Cannabidiol regulates apoptosis and autophagy in inflammation and cancer: A review

Abstract

Introduction

Cell death plays an important role in physiologic and pathophysiologic processes, implying that the pathogenesis of human diseases is closely related to this process. Cell death can be broadly divided into two categories, namely programmed cell death (PCD) and simple necrosis. PCD refers to the ontogenetic, preprogrammed, and tightly controlled death of certain cells in a multicellular organism. PCD includes apoptosis, programmed necrosis, and pyroptosis. Apoptosis is usually caused by some proapoptotic stimuli, such as endoplasmic reticulum stress and accumulation of reactive oxygen species. These stimuli activate several caspases and eventually cause a series of irreversible changes in cells. Morphologically, apoptosis is characterized by cell shrinkage, chromatin condensation, blebbing, and the formation of apoptotic bodies (Kishino et al., 2019; Hu et al., 2020). Despite this, the cell membrane remains intact and no inflammation occurs (Zhang et al., 2018). The process of apoptosis avoids the occurrence of inflammatory reactions and tissue damage to achieve a steady state while maintaining an internal environment and protecting the host.

Autophagy is a phylogenetic degradation process that depends on the formation of specialized membrane structures including phagosomes, autophagosomes, and autolysosomes. Autophagy plays a complex role in maintaining health and developing diseases (Levine and Kroemer, 2019). Eukaryotic cells maintain homeostasis and renew themselves through autophagy, which is an evolutionarily conserved mechanism (Hao et al., 2022). Autophagy can be divided into macroautophagy, microautophagy, and chaperone-mediated autophagy according to the different routes of cellular material transported to lysosomes; macroautophagy is the most common type among them. Autophagy is thought to be a major factor in determining the fate of the cells and can trigger apoptosis. This implies that autophagy can also represent a form of programmed cell death known as autophagic cell death or type II programmed cell death. Three sequential steps are involved in intact autophagy: induction, autophagosome formation, and autophagosome-lysosome fusion and degradation. Bulk and selective are the two modes of autophagy (Gatica et al., 2018), contributing to metabolic adaptation and cellular homeostasis, respectively (Kaur and Debnath, 2015).

Autophagy is closely associated with several human diseases, such as cancer, neurodegenerative diseases, and infectious diseases (Chung et al., 2017; Padman et al., 2019; Amorim et al., 2022). The endocannabinoid system is a naturally occurring lipid signaling system intricately related to a range of physiologic and disease processes. The endocannabinoid system comprises endocannabinoid receptors, endocannabinoids, and metabolic enzymes responsible for regulating the synthesis and decomposition of ligands. The endocannabinoids mainly exist in two forms in the body, arachidonoylethanolamine (AEA) and 2-arachidonoylglycerol (2-AG). Endocannabinoid receptors mainly exist in mammalian tissues with two types of G protein-coupled receptors, CB1 and CB2. CB1 is mostly distributed in the central nervous system (Ghosh et al., 2021; Pant et al., 2021), including the amygdala, cortex, hypothalamus, hippocampus, and cerebellum; the proportion of CB1 in these locations is higher than in other locations. CB2 is mainly expressed in peripheral immune cells (lymphocytes, macrophages, and splenocytes). The endocannabinoid system is important in the regulation of the autonomic nervous system, immune system, and peripheral microcirculation. The theory of clinical endocannabinoid deficiency syndrome (CEDS), proposed by Russo, suggests that some chronic diseases may occur because of a lack of endocannabinoid signaling (Silver, 2019). Therefore, whether the supplementation of exogenous cannabinoids can maintain the homeostasis of the body and improve the prognosis of some diseases is worth examining. Tetrahydrocannabinol (THC) and CBD are the most common cannabinoids used in medical cannabis preparations. However, the use of THC, which has psychoactive features, has adverse effects on the nervous system. THC-treated mice showed obvious but transient neurological changes, slow movement and breathing, hyperactivity, secondary immune dysfunction, and impaired ability to eliminate infection (Fitton and Pertwee, 1982)]. In contrast, CBD is a non-psychoactive terpenoid and has a wide range of biological effects, including anti-inflammatory, anticancer, and neuroprotective effects. Therefore, CBD has been the main focus of research on bioactive compounds for treating psychological disorders. This review aims to describe the critical role of CBD in the regulation of apoptosis and autophagy. This information may be useful in analyzing the prospects of using CBD as an anti-inflammatory agent and in innovative cancer therapy for clinical use (Figure 1).

