Expression of the lysophosphatidylinositol receptor GPR55 correlates with invasive potential of metastatic cells and bone metastasis formation of different types of tumors. These findings suggest a role for GPR55 signaling in cancer progression, including in lymphoproliferative diseases. Here, we screened a M13-phage-displayed random library using the bait of HEK293 cells that heterologously expressed full-length HA-GPR55. We selected a set of phagotopes that carried 7-mer insert peptides flanked by a pair of cysteine residues, which resulted in cyclized peptides. Sequencing of selected phagotopes dictated the primary structure for the synthetic FITC-labeled peptide P1, which was analyzed for binding specificity to immunoprecipitated HA-GPR55, and to endogenously expressed GPR55, using cells interfered or not for GPR55, as well as for co-localization imaging with HA-GPR55 at the membrane level. The peptide P1 stimulated GPR55 endocytosis and inhibited GPR55-dependent proliferation of EHEB and DeFew cells, two human B-lymphoblastoid cell lines. Our data support the potential therapeutic application of peptide ligands of GPR55 for targeting and inhibiting growth of neoplastic cells, which overexpress GPR55 and are dependent on GPR55 signaling for their proliferation.

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Increasing evidence points to the involvement of group IIA secreted phospholipase A2 (sPLA2-IIA) in pathologies characterized by abnormal osteoclast bone-resorption activity. Here, the role of this moonlighting protein has been deepened in the osteoclastogenesis process driven by the RANKL cytokine in RAW264.7 macrophages and bone-marrow derived precursor cells from BALB/cJ mice. Inhibitors with distinct selectivity toward sPLA2-IIA activities and recombinant sPLA2-IIA (wild-type or catalytically inactive forms, full-length or partial protein sequences) were instrumental to dissect out sPLA2-IIA function, in conjunction with reduction of sPLA2-IIA expression using small-interfering-RNAs and precursor cells from Pla2g2a knock-out mice. The reported data indicate sPLA2-IIA participation in murine osteoclast maturation, control of syncytium formation and resorbing activity, by mechanisms that may be both catalytically dependent and independent. Of note, these studies provide a more complete understanding of the still enigmatic osteoclast multinucleation process, a crucial step for bone-resorbing activity, uncovering the role of sPLA2-IIA interaction with a still unidentified receptor to regulate osteoclast fusion through p38 SAPK activation. This could pave the way for the design of specific inhibitors of sPLA2-IIA binding to interacting partners implicated in osteoclast syncytium formation.