👤 Michael Ploug

Apolipoprotein AV (APOA5) regulates intravascular triglyceride metabolism by binding to the angiopoietin-like protein 3/8 complex (ANGPTL3/8) and suppressing its ability to unfold the native conformation of lipoprotein lipase (LPL). LPL unfolding results in loss of catalytic activity and the detachment of LPL from the surface of cells. An Show less

Lipoprotein lipase (LPL) carries out the lipolytic processing of triglyceride-rich lipoproteins (TRL) along the luminal surface of capillaries. LPL activity is regulated by the angiopoietin-like proteins (ANGPTL3, ANGPTL4, ANGPTL8), which control the delivery of TRL-derived lipid nutrients to tissues in a temporal and spatial fashion. This regulation of LPL mediates the partitioning of lipid delivery to adipose tissue and striated muscle according to nutritional status. A complex between ANGPTL3 and ANGPTL8 (ANGPTL3/8) inhibits LPL activity in oxidative tissues, but its mode of action has remained unknown. Here, we used biophysical techniques to define how ANGPTL3/8 and ANGPTL3 interact with LPL and how they drive LPL inactivation. We demonstrate, by mass photometry, that ANGPTL3/8 is a heterotrimer with a 2:1 ANGPTL3:ANGPTL8 stoichiometry and that ANGPTL3 is a homotrimer. Hydrogen-deuterium exchange mass spectrometry (HDX-MS) studies revealed that ANGPTL3/8 and ANGPTL3 use the proximal portion of their N-terminal α-helices to interact with sequences surrounding the catalytic pocket in LPL. That binding event triggers unfolding of LPL's Show less

Movement of lipoprotein lipase (LPL) from myocytes or adipocytes to the capillary lumen is essential for intravascular lipolysis and plasma triglyceride homeostasis-low LPL activity in the capillary lumen causes hypertriglyceridemia. The trans-endothelial transport of LPL depends on ionic interactions with GPIHBP1's intrinsically disordered N-terminal tail, which harbors two acidic clusters at positions 5-12 and 19-30. This polyanionic tail provides a molecular switch that controls LPL detachment from heparan sulfate proteoglycans (HSPGs) by competitive displacement. When the acidic tail was neutralized in gene-edited mice, LPL remained trapped in the sub-endothelial spaces triggering hypertriglyceridemia. Due to its disordered state, the crystal structure of LPL•GPIHBP1 provided no information on these electrostatic interactions between LPL and GPIHBP1 acidic tail. In the current study, we positioned the acidic tail on LPL using zero-length crosslinking. Acidic residues at positions 19-30 in GPIHBP1 mapped to Lys Show less

Angiopoietin-like 3 (ANGPTL3) is a major regulator of lipoprotein metabolism. ANGPTL3 deficiency results in lower levels of triglycerides, LDL-cholesterol (LDL-C), and HDL-cholesterol (HDL-C), and may protect from cardiovascular disease. ANGPTL3 oligomerizes with ANGPTL8 to inhibit lipoprotein lipase (LPL), the enzyme responsible for plasma triglyceride hydrolysis. Independently of ANGPTL8, oligomers of ANGPTL3 can inhibit endothelial lipase (EL), which regulates circulating HDL-C and LDL-C levels through the hydrolysis of lipoprotein phospholipids. The N-terminal region of ANGPTL3 is necessary for both oligomerization and lipase inhibition. However, our understanding of the specific residues that contribute to these functions is incomplete. In this study, we performed mutagenesis of the N-terminal region to identify residues important for EL inhibition and oligomerization. We also assessed the presence of different ANGPTL3 species in human plasma. We identified a motif important for lipase inhibition, and protein structure prediction suggested that this region interacted directly with EL. We also found that recombinant ANGPTL3 formed a homotrimer and was unable to inhibit EL activity when trimerization was disrupted. Surprisingly, we observed that human plasma contained more monomeric ANGPTL3 than trimeric ANGPTL3. An important implication of these findings is that previous correlations between circulating ANGPTL3 and circulating triglyceride-rich lipoproteins need to be revisited. Show less

