The Endocannabinoid System and Cancer
The endocannabinoid system (ECS) is known to be a key regulator of homeostasis in many systems throughout the body, helping restore balance in response to environmental changes or disruptions. The ECS encompasses a network of cannabinoid receptors located on the surface of cells all throughout the body, endogenous cannabinoids (endocannabinoids) that activate these receptors, and enzymes that synthesize and degrade the endocannabinoids. Research demonstrates that the ECS is involved in metabolic homeostasis, immune homeostasis, brain function homeostasis, glucose homeostasis, and emotional homeostasis, among other regulatory functions. Given the role of the ECS in helping maintain balance in many other systems, it is reasonable that it may also assist with controlling against the growth of cancer. While complex and far from completely elucidated, there is clearly some relationship between the ECS and cancer, as demonstrated by numerous lines of evidence across many different cancer types. First, it is helpful to see how the ECS is involved in some other diseases to gain an appreciation for its widespread role in physiology.
The Endocannabinoid System Changes in Disease States
Many cells in the human body possess cannabinoid receptors, known as CB1 and CB2 receptors, with some regions of the body featuring higher concentrations of receptors than other parts. These receptors are proteins that convey signals from outside a cell to inside a cell after being activated by endocannabinoids, the natural ligands of cannabinoid receptors. Central nervous system (CNS) cells express especially high levels of CB1, while cells within the immune system have especially high levels of CB2. While receptor amounts may vary across the body, and different individuals have natural variations as well, there are more-or-less “normal” expression levels of cannabinoid receptors in different tissues. These quantities of cannabinoid receptors do not always stay the same – they change in response to different conditions, including the emergence of disease states. For example, when Parkinson’s disease patients were compared to healthy controls, CB1 receptor levels were both upregulated (increased) and downregulated (decreased) in different brain regions. In Alzheimer’s patients, CB1 expression in certain brain areas was found to increase in the early stages of the disease, but decrease in the later stages, which the authors suggested was due to the ECS trying to compensate against diminished neural function before being overwhelmed by disease progression.
Conversely, in Huntington’s disease (HD), CB1 receptors continuously decrease in certain brain areas as HD progresses, whereas CB2 receptors in brain immunity-related cells called microglia increase. In the case of HD, the CB1 decrease was clearly correlated with disease severity, while CB2 receptors appeared to confer neuroprotection, as demonstrated by the harm that came from deleting them and the benefits observed when activating them in animal studies.
In addition to receptors, the endocannabinoids themselves (as well as synthesizing and degrading enzymes) are modulated in several diseases. It may even be that endocannabinoid deficiency could be the cause of certain conditions, as explored in this 2016 article about clinical endocannabinoid deficiency. For example, lower levels of anandamide, one of the primary endocannabinoids, are associated with chronic migraines. Conversely, excessive levels of endocannabinoids may contribute to some diseases, such as obesity. Changes in cannabinoid receptor density and endocannabinoid levels have been observed in many kinds of cancer and are explored below.
The Endocannabinoid System and Brain Cancer
A comprehensive study published in 2012 by Chinese researchers analyzed receptors and endocannabinoids present in normal tissue, low-grade glioma brain tumors, and high-grade gliomas. These tumors originate from supportive glial cells in the nervous system. Normal tissue featured decreased anandamide levels compared to the gliomas, but the other primary endocannabinoid, 2-AG, was increased. Both CB1 and CB2 receptors also increased significantly, although the authors noted that other research has found little or no change in CB1 expression of glioma tumors compared to normal tissue. In any case, results from this study are shown below (N = non-tumor tissue, L = low-grade glioma tissue, H = high-grade glioma tissue).
