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Serum from partially hepatectomised rats promoted DNA synthesis in primary adult rat hepatocyte cultures. If the rats had been exposed to sub-lethal γ-irradiation immediately following operation or if their serum, collected at 3 h, was exposed to irradiation in vitro, the growth-promoting activity was destroyed. Prostaglandin E 2also stimulated DNA synthesis in the cultures; if PGE 2was irradiated in serum from intact or partially hepatectomised rats its growth-promoting activity was markedly diminished.

The hepatocyte growth factor/scatter factor (HGF/SF) receptor which is a transmembrane protein encoded by the Met oncogene, possesses intrinsic tyrosine kinase activity which transduces the mitogenic, morphogenic and the scattering effect of HGF/SF. The pluripotent signal of HGF/SF is transduced through association of the Met receptor with various intracellular adaptors. Phosphorylation of cytosolic phospholipase A2 (cPLA2) is associated with activation of this molecule which in turn leads to arachidonic acid production followed by release of prostaglandins and related compounds exerting their roles onto cell proliferation, chemotaxis and vascular motility. Arachidonic acid and its metabolites were shown to be involved in processes like liver regeneration where growth factor receptors possessing tyrosine kinase activity are implicated. In this study we examined whether stimulation of the HGF/SF-receptor’s tyrosine kinase activity would involve changes in the phosphorylation state and the activity of cPLA2 in MDCK cells, where HGF/SF is known to induce scattering responses rather than mitogenesis. The activated p145βMET was shown to associate with and to phosphorylate cPLA2on tyrosine residues, this leading to subsequent release of arachidonic acid. cPLA2 was also phosphorylated in serine residues and such a role has been so far assigned to the mitogen activated protein (MAP) kinase. Our data have also shown that MAP kinase is associated and phosphorylated on tyrosine by the activated p145βMET Immunodepletion of MAP kinase via electroporation of an anti-MAP kinase antibody, did not significantly decrease arachidonic acid release in HGF/SF-stimulated MDCK cells. It is therefore emerging that phosphorylation of cPLA2 on tyrosine by the HGF/SF receptor kinase is capable of triggering arachidonic acid release and that MAP kinase is contributing to full, but does not drive, the activity of cPLA2. The release of arachidonic acid by MDCK cells following HGF/SF stimulation is establishing this fatty acid and its metabolites as major components involved in the transduction of MET-driven signals and at the same time in the amplification of such signals.

Serum obtained from partially hepatectomized rats 1, 3 or 24 h after operation was more effective in stimulating DNA synthesis in primary adult rat hepatocytes than serum from sham-operated rats; exposure to the serum for 2h was sufficient to promote growth. Serum from the partially hepatectomized rats contained elevated levels of PGE2 and PGF2α; it promoted hepatocytes to release prostaglandins into their culture medium. Growthpromoting effects of the serum and its capacity to elicit prostaglandin release into the culture medium were inhibited by 0*1 mM-indomethacin or 1 mM-aspirin. 0–1 mM-indomethacin also prevented DNA synthesis if the inhibitor were added 4h after growth had been initiated by serum from partially hepatectomized rats, suggesting that prostaglandins continue to be important for the maintenance of hepatocyte growth for at least 6h.