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Antagonists of Growth Hormone-Releasing Hormone Inhibit the Proliferation of Experimental Non-Small Cell Lung Carcinoma

¹ Endocrine, Polypeptide and Cancer Institute, Veterans Affairs Medical Center,
² Section of Experimental Medicine, Department of Medicine, Tulane University School of Medicine, New Orleans, Louisiana 70112

Recent studies show that antagonists of growth hormone-releasing hormone (GH-RH) inhibit proliferation of various cancers indirectly through blockage of the endocrine GH-insulin-like growth factor (IGF) I axis and directly by an action on tumor cells involving the suppression of autocrine/paracrine IGF-I, IGF-II, or GH-RH. The effectiveness of therapy with GH-RH antagonist JV-1–38 and its mechanisms of action were investigated in NCI-H838 non-small cell lung carcinoma (NSCLC) xenografted s.c. into nude mice and in vitro. Treatment with GH-RH antagonist JV-1–38 significantly (P < 0.05–0.001) inhibited tumor growth as demonstrated by a 58% decrease in final tumor volume, 54% reduction in tumor weight, and the extension of tumor-doubling time from 8.5 ± 1.38 to 12 ± 1.07 days as compared with controls. Using ligand competition assays with ¹²⁵I-labeled GH-RH antagonist JV-1–42, specific high-affinity binding sites for GH-RH were found on tumor membranes. Reverse transcription-PCR revealed the expression of mRNA for GH-RH and splice variant 1 (SV₁) of GH-RH receptor in H838 tumors. Reverse transcription-PCR analysis also demonstrated that H838 tumors express IGF-I and IGF-I receptors. Tumoral concentration of IGF-I and its mRNA expression were significantly decreased by 25% (P = 0.05) and 65% (P < 0.001), respectively, in animals receiving JV-1–38, whereas serum IGF-I levels remained unchanged. In vitro studies showed that H838 cells secreted GH-RH and IGF-I into the medium. The growth of tumor cells in vitro was stimulated by IGF-I and inhibited by GH-RH antagonist JV-1–38 and a GH-RH antiserum. Our results extend the findings on the involvement of IGF-I in NSCLC and suggest that GH-RH may be an autocrine growth factor for H838 NSCLC. The antitumorigenic action of GH-RH antagonists could be partly direct and mediated by SV₁ of tumoral GH-RH receptors. The finding of GH-RH and SV₁ of GH-RH receptors in NSCLC provides a new approach to the treatment of this malignancy based on the use of antagonistic analogues of GH-RH.

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