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

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 expression is 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. Whole-cell-based screening of M13-phage-displayed random library3 was performed using as bait the GPR55 receptor heterologously expressed in HEK293 cells. The 7-mer insert peptides in M13-phage library were flanked by a pair of cysteine residues resulting in cyclized peptides. After the screening, binding specificity to GPR55-positive cells was verified using synthetic FITC-labelled peptides by FACS and confocal microscopy. In this way, 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. This analysis has been extended to lymphoblastoid leukemic cells. Subsequently, 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. The identified peptide did not affect significantly the GPR55 signalling; however, it showed a significant modulation of both basal and agonist-induced GPR55 internalization. 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.