Impact of radiation and docetaxel on anti-tumor effects of insulin-like growth factor binding protein-related protein 1 (IGFBP-rP1) in hormone-refractory human prostate cancer cells

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Introduction and Objective: We previously reported that insulin-like growth factor binding protein-related protein 1 (IGFBP-rP1) exerts tumor-suppressive activity through induction of apoptosis in human prostate cancer cells (Cancer Res. 63, 7717, 2003). The aim of the present study was to investigate the synergistic effects of radiation and an anticancer agent with IGFBP-rP1 in hormone-refractory human prostate cancer cells.

Methods: A hormone-refractory human prostate cancer cell line, PC3, was transfected with IGFBP-rP1. One of the clonal transfectants, named PC3-rP1, was used in this study. To examine the effect of radiation, PC3-rP1 cells (1,000) were seeded in 9-cm dishes, exposed to 4 or 8 Gy of radiation, and cultured in OPTI-MEM 1 medium for 14 days. The number of colonies formed was then counted. The effects of radiation on cell cycle and apoptosis were also evaluated by flowcytometry and Annexin affinity assay, respectively. To examine the effect of the anticancer agent, the cells were seeded in the same way, exposed to 0.01 or 0.1 micro M docetaxel in the same medium, and then cell viability was evaluated by MTT assay on days 1, 2 and 3.

Results: Four and 8 Gy of radiation dramatically decreased the number of PC3-rP1 cell colonies by 93.7% and 99.2%, respectively. On the other hand, the number of colonies of Mock cells bearing an empty vector was reduced by 59.5% and 81.5%, respectively. The number of colonies formed differed significantly between the PC3-rP1 and the Mock cells (P<0.0001). Annexin affinity assay revealed that 8 Gy of radiation caused apoptosis for Mock and PC3-rP1 cells by 16.6% and 38.1%, respectively. Also, at 24 hours after 8 Gy of radiation, proportion of G2/M phase was 31.2% for Mock cells, while that was 98.1% for PC3-rP1 cells (P<0.05). The mean cell viability on day 3 was 0.61 for Mock, 0.38 for PC3-rP1 without docetaxel, 0.27 for PC3-rP1+0.01micro M docetaxel and 0.21 for PC3-rP1+ 0.1micro M docetaxel. This difference was significant (P<0.0001).

Conclusions: These results indicate that radiation and docetaxel enhance the growth-suppressive activity of IGFBP-rP1 in hormone-refractory human prostate cancer cells. Combination of IGFBP-rP1 with radiation or docetaxel may provide a new avenue for gene therapy of hormone-refractory prostate cancer.