Directionally specific paracrine communication mediated by epithelial FGF9 to stromal FGFR3 in two-compartment premalignant prostate tumors

Tissue homeostasis in normal prostate and two-compartment nonmalignant prostate tumors depends on harmonious two-way communications between epithelial and stromal compartments. Within the fibroblast growth factor (FGF) family, signaling to an epithelial cell-specific FGF receptor (FGFR) 2IIIb-heparan sulfate complex from stromal-specific FGF7 and FGF10 delivers directionally specific instruction from stroma to epithelium without autocrine interference. Using a two-compartment transplantable prostate tumor model in which survival of stromal cells in vivo depends on epithelial cells, we show that signaling from epithelial FGF9 to stromal FGFR3 potentially mediates epithelial-to-stromal communication that also is directionally specific. FGF9 mRNA was expressed exclusively in the epithelial cells derived from well-differentiated, two-compartment Dunning R3327 rat prostate tumors. In contrast, FGFR3 was expressed at functionally significant levels only in the derived stromal cells. Competition binding and immunoprecipitation assays revealed that FGF9 only bound to an FGFR on the stromal cells. FGF9 also failed to covalently cross-link to clonal lines of stromal cells devoid of FGFR3 that expressed FGFR1 and FGFR2IIIc. Furthermore, FGF9 specifically stimulated DNA synthesis in stromal cells expressing FGFR3. These results demonstrate a directionally specific paracrine signaling from epithelial FGF9 and stromal FGFR3. Similar to the FGF7/FGF10 to FGFR2IIIb signaling from the stroma to the epithelium, the directional specificity from epithelium to stroma appears set by a combination of cell-specific expression of isoforms and cell-context specificity of FGFR isotypes for FGF.