Peptide-guided targeting of GPR55 for new therapeutic strategies of cancer

Introduction: Increasing evidence indicates that G-protein-coupled receptors (GPCRs) are 'druggable' targets in cancer treatment and prevention1. GPR55 was recently identified as the lysophosphatidylinositol receptor, and several aspects of its physiology are still under investigation2. GPR55 has a role in the regulation of cancer progression, being its expression correlated with the invasive potential of metastatic cells and bone metastasis formation2. In order to develop innovative cancer therapy, this study is aimed to interfere the GPR55 function in cancer cells with peptidic binders. Material and method: Whole-cell-based screening of M13-phage-displayed random library3 was performed using as bait the GPR55 receptor heterologously expressed in HEK293 cells. This method has been largely used in reverse pharmacology to identify ligands of orphan receptors. The 7-mer insert peptides in M13-phage library were flanked by a pair of cysteine residues resulting in cyclized peptides stabilized by disulfide bond. Binding specificity to GPR55-positive cells was verified using synthetic FITC-labelled peptides by FACS and confocal microscopy. The effect of peptide binding to GPR55 was analysed in lysphosphatidyinositol-triggered GPR55 signalling, including intracellular calcium, AKT phosphorylation, modulation of actin cytoskeleton, and receptor internalization. Results and discussion By screening M13-phage-displayed random library, we have selected a peptide that binds specifically to HeLa cells expressing GPR55 and not to cells interfered for GPR55. Binding specificity of the FITC-labelled peptide was further investigated in competition assays with unlabelled peptide, and confirmed by the use of scramble analogues. The identified peptide did not affect significantly the GPR55 signalling; however, it showed a significant modulation of both basal and agonist-induced GPR55 internalization. This analysis has been extended to lymphoblastoid leukemic cells. Conclusions: Based on preliminary results, we propose the use of GPR55-peptidic binders for in-vivo diagnosis of GPR55-positive tumor cells, specific delivery of chemotherapeutic agents to cancer cells, and direct modulation of receptor activities for therapeutic applications. References: 1- Lappano R, Maggiolini M (2011) Nat Rev Drug Discov 10:47-60. 2- Ross RA (2011) Trends Pharmacol Sci 32:265-9. 3- Palmieri C et al. (2010) Blood 116:226-38.