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HGF- and KGF-induced activation of PI-3K/p70 s6 kinase pathway in corneal epithelial cells: its relevance in wound healing.

In this study we have investigated the involvement of PI-3K and its downstream target p70 S6K in the signaling response of corneal epithelial cells after HGF and KGF stimulation. HGF induced three- to five-fold increase in PI-3K activity in 5-10 min, whereas KGF stimulation resulted in two- to three-fold increase in activity in 2-10 min. Both growth factors also caused the phosphorylation of p70 S6K and stimulation of its activity. HGF increased p70 S6K activity by 300% and KGF by about 200%. Protein kinase C (PKC) activator TPA also induced the phosphorylation of p70 S6K. Both the PI-3K inhibitor wortmannin and PKC inhibitor calphostin C blocked the phosphorylation of p70 S6K mediated by the growth factors. However, the mitogen-activated protein kinase (p42/44 MAPK) cascade inhibitor PD98059 had no effect on p70 S6K activation. Furthermore, HGF and KGF increased the rate of corneal epithelial wound healing in an organ culture model, and wortmannin and rapamycin (the p70 S6K inhibitor) blocked corneal epithelial wound healing promoted by the growth factors. These studies suggest that PI-3K and p70 S6K are important signal transducers in the stimulation of corneal epithelial cells by HGF and KGF. PKC is involved in the PI-3K-dependent activation of p70 S6K but not MAPK. Inhibition of wound closure by PI-3K and p70 S6K inhibitors suggests these enzymes play a significant role in corneal wound repair stimulated by HGF and KGF.