Fibroblast growth factor-8 regulation of breast cancer cell growth

Abstract: Expression of fibroblast growth factor-8 (FGF-8) is increased in a large number of breast cancers. In in vitro and in vivo breast cancer models FGF-8 promotes growth, invasion and angiogenesis. In the present work we have studied the mechanisms by which FGF-8b and its receptors (FGFR1-3) promote breast cancer cell growth using S115 murine and human MCF-7 breast cancer cell lines in in vitro and in vivo models. We have also examined FGF-8 protein level together with FGFR2, and the androgen receptor by means of immunohistochemical staining of tumour tissues from 144 women diagnosed with invasive breast cancer. We found that FGF-8b induces cell cycle progression and proliferation by upregulating cyclin D1 via the MAPK/ERK, PI3K/Akt and p38/MAPK pathways in breast cancer cells. Moreover, we found that FGF-8b promotes growth and survival of breast cancer cells by protecting against cell death. FGF-8b-regulated breast cancer cell growth was further characterised by gene expression profiling of breast cancer cells treated with FGF-8b, from 1 to 24 hours. The most up- or downregulated genes in response to FGF-8b treatment mainly belonged to the functional categories and pathways related to mitosis, cell cycle regulation, cancer, and cell death. A number of key regulatory genes of cell cycle and mitosis including Btg2, Plk1 and aurora A were identified as novel targets for FGF-8b. The role of FGF receptors 1-3 in breast cancer cell growth was studied in three different cell lines silenced for expression of FGFR1, 2 and 3, respectively. Studies on growth and signalling of the FGFR1-3 silenced cell lines in vitro as well as in nude mouse tumours revealed differential roles of FGFR1 and FGFR2 in this breast cancer cell model. While FGFR1 strongly promoted growth, FGFR2 seemed to have tumour suppressing functions. In human breast cancer specimens FGF-8 and FGFR2 protein expression was shown to correlate negatively with each other. We also found downregulation of FGFR2 protein expression by FGF-8b in vitro in breast cancer cells. In accordance with these results, FGFR2 expression correlated negatively with tumour size. These results further suggest a tumour suppressor function for FGFR2. In conclusion, FGF-8 and its receptors primarily regulate breast cancer cell proliferation and survival. The novel targets and mechanisms identified may be of importance in developing therapies against breast cancer.