Inhibition of Sphingosine Kinase Activity Enhances Immunogenic Cell Surface Exposure of Calreticulin Induced by the Synthetic Cannabinoid 5-epi-CP-55,940

Cannabinoids as Potential Cancer Therapeutics: The Concentration Conundrum

Background: Studies have reported that cannabinoids, in particular Δ9-tetrahydrocannabinol (Δ9-THC) and cannabidiol (CBD), significantly reduce cancer cell viability in vitro. Unfortunately, treatment conditions vary significantly across reports. In particular, a majority of reports utilize conditions with reduced serum concentrations (0-3%) that may compromise the growth of the cells themselves, as well as the observed results. Objectives: This study was designed to test the hypothesis that, based on their known protein binding characteristics, cannabinoids would be less effective in the presence of fetal bovine serum (FBS). Moreover, we wished to determine if the treatments served to be cytotoxic or cytostatic under these conditions. Methods: Six cancer cell lines, representing two independent lines of three different types of cancer (glioblastoma, melanoma, and colorectal cancer [CRC]), were treated with 10 μM pure Δ9-THC, CBD, KM-233, and HU-331 for 48 h (in the presence or absence of FBS). Cell viability was measured with the MTT assay. Dose-response curves were then generated comparing the potencies of the four cannabinoids under the same conditions. Results: We found that serum-free medium alone produces cell cycle arrest for CRC cells and slows cell growth for the other cancer types. The antineoplastic effects of three of the four cannabinoids (Δ9-THC, CBD, and KM-233) increase when serum is omitted from the media. In addition, dose-response curves for these drugs demonstrated lower IC50 values for serum-free media compared with the media with 10% serum in all cell lines. The fourth compound, HU-331, was equally effective under both conditions. A further confound we observed is that omission of serum produces dramatic binding of Δ9-THC and CBD to plastic. Conclusions: Treatment of cancer cells in the absence of FBS appears to enhance the potency of cannabinoids. However, omission of FBS itself compromises cell growth and represents a less physiological condition. Given the knowledge that cannabinoids are 90-95% protein bound and have well-known affinities for plastic, it may be ill-advised to treat cells under conditions where the cells are not growing optimally and where known concentrations cannot be assumed (i.e., FBS-free conditions).

Library Screening and Preliminary Characterization of Synthetic Cannabinoids Against Prostate and Pancreatic Cancer Cell Lines

Background: Our previous screening efforts with colorectal cancer cell lines suggested potential cannabinoid therapeutic leads for other solid cancers. Objectives: The aim of this study was to identify cannabinoid lead compounds that have cytostatic and cytocidal activities against prostate and pancreatic cancer cell lines and profile cellular responses and molecular pathways of select leads. Materials and Methods: A library of 369 synthetic cannabinoids was screened against 4 prostate and 2 pancreatic cancer cell lines with 48 h of exposure at 10 μM in medium with 10% fetal bovine serum using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) viability assay. Concentration titration of the top 6 hits was carried out to identify their concentration-response patterns and calculate IC50 values. Three select leads were examined for cell cycle, apoptosis, and autophagy responses. The role of cannabinoid receptors (CB1 and CB2) and noncanonical receptors in apoptosis signaling was examined with selective antagonists. Results: Two independent screening experiments in each cell line detected growth inhibitory activities against all six or a majority of cancer cell lines for HU-331 (a known cannabinoid topoisomerase II inhibitor), (±)5-epi-CP55,940, and PTI-2, each previously identified in our colorectal cancer study. 5-Fluoro NPB-22, FUB-NPB-22, and LY2183240 were novel hits. Morphologically and biochemically, (±)5-epi-CP55,940 elicited caspase-mediated apoptosis of PC-3-luc2 (a PC-3 subline with luciferase) prostate cancer and Panc-1 pancreatic cancer cell lines, each the most aggressive of the respective organ site. The apoptosis induced by (±)5-epi-CP55,940 was abolished by the CB2 antagonist, SR144528, but not modulated by the CB1 antagonist, rimonabant, and GPR55 antagonist, ML-193, nor TRPV1 antagonist, SB-705498. In contrast, 5-fluoro NPB-22 and FUB-NPB-22 did not cause substantial apoptosis in either cell line, but resulted in cytosolic vacuoles and increased LC3-II formation (suggestive of autophagy) and S and G2/M cell cycle arrests. Combining each fluoro compound with an autophagy inhibitor, hydroxychloroquine, enhanced the apoptosis. Conclusions: 5-Fluoro NPB-22, FUB-NPB-22, and LY2183240 represent new leads against prostate and pancreatic cancer cells in addition to the previously reported compounds, HU-331, (±)5-epi-CP55,940, and PTI-2. Mechanistically, the two fluoro compounds and (±)5-epi-CP55,940 differed regarding their structures, CB receptor involvement, and death/fate responses and signaling. Safety and antitumor efficacy studies in animal models are warranted to guide further R&D.

