The discovery that the physiological
The discovery that the physiological functions of LPI are associated with the activation of specific GPR55 membrane receptor  has launched a new era of research on this lysophospholipid. However, the activation of membrane receptors by lysophospholipids is extremely complex due to the multitude of potential ligands, the number of receptors identified so far, their varying specificity and possible non-specific interactions of lysophospholipids with the cell membrane. Noteworthy, under in vivo conditions not only LPI was found, but also lysophosphatidic acid, lysophosphatidylserine, lysophosphatidylethanolamine, lysophosphatidylthreonine, and the most abundant lysophosphatidylcholine (LPC) , . The latter was identified as a ligand of GPR119 receptor which has received considerable attention from the pharmaceutical industry being one of the crucial targets responsible for regulation of insulin secretion . Homology clustering analysis revealed that the closest relatives of GPR119 are CB1 and CB2 cannabinoid receptors .
LPI and LPC share common structural features: they possess a glycerol backbone, a phosphate head group at the sn-3, a hydroxyl group at the sn-2 or sn-1 and a single fatty guanabenz chain at the sn-1 or sn-2 position. Here, we show that both unmodified LPCs and their phosphorothioate derivatives synthesized by us to enhance the stability of normally unstable native ligands also interact with GPR55 and the strength of interaction is structure-dependent. Our results demonstrating that GPR55 is the functional LPC receptor should be taken into consideration for targeting and inhibiting growth of cancer cells, which overexpress GPR55 and depend on GPR55 signaling.
Materials and methods
Results and discussion
Conflict of interest
Acknowledgments This work was supported by grants from the National Science Centre (2011/01/B/ST5/06383 and 2013/11/N/NZ5/00270) and PLATON (Blue Ocean) Computational Cluster of Lodz University of Technology.
Introduction GPR55 is a membrane-bound G protein-coupled receptor (GPCR) that is highly expressed in the brain, particularly in the hypothalamus, olfactory bulb, and striatum.1, 2 It is also found in other organs and tissues such as bone marrow, neutrophils and the spleen. It has been established so far that GPR55 is a potential target for treating pain, osteoporosis and cancer. In adjuvant-induced inflammation and partial nerve ligation pain models, GPR55 knockout mice developed neither inflammatory nor neuropathic mechanical hyperplasia. The receptor-deprived mice also revealed a role of GPR55 in regulating osteoclasts’ number and function based on the finding that the number of inactive osteoclasts was higher compared to wild type mice. A recent study revealed high expression of GPR55 in myenteric colonic neurons and the involvement of the receptor in colonic motility. The role of GPR55 in cancer proliferation has been recently well-established to be proliferative rather than anti-proliferative. It was not only detected in various cancer cell lines such as breast, brain, skin, and prostate but also discovered in a number of clinical isolates from cancer patients. Furthermore, it has been shown that GPR55 activation stimulates angiogenesis and metastasis, which explains the correlation of GPR55 expression with cancer aggressiveness. Initial in vitro screening identified GPR55 as a cannabinoid receptor due to similarities with cannabinoid receptors CB1 and CB2. Yet the various cannabinoid ligands exert different pharmacology when bound to GPR55. AM251 for instance, which is a CB1 antagonist, strongly activates GPR55. Another example is the CB1 and CB2 agonist CP55940 which acts as an antagonist when bound to GPR55. In addition, the results obtained for the same ligand tested may vary depending on the assay employed and the cell type.14, 15, 16 l-α-Lysophosphatidylinositol (LPI, 1; Fig. 1) is the endogenous agonist of GPR55, but not the best candidate to be labeled for studying the receptor due to its lack of selectivity for GPR55. The need for a selective and potent ligand prompted high-throughput screening which was conducted by the Sanford-Burnham screening center. Out of approximately 290,000 different compounds tested, three chemically distinct compounds, namely CID1792197 (2),17, 18 CID1172084 (3), and CID2440433 (4), were identified as potent agonists of GPR55 with EC50’s of 0.11, 0.16 and 0.26μM, respectively. In addition to the moderate potency of these ligands is their selectivity for GPR55 over other cannabinoid receptors, >30μM activity of CB1, CB2 and the closely related atypical cannabinoid receptor GPR35, which renders them as appropriate leads to develop probes for studying GPR55.