The met proto-oncogene encodes the cell surface receptor for hepatocyte growth factor (HGF) and transmits its multifunctional signals such as regulation of cell proliferation, motility, and morphogenesis. These pleiotropic actions attributable to HGF are mainly reported on cells of epithelial derivation which express the Met receptor. The HGF gene, on the other hand, is expressed in mesenchymally derived cells including peripheral blood leukocytes. Recently, we reported that Met receptor gene expression in epithelial cells is induced by inflammatory cytokines; currently, however, little is known concerning Met gene expression in mesenchymal cells. In the present study, we have explored the role of Met expression during monocyte-macrophage differentiation using THP-1 cells, a monocytic cell line, and monocytes freshly isolated from normal human peripheral blood. We have found that untreated monocytes do not express Met mRNA and protein. Upon incubation with differentiation inducers such as 12-O-tetradecanoylphorbol-13-acetate, lipopolysaccharide, a combination of interleukin (IL) 6 plus tumor necrosis factor (TNF) alpha, or IFN-gamma plus TNF-alpha, a pronounced increase in the amounts of Met mRNA and protein are seen in THP-1 cells. The expression of Met appears to correlate with the onset of differentiation of monocytes as noted by changes in cell morphology and adherence to culture plates, and the increased accumulation of Met protein was observed only in cells that differentiated and adhered to the culture dish. Moreover, Met was found to be phosphorylated on tyrosine residues, indicating that the receptor is potentially involved in signal transduction events. Addition of exogenous HGF to the activated cells resulted in the suppression of cell proliferation and an increase in cell motility. Reverse transcription-PCR and Western blot analyses revealed that untreated THP-1 cells contain HGF transcript and protein, and that HGF expression is inducible by addition of the differentiation agents such as 12-O-tetradecanoylphorbol-13 acetate or IL-6 plus TNF-alpha. Immune serum that is specific for neutralizing HGF activity markedly inhibited monocyte differentiation (50% reduction in cell attachment and process formation) induced by IL-6 and TNF-alpha. Moreover, we also found that the mRNA for Ron, which encodes a tyrosine kinase receptor for HGF-like protein (also known as macrophage-stimulating protein), is induced in THP-1 cells during the course of their differentiation to macrophages by IFN-gamma plus TNF-alpha. These findings indicate that the HGF and Met families may indeed be physiological regulators of monocyte-macrophage differentiation/maturation.