Antibiotics and natural peptides as modulators of P2X7 functions.

Among natural antibiotics, polymyxin B (PMB) and hCAP-18/LL37 have recently been shown to modulate responses induced by stimulation of the P2X7 receptor subtype. PMB, a cationic cyclic antibiotic produced by Bacillus polymyxa, potentiates responses elicited by ATP or BzATP. Expression of P2X7 is mandatory for up-regulation of Ca2+ influx, plasma membrane permeabilization, IL-1beta secretion and cytotoxicity. The N-terminal fatty amino acid 6-methylheptanoic/octanoic-diaminobenzoic residue of PMB is also required for up-modulation of ATP- and BzATP-induced responses. Cathelicidin peptides bind to bacterial surface thus leading to microbial killing. hCAP-18/LL-37 is the only cathelicidin of humans. It is stored in granules of neutrophils, in lymphocytes, macrophages and epithelial cells from skin and mucose membranes. LL-37 stimulates the growth of murine fibroblasts through activation of P2X7. This effect does not depend on helix sense (the all-D analog is active) but requires a strong helix-forming propensity in aqueous solution (a scrambled analog and primate orthologs devoid of this property are inactive). In P2X7-expressing cells, LL-37 raises intracellular Ca2+, synergizes with BzATP to increase intracellular Ca2+ and allows cellular internalization of ethidium bromide. The activity of hCAP-18/LL-37 has an absolute requirement for P2X7 expression, as it is fully blocked by the potent P2X7 inhibitor KN-62 and is absent in cells lacking P2X7. Peculiarly, hCAP-18/LL-37 restores the pore-forming activity in cells expressing a truncated P2X7 receptor. Our results show that besides a direct effect on microbes, PMB and hCAP-18/LL-37 interact with a receptor expressed by eukariotic cells, thus adding complexity to the picture of the antibiotic-mediated effects.