Suppression of growth of H-69 small cell lung carcinoma by antagonists of growth hormone releasing hormone and bombesin is associated with an inhibition of protein kinase C signaling

We investigated the effects of antagonists of growth hormone-releasing hormone (GHRH) alone and in combination with bombesin/gastrin-releasing peptide (BN/GRP) antagonist RC-3940-11 on the growth of H-69 human small cell lung carcinoma (SCLC) xenografted into nude mice. Since the activation of the signaling pathways involving protein kinase C (PKC) and the subsequent steps involving mitogen-activated protein kinase (MAPK) and c-fos and c-jun oncogenes are known to be important mechanisms implicated in cellular growth, we investigated how the blockade of tumoral GHRI-1 receptor splice variants and BN/GRP receptors by these antagonists could interfere with these intracellular signaling pathways. Treatment with GHRH antagonists JV-1-65 or MZ-J-7-110 for 4 weeks significantly (p<0.05) decreased the tumor volume by 22.7 ± 3.0% and 36.7 ±3.6%, respectively, as compared to controls. A larger decrease in tumor volume of 73.0 ± 9.5% (p<0.01) was produced by BN/GRP antagonist RC-3940-11 and its combination with JV-1-65 caused the greatest tumor reduction of 91.0 ±9.8% (p<0.01) vs. controls. H-69 SCLC tumors expressed α-, β11-, δ- and η-PKC isoforms. Antagonists of GHRH and BN/GRP decreased significantly (p<0.05) the expression of β11- and δ-, but not of α- and η-PKC isoforms. They also inhibited MAPK levels, the effects being significant (p<0.05) in the groups that received BN/GRP antagonist. In addition, expression of c-fos and c-jun mRNA was reduced after combined treatment with JV-1-65 and RC-3940-11. The proliferation of H-69 SCLC cells "in vitro" was also significantly inhibited after incubation of cells with GHRH antagonist, PKC inhibitors or MAPK inhibitor. These findings suggest that the anti-proliferative effects of antagonists of GHRH and BN/GRP on H69-SCLC involve an inhibition of the signaling pathways of specific PKC isoforms, HAPK and c-fos and c-jun oncogenes.