While the role of the CB1 receptor appears murkier, studies seem to agree that CB2 receptor expression increases as malignancy grade (a measure of metastatic and growth potential) increases. Whether this is a protective mechanism or a malignant development is unclear, but another study published in the journal Child’s Nervous System looking at expression of the CB1 receptor gene in children with low-grade gliomas supports the protection theory at least for that receptor. Researchers performed extensive molecular analysis on primary untreated tumors from pediatric patients who underwent subtotal (incomplete) surgical removal and found that high CB1 receptor expression was ultimately associated with spontaneous tumor involution (shrinkage) or stability, as indicated by at least 10 years of follow-up with patients. It was concluded that high expression levels of CB1 may make the tumors more susceptible to the antitumor effects of endocannabinoids like anandamide, which could cause shrinkage, a theory in line with many studies showing how activation of CB1 receptors inhibits cancer. The possibility that CB2 upregulation may also be protective is bolstered by the fact that CB2 activation is linked to inhibition of glioma cell invasion.
The Endocannabinoid System and Breast Cancer
There are several forms of breast cancer differentiated by the presence or absence of different receptors (ERα, PR, and HER2) on the cancer cells. Depending on the type, CB1 and especially CB2 receptors vary. For example, in HER2+ breast cancer, a 2015 study examining hundreds of breast tumor samples determined an association between high CB2 protein expression and decreased overall survival. This was backed up by another study which indicated a positive correlation between histologic grade of breast tumors and CB2 receptor mRNA expression, while normal breast tissue had hardly any CB2 expression. It is important to note here that some studies examine the mRNA that codes for cannabinoid receptor proteins rather than the proteins themselves, although some research (described in the Discussion of the linked article) indicates a correlation between cannabinoid receptor protein levels and the associated mRNA. In this case, the fact that both high CB2 protein expression and high CB2 mRNA expression is correlated with more aggressive disease bolsters the correlation.
While CB1 receptor mRNA was also present in the above study, levels were even lower than in normal breast tissue, contrary to CB2 receptor mRNA which was significantly higher than CB1 receptor mRNA in all the tested cell types. Due to the abundance of CB2 receptors found in breast cancer subtypes resistant to conventional therapies, researchers suggested the possibility that selective targeting of CB2 receptors could be a novel treatment option, as they had demonstrated the phytocannabinoid tetrahydrocannabinol (THC) induced apoptosis in breast cancer cells via CB2 activation. In contrast to results seen in HER2+ breast cancer, a 2017 study linked higher CB2 protein expression with better recurrence-free survival in ERα+ and ERα- breast cancer patients (chart shown below), along with demonstrating that CB2 activation inhibited invasion of both cell types. This is notable as 60-70% of breast cancers are ERα+. Also of note, anandamide has been shown to inhibit proliferation of ERα+ cells, although through CB1 activation rather than CB2.
The Endocannabinoid System and Colorectal Cancer
The importance of the CB1 receptor in colorectal cancer (CRC) was illuminated in a study with mice, in which loss or inhibition of CB1 receptors through genetic or pharmacological means accelerated tumor growth and activation of those receptors reduced tumor growth. Other animal evidence has also pointed to CB1 receptors acting as tumor suppressors in CRC. These observations were further supported in a small human study conducted by Italian researchers, in which patients with metastatic and non-metastatic colorectal cancer were analyzed to determine differences in CB1 receptor protein expression. Patients with metastatic disease had a lower level of CB1 receptors than those without metastasis, indicating that dysfunctional CB1 signaling may contribute to the spread of cancer cells. The authors concluded, “drugs able to induce CB1 receptor expression can be helpful in order to set new anticancer therapeutic strategies.”
As for endocannabinoids, a study in 2003 pointed to the possible role of endocannabinoids in controlling CRC growth in humans. Biopsies of colon tissue from patients with CRC were analyzed and compared with healthy tissue, finding that 2-AG and anandamide increased significantly in both the precancerous polyps (adenomas) and tumors themselves. They also confirmed in isolated cell studies that treating CRC cells with both endocannabinoids directly, or just inhibiting their degradation as an alternative way to increase their levels, reduced cell proliferation. Due to these and other observations, researchers concluded, “These findings are in agreement with the presence of both [cannabinoid receptor] subtypes in colon normal mucosa and CRC and suggest that endocannabinoids, present in high amounts in CRCs and, particularly, colorectal adenomas, might function as endogenous inhibitors of cancer growth.”
