Phosphatidate degradation: phosphatidate phosphatases (lipins) and lipid phosphate phosphatases

Abstract

Three lipid phosphate phosphatases (LPPs) regulate cell signaling by modifying the concentrations of a variety of lipid phosphates versus their dephosphorylated products. In particular, the LPPs are normally considered to regulate signaling by the phospholipase D (PLD) pathway by converting phosphatidate (PA) to diacylglycerol (DAG). LPP activities do modulate the accumulations of PA and DAG following PLD activation, but this could also involve an effect upstream of PLD activation. The active sites of the LPPs are on the exterior surface of plasma membranes, or on the luminal surface of internal membranes. Consequently, the actions of the LPPs in metabolizing PA formed by PLD1 or PLD2 should depend on the access of this substrate to the active site of the LPPs. Alternatively, PA generated on the cytosolic surface of membranes should be readily accessible to the family of specific phosphatidate phosphatases, namely the lipins. Presently, there is only indirect evidence for the lipins participating in cell signaling following PLD activation. So far, we know relatively little about how individual LPPs and specific phosphatidate phosphatases (lipins) modulate cell signaling through controlling the turnover of bioactive lipids that are formed after PLD activation.

Figure 1. All mammalian lipin proteins exhibit PAP1 activity

Different lipins were overexpressed in HEK 293 cells and PAP1 activity was determined in cell lysates two days after transfection with the constructs [22]. Mg²⁺-dependent PAP activity was determined essentially as before except that the concentration of PA was kept constant at 200 μM and that of Triton X-100 was adjusted to provide the mol% of PA in Triton X100 that is quoted. The specific activity of PAP (relative to protein) in cells transfected with the empty vector was <1% of that for cells expressing lipin-1A and it was subtracted from all other values. The relative specific activity of the lipins was then calculated by normalizing to the corresponding Western blot of the V5 tagged [22]. The results demonstrate that each lipin has PAP activity and that the relative activities for lipin-1A and -1B are higher than for lipin-2 and lipin-3. Each lipin shows cooperative kinetics as expected from the yeast Pah1 [21]. We have not given kinetic constants since the kinetic analysis was performed with overexpressed, but impure enzymes, which are intrinsically labile.

Figure 2. Proposed roles of LPPs and lipins in the PLD signaling cascades

Green arrows indicate the stimulation of signaling targets and red lines indicate inhibition of signaling. cPLA₂, cytosolic PLA₂; PKC, protein kinase C; PI 4K, phosphatidylinositol 4-kinase; PLC, phospholipase C; SK-1, sphingosine kinase-1, SPP, specific sphingosine 1-phosphate phosphatases. The authors gratefully acknowledge support from the Canadian Institutes of Health Research to D.N.B (BC039) and the National Heart, Lung, and Blood Institute (HL-28481 and HL-90553) to K.R. D.N.B. is a recipient of a Medical Scientist Award from the Alberta Heritage Foundation for Medical Research.