title: Effects of sphingosine, albumin and unsaturated fatty acids on the activation and translocation of phosphatidate phosphohydrolases in rat hepatocytes.
abstract: The activities of two phosphatidate phosphohydrolases were measured in cultured rat hepatocytes incubated with 0.1 mM albumin. The activity, which is inhibited by N-ethylmaleimide (PAP-1) is located in the cytosolic and membrane fractions. PAP-1 activity is stimulated by Mg2+ and it can be translocated from the cytosol to the membranes by relatively low (0.5-1 mM) concentrations of fatty acids. In addition, higher concentrations (1-3 mM) of fatty acids cause an increase in the total PAP-1 activity. Translocation of PAP-1 activity in the hepatocytes is preferentially promoted by unsaturated fatty acids (C18:1, C18:2, C18:3, C20:4 and C20:5), rather than by saturated acids (C14:0, C16:0, C18:0). Increasing the extracellular concentration of albumin from 30 microM to 1 mM displaces PAP-1 activity from the membrane fraction. Sphingosine, but not staurosporine, can inhibit the redistribution of PAP-1 activity induced by oleate. The amphiphilic amines, sphingosine, chlorpromazine and propranolol, also decrease membrane-bound PAP-1 activity in the absence of fatty acids, but they do not alter, significantly, the activity of the cytosolic PAP-1. In the presence of 1 mM oleate, sphingosine, chlorpromazine and propranolol decrease the translocation of PAP-1 from the cytosol to the membranes. The phosphohydrolase activity, which is insensitive to N-ethylmaleimide (PAP-2), is specifically located in the plasma membrane (Jamal, Z., Martin, A., Gomez-Muñoz, A. and Brindley, D.N. (1991) J. Biol. Chem. 266, 2988-2996) and it is not stimulated by Mg2+. Saturated fatty acids, albumin, sphingosine and propranolol have no significant effects on PAP-2 activity. However, chlorpromazine decreases PAP-2 activity by about 14%. Linolenate, arachidonate and eicosapentaenoate at 1 mM also produced small (7-10%) decreases in PAP-2 activity. It is proposed that both PAP-1 and PAP-2 activities may be involved in signal transduction, although the main function of PAP-1 seems to be involved in the synthesis of glycerolipids.
authors.affiliation: Department of Biochemistry, University of Alberta, Edmonton, Canada.
authors.forename: A
authors.initials: A
authors.lastname: Gomez-Muñoz
authors.forename: E H
authors.initials: EH
authors.lastname: Hamza
authors.forename: D N
authors.initials: DN
authors.lastname: Brindley
mesh_terms: D000418:Albumins / Q000494:pharmacology*; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002478:Cells, Cultured; D004789:Enzyme Activation / Q000187:drug effects; D005231:Fatty Acids, Unsaturated / Q000494:pharmacology*; D008099:Liver / Q000187:drug effects* / Q000201:enzymology; D008297:Male; D000073718:Pancreatitis-Associated Proteins; D010749:Phosphoprotein Phosphatases / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D013110:Sphingosine / Q000494:pharmacology*
chemical_list: D000418:Albumins; D005231:Fatty Acids, Unsaturated; D000073718:Pancreatitis-Associated Proteins; C000612141:REG3A protein, human; D010749:Phosphoprotein Phosphatases; D013110:Sphingosine