In peripheral tissues, an endothelial cell (EC) protein, GPIHBP1, captures lipoprotein lipase (LPL) from the interstitial spaces and transports it to the capillary lumen. LPL mediates the margination of triglyceride-rich (TG-rich) lipoproteins (TRLs) along capillaries, allowing the lipolytic processing of TRLs to proceed. TRL-derived fatty acids are used for fuel in oxidative tissues or stored in adipose tissue. In mice, GPIHBP1 is absent from capillary ECs of the brain (which uses glucose for fuel); consequently, LPL and TRL margination are absent in mouse brain capillaries. However, because fatty acids were reported to play signaling roles in the brain, we hypothesized that LPL-mediated TRL processing might occur within specialized vascular beds within the central nervous system. Here, we show that GPIHBP1 is expressed in capillary ECs of human and mouse choroid plexus (ChP) and that GPIHBP1 transports LPL (produced by adjacent ChP cells) to the capillary lumen. The LPL in ChP capillaries mediates both TRL margination and processing. Intracapillary LPL and TRL margination are absent in the ChP of Gpihbp1-/- mice. GPIHBP1 expression, intracapillary LPL, and TRL margination were also observed in the median eminence and subfornical organ, circumventricular organs implicated in the regulation of food intake. Show less

Lipoprotein lipase (LPL) and multiple regulators of LPL activity (e.g., APOC2 and ANGPTL4) are present in all vertebrates, but GPIHBP1-the endothelial cell (EC) protein that captures LPL within the subendothelial spaces and transports it to its site of action in the capillary lumen-is present in mammals but in not chickens or other lower vertebrates. In mammals, GPIHBP1 deficiency causes severe hypertriglyceridemia, but chickens maintain low triglyceride levels despite the absence of GPIHBP1. To understand intravascular lipolysis in lower vertebrates, we examined LPL expression in mouse and chicken hearts. In both species, LPL was abundant on capillaries, but the distribution of Lpl transcripts was strikingly different. In mouse hearts, Lpl transcripts were extremely abundant in cardiomyocytes but were barely detectable in capillary ECs. In chicken hearts, Lpl transcripts were absent in cardiomyocytes but abundant in capillary ECs. In zebrafish hearts, lpl transcripts were also in capillary ECs but not cardiomyocytes. In both mouse and chicken hearts, LPL was present, as judged by immunogold electron microscopy, in the glycocalyx of capillary ECs. Thus, mammals produce LPL in cardiomyocytes and rely on GPIHBP1 to transport the LPL into capillaries, whereas lower vertebrates produce LPL directly in capillary ECs, rendering an LPL transporter unnecessary. Show less

Apolipoprotein AV (APOA5) deficiency causes hypertriglyceridemia in mice and humans. For years, the cause remained a mystery, but the mechanisms have now come into focus. Here, we review progress in defining APOA5's function in plasma triglyceride metabolism. Biochemical studies revealed that APOA5 binds to the angiopoietin-like protein 3/8 complex (ANGPTL3/8) and suppresses its ability to inhibit the activity of lipoprotein lipase (LPL). Thus, APOA5 deficiency is accompanied by increased ANGPTL3/8 activity and lower levels of LPL activity. APOA5 deficiency also reduces amounts of LPL in capillaries of oxidative tissues (e.g., heart, brown adipose tissue). Cell culture experiments revealed the likely explanation: ANGPTL3/8 detaches LPL from its binding sites on the surface of cells, and that effect is blocked by APOA5. Both the low intracapillary LPL levels and the high plasma triglyceride levels in Apoa5 Show less

Apolipoprotein AV (APOA5) lowers plasma triglyceride (TG) levels by binding to the angiopoietin-like protein 3/8 complex (ANGPTL3/8) and suppressing its capacity to inhibit lipoprotein lipase (LPL) catalytic activity and its ability to detach LPL from binding sites within capillaries. However, the sequences in APOA5 that are required for suppressing ANGPTL3/8 activity have never been defined. A clue to the identity of those sequences was the presence of severe hypertriglyceridemia in two patients harboring an Show less

