Bombesin and substance P analogues differentially regulate G-protein coupling to the bombesin receptor - Direct evidence for biased agonism

Substance P analogues including [D-Arg(1),D-Phe(5),D-Trp(7,) (9),Leu(11)] s ubstance P (SpD) act as "broad spectrum neuropeptide antagonists" and are p otential anticancer agents that inhibit the growth of small cell lung cance r cells in vitro and in vivo. However, their mechanism of action is controv ersial and not fully understood. Although these compounds block bombesin-in duced mitogenesis and signal transduction, they also have agonist activity. The mechanism underlying this agonist activity was examined. SpD binds to the ligand-binding site of the bombesin/gastrin-releasing peptide receptor and blocks the bombesin-stimulated increase in [Ca2+](i) within the same co ncentration range that causes sustained activation of c-Jun N-terminal kina se and extracellular signal-regulated protein kinase (ERK). The activation of e-Jun N-terminal kinase by SpD and bombesin is blocked by dominant negat ive inhibition of G(alpha 12). The ERK activation by SpD is pertussis toxin -sensitive in contrast to ERK activation by bombesin, which is pertussis to xin-insensitive but dependent on epidermal growth factor receptor phosphory lation. Spl) does not simply act as a partial agonist but differentially mo dulates the activation of the G-proteins G(alpha 12), G(i), and G(q) compar ed with bombesin. This unique ability allows the bombesin receptor to coupl e to Gi and at the same time block receptor activation of G(q). Our results provide direct evidence that Spl) is acting as a "biased agonist" and that this has physiological relevance in small cell lung cancer cells. This val idation of the concept of biased agonism has important implications in the development of novel pharmacological agents to dissect receptor-mediated si gnal transduction and of highly selective drugs to treat human disease.