The Gγ/T-15 transgenic mouse model of androgen-independent prostate cancer: Target cells of carcinogenesis and the effect of the vitamin D analogue EB 1089

Transgenic mouse models of prostate cancer provide unique opportunities to understand the molecular events in prostate carcinogenesis and for the preclinical testing of new therapies. We studied the GγT-15 transgenic mouse line, which contains the human fetal globin promoter linked to SV40 T antigen (Tag) and which develops androgen-independent prostate cancer. Using the immunohistochemistry of normal mouse prostates before tumor formation, we showed that the target cells of carcinogenesis in GγT-15 mice are located in the basal epithelial layer. We tested the efficacy of the 1,25(OH)₂D₃ analogue, EB 1089, to chemoprevent prostate cancer in these transgenic mice. Compared with treatment with placebo, treatment with EB 1089 at three different time points before the onset of prostate tumors in mice did not prevent or delay tumor onset. However, EB 1089 significantly inhibited prostate tumor growth. At the highest dose, EB1089 inhibited prostate tumor growth by 60% (P = 0.0003) and the growth in the number of metastases, although this dose also caused significant hypercalcemia and weight loss. We conducted several in vitro experiments to explore why EB 1089 did not prevent the occurrence of the primary tumors. EB1089 significantly inhibited the growth of a Tag-expressing human prostate epithelial cell line, BPH-1, and an androgen-insensitive subline of LNCaP cells [which was not inhibited by 1,25(OH)₂D₃]. Thus, neither Tag expression nor androgen insensitivity explain the absence of chemopreventive effect. Conversely, neither 1,25(OH)₂D₃ nor EB 1089 inhibited the growth of the normal rat prostate basal epithelial cell line NRP-152. It is likely that EB 1089 was not effective in delaying the growth of the primary tumor in GγT-15 transgenic mice because the target cells of carcinogenesis in these mice are located in the basal epithelial layer. We conclude that GγT-15 transgenic mice are a useful model for testing vitamin D-based therapies in androgen-insensitive prostate cancer but are not suitable for studies of vitamin D-hased chemoprevention. The superiority of EB 1089 over 1,25(OH)₂D₃ in the growth suppression of androgeninsensitive prostate cancer cells supports the use of EB 1089 in androgen-insensitive prostate cancer.