Cannabinoids and NF-κB: Hijacking a pro-cancer signal Can Be Done

See article by Volmar et al. pp 1898–1910.

Compounds from cannabis, particularly cannabidiol (CBD) and delta-9-tetrahydrocannabinol (THC), have long attracted interest for their potential medicinal properties and are often brought up by patients in our neuro-oncology clinics. Prior research has indeed suggested the activity of cannabinoids against glioma, with mechanisms including increased ceramide accumulation and induction of apoptosis, as well as increasing mitophagy and autophagy and synergy with temozolomide.^1–3 Other than legal issues—which are easing in the United States and many other countries—there is considerable appeal in applying compounds that are generally widely available and better tolerated than most agents used against brain tumors. The psychoactive effects of cannabis are mediated primarily by THC, thus offering the potential to minimize or eliminate such effects in THC-low or -free preparations that contain primarily other cannabinoids such as CBD. However, the mechanisms and activities of cannabinoids against gliomas and other brain tumors remain poorly explored, making it far more difficult to know how to apply them in neuro-oncology. A report in this issue from Volmar et al. describes a remarkable new mechanism for CBD, turning NF-κB (nuclear factor κ-light-chain enhancer of activated B cells) from pro-glioma to anti-glioma activity, shedding new light on this area and potentially indicating ways to apply these compounds.⁴

NF-κB signaling is a prominent mediator of inflammation, often working hand-in-hand with other inflammatory signaling pathways such as TGF-β to drive the particularly malignant mesenchymal phenotype in many cancers. NF-κB is activated by numerous inflammatory inputs, which in the canonical pathway trigger phosphorylation and activation of IκB kinases IKKα and IKKβ that phosphorylate IκB and trigger its ubiquitination and proteolytic degradation; this, in turn, liberates p65 (RelA) and p50 to migrate to the nucleus and drive transcription of numerous inflammatory mediators.⁵ A noncanonical NF-κB pathway also exists, with IKKα phosphorylation prompting phosphorylation of p100, which activates RelB so that it can complex with p52 to drive transcription of NF-κB targets in the nucleus.⁶ NF-κB targets drive malignant mesenchymal traits in cancers, such as increased invasiveness, increased angiogenesis, and resistance to numerous treatments. NF-κB has been specifically implicated in radiation resistance and the mesenchymal subtype in glioblastoma (GBM) stem cells.⁷ Though there has been decreased enthusiasm for therapeutic targeting of such transcriptome-defined GBM subtypes due to subtype heterogeneity and plasticity within single tumors, suppressing mesenchymal signaling and NF-κB could still represent a potent strategy for breaking treatment resistance. NF-κB is thus a high-value target for oncology in general and neuro-oncology in particular, but there are not yet any viable NF-κB inhibitors in the clinic. A few dedicated NF-κB inhibitors are in development, but blood-brain barrier (BBB) penetration is typically an issue and they may have little utility against GBM. Aspirin and statins have been reported to show some inhibitory activity against NF-κB, but it is limited.^8,9 Proteasome inhibitors yield significant NF-κB inhibition through blocking degradation of IκB, and the BBB-penetrant proteasome inhibitor marizomib is in trials for patients for GBM. Preclinical results for marizomib in GBM have been mixed, however, with modest in vivo efficacy and no radiosensitization in mouse models—possibly due to low proteasomal activity in GBM stem-like cells.¹⁰ The work by Volmar et al. is therefore particularly exciting, suggesting that CBD may even surpass NF-κB inhibition by converting it to a tumor suppressor.⁴

Volmar et al. tested a panel of cannabinoids for their cytotoxicity against a panel of transgenic murine GBM lines engineered to mimic the genetic diversity of GBM. CBD emerged as the most effective against GBM cells, a potentially fortuitous finding given that the CBD drug Epidiolex is already in the clinic as a treatment for pediatric epilepsy. A pharmacogenomics screen for genes controlling GBM sensitivity to CBD identified the NF-κB component RELA and a selenoprotein regulator of reactive oxygen species (ROS). Transcriptomic studies showed that most of the genes with CBD-induced decreased transcription were regulated by the RelA/NF-κB pathway, and further studies identified nuclear RelA as a key mediator of CBD toxicity. Importantly, CBD induced a slow but long-lived increase in RelA bound to DNA, but lacking a key phosphorylation at serine-311—and this was found to mediate its cytotoxicity in GBM cells. Testing CBD against a panel of GBM stem cell-like lines showed a wide range in this cytotoxicity, underscoring the need for a biomarker of sensitivity. The authors identified such a biomarker through further studies on the connections between NF-κB and regulation of ROS, discovering that GBM cells with low ROS levels were more sensitive to CBD. Interestingly, ROS levels acted not only as a biomarker but also a modulator of CBD cytotoxicity, as co-administration with the antioxidant N-acetylcysteine enhanced CBD effects.

This report not only reveals a unique new mechanism for CBD in GBM but also suggests potential therapeutic strategies. Furthermore, the identification of a biomarker for sensitivity to CBD is a striking finding and boosts the potential clinical applicability of this work—with the caveat that implementing this biomarker would require a new clinical test for ROS levels in GBM. There are a number of therapeutic directions suggested by this work, particularly combinations. CBD could be added to therapies such as radiation that lead to elevated NF-κB activity in GBM, or it could be administered in a sequenced fashion with CBD given after radiation. This report also indicates that adding a potent antioxidant could broaden and enhance the activity of CBD in GBM, and there may be applicable antioxidants available in the clinic. The BBB-penetrating antihypertensives captopril and fosinopril are powerful antioxidants due to the presence of sulfhydryl groups, and either of these could potentially be added to CBD. These and other strategies could easily be tested preclinically and advanced to clinical trials if justified. The implications of this report may be far-ranging and could be generalizable beyond GBM, given the prominent roles of NF-κB and the epithelial-mesenchymal transition in other cancers.