Targeting endocannabinoid signaling in tumor-associated macrophages as treatment for glioblastoma multiforme

Abstract

Glioblastoma multiforme (GBM) is the most common form of primary brain tumor and is diagnosed in approximately 15,000 people each year in the United States alone. No cure for this type of cancer exists, and the current standard of care treatments provide little benefit and are associated with debilitating side effects. Recent evidence shows that a large number of tumor-associated macrophages (TAMs) invade the GBM tumor mass and secrete factors that directly and indirectly promote tumor growth. TAMs express a panel of unique receptors that could be targeted for therapeutic benefit. One such receptor, cannabinoid receptor 2 (CB2), is a member of the endocannabinoid (eCB) signaling system, and its activation has been shown to tightly control the migration and phenotype of both macrophages and microglia. Additional receptors, also engaged by eCBs and cannabinoid-like compounds, are expressed by macrophages and microglia. These receptors also control cell migration and phenotype, but exhibit distinct pharmacological profiles and operate through a different mechanism of action. Strong evidence accumulated over the past decade indicates that membrane receptors expressed by TAMs represent novel targeting opportunities to treat GBM tumor progression. Here we review studies that significantly increased our understanding of the molecular mechanism of action of receptors engaged by eCBs and cannabinoid-like compounds expressed by GBM tumor cells and TAMs. This evidence provides a strong rationale for developing new therapeutics that target the eCB signaling of TAMs for the treatment of GBM while minimizing the typical side effects associated with standard care. WIREs Membr Transp Signal 2014,3:39-51. doi: 10.1002/wmts.101 Conflict of interest: The authors have declared no conflicts of interest for this article. For further resources related to this article, please visit the WIREs website. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.