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

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 (1 alpha and 1 beta), -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-1 alpha or -1 beta increased the synthesis and secretion of [H-3] glycerol-labeled lipids under either basal-or oleate-supplemented conditions. In the presence of oleate, the increased TAG secretion was mainly associated with VLDL1 (S-f > 100) and VLDL2 (S-f 20-100). Expression of lipin-1 alpha or -1 beta increased secretion efficiency and decreased intracellular degradation of [S-35] apolipoprotein B-100 (apoB100). Knockdown of lipin-1 using specific short interfering RNA decreased secretion of [H-3] glycerolipids and [S-35] 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-1a not only abolished nuclear localization but also resulted in impaired association with microsomal membranes. Cells expressing the cytosolic lipin-1 alpha mutant failed to promote [S-35] apoB100 synthesis or secretion, and showed compromised stimulation in [H-3] TAG synthesis and secretion. Thus, alteration in hepatic expression of lipin-1 and its compartmentalization control VLDL assembly/secretion.-Bou Khalil, M., M. Sundaram, H-Y. Zhang, P. H. Links, J. F. Raven, B. Manmontri, M. Sariahmetoglu, K. Tran, K. Reue, D. N. Brindley, and Z. Yao. The level and compartmentalization of phosphatidate phosphatase-1 (lipin-1) control the assembly and secretion of hepatic VLDL. J. Lipid Res. 2009. 50: 47-58.