Purpose: The purpose of this study was to characterize the expression of hepatocyte growth factor (HGF) and HGF receptor proteins in lacrimal gland, tears, and cornea.
Methods: The reverse transcription-polymerase chain reaction method was used to detect HGF and HGF receptor messenger RNA in human lacrimal gland tissue. HGF and HGF (c-met) receptor monoclonal antibody specificity was demonstrated with fluorescent antibody sorting of cells engineered to express HGF or HGF receptor compared with control cell lines, enzyme-linked immunoadsorbent assay (ELISA), immunoprecipitation, and immunohistology with preabsorption. Immunohistochemistry was applied to study the distribution of HGF and HGF receptor expression in rabbit lacrimal gland tissue and in wounded and unwounded rabbit cornea. An ELISA was used to detect HGF in pooled samples of human tears and individual aliquots of tears collected from patients 1 day after anterior segment surgery.
Results: Amplification products of the expected size for HGF and HGF receptor mRNAs were detected in lacrimal tissue and were confirmed to be specific by hot blotting and nucleic acid sequencing. Hepatocyte growth factor protein was detected in interalveolar and interlobular connective tissue cells adjacent to glandular alveolar (acinar) cells and associated with the cells lining the interlobular ducts. Hepatocyte growth factor receptor protein was expressed in the glandular alveolar and interlobular ductal cells in the lacrimal gland and all three cell types of the cornea. It was detected in keratocyte and endothelial cells, and expression was increased in keratocytes after epithelial wounding. Hepatocyte growth factor was not present in corneal epithelial cells, but in the unwounded cornea a strong signal was associated with the epithelial cell surface. It was detected by ELISA in pooled normal tears at levels 186 to 290 pg/ml and in individual postoperative tear samples at 453 to 619 pg/ml. In some tear samples, HGF levels were below the sensitivity of the assay (97.5 pg/ml).
Conclusions: The distribution of HGF receptor protein expression in the lacrimal gland suggests that HGF secreted by interalveolar connective tissue cells traverses the acinar cells and modulates functions in acinar and ductal epithelial cells. Hepatocyte growth factor likely collects within the interlobular ducts and becomes a component in normal tears. Thus, lacrimal gland HGF probably modulates corneal epithelial cell proliferation, motility, and differentiation. Its expression in keratocytes is upregulated after corneal epithelial wounding and probably contributes to the epithelial wound healing process.