Lending further support to this theory was a 2008 study published in the Journal of Molecular Medicine which found that chemically-inducing precancerous colorectal lesions (known as aberrant crypt foci [ACF]) in mice significantly increased 2-AG levels as well as increased anandamide levels in a nonsignificant way. The researchers suggested this was a protective event for several reasons, including that further increasing the levels of 2-AG and anandamide by inhibiting their degradation decreased formation of ACF, and both endocannabinoids also directly inhibited proliferation of colorectal carcinoma cells. These researchers also referenced an article on the anti-obesity drug Orlistat, which inhibits 2-AG biosynthesis, and its ability to increase precancerous colorectal lesions in mice. Given the results of their study, they theorized Orlistat may have caused the lesions by decreasing 2-AG and counteracting its protective function.
The Endocannabinoid System and Liver Cancer
Hepatocellular carcinoma (HCC) is the most common form of liver cancer. An extensive 2019 study in Oncology Letters compared HCC tissues with non-cancerous counterpart controls to determine differences in both cannabinoid receptor expression and endocannabinoid levels. The results showed that anandamide and 2-AG were respectively downregulated and upregulated in the cancerous tissue, while the CB1 and CB2 receptor proteins were also respectively downregulated and upregulated.
There appears to be variable function of cannabinoid receptors in HCC, as demonstrated by researchers in the United States, Germany, and South Korea. In mice with chemically-induced HCC, CB1 inactivation suppressed cancer cell proliferation, whereas CB2 inactivation increased cancer cell proliferation, suggesting that CB1 receptors exerted a pro-cancer role and whereas CB2 receptors yielded anticancer effects. Interestingly, the inactivation of CB2 receptors reduced the recruitment of specific immune cells (CD4+ T cells) with antitumor properties, indicating that CB2 receptors may mediate an anticancer function of the immune system. The pro-cancer effect of CB1 was further demonstrated in a 2015 study that showed CB1 blockade resulted in fewer HCC tumors in mice.
Despite the negative effects of CB1 receptors suggested in these studies, another one that analyzed HCC tissue and nontumor tissue from patients determined that CB1 and CB2 receptors were both upregulated in the cancerous tissue, but to different extents in different people. Those HCC patients who had higher expression of CB1 and CB2 receptors experienced significantly better disease-free survival than patients with low expression levels, leading researchers to conclude, “Our results indicate that CB1 and CB2 have potential as prognostic indicators and suggest possible beneficial effects of cannabinoids on prognosis of patients with HCC.” While there are disparate results concerning the CB1 receptor and HCC, the CB2 receptor has more consistently been shown to be upregulated and confer an anticancer effect.
The Endocannabinoid System and Lung Cancer
Depending on the subtype of a particular cancer, even within the same organ, the ECS appears to change in different ways. A study published in 2016 examined cancerous and adjacent normal tissue specimens from patients with two subtypes of non-small cell lung cancer, squamous cell carcinoma (SCC) or adenocarcinoma (AC). Substantial differences in endocannabinoids, receptor expression, and metabolizing enzymes were found between the two cancerous tissues and normal tissue. Both anandamide and 2-AG were upregulated in in SCC, but downregulated in AC. Interestingly, the metabolizing enzymes had significantly increased in SCC and AC as well, which would theoretically cause lower levels of both endocannabinoids in both cell types, yet only AC tissue was observed to present those lower levels. Of additional interest, the CB1 and CB2 receptors only increased in AC cells, whereas receptor expression in SCC cells was largely unchanged, but other receptors that interact with endocannabinoids, including TRPV1 (VR1) and GRP55, were upregulated in both types.
The researchers also discussed several studies showing anticancer effects of cannabinoid receptor activation, and stated, “Data from the literature confirm that endocannabinoids control the fundamental processes of cell homeostasis and neoplastic transformation, which agree with our results.” Indeed, in cell and animal studies, both endocannabinoid degradation inhibitors and endocannabinoids themselves, including anandamide and 2-AG, inhibited invasion of lung cancer cells and tumors.