Cannabinoid ameliorates inflammatory diseases by regulating apoptosis

Cannabis contains various complex cannabinoids, such as tetrahydrocannabinol (THC) and CBD, which affect inflammatory and lymphocyte activation pathways in the lymphoid tissue and brain. Therefore, it is increasingly being used in clinical settings. The use of cannabinoids in treating several diseases has been extensively studied in Western countries. Several studies on viral infections have revealed that cannabis inhibits pro-inflammatory pathways in HIV-infected adults and non-human primates infected with SIV (Rizzo et al., 2018; Costiniuk and Jenabian, 2019; Watson et al., 2020). Apoptosis caused the depletion of CD4⁺ and CD8⁺ T cells in HIV-1 infection, leading to an increased risk of infection and increased mortality. In a cohort study of patients with HIV, increased CD4⁺ and CD8⁺ T cell counts were associated with the use of cannabinoids (Keen et al., 2019). In an animal model of SIV-infected rhesus monkeys, the apoptosis of intestinal lymphocytes was reduced after THC treatment, suggesting that chronic THC administration can regulate duodenal T-cell populations, promote Th1/Th2 cytokine balance, and reduce intestinal cell apoptosis (Molina et al., 2014). Overall, cannabinoids exert their classical anti-inflammatory effects on the cells of monocyte lineage in HIV infection while protecting T lymphocytes from apoptosis.

Ulcerative colitis (UC) is a chronic immune-mediated inflammatory bowel disease that involves the dysfunctional immune response to the normal microbiota and dietary contents in the gastrointestinal tract. The inflammatory cascade activates T cells, which secrete excessive amounts of pro-inflammatory cytokines, including IL-1, IL-6, and TNF-α, thereby damaging healthy tissues. Endoplasmic reticulum stress and unfolded protein reaction are closely related to cell apoptosis, autophagy, and inflammatory reaction in the pathogenesis of UC (Hetz, 2012; Cao, 2016). Severe or persistent endoplasmic reticulum stress eventually triggers the internal apoptosis pathway, leading to cell death and aggravating UC (Zhang et al., 2015). In an in vitro study on inflammatory bowel disease, CBD reduced the production of reactive oxygen species and lipid peroxidation in cells, thereby reducing the occurrence of apoptosis (Borrelli et al., 2009; Esposito et al., 2013). Meanwhile, some authors suggest that CBD may indirectly activate the CB1 receptor in the intestine, thereby reducing intestinal motility and relieving UC symptoms [ (Capasso et al., 2008). Taken together, although there is definite evidence that CBD has a therapeutic effect in autoimmune diseases, its action mechanisms need to be explored further.

Several authors have confirmed that A2A, A2B, and A3 receptors participate in the antiapoptotic effect of ATP through MEK/ERK1/2, PKA, and NOS pathways. The selective antagonists of A2A, A2B, and A3 receptors limit the antiapoptotic effect of ATP against hypoxia (Feliu et al., 2019). In the LPS-induced acute lung injury animal model, CBD reduced the migration of leukocytes to the lung, thereby decreasing the concentration of albumin and pro-inflammatory cytokines in bronchoalveolar lavage fluid. The use of an A2A antagonist restored the inflammatory response, suggesting that CBD may play a role by binding with the A2A receptor (Ribeiro et al., 2012). In addition, in the inflammatory model of demyelinating disease induced by encephalomyelitis virus, the application of CBD reduced leukocyte infiltration and activated microglia in the cerebral cortex, suggesting that CBD can limit inflammatory response by increasing adenosine signal transduction (Longoria et al., 2022). Overall, we suggest that CBD can inhibit inflammation; however, whether it plays an anti-inflammatory role by binding to adenosine receptors to inhibit apoptosis needs further investigation.

Liver fibrosis is the basic pathologic change in the liver caused by chronic liver diseases. The activation of hepatic stellate cells (HSCs) leads to the accumulation of scar matrix and fibrotic liver. A basic study on HSCs revealed that CBD induced downstream activation of the IRE1/ASK1/c-Jun N-terminal kinase pathway that promoted apoptosis, leading to HSC death. The authors suggested that CBD can be used as a potential therapeutic agent for chronic hepatitis that leads to liver fibrosis by selectively inducing apoptosis of activated HSCs (Lim et al., 2011).

