2786

We have developed a murine model of breast cancer progression in which ductal, hormone-dependent (HD) carcinomas transit through different stages of hormone responsiveness, always retaining the expression of high levels of estrogen and progesterone receptors (PR). Unlike HD carcinomas, hormone-independent (HI) tumors do not need the exogenous administration of medroxyprogesterone acetate (MPA) to grow. In vitro, however, there are no differences in hormone responsiveness between both types, suggesting the involvement of host factors regulating in vivo tumor growth. In in vitro studies, we demonstrated that carcinoma associated fibroblasts (CAF) obtained from HI (CAF-HI) tumors expressed high levels of FGFs; in co-cultures they increased epithelial cell (EPI) proliferation and PR activation more efficiently than CAF-HD. The aim of this study was to evaluate a) FGFR expression in HD and HI tumors, b) MPA regulation of FGFR and c) the role of FGFR as possible mediators of CAF-induced EPI proliferation. Samples of C4-HD and C4-HI were processed for western blotting. An increase in FGFR 1-4 expression (p<0.05) was observed in C4-HI tumors and in MPA-treated as compared with untreated C4-HD tumors. Co-immunoprecipitation studies with p-tyr or FGFR antibodies and further blotting indicated that MPA induced FGFR activation. Immunohistochemical and immunofluorescence studies using paraffin-embedded or frozen tissues respectively showed FGFR expression mainly in EPI. FGFR nuclear staining was a common finding. Studies performed in a tissue microarray in which 5 HD tumors and their respective HI variants were included indicate that all MPA-induced carcinomas express high levels of FGFR, mainly FGFR2 and FGFR3. To further evaluate the functionality of FGFR2 and FGFR3, EPI C4-HI were transfected with their respective siRNA (Santa Cruz Biotech) and grown in co-cultures. The blockage of the corresponding FGFR was confirmed by Western blots. Co-cultures of CAF-HI together with wild type EPI-HI induced a significant increase in EPI proliferation (3H-thymidine uptake) which was not observed when siRNA-FGFR2, or siRNA-FGFR3 EPI transfected cells were used. BrdU labeling together with cytokeratin staining confirmed that the decrease in cell proliferation was specific for the EPI. In summary, our results confirm an active CAF participation in the HI phenotype in mammary tumors and point to FGFR 2 and 3 as key players mediating CAF-induced HI tumor growth.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA