GPIHBP1 and ANGPTL4 Utilize Protein Disorder to Orchestrate Order in Plasma Triglyceride Metabolism and Regulate Compartmentalization of LPL Activity

Abstract

Intravascular processing of triglyceride-rich lipoproteins (TRLs) is crucial for delivery of dietary lipids fueling energy metabolism in heart and skeletal muscle and for storage in white adipose tissue. During the last decade, mechanisms underlying focal lipolytic processing of TRLs along the luminal surface of capillaries have been clarified by fresh insights into the functions of lipoprotein lipase (LPL); LPL’s dedicated transporter protein, glycosylphosphatidylinositol-anchored high density lipoprotein–binding protein 1 (GPIHBP1); and its endogenous inhibitors, angiopoietin-like (ANGPTL) proteins 3, 4, and 8. Key discoveries in LPL biology include solving the crystal structure of LPL, showing LPL is catalytically active as a monomer rather than as a homodimer, and that the borderline stability of LPL’s hydrolase domain is crucial for the regulation of LPL activity. Another key discovery was understanding how ANGPTL4 regulates LPL activity. The binding of ANGPTL4 to LPL sequences adjacent to the catalytic cavity triggers cooperative and sequential unfolding of LPL’s hydrolase domain resulting in irreversible collapse of the catalytic cavity and loss of LPL activity. Recent studies have highlighted the importance of the ANGPTL3–ANGPTL8 complex for endocrine regulation of LPL activity in oxidative organs (e.g., heart, skeletal muscle, brown adipose tissue), but the molecular mechanisms have not been fully defined. New insights have also been gained into LPL–GPIHBP1 interactions and how GPIHBP1 moves LPL to its site of action in the capillary lumen. GPIHBP1 is an atypical member of the LU (Ly6/uPAR) domain protein superfamily, containing an intrinsically disordered and highly acidic N-terminal extension and a disulfide bond–rich three-fingered LU domain. Both the disordered acidic domain and the folded LU domain are crucial for the stability and transport of LPL, and for modulating its susceptibility to ANGPTL4-mediated unfolding. This review focuses on recent advances in the biology and biochemistry of crucial proteins for intravascular lipolysis.

Electrostatic sheathing of lipoprotein lipase is essential for its movement across capillary endothelial cells

“…In the current study, we show that the prevention of LPL-HSPG interactions by GPIHBP1’s AD is required for trafficking of GPIHBP1-LPL complexes across capillary ECs. The ability of HSPGs to influence receptor-ligand interactions has been recognized for years ( 33 – 35 ), but the role of HSPGs in intravascular lipolysis stands as one of the best understood, in part because GPIHBP1-LPL interactions are accessible to both biophysical analyses ( 20 ) and experimental physiology studies in mouse models ( 1 , 2 , 4 , 17 ).…”

“…GPIHBP1’s AD is not required for LPL binding; a mutant human GPIHBP1 lacking the AD forms stable interactions with LPL ( 8 , 19 ). However, biophysical studies with purified proteins raised the possibility that the AD could be important for TG metabolism ( 10 , 20 ). First, surface plasmon resonance (SPR) studies revealed that the AD of human GPIHBP1 accelerates interactions between GPIHBP1 and LPL despite having little or no effect on the stability of the binding (i.e., negligible effects on the off-rate; ref.…”

Role of Endothelial Cell Lipoprotein Lipase for Brown Adipose Tissue Lipid and Glucose Handling

“…LPL produced by activated brown adipocytes controls BAT TRL disposal, as shown in the present study using brown adipocyte-specific knockout, and previously using adipocyte-specific knockouts ( Bartelt et al, 2013 ). Another reason might be, that LPL secretion is less efficient in endothelial cells than in adipocytes, which has been shown to be sensitive to intracellular disturbances requiring specific chaperones and tight redox regulation ( Kristensen et al, 2021 ).…”

Targeting the Urokinase-Type Plasminogen Activator Receptor (uPAR) in Human Diseases With a View to Non-invasive Imaging and Therapeutic Intervention

“…Of note, human uPAR deviates from the ancestral LU domain consensus motif inasmuch it contains three consecutive LU domains and that its N-terminal domain lacks one of the five plesiotypic disulfide bond ( Figure 1A )—a feature shared among all known mammalian orthologues of uPAR. This is indeed remarkable, as that disulfide bond connecting cysteine 7 and 8 is essential for the correct folding and stability of single LU domain proteins such as SLURP-1 ( Adeyo et al, 2015 ), GPIHBP1 ( Beigneux et al, 2015 ; Kristensen et al, 2021 ), CD59 ( Petranka et al, 1996 ), and κ-bungarotoxin ( Grant et al, 1998 ). Akin to uPAR, other multidomain members of the LU gene superfamily (e.g., Haldisin, C4.4A, TEX101) also lack this particular disulfide bond, but notably only in their N-terminal LU domain ( Kjaergaard et al, 2008 ; Gårdsvoll et al, 2013 ; Jiang et al, 2020 ; Masutani et al, 2020 ).…”