Therapeutic Implications of Enhanced G0/G1 Checkpoint Control Induced by Coculture of Prostate Cancer Cells with Osteoblasts

Osteoblastic metastases are common in lethal prostate cancer. Effective therapy for bone metastases is lacking. Thus, developing an appropriate in vitro screening system is critical to prioritize which of the newly developed agents should undergo additional expensive and time-consuming in vivo evaluation in bone metastases animal models. In the past, such in vitro screening evaluated the response of prostate cancer cells to chemotherapeutic agents in monoculture without the presence of osteoblasts. In such monoculture, prostate cancer cells have a high (i.e., >90%) proliferative growth fraction. In contrast, the growth fraction (i.e., mean: 7.1 +/- 0.8%; median: 3.1%) in 117 metastatic sites of prostate cancer obtained from 11 androgen ablation failing patients at "warm" autopsy was found to be >10-fold lower. To better mimic the lower growth fraction observed clinically, LNCaP human prostate cancer cells were cocultured with membrane-separated hFOB human osteoblasts. Such coculturing significantly lowered the growth fraction of the LNCaP cells (i.e., from >90 to 90%) growth fraction cultures (i.e., cultures in the absence of osteoblast-conditioned medium), there was a dose-dependent and significant (P < 0.05) increase in apoptosis of LNCaP cells exposed to Taxol or doxorubicin. In contrast, even the highest dose of Taxol (1 microM) did not enhance apoptosis of lower growth fraction LNCaP cells cultured in osteoblast-conditioned medium. Similarly, only the highest concentration of doxorubicin (1 microM) enhanced apoptosis in lower growth fraction cells. In contrast, 100 nM TG induced high levels of apoptosis in both lower and high-growth fraction LNCaP cultures. These results demonstrate that the osteoblast/LNCaP coculture system is a better in vitro screen than monoculture to identify proliferation-independent agents for the treatment of prostate cancer bone metastases, and TG is such an agent.