Abstract P5-03-01: Role of Aromatase and Its Inhibitor in Breast Cancer-Induced Tumorigenesis and Bone Metastasis

Aromatase (Aro) is the rate-limiting enzyme that catalyzes the final step in estrogen (E2) biosynthesis. An important strategy to treat hormone-dependent BCa is suppression of estrogen receptor (ER) action by antiestrogens or aromatase inhibitors (AI). Letrozole is a very specific and potent AI. In postmenopausal women, the ovaries cease to make E2 but concentration of E2 in their BCa tissue are maintained at a certain level for survival and proliferation of BCa cells, which is dependent on local E2 formation catalyzed by Aro. Although BCa cells have been shown to express Aro, the local E2 is largely produced by adipose stromal cells in the breast. This raises the question of how ER positive (ER+) metastatic BCa cells survive after they enter blood circulation, where E2 level is very low. We cultured human ER+ BCa CAMA-1, Aro-transfected MCF-7 and ZR-75-1 (ZR) cells in suspension to mimic circulating BCa cells. Interestingly, suspension culture increased Aro expression, suggesting circulating ER+ BCa may up-regulate intracrine E2 activity for survival after leaving the E2-rich adipose stroma at primary site. The expression of Aro also enhances cell proliferation and supplementation of testosterone (T), the substrate of Aro, stimulates this proliferation further. Notably, while these cells show an increased rate of apoptosis in suspension than in adherent culture, addition of T in suspension culture significantly suppressed the rate of apoptosis and addition of letrozole blocked the T-induced cell survival in suspension culture. To investigate the importance of intracrine E2 in promoting tumorigenesis and metastasis, we implanted Aro-expressing ZR cells orthotopically and intracardiacally (I.C.) into female athymic mice; vector-transfected ZR cells were used as control. While control ZR cells were incapable of forming tumors without E2 supplementation, Aro-expressing cells generated orthotopic tumors with no E2 supplementation after 3-weeks of inoculation. More interestingly, mice with I.C. inoculated Aro-expressing cells also presented distant bone metastasis in the mandible and tibiae/femora after 2-weeks of inoculation, detected by whole mouse fluorescence and bioluminescence imaging as the cells were stably transfected with a luciferase and GFP expression vector. To determine whether growth of orthotopic tumors can be inhibited by systemic administration of an AI, we treated the mice in one group with letrozole at 10 mg/mouse/day and the other group with the vehicle as control after the average tumor volume reached 150 mm3. After 3-weeks, the tumor burden in the letrozole treatment group reduced significantly while tumor burden in the control group increased continuously. Our studies show that suspension culture increases expression of Aro mRNA in several ER+ BCa cell lines, which likely results in increased intracrine E2 signaling and contributes to the survival of these BCa cellsin suspension. This provides a mechanistic insight into how ER+ BCa cells may survive the low E2 condition in circulation and subsequently induce distant metastasis as observed in the I.C. model. Our study provides an important foundation for future investigation on how hormone-dependent BCa cells up-regulate Aro expression in circulation and induce bone metastasis. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P5-03-01.