The Endocannabinoid System and Pancreatic Cancer
Pancreatic cancer is one of the most aggressive cancers in existence, and finding new ways to target it is paramount to improving patient outcomes. In a 2006 study which demonstrated how THC induced apoptosis in pancreatic cancer cells via CB2 receptor activation, researchers also found that normal pancreatic tissue had markedly low cannabinoid receptor expression. Pancreatic tumor biopsies and isolated cancer cell lines had much higher CB1 and CB2 levels (as demonstrated by mRNA and protein analysis), yet the CB1 receptor appeared to be uninvolved in antitumor effects.
Two years later, another study by German and American researchers found that high CB1 receptor expression was correlated with shorter survival of pancreatic cancer patients, whereas CB2 receptors were not significantly implicated in survival, yet both receptors were generally upregulated in the cancerous tissue compared to normal tissue. The study also noted that low levels of the endocannabinoid metabolizing enzymes fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) were correlated with shorter survival. Since these enzymes break down the endocannabinoids anandamide and 2-AG, lower levels theoretically would facilitate higher levels of endocannabinoids. However, both anandamide and 2-AG were found unchanged between cancerous and normal tissue. The paradoxical nature of this example is made starker by the fact that 2-AG has been found to inhibit pancreatic cancer cell proliferation via CB1 receptor activation. Due to this and similar reports, researchers from the previously mentioned study believe it is worth clinically testing cannabinoids in the treatment of pancreatic cancer.
The Endocannabinoid System and Prostate Cancer
The current research demonstrates complex interactions between the ECS and prostate cancer. A 2005 study compared normal prostate cells with five different prostate cancer cell lines, finding that all the latter cells expressed higher levels of CB1 and CB2 receptor proteins than normal cells. Further experiments demonstrated that activation of CB1 and CB2 receptors was important for the induction of cell death. However, as seen in pancreatic cancer, a study conducted in 2009 linked comparatively high CB1 receptor expression with greater disease severity (more metastases, larger tumor size, and rate of cell proliferation) as well as shorter survival. The 15-year probabilities of survival from prostate cancer were substantially higher in the low CB1 expression group than the high CB1 expression group. In the chart below, “Events” corresponds to deaths specifically from prostate cancer, not other causes. CB1IR<2 corresponds to relatively low expression, while CB1IR≥2 corresponds to relatively high expression.
Another study conducted by some of the same researchers as above found similar conclusions. By analyzing data from 419 prostate cancer patients, high expression of CB1 receptors in tumors was associated with worse pathology at diagnosis and poorer disease-specific survival.
Although there was a negative association between survival and CB1 expression, both anandamide and 2-AG have been shown to inhibit prostate cancer cell growth and induce apoptosis through activation of CB1 receptors.
The Endocannabinoid System and Skin Cancer
The three most common types of skin cancer are basal cell carcinoma, squamous cell carcinoma, and melanoma. Each type originates from a particular type of skin cell. The interaction between cannabinoids and melanoma appears to be the most studied area. A 2006 study examined 61 melanoma biopsy samples, finding that 36 expressed significant levels of CB1 and CB2 receptor proteins; 10 samples each expressed significant levels for CB1 or CB2 receptors, and only 5 did not show either receptor. In cell lines, normal melanocytes had similar levels of CB1 expression as the cancerous melanoma cells, but only the latter also expressed CB2 receptors. In support of this, a 2012 study published in the Journal of Cancer Research and Therapeutics that examined melanoma and normal skin tissues found that CB2 expression was upregulated in the cancerous tissue. This may be a good thing, as a study two years later indicated that activation of CB2 receptors inhibited the metastatic potential of melanoma cells. The researchers stated, “Our data identify CB2 as a potential target in reducing the number of brain metastases originating from melanoma.”
Alternatively, another study demonstrated anandamide toxicity against melanoma cells via CB1 receptor activation, an effect that was potentiated when a FAAH inhibitor was added. The 2006 study mentioned above demonstrated that CB1 and CB2 activation reduced proliferation of melanoma cells while not affecting the growth of healthy cells.