To support in vivo and in vitro studies of intravascular triglyceride metabolism in mice, we created rat monoclonal antibodies (mAbs) against mouse LPL. Two mAbs, mAbs 23A1 and 31A5, were used to develop a sandwich ELISA for mouse LPL. The detection of mouse LPL by the ELISA was linear in concentrations ranging from 0.31 ng/ml to 20 ng/ml. The sensitivity of the ELISA made it possible to quantify LPL in serum and in both pre-heparin and post-heparin plasma samples (including in grossly lipemic samples). LPL mass and activity levels in the post-heparin plasma were lower in Gpihbp1 Show less

The lipolytic processing of triglyceride-rich lipoproteins (TRLs) by lipoprotein lipase (LPL) is crucial for the delivery of dietary lipids to the heart, skeletal muscle, and adipose tissue. The processing of TRLs by LPL is regulated in a tissue-specific manner by a complex interplay between activators and inhibitors. Angiopoietin-like protein 4 (ANGPTL4) inhibits LPL by reducing its thermal stability and catalyzing the irreversible unfolding of LPL's α/β-hydrolase domain. We previously mapped the ANGPTL4 binding site on LPL and defined the downstream unfolding events resulting in LPL inactivation. The binding of LPL to glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 protects against LPL unfolding. The binding site on LPL for an activating cofactor, apolipoprotein C2 (APOC2), and the mechanisms by which APOC2 activates LPL have been unclear and controversial. Using hydrogen-deuterium exchange/mass spectrometry, we now show that APOC2's C-terminal α-helix binds to regions of LPL surrounding the catalytic pocket. Remarkably, APOC2's binding site on LPL overlaps with that for ANGPTL4, but their effects on LPL conformation are distinct. In contrast to ANGPTL4, APOC2 increases the thermal stability of LPL and protects it from unfolding. Also, the regions of LPL that anchor the lid are stabilized by APOC2 but destabilized by ANGPTL4, providing a plausible explanation for why APOC2 is an activator of LPL, while ANGPTL4 is an inhibitor. Our studies provide fresh insights into the molecular mechanisms by which APOC2 binds and stabilizes LPL-and properties that we suspect are relevant to the conformational gating of LPL's active site. Show less

Lipoprotein lipase (LPL) is secreted into the interstitial spaces by parenchymal cells and then transported into capillaries by GPIHBP1. LPL carries out the lipolytic processing of triglyceride (TG)-rich lipoproteins (TRLs), but the tissue-specific regulation of LPL is incompletely understood. Plasma levels of TG hydrolase activity after heparin injection are often used to draw inferences about intravascular LPL levels, but the validity of these inferences is unclear. Moreover, plasma TG hydrolase activity levels are not helpful for understanding LPL regulation in specific tissues. Here, we sought to elucidate LPL regulation under thermoneutral conditions (30 °C). To pursue this objective, we developed an antibody-based method to quantify (in a direct fashion) LPL levels inside capillaries. At 30 °C, intracapillary LPL levels fell sharply in brown adipose tissue (BAT) but not heart. The reduced intracapillary LPL levels were accompanied by reduced margination of TRLs along capillaries. ANGPTL4 expression in BAT increased fourfold at 30 °C, suggesting a potential explanation for the lower intracapillary LPL levels. Consistent with that idea, Show less

Why apolipoprotein AV (APOA5) deficiency causes hypertriglyceridemia has remained unclear, but we have suspected that the underlying cause is reduced amounts of lipoprotein lipase (LPL) in capillaries. By routine immunohistochemistry, we observed reduced LPL staining of heart and brown adipose tissue (BAT) capillaries in Apoa5-/- mice. Also, after an intravenous injection of LPL-, CD31-, and GPIHBP1-specific mAbs, the binding of LPL Abs to heart and BAT capillaries (relative to CD31 or GPIHBP1 Abs) was reduced in Apoa5-/- mice. LPL levels in the postheparin plasma were also lower in Apoa5-/- mice. We suspected that a recent biochemical observation - that APOA5 binds to the ANGPTL3/8 complex and suppresses its capacity to inhibit LPL catalytic activity - could be related to the low intracapillary LPL levels in Apoa5-/- mice. We showed that an ANGPTL3/8-specific mAb (IBA490) and APOA5 normalized plasma triglyceride (TG) levels and intracapillary LPL levels in Apoa5-/- mice. We also showed that ANGPTL3/8 detached LPL from heparan sulfate proteoglycans and GPIHBP1 on the surface of cells and that the LPL detachment was blocked by IBA490 and APOA5. Our studies explain the hypertriglyceridemia in Apoa5-/- mice and further illuminate the molecular mechanisms that regulate plasma TG metabolism. Show less

