Cross talk between the bombesin neuropeptide receptor and Sonic hedgehog pathways in small cell lung carcinoma

Small cell lung carcinoma (SCLC) often features the upregulation of the Sonic hedgehog (Shh) pathway leading to activation of Gli transcription factors. SCLC cells secrete bombesin (BBS)-like neuropeptides that act as autocrine growth factors. Here, we show that SCLC tumor samples feature co-expression of Shh and BBS-cognate receptor (gastrin-releasing peptide receptor (GRPR)). We also demonstrate that BBS activates Gli in SCLC cells, which is crucial for BBS-mediated SCLC proliferation, because cyclopamine, an inhibitor of the Shh pathway, hampered the BBS-mediated effects. BBS binding to GRPR stimulated Gli through its downstream Gαq and Gα₁₂/₁₃ GTPases, and consistently, other Gαq and Gα₁₃ coupled receptors (such as muscarinic receptor, m1, and thrombin receptor, PAR-1) and constitutively active GαqQL and Gα₁₂/₁₃QL mutants stimulated Gli. By using cells null for Gαq and Gα₁₂/₁₃, we demonstrate that these G proteins are strictly necessary for Gli activation by BBS. Moreover, by using constitutively active Rho small G-protein (Rho QL) as well as its inhibitor, C3 toxin, we show that Rho mediates G-protein-coupled receptor (GPCR)-, Gαq- and Gα₁₂/₁₃-dependent Gli stimulation. At the molecular level, BBS caused a significant increase in Shh gene transcription and protein secretion that was dependent on BBS-induced GPCR/Gαq-₁₂/₁₃/Rho mediated activation of nuclear factor κB (NFκB), which can stimulate a NF-κB response element in the Shh gene promoter. Our data identify a novel molecular network acting in SCLC linking autocrine BBS and Shh circuitries and suggest Shh inhibitors as novel therapeutic strategies against this aggressive cancer type.