A new screening system for proliferation-independent anti-cancer agents

An in vitro screen for identification of novel anti-cancer agents, which can induce proliferation-independent apoptosis of prostate cancer (PCA) cells, is required, since the proliferative growth fraction of human prostate cancers in patients is usually <10%. This is possible using the PCA cell line LNCaP, which can be permanently transdifferentiated into a quiescent neuroendocrine (NE) phenotype without undergoing apoptosis by the cytokine interleukine-6 (IL-6). Transdifferentiation of LNCaP cells into a NE phenotype was documented using western blot analysis and immunohistochemistry for the NE markers, neuron-specific enolase (NSE) and beta III tubulin. Accumulation of NE cells in the G0 phase of the cell cycle was demonstrated by Ki-67 immunohistochemistry. The effects of paclitaxel, vinblastine and thapsigargin (TG) on viability and apoptosis of NE and LNCaP cells were assessed by trypan blue exclusion and 4', 6-diamidino-2-phenylindole nuclear staining assays. In proliferating LNCaP cells, there was a significant decrease in viable cells after 48 h exposure to paclitaxel and vinblastine and a dramatic increase of apoptosis as compared with the controls. On the other hand, treatment with paclitaxel or vinblastine decreased the viability of NE cells only slightly without markedly increasing their rate of apoptosis compared to controls. In contrast, both LNCaP and NE cells showed a significant and comparable decrease in cell viability and similar high levels of apoptosis when treated with TG. These results demonstrate that terminally transdifferentiated NE cells represent a useful in vitro screening system for identification of novel anti-cancer agents, like TG, that can induce apoptosis without requiring proliferation.