The level and compartmentalization of phosphatidate phosphatase-1 (lipin-1) control the assembly and secretion of hepatic VLDLs⃞

Abstract

Phosphatidate phosphatase-1 (PAP-1) converts phosphatidate to diacylglycerol and plays a key role in the biosynthesis of phospholipids and triacylglycerol (TAG). PAP-1 activity is encoded by members of the lipin family, including lipin-1 (1α and 1β), -2, and -3. We determined the effect of lipin-1 expression on the assembly and secretion of very low density lipoproteins (VLDL) using McA-RH7777 cells. Expression of lipin-1α or -1β increased the synthesis and secretion of [3H]glycerol-labeled lipids under either basal- or oleate-supplemented conditions. In the presence of oleate, the increased TAG secretion was mainly associated with VLDL1 (Sf > 100) and VLDL2 (Sf 20–100). Expression of lipin-1α or -1β increased secretion efficiency and decreased intracellular degradation of [35S]apolipoprotein B-100 (apoB100). Knockdown of lipin-1 using specific short interfering RNA decreased secretion of [3H]glycerolipids and [35S]apoB100 even though total PAP-1 activity was not decreased, owing to the presence of lipin-2 and -3 in the cells. Deletion of the nuclear localization signal sequences within lipin-1α not only abolished nuclear localization but also resulted in impaired association with microsomal membranes. Cells expressing the cytosolic lipin-1α mutant failed to promote [35S]apoB100 synthesis or secretion, and showed compromised stimulation in [3H]TAG synthesis and secretion. Thus, alteration in hepatic expression of lipin-1 and its compartmentalization control VLDL assembly/secretion.

Keywords

• fatty liver dystrophy
• hepatosteatosis
• triacylglycerol
• hypertriglyceridemia
• diabetes
• dexamethasone