A 2015 study from researchers at the University of Bonn in Germany utilized mice with genetically deleted cannabinoid receptors (known as CB1/2 receptor knockout mice) to ascertain if the ECS was involved in the pathogenesis of skin cancer. Tumors that were chemically induced in the knockout mice developed similarly to tumors in wild-type mice, which have normal expression levels of cannabinoid receptors. While this result would suggest a lack of involvement of cannabinoid receptors, THC was still effective in inhibiting tumor growth in wild-type mice with transplanted mouse melanoma cells, but not knockout mice with the same cell transplants. This indicates a cannabinoid receptor dependent effect. Therefore, although the receptors appeared uninvolved in melanoma progression or inhibition, their additional stimulation by an exogenous compound had an anticancer effect.
Counter to the above study, a 2017 study that tested human melanoma cell lines found that genetic deletion of CB1 receptors reduced the number of viable melanoma cells, migration, and colony-forming ability, leading researchers to state that “the CB1 receptor might function as tumor-promoting signal in human cutaneous melanoma.”
A comparison of normal human skin and squamous cell carcinoma tissue found that CB2 receptors were upregulated in the cancerous tissue, akin to the observation seen in melanoma. The upregulation occurred at both the mRNA and protein levels.
It is worthy to note that anandamide has also been shown to induce cell death in non-melanoma skin cancer cells, although interestingly through a mechanism of action unrelated to the cannabinoid receptors.
The Endocannabinoid System and Lymphoma and Leukemia
One of the areas where cannabinoid receptors are expressed with special abundance is the immune system, so it makes sense that cancers of this system like lymphoma and leukemia would influence ECS activity. Lymphoma and leukemia both involve white blood cells, known as leukocytes. In lymphoma, a subset of leukocytes known as lymphocytes (which include T-cells, B-cells, and NK cells) grow uncontrollably and can form solid tumors or swelling, usually in the lymph nodes, spleen, or liver. Lymphomas most commonly involve B-cells. In leukemia, there can be overproduction of either immature lymphocytes or myeloid blast cells, the latter of which can differentiate into red blood cells, platelet-producing cells, or non-lymphocyte white blood cells. The immature, abnormal cells are produced in the bone marrow, where they can eventually crowd out normal cells and cause damage associated with the lack of functioning cells. These leukemic cells can also become disseminated in the blood but do not generally form solid tumors, unlike lymphomas. Therefore, in lymphoma, cancer cells are mainly in lymph nodes or other tissues, while in leukemia, the cells are mainly in the blood or bone marrow. Further differentiations among lymphoma and leukemia are decided by the specific cell types involved and the level of maturity and aggressiveness of the involved cells. For example, acute leukemias tend to involve high numbers of immature cells that replicate rapidly, whereas chronic leukemias feature more mature cells that reproduce less quickly.
Several studies have determined that upregulation of CB1 and/or CB2 receptors occurs in lymphomas. A 2008 study by Swedish researchers examined several non-Hodgkin’s lymphoma samples including small lymphocytic lymphoma/B cell chronic lymphocytic leukemia (SLL/CLL), marginal zone lymphoma (MZL), diffuse large B cell lymphoma (DLBCL), FL, precursor‐B acute lymphoblastic lymphoma (pre‐B ALL), mantle cell lymphoma (MCL), immunocytoma (IC) and Burkitt lymphoma (BL). Most of these samples had higher CB1 and/or CB2 receptor mRNA than normal tissue. Interestingly, when an anandamide analog known as R(+)‐methanandamide was tested on MCL, CLL, and BL cell lines, it only induced cell death in the first two types, which happened to overexpress the mRNA of both cannabinoid receptors. BL had relatively low CB2 receptor mRNA levels, which may have contributed to its resistance, as it was demonstrated that activation of both CB1 and CB2 receptors was critical to the anticancer effect of R(+)‐methanandamide in the MCL and CLL cells. These conclusions agreed with results from an earlier study the researchers conducted, where MCL cells expressing CB1 and CB2 receptors were susceptible to cell death from cannabinoid receptor agonists, but cells expressing only CB1 or only CB2 were resistant.
