Transcriptional regulation of a binding protein for FGF (FGF-BP) through growth factor signaling
A necessary event for tumor growth and metastases is the process of forming new blood vessels, also called angiogenesis. The fibroblast growth factor-binding protein (FGF-BP) binds and activates fibroblast growth factors (FGFs) from the extracellular matrix, and has a rate-limiting role in tumor angiogenesis. FGF-BP expression is upregulated in squamous cell carcinoma by treatment with phorbol esters such as TPA or mitogens such as the epidermal growth factor (EGF). The focus of this thesis is to better understand the mechanisms by which FGF-BP transcription is regulated by growth factor-induced signaling in cancer.
The ability of fetal bovine serum was demonstrated to upregulate the expression of FGF-BP mRNA in ME-180 squamous cell carcinoma cells via an EGF receptor-independent, p38 MAPK-dependent signaling pathway and through CCAAT/enhancer-binding mediated-transcription. In addition, FGF-BP was expressed in invasive human breast cancer samples and in the MCF-7/Adr and MDA-MB-468 human breast cancer cell lines. In the MDA-MB-468 cell line, FGF-BP was upregulated by treatment with EGF being dependent on PKC and p38 MAPK signaling, and the AP-1 and C/EBP sites on the FGF-BP promoter. Deletion of the C/EBP site resulted in an increase in FGF-BP promoter basal activity in these cells. These data suggest that signaling through p38 MAPK and C/EBP are universal mechanisms by which FGF-BP transcription is regulated.
C/EBPbeta exists in two forms, LIP (inhibitory) and LAP (activating), translated from the same mRNA. In comparison to ME-180 SCC cells, MDA-MB-468 cells expressed higher levels of C/EBPbeta-LIP, with a C/EBPbeta-LAP/LIP ratio approaching 1. Overexpression of C/EBPbeta-LAP in MDA-MB-468 cells resulted in an increase in FGF-BP promoter basal activity of 80 fold, which was reversed when coexpressed with increasing concentrations of LIP. Inhibition of p38 MAPK was able to differentially regulate the binding of C/EBPbeta dimers to the C/EBP site in an EGF-dependent manner. These data suggest that C/EBPbeta plays a major role in the transcriptional regulation of FGF-BP, and that changes in LAP:LIP ratios results in the modulation of FGF-BP expression in breast cancer cells. C/EBPbeta may represent a prime target through which FGF-BP expression can be regulated in breast cancer.