Approximately one in five breast cancer patients exhibits triple negative tumors that are characterized by the lack of estrogen receptor (ER), progesterone receptor and HER2. While ER and HER2 positive tumors are responsive respectively to ER antagonists and anti-HER2 antibodies, triple negative breast cancer (TNBC) tumors do not benefit from any of these therapies. As a result, it is imperative to search for alternative treatments for this subtype. Since synthetic cannbinoids has been shown to possess anticancer effects in a number of other cancer types, we therefore hypothesize that synthetic cannabinoids also have anticancer effects against TNBC tumors. The current study focused on the anticancer potential against TNBC of two synthetic cannabinoids, WIN55,212-2 and JWH-133. In vitro cytotoxicity assessment demonstrated that WIN55,212-2 inhibited the proliferation in vitro of MDA-MB-231 and MDA-MB-468 TNBC cells in a dose-dependent and time-dependent manner; whereas JWH-133 showed no significant cytotoxicity up to 30 µM in MDA-MB-231 and 10 µM in MDA-MB-468 cells. Flow cytometry experiments demonstrated that treatment with WIN55,212-2 (6 µM) for 24 h, 36 h and 48 h significantly increased the proportion of MDA-MB-231 and MDA-MB-468 cells in G1 phase.
In vivo, mice bearing MDA-MB-468 xenografts and treated with WIN55,212-2 (25 µg/day i.p. for 10 weeks) showed a significant 76% reduction in tumour volume compared to vehicle treated mice, and this effect was reversed by 60% by co-treatment with CB2 antagonist AM630. In contrast, JWH-133 (25 µg/day i.p. for 10 weeks) increased tumor volume by 166% compared to vehicle treated mice and this acceleration effect was abolished by co-treatment of AM630. Toxicity profiling studies showed that mice remained healthy after long-term treatment with either WIN55,212-2 or JWH-133 confirmed by no significant change in body weight or major organ weight, and normal plasma ALT levels for each treatment group.
Assessment of receptor expression status demonstrated the presence of both CB1 and CB2 cannabinoid receptors in MDA-MB-468 xenografts. Importantly, WIN55,212-2 treatment significantly downregulated the expression of both CB1 and CB2 receptors while tended to downregulate EGFR expression. JWH-133 treatment on the other hand was associated with a tendency to activate EGFR. Furthermore, the current study demonstrated for the first time that WIN55,212-2 treatment activated p38 MAPK and inhibited NF-κB expression in the TNBC tumors; whereas JWH-133 treatment tended to activate the survival Akt and β-catenin pathways and the inhibition of p27. The results strongly suggest the diversity in biological effects and complexity in molecular mechanisms of different cannabinnoids, and that a re-consideration on the use of cannabinoids in cancer patients might be needed.