Cannabinoids promote autophagy and ameliorate chronic inflammatory diseases

Autophagy, a lysosomal catabolic process, is critical to cell homeostasis and is crucial to the neuroprotection of the central nervous system. Autophagy defects are observed in many neurodegenerative diseases (Baron et al., 2019). However, abnormal autophagy activation may aggravate the extensive ischemic and inflammatory processes caused by nerve traumatic diseases (Lu et al., 2019). CBD, a non-psychoactive cannabinoid, is becoming a promising therapeutic agent for mental disorders and inflammatory diseases (Campos et al., 2016). CBD has antidepressant (Linge et al., 2016), antiemetic (Rock and Parker, 2015), neuroprotective (Schiavon et al., 2014; Silveira et al., 2014), anticonvulsant (Pelz et al., 2017), antianxiety (Crippa et al., 2011), and antipsychotic effects (Rohleder et al., 2016). CBD has pharmacological properties in the treatment of neurological diseases. Therefore, the mechanism of CBD-mediated potential autophagy activation needs to be explored further. The neuroprotective effects of autophagy occur because it reduces the effects of inflammation (Lowin et al., 2020).

Neuronal cell death caused by chronic inflammation, which is induced by dysfunctional mitochondria, is considered the main cause of neurodegenerative diseases (Johnson et al., 2021). When mitochondrial dysfunction occurs in cells, accurate autophagy can prevent the occurrence of chronic inflammation and delay degenerative diseases. PPAR agonists restore autophagy, enhance mitochondrial ß-oxidation, and stimulate mitochondrial biosynthesis; therefore, they can be potentially used to treat patients with steatohepatitis (caused by glycogen storage disease type Ia) (Zhou et al., 2019). CBD mediates the functions regulated by PPARα and ? receptors and plays neuroprotective, anti-inflammatory, and metabolic roles (O’Sullivan, 2016). PPARγ agonists have been shown to reduce inflammatory processes associated with chronic and acute nerve injury (Kapadia et al., 2008). CBD can bind and activate PPARγ in vitro (Hegde et al., 2015); it can also protect rats from neurological and inflammatory damage caused by ß-amyloidosis (Esposito et al., 2011). In addition to its role in inhibiting inflammation, some studies have evaluated whether CBD treatment is feasible for chronic inflammatory pain (Britch et al., 2020). In another study on neurodegenerative diseases, CBD played an important role in autophagy activation by regulating ERK1/2 and AKT kinase phosphorylation and participating in ULK1 (Vrechi et al., 2021). In this study, low doses of CBD promoted autophagy flux without affecting cell viability, and CB1, CB2, and TRPV1 receptors mediated CBD-induced autophagy, which is crucial to maintain neuronal activity and function.

CBD plays a role in the treatment of neurodegenerative diseases by promoting autophagy, participating in homeostasis, and regulating the circulation and degradation of cellular proteins through the lysosome degradation pathway; however, it exerts the opposite effect in autoimmune diseases. RA is characterized by an anoxic environment in the joints accompanied by mitochondrial dysfunction. The compounds that inhibit autophagy (chloroquine or hydroxychloroquine) are being clinically used to treat RA. In an animal model of RA chronic inflammation, CBD showed anti-inflammatory and analgesic effects. In some studies, immune cells and synovial fibroblasts were reduced following CBD treatment to alleviate inflammation (Hammell et al., 2016; Lowin et al., 2020). In addition, CBD may also interact with antirheumatic drugs, such as methotrexate or JAK inhibitors to increase the absorption of cytotoxic chemotherapy drugs (Fraguas-Sánchez et al., 2020). CBD is feasible as an adjuvant treatment for RA and can be used in combination with other antirheumatic drugs. The underlying mechanism may involve the binding of CBD to β2 adrenergic receptors and then activating downstream ß-arrestin2 to inhibit autophagy (Lowin et al., 2019; Chen et al., 2020b)].

Potential application of the endogenous cannabinoid system (ECS) in cancer

In the past decade, ECS has become the focus of anticancer therapies. ECS refers to the complex network of cannabinoid receptors, endogenous cannabinoid ligands, enzymes that drive its biosynthesis, degradation, and transportation, and all cells and neural pathways involved in endogenous cannabinoid signal transduction. Presently, ECS is thought to be related to the important processes of controlling the body, assisting and maintaining the homeostasis of the body, which explains its uncertain role in tumorigenesis and tumor inhibition.

