APOPTOTIC REGRESSION OF MCF-7 XENOGRAFTS IN NUDE-MICE TREATED WITH THE VITAMIN-D-3 ANALOG, EB1089

1,25-Dihydroxyvitamin D-3 [1,25-(OH)(2)D-3] and its synthetic analog E B1089 induce characteristic morphological features of apoptosis in MCF -7 cells in vitro that coincide with up-regulation of clusterin and ca thepsin B, proteins associated with apoptosis in the mammary gland, an d with clown-regulation of Bcl-2, an antiapoptotic protein. To determi ne whether vitamin D-3 compounds could mediate apoptosis of breast tum ors in vivo, we treated nude mice carrying established MCF-7 xenograft s with the low calcemic vitamin D-3 analog EB1089 via daily injection or sustained release pellets for up to 5 weeks. The volume of tumors f rom mice treated with 45 pmol/day EB1089 was 4-fold lower than that of tumors from vehicle-treated control mice after 5 weeks. The reduced g rowth of tumors from EB1089-treated mice was associated with character istic apoptotic morphology and a marked reduction in the proportion of epithelial cells to stroma. After 5 weeks of treatment with EB1089. M CF-7 tumors exhibited a 6-fold increase in DNA fragmentation (as measu red by in situ end labeling) relative to that in control tumors. The e nhanced rate of apoptosis in tumors from EB1089-treated mice was coupl ed to a 2-fold reduction in proliferation (as measured by expression o f proliferating cell nuclear antigen) compared with that in tumors fro m control mice. The antitumor effects of EB1089 were evident at doses that had minimal effects on serum calcium and body weight. EB1089 trea tment did not alter the growth of xenografts derived from a vitamin D- 3-resistant variant of MCF-7 cells (MCF-7(D3Res) cells), which display resistance to EB1089 in, vitro, indicating that resistance to EB1089 is maintained in vivo. Tumors derived from both MCF-7 and MCF-7(D3Res) cells underwent apoptotic regression upon estradiol withdrawal, indic ating comparable estrogen dependence of tumors with differential sensi tivity to vitamin D-3 compounds. These are the first studies to demons trate apoptotic morphology and regression of human breast tumors in re sponse to treatment with a vitamin D-3 analog in vivo and support the concept that vitamin D-3 compounds can effectively target human breast cancer.