Investigating the contribution of FGF receptors in breast tumorigenesis using models of mammary cancer

Breast cancer, the main cancer in women, occurs in approximately 27% of all yearly diagnosed cancer cases. It is estimated that one out of eight women will develop breast cancer in her lifetime. Therefore it is not surprising that this disease ranks second as a cause of cancer death in women, after lung cancer (American Cancer Society, Cancer Facts and Figures 2009). Based on these numbers, many studies have been undertaken in the area of breast cancer and since 1990, death rates from breast cancer have been decreasing, mainly due to earlier detection and improved treatments. Since the 1970s, hormonal therapies targeting the estrogen receptor have been very successful for treatment of estrogen receptor positive (ER+) breast cancers that consist of about 60% of the cases. More recently, therapies targeting the ErbB2 receptor tyrosine kinase that is overexpressed in about 20% of breast cancer have shown to be of benefit for this subset of breast cancer patients. However, not every patient responds to these treatments or patients become resistant, thus for both therapies there is a high risk of relapse. Consequently, novel therapies are required and will likely arise from an improved understanding of the disease biology.
Recent studies have shown that members of the fibroblast growth factor receptor (FGFR) family of tyrosine kinase receptors are deregulated in breast cancer. Indeed FGFR1 gene is amplified in 9% of breast cancers and single nucleotide polymorphisms in FGFR2 gene are strongly associated with an increased probability to develop breast cancer. Using models of breast carcinoma (4T1, 4TO7, 168FARN and 67NR cell lines), our study aimed at a better understanding of how FGFRs contribute to breast tumorigenesis. Furthermore we analysed the effects of blocking the activity of these receptors in cell lines and primary tumors.
In vitro, we showed that 4T1, 4TO7, 168FARN and 67NR cell lines co-express FGF ligands and FGFRs, suggesting an autocrine loop activating FGFRs which leads to the constitutive basal activation of downstream signaling pathways, mainly MAPK and PI3K/AKT. Using TKI258, a tyrosine kinase inhibitor blocking FGFRs, we inhibited the basal activity of FGFRs. This inhibition resulted in a decrease of ERK1/2 and AKT activity, confirming that FGFR signaling maintains the activity of MAPK and PI3K/AKT pathways. Furthermore, interfering with autocrine FGFR signaling dramatically impaired proliferation of the four cell lines and apoptotic cell death was observed in 4T1 and 4TO7. Using constitutively active mutants, we demonstrated that Ras and AKT contribute to cell survival downstream of FGFRs and that 4T1 cells expressing these constructs are partially rescued from the effects of TKI258. Interestingly, when we combined inhibitors of MEK (UO126) or PI3K (LY294002) with TKI258, we increased the sensitivity of 4T1 cells to TKI258 induced cell death.
In vivo, we describe that TKI258 treatment inhibits FGFR signaling in mice bearing 4T1-induced tumors. In addition, daily oral treatment of mice bearing 4T1- or 67NR- induced tumors with TKI258 over 14 days, significantly reduces tumor outgrowth and decreases 4T1 lung metastasis, showing that blockade of FGFR has strong anti-tumor and anti-metastatic activities.
Two microarray analyses performed on treated 4T1 cells or 4T1 tumors, led to the identification of genes that were regulated after TKI258 treatment. Detailed analyses showed that some of these genes were known to contribute to the metastatic process (matrix metalloproteinases and extracellular matrix proteins) or to cell cycle progression (cyclins and E2F transcription factors). Comparison of these TKI258-regulated genes with publicly available databases of breast cancer patients identified a cohort of patients showing overexpression of genes down-regulated upon TKI258 treatment, and these patients have a higher probability of metastatic disease compared to the other patients. These highly expressed genes might therefore reflect activation of receptor tyrosine kinase signaling pathways like FGFR in 4T1 tumors.
In summary our results show that targeting FGFRs using a TKI has an impact on various biological characteristics of FGFR driven models of breast cancer, including proliferation and survival. In addition, our observations show that blockade of FGFR signaling can be achieved in vivo and this leads to reduction of tumor outgrowth as well as decreased metastases formation. Finally, our meta-analysis on genes that are changed in 4T1 treated cells and tumors provides evidences that results obtained using animal models of a diseases are meaningful in terms of prognostic and can be translated to breast cancer patients.