A 2007 study found that normal T-cells had low protein expression of CB2 receptors, but T-cells associated with non-Hodgkin’s lymphoma showed much higher CB2 expression. A similar observation was recorded for B-cell non-Hodgkin’s lymphoma. However, a 2011 study published in the European Journal of Haematology determined that CB2 receptor expression was not associated with treatment outcomes in patients with diffuse large B-cell lymphoma.
The potential importance of the ECS in controlling white blood cell counts was elucidated in a 2014 study that compared MCL tumor tissues with non-malignant B-cells. In agreement with other studies, CB1 and CB2 receptor mRNA was upregulated in tumors, while FAAH was downregulated, which theoretically could lead to accumulation of anandamide. Of particular interest, low CB1 receptor mRNA and high FAAH expression (and thus potentially lower anandamide levels) in a subset of cases correlated with abnormally high levels of lymphocytes and white blood cells, indicating that anandamide may reduce these excessive numbers via CB1 receptor activation.
A 2016 study from researchers at the Medical University of Vienna supported the results of previous studies, finding that B-cells associated with CLL had higher expression of CB1 and CB2 receptor mRNA than healthy B-cells. This was determined by analyzing peripheral blood samples from patients with CLL compared to healthy volunteers. Patients with comparatively high levels of CB1 receptor mRNA expression had shorter average overall survival than patients with low CB1 receptor mRNA expression; 153 months to 277 months, respectively. CB2 receptor mRNA was not correlated with survival. In this case, the researchers were unable to confirm an association of cannabinoid receptor proteins with survival, which they attributed to a problem with their protein detection compounds, but noted that other studies found a correlation between cannabinoid receptor mRNA expression and protein expression. In any case, despite the poor association of CB1 expression with survival, further tests showed that activation of CB1 or CB2 receptors with synthetic agonists reduced viability of CLL cells.
Summary
As the research demonstrates, the relationship between the ECS and various forms of cancer is far from consistent. Some cancer cells exhibit higher or lower amounts of cannabinoid receptors or endocannabinoids than their normal cell counterparts, although either CB1 and/or CB2 receptors do tend to upregulate more often in cancer. The answers to why these changes happen are not certain, and likely differ depending on the type of cancer or a patient’s unique genetic situation. In some cases, the ECS may be functioning as a protective mechanism against cancer development, or the cancer itself is somehow exploiting the ECS to enhance growth.
There have been several instances where cannabinoid receptor upregulation was associated with poorer survival outcomes, yet even in these cases, activation of those receptors by cannabinoids is repeatedly demonstrated to yield anticancer effects. It could be that in the absence of enough cannabinoids to substantially activate them, cannabinoid receptors may facilitate cancer growth, possibly by strengthening activation of survival pathways. More research is certainly needed to elucidate how cannabinoid receptors could contribute to tumor growth. However, there are also several studies showing that high receptor expression is associated with better outcomes, which further supports the theory of the ECS possessing a protective role against cancer.
As for cannabinoids as direct cancer treatments, the vast majority of studies show cannabinoid receptor activation causes anticancer effects. Many animal studies and case studies conducted in humans show that cannabinoid administration (including synthetic, endogenous, and phytocannabinoids) reduces tumor growth. If the ECS was only involved in pro-cancer effects, it makes little sense that its stimulation is so consistently shown to impede various forms of cancer. Furthermore, a lack of cannabinoid receptor expression appears correlated with resistance to cell death from cannabinoids, which lends additional support to the protection theory.
While the ECS is almost certainly not significantly dysregulated in all cancers, it is remarkable how many studies describe its entanglement with such a wide spectrum of cancers. Overall, the current research is clear that the ECS is differentially modulated depending on the form of cancer, and compounds that activate the ECS exert anticancer effects. With further research, it may be possible that cannabinoid treatments could be tailored depending on the nature of ECS dysregulation. The growing interest in this field improves hope that more research will be conducted.
