The low high: Hidden medicinal boon in Cannabis

Article by Dr. Srijay Kamat

Cannabis sativa has been demonized as a gateway drug to more potent narcotics. In contrast, recipes for medicinal preparations of the plant have been found in ancient texts. Herbal extracts of medicinal plants are complex mixtures and C. sativa is no exception as it contains a multitude of natural compounds from several chemical classes. Cannabinoids are one class of terpenophenolics from C. sativa of which Tetrahydrocannabinol (THC) is the most well known and disreputable molecule due to its psychoactive properties. Other distinguished cannabinoids also exist such as Cannabidiol (CBD), cannabinol (CBN) and cannabigerol (CBG) that are free of psychoactivity and have been studied for their anticancer activity. These molecules exert their neurotransmitter effect through the human endocannabinoid system which consists of specific cannabinoid receptors, CB1 and CB2. CB1 is found in several brain regions such as the forebrain and hippocampus but also exists in peripheral organs including the liver, thyroid gland, skeletal muscle and adipose tissue and CB2 is found in specific neuronal subpopulations, in glioma cells, circulating immune cells, on macrophage-derived cells such as osteoclasts and hepatic Kupffer cells, as well as in tonsils and spleen. These receptors are targets for anandamide and 2-arachidonoyl glycerol (2-AG), two endocannabinoids that exist throughout the body. To holistically utilize existing inherent signaling pathways, studies are currently underway to discover the potential role of CBD in cancer therapy.

CBD efficacy against glioblastoma multiforme

Glioblastoma multiforme (GBM) is a grade IV astrocytoma which accounts for 3-4% of all cancer-related deaths with a life expectancy after diagnosis of just 12-15 months. Current treatments are only palliative and consist of surgical resection? followed by a combination of radiotherapy and chemotherapy. GBM treatment is further complicated by the presence of the blood brain barrier which presents a baseline resistance to a multitude of drugs. A systematic review by Kyriakou et al., compiled a number of preclinical CBD studies in GBM either as monotherapy or in combination with other chemo- or radiotherapy. CBD in combination with THC and Temozolomide (TMZ, an FDA approved chemotherapy drug for GBM) showed that co-administration reduced tumor growth to a greater extent than treatment with individual agents upon glioma xenografts. CBD monotherapy was found to inhibit the self renewal capacity of glioma stem cells (GSC), caused a down-regulation of tumor-related proteins and also stimulated glial differentiation along with inhibition of GSCs at micromolar concentrations. CBD and TMZ combination therapy was found to increase chemosensitivity of glioma cells to TMZ and induce apoptosis at significantly reduced IC50 values of TMZ than compared to TMZ monotherapy. These studies along with the fairly safe profile of CBD suggest that future clinical trials are essential to evaluate the exact effect and bioavailability of cannabinoids on humans.

Effect of cannabinoid receptors and CB agonists in breast cancer cell survival and migration

A preclinical study by? Khunluck et al., demonstrated that the endocannabinoid system (ECS), comprised of the endogenous cannabinoids (endocannabinoids), cannabinoid receptors (CB) and enzymes pertaining to endocannabinoid biosynthesis and degradation, provided protection against breast cancer bone metastasis and its negative consequence on bone cell survival. The study showed that of the two CB agonists, either CB1 (ACEA), CB2 (GW405833) or a combination of both could reduce breast cancer cell survival by inducing apoptosis and inhibiting cell migration through the NF-κB-dependent pathway. The CB2 agonist was also able to prevent breast cancer-induced osteoblast suppression. Activation of CB1 and CB2 in breast cancer suppressed ERK1/2 and AKT/mTOR signaling pathways and both pathways have NF-κB as one of the many downstream effectors. NF-κB is associated with cancer cell survival and progression and inhibition of NF-κB activity in breast cancer treated with CB agonists was shown to enhance apoptosis corresponding to the reduced phosphorylated NF-κB levels. The study also showed that ROS production was not increased in CB agonist-exposed breast cancer cells and the ECS was capable of inducing breast cancer cell apoptosis in a pathway that did not involve ROS and its harmful effect from ROS on bone homeostasis. The authors thus provided evidence to support the novel roles of ECS in breast cancer bone metastasis.

Regulation of XIAP/Smac directed apoptosis in gastric cancer by cannabidiol?

A study by Jeong et al., elucidated a mechanism of CBD action which mediates cell death in gastric cancer cell lines through the regulation of XIAP/Smac proteins. XIAP is a potent anti-apoptotic effector among the IAP (inhibitor of apoptosis) family of proteins that directly inhibit caspases and regulate cell death. XIAP has higher expression in cancer tissues than in normal tissues. CBD induced apoptosis by suppressing X-linked inhibitor apoptosis (XIAP) protein level while increasing ubiquitination of XIAP via the ubiquitin-proteasome system as revealed through CHX chase assay studies. Second mitochondria-derived activator of caspase (Smac, HGNC gene name: DIABLO) is the main negative regulator of XIAP and in its activated form competitively blocks the caspase-binding sites of XIAP, inducing the caspase cascade, which further promotes apoptosis. Smac is found to be less expressed in cancer tissues as opposed to normal tissues.?

CBD therapy was found to enhance the interaction between Smac and XIAP, through the increase in the release of Smac from the mitochondria into the cytosol. XIAP and Smac, both demonstrate negative correlation in different cancers like non-small cell lung cancer and renal cell carcinoma. The study also found that CBD inhibited tumor growth in Balb/c-nude mice that were subcutaneously injected with MKN45 gastric cancer cells to establish a mouse gastric cancer model. Tumor size and weight were significantly less in CBD treated mice than that of the control group. Tumor tissue was isolated from mice to investigate whether CBD promotes apoptosis via visualization of DNA fragmentation using the TUNEL assay. DNA fragmentation in the tumor tissue from the CBD-treated mouse group was significantly increased along with a concomitant decrease in XIAP expression. Thus CBD was found to induce apoptotic cell death in gastric cancer cells via XIAP inhibition by Smac.?

Apart from the above described studies, there exists a vast amount of preclinical literature that demonstrates the anti-proliferative and pro-apoptotic effects of CBD on a wide variety of cancer types both in cultured cancer cell lines and in mouse tumor models. The added benefit is that CBD lacks psychoactivity and generally has a milder effect on normal cells from the same tissue/organ. Current theories suggest that CBD disrupts the cellular redox homeostasis to induce a drastic increase of ROS and ER stress which in turn affects cell cycle arrest, autophagy, and cell death. However, seminal studies are still required which are key to comprehend the interplays of CBD and the different signaling transduction pathways which lead to cancer cell death, inhibition of tumor migration, invasion, metastasis, and angiogenesis. These studies can then pave the way for extensive and well-designed clinical trials that would finally realize the goal of CBD as an oncology drug.?

References:

[1] Kyriakou, Ismini, Niousha Yarandi, and Elena Polycarpou. "Efficacy of cannabinoids against glioblastoma multiforme: A systematic review." Phytomedicine 88 (2021): 153533.

[2] Khunluck, Tueanjai, et al. "Activation of cannabinoid receptors in breast cancer cells improves osteoblast viability in cancer-bone interaction model while reducing breast cancer cell survival and migration." Scientific reports 12.1 (2022): 1-15.

[3] Jeong, Soyeon, et al. "Cannabidiol promotes apoptosis via regulation of XIAP/Smac in gastric cancer." Cell death & disease 10.11 (2019): 1-13.

Image Courtesy: Aphiwat Chuangchoem