Lipoprotein lipase (LPL), the enzyme that carries out the lipolytic processing of triglyceride-rich lipoproteins (TRLs), is synthesized by adipocytes and myocytes and secreted into the interstitial spaces. The LPL is then bound by GPIHBP1, a GPI-anchored protein of endothelial cells (ECs), and transported across ECs to the capillary lumen. The assumption has been that the LPL that is moved into capillaries remains attached to GPIHBP1 and that GPIHBP1 serves as a platform for TRL processing. In the current studies, we examined the validity of that assumption. We found that an LPL-specific monoclonal antibody (mAb), 88B8, which lacks the ability to detect GPIHBP1-bound LPL, binds avidly to LPL within capillaries. We further demonstrated, by confocal microscopy, immunogold electron microscopy, and nanoscale secondary ion mass spectrometry analyses, that the LPL detected by mAb 88B8 is located within the EC glycocalyx, distant from the GPIHBP1 on the EC plasma membrane. The LPL within the glycocalyx mediates the margination of TRLs along capillaries and is active in TRL processing, resulting in the delivery of lipoprotein-derived lipids to immediately adjacent parenchymal cells. Thus, the LPL that GPIHBP1 transports into capillaries can detach and move into the EC glycocalyx, where it functions in the intravascular processing of TRLs. Show less

Lipoprotein lipase (LPL) is the rate-limiting enzyme for intravascular processing of circulating triglyceride-rich lipoproteins (TRLs). One emerging strategy for therapeutic lowering of plasma triglyceride levels aims at increasing the longevity of LPL activity by attenuating its inhibition from angiopoietin-like proteins (ANGPTL) 3, 4 and 8. This mini-review focuses on recent insights into the molecular mechanisms underpinning the regulation of LPL activity in the intravascular unit by ANGPTLs with special emphasis on ANGPTL4. Our knowledge on the molecular interplays between LPL, its endothelial transporter GPIHBP1, and its inhibitor(s) ANGPTL4, ANGPTL3 and ANGPTL8 have advanced considerably in the last 2 years and provides an outlined on how these proteins regulate the activity and compartmentalization of LPL. A decisive determinant instigating this control is the inherent protein instability of LPL at normal body temperature, a property that is reciprocally impacted by the binding of GPIHBP1 and ANGPTLs. Additional layers in this complex LPL regulation is provided by the different modulation of ANGPTL4 and ANGPTL3 activities by ANGPTL8 and the inhibition of ANGPTL3/8 complexes by apolipoprotein A5 (APOA5). Posttranslational regulation of LPL activity in the intravascular space is essential for the differential partitioning of TRLs across tissues and their lipolytic processing in response to nutritional cues. Show less

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. Show less

The complex between lipoprotein lipase (LPL) and its endothelial receptor (GPIHBP1) is responsible for the lipolytic processing of triglyceride-rich lipoproteins (TRLs) along the capillary lumen, a physiologic process that releases lipid nutrients for vital organs such as heart and skeletal muscle. LPL activity is regulated in a tissue-specific manner by endogenous inhibitors (angiopoietin-like [ANGPTL] proteins 3, 4, and 8), but the molecular mechanisms are incompletely understood. ANGPTL4 catalyzes the inactivation of LPL monomers by triggering the irreversible unfolding of LPL's α/β-hydrolase domain. Here, we show that this unfolding is initiated by the binding of ANGPTL4 to sequences near LPL's catalytic site, including β2, β3-α3, and the lid. Using pulse-labeling hydrogen‒deuterium exchange mass spectrometry, we found that ANGPTL4 binding initiates conformational changes that are nucleated on β3-α3 and progress to β5 and β4-α4, ultimately leading to the irreversible unfolding of regions that form LPL's catalytic pocket. LPL unfolding is context dependent and varies with the thermal stability of LPL's α/β-hydrolase domain ( Show less