CB1 and CB2 are the main receptors in the ECS. They belong to the extensive class A rhodopsin-like G protein-coupled receptor family, and their main functions include completing the transduction of signal molecules from extracellular space to various cells (Pertwee et al., 2010). The signal transduction of endocannabinoids is different from that of neural signal transduction. The role of endogenous cannabinoids is mainly limited to their biosynthesis and storage in synaptic vesicles in advance and released to the receptors when required (Lu and Mackie, 2016). Endogenous and exogenous cannabinoids have other in the central nervous system and tumor tissues (Soderstrom et al., 2017), including transient receptor potential channels, ligands, voltage-gated ion channels, and other orphan G protein-coupled receptors such as GPR55, GPR18, and GPR119. The exogenous cannabinoids may cause the activation, antagonism, or reverse activation of cannabinoid and non-cannabinoid receptors because of the differences in the abundance of receptors and levels and activities of endogenous ligands (Pertwee, 2015). This may also be one of the reasons for the inconsistent effects of exogenous cannabinoids observed in many empirical studies.

CB1 and CB2 and other members of the endogenous cannabinoid-like system have become novel targets to treat various cancer subtypes because of their dual roles in tumorigenesis and inhibition of tumor growth and metastasis. Although the clinical use of cannabinoids has been widely recorded in palliative treatment, clinical trials on their use as an anticancer drug are still in progress. Cannabinoid receptors and endocannabinoids are usually upregulated in tumors, and their expression levels may be related to tumor invasiveness (Malfitano et al., 2011). Some authors have revealed that the loss of CB1 accelerates tumor growth (Wang et al., 2008), and higher levels of endogenous cannabinoid in the body lead to the reduction of precancerous lesions (Izzo et al., 2008), which confirms the theory of CEDS (occurrence of some diseases is related to the disorder of ECS signal transduction). The relationship between the distribution and expression of CB1 and CB2 on tumor cells and the tumor itself is not clear. Some authors believe that the loss of CB1 and/or CB2 expression accelerates tumor growth; however, it is likely to be related to the type of tumor cells (Pagano and Borrelli, 2017). More clinical trials are needed to confirm the accuracy of results to better understand the relationship between ECS and neoplasmic diseases.

Conclusion and outlook

PCD has been increasingly implicated as the gatekeeper of cell fate, with decisive roles in various diseases including inflammatory diseases, autoimmune diseases, degenerative diseases, and cancer. The role of cell death in the pathophysiology of inflammation and cancer needs to be explored further. Several in vitro/in vivo studies on exogenous cannabinoids have indicated that cannabinoids can affect the key functions of cells in inflammatory or cancer diseases, such as proliferation, migration, cytokine formation, differentiation, autophagy, and apoptosis. The diversity of cellular receptors of cannabinoid ligands may explain the wide range of biological effects of cannabinoid drugs in several diseases. More information on cannabinoid CB1/CB2-independent targets is needed to determine their in vitro effects and tolerance/resistance mechanisms and to properly interpret the results of available studies. Previous studies have shown that cannabinoids have an immunosuppressive effect on the immune system. Cannabinoid ligands inhibit phagocytosis, cell proliferation, antigen presentation, and other properties of immune cells, and cannabinoid may be a safe and effective antitumor drug. Nevertheless, clinical studies to evaluate the effect of cannabinoids on the human body are limited. Only animal models or in vitro cell lines have been used to evaluate the feasibility of cannabinoid treatment and large-scale clinical trials have not been conducted. Therefore, the results may vary in the clinical setup.

The prospects of cannabinoid treatment in inflammatory and cancer diseases are worth exploring. The influence of cannabinoids on cell fate should be explored further. A detailed understanding of the regulation of autophagy and apoptosis by cannabinoids will not only improve the understanding of the biology of the disease but will also be crucial in finding better therapeutic targets, thereby generating new combined therapies.

Funding Statement

This work was supported by grants from Military Medical Innovation Project (18CXZ025) and Department of General Surgery, First Medical Center of the Chinese PLA General Hospital, Beijing, China.

References