report of a chemotherapeutic effect of cannabinoids in cancer was described by Munson et al. , where Δ9-THC, Δ8-THC and cannabinol reduced Lewis lung adenocarcinoma cell growth in C57BL/6 mice. Even though these promising results were published some 30 years ago, it was not until the early 1990s, when the receptors were cloned, that there was renewed interest in this field. During the last decade there has been a gradual increase in cannabinoid research and particularly in the number of reports demonstrating the anti-neoplastic properties of plant-derived cannabinoids (see Table 1). One recent study highlighted the anti-angiogenic properties of cannabinoids . Two patients with recurrent glioblastoma multiforme were treated with Δ9-THC directly administered into the brain cavity and a reduction in angiogenic growth factors was observed .
The first endogenous cannabinoid to be isolated was AEA (N-arachidonoyl ethanolamide, anandamide). Endocannabinoids, including AEA and 2-AG (2-arachidonoyl glycerol), are present within the gastrointestinal tract (reviewed in ) and play a role in gastrointestinal homoeostasis. There is also accumulating evidence that endocannabinoids have the ability to modulate cell proliferation (see Table 1). Interestingly, the stable analogue of anandamide, Met-F-AEA (2 methyl-arachidonyl-2′-fluoro-ethylamide), reduced the growth of thyroid epithelioma cells both in vitro and in vivo . Furthermore, inhibition of AEA degradation (resulting in sustained levels of AEA) reduced thyroid tumour growth in vivo . AEA mediates its antitumour properties in many different tumour types via activation of CB receptors; for example, in breast, mouse and human lymphoblastic tumour cells, rat glioma, prostate and cervical carcinoma cells (reviewed in ). Recent evidence also suggests that the endocannabinoids AEA and 2-AG are effective anti-proliferative agents since they significantly reduce cell viability of CaCo-2 CRC cells . The reduction in cell viability observed with AEA was dependent upon CB1 receptor activation . However, there was no information available about whether the reduction in cell viability was due to induction of cell death. We have recently shown that AEA induces a type of cell death that is neither apoptosis nor necrosis in high COX-2-expressing CRC cells (H.A. Patsos and C. Paraskeva, unpublished work).
Interestingly, other than activation of CB receptors, AEA can exert its effects independently of CB receptor activation. Unlike other neurotransmitters, AEA is not stored in intracellular components; instead it is made on demand from membrane precursors involving cleavage of N-arachidonoyl phosphatidylethanolamine by N-acyl phosphatidylethanolamine-phospholipase-D into AEA (reviewed in ). Free AEA is then transported out of the cell where it can activate CB receptors as previously discussed. However, AEA may also be transported back into the cell where it is degraded into arachidonic acid and ethanolamine by fatty acid amide hydrolase (reviewed in ), or metabolized by COX-2 into PG-EAs (prostaglandin-ethanolamides)  or by LOX (lipooxygenase) enzymes into 15-hydroxyeicosatetraenoic acids . COX- and LOX-dependent metabolites of AEA may play a role in the antitumour properties of AEA, as has been suggested by Maccarrone et al. . These are potentially exciting results since COX-2 is overexpressed in the majority of CRC and it may be possible to exploit the high levels of COX-2 for selective targeting by AEA.
The Molecular Biology of Colorectal Cancer: Focused Meeting held at the UBHT Education Centre, Bristol, U.K., 10–11 March 2005. Organized and edited by T. Corfield (Bristol, U.K.), C. Paraskeva (Bristol, U.K.) and H. Wallace (Aberdeen, U.K.).
Abbreviations: AEA, arachidonoyl ethanolamide (anandamide); 2-AG, 2-arachidonoyl glycerol; CB, cannabinoid; COX, cyclo-oxygenase; CRC, colorectal cancer; Met-F-AEA, 2 methyl-arachidonyl-2′-fluoro-ethylamide; PG-EA, prostaglandin ethanolamide; THC, tetrahydrocannabinol
- © 2005 The Biochemical Society