HGF signaling through c-Met receptor and regulation of pulmonary surfactant in NCI-H441 cells

c-Met is the putative receptor for Hepstocyte Growth Factor (HGF). Since HGF is released in injury, inhibits surfactant PC metabolism, and stimulates proliferation of type II cells, we used NCI-H441 cells to study its signaling and regulation of pulmonary surfactant Western blotting and immunoprecipitation of cell lysates showed that c-met is constitutively phosphorylated on tyrosines. This tyrosine phosphorylation increases after ligand stimulation in a dose-dependent manner, with increases of 160% and 185% of control at doses of 100 and 300 ng/ml, respectively. The increase is evident within 10 s of ligand addition, and maximal phosphorylation occurs at 1 min. Serine phosphorylation of c-met was not observed. When cells were stimulated with 10 ng/ml HGF, PKC activity was increased within 10 min with a corresponding translocation of alpha and zeta isoforms to the membrane, and this was maintained through 2 h. Cytidylyltransferase activity, the rate-limiting enzyme in PC synthesis, was decreased by 50% after 1 h, but returned to basal level after 3 h. However, there was no significant difference in the intracellular or extracellular pools of either SP-A or SP-D as determined by ELISA. The results indicate that although SP-A and SP-D pools are not affected by HGF, it may have an inhibitory effect on PC synthesis through down-regulation of cytidylyltransferase.