Bombesin and gastrin-rleas peptde act as autocrine mitogens in various cancers. Bombesin ataist RC-3095 inhibited growth in some cancers and slowed the progression of premalignant lesions, possibly by downregulating epidermal growth factor (EGF) receptors. Since the EGF receptor mitogen response Involves tyrosine kinase stimulation, we tested the hypotheses that bombesin stimulates, and RC-3095 inhibits, phosphorylation; EGF and bombesin promote the phosphorylation of the same substrates; and EGF and bombesin act synergistically on phosphorylation. Therefore, in vitro assays for phosphorylation were performed in the presence or absence of EGF, bombesin, RC-3095, and combinations in samples derived from tumor, tissue surrounding tumor, cell lines, and normal and transforming tissue derived from the 9,10-dlmethyl-1,2-benzanthracene-induced squamous cell lesions of the hamster cheek pouch. Bombesin increased, and RC-3095 decreased, phosphorylation in these samples. In the human hepatoma sample and ondi tissue, these ligands altered the phosphorylation of the same substrates affected by EGF. EGF and bombesin stimulated phosphorylation synergistically in the hamster samples and the hepatoma. Bombesin-induced phosphorylation was greater in tissue surrounding the hepatoma, whereas RC-3095 was more effective in inhibiting phosphorylation in the hepatoma itself. This cancer, therefore, could be endogenously stimulated by gastrin-releasing peptide. These observations support the hypothesis that bombesn stimulates growt of tissues and tumors by amplifying the phosphorylation response to EGF. The growth inhibitory response to 1&C-3095, or other bombesin analogues, of individual tumors may be proguosed by in vitro phosphorylation assays using the samples from the patient's tumor.Bombesin is an amphibian tetradecapeptide related in structure and function to gastrin-releasing peptide (GRP), its mammalian counterpart. Bombesin and GRP produce a wide range of neuroendocrine responses: acting as secretagogues throughout the gastrointestinal tract (1), stimulating central nervous system activity (2) and muscle activity (1), and promoting chemotaxis (3). In addition to these diverse actions, bombesin and GRP have mitogenic activity [in 3T3 fibroblasts (3) and bronchial epithelium (4)]. Growthpromoting activity ofbombesin and GRP has also been found in a number of cancers, including small cell lung carcinoma (5,6), which may secrete GRP in an autocrine fashion (5, 7). The human pancreatic tumor cell line, CAPAN, produces GRP and its receptor (8). All of these effects suggest a self-stimulating, growth-promoting role for GRP. The cellular mechanism by which bombesin and GRP stimulate the growth of cancers, and conversely how their antagonist inhibits this growth, has been investigated. Treatment of nude mice bearing xenografts of MCF-7 MIII human breast cancer with RC-3095 for 7 weeks significantly reduced the epidermal growth factor receptor (EGFR) content by 85%, whereas luteinizing hormone-releasing hormone antagonist SB-7...