The binding of lipoprotein lipase (LPL) to GPIHBP1 focuses the intravascular hydrolysis of triglyceride-rich lipoproteins on the surface of capillary endothelial cells. This process provides essential lipid nutrients for vital tissues (e.g., heart, skeletal muscle, and adipose tissue). Deficiencies in either LPL or GPIHBP1 impair triglyceride hydrolysis, resulting in severe hypertriglyceridemia. The activity of LPL in tissues is regulated by angiopoietin-like proteins 3, 4, and 8 (ANGPTL). Dogma has held that these ANGPTLs inactivate LPL by converting LPL homodimers into monomers, rendering them highly susceptible to spontaneous unfolding and loss of enzymatic activity. Here, we show that binding of an LPL-specific monoclonal antibody (5D2) to the tryptophan-rich lipid-binding loop in the carboxyl terminus of LPL prevents homodimer formation and forces LPL into a monomeric state. Of note, 5D2-bound LPL monomers are as stable as LPL homodimers (i.e., they are not more prone to unfolding), but they remain highly susceptible to ANGPTL4-catalyzed unfolding and inactivation. Binding of GPIHBP1 to LPL alone or to 5D2-bound LPL counteracts ANGPTL4-mediated unfolding of LPL. In conclusion, ANGPTL4-mediated inactivation of LPL, accomplished by catalyzing the unfolding of LPL, does not require the conversion of LPL homodimers into monomers. Thus, our findings necessitate changes to long-standing dogma on mechanisms for LPL inactivation by ANGPTL proteins. At the same time, our findings align well with insights into LPL function from the recent crystal structure of the LPL•GPIHBP1 complex. Show less

Angiopoietin-like (ANGPTL) 8 is a secreted inhibitor of LPL, a key enzyme in plasma triglyceride metabolism. It was previously reported that ANGPTL8 requires another member of the ANGPTL family, ANGPTL3, to act on LPL. ANGPTL3, much like ANGPTL4, is a physiologically relevant regulator of LPL activity, which causes irreversible inactivation of the enzyme. Here, we show that ANGPTL8 can form complexes with either ANGPTL3 or ANGPTL4 when the proteins are refolded together from their denatured states. In contrast to the augmented inhibitory effect of the ANGPTL3/ANGPTL8 complex on LPL activity, the ANGPTL4/ANGPTL8 complex is less active compared with ANGPTL4 alone. In our experiments, all three members of the ANGPTL family use the same mechanism to inactivate LPL, which involves dissociation of active dimeric LPL to monomers. This inactivation can be counteracted by the presence of glycosylphosphatidylinositol-anchored HDL binding protein 1, the endothelial LPL transport protein previously known to protect LPL from spontaneous and ANGPTL4-catalyzed inactivation. Our data demonstrate that ANGPTL8 may function as an important metabolic switch, by forming complexes with ANGPTL3, or with ANGPTL4, in order to direct the flow of energy from triglycerides in blood according to the needs of the body. Show less

The intravascular processing of triglyceride-rich lipoproteins depends on lipoprotein lipase (LPL) and GPIHBP1, a membrane protein of endothelial cells that binds LPL within the subendothelial spaces and shuttles it to the capillary lumen. In the absence of GPIHBP1, LPL remains mislocalized within the subendothelial spaces, causing severe hypertriglyceridemia (chylomicronemia). The N-terminal domain of GPIHBP1, an intrinsically disordered region (IDR) rich in acidic residues, is important for stabilizing LPL's catalytic domain against spontaneous and ANGPTL4-catalyzed unfolding. Here, we define several important properties of GPIHBP1's IDR. First, a conserved tyrosine in the middle of the IDR is posttranslationally modified by O-sulfation; this modification increases both the affinity of GPIHBP1-LPL interactions and the ability of GPIHBP1 to protect LPL against ANGPTL4-catalyzed unfolding. Second, the acidic IDR of GPIHBP1 increases the probability of a GPIHBP1-LPL encounter via electrostatic steering, increasing the association rate constant ( Show less