Effects of bisphosphonates and insulin-like growth factor-1 (IGF-1) on prostate cancer cells in vitro

Abstract

4758 Background: The bisphosphonate (BP) zoledronic acid (Zol) reduces skeletal morbidity from metastatic bone disease in patients with prostate cancer, presumably through inhibition of osteolysis. Osteolysis undermines skeletal integrity and releases IGF-1, thus stimulating prostate cancer growth. In addition, BPs have been shown to inhibit cancer cell growth. Therefore, we investigated the effects of BPs on prostate cancer cell growth in vitro. Methods: The effects of clodronate (Clod), pamidronate (Pam), ibandronate (Iban), and Zol on human PC-3 cell growth and survival were investigated after 1–6 days in culture. Cells were grown in media containing 1 to 100 microM BPs in the absence or presence (for Clod and Zol only) of 100 ng/mL IGF-1. Cell survival and growth were assessed by MTT assay, DNA content, and crystal violet staining. Proportion of cells in G0/G1 phase was determined by fluorescent staining, and apoptotic cells were identified by annexin staining. Results: Cell survival was not affected by Clod, but decreased in a time- and dose-dependent manner with Pam, Iban, and Zol. The largest effect was observed after 6 days with 100 microM Zol. Clod and Pam had no significant effect on cell cycle distribution. In contrast, 100 microM Zol or Iban produced cell accumulation in G0/G1 after 1, 2, and 4 days. The percentage of apoptotic cells was increased 2-fold by both Pam and Zol, and 1.3-fold by Iban. Clod had no effect. In the presence of IGF-1, cell growth increased by 28% (standard error of the mean [SEM] = 5%) at day 2 and by 23% (SEM = 7%) at day 6 (P < .001). These mitogenic effects were blocked by 1 microM Zol. Cell proliferation in the presence of IGF-1 was blocked by 10 microM Zol, and 100 microM Zol resulted in 85% cell death at day 6. Mitogenesis by IGF-1 was blocked by 10 microM Clod, but cell growth was not inhibited even at 100 microM Clod. Conclusions: Therefore, BPs, especially Zol, have direct effects on prostate cancer cells, inhibiting growth through cytostatic, apoptotic, and antimitogenic activities. These activities could contribute to the beneficial effects of Zol in the treatment of prostate cancer-induced bone disease. No significant financial relationships to disclose.