Characterization of a soluble KGF receptor cDNA from human corneal and breast epithelial cells

PURPOSE. Keratinocyte growth factor (KGF) is a member of the fibroblast growth factor (FGF) family FGF-7. It exhibits potent mitogenic activity for epithelial cells, including corneal and mammary epithelial cells. A messenger RNA has been reported that is generated by alternative splicing of bek that putatively codes only for the extracellular ligand-binding domain of KGF receptor (soluble KGF receptor). In the present study, the expression of the mRNA coding for this alternative bek transcript was examined and the corresponding protein characterized. METHODS. Alternative messenger RNA transcripts were detected in various cell lines or tissues using reverse transcription-polymerase chain reaction (RT-PCR) and RNase protection assay. NIH/3T3 fibroblast cells and 293 kidney embryonic epithelial cells were stably transfected with soluble KGF receptor cDNA and transmembrane KGF receptor cDNA. Soluble KGF receptor protein was produced using a baculovirus- insect expression system. Soluble KGF receptor protein was detected using western and dot blot analyses. Binding assays and cross-linking labeling were used to determine the affinity anti specificity of soluble KGF receptor. A mitogenic assay was performed to examine the function of the soluble KGF receptor. RESULTS. The soluble KGF receptor mRNA was primarily expressed in epithelial cells, including cells from the cornea and breast. Cross-linking labeling and affinity-binding assays with ¹²⁵I-KGF showed that the soluble KGF receptor bound KGF (FGF-7) but not FGF-1 or FGF-2. Soluble KGF receptor was detected in the culture medium of cells stably transfected with soluble KGF receptor cDNA but not with transmembrane KGF receptor cDNA, suggesting that the soluble receptor was generated by mRNA splicing and probably not by proteolysis or posttranslational processing. Soluble KGF receptor inhibited KGF binding to transmembrane KGF receptor and DNA synthesis in BALB/MK epidermal keratinocytes in response to KGF, suggesting that soluble KGF receptor expression could provide a mechanism for the cell to downregulate responses to KGF. CONCLUSIONS. A truncated soluble KGF receptor expressed in corneal and other epithelial cells probably functions to downregulate the response of the cell to KGF.