Fanning the Flames of Endoplasmic Reticulum (ER) Stress: Can Sphingolipid Metabolism Be Targeted to Enhance ER Stress-Associated Immunogenic Cell Death in Cancer?

The three arms of the unfolded protein response (UPR) surveil the luminal environment of the endoplasmic reticulum (ER) and transmit information through the lipid bilayer to the cytoplasm to alert the cell of stress conditions within the ER lumen. That same lipid bilayer is the site of de novo synthesis of phospholipids and sphingolipids. Thus, it is no surprise that lipids are modulated by and are modulators of ER stress. Given that sphingolipids have both prosurvival and proapoptotic effects, they also exert opposing effects on life/death decisions in the face of prolonged ER stress detected by the UPR. In this review, we will focus on several recent studies that demonstrate how sphingolipids affect each arm of the UPR. We will also discuss the role of sphingolipids in the process of immunogenic cell death downstream of the protein kinase RNA-like endoplasmic reticulum kinase (PERK)/eukaryotic initiating factor 2α (eIF2α) arm of the UPR. Furthermore, we will discuss strategies to target the sphingolipid metabolic pathway that could potentially act synergistically with agents that induce ER stress as novel anticancer treatments. SIGNIFICANCE STATEMENT: This review provides the readers with a brief discussion of the sphingolipid metabolic pathway and the unfolded protein response. The primary focus of the review is the mechanism(s) by which sphingolipids modulate the endoplasmic reticulum (ER) stress response pathways and the critical role of sphingolipids in the process of immunogenic cell death associated with the ER stress response.

Efficient Synthesis for Altering Side Chain Length on Cannabinoid Molecules and Their Effects in Chemotherapy and Chemotherapeutic Induced Neuropathic Pain

(1) Background: Recently, a number of side chain length variants for tetrahydrocannabinol and cannabidiol have been identified in cannabis; however, the precursor to these molecules would be based upon cannabigerol (CBG). Because CBG, and its side chain variants, are rapidly converted to other cannabinoids in the plant, there are typically only small amounts in plant extracts, thus prohibiting investigations related to CBG and CBG variant therapeutic effects. (2) Methods: To overcome this, we developed an efficient synthesis of corresponding resorcinol fragments using the Wittig reaction which, under acid catalyzed coupling with geraniol, produced the desired side chain variants of CBG. These compounds were then tested in an animal model of chemotherapeutic-induced neuropathic pain and to reduce colorectal cancer cell viability. (3) Results: We found that all side-chain variants were similarly capable of reducing neuropathic pain in mice at a dose of 10 mg/kg. However, the molecules with shorter side chains (i.e., CBGV and CBGB) were better at reducing colorectal cancer cell viability. (4) Conclusions: The novel synthesis method developed here will be of utility for studying other side chain derivatives of minor cannabinoids such as cannabichromene, cannabinol, and cannabielsoin.

Regulatory Role of Sphingosine-1-Phosphate and C16:0 Ceramide, in Immunogenic Cell Death of Colon Cancer Cells Induced by Bak/Bax-Activation

We recently identified the sphingosine kinases (SphK1/2) as key intracellular regulators of immunogenic cell death (ICD) in colorectal cancer (CRC) cells. To better understand the mechanism by which SphK inhibition enhances ICD, we focused on the intracellular signaling pathways leading to cell surface exposure of calreticulin (ectoCRT). Herein, we demonstrate that ABT-263 and AZD-5991, inhibitors of Bcl-2/Bcl-XL and Mcl-1, respectively, induce the production of ectoCRT, indicative of ICD. Inhibition of SphK1 significantly enhanced ABT/AZD-induced ectoCRT production, in a caspase 8-dependent manner. Mechanistically, we demonstrate that ABT/AZD-induced Bak/Bax activation stimulates pro-survival SphK1/sphingosine-1-phosphate (S1P) signaling, which attenuates ectoCRT production. Additionally, we identified a regulatory role for ceramide synthase 6 (CerS6)/C16:0 ceramide in transporting of ectoCRT to the cell surface. Together, these results indicate that the sphingolipid metabolic regulators of the sphingolipid rheostat, S1P and C16:0 ceramide, influence survival/death decisions of CRC cells in response to ICD-inducing chemotherapeutic agents. Importantly, SphK1, which produces S1P, is a stress-responsive pro-survival lipid kinase that suppresses ICD. While ceramide, produced by the inhibition of SphK1 is required for production of the cell surface marker of ICD, ectoCRT. Thus, inhibition of SphK1 represents a means to enhance the therapeutic efficacy of ICD-inducing agents.