Pathway: Assembly of active LPL and LIPC lipase complexes

Reactions in pathway: Assembly of active LPL and LIPC lipase complexes :

Assembly of active LPL and LIPC lipase complexes

Lipoprotein lipase (LPL) and hepatic triacylglycerol lipase (LIPC) enzymes on the lumenal surfaces of capillary endothelia mediate the hydrolysis of triglyceride molecules in circulating lipoprotein particles.
LPL is widely expressed in the body and is especially abundant in adipocytes and skeletal and cardiac myocytes. Activation of the protein requires glycosylation, dimerization, and glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1), which delivers it to heparan sulfate proteoglycan (HSPG) associated with the plasma membrane. It is inactivated by proteolytic cleavage (Berryman & Bensadoun 1995; Sukonina et al. 2006; Young et al. 2011).
Expression of the LPL gene is transcriptionally regulated by Cyclic AMP-responsive element-binding protein 3-like protein 3 (CREB3L3), which also regulates the expression of APOA4, APOA5, APOC2, CIDEC and FGF21 (Lee et al. 2011).
Maturation of LIPC enzyme requires association with LMF1 protein (or possibly, inferred from sequence similarity, LMF2). Heparin binding stabilizes LIPC in its active dimeric form (Babilonia-Rosa & Neher 2014; Ben-Zeev et al. 2011).

Plasma lipoprotein assembly, remodeling, and clearance

Because of their hydrophobicity, lipids are found in the extracellular spaces of the human body primarily in the form of lipoprotein complexes. Chylomicrons form in the small intestine and transport dietary lipids to other tissues in the body. Very low density lipoproteins (VLDL) form in the liver and transport triacylglycerol synthesized there to other tissues of the body. As they circulate, VLDL are acted on by lipoprotein lipases on the endothelial surfaces of blood vessels, liberating fatty acids and glycerol to be taken up by tissues and converting the VLDL first to intermediate density lipoproteins (IDL) and then to low density lipoproteins (LDL). IDL and LDL are cleared from the circulation via a specific cell surface receptor, found in the body primarily on the surfaces of liver cells. High density lipoprotein (HDL) particles, initially formed primarily by the liver, shuttle several kinds of lipids between tissues and other lipoproteins. Notably, they are responsible for the so-called reverse transport of cholesterol from peripheral tissues to LDL for return to the liver.

Three aspects of lipoprotein function are currently annotated in Reactome: chylomicron-mediated lipid transport, LDL endocytosis and degradation, and HDL-mediated lipid transport, each divided into assembly, remodeling, and clearance subpathways.

Transport of small molecules

By definition cells have a critical separation between inner (cytoplasmic) and outer (extracellular) compartments. This separation provides for protection, gradient assembly, and environmental control but at the same time isolates the interior compartments of the cell from energy resources, oxygen, and raw materials. Cells have evolved a myriad of mechanisms to regulate, and enable transportation of small molecules ascross plasma membranes and between cellular organelle compartments within cells.