Abstract 2504: Preferential cell cycle expression of the bombesin receptor (BB2r) in prostate cancer cells
Authors: Christopher A. Manuel, Brienne N. Bottenus, Kathy L. Schreiber, Gary L. Sieckman, Adam F. Prasanphanich, Charles J. Smith, Timothy J. Hoffman
Journal: Cancer Research (2010)

Abstract Aberrant expression of the bombesin receptor subtype 2 (BB2r) has been found in over 80% of prostate cancers. The functional role of the receptor in cancer has been identified as a promoter of growth, morphogenesis and mutagenesis in tumor cell lines. The BB2r binds with high affinity to the bombesin peptide (BN), stimulating agonist mediated receptor-peptide internalization. Accordingly, targeting the BB2r with radioisotopes conjugated to BN represents a promising method for therapeutic radiopharmaceutical development. We look to optimize ionizing radiation damage to targeted tumor cells by combining BB2r targeted radiotherapy with the radiosensitizing chemotherapeutic docetaxel. The hallmark mechanism of docetaxel is the promotion of G2/M phase arrest, which is also the most radiosensitive phase of the cell cycle. The objectives of this study are to characterizing general BB2r expression in the cell cycle and the receptors response to cell cycle perturbations caused by docetaxel. We have synthesized, purified, and characterized two BN analogs [BN(7-14) agonist and scrambled sequence] conjugated to a fluorophore to be used in combination with cell cycle analysis in multiparameter flow cytometry. For evaluation of cell surface receptor expression or agonist mediated internalization of the BB2r, PC-3 cell experiments were performed at either 4°C or 37°C, respectively. Our data demonstrates constitutive BB2r expressed in all phases of the cell cycle. However, there is a 60% increase in fluorophore conjugated agonist binding, representative of bombesin receptor expression, in the G2/M phase as compared to the G0/G1 phase. To determine if G2/M phase preferential expression could be exploited during G2/M phase arrest, PC-3 cells were exposed to physiologically obtainable serum concentrations of docetaxel at 1, 5 or 10 nM for 48 hours. A dose dependent response in G2/M phase cell arrest was only observed at 5 and 10 nM. At docetaxel concentrations of 1, 5 and 10 nM we observed an increase of 60%, 200% and 300% in BB2r expression in the G2/M phase as compared to the G0/G1 phase. However, with increasing concentrations of docetaxel, a net decrease in BB2r expression was observed in the entire cell population with a 13%, 34% and 48% decrease in receptor expression at 1, 5 and 10 nM docetaxel as compared to control. These findings suggest preferential expression of the BB2r in the G2/M phase of the cell cycle, ideal for optimizing targeted radiotherapy. We also report an unanticipated down-regulation of the BB2r with docetaxel treatment which may contribute to chemotherapeutic efficacy by reducing mitogen stimulation through the BB2r. Overall, the disproportionate BB2r expression in the G2/M phase coupled with reduced mitogen stimulation achieved with docetaxel therapy may contribute to chemotherapeutic synergy with BB2r targeted radiotherapy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2504.