Acidic or basic fibroblast growth factors (FGF) are present at significant concentrations in most normal tissues in the adult, can stimulate angiogenesis and thus have the potential to play an important role during tumor growth and metastasis. However, both of these FGFs are immobilized in an inactive state on the extracellular matrix and it is only poorly understood how they are solubilized and activated to reach their extracellular receptors. One mechanism through which these growth factors can be mobilized is by binding to a secreted binding protein for FGF (BP) that was described by Wu et al in 1991. In recent work from our laboratory, we detected high levels of BP mRNA in majority of squamous cell cancer (SCC) samples from patients and in SCC cell lines in culture. On the other hand, we did not detect BP mRNA in normal adult human tissues or in normal adult rodent tissues as well as in a series of cultured cell lines that were not of squamous origin. In contrast with the lack of expression of BP in adult tissues, we found BP mRNA highly expressed in murine embryonic squamous epithelia of the lungs and skin during late gestation. In functional studies with non-tumorigenic cells (SW-13), we demonstrated that expression of BP can mobilize and activate bFGF leading to tumor growth and angiogenesis of the BP- transfected cells. Furthermore, in an SCC cell line expressing high levels of BP mRNA, reduction of BP expression using BP-targeted ribozymes reduced tumor growth and angiogenesis of xenografts of these cells in athymic nude mice. This suggests a potentially rate- limiting role of this protein for SCC tumor growth in vivo. In addition, we found that retinoids downregulate BP mRNA rapidly through a posttranscriptional mechanism. We propose the following experiments to study the role of BP in tumor growth as well as the mechanism(s) and therapeutic significance of its downregulation by retinoids:
Under AIM 1, we will express BP in cell lines which have a low tumorigenic potential and are negative for BP and study phenotypic changes. The expression of BP will be under the control of a tetracycline- regulated promoter that allows to regulate transfected BP in vitro or in vivo by administration of tetracycline.
Under AIM 2, we will use molecular targeting of BP mRNA with ribozymes to elucidate the contribution of BP to tumor growth of cell lines that express BP.
Under AIM 3, we will study the mechanism(s) of retinoid regulation of BP with respect to receptor subtype and the retinoid- dependent target site in the BP mRNA. Furthermore, we will study to what extent downregulation of BP by retinoids contributes to the effect of these drugs on SCC tumors.
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