PLASMA-MEMBRANE FORM OF PHOSPHATIDATE PHOSPHOHYDROLASE - A POSSIBLE ROLE IN SIGNAL-TRANSDUCTION DURING LIVER FIBROGENESIS

1. Several growth factors important in liver regeneration and fibrosis stimulate phospholipase D in plasma membranes via a receptor/G-protei n-coupled mechanism resulting in hydrolysis of phosphatidylcholine to phosphatidate. Phosphatidate can be further hydrolysed to diacylglycer ol by phosphatidate phosphohydrolase. Phosphatidate and diacylglycerol can act as 'second-messengers' and regulation of phosphatidate phosph ohydrolase activity could control the balance between them. 2. A form of phosphatidate phosphohydrolase, located in the plasma membrane and insensitive to inhibition by N-ethylmaleimide, has recently been ident ified that is distinct from the 'metabolic' form, which is present in the cytosol and microsomes and is sensitive to N-ethylmaleimide. 3. We have investigated the hypothesis that the balance between regeneratio n and fibrosis is, in part, determined by the activity of plasma membr ane phosphatidate phosphohydrolase through its effect on the phosphati date/diacylglycerol ratio. N-Ethylmaleimide-insensitive and -sensitive phosphatidate phosphohydrolase activities were measured in three hepa tic conditions characterized by regeneration and/or fibrosis: alcoholi c liver disease in humans (regeneration and fibrosis) and rat livers a fter either acute CCl-4-induced injury (regeneration) or common bile d uct ligation (fibrosis). 4. In patients with alcoholic liver disease, N-ethylmaleimide-insensitive phosphatidate phosphohydrolase activity w as higher in cirrhotic biopsies (5.82 +/- 0.3 nmol of P(i) min-1 mg-1 of protein, n = 19) than in non-cirrhotic biopsies (2.17 +/- 0.2, n = 23) or in wedge biopsies from healthy subjects undergoing routine chol ecystectomy (2.16 +/- 0.5, n = 6). N-Ethylmaleimide-insensitive phosph atidate phosphohydrolase activity was unchanged in the 10 days after C Cl4 treatment but increased progressively in common bile duct-ligated rats (e.g. day 28: 'sham' operation, 1.97 +/- 0.3, chronic bile duct l igation, 6.91 +/- 1.24 nmol of P(i) min-1 mg-1 of protein). N-Ethylmal eimide-insensitive phosphatidate phosphohydrolase activity correlated closely with the degree of fibrosis in humans and rats. N-Ethylmaleimi de-sensitive phosphatidate phosphohydrolase activity was unchanged aft er CCl4 treatment or common bile duct ligation and was not increased i n cirrhotic livers. 5. Plasma membrane N-ethylmaleimide-insensitive ph osphatidate phosphohydrolase increases in liver fibrosis but not regen eration. Stimulation of phosphatidate phosphohydrolase activity with i ts effect on the diacylglycerol/phosphatidate ratio may play a role in transduction of the fibrosis signal.