GPR55-Mediated Effects in Colon Cancer Cell Lines
Carina Hasenoehrl, David Feuersinger, Melanie Kienzl, Rudolf Schicho
Medical Cannabis & Cannabinoids, 2019, 2, 22–28
Doi : 10.1159/000496356
The cannabinoid-responsive G protein-coupled receptor GPR55 and its endogenous ligand L-α lyso-phosphatidylinositol (LPI) have been reported to play a role in several cancers. A proliferation-enhancing effect of GPR55 has been described for several cancer cell lines and LPI has been found elevated in cancer patients. The aim of this study was to investigate whether GPR55 signaling had an effect on the proliferation of colon cancer cell lines. Using cell viability assays and Western blotting, we show that stable overexpression of the GPR55 receptor led to a growth advantage of SW480 cells per se. Proliferation of native colon cancer cell lines, however, was not affected by pharmacological manipulation of GPR55. Interestingly though, GPR55 signaling was responsive to treatment with both the GPR55 agonist LPI and the antagonist CID16020046 in the overexpressing cancer cell lines. This was evident through significantly increased or decreased levels of phosphorylated ERK1/2, respectively. Taken together, our findings suggest that GPR55 is constitutively activated in overexpressing colon cancer cells affecting ERK1/2 phosphorylation and cell proliferation.
Keywords : GPR55 · Colorectal cancer · Cannabinoids · Lysophosphatidylinositol
The G protein-coupled receptor 55 (GPR55) has been found to be responsive to a variety of natural and synthetic cannabinoids, including anandamide, virodhamine, and Δ9-tetrahydrocannabinol and has, therefore, been suggested to belong to the class of cannabinoid receptors [1, 2]. GPR55, however, has no significant sequence similarity with the classical cannabinoid receptors CB1 or CB2  and is rather related to the purinergic receptor P2Y5, as well as to the G protein-coupled receptors 23 and 92 (GPR23 and GPR92), which have been shown to be lysophosphatidic acid receptors [4–6]. GPR55 does not possess the typical “cannabinoid binding pocket” but rather exhibits a deep vertical binding pocket for long, thin inverted L- or T-shaped ligands  and, accordingly, its endogenous ligand was found to be a lysophospholipid, namely L-α-lysophosphatidylinositol (LPI) . In particular, it is believed that LPI carrying an arachidonic acid moiety is the most potent endogenous agonist of GPR55 .
GPR55 signaling has been implicated in a variety of cancers. In squamous cell carcinomas, breast and pancreatic cancer, and in glioblastomas, GPR55 expression levels are upregulated, positively correlating with aggressiveness [10, 11]. In vivo, GPR55 promotes carcinogenesis in mouse models of skin, pancreatic, and colorectal cancer [10, 12, 13]. Furthermore, GPR55 expression has been demonstrated in several cancer cell lines, such as breast, prostate, ovarian, glioblastoma, and colon cancer cells [11, 14, 15].
Since LPI has been found to be the endogenous agonist of GPR55 and since GPR55 is expressed in various cancers in an aggressiveness-related manner, a role for the LPI/GPR55 axis in cancer has been postulated . Overexpression of GPR55 enhanced, whereas silencing reduced the proliferation of breast cancer, glioblastoma, and skin carcinoma cells, and the main signaling pathway through which GPR55 exerts its proliferative effects in cancer cells was identified to be ERK1/2 phosphorylation
We have recently found that GPR55 promotes colorectal carcinogenesis  and that serum LPI levels were increased in colorectal cancer patients . We, therefore, set out to investigate whether LPI had direct effects on the proliferation of colon cancer cell lines and whether GPR55 overexpression would alter signal transduction in these cells.